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Colloids and Surfaces B: Biointerfaces 88 (2011) 697– 705

Contents lists available at ScienceDirect

Colloids and Surfaces B: Biointerfaces

jou rn al h om epage: www.elsev ier .com/ locate /co lsur fb

iosorption kinetics, thermodynamics and isosteric heat of sorption of Cu(II) ontoamarindus indica seed powder

hamik Chowdhury ∗, Papita Das Sahaepartment of Biotechnology, National Institute of Technology-Durgapur, Mahatma Gandhi Avenue, Durgapur 713209, West Bengal, India

r t i c l e i n f o

rticle history:eceived 6 April 2011eceived in revised form 3 August 2011ccepted 3 August 2011vailable online 10 August 2011

eywords:u(II)

a b s t r a c t

Biosorption of Cu(II) by Tamarindus indica seed powder (TSP) was investigated as a function of temper-ature in a batch system. The Cu(II) biosorption potential of TSP increased with increasing temperature.The rate of the biosorption process followed pseudo second-order kinetics while the sorption equilib-rium data well fitted to the Langmuir and Freundlich isotherm models. The maximum monolayer Cu(II)biosorption capacity increased from 82.97 mg g−1 at 303 K to 133.24 mg g−1 at 333 K. Thermodynamicstudy showed spontaneous and endothermic nature of the sorption process. Isosteric heat of sorption,determined using the Clausius–Clapeyron equation increased with increase in surface loading show-

amarindus indica seed powdersothermsineticshermodynamicssosteric heat of sorption

ing its strong dependence on surface coverage. The biosorbent was characterized by scanning electronmicroscopy (SEM), surface area and porosity analyzer, X-ray diffraction (XRD) spectrum and Fouriertransform infrared (FTIR) spectroscopy. The results of FTIR analysis of unloaded and Cu(II)-loaded TSPrevealed that NH2, OH, C O and C O functional groups on the biosorbent surface were involvedin the biosorption process. The present study suggests that TSP can be used as a potential, alternative,

emov

low-cost biosorbent for r

. Introduction

During recent years, the surge of industrial activities has ledo tremendous increase in the use of heavy metals and inevitablyesulted in an increased flux of these metal ions in the aquatic envi-onment. Cu, Cr, Cd, Pb, Hg, Zn, and Ni are the most common heavyetals discharged into water streams from large industrial sectors

1–4]. These metallic substances are extremely stable and persis-ent environmental contaminants since they cannot be degradedr destroyed, and therefore tend to accumulate in the soils, seawa-er, freshwater and sediments. The persistence of heavy metals inhe aquatic environment is a serious environmental problem dueo their extreme toxicity towards aquatic life, human beings andhe environment.

Copper is one of the most widely used heavy metal. Copper andts compounds are extensively used in various important indus-rial applications such as electrical wiring, plumbing, gear wheel, aironditioning tubing, and roofing [5]. Its potential sources in indus-rial effluents include metal cleaning and plating baths, fertilizer,efineries, pulp, paper board mills, printed circuit board production,

ood pulp production, wood preservatives, paints and pigments,unicipal and storm water run-off, etc. [4–7]. Intake of excessively

arge doses of copper by human beings leads to severe mucosal

∗ Corresponding author. Tel.: +91 9831387640; fax: +91 3432547375.E-mail address: chowdhuryshamik@gmail.com (S. Chowdhury).

927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfb.2011.08.003

al of Cu(II) ions from aqueous media.© 2011 Elsevier B.V. All rights reserved.

irritation and corrosion, stomach upset and ulcer, wide spread cap-illary damage, hepatic and renal damage, central nervous systemirritation followed by depression, gastrointestinal irritation, andpossible necrotic changes in the liver and kidney [8]. Chronic cop-per poisoning can also result in Wilson’s disease leading to brainand liver damage [9]. Therefore, removal of copper from effluentsis essential not only to protect the water sources but also for theprotection of human health.

Various treatment technologies exist for removal of Cu(II) fromwastewater, including precipitation, ion-exchange, evaporation,oxidation, electroplating and membrane filtration [7,8,10]. How-ever, application of such technologies is restricted because oftechnical or economical constraints [10–13]. Biosorption as analternative and effective technology has been widely studied overrecent years, because of its wide range of target pollutants, highsorption capacity, excellent performance, ecofriendly nature andlow operating cost [14,15]. Recent studies on biosorption haveshown that common agricultural wastes can be used as potentialbiosorbents for the removal of heavy metals. Agricultural wastematerials being economic and ecofriendly due to their uniquechemical composition, availability in abundance, renewable bynature, low cost and high efficiency, seem to be viable option asbiosorbents for heavy metal removal [15].

