Sorption data: Enhancement of geochemical modeling by chemically evident surface speciation

Modeling geochemical scenarios for the safety analyses of disposal of hazardous radioactive and (chemo)toxic waste requires comprehensive and consistent thermodynamic data as well as sorption data for the surrounding host rocks. Whereas there are several projects running worldwide to develop at the comprehensive and consistent thermodynamic database for the aqueous phase and forming solids, the situation is much more complicated concerning the reactions on the mineral-water interface. For sorption data, there is currently no database providing quality assured thermodynamic surface complexation modeling (SCM) data. Even though spectroscopic methods to determine the actual surface species have made great progress in recent years, the SCM data still contain questionable to assuredly non-existent species. This leads to hardly comparable results in geochemical modeling. To address this problem, publicly available SCM (protolysis and sorption) data are currently being reevaluated and new reaction data are generated building on spectroscopically evidenced surface complexes. Critical data gaps shall be closed by the use of analogies (for both radionuclides’ chemistry as well as the mineral phases) or established estimation methods (e.g. linear free energy relationship). The RES³T sorption database¹, the PSI Chemical Thermodynamic Database² as well as the LLNL’s sorption raw data compilation³ provide the solid basis for this work. In combination with surface site density data from crystallographic calculations, this approach yields realistic and robust models that significantly improve sorption in geochemical calculations e.g. through so-called smart Kd values⁴. The results of this work will be published within the THEREDA framework⁵ including ready-to-use parameter files for common geochemical codes (e.g. GEMS, Geochemist’s Workbench, PHREEQC). This work is funded by BGE – the federal company for radioactive waste disposal in Germany, with the contract number TEKFuE-21-03-j