Reactivity and Role of SO5•- Radical in Aqueous Medium Chain Oxidation of Sulfite to Sulfate and Atmospheric Sulfuric Acid Generation

New insight into wastewater treatment by activation of sulfite with humic acid under visible light irradiation

Sulfite (S(IV)), as an alternative to persulfate, has demonstrated its cost-effectiveness and environmentally friendly nature, garnering increasing attention in Advanced Oxidation Processes (AOPs). Dissolved organic matter (DOM) commonly occurred in diverse environments and was often regarded as an interfering factor in sulfite-based AOPs. However, less attention has been paid to the promotion of the activation of sulfite by excited DOM, which could produce various reactive intermediates. The study focused on the activation of sulfite using visible light (VL) - excited humic acid (HA) to efficiently degrade many common organic pollutants, which was better than peroxydisulfate (PDS) and peroxymonosulfate (PMS) systems. Quenching experiments and electron paramagnetic resonance (EPR) analysis revealed that the triplet states of HA (3HA*) activated sulfite through energy transfer, resulting in the production of SO4·-, O2·-, and 1O2. The most significant active species found in the degradation of roxarsone (ROX) was 1O2, which was a non-radical pathway and exhibits high selectivity for pollutant degradation. This non-radical pathway was not commonly observed in traditional sulfite-based AOPs. Additionally, the coexistence of various inorganic anions, such as NO3-, Cl-, SO42-, CO32-, and PO43-, had little effect on the degradation of ROX. Furthermore, DOM from different natural water demonstrated efficient activation of S(IV) under light conditions, opening up new possibilities for applying sulfite-based advanced oxidation to the remediation of organic pollution in diverse sites and water bodies. In summary, this research offered promising insights into the potential application of sulfite-based AOPs, facilitated by photo-excited HA, as a new strategy for efficiently degrading organic pollutants in various environmental settings.

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.

Elucidating the enhanced role of carbonate radical in propranolol degradation by UV/peroxymonosulfate system

Carbonate radical (CO3•-) has been proved to be an important secondary radical in advanced oxidation processes due to various radical reactions involved HCO3-/CO32-. However, the roles and contributions of CO3•- in organic micropollutant degradation have not been explored systematically. Here, we quantified the impact of CO3•- on the degradation kinetics of propranolol, a representative pollutant in the UV/peroxymonosulfate (PMS) system, by constructing a steady-state radical model. Substantially, the measured values were coincident with the predictive values, and the contributions of CO3•- on propranolol degradation were the water matrix-dependent. Propranolol degradation increased by 130% in UV/PMS system containing 10 mM HCO3-, and the contribution of CO3•- was as high as 58%. Relatively high pH values are beneficial for propranolol degradation in pure water containing HCO3-, and the contributions of CO3•- also enhanced, while an inverse phenomenon was shown for the effects of propranolol concentrations. Dissolved organic matter exhibited significant scavenging effects on HO•, SO4•-, and CO3•-, substantially retarding the elimination process. The developed model successfully predicted oxidation degradation kinetics of propranolol in actual sewage, and CO3•- contribution was up to 93%, which in indicative of the important role of CO3•- in organic micropollutant removal via AOPs treatment.

Sulfite induced degradation of sulfamethoxazole by a silica stabilized ZIF-67(Co) catalyst via non-radical pathways: Formation and role of high-valent Co(IV) and singlet oxygen

The sulfite (S(IV))-based advanced oxidation process (AOP) has emerged as an appealing alternative to the traditional persulfate-based AOP for the elimination of organic contaminants from diverse water matrices. In this work, a silica reinforced ZIF-67(Co) catalyst (CZS) is fabricated, characterized and tested in the activation of S(IV) for the sulfamethoxazole (SMX) degradation. The prepared CZS demonstrates superior stability and catalytic ability for the degradation of SMX compared to ZIF-67(Co) across a broad pH range. Unlike the conventional radical-dominated oxidation systems, the CZS/S(IV) system for SMX degradation operates through a non-radical mechanism, featuring high-valent Co(IV) and singlet oxygen (1O2) as the predominated reactive species. The hydroxylated Co species exposed on the CZS surface is identified as the pivotal active site, realizing the S(IV) activation through a complexation-electron transfer process, resulting in the production of various reactive intermediates. Co(II) undergoes the conversion to Co(IV) by generated HSO5-, and 1O2 predominantly originates from the intermediate SO4•-. Profiting from the highly selective oxidation capacities of Co(IV) and 1O2, the established oxidative system demonstrates a remarkable interference resistance and exhibits an exceptional decontamination performance under real-world water conditions. In short, this work provides a sustainable S(IV)-based oxidation strategy for environmental remediation via non-radical mechanism.

Monoethanolamine enhanced iohexol degradation in the Co(II)/sulfite system: Nonnegligible role of complexation in accelerating cobalt redox cycling

Generation of sulfate radicals (SO4•-) from sulfite activation has emerged as a promising method for abatement of organic pollutants in the water and wastewater treatment. Co(II) has garnered attention due to its high catalytic activity in the sulfite activation, which is compromised by the slow Co(II)/Co(III) redox cycling. Regarding the regulation of Co(II) electronic structure via the complexation effect, monoethanolamine (MEA), a common chelator, is introduced into the Co(II)/sulfite system. MEA addition results in a significant improvement in iohexol abatement efficiency, increasing from 40% to 92%. The superior iohexol abatement relies on the involvement of SO4•-, hydroxyl radicals (HO•) and Co(IV). Hydrogen radical (•H) is unexpectedly detected, acting as a strong reducing agent, contributing to the reduction of Co(III). This enhancement of sulfite activation by MEA is due to the formation of the Co(II)-MEA complex, in which the complexation ratio of Co(II) and MEA is critical. Electrochemical characterization and theoretical calculations demonstrate that the complexation can facilitate the Co(II)/Co(III) redox cycling with the concomitant enhancement of sulfite activation. This work provides a new insight into the Co(II)/sulfite system in the presence of organic ligands.

Development of a Five-Chemical-Probe Method to Determine Multiple Radicals Simultaneously in Hydroxyl and Sulfate Radical-Mediated Advanced Oxidation Processes

Advanced oxidation processes (AOPs), such as hydroxyl radical (HO•)- and sulfate radical (SO4•-)-mediated oxidation, are attractive technologies used in water and wastewater treatments. To evaluate the treatment efficiencies of AOPs, monitoring the primary radicals (HO• and SO4•-) as well as the secondary radicals generated from the reaction of HO•/SO4•- with water matrices is necessary. Therefore, we developed a novel chemical probe method to examine five key radicals simultaneously, including HO•, SO4•-, Cl•, Cl2•-, and CO3•-. Five probes, including nitrobenzene, para-chlorobenzoic acid, benzoic acid, 2,4,6-trimethylbenzoic acid, and 2,4,6-trimethylphenol, were selected in this study. Their bimolecular reaction rate constants with diverse radicals were first calibrated under the same conditions to minimize systematic errors. Three typical AOPs (UV/H2O2, UV/S2O82-, and UV/HSO5-) were tested to obtain the radical steady-state concentrations. The effects of dissolved organic matter, Br-, and the probe concentration were inspected. Our results suggest that the five-probe method can accurately measure radicals in the HO•- and SO4•--mediated AOPs when the concentration of Br- and DOM are less than 4.0 μM and 15 mgC L-1, respectively. Overall, the five-probe method is a practical and easily accessible method to determine multiple radicals simultaneously.

