The present study investigated the adsorption behaviour of Direct Orange 34, a highly toxic dye used in textile industries in Tunisia, on modified kaolinite-rich clays. A kaolin from the Sidi Bader (SDB) area was activated with hydrochloric acid to create the activated clay referred to hearafter as SDBa, or treated with FeSO4•7H2O to obtain its Fe-saturated form, Fe-SDB. The adsorbents were characterized by X-ray diffraction, X-ray fluorescence, transmission electron microscopy, BET surface area and zeta-potential measurements. The equilibrium adsorption data were analysed using the Langmuir and Freundlich isotherms. The estimated adsorption capacities (qm) for the dye were improved in the Fe-loaded samples. The good fit (R2 = 0.99) with a pseudo-second order expression suggests that the adsorption process could be effective following a chemisorption mechanism. At acidic pH, the optimum dye-retention rate was achieved for SDB (83%) after 60 min. The uptake decreased at neutral pH and increased again in alkaline media. This behaviour might be explained by the formation of covalent bonds between the OH radicals on the external surface and the negatively charged dye molecules. On the other hand, Fe impregnation increased the zeta potential of kaolinite, leading to a greater adsorption capacity compared to its natural and acid-activated counterparts. In addition, the adsorption rate increased when increasing the suspension temperature from 283 to 313 K. The modified kaolinite-rich materials showed satisfactory affinity for adsorbing this reactive dye.

Clay-mineral type and content, bulk mineralogical composition and alteration of bentonite are very important factors for the ultra-long-term stabilization of barriers and backfills in radioactive waste disposal. This study investigates the effects of clay-mineral type and content on the swelling characteristics and permeability of bentonite–sand mixtures with clay minerals using one-dimensional swelling-pressure and constant-pressure permeability tests. The hydraulic conductivity of bentonite–sand–clay mineral mixtures increased with increasing content of non-swelling alteration products of montmorillonite. Furthermore, hydraulic conductivity was comparable to that determined with the Kozeny–Carman equation for a specific surface area, suggesting that hydraulic conductivity may be estimated based on the abundance of expected alteration products of montmorillonite. This study provides a basis for evaluation of the hydraulic conductivity of bentonite–sand mixtures with known quantities of expected alteration products of montmorillonite.

Textural, mineralogical and chemical characterization of archaeological ceramics (zellige) from El Badi Palace (Marrakech, Morocco), the main Islamic monument from the Saadian period (sixteenth century), has been performed to enhance restoration and to determine the technology of manufacturing. A multi-analytical approach based on optical and scanning electron microscopy, cathodoluminescence, X-ray fluorescence and X-ray diffraction was used. Re-firing tests on ceramic supports were also performed to determine the firing temperatures used by the Saadian artisans. A calcareous clay raw material was used to manufacture these decorative ceramics. The sherds were fired at a maximum temperature of 800°C in oxidizing atmosphere. The low firing temperature for ‘zellige’ facilitates cutting of the pieces, but also causes fragility in these materials due to the absence of vitreous phases.

In deep geological repositories, compacted bentonites have been proposed for use as barrier materials for isolating nuclear waste. The prevailing thermo-hydro-mechanical boundary conditions in the repositories may affect the swelling capacity and permeability of the compacted bentonites. In this study, the effect of preliminary desiccation on the subsequent hydro-mechanical behaviour (swelling pressure and hydraulic conductivity) of compacted Calcigel bentonite was investigated experimentally at 22°C and 80°C. In the first stage of the test, the compacted specimens were subjected to suction-controlled desiccation at 22°C and 80°C using the vapour-equilibrium technique. After the water content reached equilibrium at a given suction, the axial, radial and volumetric shrinkage strains were measured. Afterwards, constant-volume swelling-pressure tests were performed on the desiccated specimens (second test stage) by saturating the dried specimens with deionized water at 22°C and 80°C. At the end of the swelling-pressure test, the hydraulic conductivities of four saturated specimens were measured at each temperature. The volumetric shrinkage strain of the compacted bentonite during desiccation is controlled by suction instead of temperature. In addition, the preliminary desiccation increases both the swelling pressure and hydraulic conductivity of compacted bentonite, particularly if compacted bentonite undergoes extreme desiccation at an applied suction of >700 MPa.

