Oxidation of sulfoxides and arsenic(III) in corrosion of nanoscale zero valent iron by oxygen: evidence against ferryl ions (Fe(IV)) as active intermediates in Fenton reaction

Persulfate activation by biochar and iron: Effect of chloride on formation of reactive species and transformation of N,N-diethyl-m-toluamide (DEET)

Fenton-like processes using persulfate for oxidative water treatment and contaminant removal can be enhanced by the addition of redox-active biochar, which accelerates the reduction of Fe(III) to Fe(II) and increases the yield of reactive species that react with organic contaminants. However, available data on the formation of non-radical or radical species in the biochar/Fe(III)/persulfate system are inconsistent, which limits the evaluation of treatment efficiency and applicability in different water matrices. Based on competition kinetics calculations, we employed different scavengers and probe compounds to systematically evaluate the effect of chloride in presence of organic matter on the formation of major reactive species in the biochar/Fe(III)/persulfate system for the transformation of the model compound N,N‑diethyl-m-toluamide (DEET) at pH 2.5. We show that the transformation of methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2) cannot serve as a reliable indicator for Fe(IV), as previously suggested, because sulfate radicals also induce PMSO2 formation. Although the formation of Fe(IV) cannot be completely excluded, sulfate radicals were identified as the major reactive species in the biochar/Fe(III)/persulfate system in pure water. In the presence of dissolved organic matter, low chloride concentrations (0.1 mM) shifted the major reactive species likely to hydroxyl radicals. Higher chloride concentrations (1 mM), as present in a mining-impacted acidic surface water, resulted in the formation of another reactive species, possibly Cl2•-, and efficient DEET degradation. To tailor the application of this oxidation process, the water matrix must be considered as a decisive factor for reactive species formation and contaminant removal.

Non-radical oxidation driven by iron-based materials without energy assistance in wastewater treatment

Chemical oxidation is extensively utilized to mitigate the impact of organic pollutants in wastewater. The non-radical oxidation driven by iron-based materials is noted for its environmental friendliness and resistance to wastewater matrix, and it is a promising approach for practical wastewater treatment. However, the complexity of heterogeneous systems and the diversity of evolutionary pathways make the mechanisms of non-radical oxidation driven by iron-based materials elusive. This work provides a systematic review of various non-radical oxidation systems driven by iron-based materials, including singlet oxygen (1O2), reactive iron species (RFeS), and interfacial electron transfer. The unique mechanisms by which iron-based materials activate different oxidants (ozone, hydrogen peroxide, persulfate, periodate, and peracetic acid) to produce non-radical oxidation are described. The roles of active sites and the unique structures of iron-based materials in facilitating non-radical oxidation are discussed. Commonly employed identification methods in wastewater treatment are compared, such as quenching, chemical probes, spectroscopy, mass spectrometry, and electrochemical testing. According to the process of iron-based materials driving non-radical oxidation to remove organic pollutants, the driving factors at different stages are summarized. Finally, challenges and countermeasures are proposed in terms of mechanism exploration, detection methods and practical applications of non-radical oxidation driven by iron-based materials. This work provides valuable insights for understanding and developing non-radical oxidation systems.

Iron coupled with hydroxylamine turns on the "switch" for free radical degradation of organic pollutants under high pH conditions

Reducing iron by hydroxylamine (HA) can promote the generation of reactive oxygen species (ROS) in the Fenton reaction and play a crucial role in the degradation of organic pollutants. However, the performance of this system at wider environmental thresholds is still not sufficiently understood, especially in the highly alkaline environments resulting from human activities. Here, we assessed the impact of solution pH on organic pollutant degradation by goethite with the addition of HA and H2O2. The solid phase variation and ROS generation were analyzed using Mössbauer spectroscopy, X-ray absorption near edge structure spectroscopy, and electron paramagnetic resonance analysis. This study found that under alkaline conditions, the system can continuously scavenge organic pollutants through oxygen-mediated generation of free radicals. At lower pH levels, organic pollutant decomposition, exemplified by the breakdown of bisphenol A (BPA), is primarily driven by the Fenton reaction facilitated by iron. As pH increases, hydroxyl radical (•OH) production decreases, accompanied by decreased BPA removal efficiency. However, the removal efficiency of BPA increased significantly at pH > 9. At pH 12, the removal of BPA exceeded that of the acidic condition after one hour, which is consistent with observations in soil system studies. Unlike the Fenton reaction, which is not sensitive to oxygen content, the removal of BPA under alkaline conditions occurs only under aerobic conditions. H2O2 is hardly involved in the reaction, and the depletion of HA becomes a critical factor in the decomposition of BPA. Importantly, in contrast to acidic conditions, where the dramatic decomposition of BPA occurs mainly in the first 10 min, the decomposition of BPA under alkaline conditions continued to occur over the 2 h of observation until complete removal. For natural systems, the remediation of pollutants depends more on the active time of ROS than on their reactivity. Therefore, this idea can reference pollution remediation strategies in anthropogenically disturbed environments.

