Degradation of Acid Orange 7 using peroxymonosulfate catalyzed by granulated activated carbon and enhanced by electrolysis

Carbonaceous materials in structural dimensions for advanced oxidation processes

Carbonaceous materials have attracted extensive research and application interests in water treatment owing to their advantageous structural and physicochemical properties. Despite the significant interest and ongoing debates on the mechanisms through which carbonaceous materials facilitate advanced oxidation processes (AOPs), a systematic summary of carbon materials across all dimensions (0D-3D nanocarbon to bulk carbon) in various AOP systems remains absent. Addressing this gap, the current review presents a comprehensive analysis of various carbon/oxidant systems, exploring carbon quantum dots (0D), nanodiamonds (0D), carbon nanotubes (1D), graphene derivatives (2D), nanoporous carbon (3D), and biochar (bulk 3D), across different oxidant systems: persulfates (peroxymonosulfate/peroxydisulfate), ozone, hydrogen peroxide, and high-valent metals (Mn(VII)/Fe(VI)). Our discussion is anchored on the identification of active sites and elucidation of catalytic mechanisms, spanning both radical and nonradical pathways. By dissecting catalysis-related factors such as sp2/sp3 C, defects, and surface functional groups that include heteroatoms and oxygen groups in different carbon configurations, this review aims to provide a holistic understanding of the catalytic nature of different dimensional carbonaceous materials in AOPs. Furthermore, we address current challenges and underscore the potential for optimizing and innovating water treatment methodologies through the strategic application of carbon-based catalysts. Finally, prospects for future investigations and the associated bottlenecks are proposed.

Selective generation of hydroxyl and sulfate radicals under electric field regulation for micropollutants degradation: Mechanism and structure-activity relationship

Peroxymonosulfate (PMS) activation generates potent reactive oxygen species (ROS) such as sulfate radical (SO4·-) and hydroxyl radical (·OH), which play a key role in organic pollutant degradation. However, controlling the generation of these free radicals remains challenging. In this study, various metal (Co, Ni, and Cu)-doped nitrogen carbon compounds (NCs) were synthesized, and their performance in PMS activation under electric field regulation was explored to modulate ROS production for selective pollutant degradation. Bisphenol A (BPA), a readily degradable compound, and ibuprofen (IBU), a recalcitrant pollutant, were chosen as model pollutants to assess degradation efficiency. All catalysts achieved over 95 % BPA removal without the electric field, but the application of an electric field significantly accelerated BPA degradation, achieving complete removal within 3 min. In contrast, IBU degradation showed significant variation depending on the catalyst used and the electric field intensity, with Cu-NC demonstrating the highest performance, enhancing the degradation rate by 3.78-fold. Mechanistic studies revealed that the electric field altered the electron density on the catalyst surface, shifting ROS production from SO4·- to·OH in Co-NC systems. The findings could provide valuable insights into PMS activation under electric field regulation, offering a novel strategy for enhancing micropollutant removal through controlled ROS generation.

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).

Insight into the roles of microenvironment and active site on the mechanism regulation in metal-free persulfate activation process coupling with an electric field

The regulation of the persulfate activation mechanism is highly desirable and meaningful for the treatment of different wastewaters. The role of active sites for mechanism regulation in carbon-driven persulfate activation is still ambiguous due to the complex and easily neglected microenvironment (concentration distributions of organics and oxidants) nearby carbon catalyst. This work aims to reveal the critical roles of active site and microenvironment on the activation mechanism through N-doped modification and application of an electric field (AC/PS/EC). Several N-doped activated carbon catalysts were prepared by activating for different times to adjust the surface active center and adsorption selectivity under an electric field. The contribution ratio of radical pathway and non-radical pathway for organic elimination significantly varied with the concentration distribution of organics and oxidants nearby the microelectrodes. The increased electro-adsorption of persulfate anion was found to be the primary promoting factor for the radical pathway for organic oxidation, resulting in a synergistic increase in degradation rate in the AC/PS/EC system. The quantitative structure-activity relationships analysis also revealed that the electro-adsorption selectivity was dominated by the surface graphitic N and pore structure of catalyst. This study sheds new light on the oxidative pathway regulation to deal with complex wastewater in a flexible and efficient manner.

Electro-Induced Carbon Nanotube Discrete Electrodes for Sustainable Persulfate Activation

In electrochemical advanced oxidation processes (EAOPs), the rate-limiting step is the mass transfer of pollutants to the electrodes due to the limited active surface areas. To this end, we established a three-dimensional (3D) EAOP system by coupling conventional graphite electrodes with dispersed carbon nanotubes (CNTs). The electrodes (particularly the anode) induced electric field spontaneously polarized CNTs into dispersed reactive particle electrodes (CNT-PEs) in the solution, which remarkably promoted electrochemical activation of peroxydisulfate (PDS) to generate surface CNT-PDS* complexes and surface-bound radicals (SBRs). Based on the excited potential (E_CNT-PEs) at different positions in the 3D electric field, CNT-PEs were activated into three states. (i) E_CNT-PEs < E_organic, CNT-PEs are chemically inert toward DCP oxidation; (ii) E_organic < E_CNT-PEs < E_water, CNT-PEs will oxidize DCP via an electron-transfer process (ETP); (iii) E_CNT-PEs > E_water, both CNT-PDS* complexes and the anode will oxidize water to produce SBRs. Thus, DCP could be oxidized by CNT-PDS* complexes via ETP to form polychlorophenols on the CNT surface, causing rapid deactivation of the micro-electrodes. In contrast, SBRs attack DCP directly into chloride ions and hydroxylated products, maintaining the surface cleanliness and activity of CNT-PEs for long-term operations.

