Sorption Properties of Surface Modified Activated

Carbon and Polymer Hydrogels for Environmental

Remediation

By

Safia Hassan

Department of Chemical Engineering

Pakistan Institute of Engineering and Applied Sciences

Islamabad, Pakistan

November 2014

ii

A dissertation submitted to the Pakistan Institute of

Engineering and Applied Sciences in Partial Fulfillment

of the requirement for the degree of

Doctor of Philosophy (PhD) in Chemical Engineering

Department of Chemical Engineering

Pakistan Institute of Engineering and Applied Sciences

Islamabad, Pakistan

(November 2014)

iii

Sorption Properties of Surface Modified Activated Carbon

and Polymer Hydrogels for Environmental Remediation

CERTIFICATE

Certified that the work contained in this dissertation is carried out by Safia Hassan under

our supervision from the Department of Chemical Engineering (DChE), Pakistan Institute

of Engineering and Applied Sciences (PIEAS), Nilore, Islamabad, Pakistan and approved

as to style and content.

_______________________ ________________________

Dr. Tariq Yasin

Supervisor

Department of Metallurgy and

Materials Engineering,

PIEAS, Nilore, Islamabad.

Dr. Muhammad Mansha Chaudhry Co-Supervisor

Department of Chemical

Engineering,

PIEAS, Nilore, Islamabad.

________________________

Dr. Muhammad Tayyeb Javed

Head of Department

Department of Chemical

Engineering,

PIEAS, Nilore, Islamabad.

Declaration

I declare hereby that any work/material present in this thesis is not my own work and

that no material has been submitted previously and approved for the award of a degree

by this or any other university.

Signature:___________________

Author’s Name: Safia Hassan

It is certified that the work in this thesis is carried out and completed under our

supervision

Supervisor:

Dr. Tariq Yasin

Department of Metallurgy and Materials Engineering,

PIEAS, Nilore, Islamabad.

Co-Supervisor:

Dr. Muhammad Mansha Chaudhry Department of Chemical Engineering,

PIEAS, Nilore, Islamabad.

v

DEDICATED

To

The sublime love of

My Parents

Husband

Daughters

Brother Mehmood ul Hassan

And Family

vi

Acknowledgments I bow my head and thank “ALLAH ALMIGHTY” Who has granted me the

strength and audacity to complete this work. Blessings of Allah be upon the Holy

Prophet Muhammad (PBUH) and his honorable progeny, who is the basis of

knowledge and leadership for the whole world.

This research work was not possible without worthy direction and cooperation

extended by my supervisor Dr. Tariq Yasin. His valuable ideas made this work

possible.

I extend my sincere appreciation to my co-supervisor Dr. Muhammad

Mansha Chaudhry and Dr. Naseem Irfan for their kind and nice cooperation during

this work. I want to thank Prof. Laurent Duclaux who provided me a chance to work

at the Polytech’ Savoie, Université de Savoie, Campus de Savoie Technolac, France.

I feel pleasure to acknowledge the kind cooperation and help from all my

colleagues, Dr. Amjad Farooq, Dr. Atif Islam, Syeda Sitwat Batool, Arif kamal,

and Muhammad Arif who gave their valuable suggestions to me. I had creative

discussions with them throughout my research work and this gave me support to

tackle the challenges. I acknowledge especially my brother, Mehmood ul Hassan,

without whose support and kind help it would not have been possible to continue

research work.

No acknowledgement could ever effectively express my obligation to loving

husband Zahid Imran, my cute twin daughters Eshaal Fatima and Nawal Fatima,

brother Imran ul Hassan and sisters Asia Hassan, Salma Hassan and Sadia Hassan

who always stand with me in times of trouble. Their love and cooperation boosted my

moral to achieve my goal.

I extend my humble obligation to my Loving Parents for their love, and wish

to see me glittering high on the skies of achievement.

Finally, I acknowledge the financial support from Higher Education

Commission (HEC), Government of Pakistan and French Embassy, Islamabad for

the financial support for this this research work.

vii

Publications List List of Journal Publications

1. Safia Hassan, Laurent Duclaux, Jean-Marc Lévêque, Laurence Reinert, Amjad

Farooq, Tariq Yasin. Effect of cation type, alkyl chain length, adsorbate size on

adsorption kinetics and isotherms of bromide ionic liquids from aqueous solutions

onto microporous fabric and granulated activated carbons (2014). Journal of

Environmental Management, Vol:144, (1) p. 108–117.

2. Safia Hassan, Tariq Yasin. Synthesis of radiation crosslinked poly (acrylic acid)

in the presence of phenyltriethoxysilane (2014). Radiation Physics and

Chemistry, Vol:97, p. 292.

3. Safia Hassan, A Farooq, M A Ahmad, N Irfan, M Tufail, L Duclaux "Activated

Carbons Employed to Remove Ionic Liquids from Aqueous Solutions" (2011). The

Nucleus, Vol:48(3) p. 237.

4. Safia Hassan, Tariq Yasin. Role of tailored surface of activated carbon for

adsorption of ionic liquids for environmental remediation. International Journal

of Environmental Science and Technology (2014), Doi: 10.1007/s13762-014-

0678-9.

5. Saira Bibi; Tariq Yasin; Safia Hassan; Muhammad Riaz; Mohsin Nawaz,

Chitosan/CNTs Green Nanocomposite Membrane: Synthesis, Swelling and

Polyaromatic Hydrocarbon Removal (2015). Material Science and Engineering

C. Vol: 46 p. 359–365

6. S. S. Batool, Safia Hassan, Z. Imran, M. A. Rafiq, Mushtaq Ahmad, M. M.

Chaudhry, M. M. Hasan, The enhancement in photocatalytic activity of bismuth

modified silica and bismuth silicate nanofibers (2014). Catalysis

Communications Vol:49 (5) p. 39–42.

viii

List of Conference Publications 1. Amjad Farooq, Safia Hassan Muhammad Jibran, Faran Nabeel, Naseem Irfan,

Muhammad Tufail, “Treatment of activated carbon with specific compounds for

environmental safety”, International Conference on Power Generation, Systems

and Renewable Energy Technologies (PGSRET)” November 29 to December 2,

2010, Islamabad.

2. Safia Hassan, Amjad Farooq, Muhammad Awais Ahmad, Naseem Irfan,

Muhammad Tufail, Presented at “Activated Carbons Employed to Remove

Harmful Metals from Aqueous Solutions” “Current Environmental Pollution

Scenario of Pakistan: Findings and Remediation" October 27-29th, 2010,

PINSTECH, Islamabad.

3. Safia Hassan, Amjad Farooq, Laurence Reinert, Jean-Marc Leveque, Naseem

Irfan, Laurent Duclaux " Removal of ionic liquids from aqueous streams onto

microporous activated carbons: effect of surface chemistry and porous structure"

CESEP 2011, Vichy, France (9/25/2011-9/29/2011).

4. Safia Hassan, Amjad Farooq, Laurence Reinert, Jean-Marc Leveque, Naseem

Irfan, Laurent Duclaux "Removal of ionic liquids from aqueous streams onto

microporous activated carbons: Effect of surface chemistry and porous structure"

CARBON 2012, Krakow, Poland (6/17/2012-6/22/2012)

5. Amjad Farooq, Safia Hassan, Laurence Reinert, Jean-Marc Leveque, Naseem

Irfan, Laurent Duclaux "Impregnation of TEDA onto activated carbon at pilot

scale level" CARBON 2012, Krakow, Poland (6/17/2012-6/22/2012).

ix

Abstract In this work, the removal of eight ionic liquids (ILs) of types of bromide based

imidazolium, pyrrolidinium and pyridinium having different alkyl chain lengths, two

dyes (nylosan red N-2RBL, palatine orange) and copper from simulated waste water

were investigated. Three adsorbent systems were used depending upon the type of

adsorbate. These systems include inorganic system based on activated carbon (AC)

and polymer based system including both natural and synthetic polymer. Two types of

activated carbons (ACs), fabric and granulated, were used. The granulated activated

carbon was further modified using nitric acid and sodium hypochlorite as oxidizing

agents, to enhance the oxygenated functional groups on AC. They were well

characterized in terms of surface chemistry by “Boehm” titrations and pH of point of

zero charge measurements and porosity by N2 adsorption at 77 K and CO2 adsorption

at 273 K. Upon modification, AC contained carboxylic, lectonic, and phenolic type’s

functionality. The adsorptions of ILs on these ACs were studied at different

temperatures (25-55 °C) and pH range. Thermodynamic studies indicated that the

adsorption of ILs onto ACs was an exothermic process. Their removal efficiency

increased with increase in alkyl chain length, which was due to the increase in

hydrophobicity of long chain ILs cations determined with the evolution of the

calculated octanol–water constant (Kow) and negative values of free energies indicated

its spontaneous nature.

The 2nd adsorption system used in this work was based on polymer membranes

prepared from chitosan (CS). The chitosan was mixed with two different amount of

poly vinyl alcohol (PVA) and chemically crosslinked by using a new crosslinking

agent i.e. methyltrimethoxysilane (MTMS). The crosslinked CS/PVA membranes

showed hydrogel properties and swelling was decreased with increase in PVA

content. Infrared spectroscopy confirmed the crosslinking reaction between the feed

components and the existence of siloxane bond. The membrane swelling was greatly

affected by pH, ionic strength and temperature of the solution. These membranes

showed high swelling in acidic and low swelling in basic pH range. This switchable

pH response of these membranes was exploited and used to adsorb dyes from aqueous

solution. The effect of dye concentration, contact time, adsorbent amount and pH on

the selectivity and sensitivity of the removal process was investigated. The pH of

solution greatly affected the removal efficiency and maximum adsorption was

x

observed at pH 3. Thermodynamic parameters suggested that the dyes adsorption on

the membrane was spontaneous and the process was endothermic. The effect of time,

pH and salt concentration on swelling were investigated. The high adsorption of dyes

in acidic media is very useful because most of textile effluents in acidic pH range and

the membranes are quite suitable for such type of system. In 3rd system, acrylic acid

was polymerized by gamma radiations in the presence of phenyltriethoxysilane

(PTES). Different amounts of PTES were incorporated in acrylic acid and irradiated

at different doses upto maximum of 30 kGy. The crosslinked poly (acrylic acid)

(PAA) showed hydrogel properties and adsorb maximum of 246 g.g-1 of water. The

increased PTES concentration decreased the EDS of the PAA hydrogels.

Thermogravimetric analysis showed an increase in the stability of the hydrogels

having high PTES content. The swelling of the hydrogel affected by pH, ionic

strength and temperature. These hydrogels showed low swelling in acidic and basic

pH range and high swelling around neutral pH. The adsorption of copper onto these

hydrogels was studied. The pH of solution greatly affected the removal efficiency and

maximum adsorption was achieved at pH 3. The effect of contact time, dye

concentration, adsorbent amount and pH on the selectivity and sensitivity of the

removal process was investigated.

All the data of ionic liquids, dyes and copper metal were analyzed by applying

different kinetics models such as: pseudo-first order and pseudo-second order,

models, diffusion law and Boyd law. The equilibrium adsorption capacities of the

adsorbent for all adsorbates removal were measured and the experimental data was

analyzed by applying adsorption model such as: Langmuir, Freundlich and Langmuir

Freundlich isotherm models.

xi

Table of Contents Acknowledgments......................................................................................................... vi Publications List........................................................................................................... vii Abstract ......................................................................................................................... ix

Table of Contents .......................................................................................................... xi List of Figures .............................................................................................................. xv

List of Tables ............................................................................................................... xx

List of Abbreviations ................................................................................................. xxii Chapter 1 ........................................................................................................................ 1

1. Introduction ........................................................................................................ 1

1.1. Adsorption process .......................................................................................... 2

1.1.1. Adsorption mechanism ............................................................................ 3

1.1.2. Factors effecting on adsorption ................................................................ 5

1.1.3. Adsorption isotherms ............................................................................... 5

1.1.3.1. Type- I Isotherm: .............................................................................. 6

1.1.3.2. Type- II Isotherm .............................................................................. 6

1.1.3.3. Type- III and IV Isotherms ............................................................... 6

1.1.3.4. Type- V Isotherms ............................................................................ 6

1.1.4. Isotherm modeling technique ................................................................... 7

1.1.4.1. Single component adsorption isotherms models .............................. 8

(i) Langmuir model ....................................................................................... 8

(ii) Freundlich adsorption model ................................................................... 9

1.1.4.2. Multicomponent Langmuir Freundlich model ................................. 9

1.1.5. Kinetics modeling .................................................................................. 10

1.1.5.1. Pseudo-first order model ................................................................ 10

1.5.1.2. Pseudo-second order model ............................................................ 11

1.5.1.3. Boyd model..................................................................................... 11

1.5.1.4. Diffusion model .............................................................................. 11

1.2. Adsorbent material .................................................................................... 11

1.2.1. Charcoal ............................................................................................. 12

1.2.1.1. Manufacture .................................................................................... 12

1.2.1.2. Physical and chemical activation of carbon ................................... 13

1.2.1.3. Types of ACs .................................................................................. 14

1.2.1.3.1. Powdered activated carbons (PAC)............................................... 14

1.2.1.3.2. Granulated activated carbons (GAC) ............................................ 15

1.2.1.3.3. Activated carbon fiber (ACF) ....................................................... 15

xii

1.2.1.4. Surface area .................................................................................... 16

1.2.1.5. Modification of ACs surface .......................................................... 16

1.2.1.6. Pore size distribution (PSD) of AC ................................................ 18

1.2.1.7. Application of AC .......................................................................... 19

1.2.2. Polymer material ................................................................................ 21

1.2.2.1. Chitosan .......................................................................................... 21

1.2.2.1.1. Characterization of chitosan .......................................................... 23

1.2.2.2. Acrylic acid ..................................................................................... 24

1.2.2.3. Polymer modification ..................................................................... 25

1.2.2.3.1. Physical modification .................................................................... 25

1.2.2.3.2. Chemical modification .................................................................. 25

1.2.2.3.2.1. Substitution reactions ................................................................. 25

1.2.2.3.2.2. Crosslinking ............................................................................... 26

1.2.2.3.2.3. Graft copolymerization .............................................................. 27

1.2.2.3.3.4. Ionizing radiation-induced polymerization ................................ 27

1.2.2.4. Hydrogel ......................................................................................... 28

1.2.2.4.1 Physical and chemical crosslinking in hydrogels ........................... 29

1.2.2.4.2. Classification of hydrogels ............................................................ 30

1.2.2.4.3. Swelling of hydrogel ..................................................................... 32

1.3. Adsorbates ................................................................................................. 33

1.3.1. Organic adsorbates ............................................................................. 33

1.3.1.1. Ionic iiquids .................................................................................... 33

1.3.1.2. Dyes ................................................................................................ 35

1.3.2. Inorganic adsorbates ........................................................................... 36

1.3.2.1. Metals ............................................................................................. 36

Chapter 2 ...................................................................................................................... 39

2. Experimental and Characterization .................................................................. 39

2.1. Materials and Methods .............................................................................. 39

2.1.1. Chemicals and solvents ...................................................................... 39

2.1.2. Ionic liquids ........................................................................................ 39

2.1.3. Dyes .................................................................................................... 41

2.1.4. Copper metal ...................................................................................... 42

2.1.5. Polymers ............................................................................................. 42

2.1.6. Activated carbons ............................................................................... 42

2.2. Synthesis of adsorbent material ................................................................. 42

2.2.1. General Procedure for CS –PVA hydrogel membranes ..................... 42

2.2.2. General procedure for radiated acrylic acid hydrogel preparation ..... 43

xiii

2.2.3. Activated carbon ................................................................................ 45

2.2.3.1. Chemical modification of ACs ....................................................... 46

2.2.3.1.1. Oxidation with sodium hypochlorite ............................................. 46

2.2.3.1.2. Oxidation with nitric acid .............................................................. 46

2.2.4. Synthesis of ionic Iiquids ................................................................... 46

2.3. Characterization ......................................................................................... 47

2.3.1. Fourier transform infrared spectroscopy ............................................ 47

2.3.2. Thermogravimetric analysis ............................................................... 47

2.3.3. Gel content ......................................................................................... 48

2.3.4. Ash content ......................................................................................... 48

2.3.5. N2-Adsorption–desorption Studies ..................................................... 48

2.3.6. Point of zero charge............................................................................ 49

2.3.7. Titrations ............................................................................................ 49

2.4. Adsorption experiments ............................................................................. 49

2.4.1. Adsorption on ILs on fabric and granulated AC ................................ 49

2.4.2. Adsorption of ILs on modified and raw AC ...................................... 50

2.4.3. Adsorption of metal on PAA hydrogel .............................................. 51

2.4.4. Adsorption of dyes on crosslinked membranes.................................. 52

2.5. Swelling studies ......................................................................................... 53

2.5.1. Swelling in non-buffer, buffer and salt solutions ............................... 53

2.5.2. Water kinetics studies......................................................................... 54

Chapter 3 ...................................................................................................................... 55

3. Results and Discussion .................................................................................... 55

3.1. Adsorption of ionic liquids on fabric (Fab) and granulated AC ................ 55

3.1.1. Characterization ................................................................................. 56

3.1.1.1. Surface chemistry ........................................................................... 56

3.1.1.2. Porosity characterization ................................................................ 57

3.1.2. Kinetics results ................................................................................... 60

3.1.3. Adsorption isotherms ......................................................................... 63

3.1.4. Porosity of the loaded carbons ........................................................... 68

3.1.5. Thermodynamic parameters ............................................................... 71

3.2. Adsorption of ionic liquids on modified AC ............................................. 76

3.2.1. Characterization ................................................................................. 76

3.2.2. Adsorption study ................................................................................ 79

3.2.2.1. Kinetics study ................................................................................. 79

3.2.2.2. Adsorption isotherms ...................................................................... 82

3.2.2.3. Effect of IL type ............................................................................. 84

xiv

3.2.2.4. Effect of pH .................................................................................... 84

3.3. Synthesis and characterization of pH-sensitive silane crosslinked chitosan/poly (vinyl alcohol) membrane ............................................................. 88

3.3.1. Structure analysis ............................................................................... 89

3.3.2. Swelling study .................................................................................... 91

3.3.2.1. Time dependent swelling ................................................................ 91

3.3.2.2. Swelling in salt solutions ................................................................ 93

3.3.2.3. Effect of buffer media on swelling ................................................. 94

3.3.3. Application of crosslinked membranes for adsorption of dyes .......... 95

3.3.3.1. Kinetics study ................................................................................. 95

3.3.3.2. Effect of pH and adsorbent dose ..................................................... 99

3.3.3.3. Adsorption isotherm ..................................................................... 101

3.3.3.4. Thermodynamic parameters ......................................................... 103

3.3.3.5. Comparative study ........................................................................ 105

3.4. Adsorption of copper metal on modified acrylic acid hydrogel with their different properties ............................................................................................. 107

3.4.1. Characterization ............................................................................... 107

3.4.2. Thermogravimetric analysis ............................................................. 108

3.4.3. Gel content ....................................................................................... 109

3.4.4. Swelling studies................................................................................ 110

3.4.4.1. Swelling in water .......................................................................... 110

3.4.4.2. pH Effect ....................................................................................... 112

3.4.4.3. Effect of electrolytes on swelling ................................................. 113

3.4.4.4. Effect of temperature on swelling ................................................ 114

3.4.5. Application of Hydrogel in adsorption of copper metal .................. 116

3.4.5.1. pH effect ....................................................................................... 117

3.4.5.2. Effect of adsorbent dose ............................................................... 119

3.4.5.3. Adsorption kinetics ....................................................................... 119

3.4.5.4. Adsorption isotherms .................................................................... 121

Chapter 4 .................................................................................................................... 126

Conclusions ............................................................................................................ 126

Future Recommendations .......................................................................................... 128

References .................................................................................................................. 129

xv

List of Figures Figure 1.1 Schematic of molecules adsorption steps 4

Figure 1.2 Classification of adsorption isotherms 7

Figure 1.3 Granulated and Powdered activated carbons 15

Figure 1.4 Molecular screening in pores of activated carbons 17

Figure 1.5 The steps involved in oxidation of AC 18

Figure 1.6 Structure of chitin and chitosan 22

Figure 1.7 Applications of chitosan 23

Figure 1.8 Formation of crosslinked hydrogel from polymer 29

Figure 1.9 Physical and chemical hydrogel 30

Figure 1.10 Adsorption of copper on hydrogel 32

Figure 1.11 Various stimuli which affect the swelling of the hydrogel 33

Figure 1.12 Structure of anionic azo dyes used in this study 35

Figure 2.1 Schematic representation of formation of CS/P membranes 43

Figure 2.2 (a)The picture of dried membrane and (b) after dye adsortpion 43

Figure 2.3 Steps involve in synthesis of acrylic acid hydrogel 44

Figure 2.4 (a) Drried acrlic acid hydrogel (b) hydrogel after adsorption of metal

(c) activated carbon cloth

45

Figure 2.5 Steps involve in chemical modificatiob of AC 47

Figure 2.6 Calibration curves (▲) BPyBr, (♦) OMImBr and (■) Bis-DDMImBr 50

Figure 2.7 Calibration curve of (■) OMImCl, (▲) BMImCl and (♦) OPyBr. 51

Figure 2.8 Calibration curve of Copper. 52

Figure 2.9 Calibration curve of (■) NR and (♦) POr 53

Figure 2.10 (a) Dried hydrogel and (b) Swollen hydrogel after adsorption of

water

54

Figure 3.1 Conductivity measurement of granulated (□) and Fab (■) ACs in

distilled water

56

Figure 3.2 CMC measurements of Bis-DDMImBr (□) and DDMImBr (■) 57

Figure 3.3 N2 adsorption (full symbols) /desorption (empty symbols) isotherms

at 77 K for granulated (,) and Fab (,) ACs

58

Figure 3.4 Pore size distribution of (a) granulated AC and (b) Fab AC 59

Figure 3.5 Kinetics of adsorption; on Fab AC using the pseudo second-order (a)

and granulated AC using Pseudo first order model (b) and Linear

62

xvi

fitting of intra particle diffusion model on Fab AC (c) of BMImBr

(), OMImBr (), BPyBr (), OPyBr (), BMPyrrBr (),

OMPyrrBr (), DDMImBr () and Bis-DDMImBr ()

Figure 3.6 Experimental adsorption equilibrium data fitted by Langmuir for the

adsorption isotherms of BMPyrrBr (), BPyBr (), BMImBr (),

Bis-DDMImBr (), OMPyrrBr (), OPyBr (), OMImBr () and

DDMImBr () on Fab AC at 25 °C

65

Figure 3.7 Experimental equilibrium data (dots) and Langmuir fits (solid lines)

for the adsorption isotherms of BMPyrrBr (), BPyBr (), BMImBr

(), Bis-DDMImBr (), OMPyrrBr (), OPyBr (), OMImBr

() and DDMImBr () on granulated AC at 25 °C

66

Figure 3.8 Maximum IL adsorption volumic uptakes on granulated AC (full

symbols) and on fabric AC (empty symbols) versus estimated

volumes of the IL cations for BPyBr (, ), BMImBr (, ),

BMPyrrBr (, ), DDMImBr (, ), OPyBr (), OMImBr (,

), OMPyrrBr (, ), and Bis-DDMImBr (, ). The total

ultramicropore volumes of granulated AC (black continuous line)

and fabric AC (grey continuous line), and the ultramicropore

volumes accessible to the thinner IL cations (thickness of about 0.4

nm) belonging to granulated AC (black dashed line) and fabric AC

(grey dashed line) are indicated for comparison with the volumic

uptakes

67

Figure 3.9 Plot of experimental KD versus log (Kow) for the adsorption on fab

AC (hollow symbols) and granulated (filled symbols) ACs, of

pyridinium (,), methylimidazolium (,) and

methylpyrrolidinium (,) ionic liquids

69

Figure 3.10 (a) Pore size distributions on granulated AC, (b) the same granulated

AC loaded at pH 7 with BPy+, (c) BMPyrr+, (d) OMIm+ and (e)

OMPyrr+

70

Figure 3.11 (a) Pore size distributions on Fab AC, (b) the same Fab AC loaded at

pH 7 with BPy+, and (c) BMPyrr+

71

Figure 3.12 Experimental adsorption equilibrium data fitted by Langmuir for the

adsorption isotherms of BMPYrrBr (,∆,) and OMPYrrBr

72

xvii

(,●,) on Fab (red lines) and granulated (black lines) ACs at 25°C

(solid dots), 40°C (light filled dots) and 55°C (hollow dots)

Figure 3.13 Experimental adsorption equilibrium data fitted by Langmuir for the

adsorption isotherms of Bis-DDMImBr(,∆,), DDMImBr

(,●,) on Fab (red lines) and granulated (black lines) ACs at 25 °C

(solid dots), 40 °C ( light filled dots) and 55 °C (hollow dots)

72

Figure 3.14 Experimental adsorption equilibrium data fitted by Langmuir for the

adsorption isotherms of BPyBr (,, ), on Fab (red lines) and

granulated (black lines) ACs at 25 °C (solid dots), 40 °C (light filled

dots) and 55 °C (hollow dots)

73

Figure 3.15 Adsorption uptake dependence (Kow), at pH = 7, of the enthalpy

changes, ∆H° (kJ/mol) for BMPyrrBr (), BPyBr (), BMImBr

(), Bis-DDMImBr (), OMPyrrBr (), OMImBr ( ) and

DDMImBr ( ) on Fab AC at 25°C (0.4 mmol.g-1)

75

Figure 3.16 Schematic representation of adsorption of ILs onto ACs 76

Figure 3.17 Nitrogen adsorption-desorption isotherm of RAC (●), BAC (■) and

AAC (▲)

78

Figure 3.18 Kinetics curve of experimental data (a), Pseudo-first order model (b)

Pseudo-second-order model (c) and Boyd model (d) for the

adsorption of BMImCl on RAC (■), AAC (●) and BAC (▲) (initial

concentration: 1mmol.L-1; temperature: 25 °C and pH: 7)

80

Figure 3.19 (a) Kinetics curve of experimental data, (b) Pseudo-first order model

(c) Pseudo-second order model and (d) Boyd model for the

adsorption of OMImCl on RAC (■), AAC (●) and BAC (▲) (initial

concentration: 1mmol.L-1; temperature: 25 °C and pH: 7)

81

Figure 3.20 (a) Kinetics curve of experimental data, (b) Pseudo-first order

model (c) Pseudo-second order model and (d) Boyd model for the

adsorption of OPyBr on RAC (■), AAC (●) and BAC (▲) (initial

concentration: 1mmol.L-1; temperature: 25 °C and pH: 7)

82

Figure 3.21 Adsorption of BMImCl (■), OPyBr (●) and OMImCl (▲) onto RAC

(dashed lines), AAC (solid lines) and BAC (dotted lines) at different

pH

83

Figure 3.22 Equilibrium adsorption isotherm using Langmuir isotherm for 85

xviii

OMImCl (▲), OPYBr (●) and BMImCl (■) onto BAC at pH 9

Figure 3.23 adsorption of all ILs on BAC at pH9 (filled blocks), pH7 (dotted

blocks) and pH2 (blank block)

86

Figure 3.24 Difference between equilibrium adsorption uptakes (Qmax) for

BMImCl (■), OPyBr (●) and OMImCl () measured as a function

of the oxygen containing functional groups of activated carbons (at

qmaxpH 9 – qmax pH 2)

86

Figure 3.25 IR spectra of CS, PVA and CS/P05, CS/P25 membranes 90

Figure 3.26 FTIR spectra of (a) dyes and CS/P05 membrane, (b) dyes and

CS/P25 membrane before and after the adsorption of dyes

92

Figure 3.27 (a) Swelling kinetics, (b) ln (F) plotted versus ln (t) for crosslinked

CS/P05 (●) and CS/P25 ■) membranes at room temperature

94

Figure 3.28 Swellings of CS/P05 (●) and CS/P25 (■) membranes (a) in buffer

solution pH (2-10), (b) Swelling in concentrations of NaCl (solid

lines) and CaCl2 (dotted lines) solutions (concentration range 0.05 to

1 M) at room temperature

96

Figure 3.29 (a) Kinetics adsorption data, simulated by pseudo first-order (solid

lines) and pseudo second-order (dotted lines) models, (b) Boyd law

fitted for the adsorption kinetics data of NR(■□) and POr(●○) dyes

onto CS/P05 (solid points) and CS/P25 (blank points) membranes

(dye concentration = 1mmol.L-1, sample weight = 50 mg, pH = 7)

98

Figure 3.30 (a) The effect of adsorbents weight at pH = 7, (b) pH Effect on

adsorption of NR (■□) and POr (●○) on CS/P05 (solid dots) and

CS/P25 (blank dots) membranes (contact time = 24h, dye

concentration = 1mmol.L-1, sample weight = 50 mg)

100

Figure 3.31 Proposed interaction of dye with crosslinked membrane at low pH 101

Figure 3.32 Adsorption of NR (■□) and POr (●○) dyes onto crosslinked CS/P05

(solid dots) membrane fitted by Langmuir model and CS/P25 (blank

dots) fitted by Langmuir-Freundlich law model at temperature 30 °C

and pH7

102

Figure 3.33 Adsorption of NR (■□) and POr (●○) dyes onto crosslinked CS/P05

(solid dots) and CS/P25 (blank dots) membrane at temperature 30 °C

(dashed lines), 45 °C (dotted lines), and 55 °C (solid lines)

104

xix

Figure 3.34 Dependence of adsorption uptake of dyes (qe in mmol.g-1), at pH = 7

of the isosteric Gibbs free energy (solid lines) and of entropy of

adsorption (dotted lines) CS/P05 and CS/P25 membranes.

