Mg and silica release in short-term dissolution tests in bentonites

Abstract

Abstract The investigation of the release of elements/structural cations from bentonites is important for high-level radioactive waste research. Smectites, the main component of bentonites, cannot be described by thermodynamics. In the present paper, therefore, element release from 38 well characterized bentonites is investigated rather than the solubility of smectites in a thermodynamic sense. The solid/liquid ratio was 0.2 mass%, temperature and time were varied. The study focused on Mg- and Si-release because Al- and Fe-concentrations measured in solution were not reproducible probably because of polymerization, recrystallization, and/or readsorption. Bentonites contain minor amounts of minerals other than smectites which could explain differences of the concentrations of released elements. Low crystalline silica phases, however, were the only admixtures significantly affecting the results. The focus, therefore, was on samples with a low amount of low crystalline silica phases. In the temperature range between 100 and 140\u202f°C Mg and Si dissolved stoichiometrically (corresponding to the typical range of Si/Mg in dioctahedral smectites). Below 100\u202f°C more Mg compared to Si was dissolved and the opposite was observed above 140\u202f°C. Extrapolating these results to higher solid/liquid rations would indicate i) that the formation of Mg/Si-phases at the heater of HLRW canisters could be explained by partial smectite dissolution and ii) that different Mg/Si-phases could form depending on temperature. The element release tests used in the present study proved to be suitable to distinguish “reactive” (released more Mg and Si) from “less reactive” bentonites.