Removal of Cr(VI) From Waste Water Using Hyacinth Roots Kinetic, Equilibrium and Thermodynamic Studies

8/3/2019 Removal of Cr(VI) From Waste Water Using Hyacinth Roots Kinetic, Equilibrium and Thermodynamic Studies

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Removal of Cr(VI) from waste water using hyacinth roots: kinetic,

equilibrium and thermodynamic studies

Document by: Bharadwaj

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Abstract

Heavy metals such as chromium, copper, lead, cadmium, mercury, zinc etc., in wastewater

are hazardous even in extremely minute quantities to the environment. Because of their toxicity,

their pollution effect on our ecosystem presents a possible human health risk. Cr(VI) is one of thehighly toxic metal entire into the environment from mining, leather tanning, cement industries,

electroplating, production of steel and other metal alloys, photographic material and corrosive paints etc. It act as carcinogens, mutagens and teratogens in biological systems. So it is essential to

remove from the environment.

The efficiency of hyacinth roots as an adsorbent for removing of Cr(VI) ions from aqueoussolution has been investigated. Batch adsorption experiments have been carried out to optimize the

dependent parameter like pH, initial Cr(VI) ion concentration, adsorbent dosage and equilibrium

contact time on the adsorption process. Maximum metal sorption was found to occur at initial pH 2.Kinetic data were best described by pseudo-second order model. The equilibrium adsorption data

were better fitted to Freundlich isotherm model. The sorption energy calculated by using Dubinin-

Radushkevich isotherm model which indicated that the adsorption processes were chemical innature. In addition, the thermodynamics parameters like standard free energy ( 0G∆ ), standard

enthalpy ( 0 H ∆ ), standard entropy ( 0S ∆ ) of the adsorption process were determined and results

found that adsorption process is spontaneous and endothermic in nature.

Keywords: Adsorption, hyacinth roots, kinetic data, thermodynamic parameter.

1. Introduction

The presence of heavy metals in the environment is a major concern because of their toxicity

and threat to human life and to the environment. Lead, cadmium, mercury, arsenic, copper,

chromium etc. are examples of heavy metals that have been classified as priority pollutants. These pollutants tend to accumulate in bottom sediments from which they may be released by various

processes of remobilization, thereby reaching human beings where they produce chronic and acutealiments. Cr(VI) as an example of such heavy metal which is toxic to animals, humans and it is alsoknown to be carcinogenic[1].

The concentration of Cr(VI) in industrial waste water varies in the ranges from 0.5 to 270 mg/L

[2]. The tolerance limit for Cr(VI) for discharge into inland surface water is 0.1mg/L and in potable

water is 0.05 mg/L [3], [4]. In order to comply with this limit, it is essential that industries treattheir effluents to reduce the Cr(VI) concentration in water and waste water to acceptable levels

before its transport and cycling into the natural environment. Several methods are utilized to

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remove Cr(VI) from industrial waste water. These include reduction followed by chemical

precipitation, ion exchange, reduction, adsorption, electrochemical precipitation, cementation,

evaporation, reverse osmosis, foam separation, freeze separation, bio-sorption [5]-[12]. Adsorptionis by far most versatile and effective method for such removal, especially, if combined with

appropriate regeneration steps. In this study hyacinth roots were used to remove Cr(VI) from

aqueous solution. Factors affecting the adsorption characteristics such as initial pH, contact time,adsorbent dosage and initial Cr(VI) ion concentration were studied. Rate kinetics and isotherm

models were also investigated to know the adsorption behavior of the adsorbent considered for

study.

2. Experimental

2.1 Adsorbent used

The water hyacinth roots used in this study were obtained from a pond of local area of Howrah

district near Kolkata, West Bengal, India. The collected roots were extensively washed with

distilled water to remove soil and dust. It was dried to the sunlight for 7 days. The sliced material

was dried 105

0

C for 6 hr to remove the adherent moisture, sieved to obtain particle size of 250-350μm and then kept in desicators.

