Uranium is a contaminant of concern in subsurface environments impacted by mining or ore processing or in areas of naturally occurring uranium-bearing rocks. Hexavalent uranium is soluble and highly mobile, and represents a threat to water quality. Remediation approaches aim at immobilizing uranium, preferably in situ. Reductive precipitation is one such strategy and relies on microbial processes to reduce hexavalent uranium to its tetravalent state, which is largely insoluble. Two major types of U(IV) precipitates have been identified: crystalline U(IV) such as uraninite (UO2) and non-crystalline forms in which U(IV) is associated with a solid phase but coordinated with functional groups such as carboxylic or phosphato groups. The latter, referred to by some as non-crystalline U(IV) (NCU4), is not well characterized and its reactivity is not well constrained. In this work, we seek to investigate its stability under oxic and anoxic conditions. Non-crystalline U(IV) was produced through the reduction of U(VI) in a flow-through column packed with sediment from the Old Rifle CO, experimental site and receiving a cocktail of electon donors in the presence and absence of sulfate. After accumulation of U(IV) in the sediment, it was characterized and found to harbor approximately 50% NCU4. Each sediment type (with or without sulfate) was then placed in continuously stirred tank reactors through which oxic or anoxic water was flowing. The water contained either 1 or 10 mM sodium bicarbonate. The leaching of uranium was monitored and the speciation of the sediment post-leaching characterized. We found that little release of U was observed under anoxic conditions while in the presence of O2, about 30-45% of the sediment-associated U was lost. Preliminary evidence points to the preferential release of NCU4 rather than UO2 from the sediments under oxic conditions, suggesting more susceptibility to oxidation. Furthermore, batch experiments in which U(IV)-bearing sediment was allowed to incubate under anoxic conditions over a 12 month period indicate no change in the speciation of U(IV) over that time period. Thus, the results to date suggest that NCU4 may be relatively stable with respect to leaching and to speciation under anoxic conditions whereas it may be more susceptible to oxidation than UO2. Ongoing work will be presented to offer a more definitive view of the fate of NCU4 under environmentally relevant conditions.

The stability of non-crystalline U(IV) under oxic and anoxic conditions

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.