Improvement of phenol photodegradation efficiency by a combined g-C3N4/Fe(III)/persulfate system

Advances in the Removal of Organic Pollutants from Water by Photocatalytic Activation of Persulfate: Photocatalyst Modification Strategy and Reaction Mechanism

Environmental pollution caused by persistent organic pollutants has imposed big threats to the health of human and ecological systems. The development of efficient methods to effectively degrade and remove these persistent organic pollutants is therefore of paramount importance. Photocatalytic persulfate-based advanced oxidation technologies (PS-AOTs), which depend on the highly reactive SO4 - radicals generated by the activation of PS to degrade persistent organic pollutants, have shown great promise. This work discusses the application and modification strategies of common photocatalysts in photocatalytic PS-AOTs, and compares the degradation performance of different catalysts for pollutants. Furthermore, essential elements impacting photocatalytic PS-AOTs are discussed, including the water matrix, reaction process mechanism, pollutant degradation pathway, singlet oxygen generation, and potential PS hazards. Finally, the existing issues and future challenges of photocatalytic PS-AOTs are summarized and prospected to encourage their practical application. In particular, by providing new insights into the PS-AOTs, this review sheds light on the opportunities and challenges for the development of photocatalysts with advanced features for the PS-AOTs, which will be of great interests to promote better fundamental understanding of the PS-AOTs and their practical applications.

Self-polarized schorl optimizing TiO2 for photocatalytic persulfate activation and organic pollutants degradation

Photocatalytic activation of persulfate has exhibited tremendous potential in water purification because of its green and environmentally friendly process. However, this process often exhibits low activation efficiencies and difficult recovery of the photocatalyst. Herein, schorl-supported nano-TiO2 composite photocatalysts (S/TiO2) were prepared by a mechanical grinding method for efficient activation of potassium monopersulfate (PMS). The anatase TiO2 nanoparticles with particle size of approximately 30 nm was uniformly loaded on the surface of schorl via forming Si-O-Ti bonds. The S/TiO2 assisted with PMS (S/TiO2-PMS) exhibited remarkable degradation performance and stability. In this system (S/TiO2-PMS), the C/C0 value of phenol solution (10 ppm) were decreased to 0.070 and 0 after 30 min and 90 min of irradiation, where the degradation extent were 93.0% and 100% respectively. The rate of phenol degradation with S/TiO2-PMS was 12.6 times that seen with TiO2-PMS. The oxidation active species were holes and SO4•- in S/TiO2-PMS system subjected to simulated sunlight. It was demonstrated that the polarization electric field of the schorl enhanced the separation efficiency of the photoinduced electrons and holes for improving the performance of the S/TiO2-PMS. On the other hand, the transformations of Fe3+ and Fe2+ on the schorl surface further promotes the activation of PMS. This work provides a new choice for designing TiO2-based photocatalytic persulfate activation system targeting the field of advanced oxidation water treatment.

Enhanced degradation of pharmaceuticals in wastewater by coupled radical and non-radical pathways: Further unravelling kinetics and mechanism

Advanced oxidation processes based on radicals and/or non-radical catalysis are emerging as promising technologies for eliminating pharmaceuticals (PhACs) from wastewater. However, the respective contributions of different removal pathways (radicals or non-radical) for PhAC degradation still lacks quantitative investigation. Zero-valent iron and carbon nanotubes are frequently used to generate both radicals and non-radical species via the activation of persulfate (Fe⁰/SWCNT/PDS). Herein, the removal kinetics of 1 μM PhACs are depicted, and the corresponding synergistic mechanism of the Fe⁰/SWCNT/PDS process is discussed. Coupled removal pathways showed the higher degradation of PhACs than the individual pathways. Radicals quenching studies combined with electron spin resonance characterisation suggested that the radical-based removal pathway tends to attack electron-deficient organics, whereas its counterpart is more likely to work on electron-rich organics. From the perspectives of the contribution rate, the redox cycles of conjugated Fe species play a more important role in the generation of radicals than free Fe species, and the faster electron transfer in the conductive bridge offered by SWCNT is responsible for the effective corrosion of Fe⁰ and the decomposition of PDS. Six real wastewater samples were used to prove the generality of the above removal contribution, regardless of the wastewater samples, and the results suggested that identical attack patterns were obtained in all real wastewater samples, although coexistence matrix slightly suppressed PhAC removal. This work provides a deeper insight into the high-performance working mechanism on synergistic interactions and contaminant removal in a combined catalysis system.

