Visible-Light Photocatalytic Ozonation Using Graphitic C3N4 Catalysts: A Hydroxyl Radical Manufacturer for Wastewater Treatment

S-scheme electron transfer promoted by novel indium oxide quantum dot-loaded carbon nitride heterojunctions promoted using oxidized indium monomers

The graphitic carbon nitride (g-C3N4) photocatalysis has emerged as a clean method for cleaving lignin-linked bonds due to its mild and sunlight-driven reaction conditions. The fast electron-hole pair complex of g-C3N4 constrains its degradation efficiency, making the heterojunction construction a popular solution. The conventional methods of preparing g-C3N4 heterojunctions by physical mixing destroy π-conjugations in g-C3N4, reducing the adsorption of lignin containing benzene rings. In this study, a novel indium oxide (In2O3) quantum dot-g-C3N4 0D/2D heterojunction was prepared through the high-temperature oxidation of pre-prepared indium-doped g-C3N4. The introduction of In2O3 at the quantum dot level minimizes the interference with lignin adsorption capacity. The strong combination of the two (In2O3 and g-C3N4) increases the intersection interface area, promoting the S-scheme transfer route of the photogenerated electrons. Consequently, this enhances the photoelectric conversion efficiency and carrier lifetime of the heterojunction, and inhibits the rapid recombination of photogenerated electron-hole pairs in g-C3N4. The proposed heterojunction was 3 times more efficient than g-C3N4 alone for selective cleavage of lignin β-O-4 bonds after 2 h of sunlight irradiation. Combined with inhibitor experiments and gas chromatography-mass spectrometry analysis, this paper defines the reactive oxides and proposes a cleavage pathway for the lignin β-O-4 bonds in In2O3-g-C3N4 heterojunction system.

Amplifying chlorinated phenol decomposition via Dual-Pathway O2 Activation: The impact of zirconium loading on BiOCl

The effectiveness of photocatalytic molecular oxygen (O2) activation in pollutant removal relies on the targeted production of reactive oxygen species (ROS). Herein, we demonstrate the dual-pathway activation of O2 on BiOCl through zirconium (Zr) loading. The incorporation of Zr onto the surface of BiOCl not only leads to an increased generation of oxygen vacancies (OV) but also fosters a coupling between the d electrons of Zr and OV, forming dual-active sites known as Zr-oxygen vacancies (Zr-OV). Generally, OV adsorbs O2 and transfers one electron directly to form superoxide radicals (•O2-). Contrary to the conventional single-electron direct activation of O2 to form •O2-, Zr-OV exhibits more flexible coordination and superior electron-donating capabilities. It facilitates O2 conversion to peroxide radicals (O22-) and enables the subsequent generation of •O2- from O22-, significantly promotes the dechlorination and mineralization efficiency of chlorophenol under visible light. This study presents a straightforward strategy to precisely regulate ROS production by expanding pathways, shedding light on the critical role of managing ROS generation for effective pollutant purification.

Atomically dispersed low-valent Au boosts photocatalytic hydroxyl radical production

Providing affordable, safe drinking water and universal sanitation poses a grand societal challenge. Here we developed atomically dispersed Au on potassium-incorporated polymeric carbon nitride material that could simultaneously boost photocatalytic generation of ·OH and H2O2 with an apparent quantum efficiency over 85% at 420 nm. Potassium introduction into the poly(heptazine imide) matrix formed strong K-N bonds and rendered Au with an oxidation number close to 0. Extensive experimental characterization and computational simulations revealed that the low-valent Au altered the materials' band structure to trap highly localized holes produced under photoexcitation. These highly localized holes could boost the 1e- water oxidation reaction to form highly oxidative ·OH and simultaneously dissociate the hydrogen atom in H2O, which greatly promoted the reduction of oxygen to H2O2. The photogenerated ·OH led to an efficiency enhancement for visible-light-response superhydrophilicity. Furthermore, photo-illumination in an onsite fixed-bed reactor could disinfect water at a rate of 66 L H2O m-2 per day.

Synthesis and Characterization of Composite WO3 Fibers/g-C3N4 Photocatalysts for the Removal of the Insecticide Clothianidin in Aquatic Media

Photocatalysis is a prominent alternative wastewater treatment technique that has the potential to completely degrade pesticides as well as other persistent organic pollutants, leading to detoxification of wastewater and thus paving the way for its efficient reuse. In addition to the more conventional photocatalysts (e.g., TiO2, ZnO, etc.) that utilize only UV light for activation, the interest of the scientific community has recently focused on the development and application of visible light-activated photocatalysts like g-C3N4. However, some disadvantages of g-C3N4, such as the high recombination rate of photogenerated charges, limit its utility. In this light, the present study focuses on the synthesis of WO3 fibers/g-C3N4 Z-scheme heterojunctions to improve the efficiency of g-C3N4 towards the photocatalytic removal of the widely used insecticide clothianidin. The effect of two different g-C3N4 precursors (urea and thiourea) and of WO3 fiber content on the properties of the synthesized composite materials was also investigated. All aforementioned materials were characterized by a number of techniques (XRD, SEM-EDS, ATR-FTIR, Raman spectroscopy, DRS, etc.). According to the results, mixing 6.5% W/W WO3 fibers with either urea or thiourea derived g-C3N4 significantly increased the photocatalytic activity of the resulting composites compared to the precursor materials. In order to further elucidate the effect of the most efficient composite photocatalyst in the degradation of clothianidin, the generated transformation products were tentatively identified through UHPLC tandem high-resolution mass spectroscopy. Finally, the detoxification effect of the most efficient process was also assessed by combining the results of an in-vitro methodology and the predictions of two in-silico tools.

