Kinetic Studies of Adsorption of Pb(II) and Cu(II) from Aqueous Solution by Activated Carbon

Group 5

Surname and Initials Student Number

Lekgoba T.P 207059063

Subject Name : Chemical Engineering Project

Subject Code : EHCIP4B

Supervisor : Prof Ochieng Aoyi

Level of Study : B–Tech: Chemical Engineering

Date Submitted : 05 DECEMBER 2012

Copper Lead

1

Table of Contents

Acknowledgement and Declaration........................................................................................ i 1. Introduction ........................................................................................................................ 1 1.1. General Background and Motivation................................................................................. 1 1.2. Main Objective................. ................................................................................................. 2 1.3. Specific Objectives............................................................................................................. 21.4. Problem Statement............................................................................................................. 22. Literature review................................................................................................................. 32.1. Background..................................................................................................................... 3-52.2. Theory ............................................................................................................................... 5 2.2.1. Adsorption.................................................................................................................... 5-9 2.2.2. Adsorbents................................................................................................................. 9-13 2.2.3. Adsorption Kinetics...................................................................................................... 14 2.2.4. Adsorption Mechanisms. .............................................................................................. 15 2.2.5. Adsorption Isotherms .............................................................................................. 16-19 2.2.6. Thermodynamic Analysis........................................................................................ 19-203. Methodology...................................................................................................................... 213.1 Equipment and Materials ................................................................................................. 21 3.1.1. Equipment..................................................................................................................... 21 3.1.2. Materials and Chemicals............................................................................................... 21 3.2. Experimental procedure................................................................................................... 21 3.2.1 Preparation of Synthetic wastewater.............................................................................. 21 3.2.2. Preparation of 1M NaOH and 1M HCL solutions ....................................................... 21 3.2.3. Determination of the effect of contact time ................................................................. 22 3.2.4. Determination of the influence of solution pH ............................................................ 22 3.2.5 Determination of the effect of adsorbent dosage........................................................... 22 3.2.6. Determination of the effect of treatment temperature................................................... 22 3.2.7 Atomic Absorption Spectrophotometer.................................................................... 23-24 3.3. Experimental Set-up......................................................................................................... 24 3.4. Experimental Analysis..................................................................................................... 24 3.5. Experimental Design................................................................................................... 24-25 3.6. Data Analysis .......................................................................................................... 25-26 4. Results and Discussion...................................................................................................... 27 4.1. Effect of contact time on adsorption of Cu(II)................................................................. 27 4.2. Effect of solution pH on adsorption of Cu(II).................................................................. 28 4.3. Effect of Adsorbent dosage on adsorption of Cu(II)........................................................ 29 4.4. Effect of Adsorbent dosage on adsorption of Pb(II) in comparison to Cu(II)................. 30 4.5. Effect of treatment temperature on adsorption of Cu(II)................................................. 31 4.6. Adsorption Kinetics.....................................................................................................32-334.7. Adsorption Isotherms................................................................................................. 34-35 4.8. Thermodynamic Studies.................................................................................................. 35 5. Conclusion and Recommendations................................................................................. 366. References ................................................................................................................... 37-38 Appendix A-Glossary.............................................................................................................. 39 Appendix B-Safety............................................................................................................. 40-41 Appendix C-Calculations................................................................................................... 42-43

2 | P a g e

List of figures FIGURE 1: Equipment Set-up of the adsorption of Cu(II). ................................................... 24

FIGURE 2: Effect of Contact Time on adsorption of Cu(II). ................................................ 27

FIGURE 3: Effect of solution pH on adsorption of Cu(II) .................................................... 28

FIGURE 4: Effect of adsorbent dosage on adsorption of Cu(II). .......................................... 29

FIGURE 5: Effect of adsorbent dosage on adsorption of Pb(II) in comparison to Cu(II). ... 30

FIGURE 6: Effect of treatment temperature on adsorption of Cu(II). .................................. 31

FIGURE 7: Graph of the Pseudo First Order Kinetic Model. ............................................... 33

FIGURE 8: Graph of the Pseudo Second Order Kinetic Model............................................. 33

FIGURE 9: Langmuir Adsorption Isotherm Model. .............................................................. 34

FIGURE 10: Freundlich Adsorption Isotherm Model. .......................................................... 35

List of tables

TABLE 1: Comparison of Physisorption and Chemisorption.................................................. 7

TABLE 2: Standards for AAS................................................................................................ 24

TABLE 3: Kinetic Parameters for adsorption of Cu(II)......................................................... 32

TABLE 4: Langmuir and Freundlich constants...................................................................... 34

TABLE 5: Thermodynamic Parameters.................................................................................. 35

TABLE 6: MSDS for Heavy metals....................................................................................... 40

TABLE 7: MSDS for Chemicals............................................................................................ 41

List of acronyms

AAS Atomic Absorption Spectrophotometer

EPAC Enteromorpha Prolifera Activated Carbon

HSAC Hazelnut Shell Activated Carbon

KINEQL Kinetic Equilibrium

MSDS Material Safety and Data Sheet

PCAC Pine Core Activated Carbon

PLAC Polygonum Orientale Activated Carbon

3 | P a g e

ACKNOWLEDGEMENTS

I would like to pass on my sincere gratitude and appreciation to Mr Lay Shoko (Lab

Technician) and Mr Tumelo Seadira (Assistant Supervisor) for the heartfelt assistance they

gave me since I started this project. Gratitude is also sent to the sources consulted in carrying

out this project. Their role was of great significance in this regard.

DECLARATION

I Tumeletso Polite Lekgoba of student number 207059063 declare that all the information

contained in this report was compiled and written by myself. Furthermore, I declare that all

the work sourced from other writers has been clearly referenced and listed thereof in the

reference section of the report. My area of focus was adsorption of copper.

Signature: ............................................ Date : 05 December 2012

This is to certify that all the experiments were conducted in the Vaal University of

Technology B-Tech Lab J001 under the supervision of Mr Lay Shoko.

Signature:………………………………

Mr Lay Shoko

4 | P a g e

1. INTRODUCTION

1.1. General Background and Motivation

Copper and lead are among the heavy metals that are present in the earth’s crust and are

found in abundance. Copper is a heavily used metal in industries such as plating, mining and

smelting, brass manufacture, electroplating industries, petroleum and excessive use of Cu-

based agrichemicals in mining. Though these metals are used for the production of certain

goods important to human life, they bear some characteristics that make them adverse/toxic

to human beings. These industries produce much wastewater and sludge containing Cu(II)

ions with various concentrations, which have negative effects on the water environment. Lead

poisoning causes damage to the liver, kidney and reduction in hemoglobin formation, mental

retardation, infertility and abnormalities in pregnant women (Mouni et al., 2011). Excessive

intake of copper by humans may lead to severe mucosal irritation, hepatic and renal damage,

widespread capillary damage and central nervous problems (Demirbas et al., 2009). These

metals are non-biodegradable, accumulate in human beings and harmful to human beings as

well as fauna and flora even at low concentrations and it is imperative to device meaningful

ways to eliminate them from the water environment. The value of a solution to this research

is to protect the environment, people and aquatic life from being exposed to high levels of

heavy metals contamination. Over the years, many chemical techniques for removing Cu(II)

and Pb(II) from aqueous solution such as oxidation, membrane filtration, ion exchange,

reverse osmosis etc have been tried but they either proved to be ineffective in removing low

concentrations of these metals or expensive to carryout. As years passed, an alternative

chemical technique called activated carbon adsorption was discovered to be the most

cheapest, simplest, environmental friendly and the most effective technique of removing

traces of lead and copper from solutions. Activated carbon is a porous adsorbent with a high

surface area, a great adsorption capacity and an effective regeneration. Carboxylic,

carbonylic, lactonic, phenolic, aldehydic, and other organic functional groups are located at

the edges of hexagonal carbon layer planes and are responsible for surface reactivity of

activated carbon. Properties such as surface charge, type of surface functional groups,

specific surface area and pore size distribution affect the adsorption capabilities of metal ions

by activated carbon (Momcilovic et al., 2011).

1.2 Main Objective

a) To study the adsorption kinetics of Cu(II) and Pb(II) by activated carbon.5 | P a g e

1.3 Specific Objectives

a) To determine the effect of contact time on adsorption kinetics of Pb(II) and Cu(II)

from aqueous solutions by activated carbon .

b) To determine the effect of the initial solution pH on the adsorption kinetics of Pb(II)

and Cu(II) from aqueous solutions by activated carbon .

c) To determine the effect of adsorbent dosage on adsorption kinetics of Pb(II) and

Cu(II) from aqueous solutions by activated carbon .

d) To measure the effect of ionic strength on the adsorption kinetics of Pb(II) and Cu(II)

from aqueous solutions by activated carbon .

e) To analyse the kinetic models of adsorption of Pb(II) and Cu(II) from aqueous

solution by activated carbon.

