Fe2+/HClO Reaction Produces FeIVO2+: An Enhanced Advanced Oxidation Process

Tremendously enhanced catalytic performance of Fe(III)/peroxymonosulfate process by trace Cu(II): A high-valent metals domination in organics removal

Dissolved copper and iron ions are regarded as friendly and economic catalysts for peroxymonosulfate (PMS) activation, however, neither Cu(II) nor Fe(III) shows efficient catalytic performance because of the slow rates of Cu(II)/Cu(I) and Fe(III)/Fe(II) cycles. Innovatively, we observed a significant enhancement on the degradation of organic contaminants when Cu(II) and Fe(III) were coupled to activate PMS in borate (BA) buffer. The degradation efficiency of Rhodamine B (RhB, 20 µmol/L) reached up to 96.3% within 10 min, which was higher than the sum of individual Cu(II)- and Fe(III)- activated PMS process. Sulfate radical, hydroxyl radical and high-valent metal ions (i.e., Cu(III) and Fe(IV)) were identified as the working reactive species for RhB removal in Cu(II)/Fe(III)/PMS/BA system, while the last played a predominated role. The presence of BA dramatically facilitated the reduction of Cu(II) to Cu(I) via chelating with Cu(II) followed by Fe(III) reduction by Cu(I), resulting in enhanced PMS activation by Cu(I) and Fe(II) as well as accelerated generation of reactive species. Additionally, the strong buffering capacity of BA to stabilize the solution pH was satisfying for the pollutants degradation since a slightly alkaline environment favored the PMS activation by coupling Cu(II) and Fe(III). In a word, this work provides a brand-new insight into the outstanding PMS activation by homogeneous bimetals and an expanded application of iron-based advanced oxidation processes in alkaline conditions.

Complexation with Metal Ions Affects Chlorination Reactivity of Dissolved Organic Matter: Structural Reactomics of Emerging Disinfection Byproducts

Metal ions are liable to form metal-dissolved organic matter [dissolved organic matter (DOM)] complexes, changing the chemistry and chlorine reactivity of DOM. Herein, the impacts of iron and zinc ions (Fe3+ and Zn2+) on the formation of unknown chlorinated disinfection byproducts (Cl-DBPs) were investigated in a chlorination system. Fe3+ preferentially complexed with hydroxyl and carboxyl functional groups, while Zn2+ favored the amine functional groups in DOM. As a consequence, electron-rich reaction centers were created by the C-O-metal bonding bridge, which facilitated the electrophilic attack of α-C in metal-DOM complexes. Size-reactivity continuum networks were constructed in the chlorination system, revealing that highly aromatic small molecules were generated during the oxidation and decarbonization of metal-DOM complexes. Molecular transformation related to C-R (R represents complex sites) loss was promoted via metal complexation, including decarboxylation and deamination. Consequently, complexation with Fe3+ and Zn2+ promoted hydroxylation by the C-O-metal bonding bridge, thereby increasing the abundances of unknown polychlorinated Cl-DBPs by 9.6 and 14.2%, respectively. The study provides new insights into the regulation of DOM chemistry and chlorine reactivity by metal ions in chlorination systems, emphasizing that metals increase the potential health risks of drinking water and more scientific control standards for metals are needed.

The generation and transformation mechanisms of reactive oxygen species in the environment and their implications for pollution control processes: A review

Reactive oxygen species (ROS), substances with strong activity generated by oxygen during electron transfer, play a significant role in the decomposition of organic matter in various environmental settings, including soil, water and atmosphere. Although ROS has a short lifespan (ranging from a few nanoseconds to a few days), it continuously generated during the interaction between microorganisms and their environment, especially in environments characterized by strong ultraviolet radiation, fluctuating oxygen concentration or redox conditions, and the abundance of metal minerals. A comprehensive understanding of the fate of ROS in nature can provide new ideas for pollutant degradation and is of great significance for the development of green degradation technologies for organic pollutants. At present, the review of ROS generally revolves around various advanced oxidation processes, but lacks a description and summary of the fate of ROS in nature, this article starts with the definition of reactive oxidants species and reviews the production, migration, and transformation mechanisms of ROS in soil, water and atmospheric environments, focusing on recent developments. In addition, the stimulating effects of ROS on organisms were reviewed. Conclusively, the article summarizes the classic processes, possible improvements, and future directions for ROS-mediated degradation of pollutants. This review offers suggestions for future research directions in this field and provides the possible ROS technology application in pollutants treatment.

Electrochemically producing high-valent iron-oxo species for phenolics-laden high chloride wastewater pretreatment

Electrochemical advanced oxidation processes (EAOPs) have shown great promise for treating industrial wastewater contaminated with phenolic compounds. However, the presence of chloride in the wastewater leads to the production of undesirable chlorinated organic and inorganic byproducts, limiting the application of EAOPs. To address this challenge, we investigated the potential of incorporating Fe(II) and Fe(III) into the EAOPs with a boron-doped diamond (BDD) anode under near-neutral conditions. Our findings revealed that both Fe(II) and Fe(III) facilitated the generation of high-valent iron-oxo species (Fe(IV) and Fe(V)) in the anodic compartment, thereby reducing the oxidation contribution of reactive chlorine species. Remarkably, the addition of 1000 μM Fe(II) under high chloride conditions resulted in over a 2.8-fold increase in the oxidation rate of 50 μM phenolic contaminants at pH 6.5. Furthermore, 1000 μM Fe(II) contributed to a reduction of more than 66% in the formation of chlorinated byproducts, consequently enhancing the biodegradability of the treated water. Additionally, transitioning from batch mode to continuous flow mode further amplified the positive effects of Fe(II) on the EAOPs. Overall, this study presents a modified electrochemical approach that simultaneously enhanced the degradation of phenolic contaminants and improved the biodegradability of wastewater with high chloride concentrations.

High-valent metal-oxo species transformation and regulation by co-existing chloride: Reaction pathways and impacts on the generation of chlorinated by-products

High-valent metal-oxo species (HMOS) have been extensively recognized in advanced oxidation processes (AOPs) owing to their high selectivity and high chemical utilization efficiency. However, the interactions between HMOS and halide ions in sewage wastewater are complicated, leading to ongoing debates on the intrinsic reactive species and impacts on remediation. Herein, we prepared three typical HMOS, including Fe(IV), Mn(V)-nitrilotriacetic acid complex (Mn(V)NTA) and Co(IV) through peroxymonosulfate (PMS) activation and comparatively studied their interactions with Cl- to reveal different reactive chlorine species (RCS) and the effects of HMOS types on RCS generation pathways. Our results show that the presence of Cl- alters the cleavage behavior of the peroxide OO bond in PMS and prohibits the generation of Fe(IV), spontaneously promoting SO4•- production and its subsequent transformation to secondary radicals like Cl• and Cl2•-. The generation and oxidation capacity of Mn(V)NTA was scarcely influenced by Cl-, while Cl- would substantially consume Co(IV) and promote HOCl generation through an oxygen-transfer reaction, evidenced by density functional theory (DFT) and deuterium oxide solvent exchange experiment. The two-electron-transfer standard redox potentials of Fe(IV), Mn(V)NTA and Co(IV) were calculated as 2.43, 2.55 and 2.85 V, respectively. Due to the different reactive species and pathways in the presence of Cl-, the amounts of chlorinated by-products followed the order of Co(II)/PMS > Fe(II)/PMS > Mn(II)NTA/PMS. Thus, this work renovates the knowledge of halide chemistry in HMOS-based systems and sheds light on the impact on the treatment of salinity-containing wastewater.

