Molecular-level geochemical characterizaiton of organic matter heterogeneity in bentonite

Abstract:

MX-80 bentonites have been selected as the primary buffer material to be used in the deep geological repository (DGR) by the Nuclear Waste Management Organization (NWMO) and their long-term plan for the storage of used nuclear fuel. Previous studies revealed that natural organic matter (NOM) in bentonites has the potential to serve as substrate for microbial activity. However, only one sample per deposit was used in that study and it remains unclear whether NOM is similar in composition within samples collected in different lots from the same bentonite deposit and whether NOM chemistry vary within different products; thus, rendering the assessment of potential variability in the geochemistry of different bentonites from the same source crucial especially as large volumes of bentonites is required. Here, we present the results of detailed geochemical and molecular-level NOM analyses of MX-80 bentonites from different lot numbers mined from the same deposit in Wyoming in order to assess the heterogeneity in the samples and the potential of NOM in these bentonites to serve as a substrate for microbial activity within the context of long-term storage of used nuclear fuels. Our results show that total carbon is less than 1% (0.70±0.04%) and organic carbon constitutes less than one-fifth (0.10±0.01%) of the total carbon content. Statistical analyses revealed no significant difference in the NOM geochemical composition. Targeted organic compound analyses indicate a predominance of long-chain homologues among the analysed compound classes. We detected nanogram-level variabilities in select compounds among the samples. However, these variabilities are not statistically significant (at P≤0.05). Solid-state ¹³C Nuclear Magnetic Resonance (NMR) analysis showed consistency between the different samples and the predominance of aliphatic as well as aromatic compounds which suggests that the NOM has undergone extensive degradation. Overall, all analyses are consistent and indicate that the NOM composition exhibits low heterogeneity in different bentonite samples.