EXAFS Structural Analysis of Aqueous Uranium(VI) Complexes with Wood Degradation Products

To study the role of carboxylic and phenolic hydroxyl groups in the complexation of uranium(VI) with phenolic wood degradation products in aqueous solution, we prepared 1 mMol/L U(VI) complexes with the following simple model compounds in two pH ranges, i.e., 4-5 and 8-10: protocatechuic acid (3,4-dihydroxybenzenecarboxylic acid), catechol (1,2-dihydroxybenzene), pyrogallol (1,2,3-trihydroxybenzene), and vanillic acid (4-hydroxy-3-methoxybenzenecarboxylic acid). The structural parameters of these complexes were measured by uranium LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy.
In acid solutions containing 1:1 complexes with ligands having both carboxylic and hydroxyl groups, the uranium is surrounded by 5 equatorial oxygen (Oeq) at a distance of 2.44 Å and 2.46 Å for vanillic acid and protocatechuic acid, respectively. This relatively long U-Oeq bond distance is characteristic for bidentate coordination of the carboxylic group to the uranyl cation [Denecke et al., J. Phys. IV France 7, C2-637 (1997)]. If the ligands have only neighbouring hydroxyl groups as in case of catechol and pyrogallol the uranium is surrounded by 5-6 oxygen atoms at a shorter U-Oeq distance (2.39 Å). This shows that in the pH range of 4-5, the coordination behaviour of the acids is dominated by the carboxylic groups and not the hydroxyl groups. In the pH range of 8-10, the neighbouring phenolic hydroxyl groups dominate the com-plexation. This is in agreement with the observed similarities in the EXAFS structural parameters of the corresponding 1:2 and 1:3 complexes. In all these complexes the measured U-Oeq bond distance is 2.38 Å. It can be concluded that this relatively short U-Oeq bond distance is characteristic for the complexation of the uranyl ion with two or three ligands having neighbouring phenolic hydroxyl groups.