The electrochemical advanced oxidation processes coupling of oxidants for organic pollutants degradation: A mini-review

The oxidative degradation of Caffeine in UV/Fe(II)/persulfate system-Reaction kinetics and decay pathways

In this study, the degradation of caffeine was investigated by UV/Fe²⁺ /persulfate (PS) process. Caffeine (CAF) degradation in sole-UV, UV/Fe²⁺ , UV/PS, and Fe²⁺ /PS systems was also conducted to examine the contribution of isolated processes to CAF degradation. The effects of pH levels, the concentration of Fe²⁺ and PS, inorganic anions, and initial concentration of CAF on the performance of UV/Fe²⁺ /PS process were evaluated. Radical competitive reactions indicated both hydroxyl radicals and sulfate radicals played important roles in CAF degradation in UV/Fe²⁺ /PS system. Nine intermediates, among which three were detected for the first time, were identified by ultra-performance liquid chromatography/electrospray-time-of-flight mass spectrometry (UPLC/ESI-TOF-MS) and SPME (solid-phase microextraction)/GC/MS. The possible degradation pathways of CAF were proposed, among which demethylation, hydroxylation, the oxidation of olefinic double bond, and the cleavage of pyrimidine ring and imidazole ring were involved in the degradation of CAF in UV/Fe²⁺ /PS system. PRACTITIONER POINTS: Caffeine degradation by UV/Fe²⁺ /PS process was investigated. Caffeine degradation did not follow a simple pseudo-first order kinetics Chloride ions promoted CAF degradation. The anions NO₃ ^- , SO₄ ^2- , and H₂ PO₄ ^- exerted a negative influence on caffeine degradation. Nine intermediates were detected, and decay pathways were proposed.

Synergistic multiple active species driven fast estrone oxidation by δ-MnO2 in the existence of methanol

Endocrine-disrupting chemicals (EDCs) cause serious threats to human health. Five types of MnO₂ were synthesized and characterized. They exhibited different removal performances for three EDCs, i.e., estrone (E1), ethynylestradiol (EE2) and bisphenol A (BPA). Only δ-MnO₂ can completely remove E1 within 120 min at pH 3.0. Free Mn (III) was determined at the beginning of the reaction and participated in the EDCs removal process. Electron spin resonance (ESR) indicated that δ-MnO₂ could produce superoxide anions (·O₂^-) and singlet oxygen (¹O₂) in the existence of methanol. The reactive oxygen species (ROS) quenching experiments showed ¹O₂ have certain contribution to the E1 removal by δ-MnO₂. The source of ROS is mainly the lattice oxygen from δ-MnO₂, and can be replenished through the layer structure destruction caused by the reaction between Mn(III) and E1. The ROS dependent EDCs removal by δ-MnO₂ leads to a deep understanding on this well-known oxidant.

Adsorptive Separation of Brilliant Green Dye from Water by Tartaric Acid-Treated Holarrhena antidysenterica and Citrullus colocynthis Biowaste

Holarrhena antidysenterica (Ha) and Citrullus colocynthis (Cc) biowaste from indigenous Pakistan resources was used to remove toxic brilliant green (BG) dye from aqueous solutions in a batch mode. Tartaric acid chemical modification gave promising results than the nonmodified forms of biowaste. Surface characterization was carried out by FT-IR and SEM. The optimum conditions for (BG) dye elimination by Ha-tartaric acid modified (Ha-Ta) were 1.8 g, 45 minutes, 4.0 pH; 40°C, and 100 rpm and with Cc-tartaric acid modified (Cc-Ta) were 1.6 g, 45 minutes, 6.0 pH; 40°C, and 150 rpm dye solution shaking rate, which gave optimum results. Equilibrium data of raw and chemically modified biowaste followed the Langmuir model indicating monolayer chemisorption. Qmax values were found to be 55.24 mg/g, 112.35 mg/g, 50.50 mg/g, and 79.36 mg/g for Ha, Ha-Ta, Cc, and Cc-Ta. Kinetics followed the pseudo-second-order kinetic model. Dye removal involved surface and intraparticle mode of diffusion. Thermodynamic studies proved the process to be spontaneous and exothermic one. The adsorbents can be effectively utilized for the removal of toxic basic dyes, such as BG from wastewaters.

Simultaneously enhanced degradation of N, N-dimethylacetamide and reduced formation of iron sludge by an efficient electrolysis catalyzed ozone process in the presence of dissolved silicate

The generation of sludge is the main issue in iron-based electrochemical techniques. Interestingly, in this study, the effluent was totally limpid and iron sludge did not generate when dissolved silicate (Na₂SiO₃) was used as the electrolyte in an electrolysis catalyzed ozone (ECO-Na₂SiO₃) system. More importantly, the pseudo-first-order rate constants (0.112 min^-1) for DMAC degradation in ECO-Na₂SiO₃ process was much higher than those of ECO systems using other electrolytes. An inhibition film formed on the iron electrode surface was identified to inhibit excess corrosion of iron electrodes and efficiently catalyze decomposition of ozone simultaneously. It was confirmed that hydroxyl radical (^•OH) played a dominant role for the degradation of DMAC, and O₂^•- and H₂O₂ were also contained in ECO-Na₂SiO₃ system. The contributions of contained oxidative reactions in ECO-Na₂SiO₃ system were quantitatively evaluated. Finally, the degradation pathway of DMAC was proposed. This work provides an effective way for protecting electrode from corrosion in electrochemical process.

