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Synthesis and characterization of PVP/AAc copolymer hydrogel and its applications in the removal of heavy metals from aqueous solution

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Synthesis and characterization of PVP/AAc copolymer hydrogel and its applications in the removal of heavy metals from aqueous solution
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  Synthesis and characterization of PVP/AAc copolymerhydrogel and its applications in the removalof heavy metals from aqueous solution A. El-Hag Ali  a,* , H.A. Shawky  b , H.A. Abd El Rehim  a , E.A. Hegazy  a a National Center for Radiation Research and Technology, P.O. Box 29, Nasr City, Egypt b Desert Research Center, El Mataryia, Cairo, Egypt Received 5 February 2003; received in revised form 13 June 2003; accepted 27 June 2003 Abstract Chelating poly(vinylpyrrolidone/acrylic acid) (PVP/AAc) copolymer hydrogels were prepared by radiation-inducedcopolymerization. The effects of preparation parameters such as PVP content in the hydrogel and irradiation dose onthe swelling behavior of the hydrogel were studied. The pH dependent swelling was investigated. The thermal stabilityof the prepared hydrogel and the metal chelated ones was characterized by TGA. The removal of Fe(III), Cu(II), andMn(II) from aqueous solution by the prepared PVP/AAc chelating hydrogel was examined by batch equilibrationtechnique. The influence of treatment time, pH, and the initial feed concentration on the amount of the metal ionsremoved was studied. The results show that the removal of the metal ion followed the following order: Fe(III)>Cu(II)>Mn(II). The amounts of the removed metal ions increased with treatment time and pH of the medium. Tore-use the hydrogel, the metal ions were stripped by using 2 N HCl.   2003 Elsevier Ltd. All rights reserved. Keywords:  Polyvinylpyrrolidone; Acrylic acid; Metal ion chelating hydrogel; Batch equilibration; Iron; Copper; Manganese 1. Introduction Groundwater is the only source of drinking and irri-gation water in many districts and localities in theWestern Desert of Egypt. Nowadays, due to the increaseof the population, the groundwater development in suchareas becomes of great necessity, especially because thesoil in the area is suitable for cultivation. The mainproblem affect the development of the groundwater isthe high concentrations of iron and manganese [1–3].This problem affect the human health as well as it cau-ses: tastes, staining of water, discoloration of clothes andprecipitation that can reduce pipe diameter and clogvalves of water distribution.The removal of such toxic heavy metals has receivedmuch attention in recent years [4–8]. Traditional treat-ment techniques to remove the dissolved toxic metal ionsfrom waste disposal by chemical treatment, ultrafiltra-tion or combination of chemical treatment and ultrafil-tration do not always provide a sufficient contaminantremoval to meet pollution control limits. Chelating ex-changers are, in general, coordinating copolymers con-taining one or more electron donor atoms (Lewis base)such as N, S, O, and P that can form coordinatebonds with most of the toxic heavy metals (Lewis acid)[9]. Hydrogels are water-swollen network (crosslinkedstructure) of hydrophilic homopolymer or copolymers.They acquire a great interest due to the facility of the incorporation of different chelating groups intothe polymeric networks. Such polymeric hydrogels arepromising materials in the field of hydrometallurgicalapplications and water purification due to their chemical * Corresponding author. E-mail address:  elhagali_a@hotmail.com (A. El-Hag Ali).0014-3057/$ - see front matter    2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0014-3057(03)00150-2European Polymer Journal 39 (2003) 2337–2344www.elsevier.com/locate/europolj  stability especially hydrolytic and thermal stability[10,11].In this work, PVP/AAc copolymer hydrogels wereprepared by means of   c -rays induced homo- and copo-lymerization as a clean and environment friend source of initiation. The factors affect on the preparation of thehydrogel will be investigated and the chelating ability of the prepared hydrogel with Fe(III), Cu(II) and Mn(II)ions will be examined using simulated metal ion solu-tions. 