Catena 100 (2012) 120–127

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Adsorption characteristics of modified sand for the removal of hexavalent chromiumions from aqueous solutions: Kinetic, thermodynamic and equilibrium studies

Sandeep Yadav a, Varsha Srivastava b, Sushmita Banerjee c, Chih-Huang Weng d, Yogesh C. Sharma b,⁎a Department of Applied Sciences, National Institute of Foundry and Forge Technology, Ranchi 834003, Indiab Department of Applied Chemistry, Indian Institute of Technology (BHU), Varanasi 221005, Indiac Department of Chemistry, University of Allahabad, Allahabad 211002, Indiad Department of Civil and Ecological Engineering, I-Shou University, Da-Shu District, Kaohsiung, 84001, Taiwan

⁎ Corresponding author. Tel.: +91 542 6702865; fax:E-mail address: ysharma.apc@itbhu.ac.in (Y.C. Sharm

0341-8162/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.catena.2012.08.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 April 2012Received in revised form 23 July 2012Accepted 1 August 2012

Keywords:AdsorptionModified sandKineticsIsothermThermodynamics

The objective of this study is to investigate the removal of Cr(VI) from aqueous solutions by using modifiedsand as adsorbent. The modified sand was characterized by Scanning Electron Microscopy (SEM), Energy Dis-persive X-ray Analysis (EDX) and FTIR. pHzpc of the raw and modified sands was found to be 6.98 and 6.66,respectively. Removal efficiency of the modified sand was investigated by using batch adsorption experi-ments. The effect of important parameters such as initial concentration, contact time, adsorbent dosage, pHand temperature on removal of Cr(VI) was investigated. It was demonstrated that the removal efficiencyincreased from 67.24% to 80.40% by the decreasing initial metal concentration from 15 to 5 mg L−1. Effectof pH was investigated by varying the solution pH from 2.0 to 8.0. The optimum pH for adsorption ofCr(VI) on modified sand was found to be 2.5 with a maximum removal of ~80.40%. Extent of removal de-creases by increasing the temperature from 25 °C to 35 °C confirming exothermic nature. Kinetics of removalprocess was studied by applying pseudo-first order and pseudo-second order models. Pseudo-first order rateconstant was found to be 0.037 min−1 while the rate constant for pseudo-second order reaction was found tobe 0.0236 g mg−1 min−1 at 25 °C. Values of thermodynamic parameters viz. ΔG°, ΔH° and ΔS° werecalculated and found to be −3.67 kJ mol−1, −68.74 kJ mol−1and −0.243 kJ mol−1 K−1, respectively at25 °C. The values of ΔG° were found to be negative at all temperatures indicating the spontaneity of theremoval process. A negative value of ΔH° further confirms the exothermic nature of removal process. Theexperimental data were fitted to Langmuir as well as Freundlich adsorption isotherm equations. The resultsobtained in the present study show the modified sand to be a better adsorbent for removal of Cr(VI).

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Chromium has widespread industrial applications in a variety ofindustries such as electroplating, leather tanning, textile dyeing,steel fabrication and wood preservation treatment (Bansal et al.,2009; Mantcheva and Kalev, 2004; Singh et al., 2012) and is a com-mon pollutant of water and wastewater. Chromium exists in manyvalence states like Cr(0), Cr(II), Cr(III) and Cr(VI) but from the pointof view of environmental pollution, Cr(III) and Cr(VI) are important(Nriagu, 1988; Sharma et al., 2010). Cr(VI) is 500 times more toxicthan Cr(III) (Acar and Malkoc, 2004). Exposure to Cr(VI) causes healthproblems including skin irritation, epigastric pain, nausea, vomiting,severe diarrhea, hemorrhage and carcinogenicity(Gładysz-Płaska etal., 2012; Shen et al., 2012). According to the guidelines recommendedby the World Health Organization (WHO), the maximum level forCr(VI) in drinkingwater is 0.05 mg L−1 (Zhang et al., 2010). According-ly, chromium containing wastewaters must be treated to lower the

+91 542 2306428.a).

rights reserved.