Oxidation of emerging contaminants by S(IV) activated ferrate: Identification of reactive species

Studies on the Fe(VI)/S(IV) process have focused on improving the efficiency of emerging contaminants (ECs) degradation under alkaline conditions. However, the performance and mechanisms under varying pH levels remain insufficiently investigated. This tudy delved into the efficiency and mechanism of Fe(VI)/S(IV) process using sulfamethoxazole (SMX) and ibuprofen (IBU) as model contaminants. We found that pH was crucial in governing the generation of reactive species, and both Fe(V/IV) and SO4•- were identified in the reaction system. Specifically, an increase in pH favored the formation of SO4•-, while the formation of Fe(VI) to Fe(V/IV) became more significant at lower pH. At pH 3.2, Fe(III) resulting from the Fe(VI) self-decay reactedwith HSO3-to produce SO4•-and •OH. Under near-neutral conditions, the coexistance of Fe(V/IV) and SO4•- in abundance contributed to the optimal oxidation of both pollutants in the Fe(VI)/S(IV) process, with the removal exceeding 74% in 5 min. Competitive quenching experiments showed that the contributions of Fe(V/IV) to SMX and IBU destruction dimished, while the contributions of radicals increased with an increase in pH. However, this evolution was slower during SMX degradation compared to IBU degradation. A comprehensive understnding of pH as the key factor is essential for the optimization of the sulfite-activated Fe(VI) oxidation process in water treatment.

Insights into the mechanism of multiple Cu-doped CoFe2O4 nanocatalyst activated peroxymonosulfate for efficient degradation of Rhodamine B

The multiple metal catalyst as a promising nanomaterial has shown excellent activity in the peroxymonosulfate (PMS) activation for pollutant degradation. However, the role of special sites and in-depth understanding of the PMS activation mechanism are not fully studied. In this study, a Cu-doped CoFe2O4 nanocatalyst (0.5CCF) was synthesized by a sol-gel and calcination method, and used for PMS activation to remove Rhodamine B (RhB). The results showed that the Cu doping obviously enhanced the catalytic performance of CoFe2O4, with 99.70% of RhB removed by 0.5CCF while 74.91% in the CoFe2O4 within 15 min. Based on the X-ray photoelectron spectroscopy and electrochemical analysis, this could be ascribed to the more low valence of Co and Fe species generated on the 0.5CCF and faster electron transfers occurred in the 0.5CCF due to the Cu doping. In addition, Cu doping could provide more reaction sites for the 0.5CCF to activate PMS for RhB removal. The metal species and the surface hydroxyl were the reaction sites of PMS activation, and the surface hydroxyl played an important role in surface-bound reactive species generation. During the PMS activation, the Cu not only activated PMS to produce reactive oxygen species (ROS), but also regenerated Co2+ and Fe2+ to accelerate the PMS activation. The non-radical of 1O2 was the main ROS with a 99.35% of contribution rate, and the SO5•- self-reaction was its major source. This study provides a new insight to enhance the PMS activation performance of multiple metal catalysts by Cu doping in wastewater treatment.

Cellulose beads supported CoFe2O4: A novel heterogeneous catalyst for efficient rhodamine B degradation via advanced oxidation processes

In this study, a novel mechanical process was used to produce cellulose beads (CB). These beads were then doped with cobalt ferrite nanoparticles (CoFe2O4 NPs) to serve as catalysts for the degradation of rhodamine B (RhB) through peroxymonosulfate (PMS) activation. The physical and chemical properties of CoFe2O4 and CoFe2O4@CB catalysts were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) combined with energy dispersive X-ray spectrometer (EDX), scanning transmission electron microscopy (STEM) techniques, and thermogravimetric analysis (TGA). To optimize RhB degradation efficiency, Response Surface Methodology (RSM) was employed, utilizing the Box-Behnken design (BBD). Under the optimized conditions of a catalyst dosage of 0.40 g/L, PMS dosage of 0.98 mM, RhB concentration of 40 mg/L, pH of 5.27, and reaction time of 60 min, a remarkable degradation efficiency of 98.51 % was achieved at a temperature of 25 °C. In quenching experiments, 1O2, SO4•-, and HO• species are produced in the CoFe2O4@CB/PMS system, with 1O2, and SO4•- species dominating RhB degradation. Remarkably, the new CoFe2O4@CB catalyst has demonstrated exceptional stability and reusability, validated by recycling tests (up to 78 % of RhB degradation efficiency after a 5-cycle experiment) and subsequent characterizations (FTIR, SEM, and EDX) emphasizing unchanged bands, uniform distribution, and consistent composition after reuse cycles. These results demonstrate the effectiveness of mechanically produced CoFe2O4@CB catalysts for advanced oxidation processes (AOPs), with promising applications in wastewater treatment.

Atomically Dispersed Fe-N4 Site as a Conductive Bridge Enables Efficient and Stable Activation of Peroxymonosulfate: Active Site Renewal, Anti-Oxidative Capacity, and Pathway Alternation Mechanism

Atomically dispersed metal sites anchored on nitrogen-doped carbonaceous substrates (M-NCs) have emerged as promising alternatives to conventional peroxymonosulfate (PMS) activators; however, the exact contribution of each site still remains elusive. Herein, isolated Fe-N4 active site-decorated three-dimensional NC substrates (FeSA-NC) via a micropore confinement strategy are fabricated to initiate PMS oxidation reaction, achieving a specific activity of 5.16 × 103 L·min-1·g-1 for the degradation of bisphenol A (BPA), which outperforms most of the state-of-the-art single-atom (SA) catalysts. Mechanism inquiry reveals enhanced chemisorption and electron transfer between PMS and FeSA-NC, enabling an inner electron shuttle mechanism in which Fe-N4 serves as a conductive bridge. The Fe-N4 sites reduce the energy barrier for the formation of SO5* and H*, thereby transforming the reaction pathway from directly adjacent electron transfer into reactive oxygen species (ROS)-dominated oxidation. Theoretical calculations and dynamic simulations reveal that the Fe-N4 sites induce facilitated desorption of reaction intermediates (PMS*/BPA*), which collectively contribute to the renewal of active sites and eventually enhance the catalytic durability. This work offers a reasonable interpretation for the important role of the Fe-N4 moiety in altering the activation mechanism and enhancing the antioxidative capacity of NC materials, which fundamentally furnishes theoretical support for SA material design.

Novel use of ferrous iron/peroxymonosulfate for high-performance seawater desalination pretreatment under harmful algal blooms

Marine harmful algae bloom (HAB) is a growing threat to desalination plants worldwide. This work proposes ferrous iron/peroxymonosulfate (Fe2+/PMS) as a novel pretreatment technology for seawater reverse osmosis (SWRO) under HAB. Herein, Fe2+/PMS achieved a significantly higher reduction of negative charge of algae-laden seawater as compared to conventional coagulation (i.e., coagulant is Fe3+), which thereby facilitated improved flocculation to remove algal cells, turbidity and algal organics matters (AOMs), and marine Ca2+ (∼430 mg/L) could partially contribute to the enhanced coagulation performance. A new understanding of the improved coagulation efficiency achieved with Fe2+/PMS in seawater has been proposed as compared to freshwater: seawater matrix (e.g., 504 mM Cl-) was demonstrated to significantly enhance the generation of high-valent iron (FeO2+) as the main reactive intermediate instead of the long-recognized Fe3+ and free radicals, as revealed by methyl phenyl sulfoxide (PMSO) probe, radicals scavenging analysis and electron spin resonance (ESR) spectra. This new mechanism is expected to provide valuable insights for the development of more novel oxidative seawater treatment technologies. Of note, while trade-off between particles and AOMs played an important role in membrane fouling reduction by different dosages of Fe2+/PMS, Fe2+/PMS with an optimal dosage of 0.1 mM/0.05 mM achieved an unprecedentedly higher reduction (95.26%) of modified fouling index (MFI) as compared to conventional coagulation (13.28%-42.36% with 0.1-0.2 mM of Fe3+). Optical-photothermal infrared spectromicroscopy with sub-micron spatial resolution was employed to analyze membrane foulants for the first time, and Fe2+/PMS was found to mainly cause reduced cake layer resistance, which was attributed to the collectively reduced concentration of algae cells, micro-particles with sizes from 2 to 10 µm, humic substances and biopolymers. Moreover, Fe2+/PMS resulted in lower dissolved Fe3+ (<0.027 mg/L) in ultrafiltration (UF) permeate, which would make it more reliable for SWRO operation as compared to conventional coagulation. When energy-intensive dissolved air flotation (DAF) was employed to withstand HAB, Fe2+/PMS outperformed it and was instrumental in achieving reduced MFI with 56.4% lower operational cost. In this context, Fe2+/PMS would facilitate a high-performance and low-cost pretreatment technology for seawater desalination plants under HAB.

