Accepted Manuscript

Removal of Chromium(VI) from aqueous solution using guar gum-nano zinc

oxide biocomposite adsorbent

Tabrez A. Khan, Momina Nazir, Imran Ali, Ajeet Kumar

PII: S1878-5352(13)00280-3

DOI: http://dx.doi.org/10.1016/j.arabjc.2013.08.019

Reference: ARABJC 1070

To appear in: Arabian Journal of Chemistry

Received Date: 3 June 2013

Accepted Date: 25 August 2013

Please cite this article as: T.A. Khan, M. Nazir, I. Ali, A. Kumar, Removal of Chromium(VI) from aqueous solution

using guar gum-nano zinc oxide biocomposite adsorbent, Arabian Journal of Chemistry (2013), doi: http://

dx.doi.org/10.1016/j.arabjc.2013.08.019

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1

Removal of Chromium(VI) from aqueous solution using guar gum-nano zinc oxide biocomposite adsorbent

Tabrez A. Khan*, Momina Nazir, Imran Ali and Ajeet Kumar

Department of Chemistry, Jamia Millia Islamia, New Delhi 110 025, India

Abstract

Guar gum-nano zinc oxide (GG/nZnO) biocomposite was used as adsorbent for enhanced removal of Cr(VI) from aqueous solution. The maximum adsorption was achieved at 50 min contact time, 25 mg/L Cr(VI) conc., 1.0 g/L adsorbent dose and 7.0 pH. Langmuir, Freundlich, Dubinin-Kaganer-Radushkevich and Temkin isotherm models were used to interpret the experimental data. The data obeyed both Langmuir and Freundlich models (R2 =0.99) indicating multilayer adsorption of Cr(VI) onto the heterogeneous surface. The linear plots of Temkin isotherm showed adsorbent-adsorbate interactions. Moreover, the energy obtained from DKR isotherm (1.58-2.24 KJ/mol) indicated physical adsorption of the metal ions on the adsorbent surface, which implies more feasibility of the regeneration of the adsorbent. GG/nZnO biocomposite adsorbent showed improved adsorption capacity for Cr(VI) (qm = 55.56 mg/g) as compared to other adsorbents reported in the literature. Adsorption process followed pseudo-second order kinetics; controlled by both liquid-film and intra-particle diffusion mechanisms. Thermodynamic parameters (ΔGo, ΔHo and ΔSo) reflected the feasibility, spontaneity and exothermic nature of adsorption. The results suggested that GG/nZnO biocomposite is economical, eco-friendly and capable to remove Cr(VI) from natural water resources.

Keywords: Guar gum-nanozinc oxide biocomposite, chromium(VI), adsorption, isotherms,

kinetics, thermodynamics

*Corresponding author. Tel.: +91 11 26985938, Fax: +91 11 26985507.

E-mail address: takhan501@yahoo.com (T.A Khan).

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1. Introduction

The contamination of water resources due to chromium(VI) is one of the serious

environmental hazards. The industrial effluents from paints and pigments, leather and

chrome plating industries are major contamination sources. Generally, Cr(VI) exists as

stable oxoanions in aqueous solution such as chromate (CrO42−), hydrogen chromate

(HCrO4−), dichromate (Cr2O7

2−) and hydrogen dichromate (HCr2O7−). The concentration of

anionic species depends on solution pH and chromium concentration, which in turn affect its

toxicity and bioavailability (Gaballah and Kilbertus, 1998). Cr(VI) toxicities include

mutagenic (Cheng and Dixon, 1998), carcinogenic (Shumilla et al., 1999) and teratogenic

(Asmatulla et al., 1998) effects. The permissible limit of Cr(VI) is 0.05 mg/L for potable

water and 0.1 mg/L for discharge to inland surface water. Cr(VI) is reported to have

damaging effects on lungs, liver, nervous system and kidney in mammals. In view of its

severe toxicity, the removal of Cr(VI) from water bodies is an important task. The

conventional methods used for heavy metals removal from wastewater are precipitation,

coagulation, chemical reduction, ion exchange and adsorption (Ali et al., 2012; Ali, 2010;

