Available online at Nuclear Instruments and Methods in Physics Research B 265 (2007)

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1 Available online at Nuclear Instruments and Methods in Physics Research B 65 (007) NIM B Beam Interactions with Materials & Atoms Synthesis, characterization and adsorption study of the uranyl ions by hydrogels based on polyethylene glycol and methacrylic acid copolymers G.A. Mun a, *, Z.S. Nurkeeva a, G.S. Irmukhametova a, O. Guven b a Department of Chemical Physics and Macromolecular Chemistry, Kazakh National University, Karasai Batyra 95 a, Almaty, Kazakhstan b Department of Chemistry, Hacettepe University, Beytepe 0653, Ankara, Turkey Available online 8 September 007 Abstract The water swelling polymeric hydrogels were obtained by c-initiated radiation copolymerization of polyethylene glycol (PEG) with methacrylic acid (MAA) and three-ethylene glycol dimethacrylate (TEGDMA) as cross-linking agent. The main regularities of PEG MAA network formation, deformation properties, structure and thermal characteristics depending on copolymer composition, molecular weight of PEG have been studied. The sorption of uranyl ions by PEG MAA hydrogels in comparison with homopolymer PMAA was investigated. The effect of uranyl ions concentration on sorption efficiency of hydrogels was shown. The ability to regeneration in desorption process was demonstrated for PEG MAA hydrogels in comparison with PMAA gel. Ó 007 Elsevier B.V. All rights reserved. PACS: r; De; Pg Keywords: Properties of macromolecules; Thermal analysis; Macro- and polymer molecules absorption spectra; Absorption by molecules 1. Introduction * Corresponding author. Tel.: x1; fax: address: mun@kazsu.kz (G.A. Mun). Enrichment and recovery of uranium from non-conventional sources like coal, natural water and seawater is of significant interest because of anticipated exhaustion of terrestrial uranium reserves in the near future. Recently hydrogels, i.e. weakly cross-linked hydrophilic polymers, were demonstrated to be efficient absorbents of various inorganic ions [1,]. Different hydrogels were studied as potential uranyl ions sorbents. Akkas and Güven [3] studied the sorption ability of acrylamide-maleic acid hydrogels prepared by simple c- irradiation method and by prestructured reaction. It was found that the gels prestructured with UO þ ions absorbed approximately 15 0% more uranyl ions than the gels synthesized by simple polymerization in pure water. Caykara et al. [4] studied the interaction of hydrogels based on poly(n-vinyl--pyrrolidone-g-citric acid) with uranyl acetate and uranyl nitrate. Uranyl adsorption capacity of these hydrogels was found to be in the range of mg ½UO þ Š=g dry gel from aqueous solution of uranyl nitrate and 156 mg ½UO þ Š=g from aqueous solution of uranyl acetate, depending on the content of citric acid in the hydrogel. Ören and co-workers [5] studied the effect of ph, ionic strength and temperature on uranyl ion adsorption by poly(n-vinyl--pyrrolidone-g-tartaric acid) hydrogels. Uranyl adsorption capacity of the hydrogels were determined to be mg ½UO þ Š=g dry gel at ph 1.8, and ½UO þ Š=g dry gel at ph 3.8, depending on the amount of tartaric acid in the hydrogel. The importance of spatial distribution of the carboxylic acid groups and swelling ability of the hydrogels to provide the best chelating configuration for uranyl ions is shown. Kantoğlu and co-workers [6] studied the effect of external stimuli on X/$ - see front matter Ó 007 Elsevier B.V. All rights reserved. doi: /j.nimb

2 380 G.A. Mun et al. / Nucl. Instr. and Meth. in Phys. Res. B 65 (007) uranyl ions uptake by hydrogels of poly(n-vinyl--pyrrolidone/itaconic acid) hydrogels. It was shown that uranyl adsorption capacity of hydrogels increases from 6.7 to 70.0 mg ½UO þ Š=g dry gel with decreasing ph of solution. Increase of itaconic acid content, ionic strength of solution as well as temperature was found to enhance the ability of hydrogels to absorb uranyl ions. Thus based on the literature analysis it can be concluded that hydrogels containing carboxylic groups in the structure can absorb uranyl ions. The last years the systems where macrochains which able to form interpolymer complexes are joined in structure of one polymer network attract attention of the researches. Peppas and co-workers [7,8] reported about interactions between unionized carboxylic groups of MAA and ether oxygen of PEG via hydrogen bonding at low ph values. However in the cited works the authors used macromonomers with relatively