Understanding smectite to illite transformation at elevated (>100 °C) temperature: Effects of liquid/solid ratio, interlayer cation, solution chemistry and reaction time

Abstract

Bentonite (mainly smectite) has been proposed as a buffer material in a nuclear waste repository, due to its low permeability, high swelling capability, and high ion exchange capacity. To evaluate the isolation performance of the material, it is important to understand a possible hydrothermal alteration of smectite to illite. We investigated the hydrothermal alteration of smectite under various physical and chemical conditions to identify key controlling factors of the process. The experiments were performed on <2 μm fractions of Na-rich or K-exchanged smectite in DI water or 1 M KCl solution at 200 °C for various liquid/solid ratios over several time periods up to 112 days. Multiple analytical techniques were used to characterize the solid and liquid products to determine the extent of the conversion and the related mineral structural changes. Our results suggest that the conversion could be relatively fast (i.e., weeks) relative to waste disposal isolation at the optimal conditions (high K+ concentration and high liquid/solid ratio), which is much faster than previously postulated for the temperature used here. A high K+ concentration in the solution is required to trigger the structural transition of a low charge smectite layer to a high charge illite layer. This transition appears to be continuous, thus contradicting the dissolution-precipitation mechanism for smectite to illite transformation. Based on the solution chemistry measurements, with respect to silica solubility, the equilibrium constant for the transformation of K-exchanged SWy-2 smectite to illite is estimated. In addition, the overall conversion is limited, to a large extent, by the dissolved silica concentration in the solution. Given a generally low potassium content, a very low liquid/solid ratio, and an extremely slow diffusion in an actual engineered barrier system, the extent of smectite conversion to illite in a repository environment, if happening at all, would be limited. The conversion can be further inhibited by introducing brucite and amorphous silica as chemical additives to the buffer material.