Desalination 259 (2010) 179–186

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Desalination

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Removal of Cr(VI) from aqueous solution by polysulfone microcapsules containingCyanex 923 as extraction reagent

Senar Ozcan ⁎, Ali Tor, Mehmet Emin AydinSelcuk University, Department of Environmental Engineering, Campus, 42031 Konya, Turkey

⁎ Corresponding author. Tel.: +90 332 223 1983; faxE-mail address: sozcan@selcuk.edu.tr (S. Ozcan).

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.04.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 January 2010Received in revised form 31 March 2010Accepted 2 April 2010Available online 7 May 2010

Keywords:MicrocapsulesCyanex 923Chromium(VI)Sorption

This paper describes the removal of Cr(VI) from aqueous solution in batch sorption technique by usingpolysulfone microcapsules containing Cyanex 923 as extraction reagent. The microcapsules containingCyanex 923 (MCs-Cyanex 923) were prepared by solvent evaporation method and characterized by usingoptical microscope, Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR)and Thermal Gravimetric Analysis (TGA). The results showed that maximum removal of Cr(VI) was obtainedwhen ratio of Cyanex 923 to polysulfone was 1.0. The complex formation between Cr(VI) and Cyanex 923favoured at initial pH of 1.0. The equilibrium time was 30 min and it obeyed the pseudo second-order kineticmodel. The Redlich Peterson and Langmuir isotherm models better represented the sorption data incomparison to Freundlich isotherm. The Langmuir sorption capacity of MCs-Cyanex 923 for Cr(VI) was0.430 mmol g−1. The MCs-Cyanex 923 preferably sorbed Cr(VI) against various metal ions, including Cr(III),Ni(II), Pb(II), Cu(II), Zn(II), Cd(II), Co(II). The regeneration studies also showed that MCs-Cyanex 923 couldbe re-used for the adsorption of Cr(VI) from aqueous solutions over 3 cycles.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The extensive use of chromium in various industries, such as leathertanning, metallurgy, electroplating, etc. has caused the release ofchromium to the environment. Cr(III) and Cr(VI) are the chromiumoxidation states usually encountered in the environment. Oxidizingpotential of the Cr(VI) species, such as HCrO4

−, Cr2O72− and CrO4

2−, makethem highly toxic to bacteria, plants and animals [1]. Human toxicity ofCr(VI) includes lung cancer, liver, kidney and gastric damage andepidermal irritation [2]. The limit for the discharge of Cr(VI) into inlandsurface waters is 0.1 mg/L. The maximum contaminant level of Cr(VI)for the drinking water is 0.05 mg/L [3]. Therefore, the removal of Cr(VI)from waters is important for environmental health.

For the treatment of aqueous solution by adsorption, activatedcarbon is the most widely used adsorbent. However, it is veryexpensive and has high operating costs [4]. Therefore, in recent years,considerable attention has been devoted to the study of differenttypes of low-cost materials in order to remove the pollutants fromaqueous solution. Many workers have employed the brown coal [5],humic acid extracted from brown coals [6], waste tyre [7] and zeolites[8], etc., for removing metals from the aqueous solution.

Liquid–liquid extraction is one of the major techniques for theremoval and recovery of metals from the aqueous solution [9].However, this method can suffer from significant drawbacks, such as

the formation of stable emulsions and disposal of solvent waste, etc. Inorder to avoid these shortcomings, alternative methods have beendeveloped by employing the solid phase instead of organic solvent,such as supported liquid membrane [10], solvent impregnated resin[11], activated composite membrane [12], chelating fiber [13],polysulfone hollow fiber membrane [14–16] and extractant micro-capsule [17].

The main disadvantage of supported liquid membrane and solventimpregnation resin is their short lifetime due to the progressiveleakage of both extractant and solvent from the membrane and resin[10,11]. The activated composite membrane has slow transportkinetics [12]. When the chelating fiber was treated with strong acidand eluted with large volumes of water, the fiber was found to swell,as resulted in the problems for re-use [13]. The synthesis of thepolysulfone hollow fiber membrane containing chelating reagent iscomplex and needs a long time [14–16].

