A review of clay based photocatalysts: Role of phyllosilicate mineral in interfacial assembly, microstructure control and performance regulation

Nanosized tubular clay minerals as inorganic nanoreactors for energy and environmental applications: A review to fill current knowledge gaps

Modern society pays further and further attention to environmental protection and the promotion of sustainable energy solutions. Heterogeneous photocatalysis is widely recognized as one of the most economically viable and ecologically sound technologies to combat environmental pollution and the global energy crisis. One challenge is finding a suitable photocatalytic material for an efficient process. Inorganic nanotubes have garnered attention as potential candidates due to their optoelectronic properties, which differ from their bulk equivalents. Among them, clay nanotubes (halloysite, imogolite, and chrysotile) are attracting renewed interest for photocatalysis applications thanks to their low production costs, their unique physical and chemical properties, and the possibility to functionalize or dope their structure to enhance charge-carriers separation into their structure. In this review, we provide new insights into the potential of these inorganic nanotubes in photocatalysis. We first discuss the structural and morphological features of clay nanotubes. Applications of photocatalysts based on clay nanotubes across a range of photocatalytic reactions, including the decomposition of organic pollutants, elimination of NOx, production of hydrogen, and disinfection of bacteria, are discussed. Finally, we highlight the obstacles and outline potential avenues for advancing the current photocatalytic system based on clay nanotubes. Our aim is that this review can offer researchers new opportunities to advance further research in the field of clay nanotubes-based photocatalysis with other vital applications in the future.

Enhanced generation of oxysulfur radicals by the BiOBr/Montmorillonite activated sulfite system: Performance and mechanism

The easily synthesized, cost-effective, and stable photocatalysts for sulfite activation are always required for the enhancement of organic contaminants degradation. Herein, the facile coprecipitation synthesis of Bismuth oxybromide (BiOBr)/Montmorillonite (MMT) was reported, which could activate sulfite (SO32-/HSO3-) under sunlight and accelerate the catalytic performance more effectively than pristine BiOBr. After adding sulfite to the photocatalysis system, the photodegradation efficiency of atrazine (ATZ) achieved 73.7% ± 1.5% after 5 min and 94.4% ± 1.6% after 30 min of sunlight irradiation with BiOBr/MMT. The BiOBr/MMT-sulfite system also presented remarkable photocatalytic performance to eliminate various contaminants, including ciprofloxacin, sulfadiazine, tetracycline, and carbamazepine. The various features of the photocatalyst materials were studied, including their surface morphology, structure, optical properties, and composition. The results illustrated that by adding MMT, the bandgap of the pristine BiOBr was reduced and the surface area was increased, which led to an increased ability to adsorb materials. Results of various influence factors showed this enhanced system had satisfactory and stable removal performance of ATZ in the pH range of 3.0-6.5, but HPO42- had a strong negative effect on the system performance. Oxysulfur radicals (SO5·- and SO4·-), h+, and 1O2 were discovered as the prevailing active species in the BiOBr/MMT-sulfite system. The proposed degradation mechanism of this photocatalyst-enhanced system revealed that sulfite adsorption on the surface of the photocatalyst played a vital role during the initial phase, and the degradation pathway of ATZ was discussed. This study provides a new synthesis strategy of a photocatalyst for sulfite activation and expands the potential uses of Bi-based photocatalysts in degrading difficult-to-remove organic pollutants.

Sepiolite promotes photodegradation of pyrene under visible light

Mechanochemistry and photocatalysis are emergent technologies for the remediation of polycyclic aromatic hydrocarbons (PAHs) in soils. In this work, mechanochemistry and photocatalysis are combined for pyrene degradation. The photodegradation of pyrene, when in contact with sepiolite under pressure application, is studied. The mechanical treatment leads to a pyrene crystal phase transformation. In this new phase, pyrene undergoes a fast photodegradation in the 320-420 nm range. We show that sepiolite is superior as a photocatalyst in pyrene degradation to TiO2, the most exploited photocatalyst. A broad physicochemical characterization is carried out to propose a mechanism in which the photoexcitation of mechanically altered pyrene leads to an electron transfer to sepiolite matrix, which triggers the PAH degradation. Finally, we want to highlight that the pyrene/sepiolite combination is a simplified system to shed light on how PAH photodegradation may occur in soils.