Organic Covalent Interaction-based Frameworks as Emerging Catalysts for Environment and Energy Applications: Current Scenario and Opportunities

The term "covalent organic framework" (COF) refers to a class of porous organic polymeric materials made from organic building blocks that have been covalently bonded. The preplanned and predetermined bonding of the monomer linkers allow them to demonstrate directional flexibility in two- or three-dimensional spaces. COFs are modern materials, and the discovery of new synthesis and linking techniques has made it possible to prepare them with a variety of favorable features and use them in a range of applications. Additionally, they can be post-synthetically altered or transformed into other materials of particular interest to produce compounds with enhanced chemical and physical properties. Because of its tunability in different chemical and physical states, post-synthetic modifications, high stability, functionality, high porosity and ordered geometry, COFs are regarded as one of the most promising materials for catalysis and environmental applications. This study highlights the basic advancements in establishing the stable COFs structures and various post-synthetic modification approaches. Further, the photocatalytic applications, such as organic transformations, degradation of emerging pollutants and removal of heavy metals, production of hydrogen and Conversion of carbon dioxide (CO₂ ) to useful products have also been presented. Finally, the future research directions and probable outcomes have also been summarized, by focusing their promises for specialists in a variety of research fields.

Synthesis of g-C3N4 Derived from Different Precursors for Photodegradation of Sulfamethazine under Visible Light

In this study, a series of g-C3N4 nanosheets were prepared by various thermal oxidative etching times from four different precursors (urea, melamine, dicyandiamide and thiourea). The physicochemical properties of these g-C3N4 nanosheets were analyzed in detail using scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence emission spectra, Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) analysis and ultraviolet-visible diffuse reflectance. The results revealed that the g-C3N4 nanosheets obtained a thinner layer thickness and larger specific surface area, with an extension of thermal oxidative etching time. Meanwhile, sulfamethazine (SMZ), one of the most widely used sulfonamides, was used to evaluate the photocatalyst activity of the g-C3N4 nanosheets prepared in this study. Compared to other g-C3N4 nanosheets, urea-derived g-C3N4 nanosheets under 330 min thermal oxidative etching showed the highest photocatalytic activity for SMZ under visible light. In conclusion, our study provides detailed insights into the synthesis and characterization of g-C3N4 nanosheets prepared from various precursors and highlights the importance of thermal oxidative etching time in determining the photocatalytic activity of these materials.

WSe2/g-C3N4 for an In Situ Photocatalytic Fenton-like System in Phenol Degradation

An in situ photo-Fenton system can continuously generate H₂O₂ by photocatalysis, activating H₂O₂ in situ to form strong oxidizing ·OH radicals and degrading organic pollutants. A WSe₂/g-C₃N₄ composite catalyst with WSe₂ as a co-catalyst was successfully synthesized in this work and used for in situ photo-Fenton oxidation. The WSe₂/g-C₃N₄ composite with 7% loading of WSe₂ (CNW2) has H₂O₂ production of 35.04 μmol/L, which is fourteen times higher than pure g-C₃N₄. The degradation efficiency of CNW2 for phenol reached 67%. By constructing an in situ Fenton-system, the phenol degradation rate could be further enhanced to 90%. WSe₂ can enhance the catalytic activity of CNW2 by increasing electron mobility and inhibiting the recombination of photogenerated electron-hole pairs. Moreover, the addition of Fe²⁺ activates the generated H₂O₂, thus increasing the amount of strong oxidative ·OH radicals for the degradation of phenol. Overall, CNW2 is a promising novel material with a high H₂O₂ yield and can directly degrade organic pollutants using an in situ photo-Fenton reaction.

Template-based textural modifications of polymeric graphitic carbon nitrides towards waste water treatment

The composite materials based on graphitic carbon nitrides (g-C₃N₄) are remarkably better semiconductors, but the inherent photocatalytic performance in its generic synthesis form is not up to the mark. Eminence efforts have been made to improve its performance and photocatalytic efficiencies. Recently, extensive investigations have been performed to develop their texturally modified and highly porous structures to get around the big flaws of bulk g-C₃N₄. One significant disadvantage is the increase in the polycondensation while preparation at 550 °C results in g-C₃N₄ materials with restricted specific surface area (SSA) (<10 m²/g) and no textured pores. Textural modification has emerged as an efficient and progressive way to improve optical and electronic characteristics. The final texture and shape of CN are influenced by the precursor's interaction with the template. Researchers are interested in developing CN materials with high SSA and changeable textural properties (pore volume and pore size). Based on the literature review it is concluded that the soft templating approach is relatively simple, and straightforward to induce textural changes in the g-CN type materials. This review focused on improving the textural properties of bulk g-C₃N₄ via templating method, and the major advances in the modified g-C₃N₄ materials for the treatment of wastewater. The procedures and mechanisms of numerous approaches with varying morphologies are thoroughly explained.