Cr(VI) to permissible limits prior to discharge. Chemical precipitation,coagulation, solvent extraction, membrane separation, reverse osmosis,ultra filtration, electrochemical deposition, ion-exchange and adsorption(Rao and Rehman, 2010; Sharma, 2003) have been reported as thecommon methods for removal of Cr(VI) from aqueous solutions andeffluents. However, considering the high cost, long operation time andsludge generation, it is necessary to find out a more effective andeconomically viable process for the removal of Cr(VI). Among thesemethods, adsorption is generally considered to be simple, relativelylow-cost and effective in removing Cr(VI) from water and wastewater(Gupta et al., 2010; Karthikeyan et al., 2005, Sharma et al., 2007). Activat-ed carbon is themost widely used adsorbent due to its large surface area,micro-porous structure, and high adsorption capacity. However the highcost and the limited selectivity of activated carbon restrict its large scaleapplication (Altun and Pehlivan, 2012; Wang et al., 2008, 2009, 2010).Therefore, the search for a cost effective adsorbent materials is of im-mense importance for wastewater treatment. The sands and clays arenon toxic and have also been reported to be good adsorbent materials(Graf et al., 1991, Guerra et al., 2010). In the present study, sand wasselected as an adsorbent but since its reported efficiency of removal

Fig. 1. ED spectrum of raw (a) and modified sands (b).

121S. Yadav et al. / Catena 100 (2012) 120–127

was low, it wasmodified by a simple method. Effect of various importantparameters such as concentration of Cr(VI), contact time, adsorbent dose,pH and temperature. Kinetics and equilibrium studies of the removal pro-cess were also carried out.

2. Materials and methods

2.1. Preparation of adsorbent

The sand samples were collected from shallow waters near theriverbank from the river Subernarekha, Jharkhand, India. Sand sam-ples were thoroughly washed to remove all earthen impurities andthen dried in a hot air oven at 105 °C overnight. After drying, particlesize of sand samples was maintained (150 μm) using different sieves.Sand was modified by soaking in 40% H2SO4 for a period of 4 h, there-after the treated sample was washed repeatedly with distilled water.Simultaneously, the pH of supernatant was checked and washing oftreated sand was continued until the pH of the supernatant reachedthe neutral pH range. Finally, the washed sample of sand was driedin an oven and then used as the adsorbent. Modified sand was char-acterized for its surface characteristics and composition. The chemicalcomposition was analyzed by Energy Dispersive X-ray Analyzer (EDX).Themorphological characteristics of rawandmodified sandswere eval-uated using a Scanning Electron Microscope (SEM) (Jeol, Japan, JSM6390LV Microscope). The FTIR spectra of raw and modified sandswere obtained with a Fourier Transform Infrared Spectrophotometer(FTIR) (Varian 1600 FT-IR Scimitar Series) to elucidate the functionalgroups present on the surface of sand before and after modification.pHzpc of raw and modified sands were also determined for the surfacecharge. For its determination, 0.01 M NaCl was prepared and its pHwas adjusted in the range between 2 and 12 by using NaOH or HCl.Then 50 ml of 0.01 M NaCl was taken in six different flasks and 0.20 gof the sand was added in each flask containing 0.01 M NaCl solutionmaintained at different pH values (Namdeo and Bajpai, 2008). Thesame procedure was also repeated for the modified sand to study.These flasks were then kept for 48 h and after that the final pH of thesolutions was measured by using a pH meter (IKON Digital pH meter).A graph was then plotted between pHfinal vs pHinitial. The point of inter-section of ‘pHfinal vs pHinitial’ curves was recorded as pHzpc of the sandsamples.