Peroxymonosulfate enhanced photoelectrocatalytic oxidation of organic contaminants and simultaneously cathodic recycling of silver

Degradation of organic contaminants with simultaneous recycling of Ag⁺ from silver-containing organic wastewater such as photographic effluents is desired. Although photoelectrocatalysis (PEC) technology is a good candidate for this type of wastewater, its reaction kinetics still needs to be improved. Herein, peroxymonosulfate (PMS) was employed to enhance the PEC kinetics for oxidation of phenol (PhOH) at the anode and reduction of Ag⁺ at the cathode. The degradation efficiency of phenol (PhOH, 0.1 mmol/L) was increased from 42.8% to 96.9% by adding 5 mmol/L PMS at a potential of 0.25 V. Meanwhile, the Ag (by wt%) deposited on the cathode was 28.1% (Ag₂O) in PEC process, while that of Ag (by wt%) was 69.7% (Ag⁰) by adding PMS. According to the electrochemistry analysis, PMS, as photoelectrons acceptor, enhances the separation efficiency of charges and the direct h⁺ oxidation of PhOH at the photoanode. Meantime, the increasing cathode potential avoided H₂ evolution and strongly alkaline at the surface of cathode, thus enabling the deposition of Ag⁺ in the form of metallic silver with the help of PMS. In addition, PMS combined with PEC process was effective in treating photographic effluents.

Heterogeneous Metal-Activated Persulfate and Electrochemically Activated Persulfate: A Review

The problem of organic pollution in wastewater is an important challenge due to its negative impact on the aquatic environment and human health. This review provides an outline of the research status for a sulfate-based advanced oxidation process in the removal of organic pollutants from water. The progress for metal catalyst activation and electrochemical activation is summarized including the use of catalyst-activated peroxymonosulfate (PMS) and peroxydisulfate (PDS) to generate hydroxyl radicals and sulfate radicals to degrade pollutants in water. This review covers mainly single metal (e.g., cobalt, copper, iron and manganese) and mixed metal catalyst activation as well as electrochemical activation in recent years. The leaching of metal ions in transition metal catalysts, the application of mixed metals, and the combination with the electrochemical process are summarized. The research and development process of the electrochemical activation process for the degradation of the main pollutants is also described in detail.

Sulfate radicals induced from peroxymonosulfate on electrochemically synthesized TiO2-MoO3 heterostructure with Ti-O-Mo bond charge transfer pathway for potential organic pollutant removal under solar light irradiation

Here, a novel method for synthesis of heterostructured TiO₂-MoO₃ (MT) nanosheets photocatalyst by utilizing a facile electrochemical method and examined it's photocatalytic activity by the degradation of tetracycline hydrochloride (TCH), a model of organic pollutants, in the presence of peroxymonosulfate (PMS) under solar light irradiation (SL) was reported for the first time. The influence of several factors on the degradation efficiency including the initial concentration of TCH, solution pH, catalyst dosage, PMS concentration, and the existence of inorganic anions was explored. The MT-15/PMS system displayed a promising photocatalytic performance and up to 97% of TCH was degraded in 90 min the rate of the degradation reaction of MT-15/PMS was the highest (0.05299 min^-1) compared to 0.00251, 0.00337, 0.00546, 0.00735, 0.01337min^-1of TiO₂-P25, TiO₂-P25/PMS, MoO₃, MoO₃/PMS, and MT-15 respectively. The enhancement can be attributed to several reasons. First, the 2D morphology of the optimized heterostructure photocatalyst plays a significant role in providing much more active sites on its surface. Next, the boosted light absorption efficiency and higher photoproduced electron-hole pair separation ability, induced by the unique direct transformation of photogenerated electrons from the valance band of TiO₂ to the conduction band of MoO₃ via the Ti-O-Mo bond formed at the interface of MT heterostructure. Finally, the appropriate accessible reactive sites for the activation of PMS together with the synergistic effect between activation of PMS and photocatalytic processes eased the production of active species for the degradation of pollutants. Based on the scavenger experiments and EPR analysis, hydroxide and sulfate radicals were found to be the dominant free radical active species in the degradation process. Furthermore, the synergistic degradation reaction mechanism was proposed.