Iron minerals enhance Fe(II)-mediated abiotic As(III) oxidation

The abiotic oxidation of As(III) is simultaneously mediated by the oxidation of Fe(II) in microaerobic environment, but the role of Fe minerals in the Fe(II)-mediated As(III) oxidation have been neglected. This work mimicked the microaerobic environment and examined the mechanisms of Fe(II) mediated the As(III) oxidation in the presence of Fe minerals using a variety of iron minerals (lepidocrocite, goethite, etc.). The results indicated the Fe(II) and As(III) oxidation rate were improved with Fe minerals, while As(III) oxidation efficiency increased by 1.3-1.8 times in comparison to that without minerals. Fe(II) mediated the As(III) oxidation happened on Fe minerals surface in the presence of Fe minerals. The As(III) oxidation efficiency increased with increasing Fe mineral concentrations (from 0.5 to 2 g L-1) but decreased with increasing pH values. Reactive oxygen species (ROS) that play a crucial role in As(III) oxidation were Fe(IV) and ·O2-, accounting for 42.7%-47.9% and 24.1%-29.8%, respectively. The Fe minerals facilitated the oxidation of As(III) by ROS and stimulated the release of ROS through the adsorbed-Fe(II) oxidation, both of which favored As(III) oxidation. This work highlighted the potential mechanisms of Fe minerals in promoting Fe(II) mediated the As(III) oxidation in microaerobic environment, especially in terms of As(III) oxidation efficiency, shedding a valuable insight on optimization of arsenic contaminated wastewater treatment processes.

Key role of Phyllanthus emblica L. fruit extract promotes ZVI/H2O2 process: rich titratable acid, suitable chelating ability, and antioxidant capacity

Phyllanthus emblica L. fruit extract (PFE) was introduced to improve ZVI/H2O2 technology, and the efficiency and mechanism of PFE promoting ZVI/H2O2 technology were explored. With the introduction of PFE, the Norfloxacin (NOR) removal rate and kobs of the process were improved by 41.17% and 5.08 times, respectively. In the ZVI/H2O2/PFE process, the degradation of NOR by the attack of ROS is the main pathway for decontamination and is dominated by the heterogeneous reaction on the catalyst surface. PFE contains 13.92 g/L titratable acid and has good complexing ability and antioxidant ability. The mechanism of PFE promoting ZVI/H2O2 technology was based on lowering the pH, complemented by chelation and antioxidant capacity. With the introduction of PFE, the utilization rate of the reagent was significantly increased (7.56 times for ZVI and 3.21 times for H2O2), the applicable pH range was widened (6-9) and the iron sludge was reduced (32.80%). Meanwhile, the concept of UPR is proposed for the first time. The result is the key role to the selection of green promoters in the ZVI/H2O2 process depends on the abundance of titratable acid, followed by a certain chelating ability and antioxidant capacity.

Size-Dependent Catalysis in Fenton-like Chemistry: From Nanoparticles to Single Atoms

State-of-the-art Fenton-like reactions are crucial in advanced oxidation processes (AOPs) for water purification. This review explores the latest advancements in heterogeneous metal-based catalysts within AOPs, covering nanoparticles (NPs), single-atom catalysts (SACs), and ultra-small atom clusters. A distinct connection between the physical properties of these catalysts, such as size, degree of unsaturation, electronic structure, and oxidation state, and their impacts on catalytic behavior and efficacy in Fenton-like reactions. In-depth comparative analysis of metal NPs and SACs is conducted focusing on how particle size variations and metal-support interactions affect oxidation species and pathways. The review highlights the cutting-edge characterization techniques and theoretical calculations, indispensable for deciphering the complex electronic and structural characteristics of active sites in downsized metal particles. Additionally, the review underscores innovative strategies for immobilizing these catalysts onto membrane surfaces, offering a solution to the inherent challenges of powdered catalysts. Recent advances in pilot-scale or engineering applications of Fenton-like-based devices are also summarized for the first time. The paper concludes by charting new research directions, emphasizing advanced catalyst design, precise identification of reactive oxygen species, and in-depth mechanistic studies. These efforts aim to enhance the application potential of nanotechnology-based AOPs in real-world wastewater treatment.

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.