In recent years, the polymeric microcapsules (MCs) have beenextensively used for the removal and recovery of heavy metals fromthe aqueous solutions [18–21]. The MCs correspond to a porouspolymeric matrix that contains an immobilized suitable extractionreagent, which is chosen to selectively extract the desired metals. Thepotential advantages of MCs are easy phase separation, large specificinterfacial area, minimal use of organic solvent, high selectivity andmore stability. Applications of MCs have been studied in many fields,but their potential use in hydrometallurgy is remarkable [19]. Forexample, Yang et al. [22] investigated Cu(II) sorption using poly-sulfone MCs containing Di-2-ethylhexyl phosphoric acid. The extrac-tion experiments showed that the MCs can be used for recovery metal

Fig. 1. (a) and (b) Optical microscope images of MCs-Cyanex 923 (composition ofdispersed phase: polysulfone: Cyanex 923: dichloromethane =2 g: 2 g; 50 mL,continuous phase: 1 wt.% gum arabic solution; agitation speed: 600 rpm).

180 S. Ozcan et al. / Desalination 259 (2010) 179–186

ions in dilute solutions and have a sufficient stability for repeatedprocesses. In other work, Ochoa et al. [21] reported the preparation ofpolysulfone/polyvinylpyrrolidone MCs containing Aliquat 336 for theextraction of Cr(VI) from aqueous solution. They reported that MCswith Aliquat 336 prepared from 2:1 polysulfone/polyvinylpyrrolidoneratio achieved the highest Cr(VI) extractive performance (92% of Crextraction for contact time of 60 min) and the best breakthroughpoint in column tests (up to 10 h in the first cycle).

Cyanex 923 has gained prominence as an extractant during the lastdecadebecause of better selectivity, hydrolytic stability and easier phaseseparations. Its active component is the mixture of tertiary octyl andhexyl phosphine oxides. This extractant is an effective compound toremove Cr(VI) from industrial effluents and is widely used in liquid–liquid extraction process [12,23,24]. Because it has not been studied, thiswork attempts both obtaining of polysulfone MCs containing Cyanex923 as extractant and investigation of their potential use for the removalof Cr(VI) from aqueous solution. Equilibrium and kinetics informationderived from the experimental results is also analyzed and discussed. Itis fundamental to know this information to understand the basicprinciples that govern the sorption process and for scaling it up to apractical system based on continuous columns packed with MCs.

2. Experimental

2.1. Reagents and solvent

The chemical reagents used for experiments andpreparation ofMCs-Cyanex 923 were of analytical grade. CH2Cl2, HCl, K2CrO4, CrCl3·6H2O,CuCl2·2H2O, NiCl2·6H2O, ZnCl2, CoCl2·6H2O, 3CdSO4·8H2O, Pb(NO3)2,NaOH, NaCl, were from Merck Co. (Darmstadt, Germany). Cyanex 923wasobtained fromCYTEC (Canada). Composition of theCyanex 923wasas follows: dioctyl-monohexyl phosphine oxide (40–44%), mono-octyldihexyl phosphine oxide (28–32%) and tri-n-octylphosphineoxide (12–16%). Polysulfone and gum arabic were obtained fromAldrich (Steinheim, Germany).

2.2. Preparation of MCs-Cyanex 923

The MCs-Cyanex 923 were prepared by the following proceduredescribed by Bari et al. [18]. A known amount of polysulfone anddesired amount of Cyanex 923were dissolved in CH2Cl2 as a dispersedphase. The continuous phasewas prepared by dissolving 1.0 wt.% gumarabic in 500 mL distilled water. The dispersed phase was introducedinto the continuous phase under agitation speed of 600 rpm at roomtemperature for 2 h. After, CH2Cl2 was permitted to evaporatecompletely under the same conditions, the polysulfone MCs-Cyanex923 were obtained. The resulting MCs-Cyanex 923 were washed forthree timeswith distilledwater, then desiccated at room temperature.

2.3. Characterization of MCs

TGA of the MCs was performed with SETARAM thermal gravimetricanalyzer at the temperature range of 25–800 °Cwith 10 °C/min heatingramp in argon atmosphere (gas flow rate: 20 mL/min). The surfacemorphology of samples was examined by SEM (Jeol, JSM 5310, Japan).The optical miscroscope images of MCs were obtained with an opticalmicroscope equipped with a digital camera (Nikon Eclipse E-400equiped with Nikon DS camera). FT-IR analysis was carried out by asingle channel Fourier transform spectrophotometer (Perkin-Elmer1605) atwavenumber between4000and650 cm−1with a resolutionof0.01 cm−1.

2.4. Batch sorption experiments

The Cr(VI) solutions were prepared by diluting the stock solution(1000 mg/L Cr(VI) prepared from K2CrO4) to the desired concentra-

tions. Ionic strength of all solutions was 0.01 M maintained withNaNO3. Initial pH of the solutions determined by using ionmeter(Orion EA940, USA) was adjusted to the desired level with 0.1 MNaOH or 0.1 M HCl solutions. The experiments were performed byshaking a known amount of MCs-Cyanex 923 in 20 mL of Cr(VI)solutions on a temperature controlled orbital shaker (Gallenkamp,UK) at 200 rpm and 25±1 °C for an equilibrium time. Then, thesolution were filtered through a Whatman No. 42 filter paper.

