Cumberland and three co-authors reported in 2016 that uranium (U) is of enormous and of increasing global importance because of its use in energy generation, albeit with potential environmental legacies [that are often over-emphasized]. While naturally occurring U is widespread in the Earth's crust at concentrations of ~1 to 3 ppm, higher concentrations can be found, including within organic matter (OM)-rich sediments, leading to economic extraction opportunities. The primary determinants of U behavior in ore systems are pH, Eh, U oxidation state (U(IV), U(VI)) and the abundance of CO32– ions. The concentration/availability and interrelationships among such determinants vary, and the solubility and mobility of ions (e.g. OH−, CO32–, PO43−, SiO44−, SO42−) that compete for U (primarily as U(VI)) will also influence the mobility of U. In addition, the presence of OM can influence U mobility and fate by the degree of OM sorption to mineral surfaces (e.g. Fe- and Si- oxides and hydroxides). Within solid-phase OM, microbes can influence U oxidation state and U stability through direct enzymatic reduction, biosorption, biomineralization and bioaccumulation. The biogenic UO2 product is, however, reported to be readily susceptible to reoxidation and therefore more likely remobilized over longer time periods. Thus several areas of uncertainty remain with respect to factors contributing to U accumulation, stability and/or (re)mobilization.
To address these uncertainties, the authors reviewed U dynamics at both geological and molecular scales. Here they identified U-OM bond values that are in agreement, relatively strong, independent from ionic strength and which may facilitate either U mobilization or immobilization, depending on environmental conditions. The authors also examined the knowledge gaps in the literature, with U-OM solubility data generally lacking in comparison to data for U sorption and dissolution, and little information available on multi-component relationships, such as UOM-V (V as vanadate).
Furthermore, the capability of OM to influence the oxidation state of U at near surface conditions remains unclear, as it can be postulated that electron shuttling by OM may contribute to changes in U redox state otherwise mediated by bacteria. Geochemical modelling of the environmental mobility of U will require incorporation of data from multi-corporation studies, as well as from studies of U-OM microbial interactions, all of which are considered in this review.
The authors concluded, in part, that the occurrence and formation of U-colloids, within OM and the impact of U mobility is one that is worthy of further investigation. Colloids and nanoparticles, are potentially highly mobile in the environment. This has particular importance because in terms of the oxidation state, inducing reduction may therefore not render U less mobile, where U(IV) colloids are formed. Moreover, surface coating by OM could further enhance transport though aquifers.
Microbial reduction of U(VI) is of considerable interest as a mechanism for controlling U mobility in contaminated sediments. Equally, these microbial processes hold relevance for natural settings. Within microbial communities there are a plethora of groups with metal-reducing capabilities. These capabilities require further investigation to identify specific processes involved in U capture and reduction in OM-rich systems.
Predicting U mobility in the environment through SCM and RTM modelling can realistically be determined both spatially and temporally. In making viable predictions, effects such as competition between anions, and the effects of OM and microbes on U geochemistry should be incorporated.
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