Ali and Gupta, 2006). Most of these techniques are often expensive. However, the

adsorption process is an attractive alternative due to its ease of operation, economic viability

and effectiveness. Recently, adsorptive removal of Cr(VI) from wastewater using various

adsorbents has been investigated by several workers (Ren et al., 2013; Ozer et al., 2012;

Gupta et al., 2012; Shukla and Vankar, 2012; Annalisa et al., 2012; Rao et al., 2012;

Lukumoni et al., 2012; Khan et al., 2010; Rao and Rehman, 2010). It was observed that

most of these adsorbents have limited applicability due to their low adsorption capacities,

prolonged contact time and inability to work at natural pH of water. Therefore, it is

imperative to develop more efficient and economical adsorbents with higher adsorption

capacity, low contact time and ability to work at pH close to 7.

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Guar gum is a polysaccharide composed of galactomannan units. Each unit of

galactomannan consists of two units of β-D-mannopyranosyl units joined via 1→4-linkage;

with one unit of α-D-galactopyranosyl residue joined by 1→6-linkage as side chains.

Hydrogel of Guar gum has been used to remove Cu(II) in aqueous solution (Chauhan et al.,

2009). Some workers (Yan et al., 2012; Singh et al., 2009a,b) modified guar gum with

multi-walled carbon nanotubes, magnetic iron oxide nanoparticles, methyl-methycrylate,

polyacrylamide or silica for the removal of toxic dyes (neutral red and methylene blue),

Cr(VI) and Cd(II) from aqueous solution. It was observed that nano identities are also toxic

to our environment; leading to chances of further environmental contamination (Ali, 2012)

due mainly to difficulty in their separation. It was, therefore, considered worthwhile to

incorporate nano ZnO particles with guar gum, which may be easily separated after

adsorption process. In the present work, we report the adsorption efficiency of guar gum-

nano zinc oxide (GG/nZnO) biocomposite, dynamics and thermodynamics of the uptake of

Cr(VI) ions from aqueous solution.

2. Experimental

2.1. Reagents and chemicals

Guar gum (moisture content 5-6%) (CDH, India), zinc acetate (SRL, India), sodium

hydroxide, potassium dichromate, potassium bromide, diphenylcarbazide (all Merck, India),

isopropyl alcohol (IPA), absolute ethanol (S.D. Fine, India), sulphuric acid (Himedia, India)

and nitric acid (Qualigens, India) were used as received.

2.2. Preparation of Cr(VI) solution

The stock solution of Cr(VI) (100 mg/L) was prepared by dissolving K2Cr2O7 in de-ionized

water, which was diluted to desired concentrations (10 to 30 mg/L).

2.3. Preparation of adsorbent

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Guar gum-nano zinc oxide (GG/nZnO) biocomposite was prepared by mixing 50 mg guar

gum powder to 30 mL aqueous solution of isopropyl alcohol (3:1) in a 50 mL tri-necked

round-bottom flask. The mixture was magnetically stirred at 60oC to obtain a viscous gel.

To this was added zinc acetate (100 mg) and the stirring continued till a homogeneous

solution was obtained. NaOH solution (0.05M) in isopropyl alcohol (10 mL) was then added

to reduce zinc acetate to zinc oxide nanoparticles, which finally got immobilized within the

polymeric guar mesh. The reaction mixture was centrifuged at 10000 rpm for 10 min and

washed with isopropyl alcohol to remove any free particles. It was subjected to

lyophilisation so as to obtain a dry powder of GG/nZnO biocomposite, which was stored in

desiccators for adsorption studies.

2.4 Characterization of the adsorbent

The infrared spectra (4000–400 cm-1) of samples were recorded on a Perkin Elmer FT-IR

BX2 instrument. The pellets were prepared by mixing 2 mg of the powdered sample with

200 mg of spectroscopic grade KBr. For TEM analysis, 5 mg of the dry nanoparticles were

dispersed in 25 mL of ethanol in an ultra-sonicator. 10 mL of this nanoparticle dispersion was

put on a formvar coated copper grid (1% solution of formvar in spectroscopic grade chloroform)

and dried in vacuo. The dried grid was then examined under an electron microscope (Tem

Technai 300KV, Ultra twin FEI with EDAX transmission electron microscope operating at 300

kV). Electron diffraction pattern of the zinc oxide and GG/nZnO biocomposite was also

recorded. Scanning Electron Microscope (JEOL, model 3300) was used to investigate the

surface morphology of ZnO nanoparticles and GG/nZnO biocomposite.

