Spacer Monomer in Polymer Chain Influencing Affinity of Ethylene Glycol Methacrylate Phosphate toward UO22+ and Pu4+ Ions

The complexation behavior of ligating groups bearing ethylene glycol methacrylate phosphate (EGMP) units spaced in the polymer chain was studied to understand the coordination ability of segregated EGMP toward UO₂²⁺ and Pu⁴⁺ ions. The EGMP units in the polymer chain were copolymerized with a varying mol proportion of spacer monomer N-isopropylacylamide (NIPA) and methyl methacrylate (MMA) along with a cross-linker. These copolymer gels were characterized by FTIR, SEM/EDS, and thermal analysis. It was observed that the copolymer gels had homogeneity and concentration of phosphate groups systematically decreased with an increase in mol proportion of spacer monomer units. The hydrophlicity of (EGMP-co-MMA) copolymer gels decreased with an increase in mol proportion of MMA units whereas hydrophilicty of EGMP-co-NIPA copolymer increased with an increase in mol proportion of NIPA units in the copolymer gels. It was observed that UO₂²⁺ ions sorption decreased with an increase in MMA units in the polymer chain at higher HNO₃ conc. (3 mol L–¹) but did not affect Pu⁴⁺ ions sorption. This seems to suggest that EGMP units segregated sufficiently in the polymer chain by spacers exhibit a remarkable selectivity toward Pu⁴⁺ ions with respect to UO₂²⁺ ions at high HNO₃ conc. Pu(IV) ions are known to have high affinity toward nitrate ions that help in the formation of a stable complex at higher acidity with a lesser number of coordination with phosphate groups. The experiments showed that the lower affinity of poly(EGMP-co-MMA) gel toward UO₂²⁺ ions was not due to dilution of phosphate groups concentration in the copolymer gel but could be attributed to the coordination requirement of UO₂²⁺ ions to form a stable complex at higher HNO₃ concentration. The X-ray photoelectron spectroscopy (XPS) and time-resolved laser-induced fluorescence spectroscopy (TRLFS) studies of poly(EGMP) loaded with UO₂²⁺ ions from aqueous solution having pH 2 and 3 mol L–¹ HNO₃ indicated that (i) 1:2 and 1:4 (UO₂²⁺–EGMP) complexes are formed at lower acidity and higher acidity, respectively, (ii) UO₂²⁺ ion is coordinated with water molecules at pH range in addition to the ion-exchange, and (iii) the complex formed in pH range has higher stability with respect to that form at 3 mol L–¹ conc. The higher hydrophilic poly(EGMP-co-NIPA) gels at 25 °C exhibit higher UO₂²⁺ uptake in 3 M HNO₃, as phosphate units in hydrophilic gel are mobile and could form stable complex involving 3–4 phosphoryl oxygen, which is not possible in collapsed state due to hydophobicity of polymer net work above critical temperature. However, there was no effect of hydrophilicty/hydrophobicity on the UO₂²⁺ ions sorption in the copolymer gels from solutions having lower HNO₃ conc. due to a requirement of lesser number of EGMP units to form a stable complex.