2. Experimental  2.1. Materials Polyvinylpyrrolidone (PVP) of M.Wt. 1,300,000(Acros, Belgium) and acrylic acid (AAc) of purity 99.9%(Merck, Germany) were used as received. Analyticalgrade ferric chloride (FeCl 3  Æ 6H 2 O), manganese chloride(MnCl 2  Æ 4H 2 O) and cupric chloride (CuCl 2  Æ 2H 2 O) werepurchased from El-Nasr Co. for Chemical Industries,Egypt and used without further purification.  2.2. Preparation of PVP/AAc Gels PVP/AAc gels were obtained by radiation-inducedhomo/copolymerization of mixtures of different molarratio using  60 Co gamma rays at a dose rate 10.28 kGy/h.All samples were washed in excess water to remove theunreacted component then dried.  2.3. Preparation of buffer solutions of different pH  ’ s (Citric acid/trisodium citrate) and (sodium dihydro-gen phosphate/disodium hydrogen phosphate) were usedto prepare buffer solutions ranged from 3 to 5 and 6 to 7,respectively [12]. HCl was used to prepare solutions of pH 1.  2.4. Swelling study The prepared gels were cutted into disks and thenswollen in buffer solution of different pH’s ranged from1 to 7 at 37   C. The swelling ratio ( S  ) was determinedfrom the following equation: S   ¼  W   s   W   o W   o   100where  W   s  and  W   o  are the weights of the swollen and thedry hydrogel, respectively.  2.5. Thermal gravimetric analysis (TGA) Shimadzu TGA system of Type TGA-50 under ni-trogen atmosphere (20 ml/min) was used in this study.The temperature range was from ambient to 500   Cat heating rate of 10   C/min.  2.6. ICP spectrometer An ICP (POEMS III) ICP-MS spectrometer (ThermoJerral Ash) were used for the determination of metal ionconcentration. Thousand ppm (Merck, Germany) metalstandard solutions were used as stock solution for cali-bration.  2.7. Adsorption equilibrium experimental  Batch studies were proceed in all adsorption experi-ments. The adsorption equilibrium included: the deter-mination of metal ion adsorbed, rate of metal ionchelated, pH dependence of adsorption and the effect of initial feed concentration of metal ions.  2.8. Adsorption procedures Dried samples (0.5 g each) of PVP/AAc chelatinghydrogel were equilibrated by stirring for 24 h in 50 mlof 500 mg/l (ppm) Fe(III), Mn(II), and Cu(II) solutionsat 25   C. The concentrations of metal ions remaining inthe solution were detected by ICP-OES, all tested sam-ples were run in triplicate. The adsorption amount A in(mg/g) was calculated as follows:  A  ¼ ½ V   ð C  1   C  2 Þ W   where  V    is the volume of solution (l),  W    is the weight of the hydrogel (g),  C  1 ,  C  2  are the concentrations of metalions before and after the adsorption, respectively (mg/l).  2.9. Rate of metal ion adsorption Batch studies were carried out with the preparedPVP/AAc hydrogel to determine the effect of time du-ration on the chelation of metal ions. Fifty ml each of Fe(III), Mn(II), and Cu(II) solutions (500 mg/l concen-tration) were mixed with the hydrogel (0.5 g) and stirred.At regular intervals, aliquots were withdrawn from thetest solution and estimated.  2.10. pH dependence of metal chelation pH dependent metal chelation was performed by in-teracting 0.5 g of the hydrogel samples with 50 ml of 500mg/l metal ion solutions. The various of metal ions so-lution were adjusted to the desired pH value by addition 2338  A. El-Hag Ali et al. / European Polymer Journal 39 (2003) 2337–2344  of diluted HNO 3  or NaOH. The mixtures were stirredfor 24 h at 25   C. The pH values were chosen to bebelow 6.0 for Mn(II), and Cu(II) solutions (below 3.0 incase of Fe(III)) so that none of the metal hydroxideprecipitates.  2.11. Effect of initial feed concentration of metal ions This was done by equilibrating a fixed amount of thechelating hydrogels (0.5 g) with a series of graduallyincreased metal ion solutions concentration, and stirredfor 24 h at 25   C.  2.12. Re-use of the chelated hydrogel  PVP/AAc chelated hydrogel (0.5 g) were stirred withHCl (50 ml, 2 N) at 50   C for 2 h to desorbs the metalions. Thereafter, the hydrogel was neutralized with di-lute NaOH, washed with deionized water and againsubjected to chelation processes. 3. Results and discussion The effect of preparation conditions such as irradia-tion dose, molar ratio and concentration on the copo-lymerization of PVP/AAc has been studied in the light of the swelling behavior that all the prepared copolymerhydrogels possess almost 100% gelation. 