The studied parameters were initial pH of the solution (1.0–6.6),contact time (10–240 min), initial Cr(VI) concentration (0.096–6.280 mmol/L), the ratio of Cyanex 923 to polysulfone in dispersedphase (0.25–1.25 g/g) and MCs-Cyanex 923 dosage (0.5–5 g/L).Moreover, the effects of Cr(III), Ni(II), Pb(II), Cu(II), Zn(II), Cd(II), Co(II) on the sorption of Cr(VI) were investigated. The amount of Cr(VI)sorbed was calculated from Eq. (1).

q = C0−Ceð Þ:V =m ð1Þ

Where q, Cr(VI) sorbed (mmol/g); C0, initial concentration of Cr(VI) (mmol/L); Ce, concentration of Cr(VI) in solution at equilibrium(mmol/L); V, solution volume (L); m, MCs-Cyanex 923 dosage (g).

The concentration of Cr(VI) and other metal ions were determinedby using Atomic Absorption Spectrometer (ContrAA 300, AnalytikJena) in air-acetylene flame. Cr(VI) concentration in the presence of Cr(III) was determined by UV spectrophotometry [25]. Hence, Cr(III)concentration was determined by the subtracting of Cr(VI) from the

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total–chromium concentration. All experiments were carried out induplicate or triplicate and variations between replicate sampleswithin an experiment range less than 3%.

3. Results and discussion

3.1. Preparation and characterization of MCs-Cyanex 923

In the preparation of MCs, gum arabic is used as dispersant toprevent the aggregation of MCs. Bari et al. [18] reported that obtainingMCs is hard without dispersant due to the adhesive aggregation causedby the high viscosity of the polystyrene solution. They also stated idealMCs containing Cyanex 272 was obtained when 1 wt.% gum arabicsolution was used as a continuous phase. Thus, for the preparation ofMCs-Cyanex 923 in this study, the concentration of gum arabic solutionwas selected as 1 wt.%.

MCs-Cyanex 923 were prepared at various agitation speeds byusing continuous phase (1 wt.% gum arabic solution) and dispersedphase (polysulfone: 2 g; Cyanex 923: 2 g; CH2Cl2, 50 mL). Theoptimum agitation speed was determined as 600 rpm. At an agitationspeed below or above 600 rpm, the dispersed phase droplets had atrend for aggregation. Therefore, in further experiments, 600 rpmwaschosen as agitation speed for the preparation of MCs-Cyanex 923.

The optical microscope images of the MCs-Cyanex 923 show thatMCs containing Cyanex 923 are spherical (Fig. 1a,b). The diameter ofthe MCs-Cyanex 923 was determined as 273(±48) µm by taking intoaccount fifteen MCs-Cyanex 923. As seen in Fig. 2, FT-IR assignmentsof blank MCs can be summarized as follows. Band at 2967.05 cm−1

was observed due to vibrations of C–H strecthing involving methylgroup. Bands at 1583.47 and 1485.82 cm−1 were attributed to thestreching vibrations of aromatic C=C groups. Bands at 1408.52 and1363.97 cm−1 corresponded to asymetric and symetric C–H bendingdeformation of methyl group, respectively. Doublet resulting from

Fig. 2. The infrared spectrum of (a) blank MCs (containing no

asymetric O=S=O strecthing of sulfone group was seen at 1322.57and 1293.72 cm−1. Band at 1234.32 cm−1 indicated asymmetric C–O–Cstrecthing of aryl ether group. Band at 1147.28 cm−1 was attributed tosymmetric O=S=O stretching of sulfone group. Bands at 1103.13 and1078.80 cm−1 corresponded to aromatic ring vibrations. In Fig. 2a,b, thepeaks at 1205.22 cm−1 and 1206.63 cm−1, which correspond to theP=O group, demonstrated Cyanex 923 was succesfully coated on theMCs. The SEM images also indicated that coating of Cyanex 923 wasconfirmed by the morphological appearance of the MCs prepared withand without Cyanex 923 (Fig. 3a,b).

