Degradation of C. I. Acid Orange 7 in aqueous solution by a novel electro/Fe3O4/PDS process

Effect of electrolytic zero-valent iron activated sodium hypochlorite on sludge dewatering performance

Using electrolytic zero-valent iron-activated sodium hypochlorite (EZVI-NaClO) to pretreat sludge, the capillary suction time (CST) was utilized to evaluate sludge dewaterability. Ammonia nitrogen (NH4-N), dissolved phosphorus, and total phosphorus in the supernatant were used to analyze sludge disintegration. This approach aimed to evaluate the effectiveness of the pretreatment process and its impact on the sludge composition. The migration and transformation of extracellular polymeric substances (EPS), including dissolved EPS (S-EPS), loosely boundEPS, and tightly bound-EPS (TB-EPS), were analyzed by detecting protein and polysaccharide concentrations and three-dimensional fluorescence excitation-emission spectroscopy (3D-EEM). The sludge particle properties, including sludge viscosity and particle size, were also analyzed. The results suggested that the optimal pH value, NaClO dosage, current, and reaction time were 2, 100 mg/gDS (dry sludge), 0.2A, and 30 min, respectively, with a CST reduction of 43%. Protein and polysaccharide contents in TB-EPS were significantly reduced in the EZVI-NaClO group. Conversely, protein and polysaccharides contents in S-EPS increased, suggesting that EZVI-NaClO treatment could disrupt the EPS. Besides, the viscosity of the treated sludge decreased from 195.4 to 54.9 mPa·S, indicating that sludge fluidity became better. ZEVI-NaClO could enhance sludge dewaterability by destructing protein and polysaccharide structure and improving sludge hydrophobicity.

Insights into S-doped iron-based carbonaceous nanocomposites with enhanced activation of persulfate for rapid degradation of organic pollutant

In the work, S-doped iron-based carbon nanocomposites (Fe-S@CN) for activating persulfate (PS) were prepared by calcining iron-loaded sodium lignosulfonate. The characterization revealed that the main substances of Fe-S@CN were FeS and Fe₃C, which were distributed on porous carbon nanosheets in rod-like morphology. In the Fe-S@CN/PS system, carbamazepine could be completely removed within 30 min, and the relative contribution of hydroxyl radicals (OH·), sulfate radicals (SO4·-) and total singlet oxygen (¹O₂) and superoxide radicals (O2·-) for carbamazepine removal were approximated as 8.7%, 19.2% and 72.1%, respectively. Electron paramagnetic resonance spectroscopy demonstrated that S doping promoted the formation of various active species. Compared with the catalyst without S doping, Fe-S@CN exhibited higher activation performance (1.48-fold) for PS due to the enhanced electron transfer rate and facilitated Fe²⁺/Fe³⁺ cycle. Density functional theory calculations showed that S doping promoted the binding between the catalyst and PS, and enhanced the overall internal electron density of the catalyst. Fe-S@CN exhibited excellent catalytic performance over a wide pH range (3.0-11.0). The active sites of Fe-S@CN used in the cycling experiments was also largely recovered after thermal regeneration. Overall, this study shows for the first time the impact of SLS as an S dopant on enhanced PS activation.

Degradation of atrazine by electroactivation of persulfate using FeCuO@C modified composite cathode: Synergistic activation mechanism

In sulfate radical-based advanced oxidation processes (SR-AOPs), high-efficiency and perdurable materials have drawn considerable interest for use as cathodes, which can effectively degrade refractory organic contaminants through the synergistic electro-activation and transition metal activation of persulfate (PS). Here, the FeCuO@C modified composite cathode (FeCuO@C/AGF) was synthesized via the solvothermal and thermal treatment method based on the CuFe-MOF-74 structure, and the electro-activation PS process (EC/FeCuO@C/AGF/PS) was developed to effectively remove atrazine (ATZ). The surface morphology, electrochemical characteristics, chemical composition, crystal structure, and electrode surface wettability of FeCuO@C/AGF were investigated. It was found that the proposed EC/FeCuO@C/AGF/PS process can successfully remove 100% of ATZ in 20 min at a low current density (2 mA cm^-2) and a low PS concentration (0.4 mM), and PS is successfully activated by combining the electrical and transition metal synergistic activation. The FeCuO@C/AGF cathode exhibits outstanding catalytic functionality over a broad pH range (2-9) and remains stable over five successive cycles. Additionally, the active species involved in the reaction as well as the potential ATZ degradation reaction mechanisms and pathways are discussed. Electrochemical oxidation is a process in which both radicals (SO₄^·-, ·OH, and O₂^·-) and non-radical (¹O₂) participate in the degradation of ATZ. The intermediates of the ATZ degradation process were studied upon the toxicity changing, and the toxicity of the intermediates was found to be reduced during degradation. These results present a novel approach toward the establishment of an effective and reliable electrode in SR-AOPs that can efficiently treat pesticide wastewater.