Oxygen vacancy engineering of ultra-small CuWO4 nanoparticles for boosting photocatalytic organic pollutant degradation

Nanomaterials have attracted great interest in the field of photocatalytic degradation due to their larger specific surface area and efficient charge/mass transfer ability, which are beneficial for enhancing photocatalytic activity. However, the bandgap of photocatalysts would increase with the size reduction, weakening the photoabsorption ability. Thus the relationship between the size of catalysts and photoactivity should be balanced to achieve optimal photocatalytic performance. Herein, ultra-small CuWO4 nanoparticles (ca. 39 nm) with moderate oxygen vacancies (CuWO4-OVs) were synthesized by the cascade strategy (ligand confinement@fast calcination). The introduction of oxygen vacancies offset the deficiency of light absorption ability caused by the small size effect. Besides, oxygen vacancies could provide more reaction active sites, conducive to the adsorption and activation of dye molecules and H2O. Degradation experiments reveal that the optimized photocatalyst CuWO4-OVs 350 shows outstanding photocatalytic activity, and the removal ratio of methylene blue (MB) reaches over 90.26% in 70 min, exceeding that of pure CuWO4-air (37.66%). Additionally, the degradation performance of CuWO4-OVs 350 surpasses most of the other CuWO4-based photocatalytic systems. More importantly, the photocatalytic degradation activity of CuWO4-OVs 350 could remain at 88.26% even after five cycles, and high photostability was achieved. This work affords constructive inspiration for synergistic photoactivity enhancement and increase of catalyst reaction active sites to achieve eminent photocatalytic degradation performance.

Boosted elimination of florfenicol by BiOClxBr1-x solid solutions via photocatalytic ozonation under visible light

In this work, a series of BiOClxBr1-x (BCB) solid solutions are facilely designed for visible-light-driven photocatalytic ozonation (PCO) degradation of florfenicol (FF) in water environments, which could add to the library of efficient, cost-effective and robust nanocatalysts for water purification. BCB solid solutions in the structure of 2D nanosheets are achieved involving the etching of BiOBr "micro-flowers" with HCl at different concentrations, allowing a removal ratio of FF up to 97.3 % within 1 h, superior to bare BiOBr and bare BiOCl. A strengthened synergistic effect between photocatalysis and ozonation is substantiated, where the separation of photo-induced charge transfer is accelerated, the band gap is tuned and the utilization efficiency of ozone is enhanced. This facilitates the production of reactive oxygen species identified as •OH, •O2-, and 1O2 that will attack the FF molecule for degradation based on three pathways.

Progress on mechanism and efficacy of heterogeneous photocatalysis coupled oxidant activation as an advanced oxidation process for water decontamination

The rising debate on the dilemma of photocatalytic water treatment technologies has driven researchers to revisit its prospects in water decontamination. Nowadays, heterogeneous photocatalysis coupled oxidant activation techniques are intensively studied due to their dual advantages of high mineralization and high oxidation efficiency in pollutant degradation. This paved a new way for the development of solar-driven oxidation technologies. Previous reviews focused on the advances in one specific coupling technique, such as photocatalytic persulfate activation and photocatalytic ozonation, but lack a consolidated understanding of the synergy between photocatalytic oxidation and oxidant activation. The synergy involves the migration of photogenerated carriers, radical reaction, and the increase in oxidation rate and mineralization. This review systematically summarizes the fundamentals of activation mechanism, advanced characterization techniques and synergistic effects of coupling techniques for water decontamination. Besides, specific cases that lead researchers astray in revealing mechanisms and assessing synergy are critically discussed. Finally, the prospects and challenges are put forward to further deepen the research on heterogeneous photocatalytic activation of oxidants. This work provides a consolidated view of the existing heterogeneous photocatalysis coupled oxidant activation techniques and inspires researchers to develop more promising solar-driven technologies for water decontamination.

A rationally designed 3DTiO2@CdZnS heterojunction photocatalyst for effectively enhanced visible-light-driven hydrogen evolution

Hydrogen production with higher efficiency and lower cost is of great significance for the sustainable development of energy. Zinc cadmium sulfide (CZS) is gaining more attention owing to its excellent photocatalytic properties. However, its development is greatly limited due to photogenerated charge recombination. In this work, an innovative design with a unique 3D morphology was introduced by integrating 3DTiO2 into CZS to form a novel 3DTiO2/CZS heterojunction photocatalyst. As a result, the optimized composite achieved a very high hydrogen production rate of 75.38 mmol h-1 g-1 under visible light, which is 2.4 times higher than that of the original CZS. It can also be greatly demonstrated through photoelectrochemical tests that this unique 3D morphology contributes to the effective separation of electrons and holes, thus dramatically improving the photocatalytic activity of 3DTiO2/CZS composites. The 3DTiO2/CZS composite has a rational energy band structure, which makes it more favorable for the hydrogen precipitation reaction. It is believed that such a modification strategy based on 3DTiO2 can be applied to other similar photocatalysts as well for boosting hydrogen evolution.

