EDTA hydrogels

ARTICLE IN PRESS

Radiation Physics and Chemistry 74 (2005) 310–316 www.elsevier.com/locate/radphyschem

Studies on radiation synthesis of PVA/EDTA hydrogels Sanju Francis, Lalit Varshney ISOMED, Radiation Technology Development Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received 16 August 2004; accepted 10 February 2005

Abstract Aqueous solutions of polyvinyl alcohol (PVA) containing different amounts of ethylenediaminetetraacetic acid (EDTA) were gamma irradiated from a Co-60 source to form PVA–EDTA hydrogels. The effect of irradiation dose and EDTA content on the specific viscosity (in feed), gel percentage and equilibrium degree of swelling (EDS) was investigated. The amount of EDTA in the hydrogel matrix was found to be considerably lower than in the feed composition. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) technique was used to determine the amount of Pb(II), Cd(II) and Hg(II) absorbed by the hydrogel. The incorporation of EDTA moieties onto PVA network results in a hydrogel which has absorption affinity for Pb(II) and Cd(II). The maximum chelating capacities of the hydrogel were observed to be 8.5 mg/g for Pb(II) and 4.2 mg/g for Cd(II). No affinity for absorption of Hg(II) by the hydrogel was observed. The thermal stabilities of the prepared hydrogels were characterized by TGA. The efficiency of EDTA in the hydrogel matrix for chelation was observed to be approximately 10% of that of pure EDTA in solution. r 2005 Elsevier Ltd. All rights reserved. Keywords: Gamma radiation; Polyvinyl alcohol; EDTA; Chelating polymers; Hydrogels

1. Introduction Many of the known heavy metal ions are highly toxic and if they are discharged into natural water resources, can potentially contaminate the environment. Some of the metal ions currently identified for regulation under the Resource Conservation and Recovery Act (RCRA) and Safe Drinking Water Act (SDWA) are Cu2+, Ag+, Hg2+, Cd2+, Pb2+ and Tl+. These are persistent environmentally toxic substances because these cannot be rendered harmless by chemical or biological remediation (Devi and Fingermann, 1995). There are a number of ways of separating heavy metal ions from aqueous

Corresponding author. Tel.: +91 22 25595689;

fax: +91 22 25505338. E-mail address: [email protected] (L. Varshney).

media like solvent extraction, ion-exchange chromatography, chemical precipitation, etc. Metal-complexing or chelating polymers refers to polymers that bind metal ions by coordinating interaction and sometimes by ionic interactions. Chelating polymers are very useful for the purpose of selective absorption of certain metal ions from their mixtures, removal of metal ions and recovery of soluble metal ions (Geckeler, 1996). Absorption of toxic metal ions using chelating polymers has the advantage of high efficiency, easy handling, availability of different adsorbents and cost effectiveness. Removal of metal ions by chelating polymers has great importance, especially in environmental applications (Chanda and Rempel, 1997; Saglam et al., 2001). The chelating polymers are insoluble in water (has a crosslinked matrix) and are characterized by two components: the polymer backbone, which provides the stability and the functional groups, which are

0969-806X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2005.02.003

ARTICLE IN PRESS S. Francis, L. Varshney / Radiation Physics and Chemistry 74 (2005) 310–316