In-situ generation of iron activated percarbonate for sustainable sludge dewatering

Effective dewatering of sewage sludge could potentially address the issues of high energy consumption and large carbon footprint inherent in the sludge treatment process, advancing toward carbon neutrality in environmental remediation. Yet, the surface hydrophilic characteristics and water-holding interfacial affinity in sludge led to dwindled sludge-water separation performance. Here, the integration of in-situ generation of iron from zero-valent scrap iron (ZVSI) and sodium percarbonate (SPC) was attempted to attenuate the water-retaining interfacial affinity within sludge, thus achieving superior sludge dewatering performance. Results showed that under the optimal conditions, the ZVSI + SPC system led to a remarkable decline of 76.09 % in the specific resistance to filtration of the sludge, accompanied by a notable decline of 34.96 % in the water content. Moreover, the utilization of ZVSI + SPC system could be a viable alternative to the traditional strategies in terms of enhanced sludge dewaterability, offering application potential with stable operating performance, economic feasibility, and reduced carbon emissions. Investigation into dewatering mechanism revealed that ZVSI could maintain the Fe3+/Fe2+ in a stable dynamic cycle and continuously in-situ generate Fe2+, thereby efficaciously fostering the SPC activation for the ceaseless yield of reactive oxygen species. The predominant •OH and 1O2 efficiently decomposed the hydrophilic biopolymers, therefore minimizing the hydrophilic protein secondary structures, along with the hydrogen and disulfide bonds within proteins. Subsequently, the water-holding interfacial affinity was profoundly diminished, leading to intensified hydrophobicity, self-flocculation, and dewaterability. These findings have important implications for the advancement of efficacious ZVSI + SPC conditioning techniques toward sustainable energy and low-carbon prospects.

Ferrate(VI) assists in reducing cytotoxicity and genotoxicity to mammalian cells and organic bromine formation in ozonated wastewater

Ozonation of wastewater containing bromide (Br-) forms highly toxic organic bromine. The effectiveness of ozonation in mitigating wastewater toxicity is minimal. Simultaneous application of ozone (O3) (5 mg/L) and ferrate(VI) (Fe(VI)) (10 mg-Fe/L) reduced cytotoxicity and genotoxicity towards mammalian cells by 39.8% and 71.1% (pH 7.0), respectively, when the wastewater has low levels of Br-. This enhanced reduction in toxicity can be attributed to increased production of reactive iron species Fe(IV)/Fe(V) and reactive oxygen species (•OH) that possess higher oxidizing ability. When wastewater contains 2 mg/L Br-, ozonation increased cytotoxicity and genotoxicity by 168%-180% and 150%-155%, respectively, primarily due to the formation of organic bromine. However, O3/Fe(VI) significantly (p < 0.05) suppressed both total organic bromine (TOBr), BrO3-, as well as their associated toxicity. Electron donating capacity (EDC) measurement and precursor inference using Orbitrap ultra-high resolution mass spectrometry found that Fe(IV)/Fe(V) and •OH enhanced EDC removal from precursors present in wastewater, inhibiting electrophilic substitution and electrophilic addition reactions that lead to organic bromine formation. Additionally, HOBr quenched by self-decomposition-produced H2O2 from Fe(VI) also inhibits TOBr formation along with its associated toxicity. The adsorption of Fe(III) flocs resulting from Fe(VI) decomposition contributes only minimally to reducing toxicity. Compared to ozonation alone, integration of Fe(VI) with O3 offers improved safety for treating wastewater with varying concentrations of Br-.

Structural Fe(II)-induced generation of reactive oxygen species on magnetite surface for aqueous As(III) oxidation during oxygen activation

Magnetite is a reductive Fe(II)-bearing mineral, and its reduction property is considered important for degradation of contaminants in groundwater and anaerobic subsurface environments. However, the redox condition of subsurface environments frequently changes from anaerobic to aerobic owing to natural and anthropogenic disturbances, generating reactive oxygen species (ROS) from the interaction between Fe(II)-bearing minerals and O2. Despite this, the mechanism of ROS generation induced by magnetite under aerobic conditions is poorly understood, which may play a crucial role in As(III) oxidation. Herein, we found that magnetite could activate O2 and induce the oxidative transformation of As(III) under aerobic conditions. As(III) oxidation was attributed to the ROS generated via structural Fe(II) within the magnetite octahedra oxygenation. The electron paramagnetic resonance and quenching tests confirmed that O2•-, H2O2, and •OH were produced by magnetite. Moreover, density function theory calculations combined with experiments demonstrated that O2•- was initially formed via single electron transfer from the structural Fe(II) to the adsorbed O2; O2•- was then converted to •OH and H2O2 via a series of free radical reactions. Among them, O2•-and H2O2 were the primary ROS responsible for As(III) oxidation, accounting for approximately 52 % and 19 % of As(III) oxidation. Notably, As(III) oxidation mainly occurred on the magnetite surface, and As was immobilized further within the magnetite structure. This study provides solid evidence regarding the role of magnetite in determining the fate and transformation of As in redox-fluctuating subsurface environments.

