batch and dynamics modeling of the biosorption of cr(vi) from aqueous solutions by solid biomass...

Batch and Dynamics Modeling of the Biosorption of

Cr(VI) from Aqueous Solutions by Solid Biomass

Waste from the Biodiesel ProductionMuthusamy Shanmugaprakash,a Venkatachalam Sivakumar,b Manickavelu Manimaran,a andJeyaseelan Aravinda

aDepartment of Biotechnology, Kumaraguru College of Technology, Coimbatore 641049, IndiabDepartment of Food Technology, Kongu Engineering College, Perundurai, Erode 638052, India; [email protected](for correspondence)

Published online 30 April 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11781

ABSTRACT Pongamia oil cake (POC), a bio-residualwaste is obtained during the production of biodiesel as abyproduct. This is used as biosorbent to evaluate the removalof chromium (VI) ions from an aqueous synthetic solution.The effects of various process parameters such as pH, contacttime, initial chromium ions concentration and adsorbentdosage have been investigated. The FT-IR and SEM analysisof the adsorbents was done in the native- and Cr(VI)-loadedstate, to explore the position of the functional groups avail-able for binding of chromium ions and the structural mor-phology of the studied adsorbents respectively. Langmuir,Freundlich, Temkin and Dubinin–Radushkevich isothermwere used to study the adsorption mechanism, and it wasfound that the equilibrium data was better represented bythe Freundlich isotherm. The maximal removal of hexavalentchromium ion was found to be at a pH of 2.0 within 2 h.The sorption kinetic follows the pseudo second order kineticmodel. The Cr(VI) ions bound to the biosorbent could beeffectively removed, using dilute H2SO4 (0.05 mM). The abil-ity of POC to adsorb Cr(VI) ions in packed column was alsoinvestigated through the column studies. Bed Depth ServiceTime model and the Thomas model were used to analyze theexperimental data and evaluate the model parameters. POCwas shown to be a promising adsorbent for removal of Cr(VI)ions from aqueous solutions. VC 2013 American Institute of

Chemical Engineers Environ Prog, 33: 342–352, 2014

Keywords: biosorption; pongamia oil cake; hexavalentchromium; isotherms; kinetics; column studies

INTRODUCTION

The recent rapid industrialization in India has led to therelease of huge amounts of industrial effluents into the envi-ronment and the heavy metals such as Cr, Cu, Pd, Ni, Zn,and so forth, present in these effluents are more stable andpersistent environmental contaminants; since they cannot bedegraded or destroyed. Thus they tend to accumulate in thesoil, ground water, seawater and sediments, which intensifyenvironmental pollution. Chromium have been widely usedin a variety of industrial applications such as mining, leather

tanning, cement industries, steel production units, othermetal alloys industries and so on [1]. The highest level ofCr(VI) ions permitted in a discharge into inland surface andpotable waters, are 0.1 and 0.05 mg/L, respectively [2,3].Chromium, especially that is present in electroplating efflu-ents, remains in two oxidation states Cr(III) and Cr(VI), ofwhich the hexavalent form is more toxic, when compared tothe trivalent form. An increased risk of lung cancer isreported among workers exposed to chromate productionenvironment [4]. Therefore, there is a major concern for theelimination of such heavy metal from wastewater before dis-posal to the environment.

Out of the wide ranges of conventional methods whichare available for the removal of this heavy metal from aque-ous solutions, include precipitation [5], oxidation-reduction[6], ionic exchange [7], electrochemical treatment [8], andmembrane techniques [9]. However, the major drawbacks ofall these methods are that they involve high operating costs,and may result in a large volume of solid wastes [10,11]. Bio-sorption is found to be an effective treatment techniqueowing to its cost-effectiveness and environmental friendlycharacteristics [12]. In biosorption, either metabolically activeor inactive biological material is used, to concentrate andrecover or eliminate these pollutants from effluents. Com-mercially available activated carbons are found be costly andhence they have to be regenerated many times for reuse.Reduction in activity during each stage of regeneration andthe short shelf life of activated carbon affect the economic vi-ability of the process for real time operation. From the analy-sis of recent publications it was found that differentinexpensive, locally and abundantly available biosorbentshave been employed for removal of various heavy metals,for example, sugarcane bagasse, maize corn cob, jatropha oilcake [13], mustard oil cake [14], olivestone [15], sawdust, ricehusk [16], modified coconut coir pith [17], and so forth.

