The photocatalytic performance of g-C3N4 from melamine hydrochloride for dyes degradation with peroxymonosulfate

Graphitic Carbon Nitride Quantum Dots (g-C3N4 QDs): From Chemistry to Applications

Since their emergence in 2014, graphitic carbon nitride quantum dots (g-C3N4 QDs) have attracted much interest from the scientific community due to their distinctive physicochemical features, including structural, morphological, electrochemical, and optoelectronic properties. Owing to their desirable characteristics, such as non-zero band gap, ability to be chemically functionalized or doped, possessing tunable properties, outstanding dispersibility in different media, and biocompatibility, g-C3N4 QDs have shown promise for photocatalysis, energy devices, sensing, bioimaging, solar cells, optoelectronics, among other applications. As these fields are rapidly evolving, it is very strenuous to pinpoint the emerging challenges of the g-C3N4 QDs development and application during the last decade, mainly due to the lack of critical reviews of the innovations in the g-C3N4 QDs synthesis pathways and domains of application. Herein, an extensive survey is conducted on the g-C3N4 QDs synthesis, characterization, and applications. Scenarios for the future development of g-C3N4 QDs and their potential applications are highlighted and discussed in detail. The provided critical section suggests a myriad of opportunities for g-C3N4 QDs, especially for their synthesis and functionalization, where a combination of eco-friendly/single step synthesis and chemical modification may be used to prepare g-C3N4 QDs with, for example, enhanced photoluminescence and production yields.

Visible light- and dark-driven degradation of palm oil mill effluent (POME) over g-C3N4 and photo-rechargeable WO3

The investigations of real industrial wastewater, such as palm oil mill effluent (POME), as a recalcitrant pollutant remain a subject of global water pollution concern. Thus, this work introduced the preparation and modification of g-C3N4 and WO3 at optimum calcination temperature, where they were used as potent visible light-driven photocatalysts in the degradation of POME under visible light irradiation. Herein, g-C3N4-derived melamine and WO3 photocatalyst were obtained at different calcination temperatures in order to tune their light absorption ability and optoelectronics properties. Both photocatalysts were proven to have their distinct phases, crystallinity levels, and elements with increasing temperature, as demonstrated by the ultraviolet-visible spectroscopy (UV-Vis), X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) results. Significantly, g-C3N4 (580 °C) and WO3 (450 °C) unitary photocatalysts exhibited the highest removal efficiency of POME without dilution due to good crystallinity, extended light absorption, high separation, and less recombination efficiency of electron-hole pairs. Furthermore, surprisingly, the superior energy storage photocatalytic performance with outstanding stability by WO3 achieved an approximately 10% increment during darkness, compared with g-C3N4 under visible light irradiation. Moreover, it has been proven that the WO3 and g-C3N4 photocatalysts are desirable photocatalysts for various pollutant degradations, with excellent visible-light utilization and favorable energy storage application.

The synergistic degradation of pollutants in water by photocatalysis and PMS activation

In recent years, the synergistic degradation of water pollutants through advanced oxidation technology has emerged as a prominent research area due to its integration of various advanced oxidation technologies. The combined utilization of peroxymonosulfate (PMS) activation technology and photocatalysis demonstrates mild and nontoxic characteristics, enabling the degradation of water pollutants across a wide pH range. Moreover, this approach reduces the efficiency of electron hole recombination, broadens the catalyst's light response range, facilitates electron transfer of PMS, and ultimately improves its photocatalytic performance. The paper reviews the current research status of photocatalytic technology and PMS activation technology, respectively, while highlighting the advancements achieved through the integration of photocatalytic synergetic PMS activation technology for water pollutant degradation. Furthermore, this review delves into the mechanisms involving both free radicals and nonradicals in the reaction process and presents a promising prospect for future development in water treatment technology. PRACTITIONER POINTS: Degradation of water pollutants by photocatalysis and PMS synergistic action has emerged. Synergism can enhance the generation of free radicals. This technology can provide theoretical support for actual wastewater treatment.

