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 Pt4+ 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 degrees 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.