TGA and its first derivative (dTGA) curves for blank MCs and MCs-Cyanex 923 are presented in Fig. 4a,b. The blank MCs showed a weightloss of about 68% at temperatures ranging from 500 to 590 °C, mainlydue to the loss of volatiles of polysulfone. Similar result was reportedby Ball and Boettner [26] for the termal analysis of polysulfone. Theweight losses (1.01, 7.25 and16.19%) at temperature ranges of 120–160,175–275 and 280–370 °C, respectively, were due to the coated Cyanex923 on the MCs. The total weight loss (28.96%) of Cyanex 923 in thistemperature range (120–370) was in good agreement with theencapsulated amount of Cyanex 923 (26%) by MCs, prepared with thecomposition polysulfone: Cyanex 923: dichloromethane =2 g: 0.25 g:50 mL.

3.2. Sorption experiments

3.2.1. Effect of Cyanex 923 and polysulfone ratio on the sorption of Cr(VI)The effects of Cyanex 923 and polysulfone ratio on both sorption of

Cr(VI) at pH 1 and encapsulation efficiency are presented in Fig. 5. Theexperiments of this section were performed at various amounts ofCyanex 923 by keeping a constant amount of polysulfone (2 g) in 50 mLof CH2Cl2. The encapsulation efficiency can be defined as the weightratio of encapsulated extractant in the MCs. It was found thatencapsulation efficiency increased with an increase in the ratio of

Cyanex 923), (b) MCs-Cyanex 923 and (c) Cyanex 923.

Fig. 3. SEM images of (a) blank MCs and (b) MCs-Cyanex 923.

182 S. Ozcan et al. / Desalination 259 (2010) 179–186

Cyanex923 to polysulfone. Additionally, the sorption of Cr(VI) increaseswith increasing the ratio of polysulfone to Cyanex 923 up to 1.0. This isexpected because increasing the amount of Cyanex 923 increases theformation of complex between the Cr(VI) and Cyanex 923. Whilst, thesorption of Cr(VI) decreased when Cyanex 923 to polysulfone ratio wasbeyond 1.0. This result can be explained as follows: at high loading ofCyanex923 (Cyanex 923 topolysulfone ratio N1.0), crystallization of theextractant in the pore of polymeric matrix may occur and limitaccessibility to reactive sites for the interaction between the Cr(VI)and the extractant retained in the porous structure of the MCs. Similarresult was reported by Navarro et al. [27], who studied cadmiumextraction from the hydrochloric acid solutions using Amberlite XAD-7impregnated with Cyanex 921.

In addition, Cyanex 923 was coagulated when its amount wasmore than 2.5 g (Cyanex 923 to polysulfone ratio N1.25) in 50 mL ofdichloromethane. Therefore, the optimum ratio of Cyanex 923 topolysulfone was chosen as 1.0 for further experiments.

3.2.2. Effect of initial pH of solution and sorption mechanismThe chromate ions may exist in the aqueous solution in different

ionic forms (HCrO4−, CrO4

2−, Cr2O72−, HCr2O7

−), which depend on the Cr(VI) concentration and pH of the solution. The equilibrium for thespecies of Cr(VI) and related equilibrium constants (K) are presented inEqs. (2–5) [28,29].

H2CrO4⇔HCrO−4 + Hþ K1 = 1:21 ð2Þ

HCrO−4 ⇔CrO2−

4 + Hþ K2 = 3 × 10−7 ð3Þ

2HCrO−4 ⇔Cr2O

2−7 + H2O K3 = 35:5 ð4Þ

HCr2O−7 ⇔Cr2O

2−7 + Hþ K4 = 0:85 ð5Þ

According to both Eqs. (2–5) and their K values, when Cr(VI)concentration is equal or less than1x10−3 MatpH6, approximately75%of Cr(VI) exists as HCrO4

− and 25% of Cr(VI) exists as CrO42− [29]. In an

acidic solution, when Cr(VI) concentration is less than 0.02 M, HCrO4− is

the dominant species and when Cr(VI) concentration is greater than0.02 M then Cr2O7

2− is the dominant species [24]. Furthermore, Cr2O72−

converts into HCrO4− in acidic aqueous solution at a total Cr(VI)

concentration less than (1.26–1.74)×10−2 M [30]. Based on thisexplanation and Refs. [31–33], in the present study, Cr(VI) ions existas HCrO4

− in the aqueous solution because its initial concentrationranged from 9.6×10−5 to 6.28×10−3 M. Moreover, Bayrakci et al. [34]stated that themonoanion have a smaller free energy of hydration thandoes the dianionic form. It is expected that there is a smaller loss in thehydration energywhenHCrO4

− is transferred from the aqueous solutionto the extractant phase.