105

Figure 3.35 FTIR spectra of (a) poly(acrylic acid) and hydrogels, (b) AA40/15,

(c) AA60/15, (d) AA80/15

108

Figure 3.36 Thermogram of AA40/15, AA60/15 and AA80/15 hydrogels 109

Figure 3.37 (a) Swelling kinetics of crosslinked hydrogel in water (b) ln (F)

plotted versus ln (t) for crosslinked hydrogels

111

Figure 3.38 pH Effect on swelling behavior of the crosslinked hydrogel in non-

buffer solution.

113

Figure 3.39 Effect of pH on swelling behavior of the crosslinked hydrogel in

buffer

114

Figure 3.40 Swelling of crosslinked hydrogels in NaCl solution 115

Figure 3.41 Swelling of crosslinked hydrogels in BaCl2 solution 115

Figure 3.42 Swelling of crosslinked hydrogels at different temperature 116

Figure 3.43 FTIR spectra of (a) poly(acrylic acid) and hydrogels, (b) AA40/15,

and (c) AA40/15 after adsorption of Cu metal

117

Figure 3.44 pH effect on the adsorption capacity of Cu metal on PAA hydrogels

(contact time = 24 h, metal concentration = 10 mg.L-1, sample

weight = 50 mg)

118

Figure 3.45 The effect of adsorbent weight on percent removal of metal (contact

time = 24 h, metal concentration = 10 mg.L-1, sample weight = 50

mg, pH = 3, room temperature)

119

Figure 3.46 Kinetics of Cu metal on PAA hydrogels (a) Pseudo first order model

fitting (b) Pseudo second order model fitting (c) Boyd law fitted

(linearly) for the kinetics adsorption data at metal concentration = 10

mg.g-1, sample weight = 100 mg, pH = 7

122

Figure 3.47 Adsorption of metal onto PAA hydrogel at different concentration

fitted by Langmuir law

124

xx

List of Tables Table 1.1 Advantages and disadvantages of removal techniques [11] 3

Table 1.2 IUPAC classification of pore sizes [14] 5

Table 1.3 Raw materials used in the manufacturing of activated carbon [34] 13

Table 1.4 Application of natural and synthetic polymers 30

Table 1.5 The classification and chemical types of dyes [207] 36

Table 1.6 Toxic Effects of dyes and metals 37

Table 2.1 Formula, octanol/water partition coefficients of the ILs and estimated

sizes of the corresponding cations

40

Table 2.2 Properties and structure of dyes 41

Table 2.3 Composition and codes of acrylic acid hydrogel 44

Table 3.1 Boehm titration results, pHPZC and ash contents of granulated and

Fab ACs

56

Table 3.2 Textural properties of Fab and granulated ACs obtained by N2

adsorption/desorption at 77 K

59

Table 3.3 Simulated kinetics results for granulated AC (Pseudo fist order

model) and for Fab AC (Pseudo second order kinetics model)

61

Table 3.4 Comparison of speed of ionic liquids on milled and without milled

ACs (Derivated Formulas Used when (t→0)

63

Table 3.5 Parameter of Langmuir fits of isotherms of adsorption of ILs on the

Fab AC at different temperature

64

Table 3.6 Parameters (rounded values) of Langmuir fits of isotherms of

adsorption for various ILs on the granulated ACs at temperature in

the range 20-55 °C

65

Table 3.7 Textural properties of granulated AC Raw Fab AC before and after

loading of ILs

71

Table 3.8 Thermodynamic para meters of adsorption of the ILs (at constant

value of adsorption uptake (at qe=0.15 mmol.g-1) on different AC

types

74

Table 3.9 Textural properties of RAC, BAC and AAC obtained by N2

adsorption/desorption at 77 K

77

Table 3.10 Boehm Titration results of RAC, BAC and AAC. 78

Table 3.11 The kinetics fitting data of ILs on activated carbons using pseudo- 81

xxi

second order model.

Table 3.12 Adsorption parameters of ILs obtained at different pH using

Langmuir isotherms on ACs

84

Table 3.13 Composition, codes of formulations, gel content and diffusion

parameters of CS/PVA membranes

88

Table 3.14 Kinetics data of NR and POr dyes adsorbed on crosslinked CS/P05

and CS/P25 membranes obtained using pseudo-first order and

pseudo-second order kinetics models at dye concentration =

1mmol.L-1, sample weight = 50 mg, pH = 7

97

Table 3.15 Parameters (rounded values) of Langmuir and Langmuir-Freundlich

fits of isotherms of adsorption for NR and POr on the CS/P05 and

CS/P25 membranes at temperature in the range 25-55 °C

103

Table 3.16 Isosteric Gibbs free energy, enthalpy and entropy of adsorption of

dyes on membranes (at constant value of adsorption uptake)

104

Table 3.17 Comparison of Qmax with published work on the adsorption of dyes

on chitosan based system

106

Table 3.18 Thermal decomposition data of crosslinked hydrogels at various

percentage mass loss.

109

Table 3.19 Gel content and diffusion parameters of crosslinked hydrogels 110

Table 3.20 Kinetics data for the adsorption of Cu on PAA hydrogels obtained

using kinetics models at metal concentration =10 mg.L-1, sample

weight =50 mg, pH = 7

121

Table 3.21 Parameter obtained from Langmuir model for the Cu metal at pH 3 124

xxii

List of Abbreviations 2D-NLDFT Bidimensional Non Local Density Functional Theory Model

AAC Acidified activated carbon

AC Activated carbon

ACs Activated carbons

Ar–OH Phenol functional group

BAC Bleached activated carbon

BaCl2 Barium chloride

BET Brunauer–Emmett–Teller

Bis-DDMImBr Dodecane-diyl-bis(methylimidazolium bromide

BMImBr Methylimidazolium bromide

BMImCl 1-methyl-3-butylimidazolium chloride

BMPyrrBr 1-butyl-1-methylpyrrolidinium bromide

BPyBr 1-butylpyridinium bromide

CaCl2 Calcium chloride

CS Chitosan

Cu Copper

DDMImBr 1-dodecyl-3-methylimidazolium bromide

DFT Density Functional Theory

Fab Fabric

FTIR Fourier transform infrared spectroscopy

HNO3 Nitric acid

mAC Modified activated carbon

MTMS Methyltrimethoxysilane

Na2CO3 Sodium Carbonate

NaCl Sodium chloride

NaHCO3 Sodium Bi-Carbonate

NaOC2H5 Sodium ethoxide

NaOCl Sodium hypochlorite

NR Nylosan Red N-2RBL

OMImBr 1-octyl-3-methylimidazolium bromide

OMImCl 1-methyl-3-octylimidazolium chloride

xxiii

OMPyrrBr 1-octyl-1-methylpyrrolidinium bromide

OPyBr 1-octylpyridinium bromide

pHPZC pH of the point of zero charge

POr Palatine orange

PSD Pore size distribution

PTES Phenyltriethoxysilane

PVA Poly (vinyl alcohol)

RAC Raw granulated Activated Carbon

R–COOH Carboxylic functional group

R–OCO Lactone functional group

Chapter 1 1. Introduction

The clean environment of our planet demands that environmental pollution

should be kept at minimum level. The rapid industrialization has generated a number

of environmental and health issues. The most basic needs of human beings i.e. water

and air is no more pure. The improper disposal of industrial, agricultural and civic

wastes contaminated water resources, threatening both the humans and ecosystems.

Around 400 billion tons of waste is generated every year all over the world and 70

percent of the untreated industrial waste is dumped in the water bodies. As a result,

only 3% of the total water on earth is now fresh water. Therefore, the access of

general public to potable water is limited and water crisis may eventually be a critical

issue in the near future. Keeping in view, the researchers are trying to address this

issue by using multi directional approach to ensure clean water resources.

A large number of organic/inorganic compounds have been produced for the

domestic and industrial use for many years. Most of them are toxic, non-

biodegradable and stable in nature which tend to accumulate in water and living

organism, creating serious problems to both wildlife and human health [1]. These

compounds can enter in the aquatic environment from industrial and agricultural

runoff, municipal effluent and atmospheric sources. Hence, it is important to

reduce/eliminate the concentration of these compounds in the aquatic environment.

Various technologies and methods are applied in the treatment of refining industrial

effluents before their discharge into the water. These methods include membrane

processes, ion exchange and precipitation. However, some of these methods may be

impractical due to economic limitations or may be insufficient to meet regulatory

requirements. Furthermore, they may produce hazardous products which are difficult

to treat [2-4].

The process of adsorption is very vital and intelligently used in many natural

processes including water. The ancient documents specified various methods,

including: heating under the sun and boiling to obtain water for drinking. The text

also recommends water filtering through coarse gravel and sand [5]. The carbon in the

form of charcoal and now in activated carbon is a unique adsorbent materiel showing

enormous capacity for adsorption from liquid and gas phases. These materials have

2

been given special place for producing a clean environment including water

purification as well as purification and separations in many chemical industries.

Other than activated carbon, large numbers of effective, easily available and

cheaper adsorbents are also developed used for the adsorption of harmful and toxic

compounds from waste water. Bailey et al. (1999) mentioned that both natural and

synthetic materials available in large quantities can be used as adsorbents [6]. Many

natural polymers like: chitin and chitosan have the ability to adsorb a great variety of

toxic materials. Their strong affinity as absorbents is explained by the high proportion

of nitrogen sites[7]. A large number of synthetic polymers are developed and tailored

for adsorption of specific compounds [8].

1.1. Adsorption process Adsorption is the process in which a pollutant (adsorbate), in liquid or gas

phase, accumulates on a surface of adsorbent which is used in solid state [9]. The

molecules which bind on the surface are known as adsorbate while the solid material

which holds the adsorbate is known as adsorbent. Adsorption process depends on the

porosity and the surface area of the adsorbent. There are two types of adsorption;

Physical adsorption

Chemical adsorption

In physical adsorption, dipole interaction, hydrogen bonding and the weak van

der Waals forces between adsorbent and adsorbate molecules exist instead of electron

exchange between them. For physical adsorption, no activation energy is required and

in a very short time equilibrium reached. It is reversible and non-specific process [9].

Chemical adsorption is achieved by the chemical link between adsorbent and

adsorbate molecules [9]. Contrary to physical adsorption, it is irreversible as well as

specific. The electronic structure of adsorbents is changed if they undergo chemical

adsorption. If the molecules are bonded by covalent bond then it is called weak

chemical adsorption while in case of ionic bonding the adsorption is called strong

chemical adsorption.

The pollutants ions (organic/inorganic) in the aqueous solution are toxic and

harmful to human beings, animals and the environment. Therefore, these are removed

from the drinking water and wastewater. Many techniques, such as chemical

precipitation, adsorption, ion exchange, electro-dialysis and membrane separation are

3

used to remove them [10]. Table 1.1 shows the general processes of separation and

their application for effluent removal [11].

Adsorption is a widely employed and well-known technique for purification

and separation of effluents and gases. The adsorbents are the important component of

an adsorption process. Thus, good adsorbents should be easy to process, abundant,

should have large surface area, show high selectivity and have long service time [10].

Table 1.1: Advantages and disadvantages of removal techniques [11].

1.1.1. Adsorption mechanism

The adsorbate molecules are adsorbed on the adsorbent by several

mechanisms [12]. In general, adsorption of adsorbates (surfactants, dyes, metal) [13]

takes place by following mechanisms.

1. Ion pairing: The adsorbate ions (from the solution) are adsorbed onto

oppositely charged sites of adsorbent (unoccupied by counter ions).

2. Ion exchange: The adsorbate ions replace the adsorbed counter ions from the

substrate.

3. Hydrophobic bonding: Adsorption occurs between the hydrophobic group of

adsorbate and molecule of adsorbent.

4. Polarization of electrons: The adsorption between adsorbate (electron rich

aromatic nuclei) and positive sites on the adsorbent.

Technique Advantage Disadvantage

Electrodialysis Adsorbate ions removal take place with electric charge

Removal of chelated ions is not satisfactory

Chemical precipitation Effluent metal removed Complex breaks and adjustment of pH is needed

Membrane separation Excellent and easy removal of effluents

High cost in membrane fouling

Ion exchange Easy removal of effluents

Regeneration and fouling

Adsorption Efficient and economical for dilute solution

needed regeneration

4

5. Dispersion forces: Adsorption between adsorbate and adsorbent occurs by

London or Van der Waals force which increases with increasing molecular

weight of adsorbate molecule.

The process of adsorption consists of following four steps and these steps are

also presented in Figure 1.1.

1. Transfer of adsorbate molecules from liquid to the surface of the adsorbent.

2. Diffusion of these molecules from the surface into the active site (inside the

pore) and travel along the pores of the adsorbent.

3. On the interior surfaces of the pores, the adsorbate molecules are adsorbed on

the active sites of the adsorbent.

4. After adsorption, the molecule may travel in the pore surface through surface

diffusion.

Figure 1.1: Schematic of molecules adsorption steps.

5

1.1.2. Factors effecting on adsorption Number of pores, their shape and size play important role in process of

adsorption of any molecules on the materials. The International Union of pure and

Applied Chemistry (IUPAC) defined the range of pore sizes which are presented in

Table 1.2. Micropores and mesopores are considered suitable for adsorption of

effluent.

Table 1.2: IUPAC classification of pore sizes [14].

Pores Pore width (W) nm

Ultramicropores W < 0.7

Supermicropores 0.7 < W< 2

Micropores W < 0.2

Mesopores 2-50

Macropores W > 50

In addition to pore size, specific surface area of adsorbent is another important

property that determines adsorbent capacity. The nature of adsorbate also affect the

adsorption capacity [15]. The type of molecular size, polarity, surface functional

group and weight of the adsorbate also influence the adsorbent-adsorbate interactions.

Moreover, the operating conditions during adsorption process, such as: solution pH,

ionic strength and temperature also affect the adsorbent-adsorbate interaction [16].

1.1.3. Adsorption isotherms The measure of adsorbate adsorbed per unit adsorbent mass as a function of

the adsorbate equilibrium concentration is expressed by adsorption isotherm. The data

required for simulation by different models is derived from experiments. For this

purpose, a specified mass of adsorbent is equilibrated with a specific

concentration and a known volume of a adsorbate. Following mass balance

equation is used to measure the resultant equilibrium concentration of adsorbate [17]:

𝑄𝑒 = (𝐶𝑜−𝐶𝑒 )×𝑉𝑚

(1.1)

6

Where Co is initial liquid-phase concentration of adsorbate (mmolg-1), m is the mass

(g) of adsorbent and V (L) is the solution volume. Adsorption isotherms are

commonly categorized into five types which are shown in Figure 1.2 [18].

1.1.3.1. Type- I Isotherm:

This type of isotherm is used to describe the adsorption by microporous

structure. This type of adsorption can also be termed as Langmuir isotherm and is

used to describe monolayer adsorption.

1.1.3.2. Type- II Isotherm

Multilayer physical adsorptions on macroporous structure are described by

these types of isotherms and are also called sigmoid isotherms. The circled mark at

point B in Figure 1.2 expresses the formation of a monolayer adsorption and

multilayer coverage starts immediately after the point B. Materials with mixed micro-,

meso-porous and macroporous structures follow type II isotherms.

1.1.3.3. Type- III and IV Isotherms

Type III and IV isotherms are found with microporous or mesoporous

adsorbents and at the relative high concentration of adsorbate. These types of

isotherms are convex shaped. These are preferred by weak interactions between

adsorbate/adsorbent systems and strong interaction between the adsorbate molecules,

which may lead to multilayer formation.

1.1.3.4. Type- V Isotherms

These isotherms are frequently used for mesoporous materials. In this case,

the multilayer coverage starts soon after the formation of monolayer. The total pore

volume rules the upper limit of adsorption.

7

Figure 1.2: Classification of adsorption isotherms.

1.1.4. Isotherm modeling technique The actual sorption/desorption processes of pollutants with various

adsorbent phases have been described by a number of developed models [19,20].

Some models are good in theoretical utilities and have only insufficient experimental

effectiveness. Because the assumptions explained in these models, for the

improvement of the adsorbent-adsorbate relationship is only for the limited number of

adsorption processes. Other models are more practical in their derivation and tend to

be more applicable. In the second case, the theoretical basis is ambiguous. Mainly two

types of isotherm models are categorized.

(i) Single component adsorption isotherms models

(ii) Multi-component adsorption isotherms models

8

1.1.4.1. Single component adsorption isotherms models

For modeling of single component adsorption isotherms, Langmuir and

Freundlich equation are mostly used.

(i) Langmuir model

The Langmuir adsorption isotherm model explain the equilibrium between

solid and aqueous phases systems as a reversible equilibrium between systems [21].

Langmuir equations are based on the following three assumptions:

At all sites the adsorption energy is same.

Adsorption of adsorbate is on adsorbent localized sites and there is no physical

interaction between adsorbate molecules.

A monolayer adsorption is take place during adsorption.

In this case, the adsorbent surface is made up of permanent discrete sites and

molecules of organic pollutant may be chemically bounded. This can be explained

mathematically by symbolizing an unoccupied adsorbent surface site as (-A) and the

molecules of adsorbate in solution as species (B), in view of the reaction between the

two to form occupied sites (-AB) and with concentration (C),

(−𝐴) + 𝐵 → (−𝐴𝐵) (1.2)

In case of Langmuir isotherm, it is supposed that the reaction in equation 1.2

has a fixed adsorption free energy. This free energy is independent on the degree of

adsorption and also not affected by interaction among adsorbent sites. Furthermore,

each site is assumed to have the ability of binding as maximum with one adsorbate

molecule. If Qmax is the maximum number of moles (mmolg-1) of a adsorbate per unit

mass of the adsorbent (monolayer adsorption), and Qe is the number of moles of

adsorbed molecules per unit mass adsorbent at equilibrium (mmolg-1), then the law of

mass action states the reaction according to following equation:

𝑘 = [−𝐴𝐵][−𝐴][𝐵]

= � 𝑄𝑒𝑄𝑚𝑎𝑥−𝑄𝑒).𝐶𝑒

� (1.3)

where k is equilibrium constant, and Ce is the equilibrium concentration of

adsorbate in solution (mmolL-1). The rearrangement of equation gives the well-

known Langmuir equation, given as follows:

𝑄𝑒 = 𝑄𝑚𝑎𝑥𝑘𝐶𝑒(1+𝑘𝐶𝑒)

(1.4)

In order to determine the constants in the Langmuir model, two styles of linearization

can be used. In style 1 Ce/Qe is plotted versus Ce, according to equation 1.5, which is

9

used for high concentration. Style 2 is used for low concentration and represented by

plotting 1/Ce versus 1/Qe, according to equation 1.6.

𝐶𝑒𝑄𝑒

= 𝐶𝑒𝑄𝑚𝑎𝑥

+ 1𝑘 𝑄𝑚𝑎𝑥

(1.5)

1𝑄𝑒

= 1𝑄𝑚𝑎𝑥

+ 1𝑘 𝐶𝑒𝑄𝑚𝑎𝑥

(1.6)

(ii) Freundlich adsorption model

The Freundlich adsorption model is a modified form of Langmuir model

because certain adsorption phenomenon cannot be properly explained by Langmuir

model. It shows the relationship between the equilibrium concentration of the

adsorbates in solution and on the surface of adsorbent. This model can work well for

heterogeneous surfaces and multi-site adsorption. For the heterogeneous surfaces, the

energy of adsorption for all surface sites is different. The Freundlich adsorption model

[22] attempts to interpret this assumption as:

𝑄𝑒 = 𝑘1.𝐶𝑒𝑛 (1.7)

where, Ce is the equilibrium concentration in solution (mmolL-1), k1 is an equilibrium

constant indicative of adsorption strength (min-1)and n is the degree of non-linearity

(most often n <1). The linear form of above equation can be expressed as:

log𝑄𝑒 = 𝑙𝑜𝑔𝑘1 + 𝑛. 𝑙𝑜𝑔𝐶𝑒 (1.8)

If log Qe is plotted as a function of log Ce, with an intercept log k1 on the ordinate a

straight line is obtained and slope n are determined.

In addition to these two major models there is a variety of existing

theoretical models and empirical fitting functions used to assess the monocomponent

isotherms.

1.1.4.2. Multicomponent Langmuir Freundlich model

Multicomponent adsorbent systems contain more than one adsorbate

layers. It includes conflict among adsorbates to reside in the limited available

surface of adsorbent and interactions between different adsorbates. There are a

number of models available to predict multicomponent adsorption equilibrium

using data from single component system adsorption isotherms.

Redlich–Peterson isotherm is proposed for multicomponent adsorption [23] as given

by

𝑞𝑒 = 𝑘𝑅𝐶𝑒1+𝑎𝑅𝐶𝑒𝑏

(1.9)

10

where aR (L.mg-1) and kR (L.g-1) are the Redlich–Peterson isotherm constants whereas

b is the exponent ranging between 0 to 1.

The data computed by the isotherm simulations by models is used to calculate

the Gibbs energy from the following equation:

ln(Ca/Ce) = -∆G°/RT (1.10)

where T is the temperature of the solution (K), Ca is the amount of adsorbate adsorbed

on the particular adsorbent from studied solution at equilibrium (mmol.L-1). The

distribution coefficient for the adsorption is equal to Kd = Ca / Ce. Isosteric Gibbs free

energy ∆G° is calculated by using the adsorption equilibrium data for each adsorbate

cation onto adsorbent. Using the equation ∆G° = ∆H° - T∆S°, the isoteric enthalpies

and entropies of adsorption are determined graphically from the linear plot of ln(Kd)

versus 1/T. The isosteric enthalpies (∆H°) are calculated from the slope of this curve,

and the entropies of adsorption (∆S°) are calculated from the y-intercept.

1.1.5. Kinetics modeling The adsorption rate (kinetics), of an adsorption process is another important

parameter. Adsorption kinetics is influenced by mass transfer steps and adsorption

reactions that govern the transfer of adsorbate ions from the bulk of the solution to the

surface of adsorbent and inside adsorbent pores. In turn, these mechanisms depend on

the adsorbent structure, the physical form of the adsorbent (powdered, flakes,

granules, membrane, beads, etc), nature of the solution and adsorbate as well as,

conditions parameters (pH and temperature). Simplified kinetics models can be used

to simulate the experimental data and identify the adsorption mechanism. There are

four kinetics models used to find out the behaviour of adsorption process of

adsorbates on adsorbent: the pseudo-first order [24], the pseudo-second order [25],

and the Boyed Models [26] and the intra particle diffusion [27]

1.1.5.1.Pseudo-first order model

Pseudo-first order model in a liquid/solid system based on solid capacity is

given below

log(𝑞𝑒 − 𝑞𝑡) = 𝑙𝑜𝑔𝑞𝑒 − � 𝑘2.303

� 𝑡 (1.11)

where ‘qe’ and ‘qt’ are the amount of solute adsorbed (mmol.g-1) on adsorbent at

equilibrium and at time ‘t’ respectively, and ‘k’ (min-1) is the rate constant of the

pseudo-first-order adsorption process.

11

1.5.1.2.Pseudo-second order model

Pseudo-second order is expressed as follows: 𝑡𝑞𝑡

= 1𝑘

+ � 1𝑞𝑒� 𝑡 (1.12)

where ‘k’ is the rate constant (mmol.g-1.min-1), ‘qe’ and ‘qt’ (mmol.g-1) are the amount

of adsorbate at equilibrium and at time ‘t’ (min) respectively. k and qe values are

determined from the linear regression of ‘t/qt’ versus ‘t’ obtained by integration of the

equation (1.12).

1.5.1.3.Boyd model

Boyd model has following equation [28]

𝐵𝑡 = − ln �1 − 𝑞𝑡𝑞𝑒� − 0.4977 (1.13)

Where qe and qt are the amounts of adsorbates, adsorbed on the adsorbent (mg.g−1) at

equilibrium time (h) and time t (h) respectively.

1.5.1.4.Diffusion model

In order to find out the main steps at any time t, which govern the overall

adsorption rate of the adsorption process, the diffusion models is applied. This is as

follows:

𝑞𝑡 = 𝑘𝑡0.5 (1.14)

Where k is diffusion rate constant (mg.g-1 .min1/2)

1.2. Adsorbent material The adsorbents are materials with high porosity with pore size distributions

ranging from micropore of width below 20 Å up to mesopores of width from 20 to

500 Å. The adsorbent’s porosity and surface area are a function of its adsorptive

capacity. The main and most important properties of an adsorbent are selectivity,

capacity, and stability over a prolonged period of time. Selectivity depends on the

diffusion rates and equilibrium properties of the system. Pore size distribution and

surface chemical composition polarity are used to determine the ability of the

adsorbent to separate molecules with different characteristics (e.g. shape, size,

polarity etc.)

Many adsorbing agents are used for the adsorption process. These include

inorganic materials such as charcoal, silica, zeolite, alumina etc. and organic materials

such as natural and synthetic polymers etc.

12

1.2.1. Charcoal

Charcoal is the most studied inorganic adsorbent. The charcoals ability to

remove taste and odor is observed centuries ago. In circa 200 BC, the Sanskrit

manuscript describes,” it is good to keep water in copper vessels, to expose it to

sunlight and to filter it through charcoal” [29]. However, the credit goes to Raphael

von Ostrezko of developing commercial activated carbon from charcoal [30].

Charcoal is versatile and unique adsorbents because of its large microporous surface

area, high degree of surface reactivity and high adsorption capacity. Charcoal (porous

material) is used commercially for the removal of gases pollutants and liquids

effluents owing to its large surface area [31,32]. Their important applications are

related to their use in removal of odor, color, taste, and other undesirable impurities

from potable waters. It is also used in the treatment of industrial and domestic

wastewater, solvent recovery, air purification in inhabited spaces (restaurants,

chemical industries, food processing) and in a variety of gas phase applications. It has

been observed that adsorption on charcoal occurs through weak van der walls forces

and is not usually selective [33].

In addition, the surface characterization of the carbon and its porous structure

with respect to the pore size distribution, surface area and chemical composition of

the surface are of vital importance whenever quantitative data are taken for processes

occurring at the carbon surface. Manufacture of activated carbons (AC) from

charcoal, the suitability and selectivity of different raw materials are briefly described

in the following sections.

1.2.1.1. Manufacture

Any cheap material with a low inorganic minerals and high carbon content can

be used as a raw material for production of AC. In early production procedures,

younger fossil materials such as peat, wood, wastes of vegetable origin, sawdust,

nutshells and fruit stones are preferred. The high-quality AC could be activated easily

and produced reasonably from the chars which is obtained from the vegetable origin.

The raw materials used for the manufacturing of AC are described in Table 1.3. The

properties of activated carbons (ACs) in adsorption are essentially attributed to high

degree of surface reactivity, their large surface area, universal adsorption effect and

favorable pore size, which makes the accessible internal surface, enhances mechanical

strength and enhances the adsorption rate [34]. The AC can be manufactured by two

activation processes namely, chemical activation and physical activation processes.

13

Table 1.3: Raw materials used in the manufacturing of AC [34].

Raw material Carbon

(%)

Ash

(%)

Texture of AC Application of AC

Hard wood 40-42 0.3-1.2 Large pore volume,

Soft,

Adsorption in aqueous

phase

Soft wood 40-45 0.3-1.1 Large pore volume, Soft, Adsorption in aqueous

phase

Lignin Nutshells 35-40 0.3-0.4 Large pore volume,

Soft,

Adsorption in aqueous

phase

Petroleum coke 55-70 5-6 Hard, small pore

volume

Wastewater treatment

Lignite Soft coal 40-45 0.5-0.6 Hard, large micropore

volume

Vapor phase

adsorption

Hard coal 70-85 0.5-0.7 Medium hard,

medium pore

volume

Wastewater treatment

Semihard coal

65-85 2-12 Medium hard,

medium

micropore volume

Liquid vapor phase

adsorption

1.2.1.2. Physical and chemical activation of carbon

In the physical activation, the decomposition of the carbonaceous material

(carbonization) takes place. The process is normally carried out below 800 °C

temperature in multiple hearth furnaces or rotary kilns in a continuous stream of an

inert gas. The char produced is further calcinated at 1000 °C in the absence of air.

The important parameters that determine the quality and the yield of the

carbonized product are the rate of heating, final temperature, physical state and the

nature of the raw material [34]. The carbonized product is further activated to enlarge

the pores diameter and to enhance the volume of the pores which are formed during

the process of carbonization and some new porosity is created. This step is generally

carries out in the presence of suitable oxidizing gases such as carbon dioxide, steam,

air or any mixture of these gases between 800 °C and 1100 °C temperatures. In the

14

nature of the raw material and the history of its carbonization, the pores structure and

pore size distribution are largely predetermined.