necessary for metal complexation. The base can be a natural polymer such as cellulose, chitin, chitosan or synthetic such as polyamines, polyacrylamides and polyacrylic acid. The most common coordinating atoms present in the main or side chain are N, O, P and S. It is possible to make chelating polymers that have selective absorption capacity for certain metal ions by fixing desired ligand groups on to the polymer backbone. For example, polymers containing amidoxime groups have high absorption ability for uranyl ions and are being tried for the recovery of uranium from seawater (Nonaka, 1996; Badawy, 2003). The selectivity of a chelating resin is based on the different stability of metal complexes at the appropriate pH values. Ethylenediaminetetraacetic acid (EDTA) is the most widely used complexing agent in the cosmetic and pharmaceutical industry. The molecule has six potential sites for binding a metal ion, the four carboxyl groups and the two amino groups. It combines with metal ions to form stable chelates, in a 1:1 ratio regardless of the charge on the cation. Hydrogels, which are threedimensional network structures of polymers contain significant amount of water (50–90%) and therefore can have free exchange of water within the external contaminated water preparations. These are insoluble in water and can therefore be easily separated. Incorporation of EDTA in such a matrix would make the hydrogel metal ion chelating. The main objective of the present study was to ascertain if some concentration of EDTA could be incorporated in the hydrogel matrix by radiation processing, while retaining its chelating property. This could be possible due to radical–radical interactions of polyvinyl alcohol (PVA) and EDTA forming covalent bonds. Instances of the use of this technique have been reported in the literature (Bodugoz et al., 1999; El-Hag Ali et al., 2003; Inam et al., 2003). 2. Experimental 2.1. Materials PVA with an average molecular weight of 125 000 (degree of hydrolysis: 86–89%) was obtained from S.D. Fine Chemicals Ltd., Mumbai, India. EDTA and HgCl2 was obtained from British Drug Houses (India) Pvt. Ltd. Pb(NO3)2 and Cd(NO3)
4H2O were obtained from E. Merck (India) Ltd. All the reagents were of analytical grade and used without further purification. Double distilled water (conductivity o1.5 mS) was used for the synthesis of the hydrogels and all other experiments. 2.2. Preparation of the hydrogels A 5% (w/w) aqueous solution of PVA was prepared at 80 1C with constant stirring of the solution.

311

For the preparation of hydrogels containing EDTA moieties, the solution was cooled to room temperature and different quantities (1–5 g) of EDTA were dissolved in 100 ml of the PVA solution. The solution was allowed to stand overnight and then it was taken in cylindrical glass vials of 20 mm diameter. The samples were irradiated to graded doses, up to a maximum dose of 100 kGy, in a 60Co Gamma Chamber at a dose rate of 7 kGy/h. The temperature inside the Gamma Chamber was 4871 1C. The cylindrical hydrogels obtained were washed several times with distilled water (Soxhlet extraction) to remove the unreacted species and dried in an air oven at 70 1C until constant weight. 2.3. Viscosity Viscosity measurements were carried out using an Ubbelhode-type viscometer at 2471 1C. The specific viscosity (Zspp ) of the solutions containing different amounts of PVA and EDTA were calculated with respect to aqueous 5% PVA solution using the relation ½Zsp  ¼ ðt  t0 Þ=t0 ,

(1)

where t0 and t are the flow time for unirradiated aqueous 5% PVA solution and the irradiated solutions of different compositions, respectively. 2.4. Gel percentage The hydrogel samples (about 1 g) were taken in a Soxhlet thimble and extracted with distilled water for about 6 h to remove the soluble fraction. The gels were dried to constant weight to determine the insoluble fraction in the samples gravimetrically. Gel % ¼ W g =W 0 100,

(2)

where Wg is the weight of dry gel after extraction and W0 is the initial weight of the gel (calculated from the amount of PVA and EDTA in the feed solution). 2.5. Equilibrium degree of swelling (EDS) The EDS was determined gravimetrically. The gel samples, dried to constant weight after the Soxhlet procedure, were immersed in excess distilled water at room temperature for 48 h. The hydrogels were weighed after the excess water was removed with a filter paper. The EDS was calculated as: EDS ¼ ðW e  W d Þ=W d ,

(3)

where We is the weight of the swollen gel at equilibrium and Wd is the initial weight of the dried gels.

ARTICLE IN PRESS 312

S. Francis, L. Varshney / Radiation Physics and Chemistry 74 (2005) 310–316

2.6. Thermogravimetric analysis (TGA)

3. Results and discussion

TGA analyses were carried out using a Mettler TA 3000 thermal analysis system (TG 50). About 10 mg of the dried gels was weighed into an alumina crucible and the profiles were recorded from 35 to 800 1C at a heating rate of 10 1C/min. The flow rate of air was maintained at 50 ml/min.

