Transforming Wastewater Treatment: Recent Advancements in Catalytic Wet Air Oxidation with Pillared Clay Catalysts for Phenol Remediation

Catalytic Wet Air Oxidation (CWAO) has recently been recognized as a promising technique for degrading persistent organic pollutants, such as phenol, in wastewater. Among various catalysts, Pillared Interlayer Clays (PILCs) stand out due to their high specific surface area and porous nature. This review delves into the latest progress in CWAO processes utilizing PILCs for the degradation of organic contaminants such as phenol in wastewater. It meticulously assesses the synthesis of PILCs, and their structural properties, including monolithic forms, to understand their effect on catalyst efficiency. Key insights into how these attributes affect the phenol degradation rate and the CWAO process's stability are discussed, providing crucial direction for enhancing catalyst performance. The review highlights the significance of choosing catalysts that offer a balance between cost-efficiency and operational efficacy under mild conditions. The recent results are reported, summarized, and compared, thus proving the feasibility of using PILCs as promising materials for phenol removal. The reaction parameters, phenol conversion, and degradation mechanisms are highlighted. The catalytic efficiency of PILCs was significantly affected by the synthesis methods and reaction parameters, and outperformed most costly catalysts. PILCs are highlighted as especially advantageous catalysts, offering strong performance at lower costs, which boosts the up-scaling opportunities of CWAO methods. This analysis also points out research gaps and proposes directions for future studies, such as exploring innovative PILC synthesis methods to improve their catalytic effectiveness and durability further. It stresses the need to incorporate environmental and sustainability considerations into catalyst design and selection, aligning with the principles of green chemistry in wastewater treatment. Finally, we conclude that proposing new directions for PILCs in CWAO in further naturally-based surface modifications, is also a promising approach for PILCs to perform more efficiently.

Recent advances and perspectives for intercalation layered compounds. Part 2: applications in the field of catalysis, environment and health

Intercalation compounds represent a unique class of materials that can be anisotropic (1D and 2D-based topology) or isotropic (3D) through their guest/host superlattice repetitive organisation. Intercalation refers to the reversible introduction of guest species with variable natures into a crystalline host lattice. Different host lattice structures have been used for the preparation of intercalation compounds, and many examples are produced by exploiting the flexibility and the ability of 2D-based hosts to accommodate different guest species, ranging from ions to complex molecules. This reaction is then carried out to allow systematic control and fine tuning of the final properties of the derived compounds, thus allowing them to be used for various applications. This review mainly focuses on the recent applications of intercalation layered compounds (ILCs) based on layered clays, zirconium phosphates, layered double hydroxides and graphene as heterogeneous catalysts, for environmental and health purposes, aiming at collecting and discussing how intercalation processes can be exploited for the selected applications.

TiO2 Nanoparticles with Adjustable Phase Composition Prepared by an Inverse Microemulsion Method: Physicochemical Characterization and Photocatalytic Properties

Titania nanoparticles (NPs) find wide application in photocatalysis, photovoltaics, gas sensing, lithium batteries, etc. One of the most important synthetic challenges is maintaining control over the polymorph composition of the prepared nanomaterial. In the present work, TiO2 NPs corresponding to anatase, rutile, or an anatase/rutile/brookite mixture were obtained at 80 °C by an inverse microemulsion method in a ternary system of water/cetyltrimethylammonium bromide/1-hexanol in a weight ratio of 17:28:55. The only synthesis variables were the preparation of the aqueous component and the nature of the Ti precursor (Ti(IV) ethoxide, isopropoxide, butoxide, or chloride). The materials were characterized with X-ray diffraction, scanning/transmission electron microscopy, N2 adsorption-desorption isotherms, FTIR and Raman vibrational spectroscopies, and diffuse reflectance spectroscopy. The synthesis products differed significantly not only in phase composition, but also in crystallinity, textural properties, and adsorption properties towards water. All TiO2 NPs were active in the photocatalytic decomposition of rhodamine B, a model dye pollutant of wastewater streams. The mixed-phase anatase/rutile/brookite nanopowders obtained from alkoxy precursors showed the best photocatalytic performance, comparable to or better than the P25 reference. The exceptionally high photoactivity was attributed to the advantageous electronic effects known to accompany multiphase titania composition, namely high specific surface area and strong surface hydration. Among the single-phase materials, anatase samples showed better photoactivity than rutile ones, and this effect was associated, primarily, with the much higher specific surface area of anatase photocatalysts.