short PEG chains ( Da). In the present work the PEG chains used were of 000, and 0000 Da, which is anticipated to enhance the interpolymer interactions within the network. Taking into account the possibility of interpolymer complex formation within the network of PEG MAA we have synthesized by c-initiated radiation copolymerization, characterized hydrogels on base of PEG MAA copolymers and studied the sorption of uranyl ions by these networks.. Experimental part.1. Materials Polyethylene glycol (PEG) with molecular weight 000, and Da was purchased from Merck (Germany) and used without further purification. Methacrylic acid (MAA) was purified by filtration through aluminium oxide filled column. Triethyleneglycoldimethacrylate (TEGDMA) was purchased from Merck (Germany) and used without further purification. Polymethacrylic acid (PMAA) was purchased from BDH Company (UK) and used without further purification. Uranyl nitrate hexahydrate (UN) UO (NO 3 ) Æ 6H O was purchased from BDH Company (UK) and used without further purification. Sodium salicylate was also supplied by BDH and used without further purification... Synthesis of hydrogels Hydrogel PEG MAA were obtained by c-initiated radiation copolymerization of PEG with MAA in the presence of TEGDMA as a cross-linking agent. A mixture of water and ethanol (50:50 vol.%) was used as the reaction medium. In order to prevent the complexation between MAA and PEG the copolymerization was performed at ph 7.0 where the MAA is in ionized state. Prepared solutions were poured in polyvinyl chloride tubes with diameter 4 mm and irradiated on 60 Co-Gammacell 0B. After synthesis the hydrogels were repeatedly washed by distilled water for weeks. The yield of copolymers G was calculated by following equation: G ¼ m 100%; m 1 where m 1 and m the weight of dry.gel after synthesis and after removing of the unreacted monomer, respectively..3. Mechanical tests Mechanical characteristics of the networks and its coefficient of elasticity (G) were obtained by using Zwick Z010 Universal Testing Instrument..4. Swelling of hydrogels Equilibrium swelling degree of hydrogels was determined gravimetrically and calculated according to the following formula: a ¼ðm m 0 Þ=m 0 ; where m and m 0 are the weights of the gel sample in swollen and in dry state, respectively..5. Sorption of UO þ by hydrogels Dry hydrogel samples with the weight approximately 0.01 g were immersed into 50 ml solutions of UO (NO 3 ) Æ 6H O of different concentration ( ppm) for 4 days at room temperature. The concentration of residual UO þ ions in solution was determined spectrophotometrically using Philips 8715 model PU UV vis spectrophotometer at the wavelength 430 nm. Sodium salicylate (10 wt%) with adsorption maximum at 430 nm wavelength was used as a complexing agent. The weight of absorbed uranyl ions was calculated according to the following formula: A ¼ ðc i CÞ10 6 V ; m where C i and C are concentrations of uranyl ions in solution before and after sorption, respectively, V is the volume of solution and m is the weight of dry gel..6. Infrared spectra FTIR spectra of dry hydrogel samples before and after sorption were recorded in KBr discs using Nicolet 50 FTIR spectrophotometer (Nicolet Instruments, Madison, WI)..7. Thermo gravimetrical analysis Thermogravimetric analysis of solid samples, PMAA, PEG and PEG MAA copolymer was performed in a Du

3 G.A. Mun et al. / Nucl. Instr. and Meth. in Phys. Res. B 65 (007) Pont 951 Thermogravimetric Analyzer (USA) at scan rate 10 /min in nitrogen atmosphere. The mass of samples was 5 mg in average. 3. Results and discussion Using of the irradiation for initiation of the polymerization has some advantages in comparison with chemical initiation: obtained polymers are highly pure, easily regulated dose rate, safe control of reaction, possibility to interrupt irradiation process, low temperature of reaction proceeding etc. In the present work the hydrogels containing complementary chains in their structure which are able to form interpolymer complexes (IPC) were obtained by c-initiated radiation copolymerization of PEG with MAA. Influence of irradiation dose, concentration of cross-linking agent, content of initial monomer mixture, etc. onto the yield and physic-chemical characteristics of the PEG MAA gels. The maximum gelation of copolymers was observed at dose 5 kgy. In Table 1 are presented results about influence of PEG molecular weight on yield of PEG MAA copolymers of different composition. It can be seen that increasing of the molecular weight of