PVA is known to undergo radiation-induced crosslinking in aqueous solution (Rosiak and Yoshii, 1999). Hydroxyl radicals have been shown to be the main species responsible for reactivity transfer from water to polymer chains by H atom abstraction and macroradical formation (kOH ¼ 1:5 108 dm3 mol1 s1 ). The reaction of the macroradicals results in the formation of crosslinked structure. The rate constant of PVA with e aq is lower (ke ¼ 9 106 dm3 mol1 s1 ) than with the hydroxyl radical.

2.7. Composition of the hydrogels The hydrogels formed on irradiation contains both the gel fraction (crosslinked part) and the sol fraction (soluble part). The gel fraction was determined as mentioned in Section 2.4. The soluble fraction contains the EDTA and PVA which did not take part in the crosslinking reactions. The amount of EDTA in the solution was determined by simple complexometric titration with standard CaCO3 using Eriochrome Black T as the indicator. After determining the concentration of EDTA in the extract, and knowing its initial concentration in the feed, the amount of EDTA incorporated in the hydrogel was calculated. The amount of PVA in the hydrogel (after washing) was calculated from the % gel and the amount of EDTA in the hydrogel matrix.

2.8. Absorption studies About 100 mg of the dried PVA/EDTA hydrogels were transferred to a 250 ml conical flask containing 50 ml of 100 ppm (mg/l) Pb(II), Cd(II) and Hg(II) solutions. The samples were allowed to equilibrate for 3 days. The solution was decanted and then filtered through 0.45 mm filter paper (Millipore Corporation). The filtrates were analyzed for the metal ions using inductively coupled plasma-atomic emission spectrophotometer (ICP-AES) Jobin Yvon Emission, JY 2000. Blank trials without the hydrogels were also carried out for all the metal ions. The amount of metal ions absorbed per unit mass of the adsorbent (A in mg/g) was calculated as follows:   C1V 1  C2V 2 A¼ , (4) W where V1 and V2 refer to the initial and final volume of the solution (l), W is the weight of the dry hydrogel (g), C1 and C2 are the concentration of metal ions (mg/l) before and after adsorption, respectively. The loss due to filtration was insignificant and the values given in the text are the average of two readings of the same set of measurement which were within 72%.

PVAðHÞþ OH ! PVA þ H2 O;

(5)

2PVA ! PVA  PVA ðCross-linked networkÞ.

(6)

g-radiolysis of EDTA results in its degradation which is proportional to the absorbed dose. The OH attack on EDTA is the most important reaction, whereas H and e aq are mostly involved in radical–radical reaction (Krapfenbauer et al., 1999; Krapfenbauer and Getoff, 1999). The rate constants are kOH ¼ 2 109 dm3 mol1 s1 at pH ¼ 9 (Buxton et al., 1988) and ke ¼ 4:7 106 dm3 mol1 s1 at pH ¼ 10 (Buitenhuis et al., 1977). The main decomposition products observed under g-irradiation are CO2, followed by N,N0 methylacetylethylenediamine and N-hydroxymethliminodiacetic acid. Ammonia and iminodiacetic acid were also found among the degradation products (Krapfenbauer and Getoff, 1999). When an aqueous solution of PVA is g-irradiated in the presence of EDTA, free radicals are formed on the polymer as well as on EDTA. The subsequent reactions of the radicals can result in the attachment of EDTA moieties (or the intermediates) to the PVA backbone. EDTAðHÞþ OH ! EDTA þ H2 O;

(7)