In order to investigate the effect of initial pH of the feed phase on thesorptionof Cr(VI)byMCs-Cyanex923, theexperimentswereperformedat the initial pH of 1.0, 2.9, 3.9, 5.1 and 6.6. A pH deviation of ±0.1was observed for each pH measurement. The results in Fig. 6 showedthat the sorption of Cr(VI) was maximum at an initial pH of 1.0. Theobtained result can be attributed to that formation of the complexbetween Cr(VI) and Cyanex 923 mostly favours at solution pH 1.0.Similar result was reported by Agrawal et al. [24], who studied theextractive removal of Cr(VI) from industrial waste solution by usingCyanex 923. Agrawal et al. [24] also pointed out that Cr(VI) wasextracted from the hydrochloric acidmedia by Cyanex 923 according tothe following reactions:

HCrO−4 + Hþ + Cl−⇔CrO3Cl

− + H2O ð6Þ

CrO3Cl− + Hþ + 2·Cyanex−923⇔HCrO3Cl·2·Cyanex−923 ð7Þ

Overall reaction can be written as follows:

HCrO−4 +2Hþ+Cl−+2·Cyanex−923⇔HCrO3Cl·2·Cyanex−923+H2O

ð8Þ

Based on this reaction, it can be stated that rate of the complexformation depends on the solution pH. A decrease in pH increases therate of the complex formation (HCrO3Cl·2·Cyanex-923) betweenHCrO4

− and Cyanex 923. Therefore, the initial pH of the solutionshould be equal to 1.0 for an efficient sorption of Cr(VI) by MCs-Cyanex 923 (see Fig. 6). After 1 h of contact time, the equilibrium pHvalues were 0.5, 2.5, 3.6, 4.8 and 6.5 for the initial pH of 1.0, 2.9, 3.9, 5.1and 6.6, respectively.

Apart from the HCl, Agrawal et al. [24] examined the other min-eral acids (i.e., H2SO4 and HNO3) in order to adjust the pH of thesolution, hence, efficiency of Cr(VI) extraction. They reported thatthe maximum extraction efficiency was obtained when HCl wasinvolved in the extraction. Therefore, in further experiments of thisstudy, the initial pH of the feed phase was adjusted to 1.0±0.1 byusing HCl.

3.2.3. Effect of contact time and kinetic evaluationIt is seen in Fig. 7 that the sorption of Cr(VI) attained equilibrium

after 30 min without dependence of initial Cr(VI) concentration.According to Barassi et al. [19], the concentration of the extractanton the surface of MCs permits the evaluation of the rate of thechemisorption by means of pseudo second-order kinetic model.

Fig. 4. TGA and its first derivatives of (a) blank MCs (polysulfone: dichloromethane=2 g: 50 mL, continuous phase: 1 wt.% gum arabic solution) and (b) MCs-Cyanex 923 (polysulfone:Cyanex 923: dichloromethane =2 g: 0.25 g: 50 mL, continuous phase: 1 wt.% gum arabic solution).

183S. Ozcan et al. / Desalination 259 (2010) 179–186

Therefore, the experimental data was applied to the pseudo second-order kinetic model given as Eq. (9) [35].

t = qt = 1= k2:q2e

� �+t = qe ð9Þ

Where qe and qt are the amounts of Cr(VI) sorbed (mmol/g) atequilibrium and time t (min), respectively, k2 is the rate constant of

pseudo second-order chemisorption (g/(mmol. min)). For the studiedconcentrations, the rate constant (k2) and theoretical equilibriumsorption capacities, qe (calculated), were calculated from the slopeand the intercept of the linear plots of the pseudo second-orderkinetic model. The coefficients of determination (R2) and theoreticaland experimental qe values suggested that the sorption of Cr(VI) byMCs-Cyanex 923 followed the second-order type reaction kinetics(Table 1).

Fig. 5. Effect of Cyanex923 to polysulfone ratio indispersedphase on the sorption of Cr(VI)and encapsulation efficiency (concentration of Cr(VI): 0.385 mmol/L, initial pHof solution:1.0(±0.1), contact time: 1 h, shaking speed: 200 rpm, temperature: 25 °C, ionic strength:0.01 M, amount of MCs-Cyanex 923: 1.5 g/L; amount of polysulfone in dispersed phase:2 g, and volume of dichloromethane: 50 mL).

Fig. 6. Effect of initial pH of the solution on the sorption of Cr(VI) (concentration of Cr(VI):0.385 mmol/L, contact time: 1 h, amount of MCs-Cyanex 923: 1.5 g/L).

