Characterization of uranium redox state in organic-rich Eocene sediments

Susan A. Cumberland, Barbara Etschmann, Joël Brugger, Grant Douglas, Katy Evans, Louise Fisher, Peter Kappen, John W. Moreau

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33 Citations (Scopus)


The presence of organic matter (OM) has a profound impact on uranium (U) redox cycling, either limiting or promoting the mobility of U via binding, reduction, or complexation. To understand the interactions between OM and U, we characterised U oxidation state and speciation in nine OM-rich sediment cores (18 samples), plus a lignite sample from the Mulga Rock polymetallic deposit in Western Australia. Uranium was unevenly dispersed within the analysed samples with 84% of the total U occurring in samples containing >21 wt % OM. Analyses of U speciation, including x-ray absorption spectroscopy and bicarbonate extractions, revealed that U existed predominately (∼71%) as U(VI), despite the low pH (4.5) and nominally reducing conditions within the sediments. Furthermore, low extractability by water, but high extractability by a bi-carbonate solution, indicated a strong association of U with particulate OM. The unexpectedly high proportion of U(VI) relative to U(IV) within the OM-rich sediments implies that OM itself does not readily reduce U, and the reduction of U is not a requirement for immobilizing uranium in OM-rich deposits. The fact that OM can play a significant role in limiting the mobility and reduction of U(VI) in sediments is important for both U-mining and remediation.

Original languageEnglish
Pages (from-to)602-613
Number of pages12
Publication statusPublished - 1 Mar 2018


  • Mobility
  • Mulga Rock
  • Organic matter
  • Oxidation state
  • Uranium
  • X-ray absorption spectroscopy

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