Deep oxidation of norfloxacin by the electrochemical enhanced heterogeneous catalytic oxidation: The role of electric field and reaction optimization

In this study, electrochemical (EC_G-G: graphite anode and cathode, EC_I-G: iron anode and graphite cathode) enhanced heterogeneous activation of peroxymonosulfate (PMS) by CoFe₂O₄ nanoparticles for the degradation of norfloxacin (NOR) in water was investigated. Although a higher NOR removal efficiency was achieved in EC_I-G/CoFe₂O₄/PMS system, the generation of Fe³⁺ had resulted in the deposition of iron mud and affect the recovery of CoFe₂O₄. Under the optimum reaction conditions of CoFe₂O₄/PMS system, the final removal efficiency of NOR did not show significant difference in EC_G-G/CoFe₂O₄/PMS system (96.0%) and CoFe₂O₄/PMS system (95.5%), but the value of apparent rate constant significantly increased in EC_G-G/CoFe₂O₄/PMS system (0.21 min^-1) compared with CoFe₂O₄/PMS system (0.11 min^-1). Similar NOR degradation pathways were obtained in these two systems, and the TOC removal efficiency in EC_G-G/CoFe₂O₄/PMS system (28.8%) is almost as low as CoFe₂O₄/PMS system (26.0%). Therefore, it can be proposed that the applied electric field through active electrodes can accelerate the reaction of heterogeneous catalytic oxidation, but does not participate much in NOR degradation. However, the TOC removal efficiency (30 min) could be reached 68.7% as the mass ratio of PMS to CoFe₂O₄ increased to 5:1 (250 mg L^-1: 50 mg L^-1). The EC_G-G/CoFe₂O₄/PMS system is a promising low-cost technique for efficient mineralization of antibiotics in wastewater.

Carbon nanotube-based materials for persulfate activation to degrade organic contaminants: Properties, mechanisms and modification insights

Removal of harmful organic matters from environment has great environmental significance. Carbon nanotube (CNT) materials and their composites have been demonstrated to possess excellent catalytic activity towards persulfate (PS) activation for the degradation of organic contaminants. Herein, detailed information concerning the function, modification methods and relevant mechanisms of CNT in persulfate-based advanced oxidation processes (PS-AOPs) for organic pollutant elimination has been reviewed. The activation mechanism of PS by CNT might include radical and nonradical pathways and their synergistic effects. The common strategies to improve the stability and catalytic capability of CNT-based materials have also been put forward. Furthermore, their practical application potential compared with other catalysts has been described. Finally, the challenges faced by CNT in practical application are clearly highlighted. This review should be of value in promoting the research of PS activation by CNT-based materials for degradation of organic pollutants and the corresponding practical applications.

A new spent coffee grounds based biochar - Persulfate catalytic system for enhancement of urea removal in reclaimed water for ultrapure water production

The demand for ultrapure water (UPW) in the semiconductor industry has increased in recent years, while the idea to use reclaimed water instead of tap water for UPW production has also attracted more attention. However, since urea concentration in reclaimed water is higher than that in tap water, UPW production has not been efficient. To resolve this problem, this study aims to develop a new spent coffee grounds based biochar (SCG-BC)/persulfate catalytic system as a pretreatment unit. The objective is to enhance urea removal from reclaimed water so that UPW production is more effective. In this study, the biochar used was prepared from spent coffee grounds with detailed characterization. Results strongly suggested that the urea removed by SCG-BC/persulfate catalytic system was very encouraging (up to 73%). The best possible dosages for SCG-BC and persulfate for urea removal were 0.2 and 2.0 g L^-1, respectively. Furthermore, this system could remove urea effectively in a wide range of pH (3-10). Moreover, the characterizations of SCG-BC (graphite C, defective edges and functional groups, i.e. -OH, CO, carboxyl C-O) helped to activate persulfate in the catalytic process. OH^• and SO₄^• - were all involved in this process, while the SO₄^• - was the main radical for urea degradation.

A heterogeneous peroxymonosulfate catalyst built by Fe-based metal-organic framework for the dye degradation

The regulatory control on dyes is an important issue, as their discharge into the environment can pose significant risks to human health. MIL-101(Fe) prepared by a solvothermal method was used as a catalyst to generate sulfate (SO₄^•-) and hydroxyl (HO^•) radicals from peroxymonosulfate (PMS) for the treatment of orange G (OG). The structural properties of MIL-101(Fe) were assessed by a number of characterization approaches (e.g., Fourier-transform infrared spectroscopy). The factors controlling the removal of OG were explored by a response surface methodology with central composite design (RSM-CCD) plus adaptive neuro-fuzzy inference system (ANFIS). The synthetized MIL-101(Fe) had uniform octahedral nanocrystals with rough surfaces and porous structures. The maximum catalytic removal efficiency of OG with MIL-101(Fe)/PMS process was 74% (the final concentration of Fe²⁺ as 0.19 mg/L and reaction rate of 434.2 μmol/g/h). The catalytic removal of OG could be defined by the non-linear kinetic models based on RSM. The OG removal efficiency declined noticeably with the addition of radical scavengers such as ethanol (EtOH) and tert-butanol (TBA) along with some mineral anions. Accordingly, MIL-101(Fe)/PMS is identified as an effective remediation option for the dyes based on advanced oxidation process (AOPs) based on high treatment efficiency at low dosage of low cost catalyst.