2.2. Batch adsorption experiments

Batch adsorption experiments were conducted to determine theoptimum conditions for the removal of Cr(VI) from aqueous solutions.Stock solution of chromium was prepared by dissolving potassiumdichromate in 1000 mLof distilledwater, andwas used for the prepara-tion of working solutions of various concentrations. In the batch exper-iments, a fixed amount of adsorbent was added into 50 mL Cr(VI)solution of known concentrations maintained at a known pH and tem-perature. The initial pH of Cr(VI) solution was adjusted to a definitevalue using 0.1 mol L−1 NaOH and/or 0.1 mol L−1 HCl solutions.The ionic strength of the aqueous solution was maintained at1.0×10−2 M NaClO4. The ionic strength of the solutions was decid-ed on the basis of the molar concentrations of Cr(VI) taken for ex-periments. After the equilibrium time, the adsorbent was separatedfrom the aqueous solutions by centrifugation (Remi 24, New Delhi,India) at 10,000 rpm for 10 min. The residual concentration of Cr(VI)in each aliquot was determined by using Atomic Absorption Spectro-photometer (AA-7000, Shimadzu, Japan). The amount of Cr(VI) ionsadsorbed per unit mass of the adsorbent was determined from the fol-lowing expression:

qe ¼Ci−Ce

w

� �� V ð1Þ

where qe is the amount adsorbed per unit mass of the adsorbent(mg g−1), Ci and Ce are the initial and equilibrium concentrations,respectively (mg L−1),W is the mass of the adsorbent (g) and V (L) isthe volume of the solution. The percentage removal of Cr(VI) ions wascalculated using Eq. (2):

% removal of Cr VIð Þ ions ¼ Ci−Ce

Ci

� �� 100: ð2Þ

3. Results and discussion

3.1. Characterization of adsorbent

In order to understand the adsorption of Cr(VI) onto sand, physi-cochemical characterization of the adsorbent was carried out. EDSspectra of raw and modified sands are shown in Fig. 1a and b. It isclear from the EDX spectrum that silica (as Silicon Oxide, SiO2) isthe major constituent of the adsorbent. SEM of raw and modifiedsands clearly indicates the effect of the modification process on theadsorbent (Fig. 2a and b). It is clear from the SEM that the adsorbenthas a porous structure. SEM of modified sand (at 5000 magnification)shows the roughness of the surface as crests and troughs. These crestsand troughs increase the surface area and consequently the adsorp-tion capacity of the adsorbent and act as reactive adsorption centersfor adsorption of Cr(VI).

In the FTIR spectra of the raw and modified sands, the peak at759.49 cm−1 shows the presence of silica (Si\O bond) and that at1783 cm−1 it indicates the presence of 4 membered cyclic com-pounds, which are probably organic in nature (figure not given).

Fig. 2. SEM images of raw (a) and modified sands (b).

Fig. 3. Effect of concentration on the removal of Cr(VI) by adsorption on modified sand.

0 20 40 60 80 1000

10

20

30

40

50

60

70

80

90

100

1.0g 1.5g 2.0g

Rem

ova

l(%

)

Contact time(min)

Fig. 4. Effect of adsorbent dose on the removal of Cr(VI) by adsorption on modifiedsand.

122 S. Yadav et al. / Catena 100 (2012) 120–127

3.2. Effect of contact time and initial concentration of Cr(VI)

In a batch adsorption process, the initial metal ion concentrationof the solution provides the necessary driving force to overcome themass transfer resistance for adsorbate between the solution andsolid phases. The effect of initial Cr(VI) ion concentration on adsorp-tion was studied by varying Cr(VI) concentration in the range of5 to 15 mg L−1 and the results are given in Fig. 3. It is clear that byincreasing the initial concentration, removal of Cr(VI) decreasesfrom 80.40 to 67.24%. A higher Cr(VI) concentration needs more bind-ing sites on the adsorbent surface as compared with a lower initialCr(VI) ion concentration at the same dosage of the adsorbent. Theremoval increased up to 80 min, after this it became constant (Fig. 3).

3.3. Effect of adsorbent dose

The effect of adsorbent dose was studied by varying the amount ofthe adsorbent between 1.0 and 2.0 g for the same volume of the solu-tions. Removal of Cr(VI) was found to increase proportionally withthe amount of the modified sand dose until reaching a constant(Fig. 4). It can be explained as on increasing the adsorbent dose, the‘active sites’ available for Cr(VI) ions also increase and consequently

more adsorption takes place. At 1 g/50 ml of modified sand dosage,the removal of Cr(VI) was found to be 80.40% and this dose wastaken as the optimum for experiments.