New insight of Mn(III) in δ-MnO2 for peroxymonosulfate activation reaction: Via direct electron transfer or via free radical reactions

Due to the increasing of industrial plastic waste and its refractory characteristics, it is extremely urgent to develop new degradation technology and environmentally friendly catalyst for industrial plastic waste. Manganese oxides are one of the most promising candidates for the catalytic degradation of plastic wastes. However, an improved understanding of the structural properties affecting their catalytic activity is required for high-efficient wastewater treatment. We herein report the surface reactivity effects of δ-MnO₂ structural defects with regards to Bisphenol A (BPA) degradation/probe in the presence of peroxymonosulfate (PMS). Four δ-MnO_x samples with different Mn(III) contents (different Mn(III)-deficient sample) were prepared and their structural properties as well as surface reactivity were characterized by batch test, ESR and XAFS analysis. For the Mn(III)-deficient sample, BPA removal was principally affected by direct electron transfer, with the adsorbed BPA degraded following hydroxylation. In contrast, a small fraction of Mn(III) substitution in δ-MnO₂ could significantly encouraged the activation of PMS to produce SO₄^-☐and ☐OH, and a BPA degradation via beta scission. Moreover, the Mn(III)-rich δ-MnO₂ demonstrate a high BPA removal rate even with a low sample load, which performed well following a reuse of five times. Our results provide a new way for the improvement of δ-MnO₂ activity for the use of industrial plastic wastes treatment.

Bicarbonate enhanced heterogeneous activation of peroxymonosulfate by copper ferrite nanoparticles for the efficient degradation of refractory organic contaminants in water

Nowadays, the treatment of residual refractory organic contaminants (ROCs) is a huge challenge for environmental remediation. In this study, a potential process is provided by copper ferrite catalyst (CuFe₂O₄) activated peroxymonosulfate (PMS, HSO₅^-) in the bicarbonate (HCO₃^-) enhanced system for efficient removal of Acid Orange 7 (AO7), 2,4-dichlorophenol, phenol and methyl orange (MO) in water. The impact of key reaction parameters, water quality components, main reactive oxygen species (ROS), probable degradation mechanism, rational degradation pathways and catalyst stability were systematically investigated. A 95.0% AO7 (C₀ = 100 mg L^-1) removal was achieved at initial pH (pH₀) of 5.9 ± 0.1 (natural pH), CuFe₂O₄ dosage of 0.15 g L^-1, PMS concentration of 0.98 mM, HCO₃^- concentration of 2 mM, and reaction time of 30 min. Both sulfate radical (SO₄^-•) and hydroxyl radical (^•OH) on the surface of catalyst were proved as the predominant radical species through radical quenching experiments and electron paramagnetic resonance (EPR) analysis. The buffer nature of HCO₃^- was partially contributed for the enhanced degradation of AO7 under CuFe₂O₄/PMS/HCO₃^- system. Importantly, according to ¹³C nuclear magnetic resonance (NMR) and EPR analysis, the positive effect of bicarbonate may be mainly attributed to the formation of peroxymonocarbonate (HCO₄^-), which may enhance the generation of ^•OH. The magnetic CuFe₂O₄ particles can be well recycled and the leaching concentration of Cu was acceptable (<1 mg L^-1). Considering the widespread presence of bicarbonate in water environment, this work may provide a safe, efficient, and sustainable technique for the elimination of ROCs from practical complex wastewater.

Degradation of sulfamethoxazole in water by AgNbO3 photocatalyst mediated by persulfate

In this paper, silver niobate (AgNbO₃) material was synthesized by a solid-state reaction. AgNbO₃ was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (DRS), and Brunauer–Emmett–Teller (BET) measurement. The photocatalytic activity of AgNbO₃ was investigated in degradation of sulfamethoxazole (SMX) under visible light, which is a widely used antibiotic with significant threats towards health and aquatic organisms. Persulfate (PS) oxidant was found to improve the efficiency of the proposed photocatalytic removal of SMX by AgNbO₃. The different operational parameters in the AgNbO₃/PS/Vis system were investigated. The best photocatalytic performance was achieved with 0.5 g L⁻¹ AgNbO₃, 1.0 mM PS, and pH = 5.0 as the optimal conditions, achieving 98% of SMX degradation after 8 h of visible-light irradiation. Scavenger and electron spin resonance (ESR) experiments were carried out to identify the major reactive species in the SMX degradation and to propose the photocatalytic mechanism by the AgNbO₃/PS/Vis system. The photodecomposition was found to be majorly caused by holes and ˙O₂⁻ species, with ˙OH and SO₄˙⁻ radicals contributing to improve the photocatalytic process. The AgNbO₃ catalyst was stable and reusable with efficient photocatalytic activity in three successive recycling experiments and its XRD patterns remained virtually unchanged. The reported process of PS activation by the AgNbO₃ photocatalyst is promising for visible-light application in remediation of antibiotic-contaminated water.

