Uranium and
arsenic often co-occur in nature, for example, in
acid mine drainage waters. Interaction with
arsenic is thus important to understand
uranium mobility in aqueous solutions. For the present study, EXAFS spectroscopy was used to investigate the formation and identify the structure of aqueous uranyl
arsenate species at pH 2. The nearest U-As distance of 3.39 Å, observed in
shock-frozen liquid samples, was significantly shorter than that observed in solid uranyl
arsenate minerals. The shorter bond length indicated that the
solution contained a bidentate-coordinated species, in contrast to the monodentate coordination in solid uranyl
arsenate minerals. The U-As coordination number of 1.6 implied that two uranyl
arsenate species with U:As ratios of 1:1 and 1:2 formed in nearly equal proportions and that the hydrated uranyl ion was present only as a minor component. The two uranyl
arsenate species could not be differentiated spectroscopically, since their U-As distances were equal. A comparison based on DFT modeling indicated for both the 1:1 and the 1:2 species, that the bidentate
arsenates were bound to
uranium with one of the binding
oxygen atoms being protonated. Based on the present spectroscopic study, the two species that will have to be considered in acidic
uranium-
arsenic-rich solutions are thus UO(2)H(2)AsO(4)(+), and UO(2)(H(2)AsO(4))(2)(0).