2.5. Adsorption studies

Adsorption experiments were carried out at a fixed agitation speed at room temperature (303

K) under batch mode technique. 10 mL of Cr(VI) ions solution of desired concentration (10-

30 mg/L) was equilibrated with varying doses of GG/nZnO biocomposite (0.5–2 g/L) in a

5

thermostatic water bath shaker for time intervals ranging from 10 to 60 min. The adsorbent

was separated from the solution by centrifugation and the residual concentration of Cr(VI)

ions in the supernatant liquid was determined spectrophotometrically following standard

procedure (Vogel, 1969) at 540 nm (λmax). For kinetic measurements, the experiments were

performed using a fixed adsorbent dose with varying contact times at different Cr(VI) ion

concentrations. The effect of varying solution pH on adsorption was studied in the range of

2 to 10. The solution pH was adjusted with dilute HCl or NaOH solution (both 0.01M). The

experiments were repeated three times and average values were taken. The amount of

Cr(VI) ion adsorbed per unit mass of the adsorbent (qe, mg/g) was calculated using the

following equation (Eq. 1):

qe = (Co–Ce) / m (1)

where, Co is initial Cr(VI) concentration in mg/L before adsorption, Ce is equilibrium Cr(VI)

concentration in mg/L after adsorption and m is the amount of adsorbent (g/L of metal ion

solution). The percent adsorption was calculated from the relationship (Eq. 2):

% adsorption = 100 (Co–Ce) / Co (2)

3. Results and discussion

3.1. Characterization of the adsorbent

3.1.1. FTIR studies

FT-IR spectra of GG/nZnO biocomposite, guar gum and ZnO nanoparticles are given in Fig.

1. In the spectrum of guar gum, a band observed at 3400 cm-1 is ascribed to O–H stretching

vibration while the band at 2925 cm-1 is due to C–H stretching of the –CH2 groups. The

bands due to C–H bending and O–H bending appeared at 1458 and 1026 cm-1, respectively.

The band at 871 cm-1 can be attributed to the C–H deformation mode. The band at 1654 cm-1

is due to ring stretching of galactose and mannose. In addition, C–O–C and C–O stretching

bands are observed, respectively, at 1155 and 1015 cm-1. The weak bands around 770 cm-1

6

are due to ring stretching and ring deformation of α–D–(1–4) and α–D–(1–6) linkages. The

formation of crystalline ZnO nanoparticles is confirmed from the characteristic peak at 470

cm-1 with a band at 3300 cm-1 due to some absorbed water on the ZnO surface. The decrease

of the band related to hydroxyl groups in FTIR spectrum of GG/nZnO biocomposite

indicated ZnO immobilized onto guar gum.

3.1.2. Transmission Electron Microscopic Studies

TEM images of ZnO nanoparticles and GG/nZnO biocomposite [Fig. 2] showed that the

nanoparticles are mono-dispersed and are spherical in shape. This confirmed that all the nuclei

formed almost simultaneously and grew at the same rate. The result is in conformity with that of

QELS [Fig. 2(c,d)], which showed no sign of agglomeration as the particles were uniform in

shape and quite well dispersed.

3.1.3. Electron Diffraction Analysis

The electron diffraction patterns of ZnO nanoparticles and GG/nZnO biocomposite [Fig. 3]

were studied. The fringe ring pattern clearly indicated the crystalline nature of the zinc oxide

nanoparticles. The fused ring pattern revealed the amorphous nature of the GG/nZnO

biocomposite.

3.1.4. Scanning Electron Microscopic studies

SEM images of ZnO nanoparticles [Fig. 4(a,b)] and GG/nZnO biocomposite [Fig. 4(c,d)]

revealed the presence of ZnO nanoparticles immobilized onto guar gum.

