Oxidation of Refractory Benzothiazoles with PMS/CuFe2O4: Kinetics and Transformation Intermediates

Beneficial utilization of ball-milled carbon sand to activate peroxymonosulfate oxidation: Quantitation of ROS using probe-based kinetic models and mechanism insights

In this study, the oil sand was treated with an integrated process of pyrolysis and ball milling, and the obtained ball-milled carbon sand (BMCS) was utilized as peroxymonosulfate (PMS) activator to treat wastewater containing aniline (AN). Quenching experiments and electron paramagnetic resonance (EPR) confirmed the existence of sulfate radical (SO4∙-), hydroxyl radical (·OH) and singlet oxygen (O12) in the BMCS/PMS system. A probe-based kinetic model was constructed to describe the degradation process of pollutants in the BMCS/PMS system, quantified the exposure of each reactive oxygen species and their contributions to AN degradation. BMCS activated PMS to quickly produce SO4∙- and gradually generate ·OH. The O12 exposure showed a rapid increasing trend and the largest total exposure, while its contribution to AN degradation was small. Ball milling time and BMCS dosage demonstrated significant effect on the exposure of ·OH and O12. The main active sites for BMCS to activate PMS were iron oxides, defective carbon and oxygen-containing functional groups. This study provides a green and low-cost process for value-added transformation of pyrolytic residue of oil sand (PROS), so as to promote PROS treatment mode from harmless disposal to resource utilization.

Nucleophilic hydrolysis of dichloroacetonitrile and trichloroacetonitrile disinfection byproducts by peroxymonosulfate: Kinetics and mechanisms

In this work, it was found that peroxymonosulfate (PMS) could appreciably accelerate the transformation rates of dichloroacetonitrile (DCAN) and trichloracetonitrile (TCAN) in aqueous solutions, especially under alkaline pHs. The impact of reactive oxygen species scavengers (methyl alcohol for sulfate radical, tert-butyl alcohol for hydroxyl radical, and azide for singlet oxygen) and water matrices (chloride (Cl-), bicarbonate (HCO3-), and natural organic matter (NOM)) on DCAN and TCAN transformation by PMS is evaluated, revealing negligible effects. A nucleophilic hydrolysis pathway, as opposed to an oxidation process, was proposed for the transformation of DCAN and TCAN by PMS, supported by the hydrolyzable characteristics of these compounds and validated through density functional theory calculations. Kinetic analysis indicated that the transformation of DCAN and TCAN by PMS adhered to a second-order kinetic law, with higher reaction rates observed at elevated pH levels within the range of 7.0-10.0. Kinetic modeling incorporating the hydrolytic contributions of water, hydroxyl ion, and protonated and deprotonated PMS (i.e., HSO5- and SO52-) effectively fitted the experimental data. Species-specific second-order rate constants reveal that SO52- exhibited significantly higher reactivity towards DCAN ((1.69 ± 0.22) × 104 M-1h-1) and TCAN ((6.06 ± 0.18) × 104 M-1h-1) compared to HSO5- ((2.14 ± 0.12) × 102 M-1h-1) for DCAN; and (1.378 ± 0.11) × 103 M-1h-1 for TCAN). Comparative analysis of DCAN and TCAN transformation efficiencies by four different oxidants indicated that PMS rivaled chlorine but falls short of hydrogen peroxide, with peroxydisulfate displaying negligible reactivity. Overall, this study uncovers the nucleophilic hydrolysis characteristics of PMS, supplementing its recognized role as an oxidant precursor or mild oxidant, and underscores its significant implications for environmental remediation.

Efficient removal of moxifloxacin through PMS activation by CuFeS2/MXene

Due to the frequent detection and potential toxicity of moxifloxacin (MOX), its removal technology had attracted attention in recent years. In this research, CuFeS2/MXene was prepared and used to activate peroxymonosulfate (PMS) to remove MOX. The degradation efficiencies, kinetics, influences, and reaction mechanism of MOX by CuFeS2/MXene/PMS were investigated. The synergistic effect of CuFeS2 and MXene significantly enhanced PMS activation, producing SO4•-, HO•, and 1O2 as the main active species. By adding 0.12 g/L CuFeS2/MXene and 0.12 mM PMS, MOX removal efficiency reached 99.1% within 40 min, with a rate constant of 0.1073 min-1. The composite ratios of CuFeS2/MXene impacted PMS activation more significantly than catalyst dosages and PMS concentrations. Acidic conditions were favorable for the degradation of MOX, while HCO3-, HPO42-, Mn2+, and HA had the inhibitory effects. Twelve major products were detected by HPLC-MS, and DFT was used to illustrate possible degradation pathways of MOX, including the removal of nitrogen-containing heterocycle and transformations of quinolone moieties. Toxicity analysis showed that the developmental toxicity, mutagenicity, and acute toxicity of degradation products tended to decrease. CuFeS2/MXene could exhibit excellent reusability, maintaining an average MOX degradation efficiency of 90.8% in the 7-cycle experiments.

Were Persulfate-Based Advanced Oxidation Processes Really Understood? Basic Concepts, Cognitive Biases, and Experimental Details

Persulfate (PS)-based advanced oxidation processes (AOPs) for pollutant removal have attracted extensive interest, but some controversies about the identification of reactive species were usually observed. This critical review aims to comprehensively introduce basic concepts and rectify cognitive biases and appeals to pay more attention to experimental details in PS-AOPs, so as to accurately explore reaction mechanisms. The review scientifically summarizes the character, generation, and identification of different reactive species. It then highlights the complexities about the analysis of electron paramagnetic resonance, the uncertainties about the use of probes and scavengers, and the necessities about the determination of scavenger concentration. The importance of the choice of buffer solution, operating mode, terminator, and filter membrane is also emphasized. Finally, we discuss current challenges and future perspectives to alleviate the misinterpretations toward reactive species and reaction mechanisms in PS-AOPs.

Chalcopyrite functionalized ceramic membrane for micropollutants removal and membrane fouling control via peroxymonosulfate activation: The synergy of nanoconfinement effect and interface interaction

This work developed a novel chalcopyrite (CuFeS2) incorporated catalytic ceramic membrane (CFSCM), and comprehensively evaluated the oxidation-filtration efficiency and mechanism of CFSCM/peroxymonosulfate (PMS) for organics removal and membrane fouling mitigation. Results showed that PMS activation was more efficient in the confined membrane pore structure. The CFSCM50/PMS filtration achieved almost complete removal of 4-Hydroxybenzoic acid (4-HBA) under the following conditions: pH = 6.0, CPMS = 0.5 mM, and C4-HBA = 10 mg/L. Meanwhile, the membrane showed good stability after multiple uses. During the reaction, SO4•- and •OH were generated in the CFSCM50/PMS system, and SO4•- was considered to be the dominant reactive species for pollutant removal. The roles of copper, iron, and sulfur species, as well as the possible catalytic mechanism were also clarified. Besides, the CFSCM50/PMS catalytic filtration exhibited excellent antifouling properties against NOM with reduced reversible and irreversible fouling resistances. The Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory analysis showed an increased in repulsive energy at the membrane-foulant interface in the CFSCM50/PMS system. Membrane fouling model analysis indicated that standard blocking was the dominant fouling pattern for CFSCM50/PMS filtration. Overall, this work demonstrates an efficient catalytic filtration process for foulants removal and outlines the synergy of catalytic oxidation and interface interaction.