Nitrate Removal in an Electrically Charged Granular-Activated Carbon Column

Nitrate removal from groundwater remains a challenge. Here, we report on the development of a flow-through, electrically charged, granular-activated carbon (GAC)-filled column, which effectively removes nitrate. In this system, the GAC functioned as an anode, while a titanium sheet acted as a cathode. The high removal rate of nitrate was achieved through a combination of electrosorption and electrochemical transformation to N₂. The column could be readily regenerated in situ by reversing the polarity of the applied potential. We demonstrate that in the presence of chloride, the mechanism responsible for the observed nitrate removal involves a combination of electroadsorption of nitrate to the anodically charged GAC, electroreduction of nitrate to ammonium, and the oxidation of ammonium to N₂ gas by reactive chlorine and other oxidative radicals (with nearly 100% N₂ selectivity). Given the ubiquitous presence of chloride in groundwater, this method represents a ready, green, and sustainable treatment process with significant potential for the remediation of contaminated groundwater.

The role of PAC adsorption-catalytic oxidation in the ultrafiltration performance for treating natural water: Efficiency improvement, fouling mitigation and mechanisms

Powdered activated carbon (PAC) has turned out to be an efficient adsorbent in drinking water treatment, whereas its application integrated with membrane filtration is still controversial because of the combined fouling effect between organic pollutants and PAC. To this end, an integrated process of combining PAC adsorption-catalytic oxidation and membrane filtration was proposed for natural surface water treatment. The synergistic effect of PAC and peroxymonosulfate (PMS) was confirmed through the generation of reactive oxidation species, and both radical oxidative pathways (^•OH, SO₄^•- and O₂^•-) and nonradical (¹O₂ and PMS) pathways involved in the process. The removal efficiency of DOC and UV₂₅₄ was significantly strengthened by PAC/PMS, with removal rates of 56.1% and 64.9%, respectively. The integration of PAC and PMS could significantly enhance the reduction of fluorescent organics, and pollutants with varying molecular weights. The fouling condition of membrane was dramatically alleviated, with the flux increased by 38.9%, and the reversible and irreversible resistances declined by 79.7% and 48.3%, respectively. The major fouling mechanism was significantly changed, and complete pore blocking always played a dominant role, rather than cake filtration. The effectiveness of PAC/PMS was further verified by the characterization of membrane surface morphologies and functional groups. Moreover, the attractive interactions between foulants and membrane were converted to repulsive interactions with the pretreatment of PAC/PMS. The proposed synergistic process was efficient and convenient, which could significantly improve the purification efficiency of conventional PAC-UF system in drinking water treatment.

Influence of dimethylphenol isomers on electrochemical degradation: Kinetics, intermediates, and DFT calculation

Dimethylphenol isomers (DMP) pose a great threat to the environment, and the electrooxidation (EO) process proves to be an extraordinarily effective method to degrade DMP. However, the EO performance is affected by the molecular structure of DMP and the adopted experimental parameters. In this study, the effects of 2,4-DMP and 2,6-DMP on the working potential, limiting current density (J_lim), and pH were systematically analysed, with Ti-mesh plates used as the cathode and Ti/PbO₂ as the anode. The peak potentials of 2,4-DMP and 2,6-DMP were determined to be 0.83 V and 0.77 V by cyclic voltammetry, with J_lim were 2.5 mA·cm^-2 and 2.0 mA·cm^-2, respectively. The whole process exhibited pseudo-first-order kinetics, and the kinetic constants (K) for the degradation of 2,4-DMP and 2,6-DMP were determined to be 0.0041 min^-1 and 0.0150 min^-1, respectively. Additionally, the optimal initial pH value for 2,4-DMP and 2,6-DMP was 5.0, where the highest hydroxyl (OH) radical density, as determined by the electron spin technique (ESR), was achieved at a higher current density. Comparatively, the OH radical density in the 2,6-DMP solution was lower than that in 2,4-DMP. In situ Fourier infrared (FT-IR) spectroscopy, GC-MS, and density functional theory (DFT) were employed to explore three possible degradation pathways. The main intermediates for 2,4-DMP degradation were determined to be quinone and ether, while that for 2,6-DMP degradation was quinone. According to the results of this study, the molecular structure (different methyl group positions on the benzene ring) has a great influence on the EO process.

Synergetic metronidazole removal from aqueous solutions using combination of electro-persulfate process with magnetic Fe3O4@AC nanocomposites: nonlinear fitting of isotherms and kinetic models

The removal of metronidazole (MNZ) from aqueous solutions by the electro-persulfate (EC–PS) process was performed in combination with magnetic Fe3O4@activated carbon (AC) nanocomposite. In the first step, the Fe3O4@AC nanocomposites were synthesized and characterized using energy-dispersive X-ray spectroscopy (XRD), vibrating-sample magnetometer (VSM) and field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), mapping, and Fourier-transform infrared spectroscopy (FTIR) analysis. The effect of Fe3O4@AC, PS and EC processes were studied separately and in combination and finally, the appropriate process for MNZ removal was selected. The effect of key parameters on the EC–Fe3O4@AC–PS process including pH, Fe3O4@AC dosage, initial MNZ concentration, and PS concentration were investigated. Based on the results obtained, the Fe3O4@AC had a good structure. The MNZ removal in EC, PS, Fe3O4@AC, EC–Fe3O4@AC, EC–PS, EC–Fe3O4@AC–NaCl, EC–Fe3O4@AC–PS, and EC–Fe3O4@AC–PS–NaCl processes were 0, 0, 59.68, 62, 68.94, 67.71, 87.23 and 88%, respectively. Due to the low effect of NaCl insertion on the EC–Fe3O4@AC–PS process, it was not added into the reactor and optimum conditions for the EC–Fe3O4@AC–PS process were determined. Under ideal conditions, including MNZ = 40 mg/L, Fe3O4@AC dose = 1 g/L, pH = 3, PS concentration = 1.68 mM, current density (CD) = 0.6 mA/cm2 and time = 80 min, the MNZ removal was 92%. Kinetic study showed that the pseudo-second-order model was compatible with the obtained results. In the isotherm studies, the Langmuir model was the most consistent for the data of the present study, and the Q max for Fe3O4@AC dose from 0.25 to 1 g/L was 332 to 125 mg/g, respectively.

