Assessment of salinity and heat impacts on natural organic matter composition in bentonite for used nuclear fuel storage

Bentonite clay (MX-80, Wyoming, USA) is a proposed sealing material for the safe and long-term management of Canada's used nuclear fuel in a deep geological repository (DGR). Due to the physiochemical properties of bentonite, this buffer layer can limit radionuclide mobilization, and minimize air and water circulation, and hence suppress microbial activity near the used fuel container. However, natural organic matter (NOM) in mined Wyoming-type bentonite (MX-80) may serve as microbial substrates and cause further corrosion to the used fuel container. In the proposed DGR, bentonite will be exposed to high salinity due to saline groundwater and heat released from radioactive decay of radionuclide inventory of the used nuclear fuel. There is limited knowledge about how NOM quantity and quality in MX-80 may change after being exposed to the anticipated levels of salinity and heat in a DGR, especially at the molecular-level. To explore this, organic carbon contents and NOM composition (both soluble- and solid-phase) in MX-80 after exposure to different saline (NaCl) solutions (1 M, 2 M, 3 M, 4 M and 5 M) with and without heat exposure (90 °C) were analyzed. Dissolved organic carbon (DOC) solubility decreased with increasing salinity but these differences were not statistically significant when compared to the control (P < 0.05). DOC solubilization was not significantly altered with heat (P < 0.05). Dissolved organic matter (DOM) composition was analyzed with solution-state ¹H nuclear magnetic resonance (NMR) and ultraviolet–visible (UV–Vis) spectroscopy. The NMR results indicated that DOM samples with varied salinities and heat exposure had similar chemical signatures. The results from UV–Vis analysis revealed a small decrease in less unsaturated components in DOM with NaCl solution compared to deionized water. The chemistry of solid-phase NOM that remained in MX-80 after DOM isolation was similar and suggests an insensitivity to either heat and/or salinity. The solubilized DOC contributes less than 1% to the total organic carbon in MX-80, thus, the anticipated level of salinity and heat in a DGR is unlikely to enhance NOM solubilization and change NOM chemistry in the bentonite markedly. As the bentonite in this study was only exposed to the heat and salinity in a short-term experiment, further studies examining NOM composition and reactivity under long-term saline and heat conditions are recommended.