Transformation of 6PPDQ during disinfection: Kinetics, products, and eco-toxicity assessment

N-phenyl-N'-(1,3-dimethyl butyl)-p-phenylenediamine-quinone (6PPDQ) currently arouses broad concerns because of its acute lethality to coho salmon and rainbow trout at environmentally relevant concentrations and the wide occurrence in runoff-impacted water. Investigation on the fate and transformation of 6PPDQ in various treatment processes is necessary for its risk assessment and control. Here, we explored the transformation of 6PPDQ during disinfection with its precursor 6PPD as a reference, focusing on kinetics, products, and toxicity variation. 6PPDQ readily reacted with hypochlorite and chlorine dioxide with second-order rate constants of 2580 ± 143 M-1 s-1 and 614 ± 52 M-1 s-1 (pH 7.0 and 25 °C), which are slightly lower than the reactions of 6PPD. We tentatively identified thirteen transformation products for 6PPDQ and eight for 6PPD in reaction with the two disinfectants. It seems that the quinone ring of 6PPDQ and the p-phenylenediamine moiety of 6PPD are reactive sites. The transformation of these compounds probably proceeds through Cl-substitution, ring cleavage, hydroxylation, and amine oxidation and hydrolysis. Tests with zebrafish embryos revealed that the transformation products of 6PPDQ could have higher eco-toxicity than the parent compound, while the toxicity of the 6PPD products remained nearly unchanged. The increased toxicity of 6PPDQ during disinfection highlights the necessity to substantially reduce its content before the disinfection of runoff-impacted water.

2D Ni-Co bimetallic oxide nanosheets activate persulfate for targeted conversion of bisphenol A in wastewater into polymers

The selective removal of targeted pollutants from complex wastewater is challenging. Herein, a novel persulfate (PS)-based advanced oxidation system equipped with a series of two-dimensional (2D) bimetallic oxide nanosheets (NSs) catalysts is developed to selectively degrade bisphenol A (BPA) within mixed pollutants via initiating nonradical-induced polymerization. Results indicate that the Ni0.60Co0.40Ox NSs demonstrate the highest catalytic efficiency among all Ni-Co NSs catalysts. Specifically, BPA degradation rate is 47.34, 27.26, and 9.72 times higher than that of 4-chlorophenol, phenol, and 2,4-dichlorophenol in the mixed solution, respectively. The lower oxidative potential of BPA in relation to the other pollutants renders it the primary target for oxidation within the PDS activation system. PDS molecules combine on the surface of Ni0.60Co0.40Ox NSs to form the surface-activated complex, triggering the generation of BPA monomer radicals through H-abstraction or electron transfer. These radicals subsequently polymerize on the surface of the catalyst through coupling reactions. Importantly, this polymerization process can occur under typical aquatic environmental conditions and demonstrates resistance to background matrices like Cl- and humic acid due to its inherent nonradical attributes. This study offers valuable insights into the targeted conversion of organic pollutants in wastewater into value-added polymers, contributing to carbon recycle and circular economy.

Phosphorus doped cyanobacterial biochar catalyzes efficient persulfate oxidation of the antibiotic norfloxacin

In this study, cyanobacterial biochars (CBs) enriched/doped with non-metallic elements were prepared by pyrolysis of biomass amended with different N, S, and P containing compounds. Their catalytic reactivity was tested for persulfate oxidation of the antibiotic norfloxacin (NOR). N and S doping failed to improve CB catalytic reactivity, while P doping increased reactivity 5 times compared with un-doped biochar. Biochars produced with organic phosphorus dopants showed the highest reactivity. Post-acid-washing improved catalytic reactivity. In particular, 950 ℃ acid-washed triphenyl-phosphate doped CB showed the largest degradation rate and reached 79% NOR mineralization in 2 h. Main attributes for P-doped CBs high reactivity were large specific surface areas (up to 655 m2/g), high adsorption, high C-P-O content, graphitic P and non-radical degradation pathway (electron transfer). This study demonstrates a new way to reuse waste biomass by producing efficient P-doped metal-free biochars and presents a basic framework for designing carbon-based catalysts for organic pollutant degradation.

Room Temperature Anhydrous Suzuki-Miyaura Polymerization Enabled by C-S Bond Activation

The Suzuki-Miyaura cross-coupling is one of the most important and powerful methods for constructing C-C bonds. However, the protodeboronation of arylboronic acids hinder the development of Suzuki-Miyaura coupling in the precise synthesis of conjugated polymers (CPs). Here, an anhydrous room temperature Suzuki-Miyaura cross-coupling reaction between (hetero)aryl boronic esters and aryl sulfides was explored, of which universality was exemplified by thirty small molecules and twelve CPs. Meanwhile, the mechanistic studies involving with capturing four coordinated borate intermediate revealed the direct transmetalation of boronic esters in the absence of H2 O suppressing the protodeboronation. Additionally, the room temperature reaction significantly reduced the homocoupling defects and enhanced the optoelectronic properties of the CPs. In all, this work provides a green protocol to synthesize alternating CPs.

Trace Cu(II)-Mediated Selective Oxidation of Benzothiazole: The Predominance of Sequential Cu(II)-Cu(I)-Cu(III) Valence Transition and Dissolved Oxygen

Trace Cu(II), which inherently exists in soil and some water/wastewater, can trigger persulfate oxidation of some pollutants, but the oxidation capability and mechanism are not well understood, especially toward refractory pollutants. We report in this research that benzothiazole (BTH), a universal refractory pollutant typically originating from tire leachates and various industrial wastewater, can be facilely and selectively removed by peroxydisulfate (PDS) with an equimolar BTH/PDS stoichiometry in the presence of environmental-relevant contents of Cu(II) (below several micromoles). Comprehensive scavenging tests, electron spin resonance analysis, spectroscopy characterization, and electrochemical analysis, revealed that PDS first reduces the BTH-coordinated Cu(II) to Cu(I) and then oxidizes Cu(I) to high-valent Cu(III), which accounts for the BTH degradation. Moreover, once the reaction is initiated, the superoxide radical is probably produced in the presence of dissolved oxygen, which subsequently dominates the reduction of Cu(II) to Cu(I). This facile oxidation process is also effective in removing a series of BTH derivatives (BTHs) that are of environmental concern, thus can be used for their source control. The results highlight the sequential Cu(II)-Cu(I)-Cu(III) transition during PDS activation and the crucial role of contaminant coordination with Cu(II) in oxidative transformation.

