The fate of carbon dioxide generated by decomposition of the organic matter in the Boom Clay upon heating at 80 °C: The buffering role of carbonates and implications for the geological disposal of heat-emitting waste

Abstract

Short-term and long-term hydrothermal experiments were performed with the bulk Boom Clay (Rupelian, Belgium) involving all its constituents (organic matter, minerals and water) in order to follow CO2 generation primarily formed by organic matter decomposition at 80 °C, the temperature to be reached in the near-field of a repository hosting heat-emitting radioactive waste. The carbon speciation and isotopic data in the sampled gas, liquid and solid phase served as a base for mass balance calculations and evaluation of the fate of the carbon dioxide in the studied system. The maximum CO2 yields are lower (1.9 mmol CO2/kg clay after 21 days) in the short-term hydrothermal tests as compared to the long-term tests (maximum 13.3 mmol CO2/kg clay after 7 years) thus underestimating the total capacity of the clay to generate CO2. This is supported by still continuous generation of the carboxylic acids, important precursors of the released CO2 gas, after 21 days. The stable isotope data (δ13C and δ18O) confirm that CO2 gas released from the Boom Clay is of organic origin. The early released CO2 gas is isotopically lighter (−22.2 to −23.3‰) than the reference δ13C of the CO2 (g) and isotopically heavier than the parent kerogen (−25.5 ‰) in the undisturbed Boom Clay. The early CO2 gas is likely out of isotope equilibrium with either dissolved inorganic carbon or solid carbonates. However, measured and extrapolated δ13C values of CO2 gas indicate the tendency towards isotope equilibrium between CO2 gas and both dissolved and solid inorganic carbon on the longer runs (up to 7 years). The precipitation of the carbonates and recycling of the organic matter derived CO2 in the carbonates is further supported by elemental chemistry and mass balance calculations. Altogether, our results suggest that the CO2 (g) released by Boom Clay organic matter is buffered by the carbonates (both dissolved and solid) present in Boom Clay. Thus, no net accumulation of CO2 (g) from organic matter is expected.