The stability of microbially reduced U(IV); impact of residual electron donor and sediment ageing

•The sensitivity of microbially-reduced uranium(IV) to oxidative remobilisation was examined after periods of ageing.•The transformation of non-crystalline U(IV) to nanocrystalline uraninite was observed after 15 months of ageing.•Ageing did not affect the susceptibility of microbially-reduced U(IV) to oxidative remobilisation by nitrate or aeration.•The presence of residual electron donor post-biostimulation exerted a strong control on U(IV) reoxidation kinetics.

The stimulation of microbial U(VI) reduction to precipitate insoluble U(IV) has been proposed as a means of remediating mobile uranium groundwater contamination. Crucial to the success of such a remediation strategy is determining the longevity of U(IV) biominerals in the subsurface, particularly if the groundwater becomes oxidising. Here we describe experiments to assess the susceptibility of microbially-reduced U(IV) to oxidative remobilisation both via aeration and by the addition of nitrate at environmentally-relevant conditions. Additional factors examined include the possibility of biogenic U(IV) becoming more crystalline (and potentially more recalcitrant) during a period of ageing, and the role played by residual electron donor in controlling the long-term fate of the uranium. Biogenic U(IV) was precipitated as a non-crystalline U(IV) or “monomeric” phase, with a small but increasing contribution to the EXAFS spectra from nanocrystalline uraninite occurring during 15 months of ageing. Despite this, no evidence was observed for an increase in recalcitrance to oxidative remobilisation. However, the presence of residual electron donor post-biostimulation was shown to exert a strong control on U(IV) reoxidation kinetics, highlighting the importance of maintaining the presence of electron donor in the subsurface, in order to protect biogenic U(IV) from oxidative remobilisation.