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Removal of Cr(VI) by solvent impregnated resins (SIR) containing aliquat 336

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Removal of Cr(VI) by solvent impregnated resins (SIR) containing aliquat 336
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  Reactive & Functional Polymers 54 (2003) 103–115www.elsevier.com/locate/react Removal of Cr(VI) by solvent impregnated resins (SIR) q containing aliquat 336 a, b c c *N. Kabay , M. Arda , B. Saha , M. Streat a  Department of Chemical Engineering ,  Ege University ,  Izmir   35100,  Turkey b  Department of Chemistry ,  Ege University ,  Izmir   35100,  Turkey c  Department of Chemical Engineering ,  Loughborough University ,  Loughborough ,  Leicestershire ,  UK LE  11 3  TU  Abstract Solvent impregnated resins (SIR) have been prepared by a wet-impregnation technique using two different polymermatrices, Diaion HP-20 and HP-2MG. Diaion HP-20 possesses a hydrophobic styrene–divinylbenzene polymeric structurewhereas HP-2MG is a hydrophilic methacrylic based polymer. Aliquat 336 was employed as the extractant and acetone asthe solvent for impregnation. Batch sorption studies have shown that solvent impregnated resins containing Aliquat 336 canbe effectively used for the removal of hexavalent chromium from aqueous solutions. The Langmuir adsorption isotherm gavea satisfactory fit of the equilibrium data. A kinetic study has been performed for SIR at two different concentrations of  2 4  2 4 Cr(VI) (2 3 10 and 4 3 10 M). A finite-bath diffusion-control model with changing bulk concentration was used forsorption of Cr(VI) by SIR. Increasing the impregnation ratio from 0.5 to 1.0 (g Aliquat 336/g polymer adsorbent) increasedthe breakthrough capacity in a column-mode sorption study. The Cr(VI) bound by the SIR has been quantitatively desorbedusing 0.1 M NaOH–0.1 M NaCl mixture. ©  2002 Elsevier Science B.V. All rights reserved. Keywords :   Cr(VI); Solvent impregnated resins; Aliquat 336 1. Introduction  water thereby preventing the pollution of sur-face and ground water.There is considerable interest in the treatment Chromium compounds are extensively usedof polluted water generated by industrial pro- in electroplating, anodizing operations in thecesses. Various technologies have been de- surface finishing industry, corrosion control,veloped over recent years and are now available oxidation, leather industry and various otherfor the removal of toxic materials from waste- industrial applications. The two common oxida-tion states for chromium in natural water areCr(III) and Cr(VI). Cr(III) is not a significant q The authors wish to dedicate this paper to the memory of   groundwater contaminant whereas Cr(VI) is Professor Abraham Warshawsky. approximately 100 times more toxic than Cr(III) * Corresponding author. Tel.:  1 90-232-388-7776; fax:  1 90- [1]. Precipitation is traditionally used for the 232-388-7600.  E  - mail address :   kabay@eng.ege.edu.tr (N. Kabay).  treatment of Cr(VI) containing wastewater. This 1381-5148/02/$ – see front matter  ©  2002 Elsevier Science B.V. All rights reserved.PII: S1381-5148(02)00186-4  104  N  .  Kabay et al .  /   Reactive &   Functional Polymers 54 (2003) 103–115  requires that Cr(VI) should be reduced to aqueous solubility of extractants, diluents andCr(III) prior to chemical precipitation in order modifiers is a major disadvantage of solventto form the poorly soluble chromium (III) extraction. This not only adds to the cost of thehydroxide [2,3]. It was reported that the redox process, through loss of reagents, but may alsoreaction is kinetically slow at very low con- contaminate effluents with hazardous organics.centrations of Cr(VI) and the residual level of The loss of organics by evaporation and entrain-Cr(VI) is still higher than the discharge limits ment is also a potential problem. Moreover,[3]. solvent extraction is inefficient at trace metalSengupta [4] reported that commercially ion concentrations