Chemical activation is usually carried out for wood origin raw material. The

activating agent in the form of concentrated solution is used to impregnate the starting

material by mixing and kneading. Then this material is extruded and pyrolyzed in a

rotary kiln in the absence of air between 400 °C to 600 °C. In order To remove the

activating agent (which is recycled), the pyrolyzed product is then cooled and washed.

After activation, carbon possesses heterogeneous surface with a typical elemental

composition of 0.5% H, 88% C, 1% S, 0.5% N, 6-7% O, and low ash constituents

[35]. The amount of oxygen can be varied from 1-20% depending on additional

treatments, raw material and activation. At corners edges of the graphene sheet, the

heteroatoms occur which behave similarly to the functional groups (commonly found

in aromatic compounds). Depending upon the nature of the activation method, the

properties of AC, such as pore size, surface area and source material are affected [36].

The short lifespan is the disadvantages of this impregnation, in a year it may lose 75%

of its effectiveness and release of the impregnating compounds which causes different

problems [37].

1.2.1.3. Types of ACs

ACs possesses complex structure and is difficult to classify on the basis of

their surface characteristics, behavior and properties. ACs are, therefore, grouped on

the basis of their particle shapes and particle size into granulated, powdered, fabric,

pelleted or spherical ACs.

1.2.1.3.1. Powdered activated carbons (PAC)

PAC have average diameter between 15 to 25 μm with fine granulometry of

less than 100 μm. They present a small diffusion distance and a large external surface

and used preferably for adsorption from solution phase. The problems related to mass

transfer are very low and the adsorption rate is very high. These carbons are generally

prepared from sawdust using chemical activation methods shown in Figure 1.3.

15

Figure 1.3: Granulated and Powdered ACs.

1.2.1.3.2. Granulated activated carbons (GAC)

These carbons are in the form of granules having a relatively larger size of

carbon particles compared to powders carbon and consequently possess smaller

external surface. Typically mesh distribution of a fine AC may be 16×30 and a mesh

size of coarse AC is 4×8. GAC have prime importance where diffusion of the

adsorbate is a primary consideration. They are therefore preferred for adsorption of

vapours and gases as their diffusion rates are faster. It is also used for water

treatment, separation of components of flow system and deodorisation. It can be either

in the extruded form or granular form. Carbons used in effluent removal are

manufactured in granular form, usually between 12×40 and 8×30 mesh in size

because they have a large surface area, good balance of size and suitable head‐loss

characteristics. GAC can also be prepared from variety of carbonaceous materials (as

lignite and bituminous coals) by using the physical activation methods [34] shown in

Figure 1.3.

1.2.1.3.3. Activated carbon fiber (ACF)

ACF are prepared from polymeric (homogeneous) materials such as cellulose,

polyacrylonitrile or phenolic resin [38]. AC fiber provides a number of advantages over

GAC and PAC. Its microporosity is very high, and micropore opens directly to the

external surface of fiber (diameter 5 to 21 Å). Due to this property it is selectively used

for the adsorption of low molecular compounds [39]. It effectively remove containments

from aqueous solution and its adsorption capacity is higher than granulated and powdered

ACs [40]. In addition, ACF is more hydrophobic than other ACs due to the lower ash

contents and higher carbon content [41].

Since ACF are commercially available as fiber cloths, it is convenient to

incorporate these in different treatment systems by immersion them into pipes or tanks or

beaker. However, the manufacturing cost of ACF is high, which is a major barrier and

GAC PAC

16

prevents its application in wastewater treatment. GAC is comparatively cheaper than ACF

[42].

1.2.1.4. Surface area

The AC processes natural sponge like structure, having a large number of

pores in network, extending throughout the material. The wide variety of pores such

as: micropores with diameter smaller than 2 nm, mesopores with diameter between 2

and 50 nm, and macropores having diameter greater than 50 nm. These pores are not

visible without a microscope. Normally, most of adsorption takes place in the pores

having diameters less than 5 nm. The good quality ACs have internal surface areas in

the range of 1000 to 1200 m2/g. In the adsorbent, approximately half the carbon atoms

(with this degree of activation) are lying on the internal surface of the pores and

available to adsorb the adsorbate molecules [43].

Figure 1.4 illustrates the movement of the molecule in the pores. In addition,

there is little molecule capture in the larger pore sizes (macro and transitional). The

adsorbate can be easily removed from these large pores because the vapor pressure in

these pores is very low.

1.2.1.5. Modification of ACs surface

Modification of surface chemistry of AC can be performed physically by heat

treatment or chemically by impregnation or acidic treatment. Acidic treatment

enhances oxygenated groups [44]. Physical modification enhances pore volume,

surface area and oxygenated groups. The surface chemistry of AC can be manipulated

using these techniques with tailored structure for a particular function. The

oxygenated functional groups can be formed through acid treatment with the amount

of oxygen gained on the surface dependent upon the precursor and method used [45-

47].

Wet chemical oxidation uses oxidizing solutions such as nitric acid [48,49],

ozone [50], sodium hypochlorite [51] and hydrogen peroxide [52]. To increase the

total acidity in a wet chemical oxidation, nitric acid is the most widely used [53]. Wet

oxidations generally do not affect other characteristics of surface chemistry such as

pore size distribution [54]. Researchers have found that the oxidation of AC with

nitric acid reduces its total pore volume and BET surface area while increases the pore

width due to its pore collapse [55-57]. Salame is noted that mesopore volume of

modified AC is blocked with ammonium persulfate and concentration nitric acid

17

oxidation [58]. Oxidation with hydrogen peroxide and hypochloriteis is not affected

the pores volume [51,59].

Figure 1.4: Molecular screening in pores of ACs.

Functional groups (lectonic, carboxylic, ketonic, hydroxyl, carbonyl, phenolic

etc.) make the surface of carbon either acidic or basic. While oxygen-containing

functional groups increase acidity and polarity of the surface, nitrogen-containing

surface functional groups are related to the simplicity of the surface [60].

For the adsorption of aromatic molecules, functional oxygen groups play an

important role. Haydar et al. [61] reported that number of carboxylic groups are

increased with the oxidation by HNO3, reducing the p-nitrophenol uptake by

withdrawing electrons from the graphene layers. Mattson et al. [62] suggested that

donor–acceptor complexes are formed and interaction developed between 3 phenol

(acceptors) and electron donors groups. Other researchers explained that there is

formation of hydrogen bond between other oxygenated groups and phenol [63].

The presence of other adsorption sites of AC are also affect the adsorption

process. The textural properties of adsorbent particularly the small micropores

[64,65], nature of adsorption sites [66] and electron-rich graphene layers which

interact through π electrons play important role in adsorption [67]. The common steps

involved` in oxidation are shown in Figure1.5.

18

Figure 1.5: The steps involved in oxidation of AC.

1.2.1.6. Pore size distribution (PSD) of AC

ACs typically exhibit a heterogeneous (micropores, mesopores and

macropores) pore structure in PAC and GAC whereas uniform PSD is present in ACF

[38]. This is one of the important property which influence the process of adsorption.

It determines the total pore volume fraction that can be occupied by an adsorbate

molecule of a given size [68]. For too small pores, it limits the adsorption of adsorbate

molecule of a given shape and size (with decreasing pore size, adsorption strength

increases). As the pore size is decreased, the possibility of contact between the

adsorbent and adsorbate is increased [69,70]. When the pore walls (having opposite

charge) has diameter little more than the adsorbate size, the increase in adsorption

forces between adsorbates and adsorbent take place. Most of the adsorption of

adsorbates takes place within small diameters micropores, since they occupy the

major part of the surface area (internal) of carbon [68]. The micropores adsorption

mechanism is considered to be mainly pore filling (due to overlapping of pore wall

potentials) which strongly bind the adsorbate molecule. Even if there are twenty

different contact points between the adsorbent and adsorbate, larger adsorbate

molecules do not undergo this adsorption phenomenon because micropores are not

Raw

Washing (with HCl)

Washing

Drying Modified AC

Adsorption of IL

Oxidizing agent

19

accessible to large molecules. In this case, molecular sieve ability or adsorption

selectivity can develop in primary small micropores. With the increase of pore size,

the primary micropores selectivity decreases and increase for secondary micropores

[71]. For example, Le Cloirec et al. [72] studied the selectivity of ACF by performing

two adsorption experiments, one with a mixture of phenol and humic substances and

one with phenol alone. Similar isotherm curves are obtained for both experiments,

which showed humic substances are not being removed by ACF. The ACF exhibited

selectivity for the phenol (low molecular weight molecules) compared to humic

substances (macromolecules) due to its high microporosity. The adsorption of

adsorbate occurs only, when average diameter of micropore increased to about 1.7

times adsorbate’s molecule [73]. The adsorption effectiveness of an adsorbent for an

adsorbate is changed, with small change in diameter of micropore of adsorbent [38].

Similarly study of the trichloroethylene (TCE) adsorption by ACs by Dastgheib and

Karanfil [69,74] demonstrated that with increase of pore volume of AC, the

adsorption of TCE is also increased. Similarly for the adsorption of atrazine, the

optimum pore region is found to be 8-20 Å [75].

1.2.1.7. Application of AC

Highly developed porosities and internal surface areas of ACs allow them to

remove various types of chemicals from liquids [76]. They have following

applications in different fields [77].

High purity water production in medical laboratories, hospitals and electronics

manufacturing

Municipal water filtration

Wastewater treatment in industries

Recovery of solvents e.g. recovery of chlorofluorocarbon from foam industry,

gasoline vapor from gasoline loading facilities

Separation of gas mixtures such as removal of H2S from natural gas

Removal of NOX, SOX from flue gases; mercury vapor from hydrogen, air,

methane and other gases

Cigarette filters

Air conditioning systems

Military uses (respirators, gas masks and defense clothing etc.)

Automotive systems evaporation control

20

In liquid-phase applications either for color, odor, or taste removal from a

aqueous solution and recovery of a solute from solution etc.

The main physicochemical characteristics of GAC, PAC, and ACF make them

effective in specific applications. For example, adsorption rates of PAC are faster than

GAC; however, PAC show resistance against flow because of its compactness. Even

though adsorption rate of ACF is high, it has low adsorption capacity for heavy

metals. The possibility of ACF preparation as non-woven mats and woven cloth

provides good applications in small purification water systems for city and in private

houses as a refrigerator deodorizer [78]. ACF and both ACs are used in catalysis

(heterogeneous) because for many reactions, they can act as catalysts and as for

immobilization supports of different catalysts. They act as excellent supports due to

their good resistance to basic as well as acidic media, high thermal stability in

atmosphere (oxygen free), tailorable PSD and high surface area [79].

In wastewater treatment, AC is normally preferred to other adsorbents because

of its non-selectivity, i.e. it has ability to adsorb different types of containments

including dyes, metals, phenols, detergents, ionic liquids, metals and pesticides [80].

However, it is not a low-cost adsorbent. For example, in the domestic and industrial

wastewaters purification, the AC share in adsorption has approximately 26% of the

total cost treatment [81]. It is difficult to regenerate the AC. The most common

method to regenerate is high temperature treatment, which require high energy

consumption, therefore, it is expensive. Besides thermal regeneration, other known

regeneration techniques are: regeneration in solvents like acetone [82], NaOH [83],

and peroxide [84].

In recent years polymeric adsorbent is used alternative to AC. Typically, the

adsorption reaction of adsorbate onto polymeric adsorbents is undergoing exothermic

(with low enthalpy), reaction which suggests that it is physical sorption or transition

between physical and chemical sorption [85,86]. Due to this physical nature of the

sorption forces, the adsorbent regeneration can easily be possible with organic

solvents and bases [87-91]. For large scale industrial operation, usually a large

amount of adsorbent is needed. For this polymers, and its derivatives which are low

cost adsorbent are suitable. The useful features of polymer include its nontoxicity,

abundance, hydro-philicity, biodegradability biocompatibility and anti-bacterial

property [92].

21

1.2.2. Polymer material

Polymer is a macromolecule, having many repeated subunits, called

monomers. Due to their extensive properties, both natural and synthetic polymers play

an unique role in life [1,8]. The natural and synthetic polymers are produced via

polymerization of monomers.

Natural polymers are non-toxic, cheap, abundant and easily available, and

show attractive properties On the basis of their chemical structure, these can be

classified: biological polymers, polysaccharides, polyamides, polythioesters,

polyisoprenoids, polyphenols, inorganic polyesters, and organic polyesters. Larger

structure is formed in the biopolymers by the covalently bonded monomeric units.

They have well defined renewable, biocompatible and biodegradable structure [101].

Polysaccharides are naturally occurring, unique class of polymers that acquire

variety of biofunctional and structural characteristics. They are long chain

carbohydrate molecules made up of many repeated monomeric units with glycosidic

bonds [93]. These also act as promising biomaterials, carry special biological

activities and physiological functions that are helpful in different applications.

Naturally available polysaccharides are cellulose, chitin/chitosan, starch, alginates,

carrageenan, pullulan, dextran and pectin: they are used extensively in food,

agriculture, industrial, pharmaceutical, medical applications and tissue engineering.

Polysaccharides have been widely investigated in new techniques of separation,

extraction, analysis and isolation of effluents. There has been a great attention by the

relationship of function and structure especially in biologically active compounds.

While the advances in functional and structural substances over the last few years

made an increasing number of developments in different application [94]. In

polymers, biopolymers are produced by living organisms Among the different

polysaccharides, the 2nd most abundant biopolymer i.e. chitin has been extensively

studied and used in different fields [95].

The synthetic polymers are categorized as: phenol formaldehyde resin, acrylic

acid, synthetic rubber, nylon, poly(vinylchloride), neoprene, polystyrene,

polyacrylonitrile, polypropylene, silicone, polyethylene and many more.

1.2.2.1. Chitosan

The chitosan (CS) which is derivative of chitin, is a naturally harvested

(copolymer of β-[1→4]-linked 2-acetamido-2-deoxy-D-glucopyranose and 2-amino-

22

2-2-deoxy-D-glucopyranose) and amino functionalized polysaccharides, structure is

shown in Figure 1.6. It is obtained by deacetylation of chitin (crustacean shells)

depending upon the synthesis and technique source [96-101]. When the chitin have

degree of deacetylation greater than 50% , the biopolymer is termed as chitosan

[102].

CS is second largest cationic polysaccharides which is cheap, renewable and

showed high adsorption capacity to pollutants [103]. chitosan as a functional

(biological) polymer offers an interesting characteristics, including non-

biodegradability, toxicity, bioactivity and biocompatibility [104]. Chitosan has ability

to adsorb and/or interact with various compounds, flexibility of the linear chain and

cationic properties which are different and unique from other polysaccharides

[103,105]. It has been widely used for binding toxic metal ions, phenols,

polychlorined biphenyls [106], enzymes [107] and dyes [86]. Chitosan functional

groups develop interactions/links with adsorbent via electrostatic interactions [108].

Figure 1.6: Structure of chitin and chitosan.

CS is cationic in nature and the amino group become protonated in slightly

acidic environment and becomes soluble in different acids such as, HCl, HNO3,

H2SO4 etc. This solubility is the main reason that it is most widely explored polymer

materials in world. The current world wide CS annual production is in the range of

3000-10000 tons [109,110]. CS is soluble in acid environment and its pKa in the

range of 6.2-6.8 gives overall positive charge. The commercially available CS has

molecular weight in the range of 100-1000 kDa [111]. Due to its reactive amino and

hydroxyl groups, it has discrete electrical, chemical, biological and clinical properties

[112,113]. Its biocompatible, biodegradable, film forming ability, fat binding,

bioactivity, good magnetic and electrochemical properties make them more effective

23

in different applications [114,115]. Based on these properties, CS is used in different

applications shown in Figure 1.7. [116-119].

It has mucoadhesive property and connected easily to anionic surface of

bacteria. CS is further crosslinked with different polymer to increase its mechanical

strength and make it stable in acidic environment [120].

Figure 1.7: Applications of CS.

1.2.2.1.1. Characterization of chitosan

CS is existing in the form of semi-crystalline polymer obtained from chitin

after deacetylation [95]. CS has been shown to be biodegradable, biologically non-

antigenic, renewable, biocompatible, biofunctional and non-toxic polysaccharide

[121]. The main parameters for its characterization are its molecular molecular

weight, deacetylation degree (DD) and crystallinity [122]. These parameters may

affect its biological, physico-chemical and conformation in solution [123].

CS is soluble in organic acids and most mineral acids. Protonation of the –

NH2 group occur on the C-2 position of the repeated unit D-glucosamine which cause

solubilisation of CS [95]. The weight of polymer may control the CS solubility. The

molecular weight of CS is determined by viscometry and by GPC [92]. In viscometry,

the molecular weight of polymer may be found by applying the Mark-Houwink

equation as follows:

[ɳ] = 𝐾𝑀𝑋 (1.15)

where [η] is the intrinsic viscosity is (dL,g-1), M is the molecular weight (g/mol) and k

and x are experimental values which can be determined in different solvents.

24

The fraction of free amino groups in the CS is controlled through

deacetylation times that will be available for interactions with organic and inorganic

ions. To evaluate the deacetylation degree, different techniques such as NMR analysis

and Infra-red spectroscopy are the most common methods used. It is obtained by

following equations [124]:

DD = 100 − [A1700A3500

× 1001.33

] (1.16)

The partial arrangement of polymer molecular chains with each other called

polymer crystallization, when these polymer chains form ordered region by folding

together called degree of crystallinity (which typically range from 10-80%).When the

values of degree of crystallinity are higher, then material are brittle and at lower

value, the materials are soft. There are different methods to crystallize a polymer such

as: solvent evaporation, mechanical stretching or cooling from melt. The methods

used to find the degree of crystallinity are: scanning calorimetry, density

measurement, infrared spectroscopy, X-ray diffraction and nuclear magnetic

resonance. The crystallinity can be controlled by the experimental procedure,

preparation and origin of the raw material. Methods used to decrease the crystallinity,

involve the CS dissolution in acid solution and followed by direct freeze-drying of

polymer solution and process of coagulation [109].

Guibal [122] asserted that it is better to consider the total number of amino

groups (in free form) accessible to metal or dye uptake rather than the deacetylation

degree, since some amino groups are also present with hydrogen bonds.

1.2.2.2. Acrylic acid

Acrylic acid is the first effective synthetic biomaterials used in the protecting and

saving human being which are roughly commercialized 40 years ago. From that time,

these materials are used for different environmental and biomedical applications. They

are also used in different biologically devices such as: artificial muscles, to set cardiac

disorders, arteries, stunts, bone scaffolds, biosensors and contact lenses [125-127]. As

biomaterial, it is used in controlled tissue engineering applications [128], drug delivery

systems, environmental [129] wound healing and coatings materials for sensors and

cosmetic surgery [130].

Acrylic acid is an unsaturated organic molecule and polymerized into PAA. It

is usually used to produce homopolymer and copolymers with other monomers. The

cross-linked homopolymer and copolymers of acrylic acid are usually used in the

25

form of hydrogel to adsorb solvent and remove dyes and metal ions from aqueous

solutions [131]. Cavus [132] prepared cross-linked acrylic acid with its copolymer

methacrylamide using crosslinking agents like N,Ǹ- methylenebisacrylamide (MBA)

and N,N,Ǹ,Ǹ-tetramethylethylenediamine (TEMED). Xie [133] prepared acrylic acid

based hydrogel to remove copper and iron ions from aqueous solution.

1.2.2.3. Polymer modification

The chemical nature and structure of polymer gave an opportunity to modify it

by different methods. Both physical and chemical methods are used to get the desired

properties and/or functions. The chemical stability of the polymer has been changed

by different methods to enhance the adsorption efficiency in different fields of

application.

1.2.2.3.1. Physical modification

Different techniques are used to physically modify the polymer, obtaining

conditioned forms such as nanoparticles, powders and gels (membranes, beads,

honeycomb fibers). Several researchers are explained that in the uptake of metal, the

particle size plays a significant role [134].

Guibal (2004) found out that the small particle size is necessary to control the

resistance during intra particle mass transfer. However in some cases (e.g. column

system) there are limitations like hydrodynamic behavior and diffusion properties. In

order to improve these properties, hydrogel, membrane of polymers are used

[122,135].

1.2.2.3.2. Chemical modification

Polymer can be modified chemically to obtain desired properties and the

presence of the functional groups made this process easier. These chemical

modifications affect different properties such as stability, physiochemical properties,

mechanical properties and adsorption efficiency etc. There are two main aims of

chemical modification of polymers: (1) to improve the dyes and metal uptake from

solution and (2) to improve the solubility of polymer in acidic medium or water. This

includes substitution reactions on surface and chain elongation (cross-linking,

polymer networks and graft copolymerization) [136-139].

1.2.2.3.2.1. Substitution reactions

The insertion of pendent groups in polymer are –NH2 or –OH groups to

produce an amphoteric polymer, anionic derivative and cationic derivative of

26

polymer These type of polymer may have higher uptake capacity for metal and dyes

[95].

The Schiff bases which are produced by the modification of polymer may

improve its capacity to interact with dyes and metallic ions. These Schiff bases can be

obtained via reaction of aromatic aldehydes in acetic acid [140], with ketones,

aldehydes, [7] and with salicylaldehyde [141]. Other polymer derivatives produced

by substitution reactions are amphoteric polymer; an anionic derivative, an

anticoagulant; N–methylene phosphonic CS trimethyl CS ammonium and a cationic

derivative [95].

1.2.2.3.2.2. Crosslinking

Crosslinking occurs when the pendent groups or the backbone of polymer

chains form bonds (ionic/covalent/ secondary interactions) between different chains

of polymer. In physically crosslinked gels, the physical interaction between the

polymer chains takes place in order to prevent the dissolution of the polymers chains.

In chemically crosslinked gels, the interlinked bond is covalent between the polymer

chains. Different methods and techniques which are used to crosslink the polymer

chains.

In free radical polymerization mechanism, the initiator such as: ammonium

persulphate ((NH4)2S2O8), potassium persulphate (K2S2O8, KPS)and benzoyl

peroxide (BPO) and azobisisobutyronitrile (AIBN) etc. are used to crosslinked

homopolymers such as poly (vinylpyrrolidone) (PVP), poly (2-hydroxyethyl

methacrylate) (PHEMA), and 2- hydroxyethyl methacrylate (HEMA) and copolymers

of N-vinyl-2-pyrrolidone are synthesized [142].

Crosslinking can also be achieved crosslinking by ionizing radiation. In this

method electron beam or high energy gamma radiation are used in polymerization for

the preparation of hydrogel. During irradiation, free radicals are produced on the

backbone of polymer. These generated radicals react with polymer chains and with

each other and produce crosslinked networks. Ionizing radiations are used to prepare

hydrogels of carboxymethyl CS, carboxymethyl chitin and carboxymethyl cellulose

[143].

The crosslinking through condensation or step-growth polymerization is take

place by removal of small molecules traces which produce through a stepwise

intermolecular reaction. In step-growth of polymerization system, the poly (ethylene

27

glycol) is crosslinked with dithiol in order to synthesize hydrogels (chemically

crosslinked) [144].

Another type of crosslinking uses complementary functional groups such as

COOH, -OH and -NH2, for crosslinking. These groups undergo condensation

reactions through covalent interaction in polymer chains. PVA and CS are crosslinked

by complementary functional groups to form hydrogels [109].

The polymer can be modified by using crosslinking agent such as

tripolyphosphate, glutaraldehyde borate, etc. [119]. In the presence of crosslinker, the

covalent links are developed. There is a need of a steps during manufacturing to

remove the crosslinker (unreacted) which is sometimes difficult [96]. This inherent

toxicity (unreacted crosslinker) limits the crosslinker applications to pharmaceutical

and medical fields.

The possibility of uptake of metal and dyes from acidic medium has motivated the

fabrication of crosslinked polymer. The cross-linking method improves the acidic

stability of CS [145].

1.2.2.3.2.3. Graft copolymerization

The attachment of small molecules or polymerchains onto the CS backbone or

by quaternization of the amino groups called grafting. Polymer derivatives are

obtained by grafting functional groups on the surface, to: (1) increase the uptake

density of the sorption sites, (2) to modify the pH range for dye and metal uptake and

(3) change sorption selectivity site for the target dye and metal [95,122]. The grafted

polymer are insoluble in acidic medium and water and they are thermally stable than

pure polymer [146]. Additionally, the grafting of phosphonic groups onto polymer

enhances the uptake of some metals from aqueous solution.

1.2.2.3.3.4. Ionizing radiation-induced polymerization

Ionizing radiation-induced polymerization is one of the important method and

has many advantages in process products purity, controllability, etc. [147]. This is a

chain reaction in which a single ionization or excitation can produce large number of

chemical changes. There are three separate stages in polymerization of monomers, i.e.

initiation of chain, chain propagation and termination of chain.

These polymerization can be describe as follows:

Initiation:

L(solvent), M(monomer) S●(radical)

Propagation:

28

S● + M SM●

SM●+ M RM2

SMn● + M RMn + 1●

Termination:

SMn● + SMm

● R (product)

In the preparation of superabsorbent polymers (SAP) composite, monomers are

acrylic acid, acrylamide or other species.

At present, hydrophilic crosslinked SAP such as modified acrylic acid and

acrylamides to develop a variety of products for industrial applications [148].

1.2.2.4. Hydrogel

The ability of polymer (natural or synthetic) to make hydrogel is their most

important property. The hydrogel of one polymer with other polymers expanded their

application in different fields. The use of hydrogel in environmental sciences and

medical science is helpful to understand the functions of polymer which makes us to

realize the polymer function in different fields. Polymer has variety of surface

functional groups which are modified for the formation of hydrogel. Polymers are

used to obtain biodegradable and biocompatible hydrogels having low toxicity with

extracellular matrix. The inherent properties of polymer to form film, hydrogel fiber

and membrane make it an important component in the environment and biomaterials.

[46-49].

Generally, the crosslinked hydrophilic polymers are used to make hydrogel.

These hydrogels retain their three dimensional structure in swollen form. They are

soft, fragile, have different degree of crosslinking and adsorb large volume of

solvent[149]. The crosslinked hydrogel is shown in Figure 1.8.

Figure 1.8: Formation of crosslinked hydrogel from polymer.

Polymer

29

1.2.2.4.1 Physical and chemical crosslinking in hydrogels

When the polymer chains are connected with each other through physical

bonds (polyelectrolyte complexation, Van der Waals interactions, ionic

stereocomplexation, and stimulating crystallinity or hydrophobic interactions) then

the crosslinking is known as physical crosslinking. This type of crosslinking is a

common method for hydrogels preparation [150,151]. Mild environment is required

for such kind of interaction and these are reversible. Such types of hydrogels are weak

and loose structure integrity when they interact with external stimuli like temperature,

pH and ionic strength. These are also affected by the polymer concentration, type of

solvent used, and solution temperature. Boucard et al prepared natural (CS) polymer

based physical hydrogel without any crosslinking agent [152].

In chemical crosslinking, polymer chains are connected through covalent

bond. Multifunctional chemical crosslinkers are used in these reactions, followed

either by condensation or free radical mechanism. Chemical crosslinked hydrogel are

also formed by photo irradiation and high energy irradiation [127]. These chemically

crosslinked structure provides excellent chemical, mechanical, surface and thermal

properties. Hydrogels obtained from synthetic or natural polymers are often

chemically crosslinked. Islam et al. have prepared poly (vinyl alcohol) and CS blends

for biomedical applications [109]. Hydrogels crosslinked by chemical method are

mechanically strong and stable compared to physical based hydrogel. Physical and

chemical hydrogels are shown in Figure 1.9.

Physical hydrogel Chemical hydrogel

Figure 1.9: Physical and chemical hydrogels.

OH

OHOHOH

OH OH

CH3 CH3

OH

OHOHOH

OH OH

CH3 CH3

ROH

OHOHOH

O O

CH3 CH3

OH

OOHO

OH OH

CH3 CH3

30

The properties of hydrogels such as superabsorbency, hydrophilicity, selective

permeability, expandability and low interfacial tension and softness can be used in

agriculture, regenerative medicine and biotechnology applications [153-155].

In industry, they are used to uptake the metal and dye from the different

mediums. The cross-linking method improves the chemical stability of the polymer in

different mediums. However, the adsorption capacity of the sorbent may decrease

during this process, especially in chemical reactions with reactive groups [122]. In the

CS, the crosslinked polymer can be obtained by reaction of CS with different

di/polyfunctional reagents such as tripolyphosphate [156] gluteraldehyde [157] and

formaldehyde [158]. There is also a possibility to graft new functional groups on CS

in order to: (a) increase the sorption sites density (b) to increase the sorption

selectivity for the target dye or metal, the change of sorption sites (c) to change the

pH range for dye or metal sorption [122]. The crosslinked polymer is not soluble in

any solvent and is thermally more stable [146].

1.2.2.4.2. Classification of hydrogels

Hydrogels are classified on the basis of type of polymer used their responsive

behavior and charge.

In natural type of hydrogel, natural polymers like polynucleotide, polypeptides

and polysaccharides are used to produce hydrogel. There are different source of these

polymers, such as cellulose, starch, albumin, CS, gelatin and collagen. There are

different applications of these hydrogels as shown in Table1.4.

