Assessment of the molecular-level compositional heterogeneity of natural organic matter in bentonites intended for long-term used nuclear fuel storage

Abstract

Bentonite-rich clays have been chosen as the sealing material in the engineered barrier system of the deep geological repository (DGR) for the long-term storage of used nuclear fuels. Natural organic matter (NOM) found in these bentonites can serve as a source of nutrients for microbial growth. Although high density bentonite does not support microbial activity within its matrix, it is possible that microbial growth will occur at interface of the DGR rock wall and the bentonite; the availability of nutrients is therefore a design consideration. Furthermore, it is unclear if NOM is similar in composition within bentonite samples collected from different locations from the same naturally-occurring clay deposit, or whether NOM chemistry will vary within different batches.Here, we report the detailed geochemical characterisation of different nutrients and NOM in five different MX-80 bentonites from the same location, but collected in different lots. Our results show that total carbon comprises <1% and that organic carbon constitutes less than one-fifth of this total carbon content. Statistical analyses revealed no significant differences in the NOM composition, or the other geochemical characteristics, of these samples. Targeted organic compound analyses indicated a predominance of long-chain homologues in the aliphatic lipids of the bentonites, suggesting that NOM in MX-80 bentonites is primarily terrestrially-derived. We detected nanogram-level variabilities in the concentrations of selected compounds between samples, but these variabilities were not statistically significant. Analysis by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy also showed consistency between the NOM of the different MX-80 samples and a predominance of aliphatic, as well as aromatic compounds, which suggests that the NOM has undergone extensive diagenesis.Overall, all analyses consistently indicated that the NOM composition exhibits low heterogeneity between the different bentonites mined from the same source. These results provide confidence in the potential use of MX-80 as a buffer material for widespread use in DGRs and they build upon past research on the NOM composition of bentonite clays.