Fig. 7. Effect of contact time on the sorption of Cr(VI) (concentrations of Cr(VI): 0.096,0.192 and 0.962 mmol/L, initial pH of solution: 1.0(±0.1), amount of MCs-Cyanex 923:1.5 g/L).

Table 1Values of sorption rate constant for pseudo second-order kinetic model.

Cr(VI) concentration,mmol/L

qe(experimental)(mmol/g)

k2[g/(mmol. min)]

qe(calculated)(mmol/g)

R2

0.096 0.043 15.923 0.053 0.9990.192 0.090 8.128 0.095 0.9990.962 0.320 2.805 0.321 0.999

184 S. Ozcan et al. / Desalination 259 (2010) 179–186

3.2.4. Effect of initial concentration of Cr(VI) and sorption isothermmodels

The influence of initial concentration of Cr(VI) on its removal wasexamined by varying the initial Cr(VI) concentration at pH 1.0(±0.1)and 1 h of equilibrium time. The results demonstrated that increasingthe initial concentration improved the amount of the Cr(VI) sorbed.For instance, the amounts of Cr(VI) sorbed were 0.043, 0.090 and0.305 mmol/g, for the initial concentrations of 0.096, 0.192 and0.961 mmol/L, respectively. This may be due to the increasing drivingforce for mass transfer with the concentration of Cr(VI) [36].

The experimental data was analyzed with two parametersisotherm models including Langmuir and Freundlich models andthree parameters Redlich–Peterson isotherm model. The Langmuirisothermmodels the monolayer coverage of the sorption surfaces andassumes that sorption take places on a structurally homogeneoussurface of the sorbent. This isotherm is given as Eq. (10) [37].

qe = Qo:b:Ce = 1 + b:Ceð Þ ð10Þ

The linear form of the Langmuir isotherm model can be presentedas Eq. (11);

Ce = qe = 1 =Qo:bð Þ + Ce = qeð Þ ð11Þ

Where Ce is the concentration of Cr(VI) (mmol/L) at equilibrium,Q0 is the monolayer capacity of the adsorbent (mmol/g) and b is theLangmuir sorption constant (L/mmol).

The Freundlich equation is derived to model the multilayersorption and for the sorption on heterogeneous surfaces. TheFreundlich model is formulated as Eq. (12) [38].

qe = k:C1=ne ð12Þ

Linearized form of the Freundlich equation is given by the followingequation:

log qe = log k + 1= nð Þ log Ce ð13Þ

Where Ce is the equilibrium concentration (mmol/L), k is thesorption capacity (mmol/ g) and n is an empirical parameter.

The Redlich–Peterson isotherm [39] incorporates three para-meters into an empirical isotherm, and therefore, can be appliedeither in homogeneous or heterogeneous systems due to its highversatility. The Redlich–Peterson equation is:

qe = KR:Ce = 1 + aR:Cβe

� �ð14Þ

Where, KR is Redlich–Peterson isotherm constant (L/mmol), aR isalso a constant (L/mmol)β and β is the exponent which lies betweenzero and one. This isotherm model involves varying the isothermparameter, KR to obtain the maximum (R2) value for the linearregression of ln Ce vs. ln[(KR·Ce/qe)−1]. For β=1, Eq. (14) reduces toLangmuir equation and for β=0, it reduces to Henry's equation.Eq. (14) can be converted to a linear form by taking logarithms:

ln KR:Ce = qeð Þ−1½ � = ln aR + β: lnCe ð15Þ

Fig. 9. The variation of sorption of Cr(VI) with MCs-Cyanex 923 dosage (concentrationof Cr(VI): 0.962 mmol/L, initial pH of solution: 1.0(±0.1), contact time: 1 h, amount ofMCs-Cyanex 923: 1.5 g/L).

Fig. 8. Non-linear isotherm plots for the sorption of Cr(VI) by MCs-Cyanex 923 (initialpH of solution: 1.0(±0.1), contact time: 1 h, amount of MCs-Cyanex 923: 1.5 g/L).

185S. Ozcan et al. / Desalination 259 (2010) 179–186

Fig. 8 shows the Langmuir, Freundlich and Redlich–Petersonisotherm models and the experimental data for the sorption of Cr(VI)byMCs-Cyanex 923. The values of the isotherm constantswere obtainedfrom the slope and intercept of the plots of the linear form ofeach isotherm. The Langmuir sorption capacity, Q0, and the sorptionequilibrium constant, b, were 0.430 mmol/g and 4.024 L/mmol, respec-tively. The Freundlich isothermconstant, k,was0.262 mmol/g andnwas2.786; the Redlich–Peterson isotherm constant KR was 3.000 L/mmol, aRwas 8.566 (L/mmol)β and β was 0.858. The R2 values for the Langmuir,Freundlich and Redlich–Peterson isotherm models were 0.992, 0.895and 0.994, respectively, which means that sorption of Cr(VI) by MCs-Cyanex 923 could be better described by the Redlich–Peterson and theLangmuir isotherm models.