The synthetic hydrogels are obtained from synthetic polymers like vinyl

polymers acrylic acid. Precursors are used during synthesis of these hydrogel and

properties synthetic hydrogel are dependent on these Precursors. The synthetic

polymer like acrylic acid, poly (vinylalcohol), poly (ethylene glycol), Polyethylene

terephthalate, Poly (vinylpyrrolidone) and Poly (ethylene) Oxide are used for

synthesis of synthetic hydrogel.

The smart or intelligent hydrogels, respond to external stimuli such as pH

change [165], temperature [166], the electrical charges and ionic strength [167]. The

hydrogels behavior to these stimuli show similar behavior like conventional hydrogel

but they show exceptional chemical, electrical, thermal and properties [154,168]. The

external stimuli influence the mechanical strength, swelling activities, permeability

and network structure of these hydrogel, Owing to this responsive behaviour of

hydrogel, these can be used in different applications [Table1.4].

31

Table 1.4: Application of natural and synthetic polymers

Natural

polymer

Synthetic

polymer

Applications References

Chitin/ CS PU drug delivery systems, contact lens,

blood contact devices, many other

biomedical and industrial applications

[103,159,160]

Carrageenan PVA wound dressing and drug delivery,

metal uptake

[103,129]

Dextran PEG pharmaceutical application [103,161]

Gelatin PAAm dye and metal uptake and dental

implants

[103,162]

Collagen PMMA pH sensitive hydrogels Intraocular

lenses

[163,164]

The current sensitive hydrogels show response against the electric current. The

magnitude of the current, interval of the electric pulses and duration are varied to

adjust the electric field [169].

Temperature sensitive hydrogels give response with change of temperature

which changes the polymer-water and polymer-polymer interaction [170]. Examples

of these hydrogels is poly (N-isopropyl acrylamide) (PNIPAM) used in biomedical

applications [171]. The types of these hydrogels include: negative temperature,

thermally reversible and positive temperature hydrogels. Positive and negative

temperature hydrogels swell and de-swell above upper and below lower critical

solution temperature respectively e.g. CS/PVA hydrogel [172]. Poly(N,N-

diethylacrylamide) semi-IPN and kappa-carrageenan-g-poly(methacrylic acid) are the

examples of pH-sensitive and thermo responsive hydrogel [173].

Crosslinked CS and its blends show variable adsorption capacity of dye

because its amino group are protonated/deprotonated at different pH values. Yoshida

used crosslinked CS fibers to adsorb acid orange-II and maximum adsorption of dye

is observed at low pH [174]. The CS-EDGE is used to adsorb Acid Red 37 and Acid

Blue 25 [175]. Many researchers have used crosslinked CS for the adsorption of

heavy metals such as: chromium, lead etc. [176,177]. The adsorption of copper is

shown in Figure 1.10.

32

Figure 1.10: Adsorption of copper on hydrogel.

1.2.2.4.3. Swelling of hydrogel

The equilibrium and swelling kinetics are dependent on different factors such

as chemical nature of polymers, crosslinking ratio, synthesis state and ionic media,

etc. [178,179]. Highly crosslinked hydrogels show lower swelling ratio. Similarly if

hydrogel contains more hydrophilic groups, the swelling is enhanced and vice versa.

There are three steps in swelling of hydrogels: First is the solvent diffusion

into the network of hydrogel, second is the polymer chains relaxation and in third step

hydrogel network expansion. When the dry hydrogel comes in contact with the

solvent, the solvent molecules enter into the free space of the hydrogel. When the

hydrogel attains enough solvent, then the hydrogel expand and that state is called

swelled hydrogel. The solvent enters/leave the hydrogel matrix through the diffusion

process. The solvent diffusion mechanism is investigated by following relation [201].

𝑀 = 𝑘𝑡𝑛 (1.17)

Where, M is fractional uptake (Mt/M∞) at time t, k is constant and characteristic of

the polymer network and n is the exponent of the mechanism of transport. The value

of n (slope) and k (intercept) obtained from the plot of ln F versus ln t. Swelling of

hydrogel is effected by different external stimuli shown in Figure 1.11.

33

Figure 1.11: Various stimuli which affect the swelling of the hydrogel.

1.3. Adsorbates The effluents which are used to adsorb on adsorbent from the solution called

adsorbate. The adsorbates are of different nature, they may be of organic or inorganic

in nature. The details of organic and inorganic adsorbates are given below:

1.3.1. Organic adsorbates

Contaminations of organic chemicals in ground water, pose a serious threat to

on the environment. Their threat severity is due to their toxicity to humans and

animals. Environmental Protection Agency has listed a number of organic compounds

as ground water contaminants. A lot of research work has been carried out on the

removal of aromatic and aliphatic compounds like pentachlorophenol, 4-

chlorophenol, chlorobenzene, carbon tetrachloride, dichlorobenzene,

dichloroethylene, chloroform, dichloromethane and trichloroethylene and other

common organic solvents like toluene, phenols, xylenes, benzene and different dyes

[180]. But limited work has been reported on organic solvents like ionic liquids which

is now one of the important waste water containment.

1.3.1.1. Ionic iiquids

Ionic liquids (ILs) are liquid organic salts at room temperature with very low

vapor pressure. Their tenability, chemical stability, and high solvent capacity are

some important properties [181,182]. Some ILs are called “task-specific ILs” because

they have acquired properties to tune the structure of the ions for a particular

Magnetic field

Thermal field

Ionic Strength

pH

Swollen Hydrogels

Metal, Dye

Electric field

Chemical Environment

Temperature

34

application [183]. They are categorized as green solvents in comparison to the

conventional organic solvents and are used for various applications such as absorption

of SO2 [184] or CO2 [185], catalysis and synthesis [186], nanomaterial [187] and

polymer science etc. [188]. They are used in industrial scale applications [189,190]

like gas separation processes [191,192]. They contain inorganic anions (Br-, Cl-, PF6-,

BF4-, CF3COO- etc.) and organic cations (alkylpyridinium, alkylimidazolium,

alkylphosphonium etc.).

Despite their low vapor pressure ILs are considered an alternative to

conventional molecular organic solvents though they can emit noxious vapors [193].

Due to their low biodegradability some ILs are considered as emerging pollutants that

may be toxic to the environment as they can potentially accumulate in soils

[194,195]. Moreover, if their industrial use increases [196] their solubility in water

will facilitate their release into the environment via liquid effluents [197] which could

damage the aquatic life [198]. It thus appears essential to develop different techniques

for the capture of ILs, in order to prevent environmental pollution. The techniques

normally used for the removal or recovery of ILs are vacuum distillation, nano-

filtration, liquid–liquid extraction, chemical oxidation, biological treatments and

thermal degradation [199-201]. However, some of these methods may be destructive

thus preventing the recovery of the ILs, or may not be suitable at low concentration.

Therefore, there is a need to develop some methods which would be nondestructive

and able to remove low concentration of ILs from wastewater.

For the removal of ILs from water streams, adsorption is an important

technique which now-a-days is widely used. For the treatment of waste streams, AC is

the most common and successful adsorbent [44,66]. The adsorption capacity of an AC

depends on different factors: the surface chemistry (surface functional groups), the

texture (surface area, pore size distribution) and its ash content [34]. Palomar et al.

(2009) reported that the adsorption efficiency of methylimidazolium based ILs onto

AC is affected by the nature of the cation and anion of the IL and the surface of the

adsorbent [202]. They showed that adsorption properties are related both to

hydrophobicities of the anions and the cations, depending on the alkyl chain length

bonded to the methylimidazolium cation [202,203].

35

1.3.1.2. Dyes

Dyes are used to colour the products in textile, paper, plastics, and cosmetic

industries. The discharge from these industries released large amount of effluents

including dyes which are creating serious toxicological and ecological problems.

Approximately, 10,000 different pigments and dyes are used industrially all

over the world [204]. Moreover, the recalcitrant molecules are acting as oxidizing

agents and are also resistant to aerobic digestion. The biodegradability of these dyes is

also difficult due to their stable nature [205]. Synthetic dyes are classified into azo,

triaryl-methane and anthraquinon dyes, and some of them are carcinogenic in nature.

Pollution by dyes in waste water is now alarming, due to their increased use.

Most important class of industrial dyes belong to azo group e.g. basic dyes,

acid dyes, disperse dyes, mordant dyes, direct dyes, solvent dyes and reactive dyes

shown in Table 1.5. The reactive, basic and acid azo dyes are ionic in nature. They are

classified on the bases of chromophore [206]. Structure of some anionic azo dyes used

in this research are shown below [Figure 1.12].

Nylosan Red N-2RBL Palatine Orange

Figure 1.12: Structure of anionic azo dyes used in this study.

36

Table 1.5: The classification and chemical types of dyes [207].

Class Substrate types

Acid dye wool, silk, nylon,

leather and paper

azo, triphenylmethane, anthraquinone, xanthene,

azine, nitroso and nitro

Basic dye polyyacrylonitrile,

Paper, modified

nylon, inks and

polyester

hemicyanine, cyanine,diazahemicynanine,

trialrylmethane, diphenylmethane, azine, azo,

acridine, xanthene, anthraquinone and oxazine

Direct dye rayon,cotton,

paper, nylon and

leather

anthraquinone, azo, nitro styryl, and

benzodifuranone

Disperse dye polyamide, polyester,

acrylic

nitro anthraquinone, azo, benzodifuranone and

styryl

Reactive dye wool, cotton, nylon

and silk

antraquinone, azo, , formazan, basic and

phthalocyanineoxazine Solvent dye gasoline, plastics,

varnishes,

stains, inks and fats,

azo, antraquinone, triphenylmethane, and

phthacyanine

Vat dye rayon, cotton and

wool

antraquinone

1.3.2. Inorganic adsorbates

Toxic inorganic compounds such as metals, not only pollute surface water

sources (ponds, seas, lakes and reservoirs), but also underground water even in trace

amounts after leaching from the soil after snow and melting rain.

1.3.2.1. Metals

The metal ions removal from aqueous solutions, either for metal recovery or

for pollution control, is a main challenge for industries. The metallic element which

has metallic properties at room, temperature is called heavy metal. They are

poisonous/toxic at low concentrations to living things. They are natural components

of Earth's crust but in aquatic environments, their concentrations increase due to

industrial activities, mining and geochemical processes. Toxic metal ions such as

copper, cadmium, mercury, chromium and lead are essential to remove from

37

industrial wastes, as they pose threat to all the living systems and environment. The

main sources of metal ions include pigments, industrial wastes, metallurgical alloying,

electroplating, electronics and battery industry [208,209]. Metal enter water source

by:

Landfills leaching wastes naturally

Contaminated soil percolation

Direct discharge of consumer waste and industrial waste

Acid rain releasing heavy metals into rivers streams, groundwater and lakes

They become toxic for animals when body is not able to metabolized them and

metals accumulate in the body soft tissues. The effects of dyes and some heavy metals

on the human health are shown in Table 1.6.

Copper has gained attention, if a given concentration of copper is exceeded, it

can be harmful [210]. The toxic effects of copper are liver or kidney and

gastrointestinal distress [211]. Because of the presence of copper in wastewater, the

strict regulations have been established by the environmental protection agencies in

order to avoid contaminated water. The World Health Organization has limited the

copper amount in drinking water to 4.0 mg.L-1 [212].

Table 1.6: Toxic Effects of dyes and metals. Constituent Source Effect on Health

Dyes textile dyeing, color photography,

paper printing, pharmaceutical,

cosmetics, food,

carcinogenic

Copper (Cu) forest fires, decaying vegetation, metal

production, mining, phosphate and

fertilizer production

anaemia, kidney and liver

and damage, intestinal and

stomach irritation.

Cadmium

(Cd)

semiconductor and chip resistor accumulate in liver and

kidney, neural damage,

Zinc metal production, mining and soils,

which adsorbed by both plants and

animals and.

skin irritations, stomach

cramps, nausea, vomiting

and anaemia. disturb

the protein metabolism,

damage the pancreas and

respiratory disorders.

38

Objective of Work

The purpose of this study is to clean up waste water using raw and modified organic

and inorganic adsorbents.

Specific Objectives:

To chemically modify different type of AC for adsorption of ionic liquids.

To optimize parameters for the chemically crosslinked membrane (natural

polymer based) for adsorption of harmful compounds and toxic elements in

water.

To optimize crosslinked parameters for development of radiation crosslinked

synthetic polymer for the adsorption of harmful compounds and toxic

elements in water.

39

Chapter 2 2. Experimental and Characterization

This chapter includes the details of the materials, methodology and chemicals

used to modify AC and to develop hydrogel membranes using natural and synthetic

polymers.

2.1. Materials and Methods 2.1.1. Chemicals and solvents

Ethanol, acetic acid, sodium hydroxide, methyltrimethoxysilane (MTMS),

phenyltriethoxysilane (PTES), hydrochloric acid (37%), nitric acid, calcium chloride

(CaCl2), sodium hypochlorite (NaOCl), sodium chloride (NaCl), ether, cyclohexane,

ethyl acetate, nitrogenated heterocycles (1-methylimidazole, pyridine or 1-

methylpyrrolidine) and bromo-alkanes (bromobutane, bromooctane, bromododecane

or di-bromododecane) were purchased from Sigma-Aldrich and were used as

received. Distilled water (conductivity 5.1 μS/cm) and deionized water (1.15uS/cm)

was used for the preparation of materials/solution and for swelling studies and

preparation.

2.1.2. Ionic liquids

The ILs used in this study for adsorption experiment were: 1-butyl-3-

methylimidazolium bromide (BMImBr), 1-octyl-3-methylimidazolium bromide

(OMImBr), 1-dodecyl-3-methylimidazolium bromide (DDMImBr), dodecane-diyl-

bis(methylimidazolium bromide) (Bis-DDMImBr), 1-butyl-1-methylpyrrolidinium

bromide (BMPyrrBr), 1-octyl-1-methylpyrrolidinium bromide (OMPyrrBr), 1-

butylpyridinium bromide (BPyBr), 1-methyl-3-butylimidazolium chloride (BMImCl),

1-methyl-3-octylimidazolium chloride (OMImCl) and 1-octylpyridinium bromide

(OPyBr). Sizes of cations of synthesized ionic liquids were measured from simple

molecular models obtained by using ‘‘Chemsketch 3D Viewer’’ (software to obtain

the atomic coordinates, assuming a parallelepiped shape and atomic radius of 0.1 nm

for the atoms at the extremities of the parallelepiped). The sizes of ILs are mentioned

in Table 2.1.

40

Table 2.1: Formula, octanol/water partition coefficients of the ILs and estimated sizes

of the corresponding cations.

Name

Molecular formula

Designation Chemical

formula

log(Kow) L x W x T*

(Å x Å x Å)

1-butyl-3-methylimidazolium bromide

BMImBr C8H15N2Br -6.1 11.9 × 6.2 × 4

1-methyl-3-octylimidazolium bromide

OMImBr C12H23N2Br -4.5 16.9 × 6.3 × 4

1-dodecyl-3-methylimidazolium bromide

DDMImBr C16H31N2Br -2.9 21.8 × 6.2 × 4.1

Dodecane-diyl bis (methylimidazolium

bromide)

Bis-

DDMImBr

C20H36N4Br2 -11.1 26.6 × 7.5 × 4.2

1-butylpyridinium bromide

BPyBr C9H14NBr -5.6 11.2 × 6.9 × 3.8

1-octylpyridinium bromide

OPyBr C13H22NBr -4.0 16.2 × 6.9 × 3.8

1-butyl-1-methylpyrrolidinium bromide

N+

CH3

CH3

BMPyrrBr C9H20NBr -6.7 10.5 × 7.2 × 7.1

1-octyl-1-methylpyrrolidinium bromide

N+

CH3

CH3

OMPyrrBr C13H28NBr -5.2 14.6 × 9.4 × 6.3

1-methyl-3-octylimidazolium chloride

OMImCl C12H23N2Cl -4.4 14.2× 5.3 × 1.8

1-butyl-3-methylimidazolium chloride

BMImCl C8H15N2Cl -6.0 9.2× 5.4 × 1.9

* Length × Width × Thickness

N+

NCH3

CH3

N+

NCH3

CH3

N+

NCH3

CH3

N+

NCH3

N N+

CH3

N+

CH3

N+

CH3

41

2.1.3. Dyes

Nylosan Red N-2RBL (NR) and Palatine orange (POr are textile acid (anionic

in nature) dyes present in an acidic solution. These dyes are commercially used in

studio dyer to dye animal/protein fibers such as alpaca, silk, wool, angora, mohair and

some synthetics. Tables 2.2 show the properties and structure of dyes.

Table 2.2: Properties and structure of dyes.

NR

Chemical name

Sodium 6-amino-5-[[4-

chloro-3-[[(2,4-

dimethylphenyl) amino]

sulfonyl]phenyl]azo]-4-

hydroxynaphthalene- 2-

sulfonate

C.A.S. number 71873-39-7

Color index C.I. Acid Red 336

Chemical formula C24H21ClN4O6S2Na

Molecular weight (g) 588

Molecular size (nm3) 1.57×1.31×0.63

pH 9-10, 25 °C

λmax (nm) 500

POr

Chemical name Chromate(1-),[3-[2-

[4,5-dihydro-3-methyl-

5-(oxo-kO)-1-phenyl-

1H-pyrazol-4-

yl]diazenyl-kN1]-2-

(hydroxy-kO)-5-

nitrobenzenesulfonato(3

-)]hydroxy-, sodium

C.A.S. number 10127-27-2

Color index Palatine Fast Orange

GEN

Chemical formula C16H11CrN5NaO8S

Molecular weight 508.34

Molecular size (nm3) 1.47×0.71×0.57

pH 8-9,25 °C

λmax (nm) 466

NHOS

O

ONN CH3

CH3

Cl

OH

NH2

SO

OO

Na

S O

O

O

Na

NO

O

N

CrO

N

OH

N

N

CH3

H

42

2.1.4. Copper metal

Copper is an important ingredient for human health and life but like other

heavy metals, it is toxic as well. The copper in the form of copper sulfide was

supplied from Sigma-Aldrich.

2.1.5. Polymers

Chitosan (CS) (bulk density 0.15-0.30 g.(cm3)-1; product number C3646 with

viscosity 200-799 centipoise; Mw: 115000-135000 Dalton; degree of deacetylation

greater than 75 %), poly (vinyl alcohol) (PVA) (98–99 % hydrolyzed; Mw: 146000-

186000) was used in studies. CS was purchased from Sigma-Aldrich.

2.1.6. Activated carbons

Two types of microporous ACs were used; a coal based granular AC

(purchase from China) and a fabric AC (900-20 from Kuraray, Japan). The granulated

carbon was in cylindrical pellets form (radius = 1 mm, length = 3–4 mm). The fabric

(cloth) was formed of woven elemental fibers of about 10 μm diameter (shown in

Figure 2.4 c). The granulated AC was further modified into two forms bleached AC

and acidified AC

2.2. Synthesis of adsorbent material 2.2.1. General Procedure for CS –PVA hydrogel membranes

Two types of CS membranes were prepared. In first type the CS (0.95 g) was

dissolved in 0.5M acetic acid (50 mL) in a glass reactor with continuous stirring at

room temperature. PVA (0.5 g) was dissolved in distilled water at 80 °C. Both

solutions were mixed together at room temperature. Fixed amounts of MTMS (5%)

were added to this mixture under constant stirring. The final mixture was transferred

into plastic petri-dish and dried at room temperature in clean environment. After

drying, the films in the form of membrane were stored in desiccator.

In the second type of membrane formation, CS (0.75 g) was dissolved in

acetic acid (0.5 M) at room temperature with continuous stirring. PVA (0.25 g) was

dissolved in distilled water (called CS/P25 membrane). Both prepared solutions were

mixed together at constant room temperature. Then 5% of MTMS was added to the

prepared mixture under constant stirring. The final mixture of solution was transferred

into petri dish (plastic) and dried in clean environment at room temperature. The dried

membrane was stored in desiccator. The schematic representation of preparation of

43

membranes is shown in Figure 2.1. The dried film (called CS/P05 membrane) after

adsorption of dye is shown in Figure 2.2 (a and b).

Figure 2.1: Schematic representation of formation of CS/P membranes.

Figure 2.2: (a) The picture of dried membrane and (b) after dye adsorption.

2.2.2. General procedure for radiated acrylic acid hydrogel preparation

Acrylic acid was first neutralized with NaOH upto 75%. Neutralized acrylic

acid (100 ml) was taken into flask and appropriate amount of Phenyltriethoxysilane

was added slowly. The samples were irradiated by Co-60 gamma source (Gamma

facility of Pakistan Radiation Services, Model JS-7900,IR-148) with dose rate of 1.05

(a) (b)

44

kGy/h. After irradiation, the resultant hydrogel was washed and dried. The

compositions and codes of synthesized hydrogels are shown in Table 2.3 and steps

involved in synthesis of hydrogel are shown in Figure 2.3. The obtained hydrogel

after drying is shown in Figure 2.4(a) and after adsorption of metal is shown in Figure

2.4(b).

Table 2.3: Composition and codes of acrylic acid hydrogel.

CODE AA40/15 AA60/15 AA80/15 AA40/30 AA60/30 AA80/30

Dose (kGy) 15 15 15 30 30 30

PTES amount* 0.83 1.25 1.65 0.83 1.25 1.65

* µmol/100ml of acrylic acid

Figure 2.3: Steps involved in synthesis of acrylic acid hydrogel.

45

Figure 2.4. (a) Drried acrlic acid hydrogel (b) hydrogel after adsorption of metal (c)

activated carbon cloth

2.2.3. Activated carbon

The granulated carbon was in the shape of cylindrical pellets (radius = 1 mm,

length = 3–4 mm). The fabric was formed of woven elemental fibers of about 10 μm

diameter. ACs with very low conductivities in water solution was selected to enable

accurate measurements of IL solution conductivities. In order to reduce significantly

the granulated AC conductivity in water, prior to adsorption experiments, about 20 g

of AC were treated under reflux with 500 mL of a 5 M HCl solution for 2 days to

remove the metallic impurities. It was then washed with distilled water in a Soxhlet

extractor for at least 2 weeks till the pH of the extracted water was equal to the pH of

the distilled water. The granulated AC was then dried in an oven at 110°C and stored

in desiccator for later use. The conductivities of the solutions obtained from filtration

(a) (b)

(c)

46

of the dispersions (2g.L-1) of HCl treated granulated or fabric ACs in distilled water

(1.5 μS/cm) was about 5.1µS/cm.

2.2.3.1. Chemical modification of ACs

2.2.3.1.1. Oxidation with sodium hypochlorite

In order to introduce oxygen groups on the surface the AC, the granulated AC

was oxidized by sodium hypochlorite (NaOCl) called bleaching of AC. The process

was performed by the slow addition of 400 ml of 10% NaOCl solution to 150g raw

GAC, dispersed in water in three neck round bottom flask. The mixture was stirred for

24 h at room temperature. After oxidation, the suspension was washed first with HCl

for 24 h and then with distilled water of 110000 ml , so that the pH of washed solution

became equal to the pH of distilled water. This bleached AC (BAC) was filtered and

dried in the oven for 24 h at 110°C then stored in an air tight container. The steps

involved in bleaching of AC are shown schematically in Figure 2.5.

2.2.3.1.2. Oxidation with nitric acid

In this type of oxidation, the granulated AC was treated with nitric acid

(HNO3), called acidified AC (AAC). The 90 g of raw granulated AC was treated with

5 M nitric acid (boiling) for 5 h in three necked round bottom flask. The AC was

filtered and washed with distilled water of 7000 ml in Soxhlet extractor, so that the

pH of washed solution becomes equal to the pH of distilled water. Then acidified AC

(AAC) was filtered and dried in oven for 24 h at 120 °C and stored in air tight

container. The steps involved in acidification of AC are shown schematically in

Figure 2.5.

2.2.4. Synthesis of ionic Iiquids

The ionic liquids were synthesized in the laboratory using conventional

synthesis from nitrogen containing heterocycles (imidazole, 1-methylimidazole,

pyridine or 1-methylpyrrolidine) and chloro or bromo-alkanes (cholorooctane,

cholorobutane, bromobutane, bromooctane, bromododecane or di-bromododecane).

The reactions were conducted using magnetic stirring, without solvent (except for the

synthesis of Bis-DDMImBr for which ethyl acetate was added to di-bromododecane

to start the reaction), at room temperature for 24 h. Products were then purified by

successive washing in cyclohexane, ether and ethyl acetate. The final ILs product was

vacuum-dried at 10-3 mbar till constant weight to remove the solvent traces. The

product was stored prior to their use in a desiccator.

47

In parallel, the octanol/water partition coefficient (log(Kow) was computed for all

the cations, from the same software, using the method of reference [213].

Figure 2.5: Steps involved in chemical modification of AC.

2.3. Characterization 2.3.1. Fourier transform infrared spectroscopy

Infrared spectra were recorded using Fourier transform infrared spectroscopy

(FTIR, Nicolet 6700) purchased from Thermo Electron Corporation, USA. The FTIR

was used in ATR mode and diamond crystal was used in ATR assembly. Before

analysis, samples were washed with excess of distilled water and dried under vacuum.

The spectra were scanned from 4000 to 500 cm-1 at 4 cm-1 resolution and averages of

200 scans were reported.

2.3.2. Thermogravimetric analysis

The thermal behavior of the samples was studied using TGA from Mettler

Toledo, (model: TGA/SDTAEN55011) under nitrogen atmosphere (50 mL/min). The

sample (3-5 mg) was heated at a rate of 20 °C/min from 30 °C to 600 °C.

48

2.3.3. Gel content

The gel content of the hydrogels were determined by using ASTM 2765 [214].

The dried hydrogel samples were extracted with water for eight h using Soxhlet

apparatus. After extraction, the samples were dried at room temperature and then in

vacuum dried oven at 70 °C until the weight became constant. The gel content was

determined using the following equation:

Gel content (%) = (Wf / Wi ) × 100 (2.1)

Here ‘Wi’ is the initial weight of dried gel and ‘Wf’ is the weight after extraction.

2.3.4. Ash content

Ash contents in granulated and fabric ACs were determined by combusting 5 g

of material in a muffle furnace at 650 °C for 2 h and weighing the residue after

calcination [215].

2.3.5. N2-Adsorption–desorption Studies

The ACs were characterized by N2 adsorption measurements (ASAP 2020,

Micromeritics) at 77 K. Firstly degassed the carbon samples at 250 °C for 12 h under

vacuum. The specific surface areas of the ACs were measured using the BET equation

(area of one N2 molecule: 0.162 nm2) [216]. The total pore volumes were measured as

the liquid volume of nitrogen, adsorbed at relative pressure of 0.99 torr. The data at

P/P0<0.01 were found using incremental fixed doses of ~10 cm3g−1 (STP). The

interval at equilibration was set up at 300 s. The pore size distribution (PSD) was

determined by using the bidimensional Non Local Density Functional Theory Model

(2D-NLDFT) method applied on the adsorption isotherm assuming a model of finite

slit pores having a diameter-to-width aspect ratio of 6 (pores diameter from 3.5 to 250

Å) [217]. In case of ACs loaded with ILs, the DFT pore size distributions were

measured at same conditions as for raw ACs but the temperature for degassing was

88 °C for 1 day under vacuum. The values of degassing temperature were decreased

to avoid the evaporation of the loaded ILs.

The porosity of the cloth and the granulated ACs were also characterized by

CO2 adsorption at 273 K. The distribution of pores smaller than 0.8 nm (narrow

micropores or ultramicropores) was evaluated from CO2 adsorption isotherms at 273

K. For that, infinite slit pores model was assumed for CO2 adsorption (pores diameter

lower than 0.8 nm) [218]

49

2.3.6. Point of zero charge

The pH of the point of zero charge (pHPZC) of ACs was determined using the

pH drift method [219]. Sample of AC (0.15 g) was placed in 50 mL of 10 mM NaCl

deoxygenated solutions (achieved by N2 bubbling for 1h). The final pH at equilibrium

was measured after stirring the suspensions for 48 h under N2. The pHPZC was then

determined graphically by the values for which initial pH was equal to final pH. The

pHPZC value of a porous carbon was related to the oxygen surface groups’ content.

2.3.7. Titrations

To quantify the acidic and basic surface groups on the ACs, “Boehm”

titrations were performed [35]. The surface functional groups such as basic groups, P-

containing acidic groups, carboxylic (R–COOH) lactone (R–OCO), and phenol (Ar–

OH), quinone or carbonyl were determined. To quantify the surface functional groups,

it was assumed that NaOC2H5 reacted with all groups, NaOH did not react with the

RR’C=O groups, Na2CO3 did not react with R–OH and RR̀ C=O groups and that

NaHCO3 only reacted with P-containing acidic and R–COOH the groups. Typically,

50 mg of AC was poured in 150 mL of a 0.1 M aqueous reactant solution (NaOH or

Na2CO3 or NaHCO3). For NaOC2H5, 0.1g of AC was added in 50 mL of 0.01 M of

absolute ethanol in a closed polyethylene flask. The mixtures were stirred at a

constant speed of 150 rpm at room temperature for 48 h. After that, the suspensions

were filtered through 0.45 μm membrane filters (Durapore-Millipore). Back titrations

of the filtrate (20 mL) were performed with standardized HCl (0.01 M) in order to

determine the oxygen containing group’s content. The contents of basic groups were

also measured by back titration of the filtrate with NaOH (0.01 M) after stirring the

AC (0.15 g) in HCl (50 mL, 0.01 M) for 24 h.

