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Chemical Engineering Journal 175 (2011) 251– 259

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

emoval of Cd(II) and Ni(II) from aqueous solution by PVA/TEOS/TMPTMS hybridembrane

ohammad Irania, Ali Reza Keshtkarb,∗, Mohammad Ali Mousaviana

Department of Chemical Engineering, Faculty of Engineering, University of Tehran, Tehran, IranNuclear Fuel Cycle School, Nuclear Science and Technology Research Institute, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 30 July 2011eceived in revised form9 September 2011ccepted 21 September 2011

eywords:

a b s t r a c t

Functionalized poly(vinyl alcohol)/tetraethyl orthosilicate (PVA/TEOS) hybrid membranes with 3-mercaptopropyltrimethoxysilane (TMPTMS) groups have been prepared by sol–gel method and studiedfor the removal of cadmium and nickel ions from the aqueous solutions. The prepared membraneswere characterized by FTIR, SEM and BET analyses. The influence of several variables such as TEOScontent, TMPTMS content, pH, contact time, initial concentration and temperature was studied in abatch mode. The kinetic data were found to follow the pseudo-second-order model for both metal ions.

ybrid membrane-Mercaptopropyltrimethoxysilaneol–geladmiumickeldsorption

The equilibrium data were well described by the Freundlich isotherm model for cadmium and nickelions, respectively. The selectivity of cadmium and nickel sorption onto the membrane was in order ofCd(II) > Ni(II). The maximum monolayer capacity of the hybrid membrane was found to be 61.43 and10.29 mg g−1 for cadmium and nickel ions, respectively, at 45 ◦C. Thermodynamic parameters were eval-uated to understand the nature of adsorption process for both metal ions. The reusability of the membranewas also determined after five sorption–desorption cycles.

. Introduction

Nickel and cadmium wastewater is a very important factorhat affects the environment and human health. Furthermore,ickel and cadmium are widely used in industrial applications. The

ncrease in concentration of nickel and cadmium in rivers, groundater, etc. endangers man and animals [1]. Nickel and cadmium is

bserved in a variety of industrial wastes such as nickel–cadmiumatteries, organic chemicals, and pesticides [2]. Several methods

ncluding chemical precipitation [3], ion-exchange sorption [4] andolvent extraction [5,6] have been used to remove the heavy metalrom wastewater. Among all, ion-exchange sorption mechanismas been widely used for the removal of heavy metal ions fromqueous solutions, because this method is low-cost and efficientor the removal of nickel and cadmium from wastewater [4,7].

In most recent researches [8–11], the heavy metal ions wereemoved from water and wastewater using ion exchange withesins. Xiong et al. [12] have used gel-type weak acid resin for theemoval of cadmium from aqueous solutions. By comparing ion

xchange resins with ion exchange hybrid membranes, we inferhat ion exchange hybrid membranes can be a good replacementor ion exchange resins. This is because of their structural flexibility

∗ Corresponding author. Tel.: +98 021 82064399; fax: +98 021 88221127.E-mail address: akeshtkar@aeoi.org.ir (A.R. Keshtkar).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.09.102

© 2011 Elsevier B.V. All rights reserved.

and functional group performance. Moreover, some limitations arecaused by the resins such as high pressure drop and nonuniformpacking [13,14].

Silica-based adsorbents were widely used in the removal ofheavy metal ions; it is due to the large specific surface area andwell modified surface properties; moreover, it can be reused forseveral times without any significant loss in adsorption perfor-mance [15,16]. Additionally, the removal efficiency of metal ionsincreases remarkably after the adsorbent surface has been modifiedby the functional groups such as –NH2, –SH and –S– groups [15,17].Therefore, ion-exchange silica membranes can be a good choice forthe adsorption of heavy metal from aqueous solutions. Poly(vinylalcohol)/tetraethyl orthosilicate (PVA/TEOS) ion exchange silicamembrane can be used as a membrane adsorber for the removal ofheavy metals from wastewater. Traditionally, Kim et al. [18], haveused PVA/TEOS hybrid membrane, prepared by sol–gel method, formethanol fuel cell applications, but there is no study on the sorptionof nickel and cadmium onto the PVA/TEOS hybrid membranes.