Embedding electronic perpetual motion into single-atom catalysts for persistent Fenton-like reactions

In our quest to leverage the capabilities of the emerging single-atom catalysts (SACs) for wastewater purification, we confronted fundamental challenges related to electron scarcity and instability. Through meticulous theoretical calculations, we identified optimal placements for nitrogen vacancies (Nv) and iron (Fe) single-atom sites, uncovering a dual-site approach that significantly amplified visible-light absorption and charge transfer dynamics. Informed by these computational insights, we cleverly integrated Nv into the catalyst design to boost electron density around iron atoms, yielding a potent and flexible photoactivator for benign peracetic acid. This exceptional catalyst exhibited remarkable stability and effectively degraded various organic contaminants over 20 cycles with self-cleaning properties. Specifically, the Nv sites captured electrons, enabling their swift transfer to adjacent Fe sites under visible light irradiation. This mechanism accelerated the reduction of the formed "peracetic acid-catalyst" intermediate. Theoretical calculations were used to elucidate the synergistic interplay of dual mechanisms, illuminating increased adsorption and activation of reactive molecules. Furthermore, electron reduction pathways on the conduction band were elaborately explored, unveiling the production of reactive species that enhanced photocatalytic processes. A six-flux model and associated parameters were also applied to precisely optimize the photocatalytic process, providing invaluable insights for future photocatalyst design. Overall, this study offers a molecule-level insight into the rational design of robust SACs in a photo-Fenton-like system, with promising implications for wastewater treatment and other high-value applications.

Nanomaterials-Enabled Physicochemical Antibacterial Therapeutics: Toward the Antibiotic-Free Disinfections

Bacterial infection continues to be an increasing global health problem with the most widely accepted treatment paradigms restricted to antibiotics. However, the overuse and misuse of antibiotics have triggered multidrug resistance of bacteria, frustrating therapeutic outcomes, and leading to higher mortality rates. Even worse, the tendency of bacteria to form biofilms on living and nonliving surfaces further increases the difficulty in confronting bacteria because the extracellular matrix can act as a robust barrier to prevent the penetration of antibiotics and resist environmental damage. As a result, the inability to eliminate bacteria and biofilms often leads to persistent infection, implant failure, and device damage. Therefore, it is of paramount importance to develop alternative antimicrobial agents while avoiding the generation of bacterial resistance to prevent the large-scale growth of bacterial resistance. In recent years, nano-antibacterial materials have played a vital role in the antibacterial field because of their excellent physical and chemical properties. This review focuses on new physicochemical antibacterial strategies and versatile antibacterial nanomaterials, especially the mechanism and types of 2D antibacterial nanomaterials. In addition, this advanced review provides guidance on the development direction of antibiotic-free disinfections in the antibacterial field in the future.

Solar-based photocatalytic ozonation employing novel S-scheme ZnO/Cu2O/CuO/carbon xerogel photocatalyst: effect of pH, salinity, turbidity, and temperature on salicylic acid degradation

This paper proposes the study of a solar-based photocatalytic ozonation process for the degradation of salicylic acid (SA) using a novel S-scheme ZnO/Cu2O/CuO/carbon xerogel photocatalyst. The incorporation of CuO and Cu2O aims to enhance charge mobility through the formation of p-n heterojunctions with ZnO, whereas the carbon xerogel (XC) was selected due to its eco-friendly nature, capacity to stabilize S-scheme heterojunctions as a solid-state electron mediator, and ability to function as a reducing agent under high temperatures. The characterization of the composites demonstrates that the presence of the XC during the calcination step led to the reduction of a fraction of the CuO into Cu2O, forming a ternary semiconductor heterojunction system. In terms of photocatalysis, the XC/ZnO-CuxO 5% composite achieved the best efficiency for salicylic acid degradation, mainly due to the stabilization of the S-scheme charge transfer pathway between the ZnO/CuO/Cu2O semiconductors by the XC. The total organic carbon (TOC) removal during heterogeneous photocatalysis was 80% for the solar-based process and 68% for the visible light process, after 300 min. The solar-based photocatalytic ozonation process was highly successful regarding the degradation of SA, achieving a 75% increase in the apparent reaction rate constant when compared to heterogeneous photocatalysis. Furthermore, a 78% TOC removal was achieved after 150 min, which is half the time required by the heterogeneous photocatalysis to obtain the same result. Temperature, salinity, and turbidity had major effects on the efficiency of the photocatalytic ozonation process; the system's pH did not cause any major performance variation, which holds relevance for industrial applications.

Recent Progress in Multifunctional Graphene-Based Nanocomposites for Photocatalysis and Electrocatalysis Application

The global energy shortage and environmental degradation are two major issues of concern in today's society. The production of renewable energy and the treatment of pollutants are currently the mainstream research directions in the field of photocatalysis. In addition, over the last decade or so, graphene (GR) has been widely used in photocatalysis due to its unique physical and chemical properties, such as its large light-absorption range, high adsorption capacity, large specific surface area, and excellent electronic conductivity. Here, we first introduce the unique properties of graphene, such as its high specific surface area, chemical stability, etc. Then, the basic principles of photocatalytic hydrolysis, pollutant degradation, and the photocatalytic reduction of CO₂ are summarized. We then give an overview of the optimization strategies for graphene-based photocatalysis and the latest advances in its application. Finally, we present challenges and perspectives for graphene-based applications in this field in light of recent developments.

