Microbial degradation of citrate mediates sealing of cement cracks under anaerobic conditions relevant to radioactive waste disposal

In radioactive waste repositories, cement is used for construction, backfill, and waste encapsulation. Over time, cracks may form, creating potential pathways for contaminant migration. A self-sealing mechanism is through calcium carbonate (CaCO₃) precipitation, which can be driven by microbial oxidation of organic compounds. We explored microbially induced calcite precipitation facilitated by metabolism of citrate, a complexant in low- and intermediate- level radioactive waste (L/ILW). Nitrate-reducing microcosms containing cement pellets, citrate, nitrate, alkaline sediment inoculum, and synthetic groundwater (pH 11.2) were incubated in the dark (20 °C, 40 days). Aqueous geochemical data revealed complete citrate removal, denitrification, pH decrease to pH 9, and removal of Ca²⁺_(aq). Furthermore, 16S rRNA gene sequencing showed enrichment of citrate-oxidising/nitrate-reducing bacteria. Solid phase analysis (XRD, SEM-EDS, µXCT) confirmed new calcite precipitates reduced cement porosity and sealed cracks at the surface. Overall, microbial oxidation of organic ligands under alkaline conditions may reduce contaminant mobility in L/ILW repositories through calcite precipitation and crack sealing.