Desalination 249 (2009) 429–439

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Desalination

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Multicomponent isotherms for biosorption of Ni2+ and Zn2+

K. Shahzad Baig, H.D. Doan ⁎, J. WuDepartment of Chemical Engineering, Ryerson University, Toronto, ON, Canada

* Corresponding author. Tel.: +1 416 979 5000#6341E-mail address: hdoan@ryerson.ca (H.D. Doan).

0011-9164/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.desal.2009.06.052

a b s t r a c t

a r t i c l e i n f o

Article history:Accepted 28 June 2009Available online 4 October 2009

Keywords:NickelZincBiosorption multicomponent isothermsSEM and EDS

The capability of wheat straw to adsorb Ni2+ and Zn2+ was investigated using a batch system. The equilibriumremoval of metal ions was obtained between 2.5 and 5h for Ni2+ and about 3h for Zn2+ over the initialconcentration range from 5 to 150 ppm. Of the total amount of metal uptake by wheat straw, about 50% wasadsorbed in thefirst 30 min. At a low initial concentration of 5 ppm,wheat strawwas capable to reduce themetalconcentration down to less than 1 ppm. For single-metal solutions, among the three models tested, namely theLangmuir, the Freundlich and the Temkin isotherms, the Freundlich model was suitable to describe theadsorption equilibrium for Ni2+ and Zn2+. For bimetal solutions, the IAST-Freundlich multicomponent isothermbest fitted the experimental data, among the four isotherm models investigated, the modified Langmuirmulticomponent model, the Langmuir partially competitive model, the Freundlich multicomponent model andthe IAST-Freundlich multicomponent model. The negative Gibbs free energy changes obtained at lowerconcentrations indicates that the adsorption was spontaneous. However, the spontaneity of the biosorptiondecreased with increases in the metal concentration from 5 to 50 ppm. For metal concentrations higher than50 ppm, the adsorption became non-spontaneous. Scanning electronmicroscopic (SEM) images of wheat strawwere also taken to exam the surface structure of the wheat straw along with the energy dispersive spectrum(EDS) analysis. The results obtained confirmed the adsorption of Ni2+ and Zn2+onwheat straw, and showed thatthe inner surface of the wheat straw appeared to provide more adsorption sites for metal binding.

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1. Introduction

Mining and steel production are the major source of zinc pollution.Zinc is also used inmetal coating for prevention of corrosion. Some zincsalts are used industrially for wood preservatives, catalysts, photo-graphic paper and accelerators for rubber vulcanization, ceramics,textiles, fertilizers, pigments and batteries [1]. All these activitiesgenerate wastewater containing Zn2+ that is toxic to humans at levelsof 100–500 mg/day [2]. World Health Organization recommended themaximum acceptable concentration of Zn in drinking water as 5.0 ppm[3]. Canada is one of the major nickel producers in the world. EPArecommends that the nickel concentration in drinkingwater should notbe more than 0.1 ppm. Plants are known to accumulate nickel. TheInternational Agency for Research on Cancer classified nickel as apossible carcinogen to humans. American Conference of GovernmentalIndustrial Hygienists confirmed that nickel is a human carcinogen [4,5].In addition, heavy metals from industrial discharges may contaminategroundwater and can be toxic to themicrobial population at sufficientlyhigh concentrations [6–8]. The current effluentdischarge limit set by thecity of Toronto, Ontario, Canada, for both Ni2+ and Zn2+ in sanitarysewer is 2 ppm [9]. Thus, industrial wastewater must be treated beforethey are disposed to water bodies.

Several techniques have been used to remove heavy metals fromwastewater, such as: precipitation, ion exchange, adsorption, electro-dialysis and membrane technologies [10–13]. Precipitation createssludge and its disposal is a problem. Most of these techniques areexpensive and ineffective at a low metal concentration, exceptadsorption [14,15]. The most common adsorbent materials used arecoal and aluminum silicate. Coal is a non-renewable source of energy soits use as adsorbent cannot beappreciated. For aluminumsilicate, a layerof bacteria could be developed on its surface, which reduces itsefficiency significantly. Activated carbon from agricultural materialshas also been tested. However, it has not been widely used because thegeneration of activated carbon requires the use of energy.

In order to find a low cost and effective material that can be used asan adsorbent, researchers have tested many biological materials for theremoval of heavy metals from aqueous solutions [16,17]. Biosorptioncan be considered as an economical and eco-friendly practice incomparison with other available techniques [18,19]. Canada is in thetop ten countries of wheat production [20]. For every pound of wheatgrains 1.3–1.4lb of wheat straw is obtainable [21]. Wheat straw is thusabundantly available in Canada. Therefore, in the present study thepotential ofwheat strawasa biosorbent for the treatment ofwastewatercontaining Ni2+ and Zn2+ was investigated. The applicability of themulticomponent isotherm models to the biosorption of the bimetalsolutions was evaluated along with the examination of the kinetic andthermodynamic behavior of biosorption ofNi2+ and Zn2+ fromaqueoussolutions by wheat straw.