Magnetic recyclable heterogeneous catalyst Fe3O4/g-C3N4 for tetracycline hydrochloride degradation via photo-Fenton process under visible light

Antibiotic pollution of water resources is a global problem, and the development of new treatments for destroying antibiotics in water is a priority research. We successfully manufactured recyclable magnetic Fe₃O₄/g-C₃N₄ through the electrostatic self-assembly method. Selecting tetracycline (TC) as the target pollutant, using Fe₃O₄/g-C₃N₄ and H₂O₂ developed a heterogeneous optical Fenton system to remove TC under visible light. Fe₃O₄/g-C₃N₄ was systematically characterized by SEM, TEM, XRD, FTIR, XPS, DRS, and electrochemical methods. The removal efficiency of 7% Fe₃O₄/g-C₃N₄ at pH = 3, H₂O₂ = 5 mM, and catalyst dosage of 1.0 g/L can reach 99.8%. After magnetic separation, the Fe₃O₄/g-C₃N₄ photocatalyst can be recycled five times with minimal efficiency loss. The excellent degradation performance of the prepared catalyst may be attributed to the proper coupling interface between Fe₃O₄ and g-C₃N₄ which promotes the separation and transfer of photogenerated electrons. Photogenerated electrons can also accelerate the conversion of Fe³⁺ to Fe²⁺, thereby producing more ˙OH. The new Fe₃O₄/g-C₃N₄ can be used as a raw material for advanced oxidation of water contaminated by refractory antibiotics.

Intumescent flame retardancy and smoke suppression of Eucommia ulmoides gum/natural rubber blends based on synergistic g-C3N4@Fe3O4 nanocomposites

A flame retardant synergist (g-C₃N₄@Fe₃O₄) was designed through an in situ co-precipitation method by using graphitized carbon nitride (g-C₃N₄) and Fe₃O₄, and its structure was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). A series of different contents of g-C₃N₄@Fe₃O₄ and intumescent flame retardant (IFR) were simultaneously added into eucommia ulmoides gum/natural rubber (EUG/NR) blends to improve the flame retardancy and reduce the smoke release. The flame retardancy, smoke suppression performance, thermal degradation behaviors and thermal stability of EUG/NR blends were evaluated by the limiting oxygen index (LOI), UL-94, cone calorimetry test (CCT) and thermogravimetric analysis (TGA). The results showed that using g-C₃N₄@Fe₃O₄ as a flame retardant synergist, the LOI value of the 9 phr g-C₃N₄@Fe₃O₄ synergist (EUG/NR 5) blend was 29.5%, which was much higher than the 20.0% of the original composite. Moreover, the 6 phr g-C₃N₄@Fe₃O₄ synergist (EUG/NR 4) and EUG/NR 5 passed the UL-94 V-0 rating, while the pristine EUG/NR blends showed no rating. Moreover, the total heat release rate (THR), peak of heat release rate (PHRR) and total smoke production (TSP) of the EUG/NR blends in the CCT test were much lower than those of the pristine EUG/NR blends. In particular, PHRR, THR and TSP of EUG/NR 5 decreased by 37.1%, 16.2% and 18.0%, respectively. This indicated that the introduction of g-C₃N₄@Fe₃O₄ decreased the release of combustible gases. TGA results also showed that the addition of g-C₃N₄@Fe₃O₄ accelerated the thermal degradation of the EUG/NR blends and changed the thermal degradation mechanism of the EUG/NR blends, indicating the synergistic effect of g-C₃N₄@Fe₃O₄. Finally, a possible degradation mechanism of EUG/NR blend composites was proposed.