Efficiency and mechanism of the degradation of ciprofloxacin by the oxidation of peroxymonosulfate under the catalysis of a Fe3O4/N co-doped sludge biochar

A novel and recyclable composite material, Fe₃O₄/N co-doped sludge biochar (FNBC), was developed from original sludge biochar (BC) and found to have excellent stability and superior catalytic capacity during the ciprofloxacin (CIP) degradation under the action of peroxymonosulfate (PMS). In the FNBC/PMS system, an approximately complete removal of CIP was achieved within 60 min under the condition of 1.0 g/L FNBC, 3.0 mM PMS, and 20 mg/L CIP, which was about 2.08 times of that in BC/PMS system (48.01%). Besides, FNBC/PMS system could effectively remove CIP under the influence of wide pH (2.0-10.0) or inorganic ions compared with BC/PMS system. Moreover, it was found that there were radical produced under the effect of Fe element, defects, functional groups, pyridinic N and pyrrolic N and non-radical caused by graphitic N, carbon atoms next to the iron atoms and better adsorption capacity in the FNBC/PMS system. It was observed that the contribution of hydroxyl radical (•OH), sulfate radical (SO₄^•-) and singlet oxygen (¹O₂), which were the main reactive oxygen species, during the CIP degradation, were 75.80%, 11.49% and 10.26%, respectively. Furthermore, total organic carbon (TOC) variation was analyzed and the degradation pathway of CIP was speculated. The application of this material could combine the recycling of sludge with the effective degradation of refractory organic pollutant, providing an environmentally friendly and economic method.

Resourcelized conversion of livestock manure to porous cage microsphere for eliminating emerging contaminants under peroxymonosulfate trigger

Pollution and resource waste caused by the improper disposal of livestock manure, and the threat from the release of emerging contaminants (ECs), are global challenges. Herein, we address the both problems simultaneously by the resourcelized conversion of chicken manure into porous Co@CM cage microspheres (CCM-CMSs) for ECs degradation through the graphitization process and Co-doping modification step. CCM-CMSs exhibit excellent performance for ECs degradation and actual wastewater purification under peroxymonosulfate (PMS) initiation, and show adaptability to complex water environments. The ultra-high activity can maintain after continuous operation over 2160 cycles. The formation of C-O-Co bond bridge structure on the catalyst surface caused an unbalanced electron distribution, which allows PMS to trigger the sustainable electron donation of ECs and electron gain of dissolved oxygen processes, becoming the key to the excellent performance of CCM-CMSs. This process significantly reduces the resource and energy consumption of the catalyst throughout the life cycle of production and application.

FeMoS2 micoroparticles as an excellent catalyst for the activation of peroxymonosulfate toward organic contaminant degradation

The FeMoS₂ catalyst for activating peroxymonosulfate (PMS) is a promising pathway for removing organic pollutants in wastewater, however, the dominant FeS₂ phases and sulfur (S) vacancies in it are little involved. Herein, for the first time, novel bimetallic FeMoS₂ microparticles were synthesized by a simple method and then applied for PMS activation for degrading organic pollutants. The catalysts were characterized by several techniques, including X-ray diffraction and X-ray photoelectron spectroscopies. The results revealed that new FeMoS₂ microparticles containing S vacancies in the main FeS₂ phases were obtained. FeS₂ and S vacancies were found to play important roles for activating PMS by radical and nonradical pathways. More Fe²⁺ and Mo⁴⁺ were formed in the presence of S vacancies, which offered a new strategy for exploring novel heterogeneous catalysts in the activation of PMS for environmental remediation.

Rapid removal of high-concentration Rhodamine B by peroxymonosulfate activated with Co3O4-Fe3O4 composite loaded on rice straw biochar

In this study, rice straw biochar modified with Co₃O₄-Fe₃O₄ (RSBC@Co₃O₄-Fe₃O₄) was successfully prepared via calcinating oxalate coprecipitation precursor and employed as a catalyst to activate peroxymonosulfate (PMS) for the treatment of Rhodamine B (RhB)-simulated wastewater. The results indicated that RSBC@Co₃O₄-Fe₃O₄ exhibited high catalytic performance due to the synergy between Co₃O₄ and Fe₃O₄ doping into RSBC. Approximately 98% of RhB (180 mg/L) was degraded in the RSBC@Co₃O₄-Fe₃O₄/PMS system at initial pH 7 within 15 min. The degradation efficiency of RhB maintained over 90% after the fourth cycle, illustrating that RSBC@Co₃O₄-Fe₃O₄ displayed excellent stability and reusability. The primary reactive oxygen species (ROS) answerable for the degradation of RhB were ¹O₂, •OH, and SO₄^•-. Moreover, the intermediates involved in the degradation of RhB were identified and the possible degradation pathways were deduced. This work can provide a new approach to explore Co-based and BC-based catalysts for the degradation of organic pollutants.

Oxidation of tetracycline hydrochloride with a photoenhanced MIL-101(Fe)/g-C3N4/PMS system: Synergetic effects and radical/nonradical pathways

MIL-101(Fe)-based catalysts have been widely used for degradation of organic pollutants based on peroxymonosulfate (PMS) activation. Hence, a facile calcination and hydrothermal method was used in this study to prepare a MIL-101(Fe)/g-C₃N₄ composite catalyst with high activity and high stability for PMS activation to degrade tetracycline hydrochloride (TC) under visible-light irradiation. We clearly elucidated the mechanism involved in the MIL-101(Fe)/g-C₃N₄ photo Fenton-catalyzed PMS activation process by separating the PMS activation and pollutant oxidation processes. The synergetic effects of MIL-101(Fe) and g-C₃N₄ involved MIL-101(Fe) acting as an electron shuttle mediating electron transfer from the organic substrate to PMS, accompanied by redox cycling of the surface Fe(II)/Fe(III). Multiple experimental results indicated that PMS was bound to the surface of MIL-101(Fe)/g-C₃N₄ during visible irradiation and generation of sulfate radicals (SO₄^•-), hydroxyl radicals (•OH) and superoxide anion free radicals (•O₂^-) for the radical pathway and singlet oxygen (¹O₂) and holes (h⁺) for the nonradical pathway. The major degradation pathways for TC can be described as demethylation, deamination, deamidation and carbonylation. This work provides valuable information and advances the fundamental understanding needed for design and syntheses of metal-free conjugated polymers modified by metal-organic frameworks for heterogeneous photo-Fenton reactions.

Sulfate radicals-based advanced oxidation processes for the degradation of pharmaceuticals and personal care products: A review on relevant activation mechanisms, performance, and perspectives

Owing to the rapid development of modern industry, a greater number of organic pollutants are discharged into the water matrices. In recent decades, research efforts have focused on developing more effective technologies for the remediation of water containing pharmaceuticals and personal care products (PPCPs). Recently, sulfate radicals-based advanced oxidation processes (SR-AOPs) have been extensively used due to their high oxidizing potential, and effectiveness compared with other AOPs in PPCPs remediation. The present review provides a comprehensive assessment of the different methods such as heat, ultraviolet (UV) light, photo-generated electrons, ultrasound (US), electrochemical, carbon nanomaterials, homogeneous, and heterogeneous catalysts for activating peroxymonosulfate (PMS) and peroxydisulfate (PDS). In addition, possible activation mechanisms from the point of radical and non-radical pathways are discussed. Then, biodegradability enhancement and toxicity reduction are highlighted. Comparison with other AOPs and treatment of PPCPs by the integrated process are evaluated as well. Lastly, conclusions and future perspectives on this research topic are elaborated.