PEG and MAA content in monomer mixture results in higher gelation of copolymeric network. These regularities can be induced by formation of more amount of hydrogen bonds in copolymer which play role of the additional cross-links in structure of formed gel and therefore enhance the template polymerization process proceeding. Fig. 1 shows that growth of the PEG MAA copolymers gelation with increasing of PEG molecular weight is accompanied by decreasing of equilibrium swelling degree of the synthesized networks. Most probably this is due to growth of effectiveness of interpolymer complex formation process us a result of increasing of PEG chains length. Obtained results also confirm the assumption about template polymerization mechanism of copolymerization between PEG and PMAA molecules due to formation of hydrogen bonds. In the present work the influence of PEG MAA hydrogel composition on their thermal stability in temperature interval from 50 to 700 C was studied. The results of thermogravimetrical analysis of PMAA, PEG and PEG MAA hydrogels are presented in Fig. (a) and (b). On thermogram of the copolymer the two main decomposition stages are observed due to presence of PMAA chains in its structure. It is known that degradation of polyacid proceeds in two stages [9,10] unlike PEG degradation. The first weight loss is mainly due to anhydride formation reaction and the second-decomposition of polymer which is followed by decarboxylation reaction. Also it was shown that increasing of PEG content in PEG MAA copolymer results in Fig. 1. Dependence of the gelation and equilibrium swelling degree of PEG MAA hydrogels from PEG molecular weight. [PEG]: [MAA] = 40:60 mol%; [TEGDMA] = 0.1 mol%; D = 5 kgy. Table 1 Gelation of PEG MAA hydrogels [PEG]:[MAA] (mol%) Gelation, G (%) M W(PEG) = 000 M W(PEG) = M W(PEG) = : : : Dose 5 kgy. Fig.. TGA thermograms (a) and its derivative TGA curves (b) PEG MAA hydrogels. [PEG]:[MAA], mol% = 64.7:35.3 (1); 50:50 (); 40:60 (3); MM (PEG) =

4 38 G.A. Mun et al. / Nucl. Instr. and Meth. in Phys. Res. B 65 (007) Table Influence of the ph and molecular weight of the PEG onto structure parameters of PEG MAA hydrogels Structure parameters of network ph, (M W(PEG) ) = 10000) M W(PEG) m m G (kpa) m e M c(ef) (g/mol) [PEG]:[MAA] = 50:50 mol%. shifting of the maximum rate of decomposition in the low temperature range because thermal stability of PEG is smaller than PMAA. Thus hydrogel composition [PEG]: [PMAA] = 40:60 N max = 436 C, 50: C, 64.7: C and for PEG hydrogel 404 C. Mechanical properties of the swelled networks were studied by using universal test-machine Zwick Z010. In Table are presented the parameters of the network structure calculated from results of mechanical test of swelled hydrogels samples. It is shown that the coefficient of elasticity (G) of the PEG MAA copolymers increases in the solutions with lower ph and in the hydrogels samples obtained on basis of PEG with higher molecular weight. It is must be induced by increasing of the intermolecular interaction efficiency between PEG and PMAA chains which form additional cross-links in hydrogel by hydrogen bonds. At the same time, effective cross-link density (m e ) and equilibrium polymer volume fraction in gel (m m ) increase efficiently. Also the calculation of the effective molecular weight between cross-links in hydrogel (M c(ef) ) showed that in alkaline medium the value of M c(ef) significantly increase due to PMAA carboxyl groups dissociation and consequently hydrogen bonds destruction which are form additional cross-links in network structure. The structural feature of hydrogels based on PEG MAA copolymers is the possibility of specific interactions between unionised carboxylic groups of MAA and ether oxygen of PEG via hydrogen bonds at low ph values. Considering such possibility we have studied the sorption of uranyl ions by these hydrogels. For comparison the hydrogels of pure PMAA were studied additionally as a control network without interpolymer interaction. Fig. 3 shows adsorption isotherms of hydrogels of PEG MAA and PMAA obtained by plotting its adsorption capacities versus equilibrium UO þ ions concentration. The sorption of uranyl ions by hydrogels is accompanied by acquiring of yellow colour by the samples. It can be seen Fig. 3. Adsorption isotherms of UO þ ions adsorption onto PMAA (1) and PEG MAA (,3) hydrogels. [PEG]:[MAA] = 50:50 (); 40:60 (3). Fig. 4. FTIR spectra of PMAA (a) and PEG MAA (b) hydrogels before (1) and after () UO þ adsorption. [PEG]:[MAA] = 40:60, C UN = 500 ppm.