Tamarindus indica (Tamarind) is a common tree of SoutheastAsia and widely indigenous to India, Bangladesh, Myanmar, SriLanka, Malaysia and Thailand [16]. It is grown mainly for its sourfruit pulp [17,18]. The seed, a byproduct of the tamarind pulp

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98 S. Chowdhury, P.D. Saha / Colloids and

ndustry, is a typical under-utilized or waste material [19]. Inecent years, a few reports have suggested the application of T.ndica seed as a biosorbent for the removal of heavy metal ionsuch as Cr(VI) from aqueous solution [19,20]. In this work, T. indicaeed powder (TSP) was used as biosorbent for removing Cu(II)ons from aqueous solutions. The aim was to evaluate the Cu(II)inding capacity of TSP as a function of temperature using batchxperiments. The biosorbent was characterized by scanning elec-ron microscopy (SEM), surface area and porosity analyzer, X-rayiffraction (XRD) spectrum and Fourier transform infrared (FTIR)pectroscopy. Equilibrium, kinetic and thermodynamic parametersere determined to understand the biosorption mechanism.

. Materials and methods

.1. Preparation of Cu(II) solutions

Stock solution of Cu(II) (500 mg L−1) was prepared by dissolv-ng required quantity of CuSO4·5H2O (analytical reagent grade)n double-distilled water. Experimental Cu(II) solutions of differ-nt concentrations were prepared by diluting the stock solutionith suitable volume of double-distilled water. The initial pH was

djusted with 0.1 (M) HCl and 0.1 (M) NaOH solutions using a digitalH meter (ELICO) calibrated with standard buffer solutions.

.2. Preparation of biosorbent

Mature seeds of T. indica were collected from the local markets ofurgapur, West Bengal, India. The seeds were washed with distilledater to remove any adhering dust and pulp and dried at 343 ± 1 K

or 24 h in an oven drier. The raw seeds were then crushed androunded to fine powder using ball mill and sieved to a constantize (100–125 �m). It was then stored in sterile, closed glass bottlesnd used as biosorbent without any further pretreatment.

.3. Batch biosorption experiments

Biosorption of Cu(II) ions by TSP was studied by batch method.xperiments were carried out in 250 mL glass-stopperred Erlen-eyer flasks by adding 0.5 g of the biosorbent to 100 mL solution

ontaining 200 mg L−1 Cu(II) at pH 5.5. The flasks were agitated at aonstant speed of 150 rpm in an incubator shaker (Model Innova 42,ew Brunswick Scientific, Canada) at temperatures of 303, 313, 323nd 333 K, respectively until reaching equilibrium. All the experi-ents were performed at pH 5.50. Samples were collected from

he flasks at predetermined time intervals for analyzing the resid-al concentration of Cu(II) in the solution. The residual amount ofu(II) in each flask was investigated colorimetrically using UV/vispectrophotometer (Model Hitachi–2800). 1% w/v sodium diethylithiocarbamate solution (0.2 mL) and 1.5 N ammonia solution20 mL) was added to the test sample (1 mL). The absorbance ofhe resulting yellow colored solution was determined at �max of60 nm. The amount of Cu(II) biosorption at equilibrium qe (mg g−1)as calculated according to the following mass balance equation

or the metal ion concentration:

e = (Ci − Ce) V

m(1)

here Ci is the initial Cu(II) concentration (mg L−1), Ce is the equi-ibrium Cu(II) concentration in solution (mg L−1), V is the volumef the solution (L), and m is the mass of the biosorbent used (g).

The percent sorption (%) of Cu(II) was calculated using Eq. (2):

orption (%) = Ci − Ce

Ci× 100 (2)

es B: Biointerfaces 88 (2011) 697– 705

Biosorption data were subject to equilibrium modelling to havea better understanding of mechanism of biosorption of Cu(II)using two parameter isotherms such as Langmuir, Freundlich andTemkin. Kinetic rate constants were determined using pseudo-second-order and intraparticle diffusion kinetic models.

2.4. Statistical analysis

In order to ensure the accuracy, reliability, and reproducibilityof the collected data, all biosorption experiments were performedin triplicate, and the mean values were used in data analysis. Rel-ative standard deviations were found to be within ±3%. MicrosoftExcel 2007 program was employed for data processing. Non-linearregression analysis using Origin Pro 8.0 software was employed todetermine the isotherm parameters and kinetic constants.

To evaluate the applicability of isotherm equations and kineticmodels, the non-linear chi-square test was used. The Chi-squarestatistic test (�2) can be defined as:

�2 =∑ (qe,meas − qe,cal)

2

qe,cal(3)

where qe,meas and qe,cal (mg g−1) are the experimental and calcu-lated values of the equilibrium adsorbate concentration in the solidphase. The �2 will be a small number if the experimental data anddata from the model are similar and vice versa.