In this study TEOS was used as a silica precursor andPVA/TEOS ion-exchange hybrid membrane was preparedby the sol–gel method for the adsorption of nickel andcadmium ions from the aqueous solutions. Furthermore, 3-mercaptopropyltrimethoxysilane (TMPTMS) as –SH functional

group was added to the PVA/TEOS hybrid membrane for themodification of the adsorbent surface. Also the effects of bothTEOS and TMPTMS contents, pH, contact time, initial con-centration and temperature on the adsorption process were

252 M. Irani et al. / Chemical Engineering Journal 175 (2011) 251– 259

iLisptomr

2

2

(bS

siT

2

TPbiimt

dlm

Fig. 1. Formula structures of (a) PVA, (b) TEOS and (c) TMPTMS.

nvestigated. The equilibrium data were fitted to the Freundlich,angmuir and Dubinin–Radushkevich isotherm models. In order tonvestigate the sorption behavior, pseudo-first-order and pseudo-econd-order kinetic models were studied. The thermodynamicarameters were evaluated for nickel and cadmium sorption ontohe PVA/TEOS/TMPTMS hybrid membranes. Desorption behaviorf nickel and cadmium ions on the PVA/TEOS/TMPTMS hybridembrane was produced by 0.5 M HNO3/0.1 M HCl in the equal

atio solution.

. Experimental

.1. Materials

PVA polymer (99% hydrolyzed, average MW = 72,000), TEOSdensity = 940 kg m−3), TMPTMS (99%), cetyltrimethylammoniumromide (CTAB), ethanol, HNO3 and HCl were provided byigma–Aldrich. Deionized water was used throughout this work.

The solutions of cadmium and nickel ions were prepared by dis-olving weighed amounts of cadmium and nickel nitrates (Aldrich)n deionized water. The chemical structures of PVA, TEOS andMPTMS are presented in Fig. 1

.2. Preparation of membranes

In this study the PVA membrane, PVA/TEOS and PVA/TEOS/MPTMS hybrid membranes were prepared. For the preparation ofVA membrane, first, aqueous 10 wt% PVA solutions were providedy dissolving 10 g of PVA in 100 ml deionized water and then reflux-

ng at 80 ◦C for 4 h. Then, the homogeneous solution was pourednto the Petri dish and was allowed to be dried for 4 days. The dry

embranes were peeled from the Petri dish and were subjected tohermal treatment for 1 h at 80 ◦C.

The PVA/TEOS hybrid membrane, using the PVA and sol–gelerived silica, was prepared by dissolving PVA in water, and was fol-

owed by the addition of the TEOS mixture to the solution. The TEOSixture was prepared by mixing CTAB:TEOS:HCl:H2O:ethanol in

Fig. 2. The schematic diagram for the preparation of PVA/TEOS/TMPTMS hybrid.

the molar ratio of 1:4:0.1:200:50. First, CTAB was mixed with waterand ethanol for 10 min at 30 ◦C. Second, TEOS was added to thesolution and stirred continuously for 1 h at 30 ◦C. Third, HCl wasdropped slowly into the solution and was stirred for 2 h at 30 ◦Cto prepare the TEOS mixture. Fourth TEOS mixture was added tothe PVA solution and the PVA/TEOS mixture was stirred at 60 ◦C foranother 4 h. Then, the solution was poured into the Petri dish, andwas allowed to be dried for 4 days. Finally, the prepared membraneswere heated in a vacuum oven at 120 ◦C for 1 h to evaporate the sol-vent completely. For comparative TEOS contents, four PVA/TEOShybrid membranes with different TEOS contents were prepared inthe same way.

For preparation of PVA/TEOS/TMPTMS hybrid membrane,TMPTMS was added to the TEOS stirred mixture and the stirring wascontinued at 30 ◦C for 1 h. Then the prepared solution was droppedslowly into the PVA solution and was mixed at 60 ◦C for extra 4 h.Finally, the homogeneous solution was placed at room tempera-ture for 4 days and then heated in a vacuum oven at 120 ◦C for 1 hto evaporate the solvent completely. For comparative sulfur con-tents, four PVA/TEOS/TMPTMS hybrid membranes with differentTMPTMS contents were prepared in the same way. The schematicdiagram for the preparation of PVA/TEOS/TMPTMS hybrids is illus-trated in Fig. 2.

2.3. Measurements and methods

The functional groups of the PVA, PVA/TEOS andPVA/TEOS/TMPTMS membranes were determined by a FourierTransform Inferred Spectrometer (Vector22-Bruker Company,Germany) in the range of 400–4000 cm−1. The morphologicalanalyses of the selected membranes were characterized using ascanning electron microscope (SEM, JEOL JSM-6380). The averagepore diameter, specific surface area and pore volume of the pre-pared membranes were measured by the Brunauer–Emmett–Teller(BET) method. A pH meter (827-pH lab, Switzerland) was used forthe measurement of pH solutions. The concentration of cadmiumand nickel ions in the adsorption medium was determined usingan inductivity coupled plasma atomic emission spectrophotome-ter (ICP-AES, Thermo Jarrel Ash, Model Trace Scan). Analyticalwavelengths were set at 214.438 and 231.604 nm for cadmiumand nickel ions, respectively.

