Citrate is a key decontaminant in the nuclear industry and here we explore its biogeochemical fate in the presence of Ni and U under conditions relevant to low level radioactive waste (LLW) disposal. Ni and U are significant contaminants in LLW and their environmental mobility may be affected by organic ligands, such as citrate, which are also present in LLW. Under alkaline repository conditions expected, citrate can alter metal solubility via complexation (e.g. with Ni) and/or may be utilised as an electron donor in support of bioreduction reactions (e.g. U(VI) to U(IV)). Understanding Ni and U fate in the presence of citrate is of interest to inform radioactive waste management practices. To date, there has been little focus on the biogeochemical behaviour of Ni/U and citrate in alkaline, anaerobic systems relevant to LLW disposal. Here, citrate was supplied as both a metal ion complexant and electron donor in incubation experiments with Ni or U under NO3-- and SO42--reducing conditions at pH 9-10. Three concentrations of Ni or U were tested: 0.01, 0.1 or 1 mM. The NO3- or SO42--reducing inocula used were enriched from alkaline sediments. A multi-technique approach was adopted to characterise the aqueous geochemistry, solid phase mineralogy, and bacterial communities in each system. In the Ni incubations [Ni(Cit)]- was the dominant Ni species and, in the inoculated 0.01 mM Ni systems, this complex was biodegraded under NO3-- and SO42--reducing conditions in &lt; 15 days. In the 0.01 mM Ni/SO42--reducing experiment, 50 % of aqueous Ni was removed from solution over 100 days and black precipitates formed; electron microscopy suggested this was a Ni-sulfide phase. In the U experiments, citrate was biodegraded within 55 days in all systems except the 1 mM U/NO3--reducing incubations. By day 55, a dark-coloured precipitate formed in all U/ SO42--reducing experiments. In the 0.01 mM U and 0.1 mM U systems 83-98 % of the added U(VI) was removed from solution by day 15, and substantial U(VI) removal (between 50-70%), was also observed by day 55 in the 1 mM U experiment. The XAS analysis of solids indicated that they contained a mix of nanocrystalline uraninite and non-crystalline U(IV)-phosphate. Overall, the work suggests that citrate will biodegrade under conditions relevant to LLW disposal, in the presence of Ni and U, and this can promote the biogeochemical immobilisation of metals into the solid phase.

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

ACS Spring 2022

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