Interaction of anaerobic Mont Terri Opalinus Clay bacteria with plutonium(VI)

The Opalinus clay layer of the Mont Terri Underground Rock Laboratory (Switzerland) is one potential host rock tested for nuclear waste disposal [1]. Bacteria indigenous to such subterranean environments can affect the speciation and hence the mobility of actinides [2]. Thus various investigations [3-7] documented the manifold impact of bacteria on the speciation of plutonium.
Recently, we studied the interaction between Pu and Sporomusa sp. MT-2.99 cells at pH 6.1 with and without adding an electron donor (10 mM Na-pyruvate). This bacterium was isolated from Mont Terri Opalinus clay core samples. At pH 6.1, a moderate to strong impact of Sporomusa sp. cells on the Pu speciation was observed. In contrast to the electron donor free experiments, a clear enrichment of Pu(III) in the biomass (bioreduction) was observed in the presence of 10 mM Na-pyruvate. However, more information is necessary to understand the Pu interaction mechanism in the electron donor free experiments, i.e. the influence of biomass on the reduction of Pu(VI) in the biomass suspensions. In the present study, our focus lies on an improved understanding of the pH-dependent Pu redox chemistry in 0.1 M NaClO4 with and without Sporomusa sp. MT-2.99 cells. The pH range was extended to 3, 4, and 7.
The time-dependent Pu concentrations measured in the supernatants were successfully fitted with bi-exponential decay functions and the time-dependent Pu oxidation state distributions by using mono-exponential decay or growth functions. Redox potential measurements indicated that the cells generated reducing conditions. This ability is pH-dependent. A clear pH influence on both the amount of accumulated Pu and on the interaction process with the biomass could be proven.

Acknowledgement
The authors thank the BMWi for financial support (contract no.: 02E10618 and 02E10971), Velina Bachvarova and Sonja Selenska-Pobell for isolation and Monika Dudek for cultivation of the strain, as well as the BGR for providing the clay samples.

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