Hydrocalumite (HC) is a type of synthetic layered double hydroxide (LDH) that has many important industrial uses and is commonly synthesized by a co-precipitation method in a water:ethanol (2:3) mixture; however, atmospheric carbon dioxide interferes with the synthesis by decreasing the solubility of other gases in the reaction medium. The aim of the present study was to vary the temperature and aging time used in the coprecipitation method in order to mitigate the adverse effects of carbon dioxide. The water/ethanol mixture (2:3) was able to prevent atmospheric carbon dioxide contamination of the sample, as it decreased the solubility of the gas in the reaction mixture. Aging time (9–36 h) and temperature (35–95°C) were varied to modify the hydrocalumite structure, textural properties, thermal stability, and basicity. The characterization of the samples was performed using X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), nitrogen physisorption, thermogravimetric analysis (TGA), and CO2 temperature-programmed desorption (TPD-CO2) techniques. The aging time of 9 h and temperature of 95°C provided the most crystalline sample with the largest mean crystallite size (49 nm). The variation of the synthesis conditions also provided changes in the surface area (6.5–20.2 m2 g–1), pore diameter (116–148 Å), and pore volume (0.0147–0.0499 cm3 g–1). The temperature ranges for thermal decomposition of structural water and carbonate varied among the samples, indicating different thermal stabilities. The basicity (basic sites quantified by TPD-CO2) was also affected by the change in aging conditions; the sample aged for 9 h at 65°C presented the greatest basicity (1557 μmol g–1), whereas that aged for 36 h at 35°C had the least basicity (337 μmol g–1).

Titanium dioxide (TiO2, rutile) nanoparticles, inorganic ultraviolet absorbers, are used extensively in sunscreen cosmetics as an inorganic ultraviolet (UV) absorber to prevent skin damage; because of their nanotoxicity, use in combination with a support, such as montmorillonite (Mnt), rather than alone, is suggested. Mnt-supported TiO2 composites (Mnt-TiO2) for sunscreens are most suitable when the particles are spherical and of relatively uniform size, which are normally accomplished by spray drying, but this is difficult to achieve because of the naturally layered structure of Mnt. The objective of the present study was, therefore, to find the ideal characteristics of spray-drying nozzles to produce the desired spherical shape and size distribution of the Mnt-TiO2 composite particles. The starting Mnt was extracted from natural bentonite by particle-size separation. An ultrasonic nozzle in the spray dryer was selected for use in the synthesis of Mnt-TiO2 composites based on the particle-size distribution (PSD) of Mnt prepared using a two-fluid nozzle and an ultrasonic nozzle at 453 K. The incorporation of TiO2 in the final Mnt-TiO2 composites was examined by X-ray powder diffraction (XRD) and elemental analysis. With increasing TiO2 concentration, the TiO2 content and average particle size of the Mnt-TiO2 composites increased. Scanning electron microscopy (SEM) images showed that all samples prepared had uniform and nearly spherical shapes. Absorbance of UV by Mnt-TiO2 (5:1) composites was greater than that by either purified Mnts or pure TiO2. The present study demonstrated a simple method, using a spray dryer with an ultrasonic nozzle, to synthesize Mnt-TiO2 composites of uniform size and shape suitable for cosmetic application.

Conventional superplasticizers based on polycarboxylate ether (PCE) show an intolerance to clay minerals due to intercalation of their polyethylene glycol (PEG) side chains into the interlayers of the clay mineral. An intolerance to very basic media is also known. This makes PCE an unsuitable choice as a superplasticizer for geopolymers. Bio-based superplasticizers derived from starch showed comparable effects to PCE in a cementitious system. The aim of the present study was to determine if starch superplasticizers (SSPs) could be a suitable additive for geopolymers by carrying out basic investigations with respect to slump, hardening, compressive and flexural strength, shrinkage, and porosity. Four SSPs were synthesized, differing in charge polarity and specific charge density. Two conventional PCE superplasticizers, differing in terms of molecular structure, were also included in this study. The results revealed that SSPs improved the slump of a metakaolin-based geopolymer (MK-geopolymer) mortar while the PCE investigated showed no improvement. The impact of superplasticizers on early hardening (up to 72 h) was negligible. Less linear shrinkage over the course of 56 days was seen for all samples in comparison with the reference. Compressive strengths of SSP specimens tested after 7 and 28 days of curing were comparable to the reference, while PCE led to a decline. The SSPs had a small impact on porosity with a shift to the formation of more gel pores while PCE caused an increase in porosity. Throughout this research, SSPs were identified as promising superplasticizers for MK-geopolymer mortar and concrete.

