Microbial processes in bentonite transformation

A safe and long-term storage of highly radioactive waste in deep geological layers should be achieved by a multi-barrier concept consisting of geological (host rock), geotechnical (e.g. bentonite) and technical barriers (canister including the highly radioactive waste). Suitable materials for a geotechnical barrier are the so-called bentonites. These clay minerals have due to their mineralogical composition a high swelling capacity and a low solvent permeability and can therefore fulfil in this system a sealing and buffering function.
Like in all natural materials, different microorganisms inhabit bentonites. These microorganisms can influence the conditions in a potential nuclear repository by microbial transformation of bentonite. To elucidate the microbial potential within a selected bentonite, microcosms were set up, which contain 20 g bentonite and 40 ml anaerobic synthetic Opalinus-clay-pore water solution under N2/CO2-gas-atmosphere. Substrates like acetate and lactate were added for the stimulation of the potential microbial activity and anthraquinone-2,6-disulfonate was added as an electron shuttle. Microcosms were incubated in the dark, without shaking at 30°C for 98 days. The samples were taken at different time points of incubation and were analysed regarding geochemical parameters like pH, O2-concentration, redox potential, iron concentration and sulphate concentration as well as biological parameters like the consumption and formation of metabolites.
The results confirm the presence of microbial life in the selected bentonite for example by consumption of lactate and the formation of acetate and pyruvate. Moreover, a fast microbial reduction of iron was detected. The results show the importance of the selection of suitable bentonites for a safe storage of highly radioactive waste in order to avoid the transformation of bentonite by microorganisms, which could cause the loss of its barrier function.