2.5. Characterization of biosorbent

2.5.1. Fourier transform infrared (FTIR) spectroscopy analysisA qualitative and preliminary analysis of the main functional

groups on the biosorbent surface that might be involved in metalbinding can be identified by FTIR as each group has a uniqueenergy absorption band. Therefore, FTIR spectrum of TSP before andafter biosorption of Cu(II) were recorded with a FTIR spectrometer(Perkin–Elmer Spectrum BX-II Model). 0.001 g of dry sample wasmixed with 0.5 g of spectroscopic grade potassium bromide powderin an agate pestle and mortar. The powder was then compressedinto a thin KBr translucent disk under a pressure of 100 kg cm−2 for8 min with the aid of a bench press. FTIR spectra were then recordedin the wavenumber range 4000–500 cm−1 at 4 cm−1 spectral reso-lution.

2.5.2. Scanning electron microscopy (SEM) analysisThe surface structure of the biosorbent before and after biosorp-

tion was analyzed by a scanning electron microscope (ModelHitachi S-3000N) at an electron acceleration voltage of 20 kV. Priorto scanning, the unloaded and metal-loaded TSP samples weremounted on a stainless steel stab with double stick tape and coatedwith a thin layer of gold in a high vacuum condition.

2.5.3. Textural characterizationTextural characterization of native and metal bound TSP sam-

ples was carried out by Quantachrome NOVA 2200C USA surfacearea and porosity analyzer. A gas mixture of 22.9 mol% nitrogenand 77.1 mol% helium was used for this purpose. The Brunauer,Emmett, Teller (BET) surface area, pore volume and pore size werethen determined.

2.5.4. X-ray diffraction (XRD) analysisAn XRD spectrum was also recorded using Miniflex X-ray

diffraction spectrometer. The dry samples of unloaded and Cu(II)-

loaded TSP were pressed into pellets. The XRD spectra wererecorded with a CuK� radiation source (used at 30 kV and 15 mA,diffraction angle ranged from 60◦ to 10◦ with a scan speed of5◦/min).

S. Chowdhury, P.D. Saha / Colloids and Surfaces B: Biointerfaces 88 (2011) 697– 705 699

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Fig. 1. FTIR spectra of TSP (a) before biosorption (b) after biosorption of Cu (II).

. Results and discussion

.1. Characterization of biosorbent

.1.1. FTIR analysisT. indica seed mainly contains polysaccharide with fats, tan-

ins, proteins and amino acids in minimum proportion. The FTIRpectral analysis is however important to identify the differentunctional groups of the biosorbent surface which are responsi-le for biosorption of metal ions. The FTIR spectra of TSP beforeiosorption and after biosorption of Cu(II) is shown in Fig. 1. TheTIR spectra of TSP (Fig. 1(a)) shows the presence of several func-ional groups, indicating the complex nature of TSP. The broadnd strong band at 3450 cm−1 suggests the presence of OH andNH2 groups [21]. The peak at 2904 cm−1 can be attributed toCH stretching vibrations while the peak appearing at 1652 cm−1

rises from C O stretching in amide groups [21,22]. The peakt 1465 cm−1 represents CH3 [23]. The peak around 1160 cm−1

ndicates C N stretching vibration [24]. The peak at 1060 cm−1

orresponds to C O stretching vibration of alcohols and carboxyliccids [25]. After Cu(II) biosorption (Fig. 1(b)), the band at 3450 cm−1

orresponding to OH and NH groups shifts to the lower fre-uency (3390 cm−1). Thus, it can reasonably be concluded that OHnd NH2 groups may be the main binding sites for Cu(II) onto TSP.hese groups may be involved in the biosorption process as follows26,27]:

TSP OH + Cu2+ → TSP OCu+ + H+

TSP NH2 + Cu2+ → TSP NH2Cu2+

here TSP denotes the biosorbent surface.Also, the peaks at 1652 cm−1 and 1060 cm−1 shifts to 1615 cm−1

nd 1030 cm−1 respectively. This shift in peaks suggests that C Ond C O groups participate in the Cu(II) binding process. Hence,TIR spectral analysis reveal that functional groups like NH2,OH, C O and C O present on TSP surface are involved in Cu(II)iosorption.

.1.2. SEM analysis

SEM is one of the most useful tools for studying the surface

orphology of a biosorbent. The SEM images of native TSP andu(II)-loaded TSP are shown in Fig. 2. SEM image of TSP beforeetal biosorption (Fig. 2(a)) shows a rough, uneven and hetero-

Fig. 2. SEM images of (a) native TSP (b) Cu(II)-loaded TSP.

geneous surface with porous structure. The rough surface can helpincrease the surface area available for biosorption of Cu(II). How-ever, as shown in Fig. 2(b), the porous textural structure is notobserved on the surface of Cu(II)-loaded TSP. The surface morpho-logical change can be linked to precipitation/complexation of Cu(II)on the biosorbent surface.

