Microbiologically influenced corrosion (MIC) potential of bentonite microorganisms: implications for a deep geological repository for nuclear waste

The integrity of carbon steel canisters in deep geological repositories (DGRs) for high-level radioactive waste (HLW) may be compromised by microbiologically influenced corrosion (MIC) driven by bentonite-associated or naturally occurring subsurface microorganisms. This study investigates the MIC potential of anaerobic microbial consortia enriched from Czech bentonite Černý Vrch (BCV). It represents the first phase of a comprehensive research project on MIC in BCV bentonite, aimed at identifying the most corrosive environments and taxa, progressing toward mechanistic studies of microbial electron transfer and corrosion behavior under near-repository conditions. Carbon steel coupons were incubated in selective media inoculated with BCV targeting nitrate-reducing bacteria (NRB), sulfate-reducing bacteria (SRB), heterotrophs, acetogens, and methanogens, under static and dynamic flow conditions. A two-stage (2- and 3-month-long, respectively) batch experiment was performed, with the second stage employing inocula from the first to enrich MIC-active consortia. Corrosion rates were quantified, and microbial communities analyzed using qPCR and 16 S rRNA amplicon sequencing. Highest corrosion rates were observed in Nitrate Broth (targeting nitrate reducers, NRB), R2A (heterotrophs), and Postgate (sulphate reducers, SRB) media, reaching 60 μm·a⁻¹, 31 μm·a⁻¹, and 33 μm·a⁻¹, respectively. Corrosion localization was observed only in Nitrate Broth media (maximum penetration depth nearly 40 μm). Organic-rich media supported greater microbial diversity and activity. Dynamic flow conditions simulating worst-case scenarios significantly increased corrosion. In Nitrate Broth, sterile samples rose 11.8-fold (211 μm·a⁻¹) and biotic samples 6.5-fold (125 μm·a⁻¹) compared to static conditions. In R2A, sterile samples increased 11.5-fold (28 μm·a⁻¹) and biotic 9.7-fold (37 μm·a⁻¹). Clostridia, Bacilli, and SRBs were identified as key corrosion-inducing organisms in the studied systems. Our findings identify NRB as a potential corrosion threat, refine MIC risk assessments for DGRs, and improve predictions of canister longevity and repository safety.