Supported silver nanoparticles (Ag NPs) have been used extensively as antibacterial agents in biomedicine, biotechnology, and environmental remediation. However, a facile and scalable method for preparing Ag NPs dispersed homogeneously on supports remains a challenge. In this study, a novel molten salt method was developed successfully to synthesize the supported, homogeneously dispersed Ag NPs on palygorskite. Abundant pores and ample surface hydroxyl groups of palygorskite served as anchoring sites, preventing the rapid growth, aggregation, and sintering of Ag NPs. Typically, palygorskite was mixed with AgNO3 (as a precursor) and NaNO3 (as a dispersant), and then the mixture was heated slowly. During the heating process, the AgNO3 decomposed gradually into Ag NPs and the molten NaNO3 with a high concentration of ions dispersed the newly formed Ag NPs. The Ag NPs were dispersed homogeneously on the palygorskite and had very small particle sizes (~5.8 nm) even for a significant loading amount (~9 wt.%). As antibacterial agents, the Ag/palygorskite nanocomposites showed enhanced antibacterial activity, compared with those synthesized without the introduction of molten NaNO3. In addition, the key effect of the surface hydroxyl groups of palygorskite on the characteristics of the loaded Ag and the corresponding antibacterial activity were also elucidated. As such, the present work provided a novel and facile strategy for the synthesis, without a chemical reductant or surfactant, of supported, highly dispersed Ag NPs on clay minerals and this could have potential in the scalable production and practical application of Ag-based antibacterial materials.

Numerous experiments have verified that smectites can adsorb aflatoxin B1 (AfB1) effectively and the efficiency of this process depends heavily on the chemical, physical, and mineralogical characteristics of the smectite. Several relationships between these characteristics and AfB1 sorption have been determined experimentally, but the molecular mechanisms underlying these were not investigated. In the current study the effects of charge density, type of exchange cation, and charge origin (octahedral vs. tetrahedral) on AfB1 sorption on smectites were analyzed by a series of molecular simulations. The calculations confirmed the formation of water bridges between carbonyl groups of AfB1 molecules and interlayer cations. Flat orientation of AfB1 molecules on smectite surfaces was also confirmed. For larger amounts of AfB1 molecules in the intercalates, self-association of two AfB1 molecules bound by π–π interaction was shown. The thermodynamics of AfB1 sorption depends heavily on the water content in the structure, being optimal for basal distances corresponding to two layers of water. A clear preference for sorption of AfB1 on smectites with bivalent cations (Ba2+, Ca2+) and an octahedral origin of its layer charge was confirmed and this was explained as steric hindrance between hydrated ions and AfB1 molecules, which tend to lie flat on smectite surfaces devoid of ions. Ba-montmorillonite with a charge of 0.4 per half unit cell was shown to have the smallest and thus the best potential energy of adsorption compared to the other layer charges.

The hydrogen isotopic composition (δ2H) of authigenic clay minerals has been used extensively in paleoclimate studies. The separation of clay minerals from sediments/soils, using various chemicals, is a prerequisite for isotope ratio measurements, where carbonate, Fe-(oxyhydr)oxides, and organic matter are removed successively from the sediments for a greater clay yield. The commonly adopted organic matter-removal method using hydrogen peroxide (H2O2) is thought to either alter directly the pristine δ2H values of the smectite clay minerals or to introduce organic hydrogen-bearing impurities through the ineffective removal of organic matter. The objective of the present study was to test whether H2O2 treatment can alter the δ2H values of kaolinite (Kln) by comparing two organic matter-removal methods, namely, H2O2 and disodium peroxodisulfate (Na2S2O8) combined with a neutral buffer. In doing so, kaolinite-rich, old (~56 Ma) sediment samples and pure kaolinite internal laboratory reference materials were used to understand the effectiveness and suitability of the above-mentioned methods in clay-sample preparation for δ2H measurements. The δ2H values of the H2O2-treated aliquots show smaller δ2H values than those for the Na2S2O8-treated aliquots. Estimated ambient water δ18O values (−4‰) from the Na2S2O8-treated aliquots agreed well with the bio-phosphate (fish vertebrae) based environmental water δ18O estimation (−3.3‰). The present study indicated, therefore, that δ2H values obtained after Na2S2O8 treatment are likely to be more realistic for paleoclimate reconstruction.