1.4 Problem Statement

Lead and copper are important metals because they are used to produce several goods that are

useful to human life. Though useful to human life, they bear some characteristics that make

them toxic/adverse to both human and aquatic life even at low concentrations. Industries that

manufacture or use these metals produce large quantities of wastewater and sludge containing

Cu(II) and Pb(II) ions with various concentrations. These metals are non-biodegradable and

they accumulate in human beings causing unbearable health problems. It is therefore

imperative to device meaningful ways to eliminate them from the water environment as their

presence in water cannot be overlooked.

2. LITERATURE REVIEW2.1 Background6 | P a g e

Several investigations have been conducted on kinetics studies of adsorption of Pb(II) and

Cu(II) from aqueous solution by activated carbon and below presented are some observations

and conclusions drawn..

Chen et al. (1996) conducted an investigation on equilibrium and kinetic studies of copper

adsorption by activated carbon and the study’s focus was to provide an insight into metal ion

adsorption from aqueous solutions in terms of both equilibrium and kinetics. Chen reported

that the adsorption of copper strongly depends on solution pH and increases from 10 – 95% at

pH ranging from 2.3 – 8. The mass transfer controls the adsorption rate and that adsorption

occurs in the microspore region where both the external mass transfer and diffusion are

important. Chen also reports that KINEQL is able to calculate metal ion adsorption and

histories under different conditions, such as pH and concentration of metal ion and gives a

good description of the surface charge of activated carbon, metal ion adsorption kinetics.

It has been reported by Demirbas et al. (2009) who studied the adsorption kinetics and

equilibrium of copper from aqueous solutions using hazelnut shell activated carbon in which

the focus was to study adsorption capacity for Cu(II) and the effects of initial metal

concentrations, pH, adsorbent dosage and temperature on activated hazelnut shell activated

carbon under kinetic, equilibrium and thermodynamic parameters that the adsorption of

copper on activated carbon is initial metal ion, concentration, solution pH, time, adsorbent

dose and temperature dependent. Experimental results indicated that the pseudo second order

reaction kinetics provided the best description of data. The isotherm study indicated that

adsorption data correlated well with the Langmuir isothermal model. Thermodynamical

analysis (parameters) were also evaluated for the metal ion adsorbent system and revealed

that the adsorption was endothermic in nature. The study demonstrated that the HSAC could

be used as an effective adsorbent for the treatment of waste water containing Cu(II) ions.

Momcilovic et al. (2011) conducted a study on the removal of lead(II) ions from aqueous

solutions by adsorption onto pine cone activated carbon and the study was focused on

investigating the lead removal efficiency of pine core activated carbon (PCAC) by adsorption

from aqueous media. It was meant to investigate the effects of contacts time, initial

concentration of lead(II) ions, pH and adsorbent dosage. Momcilovic reports that the study

revealed that the adsorption process fit well with the Langmuir isotherm and pseudo second

7 | P a g e

order kinetic model. Removal of lead(II) ions was pH dependent and better in basic

medium ,though at pH values higher than 6.7 is ascribed to lead precipitation. It may be

concluded that activated carbon from pine cones can be considered as a potential adsorbent

used for the elimination of lead(II) ions from waste water since it is a low cost and locally

available adsorbent.

It has been reported by Wang et al. (2010) who studied the adsorption of Pb(II) on activated

carbon prepared from polygonum orientale; kinetics, isotherms, pH and ionic strength studies

which focused on evaluating the application potential of developing effective low cost

adsorbent from PL by H3PO4 activation, to investigate its ability to treat waste water that

contains toxic lead and the adsorption parameters(i.e. contact time, shaking rate, temperature,

pH and ionic strength) that H3PO4 activated PL acts as a good adsorbent to absorb Pb(II) from

aqueous solutions. Wang also reports that the adsorption follows pseudo second-order

kinetics and the equilibrium data can be well fitted with the Langmuir isotherm. The

adsorption of Pb(II) on PLAC increases with the increase in pH and it significantly decreases

with the increase in ionic strength, which indicates an ion exchange mechanisms. The study

shows that PLAC has a strong ability for regeneration therefore it has a great potential to be

used as an economical, efficient adsorbent to adsorb Pb(II) ions from aqueous solution.

Mouni et al. (2011) conducted an investigation on adsorption of Pb(II) from aqueous

solution using activated carbon developed from apricot stone and its main focus was on the

adsorption capacity of adsorbent using batch experiment and the influence of pH, contact

time, metal ion together with adsorbent concentrations and reported that the present

investigations shows that apricot stone activated carbon is an effective adsorbent for the

removal or recovery of lead from wastewater. The adsorption of lead on activated carbon is

found to be contact time, Pb initial concentration, pH and adsorbent dose dependent. The pH

influence on metal ion concentrations was high and was more noticeable at pH values

between 1.5 and 5 in particular. The kinetic study of the sorption of lead shows that pseudo-

second order model provides better correlation of the sorption data than the pseudo-first order

model and this suggests that the rate limiting step may be chemical sorption rather than

diffusion.Further research on the optimization of adsorption process and investigation on

competitive adsorption of metal ions from mixture solutions onto activated carbon still needs

to be done.

8 | P a g e

2.2 Theory

2.2.1 Adsorption

Adsorption is the adhesion of the molecules of liquids , gases and dissolved substances to the

surfaces of solids as opposed to absorption in which the molecules actually enter the

absorbing medium. It creates a film of the adsorbate on the surface of the adsorbent. It is an

equilibrium based separation process but also molecular sieving and rate based separation can

be applied. Adsorption can perform many separations that are impossible or impractical by

conventional techniques, such as distillation, absorption and even membrane-based systems.

Lately application applications for adsorption have expanded rapidly because of sharply

rising environmental or quality requirements. Advances in adsorbent technology have made it

possible to meet many of those demands and recently developed adsorbents are now available

“off the shelf,” and in most cases they can perform satisfactorily. However new adsorbents

are constantly being synthesized that have dramatically improved properties which decode

into better performance.

2.2.1.1 Two types of adsorption:

Physical adsorption or Physisorption: Is adsorption in which the forces involved are

intermolecular forces (van der Waals forces) of the same kind as those responsible for the

imperfection of real gases and the condensation of vapors, and which do not involve a

significant change in the electronic orbital patterns of the species involved. The term van der

Waals adsorption is synonymous with physical adsorption. It is well suited for a regenerable

process.

Characteristics/Features of Physisorption

a. Phenomenon is a general one and occurs in any solid/fluid system, although certain

specific molecular interactions may occur, arising from particular geometrical or

electronic properties of the adsorbent and/or adsorptive.

b. Evidence for the perturbation of the electronic states of adsorbent and adsorbate is

minimal.

c. The adsorbed species are chemical identical with those in the fluid phase, so that the

chemical nature of the fluid is not altered by adsorption and subsequent desorption.

9 | P a g e

d. The energy of interaction between the molecules of adsorbate and the adsorbent is of

the same order of magnitude as, but is usually greater than, the energy of

condensation of the adsorptive.

e. The elementary step in physical adsorption from a gas phase does not involve an

activation energy. Slow, temperature dependent, equilibration may however result

from the rate determining transport processes.

f. Equilibrium is established between the adsorbate and the fluid phase. In solid/gas

systems at not too high pressures the extent of physical adsorption increases with

increase in gas pressure and usually decreases with increasing temperature. In the case

of systems showing hysteresis the equilibrium may be metastable.

g. Under appropriate conditions of pressure and temperature, molecules from the gas

phase can be adsorbed in excess of those in direct contact with the surface (multilayer

adsorption or filling of micropores).

Chemical adsorption or Chemisorption: Is the adsorption in which the forces involved are

valence forces of the same kind as those operating in the formation of chemical compounds.

The problem of distinguishing between chemisorptions and Physisorption is basically the

same as that of distinguishing between chemical and physical interaction in general. No

absolutely sharp distinction can be made and intermediate cases exist, for example adsorption

involving strong hydrogen bonds or weak charge transfer). It is most importantly used in

heterogeneous catalysis, which involves molecules reacting with each other via the formation

of chemisorbed intermediates.