430 K.S. Baig et al. / Desalination 249 (2009) 429–439

2. Materials and methods

2.1. Preparation of wheat straw and chemicals

Wheat straw was obtained from local farms outside of the city ofToronto. The wheat straw was washed with distilled water and driedat 60 °C until constant weight was obtained. The dried wheat strawwas then cut into 5-mm particles for use in batch experiments.

All chemicals used in the present study were of analytical grade.Stock metal solutions of 1500 ppm were prepared by dissolvingZnSO4.7H2O and NiSO4.6H2O (J. T. Baker, New Jersey, USA) in distilledwater. For experiments with various metal concentrations from 5 to150 ppm, the stock solutionswere diluted furtherwith distilledwater.0.1 M solutions of HCl and NaOH were used to adjust the initial pH ofthe solutions in various experiments.

2.2. Experimental set-up

In batch biosorption experiments, 200 ml of metal ions solutionwas added into a 300-ml Erlenmeyer flask containing 1.0 g of biomass.The flask was then placed in a temperature-controlled shaker bath(Julabo SW 22, Julabo Labortech GmbH, Germany) and shaken at120 rpm. The temperature in the bath was kept at 25 °C. Watersamples were taken from the flask continually. The concentrations ofthe metal ions in the water samples were measured using an atomicabsorption spectrometer (AAnalyst 800, PerkinElmer Inc. USA).Multiple measurements of the metal concentration of each watersample were carried out. The standard deviation was in the order of3.8% of the average value. All experiments were single runs. However,in order to test the reproducibility of the experiments initially,triplicate runs were done and the metal concentrations weremeasured. The average deviation from the mean was about 5%.

The amount of metal ion uptake by wheat straw was calculatedusing the following equation:

q = ðC0 � CÞ VM

ð1Þ

where C0 is the initial metal concentration in the solution (mg L−1),C is the metal concentration remaining in the solution at a giventime (mg L−1), M is mass of biosorbent (g), q is the metal ion uptake(mg g−1) and V is the volume of solution in the flask (L).

The surface structure was also studied using a scanning electronmicroscope (SEM) (JSM-6380 LV, JEOL, Japan) with an acceleratingvoltage 20 kV, and the elemental analysis was performed by energydispersive spectrometry (EDS)using a INCAX-sight (Oxford instruments,UK).

3. Adsorption isotherm models

3.1. Models for single-component systems

For a single-metal solution, isotherms such as the Langmuir,Freundlich and Temkin models were often used to fit the data so tounderstand the adsorption process and to obtain information needed forscaling up to a larger system.

3.1.1. The Langmuir modelThe Langmuirmodel for monolayer adsorption can be expressed as

below [22]:

Ce

qe=

Ce

qL+

1qLKL

ð2Þ

where Ce is the adsorbate concentration in the solution at equilibrium,KL is the Langmuir adsorption constant, qe is the equilibrium uptake of

the adsorbate and qL is the maximum adsorbate uptake by thebiosorbent (biosorption capacity). From a plot of Ce/qe versus Ce, qLand KL can be determined from the slope and the intercept of the plot,respectively.

3.1.2. The Freundlich modelThe Freundlich model for heterogeneous surface energy can be

expressed as [23]:

qe = KFC1 = ne ð3Þ

lnðqeÞ =1nlnðCeÞ + lnðKFÞ ð4Þ

where KF is the Freundlich constant and 1/n is the biosorptionintensity.

As indicated by Eq. (4), KF and 1/n can be determined from a plotof log(qe) versus log(Ce). KF shows the adsorbent capacity and 1/nindicates the degree of non-linearity between the adsorbate concen-tration in the solution and the amount adsorbed onto the adsorbent atequilibrium.

3.1.3. The Temkin isothermThe Temkin isotherm, considering the effect of the adsorption heat

that decreases with the coverage of the adsorbent and the adsorbate-adsorbent interaction, is given as [24]:

qe =RTb

� �lnðACeÞ ð5Þ

where R is gas constant and T is the absolute temperature (K). A plotof qe versus lnCe enables the determination of the constant A and b.

3.2. Models for multicomponent systems

When several components are present, there is interference andcompetition among different components for adsorption sites. Theisothermmodels for a single-component system are thus inapplicable.A multicomponent system requires a more complex mathematicalisotherm model. Some popular isotherm models for multicomponentsystems are presented below.