Critical Role of Mineral Fe(IV) Formation in Low Hydroxyl Radical Yields during Fe(II)-Bearing Clay Mineral Oxygenation

In subsurface environments, Fe(II)-bearing clay minerals can serve as crucial electron sources for O2 activation, leading to the sequential production of O2•-, H2O2, and •OH. However, the observed •OH yields are notably low, and the underlying mechanism remains unclear. In this study, we investigated the production of oxidants from oxygenation of reduced Fe-rich nontronite NAu-2 and Fe-poor montmorillonite SWy-3. Our results indicated that the •OH yields are dependent on mineral Fe(II) species, with edge-surface Fe(II) exhibiting significantly lower •OH yields compared to those of interior Fe(II). Evidence from in situ Raman and Mössbauer spectra and chemical probe experiments substantiated the formation of structural Fe(IV). Modeling results elucidate that the pathways of Fe(IV) and •OH formation respectively consume 85.9-97.0 and 14.1-3.0% of electrons for H2O2 decomposition during oxygenation, with the Fe(II)edge/Fe(II)total ratio varying from 10 to 90%. Consequently, these findings provide novel insights into the low •OH yields of different Fe(II)-bearing clay minerals. Since Fe(IV) can selectively degrade contaminants (e.g., phenol), the generation of mineral Fe(IV) and •OH should be taken into consideration carefully when assessing the natural attenuation of contaminants in redox-fluctuating environments.

Novel electrocatalytic capacitive deionization with catalytic electrodes for selective phosphonate degradation: Performance and mechanism

Phosphonate is becoming a global interest and concern owing to its environment risk and potential value. Degradation of phosphonate into phosphate followed by the recovery is regarded as a promising strategy to control phosphonate pollution, relieve phosphorus crisis, and promote phosphorus cycle. Given these objectives, an anion-membrane-coated-electrode (A-MCE) doped with Fe-Co based carbon catalyst and cation-membrane-coated-electrode (C-MCE) doped with carbon-based catalyst were prepared as catalytic electrodes, and a novel electrocatalytic capacitive deionization (E-CDI) was developed. During charging process, phosphonate was enriched around A-MCE surface based on electrostatic attraction, ligand exchange, and hydrogen bond. Meanwhile, Fe2+ and Co2+ were self-oxidized into Fe3+ and Co3+, forming a complex with enriched phosphonate and enabling an intramolecular electron transfer process for phosphonate degradation. Additionally, benefiting from the stable dissolved oxygen and high oxygen reduction reaction activity of C-MCE, hydrogen peroxide accumulated in E-CDI (158 μM) and thus hydroxyl radicals (·OH) were generated by activation. E-CDI provided an ideal platform for the effective reaction between ·OH and phosphonate, avoiding the loss of ·OH and triggering selective degradation of most phosphonate. After charging for 70 min, approximately 89.9% of phosphonate was degraded into phosphate, and phosphate was subsequently adsorbed by A-MCE. Results also showed that phosphonate degradation was highly dependent on solution pH and voltage, and was insignificantly affected by electrolyte concentration. Compared to traditional advanced oxidation processes, E-CDI exhibited a higher degradation efficiency, lower cost, and less sensitive to co-existed ions in treating simulated wastewaters. Self-enhanced and selective degradation of phosphonate, and in-situ phosphate adsorption were simultaneously achieved for the first time by a E-CDI system, showing high promise in treating organic-containing saline wastewaters.

Freezing-enhanced chlorination of organic pollutants for water treatment

Freezing has been reported to accelerate chemical reactions and thus affect the fate of pollutants in the environment. However, little research has been conducted on the potential effects of freezing on the chlorination process. This study aimed to explore the freezing-enhanced chlorination process by comparing the oxidation of clofibric acid (CA) by chlorine in ice (at -20 °C) to the same reaction in water (at 25 °C). The degradation of CA, which was negligible in water, was significantly accelerated in ice. This acceleration can be attributed to the freeze concentration effect that occurs during freezing, which excludes solutes such as chlorine, CA and protons from the ice crystals, leading to their accumulated concentration in the liquid brine. The increased concentration of chlorine and protons in the liquid brine leads to higher rates of CA oxidation, supporting the freeze concentration effect as the underlying cause for the accelerated chlorination of CA in ice. Moreover, the chlorine/freezing system was also effective in the degradation of other organic pollutants. This highlights the environmental relevance and significance of freezing-enhanced chlorination in cold regions, particularly for the treatment of organic contaminants.

Efficient reduction and adsorption of Cr(VI) using FeCl3-modified biochar: Synergistic roles of persistent free radicals and Fe(II)

Transition metal iron and persistent free radicals (PFRs) both affect the redox properties of biochar, but the electron transfer relationship between them and the coupling reduction mechanism of Cr(VI) requires further investigation. To untangle the interplay between iron and PFRs in biochar and the influences on redox properties, FeCl3-modified rice husk biochar (FBCs) was prepared and its reduction mechanism for Cr(VI) without light was evaluated. The FBCs had higher surface positive charges, oxygen-containing functional groups, and PFRs compared with pristine rice husk biochar (BC). Phenoxyl PFRs with high electron-donating capability formed in biochar. The pronounced electron paramagnetic resonance signals showed that the PFRs preferred to form at lower Fe(III) concentrations. While a high concentration of Fe(III) would be reduced to Fe(II) and consumed the formed PFRs. Adsorption kinetics and X-ray photoelectron spectroscopy analysis indicated that the FBCs effectively enhanced the Cr(VI) removal efficiency by 1.54-8.20 fold and the Cr(VI) reduction efficiency by 1.88-9.29 fold compared to those of BC. PFRs quenching and competitive reductant addition experiments revealed that the higher Cr(VI) reduction performance of FBCs was mainly attributed to the formed PFRs, which could contribute to ∼74.0% of Cr(VI) reduction by direct or indirect electron transfer. The PFRs on FBCs surfaces could promote the Fe(III)/Fe(II) cycle through single electron transfer and synergistically accelerate ∼52.3% of Cr(VI) reduction. This study provides an improved understanding of the reduction mechanism of iron-modified biochar PFRs on Cr(VI) in environments.

Photo-self-Fenton Reaction Mediated by Atomically Dispersed Ag-Co Photocatalysts toward Efficient Degradation of Organic Pollutants

Achieving the complete mineralization of persistent pollutants in wastewater is still a big challenge. Here, we propose an efficient photo-self-Fenton reaction for the degradation of different pollutants using the high-density (Ag: 22 wt %) of atomically dispersed AgCo dual sites embedded in graphic carbon nitride (AgCo-CN). Comprehensive experimental measurements and density functional theory (DFT) calculations demonstrate that the Ag and Co dual sites in AgCo-CN play a critical role in accelerating the photoinduced charge separation and forming the self-Fenton redox centers, respectively. The bimetallic AgCo-CN exhibited excellent photocatalytic performance toward the phenol even under extreme conditions due to an efficient degradation pathway and in situ generation of the hydrogen peroxide producing the main active oxygen species (⋅OH and 1 O2 ) and showed long-term activity in a self-design photo-Filter reactor for the purification of the phenol. Our discoveries pave the way for the design of efficient single-atoms photocatalysts-based photo-self-Fenton reaction for recalcitrant pollutant treatment.