Characteristics/Features of Chemisorption

a. Phenomenon is characterised by chemical specificity.

b. Changes in the electronic state may be detectable by suitable physical means e.g. u.v,

infrared or microwave spectroscopy, electrical conductivity, magnetic susceptibility.

c. The changes in chemical nature of the adsorptive(s) may be altered by the surface

dissociation or reaction in such a way that on desorption the original species cannot be

recovered: in this sense chemisorption may not be reversible.

d. The energy of chemisorption is of the same order of magnitude as the energy change

in a chemical reaction between a solid and a fluid: thus chemisorption, like chemical

10 | P a g e

reactions in general, may be exothermic or endothermic and the magnitudes of the

energy changes may range from very small to very large.

e. The elementary step in chemisorption often involves an activation energy.

f. Where the activation energy for adsorption is large (activated adsorption), true

equilibrium may be achieved slowly or in practice not at all. For example, in the

adsorption of gases by solids the observed extent of adsorption, at a constant gas

pressure after a fixed time, may in certain ranges of temperature increase with rise in

temperature. In addition, where the activation energy for desorption is large, removal

of the chemisorbed species from the surface may be possible only under extreme

conditions of temperature or high vacuum, or by some suitable chemical treatment of

the surface.

g. Since the adsorbed molecules are linked to the surface by valence bonds, they will

usually occupy certain adsorption sites on the surface and only one layer of

chemisorbed molecules is formed (monolayer adsorption).

2.2.1.2 Comparison of Physisorption and Chemisorption

Table 1: Comparison of Physisorption and Chemisorption

Physisorption ChemisorptionForces operating in these are weak van der Waals forces

Forces operating in these cases are similar to those of a chemical bond

Heat of adsorption are low i.e. about 20-40kJ/mol

Heat of adsorption are high i.e. about 40-400kJ/mol

No compound formation takes place Surface compounds are formedProcess is reversible i.e. desorption of the gas occurs by increasing temperature or decreasing the pressure

Process is irreversible. Efforts to free the adsorbed gas give some definite compound.

Does not require activation energy Requires an activation energyDecreases with increasing temperature Increases with increasing temperature. Effect

is called activated adsorptionIt is not specific in nature i.e. all gases are adsorbed on all solids to some extent

It is specific in nature and occurs only when there is some possibility of compound formation between the gas being adsorbed and the solid adsorbent

Amount of gas adsorbed is related to the ease of liquefaction of the gas

There is no such correlation exists

Forms multimolecular layer Forms unimolecular layerAdsorption undergoes three interrelated processes:

a) Surface ionization

11 | P a g e

b) Complex formation

c) Formation and presence of an electrostatic double layer adjacent to adsorbent

surfaces.

Adsorption process

McCabe et al., (1993) reported that most adsorbents are highly porous materials and

adsorption takes place primarily on the walls of the pores or at specific sites inside the

particle. Because the pores are generally very small, the internal surface area is orders of

magnitude greater than the external area and is often 500 to 1000 m2/g. Separation occurs

because different in molecular weight, shape, or polarity cause some molecules to be held

more strongly on the surface than others or because the pores are too small to admit the larger

molecules. In many cases , the adsorbing component (or adsorbate) is held strongly enough to

permit complete removal of that component from the fluid with very little adsorption of other

components. Regeneration of the adsorbents can then be carried out to obtain the adsorbate

in concentrated or nearly pure form.

Application of vapour-phase adsorption include the recovery of organic solvents in paints,

printing inks, and the solution for film casting or fabric coating. Adsorption on carbon is used

to remove pollutants such as H2S, CS2 and other odorous compounds from air circulated in

ventilation systems, and canisters of carbon are placed in most new automobiles to prevent

gasoline vapours from being vented to the air.

Adsorption from the liquid phase is used to remove organic components from drinking water

or aqueous wastes, coloured impurities from sugar solutions and vegetable oils and water

from organic liquids. Adsorption can also be used recover reaction products that are not

easily separated by distillation or crystallization. Some of the same types of solids are used

for both vapour-phase and liquid-phase adsorption, though often adsorbents with larger pores

are preferred for use with liquids.

Applications of adsorption

12 | P a g e

Numerous purification and recovery processes for gases and liquids, Activated carbon-based

applications, Desiccation using silica gels, aluminas, and zeolites, oxygen from air by PSA

using 5A zeolite, Nitrogen from air by PSA using carbon molecular sieve, Separation on n-

and iso-parafins using 5A zeolite and Separation of xylenes using zeolite.

2.2.2 Adsorbents

2.2.2.1 Adsorbent Attributes

Many adsorbents can be used in the removal of toxic waste e.g. heavy metals from

wastewater. The most important attributes of an adsorbent for any application are: capacity,

selectivity, regenerability, kinetics, compatibility and cost.

a) Adsorption Capacity

Adsorption capacity is the most important characteristic of an adsorbent. Simply stated, it is

the amount of adsorbate taken up by the adsorbent , per unit mass of the adsorbent. It depends

on the fluid phase concentration, temperature and other condition like adsorbent condition.

Normally, adsorption capacity data are gathered at a fixed temperature and various adsorbate

concentrations. Adsorption capacity is of paramount importance to the capital cost because it

indicates the amount of adsorbent required, which also fixes the volume of the adsorber

vessels and both generally are significant if not dominant.

b) Selectivity

Selectivity is related to capacity but the simplest is the ratio of the capacity of one component

to that of another at a given fluid concentration. The ration generally approaches a constant

value as concentration drops towards zero. The closest analogy is to the relative volatility

(e.g. in distillation) in that the smaller value, the larger the required equipment. An ideal

situation occurs when the major component is not adsorbed much, which leads to a very large

selectivity (Kraebel et al., 2003).

c) Regenerability

It has been reported by Kraebel et al., (2003) that all cyclic adsorption applications rely on

regenerability, so that the adsorbent can operate in sequential cycles with uniform

performance. This means each adsorbable component must be relatively weakly adsorbed.

Regeneration might be accomplished by a thermal swing, pressure swing, chemical (e.g. by

13 | P a g e

displacement, elution or supercritical extraction), or sometimes by combination of those.

Displacement would involve introducing a species that adsorbs more strongly than the

adsorbate of interest, while elution would entail dissolving the adsorbed material by a solvent

that is weakly adsorbed if at all. The chemical methods all require a separate separation

operation that may be costly, plus a means must be found for purging the bed of the

regenerant. The regenerability of an adsorbent affects the fraction of the original capacity that

is retained and the time, energy, etc. required for regeneration.

d) Kinetics

Mass transfer kinetics is a catch all term related to intraparticle mass transfer resistance. It is

vital because it controls the cycle time of a fixed bed adsorption process. Fast kinetics

provide a sharp breakthrough curve, while slow kinetics yields a distended break through

curve. The effect of a distended break through curve can be overcome by adding adsorbent at

the product end, or by increasing the cycle time (which reduces the throughput per unit of

adsorbent).Both of these options affect the amount of adsorbent required in that the longer the

cycle time, the greater the adsorbent inventory. Normally, slow diffusion of any adsorbate is

a disadvantage and to compensate for slow diffusion, it is possible to use small particles, but

there is a corresponding sacrifice due to increased pressure drop.

e) Compatibility and Cost

Compatibility covers various possible models of chemical and physical attack that could

reduce the life expectancy of the adsorbent, such as biological fouling or attrition. Operating

conditions such as velocity, temperature, pressure and vibration should not cause undue

disintegration of the adsorbent particles. This could happen by crushing or abrasion, and there

are standard methods for measuring those (Kraebel et al., 2003). Cost is the most subtle

characteristic to understand because it may vary from time to time and from company to

company, event for the same exact material.

2.2.2.2 Classes of Adsorbents

a) Inorganic Materials

14 | P a g e

Most minerals and many synthetic inorganic materials have been tried as adsorbents and

some have been successful , despite being poor adsorbents, simply because they were so

inexpensive. Others have turned out to be immensely effective adsorbents. On the contrary,

some inorganic materials may act more as absorbents than adsorbents, but have applications

from drying to recovery of PCBs. Among these are metal chlorides (CaCl2), oxides (CaO,

MgO, ZnO for life support in the space programme), silicates (MgSiO3), sulphates (CaSO4),

kieselguhr and even sodium bicarbonate and limestone (for flue gas treatment). Below are a

few inorganic adsorbents;

i) Zeolites

Zeolites are crystalline aluminosilicates. Zeolitic adsorbents have had their water of hydration

removed by calcination to create a structure with well-defined openings into crystalline

cages. The molecular sieving properties of zeolites are based on the size of these openings.

Two crystal types are common: type A (with openings formed by 4 sodalite cages) and type

X or Y (with openings formed by 6 sodalite cages). Cations balancing charge and their

locations determine the size of opening into a crystal unit cell. Nominal openings sizes for the

most common synthetic zeolites are 0.3 nm for KA, 0.4 nm for NaA, 0.5 nm for CaA, and 1.0

nm for NaX. Further details, including effective molecular diameters, are widely available

(Perry et al., 1997).

Hence, zeolites are often used in filtration systems due to their hydraulic characteristics.