3.2.1. The modified Langmuir modelThe modified Langmuir isotherm, allowing for a complete

competition of adsorption sites by different solutes, is given as [25]:

qe;i =qoi bi Ci

1 + ∑n

j=1bj Cj

ð6Þ

where Ci and Cj are the concentrations of the adsorbates “i” and “j”remaining in liquid at equilibrium, qe,i is the equilibrium uptake of theadsorbate “i” in the multicomponent system. bi and bj are the Langmuiradsorption constants (equivalent to KL) of the adsorbates in single-component systems and qi

o is the Langmuir adsorption capacity of theadsorbate “i” (equivalent to qL) in the single-component system.

For a binary system, the modified Langmuir isotherms can bewritten as:

qe;1 =q01 b1 C1

1 + b1 C1 + b2 C2ð7Þ

qe;2 =q02 b2 C2

1 + b1 C1 + b2 C2ð8Þ

Fig. 1. Equilibrium metal uptake for Ni2+ and Zn2+ with bimetal solutions at variousinitial concentrations from 5 to 150 ppm and at 25 °C.

431K.S. Baig et al. / Desalination 249 (2009) 429–439

where the subscripts 1, 2 represent the two solutes (Ni2+ and Zn2+)in the binary system used in the present study.

3.2.2. The Langmuir partially competitive multicomponent modelThe Langmuir multicomponent isotherm assumes complete

competition among different adsorbate species while the partiallycompetitive Langmuir isotherm allows for a partial competition inadsorption of multiple adsorbates [26]. For two-adsorbate systems,the isotherm can be written as follows:

q1 =ðq01 � q02Þb1 C1

1 + b1 C1+

q01 b1 C1

1 + b1 C1 + b2 C2ð9Þ

q2 =q02 b2 C2

1 + b1 C1 + b2 C2ð10Þ

The first term of the right-hand side of Eq. (9) accounts for theamount of solute 1 adsorbed without competition. The second term,which is based on the original modified Langmuir isotherm, is for theamount of solute 1 adsorbedunder competitionwith solute 2. Eq. (10) isfor the amount of solute 2 adsorbed under competition with solute 1.

3.2.3. The Freundlich multicomponent modelThe multicomponent Freundlich isotherm can be expressed as

[27]:

qi = KiCi ∑k

j=1aij Cj

!mi�1

ð11Þ

For a binary system, the isotherms can be written as:

C1

C2=

1C2

β1 � a12 ð12Þ

C2

C1=

1C2

β2 � a21 ð13Þ

where βi =qi

Ki Ci

� �1=mi�1and aij is the competition coefficient with

aij=1/aji. The Freundlich constants Ki and mi (mi=1/ni in theFreundlich model for a single-component system) are of the single-component system while qi and Ci are the equilibrium uptake and theequilibrium concentration of the solute “i” in the multicomponentsystem, respectively. For a binary solution, these are q1, q2, C1 and C2.

3.2.4. The IAST-Freundlich isotherm modelThe ideal adsorption solution theory (IAST)-Freundlich model is a

simplification of the multicomponent Freundlich isotherm and can beexpressed as [28]:

Ci =qi

∑N

j=1qj

∑N

j=1nj qj

ni Ki

0BBB@

1CCCA

ni

ð14Þ

where i and j are the adsorbates in the multicomponent system. For abinary system, the model can be written as:

C1 =q1

q1 + q2

n1 q1 + n2 q2n1K1

� �n1ð15Þ

C2 =q2

q1 + q2

n1 q1 + n2 q2n2K2

� �n2ð16Þ

where C1 and C2 are the concentrations of adsorbates 1 and 2remaining in the solution at the equilibrium, and q1 and q2 are theequilibrium uptakes onto the adsorbent in the multicomponent

system. K1, K2, n1 and n2 are the Freundlich constants for theadsorbates 1 and 2, respectively, in the single-component systems.

4. Results and discussion

4.1. Kinetics of biosorption of Ni2+ and Zn2+

In general, the equilibriumsolute uptake bya biomass increaseswiththe concentration of the solute in the solution until it reaches amaximalvalue known as the adsorption capacity of the biomass. This was indeedthe case forwheat strawaswell. As can be seen in Fig. 1, the equilibriummetal uptake increased with the initial concentration. This is inagreement with several reported literatures [29,30]. It was also noticedthat the initial adsorption rate was higher than that at the later stage ofthe adsorption process since the number of vacant adsorption sitesavailable on the surface of the wheat straw was higher initially. Almost50% of Ni2+ and 30% of Zn2+ were adsorbed in the first 30min.

