Campbell, and 15 co-authors, reported in 2011 that reductive biostimulation is currently being explored as a possible remediation strategy for uranium (U) contaminated groundwater, and is currently being investigated at a field site in Rifle, Colorado. The long-term stability of the resulting U(IV) phases is a key component of the overall performance and depends upon a variety of factors, including rate and mechanism of reduction, mineral associations in the subsurface, and propensity for oxidation.
To address these factors, several approaches were used to evaluate the redox sensitivity of U: measurement of the rate of oxidative dissolution of biogenic uraninite (UO2(s)) deployed in groundwater at Rifle, characterization of a zone of natural bioreduction exhibiting relevant reduced mineral phases, and laboratory studies of the oxidative capacity of Fe(III) and reductive capacity of Fe(II) with regard to U(IV) and U(VI), respectively.
The authors concluded that with the goal of producing stable, insoluble reduced U(IV) phases, biostimulation is a promising strategy for contaminated groundwater remediation. The stability of products in the field depends upon a variety of factors, including rate and mechanism of reduction, mineral associations in the subsurface, and propensity for oxidation. A zone of natural bioreduction suggests that long-term stability of adsorbed U(IV) phases may be possible, potentially by sustaining locally reduced conditions, precipitating redox-buffering minerals, and even maintaining the presence of biomass. Biogenic uraninite was found to be more stable to oxidation by DO under aquifer conditions than predicted in laboratory studies, and its nanoparticulate nature does not appear to make it more susceptible to oxidation.
Kinetic limitations of chemical diffusion may extend the lifetime of U(IV) in the subsurface. However, other oxidants in the subsurface, such as Fe(III) oxides, may be important. Although further research is necessary to determine the redox balance in the field, laboratory results indicate that the favorability of Fe(III) as an oxidant over Fe(II) as a reductant is very sensitive to geochemical conditions, and may be an important consideration during and after active remediation.
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