Silver niobate was synthesized by the solid-state reaction and combined with persulfate (PS) oxidant to advance water treatment application. The AgNbO₃/PS/Vis system was applied successfully for sulfamethoxazole removal from water samples.

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.

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.

Simplified creation of polyester fabric supported Fe-based MOFs by an industrialized dyeing process: Conditions optimization, photocatalytics activity and polyvinyl alcohol removal

MIL-53(Fe) was successfully prepared and deposited on the surface carboxylated polyester (PET) fiber by an optimized conventional solvothermal or industrialized high temperature pressure exhaustion (HTPE) process to develop a PET fiber supported MIL-53(Fe) photocatalyst (MIL-Fe@PET) for the degradation of polyvinyl alcohol (PVA) in water under light emitting diode (LED) visible irradiation. On the basis of several characterizations, MIL-Fe@PET was tested for the photocalytic ability and degradation mechanism. It was found that temperature elevation significantly enhanced the formation and deposition of MIL-53(Fe) with better photocatalytic activity. However, higher temperature than 130°C was not in favor of its photocatalytic activity. Increasing the number of surface carboxyl groups of the modified PET fiber could cause a liner improvement in MIL-53(Fe) loading content and photocatalytic ability. High visible irradiation intensity also dramatically increased photocatalytic ability and PVA degradation efficiency of MIL-Fe@PET. Na₂S₂O₈ was used to replace H₂O₂ as electron acceptor for further promoting PVA degradation in this system. MIL-Fe@PET prepared by HTPE process showed higher MIL-53(Fe) loading content and slightly lower PVA degradation efficiency than that prepared by solvothermal process at the same conditions. These findings provided a practical strategy for the large-scale production of the supported MIL-53(Fe) as a photocatalyst in the future.

Urchin-like Co3O4 anchored on reduced graphene oxide with enhanced performance for peroxymonosulfate activation in ibuprofen degradation

Urchin-like Co₃O₄ anchored on reduced graphene oxide was easily prepared with hydrothermal reaction by using a cheap and green agent. First, the surface morphology and physicochemical properties of Co₃O₄-rGO were characterized. Compared with Co₃O₄, Co₃O₄-rGO possessed excellent activity in peroxymonosulfate (PMS) activation for ibuprofen (IBU) degradation. Then, the influences of Co₃O₄-rGO dosage, IBU concentration, PMS concentration and pH on IBU and TOC removal were investigated, respectively. Furthermore, both ·OH and SO₄^•- were identified to be the main active species, and SO₄^•- made the predominant contribution. In addition, residual PMS and SO₄^•- quantification demonstrated that Co₃O₄-rGO could activate PMS more effectively, and produce more SO₄^•-. The mechanistic study revealed that the valence state conversion of Co²⁺/Co³⁺ was the critical PMS activation mechanism. Moreover, the enhanced activity of Co₃O₄-rGO is accounted for the combination of multiple unique characteristics, including excellent electronic transmission (Co²⁺ to Co³⁺, Co²⁺ to PMS), more active sites, and chemical bonds between Co₃O₄ and rGO. 13 degradation products were determined and possible degradation routes were proposed based on the results of LC-MS/MS. Finally, the Co₃O₄-rGO/PMS system also exhibited satisfactory removal of IBU in real water matrices.

Using Fe-Cu/HGF composite cathodes for the degradation of Diuron by electro-activated peroxydisulfate

An iron-copper graphite felt (Fe-Cu/HGF) electrode was successfully prepared by heat treatment and impregnation of graphite felt as the support followed by calcination, and an electro-activated peroxydisulfate (E-PDS) system with Fe-Cu/HGF as the cathode was constructed to degrade Diuron. This system synergistically activated PDS through electrochemical processes and transition metal catalysis. High-valence metal ions could be converted into low-valence metal ions by reduction at the cathode, and low-valence metal ions continuously activated PDS to generate more sulfate radicals (SO₄^-) and hydroxyl radicals (OH) to accelerate Diuron degradation. The Fe-Cu/HGF composite cathode exhibited a performance superior to graphite felt (RGF) obtained using pretreatment only, including increased hydrophilicity, significantly increased number of defect sites and larger electroactive surface area. Under optimized experimental degradation conditions, Diuron could be completely removed in 35 min, at which time copper ion leaching was not detected in the solution, while the total iron ion concentration was 0.27 mg L^-1. Extending the reaction time to 6 h, the amount of total organic carbon was reduced to 32.2%. In addition, the free radicals that degraded Diuron were identified as mainly SO₄^- and OH with a slightly higher contribution of SO₄^-. The mechanism and pathways of Diuron degradation in the E-PDS system were determined. The E-PDS system was successfully applied to the degradation of other pollutants and the degradation of Diuron in different simulated water environments. In summary, the E-PDS system using Fe-Cu/HGF as the cathode is a promising treatment method for Diuron-containing wastewater.

