Investigation of ion concentration in pore water in compacted bentonite after infiltration by salt solutions

Abstract

The behavior of salt solutions infiltrated into compacted bentonite was investigated in this study, with particular attention paid to the ion concentration in the pore water, in order to improve the understanding of the bentonite behavior in geological disposal projects. A Japanese bentonite, Kunigel V1 (K_V1), was used to prepare specimens with a thickness of 2 mm and an initial dry density of 1.4 to 1.7 Mg/m³. For each density case, salt solutions (NaCl, KCl, and CaCl₂) of different amounts (0 to 2 mol/L) were supplied to the specimens. After infiltration, the basal spacing (d₀₀₁) and exchangeable cations of the montmorillonite in the bentonite and the leached cations from the bentonite were measured. Based on the test results, the ion concentration in the interlayer pore water of the montmorillonite or the interparticle pore water was discussed. The findings indicated that the infiltration capacities of the various salt solutions into the compacted K_V1 bentonite were in the order of KCl > CaCl₂ > NaCl. The K_V1 specimen with the highest initial dry density exhibited the strongest resistance to salt solution infiltration. After the infiltration of the NaCl solution into the compacted K_V1 bentonite, the increased sodium ions mainly remained in the interparticle pores, leading to an increase in the sodium ion concentration in the interparticle pore water. During the infiltration of the KCl and CaCl₂ solutions into the compacted K_V1 bentonite, the infiltrated potassium ions in the case of KC1 and the calcium ions in the case of CaCl₂ tended to penetrate the interlayer pore preferentially, thereby displacing the exchangeable sodium ions. After most of the exchangeable sodium ions that had initially existed in the montmorillonite had been replaced, the infiltrated potassium or calcium ions remained in the interparticle pores.