Tracking structural H- and O-isotope exchange in smectite during bentonite-water interactions over a thermal gradient in the Alternative Buffer Materials experiments

In order to track structural stable hydrogen (H)- and oxygen (O)-isotope exchange between smectite and water over a thermal gradient, samples were studied from the Alternative Buffer Materials (ABM) 2 and 5 experiments conducted at the Äspö Hard Rock Laboratory in Sweden for five different bentonite materials. The ABM experiments were designed to investigate the stability of different bentonite materials of interest as buffers around canisters containing spent nuclear fuel in long-term underground repositories. The ABM2 and 5 experiments consisted of compacted bentonite discs stacked vertically around a central heating system in a host-rock borehole for several years, with this configuration intended to simulate heating that occurs during radionuclide decay. The ABM experiments provided a unique opportunity to track isotopic exchange between smectite and water in a long-term, imperfect, natural but still semi-controlled environment. Clear differences were observed for H-isotope compositions between the ABM2 and ABM5 packages. O-isotope compositions showed similar trends between the two ABM packages. In both ABM packages H- and O-isotope exchange was more progressed at higher temperatures but the mechanisms for H- and O-isotope exchange proceeded differently. H-isotope compositions nearly reached modeled equilibrium values at temperatures between 130 and 150 °C in the absence of any smectite alteration, which suggests proton exchange as the exchange mechanism. The percentage of exchange calculated for oxygen showed that it was far from reaching equilibrium and was more resistant to exchange than hydrogen. A dissolution/re-precipitation mechanism involving fine smectite most simply explains calculated percentages of oxygen exchanged that range between initial and modeled equilibrium O-isotope compositions.