Structure control and quantitative evaluation of porous materials are essential for many industrial and consumer applications of clay minerals, and nanotubular halloysite (HNT) has been used extensively for such purposes; performance enhancements are still needed, however. The objective of the present study was to improve the gas-adsorption capacity of HNT by controlling the particle size and porosity. This was accomplished through acid treatment and particle-size fractionation by centrifugation. Various particle sizes were obtained and porosities ranged from macropores to mesopores. Natural halloysite nanotubes were modified by sulfuric acid in various concentrations to selectively remove the alumina composition of the tubes. X-ray diffraction and energy dispersive X-ray spectroscopy were used to verify the mineralogical and compositional changes. Surface modification by the acid treatment increased the inner space volume of the tubes and decreased the mass of the nanotubes because of the elimination of alumina. The gas adsorption capacity of both natural and modified halloysite nanotubes was measured quantitatively using N2 adsorption and the Brunauer-Emmett-Teller (BET) method, and the morphology was determined from transmission electron microscopy (TEM) images. The results showed that the modified halloysite nanotube was 7.47 times more efficient at gas adsorption than pristine halloysite. Moreover, the dealumination of the surface increased the inner space. Greatly increased porosity characteristics, including gas adsorption and macroporosity, were obtained through modification by acid treatment.

In the production of alumina (Al2O3) from clays by acid leaching, thermal activation by calcination is applied widely as a pre-treatment to improve the dissolution of aluminum. Previous studies have focused only on the thermal behavior of high-purity kaolinite and pyrophyllite, individually. However, thermal activation of complex clay ores containing several silicate minerals and their effect on aluminum extraction have not been studied. The purpose of the present study was to characterize the thermal behavior of a clay ore containing mainly pyrophyllite (Al2Si4O10(OH)2), kaolinite (Al2Si2O5(OH)4), muscovite (KAl2(AlSi3O10)(OH)2), quartz (SiO2), and kyanite (Al2SiO5) from the Pütürge clay deposits (Malatya, Turkey) for possible use in alumina (Al2O3) production by acid leaching. The ore and its calcination products obtained at various temperatures were characterized with respect to their mineral paragenesis, crystal structure, surface morphology, and thermal, calorimetric, and aluminum dissolution properties in order to understand the changes that occurred. Aluminum recovery in the leach solution increased in direct proportion to the dehydroxylation degree (Dtg) of the ore as the calcination temperature increased to 900°C. A maximum aluminum extraction of 90.57% was achieved by leaching of the product from calcination at 900°C. Aluminum extraction decreased sharply above that temperature, even though Dtg increased a little. By increasing the calcination temperature; the structures of pyrophyllite, kaolinite, and muscovite were destroyed by dehydroxylation, resulting in the exfoliation of the mineral layers, thus, a mixture of dehydroxylated phases formed. Depending mainly on the temperature range at which each of the dehydroxylated phases is durable, aluminum could be leached to some extent. The sharp decrease in the extraction of aluminum, iron, and potassium at higher temperatures was attributed to compaction of previously exfoliated layers of the minerals through re-crystallization to form mullite-like structures which seemed insensitive to acid attack during the leaching.

The efficient separation of hexane isomers from the light naphtha fraction is a significant challenge in the petrochemical industry. 5A zeolite adsorbent is used commercially to sieve alkane isomers. In this study, 5A zeolites were synthesized using a low-cost natural clay mineral precursor, i.e. palygorskite (PAL), with the addition of crystallization directing agent (CDA). By varying the mass ratio of CDA/deionized water, 5A zeolites were obtained as CDA-5%, CDA-7.5%, and CDA-10%. All products were submicron particles with an average particle size of 400–800 nm. A sieving test of CDA-induced 5A zeolites was carried out on hexane adsorbates including n-hexane (nHEX), 2-methylpentane (2MP), and 3-methylpentane (3MP). According to vapor-phase batch adsorption experiments, a significant equilibrium amount (0.149 g/g) of nHEX and only 0.0321 g/g 2MP and 0.0416 g/g 3MP were adsorbed on the 5A zeolite product with CDA-5%. The dynamic adsorption performance of 5A zeolite (CDA-5%) was evaluated by breakthrough curves of binary mixtures of nHEX/2MP and nHEX/3MP. Palygorskite 5A (PAL 5A) zeolite achieved maximum dynamic adsorption capacities of nHEX (0.16 g/g in both cases) at 200°C and 1.2 MPa total pressure. This work provided an economic alternative for the synthesis of 5A zeolites using natural clay minerals instead of chemical raw materials.