However, it is still found to be inadequate to addressthese problems, more work and research are needed to iden-tify other locally available, cheap adsorbents to eliminateCr(VI) ions from industrial wastewater samples with differentcompositions and characteristics. Therefore, there is a needto identify suitable sources of materials, which are locallyavailable in plenty with low-cost or generated as a waste

VC 2013 American Institute of Chemical Engineers

Environmental Progress & Sustainable Energy (Vol.33, No.2) DOI 10.1002/ep342 July 2014

from any process to remove Cr(VI) ions from industrialwastewater. In India, particularly in southern part, manyindustrial plants have been installed for extraction of oilfrom Pongamia seeds to prepare the biodiesel. After theextraction of oil, the cake generated from this is usually dis-carded as waste material (POC). Because of the huge volumeof cake generated during oil extraction, this waste could beused as biosorbent for removal of heavy metals due to itspotential adsorption characteristics.

In this study, an attempt has been made to explore the useof pongamia oil cake as suitable adsorbent for the removal ofCr(VI) ions from an aqueous system under different experimen-tal conditions in the batch and continuous modes. The mainprocess parameters considered in the batch studies, were pH,contact time, initial Cr(VI) ions concentration, and adsorbentdose. The Bed Depth Service Time (BDST) and the Thomasmodels were analyzed, using the column breakthrough data.

MATERIALS AND METHODS

Preparation of the Adsorbent and Synthetic WastewaterPongamia oil cake (POC) was acquired from the local pon-

gamia processing industry. The cake was crushed into fineparticles using a mortar and pestle. It was dried in a hot airoven at 363 K for 24 h. The dried cake was filtered through a250 mm size sieve. The sieved material was stored in a desic-cator for further adsorption studies. Synthetic wastewater wasprepared by dissolving K2Cr2O7.H2O (AR grade) crystals indistilled water. A stock solution of 1000 mg/L was prepared,and from this the working standards were diluted accordingly.The pH of the solution was adjusted with 0.1 M HCl and 0.1M NaOH, using the pH meter (ELICO LI120, India), calibratedwith buffers of pH 4.0, 7.0, and 9.2.

Batch Adsorption StudiesBatch experiments were carried out with various pH (2-

10), metal concentration (100–500 mg/L), adsorbent doses(1–5 g/L) maintaining a temperature of 303 6 2 K, and a stir-ring speed of 150 rpm. Samples were withdrawn at specified

time intervals and centrifuged in a refrigerated centrifuge(Kubota 3700, Japan) at 5000 rpm for 15 min. The superna-tant was collected, and the amount of Cr(VI) present in thesupernatant was determined, using DPC (1,5-diphenyl carba-zide). In this method, concentration of Cr(VI) was deter-mined by developing red-violet color with DPC andmeasuring the absorbance at 540 nm using a UV–vis Spectro-photometer(Shimadzu UV-1800, Japan). The removal effi-ciency (E %) was calculated using the following equation,

Eð%Þ5 Co2Ce

Co

� �3100 (1)

where Co and Ce are initial and equilibrium chromium con-centrations in the solution (mg/L), respectively. The adsorp-tion capacity, qe (mg/g), of an adsorbent is the amount ofchromium adsorbed per unit mass of the adsorbent, and itwas calculated using the following equation,

qe5Co2Ce

M

� �3V (2)

where M is the mass of the adsorbent (g) used and V is thevolume of the solution (L).

Desorption StudiesChemicals such as sulfuric acid (H2SO4) (0.05–0.5 mM),

hydrochloric acid (HCl) (0.01–0.1 mM) and ethylenediamine-tetraacetic acid (EDTA) (0.01–0.1 mM), were used as adesorbing agents for the recovery of Cr(VI) ions from theloaded adsorbent. The Cr(VI) loaded biomass was wettedwith distilled water and added to 100 mL of desorbing solu-tion. The flasks were incubated at 303 6 2 K in an orbitalshaker at 150 rpm.