Electro-oxidation of Ibuprofen using carbon-supported SnOx-CeOx flow-anodes: The key role of high-valent metal

Exploiting electrochemically active materials as flow-anodes can effectively alleviate mass transfer restriction in an electro-oxidation system. However, the electrocatalytic activity and persistence of the conventional flow-anode materials are insufficient, resulting in limited improvement in the electro-oxidation rate and efficiency. Herein, we reported a rational strategy to substantially enhance the electrocatalytic performance of flow-anodes in electro-oxidation by introducing the redox cycle of high-valent metal in a suitable carbon substrate. The characterization suggested that the SnOx-CeOx/carbon black (CB) featured well-distributed morphology, rapid charge transfer, high oxygen evolution potential, and strong water adsorption, and stood out among three kinds of SnOx-CeOx loaded carbon materials. Mechanistic analysis indicated that the redox cycle of Ce species played a key role in accelerating the electron transfer from SnOx to CB directionally and could continuously create the electron-deficient state of the SnOx, thereby sustainably triggering the generation of ·OH. All these features enabled the resulting SnOx-CeOx/CB flow-anode to accomplish a calculated maximum kinetic constant of 0.02461 1/min, a higher current efficiency of 47.1%, and a lower energy consumption of 21.3 kWh/kg COD compared with other conventional flow-anodes reported to date. Additionally, SnOx-CeOx/CB exhibited excellent stability with extremely low leaching concentrations of Sn and Ce ions. This study provides a feasible manner for efficient water decontamination using the electro-oxidation system with SnOx-CeOx/CB.

Unravelling the removal mechanisms of trivalent arsenic by sulfidated nanoscale zero-valent iron: The crucial role of reactive oxygen species and the multiple effects of citric acid

The remediation of arsenic-contaminated groundwater by sulfidated nanoscale zero-valent iron (S-nZVI) has raised considerable attention. However, the role of trivalent arsenic (As(III)) oxidation by S-nZVI in oxic conditions (S-nZVI/O2) remains controversial, and the comprehensive effect of citric acid (CA) prevalent in groundwater on As(III) removal by S-nZVI remains unclear. Herein, the mechanisms of reactive oxygen species (ROS) generation and multiple effects of CA on As(III) removal by S-nZVI/O2 were systematically explored. Results indicated that the removal efficiency of As(III) by S-nZVI/O2 (97.81 %) was prominently higher than that by S-nZVI (66.71 %), resulting from the significant production of ROS (mainly H2O2 and OH) under oxic conditions, which played a crucial role in promoting the As(III) oxidation. Additionally, CA had multiple effects on As(III) removal by S-nZVI/O2 system: (i) CA impeded the diffusion of As(III) towards S-nZVI and increased the secondary risk of immobilized As(III) re-releasing into the environment due to the Fe dissolution from S-nZVI; (ii) CA could significantly enhance the yields of OH from 25.29 to 133.00 μM via accelerating the redox cycle of Fe(II)/Fe(III) and increasing the oriented conversion rate of H2O2 to OH; (iii) CA could also enrich the types of ROS (such as O2- and 1O2) in favor of further As(III) oxidation. This study contributed novel findings regarding the control of As(III) contaminated groundwater using S-nZVI technologies.

Effect and mechanism of norfloxacin removal by Eucalyptus leaf extract enhanced the ZVI/H2O2 process

The conventional ZVI/H2O2 technology suffers from poor reagent utilization, excess iron sludge generation, and strong low pH dependence. Therefore, eucalyptus leaf extract (ELE) was introduced to improve ZVI/H2O2 technology, and the efficacy and mechanism of ELE promoting ZVI/H2O2 technology were deeply explored. The results showed that the norfloxacin (NOR) removal and kobs of the ZVI/H2O2/ELE process were enhanced by 35.64 % and 3.27 times, respectively, compared to the ZVI/H2O2 process. In the ZVI/H2O2 process, the production of three reactive oxygen species (ROS: 1O2，·O2-，·OH) was effectively promoted by ELE so that the reaction efficacy was significantly enhanced. Moreover, the attack and degradation of pollutants by ROS was the main way to remove pollutants. With the introduction of ELE, the reactive sites on the catalyst appearance were increased to some extent, and the Fe(III)/Fe(II) cycle was improved. The analysis showed that ELE is rich in titratable acids and the ZVI/H2O2 technology is promoted mainly by lowering the pH of the process. In addition, the chelation of ELE and the reduction in pH by the ELE synergistically enhanced the ZVI/H2O2 technology, which significantly improved the reagent utilization (4.70 times for ZVI and 3.03 times for H2O2), broadened the pH range of the technology (6-9) and was able to effectively reduce the iron sludge contamination (30.33 %) of the process. Therefore, the study offers an important value to study eucalyptus leaves in micron-scale ZVI-Fenton technology.