Beyond the formation: unveiling the atmospheric transformation of organosulfates via heterogeneous OH oxidation

Organosulfates (OSs), characterized with a sulfate ester group (R-OSO3-), are abundant constituents in secondary organic aerosols. Recent laboratory-based investigations have revealed that OSs can undergo efficient chemical transformation through heterogeneous oxidation by hydroxyl radicals (˙OH, interchangeably termed as OH in this article), which freshly derives functionalized and fragmented OSs. The reaction not only contributes to the presence of structurally transformed OSs in the atmosphere of which sources were unidentified, but it also leads to the formation of inorganic sulfates (e.g., SO42-) with profound implication on the form of aerosol sulfur. In this article, we review the current state of knowledge regarding the heterogeneous OH oxidation of OSs based on state-of-the-art designs of experiments, computational approaches, and chemical analytical techniques. Here, we discuss the formation potential of new OSs and SO42-, in light of the influence of diverse OS structures on the relative importance of different reaction pathways. We propose future research directions to advance our mechanistic understanding of these reactions, taking into account aerosol matrix effects, interactions with other atmospheric pollutants, and the incorporation of experimental findings into atmospheric chemical transport models.

The altered treatment efficiency of the bisulfite/permanganate process by chloride

Bisulfite-activated permanganate (S(IV)/Mn(VII)) process has proven to be a promising method for rapidly degrading micropollutants. Previous studies have shown that the treatment efficiency of the S(IV)/Mn(VII) process suffer from significant water matrix effects while the mechanism still remains unclear. This study systematically investigates the influence of chloride, which is a common water constituent, on the S(IV)/Mn(VII) process. Addition of chloride decreased the removal of methyl phenyl sulfoxide, phenol, benzoic acid and carbamazepine by the S(IV)/Mn(VII) process but increased dimethoxybenzene removal. The distribution of reactive species in the S(IV)/Mn(VII) process in the absence and presence of chloride was determined with relative rate method. The S(IV)/Mn(VII) process primarily relies on SO4•- and reactive manganese species (RMnS) for pollutant abatement while dosing chloride decreased the concentration of these reactive species. Reactive chlorine species (RCS), such as Cl2•- and ClO•, are formed through the reaction of SO4•- with chloride, and become more important at high concentrations of chloride. RMnS includes Mn(VI), Mn(V) and Mn(III), but none of these species are capable of oxidizing chloride. However, chloride retarded the consumption of bisulfite which reduced RMnS and RCS in turn. DOM inhibited pollutant removal by the S(IV)/Mn(VII) process while the impact mechanism was significantly altered by chloride. Additionally, the study observed a synergistic inhibition of DOM and chloride on the degradation of pollutants that are highly reactive towards Cl2•- and ClO•.

Cobalt-doped molybdenum disulfide for efficient sulfite activation to remove As(III): Preparation, efficacy, and mechanisms

The sulfite(S(IV))-based advanced oxidation process has attracted significant attention in removing As(III) in the water matrix for its low-cost and environmental-friendly. In this study, a cobalt-doped molybdenum disulfide (Co-MoS₂) nanocatalyst was first applied to activate S(IV) for As(III) oxidation. Some parameters including initial pH, S(IV) dosage, catalyst dosage, and dissolved oxygen were investigated. The experiment results show that >Co(II) and >Mo(VI) on the catalyst surface promptly activated S(IV) in the Co-MoS₂/S(IV) system, and the electron transfer between Mo, S, and Co atoms accelerated the activation. SO₄^•- was identified as the main active species for As(III) oxidation. Furthermore, DFT calculations confirmed that Co doping improved the MoS₂ catalytic capacity. This study has proven that the material has broad application prospects through reutilization test and actual water experiments. It also provides a new idea for developing bimetallic catalysts for S(IV) activation.

The overlooked role of UV185 induced high-energy excited states in the dephosphorization of organophosphorus pesticide by VUV/persulfate

Vacuum ultraviolet (VUV) based advanced oxidation processes (AOPs) recently attracted widespread interests. However, the role of UV₁₈₅ in VUV is only considered to be generating a series of active species, while the effect of photoexcitation has long been overlooked. In this work, the role of UV₁₈₅ induced high-energy excited state for the dephosphorization of organophosphorus pesticides was studied using malathion as a model. Results showed malathion degradation was highly related to radical yield, while its dephosphorization was not. It was UV₁₈₅ rather than UV₂₅₄ or radical yield that was responsible for malathion dephosphorization by VUV/persulfate. DFT calculation results demonstrated that the polarity of P-S bond was further increased during UV₁₈₅ excitation, favoring dephosphorization while UV₂₅₄ did not. The conclusion was further supported by degradation path identification. Moreover, despite the fact that anions (Cl^-, SO₄^2- and NO₃^-) considerably affected radical yield, only Cl^- and NO₃^- with high molar extinction coefficient at 185 nm significantly affected dephosphorization. This study shed light on the crucial role of excited states in VUV based AOPs and provided a new idea for the development of mineralization technology of organophosphorus pesticides.

Application of O3/PMS Advanced Oxidation Technology in the Treatment of Organic Pollutants in Highly Concentrated Organic Wastewater: A Review

The ozone/peroxymonosulfate (O3/PMS) system has attracted widespread attention from researchers owing to its ability to produce hydroxyl radicals (•OH) and sulfate radicals (SO4•−) simultaneously. The existing research has shown that the O3/PMS system significantly degrades refinery trace organic compounds (TrOCs) in highly concentrated organic wastewater. However, there is still a lack of systematic understanding of the O3/PMS system, which has created a significant loophole in its application in the treatment of highly concentrated organic wastewater. Hence, this paper reviewed the specific degradation effect, toxicity change, reaction mechanism, various influencing factors and the cause of oxidation byproducts (OBPs) of various TrOCs when the O3/PMS system is applied to the degradation of highly concentrated organic wastewater. In addition, the effects of different reaction conditions on the O3/PMS system were comprehensively evaluated. Furthermore, given the limited understanding of the O3/PMS system in the degradation of TrOCs and the formation of OBPs, an outlook on potential future research was presented. Finally, this paper comprehensively evaluated the degradation of TrOCs in highly concentrated organic wastewater by the O3/PMS system, filling the gaps in scale research, operation cost, sustainability and overall feasibility.

Degradation of geosmin and 2-methylisoborneol in UV-based AOPs for photoreactors with reflective inner surfaces: Kinetics and transformation products

Geosmin (GSM) and 2-methylisoborneol (2-MIB) are representative musty/earthy odor compounds commonly present in surface water. In present study, the degradation of GSM and 2-MIB subject to different UV-based advanced oxidation processes (AOPs), including UV/H₂O₂, UV/S₂O₈^2-, UV/chlorine, and UV/chloramine, in a phosphate-buffered saline (PBS) was conducted in a photoreactor with reflective inner surfaces and compared with that in an environmental water sample. A dynamic model to predict the degradation of GSM and 2-MIB in the photoreactor with reflective inner surfaces in the four UV-based AOPs was developed applying the second-order rate constants for the GSM and 2-MIB with primary reactive species (i.e., •OH, •Cl, and •SO₄^-) determined in this study. The model was proven to successfully simulate the degradation of GSM and 2-MIB. In addition, 8, 7, 8, and 11 degradation intermediates were detected from UV/H₂O₂, UV/S₂O₈^2-, UV/chlorine, and UV/chloramine in this study, and possible degradation pathways were proposed. This study is the first to report the degradation kinetics and formation products of GSM and 2-MIB in UV/chloramine. Research based on photoreactors with reflective inner surfaces may provide some guidance for eliminating GSM and 2-MIB in UV-based AOPs for full-scale engineering applications.