The mineralogy, texture and composition of rocks associated with the kaolin deposits in the Ahırözü-Hamidiye-Üçbaşlı area, SE of Mihalıççık, Eskişehir, Turkey, were investigated. In the study area, Triassic, blue-green schists and serpentinized ultramafic rocks are exposed. Kaolinization occurs at the contact between a Triassic granitic intrusion and metamorphic rocks. Textural and mineralogical characteristics were identified by X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) analyses. The effects of hydrothermal alteration on the whole-rock chemical composition were determined by inductively coupled plasma mass spectrometry (ICP-MS) analysis. Homogenization temperature (Th, °C), salinity, (wt.% NaCl) and melting temperature (Tm, °C) were detected by fluid-inclusion analyses.

The argillic alteration zone (Zone A) includes kaolinite, smectite, natroalunite and accessory pyrite, hematite and goethite. This paragenesis corresponds to a low–intermediate argillic alteration caused by fluids having neutral to acidic pH, at a temperature of <200°C. The propylitic alteration zone (Zone B) is characterized by epidote, chlorite, illite and pyrite and was caused by fluids with a neutral pH and a temperature of >250°C. The rocks in Zone C are mainly silicified and represent sinter formation in the study area. The rocks in the kaolin deposit are depleted in Rb, Pb and Y and enriched in Cs, U and Sr, and represent the products of epithermal hydrothermal alteration of granitic intrusions emplaced in a volcanic arc.

Compositional and microstructural analysis of mullites in porcelain whitewares obtained by the firing of two blends of identical triaxial composition using a kaolin B consisting of ‘higher-crystallinity’ kaolinite or a finer halloysitic kaolin M of lower crystal order was performed. No significant changes in the average Al2O3 contents (near the stoichiometric composition 3:2) of the mullites were observed. Fast and slow firing at the same temperature using B or M kaolin yielded different mullite contents. The Warren–Averbach method showed increase of the D110 mullite crystallite size and crystallite size distributions with small shifts to greater values with increasing firing temperature for the same type of firing (slow or fast) using the same kaolin, as well as significant differences between fast and slow firing of the same blend at different temperatures for each kaolin. The higher maximum frequency distribution of crystallite size observed at the same firing temperature using blends with M kaolin suggests a clearer crystallite growth of mullite in this blend. The agreement between thickening perpendicular to prism faces and mean crystallite sizes <D110> of mullite were not always observed because the direction perpendicular to 110 planes is not preferred for growth.

The adsorption and desorption of cesium onto layered minerals, zeolite and geochemical reference samples were studied. 0.5 g of bentonite and mica were able to adsorb 71.2 and 51.5 mg of cesium, respectively, from 50 mL of deionized water containing 200 mg/L of cesium under neutral pH condition. These amounts of cesium adsorption were greater than those reported for vermiculites (8.9 and 5.6 mg, respectively). Additionally, the cesium adsorption on mica and vermiculite remained essentially unchanged under seawater conditions, but it decreased drastically on zeolite. The cesium desorption from the layered minerals was promoted by the addition of ammonium ions, namely trioctylmethylammonium chloride and zephiramine. These ammonium ions desorb cesium from the interlayers of the minerals without destroying the mineral structure. The cesium desorption procedure using quaternary ammonium ions would be extremely useful for decontamination of soil containing the layered minerals with adsorbed radioactive cesium.

The FEBEX experiment, a full-scale, high-level radioactive waste repository test, ran for ~18 years and hence is the longest-running disposal test to date. The test consisted of a heater emplaced in an envelope of compacted bentonite at the Grimsel test site, Switzerland. The water content of the bentonite was somewhat lower near the heater and increased towards the granite. This phenomenon probably led to gradients in the exchangeable cation population at various locations within the bentonite buffer. The cement (shotcrete) bentonite interface of one block was characterized by a thin (0.1–1.0 mm) reaction zone in which bentonite constituents, carbonates and sulfates occurred. Cation exchange and a slight decrease of the cation exchange capacity were observed near the heater. Oxic corrosion was observed predominantly at the bentonite/steel liner interfaces, pointing towards a role of air entrapped between the liner and heater during emplacement. At the liner surface, intimate intergrowth of bentonite constituents, metal and corrosion products was observed. At the face of the heater, the bentonite blocks were in direct contact with the heater surface without any signs of metal corrosion. Instead, a significant increase in the Mg content was recorded, which is in agreement with previous large-scale disposal tests. The FEBEX experiment proved that the Mg increase and corrosion were independent processes. The increase in Mg may be explained by the formation of trioctahedral domains or the precipitation of neoformed silicates. For the first time, however, brucite was identified as an additional phase that formed at the metal/bentonite interface, pointing towards a special role for Mg mobility in the bentonite barrier.