Column StudiesIt is necessary to generate the data by conducting the

fixed bed column experiments in order to understand theadsorption characteristics, and also used to design the indus-trial fixed bed column. Continuous flow sorption experi-ments were conducted in a glass column (2 cm internaldiameter and 45-cm height) packed with required POC. Aknown concentration of the metal solution was pumped intothe column at a desired flow rate using peristaltic pump(Miclins PP 20 Ex, India). The residual concentration ofCr(VI) ions at the outlet of the column was determined atregular time intervals. The volume of the effluent, Vef (mL)was calculated using the following equation

Vef5Qttotal (3)

where ttotal is the total time(min), Q is the flow rate whichcirculates through the column (mL/min). The adsorptioncapacity qe (mg/g) can be determined by the equation as inbatch studies, but with slight modifications. The equation isgiven as follows

Table 1. Isotherm constants for the removal of Cr(VI) by POC.

Adsorbent

Langmuir isotherm Freundlich isotherm D-R isothermTemkin

isotherm

Qo

(mg/g)b

(L/mg) R2KF

[(mg/g)(L/mg)1/n] n R2qd

(mg/g)ED

(kJ/mol) R2A

(L/g) R2

POC 166.6 0.00233 0.91 0.984 1.33 0.99 78.0 0.286 0.91 33.33 0.95

Table 2. Comparison of adsorption capacity of otheradsorbents for Cr(VI) onto POC.

Adsorbents

Adsorptioncapacity(mg/g) pH References

Soya cake 0.28 1.0 [32]Coconut shell carbon 10.88 4.0 [33]Beech saw dust 16.13 1.0 [34]Jatropha seed press cake 22.727 2.0 [35]Rice husk carbon 48.31 2.0 [16]Saw dust carbon 53.48 2.0 [16]Modified coconut coir pith 76.3 2.0 [17]S. platensis 188.68 2.0 [36]C. varlgaris 163.68 2.0 [36]POC 166.6 2.0 Present

work

Environmental Progress & Sustainable Energy (Vol.33, No.2) DOI 10.1002/ep July 2014 343

qe5Co2Cb

M

� �3Vef (4)

where M is the mass of adsorbent (g), Cb is the breakthroughconcentration (mg/L) and Vef is the volume of effluent that isrequired to attain the exhaustion of the column (L).

Modeling of Isotherm, Kinetic and BreakthroughCurves

The various theoretical models for both batch and columnadsorption studies (Tables 1–5) were applied to find a modeladequacy to predict the isotherm, kinetic and breakthroughdata. The validity of models was evaluated based on theregression coefficient (R2).

RESULTS AND DISCUSSION

Fourier Transforms Infrared AnalysisTo determine the surface functional groups on the ad-

sorbent during biosorption and desorption processes ofCr(VI) onto POC, Fourier transform infrared (FT-IR) analysiswas done (Spectrum one, FT-IR Spectrometer). The spectraof the dry adsorbents were measured within a range of 500–4000 cm21. An intense broad spectrum observed at 3411cm21 corresponds to OAH stretching vibrations of cellulose,pectin, absorbed water and lignin in both native and de-sorbed biomass was shown in Figure 1 [23]. The sharp peakat 2989 cm21 is attributed to the CAH stretching. Bands at

2292 and 2143 cm21 are due to the CAN stretching vibration.A peak at 1629 cm21 is due to stretching of carbonyl (C@O)vibrations and broad peak at 1239 cm21 is due to CANstretching [24].

After treating the adsorbent with Cr(VI) solutions at pH2.0, there is disappearance, shifting and increase in peakintensities at 3411, 2989, 2292, 2143, 1629, 1563, 1493, and1239, 1170 cm21. The changes seen in the peak at 2989cm21 after biosorption indicate that there is possibleinvolvement between symmetric or asymmetric CAH andthe symmetric stretching vibrations of CH2. This is con-firmed by the reappearance of peak at 2989 cm21 afterCr(VI) desorption from POC. The changes observed onpeaks 1629 and 1563 cm21which reflect stretching vibra-tions of symmetrical or asymmetric ions carboxylic groups(ACOOH) present on the surface of POC. A slight increasein intensity is observed at 1473 cm21 after biosorption, sug-gesting the coordination of CH3 groups present on the sur-face of metal ions. The changes in the band at 856 cm21

are indicative of CAH bending during biosorption process.

