Thermodynamic modeling and sensitivity analysis of porewater chemistry in compacted bentonite

Ochs, M.*; Lothenbach, B.*; Shibata, Masahiro ; Yui, Mikazu

Compacted bentonite is foreseen as the buffer material in the engineered barrier system of the geological disposal system. To derive d values and diffusion coefficients of radionuclides for performance assessment, it is critical to be able to describe the chemistry of the pore solution and to understand how it is influenced by different factors. This paper presents a sensitivity analysis on the influence of important parameters on porewater chemistry in compacted bentonite. The principal parameters varied were the fractions of calcite, gypsum, and NaCl dissolving from bentonite, and pCO. Model calculations treated ion exchange reactions of smectite and protonation/deprotonation at the edge of smectite using the diffuse layer model. Some calculations consider electric double layer effects in the porespace. The model results show that two powerful pH buffer systems are operative in compacted bentonite: Amphoteric edge SOH sites and the carbonate buffer system. If pCO is imposed externally, the resulting porewater pH is mainly controlled by the carbonate buffer. If bentonite is treated as a closed system, the buffering action of the SOH sites becomes more important. In both cases, the dissolution of calcite and gypsum from the bentonite is important. If the development of an electric double layer in the bentonite pores is considered, entire porespace may be occupied with diffuse layers, leaving no space for free porewater, at higher dry densities (1200 kg/m).