3.1.3. Textural characterizationThe BET surface area, total pore volume and average pore diam-

eter of the biosorbent before metal biosorption was found to be29.46 m2 g−1, 0.0189 cm3 g−1 and 67 A, respectively. After metalbiosorption, the BET surface area, total pore volume and averagepore diameter of the biosorbent was found to be 46.72 m2 g−1,0.0097 cm3 g−1 and 17 A, respectively. The increase in the surfacearea of the biosorbent indicates that Cu(II) ions were adsorbed onthe surface of the biosorbent. The decrease in the total pore volumeas well as the average pore diameter of the biosorbent suggests thatCu(II) ions interact with the functional groups present inside thepores.

3.1.4. XRD analysisThe XRD pattern of TSP and Cu(II) loaded TSP are shown in

Fig. 3(a) and (b). The XRD pattern of TSP (Fig. 3(a)) does not showany characteristic peaks indicating amorphous nature of the biosor-bent. The amorphous nature suggests that metal ions could easily

penetrate into the surface of the biosorbent as was observed inthe case of gum kondagogu [28]. The XRD pattern of metal boundTSP (Fig. 3(b)) shows distinct and complex peaks, indicating thedeposition of Cu(II) on the surface of TSP.

700 S. Chowdhury, P.D. Saha / Colloids and Surfac

3

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Fig. 3. XRD spectrum of (a) native TSP (b) Cu(II)-loaded TSP.

.2. Effect of temperature

Temperature is a highly significant parameter governing theiosorption processes. The temperature has two main effects onhe sorption processes. Firstly, increasing temperature is knowno increase the diffusion rate of the sorbate molecules within the

ores as a result of decreasing solution viscosity. Secondly, it willlso modify the equilibrium capacity of the biosorbent for a par-icular sorbate [29]. Fig. 4 shows the effect of temperature on theiosorption of Cu(II) onto TSP. As seen from Fig. 4, the equilibrium

ig. 4. Effect of temperature on the biosorption of Cu(II) by TSP (experimental con-itions: initial Cu(II) concentration = 200 mg L−1, biosorbent dose = 1 g L−1, agitationpeed = 150 rpm, contact time = 180 min, pH 5.5, error bars represent the standardeviation at n = 3).

es B: Biointerfaces 88 (2011) 697– 705

biosorption capacity increased with an increase in temperature.The uptake equilibrium of Cu(II) was achieved after 60 min and noremarkable changes were observed for higher reaction times (notshown in Fig. 4). The removal efficiency increased from 88.24 to98.34% with rise in temperature from 303 to 333 K at the samemetal ion concentration indicating that Cu(II) removal by biosorp-tion onto TSP was favourable at higher temperatures. The increasein the Cu(II) uptake capacity with increasing temperature can beexplained by the availability of more binding sites [4]. An increasein temperature also results in an increased mobility of the metalions and a decrease in the retarding forces acting on the ions. Theseresult in the enhancement in the sorptive capacity of the biosor-bent. The increase in biosorption efficiency can also be attributedto an increase in kinetic energy of the biosorbent. The finding isin agreement with the observations of Kılıc et al. on biosorptionof Cu(II) on NaOH-pretreated Marrubium globosum ssp. globosumleaves powder [30].

3.3. Biosorption kinetics

The knowledge of the kinetics of any biosorption process isimportant since the kinetics describes the uptake rate of sorbatewhich in turn controls the residence time of the sorbate at thesolid–solution interface including the diffusion process [31]. There-fore, the results obtained from the experiments were used to studythe kinetics of Cu(II) biosorption. The kinetics of Cu(II) biosorp-tion onto TSP was analyzed using the pseudo-second-order andintra-particle diffusion models.

The pseudo-second order kinetic model is frequently used toanalyze biosorption data and is expressed as [32]:

qt = k2q2e t

1 + k2qet(4)

where qt and qe are the amount of Cu(II) sorbed at time t and at equi-librium (mg g−1) and k2 (g mg−1 min−1) is the pseudo-second-orderrate constant for the biosorption process. The initial biosorptionrate, h (mg g−1 min−1) as t → 0 can be calculated as [33]:

h = k2qe2 (5)

The parameters obtained for the pseudo-second-order model atdifferent temperatures are listed in Table 1. Low �2 values indicatethat biosorption of Cu(II) onto TSP follows the pseudo-second-ordermodel. The theoretical qe,cal values are closer to the experimentalqe,exp values reinforcing the applicability of this model. The initialbiosorption rate, h increases with the rise in temperature indicatinghigher temperature favours the biosorption process by increasingbiosorption rate and capacity. A similar trend was reported for thesorption of Cu(II) by tamarind fruit shell-cation exchanger [4]. Theapplicability of pseudo-second-order kinetic model suggests thatCu(II) biosorption by TSP was based on chemical reaction, involvingexchange of electrons between the biosorbent and metal. A com-parison of the pseudo-second-order model with the experimentalkinetic data is illustrated in Fig. 5.