Scanning Electron Microscopy of the AdsorbentsScanning electron microscopy (SEM) is a primary tool

for characterizing the surface morphology, and fundamentalphysical properties of the given adsorbents (FEI QuantaFEG 200 High resolution SEM). SEM micrographs of nativePOC and the adsorbed POC with Cr(VI) are shown inFigures 2(a) and 2(b). It is clear that, the POC has a consid-erable number of pores with high homogeneity and thereis considerable decrease in the porous nature of Cr(VI)loaded biomass. This may be due to the significant possibil-ity of the chromium ions being adsorbed onto the poresand remaining trapped [25]. Also, there are shiny bulkyparticles accumulated as a layer over the surface of adsorb-ents due to the presence of chromium ions. The specificsurface area of the POC was determined by BET methodand it is found to be 0.9 m2/g and its bulk density is givenby 0.60 kg/m3.

Effect of pHpH is an important factor which influences the surface

properties of the adsorbent, as well as the ionic form ofthe chromium ions during adsorption studies. It is seen thatthe maximum chromium removal is observed at a pH of2.0 and as pH increased there is a significant decrease inthe removal of chromium (Figure 3). This is due to the factthat chromium exists as Cr2O

-7, HCr-4, Cr3O

210, Cr4O

313, and

Cr4O3-13 in the acidic pH range and HCrO-

4 is the most pre-dominant. At pH 2.0, the surface of the adsorbent seemshighly protonated, which results in strong electrostaticattractions between the protons and the chromate ions. Ata higher pH, that is, in the alkaline range, the surface ofthe adsorbent will have hydroxyl groups which will notattract the chromate ions that compete with the hydroxylions [26].

Table 3. Kinetic parameters for the removal of Cr (VI) ions onto the by POC.

Initial conc.of metal ion(mg/L)

qe exp(mg/g)

Pseudo I order Pseudo II orderIntraparticle

diffusion constants

kI

(1/min)qe cal(mg/g) R2

KII

(g/mg/min)qe cal(mg/g) R2

ki

(mg/g time1/2) C R2

100 18.63 0.044 21.54 0.892 0.0012 22.22 0.943 1.596 0.078 0.891200 32.22 0.05 25.53 0.818 0.0022 34.48 0.994 2.351 6.491 0.862300 47.89 0.043 24.04 0.882 0.0027 50.00 0.998 3.263 14.30 0.748400 60.35 0.048 33.75 0..873 0.0022 62.50 0.998 4.042 18.49 0.751500 70.11 0.053 23.31 0.924 0.0046 71.42 0.999 4.316 28.43 0.609

Table 4. Desorption of Cr (VI) with time using different eluent.

EluantConcentrationof eluant (mM)

Desorption ofCr(VI) (%) at

20 min 40 min 60 min

H2SO4 0.00 0.04 0.25 0.250.05 22.22 24.19 26.220.1 27.88 30.2 30.90.2 33.21 34.99 36.330.3 39.71 41.77 42.810.4 46.33 48.17 59.260.5 51.36 60.28 84.65

HCl 0.00 0.05 0.27 0.270.01 5.21 6.44 7.010.05 10.25 11.88 11.880.075 30.25 32.58 35.580.1 44.21 56.63 69.46

EDTA 0.00 0.05 0.27 0.270.01 7.45 8.21 8.210.05 19.54 23.65 23.650.075 30.55 31.45 31.450.1 45.27 55.67 61.22


Effect of the Adsorbent DoseThe adsorbent dosage determines the capacity of an ad-

sorbent for an adsorbate at a given initial concentration [27].From Figure 4, it can be concluded that the removal effi-ciency increased from 48 to 81% as the adsorbent doseincreased but the amount of Cr(VI) ions adsorbed per unitmass decreases by increasing the biosorbent concentration.This is mainly due to an increase in the number of availableadsorption sites with an increase in biosorbent concentrationand resulting in the increase of adsorbed metal concentra-tion. On the contrary, the adsorption qe (mg/g) capacitydecreased from 129.3 to 69.23 as the adsorbent doseincreased from 1.0 to 5.0 g/L, mainly due to the overlappingof adsorption sites caused by the overcrowding of theadsorbent particles or unsaturation of adsorption sitesthrough the adsorption reaction [28]. Another reason may beparticle interactions, such as aggregation, resulting from highbiosorbent concentration and such aggregation would leadto decrease in total surface area of the sorbent and anincrease in diffusional path length [29]. Further increment inbiosorbent concentration from 3.0 g/L did not cause any sig-nificant improvement in adsorption. This is due to the bind-ing of almost all Cr(VI) ions to the adsorbent and theestablishment of equilibrium between the Cr(VI) ions boundto the sorbent and those remaining unadsorbed in thesolution.