PVA  PVA þ EDTA ! PVA  PVA  EDTA: (8) As the irradiation dose is further increased, crosslinking of the polymer chains results in the formation of a hydrogel. 3.1. Viscosity Fig. 1 shows the variation of specific viscosity with the radiation dose for various feed compositions of the hydrogels. For all the plots (A–D), there is a small dip in the viscosity at the initial stages of irradiation which might be due to the oxidative degradation of the polymeric chains induced by the traces of oxygen left in the solution. For pure PVA solution (A), there is a sharp increase in the specific viscosity at a dose of 3 kGy which is due to the crosslinking of the polymer chains. The specific

ARTICLE IN PRESS S. Francis, L. Varshney / Radiation Physics and Chemistry 74 (2005) 310–316

313

6.0

4.5

80 70

4.0 GEL %

SPECIFIC VISCOSITY

5.0

A B C

90

A B C D

5.5

3.5 3.0 2.5

60 50

2.0 1.5

40

1.0 30

0.5 0.0

20

1

2

3

4

5

6

30

40

DOSE(kGy)

Fig. 1. Effect of g-irradiation dose on the specific viscosity [Zspp ] at various feed composition of the hydrogels with respect to 5% PVA solution. A ¼ 5% PVA solution, B ¼ 5% PVA, 1% EDTA, C ¼ 5% PVA, 3% EDTA, D ¼ 5% PVA, 5% EDTA.

viscosity could not be measured above an irradiation dose of 3.5 kGy due to the very high viscosity of the resulting solution. Further irradiation results in the formation of a hydrogel. Plots B, C and D show the specific viscosity of the solution containing increasing amount of EDTA (1%, 3% and 5%). As can be seen from the figure, the sharp increase in the specific viscosity of the solutions is delayed in the presence of EDTA. Higher the concentration of EDTA, the more delayed the change in specific viscosity. This is because, when PVA is irradiated in presence of EDTA, a considerable fraction of the primary radicals, produced by the radiolysis of water, is consumed by the EDTA molecules. Only the remaining fractions of radicals are available for the crosslinking of PVA which results in the formation of hydrogels.

3.2. Gel percentage Fig. 2 shows the variation of gel percentage of PVA/ EDTA hydrogels with the irradiation dose and varying amount of EDTA in the hydrogels. Plot A (5% PVA, 1% EDTA) has a high gel percentage of 85% even at a dose of 25 kGy. The maximum gel percentage of 91%, for hydrogel of this composition, is obtained at a dose of 50 kGy. Further increase in irradiation dose decreases the gel percentage. Plot B (5% PVA, 3% EDTA) shows a gel percentage of 59% at 25 kGy dose which gradually increases to 69% at the maximum dose of 100 kGy. Plot C (5% PVA, 5% EDTA) shows the minimum gel percentage of 35% at 25 kGy, which gradually increases to 63% at the maximum dose of 100 kGy.

50

60

70

80

90

100

110

DOSE(kGy)

7

Fig. 2. Effect of g-irradiation dose on the gel percentage of PVA/EDTA hydrogels at different feed ratios of PVA and EDTA. A ¼ 5% PVA, 1% EDTA, B ¼ 5% PVA, 3% EDTA, C ¼ 5% PVA, 5% EDTA.

35

30

25

EDS

0

20

15

10 0

1

2

3

4

5

% EDTA

Fig. 3. Effect of EDTA concentration (in the feed) on the EDS of the hydrogels synthesized at 25 kGy.

3.3. Equilibrium degree of swelling (EDS) The swelling of the hydrogel increases with time and reaches a limiting value, the EDS in 23–25 h. Fig. 3 shows the variation in EDS of the hydrogels synthesized at 25 kGy, with the increase in the concentration of EDTA in the feed solution. Pure PVA hydrogel (without EDTA) has an EDS of 10 g/g which increases in an almost linear manner and attains the maximum value of 31 g/g when the concentration of EDTA is 5% (the highest). A similar trend was observed at other irradiation doses.

