Kinetics of rehydration in smectite and its application to water resorption in bentonites

Abstract

This study investigated the rehydration kinetics of homoionic model smectites and various bentonites following dehydration, addressing gaps in the understanding of how both dehydration and rehydration temperature and the type of interlayer cation affect this process, critical in basic studies involving swelling clays and in their industrial applications. The results showed that higher dehydration temperatures reduced the extent of rehydration and were correlated with a decrease in layer charge, particularly in smectites exchanged with divalent cations. This was linked to the fixation of small interlayer cations, resulting in the collapse of high-charge layers and, therefore, slower reaction progress. Smectites exhibited fast initial rehydration followed by slow equilibration stage. Under typical laboratory conditions (20 °C, RH = 30 %), 25 % of water was readsorbed within several minutes whereas 90 % - within four hours, emphasizing the necessity to prevent rehydration during experiments to ensure accuracy of various measurements. Regardless of the experimental conditions, reaction profiles were fitted to contraction and 2D/3D diffusion kinetic models. The activation energy for rehydration (∼50–70 kJ/mol) was comparable to that of dehydration, reflecting similar energy barriers in overcoming adsorption/desorption of tightly bound water. Notably, bentonites exhibited rehydration behaviour similar to that of smectites, with the kinetics also influenced by the layer charge of the smectites. Similarities in behaviour of model smectites and bentonites will aid in predicting the long-term rehydration behaviour of bentonite barriers in high-level radioactive waste underground repositories.