2.4. Adsorption experiments 2.4.1. Adsorption on ILs on fabric and granulated AC

The stock solutions of ILs (BMImBr, OMImBr, DDMImBr, Bis-DDMImBr,

BMPyrrBr, OMPyrrBr and BPyBr), (55 < IL < 125 mM) were prepared by dissolving

a dried IL product in osmosed water (1.51 μS cm-1). Prior to adsorption experiments,

the evolution of the conductivity of each IL was measured in osmosed water versus its

concentration and further used as calibration data. The adsorption experiments were

performed by agitating stoppered flasks containing the adsorbent (0.05 g) and the

adsorbate (25 mL) in a shaking bath of fixed temperature. The concentrations of ILs

50

in the solution (initial and final after adsorption) were determined using conductivity

meter (Radiometer Analytical CDM210) after filtration by the glass fiber filters

(PALL, Type A/E, P/N 61631, pore size: 1 µm). The kinetics was studied on raw

ACs at neutral pH and room temperature for 5 h (initial IL concentration equal to 4

mM). The adsorption isotherms of various ILs on the granulated AC and Fabric AC

were studied at three constant temperatures (25 °C, 40 °C and 55 °C) at pH 7.

The calibration curves for each ILs with known concentrations were found out

using conductivity meter at room temperature and pH 7. The calibration curve of

OMImBr, BPyBr and Bis-DDMimBr is shown in Figure 2.6.

Figure 2.6: Calibration curves (▲) BPyBr, (♦) OMImBr and (■) Bis-DDMImBr.

2.4.2. Adsorption of ILs on modified and raw AC

The adsorption experiments of ILs (BMImCl, OMImCl and OPyBr) were

carried out at 25°C and at pH 7 in 100 ml flask containing AC (50 mg) and ILs (50 ml

in distil water) at 400 rpm. The adsorption of ILs on AC was determined by UV-

Visible spectrometry (Cary50, Varian) at 211 nm for OMImCl and BMImCl at 260

nm for OPyBr. The adsorption kinetics was carried out at pH 7 for 24 h with 1mM

initial concentration of ILs. The adsorption isotherms were carried out at 25 °C using

buffer solutions of pH 9, 7 and 2 and the concentration was varied from 0.2-5mM.

The Langmuir equation was fitted to all the isotherms of ACs. The calibration of each

(mSc

m-1

)

(mmol.L-1)

51

IL on UV UV-Visible spectrometry was done and the calibration curves of IL are

shown in Figure 2.7.

2.4.3. Adsorption of metal on PAA hydrogel

The adsorption equilibrium experiment of copper metal on PAA hydrogels at

room temperature (~35 °C) was carried out in 100 ml flask at agitation rate 50 rpm.

The metal (10 μg.L-1) was prepared in distilled water. Approximately, 50 mg of

hydrogel was placed in 50 ml metal solution and agitated at room temperature for 24

h.

Figure 2.7: Calibration curve of (■) OMImCl, (▲) BMImCl and (♦) OPyBr.

In adsorption kinetics study, the 50 mg of hydrogel was placed in 50 ml of

metal solution (10 ppm) and agitated at 50 rpm at room temperature at 7 pH. The

effect of pH of metal solution on the adsorption of PAA hydrogel was carried out at

constant dye concentration 1 mM by varying pH from 3 to 10. Similarly, the effect of

adsorbent dose was investigated at 10 ppm metal concentration and adsorbent weight

was varied from 20 to 160 mg at pH 3. In adsorption isotherm study, 50 mg of

hydrogel was placed into flask (100 mL) and concentration of metal was varied from

5 to 100 ppm at pH 3 and at room temperature for 24 h in order to achieve

equilibrium.

After adsorption, the solution was centrifuged at 3000 rpm (using EBA20-

Hettich centrifuge) for 5 min and then filtered (Type A/E, PALL, P/N 61631, pore

(mmol.L-1)

52

size: 1 µm). The adsorption of the filtrate was measured by using atomic adsorption

spectrophotometer (Spectra 300+, Varian, Australia). Calibration curve of copper

using atomic absorption spectrophotometer is shown in Figure 2.8.

2.4.4. Adsorption of dyes on crosslinked membranes

Batch equilibrium method was used to study the kinetics adsorption of dyes at

room temperature (~35 °C). The dyes solution having concentration of 1 mM, was

prepared in distilled water. Approximately, 50 mg of the blend was taken, placed in

100 ml flasks then dye solution (50 ml) was added in the flasks and agitated at 50 rpm

rate at room temperature.

Figure 2.8: Calibration curve of Copper.

The adsorption isotherm study of dyes onto crosslinked membranes was

carried out at constant different concentrations (1 to 2 mM) of dye (50 ml) in aqueous

solution and 50 mg of membranes was taken and agitated at 50 rpm. The effect of pH

of solution on the adsorption was studied by varying pH from 3 to 10 at constant dye

concentration (1mM). Similarly, the effect of adsorbent dose was studied at constant

dye concentration (1 mM) and weight of adsorbent was varied from 20 to 100 mg at

pH 3. In isotherm study, 50 mg of membrane was taken into 100 mL flask and dye

concentration was varied from 0.2 to 1.7 mM pH 3 at room temperature having for

24 h.

Concentration (mg/L) (mg.L-1)

R2=0.9989

53

After each adsorption, the solution was centrifuged using EBA20-Hettich

centrifuge at 3000 rpm for 5 min and filtered. The absorbance of the filtrate was

measured by using UV-Vis spectrophotometer (model UV-1201 SHIMADZU) at a

fixed wavelength of 500 nm for NR dye and 466 nm for POr dye. The calibration

curve of both dyes is shown in Figure 2.9.

Figure 2.9: Calibration curve of (■) NR and (♦) POr.

2.5. Swelling studies The swelling response of crosslinked acrylic acid was studied under different

conditions. Sample (50 mg) of uniform size was placed in a beaker filled with

distilled water (100 mL) at given temperature. At fixed time intervals, the weight of

the swollen sample was determined after removing the excess of surface water. After

weighing, the sample was placed again in the same solution and weighed again. The

swelling of the sample was determined gravimetrically by using the following

equation [110]:

Swelling = ( Ws − Wi ) / Wi (2.2)

Here, Wi is the initial weight of the sample and Ws is the swollen weight (g) of the

sample after time t (ºC). The hydrogel before and after swelling is shown in Figure

2.10.

2.5.1. Swelling in non-buffer, buffer and salt solutions

The swelling response of hydrogels against pH is studied in non-buffer and

buffer solutions. Non-buffer solutions were prepared from the dilution of the stock

solution of HCl (0.1 M) and NaOH (0.1 M). Buffer solutions were prepared using

(mmol.L-1)

54

standard method and the pH values were rechecked by pH meter. Hydrogels response

against different salts concentration is investigated. Sodium chloride (NaCl) and

barium chloride (BaCl2) were selected for this study.

Figure 2.10: (a) Dried hydrogel and (b) Swollen hydrogel after adsorption of water.

2.5.2. Water kinetics studies

The mechanism of water diffusion can be measured by using the following

equation:

F=ktn (2.3)

Whereas F is the swelling ratio in fraction at time t (min), n diffusion exponent and

‘k’ is rate constant. The value of n is used to characterized the mechanism of release

and transport [179]. It may be Fickian diffusion (n<0.5), case-II diffusion (n=0.5),

super-case II diffusion (n>1) and non-Fickian diffusion (0.5<n<1) [220,221].

Statistical Analysis

All the reported results are the average of three readings with relative standard

deviation of ± 4.5%.

(a) (b)

55

Chapter 3 3. Results and Discussion

The ability of AC to remove ionic liquids is investigated. The AC is used in

raw and modified form for adsorption of ionic liquids. The AC is modified using

oxidizing agent in order to obtain required surface functional groups once ionic

liquids are considered green solvent but now they are considered as pollutant, because

of their stable nature and low vapour pressure.

In the second part of the work, pH-sensitive CS/PVA membrane are prepared

an crosslinked with crosslinker methyltrimethoxysilane (MTMS) using for the

adoption of dyes.

In the third part of the work, acrylic acid is polymerized with gamma radiation

in the presence of phenyltriethoxysilane for the removal of Copper metal.

This chapter contained the following discussion:

a) Adsorption of ionic liquids on AC.

b) Adsorption of ionic liquid on modified AC with different surface chemistry.

c) Adsorption of dyes on crosslinked CS/PVA membrane with their detail

explanation of properties.

d) Adsorption of copper metal on modified acrylic acid hydrogel.

3.1. Adsorption of ionic liquids on fabric (Fab) and granulated AC The conductivity of Fab AC and HCl treated granulated AC in distilled water

is measured before adsorption experiment, which is negligible 5.1 μS/cm for

granulated AC and 4.0 μS/cm for Fab AC as shown in Figure 3.1.

The ionic liquids DDMImBr and Bis-DDMImBr used in this study have long

alkyl chain as compared to other studied ILs. Their critical micelle concentration (the

concentration of IL above which micelles form and all additional IL added to the

system go to micelles) is measured shown in Figure 3.2 and adsorption experiment is

done below this concentration. The ionic liquids which have long alkyl chain are

commonly form micelle at higher concentration.

56

Figure 3.1: Conductivity measurement of granulated (□) and Fab (■) ACs in distilled

water.

3.1.1. Characterization

3.1.1.1. Surface chemistry

The pHs at point of zero charge (pHPZC) for both the ACs are basic (pHPZC =

8.0 for Fab AC and 8.7 for granulated AC). Both the ACs contain very low amounts

of oxygenated surface groups shown in Table 3.1 and are mainly due to carbonyl

functional groups (0.48 meq.g-1 for CH AC and 0.67 meq.g-1 for the Fab AC).

Granulated AC possesses a number of phenolic groups (~ 0.2 meq.g-1) which are

about ten times higher than Fab AC (0.03 meq.g-1). Ash content of the granulated AC

(0.16%) is very low compared to the one of pristine material (about 10 %) showing

that main part of the metallic impurities are successively removed by the acid

washing. In the case of Fab AC, the ash content is negligible (Table 3.1).

Table 3.1: Boehm titration results, pHPZC and ash contents of granulated and Fab

ACs.

Surface group (meq.g-1)

ACs Carboxylic Lactonic Phenolic Carbonyl Total Basic pHPZC Ash content

(%)

granulated

AC 0.00 0.20 0.20 0.48 0.53 8.70 0.16

Fab AC 0.04 0.13 0.03 0.67 0.43 8.00 0.00

0

1

2

3

4

5

6

7

0 50 100 150 200 250

Cond

uctiv

ity (µ

Scm

-1)

Time (h)

57

Figure 3.2. CMC measurements of Bis-DDMImBr (□) and DDMImBr (■).

3.1.1.2. Porosity characterization

The nitrogen adsorption-desorption isotherms for both ACs are of type I

(Figure 3.3) which is typical of a microporous texture. The determined mesoporous

volumes are negligible (≤ 0.04 cm3.g-1, Table 3.2). The knee is more pronounced for

the Fab AC compared to granulated AC in agreement with higher micropore content

in this carbon material (Table 3.2).

The SBET of both ACs are determined in the relative pressure (range from 0.01

to 0.15). In this case, SBET is rather an indication of the microporous volume than a

true surface measurement. The 1910 m2.g-1 SBET of the Fab AC is higher than the

value measured for the granulated AC, i.e. 1044 m2.g-1. The total microporous volume

is higher for the Fab AC (0.60 cm3.g-1) than for the granulated AC (0.35 cm3.g-1). The

ratio of supermicropore volume with regards to the micropore one is higher for the

granulated AC (57 %) than for the Fab (40 %). Thus the Fab possesses more

ultramiropore (in vol. %) than the granulated AC.

Concentration (m.molL-1)

(m.S

cm-1

)

58

The pore size distributions (PSD) of both ACs obtained by DFT shown in

Figure 3.4 confirm that they are mainly microporous in nature (Pore Size Distribution

from N2 adsorption isotherms at 77 K obtained by bidimensional Non-Local Density

Functional Theory (2D-NLDFT) method based on carbon finite slit pore). This Figure

presents the PSDs for pore widths below 10 nm as negligible incremental pore

volume values lower than 10-3 cm3.g-1 are measured in the (10-25 nm) range. Both

PSD curves show that the Fab and granulated AC contain two types of micropores:

supermicropores (having diameter between 0.8 and 2 nm) and ultramicropores

(having diameter lower than 0.8 nm). The granulated AC possesses few larger pores

(small mesopores) but not the Fab AC (Figure 3.4). The mesoporous volumes (pores

with diameter higher than 2 nm) of both ACs estimated by DFT are negligible (≤ 0.04

cm3.g-1) compared to the microporous ones (Figure 4). The ultramicropore volume of

the Fab AC (0.36 cm3.g-1) determined from the CO2 adsorption at 273 K (Table 2) is

higher than the one of the granulated one (0.15 cm3.g-1).

Figure 3.3: N2 adsorption (full symbols) /desorption (empty symbols) isotherms at 77

K for granulated (,) and Fab (,) ACs.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

100

200

300

400

500

600

Volum

e Ad

sorb

ed (c

m3/

g ST

P)

Relative Pressure (P/Po)

(cm

3 .g-1

)

59

Table 3.2: Textural properties of Fab and granulated ACs obtained by N2

adsorption/desorption at 77 K.

ACs BET

surface

area

(m².g-1)

Micropore

area*

(m2.g-1)

Mesopore

surface

area*(m2.g 1)

Ultramicropore

volume$

(<0.8 nm)

(cm3.g-1)

Supermicropore

volume*

0.8 nm<∅<2 nm

(cm3.g-1)

Mesopore

volume *

(cm³.g-1)

granulated 1044 1088 30 0.15 0.20 0.04

Fab 1910 2180 0 0.36 0.24 0

*: from N2 DFT; $: from CO2 DFT

Figure 3.4: Pore size distribution of (a) granulated AC and (b) Fab AC.

60

3.1.2. Kinetics results

Figure 3.5 shows the comparison of the adsorption kinetics of eight ILs on Fab

AC (Figure 3.5 a) and granulated ACs (Figure 3.5 b). The alkyl chain length grafted

to the methylimidazolium cation (BMImBr, OMImBr and DDMImBr) is found to

have an influence on the adsorption kinetics. Indeed, the plateau for butyl based ILs

() is attained earlier than for longer chain methylimidazolium ILs (, ). The

plateaus of adsorption are obtained at around 60 min for BMImBr and at more than

150 min for both OMImBr and DDMImBr. This might be attributed to slower

diffusion of the large sized ILs through microporous network of the ACs.

This Figure also shows the importance of the cation type on the adsorption

kinetics for various ILs with a butyl chain. The plateau of adsorption is obtained

earlier for BMIm+ and BMPyrr+ (around 60 min) than for BPy+ (~150 min). This

cannot be related to the cation size as the kinetics of BMPyrr+ is one of the quickest,

though this cation size is larger than the one of BMIm+ and BPy+. Thus, the slower

kinetics of BPy+ cation compared to other butyl species might be attributed to its

higher affinity with the carbon matrix which could slow its diffusion within the

microporous network because of the presence of more unsaturated carbon promoting

π stacking interactions.

The modeling study shows that whatever the ionic liquid, the best kinetics

models are quite dependent on carbon type used [222]. This suggests that the kinetics

depend on the structural properties and on the texture of the carbon for all studied ILs,

as each IL possesses different size. The ILs adsorption kinetics on granulated AC are

in good agreement with first order rate equations (R2 ~0.9989); whereas the kinetics

on Fab AC are better reproduced with second-order equation (R2 ~0.9998) as shown

in Table 3.3.

The plot of the adsorption uptake versus t1/2 (Figure 3.5 c) displays that the

beginning of the adsorption kinetics on Fab AC follows a diffusion model irrespective

of IL type. The obtained initial diffusion rate for the Fab AC is not related to the size

of cations but is in agreement with the increased hydrophobicity of the IL cations.

Thus, the initial kinetics (t<10 min) of the Fab AC could be explained by the diffusion

of the ILs molecules towards the external surface (or the more accessible surface) of

the Fab for which the diffusion speed is related to the affinity of each IL towards the

carbon surface. Moreover, the speed of the molecular diffusion [Table 3.4] can be

61

increased by milling the Fab and granulated ACs (more than four times for OMPyrr+

cation) as the access is quicker to the external porosity in agreement with the work of

Lemus et al (2013), who showed that the selection of adequate particle size of the

adsorbent can accelerate the IL adsorption. In case of adsorption on the granulated

AC, the initial kinetics (t<5 min) can also be reproduced by a diffusion law, but no

clear relations are found between the IL type and their diffusion speed [223].

In conclusion, kinetics of IL adsorption occurs in two steps. In first step (few

min) molecules diffuse towards the external surface. This step depends on the affinity

of the IL cation for the surface in relation with the hydrophobicity [Table 2.1]. Second

step is controlled by diffusion of the molecule within the pores and slow down as the

molecule interacted with the surface, i.e. when the IL cation possesses longer chains

and more unsaturated electrons.

Table 3.3: Simulated kinetics results for granulated AC (Pseudo-first order model)

and for Fab AC (Pseudo-second order kinetics model).

ILs

Granulated AC Fab AC

Qmax

(mmol.g-1) R2

Qmax

(mmol.g-1) R2

BMImBr 0.18 0.9899 0.41 0.9889

OMImBr 0.46 0.9898 1.40 0.9899

DDMImBr 0.55 0.9999 1.52 0.9899

Bis-DDMImBr 0.28 0.9898 0.92 0.9898

BMPYrrBr 0.17 0.9899 0.52 0.9888

OMPYrrBr 0.46 0.9885 1.03 0.9889

BPYBr 0.42 0.9885 0.55 0.9889

OPYBr 0.45 0.9887 0.77 0.9899

62

Figure 3.5: Kinetics of adsorption; on Fab AC using the pseudo second-order (a) and

granulated AC using Pseudo first order model (b) and Linear fitting of intra particle

diffusion model on Fab AC (c) of BMImBr (), OMImBr (), BPyBr (), OPyBr

(), BMPyrrBr (), OMPyrrBr (), DDMImBr () and Bis-DDMImBr ().

0 2 4 6 8 10 12 14 16 18 200.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Q e m

mol o

f IL/g

of AC

t1/2 (min)1/2

(c)

63

Table 3.4: Comparison of speed of ionic liquids on milled and without milled ACs

(Derivated Formulas Used when (t→0).

2nd Order, dqt/dt=ab2; 1st model, dqt/dt=ab

3.1.3. Adsorption isotherms

The adsorption isotherm curves of ILs on the two ACs at 25 °C and pH =7 are

of L type 2 according to classification of Giles et al. (1974)[224] (Figures 3.6 and

3.7).

Using a non-linear least squares algorithm, the isotherms are simulated using

the Freundlich, Langmuir, and Langmuir-Freundlich equations. A good agreement is

established between the experimental isotherms onto Fab or granulated ACs and their

Langmuir fitted plots. The parameter of Langmuir obtained after simulated of data by

Langmuir equation is shown in Table (3.5 and 3.6) (R-square values are higher than

0.9798) .

ILs Type of ACs Type of Model R2 Value Speed (ms-1)

BMPyrrBr

Granulated 1st 0.9999 0.05 Granulated-m 0.9999 1.00 Fab 2nd 0.9889 0.87 Fab-m 0.9989 5471.39

OMPyrrBr

Granulated 1st 0.9998 0.01 Granulated-m 0.9888 1.00 Fab 2nd 0.9899 0.05 Fab -m 0.9989 4715.18

BMImBr

Granulated 1st 0.9899 0.03 Granulated-m 0.9889 0.08 Fab 2nd 0.9998 4.04 Fab -m 0.9199 4.04

OMImBr

Granulated 1st 0.9898 0.01 Granulated-m 0.9858 0.50 Fab 2nd 0.9868 0.01 Fab-m 0.9955 1396.16

DDMImBr

Granulated Elovich 0.9898 0.01 Granulated-m 0.9889 0.78 Fab 2nd 0.9966 0.01 Fab-m 0.9858 5.25

Bi-DDMImBr

Granulated 1st 0.9888 0.05 Granulated-m 0.9999 0.16 Fab 2nd 0.9899 0.03 Fab-m 0.9998 34.01

BPYBr

Granulated 1st 0.9898 0.02 Granulated-m 0.9898 0.03 Fab 2nd 0.9888 0.11 Fab m 0.9788 1.72

64

The isotherms show that the Fab AC acts as a good adsorbent for ILs, showing

0.61, 0.70, 0.76, 1.31, 1.32 and 1.35 mmol.g-1 capacities for BMPyrrBr, BMImBr,

BPyBr, Bis-DDMImBr, OMPyrrBr, OMImBr and DDMImBr, respectively (Figure

3.6). The adsorption uptakes of these ILs are higher than the granulated AC (Figure

3.7). This higher value of Fab AC is in agreement with the respective porous volume

values as reported in Table 3.2, as specific surface area and micropore volume. These

figures also show that the adsorption uptake for one cation type increases as its alkyl

chain length increases (from butyl to octyl). These show that the increased

hydrophobicity of the IL cationic molecule also affects the adsorption of ILs

(Table 3.2). However, saturation is observed for chains longer than octyl as the uptake

of DDMImBr is close to OMImBr irrespective of AC. This saturation might be

attributed to increased steric hindrance which blocked the micropores and stopping

further adsorption. Similar steric hindrance is observed in Bis-DDMIm2+ cation, for

which adsorption uptakes is lowered than OMIm+, and DDMIm+, on both ACs.

According to the cation type, the adsorption uptakes of the butyl ILs

derivatives increased in the following sequence BMPyrr+ < BMIm+ < BPy+ for the

Fab AC (Figure 3.6), and BPy+ < BMPyrr+ < BMIm+ for the granulated AC (Figure

3.7). The occurrence of two different sequences on the ACs means that an adsorption

capacity depends not only on the affinities for the carbon surface (controlled by

surface chemistry) but also on the accessible porosities for each organic cation.

Table 3.5: Parameter of Langmuir fits of isotherms of adsorption of ILs on the Fab

AC at different temperature.

ILs 25 °C 40 °C 55 °C

Qmax aK R2 Qmax aK R2 Qmax aK R2

BMImBr 0.70 0.81 0.9899 0.68 0.83 0.9976 0.62 0.87 0.9799

OMImBr 1.32 1.26 0.9986 1.20 1.22 0.9987 1.16 1.22 0.9899

DDMImBr 1.35 1.39 0.9987 1.27 1.36 0.9866 1.18 1.274 0.9987

Bis-DDMImBr 1.28 1.17 0.9877 0.90 0.90 0.9788 0.87 0.90 0.9876

BMPYrrBr 0.61 0.72 0.9999 0.47 0.57 0.9877 0.39 0.50 0.9987

OMPYrrBr 1.31 0.95 0.9888 1.02 0.83 0.9987 1.0 0.60 0.9987

BPYBr 0.76 0.95 0.9899 0.74 0.95 0.9788 0.52 0.65 0.9876

Qmax=mmol. g-1, aK=mmol.g-1

65

Figure 3.6: Experimental adsorption equilibrium data fitted by Langmuir for the

adsorption isotherms of BMPyrrBr (), BPyBr (), BMImBr (), Bis-DDMImBr

(), OMPyrrBr (), OPyBr (), OMImBr () and DDMImBr () on Fab AC at 25

°C.

Table 3.6: Parameters (rounded values) of Langmuir fits of isotherms of adsorption

for various ILs on the granulated ACs at temperature in the range 20-55 °C.

ILs 25 °C 40 °C 55 °C

Qmax bK aK Qmax bK aK Qmax bK aK

BMImBr 0.45 0.57 0.56 0.42 0.44 0.55 0.39 0.27 0.58

OMImBr 0.85 4.63 0.88 0.82 3.13 0.86 0.63 3.39 0.66

DDMImBr 0.89 0.84 0.94 0.87 1.50 0.96 0.80 1.35 0.88

Bis-DDMImBr 0.61 4.05 0.64 0.56 3.01 0.59 0.44 3.47 0.46

BMPYrrBr 0.36 0.59 0.44 0.23 0.51 0.28 0.17 0.66 0.21

OMPYrrBr 0.83 1.13 0.94 0.72 1.31 0.83 0.54 1.72 0.59

BPYBr 0.21 4.69 0.21 0.18 1.75 0.19 0.18 1.27 0.18

OPYBr 0.75 5.68 0.79 0.66 9.87 0.68 0.57 9.34 0.58

Qmax= mmol.g-1, bK =mmol. g-1.min-1, aK= mmol.g-1

0 1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Qe (m

mol

e IL

/g o

f AC)

Ce (mmole/L)

Qe (m

mol

.g-1

)

66

In order to assess the pore sizes of ACs in which ILs preferentially adsorb, the

volumic adsorption capacities at equilibrium are compared with the pore volumes

(Figure 3.8) in a given size range for each AC (Table 2.1). The maximum uptakes of

all ILs, stated in volume (calculated using the sizes of the molecules) are smaller than

the available micropore volumes (0.35 cm3.g-1 for granulated AC and 0.60 cm3.g-1 for

Fab AC), except for OMPyrrBr and DDMImBr (in case of adsorption on Fab). For

these ILs, the available total porous volume appears insufficient for the estimated

volumic uptake probably because of an overestimation of the volume of the IL

cations. Thus Figure 3.8 suggests that except the bulkiest one (OMPyrrBr), the ILs are

adsorbed in the accessible micropores. Figure 3.8 also shows that only the butyl IL

cations (i.e. the smallest in volume) could adsorb into even into smaller slit pores (i.e.

ultramicropores) of the granulated AC and the cations with alkyl length higher than

butyl could accommodate in the whole micropore volume of this AC. In case of the

Fab AC, the bulkiest cations: OMPyrr+, Bis DDMIm2+ and DDMIm+ could

accommodate only in the whole micropore and the others IL cations even in the

ultramicropore.

Figure 3.7: Experimental equilibrium data (dots) and Langmuir fits (solid lines) for

the adsorption isotherms of BMPyrrBr (), BPyBr (), BMImBr (), Bis-

DDMImBr (), OMPyrrBr (), OPyBr (), OMImBr () and DDMImBr () on

granulated AC at 25 °C.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Ce (mmole/L)

Qe (

mm

ole

IL/g

of A

C)

Ce (mmol.L-1)

q e (m

mol

.g-1

)

67

Figure 3.8: Maximum IL adsorption volumic uptakes on granulated AC (full symbols)

and on Fab AC (empty symbols) versus estimated volumes of the IL cations for

BPyBr (, ), BMImBr (, ), BMPyrrBr (, ), DDMImBr (, ), OPyBr (),

OMImBr (, ), OMPyrrBr (, ), and Bis-DDMImBr (, ). The total

ultramicropore volumes of granulated AC (black continuous line) and Fab AC (grey

continuous line), and the ultramicropore volumes accessible to the thinner IL cations

(thickness of about 0.4 nm) belonging to granulated AC (black dashed line) and Fab

AC (grey dashed line) are indicated for comparison with the volumic uptakes.

More precisely, the smallest ILs of thickness of almost 0.4 nm is able to

almost completely penetrate the slit ultramicropore volumes of diameter higher than

0.6 nm and lower than 0.8 nm (taking into account atomic radius of almost 0.1 nm for

the adsorbent). This volume is estimated to 0.13 cm3 g-1 for the Fab AC and to 0.04

cm3 g-1 for the granulated one, from the PSD obtained from CO2 adsorption at 273 K.

This means that the smallest cations (BPy+ and BMIm+) which are adsorbed in lower

volumic uptake (Figure 3.8) are expected to be accommodated mainly in

ultramicropore and slightly supermicropore. For the more voluminous cations of

smallest thickness (OPy+, OMIm+, DDMIm+, and Bis DDMIm2+), while the

accessible ultramicropores are completely filled, adsorption can continue in the

supermicropores.

(cm

3 mol

.g-1

)

68

In case of MPyrr IL derivatives of larger thickness (about 0.6-0.7 nm), their

accommodation in the slit pores might be restricted to the volume fraction higher than

0.8-0.9 nm diameter, the estimated value to 0.30 cm3 g-1 for the Fab AC and 0.24 cm3

g-1 for granulated AC. Hence, theoretically, due to a lack of accessible volume the

ultramicropores modelled by DFT PSD should not be filled by the MPyrr+ cations but

only the supermicropores and mesopores.