Enhanced recovery of phosphorus from hypophosphite-laden wastewater via field-induced electro-Fenton coupled with anodic oxidation

Electrochemical recovered ferric phosphate (FePO4) precipitates from hypophosphite-laden wastewater were shown to be an efficient method for phosphorus (P) recovery. However, the influence of chloride ions (Cl-) coexisting commonly in wastewater is not known for this treatment. Herein, a field-induced electro-Fenton coupled with anodic oxidation electrochemical system consisting of a Ti-RuO2 anode, an Fe inductive electrode and an activated carbon fiber (ACF) cathode, namely Ti-RuO2/Fe/ACF(NaCl) system, was established to recover phosphorus (P) as FePO4 from hypophosphite-laden wastewater in the presence of Cl-. This system enabled a hypophosphite (H2PO2-, 1.0 mM) removal ratio of ~100% and all P was recovered within 30 min at 5.0 V under the initial solution pH of 3.0. The Faradaic efficiency and energy consumption of P recovery achieved the maximum value (~94%) and the lowest value (~16 kW h kg-1 P), respectively. Reactive oxygen species including 1O2, FeIVO2+, •O2- and •OH contribute to convert H2PO2- to PO43-, which immediately formed FePO4 with the generated Fe3+ at the optimized conditions. Therein, the contribution of non-radical 1O2 was very considerable. This system exhibited good stability. The efficiency and cost for treatment of actual hypophosphite-laden wastewater were addressed to check its applicability for P recovery.

Peroxymonosulfate activation by trace iron(III) porphyrin for facile degradation of organic pollutants via nonradical oxidation

Nonradical species with great resistance to interference have shown great advantages in complex wastewater treatment. Herein, a novel system constructed by biodegradable tetrakis-(4-carboxyphenyl)-porphyrinatoiron(III) (FeIII-TCPP) and peroxymonosulfate (PMS) was proposed for facile decontamination. Nonradical pathway is observed in FeIII-TCPP/PMS, where 1O2 and high-valent iron-oxo species play dominant roles. The genres and valence of high-valent iron-oxo species, including iron(IV)-oxo porphyrin radical-cationic species [OFeIV-TCPP•+] and iron(IV)-hydroxide species [FeIV-TCPP(OH)], are ascertained, along with their generation mechanism. The axial ligand on the iron axial site affects the ground spin state of FeIII-TCPP, further influencing the thermodynamic reaction pathway of active species. With trace catalyst in micromoles, FeIII-TCPP exhibits high efficiency by degrading bisphenol S (BPS) completely within 5 min, while Co2+/PMS can only achieve a maximum of 26.2% under identical condition. Beneficial from nonradical pathways, FeIII-TCPP/PMS demonstrates a wide pH range of 3-10 and exhibits minimal sensitivity to interference of concomitant materials. BPS is primarily eliminated through β-scission and hydroxylation. Specifically, 1O2 electrophilically attacks the C-S bond of BPS, while high-valent iron-oxo species interacts with BPS through an oxygen-bound mechanism. This study provides novel insights into efficient activation of PMS by iron porphyrin, enabling the removal of refractory pollutants through nonradical pathway.

An electrochlorination process integrating enhanced oxidation of phosphonate to orthophosphate and elimination: Verification of matrix chloridion-induced oxidation mechanism

Phosphonate used as scale inhibitor is a non-negligible eutrophic contaminant in corresponding polluted waters. Besides, its conversion to orthophosphate (ortho-P) is a precondition for realizing bioavailable phosphorus recovery. Due to the feeble degradation efficiency with less than 30 % from classical Fenton commonly used in industrial wastewater treatment and itself vulnerable to strong inhibition interference of matrix chloride ions, we proposed an electrochemical approach to transform the native salt in the solution into oxidizing substances, sort of achieving beneficial utilization of matrix waste, and enhanced the ortho-P conversion rate of 1-Hydroxyethane-1,1-diphosphonic acid (HEDP) to 89.2 % (± 3.6 %). In electrochlorination system, it was found that HEDP rapidly complexed with Fe(II) and then coordinated in-situ Fe(III) to release free HEDP via intramolecular metal-ligand electron transfer reaction. The subsequent degradation mainly rooted in the oxidation of pivotal reactive species HClO, FeIVO2+ and 1O2, causing C-P and CC bonds to fracture in sequence. Eventually the organically bound phosphorus of HEDP was recovered as ortho-P. This study acquainted the audiences with the rare mechanism of chloridion-triggered HEDP degradation under electrochemical way, as well as offered a feasible technology for synchronous transformation of organically bound phosphorus to ortho-P and elimination from phosphonates.

Treatment of landfill leachate nanofiltration concentrate by a three-dimensional electrochemical technology with waste aluminum scraps as particle electrodes: Efficacy, mechanisms, and enhancement effect of subsequent electrocoagulation

Landfill leachate nanofiltration concentrate is a kind of wastewater containing high concentrations of color and refractory organics. Herein, we proposed a novel three-dimensional electrochemical technology (3DET) with waste aluminum scraps as particle electrodes for its treatment. The planar and particle electrodes were first optimized. Ti/RuO2 and graphite were used as anodes in the two-dimensional electrochemical technology (2DET). In the light of contaminant removal (color, UV254, COD, and TOC), chlorine reduction, and energy consumption, graphite was selected as planar anodes and cathodes. Moreover, 3DET with Al particle electrodes (Al 3DET) outperformed that with conventional granular activated carbon electrodes, 2DET, and Al particles. At 120 min, the removal efficiencies of color, UV254, COD, and TOC using Al 3DET were 98.94 %, 84.72 %, 51.93 %, and 67.46 %, respectively. UV-vis and EEM spectroscopy, and GC-MS analyses indicate that macromolecular organic matter such as humic-like substances could be effectively degraded and simultaneously removed. Reactive species identification tests including free radical quenching and EPR spectra were conducted. The results indicate that in addition to anodic direct oxidation, indirect oxidation by oxidative species (H2O2, •OH, and RCS) and flocculation by Al species also played a vital role in contaminant removal. Continuous-flow experiments show that Fe EC as a post-treatment step of Al 3DET could effectively provide a neutralization effect for the 3DET effluent and enhance the removal efficiency of contaminants. The total operating cost of combined process was 1.307 USD/m3. This study shows that the Al 3DET-Fe EC process is a promising technology for the treatment of nanofiltration concentrate.

Spherical porous iron-nitrogen-carbon nanozymes derived from a tannin coordination framework for the preparation of L-DOPA by emulating tyrosine hydroxylase

L-3,4-Dihydroxyphenylalanine (L-DOPA) is widely used in Parkinson's disease treatment and is therefore in high demand. Development of an efficient method for the production of L-DOPA is urgently required. Nanozymes emulating tyrosine hydroxylase have attracted enormous attention for biomimetic synthesis of L-DOPA, but suffered from heterogeneity. Herein, a spherical porous iron-nitrogen-carbon nanozyme was developed for production of L-DOPA. Tannic acid chelated with ferrous ions to form a tannin-iron coordination framework as a carbon precursor. Iron and nitrogen co-doped carbon nanospheres were assembled via an evaporation-induced self-assembly process using urea as a nitrogen source, F127 as a soft template, and formaldehyde as a crosslinker. The nanozyme was obtained after carbonization and acid etching. The nanozyme possessed a dispersive iron atom anchored in the Fe-N coordination structure as an active site to mimic the active center of tyrosine hydroxylase. The material showed spherical morphology, uniform size, high specific surface area, a mesoporous structure and easy magnetic separation. The structural properties could promote the density and accessibility of active sites and facilitate mass transport and electron transfer. The nanozyme exhibited high activity to catalyze the hydroxylation of tyrosine to L-DOPA as tyrosine hydroxylase in the presence of ascorbic acid and hydrogen peroxide. The titer of DOPA reached 1.2 mM. The nanozyme showed good reusability and comparable enzyme kinetics to tyrosine hydroxylase with a Michaelis-Menten constant of 2.3 mM. The major active species was the hydroxyl radical. Biomimetic simulation of tyrosine hydroxylase using a nanozyme with a fine structure provided a new route for the efficient production of L-DOPA.

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.

