Pore water concentrations of low-molecular-weight organic acids – effects of thermal generation and decomposition

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Summary At elevated temperatures e.g. caused by heat-emitting, high-level radioactive waste, low-molecular-weight organic acid (LMWOA) generation and decomposition from organic matter take place simultaneously. In the process, the generation can exceed the decomposition for individual compounds – and vice versa. As a result, concentrations of LMWOA in aqueous geological fluids undergo constant, temperature-dependent changes. This study presents hydrous pyrolysis data for the generation of three LMWOA (formate, acetate, and oxalate) from a natural claystone (Opalinus Clay). Experiments were conducted between 80–200 °C and at 20 MPa over 504 to 906 hours. Thus, the material was exposed to conditions resembling diagenesis to catagenesis. The results clearly show individual generation and decomposition behaviors. As an example, at 200 °C, acetate was continuously generated, whereas formate experienced rapid generation and successive decomposition. Oxalate decomposition outcompeted its generation, which resulted in concentrations below the limit of quantification after 48 hours. The observations are consistent with calculations using kinetic parameters for thermal LMWOA decomposition. This study emphasizes the importance of simultaneous thermal LMWOA generation and decomposition in the context of mineral dissolution, metal-ion and radionuclide complexation, for sorption processes and as a pathway to sustain microbial communities e.g. in the deep biosphere.