because of excess solventavailable anion-exchange resins can be used for requirement [9]. To reduce this loss, severalthe removal of Cr(VI) from wastewater at acidic novel techniques have been studied; e.g. a non-pH in the presence of high concentrations of dispersive solvent extraction system that in-chloride and sulphate ions. Sengupta and Clif- volves contacting the effluent with a hollowford [5] obtained a gradual breakthrough with a fiber in which the extractant is circulated [10–strong base anion-exchanger during a fixed bed 12]. Very recently, chitosan hollow fiber mod-column run and they explained this behaviour as ules have been prepared and tested for thean equilibrium phenomenon characteristic of solvent extraction of Cr(VI) using Aliquat 336some unusual interactions between the anion- as the extractant [13].exchanger and the chromate species at acidic Solvent impregnated resins (SIR) have beenpH. In addition, commercial anion-exchange used in various applications for the treatment of resins cannot selectively remove Cr(VI) at metal effluents containing heavy metals [14–neutral to alkaline pH in the presence of sul- 20]. The concept of SIR is based on thephate, chloride, bicarbonate and nitrate [6]. A incorporation of a solvent extraction reagentnew polymeric ligand based exchanger was into a porous polymer by a physical impreg-introduced by Zhao et al. [6] as a potential new nation technique [21]. The advantages of SIRanion-exchanger with higher chromate affinity are wide selection of solvent extraction re-than commercially available anion-exchangers. agents, ease of preparation and continuous typeRecently, a modified poly(4-vinyl pyridine) of operation. The investigation of Cr(VI) re-coated silica gel has been prepared by Gang et moval by solvent-impregnated resins containingal. [7]. It was reported that this modified sorbent Aliquat 336 is the subject of this study. Theexhibited very fast kinetics for Cr(VI) removal. present work was directed towards the prepara-Solvent extraction is a convenient technique tion by wet-impregnation of two different poly-for the removal of chromate from aqueous mer matrices, Diaion HP-20 (hydrophobicsolutions [8]. It has been reported that the styrene–divinylbenzene) and HP-2MG (hydro-tertiary amine extractants Alamine 336 and philic methacrylic based). Batch Cr (VI) sorp-Aliquat 336 are effective reagents for the re- tion, kinetic studies, breakthrough experimentsmoval of Cr(VI). However, there has not been as well as desorption studies of these sorbentsany practical application of solvent extraction have been investigated.for chromium recovery [8]. However, Salazar etal. [8] have reported an interesting solventextraction procedure for the recovery of Cr(VI) 2. Experimental from industrial effluents using Aliquat 336 inthe organic phase. 2.1.  Materials The major drawback of solvent extraction isthe loss of extractant/solvent in large-scale Mitsubishi Co., Japan provided DIAION HP-industrial applications. The small but finite 20 and HP-2MG. The characteristics of these   N  .  Kabay et al .  /   Reactive &   Functional Polymers 54 (2003) 103–115   105Table 1 solution of K CrO solution (pH 4.0) at varying 2 4 Characteristics of the polymer adsorbents  2 4  2 4  2 4  2 3 concentrations (10 , 2 3 10 , 4 3 10 , 10 , 2 3  2 3 Characteristics HP-2MG HP-20 2 3 10 , 4 3 10 M) at 298 K for 24 h. Moisture content (%) 61.10 56.50Swelling (ml/g) 3.67 3.45 2.4.  Kinetic study 2 BET surface area (m /g) 524.16 556.81Pore volume (ml/g) 1.15 1.18 Kinetic studies were performed using 2 g of  Specific gravity 1.09 1.01 SIR (HP-2MG) prepared at an impregnationratio of 2.5 (g Aliquat 336/g-polymer adsor-bent) in 1.0 l of K CrO solution (pH 4.0) at 2 4 2 4  2 4 concentrations of 2 3 10 and 4 3 10 M at apolymeric adsorbents are summarised in Tablestirrer speed of 250 rev./min at 303 K. The1. Aliquat 336 (Tricaprylmethylammoniumstirrer was connected to a sample containerchloride CH N[(CH ) CH ] Cl) was purchased 3 2 7 3 3 made of Teflon mesh (50  m m) surrounded by afrom Fluka, Switzerland. All other chemicalsPerspex frame. This retained the resin beadswere reagent grade (Merck) and used withoutwithin the basket and provided good contactfurther purification.with the solution. The resin sample was intro-duced to the reaction flask at time zero and then 2.2.  