Enhanced organics degradation by three-dimensional (3D) electrochemical activation of persulfate using sulfur-doped carbon particle electrode: The role of thiophene sulfur functional group and specific capacitance

For further enhancing the electrochemical oxidation performance, sulfur-doped carbon particle electrode was employed in the three-dimensional (3D) electro-assisted activation of persulfate process (ACS/PS/EC). Herein, an in situ S-doped activated carbon (ACS) was prepared and applied as the particle electrode as well as catalyst in ACS/PS/EC system. Several carbon particle electrodes with different annealing temperature were prepared and characterized via EA, BET, XPS and Raman spectra. Cyclic voltammetry (CV) was perform to obtain the specific capacitance and investigate the interfacial electron transfer of ACS particle. The results of comparative experiments showed significant synergy between electric and catalytic activations of PS. Especially, the as-prepared sample treated at 850 °C (ACS-850) exhibited an outstanding catalytic performance, and the phenol degradation rate was greatly improved by nearly 100% with the application of electric field. By comparing of several carbon particle electrodes with different functional groups and specific capacitances, it is revealed that thiophene sulfur functional group is the mainly active site for both electric and catalytic activation of PS, and the specific capacitance acts as assistant factor. Quenching experiments proved that the 3D electro-assisted activation of PS proceeded through both radical and non-radical pathway. Possible mechanism for ACS/PS/EC electrochemical process was proposed.

Insight into the degradation of p-nitrophenol by visible-light-induced activation of peroxymonosulfate over Ag/ZnO heterojunction

In this report, the peroxymonosulfate activation over Ag/ZnO heterojunction under visible light (Ag/ZnO/PMS/Vis) for p-nitrophenol (p-NP) contaminant degradation was conducted in detail. Herein, the catalyst dosage was decreased, and the results showed that a dosage of 0.5 g L^-1 Ag/ZnO and 4 mM PMS almost completely degraded 30 mg L^-1 p-NP after 90 min of irradiation. In addition, the PMS activation mechanism of Ag/ZnO/PMS/Vis system was proposed by investigations of the influence of PMS concentration, the FTIR spectra, UV-Vis spectroscopy, and electrochemical analyses. Additionally, the role of SO₄^•- in the photocatalytic reaction is determined by a combination of a trapping test using isopropanol and tert-butanol as probe compounds and electron spin resonance (ESR) spectroscopy. This report provides a potential alternative to remove persistent organic contaminants in sewage using PMS incorporated with Ag/ZnO under visible light irradiation.

Electrochemical/Peroxymonosulfate/NrGO-MnFe2O4 for Advanced Treatment of Landfill Leachate Nanofiltration Concentrate

A simple one-pot method was used to successfully embed manganese ferrite (MnFe2O4) nanoparticles on the nitrogen-doped reduced graphene oxide matrix (NrGO), which was used to activate peroxymonosulfate to treat the landfill leachate nanofiltration concentration (LLNC) with electrochemical enhancement. NrGO-MnFe2O4 and rGO-MnFe2O4 were characterized by various means. This indicates that nitrogen-doped could induce more graphene oxide (GO) spall and reduction to produce more active centers, and was favorable for uniformly loading MnFe2O4 particles. The comparison between electrochemical/peroxymonosulfate/NrGO-MnFe2O4 (EC/PMS/NrGO-MnFe2O4) system and different catalytic systems shows that electrochemical reaction, NrGO and MnFe2O4 can produce synergies, and the chemical oxygen demand (COD) removal rate of LLNC can reach 72.89% under the optimal conditions. The three-dimensional (3D-EEM) fluorescence spectrum shows that the system has a strong treatment effect on the macromolecules with intense fluorescence emission in LLNC, such as humic acid, and degrades into substances with weak or no fluorescence characteristics. Gas chromatography-mass spectrometry (GC-MS) indicates that the complex structure of refractory organic compounds can be simplified, while the simple small molecular organic compounds can be directly mineralized. The mechanism of catalytic degradation of the system was preliminarily discussed by the free radical quenching experiment. Therefore, the EC/PMS/NrGO-MnFe2O4 system has significant application potential in the treatment of refractory wastewater.