Molecular dynamics (MD) simulations provide an accurate description of the mineral–fluid interface from the perspective of the atomistic level taking into account all atom interactions. This simulation approach is computationally expensive if applied to large molecular systems. Classical Fluid Density Functional Theory (f-DFT) delivers structural and thermodynamic information at comparatively small computational costs. Numerous applications of f-DFT for electrolytes neglect an explicit consideration of solvent. In this work, an unrestricted three-component model (3CM) of f-DFT was applied, which incorporates Lennard-Jones (LJ) attractions for the description of the short-range interactions of fluid–fluid and fluid–wall rather than the hard sphere repulsions, named DFT/LJ-3CM. The DFT/LJ-3CM model considers ions as charged LJ particles and treats solvent molecules as neutral LJ particles. To validate the performance of the DFT/LJ-3CM, the f-DFT calculations were compared with atomistic simulations for montmorillonite (Mnt) with various hydrated states in electrolyte solutions. This benchmarking was used to assess critically the advantages and limitations of the f-DFT model. The calibrated DFT/LJ-3CM model for Na and Ca Mnt was applied to calculate cation selectivity for the ion exchange equilibrium with effective ion radius and swelling behavior of Mnt. The predictions of the DFT/LJ-3CM model were found to be in good agreement with the atomistic simulations and experimental data under a wide range of conditions. At the same time, the DFT calculations were 3–4 orders of magnitude faster than conventional MD simulations. Thus, the DFT/LJ-3CM model can be a computationally effective alternative to atomistic simulation in providing structural and thermodynamic properties of fluid–clay mineral interfaces. The DFT/LJ-3CM model provides a robust approach, which can be used for upscaling in reactive transport simulators and modeling ion migration taking place under more complex thermo-chemo-hydro-mechanical conditions.

A lustrous appearance and interference-based colors make pearlescent pigments attractive for use in applications such as automotive paints, plastics, consumer electronics, and cosmetics. A combination of interference and absorption in the visible light spectrum improves significantly the hiding power as well as the color strength of pearlescent pigments while potentially extending their color range. The aim of the present study was to introduce synthetic fluorohectorites, having an appreciable diameter (~20 μm) and aspect ratio (~1000), as promising colored cores for pearlescent pigments. Fluorohectorites can adopt a variety of colors by ion-exchange reaction with cationic organic dyes of high absorption coefficient. Unlike related dye-exchanged natural montmorillonite clays, which undergo acid activation accompanied by release of dye at low pH, as is required for subsequent coating with TiO2 in an environment with low pH and elevated temperature, no leaching was observed with dye-exchanged synthetic fluorohectorites ([Na0.5]int.[Mg2.5Li0.5]oct.[Si4]tet.O10F2). Due to its significantly greater layer charge, more organic dye molecules were adsorbed per volume of the fluorohectorite than for montmorillonite. Consequently, the free volume available in the interlayer space for H3O+ diffusion was less for synthetic fluorohectorite than for montmorillonite. Acid attack via interlayer space was, therefore, retarded significantly for fluorohectorite. Acid attack from the external edges of synthetic fluorohectorites was in the range of conventionally applied mica pigment core (fluorophlogopite, ([K]int.[Mg3]oct.[AlSi3]tet.O10(F,OH)2) because of the comparable large diameter of the platelets. Montmorillonite, however, occurs with particle diameters typically <200 nm and the much increased relative contribution of edges to the total surface area also makes them more prone to acid attack and concomitant leaching. Aside from leaching stability, the confinement of organic dyes in the interlayer space restricts rotational and vibrational motions, which in turn stabilizes the dyes typically by ~100°C against thermal decomposition as compared to chloride salts of the dyes.