Challenges Relating to the Quantification of Ferryl(IV) Ion and Hydroxyl Radical Generation Rates Using Methyl Phenyl Sulfoxide (PMSO), Phthalhydrazide, and Benzoic Acid as Probe Compounds in the Homogeneous Fenton Reaction

Ferryl ion ([FeIVO]2+) has often been suggested to play a role in iron-based advanced oxidation processes (AOPs) with its presence commonly determined using the unique oxidation pathway from methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2). However, we show here that the oxidation products of PMSO, formed on reaction with hydroxyl radical, enhance PMSO2 formation as a result of their complexation with Fe(III) leading to the changes in the reactivity of Fe(III) species in the homogeneous Fenton reaction. As such, PMSO should be used with caution to investigate the role of [FeIVO]2+ in iron-based AOPs with these insights suggesting the need to reassess the findings of many previous studies in which this reagent was used. The other common target compounds, phthalhydrazide and hydroxybenzoic acids, were also found to modify the rate and extent of iron cycling as a result of complexation and/or redox reactions, either by the probe compound itself and/or oxidation products formed. Overall, this study highlights that these confounding effects of the aromatic probe compounds on the reactivity of iron species should be recognized if reliable mechanistic insights into iron-based AOPs are to be obtained.

Highly selective synthesis of surface FeIV=O with nanoscale zero-valent iron and chlorite for efficient oxygen transfer reactions

High-valent iron-oxo species (FeIV=O) has been a long-sought-after oxygen transfer reagent in biological and catalytic chemistry but suffers from a giant challenge in its gentle and selective synthesis. Herein, we propose a new strategy to synthesize surface FeIV=O (≡FeIV=O) on nanoscale zero-valent iron (nZVI) using chlorite (ClO2-) as the oxidant, which possesses an impressive ≡FeIV=O selectivity of 99%. ≡FeIV=O can be energetically formed from the ferrous (FeII) sites on nZVI through heterolytic Cl-O bond dissociation of ClO2- via a synergistic effect between electron-donating surface ≡FeII and proximal electron-withdrawing H2O, where H2O serves as a hydrogen-bond donor to the terminal O atom of the adsorbed ClO2- thereby prompting the polarization and cleavage of Cl-O bond for the oxidation of ≡FeII toward the final formation of ≡FeIV=O. With methyl phenyl sulfoxide (PMS16O) as the probe molecule, the isotopic labeling experiment manifests an exclusive 18O transfer from Cl18O2- to PMS16O18O mediated by ≡FeIV=18O. We then showcase the versatility of ≡FeIV=O as the oxygen transfer reagent in activating the C-H bond of methane for methanol production and facilitating selective triphenylphosphine oxide synthesis with triphenylphosphine. We believe that this new ≡FeIV=O synthesis strategy possesses great potential to drive oxygen transfer for efficient high-value-added chemical synthesis.

Ru(III)-Periodate for High Performance and Selective Degradation of Aqueous Organic Pollutants: Important Role of Ru(V) and Ru(IV)

In this study, Ru(III) ions were utilized to activate periodate (PI) for oxidation of trace organic pollutants (TOPs, e.g., carbamazepine (CBZ)). The Ru(III)/PI system can significantly promote the oxidation of CBZ in a wide initial pH range (3.0-11.0) at 1 μM Ru(III), showing much higher performance than transition metal ions (i.e., Fe(II), Co(II), Zn(II), Fe(III), Cu(II), Ni(II), Mn(II), and Ce(III)) and noble metal ion (i.e., Ag(I), Pd(II), Pt(II), and Ir(III)) activated PI systems. Probe experiments, UV-vis spectra, and X-ray absorption near-edge structure (XANES) spectra confirmed high-valent Ru-oxo species (Ru(V)=O) as the dominant oxidant in the process. Because of the dominant role of Ru(V)=O, the Ru(III)/PI process exhibited a remarkable selectivity and strong anti-interference in the oxidation of TOPs in complex water matrices. The Ru(V)=O species can undertake 1-e- and 2-e- transfer reactions via the catalytic cycles of Ru(V)=O → Ru(IV) → Ru(III) and Ru(V)=O → Ru(III), respectively. The utilization efficiency of PI in the Ru(III)/PI process for the oxidation of TOPs can approach 100% under optimal conditions. PI stoichiometrically transformed into IO3- without production of undesired iodine species (e.g., HOI and I2). This study developed an efficient and environmentally benign advanced oxidation process for rapid removal of TOPs and enriched understandings on reactivity of Ru(V)=O and Ru catalytic cycles.

Single-atom Mo-Co catalyst with low biotoxicity for sustainable degradation of high-ionization-potential organic pollutants

Single-atom catalysts (SACs) are a promising area in environmental catalysis. We report on a bimetallic Co-Mo SAC that shows excellent performance in activating peroxymonosulfate (PMS) for sustainable degradation of organic pollutants with high ionization potential (IP > 8.5 eV). Density Functional Theory (DFT) calculations and experimental tests demonstrate that the Mo sites in Mo-Co SACs play a critical role in conducting electrons from organic pollutants to Co sites, leading to a 19.4-fold increase in the degradation rate of phenol compared to the CoCl2-PMS group. The bimetallic SACs exhibit excellent catalytic performance even under extreme conditions and show long-term activation in 10-d experiments, efficiently degrading 600 mg/L of phenol. Moreover, the catalyst has negligible toxicity toward MDA-MB-231, Hela, and MCF-7 cells, making it an environmentally friendly option for sustainable water treatment. Our findings have important implications for the design of efficient SACs for environmental remediation and other applications in biology and medicine.

Periodate-based advanced oxidation processes: A review focusing on the overlooked role of high-valent iron and manganese species

Periodate-based advanced oxidation processes (AOPs) have received mounting attention in scientific research in the past two decades due to their fair oxidizing capability for satisfactory decontamination performance. Unlike iodyl (IO₃^•) and hydroxyl (^•OH) radicals are widely recognized as the predominant species generated from periodate activation, the role of high-valent metal as a dominant reactive oxidant has been proposed recently. Although several excellent reviews concerning periodate-based AOPs have been reported, there are still prevalent knowledge roadblocks to high-valent metals' formation and reaction mechanisms. Therefore, this work aims to provide a comprehensive overview of high-valent metals, especially concerning the identification methods (e.g., direct and indirect strategies), formation mechanisms (e.g., formation pathways and interpretation based on density functional theory calculation), reaction mechanisms (e.g., nucleophilic attack, electron transfer, oxygen-atom transfer, electrophilic addition, and hydride and hydrogen-atom transfer), and reactivity performance (e.g., chemical properties, influencing factors, and practical applications). Furthermore, points for critical thinking and further prospects for high-valent metal-mediated oxidation processes are suggested, emphasizing the need for parallel efforts to enhance the stability and reproducibility of high-valent metal-mediated oxidation processes in real world applications.

Selective Removal of Emerging Organic Contaminants from Water Using Electrogenerated Fe(IV) and Fe(V) under Near-Neutral Conditions

Fe(IV) and Fe(V) are promising oxidants for the selective removal of emerging organic contaminants (EOCs) from water under near-neutral conditions. The Fe(III)-assisted electrochemical oxidation system with a BDD anode (Fe(III)-EOS-BDD system) has been employed to generate Fe(VI), while the generation and contributions of Fe(IV) and Fe(V) have been largely ignored. Thus, we examined the feasibility and involved mechanisms of the selective degradation of EOCs in the Fe(III)-EOS-BDD system under near-neutral conditions. It was found that Fe(III) application selectively accelerated the electro-oxidation of phenolic and sulfonamide organics and made the oxidation system be resistant to interference from Cl^-, HCO₃^-, and humic acid. Several lines of evidence indicated that EOCs were decomposed via direct electron-transfer process on the BDD anode and by Fe(IV) and Fe(V) but not Fe(VI), besides HO^•. Fe(VI) was not generated until the exhaustion of EOCs. Furthermore, the overall contributions of Fe(IV) and Fe(V) to the oxidation of phenolic and sulfonamide organics were over 45%. Our results also revealed that Fe(III) was oxidized primarily by HO^• to Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system. This study advances the understanding of the roles of Fe(IV) and Fe(V) in the Fe(III)-EOS-BDD system and provides an alternative for utilizing Fe(IV) and Fe(V) under near-neutral conditions.

