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

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.