Editorial for Special Issue “Adsorption Properties and Environmental Applications of Clay Minerals”

Clay minerals play a fundamental role in various environmental processes, particularly in controlling the movement of various ions and molecules in soils, waters and natural and/or engineered barriers of waste storage facilities. A number of operations and applications are facilitated by clay minerals, either via direct use or after modification; these operations are carried out with the aim of improving the environmental quality. Mineralogical, chemical and geotechnical characterizations of the clay materials being studied and applied need to be conducted in order to learn the control processes determining the uptake of contaminants by clay materials. The objective of this Special Issue is to consider the various aspects involved in the characterization of clay materials and their behavior in the environment, with an aim to better understand the control processes determining the uptake of contaminants by them. Another objective of this Special Issue is to present models that might be suitable for modeling the transport of contaminants near their sources in the environment, in which clay minerals play a significant role. In the years 2024 to 2025, five contributions, which fulfilled the objectives set out, were accepted for this Special Issue. The contributions are devoted to addressing a wider spectrum of issues, from the purification and modification of different clay minerals and the modification of methodologies to applications in wastewater treatment. The study of the adsorption of selected species using standard description methods is the unifying element across all of the included studies; none of the contributions deal with transport modeling.

There are some hazardous nuclear and radiation facilities in the Russian Federation; these require special monitoring and/or decommissioning procedures. Following the decision of the USSR government in the second half of the 1950s of the 20th century, geological exploration and research were carried out to justify and create deep (underground) disposal test sites for liquid radioactive waste (LRW) for four nuclear-industry enterprises. One of these facilities was the disposal site for LRW on the territory of the JSC Siberian Chemical Plant, where specially prepared waste was injected into sand reservoirs lying at depths of 300–350 m between clayey strata. In the contribution of Krupskaja et al. (2024) (contribution 1), the samples used in the study were taken from five observation cores at a distance of 110–180 m from the injection cores; samples from buffer clay horizons underlying and overlying reservoir layers and same-age rocks outside the radioactive waste isolation site were characterized, and the authors subsequently discussed the acquired knowledge. This study examined, in detail, the features of the lithological and mineral composition of reservoir sands and aquitards. The processes of environmental transformation in reservoir sands, which led to the changes in the composition and structure of rocks, were characterized by the researchers. These processes manifested themselves in the development of leaching zones and their “healing” with newly formed smectite, the destruction of terrigenous grains, including the development of cracks, and the growth of newly formed smectite in the pore space of the reservoirs. The occurrence and localization of authigenic smectite formed as a result of technogenic impacts were described. Although the obvious impact of highly reactive solutions was observed, accompanied with the impacts of LRW, the insulating properties of the geological environment were maintained and even improved to some extent. The rocks surrounding the test site served as an additional natural barrier that could be characterized by a higher value of smectite phases with good adsorption abilities compared with reservoir sands. These characteristics were also confirmed to apply to the overlying and underlying buffer clay horizons, in which the content of smectite phases increased, including in the composition of mixed-layer clay minerals.