Effects of ferroferric oxide on azo dye degradation in a sulfate-containing anaerobic reactor: From electron transfer capacity and microbial community

Anaerobic decolorization of azo dye in sulfate-containing wastewater has been regarded as an economical and effective method, but it is generally limited by the high concentration of azo dye and accumulation of toxic intermediates. To address this problem, Fe₃O₄ was added to one of the anaerobic reactors to investigate the effects on system performances. Results showed that AO7 removal rate, COD removal rate, and sulfate reduction were enhanced with the addition of Fe₃O₄ under various influent AO7 concentrations (153 mgCOD/L - 1787 mgCOD/L). According to the proposed pathway for the degradation of AO7, more intermediates (2-hydroxy-1,4-naphthoquinone, phthalide, 4-methylphenol) were produced in the presence of Fe₃O₄. The electron transfer capacity of sludge was also increased since Fe₃O₄ could stimulate to secrete humic acid-like organics in EPS. Microbial analysis showed that iron-reducing bacteria like Clostridium and Geobacter were also enriched, which were capable of azo dye and aromatic compounds degradation.

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.

3D Self-Supported Nitrogen-Doped Carbon Nanofiber Electrodes Incorporated Co/CoOx Nanoparticles: Application to Dyes Degradation by Electro-Fenton-Based Process

We developed free-standing nitrogen-doped carbon nanofiber (CNF) electrodes incorporating Co/CoO_x nanoparticles (NPs) as a new cathode material for removing Acid Orange 7 (AO7; a dye for wool) from wastewater by the heterogeneous electro-Fenton reaction. We produced the free-standing N-doped CNF electrodes by electrospinning a polyacrylonitrile (PAN) and cobalt acetate solution followed by thermal carbonation of the cobalt acetate/PAN nanofibers under a nitrogen atmosphere. We then investigated electro-Fenton-based removal of AO7 from wastewater with the free-standing N-doped-CNFs-Co/CoO_x electrodes, in the presence or not of Fe²⁺ ions as a co-catalyst. The electrochemical analysis showed the high stability of the prepared N-doped-CNF-Co/CoO_x electrodes in electrochemical oxidation experiments with excellent degradation of AO7 (20 mM) at acidic to near neutral pH values (3 and 6). Electro-Fenton oxidation at 10 mA/cm² direct current for 40 min using the N-doped-CNF-Co/CoO_x electrodes loaded with 25 wt% of Co/CoO_x NPs led to complete AO7 solution decolorization with total organic carbon (TOC) removal values of 92.4% at pH 3 and 93.3% at pH 6. The newly developed N-doped-CNF-Co/CoO_x electrodes are an effective alternative technique for wastewater pre-treatment before the biological treatment.

A combined radical and non-radical oxidation processes for efficient degradation of Acid Orange 7 in the homogeneous Cu(II)/PMS system: important role of chloride

Trace copper ion (Cu(II)) in water and wastewater can trigger peroxymonosulfate (PMS) activation to oxidize organic compounds, but it only works under alkaline conditions. In this work, we found that the presence of chloride could significantly accelerate the oxidation of Acid Orange 7 (AO7) by the Cu(II)/PMS process at a wide pH range (4.0-9.0). The observed pseudo-first-order rate constant k for AO7 oxidation was linearly correlated with the increased Cl^- concentration (0-300 mM). An increase in mineralization rate was observed in the presence of Cl^-, while the overall mineralization was quite low. Decomposition of PMS facilitated when Cl^- concentration or pH value increased. Based on the scavenger experiments and electron paramagnetic resonance (EPR) measurement, the mechanism of Cu(II)-catalyzed PMS oxidation process in the presence of Cl^- was proposed as both the radical and non-radical pathway, and ¹O₂ was the reactive oxygen species in the Cu(II)/PMS system. Finally, a possible degradation pathway of AO7 was elucidated. The feasibility of in situ utilizing high salinity and trace cupric species to accelerate the degradation of organic pollutants by the Cu(II)/PMS process in water and wastewater was demonstrated. However, the identification of undesired chlorinated by-products reminds us of cautiousness in assessing the application of Cu(II)/PMS system under chloride-rich environment. The findings of this work provide a simple and efficient approach to apply PMS in the remediation of refractory organic contaminants in the presence of trace cupric species under a high salinity environment with a wide range of pH.

