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

Cumberland, S. A., Douglas, G., Grice, K. and Moreau, J. W. (2016) Uranium mobility in organic matter-rich sediments: a review of geological and geochemical processes. Earth Science Reviews, 159, pp. 160-185. (doi: 10.1016/j.earscirev.2016.05.010)

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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 ~ 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.

Item Type:Articles
Additional Information:This work was funded though the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Organic Geochemistry of Mineral Systems (OGMS) Cluster Flagship. Cumberland would like to thank the Australian Synchrotron and Monash University for their latter support of this manuscript through a post-doctoral fellowship.
Glasgow Author(s) Enlighten ID:Moreau, Dr John
Authors: Cumberland, S. A., Douglas, G., Grice, K., and Moreau, J. W.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
Journal Name:Earth Science Reviews
ISSN (Online):1872-6828
Published Online:27 May 2016
Copyright Holders:Copyright © 2016 The Authors
First Published:First published in Earth Science Reviews 159: 160-185
Publisher Policy:Reproduced under a Creative Commons License

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