This paper discusses the mechanism of montmorillonite structural alteration and modification of bentonites’ properties (based on samples from clay deposits Taganskoye, Kazakhstan and Mukhortala, Buriatia) under thermochemical treatment (treatment with inorganic acid solutions at different temperatures, concentrations and reaction times). Treatment conditions were chosen according to those accepted in chemical industry for obtaining acid modified clays as catalysts or sorbents. Also, more intense treatment was carried out to simulate possible influence at the liquid radioactive site repositories. A series of methods was used: XRD, FTIR, ICP-AES, TEM, nitrogen adsorption, and particle size analysis. It allowed revealing certain processes: transformation of montmorillonite structure which appears in the leaching of interlayer and octahedral cations and protonation of the interlayer and –OH groups at octahedral sheets. In turn, changes in the structure of the 2:1 layer of montmorillonite and its interlayer result in significant alterations in the properties: reduction of cation exchange capacity and an increase of specific surface area. Acid treatment also leads to a redistribution of particle sizes and changes the pore system. The results of the work showed that bentonite clays retain a significant portion of their adsorption properties even after a prolonged and intense thermochemical treatment (1 M HNO₃, 60 °C, 108 h).

This study is devoted to studying the sorption of ¹³⁷Cs on mineral sorbents at a wide pH range, from 2 to 10, as well as to studying sorption mechanisms. In order to obtain the most reliable sorption characteristics, samples of high purity were examined as sorbents: bentonite, glauconite, zeolite, and diatomite. A detailed description of their mineral composition, cation exchange capacity and specific surface of sorbents is given. XRD, XRF, FTIR, SEM, and BET adsorption methods were used for assaying. The sorption and desorption values were identified for each sorbent. As a result of the conducted research, it can be concluded that ¹³⁷Cs sorption mainly occurs through the exchange reaction on zeolite, glauconite and bentonite. The highest cesium Kd was observed on zeolite due to its high CEC and amounted to 4.05 mg/L at pH 7. The higher sorption capacity of glauconite in comparison with bentonite is primarily due to the high layer charge which is mainly localized in tetrahedral sheets, and to the existence of highly selective sorption sites (frayed edge sites) on the glauconite surface. Diatomite showed the lowest sorption capacity provided by the presence of a small quantity of smectite and kaolinite in its composition. The values of desorption increase in the following order: zeolite < bentonite ~ diatomite < glauconite.

—Complex studies revealed transformations of composition, structure, and properties of bentonite clays of the Taganskoe (Kazakhstan) and Dahskovskoe (Moscow oblast) deposits under...

This paper deals with adsorption regularities and mechanisms of nonradioactive Cs(I) and Sr(II) analogs on bentonites of different chemical and mineral composition from solutions of Cs and Sr nitrates with pH 3, 7, and 10 units at constant ionic strength. The bentonites were taken from the deposits Taganskoe (T), Dash-Salakhlinskoe (DS), Zyryanskoe (Z), and 10th Khutor (10H). The pH of bentonite aqueous suspensions, T and DS, exceeded 9 units. A less alkaline reaction was observed in bentonite suspensions Z and T with pH 8.94 and 7.70, respectively. Bentonites T and DS contained significant amounts of nonsilicate iron compounds, 1.0 and 0.5%, respectively. The recovery rate of the studied clays from aqueous solutions of Cs(I) and Sr(II) ions in concentrations from 0.25 to 5 mmol/L varied from 50% to 90% and decreased in the following order: “Ta-ganskoe” > “Dash-Salakhlinskoe” > “Zyryanskoe” > “10th Khutor” in the studied pH range. The main mechanism of Cs(I) and Sr(II) sorption in the studied pH range was cation fixation in the form of outer-sphere complexes on planar surfaces resulting from ion exchange. Increasing pH (pH > 6) enhanced pH-dependent positions, which allowed Cs(I) and especially Sr(II) ions to fix on them more firmly as inner-sphere complexes. At pH 9–10, Sr(II) could precipitate in the form of carbonates. The sorption of Cs(I) + and Sr(II) was accompanied by competitive interactions with proton at pH < 6 and Na⁺, Ca²⁺, Mg²⁺, and K⁺ cations at higher pH values. This competition was more apparent at concentrations of Cs(I)and Sr(II) in initial solutions < 0.5 mmol/L. The ability of bentonite T to sorb Cs(I) and Sr(II) in large amounts compared to the other bentonites was determined by high CEC values and charge of smectite T.

Bentonite clays have unique properties that determine their use as the main component of engineered barrier systems (EBS) for the isolation of radioactive waste. At present, the Russian Federation is elaborating the concept of deep geological disposal of radioactive waste in crystalline rocks of the Yeniseisky site, where bentonite clays play an important role in ensuring the safety of the repository for a long period of time. This work demonstrates the first results of short-term laboratory experiments (1 and 3 months) on the thermochemical interaction of bentonite and concrete in the presence of synthetic water at an elevated temperature. These experiments will help predict the effect of EBS materials on montmorillonite. Bentonite from the 10th Khutor deposit (Russia) and Portland cement were used in the experiments. At the first stage of the experiments, solutions were obtained after leaching the concrete with a synthetic groundwater solution for 1 month at 90 °C. At the second stage, the interactions of the obtained solutions with bentonite at 90 °C were studied. As a result of the experiments, the processes of concrete leaching were revealed, which changed the composition and acidity (an increase in the pH from 6.1 to 12.1) of the synthetic water and led to an increase in the porosity of the material in contact with the solution. However, no dissolution of montmorillonite was observed, and the changes were quite small. The research results show the high stability of bentonite from the 10th Khutor deposit under model conditions, which was confirmed by modeling. Thus, we can say that at pH ≈ 12 and at elevated temperatures, montmorillonite retains a stable structure for a long time, which is important for ensuring the safety of disposal in general.