Oxygen persistence in compacted bentonite and its impact on microbial activity in future deep geological repository
Résumé fourni par la source
Future Deep Geological Repositories (DGRs) are expected to evolve from initially oxic to reducing redox conditions within a relatively short period after closure (within a few weeks to months). Consequently, microbial processes relevant to canister corrosion, such as the activity of sulfate-reducing bacteria (SRB), are generally assumed to begin only after oxygen (O2) depletion. However, biological markers observed throughout the Iron Corrosion (IC-A) experiment — including the long-term per-sistence of aerobic microorganisms, the apparent O2 tolerance of SRB, commonly regarded as strictly anaerobic, and the inhibition of non-adapted SRB populations — suggest that O2 may persist and influence microbial dynamics longer than initially predicted. Here, we investigate the persistence of O2 within the bentonite by deploying modules containing Wyoming bentonite compacted to a density of 1.25 g/cm³ and carbon steel coupons for 1.5 years into the BIC-A borehole in the Mont Terri Rock Laboratory (MTRL). Prior to deployment, dry bentonite was pre-equilibrated with atmospheres containing 0%, 21%, or 100% O2 to assess the effect of oxygen. Integrated mineralogical, chemical, corrosion, and microbial analyses demonstrate that, contrary to previous assumptions, O2 promotes a temporary enrichment of bacteria, including SRB, at the bentonite-host rock interface during early saturation. This enrichment leads to localized sulfide production and reduction of structural Fe(III) in montmorillonite, yet exerts a negligible impact on carbon steel corrosion. Overall, these findings provide a more realistic assessment of early-stage O2 and its impact on microbial dynamics in DGR systems and indicate a limited but measurable influence of initial O2 on buffer and canister stability.
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