Carbon Nitride Grafting Modification of Poly(lactic acid) to Maximize UV Protection and Mechanical Properties for Packaging Applications

Due to their biobased nature and biodegradability, poly(lactic acid) (PLA) rich blends are promising for processing in the packaging industry. However, pure PLA is brittle and UV transparent, which limits its application, so the exploration of nanocomposites with improved interfacial interactions and UV absorbing properties is worthwhile. We therefore developed and optimized synthesis routes for well-designed nanocomposites based on a PLA matrix and graphitic carbon nitride (g-C3N4; CN) nanofillers. To enhance the interfacial interaction with the PLA matrix, a silane-coupling agent (γ-methacryloxypropyl trimethoxysilane, KH570) is chemically grafted onto the CN surface after controlled oxidation with nitric acid and hydrogen peroxide. Interestingly, only 1 wt % of CNO-KH570, as synthesized under mild conditions, is needed to significantly improve the UV absorption, blocking even a large part of both UV-C, UV-B, and UV-A outperforming the UV absorption performance of PLA and, for instance, polyethylene terephthalate (PET). The low nanofiller loading of 1 wt % also results in a higher ductility with an increase in elongation at break (+73%), maintaining the tensile modulus. The results on a joint optimization of UV protection and mechanical properties are supported by a broad range of experimental characterizations, including FTIR, XRD, DSC, DSEM, FETEM, XPS, FTIR, TGA, and BET N2 adsorption-desorption analysis.

Degradation of malachite green by g-C3N4-modified magnetic attapulgite composites under visible-light conditions

Water resources are seriously threatened by dye wastewater, and the removal of the dye molecules from the wastewater has garnered considerable interest. People have favored photocatalytic technology in recent years for the treatment of dye wastewater. In this work, attapulgite (ATP) was used as a carrier, Fe3O4 and g-C3N4 were grafted onto ATP, and the surface was then modified with polyethyleneimine (PEI) to produce photocatalyst ATP-Fe3O4-g-C3N4-PEI, which was used in Malachite green (MG) dye wastewater. The structure and surface properties of the composites were analyzed and characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray spectrum (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Uv-vis spectrum analysis, zeta potential measurement, and vibrating-sample magnetometry (VSM) analysis. The removal performance of ATP-Fe3O4-gC3N4-PEI for MG was studied, and the removal mechanism was explored and revealed. It has been shown that the heterojunction formed by Fe3O4 and g-C3N4 can inhibit the compounding of photogenerated electrons and holes, effectively improving the performance of the ATP-Fe3O4-g-C3N4-PEI. Electron paramagnetic resonance (EPR) analysis confirmed that ATP-Fe3O4-g-C3N4-PEI could generate hydroxyl radicals (·OH) and superoxide radicals (·O2-) to degrade the MG. It was believed that ATP-Fe3O4-g-C3N4-PEI could generate hydroxyl radicals (·OH) through the photocatalysis and the Fenton reaction of the composite materials. Under the action of H+, ·O2-, and ·OH, the removal rate of MG by ATP-Fe3O4-g-C3N4-PEI exceeded 98 % at an optimal condition. The intermediate products and degradation pathways of MG degradation were also inferred by LC-MS analysis. These results showed that the prepared photocatalyst has excellent degradation performance for MG and could be used in dye wastewater treatment.

Augmented photocatalytic degradation of Acetaminophen using hydrothermally treated g-C3N4 and persulfate under LED irradiation

Photocatalytic degradation of organic pollutants in water using graphitic carbon nitride and persulfate under visible light (g-C₃N₄/PS system) has been studied. Here, we demonstrate augmentation of photocatalytic degradation of Acetaminophen (AAP) using hydrothermally treated g-C₃N₄ and PS under 400 nm LED irradiation (HT-g-C₃N₄/PS system). A pseudo-first-order rate constant (k_obs, 0.328 min^-1) for degradation of AAP using HT-g-C₃N₄/PS system was determined to be 15 times higher compared to g-C₃N₄/PS system (k_obs, 0.022 min^-1). HT-g-C₃N₄ showed a higher surface area (81 m²/g) than g-C₃N₄ (21 m²/g). Photocurrent response for HT-g-C₃N₄ was higher (1.5 times) than g-C₃N₄. Moreover, Nyquist plot semicircle for HT-g-C₃N₄ was smaller compared to g-C₃N₄. These results confirm effective photoelectron-hole separation and charge-transfer in HT-g-C₃N₄ compared to g-C₃N₄. AAP degradation using HT-g-C₃N₄/PS system was significantly inhibited with O2.- and h⁺ scavengers compared to ¹O_2,SO4.- and HO. scavengers. ESR results revealed O2.- formation in HT-g-C₃N₄/PS system. Moreover, photocurrent measurements reveal AAP oxidation by h⁺ of HT-g-C₃N₄ was effective than g-C₃N₄. HT-g-C₃N₄ was reused for five cycles in HT-g-C₃N₄/PS system. Augmented photocatalytic degradation of AAP by HT-g-C₃N₄/PS system compared to g-C₃N₄/PS is attributed to effective photoelectron hole separation of HT-g-C₃N₄ that generates O2.- and h⁺ for oxidation of pollutant. Importantly, electrical energy per order (E_EO) was 7.2 kWh m^-3 order^-1. k_obs for degradation of AAP in simulated groundwater and tap water were determined as 0.029 and 0.035 min^-1, respectively. Degradation intermediates of AAP were proposed. AAP ecotoxicity against marine bacteria Aliivibrio fischeri was completely removed after treatment by HT-g-C₃N₄/PS system.