Activation of bisulfite by LaFeO3 loaded on red mud for degradation of organic dye

In this study, red mud (RM) was used as a support for LaFeO₃ to prepare LaFeO₃-RM via the ultrasonic-assisted sol-gel method for the removal of methylene blue (MB) assisted with bisulfite (BS) in the aqueous solution. Characterization by scanning electron microscopy and the Brunauer-Emmett-Teller method indicated that LaFeO₃-RM exhibited a large surface area and porous structure with a higher pore volume (i.e. 10 times) compared with the bulk LaFeO₃. The XRD, XPS and FTIR results revealed that the support of porous RM not only dispersed LaFeO₃ particles but also increased Fe oxidation capability, oxygen-containing functional groups and chemically adsorbed oxygen (from 44.3% to 90.3%) of LaFeO₃-RM, which improved the catalytic performance in structure and chemical composition. MB was removed through the synergistic effect of adsorption and catalysis, with MB molecules first absorbed on the surface and then degraded. The removal efficiency was 88.19% in the LaFeO₃-RM/BS system under neutral conditions but only 27.09% in the LaFeO₃/BS system. The pseudo-first-order kinetic constant of LaFeO₃-RM was six times higher than that of LaFeO₃. Fe(III) in LaFeO₃-RM played a key role in the activation of BS to produce SO 4 ⋅ - by the redox cycle of Fe(III)/Fe(II). Dissolved oxygen was an essential factor for the generation of SO 4 ⋅ - . This work provides both a new approach for using porous industrial waste to improve the catalytic performance of LaFeO₃ and guidance for resource utilization of RM in wastewater treatment.

Reutilization of waste self-heating pad by loading cobalt: A magnetic and green peroxymonosulfate activator for naphthalene degradation

The disposal and recovery of solid wastes and the remediation of polycyclic aromatic hydrocarbons (PAHs) are the key issues of environmental pollution control. In this study, micro cobalt loaded on iron-carbon-vermiculite composite (Co-ICV) was prepared for the first time by the reutilization of waste self-heating pad as a carrier of cobalt catalyst, which exhibited better performance than bulk cobalt catalyst in peroxymonosulfate (PMS) activation for the degradation of naphthalene (NAP) in water. Above 98% of NAP (2.0 mg/L) was effectively eliminated within 15 min by the Co-ICV (0.2 g/L) activated PMS (0.5 mmol/L) in a pH range of 5.0-9.0. High magnetism and very limited cobalt leaching realized the convenient separation and stable reusability of Co-ICV. Mechanism investigation indicated that Co(II) species were the main active sites to activate PMS decomposition for the generation of SO₄^•- and ^•OH, contributing to the rapid degradation of NAP. Meanwhile, the NAP degradation pathways were deduced via combining the identification of intermediates and the calculation of frontier electron densities (FEDs). Furthermore, the ability of the Co-ICV/PMS system for the NAP degradation in actual lake water and the removal of other refractory pollutants demonstrated that the combination of Co-ICV and PMS was a prospective method for the removal of PAHs. Overall, Co-ICV is a green and promising activator of PMS, and the future development will provide more insights into the comprehensive utilization of solid wastes for the remediation of wastewater containing PAHs.

Mechanistic Insights into the Markedly Decreased Oxidation Capacity of the Fe(II)/S2O82- Process with Increasing pH

The poor oxidation capacity of the Fe(II)/S₂O₈^2- [Fe(II)/PDS] system at pH > 3.0 has limited its wide application in water treatment. To unravel the underlying mechanism, this study systematically evaluated the possible influencing factors over the pH range of 1.0-8.0 and developed a mathematical model to quantify these effects. Results showed that ∼82% of the generated Fe(IV) could be used for pollutant degradation at pH 1.0, whereas negligible Fe(IV) contribution was observed at pH 7.5. This dramatic decline of Fe(IV) contribution with increasing pH dominantly accounted for the pH-dependent performance of the Fe(II)/PDS process. Unexpectedly, Fe(II) could consume ∼80% of the generated SO₄^•- non-productively under both acidic and near-neutral conditions, while the larger formation of Fe(III) precipitates at high pH inhibited the SO₄^•- contribution mildly. Moreover, the strong Fe(II) scavenging effect was difficult to be compensated for by slowing down the Fe(II) dosing rate. The competition of dissolved oxygen with PDS for Fe(II) was insignificant at pH ≤ 7.5, where the second-order rate constants for reactions of Fe(II) with oxygen were much lower than or comparable to that between Fe(II) and PDS. These findings could advance our understanding of the chemistry and application of the Fe(II)/PDS process.

An overview on surfactants as pollutants of concern: Occurrence, impacts and persulfate-based remediation technologies

Surfactants are molecules that reduce interfacial energy and increase solubility of other pollutants in water. These properties make them suitable for various domestic and industrial applications, soil remediation, pesticide formulation, among others. The increase in their use and the lack of strict regulations regarding their disposal and management is a matter of concern and requires more attention since the release and distribution of these compounds into the environment can modify important water quality parameters. As a result of these changes, different toxicological effects to aquatic organisms are discussed and exposed herein. On this basis, we provide an overview of the classes of surfactants, as well as their occurrence in different aqueous matrices. In addition, existing regulations around the world regarding their concentration limit for different environments are discussed. Current research focuses on the application of conventional treatments, such as biological treatments; notwithstanding, more toxic and bioaccumulative products can be generated. Advanced Oxidation Processes are promising alternatives and have also been widely applied for the removal of surfactants. This study provides, for the first time, an overview of the application of persulfate-based processes for surfactants degradation based on recent literature findings, as well as the various factors related to the activation of the persulfate anions. This review also highlights the challenges and opportunities for future research to overcome the obstacles to the practical application of this process.

Enhanced degradation of amiloride over Bi2FeNbO7/bisulfite process: Key factors and mechanism

Construction of Bi₂FeNbO₇/bisulfite system for abatement of pharmaceutical residue was achieved. An attempt to synthesize Bi₂FeNbO₇ through hydrothermal technique was confirmed by X-ray diffraction. The magnetic field experiment revealed that Bi₂FeNbO₇ possessed a saturation magnetization of 6.99 emu/g, indicating magnetic attributes of Bi₂FeNbO₇. Scanning electron microscopy images showed that Bi₂FeNbO₇ exhibited regular octahedra in the size of 200-300 nm. In a self-made device, the activation of sodium bisulfite using Bi₂FeNbO₇ for the disposal of amiloride has been carefully explored. The effects of solution pH, sodium bisulfite concentration, Bi₂FeNbO₇ dosage, amiloride concentration, coexisting ions, and water matrix on the performance of Bi₂FeNbO₇/bisulfite system was investigated. The catalytic performance of Bi₂FeNbO₇/bisulfite to degrade amiloride was considerably higher than that of traditional iron oxides. The maximum removal efficiency of amiloride was 97.9% in Bi₂FeNbO₇/bisulfite process. The involvement of Fe might be crucial for activating bisulfite to create active species. The dominating radical in Bi₂FeNbO₇/bisulfite process was identified as SO₃^•‒. With the help of UHPLC/MS/MS, three new degradation products of amiloride were found. Dehalogenation and deamination of amiloride might account for the formation of these transformation products. This work provides a highly efficient Bi₂FeNbO₇/bisulfite process for the disposal of pharmaceutical pollutants in water treatment.

A novel pathway of atmospheric sulfate formation through carbonate radicals

Abstract. Carbon dioxide is considered an inert gas that rarely participates in atmospheric chemical reactions. Nonetheless, we show here that CO2 is involved in some important photo-oxidation reactions in the atmosphere through the formation of carbonate radicals (CO3⚫-). This potentially active intermediate CO3⚫- is routinely overlooked in atmospheric chemistry concerning its effect on sulfate formation. The present work demonstrates that the SO2 uptake coefficient is enhanced by 17 times on mineral dust particles driven by CO3⚫-. Importantly, upon irradiation, mineral dust particles are speculated to produce gas-phase carbonate radical ions when the atmospherically relevant concentration of CO2 presents, thereby potentially promoting external sulfate aerosol formation and oxidative potential in the atmosphere. Employing a suite of laboratory investigations of sulfate formation in the presence of carbonate radicals on the model and authentic dust particles, ground-based field measurements of sulfate and (bi)carbonate ions within ambient PM, together with density functional theory (DFT) calculations for single electron transfer processes in terms of CO3⚫--initiated S(IV) oxidation, a novel role of carbonate radical in atmospheric chemistry is elucidated.