Peroxymonosulfate activated by natural porphyrin derivatives for rapid degradation of organic pollutants via singlet oxygen and high-valent iron-oxo species

The activation of peroxymonosulfate (PMS) by sodium ferric chlorophyllin (SFC), a natural porphyrin derivative extracted from chlorophyll-rich substances, was systematically investigated for facile degradation of bisphenol A (BPA). SFC/PMS is capable of degrading 97.5% of BPA in the first 10 min with the initial BPA concentration of 20 mg/L and pH = 3, whereas conventional Fe²⁺/PMS could only remove 22.6% of BPA under identical conditions. It demonstrates a prominent flexibility to a broad pH range of 3-11 with complete pollutant degradation. A remarkable tolerance toward concomitant high concentration of inorganic anions (100 mM) was also observed, among which (bi)carbonates can even accelerate the degradation. The nonradical oxidation species, including high-valent iron-oxo porphyrin species and ¹O₂, are identified as dominant species. Particularly, the generation and participation of ¹O₂ in the reaction is evidenced by experimental and theoretical methods, which is vastly different from the previous study. The specific activation mechanism is unveiled by density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. The results shed light on effective PMS activation by iron (III) porphyrin and the proposed natural porphyrin derivative would be a promising candidate for efficient abatement of recalcitrant pollutants toward complicated aqueous media in wastewater treatment.

Whose Oxygen Atom Is Transferred to the Products? A Case Study of Peracetic Acid Activation via Complexed MnII for Organic Contaminant Degradation

Identifying reactive species in advanced oxidation process (AOP) is an essential and intriguing topic that is also challenging and requires continuous efforts. In this study, we exploited a novel AOP technology involving peracetic acid (PAA) activation mediated by a Mn^II-nitrilotriacetic acid (NTA) complex, which outperformed iron- and cobalt-based PAA activation processes for rapidly degrading phenolic and aniline contaminants from water. The proposed Mn^II/NTA/PAA system exhibited non-radical oxidation features and could stoichiometrically oxidize sulfoxide probes to the corresponding sulfone products. More importantly, we traced the origin of O atoms from the sulfone products by ¹⁸O isotope-tracing experiments and found that PAA was the only oxygen-donor, which is different from the oxidation process mediated by high-valence manganese-oxo intermediates. According to the results of theoretical calculations, we proposed that NTA could tune the coordination circumstance of the Mn^II center to elongate the O-O bond of the complexed PAA. Additionally, the NTA-Mn^II-PAA* molecular cluster presented a lower energy gap than the Mn^II-PAA complex, indicating that the Mn^II-peroxy complex was more reactive in the presence of NTA. Thus, the NTA-Mn^II-PAA* complex exhibited a stronger oxidation potential than PAA, which could rapidly oxidize organic contaminants from water. Further, we generalized our findings to the Co^II/PAA oxidation process and highlighted that the Co^II-PAA* complex might be the overlooked reactive cobalt species. The significance of this work lies in discovering that sometimes the metal-peroxy complex could directly oxidize the contaminants without the further generation of high-valence metal-oxo intermediates and/or radical species through interspecies oxygen and/or electron transfer.

New insights into the mechanism of Fered-Fenton treatment of industrial wastewater with high chloride content: Role of multiple reactive species

Hydroxyl radical (OH) is considered the dominant reactive species in the electro-Fenton (EF) and Fered-Fenton (EF-Fere) processes for wastewater treatment. However, in chloride-rich media, this is arguable due to the obscure mechanisms for the oxidant speciation and pollutant degradation. Herein, the role of active chlorine and Fe(IV)-oxo species (Fe^IVO²⁺) as primary oxidizing agents in HClO-mediated Fered-Fenton (EF-Fere-HClO) process is discussed, along with the dependence of their contribution on the pollutant structure. HClO generated from anodic oxidation of Cl^- can be consumed by added H₂O₂ to form singlet oxygen (¹O₂), which is detrimental because this species is quickly deactivated by water. The reaction between HClO and Fe²⁺ was proved to generate Fe^IVO²⁺, rather than OH or Cl suggested in the literature. The yield of Fe^IVO²⁺ species was proportional to the Cl^- concentration and barely affected by solution pH. The long-lived HClO and Fe^IVO²⁺ can selectively react with electron-rich compounds, which occurs simultaneously to the non-selective attack of OH formed from Fenton's reaction. The Fe^IVO²⁺ and OH concentration profiles were successfully modelled. Although the accumulation of toxic chlorinated by-products from HClO-mediated oxidation might cause new environmental concerns, the toxicity of pesticide wastewater with 508 mM Cl^- was halved upon EF-Fere-HClO treatment.

Iron species activating chlorite: Neglected selective oxidation for water treatment

Chlorite (ClO₂ ^-) is the by-product of the water treatment process carried out using chlorine dioxide (ClO₂) as an effective disinfectant and oxidant; however, the reactivation of ClO₂ ^- has commonly been overlooked. Herein, it was unprecedentedly found that ClO₂ ^- could be activated by iron species (Fe_b: Fe⁰, Fe^II, or Fe^III), which contributed to the synchronous removal of ClO₂ ^- and selective oxidative treatment of organic contaminants. However, the above-mentioned activation process presented intensive H⁺-dependent reactivity. The introduction of Fe_b significantly shortened the autocatalysis process via the accumulation of Cl^- or ClO^- during the protonation of ClO₂ ^- driven by ultrasonic field. Furthermore, it was found that the interdependent high-valent-Fe-oxo and ClO₂, after identification, were the dominant active species for accelerating the oxidation process. Accordingly, the unified mechanisms based on coordination catalysis ([Fe ^N (H₂O) _a (ClO _x ^m-) _b ] ^{n +}-P) were putative, and this process was thus used to account for the pollutant removal by the Fe_b-activated protonated ClO₂ ^-. This study pioneers the activation of ClO₂ ^- for water treatment and provides a novel strategy for "waste treating waste". Derivatively, this activation process further provides the preparation methods for sulfones and ClO₂, including the oriented oxidation of sulfoxides to sulfones and the production of ClO₂ for on-site use.

Comparison of peracetic acid and sodium hypochlorite enhanced Fe(Ⅱ) coagulation on algae-laden water treatment

In this study, Fe(Ⅱ)/peracetic acid (PAA) and Fe(Ⅱ)/sodium hypochlorite (NaClO) systems were applied as the combined preoxidation and coagulation process to enhance algae removal. A high removal rate of algae and turbidity could be achieved, with most algal cells keeping intact when adding reasonable concentrations of PAA and NaClO to enhance Fe(Ⅱ) coagulation. The variations of chlorophyll a, malondialdehyde, and intracellular reactive oxygen species suggested that moderate oxidation with only destroying surface-adsorbed organic matter rather than cell integrity was realized. The generated organic radicals, Fe(Ⅳ), and hydroxy radical played the major roles in the Fe(Ⅱ)/PAA system for the moderate oxidation of algal cells, but direct oxidation by NaClO rather than producing reactive species in the Fe(Ⅱ)/NaClO process contributed to the preoxidation. Concurrently, the in-situ formed Fe(Ⅲ) greatly promoted the agglomerating and settling of algae. The analysis of cell integrity, biochemical compositions, and fluorescence excitation-emission matrices spectra demonstrated that excess NaClO but not PAA would seriously damage the algal cells. This might be because that NaClO would directly oxidize the cell wall/membrane, while PAA mainly permeates into the cell to inactivate algae. These results suggest that Fe(Ⅱ)/PAA is an efficient strategy for algae-laden water treatment without serious algae lysis.