Effects of Initial Metal Ion Concentration andContact Time

The adsorption of chromium onto the POC for the differ-ent initial concentrations (100–500 mg/L) as a function of thecontact time was studied, by keeping the other relatedparameters constant. The removal of chromium increasedwith an increase in the time and reached the maximal levelat 60 min with no further increase, suggesting that equilib-rium has been attained. It was seen that as the metal ionconcentration increased, the removal efficiency decreased.This is due to the fact that when the initial metal ion concen-tration is low, the ratio of the available surface area foradsorption to the concentration of chromium ions is higher;whereas in higher initial metal ion concentrations this ratio isless and so the efficiency is also lesser. However, the adsorp-tion capacity increased as the metal ion concentrationincreases as shown in Figure 5. An increase in the initialmetal ion concentration acts as a prime force in overcomingall the resistance to adsorption [30]. Moreover, it increasesthe number of collisions between the Cr(VI) ions.

Biosorption Isotherm StudiesThe adsorption isotherms are important to analyze the

equilibrium data by developing a mathematical equation, todesign and optimize an operating procedure on a large scale.It also provides the relationship between the amount of themetal ions adsorbed onto the surface, and the concentrationof the metal ions in the aqueous solution at equilibrium con-dition. The Langmuir isotherm assumes that the adsorbateforms a monolayer around the homogenous surface of theadsorbent, and there is no interaction between the adsorbedmolecules [31]. When the adsorbent surface is completelysaturated with the adsorbate, equilibrium is said to beachieved. The linear form of the Langmuir model is given by

Ce

qe5

1

Qmb1

Ce

Qo(5)

where qe is the amount of the solute adsorbed on the ad-sorbent surface at equilibrium (mg/g), Ce is the concentrationof the un adsorbed solute in solution (mg/L), Qm is the maxi-mum amount of the solute adsorbed per unit mass of theadsorbent to form a complete monolayer (mg/g) and b is aconstant related to the affinity of the binding sites (L/mg). Aplot between Ce

qeversus Ce as shown in Figure 6(a) is used to

calculate the constants Qm and b.The Freundlich model is an empirical one, which assumes

that adsorption takes place on a heterogeneous surface andalso proposes multilayer sorption with interaction among the

Table 5. Effect of Bed height for Cr(VI) adsorption by POC using BDST and Thomas Model.

The BDST model parameters

Bed height (cm) Sorbent weight (g) Breakthrough time (min) Qo (mg/g)Exhaustiontime (min)

5 6.5 105 43.53 54010 13 195 58.64 96015 19.5 300 71.61 1440

The Thomas model parameters

Flow rate (ml/min) Bed height (cm) KTh (L/mg h) Qo (mg/g) R2

10 5 1 3 1024 43.53 0.85715 5 2 3 1024 40.53 0.87520 5 2.4 3 1024 37.73 0.977

Figure 1. (A) FT-IR of native POC, (B) FT-IR of Cr(VI)loaded POC (C) FT-IR of Cr(VI) desorbed POC.


adsorbed molecules [32]. The logarithmic form of aboveequation can be written as

ln qe5ln kf11

nln Ce (6)

where KF is the Freundlich constant representing the bond-ing energy((mg/g)(L/mg)(1/n)) and n is a constant which isan indicator of the adsorption favorability. Values of n > 1favors adsorption. The values of KF and n are determinedfrom the plot Figure 6. (b) between lnqe versus lnCe.

The effects of some indirect adsorbate/adsorbent interac-tion on adsorption isotherms were studied by using isothermproposed by Temkin and Pyzhez [33] and it was found thatthe heat of adsorption of all the molecules in the layerdecreases linearly with coverage [34]. The linearized Temkinisotherm is represented by the equation

qe5RT

bln A1

RT

bln Ce (7)

where R is the gas constant (kJ/mol), T is the absolute tem-perature (K), b is a constant related to the heat of adsorp-tion, and A is the equilibrium binding constant (L/g)

Figure 2. (a) SEM images of native POC (a) and (b) SEM images of Cr(VI) loaded PO.

Figure 3. Effect of pH on adsorption of Cr (VI) by POC[Chromium concentration 5 100 mg/L; stirring speed 5 150rpm; contact time 5 120 min; adsorbent dose 5 3 g/L, tem-perature 5 30 6 2�C].