2.2 Reagent and equipments

All the chemicals and regents used in the study were obtained from E. Merck Limited, Mumbai,

India and had a pure analytical quality. Characterization of adsorbents were carried out by scanning

electron microscope (SEM) Scanning electron microscope (S-3400N, Hitachi, Japan) studies wasconducted to observe the surface texture and porosity of the adsorbents. UV-Spectrophotometer (U-

4100 spectrophotometer, Hitachi, Japan) was used to determine the Cr(VI) content in standard and

treated solutions after adsorption experiments. The pH of the solution was measured with aEUTECH make digital microprocessor based pH meter previously calibrated with standard buffer

solutions. Fig. 1 shows the scanning electron micrographs of hyacinth roots. This figure showed

that it had an irregular and porous surface.

2.3 Preparation of standard Cr(VI) solution

The stock solution containing 1000 mg/L of Cr(VI) was prepared by dissolving 3.73 g of A. R.grade K 2CrO4, 2H2O in 1000 ml of de-ionized, double distilled water.

1 ml of the above stock solution =1 mg of Cr(VI).

Required initial concentration of Cr(VI) standards were prepared by appropriate dilution of theabove stock Cr(VI) standard solution.

2.4 Batch adsorption studies

The quantitative amount of adsorbents were taken in a 250 ml stopper conical flask containing

100 ml of desired concentration of the test solution at the desired pH value, contact time and

adsorbent dosage level. The pH of the solution was measured with a 5500 EUTECH pH Meter using FET solid electrode calibrated with standard buffer solutions. The contents in the flask were

shaken for the desired contact time in an electrically thermostated reciprocating shaker @ 120-130

strokes/minute at 300C. The contents of the flask were filtered through filter paper and the filtrate



was analyzed for remaining metal ion concentration by UV visible spectrophotometer (Model No.

U-4100 spectrophotometer, Hitachi, Japan) [13].

Results and discussion

Fig. 2 shows the percentage removal of Cr(VI) as a function of pH. It is clearly evident that the

adsorption characteristics of the adsorbent are highly pH dependent. The percentage removalreached a maximum value at an initial pH of the solution at 2. The equilibrium was reached at the

contact time of 4 hr. as shown in Fig. 3. Fig. 4 shows the variation of adsorbent dosage on the

percentage removal of Cr(VI). It is clear from the graph that the optimum adsorbent dosage 10 g/Lfor hyacinth roots. The effect of initial metal ion concentration on the removal of Cr(VI) is shown

in Fig. 5. This graph indicated that the decrease in metal ion removal as the initial metal ion

concentration increases.

3.1 Adsorption kinetics study

3.1.1 Pseudo first order Lagergren model

The pseudo first order kinetic model was proposed by Lagergren [14]. The integral form of themodel generally expressed as follows,

( )303.2

loglogt K

qqq ad ee −=−

(1)

3.1.2 Pseudo second order model

The linearized form of pseudo second order kinetic equation [15] may be expressed as,

t qq K q

t

ee

112

2

+= (2)

Lagergren and Pseudo second order models were presented in Fig. 6 and 7 respectively. The

values of rate constants and correlation coefficients for each model were shown in Table 1. Pseudosecond order model was best fitted with experimental adsorption data.

3.2 Isotherm model

3.2.1 Langmuir isotherm model

The data obtained from adsorption studies were fitted to the Langmuir adsorption isotherm as

[16]

maxmax

1

q

C

bqq

C e

e

e+=

(3)

Linear plots of Ce/qe vs. Ce have been used to determine the value of qmax (mg/g) and b (L/mg) for the adsoption. The Langmuir constant along with correlation coefficients (r 2) are listed in Table 2.

3.2.2 Freundlich isotherm model

The adsorption data obtained were also fitted to the Freundlich adsorption isotherm as [17]

e f e C n

K q log1

loglog += (4)



The values for Langmuir, Freundlich constants and correlation coefficients (r 2) obtained fromFigures 8 and shown in Table 2. Experimental data on adsorption were best fitted to freundlich

adsorption isotherm.