The equilibrium uptake of Zn2+ was substantially higher than that ofNi2+ at higher initial concentrations. Also, the equilibrium uptake of eachmetal in the bimetal solution was lower than its own equilibrium uptakewith the single-metal solution. This indicates that Ni2+ and Zn2+ bisolutebiosorption was a competitive process where the two species inhibitedeachother's adsorption sites. For the conditions in thepresent study, Zn2+

was removed more preferably than Ni2+. Chong and Volesky (1995)investigated co-adsorption ofmetal ions by seaweedusing various binarysystems, such as: Cu2+ and Cd2+, Cu2+ and Zn2+, Cd2+ and Zn2+. Theauthors reported that the adsorption capacity for each metal in a binarysystem was lower than that for a single-metal system [31]. Theelectronegativities of Ni2+ and Zn2+ are 1.91 and 1.65, respectively,while theirmolecularweights are comparable (58.7 gmol-1 fornickel and65.4 g mol-1 for zinc). The wheat straw cell wall residue has oxygen-containing group (negative sites) such as carboxylic and phenolic [32].The electron clouds on oxygen atoms of those groups tend to repel thespecies with a higher electronegativity more forcibly. Therefore, Ni2+

might be repelled slightlymore by the negative sites on thewheat straw,resulting in a lower adsorbed amount than that of Zn2+. A similar trendwas observed in our previous study [33]. However, the time to reachequilibrium for Ni2+ was observed to vary between 2.5 and 5h with theinitial concentrations of 5 to 150 ppm while the equilibrium time wasestablished sooner at about 3h for all initial concentrations of Zn2+.

The effect of pH on the biosorption capacity can be attributed tochemical forms of heavy metals in the solution at a specific pH, i.e. pureionic metal or hydroxyl-metal. In addition, due to different functionalgroups on the biosorbent surface, which become active sites for themetal binding at a specific pH, the effect of pH on biosorption can varysubstantially. Therefore, theoptimumpHvaried significantlydependent

432 K.S. Baig et al. / Desalination 249 (2009) 429–439

on the types of biosorbents and metal ions. This has been shown to bethe case by several researchers [34–36]. In general, biosorption of bothZn++ and Ni++ increased with the solution pH from 4.0 to 7.0 as foundin our previous study [33]. This also indicates that cationic exchangebetween H+ attached on the adsorption sites with metal cations wasone of the mechanisms for the adsorption of Ni2+ and Zn2+ on wheatstraw. A high H+ concentration in a solution at a low pH tends to hinderthe exchange of the metal cations.

Although the equilibrium uptake of metal ions increased with theinitial metal concentration, the available adsorption sites would besimilar for the same amount of adsorbent used in all individualexperiments with variousmetal concentrations from 5 to 150 ppm. Asthe adsorption process proceeded, the adsorbent was becomingsaturated, and hence, less adsorption sites were available for furthermetal adsorption, especially at high initial concentrations. The metalions would first bind to high affinity sites of the biomass surface andthen with low affinity ones. In addition, the adsorption could behindered by the interaction of the adsorbate–adsorbent at high metalconcentrations. The surface of the adsorbent is usually negativelycharged. Positively chargedmetal ions could thereby bind the biomassparticle together, leading to the reduction in the available biomasssurface area [37,38]. Therefore, the percentage removal of the metalions decreased with the initial metal concentration. In the presentstudy, the percentagemetal removal reduced from above 80% for bothNi 2+ and Zn2+ at the initial concentration of 5 ppm to about 30% at150 ppm. Nevertheless, the percentage removal could be improved byeither an increase in the amount of biomass or a further reduction ofthe biomass particle size.

From the analysis of the kinetic data, it was also found thatbiosorption of Ni2+ and Zn2+ bywheat straw can be well described bythe pseudo-second order kinetics as below:

dqtdt

= K ðqe � qtÞ2 ð17Þ

Integration of Eq. (17) and rearrangement of the resultantequation gives:

tqt

=tqe

+1

K q2eð18Þ

Fig. 2. Pseudo-second order kinetics of biosorption of Zn2

where K is the rate constant, qe is the equilibriummetal uptake and qtis the metal uptake at a given time t.

The rate constant, K, and the equilibrium metal uptake, qe, can beobtained from a plot of t/qt versus t as shown in Fig. 2. A linearregression curve fitting was applied to the data, and the resultsobtained are presented in Table 1. The model fits the experimentaldata well, as indicated by the values of the coefficient of determina-tion, r2, close to unity for all cases with initial concentrations from 5 to150 ppm. The predicted equilibrium metal uptakes agree with theexperimental values well. It was also noticed that the adsorption rateconstant, K, decreased with the initial concentration and leveled off atthe initial concentration of about 80 ppm onward as can be seen inFig. 3. For the initial concentration of 80 ppm onwards, the rateconstants for both Ni2+ and Zn2+ became similar. However, the metaluptake rate for Zn2+ was still higher than that of Ni2+. This indicatedthat the adsorption affinity of the wheat straw toward a specific metaldictated the equilibrium uptake. In addition, this revealed that at highmetal concentrations relative to the amount of biomass in the system,the probable shortage of the adsorption sites rendered a limitingadsorption rate constant that was independent of the types of metalions.