Vitamin C-regulated CoAl- layered double hydroxide with oxygen vacancies to efficiently activate peroxydisulfate for sulfamethoxazole removal triggered via reactive oxygen and high-valent cobalt species

In this study, a vitamin C-regulated CoAl-layered double hydroxide with abundant oxygen vacancies was synthesized via a simple hydrothermal process. The resulting CoAl-layered double hydroxide was employed to activate peroxydisulfate for removal of sulfamethoxazole. The effect of the experimental parameters such as pH, catalyst dose and peroxydisulfate concentration on sulfamethoxazole removal was investigated. The current system exhibited excellent catalytic performance for sulfamethoxazole removal in a broad pH range (i.e., pH 3.0-11.0). Under the optimized condition, 94.2% of sulfamethoxazole was degraded within 15 min, accompanied by a 67.6% reduction in chemical oxygen demand. The effective sulfamethoxazole degradation could be attributed to four pathways. Firstly, the ≡ Co2+ in catalyst reacted with peroxydisulfate to generate reactive species, including SO4•-, •OH, O2•- and 1O2, which could degrade sulfamethoxazole. Secondly, the oxygen vacancies could modulate intrinsic electrons, resulted in the surface activation of catalyst and accelerated charge transfer, which was favorable for the degradation of sulfamethoxazole. Thirdly, the presence of vitamin C not only promoted the formation of oxygen vacancies but also expanded the interlayer spacing of layered double hydroxide. A large interlayer spacing facilitated the diffusion of peroxydisulfate and pollutants in the interlayer and improved the utilization efficiency of the active site. Lastly, the high-valent cobalt species exhibited excellent oxidation ability and enhanced the catalyst performance through continuously being employed as an electron acceptor. This study provided a valuable insight for the design and application of Co-based catalysts in peroxydisulfate-based advanced oxidation processes.

pH-Driven Efficacy of the Ferrate(VI)-Peracetic Acid System in Swift Sulfonamide Antibiotic Degradation: A Deep Dive into Active Species Evolution and Mechanistic Insights

In the realm of wastewater treatment, the power of ferrate (Fe(VI)) and peracetic acid (PAA) as oxidants stands out. But their combined might is where the enhancement truly lies. Their collaborative effect intensifies, but the underlying mechanics, especially across varying pH levels and pollutant types, still lurks in obscurity. Our study delved into the sophisticated oxidation interplay among Fe(VI)-PAA, Fe(VI)-H2O2, and standalone Fe(VI) systems. Notably, at a pH of 9.0, boasting a kinetic constant of ∼0.127 M-1·s-1, the Fe(VI)-PAA system annihilated the pollutant sulfamethoxazole, outpacing its counterparts by a staggering 48.73-fold when compared to the Fe(VI)-H2O2 system and 105.58-fold when using Fe(VI) individually. The behavior of active species─such as the dynamic •OH radicals and high-valent iron species (Fe(IV)/Fe(V))─shifted with pH variations, leading to distinct degradation pathways. Our detailed exploration pinpoints the behaviors of certain species across pH levels from 3.0 to 9.0. In more acidic environments, the •OH species proved indispensable for the system's reactivity. Conversely, as the pH inclined, degradation was increasingly steered by high-valent iron species. This intensive probe demystifies Fe(VI) interactions, deepening our understanding of the capabilities of the Fe(VI)-centered system and guiding us toward cleaner water solutions. Importantly, pH value, often underappreciated, holds the reins in organic wastewater decontamination. Embracing this key player is vital as we strategize for more expansive systems in upcoming ventures.

Ferryl Ion in the Photo-Fenton Process at Acidic pH: Occurrence, Fate, and Implications

Fenton processes produce reactive species that can oxidize organic compounds in natural and engineered systems. While it is well-documented that Fenton reactions produce hydroxyl radical (HO•) under acidic conditions, we demonstrated the generation of ferryl ion (FeIVO2+) in the UV/Fe(III) and UV/Fe(III)/H2O2 systems at pH 2.8 using methyl phenyl sulfoxide (PMSO) as the probe compound. Moreover, we clarified that FeIVO2+ is parallelly formed via the oxidation of Fe(III) by HO• and the O-O homolysis of [FeIII-OOH]2+ in the photo-Fenton process. The rate constant for the reaction between HO• and Fe3+ measured by laser flash photolysis was 4.41 × 107 M-1 s-1. The rate constant and quantum yield for thermal and photo O-O homolysis of [FeIII-OOH]2+ complex were 1.4 × 10-2 s-1 and 0.3, respectively, which were determined by fitting PMSO2 formation. While FeIVO2+ forms predominantly through the reaction between HO• and Fe3+ in the absence of H2O2, the relative contribution of [FeIII-OOH]2+ O-O homolysis to FeIVO2+ formation highly depends on the molar ratio of [H2O2]0/[Fe(III)]0, the level of HO• scavenging, and incident irradiance in the UV/Fe(III)/H2O2 system. Accordingly, an optimized kinetic model was developed by incorporating FeIVO2+-involved reactions into the conventional photo-Fenton model, which can accurately predict Fe(II) formation and contaminant decay in the UV/Fe(III) and UV/Fe(III)/H2O2 systems. Our study illuminated the underlying formation mechanism of reactive oxidative species in the photo-Fenton process and highlighted the role of FeIVO2+ evolution in modulating the iron cycle and pollutant abatement therein.

