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Susan Cumberland: Determining uranium geochemistry within natural organic-matter rich environments using XAS and XFM techniques

2020-August-10

 
Analysing metals within environmental samples using wet chemistry can pose analytical problems, particularly with carbon-based and solid samples. X-ray absorbance spectroscopy (XAS) analysis is one alternative approach, able to target elements of interest whilst avoiding matrix interference from low mass elements (e.g. carbon). One element that is particularly suited to XAS is uranium (U), which is commonly investigated at the L3 edge. Understanding behaviour of U within the environment has been the focus of researchers for many decades, whether in exploration, mining, contamination, remediation or nuclear. Uranium geochemistry is the same regardless of whether its source is natural, depleted or as spent fuel, and the study of U is therefore multifaceted. In general terms, of the two main oxidation states, U(IV) and U(VI), U(IV) minerals have extremely low solubility, while U(VI) can be more soluble and therefore potentially mobilised.

   In my work I have explored the nature of U within ore deposits, in particular the relationship between U and natural organic matter (OM) using XAS and X-ray fluorescence microscopy (XFM). OM may influence U mobility whether facilitating its pathway through the environment or acting as a sink for U. In the case of the latter, OM may accumulate U over millennia within sediments or wetlands. Combining techniques from two beamlines at the Australian Synchrotron, XAS (XANES, EXAFS) and XFM (element mapping, µXANES), together with laboratory µXRD, spatial and chemical information were acquired on organically hosted U sediments taken from Mulga Rock Deposit, Western Australia.
   Results from XAS on bulk samples revealed how U at Mulga Rock is predominantly U(VI) within the OM sediments. However, from using µXANES transects across U on pyrite rims, we observe U(IV) outwardly transitioning to U(VI). Analysis of other features suggest that U has immortalised bacteria as coffinite (USiO4) ellipsoids. Furthermore, I show from laboratory experiments how OM may immobilise U(VI) rapidly without change in oxidation state. Thus, challenging traditional views that reduction of U(VI) to U(IV) has to occur for U to be immobilised within OM sediments. 

 
References:
  1. 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: 160-185. https://doi.org/10.1016/j.earscirev.2016.05.010
  2. Cumberland, S.A., Etschmann, B., Brugger, J., Douglas, G., Evans, K., Fisher, L., Kappen, P. and Moreau, J.W., 2018a. Characterization of uranium redox state in organic-rich Eocene sediments. Chemosphere, 194: 602-613. https://doi.org/10.1016/j.chemosphere.2017.12.012
  3. Cumberland, S.A., Wilson, S.A., Etschmann, B., Kappen, P., Howard, D., Paterson, D. and Brugger, J., 2018b. Rapid immobilisation of U(VI) by Eucalyptus bark: Adsorption without reduction. Applied Geochemistry, 96: 1-10. https://doi.org/10.1016/j.apgeochem.2018.05.023.
  4. Cumberland, SA., Evans, K., Douglas, G., de Jonge, M., Fisher, L., Howard, D., Moreau., J. Characterisation of uranium-pyrite associations within organic-rich Eocene sediments using EM, XFM-μXANES and μXRD.  (submitted, Ore Geology Reviews)

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Date:
2020-August-10