ARTICLE IN PRESS S. Francis, L. Varshney / Radiation Physics and Chemistry 74 (2005) 310–316

314

100

A B C

30

EDTA PVA

I 80

II

Weight Loss

EDS

25

20

I

60

III 40

15

20

10

0

II

-20 100

5 20

30

40

50

60

70

80

90

100

200

DOSE(kGy)

Fig. 4. Effect of g-irradiation dose on the EDS of the hydrogels containing different concentration of EDTA. A ¼ 5% PVA, 1% EDTA, B ¼ 5% PVA, 3% EDTA, C ¼ 5% PVA, 5% EDTA.

300

400

500

600

700

800

Temperature (°C)

110

Fig. 5. TGA profiles of EDTA and g-radiation crosslinked PVA (5% PVA, 100 kGy).

100

3.4. Thermo gravimetric analysis (TGA) Fig. 5 shows the TGA profiles of EDTA and dried PVA hydrogels. Three steps (I, II and III) were observed in the profile of EDTA. EDTA starts losing weight at around 110 1C (step I). When the pure samples were heated up to 800 1C, the weight loss for EDTA was 80%. Pure PVA shows two steps (I and II) in the thermal profile, of which I step is more prominent. The weight loss for pure PVA starts at around 300 1C and at 800 1C, the weight loss was almost 100%.

80

Weight Loss

Fig. 4 shows the variation of EDS of the hydrogels containing different concentration of EDTA, as a function of the irradiation dose. The hydrogel containing the maximum amount of EDTA has the highest EDS at all the irradiation doses (25–100 kGy) in the present study. EDTA has four carboxyl groups which ionize according to the pH of the medium. The values of pK are pK 1 ¼ 2:0; pK 2 ¼ 2:7; pK 3 ¼ 6:2 and pK 4 ¼ 10:3 at 20 1C (Skoog et al., 2001). The presence of ionic groups in the hydrogel improves the swelling characteristic of the hydrogels. Moreover, the presence of EDTA in the hydrogel feed solution could decrease the crosslink density of the hydrogels, because some of the radicals responsible for crosslinking are consumed by the EDTA molecules. The EDS of the hydrogels is found to decrease with the increase in the irradiation dose. This is because the crosslink density increases with the irradiation dose and at high crosslink density the free volume available for swelling of the hydrogels is reduced by the formation of a rigid network structure.

60

40

A 20

B

C D

0 -20 100

200

300

400

500

600

700

800

Temperature (οC)

Fig. 6. TGA profiles of g-radiation crosslinked PVA/EDTA hydrogels of different compositions, synthesized at 100 kGy. A ¼ 5% PVA, B ¼ 5% PVA, 1% EDTA, C ¼ 5% PVA, 3% EDTA, D ¼ 5% PVA, 5% EDTA.

Fig. 6 shows the TGA profiles of the three washed and dried PVA/EDTA hydrogels along with the profile for pure PVA. From the figure it is clear that the profiles of PVA/EDTA hydrogels are similar to the profile of pure PVA. In fact, it is difficult to identify the different PVA/ EDTA hydrogel profiles from the figure. This means that the EDTA in the hydrogel is not independent but linked to PVA chains.

3.5. Composition of the hydrogels Table 1 shows the amount of EDTA in the feed (initial composition) as well as in the hydrogel matrix. The % of gelation is also given in the table. The amount

ARTICLE IN PRESS S. Francis, L. Varshney / Radiation Physics and Chemistry 74 (2005) 310–316

315

Table 1 Weight % of EDTA in the feed solution and in the hydrogel matrix and the % gelation of the hydrogels (synthesized at 100 kGy) Hydrogels

PVA/EDTA-1 (5% PVA, 1% EDTA) PVA/EDTA-3 (5% PVA, 3% EDTA) PVA/EDTA-5 (5% PVA, 5% EDTA)