The rheological properties of sepiolite gels in relation to solution chemistry, fiber charge, and microstructure are poorly understood. The purpose of this study was to bring more clarity to this topic by quantifying the effects of solution pH, ionic strength, and adsorbed tetrasodium pyrophosphate (TSPP) additive on rheological properties. The electrical charge on sepiolite fibers was investigated to explain the fiber interaction configuration observed in the microstructure. Fiber interaction forces and dynamics explained the ageing behavior of the gel. Sepiolite gels of only a few percent solids displayed long-time ageing behavior, which was manifested by an increasing yield stress with wait time and continued for weeks. The gel microstructure showed randomly orientated rigid fibers with cross configuration attraction. Each fiber experiences both attractive (van der Waals and heterogeneous charge) and repulsive (electric double layer) forces, and initially a net force. The repulsive force causes these fibers to orientate or move continually to achieve a state of force equilibrium and this process takes a long time. The Leong model describes this ageing behavior. For good fiber separation, high intensity probe sonication of the suspension was required. The yield stress increased with sonication time, solids loading, and temperature. The yield stress was absent at pH > 11 and increased to a maximum value at pH < 8. This maximum was insensitive to pH between 4 to 8, and ionic strength up to 1 M KCl. TSPP reduced this maximum and shifted the zero yield stress region to a lower pH, ~7. The zero yield stress state corresponded to a zeta potential with a minimum magnitude of 30 mV.

The removal of pesticide contaminants from water is a key priority in environmental remediation, and requires intensive effort; this necessitates modification of the properties of pillared clays (PILCs) such as porosity, pore-volume, surface area, and synthesis methods. The purpose of the present study was to test the ability of Al-pillared bentonite (Al-PILB), using [Al13O4(OH)24(H2O)12]7+ and [Al30O8(OH)56(H2O)24]18+ (keggin cations, Al13 and Al30) as pillars, to adsorb chlorinated pesticides from contaminated water. In order to maximize intercalation and uniformity of layer stacking, various ratios of the nitrate forms of the synthesized keggin cations were intercalated into the natural bentonite (BT). The synthesized materials (Al-PILBs) were characterized by various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), Fourier-transform infrared (FTIR) spectroscopy, UV-Vis spectroscopy, and N2 adsorption-desorption measurements. Increases in basal spacing, surface area, and pore volume were observed. The adsorption capacity of the Al-PILBs for 17 types of chlorinated pesticides from contaminated water was better than using the BT alone, e.g. for heptachlor epoxide, dieldrin, and endrin at natural pH, the maximum adsorptions obtained at equilibrium solution concentrations of 16, 20, and 20 μg/L, respectively, were 59.2, 59.15, and 60 μg/g, whereas corresponding values using pristine BT were 34.68, 39.45, and 38.9, respectively. The data were best described by the Freundlich adsorption model.

Several procedures for hydrothermal synthesis of saponite can be found in the literature. They differ in terms of the preparation conditions of the precursor and of the synthesis temperature. The objective of the present study was to investigate how these two parameters influence the structure of the final synthesis product. The precursor was prepared from Mg(NO3)2, Al(NO3)3, and Na4SiO4 in three different ways: as a gel, a dried gel, and a calcined gel. The influence of the synthesis temperature on the structure of saponite was investigated in the range 90–200°C. The results showed that the use of a calcined precursor yielded a single mineral phase, saponite, with up to 90% aluminum in tetrahedral configuration. In comparison, the use of a gel precursor resulted in a product with only 60% aluminum in the tetrahedral configuration. The synthesis temperature had no significant effect on the saponite structure. The reported synthesis method showed the possibility of obtaining saponite with superior characteristics, in terms of crystallinity, surface acidity, and thermal stability compared to the natural mineral, even at 90°C, and thus with greater potential for industrial application.