Visible light-driven NH2Cl activation by g-C3N4 photocatalysis producing reactive nitrogen species to degrade bisphenol A

The photolysis of monochloramine (NH₂Cl), a widely used disinfectant, under UVC irradiation produces different radicals for the micropollutant degradation. For the first time, this study demonstrates the degradation of bisphenol A (BPA) via the NH₂Cl activation by graphitic carbon nitride (g-C₃N₄) photocatalysis using visible light-LEDs at 420 nm, termed as the Vis₄₂₀/g-C₃N₄/NH₂Cl process. The process produces •NH₂, •NH₂OO, •NO and •NO₂ via the e_CB^-- and O₂^•--induced activation pathways and •NHCl and NHClOO• via the h_VB⁺-induced activation pathway. The produced reactive nitrogen species (RNS) enhanced 100% of the BPA degradation compared with the Vis₄₂₀/g-C₃N₄. Density functional theory calculations confirmed the proposed NH₂Cl activation pathways and further demonstrated that e_CB^-/O₂^•- and h_VB⁺ induced the cleavage of N-Cl and N-H bonds in NH₂Cl, respectively. The process converted 73.5% of the decomposed NH₂Cl to nitrogen-containing gas, compared with that of approximately 20% in the UVC/NH₂Cl process, leaving much less ammonia, nitrite and nitrate in water. Among different operating conditions and water matrices tested, of particular significance is natural organic matter of 5 mgDOC/L only reduced 13.1% of the BPA degradation compared against that of at least 46% reduction in the UVC/NH₂Cl process. Only 0.017-0.161 µg/L of disinfection byproducts were produced, two orders of magnitudes lower than that in the UVC/chlorine and UVC/NH₂Cl processes. The combined use of visible light-LEDs, g-C₃N₄ and NH₂Cl significantly improves the micropollutant degradation and reduces the energy consumption and byproduct formation of the NH₂Cl-based AOP.

Main/side chain asymmetric molecular design enhances charge transfer of two-dimensional conjugated polymer/g-C3N4 heterojunctions for high-efficiency photocatalytic sterilization and degradation

Heterojunctions based on conjugated polymers (PHJs) are of promise as photocatalysts. Here, we fabricate the two-dimensional benzodithiophene (BDT) and thieno[2,3-f]benzofuran (TBF) based conjugated polymers/g-C₃N₄ PHJs creatively using the symmetry-breaking strategy. PD1 and PD3 with the asymmetric backbone TBF have better crystallinity. Moreover, PD3 utilizing fluorinated benzotriazole as the electron acceptor unit possesses more compact π - π stacking and higher charge mobility. The conjugated polymer PD5 with asymmetric side chains in the donor unit BDT guarantees more efficient charge transfer in the corresponding PD5/g-C₃N₄ PHJ while maintaining comparable light utilization rate. Consequently, PD5/g-C₃N₄ shows the champion performance with photocatalytic sterilization rates reaching 99.1% and 97.3% for S. aureus and E. coli. Notably, the reaction rate constant for Rhodamine B degradation of PD5/g-C₃N₄ is 8 times that of g-C₃N₄, a record high among conjugated polymers/g-C₃N₄. This study aims to reveal the structure - property correlation of asymmetric conjugated polymers/g-C₃N₄ for potential photocatalysis applications.

F,O,S-Codoped Graphitic Carbon Nitride as an Efficient Photocatalyst for the Synthesis of Benzoxazoles and Benzimidazoles

Graphitic carbon nitride (g-C3N4) is a metal-free heterogeneous catalyst that has attracted attention because of its good chemical stability, cost-effectiveness, and environmental friendliness. In the work presented herein, F,O,S-codoped carbon nitride was synthesized by heating a mixture of melamine cyanurate and trifluoromethanesulfonic acid at 550 °C for 50 min. The obtained product was characterized by transmission electron microscopy, infrared spectroscopy, X-ray powder diffraction, CHNS elemental analysis, total combustion-ion chromatography, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and UV-Vis spectroscopy. Results point to an F,O,S-codoped g-C3N4. The material was applied as a photocatalyst for the formation of benzoxazoles and benzimidazoles by condensation–aromatization of 2-aminophenol or 1,2-phenylenediamine with suitable aldehydes (viz. benzaldehyde, 4-chlorobenzaldehyde, 2-naphthaldehyde, 2-hydroxybenzaldehyde, and 2-methoxybenzaldehyde), obtaining yields of up to 90% in 15 min under visible light irradiation, with good selectivity and reusability. Thus, the reported findings suggest that this F,O,S-codoped g-C3N4 may hold promise as a metal-free photocatalyst for the rapid synthesis of 2-arylbenzoxazoles and 2-arylbenzimidazoles.

Visible-Light-Induced Phenoxyl Radical-based Metal-Organic Framework for Selective Photooxidation of Sulfides

Phenoxyl radicals originating from phenols through oxidation or photoinduction are relatively stable and exhibit mild oxidative activity, which endows them with the potential for photocatalysis. Herein, a stable and recyclable metal-organic framework Zr-MOF-OH constructed of a binaphthol derivative ligand has been synthesized and functions as an efficient heterogeneous photocatalyst. Zr-MOF-OH shows fairly good catalytic activity and substrate compatibility toward the selective oxidation of sulfides to sulfoxides under visible light irradiation. Such irradiation of Zr-MOF-OH converts the phenolic hydroxyl groups of the binaphthol derivative ligand to phenoxyl radicals through excited state intramolecular proton transfer, and the excited state photocatalyst triggers the single-electron oxidation of the sulfide. No reactive oxygen species are produced in the photocatalytic process, and triplet O₂ directly participates in the reaction, endowing Zr-MOF-OH with wide substrate compatibility and high selectivity, which also proposes a promising pathway for the direct activation of substrates via phenoxyl radicals.