Peroxymonosulfate Activation by BaTiO3 Piezocatalyst

Peroxymonosulfate (PMS) plays an important role in the advanced oxidation process for environmental remediation. In this study, barium titanate (BTO) piezocatalyst was selected for the activation of PMS driven by ultrasonic power. The degradation of Rhodamine B (RhB) by BTO single component, PMS single component, and BTO/PMS double components were investigated. The results indicated that PMS can be efficiently activated by BTO under an ultrasound with an RhB degradation rate of 98% within 20 min. The ultrasound not only promoted the activation of the PMS itself, but the surface charge carriers of BTO induced by the ultrasound also contributed to the activation of PMS. ·O2−, ·OH, and ·SO4− radicals were found to be the main active species that participated in the reaction. In order to verify the reaction’s environmental applicability, amoxicillin (AMX) as a typical environmental pollutant was studied. BTO/PMS displayed 80% removal efficiency of AMX, and the products generated were less toxic as demonstrated by eco-toxicity comparison. This work provides a promising strategy to improve the utilization of ultrasonic energy and apply it to the field of environmental pollutants treatment.

Treatment of Water Contaminated with Non-Steroidal Anti-Inflammatory Drugs Using Peroxymonosulfate Activated by Calcined Melamine@magnetite Nanoparticles Encapsulated into a Polymeric Matrix

In the present study, calcined melamine (CM) and magnetite nanoparticles (MNPs) were encapsulated in a calcium alginate (CA) matrix to effectively activate peroxymonosulfate (PMS) and generate free radical species for the degradation of ibuprofen (IBP) drug. According to the Langmuir isotherm model, the adsorption capacities of the as-prepared microcapsules and their components were insignificant. The CM/MNPs/CA/PMS process caused the maximum degradation of IBP (62.4%) in 30 min, with a synergy factor of 5.24. Increasing the PMS concentration from 1 to 2 mM improved the degradation efficiency from 62.4 to 68.0%, respectively, while an increase to 3 mM caused a negligible effect on the reactor effectiveness. The process performance was enhanced by ultrasound (77.6% in 30 min), UV irradiation (91.6% in 30 min), and electrochemical process (100% in 20 min). The roles of O•H and SO4•- in the decomposition of IBP by the CM/MNPs/CA/PMS process were 28.0 and 25.4%, respectively. No more than 8% reduction in the degradation efficiency of IBP was observed after four experimental runs, accompanied by negligible leachate of microcapsule components. The bio-assessment results showed a notable reduction in the bio-toxicity during the treatment process based on the specific oxygen uptake rate (SOUR).

gCN-P: a coupled g-C3N4/persulfate system for photocatalytic degradation of organic pollutants under simulated sunlight

A coupled g-C3N4/PDS system, named gCN-P, has been put forward to degrade refractory organic pollutants under simulated sunlight which integrates photocatalysis and PS-AOPs (advanced oxidation of persulfate based on sulfate radicals). The coupled g-C3N4 and PDS showed superior synergistic effect for MO degradation under simulated sunlight. Results showed that almost all MO was removed in the gCN-P system after irradiation for 80 min under simulated sunlight. The degradation rate of gCN-P system was improved by 12.6 and 4.9 times compared to single PDS and g-C3N4 systems, respectively. And only by adding 0.01 g of persulfate into the gCN-P system. The results of quenching experiments and EPR showed that O2-, 1O2 and h+ were main active species for the degradation of MO in the gCN-P system under simulated sunlight. Application of the gCN-P system in tap water samples demonstrated its excellent performance in real-world water environment, and the gCN-P system was employed for removing other new contaminants such as bisphenol A, ciprofloxacin, and paracetamol. The results demonstrated the gCN-P system can effectively remove organic pollutants under sunlight in practices.

