Microbial sulphide production in compacted Wyoming bentonite MX-80 under in situ conditions relevant to a repository for high-level radioactive waste

The activity of sulphate reducing bacteria (SRB) in Wyoming bentonite MX-80 saturated with groundwater from 450 m underground was investigated in situ. The bentonite was compacted to densities of 1.5, 1.8, and 2.0 g cm− 3. Lactate was added to the bentonite as a source of energy and organic carbon for SRB. Radioactive sulphur (35SO42−) was used as a tracer of sulphide production. The copper sulphide (Cux35S) that was produced was localized and quantified using electronic autoradiography. The mean copper sulphide production rates observed were 1.5 × 103, 3.1 × 102, and 3.4 × 101 fmol CuxS mm− 2 day− 1 at densities of 1.5, 1.8, and 2.0 g cm− 3, respectively. The use of sterile-filtered (0.2 µm) groundwater resulted in sulphide production of 1.5 × 102 and 2.4 × 101 fmol CuxS mm− 2 day− 1 at densities of 1.8, and 2.0 g cm− 3, respectively. Additional in situ experiments were performed with sterile-filtered (0.2 µm) groundwater and bentonite that had been heated to 120 °C for 15 h. Sulphide production rates in the heated bentonite were 1.3–16 times lower than in controls treated at 25 °C. These results reveal bentonite to be a source of SRB, in addition to the groundwater. Furthermore, all experiments demonstrated that increasing bentonite density correlated with decreasing copper sulphide production rates. According to the results presented here, sulphide production rates in bentonite compacted to 2.0 g cm− 3 are hundred to thousands of times below the rate needed to corrode through the copper capsule over 100 000 years.