Mechanistic Insights into the Oxidative Degradation of Formic and Oxalic Acids with Ozone and OH Radical. A Computational Rationale

Gas-phase and aqueous oxidations of formic and oxalic acids with ozone and OH radicals have been thoroughly examined by DFT methods. Such acids are not only important feedstocks for the iterative construction of other organic compounds but also final products generated by mineralization and advanced oxidation of higher organics. Our computational simulation unravels both common and distinctive reaction channels, albeit consistent with known H atom abstraction pathways and formation of hydropolyoxide derivatives. Notably, reactions with neutral ozone and OH radical proceed through low-energy concerted mechanisms involving asynchronous transition structures. For formic acid, carbonylic H-abstraction appears to be more favorable than the dissociative abstraction of the acid proton. Formation of long oxygen chains does not cause a significant energy penalty and highly oxygenated products are stable enough, even if subsequent decomposition releases environmentally benign side substances like O₂ and H₂O.

A comprehensive review on metal based active sites and their interaction with O3 during heterogeneous catalytic ozonation process: Types, regulation and authentication

Heterogeneous catalytic ozonation (HCO) was a promising water purification technology. Designing novel metal-based catalysts and exploring their structural-activity relationship continued to be a hot topic in HCO. Herein, we reviewed the recent development of metal-based catalysts (including monometallic and polymetallic catalysts) in HCO. Regulation of metal based active sites (surface hydroxyl groups, Lewis acid sites, metal redox cycle and surface defect) and their key roles in activating O₃ were explored. Advantage and disadvantage of conventional characterization techniques on monitoring metal active sites were claimed. In situ electrochemical characterization and DFT simulation were recommended as supplement to reveal the metal active species. Though the ambiguous interfacial behaviors of O₃ at these active sites, the existence of interfacial electron migration was beyond doubt. The reported metal-based catalysts mainly served as electron donator for O₃, which resulted in the accumulation of oxidized metal and reduced their activity. Design of polymetallic catalysts could accelerate the interfacial electron migration, but they still faced with the dilemma of sluggish Me^(n+m)+/Meⁿ⁺ redox cycle. Alternative strategies like coupling active metal species with mesoporous silicon materials, regulating surface hydrophobic/hydrophilic properties, polaring surface electron distribution, coupling HCO process with photocatalysis and H₂O₂ were proposed for future research.

Manipulating Selectivity of Hydroxyl Radical Generation by Single-Atom Catalysts in Catalytic Ozonation: Surface or Solution

Hydroxyl radical-dominated oxidation in catalytic ozonation is, in particular, important in water treatment scenarios for removing organic contaminants, but the mechanism about ozone-based radical oxidation processes is still unclear. Here, we prepared a series of transitional metal (Co, Mn, Ni) single-atom catalysts (SACs) anchored on graphitic carbon nitride to accelerate ozone decomposition and produce highly reactive ·OH for oxidative destruction of a water pollutant, oxalic acid (OA). We experimentally observed that, depending on the metal type, OA oxidation occurred dominantly either in the bulk phase, which was the case for the Mn catalyst, or via a combination of the bulk phase and surface reaction, which was the case for the Co catalyst. We further performed density functional theory simulations and in situ X-ray absorption spectroscopy to propose that the ozone activation pathway differs depending on the oxygen binding energy of metal, primarily due to differential adsorption of O₃ onto metal sites and differential coordination configuration of a key intermediate species, *OO, which is collectively responsible for the observed differences in oxidation mechanisms and kinetics.

Graphitic Carbon Nitride as Visible-Light Photocatalyst Boosting Ozonation in Wastewater Treatment

Light can boost ozone efficiency in advanced oxidation processes (AOPs), either by direct ozone photolysis with UV light or by using a photocatalyst that can be excited with UV-Vis or solar light. The present review summarizes literature data on the combination of ozone and the g-C₃N₄ photocatalyst for the degradation of probe molecules in water, including oxalic, p-hydroxybenzoic and oxamic acids as well as ciprofloxacin and parabens. g-C₃N₄ is a metal-free visible-light photocatalyst based on abundant elements that establishes a synergistic effect with ozone, the efficiency of the combination of the photocatalysis and ozonation being higher than the sum of the two treatments independently. Available data indicate that this synergy derives from the higher efficiency in the generation of hydroxyl radicals due to the efficient electron quenching by O₃ of photogenerated conduction band electrons in the g-C₃N₄ photocatalyst. Given the wide use of ozonizers in water treatment, it is proposed that their implementation with g-C₃N₄ photocatalysis could also boost ozone efficiency in the AOPs of real waste waters.

Chlorophyll sensitized (BiO)2CO3/ CdWO4/rGO nano-hybrid assembly for solar assisted photo-degradation of chlorzoxazone

In the present laboratory scale experiment, we report the fabrication of chlorophyll sensitized (BiO)₂CO₃/CdWO₄/rGO (BCR) photo-catalyst. The green approach has been adopted for boosting the optical activity by chlorophyll as a sensitizer. The functionality, nature and surface compositions of synthesized photo-catalyst have been identified by FTIR, XRD and XPS instrumentation. The internal and surface morphology has been studied using FE-SEM and HR-TEM. The optical activity has been investigated by UV-vis and photoluminescence spectroscopy. The catalytic activity of chlorophyll sensitized BCR have been tested for the photo degradation of Chlorzoxazone (CZX) under simulated visible light for 90 min. The detailed comparison has been studied for the different loading amount of chlorophyll and RGO onto BCR photo-catalyst. The potential of BCR for the photo-degradation of CZX was investigated under various operational parameters such as catalysts dosage, pollutant concentration, effect of pH and ions etc. Approximately, 96.2% of CZX has been degraded over 90 min with the optimum catalyst amount 250 mgL^-1 at pH 7. The ^●OH radical has been identified as major reactive species using radical scavenging experiment. The mineralization of CZX has been evaluated in terms of HR-MS and TOC-COD analysis.