Energy efficient photocatalytic activation of peroxymonosulfate by g-C3N4 under 400 nm LED irradiation for degradation of Acid Orange 7

Photocatalytic activation of peroxymonosulfate (PMS) by graphitic carbon nitride (g-C₃N₄) is emerging as a sulfate radical anion based advanced oxidation process (S-AOP) for degradation of organic pollutants. Importantly, photocatalytic activation of PMS by g-C₃N₄ is energy intensive as light irradiation requires high electrical energy. Here, we studied activation of PMS by g-C₃N₄ under 400 nm light emitting diode (LED) irradiation (g-C₃N₄/PMS/400-LED system) for degradation of Acid Orange 7 (AO7). LED array having optical emission maximum around 400 nm (FWHM = 16 nm), with electrical input power of 1.54 W (14 V and 110 mA) was used for irradiation. Pseudo-first order rate constant (k_obs) value for degradation of AO7 by g-C₃N₄/PMS/400-LED system was determined to be 0.094 min^-1. O₂^·-, SO₄^·- were revealed by radical scavenging and ESR investigations. k_obs value in simulated ground and real tap water were determined to be 0.068 min^-1 and 0.063 min^-1, respectively. g-C₃N₄ was stable, and reused four times without any significant loss in its photocatalytic activity. Importantly, electrical energy per order (E_EO) for degradation of AO7 by g-C₃N₄/PMS/400-LED system was determined to be 24.51 kWh m^-3 order^-1. In contrast, the E_EO value for the degradation of AO7 by g-C₃N₄ activated PMS under visible light irradiation (>400 nm), using conventional xenon lamp, (g-C₃N₄/PMS/Vis-Xe system) was found to be very high as 2702 kWh m^-3 order^-1. Thus, the study highlights, LED irradiation source is promising for the development of energy efficient g-C₃N₄ photocatalytic activation of PMS for removal of organic pollutants.

N-doped graphitic C3N4 nanosheets decorated with CoP nanoparticles: A highly efficient activator in singlet oxygen dominated visible-light-driven peroxymonosulfate activation for degradation of pharmaceuticals and personal care products

CoP nanoparticle-loaded N-doped graphitic C₃N₄ nanosheets (CoP/N-g-C₃N₄) were fabricated via a facile three-step method to degrade pharmaceuticals and personal care products (PPCPs) via a visible-light-driven (VLD) peroxymonosulfate (PMS) activation system. 2 ppm carbamazepine (CBZ) can be removed completely within 10 min by the VLD-PMS system with a kinetic constant of k = 0.29128 min^-1, as 25.8 times compared to that under dark conditions (k = 0.01128 min^-1). The experimental and theoretical results showed that the doped graphitic N atoms could modulate the electronic properties of the g-C₃N₄ nanosheets. Subsequently, the Density Functional Theory (DFT) explained that CoP showed preference to bonding with the nitrogen atoms involved in the newly formed N˭N bond, and the Co‒N bond dramatically enhanced the transfer of photo-generated electrons from the N-g-C₃N₄ nanosheets. Electron paramagnetic resonance (EPR) tests show that singlet oxygen (¹O₂) plays a leading role in this case. Moreover, PMS molecules are also tended to be absorbed onto the electron-deficient carbon atoms near the newly formed N˭N bonds for PMS reduction, synergistically enhancing the degradation efficiency for CBZ and benzophenone-3 (BZP). The newly established VLD-PMS activation system was shown to treat the actual sewage in Hong Kong sewage treatment plants (STPs) very well. This work supplements the fundamental theory of radical and non-radical pathways in the sulfate radical (SO₄^•-)-based advanced oxidation process (SR-AOP) for environmental cleanup purposes.

One Spot Microwave Synthesis and Characterization of Nitrogen-Doped Carbon Dots with High Oxygen Content for Fluorometric Determination of Banned Sudan II Dye in Spice Samples

A simple microwave-assisted synthesis of nitrogen-doped carbon dots with high oxygen content (O-N-CDs) was carried out with citric acid as a carbon source and 2,4-diamino-6-methyl-1,3,5-triazine as a nitrogen source in triethylene glycol (TEG) media. It was determined by SEM analysis that O-N-CDs consisted of particles of different sizes and shapes. Transmission electron microscopy (TEM), Raman spectroscopy, and X-ray diffraction (XRD) analysis confirmed that O-N-CDs have a graphitic structure. Moreover, they showed a high fluorescence property based on the excitation wavelength. Therefore, a new fluorometric method was developed for the determination of banned food dye Sudan II by using the O-N-CDs. The proposed method was used in the determination of Sudan II in spiked spice samples. The detection limit was 0.6 mg L^-1 and the linear range was 0-8 mg L^-1.