Improvement of sludge dewaterability and disintegration efficiency using electrolytic zero-valent iron activated peroxymonosulfate

Electrolysis zero-valent iron activated peroxymonosulfate (EZVI-PMS) was applied to enhance sludge dewaterability and disintegration performance. Sludge dewaterability was characterized by capillary suction time (CST), specific resistance to filtration (SRF), and disintegration performance was explored by measuring sludge DNA content, ammonia nitrogen, chemical oxygen demand (COD), extracellular polymeric substances (EPS) and dissolved organic carbon (DOC). EPS, including soluble EPS (SB-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) were analyzed by three dimensional fluorescence excitation-emission spectrum (3D-EEM) to confirm the proteins' transformation tendency. DOC, protein and polysaccharide in EPSs were quantified to investigate the conditioning mechanism. The results showed that sludge CST and SRF were reduced significantly when the current was 0.2 A and PMS dosage was 130 mg/gDS with the reductions of 43.8% and 74.1%, respectively, and DNA was released from sludge cells to the liquid phase. Mechanically, sludge TB-EPS converted to SB-EPS with DOC in TB-EPS decreasing from 367.0 mg/L to 210 mg/L, while DOC in SB-EPS increased from 44 mg/L to 167.4 mg/L. Besides, the changes of protein and polysaccharide contents in SB-EPS and TB-EPS were similar to DOC, and protein in TB-EPS transformed to other protein-like or organic substances obviously.

Enhancement of the electro-activated persulfate process in dye removal using graphene oxide nanoparticle

This study aimed to improve the speed of the electrochemical process by graphene oxide nanoparticle as a current accelerator in Acid Blue 25 removal from aqueous solutions. To do so, the effect of different parameters including pH, dye concentration, sodium persulfate concentration, the ratio of sodium persulfate to iron (II) sulfate concentration, current density, and the distance between electrodes was investigated on dye removal. Under optimal conditions of pH = 5, dye concentration = 200 mg/L, sodium persulfate concentration = 500 mg/L, iron (II) sulfate concentration = 100 mg/L, current density = 16.67 mA/cm², and electrode distance = 2 cm, 95% of dye was removed after 60 min in the electro-activated persulfate process; while the modified electro-activated persulfate process achieved 95% dye removal after only 40 min under the same conditions. This system was able to remove 90% of dye after 60 min at a higher concentration (300 mg/L). Also, the modified electro-activated persulfate process obtained the removal of 80% of COD, and 54% of TOC after 180 min in the mentioned conditions, for the dye concentration of 300 mg/L.

Synergetic decolorization of azo dyes using ultrasounds, photocatalysis and photo-fenton reaction

In the present work, ultrasound irradiation, photocatalysis with TiO₂, Fenton/Photo-Fenton reaction, and the combination of those techniques were investigated for the decolorization of industrial dyes in order to study their synergy. Three azo dyes were selected from the weaving industry. Their degradation was examined via UV illumination, Fenton and Photo-Fenton reaction as well as ultrasound irradiation at low (20 kHz) and high frequencies (860 kHz). In these experiments, we investigated the simultaneous action of the ultrasound and UV irradiation by varying parameters like the duration of photocatalysis and ultrasound irradiation frequency. At the same time, US power, temperature, amount of TiO₂ photocatalyst and amount of Fenton reagent remained constant. Due to their diverse structure, each azo dye showed different degradation levels using different combinations of the above-mentioned Advanced Oxidation Processes (AOPs). The Photo-Fenton reagent is more effective with US 20 kHz and US 860 kHz for the azo dyes originated from the weaving industry at pH = 3 as compared to pH = 6.8. The combination of the Photo-Fenton reaction with 860 kHz ultrasound irradiation for the same dye gave an 80% conversion at the same time. Experiments have shown a high activity during the first two hours. After that threshold, the reaction rate is decreased. FT-IR and TOC measurements prove the decolorization due to the destruction of the chromophore groups but not complete mineralization of the dyes.

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.

