Bhattacharyya, et al., report that historically, some geoscientists concluded 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 the authors 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. [This is not new information at all. See: Rackley (1972) and (Campbell and Biddle (1977)]. 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.
Typical sandstone-hosted uranium (U) roll-front deposits are formed in confined aquifers at a redox front of oxidized groundwater reacting with unaltered reduced aquifer materials, often associated with organic detritus. Roll-front deposits are an economically important ore for in situ recovery (ISR) mining, whereby U is mined via solubilization of the ore by a leach solution injected in the subsurface. The reduction mechanism for U(VI) in roll-front deposits has long been thought to be abiotic electron transfer by redox active minerals (pyrite and mackinawite), aqueous sulfide and/or other reactive sulfur species. Other possible abiotic U(VI) reduction mechanisms observed in low-temperature environments include reaction with adsorbed Fe(II), structural Fe(II) in clays and reduced organic functional groups (thiols). Previous mineralogy studies of sandstone-hosted roll-front deposits have identified crystalline uraninite and coffinite as the dominant U-bearing minerals.
The role of bacteria has been poorly understood and was thought to be limited to bacterial production of reducing agents (for example, sulfide) that play a key role in U deposit formation. Over the past decade, with the advent of synchrotron radiation-based spectroscopy, the presence of short-range ordered nano-particulate uraninite, presumed to result from enzymatic reduction of U(VI), has frequently been reported in a variety of U-reducing environments. More recently, a non-uraninite U(IV) species, commonly termed non-crystalline U(IV) (but also referred to as ‘monomeric U(IV)’, ‘mononuclear U(IV)’ and ‘molecular U(IV)’) was reported to form in the presence of biofilms and specific inorganic ligands in laboratory-based studies and field settings.
A study by Bargar et al. showed the formation of non-crystalline U(IV) in a shallow U-contaminated aquifer during microbial bioremediation with acetate as an electron donor. The authors hypothesized that the reduction occurred through a combined biotic–abiotic pathway mediated by biogenic mackinawite (FeS) followed by complexation of the resulting U(IV) by extracellular polymeric substances. Further, non-crystalline U(IV) species with a structure lacking the features of a U–U pair correlation were observed within a sediment sample collected from an ISR U mine following 5 years of post-mining restoration. Finally, the heavy U isotope signature identified in groundwater and uranium ore concentrates from low-temperature sandstone ore formations suggests that biological reduction maybe a dominant process in those systems. Thus, these new developments in environmental speciation of reduced U and U isotope interpretations call for re-evaluation of the prevailing model for U ore formation in roll-front deposits in order to optimize ore exploration and exploitation, as well as mine restoration. The aforementioned studies targeted either microbial U(VI) reduction within a laboratory setting or bioremediation of U(VI) within contaminated shallow aquifer systems, which are very different from ore formation in roll-front deposits. Our study aims to characterize U naturally present in an undisturbed roll-front deposit and therein lies the novelty of this research.
Here, the authors investigated formation mechanisms and U(IV) species naturally present in undisturbed roll-front U deposits in Wyoming, USA . They observed that non-crystalline U(IV) is a dominant U species and hypothesize that it is primarily formed via direct enzymatic U(VI)reduction. These deposits occur in three superposed arkosic sandstone units separated by thin beds of mudstone, interbedded shales and high-carbonate zones3, and are ∼3 million years old according to U-Pb dating. Based on X-ray diffraction (XRD), a previous study at a location situated 10 km from our study site identified pitchblende, uraninite and coffinite as the main crystalline U-bearing minerals. However, non-crystalline U(IV) lacks long-range order, thus eluding identification via standard mineralogical characterization. They combined advanced techniques in order to fully probe the potential presence of biogenic non-crystalline U(IV) in an unmined ore zone located 200 m below ground surface (m-bgs): X-ray absorption spectroscopy (XAS) was used to determine the valence state and the average local atomic coordination environment of U; sequential extractions were used to determine the sediment fractions hosting U; a multicollector inductively coupled plasma mass spectrometer was used to measure the 238U/235U isotope ratio to discriminate between abiotic and biotic U transformation mechanisms; and DNA-based (16S rRNA) microbial community analysis was performed in order to characterize its diversity and metabolic potential. The experimental findings for this deposit may help shed light on other deposits of the same type and augment the authors' current understanding of roll-front deposit formation.
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