3.4. Effect of pH

pHzpc is an important property of an adsorbent and determines thesurface behavior of the adsorbent. pHzpc values of the raw andmodifiedsands were found to be 6.98 and 6.66 respectively (Fig. 5a and b).

At a pH below pHzpc of the adsorbent, the surface is positivelycharged and above pHzpc, the adsorbent surface is negatively charged.The results of present study are consistent with this fact.

The pH of solution plays an important role in the removal of pol-lutants from water and waste water. It controls the surface chargeand surface properties of the adsorbent surface as well as degree ofionization.

Effect of initial pH was investigated by agitating 50 ml Cr(VI) solu-tions of different pH. The removal of Cr(VI) by modified sand wasfound to be 80.40, 44.08 and 36.40 % at pH values of 2.5, 4.0 and 8.0respectively (Fig. 6). It is clear from the speciation diagram of Cr(VI)that at lower values of pH, HCrO4

− are the major species (Fig. 7). Atlower pH, OH− group is neutralized by hydrogen ions and favors

Fig. 7. Speciation diagram of Cr(VI).

Fig. 5. pHzpc of raw (a) and modified sands (b).

123S. Yadav et al. / Catena 100 (2012) 120–127

the adsorption of this species. HCrO4− is introduced in the solution by

the following reaction:

Cr2O2−7 þ H2O→2 HCrO

−4 : ð3Þ

At higher pH, Cr2O72− and CrO4

2− are present in aqueous solutions.In this condition the presence of larger number of OH− hinders thediffusion of dichromate ions so the removal decreases with increasingpH.

Fig. 6. Effect of pH on the removal of Cr(VI) by adsorption on modified sand.

It is seen that a pH of 2.5 will be quite suitable for the removal ofdominating HCrO4

− species. As the solution pH increases, not only lessfunctional groups are deprotonated but alsomore number of OH− com-pete with the HCrO4− and CrO4

2− ions for the active surface sites. Con-sequently, it is difficult for them to form complexes and the adsorbedamount decreases.

3.5. Effect of temperature

In the present investigations the effect of temperature on the removalof Cr(VI) by modified sand was studied at three different temperatures25, 30 and 35 (±0.5)°C by keeping other experimental conditions con-stant. Removal decreased from 80.40% to 62.42% with the increase intemperature from 25 to 35 °C for the initial concentration of 5 mg L−1

(Fig. 8).Removal of Cr(VI) by modified sand is an exothermic process. The

decrease in removal with increasing temperature can be attributed tothe fact that at enhanced temperature, relative escaping tendency ofCr(VI) species gets enhanced which consequently results in reductionof the boundary layer thickness. Also, solubility of Cr(VI) increaseswith increasing temperature which is also a reason of the decreasein removal at increasing value of temperature.

Fig. 8. Effect of temperature on the removal of Cr(VI) by adsorption on modified sand.

Fig. 10. Intra-particle diffusion kinetics for adsorption of Cr(VI) on modified sand atdifferent temperatures.

20 30 40 50 60

-1.6

-1.5

-1.4

-1.3

-1.2

-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

250C

300C

350C

log

(qe-

qt)

Contact time(min)

Fig. 9. Pseudo-first order kinetic plots for the adsorption of Cr(VI) on modified sand atdifferent temperatures.

124 S. Yadav et al. / Catena 100 (2012) 120–127

3.6. Adsorption kinetics

Studies were carried out in order to understand the kinetic behaviorof adsorption of Cr(VI) on modified sand. Different models viz. pseudo-first-order, pseudo-second order and intraparticle diffusion have beenapplied to understand the kinetics of adsorption of Cr(VI) on modifiedsand.

3.6.1. The pseudo-first order kinetic modelThe pseudo-first order model is expressed by following the expres-

sion (Malkoc and Nuhoglu, 2007):

dqdt

¼ k1 qe−qtð Þ: ð4Þ

The integrated form of the above expression is (Sharma et al.,2007):

log qe−qtð Þ ¼ logqe−k1

2:303t ð5Þ

where k1 (min−1) is the rate constant of pseudo-first order kineticequation, qe and q, respectively are the amounts of adsorbateadsorbed at equilibrium and at any time (mg g−1). The value of k1was determined from the slope of the linear plots of log (qe−q) vst (Fig. 9) at different temperatures. Values of pseudo-first orderrate constant and corresponding R2 at different temperatures aregiven in Table 1.