Enzymes adsorbed on clay minerals and soil colloids may exhibit lower activities compared to those of free enzymes. A particular toxic metal may affect the activity of the adsorbed enzyme less critically than that of the free form, however. This information is necessary for predicting catalytic performances of clay-immobilized enzymes in natural soils as well as in food, pharmaceutical, and chemical systems. The objective of the present study was to find out how adsorption on palygorskite and sepiolite minerals modifies the catalytic activity and the Michaelis–Menten kinetics of alkaline phosphatase (ALP). Inhibition kinetics of adsorbed ALP by Cd was also compared to that of the free enzyme. The results revealed that the affinity to the substrate and the maximum reaction velocity of ALP decreased upon adsorption on the fibrous clay minerals. The ALP adsorbed maintained a reasonably high activity recovery (AR) compared to the free enzyme. The AR of the adsorbed ALP ranged from 76.9 to 92.5% for palygorskite and from 71.2 to 90.2% for sepiolite, depending on the substrate concentration applied. The presence of Cd decreased the affinity to the substrate of both the free and the adsorbed ALP, while the maximum reaction velocity remained nearly unchanged, indicating that the inhibitory effects of Cd on both the free and adsorbed ALP activities were competitive in nature. The adsorbed enzyme, however, was inhibited less severely by Cd compared to the free enzyme. The adsorption of ALP on the fibrous clay minerals, therefore, maintains the ALP activity to a great extent and provides more resistance for the enzyme against the inhibitory effects of Cd.

Understanding clay-mineral assemblages forming in saline lakes aids in reconstructing paleoenvironments on Earth and other terrestrial planets; this is because authigenic phyllosilicates are sensitive to the prevailing geochemical conditions present during formation. In most geochemical models, evaporative concentration favors sepiolite with increasing silica and Mg2+ concentrations without considering the role of the biogenic removal of silica from solution by diatoms. In the present study, phyllosilicates occurring in the mudflats of Bolivian salars were investigated to aid in understanding the geochemical factors that control mineral assemblages forming in (SO42–)- and (Cl–)-rich environments in relation to dissolved silica. From transects across the mudflats, the physical, chemical, and mineralogical characteristics of the bulk sediment and the <2 μm fraction of each sedimentary layer were analyzed. From these analyses, three types of sediments were identified: (1) regolith sediments dominated by Al-dioctahedral smectite, illite, and chlorite; (2) detritus-rich mudflat sediments with Mg-trioctahedral smectite and Al-dioctahedral smectite along with illite and chlorite; and (3) authigenic mudflat sediments dominated by poorly formed Mg-trioctahedral smectite, kerolite, and biogenic silica. The absence of sepiolite-palygorskite in the salars is the result of excessively high Mg:Si ratios within the waters. In the surface water Mg becomes enriched relative to Si as diatoms remove dissolved Si from solution through biologically mediated uptake. The geochemical conditions present within the salars that act to preserve the diatom frustules and prevent their dissolution include: neutral–slightly alkaline pH solutions, cold temperatures, shallow water, and high salinity. Under these conditions the formation of sepiolite is restricted by the small amount of dissolved silica, despite the silica-rich environment. The formation of Mg-smectite and kerolite is favored under these conditions.

Hydrothermal alteration led to development of the Yarıkçı clay deposit within the Mesozoic chlorite-, muscovite-, chlorite-muscovite-schist, and garnet-graphite phyllite units along NW–SE- and N–S-trending faults in Mihalıççık in western central Anatolia. The geological, mineralogical, and geochemical characteristics and genesis of this economically important clay deposit have not been examined in detail previously. The present study has attempted to fill this gap. Green smectitic and cream kaolinitic claystones are abundant with smaller amounts of gray illite, dark brown Fe oxides, and silica phases occurring as stockwork/fracture infill and stain/coating. These units are covered by a dark, hard, sharp-edged, and thick silica cap. Metamorphic units exhibit cataclastic texture due to tectonic activities. Muscovite is mostly degraded to kaolinite, and feldspars show sericitization and argillization. Kaolinite typically has a platy form with irregular margins and locally sub-rounded, book-like texture suggesting hydrolysis during the hydrothermal injections. The association of Fe oxides, cristobalite/tridymite/quartz, gypsum/anhydrite, and jarosite are indicative of intense hydrothermal activities and development of kaolinite under acidic geochemical conditions. The local enrichment of SiO2, Fe2O3, S, Cu, and Au also supports this suggestion. The leaching of Sr, Rb, Ba, and Zr, and the slight increase in LREE/MREE+HREE ratios together with the negative Eu and Ce anomalies suggest the selective dissolution of muscovite, garnet, feldspar, and pyroxene by the hydrothermal fluids. Thus, abundant claystones of smectite and kaolinite were formed via the increase in Al+Fe+Mg/Si and Al±Fe/Si ratios in the alkaline and acidic environment, respectively, under the tectonic control of hydrothermal activity as seen in the alteration of chlorite, muscovite, and feldspar in metamorphic units.