Bisulfite activated permanganate for oxidative water decontamination

Recently, bisulfite-activated permanganate (MnO₄^-; Mn(VII)) process has attracted considerable attention as a novel class of advanced oxidation technology for destruction of organic contaminants in water. However, disputes over the underlying activation mechanism as well as reactive species generated in the Mn(VII)/bisulfite system remain for a long period due to the fairly complex chemistry involved in this system. This article aims to present a critical review on scientific development of the Mn(VII)/bisulfite system, with particular focus on the generation and contribution of various reactive intermediates. Both reactive manganese species (RMnS) (i.e., soluble Mn(III), Mn(V), and Mn(VI)) and radical species (primarily SO₄^•-) are identified as the oxidizing components responsible for enhanced degradation of organic contaminants by the Mn(VII)/bisulfite system. Bisulfite plays a dual role of being an activating agent for reactive intermediates generation and acting as a complexing agent to stabilize RMnS. Solution chemistry (e.g., the [Mn(VII)]/[bisulfite] molar ratio, solution pH, the type of contaminants, ligands, and water matrix components) greatly impacts the generation and consumption of RMnS and radicals, thus influencing the degradation kinetics and pathways of organics. Particularly, dissolved oxygen (DO) is a vital factor for driving the oxidation of organics since the absence of DO can block the generation of SO₄^•- and meantime causes the consumption of RMnS by excess SO₃^•- as a strong reductant. Interestingly, ferrate (FeO₄^2-, Fe(VI)) and hexavalent chromium (CrO₄^2-/HCrO₄^-, Cr(VI)) that are high-valent metal oxyanions analogous to Mn(VII) can be activated by bisulfite via a similar pathway (i.e. both high-valent metal-oxo intermediates and reactive radicals are involved). Furthermore, key knowledge gaps are identified and future research needs are proposed to address the potential challenges encountered in practical application of the Mn(VII)/bisulfite oxidation technology.

Kinetic study on degradation of micro-organics by different UV-based advanced oxidation processes in EfOM matrix

Effluent organic matter (EfOM) contains a large number of substances that are harmful to both the environment and human health. To avoid the negative effects of organic matter in EfOM, advanced treatment of organic matter is an urgent task. Four typical oxidants (H₂O₂, PS, PMS, NaClO) and UV-combined treatments were used to treat micro-contaminants in the presence or absence of EfOM, because the active radical species produced in these UV-AOPs are highly reactive with organic contaminants. However, the removal efficiency of trace contaminants was greatly affected by the presence of EfOM. The degradation kinetics of two representative micro-contaminants (benzoic acid (BA) and para chlorobenzoic acid (pCBA)) was significantly reduced in the presence of EfOM, compared to the degradation kinetics in its absence. Using the method of competitive kinetics, with BA, pCBA, and 1,4-dimethoxybenzene (DMOB) as probes, the radicals (HO^·, SO₄^-·, ClO^·) proved to be the key to reaction species in advanced oxidation processes. UV irradiation on EfOM was not primarily responsible for the degradation of micro-contaminants. The second-order rate constants of the EfOM with radicals were determined to be (5.027 ± 0.643) × 10² (SO₄^-·), (3.192 ± 0.153) × 10⁴ (HO^·), and 1.35 × 10⁶ (ClO^·) (mg C/L)^-1 s^-1. In addition, this study evaluated the production of three radicals based on the concept of R_ct, which can better analyze its reaction mechanism.

Heat/PMS Degradation of Atrazine: Theory and Kinetic Studies

The degradation effect of heat/peroxymonosulfate (PMS) on atrazine (ATZ) is studied. The results show that the heat/PMS degradation for ATZ is 96.28% at the moment that the phosphate buffer (PB) pH, temperature, PMS dosage, ATZ concentration, and reaction time are 7, 50 °C, 400 μmol/L, 2.5 μmol/L, and 60 min. A more alkaline PB is more likely to promote the breakdown of ATZ through heat/PMS, while the PB alone has a more acidic effect on the PMS than the partially alkaline solution. HO• and SO4−• coexisted within the heat/PMS scheme, and ATZ quantity degraded by HO• and SO4−• in PB with pH = 7, pH = 1.7~1. HCO3− makes it difficult for heat/PMS to degrade ATZ according to inorganic anion studies, while Cl− and NO3− accelerate the degradation and the acceleration effect of NO3− is more obvious. The kinetics of ATZ degradation via heat/PMS is quasi-first-order. Ethanol (ETA) with the identical concentration inhibited ATZ degradation slightly more than HCO3−, and both of them reduced the degradation rates of heat/PMS to 7.06% and 11.56%. The addition of Cl− and NO3− increased the maximum rate of ATZ degradation by heat/PMS by 62.94% and 189.31%.

Insight into the role of Fe in the synergetic effect of persulfate/sulfite and Fe2O3@g-C3N4 for carbamazepine degradation

In this work, the role of Fe in the synergetic effect of persulfate/sulfite and Fe₂O₃@g-C₃N₄ (FCN) for carbamazepine (CBZ) degradation was studied. Unexpectedly, Fe₂O₃ in FCN plays very different roles for sulfite [S(IV)] and persulfate (PS) activation. Specifically, since photo-generated holes (h⁺) can transform S(IV) into SO₄^-, and photo-generated electrons (e^-) can accelerate Fe(III) reduction which promotes transition metal based S(IV) activation, a synergetic effect of photocatalysis and Fe is observed in FCN/S(IV)/vis system. In contrast, in FCN/PS/vis system, both Fe(III)/Fe(II) cycle and PS activation compete for e^-. Since PS is a stronger electron acceptor, Fe(III) reduction by e^- is limited. Therefore, the contribution of Fe₂O₃ in FCN/S(IV)/vis system is 3 times higher than that in FCN/PS/vis system. Initial pH affects CBZ removal by changing surface charge of catalysts and oxidants species, while the effect varies for different catalysts and oxidants. This study provides new insight into the synergetic effect of photocatalysis and transition metal for SO₄^- generation, which contributes to catalyst design for environmental application.

High yield M-BTC type MOFs as precursors to prepare N-doped carbon as peroxymonosulfate activator for removing sulfamethazine: The formation mechanism of surface-bound SO4•- on Co-Nx site

M-BTCs (M = Fe, Co and Mn)/melamine were used to prepare N-doped carbon materials, and their performances as activator of peroxymonosulfate (PMS) for sulfamethazine (SMZ) removal were compared. M-BTC type metal-organic frameworks (MOFs) were synthesized under room temperature, with their yield about 7.5 times of ZIF-67 which is the most used MOFs to prepare N-doped carbon materials as the catalyst of persulfate-based advanced oxidation processes. Co-BTC/melamine derived N-doped carbon materials (Co-BTC/5MNC) performed the best, even better than that of ZIF-67 derived N-doped carbon materials. Initial pH (3-9), 0-10 mM inorganic anions (Cl^-, NO₃^-, HCO₃^- and H₂PO₄^2-) and humic acid (5 and 10 mg/L) had no obvious inhibition on SMZ removal with Co-BTC/5MNC as catalyst. The results of both X-ray photoelectron spectroscopy and density functional theory (DFT) calculations indicated that N-coordinated cobalt single atom site (Co-N_x) was the possible active site of Co-BTC/5MNC. Importantly, surface-bound SO₄^•- was identified as the dominant reactive oxygen species for SMZ removal. The SO₄^•- generated through the charge transfer between PMS and catalyst, and was tightly adsorbed on Co-N_x site.

Peroxymonosulfate Activation on Synergistically Enhanced Single-Atom Co/Co@C for Boosted Chemiluminescence of Tris(bipyridine) Ruthenium(II) Derivative

Tris(bipyridine) ruthenium(II)-based luminophores have been well developed in the area of electrochemiluminescence, while their applications in chemiluminescence (CL) are rarely studied due to the poor luminous efficiency and complicated CL reaction. Herein, a novel tris(bipyridine) ruthenium(II)-based ternary CL system is proposed by introducing cobalt single atoms integrated with graphene-encapsulated cobalt nanoparticles (Co SAs/Co@C) and peroxymonosulfate (PMS) as advanced coreaction accelerator and promising coreactant, respectively. On the basis of the experimental results and density functional theory calculations, it is concluded that Co@C can synergistically modulate the adsorption behavior of PMS on Co SAs and then efficiently activate PMS to produce massive singlet oxygen for remarkable CL emission. Under the optimum conditions, the as-prepared CL biosensor exhibits a good linear range, excellent sensitivity, and selectivity, holding great potential for the practical detection of prostate-specific antigen in human serum.