The Chi-square (χ2) test, defined by Eq. (16), was also employed tofind out the best-fit isotherm model for the sorption data [40].

χ2 = Σ qe–qmð Þ2 = qmh i

ð16Þ

Where qm is the equilibrium capacity obtained by calculating fromthe model (mmol/ g), and qe is experimental data of the equilibriumcapacity (mmol/g). Small number of χ2 indicates that data from themodel is close to the experimental data whereas, a large number of χ2

indicates that data from the model is different from the experimentaldata. The χ2 values for the Langmuir, Freundlich and Redlich–Peterson isotherm models were 0.022, 0.110 and 0.025, respectively.This means that the best-fit isotherm models were the Langmuir andRedlich–Peterson isotherm models. In fact, for our experimental data,the Redlich–Petersonmodel supported the Langmuir isothermmodel,meaning that the sorption process would occur in a monolayer over ahomogeneous surface. This result can be also inferred from the value(0.858) of β, which is close to unity [41].

The maximum Cr(VI) sorption capacity of the MCs-Cyanex 923was estimated as 0.430 mmol/g. The maximum Cr(VI) sorptioncapacity of the MCs containing Aliquat 336 was in the range of0.857–0.967 mmol/g [19]. The higher result may be attributed to thedifference between the sorption mechanisms of MCs containingAliquat 336 and Cyanex 923. Aliquat 336, a commercial quaternaryammonium salt [(C8–10H17–21)3·CH3]N+·Cl−, reacts with the anionicchromium species (HCrO4

−) according to the following anionexchange mechanism [19].

½ðC8–10H17–21Þ3:CH3�Nþ:Cl

−þHCrO−4 ⇔½ðC8–10H17–21Þ3:CH3�Nþ

:HCrO−4 þ Cl

−

However, the sorption of Cr(VI) by Cyanex 923 is based on thecomplexationmechanism as described in Section 3.2.2. The procedure

used for the synthesis of MCs may also affect both encapsulationefficiency and pyhsical property, hence the sorption capacity of theMCs. For example, Barassi et al. [19] used the in situ radicalpolymerization method for the preparation of MCs-Aliquat 336 with87% of encapsulation and 20 µm of mean diameter. In the presentstudy, the solvent evaporationmethodwas used in order to obtain theMCs-Cyanex 923 because it was easier than radical polymerizationmethod. The encapsulation and mean diameter of MCs-Cyanex 923were determined as 75% and 273 µm, respectively. Therefore, it isexpected that the sorption capacity of MCs-Aliquat 336 is higher thanthat of MCs-Cyanex 923 due to higher surface area and encapsulationefficiency. Nevertheless, it should be emphasized that the potentialuse of Cyanex 923 for the removal or recovery of solutes fromwater ismore extensive than that of Aliquat 336 because it has affinitytowards for both inorganic and organic solutes, i.e. phenol, carboxylicacid, ect. In addition, the major advantage of Cyanex 923 over similarextraction reagents, e.g. trioctylphosphine oxide, is that it iscompletely miscible with all common hydrocarbon diluents even atlow ambient temperatures. The major benefit of high solubility lies inthe ability to prepare concentrated, stable solvents which can recoversolutes (e.g. acetic acid) that are normally only weakly extracted bythis type of reagent [42].

MCs-Cyanex 923 showed comparable Cr(VI) sorption capacity withLewatit MP64 (0.400 mmol/g) and Lewatit MP500 (0.410 mmol/g)anion-exchange resins [43]. The sorption capacity of MCs-Cyanex 923was lower than that of commercial activated carbon (0.580 mmol/g) [6].However, the commercial activated carbon is very expensive and hashigh operating costs.

3.2.5. Effect of MCs-Cyanex 923 dosageAn increase in the MCs-Cyanex 923 dosage increased the removal

of Cr(VI) (Fig. 9). This is consistent with the expectation that highersorbent dosage results in lower q values. The concentration of Cyanex923 is related to MCs-Cyanex 923 concentration through surface sitedensity [44]. Therefore, the removal increased with increasing themicrocapsule dosage, whereas q decreased.