3.1. Effect of PVP content in the hydrogel on its swelling behavior The effect of feed solution composition on theswelling of the prepared PVP/AAc copolymer hydrogelat different pH’s was studied and the results are pre-sented in Fig. 1. It can be seen that the swelling behaviorof the copolymer is greatly influenced by its composi-tion. At low pH values (pH 1 and 4), the swelling degreeincreases as the PVP content in the copolymer gel in-creases. Whereas, at high pH values (pH 6 and 7), theswelling degree decreases as the PVP content in the co-polymer hydrogel increases. Meanwhile, the swellingdegrees for all the composition are almost the same atmoderate pH value (pH 5). These results could be ex-plained as follows: at low pH values, the AAc-rich co-polymer hydrogels possess very low swelling degree dueto the association of the carboxylic groups and the in-termolecular hydrogen bonding. As the PVP contentincreases the number of the associated groups decreasesand consequently the intermolecular hydrogen bondingalso decreases which give a space for the PVP chain toswell freely in water. In contrary, at high pH values, theAAc-rich copolymer gels possess high swelling degreethat the carboxylic groups are completely dissociated.The swelling values reach its maximum value because of the intermolecular repulsion between the carboxylategroups. As the PVP content increases the number of carboxylate groups decreases and as a result, the in-termolecular repulsion forces decreases resulting in thereduction in the free spaces valid for swelling. Theconstant swelling of all the copolymer under investiga-tion at pH 5 may be due to the balance between thenumber of carboxylic groups and its degree of dissoci-ation. 3.2. Effect of irradiation dose It is well known that degrees of conversion andcrosslinking greatly depend on the irradiation dose. Thehigher exposure dose means longer exposure time, whichconsequently prolongs the propagation step of the pro-cess of copolymerization leading to higher degrees of conversion and crosslinking.Fig. 2 shows the effect of total irradiation dose on theswelling behavior of PVP/AAc copolymer hydrogel of different compositions at buffer solution of pH 1. For allthe investigated compositions, it was found that theswelling degree of the copolymer hydrogel decreases byincreasing the total irradiation dose. These results couldbe explained in terms of crosslinking degree, which in-creases by increasing the total irradiation dose. The in-crement in the degree of crosslinking reduces the freevolume available for swelling by increasing the tightnessof the network structure. 3.3. Effect of pH of the surrounding medium The equilibrium swelling behavior of PVP/AAc co-polymer hydrogel was investigated as a function of pH.Fig. 3 shows a typical behavior of a pH-sensitive co-polymer hydrogel. The pH sensitivity of such hydrogel is Fig. 1. Effect of feed solution composition on the swellingbehavior of PVP/AAc copolymer hydrogel at different pH’s:( d ) pH 1, (  ) pH 4, ( . ) pH 5, ( O ) pH 6 and ( j ) pH 7. A. El-Hag Ali et al. / European Polymer Journal 39 (2003) 2337–2344  2339  fully dependent on its composition. All of the investi-gated copolymer hydrogels possess abrupt change in theswelling degree at pH value around pH 4, which is thep  K  a  value of AAc [13]. At pH values lower than the p  K  a value, the carboxylic groups are completely collapsedand the swelling degree is very low. At pH values higherthan the p  K  a  value, the swelling degree increased due tothe dissociation of the carboxylic groups and broken thehydrogen bonding. The increase in the AAc content inthe hydrogel leads to the increment in the carboxylategroups and consequently increases the electrostatic re-pulsion, which results in the expansion of the networkstructure. 3.4. Thermogravimetric analysis The TGA data furnished in Table 1 and Figs. 4 and 5show the general nature of the thermograms of the co-polymer hydrogel of different compositions and themetal ions chelated hydrogel. The relative thermal sta-bility of the different hydrogels was assessed by com-paring the weight loss in the temperature range 25–600  C. Fig. 4 shows the effect of PVP content in the co-polymer hydrogel on its thermal stability. Pure PVPshows high thermal stability that starts to decompose at400   C whereas pure PAAc shows its famous four de-gradation steps at 100, 250, 400 and 450   C which due tothe lose of associated water, anhydride formation, de-carboxylation and backbone degradation, respectively[14,15]. The thermal stability of the prepared PVP/AAccopolymer hydrogel increases as the more stable moiety,PVP, content increases. The Figure shows