Coordination of uranium aquo chloro and sulfato complexes under different redox conditions

Natural aquatic systems may exert large variations in actinide oxidation states because of limited oxygen diffusion, and microbially and inorganic (surface catalyzed) redox processes. Redox state and coordination hence is a key variable controlling uranium mobility at former uranium mining areas. We have developed a spectro-electrochemical cell, which allows to study the structure speciation of aqueous U(VI) and U(IV) complexes in situ by X-ray absorption spectroscopy, while applying and maintaining a constant potential [1]. Here we present data on the influence of redox state on the uranium coordination by chloride and sulfate. The reduction of U(VI) to U(IV) involves an electron transfer and a chemical reaction transforming the trans-dioxo cation to the spherically coordinated U4+ cation. With [Cl] increasing from 0 to 9 M, we observed for U(VI) the complexes UO2(H2O)52+, UO2(H2O)4Cl+, UO2(H2O)3Cl20 and UO2(H2O)2Cl3, and for U(IV) the complexes U(H2O)94+, U(H2O)8Cl3+, U(H2O)6-7Cl22+ and U(H2O)5Cl3+ [1]. Elevated sulfate concentrations have been monitored at the former uranium mine Königstein/Germany, where uranium was leached by sulfuric acid. We observed that tetra- and hexavalent uranium forms both monodentate and bidentate complexes with sulfate. However, with increasing [SO42-] concentration the bidentate coordination prevails. The observed formation of soluble U(IV) sulfato complexes may prevent uraninite precipitation even at low pH, thus increasing the mobility of uranium even under reducing conditions.