EXAFS Determinations of Uranium Structures:  The Uranyl Ion Complexed with Tartaric, Citric, and Malic Acids

U LIII-edge EXAFS spectra were measured for 1:1 mixtures of the uranyl ion with tartaric, malic, and citric acids. The room-temperature EXAFS spectra of these mixtures demonstrate the characteristic uranium−oxygen interactions of the uranyl ion UO22+ and also show a U−U interaction at 3.92 Å, indicating the presence of a uranyl dimer. On the basis of models proposed by Martell et al. from earlier potentiometric titration experiments, it is concluded that the dimer is the ligand-bridged species (UO2)2(L)2, in which bridging occurs through the α-hydroxyl groups of the polycarboxylate ligands.