The Application of Mineral Kaolinite for Environment Decontamination: A Review

Kaolinite clay mineral with a layered silicate structure is an abundant resource in China. Due to its advantages of excellent stability, high specific surface area and environmental friendliness, kaolinite is widely used in environment decontamination. By using kaolinite as a carrier, the photocatalytic technology in pure photocatalysts of poor activities, narrow spectral responses, and limited electron transport can be overcome, and the nano-Ag disinfectant’s limitation of the growth and aggregation of nanoparticles is released. Moreover, pure kaolinite used as an adsorbent shows poor surface hydroxyl activity and low cation exchange, leading to the poor adsorption selectivity and easy desorption of heavy metals. Current modification methods including heat treatment, acid modification, metal modification, inorganic salt modification, and organic modification are carried out to obtain better adsorption performance. This review systematically summarizes the application of kaolinite-based nanomaterials in environmental decontamination, such as photocatalytic pollutant degradation and disinfection, nano silver (Ag) disinfection, and heavy metal adsorption. In addition, applications on gas phase pollutant, such as carbon dioxide (CO2), capture and the removal of volatile organic compounds (VOCs) are also discussed. This study is the first comprehensive summary of the application of kaolinite in the environmental field. The review also illustrates the efficiency and mechanisms of coupling naturally/modified kaolinite with nanomaterials, and the limitation of the current use of kaolinite.

A transparent kaolinite-loaded zinc oxide nanocomposite sunscreen with UV shielding rate over 99% based on bidirectional dispersion

Nano-sized TiO₂and ZnO are the most efficient and widely used inorganic sunscreen, but they still have some drawbacks including agglomeration, delamination, clogging pores and high cost. In this study, a kaolinite-loaded zinc oxide nanocomposite sunscreen was prepared and the key technical problems in application of inorganic nano-sized sunscreens was solved. The synthesized kaolinite-loaded zinc oxide nanocomposite was characterized by XRD, SEM, EDS, XRF and UV-vis spectrophotometry. The SEM image of the nanocomposite suggests that agglomeration of nano-ZnO is avoided by bidirectional dispersion of superfine kaolinite powder and nano-ZnO. Nano-effect and UV shielding rate are enhanced and the nanocomposite sunscreen possesses UV shielding efficiency of 1 + 1 > 2. The UV shielding rate of the nanocomposite sunscreen is greater than 99%, only 10% addition of it endows ordinary skin care products with excellent UV protective efficacy. Moreover, the content of nano-ZnO is reduced by half through introduction of kaolinite, the cost of the sunscreen is lowered, delamination and pore clogging are avoided. This work provides a technical approach for producing stronger, safer and more economical popularized anti-UV skincare products.

Defect Engineering Modulated Iron Single Atoms with Assist of Layered Clay for Enhanced Advanced Oxidation Processes

Single-atom catalysts (SACs) feature maximum atomic utilization efficiency; however, the loading amount, dispersibility, synthesis cost, and regulation of the electronic structure are factors that need to be considered in water treatment. In this study, kaolinite, a natural layered clay mineral, is applied as the support for g-C₃ N₄ and single Fe atoms (FeSA-NGK). The FeSA-NGK composite exhibits an impressive degradation performance toward the target pollutant (>98% degradation rate in 10 min), and catalytic stability across consecutive runs (90% reactivity maintained after three runs in a fluidized-bed catalytic unit) under peroxymonosulfate (PMS)/visible light (Vis) synergetic system. The introduction of kaolinite promotes the loading amount of single Fe atoms (2.57 wt.%), which is a 14.2% increase compared to using a bare catalyst without kaolinite, and improved the concentration of N vacancies, thereby optimizing the regulation of the electronic structure of the single Fe atoms. It is discovered that the single Fe atoms successfully occupied five coordinated N atoms and combined with a neighboring N vacancy. Consequently, this regulated the local electronic structure of single Fe atoms, which drives the electrons of N atoms to accumulate on the Fe centers. This study opens an avenue for the design of clay-based SACs for water purification.