Roles of alkali metal dopants and surface defects on polymeric carbon nitride in photocatalytic peroxymonosulfate activation towards water decontamination

Polymeric carbon nitride (PCN) has been extensively employed in peroxymonosulfate (PMS) activation for water decontamination. However, limited photocatalytic efficiency can be achieved by pristine PCN due to its intrinsic deficiencies like high electron-hole recombination rate and resistance to charge transfer. Herein, in a two-stage thermal treatment process, the nontoxic and stable Na and K were successfully anchored among the PCN skeleton with surface defects created, leading to an elevated photocatalytic activity for PMS activation. The SO₄·^- and ¹O₂ were identified as the dominant reactive oxygen species, which were generated from electron transfer processes between PMS and catalyst. Experimental and theoretical analyses suggested that the defective structures and metal dopants improved the optical properties of catalyst, endowing it a wider light absorption range and a lower energy barrier for electron transitions. The modified structures were also beneficial to electron transfer processes due to the weaker electron confinement effect, accelerating the production of SO₄·^- on the defective sites and ¹O₂ on the metal sites. The synergy of radical and non-radical species weakened the influence of side reactions between radicals from PMS and coexisting inorganic anions in practical water, hence to promote the resistance of modified catalysts in complex water matrices.

Advances in application of g-C3N4-based materials for treatment of polluted water and wastewater via activation of oxidants and photoelectrocatalysis: A comprehensive review

Recently, graphitic carbon nitride (g-C₃N₄) has received significant attention as a non-metallic, visible-light-activated photocatalyst for treating water and wastewater by degrading contaminants. Accordingly, previous review articles have focused on the photocatalytic properties of g-C₃N₄-based materials. However, g-C₃N₄ has several other notable features, such as high adsorption affinity towards aromatic substances and heavy metals, high thermal and chemical resistances, good compatibility with various materials, and easily scalable synthesis; therefore, in addition to simple photocatalysis, it can be widely used in other decontamination systems based on activation of oxidants and electrocatalysis. This critical review provides a comprehensive summary of recent advancements in g-C₃N₄-based materials and their use in treating polluted water and wastewater via the following routes (1) activation of oxidizing agents (e.g., hydrogen peroxide, ozone, peroxymonosulfate, and persulfate): and (2) photoelectrocatalysis using fabricated g-C₃N₄-based photocathodes and photoanodes. For each route, we briefly summarize the primary mechanisms, distinctive features, and performances of various water treatment systems using g-C₃N₄-based catalysts. We also highlight the specific roles of g-C₃N₄ in improving the efficiencies of these treatment processes. The advantages and limitations of previously reported water treatment systems using g-C₃N₄-based materials are also described and compared in this review. Finally, we discuss the challenges and prospects of improving g-C₃N₄-based water purification applications.

l-cysteine-modified Fe3 O4 nanoparticles as a novel heterogeneous catalyst for persulfate activation on BTEX removal

l-cysteine-modified Fe₃ O₄ nanoparticles (l-cys@nFe₃ O₄ ) were synthesized successfully and used as catalyst to activate persulfate (PS) for benzene, toluene, ethylbenzene, and xylenes (BTEX) degradation. The composite was fully characterized, and the l-cys@nFe₃ O₄ had more protrusions and l-cys was combined on the surface of nFe₃ O₄ . The removals of BTEX were 78.2%, 85.1%, 85.3%, 81.2%, respectively, in PS/l-cys@nFe₃ O₄ system, while only 52.7% 57.8%, 60.8%, and 56.3% of BTEX removals reached under the same condition for nFe₃ O₄ chelated with l-cys in 48 h. Four successive cycles of BTEX degradation were completed in PS/l-cys@nFe₃ O₄ system. The synergistic mechanisms of BTEX degradation in PS/l-cys@nFe₃ O₄ system were investigated by electron paramagnetic resonance (EPR), benzoic acid (BA) probe and X-ray photoelectron spectroscopy (XPS) tests. SFe bond in l-cys-Fe complexes promoted the electron transfer between nFe₃ O₄ core and the solution, iron and iron at the interface, thereby promoting the Fe³⁺ /Fe²⁺ cycle and the catalytic capacity of nFe₃ O₄ . The optimal pH of PS/l-cys@nFe₃ O₄ system was 3, while HCO₃ ^- and Cl^- exhibited negative influences on BTEX degradation. Only 14.2%, 15.5%, 15.9%, and 15.6% BTEX had been removed in the presence of 0.12-M PS and 8.0 g/L l-cys@nFe₃ O₄ under the actual groundwater condition. However, expanding the dosage of PS and l-cys@nFe₃ O₄ was an effective strategy to overcome the adverse effect. PRACTITIONER POINTS: L-cys@nFe₃ O₄ were synthesized successfully and used as catalyst to activate PS for BTEX degradation. Four successive cycles of BTEX degradation were completed in PS/L-cys@nFe₃ O₄ system. lS-Fe bond in L-cys@nFe₃ O₄ promoted the electron transfer between PS and nFe₃ O₄ core.