Further, the biosorption kinetic data were analyzed using theintraparticle diffusion model to elucidate the biosorption mecha-nism [34]:

qt = kit1/2 (6)

where qt is the amount of solute on the surface of the biosor-bent at time t (mg g−1), and ki is the intraparticle diffusion rateconstant (mg g−1 min−0.5). If intraparticle diffusion is involved inthe biosorption process, then the plot of qt versus t0.5 would be

linear. However, when the plots do not pass through the ori-gin, it indicates that some degree of boundary layer control isinvolved in biosorption process. This further shows that the intra-particle diffusion is not the only rate controlling step and other

S. Chowdhury, P.D. Saha / Colloids and Surfaces B: Biointerfaces 88 (2011) 697– 705 701

Table 1Pseudo second order kinetic parameters for biosorption of Cu(II) onto TSP at different temperatures.

T (K) qe,exp (mg g−1) qe,cal (mg g−1) �2 k2(g mg−1 min−1) h (mg g−1 min−1)

303 68.36 68.77 0.5747 6.49 × 10−3 30.32313 72.36 72.87 0.4865 7.65 × 10−3 40.05323 85.87 86.29 0.6721 8.97 × 10−3 66.14333 97.34 97.91 0.6153 9.45 × 10−3 89.53

Table 2Isotherm parameters for biosorption of Cu(II) onto TSP at different temperatures.

Isotherm Parameters T (K)

303 313 323 333

Langmuir qm (mg g−1) 82.97 100.65 117.87 133.24KL (L mg−1) 1.982 2.113 2.675 2.897�2 0.2897 0.1923 0.2153 0.2436

Freundlich KF (mg g−1) (L mg−1)1/n 44.13 55.86 62.95 72.241/n 0.3261 0.4214 0.5344 0.5982�2 0.9341 0.8437 0.9532 0.9898

Temkin KT (L mg−1) 0.235 0.789 1.418 2.031B 128.71 136.49 149.25 161.87

psottcitstissbsrC[

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rocesses may also control the rate of biosorption. In the presenttudy, the plots were linear within a certain extent but not linearver the whole time range (figure not shown). All the plots hadhe same characteristic features. There were three distinct por-ions representing the different stages in biosorption: an initialurve portion followed by a linear portion and then a plateau. Thenitial curve portion was due to boundary layer diffusion charac-erized by surface biosorption and rapid external diffusion. Theecond linear portion was the gradual biosorption stage wherehe intra-particle diffusion is rate-controlled. The plateau regions the final equilibrium stage, in which the intraparticle diffusiontarts to slow down due to the low solute concentration in theolution. Therefore, although intraparticle diffusion is involved iniosorption of Cu(II) onto TSP, but it is not the sole rate-controllingtep and that some other mechanisms also play an important

ole. A similar multilinearity was observed for biosorption ofu(II) on peanut hull [5], chestnut shell [8] and pecan nutshell35].

ig. 5. Pseudo-second-order kinetics for biosorption of Cu(II) onto TSP (experimen-al conditions: initial Cu(II) concentration = 200 mg L−1, biosorbent dose = 1 g L−1,gitation speed = 150 rpm, contact time = 180 min, pH 5.5, temp. = 303 K, error barsepresent the standard deviation at n = 3).

1.6985 1.4536 1.2494

3.4. Biosorption isotherms

An isotherm describes the equilibrium relationship between theadsorbate concentration in the liquid phase and that on the adsor-bent’s surface at a given condition [36]. It is an invaluable curvedescribing the phenomenon governing the retention (or release) ormobility of a substance from the aqueous porous media or aquaticenvironments to a solid-phase at a constant temperature and pH.Isotherms are a thermodynamic basic of separation processes anddetermine the extent to which a material can be adsorbed onto aparticular surface [36]. A variety of isotherms have been developedto describe equilibrium relationships. However, no single model isuniversally applicable; all involve assumptions which may or maynot be valid in particular cases. It is therefore important to establishthe most appropriate isotherm model for the equilibrium data forevaluating the applicability of the sorption process. In the presentstudy, the Langmuir, Freundlich and Temkin isotherm models wereused to describe the equilibrium biosorption data.

