Brick-red deposits with palygorskite (Pal) as the main ingredient are widely distributed in nature, but these have not been deployed at a large scale in industry because of their inherent deep colors. In the present study, the brick-red Pal deposit was treated hydrothermally in various reaction media including water, a urea solution, and a thiourea solution. The effects of these processes on the structure, physicochemical features, and color of Pal were studied intensively to understand the structure and composition of the brick-red Pal deposit and to lay a theoretical foundation for the extension of its industrial application. The changes in structural features after hydrothermal treatment were studied by Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, magic-angle spinning nuclear magnetic resonance, and Mössbauer spectroscopy techniques. The results indicated that the color of brick-red Pal did not change after hydrothermal treatment in water or in a urea solution, and the color changed to gray-white after treatment in the thiourea solution. The rod-like crystal morphology of Pal was retained throughout the experiments and no significant change in the main associated minerals, including feldspar, muscovite, and quartz, was observed after hydrothermal treatment. The dissolution of associated hematite (α-Fe2O3 and the reduction of Fe(III) species are the main reason for the change of Pal from brick-red to gray-white.

Many types of oxidative pollutants are dangerous chemicals and may pose a health risk, but an inexpensive and effective method for mitigating those risks would offer significant advantages. The objective of this study was, therefore, to investigate the potential for Fe-pillared montmorillonite to fill that gap. Surface mediated reduction reactions by ferrous species often play an important role in governing the transport, transformation, and fate of hazardous oxidative contaminants. Compared to the untreated montmorillonite (Mnt), the synthetic polyhydroxyl-Fe pillared montmorillonite (Fe-Mnt) was found to be somewhat similar to goethite in promoting the ability of specifically adsorbed Fe(II) to reductively transform 2-nitrophenol (2-NP). The 2-NP was efficiently removed within 30 min from solutions at the optimum neutral pH in a mixed reduction system of Fe(II)/Fe-Mnt under an anoxic atmosphere. This demonstrated that the specifically adsorbed Fe(II) of Fe-Mnt can enhance 2-NP reduction. The highly enhanced 2-NP reduction by Fe(II) through Fe-Mnt surface catalysis can, therefore, be ascribed to clearly increased amounts of an adsorbed Fe(II) species surface complex, which gave rise to enhanced Fe(II) reductive activity that enabled the rapid reduction of 2-NP. The reduction processes produced a faster transformation of 2-NP in a Fe-Mnt suspension than in a Mnt suspension. The transformation kinetics were described using pseudo-first-order rate equations. Moreover, in addition to the effects of mineral surface properties, the interactions were affected by the aqueous chemistry, and the removal rates of 2-NP were increased at pHs of 6.0–7.3. In the present study, the structure and surface reactivity of Fe-Mnt was characterized in depth. The polyhydroxyl-Fe added to Mnt and the pH were determined to be the two key controlling factors to mediate the reductive transformation of 2-NP in the presence of Fe-Mnt in comparison to goethite and Mnt. Finally, the catalysis mechanism responsible for the enhanced 2-NP reduction by Fe(II) was elucidated using cyclic voltammetry.

Sepiolite is a hydrated magnesium silicate with a microporous and mesoporous structure. The fibrous morphology and the alternating blocks and tunnels along the fiber direction of sepiolite make it an ideal material to sequester a variety of organic and inorganic contaminants. The adsorption of various surfactants by organo sepiolites have been experimentally investigated. How this hydrophobic material adsorbs dye molecules at the atomic level, however, is a great mystery. For this reason, the present study focused on the adsorption of acid azo 57 dye molecules to modified sepiolite. For this purpose, the amenability of sepiolite to remove the anionic textile dye (acid azo red dye 57) was first studied in detail. Additionally, a typical cationic surfactant, hexadecyltrimethylammonium Br (HTAB), was used to modify sepiolite to increase the adsorption capacity. Zeta potential measurements on the sepiolite and the HTAB modified sepiolite were also carried out. Moreover, Density Functional Theory (DFT) studies were performed to understand the mechanism of the adsorption of dye molecules to natural and modified sepiolite surfaces. On the basis of the experimental studies, three general systems were theoretically examined: (i) HTAB molecules on sepiolite basal surfaces to represent four Si tetrahedra, (ii) neutral or charged acid azo red dye 57 molecules on sepiolite basal surfaces to represent four Si tetrahedra, and (iii) HTAB on the surface of neutral or charged acid azo red dye 57 molecules as a substrate. The results clearly indicated good agreement between the experimental studies and the theoretical computational DFT studies. For example, the double layer structure found in experimental studies was also demonstrated in DFT studies and confirmed increased adsorption in the presence of acid azo dye 57.