2.4. Determination of the point of the zero charge (pHpzc)

The pHpzc for the PVA/TEOS20%/TMPTMS15% hybrid membranewas determined by the following method. 50 ml of 0.1 M NaCl wastransferred in series of flasks; the pH of solutions was adjusted in

M. Irani et al. / Chemical Engineering Journal 175 (2011) 251– 259 253

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2

ftTtwwtodtortpcr4r

q

wiVm

3

3

Tsb

Table 1Physical properties of prepared membranes.

Sample SBET (m2 g−1) Pore volume(cm3 g−1)

Average porediameter (nm)

PVA/TEOS20% 228.2 0.435 2.82PVA/TEOS40% 176.4 0.298 2.51

ig. 3. FTIR Spectra of (a) PVA membrane, (b) PVA/TEOS20% and (c)VA/TEOS20%/TMPTMS15% hybrid membranes.

he range of 2–7 by adding 0.1 M HCl or/and 0.1 M NaOH solutions.hen 0.1 g hybrid membrane was added on the solution. After that,he solutions were shaken for 5 days at 25 ◦C. Finally, the pH of theolutions was measured. pHpzc was reported at the pH in which thenitial pH equals the final pH. Similar method is applied by Hameedt al. [19] for determining pHpzc.

.5. Batch adsorption experiments

The adsorption of cadmium and nickel ions was studied as aunction of pH, contact time, initial concentration and tempera-ure. Adsorption processes were carried out by a batch method.he effect of pH of the solution on the cadmium and nickel sorp-ion was studied in the range of 2–7. The initial pH of the solutionas adjusted with 0.1 M HCl and/or 0.1 M NaOH. The experimentsere carried out in 250 ml Erlenmeyer flasks containing 0.1 g of

he adsorbent in 100 ml of nickel and cadmium solutions at 25 ◦Cn a rotary shaker at 150 rpm for 4 h at the different pH levels. Foretermining the effect of contact time on cadmium and nickel sorp-ion, experiments were done by placing 0.1 g of adsorbent in 100 mlf metal solutions (100 mg L−1) at 25 ◦C and the optimum pH on aotary shaker at 150 rpm at definite time intervals. For examininghe effect of initial concentration of nickel and cadmium and tem-erature, 0.1 g of the membrane samples was rinsed in 100 ml ofadmium and nickel solutions with concentrations varying in theange of 10–500 mg L−1 at three different temperatures (25, 35 and5 ◦C) for 4 h. Each experiment was repeated three times and theesults were given as averages.

The amount of metal adsorbed was calculated as follows:

e = (C0 − Ce)V1000M

(1)

here qe is the adsorption capacity in mg g−1, C0 and Ce are thenitial, and equilibrium concentrations of metal solution in mg L−1,

is the volume of the solution in ml and M is the weight of the dryembrane in g.

. Results

.1. Characterization of the prepared membranes

The functional groups of samples are characterized by Fourierransform Infrared (FTIR) and the results are shown in Fig. 3. Theamples contain pure PVA membrane, PVA/TEOS20% hybrid mem-rane and PVA/TEOS20%/TMPTMS15% hybrid membrane. In all the

PVA/TEOS20%/TMPTMS5% 182.3 0.362 2.59PVA/TEOS20%/TMPTMS15% 164.7 0.305 2.11PVA/TEOS20%/TMPTMS20% 151.9 0.261 1.99

samples, there is a broad bond at around 3100–3600 cm−1, whichis assigned to O–H stretching for the hydrogen bonded hydroxylgroups which is present in the samples. The peak at around2850–2950 cm−1 is due to the alkyl stretching groups. The C–O estergroup peak was observed around 920–950 cm−1; this peak wascaused by the etherification reactions in some vicinal –OH groupsduring the dissolution of PVA in boiling water [20]. For silica mate-rial spectra, the broad bonds appeared around 1080–1200 cm−1

due to the siloxane bonds (Si–O–Si) which were observed in thespectra of the PVA/TEOS and PVA/TEOS/TMPTS hybrid membranes;moreover, the vibrations of Si–OH were observed in 1653 and3400 cm−1 indicating uncondensed silanols in the structure ofPVA/TEOS20% and PVA/TEOS20%/TMPTMS15% hybrid membranes.A weak S–H stretching peak was seen in 2550–2580 cm−1 whichindicates that mercapto groups have been successfully added tothe mesoporous silica skeleton of PVA/TEOS/TMPTMS hybrid mem-brane.