Degradation of phenol by photocatalysis using TiO2/montmorillonite composites under UV light

Composites of titanium (IV) oxide combined with montmorillonite (MMT) with various TiO₂/MMT were prepared for photocatalysis application. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance UV-visible spectroscopy, and X-ray photoelectron spectroscopy (XPS). The main influential factors such as the TiO₂/MMT dose, calcined temperature, and pH value of the solution were studied. The main intermediates of phenol degradation were determined by high performance liquid chromatography (HPLC). The results showed that the average size of TiO₂ nanoparticles was decreased from 22.51 to 10.66 nm through the immobilization on MMT. The components in the interlayer domain were replaced by titanium pillars, and the pillaring reaction proceeded in the interlayer domain, the basic skeleton of MMT was unchanged, and TiO₂ was dispersed on the surface of the MMT. When the initial concentration of phenol is 10 mg/L, the phenol solution pH is 6, and the UV light irradiation time is 240 min; the phenol degradation rate of 30%TiO₂/MMT composite is 89.8%, which is better than MMT (11.5%) and pure TiO₂ (58.8%). It shows that TiO₂ loaded on MMT improves its photocatalytic activity. The phenol reaction process detected by HPLC showed that it had undergone through hydroquinone and benzoquinone, and finally converted into maleic acid and carbon dioxide and small molecules. The possible photocatalysis mechanism is presented.

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.

Recent Breakthrough in Layered Double Hydroxides and Their Applications in Petroleum, Green Energy, and Environmental Remediation

The fast development of the world civilization is continuously based on huge energy consumption. The extra-consumption of fossil fuel (petroleum, coal, and gas) in past decades has caused several political and environmental crises. Accordingly, the world, and especially the scientific community, should discover alternative energy sources to safe-guard our future from severe climate changes. Hydrogen is the ideal energy carrier, where nanomaterials, like layered double hydroxides (LDHs), play a great role in hydrogen production from clean/renewable sources. Here, we review the applications of LDHs in petroleum for the first time, as well as the recent breakthrough in the synthesis of 1D-LDHs and their applications in water splitting to H2. By 1D-LDHs, it is possible to overcome the drawbacks of commercial TiO2, such as its wide bandgap energy (3.2 eV) and working only in the UV-region. Now, we can use TiO2-modified structures for infrared (IR)-induced water splitting to hydrogen. Extending the performance of TiO2 into the IR-region, which includes 53% of sunlight by 1D-LDHs, guarantees high hydrogen evolution rates during the day and night and in cloudy conditions. This is a breakthrough for global hydrogen production and environmental remediation.

Induced morphology orientation of α-FeOOH by kaolinite for enhancing peroxymonosulfate activation

Persulfate activation technology based on sulfate radicals is currently a hot spot in the field of environmental governance. In our work, α-FeOOH was successful in situ loaded on kaolinite surface through a simple one-step hydrothermal process. The prepared composites were systematically characterized, and the relationship between the structural properties and peroxymonosulfate activation properties was explored. Interestingly, compared to bare α-FeOOH, the introduction of kaolinite in composite induced the transformation of α-FeOOH crystal and affected the morphology, where uniformly dispersed nanoparticles rather than rod-like agglomerated crystals appeared. The received FeOOH/kaolinite composite exhibited admirable adsorption and degradation of ciprofloxacin performance with the removal efficiency of 86.1%, and the degradation rate constant was up to 5.2 times higher than that of bare α-FeOOH. In addition, the main active species in the catalytic oxidation system are surface-bound SO₄^•-, ^•OH and free ¹O₂. This work would give a deep insight into the role of natural minerals in composite catalytic materials and the construction of high-efficient mineral-based composite materials.