Activation of peroxymonosulfate by calcined electroplating sludge for ofloxacin degradation

Developing cost-effective metal/metal oxides for peroxymonosulfate (PMS) activation remains a key issue in the sulfate radical based advanced oxidation process. In this work, electroplating sludge (ES), a transition metal-rich byproduct, was anaerobic calcined and characterized. Then, calcined electroplating sludge (CES) was applied as PMS activator for degradation of ofloxacin (OFL) and CES/PMS system exhibited a nearly 90% of OFL removal in 60 min. In addition, effect of CES, PMS, the initial pH and water constituents (chloride, bicarbonate, natural organic matter (NOM) and water backgrounds) on OFL degradation were systematically studied. Moreover, radical quenching tests and electron spin-resonance spectroscopy studies manifested that both SO₄^- and HO were the ruling reactive oxygen species. X-ray photoelectron spectroscopy results of the fresh and used CES demonstrated that the PMS activation mainly occur in the transformation from Fe³⁺ (Cu²⁺) to Fe²⁺ (Cu⁺). Furthermore, liquid chromatography coupled with ion trap time-of-flight mass spectrometry was used to illustrate the possible degradation pathway of OFL. Moreover, CES showed excellent stability and reusability during reaction. This work points out a new way for value-added reuse for ES as a cost-efficient activator of PMS for organic contaminant removal.

Activation of peroxymonosulfate by phosphite: Kinetics and mechanism for the removal of organic pollutants

In this study, phosphite (HPO₃^2-) was used as a novel activator to activate peroxymonosulfate (PMS) for acid orange 7 (AO7) removal. Under the optimized conditions, the decolorization efficiency of AO7 was 82.1% within 60 min with rate constant values (k_obs) of 0.0301 min^-1. Besides, effects of the solution pH and the co-existing inorganic anions including Cl^-, HCO₃^-, HPO₄^2- and SO₄^2- on AO7 removal were also investigated. Except for SO₄^2-, other examined co-existing inorganic anions displayed favorable effects on the removal of AO7. Furthermore, the mechanism for PMS activation by the HPO₃^2- was deeply elucidated by radical scavenger including ethanol (EtOH), tert-butanol (TBA), l-histidine and tiron, and electron spin resonance (ESR) studies. It was proposed that singlet oxygen (¹O₂) would be the dominant reactive oxygen species (ROS) in the HPO₃^2-/PMS system for contamination degradation at neutral pH condition. The findings of this study provided useful information for the application of the substances in industrial wastewaters to activate PMS for organic contaminants degradation and in particular for HPO₃^2--rich electroplating wastewater treatment.

The mechanism changes during bisphenol A degradation in three iron functionalized biochar/peroxymonosulfate systems: The crucial roles of iron contents and graphitized carbon layers

Three magnetic biochar nanocomposites named as C800-1, C800-2 and C800-3 with increased iron deposition amount, decreased graphitized degree and gradually destroyed graphitized carbon layers, respectively, were prepared using potassium ferrate as activator and corn straw as biomass. C800-1, C800-2 and C800-3 exhibited much different bisphenol A degradation effect in presence of peroxymonosulfate among which C800-3 owned the best catalytic performance. For the degradation mechanism, the dominant role of electron transfer pathway was gradually replaced by the SO₄^•- pathway with the increase of iron amount and the destruction of graphitized carbon layers. This work would provide a simple and feasible method, namely changing the ratio of potassium ferrate and biochar, to manipulate the radical and nonradical degradation pathway in PMS-based organic wastewater purification.

Metal-organic frameworks derived C/TiO2 for visible light photocatalysis: Simple synthesis and contribution of carbon species

A series of in-situ carbon-doped TiO₂ (C_x/TiO₂) composites with a porous and crystalline structure were successfully synthesized via one-step and low-temperature calcination of titanium metal-organic framework (MOF), MIL-125(Ti). The resultant materials were comprehensively investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), N₂ adsorption-desorption measurements, UV-vis diffuse reflectance spectrum (DRS), photoluminescence (PL) spectra and photoelectrochemical measurements, and their photocatalytic activities for bisphenol A (BPA) degradation were assessed. Compared with the benchmark TiO₂ photocatalyst (P25), the C_x/TiO₂ composite material with high specific surface, lower band gap, and reduced photogenerated electron hole ratio exhibited outstanding photodegradation activity and durability for BPA, which could be attributed to the combined effect of co-doping of multiple carbon species (substituent carbon and carbonate) and porous structure. During BPA degradation, the holes and superoxide radicals were the primary role oxidative species in the reaction process. Therefore, this new efficient photocatalyst is promising candidate for photodegradation of organic pollutants.

