Technical Report NTB 23-05

Abstract

The ClaySor model is an implementation of the two-site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model in the GEM-Selektor soft­ware and databases. In this work, the ClaySor model was made consistent with the PSI Chemical Thermodynamic Database 2020 (TDB 2020), resulting in the updated Sorption Thermodynamic Database (STDB 2023) for illite and montmorillonite. The STDB 2023 comprises surface complexation constants and selectivity coefficients for Cs(I), Cd(II), Co(II), Fe(II), Mn(II), Ni(II), Pb(II), Ra(II), Zn(II), Am(III), Cm(III) (illite only), Eu(III), Pu(III, IV), Np(IV, V), Sn(IV), Th(IV), Nb(V), Pa(V), and U(IV, VI).

Extensive experimental data on pH edges and adsorption isotherms, collected over a few decades at the Laboratory for Waste Management (LES) at the Paul Scherrer Institute and supplemented with recently obtained datasets, were compiled and evaluated. At the same time, the ClaySor model was fitted to all datasets of certain metal/clay mineral pairs using the GEMSFITS param­eter optimisation tool to determine adjustable model parameters such as surface complexation constants and cation exchange selectivity coefficients. The performance of the updated model is demonstrated on sample pH edges and adsorption isotherms. The uncertainties of adjustable parameters were estimated by means of the 95% confidence interval determined via the Monte Carlo method as implemented in the GEMSFITS package. The re-evaluated equilibrium surface complexation constants and cation exchange selectivity coefficients (STDB 2023) are now fully consistent with the TDB 2020 and are provided in the appendix, along with the related metal hydrolysis constants. In addition, an overview of batch sorption experiments of the investigated elements on illite and montmorillonite used for the ClaySor model parameter optimisation is given in the appendix.

For some cations of interest, namely Cd(II) and Fe(II) for illite, as well as the surface com­plexation constants of Pu(III, IV), Np(IV), and U(IV) for illite and montmorillonite, the surface complex­ation constants were determined based on the linear correlations between the stability constants of aqueous and surface complexes (linear free energy relationships, LFERs). The estimated equi­lib­rium constants of Cd(II) and Fe(II) were used to predict pH edges and adsorption isotherms, which showed a good agreement with the experimental data of chemical analogues, implying the reliability of the constants found in LFERs.

The ClaySor 2023 model and STDB 2023 for 2:1 clay minerals can be used to compute the solid-liquid distribution ratios (R_d) of many cationic elements in complex systems of geological and engineered barriers in the deep geological repository, namely the illite-containing argillaceous rocks and the bentonite buffer materials, interacting with porewaters of various compositions.