Injection of CaCl2 and Na2SiO3 solutions into clay suspensions during electroosmosis often improves the cohesive strength of clays near the anode and cathode, whereas the cohesive strength of clays between the electrodes remains weak. Although the main improvement mechanism for the cohesive strength of clays near the cathode was demonstrated to be a pozzolanic reaction (formation of calcium silicate hydrate cement), the mechanism of improved cohesive strength near the anode is still not understood. The objective of the present study was to investigate the mechanism for the improvement of cohesive strength near the anode and, thus, make it possible to determine a way to enhance the range in improvement using kaolinite as the test clay. The test was performed by first injecting CaCl2 solution during electroosmosis until the optimum volume of CaCl2 was attained. This was followed by treatment with Na2SiO3 solution for different lengths of time. The results indicate that the anode region after treatment was acidic (pH = 4) because the electrolysis of water causes acidification near the anode. As Na2SiO3 solution was injected through the anode, the mechanism of cohesive strength improvement of the treated clay near the anode was attributed to the silicic acid polymerization effect provided by the Na2SiO3 solution. The silicic acid may link the clay particles together to form a gel network in a low pH environment. The clay gel network structure developed rigidity as the water content was reduced. In addition, as the volume of injected Na2SiO3 solution was increased, the cohesive strength near the anode also increased.

The Boom Clay in northern Belgium has been studied intensively over recent decades as a potential host rock in the context of disposal of radioactive waste. One of the parameters of interest is the cation exchange capacity (CEC) as it is related to the sorption potential of radionuclides to the clay host rock. In the past, the CEC was determined using various methods on a limited number of samples, leading to significant variations. To constrain the CEC of the Boom Clay better, a sample set covering the entire stratigraphy was measured using the quick copper(II) triethylenetetramine method. Part of the sample set was also measured using the cobalt(III) hexamine method, as a quality control for the results of the former method. In addition, the exchangeable cation population of the Boom Clay was quantified systematically for the first time and these results were compared to the in situ pore-water chemistry, indicating a strong coupling between the pore-water composition and the exchangeable sites of clay minerals.

The elimination of Pb2+ and recovery of lead metal during the treatment of industrial sewage is an important research topic. Montmorillonite (Mnt) is a promising material in this regard. The purpose of the present study was to improve the Pb2+ adsorption ability of Na-containing Mnt (Na-Mnt) by pillaring titania (anatase) into its interlayer spaces using a sol-gel method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The ratio of Ti to Mnt affected the crystal phase of titania-pillared Na-Mnt (Ti-Mnt), and changed the interlayer spacing of the (001) plane of Ti-Mnt and the growth of anatase. The Pb2+-adsorption capabilities of Ti-Mnt were tested using an aqueous solution of lead nitrate as a wastewater model. The Ti-Mnt prepared adsorbed >99.99% of the Pb2+; leached and activated Ti-Mnt adsorbed >95.7% of the Pb2+, indicating that Ti-Mnt could be recycled effectively. Furthermore, the Pb2+-adsorption capability of Ti- Mnt was related to the interlayer spacing of Mnt, the distribution of anatase particles pillared in Mnt, and the specific surface area, especially with respect to the relationship between the anatase particles and the interlayer spacing of the (001) plane.