Transformation of iopamidol and atrazine by peroxymonosulfate under catalysis of a composite iron corrosion product (Fe/Fe3O4): Electron transfer, active species and reaction pathways

Cast iron pipes are commonly applied in drinking water distribution systems (DWDSs); peroxymonosulfate (PMS) is a promising alternative for drinking water disinfection; organic micropollutants is still present in drinking water after waterworks' treatment. However, iron corrosion products may affect the reactions between a disinfectant and organic micropollutants. The study investigated the transformation of iopamidol (IPM) and atrazine (ATZ) by PMS under the catalysis of a composite iron corrosion product (Fe/Fe₃O₄). The pseudo-first-order rate constants (k) for the degradation of IPM and ATZ were 1.47 and 1.03 min^-1, respectively. Electron paramagnetic resonance (EPR) experiments indicated that PMS was effectively activated to yield sulfate radical (SO₄^•-) and hydroxyl radical (HO^•), mainly via the reduction by Fe component, dissolved Fe²⁺ and generated Feocta2+. SO₄^•- contributed more than HO^• to the degradation of IPM and ATZ, and the radical yield achieved 0.97 mol/mol. The k values reached maximum with Fe/Fe₃O₄ and PMS doses of 2.5 g L^-1 and 25 mg L^-1, respectively. The optimum mass fraction of Fe₃O₄ in Fe/Fe₃O₄ (MF_mag) and pH were 10% and 7.0, respectively. The k values increased with increasing temperature, while decreased in the presence of water matrix. Most of the iodine released from IPM was oxidized to IO₃^-, and NH₄⁺ was the dominant species of nitrogen released from ATZ. The identification of transformation intermediates showed that the radical chain reactions of IPM was mainly initiated from single electron transfer and radical adduct formation, while those of ATZ was primarily initiated from hydrogen atom abstraction and radical adduct formation.

Study on synergistic effect of ozone and monochloramine on the degradation of chloromethylisothiazolinone biocide

In this study, it was found that monochloramine induced the formation of reactive species during ozonation of chloromethylisothiazolinone (CMIT). CMIT was found recalcitrant to chloramine. However, chloramine promoted the degradation of CMIT by ozonation significantly. Hydroxyl radicals contributed most to CMIT degradation (87%) during ozone/chloramine synergistic oxidation process (SOP). The hydroxyl radical exposure during ozone/chloramine SOP was around 7.9 times higher than that of ozonation process. The hydroxyl radical yield of ozone/chloramine SOP was estimated to be 32%. The reaction mechanisms between ozone and chloramine were postulated to include the oxygen transfer reaction to form singlet oxygen, and the formation of hydroxyl radical by the insertion pathway or electron transfer pathway. Chloramine dosage and pH are essential influencing factors. The degradation of CMIT increased from 41% to 74% with increasing chloramine dosage (0-20 μM), and then decreased to 65% when chloramine dosage continually increased to 40 μM. Ozone/chloramine SOP showed better performance at acidic or neutral conditions than basic condition. Based on the intermediates identified, the degradation pathway of CMIT during ozone/chloramine SOP included the oxidation of sulfur atom and the substitution of chlorine group by hydroxyl group. The oxidation of sulfur atom induced lower toxicities of transformation products. The toxicities of hydroxylation products were lower to fish and algae, but higher to daphnia. Based on the GC-ECD results, only trichloromethane (1.94 μg/L) was detected after ozone/chloramine SOP, accounting for 0.17% (μM/μM) of the CMIT removal.

Enhancing the performance of Fenton-like oxidation by a dual-layer membrane: A sequential interception-oxidation process

Fenton-like oxidation for multicomponent wastewater treatment suffers from a low efficiency due to non-selective nature of produced reactive species. In this study, a multifunctional dual-layer ultrafiltration membrane (Seq-ICM) was synthesized for multiple pollutants decontamination. Characterizations of the membranes indicate that Seq-ICM comprises a skin layer for ultrafiltration, and a porous support layer loaded with ∼50% MIL-53(Fe) for catalysis. With bovine serum albumin coexisting, Seq-ICM can remove 75.7% bisphenol S (BPS), which is much higher than that of a simultaneous interception-catalysis membrane (44.2 %). For multicomponent wastewater treatment, Seq-ICM system can save ∼59%-67% oxidant dosage as well compared with catalysis alone membrane system to achieve 50% BPS removal. Furthermore, the decontamination mechanisms were investigated to explain the advantages of Seq-ICM. Sequential interception and oxidation process by Seq-ICM leads to the interception of macromolecular substances first, following by catalytic oxidation of low-molecular-weight organics. This process prevents macromolecular substances from competing for active species with low-molecular-weight organics, thereby enhancing selectivity and oxidation efficiency. Meanwhile, Seq-ICM shows satisfactory BPS removal efficiency for treatment of 2865 L/m² synthetic solution, as well as in real wastewater matrix. We believe the proposed technology based on a composite membrane is promising for the removal of multicomponent substances from wastewater.

Highly efficient removal and detoxification of phenolic compounds using persulfate activated by MnOx@OMC: Synergistic mechanism and kinetic analysis

Persulfate-based advanced oxidation technology exhibits great potential for hazardous organic pollutant removal from wastewater. Acceleration of pollutant degradation needs to be elucidated, particularly for heterogeneous catalytic systems. In this study, manganese oxide ordered mesoporous carbon composites (MnO_x@OMC) were prepared by nano-casting method and used for persulfate activation to degrade phenol. Kinetics analysis indicate that the rate of phenol degradation using MnO_x@OMC composites was improved by 34.9 folds relative to that using a mixture of MnO_x and OMC. The phenol toxicity towards Caenorhabditis elegans could be totally reduced within 8 min. The different roles of MnO_x and OMC in persulfate activation were confirmed to validate their synergistic effect. MnO_x provided major active sites for persulfate activation in accordance with the surface Mn³⁺/Mn⁴⁺ cycle to induce SO₄^•- radicals. The OMC matrix provided the adsorption sites to enrich phenol molecules on the catalytic surface and promote the interfacial electron transfer process for persulfate activation. Moreover, a novel kinetic model with two distinct kinetic stages was established to verify the effects of phenol and persulfate on phenol removal.

