Neptunium (as NpO2+) is predicted to be the most mobile transuranic element in natural and engineered environments and is a key risk-driving radionuclide in radioactive waste geological disposal. However, when compared to other radionuclides, there is a paucity of information concerning neptunium’s environmental behavior. This is particularily true when one considers potential microbial controls on Np solubility and Np interactions with microbes and minerals in the subsurface. We have characterised Np biogeochemical behaviour in a range of complex environmental systems, and in more constrained mineral and microbe studies. Taken together, data from these systems provide an unprecedented insight into the subtleties of Np biogeochemistry. Here, highlights from several experiments will be presented, including: - Results from microcosm experiments conducted with Fe/Mn rich sediments, at both circumneutral and alkaline pH, where Np removal from solution was accelerated comensurate with microbially-mediated metal reduction (Mn(II) and Fe(II) ingrowth measurable) and XAS analysis of the sediments revealed reduction of Np(V) to insoluble Np(IV). Further, the sediment associated Np was recalcitrant to oxidative remobilisation. - Results from studies with Fe(II)-bearing silicate mineral systems, where prior reaction of the minerals with metal-reducing bacteria primed them for enhanced Np(V) reduction via Fe(II) interactions, and EXAFS revealed nanoparticulate NpO2 as the reduction product. - Results from Fe mineral systems conducted across a range of pH values that show combined sorption and potential incoporation of Np(V) into Fe-mineral lattice sites. - Results from pure culture experiments where microbially-mediated reduction of Np(V) to Np(IV) was facile in comparision to U(VI) reduction to U(VI). Overall, these data provide new insights into biogeochemical redox cycling of Np, show novel pathways to incorporation of Np into Fe-oxide minerals, and highlight pure culture work showing Np(V) reduction to Np(IV) in model systems.

Controls on neptunium behaviour in the environment

251st National Meeting (2016)

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