Low-Temperature Reduction Synthesis of γ-Fe2O3-x@biochar Catalysts and Their Combining with Peroxymonosulfate for Quinclorac Degradation

Biochar loading mixed-phase iron oxide shows great advantages as a promising catalyst owing to its eco-friendliness and low cost. Here, γ-Fe₂O_3-x@biochar (E/Fe-N-BC) composite was successfully prepared by the sol-gel method combined with low-temperature (280 °C) reduction. The Scanning Electron Microscope (SEM) result indicated that γ-Fe₂O_3-x particles with the size of approximately 200 nm were well-dispersed on the surface of biochar. The CO derived from biomass pyrolysis is the main reducing component for the generation of Fe (II). The high content of Fe (II) contributed to the excellent catalytic performance of E/Fe-N-BC for quinclorac (QNC) degradation in the presence of peroxymonosulfate (PMS). The removal efficiency of 10 mg/L of QNC was 100% within 30 min using 0.3 g/L γ-Fe₂O_3-x@biochar catalyst and 0.8 mM PMS. The radical quenching experiments and electron paramagnetic resonance analysis confirmed that •OH and SO₄•^- were the main radicals during the degradation of QNC. The facile and easily mass-production of γ-Fe₂O_3-x@biochar with high catalytic activity make it a promising catalyst to activate PMS for the removal of organic pollutants.

The performance and pathway of benzothiazole degradation by electron beam irradiation

Benzothiazole (BTH) is a typical refractory heterocyclic compound that can be used as a photosensitive material in organic synthesis and conditional plant resource research. The extensive use of BTH has led to high BTH concentrations in natural environment, such as in tap water and urine, which tend to inhibit animal hormone synthesis and induce genotoxicity. Traditional wastewater treatment processes cannot effectively remove BTH. Therefore, we aimed to use the electron beam method, an emerging method for pollutant degradation, to degrade BTH in water. Experiments showed that BTH can be effectively degraded (up to 90%) when the electron beam reaches 5 kGy and irradiation conformed perfectly to the pseudo first-order kinetics model. Experimental results showed that acidic conditions are more favorable for electron beam degradation of BTH, while the degradation of most other inorganic ions is inhibited (except SO₄^2-). Hydroxyl radicals (•OH) was confirmed to play a major role in degradation by the experiment, and the mineralization rate was greatly increased by the addition of H₂O₂ and K₂S₂O₈. In addition, our experimental and theoretical calculations showed that the degradation of BTH occurred mainly through the opening of the benzene ring. Theoretical calculations showed that the toxicity of BTH decreased significantly after electron beam degradation, making it an effective way to degrade BTH.

Degradation of benzothiazole pollutant by sulfate radical-based advanced oxidation process

Benzothiazole (BTH) is an aromatic heterocyclic compound with wide industrial applications. In view of its toxicity and wide environmental presence, previous efforts have been made to decompose BTH via different degradation pathways. However, due to its recalcitrant nature, conventional biological treatment methods cannot completely degrade BTH in the wastewater. In this study, sulfate radical-based advanced oxidation process (AOP) technique has been adopted to degrade BTH in aqueous phase. Persulfate (PS) was employed as radical promotor to generate sulfate radical via heat activation. Degradation of BTH by thermally activated persulfate via AOP has been experimentally evaluated in a systematic manner. Laboratory efforts have been made to examine the impact of a number of physiochemical parameters including the type of oxidants, reaction temperature, initial concentrations of PS and BTH, solution pH, and the presence of anionic species. It shows that a higher BTH degradation rate can be achieved by lowering BTH initial concentration or increasing PS concentration. Increasing solution pH or the presence of 10 mM of Cl^-, Br^-, CO32-, or HCO3- species can decrease BTH degradation rate. Furthermore, the primary radical(s) responsible for BTH degradation have been identified as sulfate radical at an acidic aqueous condition, and hydroxyl radical and sulfate radical combined at a basic condition. This study provides the necessary theoretical and technical foundations for BTH degradation via sulfate radical-based AOP technique. The conclusions from this study can substantially promote the field application of AOP, especially sulfate radical-based AOP technique, for BTH degradation in wastewater treatment process.

Peroxymonosulfate (PMS) activation by mackinawite for the degradation of organic pollutants: Underappreciated role of dissolved sulfur derivatives

The internal Fe²⁺/Fe³⁺ cycle is important for peroxymonosulfate (PMS) activation by iron-based materials to produce the reactive oxidative species (ROS) for the breakdown of organic contaminants. Previous studies have focused on the contribution of heterogeneous sulfur species to the Fe²⁺/Fe³⁺ cycle such as lattice S(-II) and surface SO₃^2- of iron sulfides. In this study, we found that the dissolved S(-II) from mackinawite (FeS) had a substantial contribution to the Fe²⁺/Fe³⁺ cycle. Furthermore, the oxidation intermediates of the dissolved S(-II) such as S₂O₃^2- and SO₃^2- ions could convert Fe³⁺ to Fe²⁺ in solution. The elimination of target organic pollutant bisphenol A (BPA) derived from PMS activation triggered by the dissolved Fe²⁺ might be enhanced by the equivalent dissolved S(-II) in the FeS/PMS system. These results revealed that previous studies underestimated the significance of PMS activation by dissolved Fe²⁺ of iron sulfides to organic pollutant degradation. Moreover, SO₄^•- and •OH were more likely to be the main ROS for BPA degradation in the FeS/PMS system compared with FeO²⁺. Considering that the metal sulfides have been widely used to activate PMS, H₂O₂ and peroxydisulfate, this study offers a new perspective on the function of sulfur in these advanced oxidation processes.

Insights into the mechanism of redox pairs and oxygen vacancies of Fe2O3@CoFe2O4 hybrids for efficient refractory organic pollutants degradation

The core-shell Fe₂O₃@CoFe₂O₄ hybrids microspheres with abundant oxygen vacancies were synthesized through in-situ ion exchange-calcination method and employed to induce peroxymonosulfate (PMS) to eliminate organic pollutants. The superior catalytic activity and stability of Fe₂O₃@CoFe₂O₄ were attributed to the synergistic effects of M²⁺/M³⁺ (M denotes Co or Fe) redox cycles. SO₄^·-, ·OH, O₂^·- and ¹O₂ were proved to be the main reactive oxygen species (ROS) involved in the phenol degradation process through quenching experiments and EPR measurements, while the surface-bound SO₄^·- played a dominant role. Trace metal ions leached during the reaction enhanced the PMS activation, and the oxygen vacancies electron transfer process played a critical role in the formation of O₂^·-/¹O₂ and the cycle of M²⁺/M³⁺ redox pairs. The formation of ROS and function of ¹O₂ were also revealed from bulk reaction and interface reaction. This study highlighted the simultaneous evolution of PMS reduction and oxidation to generate ROS, which provided an insight into the efficient catalytic degradation of persistent organic pollutants (POPs).