Preparation of porous silicate supported micro-nano zero-valent iron from copper slag and used as persulfate activator for removing organic contaminants

Porous silicate supported micro-nano zero-valent iron (PSi@ZVI) was prepared from copper slag (CS) through carbothermal reduction technology, and used as a persulfate (PS) activator for removing organic contaminants. Results showed that the properties of the activator were greatly affected by the preparation conditions. Calcination for 20 min at 1100 °C with 20% anthracite was considered the optimum preparation condition for degradation of orange G (OG). The removal rate of OG was improved by increasing the dosages of PSi@ZVI or PS and raising the reaction temperature. Moreover, PSi@ZVI exhibited excellent PS activator ability in a wide range of initial pH, good degradation capability for eosin Y, methyl orange, acid fuchsine, and methylene blue. The reusability and safety of PSi@ZVI were verified. Electron paramagnetic resonance and radical quenching tests indicated that sulfate radical (SO₄^-) was the main active species in the PSi@ZVI/PS system. The X-ray diffraction results indicated that a high calcination temperature (1100 °C) was beneficial to the reduction of iron-bearing minerals to ZVI. Scanning electron microscopy and energy-dispersive spectroscopy results revealed that the formation of porous structure of PSi@ZVI and the generation of nano to micro-sized ZVI particles on the surface of the silicate holes. The X-ray photoelectron spectra showed that ZVI was first convert into Fe(II), which mainly activated PS and generated Fe(III) in the PSi@ZVI/PS system. Furthermore, the intermediates of OG were detected using gas chromatography-mass spectrometry, and the possible degradation pathway of OG was proposed. This study provides a novel approach for reuse of CS as a heterogeneous activator to effectively activate PS.

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.

Synthesis of Nano-Fe@NdFeB/AC magnetic catalytic particle electrodes and application in the degradation of 2,4,6-trichlorophenol by electro-assisted peroxydisulfate process

The coupling of electrolysis and the peroxydisulfate (PDS) activation was selected in this study to degrade solution-phase 2,4,6-trichlorophenol (TCP). To enhance the PDS activation efficiency and catalytic recycling ratio, a novel magnetic activator, nano iron coated on neodymium iron boron/activated carbon nanocomposite (Nano-Fe@NdFeB/AC), was synthesized and utilized as catalytic particle electrodes. To increase the mass transfer ability, a novel magnetic internal circulation electrolytic reactor (MICE) was established. The results indicated that globular Fe, with sizes ranging from 25 nm to 300 nm, is present on the surface of the catalyst. This catalyst has sufficient magnetism to be separated by the magnetic separation method and its specific saturation magnetization and residual magnetization were 1.48 and 0.26 emu/g, respectively. At the optimal condition of [pH]₀= 9.0, [Na₂S₂O₈]₀= 2.0 mmol/L, [Nano-Fe@NdFeB/AC]₀= 5.0 g/L and I = 50 mA, the TOC percentage of removal could reach 84% after 30 min of reaction. The TCP mineralization follows pseudo-first-order kinetics. The intermediate products of 2,6-dichloro-2,5-cyclohexadiene-1,4-dione, Tetrachloro-hydroquinone, and 2,3,5,6-tetrachloro-p-benzoquinone were found during the reaction. TCP mineralization was confirmed to have a hybrid mechanism involving reductive dechlorination with Fe, •OH addition oxidation and electron capture by SO•₄ ^-. This study provides a new method for the treatment of degradation-resistant pollutants.

Biochar-activated peroxydisulfate as an effective process to eliminate pharmaceutical and metabolite in hydrolyzed urine

Eliminating pharmaceutical active compounds from source-separated urine is essential for nutrient recovery and reducing the contaminant load to wastewater treatment plants. However, limited oxidation treatment processes have shown satisfactory performance due to strong scavenging effect of urine components. This study proposed a heterogeneous catalytic system by combining biochar with peroxydisulfate (PDS), which effectively removed sulfamethoxazole (SMX) and its major human metabolite, N₄-acetyl-sulfamethoxazole (NSMX) in urine. The performance of biochar/PDS was investigated in both a complete-mixing reactor and a biochar-packed column. Interestingly, urine components slightly inhibited the degradation of sulfonamides in biochar suspension but significantly improved their removal in biochar-packed column. Further investigation elucidated the PDS activation process and the effects of the main urine components, which explained the different results in biochar suspension and biochar-packed column. The biochar/PDS system mainly produced ·OH radical, singlet oxygen and surface-bound radicals (SBR), which transformed SMX to products of no apprarent antimicrobial properities. A cost-effective two-stage process was designed utilizing SBR as the major reactive species. This study may help to improve the understanding of the catalytic role of biochar and provide cost-effective treatment options for urine.