Earlier studies have shown that IL adsorption depends on the surface

chemistry of carbons which is governed by the existence of oxygen as functional

groups [222]. In this study, both the studied ACs contain very low amount of oxygen

containing surface groups, and their pHPZC values are 8.7 (granulated AC) and 8.0

(Fab AC). At neutral pH, where the isotherms are studied, the whole surface charges

of the ACs are positive, disabling the electrostatic attraction. Thus, the adsorption of

IL cations might only be promoted by dispersive forces. Among them, the

hydrophobic interactions are previously mentioned to explain the adsorption

properties of ILs on AC [202,203]. As a matter of fact, increased the chain length of

IL increases their hydrophobic character (Table 2.1) and the affinity for non-polar AC

environment with respect to the polar water medium. Thus, for each monocationic IL,

an increase in the apparent distribution coefficient at maximum uptake (Kd) is

observed together with the hydrophobicity, as shown in the plot of Kd versus logKow

(Figure 3.9). The dicationic ionic liquid i.e. Bis-DDMImBr showed higher uptake

despite its lowest hydrophobicity. Figure 3.9 confirms that hydrophobic ILs (having

long alkyl chain or hydrophobic cations) is showed higher adsorption by ACs from

aqueous solution than hydrophilic character. Whereas the IL having longer alkyl chain

than octyl, the increase in adsorption uptake is lessen may be because of steric

hindrance. But in case of Bis-DDMImBr, the lowest hydrophobic character predicted

by calculation is associated to a high retention by the ACs. Thus, the attraction of

adsorbate toward the surface of AC may be assigned not only by the hydrophobic-

hydrophobic interactions but also to other intermolecular interactions (polar, π-π-,

Van der Waals and hydrogen bonding) [202,222,225,226].

3.1.4. Porosity of the loaded carbons

In order to determine the adsorption sites of some of the organic cations, the

DFT PSDs of the granulated ACs loaded at saturation (at pH 7) with BPyBr (0.22

mmol.g-1), BMPyrrBr (0.37 mmol.g-1), OMImBr (0.87 mmol.g-1) or OMPyrrBr (0.86

mmol.g-1) ILs are studied (Figure 3.10). It is previously reported that

69

methylimidazolium cations linked with butyl chains are scarcely hydrated [227].

Moreover, recent studies [228,229] have shown that ions confined or electrosorbed in

subnanometer pores are desolated (or dehydrated when the considered solvent is

water). Thus the studied IL cations unaccompanied with any solvation sphere are

expected to diffuse into the ultramicropores due to their small thickness (<0.8 nm).

Compared to the DFT pore size distribution of pristine (Figure 3.10 a), the AC loaded

with BPy+ (Figure 3.10 b) or BMPyrr+ (Figure 3.10 c) mainly exhibit a decrease in the

ultramicropore range indicating that these butyl cations are adsorbed preferentially in

the ultramicropores as expected previously from the Figure 3.8 ignoring the size of

the adsorbent in the calculation of the accessible volume. However, ignoring this

assumption, the BMPyrr+ cation could not be penetrated in the ultramicropore (section

3.3). Thus, the difference between the predicted and experimental BMPyrr+ occupied

pore size might be attributed either to the disagreement of the real pore shape with the

slit pore model or an estimation of the volume of BMPyrr+ because of its non-

parallelepiped shape.

Figure 3.9: Plot of experimental KD versus log (Kow) for the adsorption on fab AC

(hollow symbols) and granulated (filled symbols) ACs, of pyridinium (,),

methylimidazolium (,) and methylpyrrolidinium (,) ionic liquids.

-12 -10 -8 -6 -4 -20.00

0.05

0.10

0.15

0.20

0.25

0.30

K d

log(Kow)

octylchain

butylchain

dodecylchainbis-

dodecylchain

70

After adsorption of OMIm+ (Figure 3.10 d) or OMPyrr+ (Figure 3.10e)

cations, the ultramicroporous volumes of the loaded ACs (calculated from DFT) is

become insignificant and the volume of supermicropore is greatly decreased. Similar

trend is observed with BMPyrr+ and BPy+ loaded Fab AC shown in figure 3.11 (b and

c). This confirms with the total filling or corking of the ultramicropores, and the

partial filling of the supermicropores by the octyl-cations is in agreement with

previous arguments. The effect on BET surface area and micropores after adsorption

of ILS on ACs has been described in Table 3.7.

Figure 3.10: (a) Pore size distributions on granulated AC, (b) the same granulated AC

loaded at pH 7 with BPy+, (c) BMPyrr+, (d) OMIm+ and (e) OMPyrr+.

(cm

3 mol

.g-1

)

71

Figure 3.11. (a) Pore size distributions on Fab AC, (b) the same Fab AC loaded at pH

7 with BPy+, and (c) BMPyrr+.

3.1.5. Thermodynamic parameters

The isotherms profiles for all the ILs determined in the temperature range of

25 to 55 °C are well reproduced by using the Langmuir equation. The experimental

and fitted adsorption isotherms of all ILs at pH 7 using different temperature on Fab

AC and granulated AC are shown in Figure (3.12-3.15). It can be seen from these

figures that for each IL, the adsorption is decreased as the temperature increases,

indicating that adsorption process is exothermic.

Table 3.7: Textural properties of granulated AC Raw Fab AC before and after loading

of ILs. Carbon Samples OMImCl-

granulated AC

BMPYrrBr- granulated AC

BPyBr- granulated AC

BMPYrrBr-Fab AC

BPyBr-Fab AC

Loading Of IL mmol. g-1

0.52 0.56 0.47 1.00 1.37

SBET (m².g-1) 505 765 785 1152 1112

Micropore area (m2.g-1) 434 741 633 1264 903

Mesopore surface area (m2.g-1) 8.00 7.86 38.00 0.00 0.00

Total Pore Volume (cm3.g-1) 0.31 0.41 0.39 0.49 0.46

Micropore volume (cm3.g-1) 0.19 0.31 0.29 0.42 0.37

Mesopore volume (cm³.g-1)

0.03 0.04 0.05 0.00 0.00

1 100.000.020.040.060.080.10 1 100.000.020.040.060.080.10 1 100.000.020.040.060.080.10

Pore Width(nm)

b

Incr

emen

tal P

ore

Volu

me

(cm

³/g)

a

cIn

crem

enta

l Por

e V

olum

e(cm

3 mol

.g-1

)

72

Figure 3.12: Experimental adsorption equilibrium data fitted by Langmuir for the

adsorption isotherms of BMPYrrBr (,∆,) and OMPYrrBr (,●,) on Fab (red

lines) and granulated (black lines) ACs at 25°C (solid dots), 40°C (light filled dots)

and 55°C (hollow dots).

Figure 3.13: Experimental adsorption equilibrium data fitted by Langmuir for the

adsorption isotherms of Bis-DDMImBr(,∆,), DDMImBr (,●,) on Fab (red

lines) and granulated (black lines) ACs at 25 °C (solid dots), 40 °C ( light filled dots)

and 55 °C (hollow dots).

Ce(mmol.L-1)

q e(m

mol

.g-1

) q e

(mm

ol.g

-1)

Ce(mmol.L-1)

73

Figure 3.14: Experimental adsorption equilibrium data fitted by Langmuir for the

adsorption isotherms of BPyBr (,, ), on Fab (red lines) and granulated (black

lines) ACs at 25 °C (solid dots), 40 °C (light filled dots) and 55 °C (hollow dots).

Thermodynamic parameters at 25 °C are calculated at 0.15 mmol.g-1

adsorption uptakes of each IL onto Fab or granulated AC (Table 3.8). Similar

variation of the Gibbs energy (∆G°) with IL type is found for the various ILs adsorbed

either on the Fab or on the granulated AC. The relation between ∆G and log(Kow) is

found quasi linear for the monocationic ILs. The values of ∆G are found negative

indicating spontaneous adsorption except for some butyl ILs (BPy+ and BMPyrr+).

The ∆G values are found more negative as alkyl chain length increased from butyl to

octyl. The adsorption of dodecane bismethylimidazolium (dicationic) IL has shown

the most exothermic value suggesting strong interactions of this molecule with the

carbon surface, possibly with unsaturated bond electrons (π-π interactions).

Moreover, for a similar chain length, the Gibbs energy values are found to vary

clearly with the ILs cation type. In fact, the cations containing more unsaturated

bonds are adsorbed more spontaneously suggesting that the involvement of π electron

in the adsorption forces. Thus, ∆G values for the same chain length IL are found to

decrease according to the following trend: MPyrr+ > MIm+ > Py+.

Heats of adsorption (∆H) values (Table 3.8) are found exothermic and typical

of physisorption. Whatever the AC, the trend of variation of the adsorption enthalpy

q e(m

mol

.g-1

)

Ce(mmol.L-1)

74

(∆H) versus the IL type (or log (Kow) is very similar to the ∆G variation (Figure 3.15).

As hydrophobicity of each monocationic IL increases, the enthalpy gets more

exothermic except for the chains longer than octyl (saturation) and Bis-DDMImBr

(Figure 3.15). Thus, the values of enthalpies are found more negative, as proceeding

from smaller to longer alkyl chain in same IL cation family (Table 3.8).

The entropy values (∆S) are quite similar for the monocationic ILs. This has

confirmed an increase of order after adsorption due to the local organization of the

molecules inside the pores. The lowest value of entropy for the Bis-DDMImBr

adsorption suggested a more ordered arrangement at the surface of AC for this

molecule compared to other ILs.

Table 3.8: Thermodynamic para meters of adsorption of the ILs (at constant value of

adsorption uptake (at qe=0.15 mmol.g-1) on different AC types.

IL type BPyBr OPyBr BMImBr OMImBr DDMImBr Bis-

DDMImBr BMPyrrBr OMPyrrBr

ΔH°

(granulated)

(kJ.mol-1)

-49 -15 -18 -18 -23 -16 -40 -5.00

ΔH° (Fab.)

(kJ.mol-1) -28 - -27 -37 -26 -116 -27 -29

ΔS°

(granulated)

(J.K-1.mol-1)

-171 -28 -67 -45 -63 -44 -145 -11

ΔS° (Fab.)

(J.K-1.mol-1) -89 - -88 -98 -66 -325 -92 -79

ΔG°

(granulated)

(kJ.mol-1)

-2.00 -6.50 2.00 -5.00 -4.00 -3.00 3.00 -1.50

ΔG° (Fab.)

(kJ.mol-1) -1.30 - -1.00 -8.00 -6.00 -19 0.00 -5.00

75

Figure 3.15: Adsorption uptake dependence (Kow), at pH = 7, of the enthalpy changes,

∆H° (kJ/mol) for BMPyrrBr (), BPyBr (), BMImBr (), Bis-DDMImBr (),

OMPyrrBr (), OMImBr ( ) and DDMImBr ( ) on Fab AC at 25°C (0.4 mmol.g-1).

In conclusion; Factors which strongly affect the adsorption capacities are the

IL cation size, the hydrophobic nature of the IL cations (which depends on the length

of the IL alkyl chains) and also on the cation type. The comparison of the sizes and

volumes of the studied IL molecules, and the porous volume of ACs have suggested

that the smaller ILs (with butyl chain) are preferentially adsorbed in the

ultramicropore volume as unsolvated pair of ions (cation and anion). For the bulkier

cations, they are found to be adsorbed both in the largest ultramicropores and in the

supermicropores. It is confirmed that more spontaneous adsorption took place for

longer chain length ILs having hydrophobic cations than for lower chain length ILs.

Results also show that some steric effects restrict the adsorption of ILs with large

molecular volume.

-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2

-120

-100

-80

-60

-40

-20

0

∆H° (

KJ/m

ol)

log(Kow)

∆Hº (

kJ.m

ol-1

)

76

3.2. Adsorption of ionic liquids on modified AC In this section, the adsorption of ILs onto modified ACs (mACs) is presented.

The ILs has two type of cation with variable length of alkyl chain are selected. The

effect of pH of the solution, alkyl chain length of ILs and cation effect of ILs on the

adsorption is studied. The analysis is done by using UV-Vis spectrophotometer. Raw

granulated AC (RAC) is further oxidized with NaOCl (called BAC) and HNO3

(called AAC). The schematic representation of adsorption of ILs onto ACs are shown

in Figure 3.16.

Figure 3.16: Schematic representation of adsorption of ILs onto ACs.

3.2.1. Characterization

Figure 3.17 shows the nitrogen adsorption–desorption isotherms of raw and

mACs. The surface area and textural properties of the ACs are shown in Table 3.9. In

case of AAC, the difference in micropore volume from RAC is 0.12 cm3.g-1 and

difference in total pore volume is 0.11cm3g-1 and in case of BAC the difference in

microspore volume from RAC is 0.05 cm3g-1 and difference in total pore volume is

0.02 cm3g-1. This decrease is more in AAC as compared to BAC because after HNO3

modification, there is a chance that the smaller microspores are blocked with complex

77

functional groups containing mainly carboxylic acids with small amount of phenol,

anhydrides and lactones [230,231] generated during nitric acid treatment [232]. In

BET measurements of dry ACs sample, these pores are not assessed, in spite of the

very small size of the molecules of nitrogen gas. As there is no particular change in

the textural properties of modified sample, this suggested the change in adsorption

capacity is due to change in surface properties only [53,233]. The isotherms given by

all ACs is L type isotherm. These results show that the ACs is typical microporous

in nature and there is no major change take place in their textural properties.

However, the specific surface areas (SBET) and microporous volume of mAC are

decreased. These results also show that acidification with HNO3 and NaOCl is an

efficient technique to produce functional groups on surface of ACs. The pH values of

these ACs are ranging from 7.1 to 8 and small decreases in pHPZC take place.

The cations size of IL are compatible with the volume of micropores of AC,

i.e., due to the lower size of IL cations, the micropore volume of AC is accessible

(molecules length of IL < 1.5 nm). In fact the thickness of all molecules is very small

(<0.2 nm) shown in Table 3.9, which could adsorb even into the smaller slit pores,

i.e., ultramicropores. The knees at low adsorption on the isotherms (Figure 3.17) are

in agreement with the presence of ultramicropores in the ACs. This suggests that

adsorption is take place mainly in the small micropores of ACs. The same trend is

found in work of Farooq et al. [234].

Table 3.9: Textural properties of RAC, BAC and AAC obtained by N2

adsorption/desorption at 77 K.

Carbon

Samples

BET surface

area (m².g-1)

Total

Pore Volume

(cm3.g-1)

Microspore volume

(cm3.g-1) pH pHPZC

RAC 984 0.52 0.25 8.5 9.5

BAC 880 0.50 0.20 8.0 9.0

AAC 819 0.41 0.13 7.1 8.5

78

Figure 3.17: Nitrogen adsorption-desorption isotherm of RAC (●), BAC (■) and AAC

(▲).

The surface functional groups of ACs are measured by Boehm titration

method [235] shown in Table 3.10. When AC is treated with NaOCl or HNO3, the

decreased in pH values take place due to the increase in the concentrations of oxygen

containing surface groups from 0.18 to ~0.56 meq/g (for NaOCl) and 0.18 to 0.67

meq/g (for HNO3). The value of pHPZC slightly decreased for the BAC (from 9.5 to

9.0) but more decreased for AAC (from 9.5 to 8.5). This decrease in pHPZC could be

ascribed to the higher amount of lactonic groups (0.02 and 0.04 meq/g for BAC and

AAC, respectively and carbonyl groups (0.50 and 0.51 meq/g for BAC and AAC

respectively) and possess high pKa values (pKa lactonic ~8.2pKa-carbonyl ~16–20).

This table shows an incremental increase of functional groups and higher value higher

values are obtained for AAC. The total acidic groups of RAC are 0.18 meq/g which

are increased three times (0.67 meq/g) in AAC. The amount of basic groups is

remaining constant in RAC and BAC whereas minimum value is observed in AAC.

Table 3.10: Boehm titration results of RAC, BAC and AAC.

ACs

Carboxylic

group

(meq.g-1)

Lactone

group

(meq.g-1)

Carbonyl

Groups

(meq.g-1)

Phenol

group

((meq.g-1)

Total acidic

groups

(meq.g-1)

Basic

groups

(meq.g-1)

RAC 0.01 0.00 0.15 0.02 0.18 0.08

BAC 0.01 0.02 0.50 0.03 0.56 0.08

AAC 0.04 0.04 0.51 0.08 0.67 0.01

0.0 0.2 0.4 0.6 0.8 1.0 1.20

70

140

210

280

350

Quan

tity

Adso

bed

(cm

3 /g S

TP)

Relative Pressure (P/Po)

Qua

ntity

ads

orbe

d (c

m3 .g

-1)

79

3.2.2. Adsorption study

3.2.2.1. Kinetics study

To study the adsorption kinetics of ILs on ACs, 1 mmol.L-1 initial

concentrations of corresponding IL solutions are used. At the beginning, the

adsorption of ILs on AC is found to be rapid (∼30 min) and then with the increase in

contact time (∼1 to ∼24 h) it become slow and a plateau is obtained after 24 h as

shown in Figure 3.18(a) - 3.20(a).

This figure shows that the ILs removal rate is initially rapid and gradually

decreased until equilibrium obtained. This phenomenon is attributed to the fact that at

initial stage a large number of vacant adsorption sites are available for adsorption and

after some time, it become difficult for IL molecules to occupy the reaming vacant

spaces because of repulsion between solute molecule in bulk and on the solid

[236,237]. For practical applications, contact time is one of the major parameters. It is

found that the AC shows much higher adsorption rate [1]. Which might be due to its

desirable microporous structure, π−π stacking interaction effect [238] or electrostatic

attraction [239] particularly for adsorption of compatible molecule having aromatic

ring.

Different models are applied on the obtained kinetics data, to understand the

adsorption characteristics of ILs onto ACs. Pseudo-first order and pseudo second-

order models are better to fit to the experimental kinetics data as shown in Figure 3.18

- 3.20 (b and c) respectively. The determination coefficients (R2) and the obtained

kinetics parameters of kinetics models are shown in Table 3.11. The R2 values of

pseudo-second order kinetics model is 0.9998 , very close to close to the calculated qe,

this model express that the adsorption of ILs onto ACs involves mass transfer of a

adsorbate to the adsorbent surface from liquid phase with physicochemical process

In porous adsorbent, if the movement of adsorbate to the solution surrounding

the adsorbent is ignored then the process of adsorption might be separated into the

following three stages follows: (1) membrane diffusion (outer diffusion or boundary

layer diffusion) in which adsorbate is diffused from the liquid to the external surface

of adsorbent; (2) intraparticle diffusion or inner diffusion in which adsorbate is

transported from the external surface of adsorbent in to internal pores/capillaries; (3)

the adsorbate is adsorbed onto adsorbent’s active sites of (in outer pores surfaces and

inner pores surface) [240]. The third step is very fast which cannot be called as rate

80

limiting step. Normally, the inner and outer diffusion or both, control the adsorption

rate. So, Boyd model is applied, to find out the actual rate controlling step involved in

the ILs sorption process. The plotted Bt against time t is shown in Figure 3.18-3.20

(d). In figure 3.18 (d) plots are linear and did not pass through the origin, confirming

that in adsorption process, involvement of external mass transfer is taking place.

While in case of figure 3.19-3.20 (d) the straight line did not pass through the origin

which conform that in adsorption process external mass transfer take place [241]. The

linearity of the plots provides useful information to distinguish between intraparticle

diffusion and external mass transfer mechanism of adsorption [242].

Figure 3.18. Kinetics curve of experimental data (a), Pseudo-first order model (b)

Pseudo-second order model (c) and Boyd model (d) for the adsorption of BMImCl on

RAC (■), AAC (●) and BAC (▲) (initial concentration: 1mmol.L-1; temperature:

25 °C and pH: 7).

(b)

q e (m

mol

.g-1

) t.q

t-1

Time (h)

Time (h) Time (h)

Time (h)

81

Figure 3.19: (a) Kinetics curve of experimental data, (b) Pseudo-first order model (c)

Pseudo-second order model and (d) Boyd model for the adsorption of OMImCl on

RAC (■), AAC (●) and BAC (▲) (initial concentration: 1mmol.L-1; temperature:

25 °C and pH: 7).

Table 3.11: The kinetics fitting data of ILs on ACs using pseudo- second order model.

ILs RAC AAC BAC

qe qexp R2 qe qexp R2 qe qexp R2

BMImCl 0.23 0.24 0.9989 0.26 0.26 0.9989 0.27 0.28 0.9989

OMImCl 0.25 0.28 0.9989 0.26 0.29 0.9968 0.27 0.30 0.9968

OPyBr 0.24 0.25 0.9986 0.25 0.27 0.9988 0.26 0.29 0.9978

qe= mmol.g, k =mmol. g-1.min-1

(b)

q e (m

mol

.g-1

) t.q

t-1

82

Figure 3.20: (a) Kinetics curve of experimental data, (b) Pseudo-first order model (c)

Pseudo-second order model and (d) Boyd model for the adsorption of OPyBr on RAC

(■), AAC (●) and BAC (▲) (initial concentration: 1mmol.L-1; temperature: 25 °C and

pH: 7).

3.2.2.2. Adsorption isotherms

The isotherm adsorption equilibrium characteristics are analyzed using the

Langmuir-Freundlich, Freundlich and Langmuir isotherm models but the Langmuir

model is best fitted to the experimental data. The estimated Langmuir model

parameters are reported in Table 3.12.

Figure 3.21 show the effect of the surface chemistry of three ACs on

adsorption of all ILs at different pH at room temperature. This figure shows that the

modified ACs adsorbed higher amount of ILs compared to raw AC. This is because of

more surface functional groups of modified ACs. The adsorption of OMImCl on

BAC, AAC and RAC is 0.94, 0.92 and 0.90 mmol.g-1 respectively. All the other ILs

showed same adsorption bhaviour on ACs. Moreover, the adsorptions of ILs on

mACs are higher which suggests that modification increase the surface functional

groups which facilitate strong interaction between ILs and adsorbents. The plateau of

adsorption of mACs compared to RAC at high concentration showed the presence of

strong adsorption sites on the mACs surface.

Time (h)

q e (m

mol

.g-1

) t.q

t-1

Time (h) Time (h)

Time (h)

83

Figure 3.21. Adsorption of BMImCl (■), OPyBr (●) and OMImCl (▲) onto RAC

(dashed lines), AAC (solid lines) and BAC (dotted lines) at different pH.

pH=2

pH=7

pH=9

Concentration (mmol.L-1)

q e (m

mol

.g-1

) q e

(mm

ol.g

-1)

q e (m

mol

.g-1

)

84

Table 3.12: Adsorption parameters of ILs obtained at different pH using Langmuir

isotherms on ACs.

Type

of

ACs pH

BMImCl OMImCl OPyBr

RA

C

qe b qmax R2 qe b qmax R2 qe b qmax R2

2 0.42 0.44 0.43 0.9989 0.90 2.02 0.90 0.9998 0.87 3.01 0.90 0.9999

7 0.45 0.25 0.46 0.9998 0.92 1.63 0.95 0.9988 0.90 2.80 0.92 0.9989

9 0.53 0.66 0.54 0.9998 0.99 3.07 0.98 0.9998 0.97 3.13 0.98 0.9988

AA

C 2 0.45 0.43 0.48 0.9988 0.92 2.55 0.92 0.9958 0.90 3.52 0.91 0.9989

7 0.46 0.36 0.48 0.9998 0.93 2.98 0.93 0.9988 0.91 4.10 0.94 0.9988

9 0.60 1.35 0.60 0.9988 1.1 5.54 1.09 0.9989 1.04 5.22 1.02 0.9999

BA

C

2 0.50 1.2 0.50 0.9998 0.94 3.9 0.94 0.8988 0.91 10.1 0.96 0.9998

7 0.52 1.7 0.52 0.8999 1.12 8.2 1.08 0.9877 0.98 29.5 0.98 0.9989

9 0.60 2.2 0.60 0.9878 1.16 19.6 1.16 0.9899 1.04 30.5 1.04 0.9988

qe=mmol.g-1, b=mmol. g-1.min-1, qmax=mmol.g-1

3.2.2.3. Effect of IL type

Figure 3.22 show the effect of alkyl chain length of ILs on equilibrium

adsorption of ILs on ACs at pH 9. The adsorption value of OMImCl is 1.16 mmol.g-1

and OPyBr is 1.08 mmol.g-1. Both these ILs show higher adsorption uptake onto ACs

compared to BMImCl (0.62 mmol.g-1). This might be due to the smaller alkyl chain

length of BMImCl which is decreased its hydrophobicity and the hydrophobic–

hydrophobic interactions between AC and IL could be the main force of attraction

required for adsorption. Similar behavior of adsorption is already reported in literature

[234]. According to Giles classification [224] ‘L’ type of isotherm is observed for the

adsorption of ILs onto ACs.

3.2.2.4. Effect of pH

It has been reported earlier that the modifications of AC greatly change the

surface chemistry of AC. The surface chemistry of ACs plays an important role in

adsorption. The effect of pH on the adsorption of ILs on different types of ACs is

shown in Figure 3.23.

85

Figure 3.22: Equilibrium adsorption isotherm using Langmuir isotherm for OMImCl

(▲), OPYBr (●) and BMImCl (■) onto BAC at pH 9.

This figure shows that adsorption of ILs on ACs at basic pH is higher. This

trend shows that at basic pH acid type functional groups (phenol and carboxyl) are

activated into their corresponding anions. Similarly the AC which contain higher

amount of oxygen containing functional groups show lower pHPZC, Among the

studied ACs, mACs contained higher amount these groups compared to RAC. Thus,

they possesses the lower pHPZC value hence show higher adsorption for ILs.

Moreover, effects of pH on adsorption also suggest that two kinds of forces

existed between ILs and ACs surface: electrostatic interactions (where the functional

groups become negatively charged) predominated at basic pH (pH = 9) and dispersive

interactions at lower pH.

Figure 3.24 shows the difference between equilibrium adsorption uptake

values of ILs at pH 2 and 9. This figure shows that maximum adsorption is

proportional to the amount of oxygen containing functional groups on respective AC.

These oxygen groups affect the strength of electrostatic force of interaction and

maximum interaction is observed at pH = 9. The subtraction value of adsorption

uptake at pH 9 to 2 is expressed the following trend OMImCl < OPyBr < BMImCl on

the ACs. This trend shows that the adsorption of IL on ACs was through weaker

dispersive interactions and stronger electrostatic interactions.

0 1 2 3 40.0

0.2

0.4

0.6

0.8

1.0

1.2

q e(m

mol

/g)

Ce(mmol/L)

q e (m

mol

.g-1

)

Ce (mmol.L-1)

86

Figure 3.23: Adsorption of all ILs on BAC at pH 9 (filled blocks), pH 7 (dotted

blocks) and pH 2 (blank block).

Palomar et al. has also tailored the surface of AC and show that the adsorption

of OMImPF6 (1-methyl-3-octylimidazolium hexafluorophosphate) increase with

increase of the oxygen content on the AC surface [202]. So it is also clear from this

study, with the increase of surface oxygen groups of AC, electrostatic attractions with

adsorbate are enhanced.

Figure 3.24: Difference between equilibrium adsorption uptakes (Qmax) for BMImCl

(■), OPyBr (●) and OMImCl () measured as a function of the oxygen containing

functional groups of ACs (qmax at pH 9 – qmax at pH 2).

q e (m

mol

.g-1

)

Ce (mmol.L-1)

q e (m

mol

.g-1

)

(mmol.g-1)

87

As the textural properties of ACs are not changed, the difference in ILs

uptakes on RAC and mACs may be described by the difference in their surface

chemical nature.

In conclusion, modification of AC by oxidation, affect the surface chemistry

of AC which in turn affects the uptake of ILs adsorption. By selecting the

approximate chemical for oxidation, one can also control the amount and type of

functional groups on AC. In this study, mAC showed good results for the uptake of

ILs.

88

3.3. Synthesis and characterization of pH-sensitive silane

crosslinked chitosan/poly (vinyl alcohol) membrane The crosslinked CS membrane is usually brittle. In order to enhance its

flexibility other polymers are normally added. In this study poly (vinyl alcohol) is

added at two different ratios with CS. The CS is blended with two different amounts

of PVA and crosslinked with fixed MTMS. The resulted polymer network structure is

quite stable in different environments. The crosslinked membranes are characterized

by infrared spectroscopy. These crosslinked membranes are used for the adsorption

studies of anionic dye and batch adsorption experiments are conducted at different

time, pH, and temperature range. Codes of formation, composition, of CS/PVA

membrane is shown in Table 3.13. CS/P05 and CS/P25 are codes used to express the

CS/PVA membranes containing 5 and 25 % PVA, respectively. All the prepared

membranes shows pH sensitive swelling and PVA contents also affect their swelling.

Table 3.13: Composition, codes of formulations, gel content and diffusion parameters

of CS/PVA membranes.

Samples MTMS

(%)

PVA

(%)

CS

(%)

Gel Content

(%)

n k×10-1

CS/P05 5 5 95 39.19 0.54 3.4

CS/P25 5 25 75 46.15 0.38 4.1

The possible chemical reactions and proposed mechanism between PVA, CS and

MTMS are shown in the scheme below. It contained three steps; in first step the

hydrolysis of MTMS is taken place in the presence of acid/alcohol medium and

methyl silanol Me (SiOH3) groups are formed leaving ethanol. This silanol group

reacts by condensation reaction with –OH and/or –NH2 groups of PVA and CS to

form -Si-O-C- and -Si-NH-C- bonds. In final step, crosslinking occurs and siloxane

(-Si-O-Si-) linkage is formed between CS and also PVA and between two CSs.