3.2.3 Dubinin-Radushkevich (DR) isotherm

The linear from Dubinin- Radushkevich isotherm model (18) was described as,2

ln lnabs m

C X λε = − (5)

ε is the Polanyi potential (19) which is equal to:

1ln(1 )

e

RT C

ε = + (6)

From the plot of abcC vs. 2ε gave a straight line from which the values of λ and m X for

hyacinth roots was calculated. Using the value of λ , the mean sorption energy, E, was evaluated as

1

2

E

λ =

− (7)

If E < 8 KJ/mol, the adsorption process is physical in nature and in the ranges from 8 to 16

KJ/mol, it is chemical in nature (20). The estimated value of E for Cr(VI) sorption was 11.5942

KJ/mol for hyacinth roots, which suggests the adsorption process is carried out chemical in nature.

3.3 Thermodynamic parameters for adsorption

Adsorption experiments to study the effect of temperature were carried out at 30, 40 and 550Cat optimum pH value of 2 and adsorbent dosage level 10 g/L. The equilibrium contact time for

adsorption was maintained at 4 hr.

The thermodynamic equilibrium constant (

0

c K ) was calculated by determining the apparentequilibrium constant,

'

c K at different initial concentration of Cr(VI) and extrapolating to zero.

' ac

e

C K

C = (8)

The Gibbs free energy, ∆G0, enthalpy, ∆H0, and entropy, ∆S0 were computed using following

equation0 0ln c

G RT k ∆ = − (9)0 0

ln c

H S k

RT R

∆ ∆= − + (10)

The value of standard free energy, ∆G0, was calculated using Equation 9. The value of slope and

intercept of the plot ln k c0 vs. 1/T , gave standard enthalpy, ∆H0, and standard entropy, ∆S0,

respectively (Table 4).

Conclusions

The optimum pH for removal was found to be 2. Increase in the concentration of adsorbent,

initial Cr(VI) concentration and contact time were found to be increase the percentage removal of

Cr(VI). The kinetics of the Cr(VI) adsorption was found to follow pseudo second order rate



mechanism. Adsorption isotherm of Cr(VI) was better described by Freundlich adsorption isotherm

model. The negative values of Gibbs free energy for the adsorption process reveal that the process

is spontaneous. The standard enthalpy change for the adsorption indicated that the process isendothermic.

Reference[1] Ranji, C. and Anirudhan, T.S. 1998. Batch Cr(VI) removal by polyacrylamide-grafted sawdust:

Kinetics and Thermodynamics. Water Res., 32(12), 3772-378.

[2] Patterson J. W. (1985). Industrial Wastewater Treatment Technology, 2nd Edn. Butterworth-Heinemann, London).

[3] EPA (Environmental Protection Agency), Environmental Pollution Control Alternativesc

Cincinnati, USA.

[4] Indian standard. 1991.Drinking water- specification, first revision. IS 10500.[5] Zhou, X. Korenaga, T. Takahashi, T. Moriwake, T. and Shinoda, S. 1993. A process

monitoring/ controlling system for the treatment of wastewater containing chromium (VI).Water Res., 27, 1049-1054.

[6] Tiravanti, G. Petruzzelli, D. and Passiono, R. 1997. Pretreatment of tannery wastewaters by anion exchange process for Cr(III) removal and recovery. Water Sci. Technol., 36, 197-207.

[7] Seaman, J. C. Bertsch, P. M. and Schwallie, L. 1999. In situ Cr(VI) reduction within coarse – textured, oxide-coated soil and aquifer systems using Fe(II) solutions. Env. Sci. Technol ., 33,

938-944.

[8] Dabhi, S. Azzi, M. and de la Guardia, M. 1999. Removal of Hexavalent chromium from

wastewaters by bone charcoal. Fresen. J. Anal. Chem., 363, 404-407.[9] Kongsricharoern, N. and Polprasert, C. 1996. Chromium removal by a bipolar electrochemical

precipitation process. Water Sci. Technol., 34, 109-116.

[10] Lin, C. F. Rou, W. and Lo, K. S. 1992. Treatment strategy for Cr(VI) bearing wastes. Water

Sci. Technol., 26, 2301-2304.