The Langmuir isotherm is valid for monolayer biosorption ontoa completely homogeneous surface with a finite number of bindingsites and is given by [36]:

qe = qm KL Ce

1 + KL Ce(7)

where qe(mg g−1) and Ce(mg L−1) are the solid phase concentra-tion and the liquid phase concentration of sorbate at equilibriumrespectively, qm(mg g−1) is the maximum biosorption capacity, andKL(L mg−1) is the biosorption equilibrium constant better knownas the Langmuir constant that quantitatively reflects the affinitybetween the sorbate and the biosorbent.

The Freundlich isotherm is an empirical equation applicable tonon-ideal biosorption on heterogeneous surfaces and is expressedby the following equation [36]:

qe = KF C1/n (8)

where qe is the equilibrium sorbate concentration on the biosorbent(mg g−1), Ce is the equilibrium sorbate concentration in solution(mg L−1), KF(mg g−1) (L g−1)1/n is the Freundlich constant related tosorption capacity and n is the heterogeneity factor.

702 S. Chowdhury, P.D. Saha / Colloids and Surfaces B: Biointerfaces 88 (2011) 697– 705

Table 3Sorption capacity of different low cost sorbents for the uptake of Cu(II) from its aqueous solution.

Sorbent pH Sorbent dose(g L−1)

Temperature (K) Sorption capacity(mg g−1)

Reference

Cinnamomum camphora leaves powder 4.0 2.0 303.2 16.75 [24]313.2 17.08323.2 17.43333.2 17.87

NaOH-pretreated Marrubium globosumssp. globosum leaves powder

5.5 2.0 293 16.23 [30]

Pecan nutshell 5.5 5.0 298 85.9 [35]

Tea waste 5.5 5 295 48.00 [42]

Lentil shell 6.0 10 293 8.98 [43]313 9.51333 9.59

Wheat shell 6.0 10 293 7.39 [43]313 16.08333 17.42

Rice shell 6.0 10 293 1.85 [43]313 2.31333 2.95

Groundnut shell 5.24 20 308 7.60 [44]

Sugar beet pulp 4.0 1.0 298 28.5 [45]

Base treated rubber leaves 4.0 2.0 300 14.97 [46]

Hazelnut shell activated carbon 6.0 3.0 293 48.64 [47]303 51.52313 55.40323 58.27

Rose waste biomass 5.0 1.0 303 ± 1 55.79 [48]

Potato peels charcoal 6.0 10.0 303 0.3877 [49]313 0.1529323 0.1430

Thermal power plants ash 4.5–5.0 10.0 277 4.59 [50]291 5.75333 7.61

Litter of poplar forests 5.0 4.5 298 19.53 [51]

Spent grain 4.2 – – 10.47 [52]

Sour orange residue 4.5 10.0 301 52.08 [53]

Sour orange residue (NaOH treated) 4.5 10.0 301 23.47 [53]

Pretreated Aspergillus niger 5.0 2.0 – 2.61 [54]

Pre-treated arca shell biomass 4.5 5.0 298 ± 2 26.88 [55]

Gelidium 5.3 2.0 293 33 ± 2 [56]308 45 ± 4

Activated sludge 4.0 1.0 – 19.06 [57]

Oak sawdust 4.0 40.0 293 3.22 [58]303 3.38313 3.60

Water hyacinth roots 5.5 0.4 298 22.7 [59]

T. indica seed powder 5.5 5.0 303 82.97 Present study313 100.65323 117.87

da

q

wbs

The Temkin isotherm model is based on the assumption that theecline of the heat of sorption as a function of temperature is linearnd has the following formulation [36]:

e = BT ln (KT Ce) (9)

here qe is the equilibrium adsorbate concentration on the biosor-ent (mg g−1), Ce is the equilibrium adsorbate concentration inolution (mg L−1) and KT(L g−1) and BT are the Temkin constants.

333 133.24

The isotherm constants determined at different temperaturesare tabulated in Table 2. Fig. 6 shows the plots comparing theLangmuir, Freundlich and Temkin isotherm models with the exper-imental data for the biosorption of Cu(II) onto TSP at a temperatureof 303 K.

Low �2 values (Table 2) for the Langmuir model at all tem-peratures indicate that the biosorption of Cu(II) onto TSP followsthe Langmuir model. The excellent fit of the Langmuir isotherm tothe experimental biosorption data confirms that the biosorption

S. Chowdhury, P.D. Saha / Colloids and Surfaces B: Biointerfaces 88 (2011) 697– 705 703

Fig. 6. Comparison between the measured and modelled isotherm profiles forbiosorption of Cu(II) by TSP (conditions: initial Cu(II) concentration = 200 mg L−1,biosorbent dose = 1 g L−1, agitation speed = 150 rpm, contact time = 180 min, pH 5.5,t

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Table 4Thermodynamic parameters for biosorption of Cu(II) onto TSP.