Zn-CNTs-Cu catalytic in-situ generation of H2O2 for efficient catalytic wet peroxide oxidation of high-concentration 4-chlorophenol

4-chlorophenol (4-CP) with high concentration is difficult to degrade thoroughly by traditional treatment methods due to its high biotoxicity and refractory to bio-degradation. A novel catalytic wet peroxide oxidation (CWPO) system based on Zn-CNTs-Cu catalysts through the in-situ generation of H₂O₂ was constructed and investigated for the degradation of high-concentration 4-CP for the first time. Zn-CNTs-Cu composite was prepared by the infiltration melting-chemical replacement method. The operational factors effect, mechanism, and pathways of Zn-CNTs-Cu/O₂ system for high concentration of 4-CP degradation were systematically performed and discussed. At the optimal experimental conditions, the degradation efficiency of 4-CP through CWPO system with Zn-CNTs-Cu/O₂ achieved 100 %, which was 689 % higher than that of wet oxidation system with O₂ alone. According to the mainly in-situ generated H₂O₂, the strong oxidative OH radical and wet-oxidation effect of O₂, high concentration of 4-CP degraded into small molecular organic matter, even been mineralized into carbon dioxide and water in the Zn-CNTs-Cu/O₂ based CWPO system. Overall, Zn-CNTs-Cu/O₂ CWPO system can efficiently degrade high-concentration 4-CP through the in-situ generation of H₂O₂ without extra replenishment, and it provides a novel method and strategy to the efficient treatment of refractory chlorophenols wastewater.

A new insight on enhanced Pb(II) removal by sludge biochar catalyst coupling with ultrasound irradiation and its synergism with phenol removal

The applicability of sludge biochar catalyst (SBC) coupling with ultrasound (US) irradiation for the simultaneous removal of Pb(II) and phenol was firstly investigated in this study. Results indicate that Pb(II) removal of SBC/US process was superior to that of SBC without US. The inhibitory order of the coexisting anions on Pb(II) removal was PO43- > HCO3- > NO3- > F- > SO42- > Cl-. Also, several coexisting metals ions inculding Cr(VI), Ni(II) and Cu(II) could be removed in a simultaneous manner with Pb(II). A high removal performance of Pb(II) by SBC/US process and its synergism with phenol oxidation had been successfully achieved. The simultaneous removal efficiencies of Pb(II) and phenol were high up to 95% within 60 min at optimum reaction conditions. Four kinds of Pb species inculding Pb0, PbCO3, PbO and Pb(OH)2 were formed during the reaction, whereas five kinds of transformation compounds of phenol such as 1,4-benzoquinone, acetic acid, formic acid, maleic acid and propionic acid were detected. Both HO and O2- contributed to the oxidation of phenol by SBC/US process, but HO was dominant radical. A reaction mechanism for the synergistic removal of Pb(II) and phenol by SBC/US process involving in four stages-namely adsorption, precipitation, reduction and Fenton-like oxidation processes was proposed. This study demonstrates that SBC/US process could be considered as a potential candidate for the remediation of real wastewaters containing Pb(II) and phenol.

Synergistic activation of peroxymonosulfate via in situ growth FeCo2O4 nanoparticles on natural rectorite: Role of transition metal ions and hydroxyl groups

Developing low-cost, high-efficiency catalysts for advanced oxidation processes remain a key issue for the degradation of organic pollutants. In this study, a novel FeCo₂O₄/rectorite composite was synthesized via a facile combustion process and employed to activate peroxymonosulfate (PMS) for dealing with atrazine (ATZ). The addition of rectorite could result in higher specific surface area, smaller pore size and more hydroxyl groups, which were beneficial to enrich pollutants to the adsorption sites and provide sufficient reactive sites. After meticulous evaluation, the degradation efficiency of FeCo₂O₄/rectorite composite towards ATZ exhibited improved PMS activation efficiency which was about 2.6 times than that of pure FeCo₂O₄. Based on the characterization results, the sulfate radicals and hydroxyl radicals were considered to be the main free radicals which were involved into the circulation of Co(II)-Co(III)-Co(II) as well as the oxidation of ≡Fe(II), which was responsible for the remarkable catalytic efficiency. In addition, the chemical stability and superior catalytic performance of FeCo₂O₄/rectorite should also be attributed to the chemical combination between metal ions and the surface hydroxyl groups of rectorite. Overall, these findings are beneficial for understanding the mechanism of PMS activation by natural mineral-based catalysts and contributing to the practical application of sulfate-based technology for organic wastewater treatment.

Visible-light activation of peroxymonosulfate by NiCo2O4/Bi24O31Br10 to accelerate tetracycline degradation

Possible degradation mechanism with NiCo₂O₄/Bi₂₄O₃₁Br₁₀ in a PMS/vis system.

Degradation of sulfamethoxazole by cobalt-nickel powder composite catalyst coupled with peroxymonosulfate: Performance, degradation pathways and mechanistic consideration

In this work, cobalt-nickel powder (Co-NP) synthesized through electroless plating was used as a heterogeneous bimetallic catalyst for peroxymonosulfate (PMS) activation to degrade sulfamethoxazole (SMX) in the aqueous phase. Compared with different oxidation systems and different catalysts, it was proved that Co-NP had excellent catalytic ability, fast electron transfer rate and good stability. The effects of key parameters (initial pH, SMX concentration, catalyst and PMS dosages) on SMX degradation and ion leaching were studied in detail. Furthermore, the sensitivity of inorganic anions and different pollutants towards the Co-NP/PMS system was studied to investigate its application in natural water bodies. Quenching test showed that the main reactive oxygen species generated during the reaction were OH and SO₄^-, of which SO₄^- played a predominating role. Combined with XPS analysis, the activation mechanism was concluded that free radicals were mainly generated through the valence change of Co and Ni. Based on the seven intermediates measured through ultrahigh performance liquid chromatograph-mass spectrometry (UPLC-QTOF-MS/MS), the possible degradation pathways of SMX were proposed. Notably, this study provided a new kind of supporter for advanced oxidation processes and electroless plating technology.