Carbamazepine Removal by Clay-Based Materials Using Adsorption and Photodegradation

Carbamazepine (CBZ) is one of the most common emerging contaminants released to the aquatic environment through domestic and pharmaceutical wastewater. Due to its high persistence through conventional degradation treatments, CBZ is considered a typical indicator for anthropogenic activities. This study tested the removal of CBZ through two different clay-based purification techniques: adsorption of relatively large concentrations (20–500 μmol L−1) and photocatalysis of lower concentrations (<20 μmol L−1). The sorption mechanism was examined by FTIR measurements, exchangeable cations released, and colloidal charge of the adsorbing clay materials. Photocatalysis was performed in batch experiments under various conditions. Despite the neutral charge of carbamazepine, the highest adsorption was observed on negatively charged montmorillonite-based clays. Desorption tests indicate that adsorbed CBZ is not released by washing. The adsorption/desorption processes were confirmed by ATR-FTIR analysis of the clay-CBZ particles. A combination of synthetic montmorillonite or hectorite with low H2O2 concentrations under UVC irradiation exhibits efficient homo-heterogeneous photodegradation at μM CBZ levels. The two techniques presented in this study suggest solutions for both industrial and municipal wastewater, possibly enabling water reuse.

Halloysite-Zinc Oxide Nanocomposites as Potential Photocatalysts

The synthesis and structural characterization of synthetic zinc oxide and halloysite-based zinc oxide nanocomposites (with 2–28 m/m% ZnO content) are presented. The chemical precipitation of zinc hydroxide precursors and its subsequent drying at 80 °C yielded dominantly zinc oxide (zincite). Thermal treatment at 350 °C completely transformed the remaining precursor to ZnO without causing structural dehydroxylation of the halloysite support. The procedure yielded zinc oxide nanoparticles with 10–22 nm average size having quasi-spherical scale-like morphology. The specific surface area of the synthetic zinc oxide was found to be low (13 m2/g), which was significantly enhanced after nanocomposite preparation (27–47 m2/g). The photocatalytic activity of the prepared nanocomposites was probed by the degradation of a phenolic compound (4-nitrophenol) upon UV irradiation in liquid phase. Compared to their individual constituents, an increased activity of the nanocomposites was observed, while the SSA-normalized photocatalytic activity revealed a synergic effect in nanocomposites above 9 m/m% ZnO content. The nanocomposites were found to be stable at pH = 5.6, with a minor and major mobilization of zinc ions at pH = 12.4 and pH = 1.9, respectively. The toxicity of leachates in different pH environments by Vibrio fischeri bioluminescence indicated low toxicity for ZnO nanoparticles and insignificant toxicity for the nanocomposites. The enhanced photocatalytic activity together with the lower toxicity of the halloysite-ZnO nanocomposites highlight their application potential in water treatment.

A novel geo-photocatalyst, an iron-containing layered clay mineral, for photocatalytic H2 evolution from water

An iron-containing layered clay mineral was discovered as a novel geo-photocatalyst found in nature for hydrogen evolution from water.

Degradation of hexavalent chromium and methyl orange by the synergistic system of graphitic carbon nitride and electron beam irradiation

Hexavalent chromium (Cr(VI)) and methyl orange (MO) are highly toxic and difficult to treat. Electron beam irradiation (EB) can produce ·OH, H·, ·O₂^-, hydrated electron (e_aq^-) and other active substances, which have strong redox ability to pollutants. However, the penetration capacity of EB is limited (the penetration depth of water is 10 cm). Therefore, the photocatalytic method of graphitic carbon nitride (CN) was used as the synergistic method of EB in this project to degrade Cr(VI) and MO. The results showed that the maximum treatment efficiency of 100 mg L^-1 Cr(VI) and 50 mg L^-1 MO with liquid surface height of 5 cm was 95.0% and 99.1%, respectively, which was much higher than that of single photocatalytic method (39.5%, 23.4%) and EB (79.6%, 92.1%), and the efficiency of synergistic treatment was higher under acidic condition. When the liquid depth increased to 30 cm, the efficiency of synergistic system decreased by 14.7% and 15.2% for the degradation of Cr(VI) and MO, respectively, less than the single EB treatment (47.2%, 45.7%). Additionally, the performance of the morphology, the light absorption performance, and the separation of photogenerated electron-hole pairs of the CN were evaluated before and after the synergistic system. Lastly, the mechanism illustrates that the electron and thermal effects of EB, e_aq^-, photogenerated electrons played key roles for the Cr(VI) reduction, and the electron and thermal effects of EB, ·O₂^-, photogenerated holes played key roles for the MO degradation. This study provides a new opportunity for the synergistic system of photocatalyst and EB in the treatment of pollutants.