4.2. Adsorption isotherms

4.2.1. Isotherms for single-component solutionsThe Langmuir and Freundlich isotherms are the classical isotherm

models that are widely used for single-component adsorption data.Therefore, thosemodelswere adopted to analyze the experimental dataobtained in the present study. The data for single-component solutionscontainingNi2+ or Zn2+were fitted to the isothermmodels and plottedin Figs. 4, 5. Among the threemodels, the Freundlich isothermappears tobest fit the datawith the values of the coefficient of determination, r2, of0.97 and 1.0 for Ni2+ and Zn2+, respectively. The Langmuir model fitsthedata reasonablywellwith the r2 values of 0.98 and0.96, respectively.The constants of the Langmuir and Freundlich isotherms for Ni2+ andZn2+ were obtained from linear regression are given in Table 2. Thevalue of the Freundlich constant, KF, for Zn2+ is about 15% higher thanthat for Ni2+, indicating that wheat straw has a higher biosorptioncapacity for Zn2+ thanNi2+. The values of the adsorption intensity 1/nof0.46 for Ni 2+ and 0.49 for Zn2+, which are substantially higher thanzero, indicate the favorability of the biosorption process. The adsorption

+ on wheat straw (single-metal solution, T=25 °C).

Table 1Experimental equilibrium uptakes and predicted values from the pseudo-second orderkinetics for biosorption of Ni2+ and Zn2+ by wheat straw (single-ion solutions).

Initialconcentration(ppm)

Ni2+ Zn2+

Expt. qe(mg g−1)

Cal. qe(mg g−1)

r2 Expt. qe(mg g−1)

Cal. qe(mg g−1)

r2

5 0.88 0.98 0.994 0.86 0.90 0.99625 2.5 2.8 0.968 3.2 3.3 0.99950 5.0 5.3 0.996 5.2 5.3 0.999100 6.2 6.5 0.999 7.8 8.0 0.999150 7.0 7.4 0.999 10.0 10.4 0.999

433K.S. Baig et al. / Desalination 249 (2009) 429–439

intensity is also an indicationof the adsorptionenergy. Since thevalue of‘n’ forNi2+ is higher than that of Zn2+,Ni2+would be attracted tohigherenergy sites, resulting in a lower uptake amount than that of Zn2+.

4.2.2. Isotherms for the binary system

4.2.2.1. The modified Langmuir isotherm and the Langmuir partiallycompetitive isotherm. Although the Langmuir isotherm represented theexperimental data for the single-component system reasonably well,themodified Langmuir isotherm underestimated the equilibriummetaluptake for Zn2+ over the whole range of initial metal concentrationsused in the present study as can be seen in Fig. 6. At the initial con-centration of 150 ppm, the experimental uptake of Zn2+was 8 mg g−1,which is almost twice the predicted value by the model. The isothermmodel also underestimated the adsorption of Ni2+ at the initialconcentration of 100 ppm or higher. On the other hand, at the initialconcentrations between 25 and 50 ppm, the model overestimated theequilibrium uptake of Ni2+ significantly. The underestimation of theequilibrium metal uptake by the modified Langmuir model could beattributed to the assumption of fully competitive adsorption incorpo-rated in the model while the competition of Ni2+ and Zn2+ for adsorp-tion sites on the wheat straw could have been much less.

The equilibrium uptakes for Zn2+ and Ni2+ estimated from theLangmuir partially competitive isotherm are plotted along with theexperimental data in Fig. 6. The model prediction was somewhatimproved as compared with that of the modified Langmuir model.

Fig. 3. Variation of the second-order rate c

However, the predicted values for Zn2+ were still consistently lowerthan the experimental data, especially at high initial concentrations.The trend of the predicted equilibrium uptake for Ni2+ remains thesame as the case with the modified (fully competitive) Langmuirmodel since the model equations for Ni2+ in the modified Langmuirisotherm and the Langmuir partially competitive isotherm were thesame. The assumptions used in the development of the Langmuirpartially competitive isotherm are the same as those for the modifiedLangmuir isotherm for a multicomponent system with the additionalassumption of the partial competition of the components. This mightbe the reason for some improvement in the predicted values, whichare closer to the experimental data for Zn2+ than those predicted bythe modified Langmuir model. For example, at the initial concentra-tion of 150 ppm, the equilibrium uptake of Zn2+ was estimated to be5.8 mg g−1 by the Langmuir partially competitive isotherm while itwas predicted to be 4.1 mg g−1 by the modified multicomponentLangmuir isotherm.