The contribution of Koroleva et al. (2024) (contribution 2) was also dedicated to the investigation of clay materials in radioactive waste repositories, based on a detailed study of samples from three industrial bentonite deposits situated in Russia and Kazakhstan. The work was generally focused on bentonite—the most promising candidate material for a buffer layer in engineered barrier systems (EBSs) of the geological disposal facility. The buffer was responsible for providing favorable conditions for the isolation of radioactive waste and protecting the packaging of radioactive waste. Thus, the buffer was assigned a safety function to limit advective mass transfer and microbial activity, protect and keep the canister in position, and resist transformation. The layer of compacted bentonite contributed to limiting the transport of radioactive contaminants in the spent fuel repository, thereby ensuring safety. The samples of the studied bentonite were subjected to stepwise purification from carbonates, organic matter, and the non-clay iron bearing phase. The results of Mossbauer spectroscopy indicated that the oxidation of Fe²⁺ into Fe³⁺ had already occurred after the removal of carbonates with acetate buffers for bentonite from the Taganskoe and Zyryanskoe deposits. However, the pattern of changes in iron states in the 10H sample was more complex. For all bentonite samples, a noticeable increase in specific surface area was observed. With regards to the Taganskoe and Zyryanskoe bentonite samples, an increase in surface area occurred due to an increase in both micro- and mesoporosity. For 10th Khutor bentonite, the observed increase in specific surface area was related to the increase in the volume of the mesopores, representing a type of clay that was more resistant to various thermal and chemical treatments in comparison with the other bentonite samples. During adsorption experiments, it was revealed that there was no dependence of sorption characteristics on the specific surface area, which indicated a predominant influence of the mineral composition of the studied bentonites on the sorption of Cs-137. The stepwise-purified bentonite samples from the Zyryanskoe deposit presented the highest sorption characteristics toward the Cs-137 radioisotope. The revealed characteristics of cesium sorption were explained by the changes in the value and localization of the layer charge. The preferential localization of charge in the tetrahedral sheet of smectite from the Zyryanskoe bentonite contributed to the increase in cesium sorption. It was found that the stepwise purification of bentonites resulted in the improvement in the sorption characteristics. This outcome was explained by the simultaneous influence of several factors: the increase in the relative content of smectite was due to the removal of non-clay components, the substitution of the exchangeable cations with Na⁺ and the effect of its dispersion. Thus, the bentonite products obtained after purification differed from the initial samples in their composition, structure, and properties. The changes in the minerals, chemical composition, composition of the smectite-exchangeable cations, sorption properties, specific surface area and characteristics of the pore space occurred as a result of the removal of impurities that affected these parameters. Therefore, the modified materials were obtained according to the results of the purification, which should be followed up on in further research.

Clayey soils exhibit a fragile structure and completely lose their undrained shear strength upon remolding; these soils can be designated as sensitive clays. The degree of sensitivity is quantified by the ratio between the undrained shear strength of an undisturbed specimen and the strength of the same specimen with the same water content in a remolded condition. The contribution of Di Sante et al. (2024) (contribution 3) analyzed the physical–chemical and geotechnical characteristics of four Scandinavian sensitive soils formed under different environmental depositional conditions; this endeavor led to an acknowledgment and assessment of the potassium sorption capacity among the investigated sensitive soils, as a basic characteristic with which to evaluate the effectiveness of treatment with KCl and to analyze potassium migration in such soils. The potassium sorption capacity was experimentally investigated through batch tests specifically performed on the sensitive soils to quantify the maximum sorption capacity and to identify the main affecting factors. Although the chemical compositions of the four soils were remarkably similar, their sensitivity was significantly different. Based on the literature linking the specific surface, mineralogy, and plasticity of sensitive clays from Eastern Canada, the correlation between these factors was found to be qualitatively valid for the investigated Scandinavian sensitive clays, too. The highest value of the sensitivity index among the tested soils was found to be related to the lowest cation exchange capacity and a limited amount of amorphous minerals. These characteristics contributed to an explanation of the highly sensitive behavior of the soil, the structure formation of which was affected during the deposition stage. The potassium sorption capacity (S_lim) limit of the investigated soils was found to be in the range of 0.6–0.9 based on the maximum value estimable based on of the cation exchange capacity. An equilibrium potassium concentration in solution of at least 0.4 g/L was necessary to determine the sorption capacity limit. Since the actual conditions of in situ treatments may be less favorable for sorption than those in the tests performed, the assessed values of S_lim were considered the upper limit of the effective sorption capacity of a type of soil. Furthermore, the equilibrium potassium concentration necessary to reach the limit sorption capacity may, in reality, be higher than 0.4 g/L. The authors concluded that the maximum potassium sorption capacity was always lower than that estimated by the cation exchange capacity, and it increased with the cation exchange capacity, plasticity index, and activity of the soils, as well as with the amount of phyllosilicates and amorphous minerals.