The effects of TEOS and TMPTMS weight percentages in hybridmembranes were studied by scanning electron micrographs (SEMs)and the results are presented in Fig. 4. The morphology of thepure PVA membrane is shown in Fig. 4a. As can be seen, thesurface of the pure PVA membrane is compact and smoothwith no aggregation of different components. SEM images of thePVA/TEOS hybrid membranes with different TEOS contents of 20and 40 wt% are shown in Fig. 4b and c. The SEM images clearlyshow that the hybrid membrane with 20 wt% TEOS content is moreuniformly dispersed in smaller size compared with the hybridmembrane with 40 wt% TEOS. Fig. 4d and e shows the morphol-ogy of PVA/TEOS20%/TMPTMS15% and PVA/TEOS20%/TMPTMS20%hybrid membranes. It can be seen that the higher TMPTMS con-centration is disadvantageous to the membrane homogeneity. AlsoSEM image of PVA/TEOS20%/TMPTMS15% hybrid membrane aftercadmium adsorption is shown in Fig. 4f. The change in the sur-face of the PVA/TEOS20%TMPTMS15% indicates that cadmium ionssuccessfully interact with the functional groups of membrane.

In Fig. 5 the pore size distribution of PVA/TEOS40% membranewas compared with the curve for PVA/TEOS20% membrane. Itcan be seen that the peak position of PVA/TEOS40% membraneshifted toward the lower pore diameter in comparison with thePVA/TEOS20% membrane. Also, according to the results observedin Table 1, the SBET, pore volume and average pore diameter ofPVA/TEOS20%/TMPTMS with the increase in percentage of TMPTMSis decreased. Increasing of sulfur atoms of mercapto groups on thesurface of the membranes gradually decreases the adsorption ofnitrogen molecules.

3.2. Effect of TEOS weight percentage in PVA/TEOS hybridmembrane

To clarify the effect of TEOS on adsorption capacity of nickel andcadmium ions onto the fabricated membranes, PVA/TEOS hybridmembranes were sensitized at TEOS amounts 10, 20, 30 and 40 wt%

with respect to the polymer weight. In Fig. 6 the adsorption capac-ity of nickel and cadmium ions onto the hybrid membranes with apH of 5, initial concentration 100 mg L−1, adsorbent concentration1 g L−1 and at 25 ◦C was shown. It can be seen that the adsorption

254 M. Irani et al. / Chemical Engineering Journal 175 (2011) 251– 259

Fig. 4. The SEM graphs of membranes (a) PVA membrane, (b) PVA/TEOS20%, (c) PVA/TEOS40%, (d) PVA/TEOS20%/TMPTMS15%, (e) PVA/TEOS20%/TMPTMS20% and (f)PVA/TEOS20%/TMPTMS15% hybrid membrane after adsorption of cadmium.

Fig. 5. BJH desorption pore size distribution plot for the prepared membranes.Fig. 6. Effect of the TEOS weight percentage in PVA/TEOS hybrid membrane foradsorption of nickel and cadmium.

M. Irani et al. / Chemical Engineering Journal 175 (2011) 251– 259 255

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PotccTcTlbtpatwa–tsgif

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ig. 7. Effect of the TMPTMS weight percentage in PVA/TEOS20%/TMPTMS hybridembrane for adsorption of nickel and cadmium.

apacity of nickel and cadmium ions increases with the decreasef TEOS amount up to 20%. Further increase in TEOS amount leadso decrease in adsorption capacity of hybrid membranes for both

etal ions. This can be attributed to the agglomeration of SiO2articles which decreases the available surface of adsorption. Fur-hermore, the agglomeration of particles is caused to the metal ionsere difficulty diffused into the pores onto the internal surface of

he membrane. Similar trends were reported by other researchers21,22]. Therefore, PVA/TEOS20% membrane was used in the nexttage of adsorption study.

.3. Effect of TMPTMS weight percentage inVA/TEOS20%/TMPTMS hybrid membrane

In this section, PVA/TEOS20% membrane is functionalized withhe mercapto groups. The sulfur atoms of the mercapto groups have

high affinity to interact with metal ions which causes an increasen adsorption capacity of metal ions by the prepared membrane.