Table 1Values of pseudo-first order, pseudo‐second order and intra-particle diffusion con-stants for the adsorption of Cr(VI) on modified sand at different temperatures.

Temperature (°C) k1 (min−1) R2

Pseudo first orderrate constants

25 0.037 0.9930 0.035 0.9935 0.023 0.98

Temperature (°C) k2 (g mg−1 min−1)

Pseudo-second orderrate constants

25 0.024 0.9730 0.040 0.4935 0.068 0.97

Temperature (°C) kdif (mg g−1 min−1/2)

Intra-particle diffusionconstants

25 0.028 0.9930 0.035 0.9935 0.030 0.94

3.6.2. Pseudo-second order kinetic modelThe adsorption data were also analyzed using a pseudo second

order kinetic equation. The second order model assumes that the ad-sorption process is of pseudo second order and the rate limiting stepis chemisorption. The mechanism may involve sharing of valenceforces or through the exchange of electrons between the adsorbentand the adsorbate. This can be expressed as follows (Ho and Mckay,1999):

dqdt

¼ k2 qe−qtð Þ2: ð6Þ

Where k2 (g mg−1 min−1) is the rate constant of pseudo‐secondorder kinetic equation. The integrated form of the above reactioncan be expressed as follows:

tqt

¼ 1k2qe

2 þ1qt

t ð7Þ

h ¼ k2qe2 ð8Þ

h is known as the initial sorption rate. The value of qe and k2 can bedetermined by the slopes and intercepts of the straight line of theplots ‘t/qt vs t’, respectively (figure not given). The values of pseudo-second-order rate constants k2 and the corresponding linear regressioncorrelation coefficients R2 are given in Table 1. On comparison of R2

values, it can be revealed that pseudo-first order model better fits the

0 1 2 3 4 5 6 70

2

4

6

8

10

12

14

16

18

20

250C 300C 350C

Ce/

qe

Ce

Fig. 11. Langmuir isotherm plot for adsorption of Cr(VI) on modified sand at differenttemperatures.

Table 2Values of Langmuir's and Freundlich constants for adsorption of Cr(VI) on modifiedsand at different temperatures.

Temperature (°C) Q0 (mg g−1) b (L mg−1) R2

Langmuir parameters 25 1.19 0.14 0.9930 1.06 0.12 0.9935 1.10 0.08 0.99

Temperature (°C) Kf (mg g−1) n

Freundlich parameters 25 0.17 1.42 0.9930 0.14 1.45 0.9935 0.10 1.34 0.98

125S. Yadav et al. / Catena 100 (2012) 120–127

kinetic data as comparedwith. It is also clear from Table 1, that values ofk1 decreasewith increasing temperature confirming exothermic natureof Cr(VI) adsorption on modified sand.

3.6.3. Intra-particle diffusion modelThe intra-particle diffusion rate constant, (kdif) was determined

using the following equation (Weber et al., 1963):

qt ¼ kdifffiffit

pþ C ð9Þ

where kdif is the intra-particular diffusion rate constant (mg g−1 min−1/2).If intra-particle diffusion is the rate-limiting step then plots of ad-sorbate uptake qt versus the square root of time (t1/2) would resultin a linear relationship and kdif and C values can be obtained fromthese plots (Fig. 10) The first step in diffusion model is the masstransfer of adsorbate molecules from bulk to the adsorbent surfaceand second stage is the intra-particle diffusion in the adsorbent.Moreover, the intra particle diffusion would be the rate controllingstep if the lines pass through the origin, but here it is not the ratecontrolling step as concluded by Fig. 10. The value of intraparticlediffusion constant is given in Table 1.