Prediction of Second-Order Rate Constants of Sulfate Radical with Aromatic Contaminants Using Quantitative Structure-Activity Relationship Model

Predicting the second-order rate constants between aromatic contaminants and a sulfate radical (kSO4•−) is vital for the screening of pollutants resistant to sulfate radical-based advanced oxidation processes. In this study, a quantitative structure-activity relationship (QSAR) model was developed to predict the values for aromatic contaminants. The relationship between logkSO4•− and three molecular descriptors (electron density, steric energy, and ratio between oxygen atoms and carbon atoms) was built through multiple linear regression. The goodness-of-fit, robustness, and predictive ability of the model were characterized statistically with indicators showing that the model was reliable and applicable. Electron density was found to be the most influential descriptor that contributed the most to logkSO4•−. All data points fell within the applicability domain, and no outliers existed in the training set. The comparison with other models indicates that the QSAR model performs well in elucidating the mechanism of the reaction between aromatic compounds and sulfate radicals.

Origins of Electron-Transfer Regime in Persulfate-Based Nonradical Oxidation Processes

Persulfate-based nonradical oxidation processes (PS-NOPs) are appealing in wastewater purification due to their high efficiency and selectivity for removing trace organic contaminants in complicated water matrices. In this review, we showcased the recent progresses of state-of-the-art strategies in the nonradical electron-transfer regimes in PS-NOPs, including design of metal and metal-free heterogeneous catalysts, in situ/operando characterization/analytical techniques, and insights into the origins of electron-transfer mechanisms. In a typical electron-transfer process (ETP), persulfate is activated by a catalyst to form surface activated complexes, which directly or indirectly interact with target pollutants to finalize the oxidation. We discussed different analytical techniques on the fundamentals and tactics for accurate analysis of ETP. Moreover, we demonstrated the challenges and proposed future research strategies for ETP-based systems, such as computation-enabled molecular-level investigations, rational design of catalysts, and real-scenario applications in the complicated water environment. Overall, this review dedicates to sharpening the understanding of ETP in PS-NOPs and presenting promising applications in remediation technology and green chemistry.

Sulfur vacancies on MoS2 enhanced the activation of peroxymonosulfate through the co-existence of radical and non-radical pathways to degrade organic pollutants in wastewater

The enhancement of vacancies in catalysts involving Fenton-like reactions is a promising way to remove organic pollutants in wastewater, but sulfur vacancies are rarely involved. In this work, MoS₂ containing defect sites were synthesized by a simple high-temperature treatment and then applied for activating peroxymonosulfate to eliminate organic pollutants in wastewater. The structure was characterized by several techniques such as XRD, BET, and XPS. Important influencing factors were systemically investigated. The results indicated that MoS₂ with sulfur vacancies possessed a higher catalytic activity than that of the parent MoS₂. The annealing temperature of the catalyst had a great effect on the removal of organic pollutants. Besides, the catalytic system had a wide pH range. Quenching and electron paramagnetic resonance (EPR) experiments indicated that the reaction system contained radical and non-radical species. The characterization results revealed that the defect sites in catalysts mainly strengthened the activity of catalysts. This study offers a new heterogeneous catalyst for the removal of organic pollutants via the peroxymonosulfate-based Fenton-like reactions.

Sulfur vacancies on MoS2 enhanced the activation of peroxymonosulfate to remove organic pollutants in wastewater.

Facile synthesis of nitrogen and sulfur co-doped hollow microsphere polymers from benzothiazole containing wastewater for water treatment

Advanced oxidation is a very efficient method in wastewater treatment, but it is a waste of resources to directly oxide the high concentration of valuable organics into carbon dioxide and water. In this paper, the combination of persulfate and wet air oxidation was developed to recover organics from high concentration of wastewater, along with high mineralization of the residual organics. Nitrogen and sulfur co-doped hollow spherical polymers with narrow size distribution was recovered from the simulated benzothialzole (BTH) wastewater in this facile way, along with chemical oxygen demand (COD) removal rate higher than 90%. The formation route of the polymers was intensively studied based on detailed analysis of different kinds of reaction intermediates. The polymers can be further carbonized into co-doped hollow carbon microsphere, which showed better performance in organic contaminants removal than a commercial activated carbon both in adsorption or catalytic persulfate oxidation. This proposed a new strategy to simultaneously combine oxidation and polymerization for resource recovery from wastewater with high concentration of heterocyclic compounds.

Hydrothermal synthesis of magnetic nano-CoFe2O4 catalyst and its enhanced degradation of amoxicillin by activated permonosulfate

Advanced oxidation process (AOP) has attracted widespread attention because it can effectively remove antibiotics in water, but its practical engineering application is limited by the problems of the low efficiency and difficult recovery of the catalyst. In the study, nano-spinel CoFe₂O₄ was prepared by hydrothermal method and served as the peroxymonosulfate (PMS) catalyst to degrade antibiotic amoxicillin (AMX). The reaction parameters such as CoFe₂O₄ dosage, AMX concentration, and initial pH value were also optimized. The reaction mechanism was proposed through free radical capture experiment and possible degradation pathway analysis. In addition, the magnetic recovery performance and stability of the catalyst were evaluated. Results showed that 85.5% of AMX could be removed within 90 min at optimal conditions. Sulfate radicals and hydroxyl radicals were the active species for AMX degradation. Moreover, the catalyst showed excellent magnetism and stability in the cycle experiment, which has great potential in the AOP treatment of antibiotic polluted wastewater.

Iron molydate catalyzed activation of peroxymonosulfate for bisphenol AF degradation via synergetic non-radical and radical pathways

Though molybdate oxides have been demonstrated as desirable catalysts for environmental remediation, the mechanism of catalytic activation of peroxymonosulfate (PMS) by iron (II) molybdate (FeMoO₄) remains unclear. In this study, FeMoO₄ was synthesized and applied for the activation of PMS to degrade bisphenol-AF (BPAF). FeMoO₄ showed excellent catalytic activity, high stability, and superior mineralization. The influence of operation parameters (i.e., FeMoO₄ dosage, PMS concentration, initial pH, co-existing anions, and temperature) on the removal of BPAF were also investigated in detail. Furthermore, the possible oxidation mechanism was proposed via the chemical quenching tests and electron spin resonance (ESR) analysis, which certified that both free radical (SO₄^-• and ^•OH) and non-radical (¹O₂) were the main reactive oxygen species for degrading BPAF. X-ray photoelectron spectroscopy (XPS) analysis indicated that the radicals were mainly generated via the continuous circulation of Fe³⁺/Fe²⁺ and Mo⁶⁺/Mo⁴⁺ redox cycles to enhance PMS activation. Finally, the degradation pathways of BPAF was proposed based on LC/MS results. This work showed the notable potential of the FeMoO₄/PMS system for degrading organic contaminants in the environment remediation and would promote the understanding of the mechanism of Fe-based molybdate in advanced oxidation.