3.2.6. Effect of other metal ions on the removal of Cr(VI)The effects of othermetal ions, including Cr(III), Cu(II), Pb(II), Zn(II),

Ni(II), Co(II) and Cd(II), on the removal of Cr(VI) by MCs-Cyanex 923were investigated. The results showed that Cr(III), Cu(II), Pb(II), Ni(II),

Table 2Effect of other metal ions on the removal of Cr(VI) by MCs-Cyanex 923. (Experimentalconditions: concentration of each metal ion: 10 mg/L, dosage of MCs-Cyanex 923: 5 g/L,initial pH: 1.0(±0.1) and contact time: 1 h).

Metal ion in aqueous solution Removal, %

Cr(III) No removalNi(II) No removalCu(II) No removalCo(II) No removalPb(II) No removalZn(II) 10Cd(II) 5Cr(VI) 89

186 S. Ozcan et al. / Desalination 259 (2010) 179–186

and Co(II) were not removed from the solution (Table 2). Contrary, 10%of Zn(II) and 5% of Cd(II) were removed from aqueous solution byMCs-Cyanex 923. This can be attributable to that Cr(III), Cu(II), Pb(II), Ni(II),and Co(II) are present as their cationic forms in the aqueous solutionused in the presented study, and thus they were not removed by MCs-Cyanex923. Thiswasnot the case for Zn(II) andCd(II)whichare presentin the solution as the corresponding chloro-complexes, andwere sorbedbyMCs-Cyanex 923. Similar resultswere reported by Alguacil et al. [45],who studied the facilitated transport of Cr(VI) through supported liquidmembrane containingCyanex921 andCyanex923. In addition, Agrawalet al. [24] investigated the effect of variousmetal ions (i.e., Cr(III), Ni(II),Ca(II), Mg(II), Fe(II), Zn(II), Cu(II), etc.) on the extraction of Cr(VI) byCyanex 923 and they reported that only Zn(II) was found to beinterfering species.

The results also indicated that Cr(VI) was preferably removed byMCs-Cyanex 923 against other metal ions. However, sorptionpercentage of Cr(VI) in the presence of these ions was 89%. Thisvalue was lower than that (95%) of the prensence of only-Cr(VI) in theaqueous solution. The result may be because of a decrease in nominalCyanex 923 concentration of the MCs due to the competitive sorptionof Zn(II), Cd(II) and Cr(VI) [45,46]. Similar findings were reported inthe related literatures [12,24].

3.2.7. Regeneration and reusability of MCs-Cyanex 923The regeneration experiments were carried out to see whether the

MCs-Cyanex 923 loaded with Cr(VI) could be chemically regenerated.According to the literature, the most efficient solution was NaOH forstripping of Cr(VI) from Cr(VI)–Cyanex 923 complex [24]. Therefore, inthepresented study, 0.5 MNaOH(10 mL)wasused for thedesorptionofCr(VI) from the loadedMCs-Cyanex 923. After desorption of Cr(VI), theMCs-Cyanex 923 were washed with distilled water until neutralizationand then desiccated at room temperature. The adsorption process wasrepeated by using the regenerated MCs. It was found that after 3 cyclesthere is no change in the Cr(VI) sorption capacity of the MCs-Cyanex923. In desorption experiments, it was observed that almost the totalrecovery of Cr(VI) occurs, suggesting that the MCs-Cyanex 923 areregenerable and can be used several times.

4. Conclusion

The following conclusions can be made from the presented study.The results from the FT-IR, TGA, SEM of the MCs clearly indicated thatimmobilization of Cyanex 923 onto the MCs was successfullyperformed. The MCs-Cyanex 923 exhibited significant sorptioncapability for Cr(VI). The maximum removal of Cr(VI) was obtainedwhen the ratio of Cyanex 923 to polysulfone was 1.0 at an initial pH of1.0. The sorption of Cr(VI) by MCs-Cyanex 923 was presented bymeans of the complexation mechanism. The sufficient time for the

sorption equilibrium was 30 min and it obeyed the pseudo second-order kinetic model.

The Langmuir sorption capacity of MCs-Cyanex 923 for Cr(VI) wasestimated as 0.430 mmol/g. The Cr(VI) was preferably sorbed byprepared microcapsules against the other metal ions [i.e., Cr(III), Ni(II),Cu(II), Zn(II), Cd(II), Co(II) and Pb(II) etc.]. The regeneration studies alsoshowed thatMCs-Cyanex 923 can be used several times for the sorptionof Cr(VI) from the aqueous solutions.

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