For optimization amount of TMPTMS into theVA/TEOS20%/TMPTMS hybrid membranes, 5, 10, 15 and 20 wt%f TMPTMS with respect to the polymer weight are added intohe fabricated hybrid membranes and the adsorption capacity ofadmium and nickel ions are studied with a pH of 5, initial con-entration 100 mg L−1, adsorbent concentration 1 g L−1, and 25 ◦C.he results are shown in Fig. 7. It can be seen that the adsorptionapacity of nickel and cadmium ions increases with the increase ofMPTMS amount up to 15%; further increase in TMPTMS amounteads to the decrease of adsorption capacity of the hybrid mem-ranes for both metal ions. Increasing adsorption capacity withhe increase of TMPTMS is due to the unique large surface, regularore structure and well modified surface properties. Reduction indsorption capacity of nickel and cadmium ions in mercapto higherhan 15% can be due to decrease in surface area and pore diameterhich reduce the available active sites for adsorption process. Xue

nd Li [15] used functionalized SBA-16 mesoporous silica withSH groups for the removal of Cu(II) from aqueous solution andhe results showed that with the further addition of TMPTMS, theurface area and pore diameter of the mesoporous silica decreasedradually. Therefore, the optimum weight percentage of TMPTMSn the PVA/TEOS20% hybrid membrane is selected as 15% forurther experiments.

.4. Effect of pH

In the adsorption process, the pH of solution is verymportant variable that affects the metal sorption by proto-ation/deprotonation of the functional groups of adsorbent.

Fig. 8. Effect of pH on the nickel and cadmium sorption onto thePVA/TEOS20%/TMPTMS15% membrane.

Consequently, the effect of pH on the nickel and cadmium sorptiononto the hybrid membrane is investigated in the pH range of 2–7for the initial concentration of 100 mg L−1, adsorbent concentration1 g L−1 and 25 ◦C. The results are presented in Fig. 8.

Since at pH more than 7, precipitation occurs in the cadmiumand nickel solutions [23,24], the experiments are not conductedbeyond a pH of 7. As can be seen from Fig. 8, the adsorption capacityincreases with increasing pH for nickel and cadmium ions; more-over, with a pH of 5, the adsorption capacity reaches the maximumvalues of 24.88 and 5.9 mg g−1 for cadmium and nickel, respec-tively, and then declines with the increase of pH values for bothmetal ions. It seems that the –SH groups of the membrane hasa stronger affinity for chelating with cadmium ions in compar-ison with nickel ions. The low adsorption capacity of cadmiumand nickel at lower pH values is due to the protonation of thesilane and mercapto groups which reduces the number of activesites of the hybrid membrane for adsorption of the metals. Withthe increase of pH values, the positive charge density of the func-tional groups is decreased which leads to the increase of adsorptioncapacity of nickel and cadmium onto the membrane. The effect ofpH on the cadmium and nickel adsorption could be also explainedwith calculation of pHpzc value of the membrane which is obtained3.1. At pH < pHpzc the membrane surface is positive. There is anelectrostatic repulsion between positive charge membrane sur-face and the metal ions, which leads to the lower sorption. WhenpH > pHpzc, the membrane surface becomes negatively charged; itcauses more attraction of the metal ions into the surface mem-brane and increases the adsorption capacity of cadmium and nickel,respectively. The decrease in the adsorption cadmium and nickelions at pH more than 5 is due to the formation of hydroxylatedcomplexes of the cadmium and nickel ions (cadmium in the formof Cd(OH)2 and nickel in the form of Ni(OH)2) that ruins the surfaceof the membrane [12]. Consequently, the optimum pH for furtheradsorption studies is selected as pH 5.

3.5. Effect of contact time and kinetic models

The effect of contact time is very important in modeling anddesigning the adsorption process in industry. Therefore, the effectof contact time on cadmium and nickel sorption onto the hybridmembrane in the initial concentration 100 mg L−1 and 25 ◦C isshown in Fig. 9. As can be seen from Fig. 9 that the adsorptionof nickel and cadmium ions into the membrane reaches the equi-librium time after 4 h. More than 90% of the total adsorption of

cadmium and nickel occurs within the first 2 h. The fast sorptionat the first 2 h is due to a large number of vacant surface sites ofthe membrane which is available for the adsorption of cadmiumand nickel. After 2 h almost the active external sites are saturated,

256 M. Irani et al. / Chemical Engineering Journal 175 (2011) 251– 259

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aine

toisctTo

oofmo

soh

g

q

wtp

i

q

wpea

TK

ig. 9. Adsorption kinetics of nickel and cadmium onto theVA/TEOS20%/TMPTMS15% membrane.

nd the adsorption process needs more time to take place by activentra-particular sites. After 4 h, almost all of the internal and exter-al active sites are saturated and the system reaches the sorptionquilibrium.

