Neptunium and manganese biocycling in nuclear legacy sediment systems

Thorpe, Clare L. and Morris, Katherine and Lloyd, Jonathan R. and Denecke, Melissa A. and Law, Kathleen A. and Dardenne, Kathy and Boothman, Christopher and Bots, Pieter and Law, Gareth T.W. (2015) Neptunium and manganese biocycling in nuclear legacy sediment systems. Applied Geochemistry, 63. pp. 303-309. ISSN 1872-9134 (https://doi.org/10.1016/j.apgeochem.2015.09.008)

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Abstract

Understanding the behaviour of the highly radiotoxic, long half-life radionuclide neptunium in the environment is important for the management of radioactively contaminated land and the safe disposal of radioactive wastes. Recent studies have identified that microbial reduction can reduce the mobility of neptunium via reduction of soluble Np(V) to poorly soluble Np(IV), with coupling to both Mn- and Fe(III)- reduction implicated in neptunyl reduction. To further explore these processes Mn(IV) as δMnO2 was added to sediment microcosms to create a sediment microcosm experiment "poised" under Mn-reducing conditions. Enhanced removal of Np(V) from solution occurred during Mn-reduction, and parallel X-ray absorption spectroscopy (XAS) studies confirmed Np(V) reduction to Np(IV) commensurate with microbially-mediated Mn-reduction. Molecular ecology analysis of the XAS systems, which contained up to 0.2 mM Np showed no significant impact of elevated Np concentrations on the microbial population. These results demonstrate the importance of Mn cycling on Np biogeochemistry, and clearly highlight new pathways to reductive immobilisation for this highly radiotoxic actinide.

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