Degradation of atrazine by Bi2MoO6 activated peroxymonosulfate under visible light irradiation

In this study, bismuth molybdate (Bi₂MoO₆) nanosheets were prepared via a facile hydrothermal method and applied to activate peroxymonosulfate (PMS) for atrazine (ATZ) elimination under visible light irradiation. An effective photocatalytic synergy was observed in vis/Bi₂MoO₆/PMS system. The Bi₂MoO₆ catalyst showed an excellent stability and extremely low metal leaching in the vis/Bi₂MoO₆/PMS system which were characterized by XRD, XPS, HRTEM, SEM and FT-IR technologies. Besides, the influence of operation parameters (i.e., Bi₂MoO₆ dosage, PMS concentration, initial pH, ATZ concentration, co-existing anions, and humic acid) on the removal of ATZ were also investigated in detail. The best ATZ degradation efficiency (> 99 %) was obtained with Bi₂MoO₆ dosage of 0.6 g/L, PMS concentration of 0.8 mM, and ATZ concentration of 2.5 mg/L after 60 min. UV-vis DRS and Mott-Schottky analysis were taken to evaluate the band structure of Bi₂MoO₆. Trapping experiments and EPR detection demonstrated that superoxide radicals, hydroxyl radicals, sulfate radicals and h⁺ both contributed to ATZ degradation, of which hydroxyl radical OH) and h⁺ were the major species. Besides, the performance of vis/Bi₂MoO₆/PMS system for different pollutant removal was also investigated. Finally, the possible degradation products were monitored by UPLC-QTOF-MS/MS and the pathways of ATZ degradation were put forward. Overall, the vis/Bi₂MoO₆/PMS system shows the potential for the removal of organic pollutants in environmental remediation.

Development of oxygen vacancies enriched CoAl hydroxide@hydroxysulfide hollow flowers for peroxymonosulfate activation: A highly efficient singlet oxygen-dominated oxidation process for sulfamethoxazole degradation

In this study, oxygen vacancies enriched cobalt aluminum hydroxide@hydroxysulfide (CoAl-LDH@CoS_x) hollow flowers was synthesized by in-situ etching of CoAl-LDH using sodium sulfide solution. The analysis of SEM, EDS, XRD, and XPS were used to characterize the samples. The as-synthesized 0.2CoAl-LDH@CoS_x displayed higher catalysis performance of sulfamethoxazole (SMX) degradation via the activation of PMS than the pristine CoAl-LDH. 98.5 % of SMX (40 μM) was eliminated with 0.1 g/L 0.2CoAl-LDH@CoS_x and 0.3 mM PMS at pH 6.0 in 4 min. The degradation fitted with the pseudo-first-order reaction kinetics well with rate constant of 0.89 min^-1 for 0.2CoAl-LDH@CoS_x/PMS system and 0.55 min^-1 for CoAl-LDH/PMS system. Singlet oxygen (¹O₂) was verified as dominant reactive oxygen species responsible for SMX degradation via quenching tests. Mechanism investigation suggested that the oxygen vacancies, redox cycles of Co(II)/Co(III) and S₂^2-/(S^2- and sulfate species) on the surface of 0.2CoAl-LDH@CoS_x were crucial for PMS activation. In addition, the plausible degradation pathways of SMX were proposed by analysis of the SMX degradation intermediates. This study not only reveals that 0.2CoAl-LDH@CoS_x is an efficient catalyst to activate PMS for SMX degradation, but also shed a novel insight into development of heterogeneous catalysts with oxygen vacancies.

Synthesis and application of magnetic materials-barium ferrite nanomaterial as an effective microwave catalyst for degradation of brilliant green

In this work, magnetic separably barium ferrite nanomaterial (BaFeO) was synthesized via citrate acid assisted sol-gel combustion method. Subsequently, X-ray diffraction (XRD), scanning electron microscopy-energy dispersion spectroscopy (SEM-EDS), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) were applied for its structural, morphological, and electromagnetic characterization. In addition, microwave (MW) absorption and thermal conversion test results indicated the BaFeO had electrothermal rather than magnetothermal conversion capacity. Meanwhile, the synthesized BaFeO showed satisfactory performance in both eliminating and mineralization of a typical triphenylmethane dye, brilliant green (BG), in MW-induced catalytic oxidation (MICO) process without extra oxidant addition. Besides, changes in element valence and content of BaFeO before and after MICO process investigated with XRD, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) showed its relatively stable properties. Furthermore, transition oxygen species involved in MICO process was deduced as lattice oxygen species. Then, the possible degradation pathway of BG was proposed as demethylation, open-loop of triphenylmethane, releasing one ring, formation of the benzene ring and the ultimate mineralization based on the degradation intermediates tentatively identified by gas chromatography mass spectrometry (GC/MS) and liquid chromatography mass spectrometry (LC/MS), respectively. Finally, ecotoxicity analysis by ecological structure activity relationships (ECOSAR) showed that both the acute and chronic toxicity of these intermediates were lower than that of parent BG. These findings are important regarding the development of efficient catalysts in MICO process for degradation of BG analogues in wastewater.