4. Equilibrium adsorption studies

4.1. Effect of contact time

The effect of varying contact time on the percent removal and qe of Cr(VI) was studied at a

constant adsorbent dose of 1 g/L GG/nZnO biocomposite and a concentration of 25 mg/L

[Fig 5(a)]. It was observed that with the increase in contact time from 10 to 60 min., the qe

and % removal increased till it reached equilibrium in 50 min., which was selected as

7

optimum contact time. The maximum percentage removal was 95.6 %. The increased uptake

of Cr(VI) ion with the passage of time might be due to the increased access of ions to active

sites on the surface of adsorbent, resulting in an increase in the adsorption rate (Shen et al.,

2009).

4.2. Effect of adsorbent dose

The percent uptake of Cr(VI) ion (25 mg/L) with varying dosage of GG/nZnO biocomposite

is shown in Fig. 5(b). The percent removal increased with increase in adsorbent dose and

equilibrium was attained at an adsorbent dose of 1.0 g/L. The maximum percent removal at

1.0 g/L was 95.26 %. No considerable increase in percent removal was seen thereafter. The

increase in percent removal of adsorbate ions with increase in the adsorbent dose could be

attributed to greater availability of adsorption sites. At equilibrium, the percent removal

became constant probably because of the saturation of the available adsorption sites.

4.3. Effect of initial Cr(VI) concentration

The effect of varying initial Cr(VI) ion concentration (10-30 mg/L) on the adsorption

capacity of the adsorbent was studied at constant adsorbent dose and contact time [Fig. 5

(c)]. The percent removal increased with increase in the initial concentration of Cr(VI) ion

reaching equilibrium at 25 mg/L. The maximum amount adsorbed under these conditions

was 95.62 %.

4.4. Effect of pH

pH is an important factor controlling the adsorption process. The effect of pH on the

adsorption efficiency of GG/nZnO biocomposite, in the pH range 2-10, is depicted in Fig. 5

(d). The figure showed that percent removal of Cr(VI) increased with increase in pH, with

maximum adsorption occurring at pH 7. The percent removal steadily decreased thereafter.

Similar variation in the adsorption of Cr(VI) with increase in pH has been reported by

Rengaraj et al. (2003), Bhattacharyya and Gupta, (2006) with maximum adsorption

8

occurring at pH 6 and 7, respectively. Cr(VI) exists in various oxoanionic forms in aqueous

solution depending upon the solution pH. Below pH 1.0, the predominant species is H2CrO4.

Between pH 1 to 6, Cr(VI) exists in the form of HCrO4−, which dimerizes to Cr2O7

2− with

the release of a water molecule. Above pH 6, Cr(VI) exists in the form of CrO42−. It has

been suggested by Bhattacharyya and Gupta (2006) that at low pH the molecular form is

predominantly adsorbed species, whereas at higher pH values the ionized form is

preferentially adsorbed. The increase in adsorption up to pH 7 may be accounted on the

basis of the fact that at low pH the species HCrO4− gets adsorbed as the adsorbent surface is

positively charged.

2H 2HCrO4 2H2CrO4 Cr2O72 H2O 2CrO4

2+ + + H2O 2H+

2-5 pH pH = 6 pH>6

The significance of using GG/nZnO biocomposite as adsorbent lies in the fact that a

optimum amount of adsorption of Cr(VI) was adsorbed at pH 7, which is the natural pH of

about all natural water bodies.

4.5. Adsorption isotherms

The equilibrium data for adsorption of Cr(VI) onto GG/nZnO biocomposite was fitted into

various isotherm models. The isotherm parameters and the regression coefficients (R2)

corresponding to each model are given in Table 1.