Electrochemical-based approaches for the treatment of pharmaceuticals and personal care products in wastewater

In recent times, emerging contaminants (ECs) like pharmaceuticals and personal care products (PPCPs) in water and wastewater have become a major concern in the environment. Electrochemical treatment technologies proved to be more efficient to degrade or remove PPCPs present in the wastewater. Electrochemical treatment technologies have been the subject of intense research for the past few years. Attention has been given to electro-oxidation and electro-coagulation by industries and researchers, indicating their potential to remediate PPCPs and mineralization of organic and inorganic contaminants present in wastewater. However, difficulties arise in the successful operation of scaled-up systems. Hence, researchers have identified the need to integrate electrochemical technology with other treatment technologies, particularly advanced oxidation processes (AOPs). Integration of technologies addresses the limitation of indiviual technologies. The major drawbacks like formation of undesired or toxic intermediates, s, energy expenses, and process efficacy influenced by the type of wastewater etc., can be reduced in the combined processes. The review discusses the integration of electrochemical technology with various AOPs, like photo-Fenton, ozonation, UV/H2O2, O3/UV/H2O2, etc., as an efficient way to generate powerful radicals and augment the degradation of organic and inorganic pollutants. The processes are targeted for PPCPs such as ibuprofen, paracetamol, polyparaben and carbamezapine. The discussion concerns itself with the various advantages/disadvantages, reaction mechanisms, factors involved, and cost estimation of the individual and integrated technologies. The synergistic effect of the integrated technology is discussed in detail and remarks concerning the prospects subject to the investigation are also stated.

Enhanced arsenic(III) sequestration via sulfidated zero-valent iron in aerobic conditions: Adsorption and oxidation coupling processes

Sulfidated zero-valent iron (S-ZVI) has shown significant potential for the removal of arsenic(III). However, little attention has been paid to the mechanism of As(III) sequestration enhancement and how the phase transformation for S-ZVI strengthens this process in aerobic conditions. In this work, sulfidated ZVI was created by ball-milling (S-ZVIbm) and liquid-mixing (S-ZVIlm) of ZVI with elemental sulfur(S0) to investigate the performance and mechanisms of As(III) sequestration in air-saturated water. Sulfidation was found to significantly enhance the As(III) removal rate constant, which was 2.8 ∼ 6.7 times (S-ZVIbm) and 3.1 ∼ 17.1 times (S-ZVIlm) higher than that without sulfidation. FeS was identified as the predominant sulfur species in the S-ZVI samples using S K-edge XANES spectra. The enhanced electron transfer and ZVI corrosion after sulfidation were verified via electrochemical tests. XANES and Mössbauer spectra suggested that lepidocrocite(γ-FeOOH) was the predominant corrosion product generated on the ZVI surface with the presence of oxygen, and DFT calculations further confirmed the improved performance of γ-FeOOH for As(III) sequestration. Besides, As(III) oxidation occurred dominantly on the heterogeneous surface rather than in solution, and the As(III) sequestration pathway of adsorption followed by oxidation was proposed. This study provides new insight into the enhanced As(III) sequestration by S-ZVI in aerobic conditions.

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.

Molecular structure-dependent contribution of reactive species to organic pollutant degradation using nanosheet Bi2Fe4O9 activated peroxymonosulfate

Iron-based catalysts have attracted increasing attention in heterogeneous activation of peroxymonosulfate (PMS). However, the activity of most iron-based heterogenous catalysts is not satisfactory for practical application and the proposed activation mechanisms of PMS by iron-based heterogenous catalyst vary case by case. This study prepared Bi₂Fe₄O₉ (BFO) nanosheet with super high activity toward PMS, which was comparable to its homogeneous counterpart at pH 3.0 and superior to its homogeneous counterpart at pH 7.0. Fe sites, lattice oxygen and oxygen vacancies on BFO surface were believed to be involved in the activation of PMS. By using electron paramagnetic resonance (EPR), radical scavenging tests, ⁵⁷Fe Mössbauer and ¹⁸O isotope-labeling technique, the generation of reactive species including sulfate radicals, hydroxyl radicals, superoxide and Fe (IV) were confirmed in BFO/PMS system. However, the contribution of reactive species to the elimination of organic pollutants very much depends on their molecular structure. The effect of water matrices on the elimination of organic pollutants also hinges on their molecular structure. This study implies that the molecular structure of organic pollutants governs their oxidation mechanism and their fate in iron-based heterogeneous Fenton-like system and further broadens our knowledge on the activation mechanism of PMS by iron-based heterogeneous catalyst.