Hydrolysis of MTMS( Si=silane, Me= methyl)

𝑀𝑒𝑆𝑖(𝑂𝐶𝐻3)3 𝑀𝑒𝑆𝑖(𝑂𝐻)3 + 3𝐶𝐻3𝑂𝐻

Grafting Reactions of MTMS

𝐶𝑆─𝑂𝐻 + 𝑀𝑒𝑆𝑖(𝑂𝐻)3 𝐶𝑆─𝑂─𝑆𝑖𝑀𝑒(𝑂𝐻)2

𝐶𝑆─𝑁𝐻2 + 𝑀𝑒𝑆𝑖(𝑂𝐻)3 𝐶𝑆─𝑁𝐻─𝑆𝑖𝑀𝑒(𝑂𝐻)2

-H2O -H2O

89

𝑃𝑉𝐴 − 𝐶𝐻2 − 𝐶𝐻2𝑂𝐻 − +𝑀𝑒𝑆𝑖(𝑂𝐻)3 𝑃𝑉𝐴 − 𝑂 − 𝑆𝑖𝑀𝑒(𝑂𝐻)2

Crosslinking Reactions

2𝐶𝑆─𝑂𝑀𝑒𝑆𝑖(𝑂𝐻)3 𝐶𝑆─𝑂𝑀𝑒𝑆𝑖(𝑂𝐻)─𝑂─(𝑂𝐻)𝑆𝑖𝑀𝑒𝑂─𝐶𝑆

𝐶𝑆─𝑂𝑀𝑒𝑆𝑖(𝑂𝐻)3

+

𝑃𝑉𝐴─𝑂𝑀𝑒𝑆𝑖(𝑂𝐻)2 𝐶𝑆─𝑂𝑀𝑒𝑆𝑖(𝑂𝐻)─𝑂─(𝑂𝐻)𝑆𝑖𝑀𝑒𝑂─𝑃𝑉𝐴

3.3.1. Structure analysis

The infra-red spectra of PVA, CS, CS/P05 and CS/P25 membranes are shown

in Figure 3.25, which shows the presence of incorporated components in the

membranes and also confirm the effect of increasing PVA content in the membrane.

The broad band between 3500 to 3250 cm-1 showing O−H stretching due to inter and

intra -molecular hydrogen bonds the symmetric N–H vibration stretching is also

present in this region [168,243,244] This broad region is increased as the PVA

content is increased from 5 to 25 % in the membranes.

It is clear from Figure 3.25 that increasing PVA content, the shifting of amide

I peak from 1653 to 1646 cm-1 is take place which showed the presence of hydrogen

bond between the components (incorporated in membranes). The peak of Amide-III is

a stable peak but as the amount of PVA is increase its intensity is also increased. The

peaks in the membranes ranging from 1580 to 1510 cm-1, corresponds to amide II

which confirm presence of CS (partially deacetylated). The increase in peak intensity

of amide II (at 1546 cm-1) is due to increase amount of PVA in the membrane

(CS/P25). The vibrational stretching of alkyl groups (C-H) is observed at 3000-2840

cm-1. The sharp peaks at 1410 and 1392cm-1 are correspond to deformation modes and

symmetrical vibrational of CH2 respectively. There are shift in band and increase in

intensity from 1410 to 1401 cm-1 is due to bending vibration of CH2 in both

membranes. The amide IV and V bending vibration is also examined at peaks 650 and

610 cm-1 confirm the presence of hydrogen bonding (intermolecular) in both

membranes.

-H2O

-H2O

-H2O

90

The peaks at 1100-1020 cm-1 in the membranes conform the siloxane bond

(Si-O-Si) resulted due to MTMS [25-26, 78]. These peaks are not present in the

spectrum of pure PVA and CS. The peaks intensity of siloxane bonds remains the

same due to fix amount of crosslinker (MTMTS) in the membranes. The gel content

of CS/P05 and CS/P25 membranes is around 40 % (Table 3.13) which also shows the

crosslinked network in the membranes resulted by MTMS.

Figure 3.25: IR spectra of CS, PVA and CS/P05, CS/P25 membranes.

FTIR spectra of pure dyes NR, POr and crosslinked CS/P05 CS/P25

membranes before and after dyes adsorption is shown in Figure 3.26 (a and b). IR

spectra are used to know the possible sites of the binding of dye with the membrane.

A substantial decrease in transmittance is observed in these vibrations after dye

adsorption, which shows that these vibrations are engaged during adsorption of dye.

Other important changes in transmittances of adsorbed CS/P05 membrane are at 1515,

1454, 1410, 1370 and 1270 cm-1 and important transmittance changes in case of

adsorbed CS/P25 membrane are at 1507, 1480, 1312 and 1153 cm-1 which is mostly

related to the N-H bending and C-N stretching bands. These results confirmed that the

main adsorption sites in membranes are nitrogen atoms of CS. The N-H bending

3500 3000 2500 2000 1500 1000 500

(CS/P25)

(CS/P05)(PVA)

(CS)

Tran

smitt

ance

(a.u)

Wavenumber (cm-1)

91

vibration at 1631 and 1546 cm-1 before dyes adsorption onto CS/P05 membrane are

shifted to 1625 and 1536 cm-1 after dyes adsorption while in 1646 and 1550 cm-1 N-H

bending vibrations are shifted to 1635 and 1540 cm-1 after dyes adsorption onto

CS/P25 membranes. Some new peaks are also appeared at 1505, 1454 and 1410 cm-1

in NR adsorbed CS/P05 membranes and 1507, 1480 cm-1 in NR adsorbed CS/P25

membrane and 1311, 1454 cm-1 in case of POr adsorbed CS/P05 membranes and

1312, 1153 cm-1 in case of POr adsorbed CS/P25 membrane which might be resulted

from NR and POr dyes respectively.

3.3.2. Swelling study

3.3.2.1. Time dependent swelling

The time dependent swelling behavior of crosslinked CS/P05 and CS/P25

membranes in distilled water is shown in Figure 3.27. Figure 3.27 (a) shows that the

initial absorption of water by the crosslinked membranes is rapid and consistent the

equilibrium is reached around 6 h in both cases. The maximum swelling of 63 g.g-1 is

observed in CS/P05 membrane and 17g.g-1 in CS/P25 membrane. CS/P05 showed

higher swelling at all the time intervals as compared to CS/P25 which contains 25 %

PVA content. This decrease in swelling and its rate with increase of PVA amount is

because of the increase in hydrophobic characteristic in the membranes and the

formation more complex and compact structure.

Generally, the absorption mechanism of water in the membranes is caused by

the diffusion process and both the external media and the polymer chains play

important role in this process [245]. The physical behavior , its hydrophobic character

[110] and the hydrogen bonding (both the intra-molecular and intermolecular) of the

membranes also play an significant role in the swelling process [246].

The mechanism of water diffusion can be measured by using the equation 2.3.

In this study, the value of ‘n’ obtained from data of swelling of CS/P05 and CS/P25

membranes in the water. The values of diffusion parameters (‘k’ and ‘n’) are obtained

from the swelling data of membranes in water. Figure 3.27 (b) shows the plot of ln t

versus ln F and the diffusion parameters values are given in Table 3.13. The CS/P25

shows Fickian diffusion while CS/P05 shows non-Fickian diffusion mechanism.

While the value of ‘n’ expresses a linear response of diffusion with the increase of

PVA content in complexes. Both membranes have same amount of MTMS

(crosslinking density) but have different PVA content. This shows that the PVA

content affects the diffusion parameters of membrane.

92

Figure 3.26: FTIR spectra of (a) dyes and CS/P05 membrane, (b) dyes and CS/P25

membrane before and after the adsorption of dyes.

3500 3000 2500 2000 1500 1000 500

(CS/P05-POr)

(CS/P05-NR)

(CS/P05)

(POr)(NR)

Tran

smitta

nce (

a.u)

Wavenumber (cm-1)

(a)

3500 3000 2500 2000 1500 1000 500

(CS/P25-NR)

(CS/P25-POr)

(CS/P25)

(POr)(NR)

Trans

mitta

nce (

a.u)

Wavenumber (cm-1)

(b)

93

The ‘n’ value indicates the release or transport mechanism and mechanism of

diffusion [179]. It may be Fickian diffusion (n<0.5), case II diffusion (n=0.5), super-

case II diffusion (n>1) and non-Fickian diffusion (0.5<n<1)[245,247] .

3.3.2.2. Swelling in salt solutions

The swelling of CS/PVA membranes in two different salts i.e. CaCl2 and NaCl

have been investigated. I both the salts the anion (Cl-1) is same but cation and its

charge is different. The concentration effect of both salts on swelling behaviour of

CS/P05 and CS/P25 membranes is shown in Figure 3.28 (a). The swelling of

membranes in both salts decreased with increase of salt concentration. This may be

attributed to the decrease in difference in osmotic pressure between the salt solution

and membrane with increase in ionic strength of surrounding medium. Therefore, the

solvent diffusion into the membrane is decreased which reduced its swelling. The

swelling of membrane in CaCl2 is much lower as compared to NaCl solution[248] .

The swelling of membrane (in ionic environment) results in screening effects

(with cations) which cause electrostatic repulsion between a non-perfect anion-anion.

This creates osmotic pressure between membrane and the external media [143]. These

membranes are ionic in nature and their swelling behaviour depends on the

surrounding ions present in the media and its chemical nature. The charge on calcium

(Ca+2) is twice as compared to sodium (Na+) which also increase its ability to form

complexation of CaCl2 solution with the other ionic groups of the membranes. Due to

this complex with two donor atoms on the surface of the membranes , as a result more

compact structure is formed which cause less swelling as compared to NaCl solution

at same concentration [249] .

94

Figure 3.27: (a) Swelling kinetics, (b) ln (F) plotted versus ln (t) for crosslinked

CS/P05 (●) and CS/P25 ■) membranes at room temperature

3.3.2.3. Effect of buffer media on swelling

The swelling of

CS/P05 and CS/P25 in buffer media is shown in Figure 3.28 (b). This figure

expresses that both membranes have high swelling in acidic pH while low swelling in

basic and neutral pH of the medium. The CS/P05 and CS/P25 membranes exhibited

0 2 4 6 8 100

10

20

30

40

50

60

70

Swell

ing (g

/g)

Time (h)

(a)

-2 -1 0 1 2

-1.5

-1.2

-0.9

-0.6

-0.3

0.0

ln (F

)

Ln (t)

(b)

Swel

ling

(g.g

-1)

Time (h)

ln (t)

95

85.2 g.g-1 and 65g.g-1 swelling at pH 2 while 33.5 g.g-1 and 13.5 g.g-1 at pH 10

respectively.

When the pH of the surrounding medium is changed, it changes the charge

balance in the polymer complex which in turn affects the degree of interaction of the

polymer chains with water. The increased in swelling of the both membranes in acidic

pHs may be attributed to the amide linkages hydrolysis of crosslinked CS network

with acid, as a result amine groups get regenerated in the network [250]. As of the fact

that the CS-amino groups which reformed in the membrane-network are protonated in

the acid, so the swelling equilibrium ratio of the membranes in acid medium is larger

compared to neutral and basic solution. It would be a used for controlled-release

system and adsorption of effluents from waste water. These membranes are, therefore,

of general interest for environmental and biomedical applications [251] .

3.3.3. Application of crosslinked membranes for adsorption of dyes

3.3.3.1. Kinetics study

The effect of contact time on adsorption of dyes is shown in Figure 3.29 (a).

The contact time is varied up to maximum of 300 min and the initial concentration of

dyes is fixed at 1mmol.L-1. It can be seen from the figure that the time required to

achieve the equilibrium adsorption for NR and POr dyes on crosslinked membranes

CS/P05and CS/P25 are around 120 and 60 min respectively at room temperature.

Such kind of high adsorption capacity with short equilibrium times shows a high

degree of affinity between crosslinked membranes and anionic dyes.

The experimental kinetics data are simulated by using different kinetics

models. The mostly used equation is the first-order for the adsorption of effluent from

solution (equation 1.11) [18]. The values of ‘qe’ and ‘k1’ are calculated from the

nonlinear regression of ‘qt’ versus ‘t’ obtained by integration of the equation and the

obtained kinetics data using these models is given in Table 3.14. This table showed

that the calculated equilibrium removal percent qmax cal. for NR and POr calculated

from pseudo- first order model is consistent with the experimental data compared to

pseudo-second order model. The value of ‘R2’are above 0.9898. This model showed

that the reaction between solid and liquid at equilibrium are reversible [25].

96

Figure 3.28: Swellings of CS/P05 (●) and CS/P25 (■) membranes (a) in buffer

solution pH (2-10), (b) Swelling in concentrations of NaCl (solid lines) and CaCl2

(dotted lines) solutions (concentration range 0.05 to 1 M) at room temperature

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40

15

30

45

60

75

90

Swel

ling

(g/g

)

Concentration (M)

(b)

2 4 6 8 10 12

30

45

60

75

90

Swel

ling

(g/g

)

pH

(a)

Swel

ling

(g.g

-1)

Swel

ling

(g.g

-1)

97

The adsorption process of dyes on the crosslinked membranes could be take

place in three steps: (1) outer diffusion of dyes (boundary layer diffusion) in which

dyes transfer from the aqueous solution to the external surface of membrane; (2) inner

diffusion (intraparticle diffusion), dye adsorbed from the surface of membrane to the

internal pores of membrane (3) The dyes interact with active sites of internal pores of

membrane [240]. Third step is very fast, so it is not considered as rate controlling

step. Generally the outer or inner or both adsorptions are considered as rate

controlling steps. In order to find the actual rate controlling step, the Boyd law

(Equation 1.13) is applied to the kinetics date of adsorption of dyes onto the

membranes [28].

The ‘Bt’ values are calculated from the kinetics date and plotted verses time ‘t’

shown in Figure 3.29 (b). The plot linearity provide important information regarding

the mechanism of adsorption to distinguish between external mass transfer and

intraparticle diffusion between liquid and membrane [242]. The linear line in the

graph is not passed through the origin in both dyes adsorption, confirming the

involvement of external mass transfer in the adsorption process [241].

Table 3.14: Kinetics data of NR and POr dyes adsorbed on crosslinked CS/P05 and

CS/P25 membranes obtained using pseudo-first order and pseudo-second order

kinetics models at dye concentration = 1mmol.L-1, sample weight = 50mg, pH = 7.

Samples Dyes

Qe exp.(mmol.g-1)

Pseudo-First Order Pseudo-Second order K1,

(min-1)

qmax calc.

(mmol.g-1) R2

K1,

(min-1)

qmax calc.

(mmol.g-1) R2

CS/P05 NR 0.46 0.029 0.46 0.9899 0.07 0.51 0.9899

POr 0.62 0.14 0.61 0.9898 0.29 0.65 0.9889

CS/P25 NR 0.35 0.036 0.36 0.9999 0.075 0.45 0.9899

POr 0.56 0.094 0.56 0.9998 0.19 0.63 0.9888

98

Figure 3.29: (a) Kinetics adsorption data, simulated by pseudo first-order (solid lines)

and pseudo second-order (dotted lines) models, (b) Boyd law fitted for the adsorption

kinetics data of NR(■□) and POr(●○) dyes onto CS/P05 (solid points) and CS/P25

(blank points) membranes (dye concentration = 1mmol.L-1, sample weight = 50 mg,

pH = 7)

0 50 100 150 200 250 300 350-1

0

1

2

3

4

5

6

7

B t

Time (min)

(b)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 30 60 90 120 150 180

qe (m

mol

/g)

Time (min)

(a)

q e(m

mol

.g-1

)

99

3.3.3.2. Effect of pH and adsorbent dose

Figure 3.30 (a) shows the effect of adsorbent weight on the adsorption of dyes.

The amount of adsorbent is varied from 20 mg to 100 mg and concentration of dye is

kept at 1mmol.L-1. This figure showed that the increase in the weight of membrane,

the removal of dye increased linearly. After 50 mg of adsorbent weight, no increase in

adsorption of dyes is observed and equilibrium is established. Therefore, the number

of free ions and the ions bound to the adsorbent remains constant and no increase in

dye adsorption is observed with further increase of adsorbent dose. Hence 50 mg is

considered as an optimum adsorbent (membrane) weight for dyes adsorption. These

results showed that even a small amount of membranes can remove higher amount of

dye. The maximum removal of NR and POr are 0.90 and 1.23 mol/g at 50 mg CS/P05

adsorbent dose respectively while 0.75 and 0.83 mmol.g-1 on CS/P25 respectively.

Similar phenomenon is observed in the adsorption of textile dye on the adsorbents

apple pomace and saw dust [252].

The effect of pH on dyes adsorption onto membranes is studied and results are

shown in Figure 3.30 (b). This figure shows that the adsorption capacities of the blend

are significantly affected by pH. Highest adsorption is observed in acidic range as

compared to neutral or alkaline range. The capacities of adsorption of NR and POr

dyes on the blend increased from 0.63 to 1.59 mmol.g-1 and from 1.30 to 2.42

mmol.g-1 on CS/P05 while 0.42 to 1.00 mmol.g-1 and 0.50 to 1.32 mmol.g-1 onto

CS/P25 respectively, as the pH of the solution decreased from 10 to 3.

This unique adsorption behavior of dyes is due to the structure of CS. At low

pH, the available proton in the medium protonate the amine groups of CS and form

-NH3+ ions on the CS chain [253]. These positively charged adsorption sites of CS

attract the negatively charged dye, which in turn increased the adsorption of dye on

the membranes. Whereas, at neutral and basic pH range, the proportional decrease in

dye adsorption took place because of deprotonation of -NH3+ groups on CS. The

electrostatic repulsion between anionic dyes (NR and POr) (negatively charged) and

the deprotonated groups of membranes have been occurred.

100

Figure 3.30: (a) The effect of adsorbents weight at pH = 7, (b) pH Effect on

adsorption of NR (■□) and POr (●○) on CS/P05 (solid dots) and CS/P25 (blank dots)

membranes (contact time = 24h, dye concentration = 1mmol.L-1, sample weight = 50

mg).

0.02 0.04 0.06 0.08 0.100.6

0.8

1.0

1.2

1.4

q e (m

mol

/g)

Weight (g)

(a)

2 4 6 8 10

0.4

0.8

1.2

1.6

2.0

2.4

q e (m

mol

/g)

pH

(b) (b)

q e (m

mol

.g-1

) q e

(mm

ol.g

-1)

101

The interactions between dyes and CS at lower pH can be: dipole/induced

dipole forces, ion/ion interactions and hydrogen bonds. The interaction is given in

Figure 3.31.

Figure 3.31: Proposed interaction of dye with crosslinked membrane at low pH.

3.3.3.3. Adsorption isotherm

Figure 3.32 shows the adsorption isotherms of the two dyes (NR, POr) onto

crosslinked membranes CS/P05 and CS/P25 at 30 °C. As the dye concentration

increased, the equilibrium adsorption density (qe) is increased. According to Giles et

al. isotherms of physical adsorption [18], the shape of isotherm of dyes adsorption

onto crosslinked membranes is L type. It indicates that the membranes show high

density of adsorption even at low dye equilibrium concentrations.

Langmuir, Langmuir- Freundlich and Freundlich isotherms are applied to this

experimental isotherm data but Langmuir- Freundlich shows good fitting with

experimental data of adsorbed dyes onto CS/P05 and Langmuir shows good fitting

with the experimental data of adsorbed dyes onto CS/P25 membranes. The parameters

which are determined from models are shown in Table 3.15. All isotherms showed

very good fitting having ‘R2’ 0.9998.

The maximum adsorption uptake ‘qmax’ of CS/P05 and CS/P25 for NR is 1.30

and 0.76 mmol.g-1 and while for POr is 1.31 mmol.g-1 and 0.80 mmol.g-1 respectively

at room temperature which increase with increasing temperature. It is clear from these

102

values that the adsorption of POr is higher compared to NR dye on both membranes

because the molecular size of POr (1.47×0.71×0.57 nm3) is smaller than that of NR

(NR is 1.57×1.31×0.63 nm3) and due to smaller size more molecules of POr can be

adsorbed per unit weight of the membranes.

By accounting the degree of deacetylation (75%) of CS and the molecular

weight (161.16 g mol-1) of repeating unit of the CS, the mole of the adsorption site –

NH3 is 4.66 mmol.g-1, –NH3 (which is used in crosslinking) is not include in this

calculation. Then by is roughly estimated, the coverage of dyes POr is 51% and NR

30% onto each membrane. The difference in coverage is due to moleculer size of

dyes, NR is larger than that of POr. It is difficult for the large NR molecule to adsorb

into very small pores in the crosslinked membrane, and one adsorbed molecule of NR

blocks number of adsorption sites –NH3 on the CS/PVA membrane which reduce the

uptake of another dye molecules. While the adsorption of dyes onto CS/P05 is higher

compared to CS/P25 membrane due to increase of higher amount of PVA content,

which increase hydrophobic characteristic and form complex and compact structure of

membrane and might be increase in the pore size thus reducing the adsorption of dye.

Figure 3.32: Adsorption of NR (■□) and POr (●○) dyes onto crosslinked CS/P05

(solid dots) membrane fitted by Langmuir model and CS/P25 (blank dots) fitted by

Langmuir-Freundlich law model at temperature 30 °C and pH 7.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.5 1 1.5 2

qe (m

mol

/g)

Time (h)

q e (m

mol

.g-1

)

103

Table 3.15: Parameters (rounded values) of Langmuir and Langmuir-Freundlich fits

of isotherms of adsorption for NR and POr on the CS/P05 and CS/P25 membranes at

temperature in the range 25-55 °C.

Temp.

(°C)

CS/P05 adsorption-Langmuir-Freundlich

parameters

CS/P25 adsorption-Langmuir

Parameters

NR POr NR POr

qe bK qmax R2 qe bK qmax R2 qe bK qmax R2 qe bK qmax R2

30 1.30 2.17 1.31 0.9988 1.31 2.16 1.28 0.9999 0.76 5.79 0.76 0.9989 0.80 8.75 0.80 0.9998

45 1.34 2.54 1.34 0.9988 1.37 2.55 1.35 0.9998 0.96 5.56 0.94 0.9988 1.02 10.62 1.01 0.9988

55 1.40 2.76 1.40 0.9998 1.43 2.83 1.42 0.9998 1.09 5.59 1.14 0.9999 1.14 15.53 1.18 0.9999

qe- mmol.g-1, bK -mmol. g-1.min-1, qmax-mmol.g-1

3.3.3.4. Thermodynamic parameters

The temperature effect on the adsorption of dyes onto crosslinked membranes

is also studied shown in Figure 3.33 which shows that the adsorption uptake of dyes

on the blend is increased, with the increase of solution temperature, which shows that

the adsorption is endothermic. The increase in ‘bK’ values with the increase of

temperature showed (Table 3.15) that the heat of adsorption increased with rising

temperature. Temperature plays an important role in adsorption process, and also

helps to determine the nature of adsorption by calculating the thermodynamic

parameters such as: Gibb’s free energy, entropy and enthalpy are shown in Table

3.16]. The results of the simulation by Langmuir and Langmuir-Freundlich obtained

with an R factor (higher than 0.99) are used to find out the isosteric Gibbs Free energy

(∆G°) of adsorption for each dye onto crosslinked membrane using the equation

below:

ln (Ca/Ce) = -∆G°/RT (3.1)

where ‘R’ is the gas constant (8.314 J /mol/K), ‘T’ is an absolute temperature (K), ‘Ce’

and ‘Ca’ are the values of solid phase concentration and liquid phase concentration at

equilibrium. The isosteric entropy (∆S°) and enthalpy (∆H°) of adsorption are

calculated from origin point and slope of the plot of 1/T versus Ln(Ka/Ce) using the

following equation:

𝛥𝐺○ = 𝛥𝐻○ − 𝑇𝛥𝑆○ (3.2)

104

The negative value of ΔG° (Table 3.16) in NR and POr adsorption on both

membranes indicates that the reaction is spontaneous and the process is possible

because more favorable at higher temperatures value. The positive value of enthalpy

(Table 3.16) shows that the adsorption process is endothermic. The entropy value

(Table 3.16) confirms that during adsorption process randomness at the solid-liquids

interface is increased and process is favored ectopically.

Figure 3.33: Adsorption of NR (■□) and POr (●○) dyes onto crosslinked CS/P05

(solid dots) and CS/P25 (blank dots) membrane at temperature 30 °C (dashed lines),

45 °C (dotted lines), and 55 °C (solid lines)

Table 3.16: Isosteric Gibbs free energy, enthalpy and entropy of adsorption of dyes on

membranes (at constant value of adsorption uptake).

IL type CS/P05 SC/P25

Dyes NR POr NR POr

ΔG° (kJ.mol-1) -0.61 -0.05 -2.27 -3.41

ΔH° (kJ.mol-1) 9.36 18.88 33.01 32.70

ΔS° (kJ.K-1.mol-1) 33.49 63.54 118.40 121.20

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.5 1 1.5 2

qe (m

mol

/g)

Time (h)

q e (m

mol

.g-1

)

105

The values of isosteric enthalpy and Gibb’s free energy of dyes adsorption onto

crosslinked membranes are plotted against qe in Figure 3.34. The adsorption process

for both dyes on both membranes is found to be spontaneous. The values show a

typical phenomenon of a physisorption process. It is also confirmed that the dyes

possessed a higher affinity with CS/P05 surface (in terms of dispersive interaction).

0.1 0.2 0.3 0.4 0.5-5

0

5

10

15

20

25

30

35

-5

0

5

10

15

20

25

30

35

∆Ηο (J

/K/m

ole)

∆ G

o (J/K

/mol

e)

qe (mmol/g)

POr-CS/P25 NR-CS/P25 POr-CSP05 NR-CS/P05 POr-CS/P25 NR-CS/P25

POr-CS/P05 NR-CS/P05

Figure 3.34: Dependence of adsorption uptake of dyes (qe in mmol.g-1), at pH = 7 of

the isosteric Gibbs free energy (solid lines) and of entropy of adsorption (dotted lines)

CS/P05 and CS/P25 membranes.

3.3.3.5. Comparative study

A comparative study has also been carried out and the obtained results are

compared with some of published results on similar CS based system. In Table 3.17,

the maximum equilibrium adsorption capacity (Qmax) of chemically crosslinked

membranes calculated from the Langmuir and Langmuir Freundlich isotherm model

is compared with literature values of Qmax of other adsorbents for different dyes.

These adsorbents showed variable adsorption values.

qe (mmol.g-1)

∆Gº (

J.K

-1. g

-1)

∆Hº (

J.K

-1. g

-1)

106

Table 3.17: Comparison of Qmax with published work on the adsorption of dyes on CS

based system.

Adsorbent Dye Qmax (mg.g-1) Reference

monmorillonite CSbeads Congo red 12.7 [254]

CS -Monmorillonite

nanocomposite

Congo red 54.5 [255]

CS beads Congo red 93.7 [256]

CS-EGDE Acid Blue 25 125 [175]

N,O Carboxymethyl CS Congo red 330.6 [257]

CTAB modified CS beads Congo red 433.1 [258]

crosslinked CS/P05 membrane Nylosan red 935 Present work

crosslinked CS/P05membrane Palatine orange 1230 Present work

crosslinked CS/P25 membrane Nylosan red 588 Present work

crosslinked CS/P25membrane Palatine orange 671 Present work

In conclusion, CS is successfully crosslinked with poly (vinyl alcohol) using

methyltrimethoxysilane crosslinker. Membranes showed maximum swelling at pH 3

and show hydrogel properties. The membranes show high adsorption capacities for

dyes in acidic pH. The adsorption capacities of membranes are significantly affected

by the solution pH and PVA content. The electrostatic interaction between dye and

CS (NH3 groups) might be used to explain the high adsorption capacity of both

anionic dyes onto crosslinked membranes. The high adsorption of dyes on membranes

in acidic range is very useful because most of textile effluent is in acidic range and

this adsorbent system is quit suitable for such type of system. These conclusions are

in good agreement with literature.

107

3.4. Adsorption of copper metal on modified acrylic acid hydrogel

with their different properties Radiation technology has been effectively used to prepare hydrogel by

crosslinking hydrophilic monomer. The properties of these hydrogels can be

controlled by changing the dose which influences the structure and crosslinking

density of final product [259]. The hydrogel of acrylic acid and its crosslinking with

different dose of gamma rays and different amount of PTES (crosslinking agent) gave

different crosslinking density of hydrogel. Novelty in this work is the use of new

PTES (as silane crosslinking agent) in gamma radiated hydrogel. The effect of PTES

and gamma dose on various properties of the hydrogel is explained in this work. The

discussion of this chapter include the structural investigation by TGA, FTIR and gel

content analysis and different swelling properties of the this hydrogel such as

swelling in water, buffer, non-buffer, CaCl2, NaCl and temperature effect. In

application, the adsorption of Cu metal is studied. The amount of PTES, dose rate

and hydrogels named are expressed in Table 2.3. 3.4.1. Characterization

The FTIR spectra of the crosslinked hydrogels are shown in Figure 3.35. The

spectra of crosslinked hydrogels show a very intense broad absorption band centered

at 3330 cm-1 is due to the associated OH groups. Highest intensity of this band is

observed in AA80/15 containing higher amount of PTES, which might be due to the

presence of high concentration of Si-OH groups. Three bands at 2949, 2922 and 2841

cm-1 are due to different CH stretching bands present in the sample. The 1695 cm-1

band of acrylic acid (AA) representing the C=O stretching is reduced and shifted to

1712 cm-1. The intensity of C–O stretching vibration of carboxylic group at ~1168

cm-1 is also reduced. A new band at 1560 cm-1 is appeared in PAA which represents

the C-C stretching and confirmed the conversion of AA into poly (acrylic acid). Two

sharp bands at 1400 cm-1 and 1180 cm-1 are due to the Si-phenyl bond. The bands at

1068 cm-1 indicated the presence of Si-O-Si linkage, while the bands at 1000, 857 and

502 cm-1 correspond to asymmetric, symmetric and bending modes of Si-O-Si [264].