4.5.1. Langmuir and Freundlich isotherm models

The linearized Langmuir and Freundlich adsorption isotherms are expressed as (Langmuir,

1918), (Freundlich, 1906):

1/qe = 1/qm + 1/qmbCe (3)

log qe = log Kf +1/nf log Ce (4)

9

where qm (mg/g) is the maximum adsorption capacity to form monolayer, b is the

equilibrium constant, and were obtained from slope and intercept of Langmuir plot,

respectively [Fig. 6(a)]. Kf (L/g) is related to adsorption capacity and nf is the exponent for

favourable adsorption. Kf and nf were calculated from intercept and slope of Freundlich

isotherm, respectively [Fig. 6(b)]. Langmuir adsorption isotherm assumes the occurrence of

adsorption at a fixed number of definite localized sites with each site being homogenous and

holding only one adsorbate molecule (monolayer) (Mckay et al., 1982). The Freundlich

adsorption isotherm, on the other hand, assumes that adsorption occurs on a heterogeneous

surface with the stronger binding sites occupied first and that the binding strength decreases

with increasing degree of site occupation. The R2 values for Langmuir and Freundlich

isotherm models (Table 1) were closer to unity (0.99), which indicated that both Langmuir

and Freundlich isotherm models were obeyed. This led to the conclusion that the adsorption

of Cr(VI) onto the adsorbent was homogeneous and multilayer in nature.

It has been suggested that for favorable adsorption the values of the dimensionless

separation factor, RL [=1 / (1 + b Co)] should be 0 < RL < 1. The value of RL represent

whether adsorption is favorable (0 < RL < 1), unfavorable (RL >1), linear (RL =1) or

irreversible (RL = 0). In present study, the values of RL were 0.53 to 0.63 (Table 1);

supporting favorable adsorption process (Hall et al., 1966). At 303 K the adsorption capacity

of the biocomposite was found to be 55.56 mg/g.

4.5.2. Temkin Isotherm model

This isotherm takes into account indirect adsorbate-adsorbent interactions on adsorbent

surface. Temkin noted experimentally that heat of adsorption more often decreases with

increasing coverage. Temkin adsorption isotherm is expressed as (Temkin and Pyzhev,

1940):

qe = RT/b. ln Kt + RT/b. ln Ce (5)

10

where, R is gas constant, T is temperature in Kelvin, b is a constant related to the heat of

adsorption and Kt is equilibrium binding constant (L/g) corresponding to maximum binding

energy. The linear plot of qe versus ln Ce at three different temperatures (303, 308 and 313

K) [Fig. 6(c)] for the adsorption system indicated the applicability of Temkin isotherm. The

values of b and Kt were determined, respectively, from the slope and intercept of the plot

(Table 1). Kt values at 303, 308, 313 K were 3.53, 3.92, 5.16 L/g, respectively; indicating

considerable adsorbent-adsorbate interactions at all temperatures. The values of b at these

three temperatures were 0.21, 0.21 and 0.24, respectively, indicating small difference in heat

of adsorption with change in temperature.

4.5.3. Dubinin-Kaganer-Radushkevich (DKR) isotherm model

DKR model is significant for calculating the apparent energy of adsorption, which predicts

type of adsorption i.e. physisorption or chemisorption. The model can be represented as

(Dubinin and Radushkevich, 1947):

ln qe = ln qD – β∈2 (6)

where ∈ = RT ln (1+1/ Ce) is Polanyi Potential. The plot of ln qe vs ∈2 yielded straight line,

thereby confirming the applicability of the model [Fig. 6(d)]. The magnitude of β was

calculated from the slope of the plot which was used to calculate the magnitude of

adsorption energy, E [=1 / (2β)1/2]. E values are useful in estimating the type of adsorption.

The energy values in the range 1.0 to 8.0 and 8.0 to 16.0 kJ/mol indicate physical and

chemical adsorption, respectively. E values for the adsorption of Cr(VI) onto GG/nZnO

biocomposite (1.58–2.24 kJ/mol) indicated physical adsorption.

4.6. Adsorption dynamics

11

In order to predict the kinetics of adsorption and the rate controlling steps, Lagergren’s

pseudo-first order, pseudo-second order, liquid-film and intra-particle diffusion models were

studied. The values of different constants of these models are given in Table 2.

