Evaluation of dry density compaction on the geochemical composition of organic matter in bentonite clays for used nuclear fuel storage

Long-term used nuclear fuel storage involves the use of bentonite clays but these deposits contain natural organic matter (NOM) which might be used as a microbial substrate. Recent short-duration (40-90 days) studies with highly compacted bentonites have shown that high uniform dry density ( 1.6 g/cm3) results in low water activity (<0.96), high swelling pressure (> 2 MPa), and average pore size of <0.02 m; conditions that create microbiologically unfavourable environments that limit microbial activity near used nuclear fuel containers in a deep geological repository setting. However, because high dry density only suppresses microbial activity but not necessarily eliminates the microbial population present in these bentonites as well as the fact that a reduction in the dry density of highly compacted bentonites may occur at myriad interface locations in the repository; it is imperative to assess how a reduction in dry density (for instance from 1.6 to 1 g/cm3) may alter the biogeochemical composition of NOM in compacted bentonites. This study used total organic carbon (TOC) and compound level characterization of MX-80 bentonites to examine how compaction may alter NOM chemistry with varying densities and durations. Our results showed that the bentonites exhibit low TOC content (< 1%) and there was no statistically significant variation in the TOC values of compacted samples compared to the uncompacted reference bentonite. Solvent extraction of the NOM revealed a preponderance of long-chain homologues among the analysed compound classes, suggesting the predominance of more chemically persistent fraction of NOM in the bentonites. Solid-state 13C nuclear magnetic resonance (NMR) analysis provided further insights into the molecular composition of NOM and revealed no marked difference in the biogeochemistry of NOM pre- and post- compaction. Statistical analyses also showed no significant difference in the TOC and molecular level compositions of NOM in compacted and uncompacted bentonites. These results suggest that dry density compaction produced little to no discernible alteration in the composition of NOM at the bulk and molecular level, consistent with other studies that found no alteration in the microbiological profile of bentonites as a result of compaction.