Figure 4. Effect of adsorbent dose on Cr (VI) removal effi-ciency (%) and biosorption capacity (qe) by POC [Chromiumconcentration 5 500 mg/L; stirring speed 5 150 rpm; contacttime 5 120 min; pH 5 2.0; temperature 5 30 6 2�C].

Figure 5. Effect of initial metal ion concentration and con-tact time on Cr (VI) removal by POC [Adsorbent dose 5 3 g/L, stirring speed 5 150 rpm, contact time 5 120 min, pH 52.0; temperature 5 30 6 2�C].


corresponding to the maximum binding energy. The valuesof the constants can be determined from the plot between qe

and lnCe in Figure 6c.Another important equation for the analysis of the iso-

therm of a high degree of rectangularity is the Dubinin–Radush Kevich isotherm [35]. The D–R isotherm model doesnot assume a homogenous surface, and is expressed as

ln qe5ln qD-2BDR2T 2ln 111CeÞ2

��(8)

where qD is the theoretical saturation capacity (mg/g) andBD is a constant related to the adsorption energy (mol2/kJ2),R is the gas constant (J/mol/K), and T is the temperature (K).The values of Bd and qD can be calculated from the slopeand intercept correspondingly from the plot between lnqe

and ln(1 1 1/Ce)2 as shown in Figure 6d. The constant BD is

a representative of the mean free energy ED (kJ/mol) of theadsorption per molecule of the adsorbate. It is calculatedfrom the following equation

ED51ffiffiffiffiffiffiffiffiffiffiffiffið2BDÞ

p (9)

According to the data from Table 1 it seems, thatthe Freundlich model fits better with the experimental data

than the Langmuir model. The regression coefficient ofFreundlich model is found to be near unity (R2 value of0.995). It can be seen from Table 1 that the value of ED is0.286 kJ/mol and it is known that when ED value is lesserthan 8 kJ/mol then the adsorption process follows physicaladsorption [36]. In the present study, POC has been com-pared with other adsorbents based on their maximumadsorption capacity (qm) for Cr(VI) ions (Table 2). Accord-ingly, this data indicates that POC could be considered aspromising adsorbent in comparison to other availableadsorbents.

Biosorption Kinetics StudiesFor constructing a waste water treatment plant, it is impor-

tant to predict the solute uptake rate, which determines theresidence time of the sorbate at the solid–liquid interface. Inorder to investigate the mechanism of adsorption, the charac-teristic constants of the adsorption rate are determined using apseudo first-order equation of Lagergren, based on solidcapacity, and pseudo second-order equation based on solidphase adsorption, and an intraparticle diffusion model [37,38].The mathematical values of these models are given in Table 3.

The pseudo first order and pseudo second order kineticmodels are applied to study the sorption kinetics of chro-mium on the POC and the extent of the uptake in the bio-sorption process.

Figure 6. Linear isotherms for Cr (VI) removal by POC at 30 6 2�C. (a) Langmuir isotherm, (b) Freundlich Isotherm, (c) Tem-kim Isotherm, and (d) D–R isotherm.


The pseudo first order kinetic model was proposed byLagergren [39] and is expressed as

dqt

dt5kIðqe-qtÞ (10)

The integrating Eq. (10) between the boundary conditions t5 0 to t 5 t, gives

lnðqe-qtÞ5ln qe-kIt (11)

where kI is the rate constant (min21), qe is the amount ofchromium adsorbed on the surface at equilibrium(mg/g),and qt is the amount of chromium adsorbed on the surfaceat any time t (mg/g). A plot is drawn between ln (qe 2 qt)versus time, and the values of k1 and qe are determined fromthe slope and intercept, respectively.

The pseudo second order kinetic model [40] is expressedby the equation

t

qt5

1

q2ekII

11

qet (12)

where kII is the pseudo second order rate constant (g/mg/min), which can be determined from the plot between t/qt

versus t. It was seen that the qe values calculated from thepseudo first order kinetic model do not correlate well withthe experimental qe values (Table 3). It was also noted thatthe coefficients in this model are low, while comparing thevalues derived from the pseudo second order kinetic model.The calculated qe values from the pseudo second order ki-netic model agree well with the experimental qe values. It isobvious that the graph is more linear in the pseudo secondorder kinetic model, and based on the above observation itcan be concluded that the pseudo second order kineticmodel fits well in this sorption study [Figures 7(a) and 7(b)].Thus the adsorption of chromium ions onto the POC is pre-sumably physisorption and the surface functional groups ofthe adsorbent. Similar results have been reported in the bio-sorption of Cr (VI) onto the activated carbon derived fromagricultural waste material [41].