Determination methods for steady-state concentrations of HO• and SO4•- in electrochemical advanced oxidation processes

Competitive kinetics and scavenging assay are commonly used for radical quantification. However, the accuracy of the two methods has been challenged in electrochemical advanced oxidation processes (EAOPs) since the strong reactivity of electrode against organic indicators may disrupt the quantitative relationship between indicator consumption and radical concentration. The present study focused on screening suitable indicators and developing suitable methods for determining the steady-state concentrations of SO₄^•- and HO^• ([SO₄^•-]_ss and [HO^•]_ss) in several EAOPs for water treatment based on competitive kinetics and scavenging assay. The applicability of the modified methods and available indicators were investigated through experimental and kinetic analysis. In anode alone process, the competitive kinetics was more appropriate than scavenging assay and benzoic acid (BA) met the basic requirement of being a competitor to determine the [HO^•]_ss. In cathode alone process, BA was more resistant to interfering factors than other competitors (ibuprofen, atrazine and nitrobenzene) and its reaction rate involved only the radical oxidation even when the reaction conditions varied over a wide range. Therefore, the [HO^•]_ss could be obtained by the competitive kinetic equation of BA when HO^• existed alone. When HO^• coexisted with SO₄^•-, a two-step method combining scavenging assay and competitive kinetics was proposed to measure [SO₄^•-]_ss and [HO^•]_ss, in which tert-butyl alcohol and BA were added as scavenger and competitor, respectively. Furthermore, the reliability of each approach was verified by the experimental results and kinetic analysis.

Catalytic potential of CuFe2O4/GO for activation of peroxymonosulfate in metronidazole degradation: study of mechanisms

Application of magnetite nanoparticles (CuFe2O4/GO) were anchored on graphene oxide (GO), as a Heterogeneous nanocomposite for activating of peroxymonosulfate (PMS) into Metronidazole (MNZ) destruction. The effect of solution pH, reaction time, effectiveness of water matrix components and trapping factors, different catalyst concentrations, PMS and contaminants were evaluated as operating factors on the efficiency of MNZ degradation. Also, mineralization, stability, reactivity and Recycling tests of the catalyst, and the degradation kinetics were performed. MNZ degradation and mineralization were obtained under optimal conditions (0.2 g/L catalyst, pH = 5, 30 mg/L MNZ and 2 mM PMS), 100% and 41.02%, respectively over 120 min. Leaching of Fe and Cu was found <0.2 mg/L for CuFe2O4/GO showed a high stability of catalyst, and a significant recyclability was achieved CuFe2O4/GO within 5 times consecutive use. MNZ degradation affected by anions was reduced as follows: HCO3 - > NO3 - > Cl- > SO4 2-. The experimental data were very good agreement with pseudo-first-order kinetic model, and during quenching tests SO4 •- radicals played a dominant role in the degradation process of MNZ. As a result, the CuFe2O4/GO/PMS system can be described as a promising activation of PMS in MNZ degradation, due to its high stability, reusability and good catalyst reactivity, and the production of reactive species simultaneously.

A Review Study on Sulfate-Radical-Based Advanced Oxidation Processes for Domestic/Industrial Wastewater Treatment: Degradation, Efficiency, and Mechanism

High levels of toxic organic pollutants commonly detected during domestic/industrial wastewater treatment have been attracting research attention globally because they seriously threaten human health. Sulfate-radical-based advanced oxidation processes (SR-AOPs) have been successfully used in wastewater treatment, such as that containing antibiotics, pesticides, and persistent organic pollutants, for refractory contaminant degradation. This review summarizes activation methods, including physical, chemical, and other coupling approaches, for efficient generation of sulfate radicals and evaluates their applications and economic feasibility. The degradation behavior as well as the efficiency of the generated sulfate radicals of typical domestic and industrial wastewater treatment is investigated. The categories and characteristics of the intermediates are also evaluated. The role of sulfate radicals, their kinetic characteristics, and possible mechanisms for organic elimination are assessed. In the last section, current difficulties and future perspectives of SR-AOPs for wastewater treatment are summarized.

Efficient degradation of industrial pollutants with sulfur (IV) mediated by LiCoO2 cathode powders of spent lithium ion batteries: A "treating waste with waste" strategy

Strategies to maximize the reuse of electronic and industrial wastes have scientific, economic, social and environmental implications. We herein propose a strategy of "treating waste with waste" using LiCoO₂ cathode powders from spent lithium ion batteries to eliminate industrial pollutants led by sulfur (S) (IV) in waste water. By radical scavenging experiments and electron spin resonance (ESR) analysis, we identified singlet ¹O₂ as the dominant species while SO₄^- and OH as the secondary species for decontamination during the oxidization process mediated by LiCoO₂ powders. The intrinsic mechanism of S(IV) conversion was revealed to be two-step hydrogen migrations from HSO₃^- to O₂ occurring on LiCoO₂ surface by density functional theory (DFT) calculations. The surface of LiCoO₂ powders plays a key role in anchoring sulfur species and forming surface complex as an excellent medium, which is found to be stable and reusable by material characterizations and the recycling experiment. Free Co(II) ions in solvents have no catalysis effect on the conversion of pollutants. Our work offers a particularly vivid example for rational reuse of electronic wastes to eliminate industrial pollutants, and may raise economic benefits in environmental practice due to two aims achieved in once action.