As can be seen in Fig. 9, the removal of cadmium was higherhan that of nickel ions by the membrane. The electronegativityf nickel is 1.91, which is higher than cadmium electronegativ-ty of 1.69. The PVA/TEOS20%/TMPTMS15% hybrid membrane hasiloxane, silanol and mercapto groups (negative sites). The electronlouds on oxygen, silicon and sulfur atoms of those groups tendo repel the species with a higher electronegativity more forcibly.hus, nickel might be repelled slightly more by the negative sitesn the membrane.

In addition, from the mass transfer standpoint, the diffusivityf cadmium (7.19 × 10−6 cm2 s−1) was about 17% higher than thatf nickel (6.13 × 10−6 cm2 s−1) [25,26]. Therefore, the mass trans-er rate of cadmium from the bulk liquid to the surface of the

embrane was higher than that of nickel; hence, the adsorptionf cadmium was enhanced.

Kinetic models, namely the pseudo-first-order and pseudo-econd-order models are used to describe the adsorption kineticsf nickel and cadmium ions onto the PVA/TEOS20%/TMPTMS15%ybrid membrane.

The pseudo-first-order kinetic model by the Lagergren [27] isiven as:

t = qe(1 − exp(−k1t)) (2)

here qt and qe (mg g−1) are the amounts of metal adsorbed atime t and the equilibrium time, respectively, and k1 (min−1) is theseudo-first-order model rate constant.

The pseudo-second-order kinetic model by Ho and McKay [28]s given as:

t = k2q2e t

1 + k2qet(3)

−1 −1

here k2 is the adsorption rate constant (g mg min ). Kineticarameters of pseudo-first-order and pseudo-second-order mod-ls were obtained from the plot of qt versus t (Fig. 9), and the resultsre presented in Table 2.

able 2inetic parameters of metal sorption onto the PVA/TEOS20%/TMPTMS15% hybrid membr

Metal qe (mg g−1) Pseudo-first-order model

qe (mg g−1) k1 (min−1)

Cadmium 24.88 24.21 0.02949

Nickel 5.90 5.83 0.02215

Fig. 10. Freundlich isotherm plots for nickel and cadmium sorption onto thePVA/TEOS20%/TMPTMS15% membrane.

By contrasting correlation coefficient for pseudo-first-order(R2 > 0.992) and pseudo-second-order (R2 > 0.997), it was foundthat the pseudo-second-order model fitted better than pseudo-first-order model with kinetic data of nickel and cadmiumsolutions.

The equilibrium data of cadmium and nickel ions sorption ontothe PVA/TEOS20%/TMPTMS15% hybrid membrane at three differenttemperatures (25, 35, and 45 ◦C) have been described by isothermmodels, namely Freundlich, Langmuir and Dubbin–Radushkevich(D–R).

Freundlich isotherm is an equation which indicates heteroge-neous surface adsorption with nonuniform energies of active sites.This model can be expressed as follows [29]:

qe = kFC1/ne (4)

where kF (mg g−1) and n are Freundlich parameters related to thesorption capacity and the sorption intensity. The parameters of theFreundlich model were calculated by plotting qe versus Ce at dif-ferent temperatures (Fig. 10). The results are presented in Table 3.It can be seen from Table 3 that values of 1/n less than one at allthree temperatures indicate the favorability of nickel and cadmiumsorption onto the membrane. The values of correlation coefficientgreater than 0.978 show that the Freundlich isotherm fitted wellwith equilibrium data of nickel and cadmium ions at different stud-ied temperatures.

Langmuir isotherm model is an equation which indicates ahomogeneous surface adsorption with uniform energies of activesites. This model can be expressed as follows [30]:

qe = qmbCe

1 + bCe(5)

where qm (mg g−1) and b (mg−1) are the Langmuir model constants.qm is the maximum value of metal ion adsorption per unit weightof membrane that is related to the monolayer adsorption capac-ity and b represents the enthalpy of adsorption. The parametersof the Langmuir model were calculated by plotting qe versus Ce atdifferent temperatures (25, 35 and 45 ◦C), the results of which are

presented in Table 3. It can be seen that the maximum adsorp-tion capacity of the hybrid membrane increases from 55.05 to61.43 mg g−1 for cadmium ions and from 10.29 to 10.39 mg g−1 fornickel ions with the increase of the temperature from 25 to 45 ◦C.

ane.

Pseudo-second-order model

R2 qe (mg g−1) k2 (g mg−1 min−1) R2

0.993 24.35 0.00139 0.9970.998 6.09 0.00401 0.998

M. Irani et al. / Chemical Engineering Journal 175 (2011) 251– 259 257

Table 3Isotherm parameters for metal adsorption onto the PVA/TEOS20%/TMPTMS15% hybrid membrane at different temperatures.