Analysis of Photocatalytic Degradation of Phenol with Exfoliated Graphitic Carbon Nitride and Light-Emitting Diodes Using Response Surface Methodology

Response surface methodology (RSM) involving a Box–Benkhen design (BBD) was employed to analyze the photocatalytic degradation of phenol using exfoliated graphitic carbon nitride (g-C3N4) and light-emitting diodes (wavelength = 430 nm). The interaction between three parameters, namely, catalyst concentration (0.25–0.75 g/L), pollutant concentration (20–100 ppm), and pH of the solution (3–10), was examined and modeled. An empirical regression quadratic model was developed to relate the phenol degradation efficiency with these three parameters. Analysis of variance (ANOVA) was then applied to examine the significance of the model; this showed that the model is significant with an insignificant lack of fit and an R2 of 0.96. The statistical analysis demonstrated that, in the studied range, phenol concentration considerably affected phenol degradation. The RSM model shows a significant correlation between predicted and experimental values of photocatalytic degradation of phenol. The model’s accuracy was tested for 50 ppm of phenol under optimal conditions involving a catalyst concentration of 0.4 g/L catalysts and a solution pH of 6.5. The model predicted a degradation efficiency of 88.62%, whereas the experimentally achieved efficiency was 83.75%.

Efficient Advanced Oxidation Process (AOP) for Photocatalytic Contaminant Degradation Using Exfoliated Metal-Free Graphitic Carbon Nitride and Visible Light-Emitting Diodes

The photocatalytic performance of metal-free graphitic carbon nitride (g-C3N4) was examined using visible light-emitting diodes (LEDs). A comparative and parametric study was conducted using the photocatalytic degradation of phenol as a model reaction. The g-C3N4 photocatalyst was synthesized from melamine using thermal condensation, followed by a thermal exfoliation that increases the catalyst surface area from 11 to 170 m2/g. Different characterization techniques, namely X-ray powder diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption using the Brunauer–Emmett–Teller method, ultraviolet-visible (UV–vis) spectroscopy, transmission electron microscopy, photoluminescence spectroscopy (PL), and zeta potential analysis, were used to characterize the photocatalyst. A comparison of the photodegradation experiments conducted with a full-spectrum xenon lamp and a custom-made single-wavelength LED immersion lamp showed that the photocatalyst performance was better with the LED immersion lamp. Furthermore, a comparison of the performance of exfoliated and bulk g-C3N4 revealed that exfoliated g-C3N4 completely degraded the pollutant in 90 min, whereas only 25% was degraded with bulk g-C3N4 in 180 min because the exfoliated g-C3N4 enhances the availability of active sites, which promotes the degradation of phenol. Experiments conducted at different pH have shown that acidic pH favors the degradation process. The exfoliated g-C3N4 has shown high photocatalytic performance in the photodegradation of other phenolic compounds, such as catechol, m-cresol, and xylenol, as well.

Graphitic carbon nitride-based materials in activating persulfate for aqueous organic pollutants degradation: A review on materials design and mechanisms

With the increasingly serious water environment problem, the persulfate-based advanced oxidation process (PS-AOP) has attracted considerable attention in water pollution treatment. To date, graphitic carbon nitride (g-C₃N₄) has been greatly favored by researchers in activating PS for its capability and unique superiorities. Though g-C₃N₄-based PS-AOP exhibits huge development prospects in removing organic pollutants, the review about its research progress has not been reported. Herein, this paper reviews the modification of g-C₃N₄ on the basis of its applications and properties for PS activation systematically. The activation mechanisms of g-C₃N₄-based modified materials are analyzed in detail, and the main formation pathways of radicals and non-radicals and their interaction mechanism with pollutants are thoroughly summarized. Finally, the existing challenges and future development directions of the PS-AOP driven by g-C₃N₄-based materials are critically discussed. The key purpose is to provide a reference for promoting the further popularization of this novel and efficient cooperative AOP in water purification industries, as well as multidisciplinary inspirations for g-C₃N₄-involved fields.