Composites of Montmorillonite and Titania Nanoparticles Prepared by Inverse Microemulsion Method: Physico-Chemical Characterization

TiO₂/montmorillonite composites were synthesized using inverse micellar route for the preparation of titania nanoparticles (4-6 nm diameter) in 1-hexanol and for the dispersion of one of the clay components. Two series of composites were obtained: one derived from cetyltrimethylammonium organomontmorillonite (CTA-Mt), exfoliated in 1-hexanol, and the other from sodium form of montmorillonite (Na-Mt) dispersed by formation of an inverse microemulsion in 1-hexanol. The TiO₂ content ranged from 16 to 64 wt.%. The composites were characterized with X-ray diffraction, scanning/transmission electron microscopy/energy dispersive X-ray spectroscopy, thermal analysis, and N₂ adsorption-desorption isotherms. The Na-Mt-derived component was shown to undergo transformation to CTA-Mt, as indicated by basal spacing of 17.5 nm, due to the interaction with the CTABr surfactant in inverse microemulsion. It was also better dispersed and intermixed with TiO₂ nanoparticles. As a result, the TiO₂/Na-Mt series displayed superior textural properties, with specific surface area up to 256 m²g^-1 and pore volume up to 0.247 cm³g^-1 compared with 208 m²g^-1 and 0.231 cm³g^-1, respectively, for the TiO₂/CTA-Mt counterpart. Members of both series were uniformly mesoporous, with the dominant pore size around 5 nm, i.e., comparable with the dimensions of titania nanoparticles. The advantage of the adopted synthesis method is discussed in the context of other preparative procedures used for manufacturing of titania-clay composites.

Mineral-Supported Photocatalysts: A Review of Materials, Mechanisms and Environmental Applications

Although they are of significant importance for environmental applications, the industrialization of photocatalytic techniques still faces many difficulties, and the most urgent concern is cost control. Natural minerals possess abundant chemical inertia and cost-efficiency, which is suitable for hybridizing with various effective photocatalysts. The use of natural minerals in photocatalytic systems can not only significantly decrease the pure photocatalyst dosage but can also produce a favorable synergistic effect between photocatalyst and mineral substrate. This review article discusses the current progress regarding the use of various mineral classes in photocatalytic applications. Owing to their unique structures, large surface area, and negatively charged surface, silicate minerals could enhance the adsorption capacity, reduce particle aggregation, and promote photogenerated electron-hole pair separation for hybrid photocatalysts. Moreover, controlling the morphology and structure properties of these materials could have a great influence on their light-harvesting ability and photocatalytic activity. Composed of silica and alumina or magnesia, some silicate minerals possess unique orderly organized porous or layered structures, which are proper templates to modify the photocatalyst framework. The non-silicate minerals (referred to carbonate and carbon-based minerals, sulfate, and sulfide minerals and other special minerals) can function not only as catalyst supports but also as photocatalysts after special modification due to their unique chemical formula and impurities. The dye-sensitized minerals, as another natural mineral application in photocatalysis, are proved to be superior photocatalysts for hydrogen evolution and wastewater treatment. This work aims to provide a complete research overview of the mineral-supported photocatalysts and summarizes the common synergistic effects between different mineral substrates and photocatalysts as well as to inspire more possibilities for natural mineral application in photocatalysis.

Rehydration Driven Acid Impregnation of Thermally Pretreated Ca-Bentonite—Evolution of the Clay Structure

A new approach to acid activation of raw Ca-bentonite was explored. The method consisted in dehydration of clay by thermal pretreatment at 200 °C, followed by immediate impregnation with H₂SO₄ solution. The acid concentration was 1.5 × or 2.0 × cation exchange capacity (CEC) of clay. The volume of the liquid was adjusted so as to leave the material in the apparently dry state. Structural evolution of the activated solids after 1, 2, 3, and 4 weeks of storage was monitored with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR), and chemical analysis. In the macroscopically dry solids, the rehydrated interlayer Ca²⁺ underwent rapid exchange with H₃O⁺ and formed extra-framework gypsum. Acid attack on montmorillonite structure resulted in continuous removal of layer forming Mg, Al, and Fe cations, with Mg²⁺ being eliminated most efficiently. No significant damage to the montmorillonite lattice was observed. Al was extracted both from the tetrahedral and the octahedral sheets. Under less acidic conditions, the monohydrated H-montmorillonite changed upon storage to bi-hydrated form, as a result of clay auto-transformation. Higher concentrations of acid in the pore network of clay stabilized the H-form of montmorillonite. The data indicate that compositional transformation of acid impregnated bentonite extended beyond the one month of aging investigated in the present work.

Clay-Supported Metal Oxide Nanoparticles in Catalytic Advanced Oxidation Processes: A Review

Advanced oxidation processes (AOPs) utilizing heterogeneous catalysts have attracted great attention in the last decade. The use of solid catalysts, including metal and metal oxide nanoparticle support materials, exhibited better performance compared with the use of homogeneous catalysts, which is mainly related to their stability in hostile environments and recyclability and reusability. Various solid supports have been reported to enhance the performance of metal and metal oxide catalysts for AOPs; undoubtedly, the utilization of clay as a support is the priority under consideration and has received intensive interest. This review provides up-to-date progress on the synthesis, features, and future perspectives of clay-supported metal and metal oxide for AOPs. The methods and characteristics of metal and metal oxide incorporated into the clay structure are strongly influenced by various factors in the synthesis, including the kind of clay mineral. In addition, the benefits of nanomaterials from a green chemistry perspective are key aspects for their further considerations in various applications. Special emphasis is given to the basic schemes for clay modifications and role of clay supports for the enhanced mechanism of AOPs. The scaling-up issue is suggested for being studied to further applications at industrial scale.