Impregnation procedure a specific amount of solution phase was col-lected at various time intervals. The samplesIn this study 1 g of dry resin (Diaion HP-20were analysed by atomic absorption spec-or HP-2MG) was immersed into 5 ml of acetonetrophotometer for metal ion concentration. Thecontaining different amounts of Aliquat 336. ¯  fractional attainment of equilibrium,  X  , wasThe mixture was shaken at 303 K for 24 h. Thedetermined by calculating the ratio of the differ-polymer beads were then separated by filtrationence between initial solution concentration ( c  )using a sintered glass funnel and washed with  o and the concentration at any time ( c  ) with thedeionized water. The resins obtained were first  t difference between the initial solution concen-air-dried and then dried under vacuum at 313 K.tration ( c  ) and the final solution concentration o ( c  ). e 2.3.  Batch Cr  (  VI   )   sorption¯  Fractional attainment of equilibrium 5  X  For the determination of the effect of pH on c  2 c o t Cr(VI) sorption, 40 mg of SIR was immersed  ]] ] 5  (1) 2 4  c  2 c o e into 25 ml of 10 M K CrO solution at 2 4 different pHs. The mixture was shaken at 2.5.  Column sorption study 298 6 2 K for 24 h. After sorption, the SIR wereeluted with a mixture of 0.1 M NaOH–0.1 M About 1 g dry SIR (HP-2MG) prepared atNaCl at 298 6 2 K for 24 h. The amount of two different impregnation ratios [0.5 and 1Cr(VI) sorbed on SIRs was calculated by de- (g-Aliquat 336/g-polymer adsorbent)] was im-termining both the amount of Cr(VI) in the mersed in deionised water overnight and thensupernatant after the sorption and the amount in packed into a mini-column with an internalthe eluant after the elution. diameter of 0.9 cm (Isolute SPE columns sup-Adsorption isotherms were studied using 50 plied by Jones chromatography Ltd, UK) fittedmg of SIR at two different impregnation ratios with 20  m m polyethylene frits as bed support). 2 4 [0.5 and 3 (g Aliquat 336/g polymer adsorbent) The solution of K CrO (4 3 10 M) at pH 4.0 2 4 for HP-20; 0.5 and 2.5 (g Aliquat 336/g was passed through the column using a peri- 2 1 polymer adsorbent) for HP-2MG] in 25 ml staltic pump at a flow rate of about 23 ml h .  106  N  .  Kabay et al .  /   Reactive &   Functional Polymers 54 (2003) 103–115  Samples were collected continuously for eachexperiment by fraction collector and analysedregularly to monitor the metal concentrationleaving the column. This was continued untilthe concentration of metal leaving the columnwas very close to the feed. The Cr(VI)-loadedSIR was eluted by column method at a flow rate 2 1 of 23 ml h using 0.1 M NaOH–0.1 M NaClmixture. 2.6.  Analysis Chromium was analysed using an atomic Fig. 1. Impregnation ratio vs. weight change plots. absorption spectrophotometer (Varian SpectraAA 200 Model) in flame absorption mode at357.9 nm wavelength. The solution pH was 2 4 sorption tests with 10 M Cr(VI) at pH 7.0 formeasured using a Mettler Toledo 340 pH meter.both HP-20 and HP-2MG based SIR. Thepercentage removal of Cr(VI) vs. impregnationratio results are given in Fig. 2. HP-2MG based 3. Results and discussion SIR gave a nearly quantitative removal of Cr(VI) (more than 90%) at all impregnation 3.1.  