Microbial community of MX80 bentonite and their interaction with iron

Abstract

MX80 bentonite has been selected as the buffer and backfill in a proposed method of long-term deep geological storage of nuclear waste. Extensive studies have been carried out on the geomechanical properties of MX80; however, it is not clear what effect microbes will have on its ability to function as an effective barrier. Specifically, in the UK, as carbon steel waste canisters will contribute iron oxides and rust products to the immediate environment, iron-reducing bacteria are of interest. Iron-reducing bacteria can reduce structural or external Fe (III) to Fe (II) and some species are adapted to high temperatures and low water availability, in keeping with conditions within the waste repository. Indigenous iron-interacting bacteria have been identified in compacted MX80 and microbially-influenced iron-reduction was observed in groundwater salinity up to 0.45M NaCl. Experiments investigating gas production, and silica-solubilising abilities of this community were carried out. Further experiments in pressurised test cells investigated microbial activities at the clay / steel interface. Significant increases in hydrogen production were observed when microbes were present, and biogenically influenced changes in structure and appearance of MX80 were seen in all experiments. Additionally, silica release occurred, likely coupled to metal / microbe interactions. Corrosion products differed depending on microbial presence following incubation in test cells. Biogenic transformation of clay minerals through iron reduction or release of silica to groundwater could significantly impact the geomechanical properties of MX80, as indicated by observed changes in clay plasticity, and ultimately this could affect the behavior of the material as a barrier.