Deciphering co-catalytic mechanisms of potassium doped g-C3N4 in Fenton process

Conventional Fenton reaction for the pollutant removal is restricted by incomplete H₂O₂ decomposition due to the low efficient Fe(III)/Fe(II) cycle. In this study, the co-catalytic Fenton processes with g-C₃N₄ and the roles of potassium doping in the diverse mechanisms were comprehensively investigated. The degradation rate of enrofloxacin (ENR) in g-CN-3.9 %K/Fe(III)/H₂O₂ was 204 times higher than that in conventional Fenton reaction. This significant enhancement was ascribed to the readily formed complex between Fe(III) and K doped g-C₃N_4. The K doping facilitated the transfer of photoexcited e^- from g-CN-3.9 %K surface to Fe(III), leading to an accelerated Fe(III) reduction to Fe(II). In addition, this complex was coordinated and oxidized by H₂O₂, resulting in the formation of Fe(V) which quickly degraded ENR. Without K doping, on the other hand, only O₂ dominated the degradation of ENR in g-CN/Fe(III)/H₂O₂ due to the lack of Fe(III) complexation. This study provides a new perspective for regulating the transfer directions of the photoexcited e^- with K doping in g-C₃N₄/Fenton coupled catalytic system for water purification.

Graphene quantum dots on stainless-steel nanotubes for enhanced photocatalytic degradation of phenanthrene under visible light

A novel nanocomposite of stainless-steel nanotubes with graphene quantum dots (SSNT@GQD) was synthesized to degrade phenanthrene photocatalytically under visible light. Photocatalytic performance of bare stainless-steel nanotubes (SSNT) is not satisfactory due to the fast recombination of photoinduced electron-hole pairs. This phenomenon was effectively overcome by inclusion of GQDs and addition of persulfate as an external electron acceptor to improve charge separation. The pseudo-first-order rate constant of phenanthrene degradation by SSNT@GQD with persulfate under visible light was 0.0211 ± 0.0006 min^-1, about 42 times higher than that of persulfate and visible light, 0.0005 ± 0.0000 min^-1. Effects of different water quality parameters were investigated, including levels of initial pH, natural organic matters, bicarbonate, and chloride. Sulfate radicals, superoxide radicals, and photo-generated holes were the key reactive species in this photocatalytic process. Based on the analysis of intermediates using purge and trap-GC-MS, possible photocatalytic degradation pathways of phenanthrene in this process were proposed. The SSNT@GQD showed high figure of merit (99.5 without persulfate and 78.7 with persulfate) and quantum yield (1.56 × 10^-5 molecules photon^-1 without persulfate and 4.64 × 10^-5 molecules photon^-1 with persulfate), indicating that this material has excellent potential for practical photocatalysis applications.

Sulfonated polyaniline-encapsulated graphene@graphitic carbon nitride nanocomposites for significantly enhanced photocatalytic degradation of phenol: a mechanistic study

The photocatalytic degradation of phenol through -s-PANI@g-C₃N₄ and its nanocomposites with graphene.

Recent Strategies for Hydrogen Peroxide Production by Metal-Free Carbon Nitride Photocatalysts

Hydrogen peroxide (H2O2) is a chemical which has gained wide importance in several industrial and research fields. Its mass production is mostly performed by the anthraquinone (AQ) oxidation reaction, leading to high energy consumption and significant generation of wastes. Other methods of synthesis found in the literature include the direct synthesis from oxygen and hydrogen. However, this H2O2 production process is prone to explosion hazard or undesirable by‑product generation. With the growing demand of H2O2, the development of cleaner and economically viable processes has been under intense investigation. Heterogeneous photocatalysis for H2O2 production has appeared as a promising alternative since it requires only an optical semiconductor, water, oxygen, and ideally solar light irradiation. Moreover, employing a metal-free semiconductor minimizes possible toxicity consequences and reinforces the sustainability of the process. The most studied metal‑free catalyst employed for H2O2 production is polymeric carbon nitride (CN). Several chemical and physical modifications over CN have been investigated together with the assessment of different sacrificial agents and light sources. This review shows the recent developments on CN materials design for enhancing the synthesis of H2O2, along with the proposed mechanisms of H2O2 production. Finally, the direct in situ generation of H2O2, when dealing with the photocatalytic synthesis of added-value organic compounds and water treatment, is discussed.