Other applications of zeolite entail it being used as a permeable membrane for the

purification of water. In addition, other authors studied the use of zeolites in environmental

applications, mainly to remove ions from wastewater by adsorption ion exchange processes

(Rubio et al., 2006). The advantages of such an application are that zeolite can possibly be

made selective to target certain contaminants, large quantities of minerals are deposited on it

and it is low cost (Lemić et al., 2007).

The problem with zeolite is that it becomes unstable at high pH (Basu, et al., 2006) and for

this reason; chemicals are added to adjust the pH, which makes this process expensive.

Although zeolite and activated carbon are known to be a good adsorbent for heavy metals at

both high and low concentrations, there are high manufacturing costs involved.

15 | P a g e

ii) Aluminas

Activated alumina is produced from hydrated alumina, Al2O3 .n H2O where n = 1 or 3 by

dehydration (calcining) under careful controlled conditions. It is a white or tan opaque

material that has a chalky appearance. The distinctions are in the crystal structure of the

alumina. Stable crystalline forms are usually not thought of as adsorbents dues to their low

surface areas. On the contrary, transitional forms such as gamma and eta alumina have defect

spinel forms which lead to higher concentrations of surface acid sites. The two widest uses of

activated alumina are as a catalyst(or catalyst support) and as a desiccant. Ancillary uses as

an adsorbent are for removal of; oxygenates and mercaptants from hydrocarbon feed streams,

fluoride ions in water, HCL from hydrogen in catalytic reforming and others (Kraebel et al.,

2003).

b) Organic Materials

These are adsorbents that are based on organic material, whether synthetic or naturally

occurring. A wide range of organic materials have been used as for “sorption,” besides

activated carbon or charcoal. Some might function as solid absorbents rather than adsorbents

and in the midst of these are cellulose (the most abundant biopolymer in nature), chitin,

collagen, wool, polysaccharides derived from corn and miscellaneous forms of biomass.

Below are a few organic adsorbents;

i) Activated Carbon

Activated carbon is used as an adsorbent in many processes such as effluent treatment,

solvent recovery, air treatment, metal ores processing, decolourisation and water purification

(McKay et al., 1996). Activated carbon is produced from carbonaceous materials such as

wood, coal and coconut shells or bones decomposed in an inert atmosphere at a temperature

of about 800 K. Because the product will not be porous, it needs additional treatment or

activation to generate a system of fine pores. The carbon may be produced in the activated

state by treating the raw material with chemicals, such as zinc chloride or phosphoric acid,

before carbonising. Alternatively, the carbon from the carbonising stage may be selectively

oxidised at temperatures in excess of 1000 K in atmospheres of materials such as steam or

carbon dioxide. Activated carbon has a typical surface area of 106 m2/kg, mostly associated

with a set of pores of about 2 nm in diameter. There is likely to be another set of pores of

16 | P a g e

about 1000 nm in diameter, which do not contribute to the surface area. There may even be a

third, intermediate set of pores that are particularly developed in carbons intended for use

with liquids (Coulson et al., 2002). Activated carbon is widely used as a commercial

adsorbent; however, under practical conditions and in the presence of natural organic matter,

the bed life of activated carbon is reduced.

ii) Polymers

Polymeric adsorbents tend to be opaque spherical bead, but the color depends strongly on the

product. Most commonly they are white or tan, but some are brown, orange or black. The

first materials were originally the inert particles that would otherwise have been further

treated to make macroporous or macro reticular ion exchange resins. As such, they were

typically polystyrene benzene copolymers having spherical shape and high pore volume.

Internally the polymer beads contain “microbeads” that are joined together at a few points

each, creating a macropore structure. Instead of being limited to styrene/divinylbenzene,

polymeric adsorbents are also made from polymethacrylate, divinylbenzene or

ethylvinybenzene and are sometimes sulfonated or chloromethylated, much as are ions

exchange resin. Consequently some are sufficiently hydrophilic to be used as a desiccant,

while others are quite hydrophobic. The effective surface area is usually smaller than for

activated carbon. A minor drawback of these materials is that they tend to shrink and swell

upon cyclic use, and for gas phase application they may require conditioning prior to use.

The range of application is somewhat restricted, since the cost of most polymeric adsorbents

is typically about 10× that of other that are available. In some instances other adsorbents

simply cannot perform, so polymeric materials are the only choice and in other instances they

compensate for cost differential by yielding much better performance, especially for high

value added uses. Current application include; recovery and purification of antibiotics and

vitamins, decolourizations, decaffeination, hem perfusion and separation of halogenated light

organics from water (Kraebel et al., 2003).

2.2.3 Adsorption kinetics

17 | P a g e

Kinetic study is important to an adsorption process because it depicts the uptake rate of

adsorbate, and controls the residual time of the whole adsorption process. Chemical kinetic describes sorption reaction pathways, along times to reach the equilibrium whereas chemical equilibrium gives no information about pathways and reaction rates. Sorption kinetics show a large dependence on the physical and/or chemical characteristics of the sorbent material which

also influences the sorption mechanism. In order to investigate the mechanism of sorption,

various sorption models have been used at different experimental condition for sorption

processes (Kocaoba et al., 2007). In investigating the controlling mechanism of adsorption

processes, the pseudo first-order and pseudo second-order models will be compared.

2.2.3.1 Pseudo first order model

This was the first equation for the sorption of liquid/solid system based on solid capacity. In

most cases, the pseudo-first order equation does not fit well for the whole range of contact

time. This model is represented by:

2.2.3.2 Pseudo second order model

Pseudo-second order reaction model is based on sorption equilibrium capacity.It is based on

the assumption that the rate-limiting step may be chemical sorption or chemical sorption

involving valence forces through sharing or exchange of electrons between sorbent and

sorbate. The surface site sorbate reaction may be represented as follows.

Active surface site + Sorbate Surface site sorbate surface complex

It is assumed that the sorption capacity is proportional to the number of active sites occupied

on the sorbent. The model equation is expressed as:

2.2.4 Adsorption Mechanisms

18 | P a g e

For a solid liquid adsorption process, to analyse the rate controlling steps such as mass

transport and chemical reaction process is very beneficial for elaborating the adsorption

mechanism. The adsorption reaction is usually divided into these steps:

a. Metal ion from the bulk liquid to the liquid film or boundary layer surrounding the

adsorbent.

b. Transport of solute ions from the boundary film to the external surface of the

biosorbent (film diffusion).

c. Transfer of ions from the surface to the intra-particular active sites (particle

diffusion).

d. Adsorption of ions by the active sites of adsorbent.

Because the first step is not involved with the adsorbent and the fourth step is a very rapid

process, they do not belong to the rate controlling steps. Therefore the rate controlling steps

mainly depend on either film diffusion or particle diffusion. Weber and Morris model is

widely used intra-particle diffusion model to predict the rate controlling step. The rate

constants of intra-particle diffusion(kid) at this stage i is determined using the following

equation:

where qt is the amount adsorbed Pb(II) ion at

time t, t is the reaction time, Ci is the intercept at stage i. The value of Ci is related to the

thickness of the boundary layer. The plots of qt vs. t at different initial Pb(II) ion

concentrations show multi-linearity characterizations), indicating that two steps occurred in

the adsorption process. The first sharp section is the external surface adsorption or

instantaneous adsorption stage. The second subdued portion is the gradual adsorption stage,

where intra-particle diffusion is rate-controlled. The larger slopes of the first sharp sections

indicate that the rate of metal removal is higher in the beginning stage due to the

instantaneous availability of large surface area and active adsorption sites. The lower slopes

of the second subdued portion are due to that the decreased concentration gradients make

Pb(II) ion diffusion in the micropores of adsorbent take long time, thus leading to a low

removal rate. The obvious two steps of the plots as well as their deviation from the origin

suggests that the intra-particle diffusion is not the only rate

controlling step for the adsorption of Pb(II) ions onto EPAC (Li et al., 2010).

2.2.5 Adsorption isotherms

19 | P a g e

Adsorption isotherms or known as equilibrium data, are the fundamental requirements for the

design of adsorption systems. In the perspective, adsorption isotherms describe how

pollutants interact with the adsorbent materials, and thus are critical for optimization of the

adsorption mechanism pathways, expression of the surface properties and capacities of

adsorbents, and effective design of the adsorption systems (Foo and Hameed et al., 2010).

Equilibrium relationships between adsorbent and adsorbate are also described by adsorption

isotherms.

An adsorption isotherm is an invaluable curve describing the phenomenon governing the

retention (or release) or mobility of a substance from the aqueous porous media or aquatic

environments to a solid-phase at a constant temperature and pH. Adsorption equilibrium (the

ratio between the adsorbed amount with the remaining in the solution) is established when an

adsorbate containing phase has been contacted with the adsorbent for sufficient time, with its

adsorbate concentration in the bulk solution is in a dynamic balance with the interface

concentration. Typically, the mathematical correlation, which constitutes an important role

towards the modeling analysis, operational design and applicable practice of the adsorption

systems, is usually depicted by graphically expressing the solid-phase against its residual

concentration. Its physicochemical parameters together with the underlying thermodynamic

assumptions provide an insight into the adsorption mechanism, surface properties as well as

the degree of affinity of the adsorbents (Foo and Hameed et al., 2010). The Freundlich and

Langmuir models are the most frequently used models to describe the experimental data of

adsorption isotherms.