Well-dispersed iron and nitrogen co-doped hollow carbon microsphere anchoring by g-C3N4 for efficient peroxymonosulfate activation

Developing single-atom Fenton-like catalysts with the maximum utilization of active sites present an attractive potential in environmental remediation. Herein, the single-atom Fe and N co-doped hollow carbon microsphere loaded g-C₃N₄ catalyst (HFeNC-g-C₃N₄) was prepared by an innovative cascade anchoring strategy using polystyrene as the hard template, iron phthalocyanine, polydopamine and urea as the Fe, N and C precursor, in which the in-situ generated g-C₃N₄ could not only effectively anchor Fe atom to create the well-dispersed Fe-N_x active sites, but also accelerate the electron transfer in peroxymonosulfate (PMS) activation. Taking advantages of such sequential protecting strategy, the as-synthesized HFeNC-g-C₃N₄ catalyst with single-atom Fe-N_x active sites, verified by XRD, XPS and HAADF-STEM, could work as an efficient Fenton-like catalyst for PMS activation, which achieved almost 100% removal of 4-chlorophenol (4-CP) in 5 min with the turnover frequency calculated to be 34.6 times higher than that of the homogeneous Fe²⁺ catalyst. The mechanism of O₂^•- dominated radical combined with nonradical ¹O₂ pathway was confirmed by quenching experiments and ESR analysis, which might be interrelated to the improvement of pH adaptability and interference immunity of HFeNC-g-C₃N₄/PMS system. Overall, the present findings provided an innovation strategy for the synthesis of excellent single-atom Fe based catalyst in wastewater purification.

Microdroplet confined assembly enabling the scalable synthesis of titania supported ultrasmall low-valent copper catalysts for efficient photocatalytic activation of peroxymonosulfate

The synthesis of highly dispersed low-valent copper catalysts is very challenging because they are prone to oxidation and sintering. Herein, scalable synthesis of ultrafine Cu(0)/Cu(i) catalysts supported on mesoporous titania microspheres is enabled by a one-step microdroplet confined assembly method. The extremely fast solute assembly in the microdroplet induces excellent metal precursor dispersion, reduces sol-gel crosslinking, and creates wrinkled microspheres with surface crusts and hollow cavities. This structural architecture allows the generation of an inner reductive gas environment during calcination in air to reduce Cu(ii) and create oxygen vacancy (O_V) sites in titania. The obtained catalysts exhibit excellent performance in the photocatalytic activation of peroxymonosulfate (PMS) for pollutant degradation. The Cu(0) species with a surface plasmon resonance effect and O_V-rich anatase facilitate efficient solar light utilization and charge separation. The intimate interface between Cu(i)/Cu(0) and anatase enables fast electron transfer and timely copper redox cycling to promote the activation of PMS.

Greatly enhanced oxidative activity of δ-MnO2 to degrade organic pollutants driven by dominantly exposed {-111} facets

The reactivity of oxidizing materials is highly related to the exposed crystal facets. Herein, δ-MnO₂ with different exposure facets were synthesized and the oxidative activities of the as-prepared materials were evaluated by degrading phenol in water without light. The degradation rate of phenol by δ-MnO₂-{-111} was significantly higher than that by δ-MnO₂-{001}. δ-MnO₂-{-111} also displayed high degradation efficiency to a variety of other organic pollutants, such as ciprofloxacin, bisphenol A, 3-chlorophenol and sulfadiazine. Comprehensive characterization and theoretical calculation verified that the {-111} facet had high density of Mn³⁺, thus displaying enhanced direct oxidative capacity to degrade organic pollutants. In addition, the dominant {-111} facet promoted adsorption/activation of O₂, thus favored the generation of superoxide radical (O₂^•-), which actively participated in the degradation of pollutants. The phenol degradation kinetics could be divided into two distinct phases: the rapid phase (k1_obs = 0.468 min^-1) induced by Mn³⁺ and the slower phase (k2_obs = 0.048 min^-1) dominated by O₂^•-. The synergistically promoted non-radical and radical based reactions resulted in greatly enhanced the oxidative activity of the δ-MnO₂-{-111}. These findings deepen the understanding of facet-dependent oxidative performance of materials and provided valuable insights into the possible practical application of δ-MnO₂ for water purification.

π-π conjugation driving peroxymonosulfate activation for pollutant elimination over metal-free graphitized polyimide surface

A novel metal-free catalyst consisting of typical flower-like graphitized polyimide (g-PI) is first synthesized via an enhanced hydrothermal polymerization process, and it exhibits excellent performance for pollutant removal through peroxymonosulfate (PMS) activation over a wide pH range (3-11). The catalyst is especially effective for attacking the endocrine disruptor bisphenol A (BPA), which can be completely degraded in a short time. Based on the results of characterization, g-PI is consisted of abundant aromatic frameworks with π conjugates based on C-O-C linkages and N-hybrid rings, which play essential roles in the subsequent degradation of pollutants. In the g-PI/PMS/BPA system, BPA (rich in π bonds) is preferentially adsorbed to the catalyst surface through π-π interactions, accompanied by a decrease in its activation energy to produce surface-adsorbed BPA*. This species can be directly attacked and degraded by PMS without the need for the radical processes, which saves the energy required for the intermediate activation process of PMS. On the other hand, the electrons obtained from pollutants are rapidly transferred to the O center, driving PMS activation to generate free radicals. The synergetic interface process offers excellent potential for practical wastewater purification.

Recycling application of modified waste electrolytic manganese anode slag as efficient catalyst for PMS activation

Electrolytic manganese anode slag (EMAS) is the waste residue produced by electrolytic manganese metal industry. At present, no mature recycling system has been established, which causes a waste of resources and threatens the environment. Therefore, the resource utilization of EMAS has attracted increased attention. In this paper, the in-situ resource utilization of EMAS can be realized by pickling treatment was reported. Specifically, EMAS after pickling treatment (PEMAS) was first used as catalyst to activate PMS to degrade tetrachlorophenol (4-CP). Pickling could remove the inert inorganic components on EMAS and increase the specific surface area, pore volume and Mn distribution of the catalyst, thus improving the catalytic performance of the catalyst. Under the conditions of 4-CP of 40 ppm, PMS of 1 mM and PEMAS of 0.3 g L^-1, 85% of 4-CP could be degraded within 50 min. Mechanism studies proved that the main active species were O₂^- and ¹O₂. Some O₂^- contributed to the generation of ¹O₂ and some O₂^- directly contributed to the degradation of 4-CP. During the reaction, the valence state of Mn transformed between Mn(III)/Mn(IV) and Mn(II)/Mn(III) and kept the cycle. Moreover, PEMAS/PMS system exhibited excellent independence of the solution pH, resistance to the versatile inorganic ions and background organic matters, and stability of recycling. In a word, this study has achieved the resource utilization of EMAS and the goal of treating waste with waste, which is a win-win strategy of economic and environmental benefits.

