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

## TL;DR

Microbes can help seal cracks in cement used for radioactive waste storage by producing calcite under anaerobic conditions.

## Contribution

Demonstrates microbial calcite precipitation in cement under conditions relevant to radioactive waste disposal.

## Key findings

- Citrate was completely removed and denitrification occurred in the microcosms.
- Calcite precipitation reduced cement porosity and sealed cracks.
- Citrate-oxidizing/nitrate-reducing bacteria were enriched in the system.

## Abstract

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.