This work presents the synthesis of minerals with a layered structure in supercritical water/ethanol mixtures to decrease the critical coordinates of the solvent regarding water. Depending on the water/ethanol ratio of the solvent, we obtained three different minerals adopting an octahedral brucitic sheet: (1) without a Si-tetrahedral sheet (O); (2) associated with one Si-tetrahedral sheet (T-O); or (3) intercalated between two Si-tetrahedral sheets (T-O-T). We have thus shown that ethanol in a supercritical water/ethanol mixture changes the solubility of silicon with a direct consequence on the formation of the tetrahedral silicon sheets and thus makes it possible to control the structure of the synthesized layered material.

The present study investigates the use of magnesium sulfate (MgSO4) instead of (NH4)2SO4 as a lixiviant in the recovery of rare earth elements (REEs) from clays. Experiments were carried out to investigate the influence of leaching conditions such as leaching time, lixiviant concentration and liquid:solid ratio on the leaching efficiency. The optimum leaching conditions, leading to 75.48% of total REE leaching efficiency, required a stirring speed of 500 rpm, a leaching time of 30 min, a lixiviant concentration of 3 wt.% and a liquid:solid ratio of 3:1. After extension of the leaching process by a second stage, the leaching efficiency may reach up to 96.19%, which is slightly higher than that obtained by (NH4)2SO4. Leaching varies from element to element, with Ce presenting the lowest leaching efficiency, and the partition in leaching solution is in agreement with that in raw ore other than for Ce. Based on these findings, MgSO4 lixiviant is an excellent alternative leaching agent for a sustainable REE industry because it reduces or eliminates NH4+–N pollution.

The Gümüşköy kaolin clay deposit, located ~40 km northwest of Kütahya, Turkey, has promising reserves for ceramic production. In order to evaluate the potential use of this region, five representative clay samples were collected and characterized with X-ray diffraction, polarized light optical microscopy, X-ray fluorescence spectrometry, differential thermal analysis and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. According to the Chemical Index of Alteration and Plagioclase Index of Alteration, the kaolin samples were formed by strong chemical alteration of the Tavşanlı volcanites. The clays contain kaolinite, illite, pyrophyllite, montmorillonite and minor halloysite, as well as quartz, plagioclase (andesine), biotite, sanidine, rutile and goethite. The clay samples were used for thermal and physical tests. The plasticity, particle-size distributions firing properties (water absorption, apparent porosity, linear shrinkage, dry bending strength, fired bending strength) and colour parameters of the fired samples were determined. The kaolinitic clay samples from Gümüşköy are suitable for making wall tiles, floor tiles and kiln wares after shaping by dry pressing and extrusion.

The adsorption capacities of nano-sized organoclays composed of a stevensite-rich clay (R), phosphorus dendrimers (GC1 and GC2) and Na+-saturated clay were evaluated for their capacity to adsorb chromate and methylene blue (MB) in the range of 298–318 K. The adsorption kinetics and the isotherms were analysed based on kinetic equations and isotherm models and by adopting a non-linear regression procedure. In addition, the organoclays and the Na+-saturated clays were characterized principally by solid-state nuclear magnetic resonance spectroscopy. The pseudo-second-order rate equation described kinetics data well, and the adsorption rates were not limited by the intraparticle diffusion or by the liquid film diffusion. Both chemical species were adsorbed spontaneously (–31 < ΔG°T< –10 kJ/mol), but the adsorbents had a high affinity for MB species. The adsorption isotherms of chromate were fitted better by the Freundlich model, while those of MB followed the Langmuir model. Chromate adsorption took place at the edges and the free surfaces of stevensite, particularly at the protonated aluminols. MB was adsorbed as MBH2+ and MB+. The MB protonation occurred at the clay surfaces, and MB+ ions were located at the planar surfaces of stevensite as well as at the external surfaces of aggregates. Moreover, the tetrahedral sheet of stevensite involved in the formation of GC1-based organoclays was the subject of a partial chemical modification.