Efficient degradation of sulfamethoxazole by NiCo2O4 modified expanded graphite activated peroxymonosulfate: Characterization, mechanism and degradation intermediates

Expanded graphite (EG) immobilized nickel ferrite (NiCo₂O₄) was successfully constructed by a simple hydrothermal approach and applied for the degradation of sulfamethoxazole (SMX) in model wastewater by peroxymonosulfate (PMS) activation. The features of prepared catalysts were characterized by SEM, TEM, EDS, XRD, BET, TPD and XPS techniques. The influences of several critical parameters including the prepared NiCo₂O₄-EG dosages, PMS concentrations, temperature, initial solution pH and inorganic ions on SMX removal were studied in details. In particular, the synthesized NiCo₂O₄-EG exhibits excellent catalytic performances for SMX depredation over a wide pH range (pH 3.0-11.0). Besides, the transformation of various reactive oxygen species (SO₄^-, HO, O₂^- and ¹O₂) with the change of initial pH was investigated by the electron paramagnetic resonance (EPR) and quenching tests. In addition, twelve major degradation intermediates of SMX were detected by UPLC-QTOF-MS/MS. Finally, the PMS activation mechanism in NiCo₂O₄-EG/PMS system by the synergistic coupling of EG and NiCo₂O₄ were put forward. In brief, this work provided a promising and potential catalyst for PMS activation to remove SMX from wastewater.

Kinetic and mechanism study of UV/pre-magnetized-Fe0/oxalate for removing sulfamethazine

In this study, UV irradiated photochemical reactions of oxalate (Ox) with premagnetized-Fe⁰ (pre- Fe⁰) as the catalyst was used to degrade sulfamethazine (SMT). Magnetic field promoted the release of iron ion from Fe⁰ thus enhanced SMT and Ox removal in UV/pre- Fe⁰/Ox process. X-ray photoelectron spectroscopy demonstrated that the presence of UV and Ox promoted the transformation of Fe³⁺ to Fe²⁺ on Fe⁰, which enhanced the surface bound •OH (•OH_surf) generation. Ox inhibited the formation of iron (hydro)xides and enhanced the hydroxylation of Fe⁰ surface. •OH_surf was mainly responsible for SMT removal (44%), while UV direct photolysis and •OH in the solution both caused around 28% SMT removal. The process with Ox exhibited much higher efficiency in SMT degradation than that added with H₃PO₄, citric acid and ethylenediaminetetraacetic acid, which greatly expanded the chelate-modified Fenton processes and their treatment efficiency.

Recovery of phosphorus and metallic nickel along with HCl production from electroless nickel plating effluents: The key role of three-compartment photoelectrocatalytic cell system

A three-compartment photoelectrocatalytic (PEC) cell system combined with ion exchange and chemical precipitation was proposed to recover phosphorus and nickel from electroless nickel plating effluents containing hypophosphite (H₂PO₂^-) and nickel ions (Ni²⁺). Ion exchange was used to concentrate and separate Ni²⁺ and H₂PO₂^-. As a key unit, the established PEC system consisted of TiO₂/Ni-Sb-SnO₂ photoanode and Ti cathode. With 25.8 mM NaH₂PO₂ and 500 mM NiCl₂, 100 % H₂PO₂^- was oxidized to PO₄^3- in the anode cell, 78 % Ni²⁺ was recovered as metallic Ni in the cathode cell, and 900 mM HCl was obtained in the middle cell within 24 h at 3.0 V. Based on quenching experiments and ESR technique, OH radicals were mainly responsible for H₂PO₂^- oxidation. In situ Raman spectroscopy indicated that Ni²⁺ initially reacted with OH^- to form α-Ni(OH)₂, which was gradually reduced to metallic Ni. Fortunately, a slight pH decrease in the cathode cell in the three-compartment cell system was beneficial for Ni²⁺ reduction to Ni°. The obtained PO₄^3- was recovered by chemical precipitation. Finally, recovery of phosphorus and metallic nickel along with HCl production from an actual electroless nickel plating effluents in terms of efficiency, cost-benefit, and stability assessment were demonstrated.

Multi-walled carbon nanotubes-CoFe2 O4 nanoparticles as a reusable novel peroxymonosulfate activator for degradation of Reactive Black 5

In this study, CoFe₂ O₄ nanoparticles supported on multi-walled carbon nanotubes (MWCNTs), as novel peroxymonosulfate (PMS) activator, were synthesized for degradation of Reactive Black 5 (RB5). The results showed that the maximum removal efficiencies of RB5 (100%), chemical oxygen demand (83.12%), and total organic carbon (65.5%) could happen at pH of 7, catalyst dosage of 100 mg/L, PMS dosage of 2 mM, RB5 concentration of 50 mg/L, and time of 30 min. The results of the temperature effect showed that the activation energy (E_a  = 20.92 kJ/mol) for the synthesized catalyst is much lower compared to other studies. The PMS/MWCNTs-CoFe₂ O₄ system had higher decolorization efficiency and kinetic rates compared to other adsorption and oxidation systems. Quenching experiments proved that RB5 was degraded by sulfate and hydroxyl radicals. The MWCNTs-CoFe₂ O₄ catalyst showed suitable stability and reusability even after five consecutive catalytic reactions. The continuous treatment of RB5 in real water resources was performed using catalyst packed in a column reactor, and its results showed the high efficiency of the column in the catalytic treatment of the dye at long reaction time. Based on the proposed degradation pathway, the azo bands and the naphthalene structure of RB5 are oxidized to compounds with low molecular weight. PRACTITIONER POINTS: MWCNTs-CoFe₂ O₄ was used as a novel recyclable catalyst for the activation of peroxymonosulfate and dye degradation. The rate of dye degradation and peroxymonosulfate activation by MWCNTs-CoFe₂ O₄ was much higher than that of the catalysts alone. Radical SO 4 · - , with contribution percentage of 73.20%, was the main agent for degradation of Reactive Black 5 dye. MWCNTs-CoFe₂ O₄ in the dye degradation process showed excellent stability and reusability, lower activation energy, and easier separation. The dye degradation products were identified by gas chromatography and UV-vis spectrophotometric analyses, and their degradation pathway was suggested.

