Abstract
Current research on bioremediation of uranium-contaminated groundwater focuses on supplying indigenous metal-reducing bacteria
with the appropriate metabolic requirements to induce microbiological reduction of soluble uranium(VI) to poorly soluble uranium(IV).
Recent studies of uranium(VI) bioreduction in the presence of environmentally relevant levels of calcium revealed limited and slowed
uranium(VI) reduction and the formation of a Ca-UO2-CO3 complex. However, the stoichiometry of the complex is poorly defined
and may be complicated by the presence of a Na-UO2-CO3 complex. Such a complex might exist even at high calcium concentrations,
as some UO2-CO3 complexes will still be present. The number of calcium and/or sodium atoms coordinated to a uranyl carbonate complex
will determine the net charge of the complex. Such a change in aqueous speciation of uranium(VI) in calcareous groundwater may
affect the fate and transport properties of uranium. In this paper, we present the results from X-ray absorption fine structure (XAFS)
measurements of a series of solutions containing 50 lM uranium(VI) and 30 mM sodium bicarbonate, with various calcium concentrations
of 0-5 mM. Use of the data series reduces the uncertainty in the number of calcium atoms bound to the UO2-CO3 complex to
approximately 0.6 and enables spectroscopic identification of the Na-UO2-CO3 complex. At nearly neutral pH values, the numbers
of sodium and calcium atoms bound to the uranyl triscarbonate species are found to depend on the calcium concentration, as predicted
by speciation calculations.
