Fixation and liberation of smectite interlayer cations under dry and wet conditions

The aim of the present study was to examine changes in the layer charge (LC) of smectite heated for an extended period of time under two contrasting conditions: wet (hydrothermal experiments) and dry (oven heating). Understanding smectite's behavior at elevated temperatures is crucial, e.g., in the case of high-level radioactive waste storage facilities, where bentonites (smectite-rich rocks) are planned to be used as one of the barriers. A set of two montmorillonites and one beidellite in four cationic forms, Mg²⁺, Ca²⁺, Fe²⁺, and K⁺, was heated at 100 or 200 °C for up to 157 days. In addition, LC regeneration tests involving cation exchange and hydrothermal treatment of oven-dried samples were performed. LC was measured using the spectroscopic OD method, whereas X-ray diffraction was used to verify the changes detected by the OD method. In the dry-heating experiments, most samples showed stable LC values up to 146 days of heating, except the LC decrease for Fe²⁺ and Mg²⁺-exchanged montmorillonite, which was interpreted as resulting from the formation of inner-sphere complexes and/or penetration of these cations into the octahedral sheet. The LC values of samples after hydrothermal experiments were more variable, possibly due to the cation exchange, and dissolution and reprecipitation processes. In the case of Fe²⁺-exchanged samples reacted hydrothermally at 200 °C, kaolinite was formed at the expanse of smectite, and collapsed interlayers formed in K⁺-exchanged samples reacted under the same conditions. Cation exchange and hydrothermal treatment were not always successful in restoring the initial LC of samples, however, hydrothermal rehydration reaction was efficient in the case of samples dried at 100 °C. The paper presents the mechanism of interlayer cations fixation and liberation, which change the properties of smectite-rich bentonite barriers for nuclear waste disposal under partially dry vs. water-saturated conditions. Reversible after drying at 100 °C and irreversible after 200 °C cation fixation match common temperature limits of bentonite exposure to radioactively-produced heat.