Performance and mechanism of methylene blue degradation by an electrochemical process

An exciting electrochemical oxidation (EO) process has been developed. Compared with electro-Fenton (EF) and electro-coagulation (EC) processes, this process had more advantages in the degradation of methylene blue. It is observed that methylene blue can be quickly degraded by EO, in which an iron rod is used as an anode, graphite is used as a cathode, and fly ash-red mud particles are used as particle electrodes. Compared to EC and EF processes that are affected by specific pH values, EO has excellent performance in the pH range of 3.0-11.0. In addition, the electric energy consumption (EEC) of EF, EC and EO is 81.51, 36.55 and 21.35 kW h m-3 respectively, suggesting EO is more economical. The free radical scavenging mechanism of i-PrOH is studied, and the contribution of EC, EF and fly ash-red mud particle electrodes in EO is inferred. Particle electrodes before and after use are characterized by SEM, EDS and BET to illustrate the role of particle electrodes in the EO system. Analysis of flocs and solutions by FTIR and GC-MS proves that EO can effectively degrade methylene blue, and the degradation route of methylene blue is speculated. The particle electrode dissolution experiment shows that the prepared fly ash-red mud particle electrode is considered to be suitable and safe for wastewater treatment. Finally, in actual surface water experiments, the EO process still has great potential.

Removal of Textile Dyes by Chemically Treated Sawdust of Acacia: Kinetic and Equilibrium Studies

Sawdust of acacia tree has been successfully used to remove textile dyes from wastewater due to its good sorption properties and its good chemical stability. Two materials are prepared by chemical treatment, including acidic and basic sawdust of acacia. The biosorption tests were carried out on two synthetic dyes of textile which are methylene blue (MB) and brilliant blue (BB). Efficient removal of the both dyes has been achieved by the basic treated sawdust acacia. The modeling of biosorption kinetic shows that the biosorption of MB and that of BB are well described by the pseudo-first-order model for both the chemically treated biosorbents. Equilibrium data have also established using Langmuir and Freundlich isotherm models. Langmuir biosorption capacities are 8.13 and 267.04 mg/g onto basic sawdust acacia and 6.19 and 230.76 mg/g onto acidic sawdust acacia, respectively, for BB and MB sorption. A real final effluent of a textile industry was treated by sorption on both biosorbent basis of sawdust acacia. In fact, the kinetic sorption was rapid with a mass ratio of 1 g/L. However, the biosorption process combined with a biological treatment provides a better result through the physicochemical characteristics of the studied effluent.

Low-Temperature Removal of Refractory Organic Pollutants by Electrochemical Oxidation: Role of Interfacial Joule Heating Effect

Low temperature presents a challenge to wastewater treatment in the winters of cold regions. In the electrochemical oxidation (EO) process, the interfacial Joule heating (IJH) effect results in interfacial temperature higher than that of bulk electrolytes, which would alleviate the negative impact of low water temperature on organic oxidation occurring within the boundary layer of the anode. This study investigated the electrochemical oxidation of the representative recalcitrant organic pollutant, i.e., phenol, p-chlorophenol (p-CP), and 2,4-dichlorophenoxyacetic acid (2,4-D) on titanium suboxide (TiSO) anode at a low water temperature (8.5 ± 1 °C). At a low current density of 2 mA cm^-2, the IJH effect was insignificant and thus had a slight impact on interfacial temperature, leading to a low-efficiency and incomplete organic removal via direct electron transfer (DET) oxidation. Increasing the current density to 20 mA cm^-2 promoted the working up of the IJH effect and thus resulted in a dramatic increase in the interfacial temperature from 8.1 to 38.7 °C. This almost eliminated the negative impact of low temperature on the abatement of organic pollutants as though the low temperature of the bulk solution did not interact with interfacial reactions at all. This was indicated by the oxidation rates of 0.158 min^-1 (phenol), 0.084 min^-1 (p-CP), and 0.070 min^-1 (2.4-D) at a temperature of 8.5 ± 1 °C, the values being almost comparable to that obtained at room temperature (23.5 ± 1 °C). Both theoretical and experimental results demonstrated that the extent to which the low- and room-temperature cases deviated from each other was positively correlated with the activation energy of organic pollutants when reacting with ^•OH. The improvement of organic oxidation at low temperature should result from the compensation of the IJH effect, giving rise to higher ^•OH reactivity, more activated organic molecules, and enhanced mass transfer. This study may prompt new possibilities to develop an IJH effect-based electrochemical manner for decentralized water decontamination in cold regions.