Metal T (◦C) Freundlich isotherm Langmuir isotherm D–R isotherm

kF (mg g−1) n R2 qmax (mg g−1) kL (L mg−1) R2 qDR (mmol g−1) BDR (mol2 J−2) R2

Cadmium 25 4.593 2.911 0.979 55.05 0.01131 0.954 0.2837 7.259 × 10−9 0.87935 4.094 2.891 0.978 59.12 0.01136 0.961 0.3030 7.171 × 10−9 0.89245 3.863 2.902 0.980 61.43 0.01145 0.963 0.3151 7.038 × 10−9 0.904

−8

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q

wcRtca

b

E

Tnoaw8fmnt

tePm(

3

wo

TT

Nickel 25 1.501 3.267 0.987 10.29

35 1.837 3.618 0.986 10.37

45 2.070 3.893 0.984 10.39

y comparing the qm values obtained from the Langmuir model, its observed that cadmium adsorption on the hybrid membrane is

ore than fivefold greater to that of nickel ions, which indicateshat the functional groups on the surface of the hybrid membraneas a relatively stronger affinity for cadmium ions than nickel ionsnd the potential of the adsorption for cadmium and nickel ions onhe hybrid membrane is in the following order: cadmium > nickel.

The D–R isotherm model is expressed as [31]:

e = qDR exp(−BDRε2DR) (6)

here qDR (mg g−1) and BDR (mol2 J−2) are the D–R isothermonstants and εDR is the Polanyi potential that is equal toT ln(1 + (1/Ce)). R is the gas constant (8.314 J mol−1 K−1) and T ishe absolute temperature (K). Parameters of the D–R model werealculated by plotting qe versus Ce at different temperatures (25, 35nd 45 ◦C) and the results are presented in Table 3.

The value of BDR is related to the adsorption free energy that cane calculated from the following equation:

= 1√2BDR

(7)

he value of free energy determines the type of adsorption mecha-ism. Physisorption processes have adsorption energy in the rangef 1–8 kJ mol−1, if E value lies between 8 and 16 kJ mol−1, thedsorption process is ion exchange [32]. The adsorption free energyas calculated from 3.9 kJ mol−1 to 4.83 kJ mol−1 and from 8.30 to

.43 kJ mol−1 with the increase at temperature from 25 ◦C to 45 ◦Cor the adsorption of nickel and cadmium ions onto the hybrid

embrane, respectively. These results indicate that the mecha-ism of nickel sorption onto the membrane is physisorption andhe adsorption of cadmium onto the membrane is ion exchange.

By comparing the correlation coefficients, it was foundhat the Freundlich isotherm model (R2 > 0.979) fitted thequilibrium data of nickel and cadmium sorption onto theVA/TEOS20%/TMPTMS15% hybrid membrane better than Lang-uir isotherm model (R2 > 0.915) and D–R isotherm model

R2 > 0.816).

.6. Thermodynamics of adsorption

Temperature is an important factor in the adsorption processhich determines the nature of the adsorption with the calculation

f the thermodynamic parameters.

able 4hermodynamic parameters for metal adsorption onto the PVA/TEOS20%/TMPTMS15% hy

Metal kC �H◦ (kJ mol−1)

25 ◦C 35 ◦C 45 ◦C

Cadmium 12.91 14.56 16.29 9.170

Nickel 0.413 0.554 0.654 18.125

0.01711 0.948 0.1271 3.287 × 10 0.8160.02164 0.926 0.1315 2.527 × 10−8 0.8790.02548 0.915 0. 1359 2.143 × 10−8 0.862

The Gibbs free energy change of adsorption process is calculatedby the following equation:

�G◦ = −RT ln kC (8)

where kC is the adsorption equilibrium constant which can be cal-culated using the following equation:

kC = limCel→0

Ces

Cel(9)

where Ces and Cel are the values of solid phase concentration and liq-uid phase concentration at equilibrium in mg L−1. The values of kCat different temperatures (25–45 ◦C) were obtained from the plotof the experimental data of Ces/Cel versus Cel according to Eq. (9)(figure is not presented). Then according to Eq. (8), the Gibbs freeenergy was calculated and the results were presented in Table 4for both metal ions. It can be seen that the negative values of theGibbs free energy changes indicate the feasibility and spontaneousnature of cadmium sorption onto the hybrid membrane. Also themore negative value of (�G◦) by raising the temperature indicatesthat the higher temperature proceed earlier than the adsorptionof cadmium ions by the membrane. Furthermore, the positive val-ues of the Gibbs free energy changes for nickel ions show that theadsorption of nickel onto the membrane is not spontaneous.