4.6.1. Lagergren’s pseudo-first order and pseudo-second order models

The experimental data were evaluated using Lagergren’s pseudo-first order (Lagergren,

1898) and pseudo-second order rate (Ho and McKay, 1999) equations:

log (qe – qt) = log qe – k1.t /(2.303) (pseudo-first order) (7)

t/qt = 1/( k2qe2) + (1/qe).t (pseudo-second order) (8)

where, qe and qt are the amount of Cr(VI) ions adsorbed per unit mass at equilibrium and at

any time t, k1 and k2 are pseudo-first order and pseudo-second order rate constants,

respectively. The kinetic parameters were calculated at different initial Cr(VI)

concentrations from the intercept of the respective kinetic plots. The unity values of

correlation coefficients for the pseudo-second order kinetic model [Fig. 7(a)] together with

close agreement between calculated qe

and experimental qe

values (Table 2) suggested that

pseudo-second order kinetic model best correlated the experimental data.

4.6.2. Intra-particle and liquid film diffusion models

The variation in the amount of adsorption with time at different initial Cr(VI) concentration

was analyzed for evaluating the role of diffusion in the adsorption process. Adsorption is

considered to be a three step process. The first step involves transport of the adsorbate

molecules from the aqueous phase to the film surrounding the adsorbent. In the second step

diffusion of the solute molecules from the film to the adsorbent surface takes place. Finally,

in the third step the adsorbate molecules diffuse into the pore interiors. The first step is bulk

12

diffusion, the second is external mass transfer resistance and the third is intraparticle mass

transfer resistance. When the intraparticle mass transfer resistance is the rate limiting step,

then the adsorption process is described as being intraparticle diffusion

controlled (Igwe and Abia, 2007). The intra-particle diffusion (Weber and Morris, 1964) and

liquid film diffusion models (Boyd et al., 1949) are expressed as:

qt = ki t0.5 + Ci (9)

–ln (1–F) = Kfd t (10)

where, ki (mg/g min0.5), Kfd (1/min), Ci and F(= qt/qe) are rate constants, boundary layer

thickness and the fractional attainment of equilibrium at time t, respectively.

The plots of qt vs t0.5 and ln (1–F) vs t [(Fig. 7(b), (c)] for the adsorption system were linear.

This indicated that both intraparticle diffusion and liquid film diffusion controlled the

adsorption process. However, the plots did not pass through the origin. The deviation of

liquid film diffusion plot from the origin may be attributed to high speed of agitation used

during kinetic experiments or may also be due to the difference between rate of mass

transfer in the initial and final steps of adsorption (El-Ashtoukhy et al., 2008). The deviation

of intraparticle diffusion plot from the origin indicated that the effect of external film

resistance was not negligible.

4.7. Thermodynamic studies

The thermodynamic parameters are important to understand the energy and entropy changes

during adsorption process. The changes in enthalpy (ΔHo) and entropy (ΔSo) were

determined from the slope and intercept, respectively of vant Hoff’s linear plots of log

(qe/Ce) vs 1/T [Fig. 8] using the following equation:

log (qe/Ce) = –ΔHo/2.303 RT + ΔSo/2.303 R (11)

The free energy change (ΔGo) was calculated using the following equation:

13

ΔGo = ΔHo – TΔSo (12)

The calculated values are presented in Table 3.The positive values of ΔHo at different initial

concentrations of Cr(VI) ions indicated the endothermic adsorption process. The magnitude

of ΔHo being lower than 40 KJ/mol suggested physisorption of Cr(VI) onto GG/nZnO

biocomposite (Bhatnagar et al., 2009). The positive ΔSo indicated an increased randomness

at the solid-solution interface. The randomness might be attributed to the displaced water

molecules gaining more translational entropy as compared to that lost by the metal ions

during adsorption (Gopal and Elango, 2007). Gibbs’ free energy change (ΔGo) was negative

indicating the adsorption process to be spontaneous in nature. The –ΔGo values increased

with increasing temperature indicating thereby that adsorption was more favorable at higher

temperature.