Photoinduced transformation of ferrihydrite in the presence of aqueous sulfite and its influence on the repartitioning of Cd

The photoinduced transformation of ferrihydrite is an important process that can predict the geochemical cycle of Fe in anoxic environments as well as the fate of trace elements bonded to Fe minerals. We report that the photooxidation of sulfite by UV irradiation produces hydrated electrons (super-reductants), which significantly promote ferrihydrite reduction to Fe(II), and SO₃^•- (a moderate oxidant), enabling its further oxidation to more crystalline Fe(III) products. The experimental results show that the concentration of sulfite was key in influencing the rate and extent of surface-bound Fe(II) formation, which ultimately determined the distribution of individual products. For example, fitting of the Mössbauer spectroscopy data revealed that the relative abundances of mineral species after 8 h of treatment in the UV/sulfite systems were 41.9% lepidocrocite and 58.1% ferrihydrite at 2 mM SO₃^2-; 41.8% goethite, 28.2% lepidocrocite, and 29.1% ferrihydrite at 5 mM SO₃^2-; and 100% goethite at 10 mM SO₃^2-. The combined results of the chemical speciation analysis and the Cd K-edge EXAFS characterization provided compelling evidence that Cd was firmly incorporated into the structure of newly formed minerals, particularly at high sulfite concentrations. These findings provide an understanding of the role of UV/sulfite in facilitating ferrihydrite transformation and promoting Cd stabilization in oxygen-deficit soils and aquatic environments.

Constant oxidation of atrazine in Fe(III)/PDS system by enhancing Fe(III)/Fe(II) cycle with quinones: Reaction mechanism, degradation pathway and DFT calculation

Quinones are potential pollutants and redox active compounds widely distributed in environmental media. In this study, methyl-p-benzoquinone (MBQ) was introduced into Fe(III)/peroxydisulfate system (Fe(III)/PDS) to expedite the conversion of Fe(III) to Fe(II) and the degradation of atrazine (ATZ), ultimately establishing an environmentally friendly system of "treating pollution with pollution". MBQ/Fe(III)/PDS system showed superior performance to traditional Fe(II)/PDS system in pH range of 2-7. Sulfate radical (SO₄^•-) and hydroxyl radical (^•OH) were confirmed to exist in MBQ/Fe(III)/PDS system according to alcohol quenching experiments and ESR tests. Meanwhile, stable 80% of η[PMSO₂] (i.e., the molar ratio of PMSO₂ generation to PMSO consumption) was achieved and manifested that highly reactive substance Fe(IV) also participated in MBQ/Fe(III)/PDS system. The spontaneous transformation of MBQ and methyl-hydroquinone (MHQ) drove Fe(III)/Fe(II) cycle, during which MHQ induced Fe(III) reduction and Fe(II) regeneration. Transformation pathways of ATZ were proposed based on HPLC-MS detection and DFT calculation and ATZ degradation could be initiated by lateral chain oxidation and dechlorination-hydroxylation. The acute toxicity, bioaccumulation factor, developmental toxicity and mutagenicity of ATZ and its degradation intermediates were evaluated by Toxicity Estimation Software Tool, and the luminescent bacteria test was conducted to investigate the acute toxicity variation of the reaction solution. Cl^- obviously inhibited ATZ degradation and three main by-products generation, while humic acid (HA) had little effect on them probably due to the established balance between inhibition (some components in HA competed to consume reactive species) and acceleration (quinone units in HA also facilitated Fe(III)/Fe(II) cycle).

Generation of FeIV =O and its Contribution to Fenton-Like Reactions on a Single-Atom Iron-N-C Catalyst

Generating Fe^IV =O on single-atom catalysts by Fenton-like reaction has been established for water treatment; however, the Fe^IV =O generation pathway and oxidation behavior remain obscure. Employing an Fe-N-C catalyst with a typical Fe-N₄ moiety to activate peroxymonosulfate (PMS), we demonstrate that generating Fe^IV =O is mediated by an Fe-N-C-PMS* complex-a well-recognized nonradical species for induction of electron-transfer oxidation-and we determined that adjacent Fe sites with a specific Fe₁ -Fe₁ distance are required. After the Fe atoms with an Fe₁ -Fe₁ distance <4 Å are PMS-saturated, Fe-N-C-PMS* formed on Fe sites with an Fe₁ -Fe₁ distance of 4-5 Å can coordinate with the adjacent Fe^II -N₄ , forming an inter-complex with enhanced charge transfer to produce Fe^IV =O. Fe^IV =O enables the Fenton-like system to efficiently oxidize various pollutants in a substrate-specific, pH-tolerant, and sustainable manner, where its prominent contribution manifests for pollutants with higher one-electron oxidation potential.

Revealing the Generation of High-Valent Cobalt Species and Chlorine Dioxide in the Co3O4-Activated Chlorite Process: Insight into the Proton Enhancement Effect

A Co₃O₄-activated chlorite (Co₃O₄/chlorite) process was developed to enable the simultaneous generation of high-valent cobalt species [Co(IV)] and ClO₂ for efficient oxidation of organic contaminants. The formation of Co(IV) in the Co₃O₄/chlorite process was demonstrated through phenylmethyl sulfoxide (PMSO) probe and ¹⁸O-isotope-labeling tests. Both experiments and theoretical calculations revealed that chlorite activation involved oxygen atom transfer (OAT) during Co(IV) formation and proton-coupled electron transfer (PCET) in the Co(IV)-mediated ClO₂ generation. Protons not only promoted the generation of Co(IV) and ClO₂ by lowering the energy barrier but also strengthened the resistance of the Co₃O₄/chlorite process to coexisting anions, which we termed a proton enhancement effect. Although both Co(IV) and ClO₂ exhibited direct oxidation of contaminants, their contributions varied with pH changes. When pH increased from 3 to 5, the deprotonation of contaminants facilitated the electrophilic attack of ClO₂, while as pH increased from 5 to 8, Co(IV) gradually became the main contributor to contaminant degradation owing to its higher stability than ClO₂. Moreover, ClO₂^- was transformed into nontoxic Cl^- rather than ClO₃^- after the reaction, thus greatly reducing possible environmental risks. This work described a Co(IV)-involved chlorite activation process for efficient removal of organic contaminants, and a proton enhancement mechanism was revealed.

Electrocoagulation coupled with electrooxidation for the simultaneous treatment of multiple pollutants in contaminated sediments

In situ and simultaneous remediation of a variety of pollutants in sediments remains a challenge. In this study, we report that the combination of electrocoagulation (EC) and electrooxidation (EO) is efficient in the immobilization of phosphorus and heavy metals and in the oxidation of ammonium and toxic organic matter. The integrated mixed metal oxide (MMO)/Fe anode system allowed the facile removal of ammonium and phosphorus in the overlying water (99% of 10 mg/L NH₄⁺-N and 95% of 10 mg/L P disappeared in 15 and 30 min, respectively). Compared with the controls of the single Fe anode and single MMO anode systems, the dual MMO/Fe anode system significantly improved the removal of phenanthrene and promoted the transition of Pb and Cu from the mobile species to the immobile species. The concentrations of Pb and Cu in the toxicity characteristic leaching procedure extracts were reduced by 99% and 97% after an 8 hr operation. Further tests with four real polluted samples indicated that substantial proportions of acid-soluble fraction Pb and Cu were reduced (30%-31% for Pb and 16%-23% for Cu), and the amounts of total organic carbon and NH₄⁺-N decreased by 56%-71% and 32%-63%, respectively. It was proposed that the in situ electrogenerated Fe(II) at the Fe anode and the active oxygen/chlorine species at the MMO anode are conducive to outstanding performance in the co-treatment of multiple pollutants. The results suggest that the EC/EO method is a powerful technology for the in situ remediation of sediments contaminated with different pollutants.

