Microbial Reduction of Fe(III) under Alkaline Conditions Relevant to Geological Disposal

Abstract

To determine whether biologically mediated Fe(III) reduction is possible under alkaline conditions in systems of relevance to geological disposal of radioactive wastes, a series of microcosm experiments was set up using hyperalkaline sediments (pH ϳ11.8) surrounding a legacy lime working site in Buxton, United Kingdom. The microcosms were incubated for 28 days and held at pH 10. There was clear evidence for anoxic microbial activity, with consumption of lactate (added as an electron donor) concomitant with the reduction of Fe(III) as ferrihydrite (added as the electron acceptor). The products of microbial Fe(III) reduction were black and magnetic, and a range of analyses, including X-ray diffraction, transmission electron microscopy, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism confirmed the extensive formation of biomagnetite in this system. The addition of soluble exogenous and endogenous electron shuttles such as the humic analogue anthraquinone-2,6-disulfonate and riboflavin increased both the initial rate and the final extent of Fe(III) reduction in comparison to the nonamended experiments. In addition, a soluble humic acid (Aldrich) also increased both the rate and the extent of Fe(III) reduction. These results show that microbial Fe(III) reduction can occur in conditions relevant to a geological disposal facility containing cementbased wasteforms that has evolved into a high pH environment over prolonged periods of time (>100,000 years). The potential impact of such processes on the biogeochemistry of a geological disposal facility is discussed, including possible coupling to the redox conditions and solubility of key radionuclides.

Microbial Reduction of U(VI) under Alkaline Conditions: Implications for Radioactive Waste Geodisposal

“…To explore the biogeochemistry of uranium under anoxic conditions at pH 10−10.5, microcosms were established using calcite dominated sediment and groundwater (SI Table S1 and ref 18 for mineralogical information) from a high pH lime workings site in the U.K. and similar to past work. 18 There was a progression of microbial redox processes in both the no added Fe(III) and with added Fe(III) microcosms which were maintained at pH 10−10.5 ( Figure 1A,B). For the no added Fe(III) system, nitrate, which was present at low but measurable concentrations (53 μM), dropped rapidly after incubation, and was below the limit of detection after 3 days.…”

Extracellular Electron Transport-Mediated Fe(III) Reduction by a Community of Alkaliphilic Bacteria That Use Flavins as Electron Shuttles

“…Thus, it is speculated that the iron reduction mechanisms of alkaliphilic bacteria must be extremely efficient. Recently, it has been shown that adding riboflavin to a community of alkaliphilic soil bacteria grown in vitro at pH 10 increased the rate at which Fe(III) was reduced, suggesting that members of the community might be able to use riboflavin as an electron shuttle under alkaline conditions (46). However, as electron shuttle-catalyzed reactions are very pH sensitive (38,39), it may not be appropriate to extrapolate what is known about the process from studies performed at nearly neutral pH to high-pH environments.…”

Bacterial Diversity in the Hyperalkaline Allas Springs (Cyprus), a Natural Analogue for Cementitious Radioactive Waste Repository

“…Recently it was shown that a clear succession of electron acceptor utilization existed in alkaline microcosm cultures up to pH 11, as rapid nitrate reduction was followed by slower soluble Fe(III)-citrate, insoluble Fe(III)-oxyhydroxide, and sulfate reduction (Rizoulis et al 2012). Further sediment microcosm studies from our group have also shown that alkaline bioreduction of Fe(III)-oxyhydroxide can lead to the formation of magnetite (Williamson et al 2013) and that microbially mediated reduction of U(VI) can occur at pH 10 (Williamson et al 2014).…”