Uranium mobility in organic matter-rich sediments: A review of geological and geochemical processes

Uranium (U) is of enormous global importance because of its use in energy generation, albeit with potential environmental legacies. While naturally occurring U is widespread in the Earth's crust at concentrations of similar to 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 behaviour 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, biomineralisation and bioaccumulation. The biogenic UO2 product is, however, reported to be readily susceptible to reoxidation and therefore more likely remobilised over longer time periods. Thus several areas of uncertainty remain with respect to factors contributing to U accumulation, stability and/or (re)mobilisation. To address these uncertainties, this paper reviews U dynamics at both geological and molecular scales. Here we identify U-OM bond values that are in agreement, relatively strong, independent from ionic strength and which may facilitate either U mobilisation or immobilisation, depending on environmental conditions. We also examine 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 U OM-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. (C) 2016 The Authors. Published by Elsevier B.V.