Fire resistance performance is one of the most important requirements in geological storage conditions in order to improve the resistance of storage packages to high thermal constraints (in the case of a fire for example). With the need to develop new fire-resistant materials, the aim of the present study was to develop fire-resistant geopolymer binders based on Callovo-Oxfordian (COx) argillite. Two types of kaolin with different degrees of purity were mixed with argillite in various proportions. These mixtures were calcined at 600 or 750°C. In order to assess the fire resistance of activated materials, thermal treatment at 1000°C was performed. The compressive strength and mineralogical composition of the samples were investigated before and after heat treatment. The results showed that the addition of argillite improved significantly the thermomechanical properties of kaolin-based geopolymers containing impurities, especially the mixture containing 67% argillite and calcined at 750°C. This phenomenon was not observed for the pure-kaolin geopolymer. Improvement of fire resistance was due to the formation in situ of leucite and zeolite-type phases (KAlSi2O6 and KAlSiO4) and of wollastonite (CaSiO3) at high temperature, which is linked to the Ca available in the raw materials.

The present study considered the problem of halloysite nanoscroll synthesis by energy modeling of the formation of chrysotile and halloysite particles. The main aim of the study was to reveal an energy preference between scrolled and platy morphologies of the particles. Both hydrosilicates possess the ability to scroll spontaneously but relatively facile hydrothermal synthesis of the nanoscrolls is available only to the former, whereas halloysite forms mainly plates under the same conditions. This issue was investigated by a phenomenological energy model, taking into account: (1) strain energy due to the size difference between metal oxide and silica sheets; (2) surface-energy difference on the opposite sides of the layer; and (3) adhesion energy. Calculations showed that the halloysite layer had a significant scrolling potential due to the first energy component, but the surface-energy difference acted in the opposite direction and tried to unbend the layer. In contrast, these two actions were co-directional in chrysotile layers. In both cases, the formation of multi-layered plates became more energy favorable when the specific surface energy of the edges decreased. In the range 0.5–3 J/m2 for the specific surface energy, only halloysite layers showed an energy preference for platy particles over nanoscrolls, especially at small layer sizes. Certain processes, such as hydration, could reduce the corresponding specific surface energy value and, as a result, could stabilize the platy morphology of halloysite at the earliest stages of particle growth under hydrothermal conditions.

Because long-term leachate migration through a hydraulic barrier is inevitable, compacted clay and cementitious liners are commonly used as ‘active-passive’ liners to attenuate percolated leachate. The scarcity of suitable clay and because of the CO2 emitted during the production of Portland cement as well as drying shrinkage, flow rate due to consolidation, limited attenuation capacity, and chemical instability may mean that these are not the best choices of materials to use for this purpose. An environmentally friendly method to improve the properties of local clay and provision for a long-term physical and chemical containment are essential. Geopolymers can be environmentally friendly substitutes for Portland cement to improve soil properties, not just because of the reduced carbon dioxide emission, but also because of its superior physical and chemical properties, as well as significant early strength, reduced shrinkage, freeze-thaw resistance, long-term durability, and attenuation capacity. According to previous studies, class-F fly ash-based geopolymers activated with NaOH exhibit superior attenuation capacity and long-term durability. The presence of silica, alumina, and iron oxides and the lack of calcium oxide play pivotal roles in the acceptable attenuation capacity and chemical stability of class-F fly ash. Accordingly, a clay-fly ash geopolymer may also work as a sustainable liner with appropriate physical and chemical performance. Clay can also participate in the geopolymerization process as an alumino-silicate precursor. All components of clay-fly ash geopolymers possess acceptable adsorption capacity. The type and percentage of the constituent raw materials control the attenuation capacity and physical properties of final products, however. The porosity and conductivity of typical geopolymers are related to the activator type and concentration, water content, and curing condition. Furthermore, the properties of liner materials can be adjusted with respect to the target contaminants. The present study aimed to present a comprehensive review of the relevant studies to highlight the properties required.

