Biogenic non-crystalline U ^(IV) revealed as major component in uranium ore deposits

Abstract

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.

Abstract

Crystalline uraninite is believed to be the dominant form in uranium deposits. Here, the authors find that non-crystalline U ^(IV) generated through biologically mediated U ^(VI) reduction is the predominant U ^(IV) species in ore deposits, implying that biogenic processes are more important than previously thought.