Removal of Acid Yellow 17 from Textile Wastewater by Adsorption and Heterogeneous Persulfate Oxidation

Azo dyes commonly used in various industries have a stable and toxic structure. Wastewater containing AY17 dye as a model contaminant was investigated in terms of color and COD removal by both adsorption and persulfate oxidation activated with the PAC. In this study, the effects of temperature (25-50 °C), pH (3-10), persulfate concentration (1000-4000 mg/L), adsorbent dosage (0.1-0.5 g), reaction time (5-60 min), dye concentration (300-1000 mg/L) and NaCI concentration (0-1000 mg/L) on both color and COD removals from wastewater containing AY17 dye were examined. As a result of the study, it was seen that the dosage of adsorbent, pH and reaction time are important parameters in both systems. The use of the PAC as an adsorbent caused to shortening of the reaction time in the HPS system. It also showed that acidic and neutral pH values are more suitable for the removal of AY17 with both systems. Color and COD removal were determined as 100-88.4% and 100-96.6%, respectively, at optimum values obtained for the adsorption and HPS system. An experimental design was applied for various operating parameters in order to analyze experimental data. Models have been proposed for both color removal and COD removal estimates for both systems.

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.

Review on the treatment of organic wastewater by discharge plasma combined with oxidants and catalysts

Discharge plasma technology is a new advanced oxidation technology for water treatment, which includes the effects of free radical oxidation, high energy electron radiation, ultraviolet light hydrolysis, and pyrolysis. In order to improve the energy efficiency in the plasma discharge processes, many efforts have been made to combine catalysts with discharge plasma technology. Some heterogeneous catalysts (e.g., activated carbon, zeolite, TiO₂) and homogeneous catalysts (e.g., Fe²⁺/Fe³⁺, etc.) have been used to enhance the removal of pollutants by discharge plasma. In addition, some reagents of in situ chemical oxidation (ISCO) such as persulfate and percarbonate are also discussed. This article introduces the research progress of the combined systems of discharge plasma and catalysts/oxidants, and explains the different reaction mechanisms. In addition, physical and chemical changes in the plasma catalytic oxidation system, such as the effect of the discharge process on the catalyst, and the changes in the discharge state and solution conditions caused by the catalysts/oxidants, were also investigated. At the same time, the potential advantages of this system in the treatment of different organic wastewater were briefly reviewed, covering the degradation of phenolic pollutants, dyes, and pharmaceuticals and personal care products. Finally, some suggestions for future water treatment technology of discharge plasma are put forward. This review aims to provide researchers with a deeper understanding of plasma catalytic oxidation system and looks forward to further development of its application in water treatment.

Synergy between Iron and Selenide on FeSe2(111) Surface Driving Peroxymonosulfate Activation for Efficient Degradation of Pollutants

In this study, iron selenide nanoparticles (FeSe₂) were synthesized and applied in Fenton-like reactions for degradation of pollutants. It was found that FeSe₂ exerts excellent catalytic reactivity toward different oxidants including peroxymonosulfate (PMS), peroxydisulfate, and H₂O₂, which can degrade a wide range of pollutants such as 2,4,4'-trichlorobiphenyl, bisphenol A, sulfamethoxazole, chlortetracycline, and perfluorooctanoic acid, with the degradation efficiency and TOC removal of pollutants reaching 55-95 and 20.3-50.9%, respectively. The mechanism of PMS activation by FeSe₂ was elucidated, and the synergistic effect between Fe and Se for PMS activation was discovered to be the dominant catalytic mechanism, as evidenced by free-radical quenching, electron paramagnetic resonance, and density functional theory studies. Briefly, the Fe(II) site on the FeSe₂ surface (111) accounted for PMS activation, while the reducing Se species on the surface not only acted as an electron donor contributing to Fe(II) regeneration but also produced Se vacancies further facilitating Fe(II) regeneration to improve the performance of PMS activation. In addition, FeSe₂ exhibited high catalytic activity and stability for PMS activation with different pH, and can degrade PCBs efficiently in the presence of anions, natural organic matter water matrices or in complex soil eluents. This study presents the development and evaluation of FeSe₂ as a novel and highly efficient activator that exhibits promise for practical applications for the degradation of pollutants in wastewater and soil wash eluent with Fenton-like reactions.

The removal of azo dye from aqueous solution by oxidation with peroxydisulfate in the presence of granular activated carbon: Performance, mechanism and reusability

Granular activated carbon (GAC) was used as catalyst for the activation of peroxydisulfate (PDS) to decolorize and degrade Acid Orange 7 (AO7) in water. EPR spectra and radical quencher experiments were employed to identify the active species for AO7 oxidation in the PDS/GAC system. Linear sweep voltammetry (LSV) and chronoamperometry test were carried out to identify the contribution of nonradical mechanism for AO7 decay. The investigation of crucial operational parameters on the decolorization indicated 100 mg/L AO7 can be almost totally decolorized in a broad range of pH. Common inorganic anions adversely affect the AO7 decolorization process and the inhibition was in the order of: HCO₃^- > H₂PO₄^- > SO₄^2- > Cl^- > NO₃^-. UV-vis spectra showed the destruction of the aromatic moiety of AO7 molecule during the oxidation reaction of the PDS/GAC system. The transformation of nitrogen related to the azo bond in AO7 molecule in this system was observed by monitoring the released N-containing inorganic ions. Recycle experiments showed GAC cannot be reused directly but its catalytic ability can be restored by using electrochemical method.