4.2.2.2. The Freundlich multicomponent isotherm and the IAST-Freun-dlich isotherm. A comparison of the equilibrium concentration of Ni2+

and the Zn2+ remaining in the solution and the predicted values fromthe multicomponent Freundlich isotherm is given in Fig. 7. The datafor Zn2+ agrees fairly well with the model prediction at initial con-centrations less than or equal to 50 ppm. However, at initial con-centrations of 100 ppm or higher, the model clearly underestimatedthe equilibrium uptake of Zn2+. For adsorption of Ni2+, the modelunderestimated the metal uptake significantly as shown by the pre-dicted equilibrium Ni2+ concentration of about 1.8 times the actualequilibrium concentration for the case with an initial concentration of150 ppm. On the other hand, the IAST-Freundlich model representedthe data for the binary solutions very well, as can be seen Fig. 7.

The comparison between the experimental data and the predictedvalues was based on either Ce or qe, dependent on the model underinvestigation. The magnitudes of Ce and qe are much different. There-fore, an averaged error was used to evaluate the model's goodness offit to the data as following:

Avg:error ð%Þ = 100⋅ 1n∑n

i=1

jx−xexp jxexp

!ð19Þ

onstant with the initial concentration.

Fig. 4. The Langmuir model for Ni2+ and Zn2+ in a single-metal system at 25 °C.

434 K.S. Baig et al. / Desalination 249 (2009) 429–439

where xmod and xexp are the model prediction and the experimentaldata, respectively, and n is the number of data points. The calculatederrors for different models are given in Table 2. Among the isothermsused to analyze the experimental data obtained in the present study,namely the Langmuir multicomponent isotherm, the Langmuirpartially competitive isotherm, the Freundlich multicomponentisotherm and the IAST-Freundlich isotherm, the IAST-Freundlichisotherm can be considered the best model to represent theadsorption equilibrium of Ni2+ and Zn2+ in a bimetal solution, asindicated by the lowest errors of less than 10% compared with theerrors of 17 to 49% for other models (Table 3). This is consistent withthe results for single-metal solutions, of which the simple Freundlichmodel for a single-component system was also found to best fit theexperimental data.

4.3. Mechanism of metal adsorption by wheat straw

4.3.1. Free energy of biosorptionThe thermodynamic parameter such as the Gibbs free energy

change indicates the degree of spontaneity of a process. A higher and

Fig. 5. The Freundlich model for Ni2+ and Zn2+ in a single-metal system at 25 °C.

negative value indicates a more energetically favorable process.Therefore, it can be used to evaluate the thermodynamic feasibilityof the adsorption of metal on wheat straw. The Gibbs free energychange (ΔG) in the adsorption process can be estimated from thevan't Hoff equation as below [39]:

ΔG = −RT lnKc ð20Þ

where ΔG is the Gibbs free energy change, Kc is the equilibriumconstant, R is the gas constant and T is the absolute temperature.

The equilibrium constant of biosorption is defined as:

Kc =C0 � Ce

Ceð21Þ

where Ce and C0 are the equilibrium and initial concentrations of themetal ion in the solution.

At the initial concentration of 5 ppm, the Gibbs free energy changesfor Ni2+ and Zn2+ biosorptionwere estimated to be−4.9 KJmol−1 and−4.5 KJ mol−1, respectively. The negative value ofΔG indicates that theadsorption was spontaneous. The spontaneity of the adsorption processdecreased as the initial concentration was increased. When the initialconcentrationwashigher than50 ppm, thebiosorptionofNi2+andZn2+

became non-spontaneous since ΔG became positive. At the initialconcentration of 150 ppm, the values of ΔG for biosorption of Ni2+ andZn2+ were 1.7 KJ mol−1 and 2.9 KJ mol−1, respectively. A similar trendof thevariation ofΔGwith the initial concentrationwas also observed foradsorption of bimetal solutions. At 5 ppm, theGibbs free energy changesof−3.7 KJ mol−1 and−3.4 KJ mol−1 were obtained for Ni2+ and Zn2+,respectively, and they increased to 4.0 KJ mol−1 and 2.5 KJ mol−1 at

Table 2The Langmuir and Freundlich constants for adsorption of Ni2+ and Zn2+ to wheat strawwith single-metal solutions.

Metalions

The Langmuir model The Freundlich model

qL(mg g−1)

KL

(L mg−1)r2 1/n

(L g−1)KF

(mg g−1)r2

Ni2+ 7.9 0.07 0.98 0.46 0.88 1.0Zn2+ 11.4 0.05 0.96 0.49 1.05 0.97

Fig. 6. Comparison of qe values predicted from the competitive and partially competitive Langmuir models and experimental data of the bimetal system at 25 °C.