3.7. Adsorption isotherm

3.7.1. Langmuir adsorption isothermLangmuir model assumes uniform adsorption on the surface and is

valid for a monolayer sorption with a homogeneous distribution ofthe sorption sites and sorption energies. The linearized expression of

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

250C 300C 350C

log

qe

logCe

Fig. 12. Freundlich isotherm plot for adsorption of Cr(VI) on modified sand at differenttemperatures.

Langmuir model can be expressed as follows (Hameed et al., 2007;Sharma et al., 2007):

Ce

qe¼ 1

Q0bþ Ce

Q0 ð10Þ

where Ce (mg L−1) and qe (mg g−1) are the concentrations of adsorbateand the amount of adsorbate adsorbed at equilibrium, respectively. Q0

(mg g−1) and b (L mg−1) are the terms related to capacity and energyof adsorption, respectively, and are known as Langmuir's constants.

From Langmuir isotherm, a dimensionless separation factor canalso be expressed by the following equation (Basha et al., 2008):

RL ¼1

1þ bC0ð Þ ð11Þ

where C0 is the initial solute concentration (mgL−1) and b is theLangmuir adsorption equilibrium constant (L mg−1).

The dimensionless separation factor, RL is used to test whether theadsorption is favorable or not. The value of RL indicates the type of theisotherm to be either unfavorable (RL>1), linear (RL=1), favorable(0bRLb1) or irreversible (RL=0). Langmuir capacities were calculat-ed by using Eq. (10).

The data has been plotted as ‘Ce/qe vs Ce’ (Fig. 11) and the values ofthe Langmuir's constants, Q0 and b were determined from the slopesand intercepts of this figure. The calculated values of Q0, and b at dif-ferent temperatures are given in Table 2. The values of RL for the pres-ent system were in the range of >0 and b1 at all temperatures whichconfirm the favorable uptake of Cr(VI) on the modified sand. Thevalues of RL at 25, 30 and 35 °C were 0.18, 0.15 and 0.24 respectively.

3.7.2. Freundlich adsorption isothermFreundlich isotherm model can be used to describe the sorption on

heterogeneous surfaces as well as a multilayer sorption. It assumesthat the uptake of adsorbate ions occurs on a heterogeneous adsorbentsurface. Freundlich equation indicates the adsorptive capacity or load-ing factor on the adsorbent surface. Freundlich model is expressed asfollows (Ewecharoen et al., 2008):

qe ¼ KfC1=n

: ð12Þ

The logarithmic form of the equation is expressed as follows(Ewecharoen et al., 2008):

log Qe ¼ log Kf þ1nlog C ð13Þ

where Kf is the Freundlich constant denoting adsorption capacity(mg g−1) and 1/n is the empirical constant indicating adsorption

Table 3Comparison of adsorption capacities of different adsorbents for the removal of Cr(VI).

Adsorbents Adsorption capacity(mg g−1)

References

Raw rice bran 0.07 Oliveira et al. (2005)Teak (Teclona grandisLinn. f)

0.89 Sumathi and Naidu (2005)

Wollastonite 0.83 Sharma (2001)Fly ash 1.4 Banarjee et al. (2004)Riverbed sand 0.15 Sharma and Weng (2007)Spent activated clay 1.42 Weng et al. (2008)Rice husks 0.6 Sumathi and Naidu (2005)Activated rice huskcarbon

0.8 Bishnoi et al. (2004)

Fly ash–wollastonite 2.9 Panday et al. (1984)Sawdust 1.5 Sumathi and Naidu (2005)Olive stones 0.012 Fiol et al. (2003)Modified riverbed sand 0.79 Present study

Table 4Values of different thermodynamic parameters for Cr(VI) adsorption on modified sand.

Temperature (°C) ΔG° (kJ mol−1) ΔH° (kJmol−1) ΔS° (kJ mol−1 K−1)

25 −3.67 −68.74 −0.24330 −4.8835 −6.10

126 S. Yadav et al. / Catena 100 (2012) 120–127

intensity (L mg−1) and depends on the temperature and propertiesof the adsorbate and the adsorbent. They are measures of adsorptioncapacity of the adsorbent and adsorption intensity respectively. Ce isthe residual concentration of solute remaining in the solution(mg L−1), qe is the amount of adsorbate adsorbed by a unit mass ofadsorbent at equilibrium (mg g−1). The values of Kf and 1/n werecalculated by the slopes and intercepts of the plots of ‘log Ce vs logqe’ (Fig. 12). The values of the Freundlich constants Kf and n for thesystem have been given in Table 2. Linear plots confirm the applica-bility of Freundlich isotherm.