5 G.A. Mun et al. / Nucl. Instr. and Meth. in Phys. Res. B 65 (007) Fig. 5. Changing of adsorption ability of UO þ ions by PMAA and PEG MAA hygrogels in reuse. PEG MAA (00 ppm); (100 ppm); PMAA (100 ppm) [PEG]:[MAA] = 40:60 mol%. from figures that adsorption becomes more pronounced at higher concentration of the uranyl ions. The copolymers composition almost do not influence onto adsorption capacity of the networks. The shapes of both curves represent the S-type adsorption isotherm according to the Giles classification for adsorption of solutes from solutions [11]. It can be seen from the figure that both hydrogels adsorb almost the same amount at low concentration of uranyl ions ( ppm) in spite of the lower content of carboxylic groups of MAA in copolymer in comparison with PMAA hydrogel. It can be attributed to conformational peculiarities of the first network. It is known from the literature that pk a of MAA is 4.3 [1] and ph of the sorption medium at these concentrations is around 4.1, so MAA- MPEG hydrogel is in complexed state and probably hydrogen bonds of polycomplex make pores, which can be additional sites for uranyl ion adsorption. Besides the coordination binding interaction of ether oxygen with UO þ can play some role in adsorption. The sorption of uranyl ions by the hydrogels can be followed by FTIR spectroscopic analysis of dried samples. Fig. 4(a) and (b) shows the FTIR spectra of initial PMAA and PEG MAA hydrogels as well as of samples after sorption of uranyl ions. It can be seen the appearance of new band at 1539 and 153 cm 1 in the spectra of PMAA and PEG MAA hydrogels saturated by UO þ ions, respectively, responsible for asymmetric vibration signals of carboxylate anions indicates the ionization of carboxylic groups caused by formation of ionic bonds with uranyl ions. A new specific band at 937 cm 1 belongs to deformation vibrations of O U O and confirms binding of metal ions by networks. The possibility of the reuse of PEG MAA copolymers in the adsorption and desorption processes in comparison with PMAA hydrogel was investigated. Hydrochloric acid usually is used for regeneration of hydrogels in process of metal ions desorption and in some cases it is led to destruction of polymeric hydrogels and restrict their reuse. In the present work the hydrochloric acid was also used for desorption process study. From Fig. 5 it can be seen that PMAA hydrogel sample destruct in aqueous solution of acid after the second cycle of sorption desorption process. At the same time copolymer hydrogel samples keep their adsorption ability after five cycles of reuse. Probably, intermolecular interactions with hydrogen bonds formation between carboxyl group of PMAA and ether oxygen of PEG enhance mechanical and deformation characteristics of PEG MAA gel and promote its higher firmness to the hydrochloric acid action in comparison with PMAA hydrogel. 4. Conclusions The water swelling polymeric hydrogels were obtained by c-initiated radiation copolymerization of PEG with MAA and TEGDMA as cross-linking agent. The main regularities of PEG MAA network formation, deformation properties, structure characteristics of cross-linked copolymers and thermal characteristics depending on their composition, molecular weight of PEG have been studied. It was shown that increasing of molecular weight of PEG and MAA content in monomer mixture results in higher gelation and cross-linking density of copolymeric network. The sorption of uranyl ions by PEG MAA hydrogels in

6 384 G.A. Mun et al. / Nucl. Instr. and Meth. in Phys. Res. B 65 (007) comparison with homopolymer PMAA has been studied. It was demonstrated that they can be used as effective uranyl ions sorbents and their adsorption capacity depends on uranyl ions concentration in surrounding solution. Also it was found that because of the presence of PEG chains, which causes the formation of IPC in copolymeric hydrogels structure, PEG MAA hydrogels are characterized by ability for regeneration in desorption process in comparison with homopolymer of PMAA gel. References [1] V.V. Khutoryanskiy, P. Kujawa, Z.S. Nurkeeva, J.M. Rosiak, Macromol. Chem. Phys. 0 (001) [] K. Yamashita, T. Nishimura, M. Nango, Polym. Adv. Technol. 14 (003) 189. [3] P. Akkas, O. Güven, J. Appl. Polym. Sci. 78 (000) 84. [4] T. Caykara, S. Ören, Ö. Kantoğlu, O. Güven, J. Appl. Polym. Sci. 77 (000) [5] S. Ören, T. Caykara, Ö. Kantoğlu, O. Güven, J. Appl. Polym. Sci. 78 (000) 19. [6] Ö. Kantoğlu, M. Sßen, O. Güven, Nucl. Instr. and Meth. B 151 (1999) 18. [7] J. Klier, A.B. Scranton, N.A. Peppas, Macromolecules 3 (1990) [8] A.M. Lowman, N.A. Peppas, Macromolecules 30 (17) (1997) [9] H.G.J. Schild, Polym. Sci.: Part A: Polym. Chem. 31 (1993) 403. [10] B. Ho, Y. Lee, W.J. Chin, Polym. Sci.: Part A: Polym. Chem. 30 (199) 389. [11] C.H. Giles, T.H. MacEwan, S.N. Nakhwa, D. Smith, J. Chem. Soc. (1960) [1] M.A. Lagutina, G.V. Rakova, N.V. Yarygina, S.A. Dubrovskii, K.S. Kazanskii, Vysokomolek. Soed. A. 44 (8) (00) 195.