New insights into the degradation of micro-pollutants in the hydroxylamine enhanced Fe(II)/peracetic acid process: Contribution of reactive species and effects of pH

The hydroxylamine-enhanced Fe(II)/peracetic acid (PAA) process is a promising advanced oxidation process (AOP) with the generation of reactive species (RS) including RO•, •OH and Fe(IV). Nevertheless, it is still challenging to identify which RS is the major intermediate oxidant, and the reasons why the optimal condition is pH 4.5 rather than 3.0 are also unclear. Herein, the generation of RS and their contribution to the degradation of three micro-pollutants were explored. The quenching experiments and pseudo first-order kinetic model demonstrated that RO• rather than the other two RS were predominant. Then the overall generation and evolution pathways of RS were depicted. The elevation of pH (3.0-4.5) would accelerate the Fe(II)/Fe(III) redox cycle through the enhanced reduction of Fe(III) by hydroxylamine and induce the conversion of Fe(IV) to RO•, which benefited naproxen degradation. While the adverse Fe(III) precipitation would dominate the reduced degradation performance with the solution pH higher than 4.5. The elevation of PAA and Fe(II) dosages sped up the PAA activation, while excess hydroxylamine could consume the formed RS and exhibited an inhibitory effect. This study helps further understand the role of HA and differentiate the contribution of RS in the emerging PAA-based AOPs.

Cooperation of multiple active species generated in hydrogen peroxide activation by iron porphyrin for phenolic pollutants degradation

The narrow acid pH range and the nonselectivity of the dominant •OH limit the Fenton systems to remediate the organic wastewater. Inspired by the role of heme in physiological processes, we employed iron porphyrin as a novel homogeneous catalyst to address this issue. Multiple active species are identified during the activation of H₂O₂, including high-valent iron porphyrin ((por)Fe(IV)) species ((por)Fe(IV)-OH, (por)^+•Fe(IV)=O) and oxygen-centered radicals (•OH, HO₂•/•O₂^-), as well as atomic hydrogen (*H) and carbon-centered radicals. With the cooperation of these active species, the degradation of pollutants could be resistant to the interference of concomitant ions and proceed over a wide pH range. This cooperative behavior is further verified by intermediates identified from bisphenol A degradation. Specifically, the presence of *H could facilitate the cleavage of the C-C bond and the addition of unsaturated or aromatic molecules. (Por)^+•Fe(IV)=O could hydroxylate substrates with an oxygen rebound mechanism. Hydrogen atom abstraction of contaminants could be performed by (por)Fe(IV)-OH to form desaturated products by attacking oxygen-centered radicals. The ecotoxicity of bisphenol A could be significantly decreased through degradation. This study would provide a new approach to wastewater treatment and shed light on the interaction between metalloporphyrin and peroxide in an aqueous solution.

An efficient Fe2+ assisted UV/electrogenerated-chlorine process for carbamazepine degradation: The role of Fe(IV)

To improve the performance and solve the restrictions of UV/chlorine process (e.g., the narrow pH application range and high disinfection by-products (DBPs) formation), a Fe²⁺ assisted advanced oxidation process with electrochemically generated chlorine (UV/E-Cl/Fe²⁺) was proposed for carbamazepine (CBZ) degradation, which eliminated CBZ (5 mg/L) within 4 min under the optimal conditions. Compared with UV/electro-generated chlorine (UV/E-Cl) and anodic oxidation-chlorination/Fe²⁺ (AO-Cl/Fe²⁺) processes, the apparent first-order kinetics constant in UV/E-Cl/Fe²⁺ increased by 2.56 and 3.18 times respectively, and the energy consumption was lower (1.15 kWh/m³-log). Simultaneously, the pH application range could be expanded to 9, and DBPs formed in this process were 17.1% less than those in UV/E-Cl. Through quenching tests, electron paramagnetic resonance (EPR) experiments, measurement of ^•OH concentration, quantification of methyl phenyl sulfoxide (PMSO) and benzosulfone (PMSO₂) and processes comparison, possible CBZ degradation pathways and mechanism of UV/E-Cl/Fe²⁺ were proposed, in which Fe(IV) played the dominant role in the early stage, while the production of radicals (i.e., ^•OH and Cl^•) was enhanced with the increase of chlorine generation, accelerating the CBZ removal. Furthermore, this process demonstrated wide application prospect in treating various contaminants and real wastewaters. In conclusion, this study offers an effective and energy-efficient method for organic pollutants degradation.

Chemiluminescence quenching capacity as a surrogate for total organic carbon in wastewater

Total organic carbon (TOC) is a valuable indicator to evaluate the degree of organic pollution in wastewater. Real-time analysis of TOC in wastewater can allow the wastewater treatment plants to manage the treatment process efficiently, avoid violations of the discharge regulations, and eliminate overtreatment. However, traditional methods for TOC determination are time-consuming. Benefitting from the rapid generation of SO₄^•- in the iron(II)-activated peroxymonosulfate (Fe(II)/PMS) system and the high reactivity of SO₄^•- towards naproxen as a chemiluminescence (CL) probe, a surrogate for TOC based on the determination of CL quenching capacity (CLQC) of organics in the Fe(II)/PMS-naproxen system was developed. According to the derived equation by considering both non-fluorescent and fluorescent quenching, the CLQC of organics in the Fe(II)/PMS-naproxen system was highly dependent on their TOC, making it to be a potential surrogate for TOC. The interferences of ubiquitous inorganic ions in wastewater on the determination of CLQC were leveled by adjusting electrical conductivity and adding mercury ions. Finally, the feasibility of CLQC as a surrogate for TOC in two real wastewaters containing different concentrations of inorganic anions was confirmed. This work can provide a TOC value within several seconds by determining the CLQC of wastewater with Fe(II)/PMS-naproxen system.

Reactive species conversion into 1O2 promotes substantial inhibition of chlorinated byproduct formation during electrooxidation of phenols in Cl--laden wastewater

The generation of chlorinated byproducts during the electrochemical oxidation (EO) of Cl^--laden wastewater is a significant concern. We aim to propose a concept of converting reactive species (e.g., reactive chlorines and HO^• resulting from electrolysis) into ¹O₂ via the addition of H₂O₂, which substantially alleviates chlorinated organic formation. When phenol was used as a model organic compound, the results showed that the H₂O₂-involving EO system outperformed the H₂O₂-absent system in terms of higher rate constants (5.95 × 10^-2 min^-1vs. 2.97 × 10^-2 min^-1) and a much lower accumulation of total organic chlorinated products (1.42 mg L^-1vs. 8.18 mg L^-1) during a 60 min operation. The rate constants of disappearance of a variety of phenolic compounds were positively correlated with the Hammett constants (σ), suggesting that the reactive species preferred oxidizing phenols with electron-rich groups. After the identification of ¹O₂ that was abundant in the bulk solution with the use of electron paramagnetic resonance and computational kinetic simulation, the routes of ¹O₂ generation were revealed. Despite the consensus as to the contribution of reaction between H₂O₂ and ClO^- to ¹O₂ formation, we conclude that the predominant pathway is through H₂O₂ reaction with electrogenerated HO^• or chlorine radicals (Cl^• and Cl₂^•-) to produce O₂^•-, followed by self-combination. Density functional theory calculations theoretically showed the difficulty in forming chlorinated byproducts for the ¹O₂-initiated phenol oxidation in the presence of Cl^-, which, by contrast, easily occurred for the Cl^•-or HO^•-initiated phenol reaction. The experiments run with real coking wastewater containing high-concentration phenols further demonstrated the superiority of the H₂O₂-involving EO system. The findings imply that this unique method for treating Cl^--laden organic wastewater is expected to be widely adopted for generalizing EO technology for environmental applications.