Bands at 1060, 976, 924, 805 and 635 cm-1 present in pure AA are disappeared due to

the overlapping of similar bands.

108

4000 3500 3000 2500 2000 1500 1000 500

Tran

smitta

nce (

a.u)

Wavenumber (cm-1)

17121553

1398

1168

1068

(a)

(b)(c)

(d)

Figure 3.35: FTIR spectra of (a) poly(acrylic acid) and hydrogels, (b) AA40/15, (c)

AA60/15, (d) AA80/15

3.4.2. Thermogravimetric analysis

The TGA thermogram of crosslinked hydrogels are shown in Figure 3.36. All

thermograms of crosslinked hydrogels show similar degradation behavior containing

two decomposition stages. The first decomposition stage is between 50 to 200 °C,

which is mainly attributed to the loss of free and bound water. The mass loss of

crosslinked hydrogels is more than 40 %. The second stage which is due to

decarboxylation and decomposition of the residual polymer started above 200 °C. The

thermal decomposition data at various percentages of mass losses is given in Table 2.

This table shows that the hydrogels containing higher concentration of PTES are

thermally more stable. High thermal stability of the hydrogel is observed when

amount of PTES is increased from 0.83 to 1.25 μmol. Further increase in PTES,

lowers the thermal stability of hydrogel. The adsorbed dose did not show any effect in

sample having same PTES amount. The crosslinked hydrogel containing same

amount of crosslinker but irradiated at higher dose showed more residue which is due

to the higher crosslinking of PAA.

109

0 100 200 300 400 500 6000

20

40

60

80

100

120

Mas

s (%

)

Temperature (oC)

AA80/15 AA60/15 AA40/15

Figure 3.36: Thermogram of AA40/15, AA60/15 and AA80/15 hydrogels.

Table 3.18: Thermal decomposition data of crosslinked hydrogels at various

percentage mass loss.

Sample T40%(ºC) T50%(ºC) T60%(ºC) Residue at 525 °C

(%)

AA40/15 122.0 216.0 392.5 24.1

AA40/30 123.1 341.0 430.5 26.7

AA60/15 157.8 164.8 315.4 19.7

AA60/30 185.0 182.2 377.3 22.3

AA80/15 107.0 191.6 381.6 21.5

AA80/30 108.2 349.1 431.3 22.7

T40%: temperature at 40% mass loss

3.4.3. Gel content

The effect of absorbed dose on gel fraction of PAA is shown in Table 3.19. It

is observed that the gel fraction is slightly affected by PTES concentration but greatly

affected by irradiation doses (15, 30 kGy). The increase of irradiation dose from 15 to

30 kGy almost doubles the gel fraction even at low PTES concentration. This shows

that at high dose, more PTES chains are incorporated in the growing polymer chain

which subsequently gives network of PAA forming a complex between PAA and

PTES [260,261].

110

Table 3.19: Gel content and diffusion parameters of crosslinked hydrogels.

3.4.4. Swelling studies

3.4.4.1. Swelling in water

The swelling behavior of crosslinked PAA hydrogel in water is studied against

time and the results are shown in Figure 3.37. This figure shows that the swelling of

hydrogel is increased linearly with increase in time and reached the equilibrium value

between 25–30 h. It is also observed that at a given dose, the swelling of hydrogel

decreased as the concentration of PTES increased. This shows that the higher PTES

amount increases the number of links between polymer chains. As a result, the size of

pores in the hydrogels decreased which resulted in marked decrease in the swelling.

For example, the swelling of AA40/15 is 246 g.g-1 which is decreased to 198 g.g-1 in

AA80/15. Similarly the swelling is also affected by amount of dose. It is decreased

with increase in dose from 15 to 30 kGy in samples having same amount of

crosslinker. At high dose, more radicals are generated in the sample, which form more

links between the polymer chains. Thus, reduces the mobility of polymer chain which

in turns reduces the EDS value of the hydrogel. Similar decreasing trend in EDS

values is observed in poly(acrylic acid) hydrogel prepared by electron beam [262].

The diffusion of solvent molecules from outer medium into the hydrogel structure

is the source of swelling. The diffusion mechanism of solvent in the hydrogel can be

explained by using the equation 2.3.

Samples Gel content (%) n K*102

AA40/15 35 0.88 6.1

AA60/15 36 0.85 6.2

AA80/15 38 0.81 8.1

AA40/30 55 0.78 6.1

AA60/30 65 0.73 6.2

AA80/30 82 0.71 8.1

111

Figure 3.37: (a) Swelling kinetics of crosslinked hydrogel in water (b) ln (F) plotted

versus ln (t) for crosslinked hydrogels.

0 1 2 3-4

-3

-2

-1

0

1

ln (F

)

ln (t)

AA40/15 AA60/15 AA80/15 AA40/30 AA60/30 AA80/30

(b)

0 10 20 30 400

50

100

150

200

250

Swel

ling

(g/g

)

Time (h)

AA40/15 AA60/15 AA80/15 AA40/30 AA60/30 AA80/30

(a)Sw

ellin

g (g

.g-1

)

112

The mechanisms of diffusion, release and transport are explained by the value

of ‘n’ [263]. The swelling data from Figure 3.37 (a) is used to obtain the values of ‘n’

and ‘k’. These values of ‘n’ and ‘k’ are given in Table 3 and plot of ‘ln F’ versus ‘ln t’

is shown in Figure 3.37 (b). It is observed that the value of ‘n’ is decreased with

increasing amount of PTES in the sample (Table 3.19). The diffusion of water in all

hydrogel follows the non-Fickian mechanism. For non-Fickian diffusion the value of

‘n’ is 0.5 < n < 1 [221,263].

3.4.4.2. pH Effect

The swelling response of ionic hydrogel is greatly affected by pH of the

external medium. The changes in pH of the external medium disturb the charge of the

ionizable groups present in the hydrogel network. Their ionization control the

penetration of solvent molecules into hydrogels structure [221]. In industrial and

biomedical applications, the swelling response of hydrogels (at different pH) plays a

significant role [264]. In this study, the swelling response of the hydrogels against pH

in non-buffer and buffer media is investigated.

The effect of pH on hydrogel swelling in non-buffer media is studied and

results are shown in Figure 3.38. This figure shows switchable trend in the swelling of

hydrogel at different pH. Maximum swelling is obtained around neutral pH while low

swelling in basic and acidic pH range. The swelling of ‘AA40/15’ hydrogels

is negligible at pH 2 and gradually increases and reaches the maximum value of

247 g.g-1 at pH 7. Further increase in pH lowers the swelling of the hydrogel and

reached to 68 g.g-1 at pH 10. This pH dependent swelling and deswelling is probably

due to the ionization of carboxylic groups of AA. When the pH of the medium is less

than the pKa of AA (pKa = 4.7), the ionization of carboxylic acid group is suppressed

due to presence of higher amount of H+ ions [265]. As a result, a compact structure is

formed by intra and inert hydrogen bond within the hydrogel. However, when the pH

of medium is around the pKa value of the AA then the carboxylic group dissociates

into carboxylate and hydrogen ions. The repulsion between the carboxylate ions

increased the free spaces and resulted high swelling of the hydrogel. Whereas in basic

pH range, all carboxylic groups are deprotonated and the concentration of negative

ions is maximum on the chain. As a result, maximum repulsion between ions is

observed and water molecules are not able to stand in the pores which reduced the

swelling [129].

113

Figure 3.38: pH Effect on swelling behavior of the crosslinked hydrogel in non-buffer

solution.

The swelling behavior of hydrogels as a function of buffer pH is shown in Figure

3.39. It shows that the swelling behavior is similar as observed in non-buffer media.

All the hydrogels showed maximum swelling at neutral pH and low swelling in basic

and acidic pH range. However at same pH, the swelling values for these hydrogels are

three times less as compared to the swelling in non-buffer media. The swelling ratio

of ‘AA40/15’ is 47 g.g-1 in buffer media while 247 g.g-1 in non-buffer at pH 7. The

possible reduction in swelling in buffer media might be due to the high ionic strength

of the buffer media as compared to the non-buffer media [129].

3.4.4.3. Effect of electrolytes on swelling

The effect of nature of cation on the swelling behavior of the hydrogel is

depicted in Figure 3.40 and 3.41. NaCl and BaCl2 are used as an electrolyte in this

swelling study. These figures show that the swelling of the hydrogels decreased with

increase in the ionic strength of salt solution. This swelling behavior of the hydrogel

is related with change in osmotic pressure which is developed because of unequal

distribution of ions between polymer network and the external medium. The ions

formed on the polymer chains are fixed and separated from the external solution.

Therefore osmotic pressure is developed when the hydrogels are placed in solution. In

NaCl solution, the osmotic pressure of the external media is high due to Na+ and Cl¯

0 2 4 6 8 100

50

100

150

200

250

Swel

ling

(g/g

)

PH

AA40/15 AA60/15 AA80/15 AA40/30 AA60/30 AA80/30

Swel

ling

(g.g

-1)

114

ions, which deswell hydrogel. At same electrolyte concentration, low swelling is

observed in BaCl2 solution. This decrease in swelling is related to the divalent nature

of barium ions. The divalent barium cation acts as ionic crosslinking agents and forms

complexes between different polymer chains. This ionic crosslinking of the polymer

chains further contributes thus the swelling of hydrogel is reduced [266].

Figure 3.39: Effect of pH on swelling behavior of the crosslinked hydrogel in buffer.

3.4.4.4. Effect of temperature on swelling

The effect of temperature on the swelling of hydrogel is studied from 25 to 40 °C

and results are shown in Figure 3.42. This figure shows that swelling of all hydrogels

increase with increase in temperature. This reveals that expansion of the polymer

chain at high temperature creates more space for penetration of water molecule

resulting in increased swelling. The thermo responsive behavior of these hydrogels

can be exploited in environmental and medical applications.

0 2 4 6 8 10 120

10

20

30

40

50

Swel

ling

(g/g

)

pH

AA40/15 AA60/15 AA80/15 AA40/30 AA60/30 AA80/30

Swel

ling

(g.g

-1)

115

Figure 3.40: Swelling of crosslinked hydrogels in NaCl solution

Figure 3.41: Swelling of crosslinked hydrogels in BaCl2 solution.

0.0 0.2 0.4 0.6 0.8 1.0 1.20

3

6

9

12

15

18

Swel

ling

(g/g

)

Concentration (M)

AA40/15 AA60/15 AA80/15 AA40/30 AA60/30 AA80/30

0.0 0.2 0.4 0.6 0.8 1.0 1.25

10

15

20

25

30

35

40

Swel

ling

(g/g

)

Concentration (M)

AA40/15 AA60/15 AA80/15 AA40/30 AA60/30 AA80/30

Swel

ling

(g.g

-1)

Swel

ling

(g.g

-1)

116

20 24 28 32 36 40

120

160

200

240

280

Swel

ling

(g/g

)

Temperature (oC)

AA40/15 AA60/15 AA80/15 AA40/30 AA60/30 AA80/30

Figure 3.42. Swelling of crosslinked hydrogels at different temperature.

3.4.5. Application of Hydrogel in adsorption of copper metal

In the adsorption, the main challenge is to identify clearly the mechanism of

adsorption. Normally the adsorption process follow the following three phases. (1)

The particles of adsorbate move to the adsorbent surface, (2) The adsorbate particle

diffuses into the adsorbent pores and (3) The chemical reaction take place between

adsorbate particle and the functional groups of the adsorbent. The process of

adsorption is controlled normally by the first or the second phases.

In this study, the Cu ions adsorption on to the hydrogel surface mainly takes

place through the chelation between metal ions (positively charged) and carboxylate

groups of the hydrogel. In the case of metal ions and superabsorbent hydrogel,

interaction is assumed to take place by coordinate bonds and electrostatic forces.

The FTIR spectra of hydrogel AA40/15 before and after adsorption of copper

is investigated and shown in Figure 3.43. Some important facts are apparent from the

spectra, such as the broad band at 3330 cm-1 (Figure 3.43(b)), which is more

broadened and appeared at 3320 cm-1 after meta adsorption, because of interaction of

Cu ions with –OH or –NH2 groups of hydrogel. The metal loaded IR spectra also

show that the peak at 1718 cm-1 which is due to carbonyl groups are weakened and

shifted to 1708 cm-1 after metal adsorption. The absorption peak due to ether groupsat

Swel

ling

(g.g

-1)

117

1279 cm-1 does not appear in hydrogel after adsorption of metal. Moreover, peak at

1556 cm-1 (corresponding to carboxylate symmetric stretching) are weakened and

shifted to 1547 cm-1 after adsorption of metal and the peak at 100 cm-1 due to

asymmetric stretching of the carboxylate are shifted to 1420 cm-1 after metal

adsorption. On the basis of IR spectral study of unloaded and Cu ion loaded AA40/15

hydrogel, the conclusion is that –OH, –NH2, COOH and COO groups are probably

involved in the adsorption of metal ion onto hydrogels.

3500 3000 2500 2000 1500 1000 500

(1279)(1708)

(1718)

(1547)

(1556)(1400)

( 3320)

Tran

smitta

nce (

a.u)

Wavenumber (cm-1)

(a)

(b)

(c) (3330)

(1480)

Figure 3.43: FTIR spectra of (a) poly(acrylic acid) and hydrogels, (b) AA40/15, and

(c) AA40/15 after adsorption of Cu metal.

3.4.5.1. pH effect

The effect of pH on the system is an important parameter which affects the

metal ions chelation and its adsorption on the polymeric adsorbents [267,268]

particularly effects the coordination bond between functional groups and metal ions. It

also affects the metal ligand complexes stability and chelating ligand ionization. The

pH of solution is effect the adsorption uptake of metal ions onto PAA hydrogel. The

effect of pH on Cu ion uptake on PAA hydrogels has been studied at a fixed initial

concentration of Cu ions (10 ppm, pH < 5.0).

Figure 3.44 shows the pH effect on the Cu uptake (qe) on PAA hydrogels. The

uptake of Cu2+ is small at pH 2, and the adsorption increase to almost constant value

118

at pH 3. This behavior of Cu adsorption is an agreement with work reported by Li et

al. [268]. This is due to hydrolysis metal ions at higher solutions pH and higher

concentration of the solution. The behavior of adsorption onto PAA hydrogels

depends on different parameters such as the solution pH, the charge valency on

adsorbent and the metal ions solvation chemistry (e.g. formation of polynuclear

species after hydrolysis [269]. The pH affects the capacity of binding by the

equilibrium shifting of ion-exchange ability and coordination reaction in following

ways: changing the active ligands concentration and the soluble metal ions

concentration. At pH 2, there is concentration of H+ which protonates the carboxylic

groups (–COO–) of PAA hydrogel which repels the metal ion and adsorption

decrease at pH 2 [268].

As the pH increases from neutral to 12, the Cu2+ ions hydrolyze to form

Cu2(OH)2+ and even in more alkaline solutions it form Cu(OH)4

2-. The experiments

are carried out below pH 5, where copper ions are found mainly in the Cu2+ form and

the hydrolysis of Cu2+ is not occur significantly under these pH condition (i.e. 3)

[270].

Figure 3.44: pH effect on the adsorption capacity of Cu metal on PAA hydrogels

(contact time = 24 h, metal concentration = 10 mg.L-1, sample weight = 50 mg).

q e (m

g/g)

q e

(mg.

g-1)

119

3.4.5.2. Effect of adsorbent dose

Figure 3.45 shows the effect of increasing weight of PAA hydrogels on

percent removal of Cu metal. The weight of PAA hydrogel is increased from 20 to

150 mg at fixed concentration of 10 ppm Cu at room temperature. The removal

percent of metal by AA40/15 hydrogel increases from 25.3 to 91.1% with increase of

hydrogel weight from 25 to 50 mg respectively. No further increases of adsorption

take place with increase of adsorbent dose. Similar trend is found on other hydrogels

as shown in Figure 3.45. The increase of removal with increase of adsorbent dose may

be due to concentration gradient of metal concentration in the solution and onto PAA

hydrogels.

Figure 3.45: The effect of adsorbent weight on percent removal of metal (contact time

= 24 h, metal concentration = 10 mg.L-1, sample weight = 50 mg, pH = 3, room

temperature).

3.4.5.3. Adsorption kinetics

The time of agitation is one of the important parameters, which reflects the

adsorbent kinetics at a given adsorbate concentration. The Cu ions adsorption kinetics

on PAA hydrogels are investigated as shown in Figure 3.46. It can be seen that the

adsorption uptakes of Cu ions increase with increase of time, but the rates of

adsorption obviously decrease as the time increases, and equilibrium gradually

Adsorbent Weight (mg)

Rem

oval

(%) o

f met

al

120

reaches around 5 h. It can be explain as follows, when the hydrogel is immersed in a

solution, molecules of water quickly penetrate into the hydrogel, resulting in a

swelling of polymeric networks. Then, the concentration gradient of Cu ions is

formed at water-hydrogel gel interface, and it starts diffusion onto the PAA hydrogel.

As hydrogel contain large amount –COO─ groups, which adsorb and tape the Cu from

the external solution and equilibrium reach after some time.

In order to study the adsorption mechanism which controls the adsorption

processes (such as chemical reaction and mass transfer), the kinetics models (pseudo

second-order model) are used to simulate the experimental data and best fitted model

to the experimental kinetics data is shown in Figure 3.46 (a and b).

The values of ‘qe’ and ‘k1’ are calculated from the nonlinear regression of ‘qt’

versus ‘t’, of both models which is obtained by integration of their respective

equations explained previously (in chapter 1) and kinetics data thus obtained using

these models are given in Table 3.20. This table shows consistency of theoretical

equilibrium (qe) with calculated experimental equilibrium (qmax cal) adsorbed by PAA

hydrogels for Cu metal ions.

This study showed both intraparticle and external diffusion in the real process.

However the actual rate limiting step is important to determine. In order to know the

actual rate limiting step, the Boyed law is used, which explain the mechanism of

diffusion of particle into PAA hydrogels. This law proposes that the solute diffusion

in the pores of adsorbent, depend on the concentration of adsorbent and not on the

solute concentration, which help to know about the rate limiting step in liquid-solid

adsorption. The ‘Bt’ values obtained from kinetics data are plotted against time ‘t’ as

shown in Figure 3.46 (c). The plot linearity give important information about

adsorption mechanism and help to distinguish between solute intraparticle diffusion

and solute external mass transfer controlled [242]. The linear lines of all PAA

hydrogel do not pass through the origin in Cu metal adsorption, confirming that there

is involvement of external mass transfer in the process of adsorption [241].

121

Table 3.20: Kinetics data for the adsorption of Cu on PAA hydrogels obtained using

kinetics models at metal concentration =10 mg.L-1, sample weight =50 mg, pH = 7.

Samples qe

exp.(mg.g-1)

Pseudo-First Order Pseudo-Second order k1,

(min-1)

qmax calc.

(mg.g-1)

R2 K1,

(min-1)

qmax calc.

(mg.g-1)

R2

AA40/15 1.65 0.68 1.66 0.9899 0.39 1.63 0.9789

AA60/15 1.61 0.65 1.60 0.9998 0.36 1.66 0.9899

AA80/15 1.22 0.33 1.21 0.9988 0.14 1.20 0.9989

AA40/30 1.60 0.46 1.60 0.9898 0.24 1.58 0.9688

AA60/30 1.56 0.36 1.58 0.9888 0.14 1.56 0.9889

AA80/30 1.14 0.14 1.16 0.9898 0.026 1.05 0.9899

3.4.5.4. Adsorption isotherms

To study the Cu metal adsorption, PAA hydrogels are dipped in the solutions

of Cu for 24 h at room temperature. After 24 h, PAA hydrogels in Cu solutions

showed blue coloration. The adsorption (mg.g-1) of PAA hydrogels are calculated by

following equation:

Adsorbed metal (mg.g-1 dry hydrogel)= 𝑞𝑒 = 𝐶𝑖−𝐶𝑒𝑚

𝑉 (3.3)

Where qe is the adsorbed metal onto PAA hydrogel (mg.g-1), Ce and Ci are the

equilibrium and initial concentrations of solution of metal, V is the total solution total

volume of the metal and m is the hydrogel dry mass.

Table 3.21 illustrates that metal adsorption capacity of PAA hydrogels are

decrease with increase of gamma irradiation dose and PTES amount. Since with the

increase of PTES amount and increase of radiation dose, the cross-linking density is

increased, the hydrogel molecular pores become smaller and the molecule of metal

could not penetrate sufficiently inside the network of hydrogel.

122

Figure 3.46: Kinetics of Cu metal on PAA hydrogels (a) Pseudo first order model

fitting (b) Pseudo-second order model fitting (c) Boyd law fitted (linearly) for the

kinetics adsorption data at metal concentration = 10 mg.g-1, sample weight = 100 mg,

pH = 7.

q e (m

g/g)

q e

(mg.

g-1)

q e (m

g.g-1

) q e

(mg.

g-1)

123

Figures 3.47 shows the uptake of copper sulfate as a function of copper equilibrium

concentration in the copper sulfate solution. At relatively low copper ion

concentration in the solution, the uptake of copper drastically increases with an

increase in the copper ion concentration. For example, with increase of equilibrium

concentration of copper ion from 5 to 100 mg.g-1 leads to an increase in copper uptake

from 0.8 to 143 mg.g-1 of AA40/15 hydrogel, same behavior is found in other

hydrogels. However, when the concentration of copper is greater than 100 mg.g-1, the

copper sulfate equilibrium sorption uptake does not increase significantly. This is easy

to understand. Considering the chelating complexation of copper ions adsorbed with

the active functional groups in the hydrogel. The PAA hydrogels has a high power to

adsorb the copper ions (even at a low concentration). However, when almost all

functional active sites in the hydrogels are filled by copper ions, the PAA hydrogel

will not be able to continue adsorbed effectively additional copper ions, in spite of

higher concentration of copper ion in the solution [253,264]. While copper ions are

adsorbed to the hydrogel active sites, the uptake of water decreases, as expected. The

chelating complexation of copper ion during adsorption occupies the functional

groups which reduces the water uptake. The network of the hydrogel becomes more

tight and ridged with complexation of copper with functional groups of hydrogel.

The adsorption data of isotherm are simulated with Langmuir model with a

correlation coefficient (R2) of 0.9899 (Figure 3.47). This model is based on adsorption

on monolayer active sites of adsorbent, which represents chemisorption on localized

adsorption sites. It assumes negligible forces between adsorbate molecules, and once

an adsorbate molecule is adsorbed on an adsorbent site, no further adsorption takes

place. It is noted that active sites of adsorption on PAA (carboxyl groups), form

chelating complexes (with one or two ligands) with copper ions [132,271]. Based on

Langmuir isotherm, the maximum sorption capacity is shown by the AA40/15

hydrogel among all hydrogels (qmax is 143.4 mg.g-1 with correlation coefficient 0.99).

124

Table 3.21: Parameter obtained from Langmuir model for the Cu metal at pH 3

Samples qe exp.(mg.g-1) Langmuir model parameters

qmax(mg.g-1) R2

AA40/15 143.4 146.3 0.9989

AA60/15 141.3 1.39.5 0.9899

AA80/15 113.4 112.7 0.9758

AA40/30 131.0 132.8 0.9685

AA60/30 127.3 126.9 0.9786

AA80/30 106.0 107.0 0.9777

Figure 3.47. Adsorption of metal onto PAA hydrogel at different concentration fitted

by Langmuir law.

Concentration (mg/L)

q e (m

g.g-1

)

Concentration (mg.L-1)

125

In conclusion, crosslinked poly (acrylic acid) hydrogel are successfully prepared

by incorporating various concentrations of PTES. The thermodynamic stability of

hydrogel increases at radiation dose and low PTES amount. They show high swelling

around neutral pH while low swelling at basic and acidic pH range. This switchable

pH response of these hydrogels can be exploited in environmental applications.

These hydrogels show good adsorption capacity of Copper from aqueous solution.

The adsorption of copper is affected by the solutions pH.

126

Chapter 4 Conclusions

The possibility of removing pyridinium, imidazolium and pyrrolodinium-

based ILs from water by effective adsorption on ACs (Fab and granulated) was

established with the help of equilibrium adsorption study at different temperatures.

The factors which strongly affect the adsorption capacities were the IL cation type

and size, its hydrophobic nature which mainly depends on the length of the alkyl

chains. The comparison of the size and volume of the IL molecules and the porous

volume of ACs have suggested that the smaller ILs (with butyl chain) are

preferentially adsorbed in the ultramicropore volume as unsolvated pair of ions

(cation and anion). Whereas, the bulkier cations are preferably adsorbed in the

ultramicropores and supermicropores of ACs. The spontaneity of adsorption follows

the trend: pyridinium > methylimidazolium > methylpyrrolidinium. The calculated

thermodynamic parameters and experimental equilibrium data are consistent with a

spontaneous and exothermic adsorption mechanism, which is controlled by π-π

interactions and attractive van der waals between adsorbate–adsorbent interactions. It

was confirmed that more spontaneous adsorption took place for longer chain length

ILs having hydrophobic cations than for lower chain length ILs.

Granulated AC was modified by treating with HNO3 and NaOCl. BET results

showed that its surface area was decreased without affecting its textural properties.

Boehm titration results showed that surface of AAC and BAC contains carboxylic,

lactones, phenol and basic groups. Adsorption study of ILs on ACs showed that IL of

higher chain showed higher adsorption and adsorption increased at pH 9 than pH 2, as

surface of AC got positive at higher pH. In mACs, the adsorption of ILs was

enhanced compared to RAC. Kinetics data was successfully simulated by Pseudo first

order and pseudo second order model and isotherm data was best fitted by Langmuir

model. This finding indicated that oxidation of AC could be useful method to increase

the adsorption uptake of ILs. Therefore, the ACs may be a capable adsorbent for

organic pollutants in water. Oxidation with NaOCl is better than HNO3 because

oxidation with HNO3 blocked the pores of AC with enhancement of functional

groups. In case of NaOCl oxidation microporous volume is not significantly affected.

127

CS was successfully and chemically crosslinked with poly (vinyl alcohol)

using methyltrimethoxysilane crosslinker. Membranes showed maximum swelling at

pH 3 and in low concentration of ionic solution and gel content around 40%.

Membranes showed hydrogel properties and there was peaks shifting in FTIR spectra

of the membranes after dye adsorption. The cross-linked membranes had high

adsorption capacities for anionic dyes which was 2.42 mmol.g-1 (1230 mg.g-1) and

1.32 mmol.g-1 (671 mg.g-1) for POr onto CS/P05 and CS/P25 membrane and 1.59

mmol/g (935 mg.g-1) and 1 mmol.g-1 (588 mg.g-1) for NR onto CS/P05 and CS/P25

membrane respectively at pH 3 and at room temperature. The adsorption capacities

were significantly affected by the pH, PVA content in membrane and initial dye. The

uptake decrease with increase in pH increased with increase in initial concentration of

dye. The strong electrostatic interaction between the CS (NH3 groups) and

concentration of dye anions might be used to explain the high adsorption capacity of

both anionic dyes onto crosllinked membranes. The adsorption kinetics data was

successfully simulated by pseudo-first order and pseudo-second order rate model.

Thermodynamic parameters showed, adsorption on the membranes was a spontaneous

and endothermic. The high adsorption of dyes on membranes in acidic range is very

useful because most of textile effluent is in acidic range and this adsorbent system is

quit suitable for such type of system. The crosslinked membranes showed good

adsorption of NR and POr among previously reported results in the literature.

Acrylic acid based superabsorbent hydrogel was prepared using different

amounts of PTES and irradiated at different doses upto maximum of 30 kGy. It was

observed that the increased PTES concentration decreased the EDS of the hydrogels.

Infrared spectroscopy confirmed the crosslinking reaction between the feed

components and siloxane bond. Thermogravimetric analysis expressed an increase in

the hydrogels stability having higher PTES content. These hydrogels showed highest

swelling around neutral pH. The hydrogel had high adsorption capacities to remove

copper metal at low pH, the adsorption capability were significantly affected by the

PTES content, pH and gamma doses in hydrogel and initial metal concentration. The

uptake decrease with increase in pH. This switchable pH response of these hydrogels

can be exploited in environmental and biomedical applications.

128

Future Recommendations Investigation of these adsorbents on industrial waste water.

Adsorption study of different adsorbates on one adsorbent system and vice versa.

Modification of adsorbent systems suitable for closed loop applications.

129

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