An indigenous iron-reducing microbial community from MX80 bentonite - A study in the framework of nuclear waste disposal

Abstract

Highly compacted MX80 bentonite has been selected as the engineered buffer and backfill material in several proposed concepts for long-term deep geological storage of nuclear waste. Iron-reducing bacteria reduce Fe (III) to Fe (II) and some are adapted to high temperatures and desiccated environments, in keeping with periods of less habitable conditions within the repository. In one potential UK repository concept, iron from carbon steel canisters may contribute to an iron-rich environment at the clay-canister interface. This could lead to changes in the mineralogy and iron-content of MX80 bentonite due to variation of the redox state and solubility, which in turn could alter the geomechanical properties of the clay. To investigate the potential role of iron-reducing bacteria in this process enrichments were carried out with both commercially available MX80 bentonite powder and compacted MX80 bentonite to identify the presence of an indigenous iron-interacting community in the clay. Throughout these enrichments Fe (II) soluble, Fe (II) total, and pH were measured, and the enrichments were subjected to 16S rRNA community analysis. Concentrations of Fe (II) total peaked at day 28 in all enrichments; however, the concentration was overall higher when accompanied by bacterial growth. Fe (II) soluble remained low throughout. 16S rRNA gene sequencing revealed the presence of several putative iron-interacting bacteria, as well as thermotolerant and spore-forming species. The indigenous community was largely comprised of firmicutes, including iron-reducers and spore-forming bacteria such as Desulfosporosinus. Therefore, MX80 bentonite inherently carries a viable microbial community which could potentially interact with structural iron present within MX80 bentonite or other mineral components, such as a carbon steel waste canister. Various research has shown that microbial activity is unlikely within the bulk bentonite provided high compaction is maintained. The importance of this high compaction is highlighted by the finding here of a viable, robust and functionally diverse community within the clay and activity may be possible anyway at edge sites and interfaces where, locally, swelling pressures might not fully develop.