A wick structure is the core part of a heat pipe that produces capillaries to move liquid from a condenser to an evaporator. The purpose of the current study was to develop a wick structure from zeolite and kaolin using various sintering methods. Due to significant porosity and water-adsorption properties, zeolite and kaolin can produce a large capillary force inside the heat pipe. A porous wick specimen is developed from pure zeolite together with a mixture of zeolite and kaolin by using pressureless (loosely packed) and conventional pressurized sintering for thermosiphon heat-pipe applications. Major properties such as porosity, water adsorption, and permeability were noted to be better under pressureless sintering compared to pressurized sintering. Significant and uneven shrinkage in both radial and linear directions is a major problem in loosely packed sintering of pure zeolite. However, the addition of kaolin helps to overcome the problem of porosity and shrinkage in pure zeolite; but the permeability and strength of the wick structure are reduced with the addition of kaolin. A general trend is that increasing porosity causes increasing permeability. Due to grain size and compaction, however, permeability is reduced with the addition of kaolin. Based on the experimental results for porosity and permeability, the wick structure formed from zeolite with 5–10% of kaolin has better thermal properties for heat-pipe applications.

Sepiolite-based composites have great potential for application as flame-retardant and thermal-insulation material but their application and development are limited by poor mechanical properties. The objective of the present study was to combine polyvinyl alcohol (PVA) and 3-aminopropyltriethoxysilane (KH-550) with sepiolite (Sep) to improve its aerogel strength. A universal testing machine, thermogravimetry, and microcalorimetry were used to investigate the mechanical properties, thermal-stability, and flame-retardant properties, respectively, of aerogels. The results indicated that KH-550 can enhance effectively the mechanical properties and flame retardancy of aerogels. The compressive modulus of PVA/Sep vs KH-550/PVA/Sep aerogel was 209.28 vs. 474.43 kPa, the LOI index changed from 26.4 to 30.4%. The porosity of the aerogels was > 96% and the density was < 0.05 g/cm3. The thermal conductivity remained at between 0.0340 and 0.0390 W/(m·K), and the aerogel could recover to > 85% after a 50% compressive deformation. These data indicated that Sep-based aerogel would act as a flame retardant and a thermal insulating material with excellent mechanical properties.

As 1:1 dioctahedral clay minerals, kaolinite and halloysite have similar chemical compositions. However, halloysite often possesses a nanotubular structure and special surface reactivity compared to platy kaolinite. The objective of this current work was to determine the effect of the SiO2/Al2O3 ratio on the microstructure and properties of geopolymers derived from two kinds of kaolin: platy kaolinite and nanotubular halloysite. The chemical structures and compositions of the geopolymers obtained were characterized through X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR), whereas the microstructural analysis was performed by scanning electron microscopy (SEM), the Brunauer–Emmett–Teller (BET) method, and N2 physisorption analysis. The results indicated that calcined halloysite showed greater geopolymerization reactivity than calcined kaolinite. In addition, the mechanical properties of the clay-based geopolymers depended not only on the SiO2/Al2O3 ratio but also on the morphology of the clay. Crystalline zeolite A and geopolymer were produced after alkali-activation of kaolin with a SiO2/Al2O3 ratio of 2.5; these products possessed porous and heterogeneous microstructures having poor compressive strength. As SiO2/Al2O3 ratios increased to >2.5, geopolymers with compact microstructure and high compressive strength were produced after alkali-activation of kaolin. Notably, at a given condition, halloysite-based geopolymers exhibited greater early compressive strength, more compactness, and more homogeneous microstructure than kaolinite-based geopolymers. This can be attributed to the nanotubular microstructure of halloysite, which can release more Si and Al during alkali activation than platy kaolinite. These results indicated that the various morphologies and microstructures among clays have significant impact on the microstructure and compressive strength of geopolymers.

Reducing the environmental footprint of cement is an absolute necessity to meet the commitments of COP26 and to limit global warming to + 1.5°C compared to the pre-industrial level. In this context, particular interest has developed in recent years in the use of calcined clays as supplementary cementitious materials (SCMs). Due to their high reactivity, large reserves and homogeneous distribution on the earth's surface, calcined clays represent a viable alternative to conventional SCMs. Clay minerals are highly variable and numerous, each with their own characteristics. As a result, not all of them have potential for use as SCMs. The present paper investigated the use of palygorskite (a clay that has been relatively poorly studied) as an SCM. Two commercial palygorskites of different grades were selected and their calcination was studied by X-ray diffraction and pozzolanic activity tests. Blended cements incorporating 20% of each calcined palygorskite were prepared and the mechanical performance and resistivity of the mortars measured. The results show that the optimum calcination temperature is 800°C (allowing complete amorphization of the clay fraction and the highest pozzolanic reactivity) for both clays. Mortars made with 80% ordinary Portland cement (OPC) blended with 20% of 800°C calcined palygorskite allowed a significant increase in compressive strength and electrical resistivity compared to the reference (100% OPC). The clay sample with palygorskite as the dominant mineral exhibited the greatest pozzolanic reactivity and mechanical performance in cementitious systems, confirming that palygorskite is a clay mineral with a significant potential for a use as a SCM. The second sample with smaller palygorskite content also allowed a significant increase in mechanical performance. This demonstrated that it is not necessary to use high-purity samples and enhances the value of this type of material.