Enhanced the synergistic degradation effect between active hydroxyl and reactive oxygen species for indoor formaldehyde based on platinum atoms modified MnOOH/MnO2 catalyst

Maintaining high activity during prolonged catalysis is always the pursuit in catalytic degradation of organic pollutants. For indoor formaldehyde (HCHO) degradation, the accumulation of intermediates is the major factor limiting the conversion of HCHO to final product CO₂ (HCHO-to-CO₂ conversion) and long-lasting catalysis. Herein, a three-dimensional radialized nanostructure catalyst self-assembled by MnOOH/MnO₂ nanosheets anchored with Pt single atoms (Pt_SA-MnOOH/MnO₂ with a trace platinum loading amount of 0.09%) is developed by thermally assisted two-step electrochemical method, which achieves enhanced CO₂ production in catalytic HCHO degradation at the room temperature by the collaborative action of active hydroxyl (OH*) and active oxygen species (O₂*). By boosting intermediates' decomposing, the catalyst implements real-time HCHO-to-CO₂ conversion (∼85.7%) and long-term continuous HCHO removal (∼98%) during 100 h in a 15 ppm HCHO atmosphere at 25 °C under a weight hourly space velocity of 30000 mL/g_cat∙h. Density functional theory calculation shows that the formation energy of O₂* from O₂ over Pt_SA-MnOOH/MnO₂ is nearly half lower than that over Pt-MnO₂ catalyst. And decomposing accumulated intermediates gives the credit to OH* species sustainably generated by the combined action of MnOOH and O₂*. The synergistic action between Pt_SA and MnOOH contributes to the continuous production of O₂* and OH* for enhancing CO₂ production in indoor catalytic formaldehyde degradation.

Complete mineralization of phenolic compounds in visible-light-driven photocatalytic ozonation with single-crystal WO3 nanosheets: Performance and mechanism investigation

Complete mineralization of phenolic compounds into CO₂ and H₂O is desirable for removing them in wastewater, but it is challenging due to the generated recalcitrant intermediates, which requires highly effective advanced oxidation process with proper catalysts. Herein, we found that single-crystal WO₃ nanosheets (NSs)-based photocatalytic ozonation (PCO) can realize complete mineralization of phenols (phenol and 2-chlorophenol) under visible light irradiation. Almost 100% mineralization ratio of phenols was achieved through WO₃ NSs-based PCO system within short time. By comparing their performances with those of polycrystalline WO₃ nanoparticles, detecting and analyzing the intermediates, identifying the dominant radicals and conducting some electrochemical characterizations, the origin of superior catalytic activity of WO₃ NSs was uncovered, the mineralization pathways and the overall mechanism were proposed. The excellent PCO performance of WO₃ NSs was contributed to their nanosheet morphology with single-crystal microstructure and good dispersion, which can provide continuous interior channels for the photogenerated charge transport from the bulk to surface of WO₃ NSs and enough active sites for the surface reactions triggered by these charges. This work puts forwards new ideas to design highly active photocatalysts for PCO and helps deepen understanding of the catalytic mechanism of PCO.

Flexible construct of N vacancies and hydrophobic sites on g-C3N4 by F doping and their contribution to PFOA degradation in photocatalytic ozonation

N vacancies, hydrophobic sites and electron rich zone were simply regulated by doping F into g-C₃N₄ (CN) to accelerate photocatalytic ozonation of PFOA. Activity of F-CN was superior to that of CN, with 74.3% PFOA removal by F-CN/Vis/O₃ but only 57.1% by CN/Vis/O₃. Experimental results and theory simulations suggested that the photogenerated hole (h_vb⁺) oxidation with the help of N vacancies was vital for PFOA degradation. N vacancies on both CN and F-CN would trap O atom of PFOA and seize electron from α -CF₂ group, which made PFOA more easily to be oxidized. Doping of F narrowed band gap, lowered the valence band position and enhanced the oxidation potential of h_vb⁺. The hydrophobic sites would accelerate the mass transfer of O₃ and PFOA, enhance O₃'s single electron reduction with e_cb^- to generate hydroxyl radicals (•OH) and reduce the recombination of h_vb⁺-e_cb^-. Under the joint function of h_vb⁺, N vacancies and •OH, PFOA degradation in F-CN/Vis/O₃ proceeded through the gradually shortening of perfluoroalky chain and loss of CF₂ unit. The acute and chronic toxicity of generated short-chain perfluorocarboxylic acid toward fish, green algae daphnid were predicted by ECOSAR. And the toxicity change of solutions was examined by luminescent bacteria.