also that thecopolymer hydrogel of molar composition (80/20 mol%)(PVP/AAc) possesses the highest thermal stability,which is almost similar to that of pure PVP. Fig. 5 showsthe effect of the chelated metal ions on the thermal sta-bility of the copolymer hydrogel. Both Fe and Mn donot affect the thermal stability of the host copolymer, Fig. 2. Effect of total irradiation dose on the swelling behaviorof PVP/AAc copolymer hydrogel of different compositions atbuffer solution of pH 1: ( M ) 10, (  ) 20 and ( d ) 30 kGy.Fig. 3. The equilibrium swelling behavior of PVP/AAc co-polymer hydrogel of different (PVP/AAc) compositions as afunction of pH: ( d ) 50/50, (  ) 60/40, ( . ) 70/30, ( O ) 80/20.Table 1Phenomenological data of the thermal decomposition of PVP/AAc of different composition and (80/20 mol %) PVP/AAc chelated withdifferent metalsTemperature(  C)PVP content (mol%) Metal chelated PVP/AAc (80/20 mol%)100 80 50 30 0 Fe Mn Cu100 91 94 99 96 86 93 93 93150 87 90 96 96 85 88 92 89200 87 89 93 95 83 86 88 87250 87 87 92 88 75 85 88 87300 87 86 83 74 71 83 85 84350 86 83 77 68 59 81 84 81400 83 82 71 60 46 77 78 69450 29 41 36 29 43 38 44 30500 7 14 15 19 22 18 22 18550 3 9 11 6 15 7 13 8600 0 9 7 3 9 4 0 42340  A. El-Hag Ali et al. / European Polymer Journal 39 (2003) 2337–2344  whereas the presence of Cu ions reduces the thermalstability of the copolymer. The reduction in the thermalstability of Cu chelated hydrogel may be due to thewater coordinated to the Cu ions, which were lost, re-sulting in little higher weight loss in the initial stages of decomposition [16]. 3.5. Metal ion chelation behavior of PVP/AAc hydrogel  The relation between the nature of the copolymer andits metal chelation behavior is generally complicated bymany possible interactions. Oxygen in the carbonylgroup of PVP and AAc are responsible for the interac-tion of the metal ion with the hydrogel. Since the mobile p -electrons are pulled strongly towards oxygen. Car-bonyl carbon is electron deficient and carbonyl oxygen iselectron rich, the metal ions act as electron acceptorsand taken up by coordination to the donor oxygen of the carbonyl group. 3.6. Effect of the extent composition of PVP  The chelation of PVP/AAc hydrogels of differentcompositions was investigated towards Fe(III), Mn(II),and Cu(II) by the batch equilibration method. Theamount of metal ions chelated by the different hydrogelsis given in Table 2. From the table, the amount of metalion uptake increases with increasing PVP content until itreach its maximum value at the composition of (80/20mol%) (PVP/AAc), then it decreases sharply.The results show a clear dependence of metal ionuptake on the copolymer hydrogel composition. AAc-rich compositions did not possess high metal uptake thatit possess low degree of swelling at low pH values of metal ion feed solutions, which prevent the diffusion of the metal ions inside the hydrogel i.e. to reach to thechelating functional groups of the hydrogel. As the AAccontent in the hydrogel decreases, the swelling ability of the hydrogel at low pH values increases, as shown inFig. 3, and as a result, the possibility of the metal ionsto reach the chelating functional groups increases. Theoptimum composition of the hydrogel was (80/20 mol%)(PVP/AAc) at which AAc possess the lowest pH sensi-tivity and maximize the ability of the hydrogel for metalion uptake.The results showed also that ability of adsorption of PVP/AAc hydrogels decreases in the order of Fe(III)>Cu(II)>Mn(II). These results are in a very good agree-ment with relative size and crystal field radii of thesehexa-coordinated metal ions [17]. 3.7. Rate of metal ion chelates Time course metal ions chelation by PVP/AAcof composition (80/20 mol%), which possesses the Fig. 4. TGA thermograms of the copolymer hydrogel of dif-ferent (PVP/AAc) compositions: (––) 100/0, (    ) 80/20, (–––)50/50, (–     ) 30/70.Fig. 5. TGA thermograms of metal ions chelated PVP/AAc(80/20 mol%) hydrogel. (––) Pure (PVP/AAc) hydrogel, (    ) Cuchelated hydrogel, (–––) Fe chelated hydrogel, (–      –) Mnchelated hydrogel.Table 2Effect of changing hydrogel composition on the metal uptake(mg/g)Hydrogel compositionPVP/AAc (mol%)Metal ion uptake (mg/g)Fe(III) Mn(II) Cu(II)30/70 6 – 240/60 14 – 850/50 20 1 1160/40 29 7 1870/30 33 12 2280/20 36 14 2390/10 3.7 – 1100/0 3.1 – –  A. El-Hag Ali et al. / European Polymer Journal 39 (2003) 2337–2344  2341
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