Sustainable strategies are required to mitigate elevated atmospheric CO₂ levels, especially using clay-based adsorbents. These are even more promising when these adsorbents are obtained via low-cost modifications. Clays represent desirable alternative adsorbents for carbon capture due to their natural abundance, low cost, and favorable textural properties. Among the clay minerals, halloysite nanotubes (HNTs) have emerged as a material of interest due to their tunable surface chemistry, surface area, porosity, excellent mechanical properties, and biocompatibility. Such materials often possess relatively high surface areas and porosity, potentially enhancing their ability to adsorb gases. However, their adsorption capacities may vary considerably as a result of specific structural and chemical characteristics, which can be influenced by various physicochemical activation methods. In the contribution by Dawoodi et al. (2025) (contribution 4), the effect of the ball milling of Australian HNT-rich kaolin samples on carbon capture performance was studied using two types of samples: one without iron impurities (Hal) and the other with iron impurities (HalFe). The tested materials were ball-milled for 30 and 60 min, and their CO₂ sorption was assessed at various pressures and temperatures. The crystallography, electronic microscopy, and surface area and charge characterization revealed a reduced length and an increased width of tubular structure following ball milling, leading to a higher specific surface area without compromising crystallinity. The CO₂ sorption of Hal increased by 14% at 20 bar and at 15 C after 60 min milling, with a ~300% rise at near-atmospheric pressures. Conversely, milling negatively affected the CO₂ sorption of HalFe, due to iron/illite-mica-related damage during milling. Since direct disposal of CO₂-laden materials would have gone against sustainability principles, these materials were assessed for methylene blue removal from aqueous solutions, achieving ~83% (Hal) and ~91% (HalFe) removal efficiencies. This highlighted HNT-rich kaolin’s valorization potential for carbon capture and utilization. The authors also identified some limitations of their study, including the role of moisture content in materials for CO₂ sorption and the reusability of CO₂-sorbed dye-laden material. The study of the long-term stability of the captured CO₂ within the materials and the life cycle assessment of the used materials were useful for assessing the effective sequestration, potential utilization, and the overall cost–benefit aspects of this approach. Future research should focus on the factors mentioned to develop a broader understanding of the materials studied in the context of carbon capture.

Clay minerals are promising candidates for caffeine removal due to their environmental friendliness and natural abundance. In the study by Goldner et al. (2025) (contribution 5), a commercially available bentonite sample from Sigma-Aldrich (Lot MKCR 5849, CAS 1302-78-9), with 100% of its particles smaller than 74 μm and a mean particle size of 5.45 μm), was modified by Na⁺ exchange. Caffeine adsorption was rapid, reaching equilibrium within 15 min. Adsorption isotherms for caffeine and its metabolites (theobromine, paraxanthine, and theophylline) in pure water were analyzed at 25.0 ± 0.5 °C using Langmuir and Freundlich models (the evaluation method used provided estimates of the standard deviations of the parameters of the used nonlinear isotherms), both individually and in mixtures. Only caffeine exhibited favorable adsorption behavior, fitting the Langmuir equation, which allowed for the determination of a maximum adsorption capacity of 20 ± 3 mg/g, regardless of metabolite presence. The removal exceeded 85% of the caffeine from a 5.0 mg/L solution. The adsorption affinity of the studied compounds toward Na⁺-exchanged bentonite followed the order caffeine >>> theobromine > paraxanthine ~ theophylline. The modified bentonite was then assessed for caffeine removal from beverages and synthetic urine, achieving removal efficiencies exceeding 87%. The used Na⁺-exchanged bentonite was characterized as an efficient adsorbent for removing caffeine from aqueous solutions, including complex matrices such as beverages and artificial urine, despite having poor desorption rates, leading to scarce reusability without further treatment. Consequently, this modified bentonite offers a promising alternative for sample decaffeination and wastewater treatment.