Aqueous Iron(IV)-Oxo Complex: An Emerging Powerful Reactive Oxidant Formed by Iron(II)-Based Advanced Oxidation Processes for Oxidative Water Treatment

High-valent iron(IV)-oxo complexes are of great significance as reactive intermediates implicated in diverse chemical and biological systems. The aqueous iron(IV)-oxo complex (Fe_aq^IVO²⁺) is the simplest but one of the most powerful ferryl ion species, which possesses a high-spin state, high reduction potential, and long lifetime. It has been well documented that Fe_aq^IVO²⁺ reacts with organic compounds through various pathways (hydrogen-atom, hydride, oxygen-atom, and electron transfer as well as electrophilic addition) at moderate reaction rates and show selective reactivity toward inorganic ions prevailing in natural water, which single out Fe_aq^IVO²⁺ as a superior candidate for oxidative water treatment. This review provides state-of-the-art knowledge on the chemical properties and oxidation mechanism and kinetics of Fe_aq^IVO²⁺, with special attention to the similarities and differences to two representative free radicals (hydroxyl radical and sulfate radical). Moreover, the prospective role of Fe_aq^IVO²⁺ in Fe_aq²⁺ activation-initiated advanced oxidation processes (AOPs) has been intensively investigated over the past 20 years, which has significantly challenged the conventional recognition that free radicals dominated in these AOPs. The latest progress in identifying the contribution of Fe_aq^IVO²⁺ in Fe_aq²⁺-based AOPs is thereby reviewed, highlighting controversies on the nature of the reactive oxidants formed in several Fe_aq²⁺ activated peroxide and oxyacid processes. Finally, future perspectives for advancing the evaluation of Fe_aq^IVO²⁺ reactivity from an engineering viewpoint are proposed.

Roles of Sulfites in Reverse Osmosis (RO) Plants and Adverse Effects in RO Operation

More than 60 years have passed since UCLA first announced the development of an innovative asymmetric cellulose acetate reverse osmosis (RO) membrane in 1960. This innovation opened a gate to use RO for commercial use. RO is now ubiquitous in water treatment and has been used for various applications, including seawater desalination, municipal water treatment, wastewater reuse, ultra-pure water (UPW) production, and industrial process waters, etc. RO is a highly integrated system consisting of a series of unit processes: (1) intake system, (2) pretreatment, (3) RO system, (4) post-treatment, and (5) effluent treatment and discharge system. In each step, a variety of chemicals are used. Among those, sulfites (sodium bisulfite and sodium metabisulfite) have played significant roles in RO, such as dechlorination, preservatives, shock treatment, and sanitization, etc. Sulfites especially became necessary as dechlorinating agents because polyamide hollow-fiber and aromatic thin-film composite RO membranes developed in the late 1960s and 1970s were less tolerable with residual chlorine. In this review, key applications of sulfites are explained in detail. Furthermore, as it is reported that sulfites have some adverse effects on RO membranes and processes, such phenomena will be clarified. In particular, the following two are significant concerns using sulfites: RO membrane oxidation catalyzed by heavy metals and a trigger of biofouling. This review sheds light on the mechanism of membrane oxidation and triggering biofouling by sulfites. Some countermeasures are also introduced to alleviate such problems.

Fate and transformation of iodine species during Mn(VII)/sulfite treatment in iodide-containing water

During oxidative treatment of iodide (I^- )-containing waters, I^- is easy to be oxidized into hypoiodous acid (HOI) by various oxidants and the further reaction of HOI with organic compounds can lead to the formation of iodinated disinfection by-products (I-DBPs). Oxidation of HOI to iodate (IO₃ ^- ) or reduction of HOI to I^- has been proposed to reduce the formation of I-DBPs. Because the reaction of HOI with sulfite proceeds rapidly, this study examined the fate of iodine and the formation of I-DBPs in Mn(VII)/sulfite process. Results showed that I^- was oxidized to HOI but the further formation of IO₃ ^- was suppressed due to the fast reduction of HOI to I^- by sulfite. The reactions of HOI with SO₃ ^2- and IO^- with SO₃ ^2- are the major pathways with species-specific second-order rate constants determined to be 1.12 × 10⁵  M^-1  s^-1 and 9.43 × 10⁷  M^-1  s^-1 , respectively. The rapid reaction of HOI with sulfite plays an essential role in minimizing the formation of iodinated products in HOI- and phenol-containing solutions. The toxic risk analysis showed that the toxicity of the generated DBPs from Mn(VII)/sulfite pre-oxidation followed by chlorination only changed slightly. PRACTITIONER POINTS: The decay of I^- was negligible in Mn(VII)/sulfite process. The rapid reaction of HOI with SO₃ ^2- resulted in the negligible generation of IO₃ ^- . Mn(VII)/sulfite process exerted slight influence on the formation of I-DBPs. Mn(VII)/sulfite process is promising for the pretreatment of I^- -containing water.

Degradation of trimethoprim by sulfate radical-based advanced oxidation processes: kinetics, mechanisms, and effects of natural water matrices

In this study, we investigated the removal efficiency of a broad-spectrum antimicrobial agent trimethoprim (TMP) in a UV-activated persulfate system (UV/PS). The pseudo-first-order reaction kinetic model based on the steady-state hypothesis was used to explain TMP degradation behavior in UV-activated persulfate system. Due to the low quantum yield and molar absorptivity of TMP at 254 nm, the direct photolysis of TMP was slower. Since the free radicals generated by adding H2O2 or PS to the system can react with TMP, the degradation rate was significantly accelerated, and[Formula: see text] played a dominant role in the UV/PS system. [Formula: see text] and [Formula: see text] were determined by the pseudo-first-order reaction kinetic model to be 6.02×109 and 3.88×109 M-1s-1, respectively. The values were consistent with competitive kinetic measurements. The pseudo-first-order reaction kinetics model can predict and explain the effect of PS concentration, natural organic matter, and chloride ion on the TMP degradation in the UV/PS system. The observed pseudo first-order rate constants for TMP degradation (kobs) increased with the persulfate concentration, but it significantly decreased in the presence of NOM and chloride. [Formula: see text] has no effect on the degradation of TMP, while [Formula: see text] promotes the degradation and [Formula: see text] inhibits the degradation. The common transition metal ion (such as Cu2+, Zn2+, and Co2+) in industrial wastewater has a synergistic effect on the TMP degradation in the UV/PS system, but excessive metal ions will lead to a decrease of the degradation rate.

Novel Nonradical Oxidation of Sulfonamide Antibiotics with Co(II)-Doped g-C3N4-Activated Peracetic Acid: Role of High-Valent Cobalt-Oxo Species

Herein, we report that Co(II)-doped g-C₃N₄ can efficiently trigger peracetic acid (PAA) oxidation of various sulfonamides (SAs) in a wide pH range. Quite different from the traditional radical-generating or typical nonradical-involved (i.e., singlet oxygenation and mediated electron transfer) catalytic systems, the PAA activation follows a novel nonradical pathway with unprecedented high-valent cobalt-oxo species [Co(IV)] as the dominant reactive species. Our experiments and density functional theory calculations indicate that the Co atom fixated into the nitrogen pots of g-C₃N₄ serves as the main active site, enabling dissociation of the adsorbed PAA and conversion of the coordinated Co(II) to Co(IV) via a unique two-electron transfer mechanism. Considering Co(IV) to be highly electrophilic in nature, different substituents (i.e., five-membered and six-membered heterocyclic moieties) on the SAs could affect their nucleophilicity, thus leading to the differences in degradation efficiency and transformation pathway. Also, benefiting from the selective oxidation of Co(IV), the established oxidative system exhibits excellent anti-interference capacity and achieves satisfactory decontamination performance under actual water conditions. This study provides a new nonradical approach to degrade SAs by efficiently activating PAA via heterogeneous cobalt-complexed catalysts.