T (K) �G0, kJ mole−1 �H0, kJ mol−1 �S0, J mol−1 K−1

303 −8.61 64.086 240.50313 −11.24

−1

emp. = 303 K, error bars represent the standard deviation at n = 3).

s monolayer; biosorption of each molecule has equal activa-ion energy and that sorbate–sorbate interaction is negligible. The

aximum monolayer biosorption capacity (qm) increases from2.97 mg g−1 at 303 K to 133.24 mg g−1 at 333 K. The Langmuir con-tant, KL also increases with increase in temperature. Seen overall,he information thus obtained specifies an endothermic nature ofhe existing process.

The empirical Freundlich model also shows good fit to the exper-mental equilibrium data at all temperatures studied. The sorptionapacity (KF) increases with increase in temperature. The magni-ude of 1/n gives a measure of favourability of biosorption. Valuesf 1/n less than 1 represent a favourable biosorption process. Forhe present study the value of 1/n also presents the same trendTable 2) representing a beneficial biosorption process.

It is to be noted that the applicability of both Langmuir and Fre-ndlich isotherm models to Cu(II) biosorption onto TSP suggestshat both monolayer sorption and heterogeneous energetic distri-ution of active sites on the surface of the biosorbent was possible. Auite similar finding has been reported for biosorption of Cu(II) ontospergillus niger [37], seeds of Capsicum annuum [38] and oysterhell powder [39].

Compared with the Langmuir and Freundlich isotherm models,he Temkin isotherm model did not fit well with the experimen-al data. However, as seen from Table 2, the Temkin constantT increases with increasing temperature. Furthermore, higher KT

alue at all temperatures suggests strong interaction between theetal ion and the biosorbent surface [40].A comparative study of the maximum Cu(II) uptake capacity of

SP has been carried out with other reported sorbents. It is to beoted that the maximum amount of metal uptake by various sor-ents varies as a function of experimental conditions. Especially theH, temperature and sorbent dose have a very important effect onhe estimation of the maximum amount of metal uptake per unitorbent [14,41]. Therefore, for a direct and meaningful comparison,he maximum amount of Cu(II) sorbed on TSP has been comparedo the maximum Cu(II) sorption capacity of other reported sor-ents under different conditions and are presented in Table 3. Fromable 3 it is observed that the maximum sorption capacity of TSPor Cu(II) is comparable and moderately higher than that of many

orresponding sorbent materials. Differences of metal uptake areue to the properties of each sorbent material such as structure,unctional groups and surface area. The easy availability and cost

323 −14.02333 −15.70

effectiveness of TSP are some additional advantages, which makeit better biosorbent for treatment of copper wastes.

3.5. Biosorption thermodynamics

Temperature dependence of the biosorption process is asso-ciated with several thermodynamic parameters. Thermodynamicconsideration of a biosorption process is necessary to concludewhether the process is spontaneous or not. The Gibbs free energychange (�G0) is a critical factor for determining the spontaneity ofa process and can be computed by the classical Van’t Hoff equation[4,8]:

�G0 = −RT ln KC (10)

where R is the universal gas constant (8.314 J mol−1 K−1), T is theabsolute temperature (K) and KC is the distribution coefficient forbiosorption defined as:

KC = Ca

Ce(11)

in which Ca is the equilibrium sorbate concentration on the biosor-bent (mg L−1) and Ce is the equilibrium sorbate concentration insolution (mg L−1).

It is also known that �G0 is a function of change in enthalpy(�H0, kJ mol−1) as well as change in standard entropy (�S0,J mol−1 K−1) according to the following equation:

�G0 = �H0 − T�S0 (12)

A plot of �G0 versus temperature, T will be linear with the slopeand intercept giving the values of �H0 and �S0.

The calculated �G0 values at all temperatures are listed inTable 4. The negative value of �G0 at different temperatures indi-cates spontaneous nature of the biosorption process. Furthermore,decrease in the negative value of �G0 with increasing tempera-ture suggests that the biosorption process was more favourable athigher temperatures.

�H0 and �S0 were determined from the intercept and slope ofthe plot of �G0 versus T (Fig. 7). The value of �H0 was estimated as64.08 kJ mol−1, and 240.05 J mol−1 K−1 for �S0. The positive valueof �H0 is indicative of the fact that the biosorption reaction wasendothermic. The metal ions had to displace more than one watermolecule for their biosorption and this resulted in the endother-micity of the biosorption process. Similar conclusions have beenproposed by Liang et al. for biosorption of Cu(II) by Mg2+/K+ typeorange peel adsorbents [60]. The positive value of �S0 reflects theaffinity of TSP for Cu(II) ions and an increased randomness at thesolid–solution interface during biosorption [61]. It also implies anincrease in degree of freedom of the adsorbed species [62]. Similarresults have been reported for biosorption of Cu(II) onto Pycnoporussanguineus [63], palm kernel fiber [64], and Cinnamomum camphoraleaves powder [24].