435K.S. Baig et al. / Desalination 249 (2009) 429–439

150 ppm. The higher ΔG for the case with the bimetal solution could bedue to the competition of adsorption sites of two different metallicspecies, Ni2+ and Zn2+. The Gibbs free energy changes for physical andchemical biosorption are usually in the range of 0.0 to 20 kJ mol−1 and80 to 400 kJ mol−1, respectively. Therefore, the biosorption of Ni2+andZn2+onwheat straw can be considered to followphysical adsorption. Inaddition, the metal uptake capacity of wheat straw, at a relatively largeparticle size (0.5 cm), is comparable to other agricultural-wastebiomasses and some fungus [8,19,40]. The adsorption capacity ofwheat straw could be improved by using smaller wheat straw particles.

Fig. 7. Comparison of Ce values predicted from the Freundlich and IAST-Freundlich

Moreover, at a low initial concentration of 5 ppm, wheat straw wascapable of reducing the metal concentration down to less than 1 ppm.Wheat straw can thus be considered as a promising biosorbent for theremoval of heavy metals in wastewater.

4.3.2. Effect of wheat straw surface structure on adsorptionAfter the completion of adsorption experiments, microscopic

images of the wheat straw were taken using a scanning electronmicroscope (SEM) and the elemental spectra were obtained with anenergy dispersive spectrum (EDS) device. The SEM images and the

multicomponent models and experimental data of the bimetal system at 25 °C.

Table 3Averaged errors between the predicted values from different isotherm models formulticomponent systems and experimental data of the bimetal system.

Model Average error (%)

Ni2+ Zn2+

The modified Langmuir model 26.9 31.8The Langmuir partially competitive model 26.9 16.7The Freundlich multicomponent model 49.2 26.4The IAST-Freundlich multicomponent model 9.6 7.1

436 K.S. Baig et al. / Desalination 249 (2009) 429–439

EDS analyses are shown at the top and the bottom, respectively, inFigs. 8 and 9. Spectrum 3 in Fig. 8 shows the adsorption of Ni2+ on theouter surface of wheat straw where the fibrils are about 10 µm long,and spectrum 5 is in the region where the fibril length is about 5 µm.Fig. 9 shows the biosorption of Ni2+ on the inner surface of wheatstraw. Spectrum 1 is an analysis of a fiber, which shows the attach-ment of Ni2+ onto wheat straw. The EDS spectrum also shows thepeaks of C, O, Si and Al that were naturally present in wheat straw.Spectrum 3 covers an area of about 100×100 µm. Ni2+ was alsopresent in that area. Similarly, biosorption of Zn2+ on the inner andthe outer surfaces of wheat straw was also observed in the SEMimages and EDS analyses of the wheat straw after adsorption.

The SEM images helped further understanding the adsorption ofheavy metals at different locations on wheat straw. For example, at aninitial metal concentration of 150 ppm, the amount of Ni2+ adsorbedat stomata was almost doubled that of Zn2+. On the other hand, at theholes (a characteristic feature on the outer surface of wheat straw),the amount of Zn2+ adsorbed was three times that of Ni2+. However,the fibrous structure yielded similar adsorbed amounts for both Ni2+

and Zn2+. In general, the EDS analyses showed that the inner surfaceof wheat straw had a higher adsorption capacity than the outer sur-face. The inner surface had a fibrous structure that allowed liquid topenetrate deeper into the wheat straw where more adsorption siteswere available for the binding of metal ions.

Fig. 8. SEM image and EDS spectrum for biosorptio

4.3.3. Transport mechanismThe prediction of the rate-limiting step is an important factor to be

considered in the biosorption process. It is governed by the biosorptionmechanism. The knowledge of the limiting step would allow an appro-priate manipulation of the process parameters so to obtain betteradsorption efficiency; hence, it is vital for the design of an adsorptionprocess. An adsorption process usually involves three steps: the trans-port of the solute from the bulk liquid to the adsorbent surface, adsorp-tion of the solute onto the adsorbent surface and the solute transportinto the pores of the adsorbent (the intraparticle diffusion for the case ofa porous adsorbent). For wheat straw, the adsorbent particles are notporous. Therefore, the adsorption process would only have the first twosteps. When the external transport is by diffusion, the amount of soluteadsorbed with time can be described as:

qt = kiffiffit

pð22Þ

where qt is the amount of metal adsorbed at a given time t, and ki isthe rate constant.

After the initial adsorption over the first 30min, the amount ofmetal adsorbed increased linearly with the square root of time as canbe seen in Figs. 10 and 11. This indicates that the biosorption of Ni2+

and Zn2+ by wheat straw in the batch system indeed followed theexternal transfer limiting step. For both Ni2+ and Zn2+, the diffusionrate constant ki increased with the initial metal concentration. Theincreasing trend of the rate constant with the initial solute concen-tration was also observed by other investigators [40–42].