Sulfite Activation by Glucose-Derived Carbon Catalysts for As(III) Oxidation: The Role of Ketonic Functional Groups and Conductivity

In this study, a series of glucose-derived carbon catalysts were developed and applied for the activation of sulfite for the oxidation of As(III). The process of sulfite activation with the carbon catalysts is based on the production of oxysulfur free radicals such as SO₃^•-, SO₅^•-, and SO₄^•-. The factors responsible for the sulfite activation performance of carbon catalysts are conductivity and ketonic functional groups. A complex is formed between the sulfite and carbon catalysts, and the electron transfer that takes place within the complex leads to the generation of semiquinone and oxysulfur radicals, and finally, the oxysulfur radicals are converted into SO₄^•- by means of O₂, which results in the As(III) oxidation. The efficiency of the sulfite/carbon system is enhanced under normoxia conditions due to the reversible transformation cycle occurring among C═O/C-O^•/C-OH triads. The present study is of great environmental significance as sulfite is a source of SO₄^•- generated, and the activation is achieved by a metal-free carbon material, which makes the process viable and environmentally friendly.

Peroxymonosulfate activation by cobalt(II) for degradation of organic contaminants via high-valent cobalt-oxo and radical species

The reaction between Co(II) and PMS is an appealing advanced oxidation process (AOP), where multiple reactive oxidizing species (ROS) including high-valent cobalt-oxo [Co(IV)], sulfate radical (SO₄^•-), and hydroxy radical (^•OH) are intertwined together for degrading pollutants. However, the relative contribution of various ROS and the influences of nontarget matrix constituents, on the degradation process are still unclear and yet to be answered. In this study, we confirmed the generation Co(IV) as dominant intermediate oxidant at acid medium by using methyl phenyl sulfoxide (PMSO) as a probe compound. Using chemical scavenging methods, the role of SO₄^•- and ^•OH was also identified, and the major ROS were converted from Co(IV) to radical species with the increase of PMS/Co(II) molar ratio as well as pH value. In addition, we found that their contributions to the abatement of organic contaminants are highly dependent on both their available amount and substrate-specific reactivity. Generally, organic substrates with low ionization potential (IP) are prone to react with Co(IV). More interestingly, in contrast to radical-based oxidation, Co(IV) exhibited the great resistance to humic acid (HA) and background ions. This study might shed new light on the PMS activation by cobalt(II) for degradation of organic contaminants.

Enhanced Rates of Transition-Metal-Ion-Catalyzed Oxidation of S(IV) in Aqueous Aerosols: Insights into Sulfate Aerosol Formation in the Atmosphere

The oxidation of S(IV) is a critical step in the fate of sulfur dioxide emissions that determines the amount of sulfate aerosol in the atmosphere. Herein, we measured accelerated S(IV) oxidation rates in micron-sized aqueous aerosols compared to bulk solutions. We have investigated both buffered and unbuffered systems across a range of pH values in the presence of atmospherically relevant transition-metal ions and salts and consistently found the oxidation rate to be accelerated by ca. 1-2 orders of magnitude in the aerosol. This enhancement is greater than can be explained by the enrichment of species in the aerosol compared to the bulk and indicates that surface effects and potentially aerosol pH gradients play important roles in the S(IV) oxidation process in the aqueous aerosol. In addition, our experiments were performed with dissolved S(IV) ions (SO₃^2-/HSO₃^-), allowing us to demonstrate that acceleration occurs in the condensed phase showing that enhanced sulfate formation is not exclusively due to gas-aerosol partitioning or interfacial SO₂ oxidation. Our findings are an important step forward in understanding larger than expected sulfate concentrations observed in the atmosphere and show that inorganic oxidation processes can be accelerated in micron-sized aqueous droplets compared to the bulk solution.

Efficient destruction of emerging contaminants in water by UV/S(IV) process with natural reoxygenation: Effect of pH on reactive species

UV/sulfite systems with oxygen have recently been considered as advanced oxidation processes in view of the participation of oxysulfur radicals. However, the contribution of •OH and the efficiency of destructing emerging contaminants (ECs) in water remain largely unclear. Here, the UV/S(IV) process was applied with natural reoxygenation to degrade two typical ECs, diethyl phthalate (DEP) and bisphenol A (BPA) showing different properties. Solution pH played the key role in determining the reactive species, and both DEP and BPA were more favorably degraded at more alkaline conditions with higher utilization efficiency of SO₃^2-. Specifically, the H•, O₂^•-, •OH and SO₃^•- were identified at acidic condition, but the amount of •OH accumulated significantly with the elevation of pH. Competitive quenching experiments showed that e_aq^- and •OH dominated the degradation of DEP and BPA at alkaline condition, respectively. Besides, DEP showed higher quantum efficiency for the indirect photolysis and mineralization degree than that of BPA at pH 9.2 mainly due to the direct use of the primary photoproduct. The possible transformation mechanisms of S(IV) and mineralization routes of both pollutants were proposed. This study may provide new insights into the mechanisms involved in UV/S(IV) process and a promising alternative for efficient removal of ECs in water.

Trace Co2+ coupled with phosphate triggers efficient peroxymonosulfate activation for organic degradation

Cobalt-mediated activation of peroxymonosulfate (PMS) has been widely used to remove the refractory organic pollutants from contaminated waters. However, the residual cobalt usually at a trace level inevitably brings about secondary pollution to be disposed of. In this study we found that the presence of phosphate could trigger a more efficient catalytic activation of PMS at trace Co²⁺ dosages (0.17-1.7 μM). Fast degradation of atrazine (ATZ) was observed in the Co²⁺/PMS/phosphate system, with the pseudo first-order kinetic rate constant as high as 5.4 and 15.4 times that in Co²⁺/PMS and phosphate/PMS systems respectively under otherwise similar conditions. The presence of phosphate promoted the production of sulfate radical (SO₄·^-), accompanying the enhanced formation of by-product ¹O₂ simultaneously. Using a competition reaction kinetics approach, the contribution of SO₄·^- to ATZ oxidation was determined as 96.5%, suggesting that SO₄·^- was the main reactive species responsible for ATZ removal. Such favorable effect was partially ascribed to the specific ligand structure of six coordination structure between phosphate and cobalt, which facilitated electron transfer in the Co^III/Co^II reduction. In addition, it was dependent upon the aqueous phosphate levels, and low level (< 0.5 mM) was insufficient to drive the Co^III/Co^II cycle, whereas the higher level (> 15 mM) showed negative effect since the excessive phosphate could quench SO₄·^- and·OH. This study is believed to advance the fundamental understanding of the ligand effect on the cobalt-mediated sulfate radicals-based advanced oxidation process.

Uncertainty and misinterpretation over identification, quantification and transformation of reactive species generated in catalytic oxidation processes: A review

The identification of reactive radical species using quenching and electron paramagnetic resonance (EPR) tests has attracted extensive attention, but some mistakes or misinterpretations are often present in recent literature. This review aims to clarify the corresponding issues through surveying literature, including the uncertainty about the identity of radicals in the bulk solution or adsorbed on the catalyst surface in quenching tests, selection of proper scavengers, data explanation for incomplete inhibition, the inconsistent results between quenching and EPR tests (e.g., SO₄^•- is predominant in quenching test while the signal of ^•OH predominates in EPR test), and the incorrect identification of EPR signals (e.g., SO₄^•- is identified by indiscernible or incorrect signals). In addition, this review outlines the transformation of radicals for better tracing the origin of radicals. It is anticipated that this review can help in avoiding mistakes while investigating catalytic oxidative mechanism with quenching and EPR tests.