Preparation of SIRs ratios whilst the HP-20 based SIR showed theAmines of high molecular weight (Alamine same sorption performance at an impregnation336 and Aliquat 336) have been suggested for ratio of   $ 0.5 (g Aliquat 336/g polymer ad-the removal of Cr(VI) from aqueous solutions sorbent). A better contact was obtained between[8,10]. However, there is no practical applica- the Cr(VI) solution and the more hydrophiliction as yet of conventional solvent extraction for HP-2MG based SIR.chromium recovery [8]. Very recently, Vincentand Guibal [13] prepared chitosan hollow fibermodules and evaluated these for solvent ex-traction of Cr(VI) from dilute solutions usingAliquat 336 as the carrier.In the present study, polymer adsorbentsDiaion HP-20 and HP-2 MG were impregnatedwith Aliquat 336 in order to produce selectiveadsorbents for the removal of Cr(VI). Theweight changes against the impregnation ratio(g Aliquat 336/g dry polymer adsorbent) plotsare shown in Fig. 1. A plateau was reached at animpregnation ratio of 2.5 (g Aliquat 336/gpolymer adsorbent) for HP-2MG whereas anoptimum impregnation ratio of 4.0 (g Aliquat336/g polymer adsorbent) was obtained for HP-20. The effect of impregnation ratio on Cr(VI) Fig. 2. Effect of impregnation ratio on the removal of chromate at removal was checked in a series of batch  pH 7.3.   N  .  Kabay et al .  /   Reactive &   Functional Polymers 54 (2003) 103–115   107 3.2.  Effect of pH   plateau at pH values greater than 2.0 for SIR.Vincent and Guibal [13] reported that solutionIt is necessary to understand the solution pH is a key parameter for the efficient ex-chemistry of hexavalent chromium in order to traction of Cr(VI) using Aliquat 336 as theexplain the binding mechanism of chromate by carrier in a hollow fiber module and, moreover,various adsorbents or ion-exchange materials stated that pH should be maintained below 4.5.[4,22]. The distribution of Cr(VI) species is According to our results, Cr(VI) removal re-dependent on both the total concentration of  mained almost constant at equilibrium pH val-Cr(VI) and pH of the equilibrium solution. The ues between 3.2 and 8.1. Vincent and Guibalfollowing equations describe the distribution of  [13] noted that the decrease in extraction ef-chromium species in the aqueous solution [22]: ficiency above pH 5 is related to the speciationof Cr(VI) and especially to a decrease in the 1 22  2 2 H CrO  ↔ H  1 HCrO (K :1.21) fractions of HCrO and Cr O with increasing 2 4 4 14 2 7 pH. Since we did not obtain any decrease in 2 1  2 2 2 7  Cr(VI) removal above pH 5, we conclude thatHCrO  ↔ H  1 CrO (K :3 3 10 ) 4 4 2 SIR can easily remove Cr(VI) above neutral pHconditions when chromate is mainly present as 2  2 2  2 2 2HCrO  ↔ Cr O  1 H O (K :35.5) the CrO ion [4,6]. 4 2 7 2 3 4 Cr(VI) is a strong oxidizing agent and theoxidation power of Cr(VI) is influenced by the 2 1  2 2 HCr O  ↔ H  1 Cr O (K :0.85) 2 7 2 7 4 structure of the reagent, the nature of thereaction medium and its pH [22]. It is wellA predominance diagram showing the rela- known that strong acids enhance the oxidizingtive distribution of different Cr(VI) species in power of Cr(VI) [22]. We observed a distinctwater as a function of pH and total Cr(VI) decrease in Cr(VI) removal at an equilibriumconcentration is reported in the literature [4,6]. pH below 2.1 (Fig. 3).Vincent and Guibal [13]Since the distribution of anionic species of also observed this decrease in acidic solutionCr(VI) is pH-dependent, this could be the main and explained it by considering that protonatedvariable for the removal of Cr(VI) by the SIR. amine groups are saturated by counter anionsFig. 3 shows that Cr(VI) removal reaches a present in the solution and also the reduction of Cr(VI) to Cr(III) is strongly increased. We canalso expect similar phenomena to occur withcrosslinked polymer beads. In addition, thedecrease in Cr(VI) extraction at acidic pH maybe explained by the fact that Cr(VI) will formH CrO as a non anionic species and therefore 2 4 does not take part in the anion-exchange pro-cess.The anion-exchange reactions involved in theprocess depend on chromate speciation in solu-tion as given below [4]: 1 2 2 1 2 2 R N X  1 HCrO  ↔ R N HCrO  1 X 3 4 3 4 Fig. 3. Effect of pH on the removal of chromate by SIR 1 2  2 2 1  2 2 2 (Impregnation ratio: 1 g Aliquat 336/g-resin).  2(R N X ) 1 CrO  ↔ (R N ) CrO  1 2X 3 4 3 2 4
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