Effects of Fe(II) on As(III) oxidation in Fe(II)-As(III) co-oxidation: Limiting and driving roles

The co-oxidation of Fe(II) and As(III) occurs under aerobic conditions, and Fe(II) may largely determine the fate of As(III), but the effect of Fe(II) on the As(III) oxidation is barely explored. In this research, the limiting and driving roles of Fe(II) in As(III) oxidation were systematically studied through batch kinetic studies in combination with X-ray photoelectron spectroscopy (XPS) depth profiling, scanning electron microscopy and energy dispersive X-ray spectrometry (SEM-EDS), and quenching experiments. The results showed that As(III) oxidation efficiency increased with the increase of Fe/As molar ratio (from 63.1% to 98.3%), but decreased with the increase of pH (from 96.0% to 44.2%) and the increase of air flow rate (from 88.1% to 75.1%). The Fe(II) oxidation rate increased with the increase of pH and air flow rate. When Fe(II) was oxidized rapidly, As(III) was more likely to be immobilized in the "inner sphere" of formed Fe (hydr)oxides, limiting As(III) oxidation. On the other hand, Fe(II) was oxidized to produce Fe (hydr)oxides to adsorb or fix As(III); meanwhile, the ROS generated by Fenton-like reaction of Fe(II) promoted As(III) oxidation, especially, •O₂^- and H₂O₂ were important ROS that drove the As(III) oxidation. These findings might provide a new insight for Fe(II) and As(III) geochemistry cycling in naturally occurring environment.

Assessing the Use of Probes and Quenchers for Understanding the Reactive Species in Advanced Oxidation Processes

Advanced oxidation processes (AOPs) are increasingly applied in water and wastewater treatment. Understanding the role of reactive species using probes and quenchers is one of the main requirements for good process design. However, much fundamental kinetic data for the reactions of probes and quenchers with reactive species is lacking, probably leading to inappropriate probe and quencher selection and dosing. In this work, second-order rate constants for over 150 reactions of probes and quenchers with reactive species such as ^•OH, SO₄^•-, and Cl^• and chemical oxidants such as free chlorine and persulfate were determined. Some previously ill-quantified reactions (e.g., furfuryl alcohol and methyl phenyl sulfoxide reactions with certain chemical oxidants, nitrobenzene and 1,4-dioxane reactions with certain halogen radicals) were found to be kinetically favorable. The selection of specific probes can be guided by the improved kinetic database. The criteria for properly choosing dosages of probes and quenchers were proposed along with a procedure for quantifying reactive species free of interference from probe addition. The limitations of probe and quencher approaches were explicated, and possible solutions (e.g., the combination with other tools) were proposed. Overall, the kinetic database and protocols provided in this work benefit future research in understanding the radical chemistry in AOPs as well as other radical-involved processes.

Mechanistic insights into the removal of As(III) and As(V) by iron modified carbon based materials with the aid of machine learning

The machine learning (ML) technique was used to examine the effects of different microscopic material features on the ability of iron modified carbon-based materials (Fe-CBMs) to remove As(V) and As(III). The findings showed that specific CBMs and Fe-CBMs features (such as surface functionality) from sophisticated microscopic and spectroscopic techniques led to models that were more accurate than those constructed using more basic information, such as bulk elemental composition and surface area (the root-mean-square error fell by 44.7% for As(V) and 56.9% for As(III), respectively). The high non-polar carbon (NPC) content of CBMs and Fe-CBMs had a detrimental influence on As(V) and As(III) removal capability, whereas surface oxygen-containing functional groups (SOFGs) contents on CBMs and Fe-CBMs played an essential role in arsenic removal based on ML approaches. The relative importance of CO was greater by 77.8% and 40.6% than that of C-O on the elimination of As(V) and As(III), respectively. The accurate ML models are helpful for the future design of Fe-CBMs and the relative importance and partial dependence plot analysis can direct the use of Fe-CBMs for arsenic removal in a sensible manner under different application situations.