2.2.5.1 Adsorption isotherms models

Over the years, a wide variety of equilibrium isotherm models (Langmuir, Freundlich,

Brunauer–Emmett–Teller, Redlich–Peterson, Dubinin–Radushkevich, Temkin, Toth, Koble–

Corrigan, Sips, Khan, Hill, Flory–Huggins and Radke–Prausnitz isotherm), have been

formulated in terms of three fundamental approaches, however Freundlich and Langmuir

models are the most frequently used models as alluded before.

Foo and Hameed et al., (2010) reported that, kinetic consideration is the first approach to be

referred. Hereby, adsorption equilibrium is defined being a state of dynamic equilibrium,

with both adsorption and desorption rates are equal. Whereas, thermodynamics, being a base

20 | P a g e

of the second approach, can provide a framework of deriving numerous forms of adsorption

isotherm models , and potential theory, as the third approach, usually conveys the main idea

in the generation of characteristic curve.

a. Langmuir model

The Langmuir isotherm was originally developed by Irving Langmuir in 1916 to describe

gas–solid-phase adsorption onto activated carbon, has traditionally been used to quantify and

contrast the performance of different bio-sorbents. In its formulation, this empirical model

assumes;

a) The surface of the adsorbent is uniform, i.e. all the adsorption sites are equivalent.

b) Adsorbed molecules do not interact.

c) All adsorption occurs through the same mechanism

d) At the maximum adsorption, only a monolayer is formed: molecules of the adsorbate

do not deposit on the other, already adsorbed, molecules of the adsorbate, only on the

free surface of the adsorbent.

In its derivation, Langmuir isotherm refers to homogeneous adsorption, which each molecule

possess constant enthalpies and sorption activation energy (all sites possess equal affinity for

the adsorbate) , with no transmigration of the adsorbate in the plane of the surface.

Graphically, it is characterized by a plateau, an equilibrium saturation point where once a

molecule occupies a site, no further adsorption can take place. Moreover, Langmuir theory

has related rapid decrease of the intermolecular attractive forces to the rise of distance (Foo

and Hameed et al., 2010). The mathematical expression of Langmuir Isotherm is described

by:

A dimensionless constant, commonly known as separation factor (RL) defined by Webber and

Chakkravorti can be represented as:

where KL (L/mg) refers to the

Langmuir constant and Co is denoted as the adsorbate initial concentration (mg/L). In this

context, lower RL value reflects that adsorption is more favorable. In a deeper explanation, RL

21 | P a g e

value indicates the adsorption nature to be either unfavorable (RL > 1), linear (RL = 1),

favorable (0 < RL < 1) or irreversible (RL = 0).

b. Freundlich model

Derived empirically in 1912 (Metcalf and Eddy et al., 2003), Freundlich isotherm is the

earliest known relationship describing the non-ideal and reversible adsorption, not restricted

to the formation of monolayer. This empirical model can be applied to multilayer adsorption,

with non-uniform distribution of adsorption heat and affinities over the heterogeneous

surface. Historically, it is developed for the adsorption of animal charcoal, demonstrating that

the ratio of the adsorbate onto a given mass of adsorbent to the solute was not a constant at

different solution concentrations. The Freundlich model assumes:

i) The heterogeneity in the surface binding process

ii) At the maximum adsorption , only a monolayer is formed: molecules of the

adsorbate do not deposit on the other , already adsorbed , molecules of the

adsorbate , only on the free surface of the adsorbent.

At present, Freundlich isotherm is widely applied in heterogeneous systems especially for

organic compounds or highly interactive species on activated carbon and molecular sieves.

The slope ranges between 0 and 1 is a measure of adsorption intensity or surface

heterogeneity, becoming more heterogeneous as its value gets closer to zero. Whereas, a

value below unity implies chemisorptions process where 1/n above one is an indicative of

cooperative adsorption. Recently, Freundlich isotherm is criticized for its limitation of

lacking a fundamental thermodynamic basis, not approaching the Henry’s law at vanishing

concentrations (Foo and Hameed et al, 2010). The mathematical expression of Freundlich

Isotherm is described by:

c. Temkin isotherm model

It has been reported by (Foo and Hameed et al., 2010) that Temkin isotherm is the early

model describing the adsorption of hydrogen onto platinum electrodes within the acidic

solutions. The isotherm contains a factor that explicitly taking into the account of adsorbent–

adsorbate interactions. By ignoring the extremely low and large value of concentrations, the

model assumes that heat of adsorption (function of temperature) of all molecules in the layer

22 | P a g e

would decrease linearly rather than logarithmic with coverage. Temkin equation is

exceptional for predicting the gas phase equilibrium (when organization in a tightly packed

structure with identical orientation is not necessary),conversely complex adsorption systems

including the liquid-phase adsorption isotherms are usually not appropriate to be represented.

The linear mathematical expression of Temkin Isotherm is described by the following

equation;

2.2.6 Thermodynamic study

Thermodynamic parameters can be calculated from the variation of the thermodynamic

equilibrium constant K0 with the change in temperature. For adsorption reactions, Kc is

defined as:

whereas is the activity of

adsorbed Pb(II) ions, Cae is the activity of the Pb(II) ions in solution at equilibrium, Cs is the

amount of Pb(II) ions adsorbed by per mass of EPAC, vs is the activity coefficient of the

adsorbed Pb(II) ions and ve is the activity coefficient of the Pb(II) ions in solution. As the

Pb(II) ion concentration in the solution decreases and approaches to zero, K0 can be obtained

by plotting ln(Cs/Ce) vs. Cs and extrapolating Cs to zero. The straight line obtained is well

fitted to the points based on a least-squares analysis and its intercept with the vertical axis

gives the values of Kc.

The adsorption standard free energy changes (ΔGo) can be calculated according to:

where R is the universal gas

constant (1.987 cal/K.mol) and T is the temperature in Kelvin.

The average standard enthalpy change (ΔHo) is obtained from Van’t Hoof equation:

where T3 and T1

are two different temperatures.

The standard entropy change (ΔS0) can be obtained by:

23 | P a g e

Positive values of ΔH0 suggests that the interaction of Pb(II) ions adsorbed by EPAC is

endothermic process, which supported by the increasing adsorption of Pb(II) ions with the

increase in temperature. The negative values of ΔG0 revealed the fact that adsorption process

was spontaneous. The positive value of ΔS0 indicates increased randomness at the

adsorbent/solution (Li et al., 2010).

3. METHODOLOGY3.1 Equipment and Materials

3.1.1 Equipment

24 | P a g e

These are the equipment that were used: Orion star A111 pH meter was used to measure the

pH of both Cu(II) and Pb(II) solution, atomic absorption spectrometer (Thermo) was used to

measure the concentration of the filtrates and a shaking incubator (Merck) was used to shake

samples to ensure good contact between activated carbon and aqueous solution.

3.1.2 Materials and Chemicals

Copper sulphate (CuSO4.5H2O) and lead nitrate Pb(NO3)2 were used to prepare the synthetic

waste water for use in the experiment and distilled water was used for all dilutions. The pHs

of the solutions were adjusted using 1M of NaOH (sodium hydroxide) or 1M HCL

(hydrochloric acid).

3.2 Experimental Procedure

3.2.1 Preparation of synthetic waste water

A stock solution of 1000 mg/l for copper was prepared by dissolving 3.93g of CuSO4.5H2O in

a litre of distilled water and was shaken until completely dissolved. In preparing the solutions

of concentrations of 20 and 40 mg/l for use in the adsorption experiments except for

temperature which ranges from 20 – 100 mg/l in a step size of 20 mg/l, a ratio of 1 ppm: 0.1

ml/100 ml distilled water was used to dilute the stock solution to the afore mentioned

concentrations.

A 1000 mg/l stock solution for lead was prepared by dissolving 1.95 g of Pb(NO3)2 in a litre

of distilled water. Solution of the following concentrations 20 and 40 mg/l were prepared for

use in the adsorption experiments except for temperature that ranged from 10 – 50 mg/l in a

step size of 10 mg/l. In preparing them required amounts of the stock solution were diluted to

the required concentration using a dilution ratio of 1 ppm: 0.1 ml/100 ml distilled water.