The observed increases in ki with the initial metal concentrationwere due to a higher concentration gradient between the bulk liquidand the liquid–solid interface at a higher initial metal concentration,which facilitated a higher mass transfer rate from liquid to theadsorbent. The diffusivity coefficient of Ni2+ (6.13×10−10m2 s−1) isabout 14% lower than that of Zn2+ (7.02×10−10m2 s−1). On the otherhand, themolecularweight of zinc (65.4 gmol−1) is slightlyhigher thanthat of nickel (58.7 g mol−1). At the same initial mass concentration of

n of Ni2+on the outer surface of wheat straw.

Fig. 9. SEM image and EDS spectrum for biosorption of Ni2+on the inner surface of wheat straw.

437K.S. Baig et al. / Desalination 249 (2009) 429–439

both Ni2+ and Zn2+, the molar concentration of Ni2+ was about 11%higher than that of Zn2+. Therefore, themass transfer rate from the bulkliquid to the adsorbent and the metal uptake of both Ni2+ and Zn2+

would have been similar if the external mass transfer were the limitingstep of the overall adsorption process. However, the ki values obtainedfor Zn2+ (0.07 to 0.43) are substantially higher than those for Ni2+ (0.07to 0.25). This suggests that both the external mass transport and the

Fig. 10. Variation of Ni2+ uptake with adsorptio

adsorption of metal ions onto the adsorbent surface contributed to theoverall metal uptake process by the wheat straw, and the wheat strawhad a higher affinity for Zn2+. Also, the higher initial rates (over the first30min) for Zn2+ than those for Ni2+ reiterated that the biosorbent'saffinity to a specific adsorbate differentiated the initial metal uptake ofNi2+ and Zn2+. It is also relevant to note that after 180min (t0.5 of 13.4)the concentration of themetal remaining in the solution started to level

n time for a single-metal system at 25 °C.

Fig. 11. Variation of Zn2+ uptake with adsorption time for a single-metal system at 25 °C.

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off, indicating that the surface adsorption was the predominant step inthe later stage of the overall adsorption process when most ofadsorption sites had been occupied by metal ions and the adsorptionwas close to exhaustion.

5. Conclusions

The biosorption of Ni2+ and Zn2+ on wheat straw was studied overthe initial metal concentration range of 5 to 150 ppm. The adsorptionequilibriumwas achieved in 2.5 to 5h for Ni2+ and about 3h for Zn2+ atall initial concentrations. The percentage metal removal of above 80%was obtained. In addition, at a low initial concentration, wheat strawwas capable of reducing the metal concentration down to less than1 ppm,which is rather difficult to achievewith other techniques.Wheatstraw can thus be considered as a potential adsorbent for the treatmentof wastewater containing heavy metal ions.

For single-metal solutions, the Freundlich isotherm fits to theexperimental data for both Ni2+ and Zn2+ better than the Langmuirmodel. For the bimetal solutions, the IAST-Freundlich multicompo-nent isotherm appeared to be themost suitablemodel among the fourmulticomponent isotherms, namely the Langmuir multicomponentisotherm, the Langmuir partially competitive isotherm, the Freundlichmulticomponent isotherm and the IAST-Freundlich isotherm, torepresent the biosorption of Ni2+ and Zn2+ in a binary system.

The thermodynamic evaluation of the biosorption of Ni2+ and Zn2+

onto wheat straw showed that the process was spontaneous at a lowinitial concentration of 5 ppm. The spontaneity decreased with themetal concentration. For initial concentrations higher than 50 ppm, themetal removal processbecamenon-spontaneous. Also, theoverallmetaladsorption process bywheat strawwas found to be contributed by boththe external mass transfer and the surface adsorption. The affinity ofwheat straw to a metal played a significant role in differentiating themetal uptake for Ni2+ and Zn2+.

Notationsaij, aji Competition coefficients for the binary systembi, bj The Langmuir adsorption constants of the metal ions in the

single-metal systemsC0 Initial metal concentration in the solution (mg L−1)Ce Metal concentration remaining in the solution at equilibrium

(mg L−1)

Ci, Cj Concentrations of the metal species remaining in liquid atequilibrium (mg L−1)

K The pseudo-second-order kinetics rate constant (g mg−1

min−1)ki The diffusion rate constant (mg g−1 min−0.5)KL The Langmuir biosorption constant (L mg−1)KF The Freundlich biosorption constant (mg g−1)1/n The Freundlich biosorption intensity (L g−1)qe The equilibrium uptake of metal ions (mg g−1)qL Themaximummetal uptake (biosorption capacity) (mg g−1)qt The metal uptake at a given time (mg g−1)qe,i The equilibrium uptake of a species “i” (mg g−1)qio The Langmuir adsorption capacity of the species “i” (mg g−1)

R Gas constant (8.314 J mol−1 K−1)T Temperature (K)V The volume of solution (L)ΔG Gibbs free energy change (J mol−1)

Acknowledgement

Financial support from the Natural Sciences and EngineeringResearchCouncil of Canada (NSERC) to this project is highly appreciated.

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