Na-montmorillonite dissolution rate determined by varying the Gibbs free energy of reaction in a dispersed system and its application to a coagulated system in 0.3 M NaOH solution at 70 °C

Abstract

Na-montmorillonite (Na+Mt) dissolution in a 0.3moldm−3 NaOH solution has been investigated at a pH of 12 at 70°C using a combination of flow-through and batch-type experiments to constrain a predictive geochemical model. The flow-through dissolution experiments were conducted in a dispersed system (initial water/solid ratio=1000cm3g−1) with varying concentrations of Si and Al to derive a Na+Mt dissolution rate as a non-linear function of the Gibbs free energy of reaction, ΔGr,mont. This rate equation was used to simulate the batch-type Na+Mt reaction experiments conducted in a coagulated system (initial water/solid ratio=20cm3g−1) in order to examine the applicability of the ΔGr,mont rate equation to higher ΔGr,mont conditions and to understand the effect of secondary mineral precipitation on the dissolution rate.The model simulation of the batch-type experiment adopting the empirical rate equations of Na+Mt dissolution and secondary mineral analcime precipitation was able to reproduce the measured changes in the amount of dissolved Na+Mt and concentrations of Si and Al in solution. The results showed that the empirical rate equation of Na+Mt dissolution determined in the far from equilibrium dispersed system was applicable to the coagulated system over a higher ΔGr,mont range and that the concentrations of Si and Al in the batch experiment were controlled by the precipitation of analcime. This implies that the precipitation of secondary minerals will strongly influence the rate of Na+Mt dissolution in the coagulated system. The effects of secondary mineral precipitation on the montmorillonite (Mt) dissolution rate will be, therefore, important in the high density, low water/solid system such as the intended bentonite buffer to be used as the primary barrier in a purpose built repository for the geological disposal of nuclear waste.