Characterization of natural organic matter in bentonite clays for potential use in deep geological repositories for used nuclear fuel

Abstract

The Nuclear Waste Management Organization (NWMO) is developing a Deep Geological Repository (DGR) to contain and isolate used nuclear fuel in a suitable rock formation at a depth of approximately 500m. The design concept employs a multibarrier system, including the use of copper-coated used fuel containers, surrounded by a low-permeability, swelling clay buffer material within a low permeability, stable host rock environment. The natural organic matter (NOM) composition of the bentonite clays being considered for the buffer material is largely uncharacterized at the molecular-level. To gain a better understanding of the NOM in target clays from Wyoming and Saskatchewan, molecular-level methods (biomarker analysis, solid-state 13C NMR and solution-state 1H nuclear magnetic resonance (NMR)) were used to elucidate the structure and sources of NOM. Organic carbon content in three commercially available bentonites analyzed was low (0.11–0.41%). The aliphatic lipid distribution of the clay samples analyzed showed a predominance of higher concentration of lipids from vascular plants and low concentrations of lipids consistent with microbial origin. The lignin phenol vanillyl acid to aldehyde ratio (Ad/Al) for the National sample indicated an advanced state of lignin oxidation and NOM diagenesis. The 13C NMR spectra were dominated by signals in the aromatic and aliphatic regions. The ratio of alkyl/O-alkyl carbon ranged from 7.6 to 9.7, indicating that the NOM has undergone advanced diagenetic alteration. The absence lignin-derived phenols commonly observed in CuO oxidation extracts from contemporary soils and sediments as well as the lack of amino acids suggests that the material corresponding to the aromatic signal is not composed of lignin or proteins but may be derived from another source such as black carbon or some other non-extractable aromatic-rich NOM. The aliphatic signal appears to correspond to long-chain compounds with little side branching based on the results of the one-dimensional (1D) and two-dimensional (2D) solution-state 1H NMR analyses. Overall, the organic geochemical analyses suggest that the NOM is composed mainly of plant-derived waxes and highly aromatic carbon with low contributions from small molecules. The compounds identified by the molecular-level analysis of NOM in the clay samples are hypothesized to be recalcitrant but future studies should examine if these compounds may serve as a microbial substrate to further test the observations of this study. Furthermore, our study suggests that the NOM has undergone diagenesis and that marine NOM signatures are no longer recognizable or detectable. As such, future work may also examine the diagenesis of these deposits to further understand the NOM geochemistry and paleoenvironmental conditions in bentonite deposits.