The enthalpy change (�H◦) and entropy change (�S◦) weredetermined from the Van’t Hoff equation:

ln kC = �S◦

R− �H◦

RT(10)

�H◦ and �S◦ were obtained from the slope and intercept of ln kCversus 1/T plot whose results are listed in Table 4. As can bepresented in Table 4, the positive values of enthalpy change forboth metal ions imply that the adsorption of nickel and cadmiumby PVA/TEOS20%/TMPTMS15% hybrid membrane is endothermic.The entropy change of the process was found to be 0.053 and0.052 kJ mol−1 K−1 for the adsorption of nickel and cadmium ions,respectively. These results indicate the increased disorder and ran-domness at the solid-solution interface of nickel and cadmium ionswith the membrane.

3.7. Regeneration of membrane

The regeneration of the adsorbent is a very important inthe adsorption process, because the reuse of the adsorbent is

a key factor in improving process economics. Therefore, fivecycles of adsorption–desorption of nickel and cadmium ions ontoPVA/TEOS20%/TMPTMS15% hybrid membrane were gone throughin the initial metal ion concentration of 100 mg L−1 and 25 ◦C. The

brid membrane.

�S◦ (kJ mol−1 K−1) �G◦ (kJ mol−1)

25 ◦C 35 ◦C 45 ◦C

0.052 −6.338 −6.858 −7.3780.053 2.188 1.515 1.123

258 M. Irani et al. / Chemical Engineering

FH

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4

poodstTPcpeiwtpwTmcoscftbstfps

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ig. 11. Five cycles of nickel and cadmium adsorption–desorption with 0.5 MNO3/0.1 M HCl in equal ratio solution.

esults are shown in Fig. 11. The desorption step of metal ions fromhe membrane was treated with 0.5 M HNO3/0.1 M HCl in equalatio solution. The desorption time was fixed as 4 h throughouthe adsorption period. It can be seen that the adsorption capac-ty of cadmium and nickel ions onto the membrane from 24.88 and.9 mg g−1 in the first step decreased to 20.98 and 4.25 mg g−1 in thefth step, respectively. This shows that the hybrid membrane can beeused frequently without almost any significant loss in adsorptionerformance and can be utilized extensively in industrial activities.vidently, the reduction of the adsorption capacity of the mem-rane is because of physically losing some –SH groups by the acidleavage [16].

. Conclusions

In this study, PVA/TEOS/TMPTMS hybrid membranes were pre-ared by sol–gel method as an adsorbent to examine the removalf nickel and cadmium ions from aqueous solutions. FTIR spectraf PVA/TEOS/TMPTMS indicated that the TMPTMS have been intro-uced successfully into the PVA/TEOS structure. The BET analysishowed that the SBET, pore volume and average pore diame-er of PVA/TEOS20%/TMPTMS with the increase in percentage ofMPTMS is decreased. The results of metal ion adsorptions onto theVA/TEOS/TMPTMS hybrid membranes with different weight per-entages of TEOS and TMPTMS showed that the optimum weightercentage are 20% and 15%, respectively. The effect of param-ters such as pH (2–7), contact time (10–360 min), initial metalon concentration (10–500 mg L−1) and temperatures (25–45 ◦C)

ere investigated for removing nickel and cadmium ions fromhe aqueous solutions by PVA/TEOS20%/TMPTMS15%. Optimumarameters for nickel and cadmium sorption onto the membraneere found to be pH 5, contact time 240 min and temperature 45 ◦C.

he kinetic data were best described by the pseudo-second-orderodel (R2 > 0.997) for both metal ions. The free energy values, cal-

ulated from the D–R isotherm model, indicated that the adsorptionf cadmium onto the membrane is ion exchange and the nickelorption mechanism is physisorption. The maximum adsorptionapacity of cadmium and nickel ions onto the membrane wasound to be 61.43 and 10.39 mg g−1, respectively, which indicatedhat the adsorption of cadmium and nickel ions by the mem-rane followed the descending order: Cd(II) > Ni(II). Also the resultshowed that the Freundlich model fitted the equilibrium data bet-

er than Langmuir and Dubbin–Radushkevich isotherm modelsor both metal ions. The thermodynamic data of the adsorptionrocess showed the endothermic nature of nickel and cadmiumorption onto PVA/TEOS20%/TMPTMS15% hybrid membrane. Also,

[

Journal 175 (2011) 251– 259

the results showed that the cadmium and nickel sorption ontothe membrane is spontaneous and non-spontaneous, respectively.The PVA/TEOS20%/TMPTMS15% hybrid membrane was regener-ated with 0.5 M HNO3/0.1 M HCl in equal ratio solution and was notobserved to be significantly lost in adsorption performance afterfive adsorption–desorption cycles.

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