Determination of uranyl incorporation into biogenic manganese oxides using x-ray absorption spectroscopy and scattering

Abstract

Uranium is a toxic and radioactive contaminant in many settings, such as groundwaters and sediments. In oxidizing environments, uranyl (U(VI)) is thermodynamically the most stable oxidation state. Sorption or incorporation of U(VI) into reactive mineral phases are processes of major importance because they retard its transport. Biogenic manganese oxides are an important source of reactive mineral surfaces in the environment and may be potentially enhanced in bioremediation cases to improve natural attenuation. Experiments were performed in which U(VI) at various concentrations was present during manganese oxide biogenesis. At all concentrations there was strong uptake of U onto the oxides. Synchrotron based x-ray studies were carried out to determine the manner in which uranyl is incorporated into the oxide and how this incorporation affects the resulting manganese oxide structure and mineralogy. The EXAFS experiments show that uranyl does not appear to substitute into the lattice of the oxides, and is rather present as a strong surface complex. However, the presence of U(VI) on the Mn-oxide layers modifies the lattice constants and coherence lengths of the oxides. These results suggest a complex mechanism in which U transport is retarded by sorption and the surface area of the sorbent is increased.