Weight % of EDTA

% Gelation

In feed

In hydrogel

16.7 37.5 50.0

6.1 18.8 38.5

of EDTA in the hydrogel was determined as mentioned in Section 2.7 and the amount of PVA in the hydrogel (after washing) was calculated from the % gel and the amount of EDTA in the hydrogel matrix For example, the amount of EDTA in PVA/EDTA-1 feed is 16.7% (% EDTA ¼ 1/(1+5) 100, if the water is removed). The amount of EDTA estimated in the hydrogel matrix is only 6.1%. Similar results were obtained for PVA/ EDTA-3 and PVA/EDTA-5, and are shown in Table 1. G(-EDTA) ¼ 2 is reported in the literature for EDTA/ water system (Krapfenbauer et al., 1999; Krapfenbauer and Getoff, 1999). But in the present study, PVA protect the degradation of EDTA to some extent and EDTA is incorporated in the PVA hydrogel matrix though the amount is considerably less than the amount present in the initial feed solution. 3.6. Absorption studies The metal absorption by polymeric resins depends on several factors like the composition of the absorbent, pH of the solution, ionic charge, concentration of metal, etc. (Kesenci et al., 2002). The results of the preliminary investigation are summarized in Table 2. Pure PVA hydrogels did not show any uptake of Pb(II), Cd(II) or Hg(II) under the conditions of the experiment. The binding capacities (single component chelation) of PVA/ EDTA-3 (5% PVA, 3% EDTA) hydrogel synthesized at 100 kGy are 8.5 mg/g for Pb(II) and 4.2 mg/g for Cd(II). The hydrogel did not show any affinity for Hg(II) under the conditions of the experiment. The affinity order of the metal ions at 100 ppm is Pb(II)4Cd(II). Table 2 shows that the weight % of EDTA in the PVA/EDTA-3 hydrogel is 18.8% which means that the hydrogel should theoretically adsorb 0.5 mM (100 mg of Pb(II), 55 mg of Cd(II) and 100 mg of Hg(II)) of the metal ions (per gram) of the hydrogel. But this value is not realized in the present study which might be attributed to the inaccessibility of the sorption sites in the interior of the hydrogel matrix due to the rigid network of the hydrogel matrix (Chanda and Rempel, 1997). The efficiency of EDTA in the hydrogel matrix for chelation was observed to be approximately 10% of that of pure EDTA in solution.

76 69 63

Table 2 Absorption of metal ions (mg/g dry gel) by PVA/EDTA-3 hydrogel (synthesized at 100 kGy) Absorption

Metal ions

Metal uptake

pH

Single component chelation

Pb+2 Cd+2 Hg+2

8.5 4.2 —

5.1 5.7 4.5

Multicomponent chelation

Pb+2 Cd+2 Hg+2

5.6 4.3 —

5.2

When a mixture of Pb(II), Cd(II) and Hg(II) was used at a concentration of 100 ppm each (multicomponent chelation), the hydrogel showed the same order of affinity. The binding capacities were 5.6 mg/g for Pb(II) and 4.3 mg/g for Cd(II). No absorption affinity for Hg(II) was observed. This means that the hydrogel could be used to selectively adsorb Pb(II) or Cd(II) from solutions containing Hg(II).

4. Conclusion Ethylenediaminetetraacetic acid (EDTA) moieties were introduced into polyvinyl alcohol (PVA) hydrogel by gamma irradiation from a Co-60 source. Viscosity studies show that the onset of crosslinking in the hydrogels is delayed in the presence of EDTA. The gel percentage was found to increase almost linearly with the irradiation dose for PVA/EDTA-3 (5% PVA, 3% EDTA) and PVA/EDTA-5 (5% PVA, 5% EDTA) hydrogels. The PVA/EDTA-1 (5% PVA, 1% EDTA) hydrogel showed the maximum gel percentage at 50 kGy, which further reduced with the increase in dose up to 100 kGy. The swelling studies show that all the hydrogels attain the equilibrium value in 23–25 h and the EDS was found to increase with the concentration of EDTA in the feed. TGA analyses indicate the thermal properties of PVA/EDTA hydrogels were similar to that of pure PVA hydrogel. Though a relatively high concentration of EDTA was taken in the feed, the

ARTICLE IN PRESS 316

S. Francis, L. Varshney / Radiation Physics and Chemistry 74 (2005) 310–316

fraction of EDTA in the hydrogel matrix was considerably low. Absorption studies show that the incorporation of EDTA moieties results in a hydrogel which shows affinity for Pb(II) and Cd(II). However, no absorption of Hg(II) by the hydrogel was observed. The chelating efficiency of EDTA in the hydrogel matrix was observed to be approximately 10% of that of pure EDTA in solution.

