Comparison of complexed species of Eu in Alumina-bound and free Polyacrylic Acid. A spectroscopic study.

The speciation of Eu complexed with polyacrylic acid (PAA) and alumina-bound PAA
(PAAads) was studied at pH=5 in 0.1M NaClO4. Structural parameters were obtained from 7F0→5D0 excitation spectra measured by Laser Induced Fluoresence ectroscopy as well as from Eu LIII edge Extended X-ray Absorption Fine Structure (EXAFS) spectra. The mode of coordination was also investigated by Infra Red pectroscopy. To elucidate the nature of the complexed species, Eu-acetate complexes were used as references. The spectroscopic techniques show that two carboxylate groups with 2-3 (EuPAA) and 4-5 (EuPAAads) hydration water molecules are coordinated to Eu in the first coordination sphere. For EuPAAads, the coordination between carboxylate groups and Eu appears to be bidendate. A similar coordination is probable for EuPAA but the EXAFS data indicate a sligthly distorted coordination. The results show that the degree of freedom of carboxylate groups is not the same for free or adsorbed PAA. For PAA, the degree of freedom is constrained by the flexibility of the methylene chain. When PAA is adsorbed on alumina, the polymer chains cannot any more be treated as independent chains. One may rather assume formation of aggregates that form an organic layer at the mineral surface presenting a complex arrangement of carboxylate groups.