3.6. Isosteric heat of sorption

Isosteric heat of sorption (�HX, kJ mol ) is defined as the heatof sorption determined at constant amount of sorbate adsorbed.It is one of the basic requirements for the characterization and

704 S. Chowdhury, P.D. Saha / Colloids and Surfaces B: Biointerfaces 88 (2011) 697– 705

oapgioC

�d

cTmp(�E��hfbuodogwahcb

TI

Fig. 8. (a) Plots of ln Ce against 1/T for biosorption of Cu(II) onto TSP (b) variation of

Fig. 7. Plot of Gibb’s free energy change versus temperature.

ptimization of a biosorption process and is a critical design vari-ble in estimating the performance of a biosorption separationrocess. It also gives some indication about the surface ener-etic heterogeneity. Knowledge of the heats of sorption is verymportant for equipment and process design. The isosteric heatf sorption was calculated at constant surface coverage using thelausius–Clapeyron equation [4]:

d(ln Ce)

dT= −�HX

RT2(13)

HX is calculated from the slope of a plot of lnCe versus 1/T forifferent amounts of adsorbate onto biosorbent.

In the present investigation, the isosteric heat of sorption wasalculated at constant surface coverage (qe = 12, 14, 16, 18 mg g−1).he equilibrium concentration (Ce) at constant amount of adsorbedetal ions was obtained from the isotherm data at different tem-

eratures. The plots of lnCe versus t were found to be linearFig. 8(a)). The R2 values of the isosteres and the corresponding

HX values are listed in Table 5. From Table 5, it can be seen thatq. (13) represented the experimental data well. The variation ofHX with surface loading is presented in Fig. 8(b). It is observed thatHX increased steadily with increase in qe indicating that TSP was

aving heterogeneous surfaces. The dependence of �HX on sur-ace coverage can be due to sorbate–sorbate interaction followedy sorbate–biosorbent interaction. In the range of lower qe val-es, sorbate–sorbate interactions are strong resulting in low heatsf sorption. With increase in qe, the sorbate–sorbate interactionecreases due to the existence of highly active sites on the surfacef the biosorbent. Sorption occurs on the most active available sitesiving rise to high interaction energy. The active sites are coveredith adsorbate molecules forming a mono-molecular layer. Thus

s qe increases, sorbate–biosorbent interaction occurs resulting inigh heats of sorption. This variation in �H with surface loading

X

an also be attributed to the possibility of having lateral interactionsetween the adsorbed metal ions.

able 5sosteric heat of sorption of Cu(II) onto TSP.

qe (mg g−1) �HX (kJ mol−1) R2

12 70.87 0.988614 76.42 0.988916 85.76 0.998418 89.81 0.9989

isosteric heat of sorption with surface loading of Cu(II) onto TSP.

4. Conclusion

In this study, the effect of temperature on the biosorption ofCu(II) from aqueous solution using TSP was investigated. The fol-lowing conclusions are made based on the results of present study:

• Temperature has strong influence on the biosorption process. Thebiosorption capacity increases with increasing temperature.

• The biosorption kinetics follows the pseudo-second-order kineticmodel. Intra-particle diffusion is not the sole rate-controlling fac-tor.

• Experimental equilibrium data of Cu(II) onto TSP follows bothLangmuir and Freundlich isotherm models implying that bothmonolayer sorption and heterogeneous energetic distribution ofactive sites on the surface of the biosorbent was possible. Themaximum monolayer Cu(II) biosorption capacity increases from82.97 mg g−1 at 303 K to 133.24 mg g−1 at 333 K.

• Thermodynamic study shows spontaneous, endothermic as wellas entropy favourable sorption behavior.

• Isosteric heat of sorption calculated by applying theClausius–Clapeyron equation increases with increase in surface

loading indicating that TSP was having energetically hetero-geneous surface and there may be some lateral interactionsbetween the sorbed metal ions.

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[60] S.S. Liang, X. Guo, N. Feng, Q. Tian, J. Hazard. Mater. 174 (2010) 756–762.

S. Chowdhury, P.D. Saha / Colloids and

The results suggest that TSP is a promising biosorbent for Cu(II).s real industrial effluents contain several other pollutants andeavy metal ions, TSP should further be investigated for its effi-iency for removal of Cu(II) using real effluents from industries.

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