References Badawy, S.M., 2003. Uranium isotope enrichment by complexation with chelating polymer adsorbent. Radiat. Phys. Chem. 66, 67–71. Bodugoz, H., Pekel, N., Guven, O., 1999. Preparation of poly(vinyl alcohol) hydrogels with radiation grafted citric acid and succinic acid groups. Radiat. Phys. Chem. 55, 667–671. Buitenhuis, R., Bakker, C., Stock, F., Louwrier, P.W.F., 1977. Rate constant for the reaction of e aq with EDTA and some metal–EDTA complexes. Radiochim. Acta 24, 189–192. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals d d ( OH/O ) in aqueous solution. J. Phys. Chem. Ref. Data 17 (2), 513–886. Chanda, M., Rempel, G.L., 1997. Chromium (III) removal by poly (ethyleneimine) granular sorbents made by a new process of templated gel filling. React. Funct. Polym. 35, 197–207. Devi, M., Fingermann, M., 1995. Inhibition of acetylcholinesterase activity in the central nervous system of the red swamp crayfish, Procambarus clarkii, by mercury, cadmium, and lead. Bull. Environ. Contam. Toxicol. 55, 746–756.

El-Hag Ali, A., Shawky, H.A., Abd El Rehim, H.A., Hegazy, E.A., 2003. Synthesis and characterization of PVP/AAc copolymer hydrogel and its applications in the removal of heavy metals from aqueous solution. Eur. Polym. J. 39, 2337–2344. Geckeler, K.E., 1996. Metal complexation polymers. In: The Polymeric Materials Encyclopedia. CRC Press, Inc., Boca Raton, FL. Inam, R., Caykara, T., Kantoglu, O., 2003. Polarographic determination of uranyl absorption onto poly(acryamide-gethylenediaminetetraacetic acid) hydrogels in the presence of cadmium and lead. Nucl. Instrum. Methods Phys. Res. B 208, 400–404. Kesenci, K., Say, R., Denizli, A., 2002. Removal of heavy metal ions from water by using poly(ethyleneglycol dimethacrylate-co-acrylamide) beads. Eur. Polym. J. 38, 1443–1448. Krapfenbauer, K., Getoff, N., 1999. Comparative studies of photo and radiation-induced degradation of aqueous EDTA. Synergistic effects of oxygen, ozone and TiO2. Radiat. Phys. Chem. 55, 385–393. Krapfenbauer, K., Robinson, M.R., Getoff, N., 1999. Development and testing of TiO2-catalysts for EDTA-radiolysis using g-rays (part 1). J. Adv. Oxid. Technol. 2, 213–217. Nonaka, T., 1996. Adsorptive resins. In: The Polymeric Materials Encyclopedia. CRC Press, Inc., Boca Raton, FL. Rosiak, J.M., Yoshii, F., 1999. Hydrogels and their medical applications. Nucl. Instrum. Methods Phys. Res. B 151, 56–64. Saglam, A., Bektas, S., Patir, S., Genc, O., Denizli, A., 2001. Novel metal complexing ligand: thiazolidine carrying poly(hydroxyethylmethacrylate) microbeads for removal of cadmium(II) and lead(II) ions from aqueous solutions. React. Funct. Polym. 47, 185–192. Skoog, D.A., West, D.M., Holler, F.J., 2001. Complexformation titrations. In: Fundamental of Analytical Chemistry. Harcourt Asia PTE Ltd., India, p. 280.