Historically, it is believed that crystalline uraninite, produced via the abiotic reduction of hexavalent uranium (U(VI)) is the dominant reduced U species formed in low-temperature uranium roll-front ore deposits. Here we show that non-crystalline U(IV) generated through biologically mediated U(VI) reduction is the predominant U(IV) species in an undisturbed U roll-front ore deposit in Wyoming, USA. Characterization of U species revealed that the majority (∼58-89%) of U is bound as U(IV) to C-containing organic functional groups or inorganic carbonate, while uraninite and U(VI) represent only minor components. The uranium deposit exhibited mostly 238U-enriched isotope signatures, consistent with largely biotic reduction of U(VI) to U(IV). This finding implies that biogenic processes are more important to uranium ore genesis than previously understood. The predominance of a relatively labile form of U(IV) also provides an opportunity for a more economical and environmentally benign mining process, as well as the design of more effective post-mining restoration strategies and human health-risk assessment.
Geologists now believe uranium is produced biologically, in a series of chemical reactions in Earth’s crust that take place over millions of years.
A team of biogeochemists has spotted promising signs that living microorganisms can also produce uranium, albeit in a different form than in the mineral uraninite. By analyzing the composition of uranium from 650-foot-deep samples mined in Wyoming – and using synchotron radiation-based spectroscopy and isotope fingerprinting – they found that 89 per cent of the uranium was bound to inorganic carbonate instead of being in uraninite ore.
The deposits match up to a series of biochemical reactions present in dissimilatory metal-reducing bacteria, a class of microbes that oxidize organic matter and produce metals in the process of anaerobic respiration. In other words, the bacteria use uranium instead of oxygen for energy.