UV-Catalyzed Persulfate Oxidation of an Anthraquinone Based Dye

Wastewater from the textile industry has a substantial impact on water quality. Synthetic dyes used in the textile production process are often discharged into water bodies as residues. Highly colored wastewater causes various of problems for the aquatic environment such as: reducing light penetration, inhibiting photosynthesis and being toxic to certain organisms. Since most dyes are resistant to biodegradation and are not completely removed by conventional methods (adsorption, coagulation-flocculation, activated sludge, membrane filtration) they persist in the environment. Advanced oxidation processes (AOPs) based on hydrogen peroxide (H2O2) have been proven to decolorize only some of the dyes from wastewater by photocatalysis. In this article, we compared two very different photocatalytic systems (UV/peroxydisulfate and UV/H2O2). Photocatalyzed activation of peroxydisulfate (PDS) generated sulfate radicals (SO4•−), which reacted with the selected anthraquinone dye of concern, Acid Blue 129 (AB129). Various conditions, such as pH and concentration of PDS were applied, in order to obtain an effective decolorization effect, which was significantly better than in the case of hydroxyl radicals. The kinetics of the reaction followed a pseudo-first order model. The main reaction pathway was also proposed based on quantum chemical analysis. Moreover, the toxicity of the solution after treatment was evaluated using Daphnia magna and Lemna minor, and was found to be significantly lower compared to the toxicity of the initial dye.

Mechanisms of electro-assisted persulfate/nano-Fe0 oxidation process: Roles of redox mediation by dissolved Fe

Mechanisms involved in an electrochemically assisted oxidation process using persulfate and nanosized zero-valent iron (NZVI) were elucidated. Initially, Fe⁰ acted as a source of Fe²⁺ to activate the persulfate, then Fe²⁺/Fe³⁺ redox mediation between cathode and persulfate played a decisive role in persulfate activation at a current density low enough not to inhibit Fe⁰ corrosion. An excessive current density which resulted in a low cathodic potential limited Fe⁰ corrosion and therefore limited the supply of dissolved Fe to activate the persulfate. Direct oxidation of phenol at the anode therefore became more important under the excessive current density than oxidation by sulfate radicals. At a low current density, Fe⁰ in the NZVI particles was completely transformed into iron (oxyhydr)oxides such as ferrihydrite, lepidocrocite, and magnetite. Fe⁰ was transformed into Fe²⁺ little when the current density was high. Increasing the current density increased the energy cost by increasing the amount of electrical energy dissipated in side reactions that decreased sulfate radical formation. The results indicated that a low current density can generally be used to give a high reaction rate and a high energy efficiency and that a high current density can be used when the NZVI particles need to be preserved.

Electrochemical oxidation of Acid Orange 7 azo dye using a PbO2 electrode: Parameter optimization, reaction mechanism and toxicity evaluation

In this study, electrochemical oxidation of Acid Orange 7 (AO 7) azo dye has been investigated using a Fe-doped PbO₂ electrode. The degradation of AO 7 followed pseudo-first-order reaction kinetics. The removals of AO 7, chemical oxygen demand (COD) and total organic carbon (TOC) were 87.15%, 49.88% and 44.94% after 60 min of electrolysis at the optimal conditions (Na₂SO₄ concentration 0.1 M, initial pH 5, initial AO 7 concentration 100 mg L^-1 and applied current density 20 mA cm^-2), respectively. And the corresponding degradation rate constant was 0.035 min^-1. The intermediates formed during electrochemical process were identified, and a possible degradation pathway was proposed, which was initiated by the oxidation of azo bond (-NN-), hydroxylation and substitution reaction of -NH₂ and -SO₃H under the attack of OH, and ended with the formation of mineralization products such as NH₄⁺, NO₃^-, SO₄^2-, CO₂ and H₂O. The toxicity of treated AO 7 solution towards Vibrio fischeri increased slightly at first and then rapidly reduced to non-toxicity with prolonging time. The results indicate that electrochemical oxidation of AO 7 using Fe-doped PbO₂ electrode is a promising way.

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.

Sustainable synthesis of modulated Fe-MOFs with enhanced catalyst performance for persulfate to degrade organic pollutants

In this study, four typical modulators (NH₄OH(A), CH₃COOH(B), CH₃COONa(C) and CH₃COONH₄(D)) were applied to modulate the microwave-assisted synthesis of Fe-MOFs. The effects of various modulators on the yield, electrochemistry activity and PS activation capacity of prepared catalysts were systematically investigated. The ideal modulator was revealed as the 7.5 mM CH₃COONH₄. Contributed by the defects caused by the dual effects of CH₃COONH₄, Fe-MOFs-D-7.5/PS system showed excellent orange G (OG) degradation with high reaction stoichiometric efficiency (RSE) and desirable recycling performance. The main radicals should be SO₄^·- and O₂^·- which were confirmed by EPR and chemical quenchers. Furthermore, the frontier molecular orbital (FMO) theory and dual descriptor (DD) method were employed in predicting radical attacking sites of OG. According to the results of theoretical computations and experimental detection, degradation pathways of OG in Fe-MOFs-D-7.5/PS system were proposed. Similar to the function of the battery, this study gives new insight into the possible mediatory roles of Fe-MOFs-D-7.5 in PS activation by transferring the electrons between PS and the unsaturated metal sites (CUS). The Fe-MOFs-D-7.5/PS system is a promising process for environmental 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.