BiOBr/MoS2 catalyst as heterogenous peroxymonosulfate activator toward organic pollutant removal: Energy band alignment and mechanism insight

Utilization of heterogenous catalysts to trigger peroxymonosulfate (PMS) activation is considered an efficient strategy for environmental decontamination. Herein, a tightly bonded flake-like 2D/2D BiOBr/MoS₂ heterojunction was successfully designed through co-precipitation process. By virtue of matched energy levels and intimate interfacial coupling, the Type-II BiOBr/MoS₂ heterojunction significantly expedited charge carrier transfer and thereby promoted the catalytic performance for organic dye oxidation and Cr(VI) reduction. The specially designed BiOBr/MoS₂ heterojunction is also conducive to split PMS and continuously generated highly active species (SO₄^-, OH and O₂^-) in a photo-Fenton system, achieving extraordinary catalytic capacity for various emerging organic pollutants (including phenol, bisphenol A and carbamazepine). The photoexcited electron with prolonged lifetime and exposed Mo sites with multivalence and multiphase nature can effectively activate PMS, which further promotes the oxidation efficiency of holes, as confirmed by scavenging experiments. The excellent stability and oxidative properties could justify scale up using BiOBr/MoS₂ to a small pilot test, implementing the potential value in practical applications. This study would provide novel insight and cognition of PMS activation via a superior heterojunction for complex polluted wastewater treatment.

UV/chlorine process for degradation of benzothiazole and benzotriazole in water: Efficiency, mechanism and toxicity evaluation

Benzothiazole (BZA) and benzotriazole (BTZ) as emerging contaminants were found persistent in aquatic environments and toxic to aquatic organisms. The degradation of BZA and BTZ by UV/chlorine was systematically investigated in this study, and the results showed that BZA and BTZ can be remarkably removed by UV/chlorine compared with UV alone and dark chlorination. The radical quenching tests showed that degradation of BZA and BTZ by UV/chlorine involved the participation of reactive chlorine species (RCS), hydroxyl radical (HO·), and UV photolysis. HO· dominated BZA degradation at neutral and alkalinity, while RCS dominated BTZ degradation. The second-rate order constants for ClO· and BZA and BTZ were 2.22 × 10⁸ M^-1 s^-1, and 2.40 × 10⁸ M^-1 s^-1, respectively. Besides, the second-order rate constants for HO· and BZA and BTZ were also determined at pH 5.0, 7.0, and 9.0, respectively. The degradation efficiency of BZA by UV/chlorine was substantially promoted at acidic conditions, while the degradation efficiency of BTZ was promoted at both acidic and specific alkaline range mainly due to the reactivity of radical species and deprotonated form. The influence of Cl^- was negligible, but the suppression effect of humic acid was slight during the BZA and BZT degradation by UV/chlorine. The transformation products were detected and the possible pathways were proposed. Seven disinfection by-products (DBPs) were identified both in BZA and BTZ degradation and trichloromethane was the main DBP. The toxicity assessment performed by luminescent bacteria and ECOSAR analysis indicated that the detoxification of BZA could be achieved by UV/chlorine, whereas the toxicity of BTZ was increased mainly due to the formation of intermediates. The findings from this study demonstrated UV/chlorine is likewise efficient for BZA and BTZ removal but the toxicity should be considered in the BTZ degradation.

A novel Co(OH)2/Cu2O nanocomposite-activated peroxydisulfate for the enhanced degradation of tetracycline

Football like Co(OH)2/Cu2O nanospheres were developed and used as efficient catalysts in the catalytic system of PDS activation for environmental protection.

Efficient degradation of bisphenol A in water by heterogeneous activation of peroxymonosulfate using highly active cobalt ferrite nanoparticles

Cobalt ferrite CoFe₂O₄ catalyst was fabricated and systematically investigated as an efficient peroxymonosulfate (PMS, HSO₅^-) activator for the degradation of recalcitrant organic contaminants (ROCs) in water treatment. Both SO₄^- and OH on the surface of catalyst were unveiled to be primarily responsible for bisphenol A (BPA) degradation by a comprehensive study using electron paramagnetic resonance (EPR), radical scavengers and quantification of SO₄^-, and the negligible contribution of singlet oxygen (¹O₂) was also observed. BPA degradation was accelerated in the presence of humic acid, and it increased first but then decreased with the further addition of fulvic acid. Moreover, the presence of chloride and bicarbonate ions can enhance both BPA and TOC removal. The toxicity of the target aqueous solution ascended slowly at the early stage but then declined dramatically and almost vanished as the reaction proceeded. The removal efficiencies of other typical ROCs (clofibric acid, 2,4-dichlorophenol, etc.) and the decontamination of natural surface water spiked with BPA were also evaluated. This CoFe₂O₄/PMS process could be well applied as a safe, efficient, and sustainable approach for ROCs remediation in complex wastewater matrix.

Nonradical Oxidation of Pollutants with Single-Atom-Fe(III)-Activated Persulfate: Fe(V) Being the Possible Intermediate Oxidant

When applied for the remediation of polluted water/soil, peroxydisulfate (PDS) usually needs to be effectively activated to generate sulfate radical as the working oxidant. However, a significant part of the oxidation capacity of PDS is lost in this way because sulfate radical unselectively reacts with most of the substances in water/soil. PDS activation without generating radicals is preferred to maximize its oxidation capacity for targeted pollutants. Here, we report that single-atom Fe(III)- and nitrogen-doped carbon (Fe-N-C) can efficiently activate PDS to selectively remove some organic pollutants following an unreported nonradical pathway. The single-atom Fe(III) coordinated with pyridinic N atoms was confirmed to be the active site for the catalytic decomposition of PDS. However, the PDS decomposition did not produce radicals or reactive oxygen species. It is very likely that the coordinated Fe(III) is readily converted to Fe(V) through two-electron abstraction by PDS, and Fe(V) is responsible for the selective degradation of organic pollutants. The PDS/Fe-N-C-coupled process utilizes more oxidation capacity of PDS than both radical oxidation and other reported nonradical oxidation like PDS/CuO under the same experimental conditions. This process provides a new approach to selectively degrade some organic pollutants through PDS activation.