3.2.2 Preparation of 1M NaOH and 1M HCL solutions

In preparing a 1M solution of NaOH, 40.0 g of NaOH pellets was dissolved in a certain

volume of distilled water and was shaken until all the pellets were completely dissolved and

the volume of the solution was topped up to make 1 litre. Similarly, a relation of 1 litre = 1.16

kg was used in preparing a solution of 1M HCL, because HCL was in liquid form the afore

mentioned relation helped in calculating the volume needed for a 36.5 g of HCL. 31.5 ml of

HCL was mixed with 968.5 ml of distilled water to make a 1M solution of HCL.

25 | P a g e

3.2.3 Determination of the effect of Contact time

The effect of contact time on Pb(II) ions adsorption was conducted by adding 0.1 g of

adsorbent into 100 ml solution with different Pb(II) ion concentrations (20 and 40 mg/l) in a

250 ml Erlenmeyer flask and the time range was between 20 and 100 minutes. At pre-

determined time intervals (of 20 minutes), samples were collected and thereafter filtered.

Samples were then analysed using an atomic absorption spectrophotometer. The same

procedure applies for Cu(II) ions.

3.2.4 Determination of the influence of solution pH

For the determination of the influence of solution pH on the adsorption capacity, 0.1g of

activated carbon was contacted with solutions of Pb(II) and Cu(II) at concentrations of 20 and

40 mg/l at pH values ranging from 3 to 11 in a sealed Erlenmeyer flask and shaken for 60

minutes. Suspensions were filtered and the concentrations of the ions in the filtrates were

analysed utilizing the atomic absorption spectrometer. pH of the samples was adjusted using

1M HCL and 1M NaOH solutions. Drop-by-drop addition of 1M HCL and 1M NaOH

resulted in the desired adjustment in pH.

3.2.5 Determination of the effect of adsorbent dosage

The effect of adsorbent dose on Pb(II) and Cu(II) removal was studied by agitating 100 ml of

(20 and 40 mg/l) solutions of Pb(II) and Cu(II) ions containing different dosages of adsorbent

ranging from 0.04–0.12 g for a period of 60 minutes. After 60 minutes of shaking, samples

were filtered and then analysed using an atomic absorption spectrophotometer.

3.2.6 Determination of the effect of Temperature

The effect of temperature on Pb(II) ions adsorption was conducted by adding 0.1 g of

adsorbent into 100 ml solution with different Pb(II) ion concentrations of ranges between 20

and 100 mg/l in a 250 ml Erlenmeyer flask and the temperature range was between 298K and

313K. Samples were filtered and then analysed using an atomic absorption spectrometer. The

same procedure applies for Cu(II) ions.

3.2.7 The Atomic Absorption Spectrophotometer

26 | P a g e

The AAS is a machine that makes use of absorption spectrometry to assess the concentration

of analyte sample. It requires standards with known analyte content to establish the relation

between the measured absorbance and the analyte concentration. The AAS that was uses

flame atomizers, which uses air-acetylene flame with a temperature of about 2300°C and the

nitrous oxide-acetylene flame with a temperature of about 2700°C.The latter flame, in

addition, offers a more reducing environment being ideally suited for analytes with high

affinity to oxygen.

AAS was used to measure the concentration of the heavy metal ions in the wastewater, which

were copper and lead, and it can only measure one metal concentration at a time. Each metal

has its own specification sheet and lamp. The fixed conditions would be lamp current, fuel,

support and flame stoichiometry. The variable working conditions would be the wavelength

and slit width. This is dependent on the optimum working range measured in ppm. The

sample to be analysed should fall into this range and is the factor that determines the range to

work in. After entering the information into the machine, calibration step is undertaken.

Calibration

Every time before the samples were analysed, the AAS was calibrated with ultra pure water

and a range of standards that were prepared based on the samples to be analysed were

analysed then followed the samples, only if the prepared standards were on specification.

Preparation for standards to be used with AAS

27 | P a g e

In preparing the standards for both of the metals, a ratio of 1 ppm: 0.1/100 was used to dilute

a stock solution of 1000 mg/l to the required concentrations. The table below shows the

standards prepared for each metal.

Table 2: Standards for AAS

Heavy metal ion Concentration in ppm

Pb(II) 1 7 9

Cu(II) 1 10 20

3.3 Experimental Set up

Figure 1: Equipment set-up for experiments on adsorption of copper on activated carbon

3.4 Experimental Analysis

Analysis of the concentration of the filtrates from the tested parameters was done using the

atomic absorption spectrophotometer. pH meter was used for pH analysis for the parameters

to be tested and tested parameters.

3.5 Experimental Design

28 | P a g e

AdsorbentWastewater

Mechanical shakerConical Flask

filtration

AAS

An experimental design is a method that is used to determine the number of runs required to

carryout a particular experiment. A one-at-a-time experimental design method was used for

the investigation of the various objectives under consideration and very many levels of

parameters are considered. This is a very easy method to use/carryout but very costly and

time consuming as compared to the factorial method because each parameter is done on a

separate basis concerning the varied parameter. In this method, two parameters are compared

but one of them is held constant (response parameter) while the other is varied. The effects of

initial solution pH, contact time, adsorbent dosage and ionic strength were investigated. The

interactions of the above effects were considered. The adsorption capacity was measured

against time, pH, adsorbent dosage and temperature at varied initial concentrations.

Data Analysis

The adsorption capacity in (mg/g) was calculated by:

where Co and is the

initial and concentration of Pb(II) and Cu(II) ions, V is the volume of the solution ,m is the

mass of activated carbon and Ce is the residual/equilibrium concentrations of the ions at any

time (t). The experimental data will be fitted in the kinetic models and the best fit model

indicates the most probable adsorption mechanism.

For adsorption isotherms analysis, the Langmuir and Freundlich models were compared.

These models were used to investigate how Pb(II) and Cu(II) ions interact with the adsorbent,

and they are applied to describe the isotherm data obtained at varied temperatures.

The linear form of the Langmuir equation is described in equation 4, where Ce is the

residual/equilibrium concentrations, qe is the residual/equilibrium adsorption capacity, qmax is

the final adsorption capacity and KL is the Langmuir affinity constant. The logarithmic form

of the Freundlich model is given by equation 5 where Ce is the residual/equilibrium

concentration of the system, qe is the maximum adsorption capacity of the material, KF is the

Freundlich constant related with adsorption capacity and n is the Freundlich exponent.

29 | P a g e

For adsorption kinetics analysis, the pseudo first order and pseudo second order models were

compared. These models were used to investigate the mechanisms of adsorption. The integral

method of the pseudo-first -order model described by equation 1 where qt is the amount

adsorbed at a time (mg/g), qe is the amount adsorbed at equilibrium (mg/g), Kpf is the

equilibrium rate constant of the pseudo-first-order (min-1). Pseudo-second-order model

equation is expressed by equation 2 where qe is the amount adsorbed at equilibrium (mg/g),

Kps is the equilibrium rate constant of the pseudo-second-order (min-1).

30 | P a g e

4. RESULTS AND DISCUSSIONFollowing the specific objectives, a series of parameters were tested to determine their effect

on the adsorption of Cu(II) ions. These parameters include solution pH, contact time,

adsorbent dosage, temperature and the kinetics studies.

4.1 Effect of Contact time on Adsorption of Cu(II)

Contact time is one of the important parameters in the process of adsorption as it gives an

idea on the time in which the process reaches equilibrium and the trend in which the capacity

goes. A study on the effect of contact time on the adsorption of Cu(II) by activated carbon at

concentrations of 20 and 40 mg/l was performed and the results are shown in Figure 2. The

results indicate that with an increase in contact time, an increase in adsorption capacity also

results. There is a slight increase of adsorption of both metal ions though the expected results

were a rapid increase for the first 20-40 minutes. With further increase in time, the uncovered

surface area and active sites on the adsorbent minimizes therefore a decrease in the driving

force is registered leading to reach into equilibrium state (Li et al., 2010). As the adsorption

reaches an equilibrium state at 60 minutes, the adsorption capacity becomes constant mainly

due to slow adsorption rate. It takes about 60 minutes to reach equilibrium.

Figure 2: Effect of contact time on the adsorption of Cu(II) ions onto activated carbon (pH=5.5, Dosage=0.1 g, Temperature=25°C, Volume=100 ml, Shaking speed = 150 rpm)

31 | P a g e

4.2 Effect of solution pH on Adsorption of Cu(II)

The pH of the solution is an imperative parameter such that its impact on the uptake of heavy

metals is crucial, since it determines the surface charge of the adsorbent, degree of ionization

and the speciation of the adsorbate (Li et al., 2010). The natural pH of the solution was found

to be 5.5. From Figure 3, an increase in pH particularly in the acidic region resulted in a rapid

increase in adsorption capacity and this can be explained by considering the surface charge

on the adsorbent material. Activated carbon acts as a negative surface and attracts the

positively charged metal ion (Demirbas et al., 2009). There is a rapid increase of adsorption

capacity from pH 3 to pH 7 for Cu(II). Looking at the figure, the adsorption process reaches

stability at pH 7 and at that point there is a minimum or slow increase in adsorption capacity.