Facile synthesis of graphitic carbon nitride from acetic acid pretreatment to activate persulfate in presence of blue light for photocatalytic removal of metronidazole

Activation of persulfate (PS) in presence of blue LED light (λ_max ∼454 nm) using acetic acid modified graphitic carbon nitride (ACN) was investigated. Usage of acetic acid had improved the specific surface area (SSA, 21.89 m² g^-1) of ACN compared with pristine graphitic carbon nitride (GCN) and it also reduced interfacial charge transfer resistance in ACN. Subsequently, photocatalytic removal of metronidazole (MET) was investigated using ACN. It was observed that upward shift in the conduction band (CB) in ACN produced the reduction of PS to form sulfate radicals (SO₄^.-) (CB of ACN (-1.25 V vs normal hydrogen electrode (NHE); Bandgap = 2.77 eV) and GCN (-1.23 V vs NHE; Bandgap = 2.73 eV)), which enhanced the MET removal. Moreover, batch experiments were conducted to quantify the effects of PS dosage (0.08-0.40 g L^-1), ACN dosage (0.20-2 g L^-1), light intensity (15-45 W), and pH (2-13.50). ACN (1 g L^-1) and GCN (1 g L^-1) with 0.16 g L^-1 of PS have shown 100% and 76.1% MET (C_o-10 mg L^-1) removal within 300 min, respectively, and the removal followed zero-order kinetics (k ∼2.39 mg L^-1 h^-1). However, MET mineralization was approximately 30% with ACN. MET removal had decreased with increase in pH and almost complete inhibition was observed at pH ∼12. Overall, it was identified that SO₄^.- was the major reactive species whereas holes (h⁺) in the valence band (VB) of ACN (1.52 V vs NHE) played a minor role in MET removal.

Advanced activation of persulfate by polymeric g-C3N4 based photocatalysts for environmental remediation: A review

Photocatalytic materials for photocatalysis is recently proposed as a promising strategy to address environmental remediation. Metal-free graphitic carbon nitride (g-C₃N₄), is an emerging photocatalyst in sulfate radical based advanced oxidation processes. The solar-driven electronic excitations in g-C₃N₄ are capable of peroxo (O‒O) bond dissociation in peroxymonosulfate/peroxydisulfate (PMS/PDS) and oxidants to generate reactive free radicals, namely SO₄^•- and OH^• in addition to O₂^•- radical. The synergistic mechanism of g-C₃N₄ mediated PMS/PDS photocatalytic activation, could ensure the generation of OH^• radicals to overcome the low reductive potential of g-C₃N₄ and fastens the degradation reaction rate. This article reviews recent work on heterojunction formation (type-II heterojunction and direct Z-scheme) to achieve the bandgap for extended visible light absorption and improved charge carrier separation for efficient photocatalytic efficiency. Focus is placed on the fundamental mechanistic routes followed for PMS/PDS photocatalytic activation over g-C₃N₄-based photocatalysts. A particular emphasis is given to the factors influencing the PMS/PDS photocatalytic activation mechanism and the contribution of SO₄^•- and OH^• radicals that are not thoroughly investigated and require further studies. Concluding perspectives on the challenges and opportunities to design highly efficient persulfate-activated g-C₃N₄ based photocatalysts toward environmental remediation are also intensively highlighted.