Remediation of phenanthrene contaminated soil by g-C3N4/Fe3O4 composites and its phytotoxicity evaluation

This work is a premier demonstrating the technical feasibility of remediation of PAHs-contaminated soil by g-C3N4/Fe3O4. g-C3N4/Fe3O4 has been synthesized by typical two steps involved the synthesis of g-C3N4 and the subsequent in situ co-precipitation of Fe3O4 nanoparticles. g-C3N4/Fe3O4 exhibits excellent visible-light-driven photocatalytic activity for the degradation of phenanthrene in soil at circumneutral pH. The enhanced photocatalytic activity of g-C3N4/Fe3O4 should be attributed to the hybrid of Fe3O4 and g-C3N4 and appropriate Fe3O4 loading amount can improve not only the visible light absorption ability but also the separation of the photo-induced electron-hole pairs. The phytotoxicity evaluation, a preliminary ecological risk assess, was conducted on lettuce cultivation experiments. Base on the data of growth indexes including seeds germination percentage, root length, leaf length, and fresh weight of lettuce, it can be conclude that photocatalytic oxidation based on g-C3N4/Fe3O4 provide a mild oxidation process to degrade the phenanthrene from contaminated soil and there is no negative impact on the growth of lettuce. This work definitely demonstrates that this soil remediation method based on g-C3N4/Fe3O4 is technologically feasible and has immense potential in the application of remediation of organic pollutant contaminated soils.

A comparative study on the photo-catalytic degradation of Cytarabine anticancer drug under Fe3+/H2O2, Fe3+/S2O82-, and [Fe(C2O4)3]3-/H2O2 processes. Kinetics, identification, and in silico toxicity assessment of generated transformation products

Cytarabine (CY) is an anticancer drug which has been identified in wastewater influents, effluents, and surface waters. In the present study, the degradation of CY under simulated solar light (SSL), by photo-Fenton (Fe³⁺/H₂O₂/SSL) and photo-Fenton-like processes (Fe³⁺/S₂O₈^2-/SSL and [Fe(C₂O₄)₃]^3-/H₂O₂/SSL), was investigated. The major parameters affecting the applied treatments (e.g., concentration of CY, Fe³⁺, H₂O₂, and S₂O₈^2-) were optimized and CY's complete removal was achieved within 45 min for all techniques used. Mineralization studies indicated that [Fe(C₂O₄)₃]^3-/H₂O₂/SSL treatment was the most efficient procedure since faster kinetics are achieved and higher mineralization percentage is reached compared to the other techniques used. Furthermore, 12 transformation products (TPs) were identified during the applied processes, by high resolution mass spectrometry, four of which were identified for the first time, indicating that CY molecule undergoes hydroxylation and subsequent oxidation, during the applied processes. Moreover, predictions of acute and chronic ecotoxicity of CY and its TPs on fish, daphnia, and green algae were conducted, using in silico quantitative structure activity relationship (QSAR) calculations. According to these predictions, the TPs generated during the studied treatments may pose a threat to aquatic environment. Finally, the efficiency of CY degradation by photo-Fenton and photo-Fenton-like treatment in real wastewater was evaluated, under the optimized conditions, which resulted in lower degradation rate constants compared to ultrapure water.

Efficient phenol removal from petrochemical wastewater using biochar-La/ultrasonic/persulphate system: characteristics, reusability, and kinetic study

This research has analysed the physiochemical properties of a catalyst that has been developed - biochar-La, including BJH, BET, EDX, SEM, FTIR, pHpzc, and iodine number. The catalyst consisted of effective functional groups, including C=S, C-O, C=C, -COOH and O-H, with a specific surface area of 31.2 m²/g. The catalyst was used in the biochar-La/ultrasonic/persulphate system to remove phenol from wastewater. The kinetics, mechanism, and reusability of the catalyst for the phenol removal from synthetic wastewater were determined. The results suggested that phenol removal kinetics follows pseudo-first-order model (k = 0.0386 1/min), and the catalyst can be reused three times. The potential of operation of the biochar-La/ultrasonic/persulphate system - with the effective removal of phenol and other organic compounds from real petrochemical wastewater - was tested. The results indicated that the removal of phenol from the petrochemical wastewater with a relatively high total dissolved solid is >99%. The gas chromatography-mass spectrometry (GC-mass) test revealed that the complete decomposition of some contaminants in the petrochemical wastewater had occurred, as H₂O and CO₂ were detected. The contribution of a heterogeneous mechanism for phenol oxidation by biochar-La/ultrasonic/persulphate was calculated to be 60%. Overall, the results showed that the biochar-La/ultrasonic/persulphate system is very effective and promising for the removal of phenol from the petrochemical wastewater.

Electronic, structural, optical, and photocatalytic properties of graphitic carbon nitride

Graphitic carbon nitride (g-C₃N₄)-based materials exhibit an organized layered porous structure and a band position optimum for the development of various optoelectronic devices and photocatalysts.