5. Evaluation of the adsorbent

Table 4 shows the comparison of the maximum adsorption capacities of various adsorbents

for the removal of Cr(VI) from aqueous solution. The relatively higher adsorption capacity

indicated that GG/nZnO is an efficient adsorbent for the removal of Cr(VI) from aqueous

solution. Moreover, lesser contact time, lower adsorbent dosage, high percent removal,

ability to work at pH 7 together with the bio-compatible and non-hazardous nature of the

GG/nZnO biocomposite make this adsorbent more efficient and useful.

The larger adsorption capacity (55.56 mg/g) together with higher removal efficiency

(98.63%) indicated that GG/nZnO is an efficient adsorbent for the removal of Cr(VI) from

aqueous solution. Moreover, lower contact time (50 min.) and adsorbent dose (1.0 g/L) the

ability to work at pH 7.0, which is close to pH of most water bodies suggested that it is a

promising adsorbent as compared to most of other adsorbents.

6. Conclusions

14

The low contact time (50 min.) and adsorbent dose (1.0 g/L) together with high removal

efficiency (98.63%) indicated that GG/nZnO biocomposite is a useful adsorbent for rapid

and effective removal of Cr(VI) from aqueous solution. At pH 7, 98.63% Cr(VI) was

removed with a contact time of 50 min. and an adsorbent dose 1.0 g/L. The maximum

adsorption capacity calculated from Langmuir adsorption isotherm was found to be 55.56

mg/g at 303K. The adsorption data best followed the Langmuir and Freundlich isotherm

models. The pseudo-second order kinetic model described the data better than pseudo-first

order, which is evident from the high correlation coefficient values and close agreement

between experimental and calculated qe values. Thermodynamic parameters (ΔGo and ΔHo)

suggested the adsorption process to be spontaneous and endothermic. Both liquid-film and

intra-particle diffusions controlled the overall kinetics of the adsorption process. The

developed adsorption system is inexpensive and advantageous for the removal of Cr(VI)

from aqueous solution due to its small adsorption time, good adsorption capacity and ability

to work at pH 7; a natural pH of most water bodies. This elucidates the practical

applicability of the prepared adsorbent in the removal of Cr(VI) from natural water bodies.

Acknowledgement

One of the authors, Momina Nazir, is thankful to the University Grants Commission (UGC),

New Delhi, India for financial assistance.

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Captions of Figures

Fig. 1 FT-IR spectra of (a) GG/nZnO biocomposite, (b) Guar-Gum (GG), and (c) ZnO nanoparticles.

Fig. 2 TEM micrograph of (a) nano zinc oxide (b) GG/nZnO biocomposite. QELS analysis of (c) nano zinc oxide (d) GG/nZnO biocomposite Fig. 3 Electron diffraction of (a) nano zinc oxide (b) GG/nZnO biocomposite. Fig. 4 SEM micrograph of (a, b) nano zinc oxide (c, d) GG/ nZnO biocomposite. Fig. 5 Effect of (a) contact time (b) adsorbent dose (c) concentration (d) pH on adsorption of Cr(VI) onto GG/nZnO biocomposite. Fig. 6 (a) Langmuir (b) Freundlich (c) Temkin (d) DRK adsorption isotherm models for adsorption of Cr(VI) onto GG/nZnO biocomposite Fig. 7 (a) Pseudo-second order (b) Intraparticle Diffusion (c) Liquid Film Diffusion plots for adsorption of Cr(VI) ions onto GG/nZnO biocomposite

Fig. 8 Thermodynamic plots for adsorption of Cr(VI) ions onto GG/nZnO biocomposite

23

Fig. 1

24

Fig. 2

25

Fig. 3

26

Fig. 4

27

Fig. 5

[d]

28

Fig. 6

[b]

29

Fig.8

Fig. 7

30

Fig. 8

31

Table 1

Adsorption isotherm parameters for Cr(VI) onto GG/nZnO biocomposite

Isotherms

Isotherm constants

GG/nZnO biocomposite

303 K 308 K 313 K

Langmuir qm (mg/g) 55.56 55.86 47.39 b (L/mg) 0.36 0.39 0.58 RL 0.59 0.58 0.50 R2 0.99 0.99 0.99

