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

AbstractThe 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 15months 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

A carbonised wood sample from Rakata and its Radio-carbon Assay. I. Stratigraphic Observations

Provides a stratigraphical setting and discusses possible origins of the carbonised timber which has been dated by 14 C to AD 1710. The timber was found at c.4 m depth under a layer of ash with pumice and a topsoil

Biostimulation by Glycerol Phosphate to Precipitate Recalcitrant Uranium(IV) Phosphate

Stimulating the microbial reduction of aqueous uranium­(VI) to insoluble U­(IV) via electron donor addition has been proposed as a strategy to remediate uranium-contaminated groundwater in situ. However, concerns have been raised regarding the longevity of microbially precipitated U­(IV) in the subsurface, particularly given that it may become remobilized if the conditions change to become oxidizing. An alternative mechanism is to stimulate the precipitation of poorly soluble uranium phosphates via the addition of an organophosphate and promote the development of reducing conditions. Here, we selected a sediment sample from a U.K. nuclear site and stimulated the microbial community with glycerol phosphate under anaerobic conditions to assess whether uranium phosphate precipitation was a viable bioremediation strategy. Results showed that U­(VI) was rapidly removed from solution and precipitated as a reduced crystalline U­(IV) phosphate mineral similar to ningyoite. This mineral was considerably more recalcitrant to oxidative remobilization than the products of microbial U­(VI) reduction. Bacteria closely related to Pelosinus species may have played a key role in uranium removal in these experiments. This work has implications for the stewardship of uranium-contaminated groundwater, with the formation of U­(IV) phosphates potentially offering a more effective strategy for maintaining low concentrations of uranium in groundwater over long time periods