Sulfate radical induced degradation of Methyl Violet azo dye with CuFe layered doubled hydroxide as heterogeneous photoactivator of persulfate

Persulfate (PS)-based advanced oxidation processes have aroused considerable attentions due to their higher efficiency and wider adaptability to the degradation of bio-recalcitrant organic contaminants. In this study, Cu-Fe layered doubled hydroxide (CuFe-LDH) was employed to degrade Methyl Violet (MV) through heterogeneous photo-activation of PS under visible-light irradiation. The reaction kinetics, degradation mechanism, catalyst stability were investigated in detail. Under the conditions of CuFe-LDH (3:1) dosage 0.2 g/L, PS concentration 0.2 g/L and without initial pH adjustment, 20 mg/L MV was almost completely degraded within 18 min. Electron Spin Resonance (ESR) test and radical quenching experiment indicated that sulfate radicals (SO₄^-) were the dominant reactive oxidants for the MV decolorization, while hydroxyl radicals (OH) were also involved. The CuFe-LDH/PS/Vis system was applicable at wide range of pH level (3-9). However, extreme pH level would lead to the reduction or transformation of SO₄^-. The catalyst CuFe-LDH exhibited excellent stability and maintained relatively high catalytic activity to PS even after four recycles. Mechanism study revealed that the redox cycle of Fe³⁺/Fe²⁺ and Cu²⁺/Cu³⁺ assisted by visible-light irradiation accounted for the enhanced generation of radicals in CuFe-LDH/PS/Vis system, resulting in the improved degradation of organic contaminants. Overall, the CuFe-LDH/PS/Vis process could be a promising approach for the removal of refractory organic pollutants in wastewater.

Visible light photocatalytic degradation of MB using UiO-66/g-C3N4 heterojunction nanocatalyst

A unique hybrid of Zr-based metal-organic framework (UiO-66) with graphitic carbon nitride (g-C₃N₄) nanosheets was synthesized by a facile annealing method. Photocatalytic effect was measured by the photodegradation of methylene blue (MB) under visible light irradiation. The morphology, structure, and porous properties of the as-synthesized composites were characterized by using transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the thermogravimetric and differential scanning calorimetry analysis (TG-DSC), diffuse reflectance UV-vis spectroscopy (UV-vis DRS), photoluminescence (PL), and N₂ sorption-desorption isotherms (BET). The results showed that about 100% of MB (200 mL of 10 mg L^-1) photodegradation was achieved by the UiO-66/g-C₃N₄ hybrids (UC 10:10) in 240 min under visible light. The enhanced photocatalytic activity could be attributed to the heterojunction between UiO-66 and g-C₃N₄ therefore the photoelectron transfers efficiently from the conduction band (CB) of g-C₃N₄ to the CB of UiO-66 through the inner electric field generated by the heterojunction resulting the decreasing of recombination of electron/hole and the porous structures which enhance adsorption of the dye molecules on the catalyst surface thereby facilitates the electron/hole transfer within the framework. The trapping experiment and electron spin resonance (ESR) results showed that superoxide radicals (•O₂^-) was the main oxidative species in the photodegradation of MB and the enhanced photocatalytic mechanism of UiO-66/g-C₃N₄ heterojunction hybrids was also proposed.

Persulfate enhanced photocatalytic degradation of bisphenol A by g-C3N4 nanosheets under visible light irradiation

The enhancement of g-C₃N₄ photocatalytic degradation of bisphenol A (BPA) via persulfate (PS) addition was investigated under visible light irradiation. The effects of various parameters on the BPA degradation were investigated, such as catalysts dosage, PS concentrations, initial pH value and BPA concentration. The results showed that g-C₃N₄ nanosheets exhibited superior photocatalytic activity toward BPA degradation as compared with bulk g-C₃N₄. The addition of PS can further improve the g-C₃N₄ photocatalytic performance for BPA degradation. With 5 mM PS, the degradation rate of BPA was increased from 72.5% to 100% at 90 min, and the corresponding first-order kinetic constants were increased from 0.0028 to 0.0140 min^-1. The removal efficiency of BPA increased with the decrease of solution pH value. The active radicals in the reaction system were tested by electron spin resonance (ESR) and radicals quenching experiments. Instead of persulfate radicals' oxidation, it was proposed that the main active radicals for BPA degradation were superoxide radicals and the photogenerated holes.