Bentonite is a potential material for use in the engineered barrier of radioactive waste repositories because of its low hydraulic permeability, self-sealing capability and retention capacity. It is expected that bentonite would react at the elevated temperatures accompanying the radioactive decay in the nuclear waste. The presented study was started in order to improve understanding of the coupled influence of temperature and (pore) water on the physicochemical and mineralogical properties of bentonite during thermal treatment under near-field relevant conditions. Granular Na-bentonite MX-80 was differently saturated (Sr = 1–0.05) and stored at different temperatures (50–150°C) in a closed system. Upon dismantling after different periods of time (3 to 18 months), mineralogical characteristics, cation exchange capacity and content of leachable cations, as well as physicochemical properties such as surface area and water adsorption were investigated.

The results showed a high mineralogical stability. A slight conversion from the sodium to an earth alkali form of the bentonite was observed. However, considerable changes in the physicochemical properties of the bentonite were observed, particularly by treatment above the critical temperature of 120°C. The cation exchange capacity decreased during heating at 150°C by approximately. 10%. The specific surface area dropped by more than 50%. The water uptake capacity under free swelling conditions showed a slight tendency to lower values especially for samples heated for more than 12 months. The water vapour adsorption ability in contrast drops by 25% already within three months at T = 120°C. These changes are mostly related to the variations in the interlayer cation composition and to smectite aggregation processes. The observed alterations are rather subtle. However, temperatures ⩾ 120°C had a remarkable negative influence on different properties of MX-80.

The use of clays as industrial catalysts requires optimization of modification methods during their preparation. The objective of this paper was to improve the traditional method of bentonite impregnation using ultrasound. For this purpose, three series of samples with Fe3+/clay molar ratios of 0.6–11 mmol/g were prepared, which differed in terms of preparation procedure. The first batch of samples was subjected to the conventional method of impregnation, where total synthesis of catalysts took 4 h. The other two series of samples were prepared with impregnation improved by ultrasound: preparation of Fe-polycation (5 min) and then its incorporation with a clay suspension for 5 and 10 min. The effect of clay preparation method on the catalyst stability and efficiency in a heterogeneous Fenton process was studied on aqueous solutions of synthetic Reactive Blue 4 dye. The catalysts prepared by the conventional method and the improved ultrasound method achieved high efficacy (91–97%, respectively), but their stability was different in the Fenton process. The catalysts prepared using ultrasound for 10 min exhibited greatest stability in the Fenton process. The catalysts synthesized with different Fe loadings displayed an increase in specific surface area and mesoporosity. Samples prepared by the improved impregnation method are comparable in terms of their characteristics with their counterparts prepared by the conventional method. It is thus possible to reduce the time taken by traditional catalyst synthesis by using optimized exposure time to ultrasonic waves.