Thiosulfate enhanced degradation of organic pollutants in aqueous solution with g-C3N4 under visible light irradiation

Developing new strategies to design more practicable and efficient g-C₃N₄ based photocatalysts is important to solve the environmental issues. Thiosulfate (STS) is a common residual product found in wastewater and removal of STS remains a matter of great environmental concern. In this work, however, STS is activated by g-C₃N₄ under visible light irradiation, resulting in a fast degradation of Rhodamine B (RhB) and other pollutants. The performance of g-C₃N₄ prepared from urea was much higher than that from melamine, due to the higher surface area and more negative conduction band potential of the former catalyst. In addition, comparison with other oxidants and reductants such as peroxymonosulfate, peroxydisulfate, hydrogen peroxide and sulfite, the use of STS in g-C₃N₄/Vis system showed the highest efficiency for RhB degradation. During ten successive cycles, the excellent reusability of the catalyst was also obtained. The effect of different concentrations of STS and g-C₃N₄, and initial solution pH on the performance of the system were also studied. The mechanism study suggests that STS is first oxidized to S₂O₃^- radicals by photohole, which will be transformed to other oxysulfur radicals such as SO₃^- and finally to SO₄^2- ions. At the same time, the rate of O₂ reduction by photoelectrons to O₂^- radicals as well as RhB degradation increases. The finding of this study provides a promising advanced oxidation process for organic pollutants degradation via STS activation.

Immobilization of Exfoliated g-C3N4 for Photocatalytical Removal of Organic Pollutants from Water

Graphitic carbon nitride (g-C3N4) was synthesized from melamine and exfoliated by thermal treatment. Exfoliated g-C3N4 particles were immobilized by electrophoretic deposition from an ultrasonically treated ethanolic suspension aged up to 12 weeks. During the aging of the suspension, the separation of particles bigger than 10 μm was observed. The separated stable part of the suspension contained particles with a relatively uniform size distribution, enabling the fabrication of g-C3N4 films that were stable in a stirred aqueous solution. Such stable immobilized particles of exfoliated g-C3N4 are reported for the first time. The photocatalytic activity of such layers was evaluated using aqueous solutions of Acid Orange 7 (AO7) and 4-chlorophenol (4-CP). The photocatalytic decomposition of AO7 was faster in comparison with the decomposition of 4-CP. Mineralization was observed in the case of AO7, but not in the case of 4-CP, where the decrease of 4-CP concentration is due to 4-CP polymerization and the formation of a dimer, C12H8Cl2O2. This indicates that the use of g-C3N4 as a photocatalyst for oxidative degradation of organic compounds in water is limited.

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.

Peroxymonosulfate-enhanced photocatalysis by carbonyl-modified g-C3N4 for effective degradation of the tetracycline hydrochloride

In this work, carbonyl-modified g-C₃N₄ (CO-C₃N₄) is prepared through one-step calcination of the melamine-oxalic acid aggregates. The visible light-assisted photocatalytic degradation efficiency of the tetracycline hydrochloride (TCH) for CO-C₃N₄ is significantly enhanced by introducing the peroxymonosulfate (PMS), and the apparent rate constant is greatly increased from 0.01966 min^-1 in CO-C₃N₄/vis system to 0.07688 min^-1 in CO-C₃N₄/PMS/vis system. It is found that carbonyl for CO-C₃N₄ might offer possible reactive sites for PMS activation and collection sites of photo-generated electrons, greatly accelerating carrier's separation for PMS activation. The favorable conditions, such as the higher catalyst dosage, higher PMS amount and alkaline pH, contribute to TCH degradation. The deleterious effects of co-existing anions on the TCH degradation efficiency are ranked in a decline: H₂PO₄^- > SO₄^2- > HCO₃^- > NO₃^- > Cl^-, and it may be affected by the type and amounts of anions and active radicals generated. The radical trapping tests and electron spin resonance (ESR) detection display that the O₂^-, h⁺, ¹O₂, OH and SO₄^- all contribute to TCH degradation. Meanwhile, possible degradation mechanism, intermediates and degradation pathway of TCH are revealed in CO-C₃N₄/PMS/vis system. This study will offer a new insight for constructing PMS activation with carbonyl modified g-C₃N₄ photocatalysis system to achieve effective treatment of organic wastewater.

Emerging Chemical Functionalization of g-C3N4: Covalent/Noncovalent Modifications and Applications

