Anaerobic biodegradation of citric acid in the presence of Ni and U at alkaline pH; impact on metal fate and speciation

Citrate is a key decontaminant in the nuclear industry and here we explore its biogeochemical fate in the presence of Ni2+ and U(VI)O22+ under conditions relevant to low level radioactive waste (LLW) disposal. Anaerobic microcosm experiments were performed under nitrate- and sulfate-reducing conditions at between pH 9 and 10. Citrate (1 mM) was supplied as both an electron donor and a potential metal ion complexant. Incubation experiments with citrate, and inoculated with nitrate- or sulfate- reducing microbial consortia, were challenged with three different concentrations of Ni or U : 0.01, 0.1 or 1 mM. The nitrate and sulfate-reducing inocula were enriched from well characterised alkaline sediments obtained from a high pH lime-workings. A multi-technique approach was adopted to characterise the aqueous geochemistry, solid phase mineralogy, and bacterial communities in each incubation system.