The compressibility behavior of clays is governed by the electrical double layer formed around the clay particles. The Gouy-Chapman diffuse double layer theory is often utilized to predict the compressibility behavior of clay minerals. The theory does not consider the effect of the size of the cations, however, and thus predicts unrealistically small void ratios for compacted bentonites under large mechanical pressures expected in high-level nuclear waste-repository applications. In this study, the Stern layer was introduced to incorporate the cation size effect in the prediction of the compressibility behavior of bentonites. The overall diffuse double-layer thickness at large pressures was much smaller than the initially assumed Stern layer thickness based on the exchangeable cation size for all the bentonites studied. A compressible Stern layer was, therefore, considered for the first time in the prediction of the compressibility behavior of bentonites. The compression behavior of the Stern layer under the applied loading is influenced by the ratio of the mid-plane to the Stern potential, which is dependent on the type and composition of the exchangeable cations on the clay surface. Stern layer compression was initiated when the potential ratio was in the range 0.65–0.75 for bentonites with various surface cation characteristics. The incorporation of cation size and a compressible Stern layer provided significant improvements over the existing models in predicting the compressibility behavior of bentonites over a wide pressure range. The compressibility data predicted by the proposed model showed very good agreement with the data measured for five bentonites from the literature in the pressure range 0.1–42 MPa.

Compacted bentonite–sand mixture is proposed widely as backfill in geological repositories for disposal of radioactive waste in many countries because this material has significant swelling capacity and low water permeability. Development of the swelling pressure of backfills upon hydration is related closely to the stability of the host rock in the geological repository. No systematic experimental studies have been carried out to explore the effect of a water phase on the swelling pressure and water retention of bentonite–sand mixtures with insignificant osmotic suction. The objective of the current study was to examine experimentally the influence of a water phase involving liquid water and water vapor on swelling pressure and water retention of a bentonite–sand mixture with insignificant osmotic suction. Swelling-pressure tests with suction control and water-retention measurements under constant-volume conditions were performed on the compacted bentonite–sand mixture with a dry density of 1.80 g/cm3. Osmotic and vapor equilibrium techniques were used to make identical specimens adsorb liquid water and water vapor, respectively. The experimental results showed that the water phase had almost no effect on the swelling-pressure patterns of the unsaturated bentonite–sand mixture upon hydration over a suction range from 27 to 3 MPa. The swelling pressure increased significantly with decreasing suction from 27 to 3 MPa, regardless of the mixture adsorbing either the liquid water or water vapor. Nevertheless, the water phase had a considerable impact on both the swelling pressure and water retention of the unsaturated bentonite–sand mixture upon hydration over the same suction range. For a given value of suction in the range above, the swelling pressure and the water content of the bentonite–sand upon adsorption of liquid water were greater than those upon adsorption of water vapor. The influence of the water phase on the swelling pressure and the water retention of the bentonite–sand mixture with insignificant osmotic suction is related mainly to the hydration or swelling mechanism of Ca-rich bentonite.

Smectites are a group of minerals traditionally analyzed by thermal methods due to their exceptionally large adsorbed-water contents and the presence of OH groups, which makes them unique among all common soil- and rock-forming minerals. The dehydration reaction of smectite is a low-temperature endothermic effect that ends typically below 200°C. Although the removal of bulk interlayer water requires activation energy (Ea) of just above 30 kJ/mol, the removal of the last few H2O molecules attached strongly to interlayer cations requires Ea > 100 kJ/mol. Dehydroxylation is the loss of structural OH groups that proceeds as evolution of H2O molecules out of the smectite structure and occurs in the 300–900°C range. In trioctahedral species, dehydroxylation is combined with recrystallization and proceeds usually at > 700°C. In dioctahedral species, the temperature of dehydroxylation is controlled by the type of octahedral vacancy, having trans-vacant and cis-vacant distinguished by the boundary at ~ 600°C, and by the octahedral cation–OH bond strength, following the order Mg > Al > Fe. The Ea of dehydroxylation correlates linearly with the temperature of maximum dehydroxylation; from > 170 kJ/mol for Cs+-exchanged beidellite and nontronite, through ~ 300 kJ/mol in Mg-rich montmorillonite, to > 500 kJ/mol in trioctahedral saponite. Dehydration and dehydroxylation of smectites can be accompanied by a number of other phenomena, such as dehydrogenation or defluorination. At high temperatures, smectite amorphization and recrystallization occurs. Unless amorphized and/or recrystallized, smectites can undergo rehydration and rehydroxylation, which are opposite reactions to dehydration and dehydroxylation, respectively. This review discusses the details of the above-mentioned thermal reactions of smectites, focusing on thermogravimetric methods, evolved gas analysis, and structural alterations. Factors affecting the accuracy and precision of thermal analysis of smectite are discussed along with examples of best laboratory practices. The paper also provides the most recent description and critical evaluation of smectite reaction kinetics.