Atomically 2D thin-layered structures, such as graphene nanosheets, graphitic carbon nitride nanosheets (g-C₃N₄), hexagonal boron nitride, and transition metal dichalcogenides are emerging as fascinating materials for a good array of domains owing to their rare physicochemical characteristics. In particular, graphitic carbon nitride has turned into a hot subject in the scientific community due to numerous qualities such as simple preparation, electrochemical properties, high adsorption capacity, good photochemical properties, thermal stability, and acid-alkali chemical resistance, etc. Basically, g-C₃N₄ is considered as a polymeric material consisting of N and C atoms forming a tri-s-triazine network connected by planar amino groups. In comparison with most C-based materials, g-C₃N₄ possesses electron-rich characteristics, basic moieties, and hydrogen-bonding groups owing to the presence of hydrogen and nitrogen atoms; therefore, it is taken into account as an interesting nominee to further complement carbon in applications of functional materials. Nevertheless, g-C₃N₄ has some intrinsic limitations and drawbacks mainly related to a relatively poor specific surface area, rapid charge recombination, a limited light absorption range, and a poor dispersibility in both aqueous and organic mediums. To overcome these shortcomings, numerous chemical modification approaches have been conducted with the aim of expanding the range of application of g-C₃N₄ and enhancing its properties. In the current review, the comprehensive survey is conducted on g-C₃N₄ chemical functionalization strategies including covalent and noncovalent approaches. Covalent approaches consist of establishing covalent linkage between the g-C₃N₄ structure and the chemical modifier such as oxidation/carboxylation, amidation, polymer grafting, etc., whereas the noncovalent approaches mainly consist of physical bonding and intermolecular interaction such as van der Waals interactions, electrostatic interactions, π-π interactions, and so on. Furthermore, the preparation, characterization, and diverse applications of functionalized g-C₃N₄ in various domains are described and recapped. We believe that this work will inspire scientists and readers to conduct research with the aim of exploring other functionalization strategies for this material in numerous applications.

Synergistic effect of persulfate and g-C3N4 under simulated solar light irradiation: Implication for the degradation of sulfamethoxazole

A method combining g-C₃N₄ and potassium peroxydisulfate (PDS) under simulated sunlight was put forward to effectively degrade sulfamethoxazole (SMX). The SMX removal efficiency was substantially improved compared with the processes involving only g-C₃N₄ or PDS. The kinetic constants for the g-C₃N_4, PDS and g-C₃N₄/PDS systems were 0.0023, 0.0239 and 0.068 min^-1, respectively. The g-C₃N₄/PDS process reached an SMX removal rate of 98.4 % after 60 min of simulated sunlight; in addition, the proposed system showed desirable efficiency for SMX degradation in two different actual water samples as well. The reaction mechanism was illustrated by trapping experiments, which showed that g-C₃N₄ can promote S₂O₈^2- to transfer SO₄^-, S₂O₈^2- favored the generation of O₂^-, and O₂^-, SO₄^- and holes (h⁺) were the main oxidative species for the SMX degradation in the combined reaction process under simulated sunlight. Then, to further explore this mechanism, the intermediates generated during the combined reaction process were analyzed by LC/MS and possible degradation pathways were proposed. The result showed that the breaking of the SN and C-S bonds, the hydroxylation of the benzene ring and the oxidation of the amino group were identified as the main pathways in the SMX degradation process by the g-C₃N₄/PDS system under simulated sunlight.

Electrochemical sensor based on an electrode modified with porous graphitic carbon nitride nanosheets (C3N4) embedded in graphene oxide for simultaneous determination of ascorbic acid, dopamine and uric acid

Two-dimensional porous graphitic carbon nitride (g-C₃N₄) nanosheets were synthesized by low-cost and direct thermal oxidation. Porous g-C₃N₄ assembled with graphene oxide (GO) was immobilized on a glassy carbon electrode. The sensor was applied to simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA) with high performance. Cyclic voltammetry and differential pulse voltammetry were used to investigate electrochemical and electrocatalytic properties. The results indicate that the electrochemical sensor possesses high specific surface area, hierarchical pore structure and excellent signal response to AA, DA and UA. The oxidation potentials are well separated at around 0.15, 0.34 and 0.46 V for AA, DA and UA respectively. The determination limits for AA, DA and UA are 3.7 μM, 0.07 μM and 0.43 μM, respectively. The sensor was applied to tracking the three analytes in spiked serum samples with recovery 95.1~105.5% and relation standard deviations of less than 5%. Graphical abstract Schematic representation of porous graphitic carbon nitride nanosheet embedded in graphene oxide for simultaneous determination of ascorbic acid, dopamine and uric acid.