Molten salt induced nitrogen-doped biochar nanosheets as highly efficient peroxymonosulfate catalyst for organic pollutant degradation

Advanced oxidation processes based on carbon catalysis is a promising strategy possessing great potential for environmental pollution degradation. Herein, nitrogen-doped biochar nanosheets (NCS-x) were synthesized using a nitrogen-rich biomass (Candida utilis) as sole precursor. The involvement of environmental-friendly molten salt (NaCl and KCl) in pyrolysis process not only facilitated the exfoliation of biochar, but also favored the retention of N element in biochar. When applying as catalyst for peroxymonosulfate activation, the as-obtained NCS-6 exhibited outstanding performance in catalytic degradation of bisphenol A (BPA). A 100% removal efficiency was observed in 6 min with fast reaction kinetic (k = 1.36 min^-1). Based on quenching test and in-situ electron paramagnetic resonance analysis, both radical pathway and non-radical pathway were suggested to be involved in BPA degradation, while singlet oxygen was identified as the dominant reactive oxygen species. Furthermore, the ecotoxicity evaluation using Chlorella vulgaris as ecological indicator indicated that BPA solution after degradation was less toxic than the original solution. It is expected that this green and facile strategy holds great promise for value-added conversion of nitrogen-rich biomass to highly efficient biochar nanosheets for environment remediation.

Removal of sulfamethoxazole in water by electro-enhanced Co2+/peroxydisulfate system with activated carbon fiber-cathode

The ACF-cathode/Co²⁺/PDS system was validated to effectively remove sulfamethoxazole (SMX), a typical carcinogenic and refractory antibiotic from the aqueous environment. The experimental conditions such as initial pH, PDS, Co²⁺ concentrations, and current density were optimized. Moreover, the system exhibited excellent ability for SMX degradation in surface water and tap water. It was proved that ACF-cathode/Co²⁺/PDS consumes much less electrical energy per order (E_EO) values than Pt-cathode/Co²⁺/PDS processes. More importantly, due to the cathodic protection, the removal rate of SMX within 30 min was satisfactory even after ACF used for 10 cycles continuously. In addition, the cobalt residue in the ACF-cathode/Co²⁺/PDS process was much lower than that of Pt-cathode/Co²⁺/PDS system due to the deposition of cobalt on ACF surface. The catalytic system not only had high catalytic performance, but also had less cobalt residue in the solution and lower power consumption. Therefore, the study provided a novel technology for the removal of refractory pollutants in water.

Structure-dependent catalysis of cuprous oxides in peroxymonosulfate activation via nonradical pathway with a high oxidation capacity

Research interests have been recently thrust into the nonradical reactions in persulfate-based advanced oxidation processes (AOPs), whilst the underlying mechanism of the nonradical pathway remains ambiguous especially in metal-based AOPs systems. In this study, we investigated the reactivity of cuprous oxide (Cu₂O) for activating peroxymonosulfate (PMS) to decompose diverse organic contaminants. Cu₂O exhibited a strong catalytic dependence on the crystal morphology, and cubic Cu₂O was more reactive than the octahedral and rhombic dodecahedral structures for catalytic degradation of bisphenol A with PMS. Chemical quenching tests, electron paramagnetic resonance (EPR), solvent exchange and selective oxidation experiment were corporately conducted to illustrate that Cu₂O-catalyzed PMS did not produce free radicals or singlet oxygen. In contrast, a surface-confined metastable intermediate would be formed via outer-sphere interactions between PMS and Cu₂O, which directly attacked the organic substrate. Such a reaction pathway is intrinsically distinct from the electron-shuttling regime in carbon (or noble metal)/persulfate systems via the conductive surface of the catalyst, and the outer-sphere interactions let the activated PMS demonstrate a higher oxidizing capacity toward organic contaminants. Therefore, this study dedicates to providing new insights into the copper-catalyzed AOPs and vital supplementary to the ongoing dialogue of the nonradical catalysis in persulfate-based oxidation.

Electrochemical/Fe3+/peroxymonosulfate system for the degradation of Acid Orange 7 adsorbed on activated carbon fiber cathode

Acid Orange 7 (AO7), as a most common and widely used synthetic dyes in the printing and dyeing industry, was hardly degradable by traditional wastewater treatment methods. Here, activated carbon fiber (ACF) as an in-situ regenerated cathodic adsorbent in the electrochemical/Fe³⁺/peroxymonosulfate process (EC/ACF/Fe³⁺/PMS) was firstly investigated for AO7 removal and compared with several different processes. The results indicated that the effective adsorption of AO7 on ACF can be enhanced under electrolytic conditions, while the adsorbed AO7 on ACF can be completely degraded and mineralized in EC/ACF/Fe³⁺/PMS process resulting in the in-situ regeneration of ACF. Besides, the electrical energy per order values were investigated, which showed an apparent reduction of electrical energy consumption from 0.42831 to 0.09779 kWh m^-3 when ACF-cathode replaced Pt-cathode. Further study revealed that higher conversion rate of Fe²⁺ from Fe³⁺ was observed with ACF-cathode. It deserved to be mentioned that the removal efficiency of AO7 was satisfactory and stable even after reusing ACF cathode for 10 times. Furthermore, structure and elements of ACF surface were investigated, which indicated the structure of ACF was intact in EC/ACF/Fe³⁺/PMS due to inhibition of ACF corrosion by electron migration at cathode. In addition, the total iron content of the effluent in EC/ACF/Fe³⁺/PMS was lower than that of EC/Fe³⁺/PMS due to the deposition of iron on ACF-cathode surface. Therefore, advantages of EC/ACF/Fe³⁺/PMS for AO7 degradation were not only a much higher oxidation efficiency and in-situ regenerated cathodic adsorbent, but also a lower electrical energy consumption and lesser iron ions contents in the effluent.