Precise control of iron activating persulfate by current generation in an electrochemical membrane reactor

Activated persulfate (PS) oxidation is promising for contaminant removal but a lack of controllable activation can lead to a loss of reagents and thus low contamination degradation. Herein, we have proposed and investigated an innovative method to control PS activation by introducing ion exchange membrane into electrochemically activated PS. This electrochemical membrane reactor (EMR) could precisely control PS activation by adjusting electrical current for slow release of Fe²⁺, and also avoid direct contact between PS and a sacrificial anode electrode (iron electrode)/an alkaline cathode solution. It was found that the PS decomposition rate constant was linearly increased by increasing the applied current (R² = 0.988). The rate of the released Fe²⁺ also exhibited a linear relationship with the applied current (R² = 0.995). Compared to one-time dosage of Fe²⁺, the EMR-based slow-release process had higher contamination degradation and better PS utilization (molar ratio of the decomposed PS to the migrated Fe, 1.04 ± 0.01:1), thereby minimizing the waste of both reaction reagents and generated radicals. The EMR was also employed to degrade a representative dye contaminant in a controllable manner and achieved 95.7 ± 0.7% removal percentage with PS dosage of 3.0 g L^-1 within 20 min. This study is among the earliest to explore effective approaches for precisely controlling PS activation and subsequent oxidation of contaminants.

Construction of ZnFe2O4/rGO composites as selective magnetically recyclable photocatalysts under visible light irradiation

This paper reports on highly active ZnFe₂O₄/reduced graphene oxide (ZnFe₂O₄/rGO) nanocomposites synthesized by a modified sol-gel method. The as-prepared samples have been characterized by XRD, TEM, XPS and other detection methods, which demonstrate that ZnFe₂O₄ nanoparticles (NPs) with a diameter of 15 ∼ 50 nm were densely grown on the rGO substrates. The photocatalytic activities of ZnFe₂O₄/rGO catalysts were evaluated by the degradation of Methylene blue (MB) under visible light. The results showed that the ZnFe₂O₄/rGO catalysts had high photocatalytic activity, and the degradation efficiency of MB was almost 100% within 180 min. Moreover, the ZnFe₂O₄/rGO catalysts also had a great removal effect on Rhodamine B (RhB) and Methyl orange (MO). Mechanistic studies revealed that the rGO acted as a stabilizer to prevent ZnFe₂O₄ from aggregation and improved the separation of photo-generated electrons. The high efficiency for dye degradation was attributed to the generation of hydroxyl radicals (·OH) via the photochemical decomposition of H₂O₂ on ZnFe₂O₄/rGO catalysts, which was responsible for the oxidation of the dyes. Of note, the ZnFe₂O₄/rGO catalyst maintained an efficiency of over 90% after five cycles. The XRD, XPS and VSM characterization revealed that the ZnFe₂O₄/rGO catalysts had a stable crystal structure and can be easily separated.

Persulfate activation by Fe(III) with bioelectricity at acidic and near-neutral pH regimes: Homogeneous versus heterogeneous mechanism

The combination of persulfate (PS) activation by iron ions with electrochemical process (electro/Fe³⁺/PS) is a promising advanced oxidation process. However, almost all these systems were performed in an unbuffered solution and actually under acidic pH condition, with the electricity being frequently supplied by external power. Considering the high buffering capacity of wastewater and energy saving, peroxydisulfate (PDS) activation by Fe(III) species with bioelectricity provided by microbial fuel cell (MFC) for bisphenol A (BPA) oxidation was investigated at fixed near-neutral pH as well as acidic pH. The results indicate that 90.8% of BPA could be removed at pH 2.5. Though the iron existed in the form of precipitate, BPA could still be efficiently removed at pH 6.0. The precipitate formed in the system at pH 6.0 was identified as the amorphous iron oxyhydroxides. Sulfate radicals in the bulk solution and that adsorbed on the precipitate were the dominant reactive species responsible for the oxidation of BPA in the homogeneous and heterogeneous MFC/Fe(III)/PDS processes, respectively. The mechanisms of BPA degradation at both pH values were proposed via EPR and quenching tests as well as XPS analysis. The effects of operating parameters, the mineralization, the mineralization current efficiency and energy consumption were also explored.