EXAFS structural analysis of aqueous uranium(VI) complexes with lignin degradation products

Uranium LIII-edge extended X-ray absorption fine structure (EXAFS) analysis was carried out on aqueous uranium(VI) complexes with monomeric intermediates of the natural wood-degradation process such as protocatechuic acid (3,4-dihydroxybenzoic acid), catechol (2-hydroxyphenol), pyrogallol (1,2,3-trihydroxybenzol), and vanillic acid (4-hydroxy-3-methoxybenzoic acid). The structural parameters of the 1:1 complexes with protocatechuic acid and vanillic acid in weakly acidic solution indicate that the carboxylic group coordinates the uranyl cation in a bidentate mode in the equatorial plane. Based on the identical structural parameters observed for protocatechuic acid, catechol and pyrogallol in alkaline media, we conclude that the coordination with uranium occurs via the neighboring OH groups of the phenols under formation of a five-membered ring.