UV/ peroxymonosulfate process for degradation of chloral hydrate: Pathway and the role of radicals

In this study, kinetics, influencing factors and potential mechanisms involved in the degradation of chloral hydrate (CH) by UV/peroxymonosulfate (PMS) process were demonstrated. The degradation rate of CH could reach 89.6% by UV₂₅₄/PMS process, significantly exceeding UV₃₀₀/PMS (0.7%), UV₃₅₀/PMS (6.3%), UV₂₅₄ direct photolysis (9.0%) and PMS alone (0.0%) processes. CH degradation in UV₂₅₄/PMS system followed pseudo first-order degradation kinetics with an apparent rate constant of 0.186 min^-1, which was suppressed by Cl^- and HCO₃^-. The optimal pH for CH degradation was around 5.0. Direct mineralization accounted for the CH degradation in UV/PMS system. Interestingly, the addition of PMS at the neutral condition before UV irradiation transferred CH into trichloroacetic acid (TCAA). The transformation efficiency of CH into TCAA at 10 min was enhanced from 2.17%-40.38% with the elevation of initial pH from 7.0-8.0. The subsequent exposure of UV lamps ceased the transformation of CH into TCAA and facilitated the direct mineralization of CH, but it did not work in the refractory TCAA degradation. Finally, it was revealed that HO predominantly participated CH degradation in UV/PMS process, while O₂^- was responsible for the transformation of CH into TCAA by addition of PMS before UV irradiation.

Degradation of Atrazine, Simazine and Ametryn in an arable soil using thermal-activated persulfate oxidation process: Optimization, kinetics, and degradation pathway

This study examined the feasibility of applying thermal-activated persulfate (PS) oxidation for remediation of soil co-contaminated with s-triazine herbicides including Atrazine (ATZ), Simazine (SIM) and Ametryn (AME). Homogeneous activation using heating method (50 °C) was selected. Results showed that thermal-activated PS oxidation process may successfully degrade ATZ in soil and degradation efficiency was increased along the arising activation temperature. Higher PS dosages and depressed initial pH were beneficial for degradation while increasing initial ATZ concentration may hamper the degradation. The oxidation process may lead to changes of surface functional groups on soil. The presence of Cl^-, HCO₃^- and H₂PO₄^- at both of low and high concentrations may inhibit the degradation of ATZ. Soil depths may apparently influence the ATZ degradation which followed 0-10 < 10-30 < 30-60 cm mainly depending on the soil organic matter (SOM) contents. Thermal-activated PS may effectively degrade ATZ, SIM and AME under co-contaminated condition and the more favorable of ethyl group towards SO₄^- than isopropyl and methylation groups was detected. Both of SO₄^- and HO were identified to be responsible for degradation. Finally, degradation intermediates of ATZ, SIM and AME were identified by LC-Q-TOF-MS and detailed transformation pathways for three pesticides were proposed, respectively.

Ultraviolet-mediated peroxymonosulfate diminution of earthy and musty compound trichloroanisole in water

Taste and odor (T&O) problem in water is one of the main obstacles to improve the quality of drinking water, and efficient water treatment processes are urgently needed to control T&O compounds. Ultraviolet-mediated peroxymonosulfate (UV/PMS) diminution of trichloroanisole (TCA) in water was investigated in this paper. The treatment of 2,3,6-trichloroanisole (2,3,6-TCA) by three advanced oxidation processes (UV, UV/H₂O₂ and UV/PMS) was compared, and UV/PMS stood out. SO₄^•- and HO• were produced in the UV/PMS, and their specific contributions to 2,3,6-TCA oxidation were investigated. The competitive kinetic model was applied to determine the second-order reaction rate between 2,3,6-TCA and SO₄^•- or HO•. The products of 2,3,6-TCA generated in UV/PMS were analyzed with gas chromatography/high resolution-mass spectrometry (GC/HR-MS), and the degradation mechanism was proposed. The effects of water matrices (chloride, bicarbonate and humic acid) on UV/PMS performance were studied, and the decontamination of 2,3,6-TCA in real water was carried out. The disinfection byproducts (DBPs) alteration from 2,3,6-TCA by UV/PMS - chlorination treatment was explored. Overall, UV/PMS can effectively deal with the T&O pollution of TCA in water.

Efficient degradation of lomefloxacin by Co-Cu-LDH activating peroxymonosulfate process: Optimization, dynamics, degradation pathway and mechanism

In this study, bimetal layered double hydroxides (Co_xCu_y-LDHs) containing a carbonate interlayer were synthesized using coprecipitation with a variety of Co/Cu mole ratios. Meanwhile, the corresponding layered double oxides (Co_xCu_y-LDOs) were prepared as controls. In this study, Electrical energy per order was performed to evaluate economic analysis. Correspondingly, we found that Co_xCu_y-LDHs possessed a significantly better PMS activation capability than the corresponding metal oxide composite (Co₃O₄/CuO). Compared with other Co_xCu_y-LDHs, Co₂Cu₁ LDH possessed the best PMS activation capability for LOM degradation and the lowest electrical energy per order (EE/O) value during the reaction. Additionally, Co₂Cu₁ LDH presented an excellent stability and worked over a wide pH range. The hydroxide states of Co(III), Co(II), Cu(I) and Cu(II) were all able to activate PMS, indicating that there were many active sites on the surface of Co₂Cu₁ LDH. The involvement of radicals in this reaction system was determined via scavenger experiments and electron paramagnetic resonance (EPR). Meanwhile, it's worth noting that a mathematical model was developed to quantify the involvement of SO₄^- and OH. Subsequently, we determined PMS activation mechanism and LOM decomposition pathway for the PMS/Co₂Cu₁ LDH system.

Electrochemical degradation of ibuprofen using an activated-carbon-based continuous-flow three-dimensional electrode reactor (3DER)

We developed a continuous-flow three-dimensional electrode reactor (3DER) to remove ibuprofen (IBP) from water. The effects of the operating parameters on the 3DER performance were investigated. The 3DER was constructed by filling a conventional two-dimensional electrode reactor with granular activated carbon, which acted as particle electrodes. The IBP removal efficiency of the 3DER was 98% in 4 h, which was 2.5 times higher than the removal efficiency for the two-dimensional electrode reactor. IBP removal kinetics tests indicated that the current density (1-20 mA/cm²) correlated better than the other operating parameters with the first-order rate constant (k). The flow rate affected the IBP removal kinetics to a small degree. Chloride and sulfate supporting electrolyte concentrations between 17 and 100 mM affected the IBP removal kinetics in opposite ways. Increasing the chloride concentration increased k, but increasing the sulfate concentration decreased k. Radical quenching experiments indicated that much more IBP degradation occurred through both indirect and direct oxidation mechanisms in the 3DER than in the two-dimensional electrode reactor. The particle electrodes caused hydroxyl radicals to form when the 3DER treatment was started, but the particle electrodes later acted as third electrodes and favored direct oxidation of IBP.

UV/sulfite chemistry to reduce N-nitrosodimethylamine formation in chlor(am)inated water

The disinfection by-product N-nitrosodimethylamine (NDMA) is a major concern in water quality management due to its carcinogenicity. Thus, a proper pretreatment is necessary to mitigate NDMA formation upon periodic chloramination by removing precursors, such as ranitidine (RNT). This study investigated the effect of UV/sulfite pretreatment on NDMA formation from an RNT-spiked tap and chloraminated synthetic swimming pool (SSP) water. At UVC intensity of 2.1 mW cm^-2 and 0.5 mM of sulfite, UV/sulfite chemistry showed complete degradation of 20 µM RNT within 30 min. It was found that SO₄^•- primarily reduced the NDMA formation potential (FP) of RNT, while hydrated electrons effectively mitigated the pre-formed NDMA in the SSP water. The UV/sulfite pretreatment alleviated NDMA formation during post-chloramination (24 h) by up to 82%, outperforming the commonly employed advanced oxidation processes such as UV/H₂O₂. However, in the presence of bromide ions, the effectiveness of UV/sulfite pretreatment was seriously deteriorated, although the bromide ion itself was found to inhibit the NDMA formation from RNT especially at pH < 8 during chloramination. Mass spectrometric analysis indicated that the NDMA-FP of RNT could be removed by UV/sulfite principally via N-methylation, dealkylation, and oxygen transfer pathways. Consequently, UV/sulfite could be used as an alternative unit process for water treatment with reduced NDMA formation.