Factors influencing uranium reduction and solubility in evaporation pond sediments

Evaporation ponds in the San Joaquin Valley (SJV), CA, USA that are used for the disposal of irrigation drainage waters, contain elevated levels of U that may be a threat to pond wildlife. The ponds support euryhaline algae, which become incorporated in the sediments as depositional organic matter (OM) – facilitating reducing conditions. Our earlier studies have shown that U in one SJV sediment was primarily present as the highly soluble U(VI) species (as opposed to the less soluble U(IV) species), despite the presence of volatile sulfides. In this research, we investigated the effects of native pond algae (Chlorella) and potential reducing agents on U redox chemistry of SJV pond sediments. San Joaquin Valley pond sediments were equilibrated with natural and synthetic pond inlet waters containing approximately 10 mg U(VI) L-1 to which reducing agents (acetate, sucrose, and alfalfa shoot) were added. The equilibrations were done under oxic (Chlorella only) and O2-limiting conditions (remaining treatments). Sediments were examined for changes in average U oxidation state by X-ray near- edge absorption structure (XANES) spectroscopy and U concentration by ICP-MS.For the alfalfa treatments, a 95 percent loss of U(VI) from solution, the presence of sulfides, and results from the XANES studies suggest U(VI) was reduced to U(IV). Upon exposure to air, the precipitated U was readily oxidized, suggesting the reduced U is susceptible to oxidation. Much less reduction of U(VI) was observed in the other 3 treatments and the solid phase was dominated by U(VI) as in the natural pond sediments. A second study was conducted with pond sediment-water suspensions to determine the effects of controlled PCO2 and low redox potential (Eh) on U solubility. These suspensions were equilibrated at 0.22 and 5.26 kPa PCO2 and allowed to free-drift from an oxidized to a reduced state. At high Eh and high PCO2, dissolved U concentrations were higher than in the low PCO2 systems due to greater complexation with CO3. Dissolved U concentrations decreased only under intense sulfate reducing conditions, even at low Eh conditions. It appears that U reduction occurred by chemical reduction via sulfide ion. Comparing the XANES data of the pond sediments with the laboratory-produced solids we conclude that biosorption by algae and bacteria is the dominant mechanism depositing U in the sediments. Even though there are organisms that can use U(VI) as a terminal electron acceptor, we found that sulfate reduction was preferred in these high- SO4 waters. Mixed oxidation state U-solids were preferentially formed in the pond sediments and in the lab except under intense SO4 reducing conditions.