[11] Aksu, Z. Kutsal, T. 1990. A comparative study for biosorption characteristics of heavy metal

ions with C. vulgaris. Env. Technol., 11, 979-987.[12] Aksu, Z. Ozer, D. Ekiz H, Kutsal, T. and Calar, A. 1996. Investigation of biosorption of

chromium(VI) on C. crispate in two staged batch reactor. Env. Technol., 17, 215-220.

[13] Standard methods for examination of water and wastewater, 1998. 20th edition, APHA,AWWA. Washington D.C., New York.

[14] Lagergren, S. 1898. Zur theorie der sogenannten adsorption geloster stoffe. Kungliga Sevenska

Vetenskapasakademiens. Handlingar, 24, 1-39.

[15] Ho, Y. S. McKay, G. Wase, D. J. and Foster, C. F. 2000. Study of the sorption of divalentmetal ions on to peat. Ads. Sci. and Tech., 18, 639 – 650.

[16] Langmuir, I. 1918. The adsorption of gases on plane surfaces of glass, mica, and platinum. J. Am. Chem. Soc., 40, 1361-1368.

[17] Freundlich, H. 1906. Adsorption in solution. Phy. Chem. Soc., 40, 1361-1368.

[18] Dubinin, M.M., Zaverina, E.D., Radushkevich, L.V., 1947. Sorption and structure of active

carbons I. Adsorption of organic vapors. Zhurnal Fizicheskoi Khimii 21, 1351-1362.[19] Polanyi, M.,1932. Theories of the adsorption of gases. A general survey and some additional

remarks, Transactions of the Faraday Society. 28, 316-333.



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166-173

Table 1 Rate Kinetics for the adsorption of Cr(VI) ion onto hyacinth roots

Table 2 Langmuir and Freundlich adsorption isotherm constants for Cr(VI) removal

Langmuir constants Freundlich constants

qmax

mg/g

b

L/mg

r 2 K f n r 2

14.3575 0.2268 0.9659 2.923

5

3.0262 0.9927

Table 3 Dubinin-Radushkevich (D-R) isotherm parameter and activation energy

for the removal of Cr(VI)

D-R constant

λ

Activation energy

KJ/mol

r 2

-0.00372 11.5942 0.9759

Table 4 Thermodynamic parameters for the sorption of Cr(VI) onto adsorbents

Temperatur

eK

Thermodynamic parameters

-∆ G°

KJ/mol

∆ H°

KJ/mol

∆ S °

KJ/mol

303 5.2345

48.874 0.2835313 7.2084

328 9.8501

Lagergren 1st order model Pseudo 2nd order model

K ad

(min-1)

r 2 K 2[(mg/g)min]

r 2

0.02655 0.87512 0.01238 0.99728



0 2 4 6 8 1030

40

50

60

70

80

90

100

Fig.2. Effect of pH on Cr(VI) adsorption

% r

e m o v a l o f C r ( V

I )

Initial pH

Fig. 1. Scanning Electron Micrograph (SEM) of hyacinth roots

0.0 3.5 7.0 10.5 14.020

40

60

80

100

Fig. 4. Effect of adsorbent dosage on Cr(VI) adsorption

% r

e m o v a l o f C r ( V I )

Adsorbent dosage, g/L0 70 140 210 280 35060

70

80

90

100

Fig. 3. Effect of contact time on Cr(VI) adsorption

P e r c e n t a g e r e m o v a l o f C r ( V I ) , %

Contact time, min



0 60 120 180 240 300-3.0

-2.4

-1.8

-1.2

-0.6

0.0

0.6

Fig. 6. Lagergren plot for Cr(VI) adsorption

l o g ( q

e - q t )

Contact time, min



0 70 140 210 280 3500

25

50

75

100

125

Fig. 7. Pseudo second-order plot for Cr(VI) adsorption

t / q

Time, min-2 -1 0 1 2 3

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Fig. 8. Freuindlich plot for adsorption of Cr(VI)

l o g q e

log Ce

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Removal of Cr(VI) From Waste Water Using Hyacinth Roots Kinetic, Equilibrium and Thermodynamic Studies