Recent Advances in g-C3 N4 -Based Photocatalysts for Pollutant Degradation and Bacterial Disinfection: Design Strategies, Mechanisms, and Applications

Emerging photocatalytic technology promises to provide an effective solution to the global energy crisis and environmental pollution. Graphite carbon nitride (g-C₃ N₄ ) has gained extensive attention in the scientific community due to its excellent physical and chemical properties, attractive electronic band structure, and low cost. In this paper, research progress in design strategies for g-C₃ N₄ -based photocatalysts in the past five years is reviewed from the perspectives of nanostructure construction, element doping, and heterostructure construction. To clarify the relationship between application requirements and structural design, variations in the morphology, electronic energy band structure, light absorption capacity, as well as interfacial charge transfer caused by various modification strategies are discussed in detail. The recent applications of g-C₃ N₄ -based photocatalysts for pollutant degradation and bacterial disinfection are reviewed, as well as the antimicrobial activity and degradation mechanisms. Finally, current challenges and future development directions for the practical application of g-C₃ N₄ -based photocatalysts are tentatively discussed.

Photocatalytic ozonation in an immersion rotary body reactor for the removal of micro-pollutants from the effluent of wastewater treatment plants

Carrier-bound titanium dioxide catalysts were used in a photocatalytic ozonation reactor for the degradation of micro-pollutants in real wastewater. A photocatalytic immersion rotary body reactor with a 36-cm disk diameter was used, and was irradiated using UV-A light-emitting diodes. The rotating disks were covered with catalysts based on stainless steel grids coated with titanium dioxide. The dosing of ozone was carried out through the liquid phase via an external enrichment and a supply system transverse to the flow direction. The influence of irradiation power and ozone dose on the degradation rate for photocatalytic ozonation was investigated. In addition, the performance of the individual processes photocatalysis and ozonation were studied. The degradation kinetics of the parent compounds were determined using liquid chromatography tandem mass spectrometry. First-order kinetics were determined for photocatalysis and photocatalytic ozonation. A maximum reaction rate of the reactor was determined, which could be achieved by both photocatalysis and photocatalytic ozonation. At a dosage of 0.4 mg O₃/mg DOC, the maximum reaction rate could be achieved using 75% of the irradiation power used for sole photocatalysis, allowing increases in the energetic efficiency of photocatalytic wastewater treatment processes. The process of photocatalytic ozonation is suitable to remove a wide spectrum of micro-pollutants from wastewater.

Protective Strategy to Boost the Stability of Aminated Graphene in Fenton-like Reactions

Improving the stability of aminated metal-free catalysts is a big challenge in Fenton-like reactions. Herein, trinuclear iron cluster (Fe₃ cluster)-protected aminated graphene (Fe₃-NH₂-GR) is designed by a protective strategy. By protecting with the Fe₃ cluster, the lone pair electrons of amino groups are protected and the N content of Fe₃-NH₂-GR can be fixed steadily. In peroxymonosulfate (PMS)-based Fenton-like reactions with a fixed-bed reactor, the lifetime of Fe₃-NH₂-GR is two times longer than that of aminated graphene (NH₂-GR) under the same conditions. The deactivation kinetics shows that both Fe₃-NH₂-GR and NH₂-GR follow zero-order kinetics and the deactivation rate constants of Fe₃-NH₂-GR are lower than that of NH₂-GR at every period. Moreover, Fe₃-NH₂-GR still maintains 50% phenol degradation after 40 h rather than being constantly deactivated as NH₂-GR. This stable activity is attributed to the formation of -O-NO₂, while the N content will be lost in NH₂-GR. This protective strategy by the Fe₃ cluster provides a reliable method to enhance the efficiency and stability of carbon catalysts in Fenton-like reactions.

Efficient degradation of tetracycline hydrochloride by photocatalytic ozonation over Bi2WO6

Photocatalytic ozonation technique for wastewater treatment has received much attention for their efficient capability in the mineralization of persistent organic pollutants. In this study, nanostructured Bi₂WO₆ was prepared by hydrothermal method and applied in the photocatalytic ozonation process for tetracycline hydrochloride (TCH) degradation under simulated solar light irradiation. Bi₂WO₆ triggered an effective synergy between photocatalysis and ozonation, and it showed a good activity and adaptability in the degradation of organic compounds. Besides, the influence of experimental factors on the total organic carbon removal (including catalyst dosage, ozone concentration, initial pH, reaction temperature and coexisting ions) was also investigated comprehensively. Spin-trapping electron paramagnetic resonance measurements and quenching experiments demonstrated that O₂^-, OH, ¹O₂ and h⁺ contributed to TCH degradation. The possible degradation pathways of TCH were proposed by identifying the intermediates with liquid chromatography-mass spectroscopy.

Radical-Driven Decomposition of Graphitic Carbon Nitride Nanosheets: Light Exposure Matters

Understanding the transformation of graphitic carbon nitride (g-C₃N₄) is essential to assess nanomaterial robustness and environmental risks. Using an integrated experimental and simulation approach, our work has demonstrated that the photoinduced hole (h⁺) on g-C₃N₄ nanosheets significantly enhances nanomaterial decomposition under ^•OH attack. Two g-C₃N₄ nanosheet samples D and M2 were synthesized, among which M2 had more pores, defects, and edges, and they were subjected to treatments with ^•OH alone and both ^•OH and h⁺. Both D and M2 were oxidized and released nitrate and soluble organic fragments, and M2 was more susceptible to oxidation. Particularly, h⁺ increased the nitrate release rate by 3.37-6.33 times even though the steady-state concentration of ^•OH was similar. Molecular simulations highlighted that ^•OH only attacked a limited number of edge-site heptazines on g-C₃N₄ nanosheets and resulted in peripheral etching and slow degradation, whereas h⁺ decreased the activation energy barrier of C-N bond breaking between heptazines, shifted the degradation pathway to bulk fragmentation, and thus led to much faster degradation. This discovery not only sheds light on the unique environmental transformation of emerging photoreactive nanomaterials but also provides guidelines for designing robust nanomaterials for engineering applications.