Weber and Morris [42] proposed the intra particle diffu-sion model, which describes the diffusion mechanism. Themodel is expressed as

qt5kit1=21C (13)

where ki is the intraparticle diffusion rate constant ((mg/g)(time)1/2), and C is a constant which gives an idea aboutthe thickness of the boundary layer, which can be calculatedfrom the plot qt versus t1/2. If this plot yields a linear graphwhich passes through the origin, then the sorption process iscontrolled only by intraparticle diffusion. It can be seen fromFigure 8 that the graph is not completely linear, and alsodoes not pass through the origin, suggesting that there areother kinetic factors involvedapart from Intraparticle diffu-sion, that control the adsorption rate.

Desorption StudiesDesorption studies were carried out using three desorbing

agents (Table 4). Among the three agents tried, for the de-sorption of Cr(VI) from a laden biosorbent, 0.5 mM H2SO4

showed the maximum elution (84.65%) of Cr(VI) in 60 min,

Figure 7. (a) Pseudo first order and (b) second order kinetic plot (b) for Cr (VI) ions removal onto the POC at 30 6 2�C.

Figure 8. Intraparticle diffusion plot for Cr(VI) ions removalonto the POC at 30 6 2�C.


which clearly indicates that desorption is a physic-chemicalsequester on the surface [43].

COLUMN STUDIES

Effect of The Bed Height on The Adsorption of Cr(VI)by the POC

The effect of the bed height was studied by varying col-umn height and by maintaining all other parameters, such asthe flow rate, pH and initial metal ion concentration and rateconstant (Figure 9). The effect of bed height is explained,using the Bed Depth Service Time (BDST) model [44]. TheBDST model is expressed as

t5Noz

Com-

1

KaColn

Co

Cb-1

� �(14)

where t is the service time that can be taken as the time(min), where the outlet concentration of the column is 5 mg/

L. Cb is the breakthrough time taken for the outlet concentra-tion in the column to reach 5 mg/L. The time required toachieve the breakthrough concentration is called as thebreakthrough time. The time required to achieve the initialmetal concentration, which is fed into the column at the out-let of the column, is called the exhaustion time [45,46]. Itmeans that the adsorbent in the column cannot absorb anyheavy metal beyond the exhaustion time. No is the adsorp-tion capacity of the column (mg/L), Co is the initial metal ionconcentration which (here) is 100 mg/L, v is the velocity incm/min, Z is the bed height in cm and ka is a rate constant(L/mg/min). It is observed from Figure 10, as the bed heightincreases both the breakthrough and exhaustion timeincrease and also it is seen that the R2 value for the BDSTplot is high, which suggests that BDST model fits well andthus the column studies are feasible. Only when the bedheight is the maximum, the adsorption capacity is more, so itis desired to perform column studies with the maximum bedheight. This value is given in Table 5 and it should also betaken into account, that when the bed height increases theprocess becomes time consuming.

The parameters of the BDST model No and the rate con-stant Ka, were calculated from the BDST plot shown in Fig-ure 10 and were found to be 3923.4 mg/L and 5.88 3 1023

L/mg/min, respectively. The rate constant Ka, which is ameasure of the rate transfer of the solute from the fluidphase to the solid phase, largely influences the breakthroughphenomenon in the column study. For a smaller value of Ka,a relatively longer bed is required to avoid a breakthrough,whereas the breakthrough can be eliminated even in smallerbed heights when the value of Ka is high [43,46].

Effect of the Flow Rate on the Adsorption ofCr (VI) by POC

The flow rate is one of the important characteristics inevaluating the sorbents for continuous treatment of themetal-laden effluents on an industrial scale. Adsorption wasstudied for various flow rates in the range of 10–20 mL/minfor the initial metal ion concentration of 100 mg/L and a bedheight of 5 cm. From Figure 11 it can be observed that asthe flow rate increases both the exhaustion time and break-through time decrease. The uptake values were found to be

Figure 9. Effect of bed height for Cr(VI) adsorption onto thePOC (flowrate 5 10 mL/min, Co 5 100 mg/L, Bed height 55 cm, pH 5 2.0 and temperature 5 30 6 2�C).