Enhancement of photocatalytic activity of g-C3N4 by hydrochloric acid treatment of melamine

Modified g-C₃N₄ samples (g-X, where X corresponds to the number of hours of acid treatment of the melamine) with outstanding photocatalytic performance were prepared by using hydrochloric acid-treated melamine as a precursor and calcining at 550 °C for 2 h. An x-ray diffractometer, field-emission scanning electron microscope, infrared spectrometer, N₂ adsorption-desorption test, x-ray photoelectron spectroscopy, and ultraviolet-visible diffuse-reflectance spectroscopy analysis were carried out to characterize the phase composition, microstructure, chemical structure, specific surface area (SSA), chemical states, elemental composition and optical properties of the samples, respectively. The photocatalytic performance of the samples was evaluated by degrading the Rhodamine B (RhB) aqueous solution. The results showed that the crystal structure and vibration bands of melamine changed due to the reaction with hydrochloric acid. The crystallinity and grain size of g-C₃N₄ in g-X (X = 1, 2, 4, 6, 8, 10) reduced, and the SSA values of g-X increased compared to that of the g-0 sample, which was synthesized from pristine melamine. The g-X samples exhibited excellent photocatalytic activity towards degradation of RhB compared to g-0. The photocatalytic activity of the g-X samples increased gradually as the acid treatment time of the melamine increased from 1 h to 2 h, and then decreased gradually with the extension of the acid treatment time. The rate constant (k) values of g-X are higher than that of g-0. g-2 presented the highest rate constant (k = 0.052 min^-1), which was 5.5 times higher than that of g-0. The improved photocatalytic activity of the g-X samples was attributed to the higher SSA value, the appearance of surface defects, the outstanding photo-carrier separation efficiency and stronger light harvesting ability of g-X, with the last two factors being more significant. Acid treatment of melamine is helpful in the preparation of high performance g-C₃N₄ photocatalyst, and the microstructure and photocatalytic performance of g-C₃N₄ were affected significantly by the acid treatment time.

Peroxymonosulfate improved photocatalytic degradation of atrazine by activated carbon/graphitic carbon nitride composite under visible light irradiation

The photocatalytic degradation of atrazine by activated carbon/graphitic carbon nitride composites with peroxymonosulfate (PMS) was investigated under visible light irradiation. The photocatalysts were prepared at different activated carbon (AC) loaded percentages and characterized by XRD, FT-IR, BET surface area, SEM, UV-Vis absorbance, photocurrent response and EIS. Several parameters which might influence the degradation efficiency were studied including PMS concentration, solution pH, catalyst dosage, initial atrazine concentration as well as water matrix effect. The results indicated that incorporation of AC contributes effectively in suppressing the recombination of electron-holes pairs and enhancing the photocatalytic performance of graphitic carbon nitride. More significantly, the degradation efficiency of atrazine showed remarkable improvement with PMS addition under visible light irradiation. The reaction rate constant of the 10% AC/g-C₃N₄/Vis/PMS system (0.0376 min^-1) was approximately 2.9 times higher than that of g-C₃N₄/Vis/PMS system (0.0128 min^-1). Results from quenching tests revealed that both sulfate and hydroxyl radicals were involved in the degradation of atrazine, while the latter is the main contributor. This paper constitutes an insight for the metal-free catalyst activation of PMS by photocatalysis for environmental remediation.

Photocatalytic degradation of organic pollutants in wastewater with g-C3N4/sulfite system under visible light irradiation

To develop low cost and high efficient sulfate radical (SO₄^-) based advanced oxidation processes (AOPs) for rapid remediation of contaminated waters is of great interest. In this study, a green and novel SO₄^- based AOPs, in situ visible light activation of sulfite by graphitic carbon nitride (g-C₃N₄), for the degradation of organic pollutants is reported. The g-C₃N₄+HSO₃^- + Vis system could achieve remarkably enhanced degradation of organic pollutants such as organic dyes and phenol in aqueous solution. The excellent reusability of the metal free catalyst was also observed during ten successive cycles. The efficiency of the system was dependent on the reaction conditions, which first increased and then decreased with the increase of HSO₃^- concentration and initial solution pH. The addition of HCO₃^- stimulated the pollutant degradation, but other water matrix components such as Cl^- and humic acid showed nearly no influence on the reaction. The mechanism investigations suggested that sulfite is oxidized in the system to sulfite radicals, which then react with dioxygen and superoxide radicals to form SO₅^- radicals and HSO₅^- respectively. SO₅^- radicals can be also reduced by sulfite or photoelectron to HSO₅^-. SO₄^- radicals were then produced from HSO₅^- reduction by photoelectron, and contributed to dye degradation in the system together with superoxide radicals. This study provides a novel new approach for efficient degradation of organic degradation via sulfite activation.

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.