To compare the adsorption efficiency of modified sand, a compar-ison of the adsorption capacity of different adsorbents for the Cr(VI)was carried out. Adsorption capacity of different adsorbents forCr(VI) is listed in Table 3 (Oliveira et al., 2005; Sumathi and Naidu,2005; Sharma, 2001; Banarjee et al., 2004; Sharma and Weng ,2007;Weng et al., 2008; Bishnoi et al., 2004; Panday et al., 1984; Fiol etal., 2003). It is clear that the modified sand used in the presentstudy has a significant adsorption capacity in comparison to otheradsorbents (Table 3) and can be a better alternative for the Cr(VI) ad-sorption. On perusal of Table 3, it is clear that the values of R2 at allthe three temperatures are higher for Freundlich isotherm. Thus, theexperimental data establishes Freundlich isotherm better.

3.8. Thermodynamic parameters

In any adsorbate–adsorbent system, the values of thermodynamicparameters are important indicators for practical application. Ther-modynamic parameters can be estimated using equilibrium constantsas a function of temperature. The thermodynamic parameters namelychange in standard free energy (ΔG°), enthalpy (ΔH°) and entropy(ΔS°) were determined by using the following equation (Dey andAiroldi, 2008; Guerra et al., 2010):

log Kc ¼ ΔS02:303R

− ΔH02:303RT

ð14Þ

ΔGads¼ ΔHads−TΔSads: ð15Þ

Where R (8.314 J mol−1 K−1) is the gas constant, T (K) is the ab-solute temperature and kc (L g−1) is the standard thermodynamicequilibrium constant defined by qe/Ce. The values ΔH° and ΔS° werecalculated from the slopes and intercepts of a graph “log kc versus1/T” (figure not given). Values of different thermodynamic parame-ters for Cr(VI) adsorption on the modified sand are given in Table 4.

The free energy change, ΔG° was found to be negative at all tem-peratures which indicates that the adsorption process is spontaneousin nature. The negative value of the enthalpy change ΔH° confirms theexothermic nature of the process of adsorption. The negative value ofΔS° suggests that there is a decrease in randomness at the solid/solu-tion interface occurring at the internal structure of the adsorption ofCr(VI) on the modified sand.

4. Conclusions

• The process of modification of sand is simple and economically via-ble. SEM revealed that modification enhanced the active sites onsurface. The ED spectra shows presence of silica in sand sample, itmight be in the form of quartz (SiO2).

• Significant removal of Cr(VI) from aqueous solutions with higherremoval at low concentration ranges has been obtained.

• The process is highly pH dependent. The removal of Cr(VI) wasfound to be maximum at pH 2.5.

• The value of the adsorption capacity of modified sand for Cr(VI) wasfound to be significant, which indicates that it can be successfullyused for the removal of Cr(VI).

• The kinetic study shows that adsorption of Cr(VI) followspseudo-first order kinetic model.

• The process of removal is exothermic and consequently low tem-peratures favor the removal of chromium. The maximum removalwas found to be 80.40% at 25 °C.

• Values of different thermodynamic parameters viz. ΔG°, ΔH° andΔS° were calculated and found to be −3.67 kJ mol−1, −68.74 kJmol−1and −0.243 kJ mol−1 K−1, respectively at 25 °C. The negativevalues of ΔG° and ΔH° show spontaneous and exothermic nature ofthe adsorption process respectively.

Further, sand is a naturally available material and may provide anon-toxic process of Cr(VI) removal. The modified sand can make agood choice for treatment of Cr(VI) rich wastewaters and industrialeffluents.

Acknowledgments

The authors are thankful to Dr. P.K. Panda, Professor of English andProfessional Communication, Indian Institute of Technology (BHU),Varanasi for improving the language of the manuscript.

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