Synthesis of polyimide-modified carbon nanotubes as catalyst for organic pollutant degradation via production of singlet oxygen with peroxymonosulfate without light irradiation

Polyimide-modified carbon nanotubes (PI/CNTs) were synthesized via a solvent-free thermal method and used as a metal-free catalyst to activate peroxymonosulfate for organic contaminant degradation without light irradiation. The characterization results suggested that PI was loaded onto the surface of CNTs. The catalytic ability of the PI/CNTs was strongly correlated with the content of PI in the catalysts. The PI/CNTs (22% of PI) showed the highest catalytic efficiency for organic pollutant degradation at room temperature. The degradation efficiency of acid orange 7 (AO7) dye was significantly enhanced to 98.9% within 15 min, compared to the efficiency of 2.2% exhibited by pure PI. The radical quenching tests and electron paramagnetic resonance spectrometry proved that singlet oxygen, instead of hydroxyl radicals or sulfate radicals, played a dominant role during the catalytic oxidation of AO7. The influences of operation parameters including temperature and catalyst amount were investigated. The PI/CNTs metal-free catalyst exhibited high catalytic activity under a broad range of pH values. The recycling study of four repeated reactions demonstrated good stability of the PI/CNTs. This work provided a promising metal-free catalyst for degradation of organic pollutants in aqueous solutions, contributing to the development of green materials for sustainable remediation.

Heterogeneous degradation of organic contaminant by peroxydisulfate catalyzed by activated carbon cloth

An eco-friendly material, activated carbon cloth (ACC) was used as the heterogeneous catalyst in activation of peroxydisulfate (PDS) for the efficient degradation of organic pollutant in water. Besides, the effects of several parameters in the ACC/PDS process including initial pH, PDS concentration, reaction temperature, stirring speed and co-existing anions were investigated. Under optimum conditions, almost complete removal (98.6%) of AO7 in 60 min and 67.4% of total organic carbon (TOC) removal within 180 min were obtained, accompanied by the remarkable destruction of azo band and naphthalene ring on AO7. The electron paramagnetic resonance and radical quenching experiments were carried out to identify the reactive radicals in the ACC/PDS process. Surface characteristic techniques such as XRD, BET, SEM, FTIR, XPS were applied to analysis the change of crystal structure, surface area, surface morphology, functional groups on the surface of fresh and spent ACC samples. Hydroxyl groups (C‒OH) and π-π transitions significantly affected the catalytic activity of ACC. The intermediate products of AO7 oxidation were identified by LC-MS and the corresponding degradation pathway was proposed.

Wood-based biochar as an excellent activator of peroxydisulfate for Acid Orange 7 decolorization

Wood-based biochar, as a metal-free heterogeneous activator of peroxydisulfate (PDS), was successfully prepared by pyrolysis of polar sawdust for efficient removal of Acid Orange 7 (AO7). The results demonstrate PDS could be effectively activated by wood-based biochar, and AO7 was rapidly eliminated in a wide range of pH value (3.0-10.0) with AO7 removal achieved ≥ 99.3% after 14 min reaction. The dominant reactive species in the biochar/PDS system were verified via radical quenching tests and electron paramagnetic resonance (EPR) technique. It is speculated that sulfate radicals (SO₄^•-) and hydroxyl radicals (^•OH) were formed on the surface of biochar. Based on the results of X-ray photoelectron spectroscopy (XPS), π-electron density and oxygen-containing functional groups (especially C-OH) on biochar surface were active centers for the catalytic reaction. Recycle experiments of biochar for 4 runs were carried out and the regeneration method of the catalyst was also studied.

Polyaniline: A New Metal-Free Catalyst for Peroxymonosulfate Activation with Highly Efficient and Durable Removal of Organic Pollutants

Metal-free heterogeneous catalysts are receiving more and more attention for wastewater remediation by activating peroxymonosulfate (PMS) due to their environmental benign. However, carbon-based materials as the most typical metal-free heterogeneous always suffer from poor durability. Inspired by the fact that a conjugated system may facilitate the electron transfer during PMS activation, we innovatively select polyaniline (PANI) as a new PMS activator and investigate its catalytic performance in detail. It is found that PANI can display better catalytic performance than traditional metal-based catalysts and popular N-doped carbocatalysts in methyl orange (MO) degradation. More importantly, PANI is not only universal for various pollutants degradation but also maintains its catalytic performance in repeated degradation experiments. The stable N sites in the conjugated chains and the oxidation-resistance benzene rings as the building units are considered to be responsible for such an excellent durability. In addition, the influences of some routine factors and actual water backgrounds are comprehensively checked and analyzed. The quenching experiments and electron paramagnetic resonance confirm that MO degradation is achieved through both radical and nonradical pathways, where SO₄^•- and ¹O₂ are primary reactive species. The reaction mechanism is also proposed with the assistance of X-ray photoelectron spectroscopy.