Dynamic Adsorption of Sulfamethoxazole from Aqueous Solution by Lignite Activated Coke

In this paper, lignite activated coke was used as adsorbent for dynamic column adsorption experiments to remove sulfamethoxazole from aqueous solution. The effects of column height, flow rate, initial concentration, pH and humic acids concentration on the dynamic adsorption penetration curve and mass transfer zone length were investigated. Results showed penetration time would be prolonged significantly by increasing column height, while inhibited by the increasement of initial concentration and flow rate. Thomas and Yoon-Nelson model and the Adams-Bohart model were used to elucidate the adsorption mechanism, high coefficients of R² > 0.95 were obtained in Thomas model for most of the adsorption entries, which revealed that the adsorption rate could probably be dominated by mass transfer at the interface. The average change rates of mass transfer zone length to the changes of each parameters, such as initial concentration, the column height, the flow rate and pH, were 0.0003, 0.6474, 0.0076, 0.0073 and 0.0191 respectively, revealed that column height may play a vital role in dynamic column adsorption efficiency. These findings suggested that lignite activated coke can effectively remove sulfamethoxazole contaminants from wastewater in practice.

Trace Cupric Species Triggered Decomposition of Peroxymonosulfate and Degradation of Organic Pollutants: Cu(III) Being the Primary and Selective Intermediate Oxidant

Activation of persulfates to degrade refractory organic pollutants is currently a hot topic of advanced oxidation. Developing simple and effective activation approaches is crucial for the practical application of persulfates. We report in this research that trace cupric species (Cu(II) in several μM) can efficiently trigger peroxymonosulfate (PMS) oxidation of various organic pollutants under slightly alkaline conditions. The intermediate oxidant dominating this process was investigated with electron paramagnetic resonance (EPR), chemical probing, and in situ Raman spectroscopy. Unlike conventional PMS activation, which generates sulfate radical, hydroxyl radical, or singlet oxygen as major oxidants, Cu(III) was confirmed to be the primary and selective intermediate oxidant during the Cu(II)/PMS oxidation. Hydroxyl radical is the secondary intermediate oxidant formed from the reaction of Cu(III) with OH^-. Hybrid oxidation by the two oxidants imparts Cu(II)/PMS with high efficiency in the degradation of a series of pollutants. The results of this work suggest that, with no need of introducing complex catalysts, trace Cu(II) inherent in or artificially introduced to some water or wastewater can effectively trigger PMS oxidation of organic pollutants.

Reactivity of unactivated peroxymonosulfate with nitrogenous compounds

A recent investigation has demonstrated that peroxymonosulfate (PMS), a peroxide commonly applied as a radical precursor during advanced oxidation processes (AOPs), can degrade organic contaminants without the involvement of radicals. However, little is known about this non-radical reaction mechanism. In this study, the reactivity of PMS with several nitrogenous compounds was investigated. Fluoroquinolone antibiotics (except for flumequine) were rapidly degraded by direct PMS oxidation, followed by aliphatic amines (e.g., metoprolol and venlafaxine) and nitrogenous heterocyclic compounds (e.g., adenine and caffeine) at pH 8. The degradation rate of fluoroquinolones followed a second-order kinetic and was highly pH and structure-dependent. Unlike the radical-based AOPs, the direct degradation of contaminants by PMS was less influenced by the scavenging effect of the water matrix. High-Resolution Mass Spectrometry (HRMS) analysis demonstrated that the piperazine ring of fluoroquinolones was the main reaction site. Results showed that the direct electron-transfer from nitrogenous moieties (piperazine ring) to PMS can produce amide and aldehyde compounds. An amide-containing transformation product of ciprofloxacin (m/z 320), showing the highest signal intensity on HRMS, was previously recorded during ozonation. Moreover, the hydroxylamine analogue of ciprofloxacin and enrofloxacin N-oxide were tentatively identified, and the formation of the latter was not impacted by the dissolved oxygen in water. These results suggested that PMS also reacts with nitrogenous compounds via oxygen transfer pathway. Agar disk-diffusion tests indicated that PMS treatment efficiently removed the antibacterial activity of ciprofloxacin with the complete degradation of parent antibiotic, except for the transformation products in an earlier stage, which might still exert antibacterial potency.

Metallic Active Sites on MoO2(110) Surface to Catalyze Advanced Oxidation Processes for Efficient Pollutant Removal

Advanced oxidation processes (AOPs) based on sulfate radicals (SO₄^⋅−) suffer from low conversion rate of Fe(III) to Fe(II) and produce a large amount of iron sludge as waste. Herein, we show that by using MoO₂ as a cocatalyst, the rate of Fe(III)/Fe(II) cycling in PMS system accelerated significantly, with a reaction rate constant 50 times that of PMS/Fe(II) system. Our results showed outstanding removal efficiency (96%) of L-RhB in 10 min with extremely low concentration of Fe(II) (0.036 mM), outperforming most reported SO₄^⋅−-based AOPs systems. Surface chemical analysis combined with density functional theory (DFT) calculation demonstrated that both Fe(III)/Fe(II) cycling and PMS activation occurred on the (110) crystal plane of MoO₂, whereas the exposed active sites of Mo(IV) on MoO₂ surface were responsible for accelerating PMS activation. Considering its performance, and non-toxicity, using MoO₂ as a cocatalyst is a promising technique for large-scale practical environmental remediation.

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The degradation rate of PMS/Fe(II)/MoO₂ system is 50 times higher than that without MoO₂

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Fe(III)/Fe(II) cycle on (110) surface of MoO₂ in PMS/Fe(II)/MoO₂ system was confirmed

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The metal active sites exposed to MoO₂ (110) surface are responsible for PMS activation

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Compared with MoS₂, MoO₂ co-catalytic system has less toxicity and no release of H₂S

Reactivity of chromophoric dissolved organic matter (CDOM) to sulfate radicals: Reaction kinetics and structural transformation

Sulfate radical (SO₄^•-) has been extensively studied as a promising alternative in advanced oxidation processes (AOPs) for water treatment. However, little is known about its reactivity to the ubiquitous dissolved organic matter (DOM) in water bodies. SO₄^•- would selectively react with electron rich moieties in DOM, known as chromophoric DOM (CDOM), due to its light absorbing property. In this study, the reactivity and typical structural transformation of CDOM with SO₄^•- was investigated. Four well characterized hydrophobic DOM fractions extracted from different surface water sources were selected as model CDOM. SO₄^•- was produced through the activation of peroxymonosulfate (PMS) by Co(II) ions at pH 8 in borate buffer. The reactivity of CDOM was studied based on the decrease in its ultraviolet absorbance at 254 nm (UVA₂₅₄) as a function of time. The reactivity of CDOM changed with time where fast and slow reacting CDOMs (i.e., CDOM_fast and CDOM_slow) were clearly distinguished. A second-order rate constant of CDOM_fast with SO₄^•- was calculated by plotting UVA₂₅₄ decrease versus PMS exposure; where a R_ct value (i.e., ratio of sulfate radical exposure to PMS exposure) was calculated using pCBA as a probe compound. The transformation of CDOM was studied through the analysis of the changes in UVA_254, electron donating capacity, fluorescence intensity, and total organic carbon. A transformation pathway leading to a significant carbon removal was proposed. This new knowledge on the kinetics and transformation of CDOM would ultimately assist in the development and operation of SO₄^•--based water treatment processes.