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.

Boosting the oxidative capacity of the Fe(0)/O2 system via an air-breathing cathode

The corrosion of Fe(0) in the presence of O₂ in nature can lead to the oxidation of organic compounds, but the efficiency is very limited. Herein, attempts were made to establish a galvanic system that separates the anodic Fe(0) oxidation reaction and the cathodic O₂ reduction reaction using an air-breathing cathode. Compared with the chemical Fe(0)/O₂ system, it exhibited a substantially higher capability of destroying a variety of pollutants, such as organic dyes (12 types), phenol, nitrobenzene, acetaminophen, phenol, and ethylenediaminetetraacetic acid. The degradation rate constant of a model dye (i.e., Rhodamine B) increased from 0.047 min^-1 (chemical) to 1.412 min^-1 (galvanic) under the passive air-breathing condition. The electric circuit design promoted Fe(0) dissolution to Fe(II) and triggered electron transfer that drives O₂ reduction to H₂O₂, two important species responsible for the generation of HO^• at high abundance. In addition, the galvanic Fe(0)/O₂ system produces electricity while destroying pollutants. Tests with real Ni plating wastewater further demonstrated the capability of the system to oxidize complexed organics and phosphite. This study provides a new strategy for boosting the oxidative capacity of the Fe(0)/O₂ system, which shows promise for acid wastewater treatment.

Degradation of Organic Contaminants by Reactive Iron/Manganese Species: Progress and Challenges

Many iron(II, III, VI)- and manganese(II, IV, VII)-based oxidation processes can generate reactive iron/manganese species (RFeS/RMnS, i.e., Fe(IV)/Fe(V) and Mn(III)/Mn(V)/Mn(VI)), which have mild and selective reactivity toward a wide range of organic contaminants, and thus have drawn significant attention. The reaction mechanisms of these processes are rather complicated due to the simultaneous involvement of multiple radical and/or nonradical species. As a result, the ambiguity in the occurrence of RFeS/RMnS and divergence in the degradation mechanisms of trace organic contaminants in the presence of RFeS/RMnS exist in literature. In order to improve the critical understanding of the RFeS/RMnS-mediated oxidation processes, the detection methods of RFeS/RMnS and their roles in the destruction of trace organic contaminants are reviewed with special attention to some specific problems related to the scavenger and probe selection and experimental results analysis potentially resulting in some questionable conclusions. Moreover, the influence of background constituents, such as organic matter and halides, on oxidation efficiency of RFeS/RMnS-mediated oxidation processes and formation of byproducts are discussed through their comparison with those in free radicals-dominated oxidation processes. Finally, the prospects of the RFeS/RMnS-mediated oxidation processes and the challenges for future applications are presented.

Performance investigation of electrochemical assisted HClO/Fe2+ process for the treatment of landfill leachate

The feasibility of removal of chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) from landfill leachate by an electrochemical assisted HClO/Fe2+ process is demonstrated for the first time. The performance of active chlorine generation at the anode was evaluated in Na2SO4/NaCl media, and a higher amount of active chlorine was produced at greater chloride concentration and higher current density. The probe experiments confirmed the coexistence of hydroxyl radical (•OH) and Fe(IV)-oxo complex (FeIVO2+) in the HClO/Fe2+ system. The influence of initial pH, Fe2+ concentration, and applied current density on COD and NH4+-N abatement was elaborately investigated. The optimum pH was found to be 3.0, and the proper increase in Fe2+ dosage and current density resulted in higher COD removal due to the accelerated accumulation of •OH and FeIVO2+ in the bulk liquid phase, whereas, the NH4+-N oxidation was significantly affected by the applied current density because of the effective active chlorine generation at higher current but was nearly independent of Fe2+ concentration. The reaction mechanism of electrochemical assisted HClO/Fe2+ treatment of landfill leachate was finally proposed. The powerful •OH and FeIVO2+, in concomitance with active chlorine and M(•OH), were responsible for COD abatement, and active chlorine played a key role in NH4+-N oxidation. The proposed electrochemical assisted HClO/Fe2+ process is a promising alternative for the treatment of refractory landfill leachate.

Minimizing toxic chlorinated byproducts during electrochemical oxidation of Ni-EDTA: Importance of active chlorine-triggered Fe(II) transition to Fe(IV)

The formation of chlorinated byproducts represents a significant threat to the quality of the effluent treated using electrochemical advanced oxidation processes (EAOPs), thus spurring investigation into alleviating their production. This study presents a new strategy to minimize the release of chlorinated intermediates during the electrochemical oxidation of Ni-EDTA by establishing a dual mixed metal oxide (MMO)/Fe anode system. The results indicate that the dual-anode system achieved a substantially higher rate (0.141 min^-1) of Ni-EDTA destruction and accordingly allowed a more pronounced removal of aqueous Ni (from 39.85 to 0.63 mg L^-1) after alkaline precipitation, compared with its single MMO anode (0.017 min^-1 of Ni-EDTA removal, with 14.38 mg L^-1 Ni remaining) and single Fe anode (insignificant Ni-EDTA removal, with 38.37 mg L^-1 Ni remaining) counterparts. Compared to reactive chlorine species (RCS) produced from the single MMO anode system, Fe(IV) was in situ generated from the dual-anode system and was predominantly responsible for the attenuation of chlorinated byproducts and thus the decrease in the acute toxicity of the treated solution (evaluated using luminescent bacteria). The Fe(IV)-dominated dual-anode system also exhibited superior performance in removing multiple pollutants (including organic ligands, Ni, and phosphite) in the real electroless plating effluent. The findings suggest that the strategy for Fe(II) transition to Fe(IV) by active chlorine paves a new avenue for yielding less chlorinated products with lower toxicity when EAOPs are used to treat chloride-containing organic wastewater.

Activation of hydrogen peroxide, persulfate, and free chlorine by steel anode for treatment of municipal and livestock wastewater: Unravelling the role of oxidants speciation

Despite the extensive application of electrochemical advanced oxidation processes (EAOPs) in wastewater treatment, the exact speciation of oxidants and their effects on pollutants removal efficiency, by-products formation, and effluent toxicity are largely unknown. In this study, galvanostatic steel anodes were used to drive the electrochemical activation of hydrogen peroxide (EAHP), persulfate (EAP), and free chlorine (EAFC), for industrial-scale treatment of municipal and livestock wastewater with a focus on micropollutants and transformation products (MTPs) and effluent toxicity. Response surface methodology determined the optimized conditions for each treatment towards total organic carbon ([TOC]₀ = 180 mg/L) removal at pH 3.0: persulfate dose = 0.12 mmol/min, 26.5 mA/cm²; free chlorine dose = 0.29 mmol/min, 37.4 mA/cm²; H₂O₂ dose = 0.20 mmol/min, 45 mA/cm². Probe-compound degradation revealed that HO^•, SO₄^•- and Fe^IVO²⁺ species were simultaneously generated in EAP, whereas HO^• and Fe^IVO²⁺ were the principal oxidants in EAHP and EAFC, respectively. Samples were analyzed via liquid and gas chromatography in non-target screening (NTS) mode to monitor the generation or removal of MTPs and by-products including compounds that have not been reported previously. The speciation of oxidants, shifted in presence of halide ions (Cl^-, Br^-) in real wastewater samples, significantly affected the mineralization efficiency and by-product formation. The production of halogenated by-products in EAFC and EAP substantially increased the effluent toxicity, whereas EAHP provided non-toxic effluent and the highest mineralization efficiency (75 - 80%) to be nominated as the best strategy.