Bentonite - a natural habitat for sulfate-reducers

Concerning the deep geological disposal of high-level radioactive waste (HLW), bentonite plays a crucial role by using it as a barrier und buffer material in between the steel-canister, containg the HLW, and the surrounding host rock. In order to analyze the potential influence of natural occuring microorganisms within the bentonite on the bentonite barrier, we set up microcosm-experiments. Therefor, two different Bavarian bentonites (a natural and an industrial one) were supplied with an anaerobic, synthetic Opalinus-clay pore water solution under an N2/CO2-atmosphere and were incubated for one year at 30 °C and 60 °C. To some set ups organics (acetate or lactate) or H2 were supplemented. During the incubation time samples were taken and analyzed for several bio-geochemical parameters and the evolution of microbial diversity.
Our results clearly demonstrate, that natural accuring microbes affect geochemical parameters. Set ups containing the industrial bentonite supplemented with lactate show the most striking effects. The microbial diversity changed completely within 6 month. The respective batches were dominated (up to 81 %) by Desulfosporosinus spp. – spore-forming, strictly anaerobic, sulfate-reducing organisms, able to survive under very harsh conditions. Concomitantly, an increase of ferrous iron and a simoultaneous decrease of ferric iron was observed. Furthermore, the lactate and sulfate concentration decreased, whereas pyruvate and acetate were formed. Similar observations were also made in setups containg H2. Desulfosporosinus spp. also dominated the microbial population in the respective batches. Desulfosporosinus spp. are known to metabolize a brought range of substrates including H2 and organic acids, thereby reducing sulfate or iron to form H2S, acetate or ferric iron – metabolites that could effect different properties of the barrier system of an HLW.