Figure 10. BDST model for Cr(VI) adsorption by POC.

Figure 11. The experimental and modeled breakthroughcurves from Thomas Model for the adsorption of Cr(VI) ontothe POC at different flow rates.(Co 5 100 mg/L, Bed height5 5 cm, pH 5 2.0 and temperature 5 30 6 2�C).


43.53, 40.43, and 37.73 mg/g for the flow rates 10, 15, and20 mL/min respectively, which shows the influence of theflow rate on the sorption capacity. The reason for thedecreased sorption capacity at a higher flow rate may be dueto the unavailability of sufficient retention time, and the lim-ited diffusivity of the solute into the sorptive sites or pores ofthe biomass.

The Thomas model is a simple and generally used modelfor the prediction of the concentration versus time profile, orthe breakthrough curve for the effluent with basic assump-tion. The adsorption is described by a pseudo second orderreaction rate principle which reduces a Langmuir isotherm atequilibrium; constant column void fraction; constant physicalproperties of the biomass (solid-phase) and the fluid phase;isothermal and isobaric process conditions; the intraparticlediffusion and external resistance during the mass transferprocesses are considered to be negligible.

On the basis of the above assumption, the Thomas model[47] can be expressed in its linear form as

lnCo

C-1

� �5

KTHQoM

F-KTHCo

F(15)

where kTh is the Thomas model constant (L/mg h), Qo is themaximum solid phase concentration of the solute (mg/g)and V is the throughput volume (L). A plot of ln Co

C -1� �

against t (where t 5 V/F) for a given flow rate is used todetermine the model constants. The Thomas model constantsare given in Table 5 and it seen that a high correlation existsbetween the experimental data and the predicted value; boththe experimental and the Thomas model predicted break-through curves are shown in Figure 11. From Table 5, it canbe inferred that the bed capacity Qo decreases with anincreasing flow rate, while the Thomas constants KTH

increased with an increasing flow rate of 10–20 ml/min.Similar trends have been observed by other researchers forvarious adsorbents [48,49].

Desorption and Column Regeneration StudiesFrom the economic point of view, the reusability of the ad-

sorbent material is very important in the field of adsorptiontechnology. In our batch studies, the results showed thatH2SO4 (0.5 mM) has good elution capacity, compared withother eluant (Table 4). Therefore, the 0.5 mM H2SO4 solutionwas used as the eluting agent at a constant flow rate of 10mL/min and preloaded with Cr(VI)biomass, of a bed height of10 cm. After the recovery of the metal ion, the regeneratedPOC was thoroughly washed with distilled water and againloaded with the Cr(VI) solution of a concentration of 100 mg/L. Five adsorption–desorption studies were carried out repeat-edly by maintaining the same operating conditions. From

Table 6, it can be interpreted that the efficiency of the Cr(VI)adsorption on the regenerated POC in the fourth and fifthcycle, was beginning to drop. This is due to the gradual dete-rioration of the POC, because of continuous usage. As theadsorption and desorption cycle proceeds, the accessibility ofthe metal ions toward the binding sites get reduced [43].

CONCLUSION

The amount of metal adsorbed was found to vary withthe pH, contact time, initial chromium ions concentrationand dosage concentration. From the FT-IR analysis it wasfound, that the hydroxyl and CAH groups present in the sur-face of the adsorbent contribute to chromium adsorption.The adsorption equilibrium data were found to fit theFreundlich isotherm, indicating that the surface is heteroge-neous. The pseudo-second order model can be used to pre-dict the adsorption kinetics. The breakthrough curves wereanalyzed at different flow rates and bed heights. The BDSTand the Thomas Model were used to evaluate the experi-mental data and both the models were valid for accurateevaluation. The Cr(VI) loaded POC was eluted with a 0.5mM H2SO4 solution through column studies, and reused forfive adsorption–desorption studies. The present study sug-gests that pongamia oil cake, an abundant agricultural wastefrom biodiesel production, can be used as an adsorbent forthe removal of chromium (VI) from aqueous solutions.

ACKNOWLEDGMENTS

The authors (M.Shanmugaprakash, M.Manimaran, J. Aravind)are thankful to the Management, Kumaraguru College of Tech-nology, Coimbatore, for providing the facilities to carry out thisresearch.

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batch and dynamics modeling of the biosorption of cr(vi) from aqueous solutions by solid biomass...

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