The net positive charge decreases with increasing pH value that leads to a decrease in the

repulsion between the adsorbent surface and metal ion and thus enhances the adsorption

capacity (Demirbas et al., 2009). At pH in the basic region, Copper starts to precipitate

forming ions such as Cu(OH)2.

Figure 3: Effect of solution pH on the adsorption of Cu(II) ions onto activated carbon (Dosage=0.1g, Temperature=25°C and Experiment time=60 minutes, Volume=100 ml, Shaking speed = 150 rpm)

32 | P a g e

4.3 Effect of adsorbent dosage on Adsorption of Cu(II)

Adsorbent dosage is a vital parameter as it determines the capacity of an adsorbent for a

given initial concentration of adsorbate (Demirbas et al., 2009). In determining the effect of

dosage a study was carried out on Cu(II) ions adsorption from aqueous solution by varying

the amount of adsorbent from 0.04-0.12g while keeping other parameters constant. Figure 4

shows the study on the specific objective of adsorbent dosage and it shows that at different

concentrations the adsorption capacity decreases with increasing adsorbent dosage. This is

due to the active sites of the adsorbent that are all occupied and the increase in dosage does

not provide a higher uptake of these metal ions. The figure also shows that the percentage

adsorption of copper increases with increasing adsorbent dosage, this may be due to the

greater adsorbent surface area and pore volume available at high adsorbent dosage providing

active adsorption sites that results in higher metal ions removal percentage (Li et al., 2010).

Figure 4: Effect of adsorbent dosage on the adsorption of Cu(II) ions onto activated carbon (pH=7, temperature=25°C, Experimental time=60 minutes, Volume=100 ml, Shaking speed = 150 rpm)

33 | P a g e

4.4 Effect of adsorbent dosage on adsorption of Pb(II) in comparison to that of Cu(II)

Both copper and lead were subjected to the same methodology in determining the effect of

adsorbent dosage in both metals. The range of dosages was between 0.04 and 0.12 g for

concentration of 20 ppm and 40 ppm. Figure 4 and 5 both shows the trend of the two metals

and the figures indicate that both metals record the same trend for both the adsorption

capacity and the percentage removal. With an increase in the adsorbent dosage there is a

decrease in the adsorption capacity for both metals and as for percentage removal, an increase

in the adsorbent dosage means an increase in the percentage removal of both metals. Looking

at the concentrations, 20 ppm registers a lower adsorption capacity and removal percentage as

compared to that of 40 ppm for both metals. Generally, Pb(II) is adsorbed at higher rates

than Cu(II), and this maybe due to the fact that Pb(II) ions are more attractive that those of

Cu(II) ions hence they attach to the adsorbent more easily.

Figure 5: Effect of adsorbent dosage on the adsorption of Pb(II) ions onto activated carbon (pH=7, temperature=25°C, Experimental time=60 minutes, Volume=100 ml, Shaking speed = 150 rpm)

34 | P a g e

4.5 Effect of treatment temperature on adsorption of Cu(II)

Two different temperatures were studied in order to determine the effect of temperature on

the adsorption process. As presented in Figure 6, an increase in temperature resulted in an

increase in adsorption of copper and this may be due to the increase in the diffusion rate of

Cu(II) ions and the number of sorption sites (Li et al., 2010). Also, this trend can be

explained by the fact that at higher temperatures, the kinetic energy of Cu2+ cations is high

therefore contact between Cu2+ and active sites of activated carbon is sufficient (Demirbas et

al., 2009). Temperature plays a significant role in diffusion rate of a process because

increasing the temperature rapids the movement of molecules hence increasing the rate of

diffusion. From the figure, at a temperature of 298K, there was a rapid increase of adsorption

from a concentration of 0-20 ppm and stability was observed at that point up to a

concentration of 20 thereafter a slight increase and some stability again until the last

concentration. This fluctuation maybe due to the fact that, at times the metal ions do lose

attraction to the adsorbent and that leads to a slight drop in the adsorption of the metal ion.

Figure 6: Effect of treatment temperature on the adsorption of Cu(II) ions onto activated carbon (pH=7, Dosage=0.1g, Experimental time=60 minutes, Volume=100 ml, Shaking speed = 150 rpm)

35 | P a g e

4.6 Adsorption Kinetics

Several models are presently used in studying the controlling mechanisms of an adsorption

process but in most cases there are two that are commonly used. The pseudo first order model

and the pseudo second order model have been used by many investigators in the adsorption

process hence in this report the same accord was awarded to these models.

In investigating the model that best fits the process, it was found that the pseudo second order

model takes precedence over the pseudo first order as it fits best with R2 = 0.995 and 0.998

respectively for initial concentrations of 20 and 40 mg/l respectively. Figure 7 and 8 shows

the pseudo first order model and the pseudo second order model respectively. Predominance

of the pseudo second order model implies that adsorption process is controlled by

chemisorption, which involves sharing of electrons between the adsorbate and the surface of

the adsorbent (Bouhamed et al., 2012). The unfitting of the pseudo first order model maybe

due to a boundary layer controlling the beginning of the adsorption process (Li et al., 2010).

Table 3 shows the comparison of the two models and it clearly confirms that the pseudo

second order is predominant over the pseudo first order model.

Table 3: Kinetic Parameters for the adsorption of Cu(II) using activated carbon

Co(mg/l)   First Order Kinetic Model   Second Order Kinetic Model

    Kpf(1/min)   R2 Kps(g/mg.min)   R2

20 0.071 0.969 0.082 0.995

40   0.075   0.910 0.044   0.998

36 | P a g e

Figure 7: Pseudo First Order Kinetic Model as a kinetic parameter for adsorption of Cu(II) ions onto activated carbon

Figure 8: Pseudo Second Order Kinetic Model as a kinetic parameter for adsorption of Cu(II) ions onto activated carbon

37 | P a g e

4.7 Adsorption Isotherms

The Langmuir and the Freundlich isotherm models are the frequent methods used to describe

the experimental data of the adsorption isotherms. The models were compared in this

experiment to find out the model that best fits the process. Figure 8 and 9 shows the graphs

for both the Langmuir and the Freundlich.

Table 4 shows the comparison of the two isotherm models studied and it indicates that

Freundlich is prevalent over that of the Langmuir. This maybe due to the fact that the overall

adsorption process seems to be controlled by the chemical process through sharing of

electrons or by covalent forces through exchanging of electrons between adsorbent and

adsorbate.

Table 4: Freundlich and Langmuir Constants for adsorption of Cu(II) ions onto activated carbon

Temp   Freundlich   Langmuir

    KF 1/n R2 qmax KL R2

298 1.478 0.83 0.972 30.11 0.606 0.911

313   1.59 0.83 0.944 36.69 0.179 0.888

Figure 9: Langmuir Adsorption Isotherm Model

38 | P a g e

Figure 10: Freundlich Adsorption Isotherm Model

4.8 Thermodynamic Studies

Thermodynamic parameters were calculated to evaluate the feasibility of the sorption process and to confirm its nature (Bouhamed et al., 2012). Table 5 shows the thermodynamic parameters for the adsorption of Cu(II) ions and it shows that the Gibbs free energy values are negative whereas the enthalpy and entropy values are positive. Negative values of ΔG° indicate the spontaneity of the the sorption process and the higher the negative values reflect a more energetically favourable sorption. Positive values of ΔH° shows that the sorption process was endothermic in nature and positive values of ΔS° show the randomness at solid-liquid interface during the reaction (Bouhamed et al., 2012).

Table 5: Thermodynamic parameters for adsorption of Cu(II) ions onto activated carbon

Thermodynamic Constant

    Temperature(K)    298   313

Equilibrium Constant, KC 1.68 1.74ΔG°(J/mol) -1285.4 -1441.4ΔH°(J/mol) 1812.13 1812.13

ΔS°(J/mol.K)     10.39   10.39

39 | P a g e

5. CONCLUSION AND RECOMMENDATIONMany processes are available for the removal of copper ions in aqueous solutions and of the

many processes adsorption has proven to be more effective and cheaper. In this study, the

wastewater was represented by the aqueous solution that was prepared using the heavy metal

studied. The success of this experiment was basically based on the results that were obtained.