The purification of clay minerals prior to their use as catalysts can escalate processing costs so methods are needed whereby less purification is necessary. One such potential method is acid treatment of the unpurified clay minerals. The main objectives of the present study were to develop the optimal acid-treatment conditions and to determine how the acidic properties of the modified clay samples influenced their catalytic capability toward the dehydration of ethanol and methanol. Clay mineral samples – allophane, palygorskite, and sepiolite, without purification – were acid treated (0.8 M HNO3; 95°C; 2, 8, or 24 h) and after calcination (500°C; 6 h) tested as catalysts for the conversion of methanol to dimethyl ether and of ethanol to diethyl ether and ethene. The changes in chemical and structural compositions as well as surface acidity of the mineral samples were analyzed and correlated with their catalytic performance. Among the samples studied, allophane was the most catalytically active in the dehydration of methanol to dimethyl ether. Acid treatment of this mineral sample decreased methanol conversion slightly. An opposite effect was found for ethanol dehydration to diethyl ether, where acid treatment increased catalytic activity of allophane. The differences in catalytic performance of the mineral samples were discussed with respect to the nature and concentration of acid sites.

The adsorptive behavior of clinoptilolite (Cpt) zeolite in its raw and nickel-exchanged form towards m-xylene/p-xylene/ethylbenzene unary, binary, and ternary mixtures was investigated. The motivation behind the research was to elucidate whether Cpt in its raw or ion-exchanged form could exhibit distinctive selective adsorption behavior. The natural Cpt (Si/Al atomic ratio = 4.59) was ion-exchanged twice with 0.5 M Ni(NO3)2 solution at 80°C for 6 h. Adsorption experiments were done at 40°C, using a 4 vol.% solution of the aromatic materials in iso-octane. The maximum amount of the total specific adsorption capacity for binary solute systems was ~0.8 and ~2.0 mmol g–1 for the raw and ion-exchanged Cpt, respectively. For the ternary solute systems, unexpectedly, this capacity increased to ~2.0 and ~3.0 mmol g–1, respectively. For binary mixtures, both forms of Cpt were selective for p-xylene. For ternary mixtures, both forms exhibited a clear selectivity for m-xylene but the Ni-exchanged Cpt was significantly higher over the concentration range studied. The unexpected increase in the adsorption capacity in ternary systems was attributed to the expulsion of tightly bound trace water molecules hindering the access of xylene molecules to the 10-member ring channels of the zeolite framework. Substitution of Mg2+ by Ni2+ in the 10-member ring channels enhanced the adsorption capacity by providing more space and stronger electrostatic interactions between the new cation and the polar m-xylene molecule.

The harm caused by mine tailings has become increasingly problematic in recent years, so efforts are needed to dispose of or reutilize them in environmentally friendly ways. The objective of the present study was to find out if the hydration properties of muscovite contained in mine tailings are suitable for it to be used as a pozzolan in cement, after undergoing mechanical activation. Aqueous suspensions of mechanically activated muscovite were blended with 10, 20, or 30 wt.% calcium oxide and then allowed to harden. The hardened paste samples were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) techniques. The results revealed that the mechanically activated muscovite exhibited pozzolanic reaction activity in the alkaline environment provided by calcium oxide, and the activated muscovite possessed a capacity to react with calcium hydroxide to form hydration products. The calcium oxide content affected significantly the quantities and kinds of hydration products, non-evaporable water content, and the compressive strength development of the paste samples. The hydration products of the 10% calcium oxide-activated, mechanically activated muscovite pastes were Al-containing hydrated calcium silicate (C-A-S-H) gel, stratlingite, and, upon addition of 20% and 30% calcium oxide, Ca-Al hydrotalcite-like (Ht) phases. The present study was helpful in evaluating the hydration properties of the mechanically activated muscovite and also provided a research basis for evaluating the hydration properties of mine tailings containing muscovite after mechanical activation. The results provided a theoretical basis for muscovite-containing mine tailings to be used as a cement additive, and was conducive to the large-scale utilization of mine tailings.