Freundlich nf 1.39 1.39 1.54 Kf (L/g) 14.17 15.31 16.84R2 0.99 0.99 0.99

DKR

qD (mg/g) 27.48 27.72 27.22 E (kJ/mol) 1.58 1.58 2.24 R2 0.93 0.94 0.95

Temkin Kt (L/g) 3.53 3.92 5.16

R2 0.97 0.97 0.98 b 0.21 0.21 0.24

32

Table 2

Pseudo-first order, Pseudo-second order, Liquid-film diffusion and Intra-particle diffusion rate constants for the adsorption of Cr(VI) onto GG/nZnO biocomposite

Conc. Pseudo-first order

Pseudo-second order

Liquid-film diffusion

Intra-particle diffusion

(mg/L)

k1

(1/min) qe (calc ) (mg/g)

R2 k2 (g/mg min)

qe (calc ) (mg/g)

qe (exp) (mg/g)

R2 Kf d × 10-2

(1/min) R2 Ki

(mg min0.5/g)

Ci R2

10 0.08

0.85 0.98 0.18 9.44 9.35 1.0 7.61 0.98 0.12 8.61 0.91

15 0.05 0.88 0.99 0.15 11.09 11.07 1.0 4.89 0.99 0.13 10.14 0.99

20 0.04 1.36 0.83 0.10 12.5 13.01 1.0 4.25 0.90 0.24 11.3 0.87

33

Table 3

Thermodynamic parameters for adsorption of Cr(VI) onto GG/nZnO biocomposite

Conc. (mg/L)

ΔHo (kJ/mol)

ΔSo (KJ/mol K)

-ΔGo (kJ/mol)

303 K 308 K 313 K

10 13.20 0.064 6.37 6.69 7.02

15 34.90 0.13 5.85 6.52 7.19

20 24.00 0.10 5.27 5.75 6.23

25 12.70 0.06 5.22 5.52 5.81

30 5.50 0.03 4.73 4.90 5.06

34

Table 4

Comparison of adsorption capacities of various adsorbents for removal of Cr(VI) ions.

Adsorbents pH Contact

time (h) Dose of

adsorbent (g/L)

Isotherms Kinetics Thermodynamics

Adsorption capacity (mg/g)

References

Activated carbon-based iron- containing adsorbents

2 48.00 0.6 Langmuir, Freundlich

_ _ 68.49 Liu et al.(2012)

GG/n ZnO biocomposite

7 0.83 1 Freundlich Pseudo-second order, Liquid film diffusion, intraparticle diffusion

Endothermic, spontaneous

55.56 This work

Hevea Brasilinesis sawdust activated carbon

2 5 0.1 Langmuir Pseudo-second order, intraparticle diffusion

Endothermic, spontaneous

44.05 Karthikeyan et al.(2005)

modified, cationic surfactant spent mushroom

3.39 0.75 5 Langmuir Pseudo-first order _ 43.86 Jing et al. (2011)

Pine needles

3 0.75 10 Langmuir _ _ 40.00 Hadjmohammadi et al. (2011)

Eichhornia crassipes root biomass-derived activated carbon

4.5 0.5 7 Langmuir Pseudo-second order Endothermic, spontaneous

36.34 Giri et al. (2012)

Immobilized mycelia in carboxymethyl- cellulose (CMC)) of Lentinus sajor-caju

2 2 25 Langmuir, Freundlich

Pseudo-second order _ 32.20 Arıca and Bayramoglu, (2005)

Aspergillus niger fungi

3 12 0.2 Freundlich Pseudo-second order Exothermic, spontaneous

30.10 Mungasavalli et al. (2007)

Poly- (methylacrylate) functionalized guar gum

1 24 4 Freundlich Pseudo-second order Exothermic, spontaneous

29.67 Singh et al. (2009a)

Terminalia arjuna nuts activated with zinc chloride

1 10 2 Langmuir, Freundlich

Pseudo-first order _ 28.43 Mohanty et al. (2005)

Oxidized activated carbon from peanut shell

4 24 0.1 Langmuir Pseudo-first order, Pseudo-second order

Endothermic, spontaneous

14.54 AL-Othman et al. (2012)