The study indicates that activated carbon is an effective adsorbent in the removal of Cu(II)

ions after it was tested under different parameters such as pH, temperature, contact time and

adsorbent dosage which the process proved to be dependent upon. For the effect of contact

time, an increase in time resulted in an increase in the adsorption of copper and it reached

equilibrium at 60 minutes. As for the pH, there was a rapid increase in adsorption in pH range

from 3-5 and it reached stability at pH 7 where the adsorption rate became constant. The

results indicate that an increase in temperature resulted in an increase in the adsorption

capacity. Adsorbent dosage had a different trend from other parameters, as the dose

increased, the adsorption capacity decreased however the reverse is observed for the

percentage adsorption. The pseudo second order model was found to be the best fit implying

that the adsorption process is controlled by chemisorption. For adsorption isotherm models,

the Freundlich model was the best-fit model for the process and this shows that the process is

heterogeneous and monolayer which is often a rare case for the Freundlich model. From the

thermodynamic studies, the process was found to be endothermic as shown by the positive

values of ΔH° and because the enthalpy values is below 40 KJ/mol shows the process favours

physisorption. An increase in temperature resulted in an increase in the negative values of the

Gibbs free energy which indicates an increased degree of spontaneity in the sorption process.

The positive values of ΔS° suggest the probability of a favourable sorption and increased

randomness.

In recommendation, adsorption is a very effective method as proven by the results. The

preparation of the adsorbent was not done as it was readily available but in future, there is

need to consider preparing the adsorbent so as to familiarise the students with the procedure

and the steps taken to indeed prepare that.

40 | P a g e

6. REFERENCESBasu, A., Mustafiz, S., Islam, M.R., Bjorndalen, N., Rahaman, M. S. and Chaalal, O., (2006).

A comprehensive Approach for Modelling Sorption of Lead and Cobalt Ions through Fish

Scales as an Adsorbent.Chemical Engineering Communications, (193): 580–605

Bouhamed, F., Elounear, Z., Bouzid, J., (2012). Adsorptive removal of copper(II) from

aqueous solution on activated carbon prepared from Tunisian date stones: Equilibrium,

Kinetics and Thermodynamics: Elsevier B.V

Chen, J., Yiacoumi, S., and Blaydes, T.G., (1996). Equilibrium and kinetic studies of copper

adsorption by activated carbon: Separation Technology 6(1996) 133-146: Elsevier B.V

Coulson, J.M., Richardson, J.F., Backhurst, J.R., and Hacker, J.H., (2002). Chemical

Engineering Volume 2, 4th Ed.Oxford: B utter worth -Heinemann

Demirbas, E., Dizge, N., Kobya, M., and Sulak, M.T., (2009). Adsorption kinetics and

equilibrium of copper from aqueous solution using hazelnut shell activated carbon: Chemical

Engineering Journal 148 (2009) 480-487: Elsevier B.V.

Foo, K.Y and Hameed, B.Y., (2010). Insights into modelling of Adsorption Isotherms

Systems: Chemical Engineering Journal 156 (2010) 2–10

Kocaoba, S., Orhan, Y., and Akyuz, T., (2007). Kinetics and equilibrium studies of heavy

metals ion removal by use of natural zeolite: Desalination 214 (2007) 1–10

Kraebel, K.S., (2003). Asorption research,Inc: Dublin, Ohio 43016

Lemić, J., Tomašević-Čanović, M., Adamović, M., Kovačević, D., and Milićević, S., (2007)

Competative adsorption of polycyclic hydrocarbons on organo-zeolites: Microporous and

Mesoporous Materials.doi:10.1016/j.micromeso.2007.04.014

Li, Y., Ju, Q., Wand, D., Wang, X and Zhang, P., (2010). Removal of lead from aqueous

solution by activated carbon from enteromorpha prolifera by Zinc Chloride activation:

Journal of Hazardous Materials 183(2010) 583-589: Elsevier B.V.

McCabe, W.L., Smith, J.C and Harriot, P., (1993). Unit Operations for Chemical

Engineering,5th Ed.Singapore: The McGraw-Hill Companies

41 | P a g e

Momčilović, M., Bojic, A., Purenovic, M., Randelovic, M and Zarubica, A., (2011).

Adsorption of Pb(II) from aqueous solutions using activated carbon developed from pricot

stone: Desalination 276 (2011) 148–153

Mouni, L., Belkhiri, L., Bouzaza, A and Merabet, D., (2011). Adsorption of Pb(II) from

aqueous solution using activated carbon developed from apricot stone: Desalination

276(2011) 148-153: Elsevier B.V.

Perry, R.H and D.W. Green., (1997). Perry’s Chemical Engineers Handbook, 7th ed.

Newyork: McGraw-Hill

Rubio, J., and Oliveira, C.R., (2006). New basis for adsorption of ionic pollutants onto

modified pollutants: Minerals engineering (20): 552 – 558

Wang, L., Zhang, J., Zhao, R., Li, Y., Li, C and Zhang, C., (2010). Adsorption of Pb(II) on

activated carbon from Polygonum Orientale Linn: Kinetics, isotherms, pH and ionic studies:

Biosource Technology 101(2010) 5808-5814: Elsevier B.V.

42 | P a g e

APPENDIX A – GLOSSARY

Adsorption – Is the adhesion of the molecules of liquids, gases and dissolved substances to

the surfaces of solids as opposed to absorption in which the molecules actually enter the

absorbing medium.

Adsorbate – A substance that has been or is to be adsorbed on a surface.

Adsorbent – A material having the capacity or tendency to adsorb another substance.

Adsorption Kinetics – describes the uptake rate of the adsorbate and controls the residual

time of the process.

Adsorption Isotherms – describe how pollutants interact with the adsorbent materials.

Adsorption Mechanisms – analyse the rate controlling steps such as mass transport.

Chemisorption – Is a type of adsorption in which the forces involved are valence forces of

the same kind as those operating in formation of chemical compounds.

Cost – An amount paid or required in payment for a purchase.

Compatibility – Is being able to exist or work together.

Desiccation – Is the state of extreme dryness or the process of extreme drying.

Diffusion – Is the process by which molecules intermingle as a result of their kinetic energy

of random motion.

Heterogeneity – Is the quality or state of being heterogeneous.

Hysteresis – It is the dependence of a system not only on its current environment but also on

its past environment.

Metastable – (of physical systems) continuing in its present state of equilibrium unless

sufficiently disturbed to pass to a more stable state of equilibrium.

Physisorption – Is a type of adsorption in which the forces involved are intermolecular

forces (Van der Waals forces) of the same kind as those responsible for the imperfection of

real gases and the condensation of vapours and which do not involve a significant change in

the electronic orbital patterns of the species involved.

Regenerability – The ability to give energy or to revitalise.

Selectivity – The state or quality of being selective.

Thermodynamics – Is the branch of natural science concerned with heat and its relation to

other forms of energy and work.

43 | P a g e

APPENDIX B – SAFETY

Safety and HazopSafety considerations are imperative as it serves as a preventative measure against

accidents in any laboratory investigation. The laboratory contains potential hazards and

depending on the material selection, steps can be taken to eliminate as many risks as

possible. During the running of the experiments, safety will be enforced and adhered to at

all times.

Safety rules and procedures

The two most imperative steps to safety are;

What can hurt me?

What can i do to prevent it?

Chemical Safety

It is also important to know the risks and dangers associated with the materials that were

used when running the experiments. This can be prevented by always ensuring that he/she

reads and understand the MSDS (material safety data sheet) of the chemicals to be used.

MSDS’ contain relevant information concerning the hazards and risks pertaining to a

particular chemical. Table 6 contains MSDS for the heavy metals and hazardous

chemicals that will be used.

Table 6: MSDS for Heavy Metals

Name of heavy

metal

Ratings

health flammability reactivity

Copper(II) 1 1 0

Lead(II) 3 1 0

Key: 3=High, 2=Moderate, 1=Slight, 0=None

44 | P a g e

Table 7: MSDS for Chemicals

Name of

chemical

Ratings

Flammable Toxic Corrosive Harmful Irritant

HCL No Yes Yes Yes Yes

NaOH No Yes Yes Yes Yes

Pb(NO3)2 Yes Yes No Yes No

CuSO4.5H2O No No No Yes No

General laboratory safety

Laboratory safety is a very important aspect on every experiment that is carried out either

being in a laboratory or in an industrial plant. It is always important to understand the

safety policies and procedures before an experiment can be carried out to avert any

fatalities or injuries that may be incurred. Such safety procedures may include:

i. Carrying out a hazard and operability study (hazop) to identify any potential

hazards and ways to pre-empt and mitigate them.

ii. Wearing of protective clothing e.g. safety boots, lab coats, safety goggles e.t.c.

iii. Careful handling of chemicals in a lab to avoid any spills.

iv. Not running around and eating in the laboratory.

v. Being extra vigilant for any arising hazards.

vi. Ensuring that all equipment is in good working condition.

45 | P a g e

APPENDIX C - CALCULATIONS

Adsorption Capacity and Percentage Removal of Copper

Kinetic Models

Isotherm Models

46 | P a g e

Thermodynamic Analysis

47 | P a g e