Boosting the activation of molecular oxygen and the degradation of tetracycline over high loading Ag single atomic catalyst

Photocatalytic activation of molecular oxygen (O₂) is a promising way in oxidative degradation of organic pollutants. However, it suffers from low efficiency mainly due to the limited active sites for O₂ activation over traditional photocatalysts. Therefore, we established a single atomic Ag-g-C₃N₄ (SAACN) catalyst with 10 wt% loading of Ag single sites for boosting the O₂ activation during the degradation of tetracycline (TC), and 10 wt% loading of nanoparticle Ag-g-C₃N₄ (NPACN) was studied as a comparison. When using SAACN, the accumulative concentration of superoxide (•O₂^-), hydroxyl radical (•OH), singlet oxygen (¹O₂) reached up to 0.66, 0.19, 0.33 mmol L^-1h^-1, respectively, within 120 min, 11.7, 5.7 and 4.9 times compared with those using NPACN, representing 17.24% of dissolved O₂ was converted to reactive oxygen species (ROS). When additionally feeding air or O_2, the accumulative concentrations of •O₂^-, •OH, ¹O₂ were even higher (air: 4.21, 0.97, 2.02 mmol L^-1 h^-1; O₂: 17.13, 1.32, 9.00 mmol L^-1 h^-1). The rate constants (k) for degrading the TC were 0.0409 min^-1 over SAACN and 0.00880 min^-1 over NPACN, respectively (mineralization rate: 95.7% vs. 59.9% after 3 h of degradation). Moreover, the degradation ability of SAACN did not decrease in a wide range of pH value (4-10) or under low temperature (10 °C). Besides the high exposure of Ag single sites, other advances of SAACN were: 1(O₂ was more energetic favorable to adsorb on single atomic Ag sites; 2) Positive Ag single sites were easier to obtain the electrons from the surrounding N atoms, and facilitated electron transfer towards adsorbed O₂.

The Role of Catalytic Ozonation Processes on the Elimination of DBPs and Their Precursors in Drinking Water Treatment

Formation of disinfection byproducts (DBPs) in drinking water treatment (DWT) as a result of pathogen removal has always been an issue of special attention in the preparation of safe water. DBPs are formed by the action of oxidant-disinfectant chemicals, mainly chlorine derivatives (chlorine, hypochlorous acid, chloramines, etc.), that react with natural organic matter (NOM), mainly humic substances. DBPs are usually refractory to oxidation, mainly due to the presence of halogen compounds so that advanced oxidation processes (AOPs) are a recommended option to deal with their removal. In this work, the application of catalytic ozonation processes (with and without the simultaneous presence of radiation), moderately recent AOPs, for the removal of humic substances (NOM), also called DBPs precursors, and DBPs themselves is reviewed. First, a short history about the use of disinfectants in DWT, DBPs formation discovery and alternative oxidants used is presented. Then, sections are dedicated to conventional AOPs applied to remove DBPs and their precursors to finalize with the description of principal research achievements found in the literature about application of catalytic ozonation processes. In this sense, aspects such as operating conditions, reactors used, radiation sources applied in their case, kinetics and mechanisms are reviewed.

Efficient decontamination of organic pollutants under high salinity conditions by a nonradical peroxymonosulfate activation system

Peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs) for wastewater treatment have recently attracted widespread interests. However, the degradation of organic pollutants via traditional radical-dominated pathway is severely limited by the side reactions between radicals and the co-existing inorganic anions, especially under high salinity conditions. Herein, an efficient Fe/O co-doped g-C₃N₄nanosheet catalyst was synthesized to dominantly activate PMS through a dual non-radical pathway with the singlet oxygen and high-valent iron-oxo species (Fe(V)=O). The rapid degradation of model pollutant bisphenol A (BPA) was achieved by dosing PMS (1 mM), catalyst (0.1 g/L) in a simulated high-salt wastewater (≥200 mM) of the developed Fe/O-doped g-C₃N₄+PMS system with a reaction rate constant of 1204-fold higher than that in g-C₃N₄+PMS system. The O and Fe co-dopants could reconfigurate the electronic structure of pristine g-C₃N₄ to produce more non-radical active species. The formed Fe(V)=O played a main role in the BPA degradation by promoting electron transfer from BPA molecule to the "metastable PMS/catalyst complex", which was verified by electrochemical tests and density functional theory calculations. The auxiliary transient productions of ·OH+SO₄·^- species were also favorable for the pollutant degradation. Excellent reusability in a wide pH range confirmed the practical application prospects of the Fe/O-doped g-C₃N₄+PMS system. The successive addition of PMS with a low dosage into the system rich in pollutants was confirmed to favor the PMS utilization. Our work unveils the potential applications of a non-radical dominated process for the decontamination of organic pollutants in saline water.

A strong hydrangea-like Au–TiO2 catalyst for round-the-clock degradation of oxalic acid in the presence of ozone

We report a strong hydrangea-like Au–TiO₂ catalyst for round-the-clock degradation of oxalic acid by either photocatalytic ozonation or catalytic ozonation.