Effect and mechanism of norfloxacin removal by guava leaf extract in the ZVI/H2O2 system

To overcome the bottlenecks of the conventional zero-valent iron Fenton-like (ZVI/H₂O₂) process, such as low reagent utilization, low applicable pH, and iron sludge contamination, guava leaf extract (GLE) was used as a green promoter to enhance ZVI/H₂O₂ process in this study. Compared with the ZVI/H₂O₂ system, the removal rate and k_obs of norfloxacin by the ZVI/H₂O₂/GLE system were increased by 33.76% and 2.19 times, respectively. The experimental investigation of the mechanism showed that the attack of reactive oxygen species was the main pathway for the removal of pollutants, and three types of reactive oxygen species (¹O₂, O₂^-,·OH) generations in the ZVI/H₂O₂/GLE system were effectively promoted by the introduction of GLE. The reactivity improvement was mainly due to the decrease of pH. At the same time, the chelation of iron ions by GLE promoted the Fe(III)/Fe(II) cycle on the catalyst surface was also a minor mechanism to improve the reactivity. This study provides a crucial reference for the practical application of guava leaf to promote the ZVI/H₂O₂ process in environmental pollution control.

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.

Generation of high-valent iron-oxo porphyrin cation radicals on hemin loaded carbon nanotubes for efficient degradation of sulfathiazole

Hemin has attracted considerable interest as an efficient catalyst recently, however, its direct application is inefficient due to severe molecular aggregation. Immobilizing hemin on various supports is a feasible approach to address this issue. In this work, a CNTs-hemin catalyst was prepared by loading hemin onto multiwalled carbon nanotubes (CNTs) through ball milling. Compared with hemin, CNTs-hemin demonstrates remarkably enhanced performance in the peroxymonosulfate system, with a 650-fold improvement of apparent rate constant, reaching 97.8% degradation of sulfathiazole in 5 min. High-valent iron-oxo porphyrin cation ((Porp)^+•Fe^IV=O) radicals are proposed as the dominant reactive species in the CNTs-hemin/peroxymonosulfate system instead of sulfate radicals (SO₄^•-), hydroxyl radicals (•OH), superoxide radicals (O₂^•-) and singlet oxygen (¹O₂). More in-depth mechanisms reveal that the strong electron transfer between CNTs and hemin promotes the generation of (Porp)^+•Fe^IV=O radicals through a heterolysis pathway. This research enriches the understanding of the catalytic mechanism of supported biomimetic catalysts for PMS activation and provides a perspective on the role of support materials for catalytic activity.

Iron boride boosted Fenton oxidation: Boron species induced sustainable FeIII/FeII redox couple

The regeneration of Fe(II) is the rate-limiting step in the Fenton/Fenton-like chain reactions that seriously hinder their scientific progress towards practical application. In this study, we proposed iron boride (FeB) for the first time as a new material to sustainably decompose H₂O₂ to generate hydroxyl radicals, which can non-selectively degrade a wide array of refractory organic pollutants. Fe(II) can be steadily released by the stepwise oxidation of FeB to stimulate Fenton reaction, meanwhile, B-B bonds as electron donors on the surface of FeB effectively promote the regeneration of Fe(II) from Fe(III) species and significantly accelerate the production of hydroxyl radicals. The low generation of toxic by-products and the high utilization rate of iron species validly avoid the secondary organic/metal pollution in the FeB/H₂O₂ system. Therefore, FeB mediated Fenton oxidation provides a novel strategy to realize a green and long-lasting environmental remediation.

A Comparison Study between Wood Flour and Its Derived Biochar for the Enhancement of the Peroxydisulfate Activation Capability of Fe3O4

In this study, both wood flour (WF) and wood flour-derived biochar (WFB) were used as supports for Fe3O4 to activate peroxydisulfate (PDS). The role of different carriers was investigated emphatically from the aspects of catalyst properties, the degradation kinetics of bisphenol A (BPA), the effects of important parameters, and the generation of reactive oxygen species (ROS). Results showed that both WF and WFB could serve as good support for Fe3O4, which could control the release of iron into solution and increase the specific surface areas (SSAs). The WFB/Fe3O4 had stronger PDS activation capability than WF/Fe3O4 mainly due to the larger SSA of WFB/Fe3O4 and the PDS activation ability of WFB. Both radical species (•OH and SO4•−) and non-radical pathways, including 1O2 and high-valent iron-oxo species, contributed to the degradation of BPA in the WFB/Fe3O4–PDS process. Moreover, the WFB/Fe3O4 catalyst also showed stronger ability to control the iron release, better reusability, and higher BPA mineralization efficiency than WF/Fe3O4.

Oxoiron(IV)-dominated Heterogeneous Fenton-like Mechanism of Fe-Doped MoS2

Oxoiron(IV) species are a critical intermediate in the Fe-based Fenton-like process at circumneutral pH, and its oxidative reactivity is closely related to the ligands. An optional inorganic host material, MoS₂ , is selected to construct a highly reactive sulfur ligand coordinated Fe species in this work. The Fe species doped in MoS₂ is presented as the Fe^II centre and triggers the transformation of the 2H phase to the octahedral 1T phase MoS₂ . The role of the interaction between doped Fe and the MoS₂ host lattice on the formation of oxoiron(IV) is studied. A significant Fenton-like reactivity and a remarkable accumulation of oxoiron(IV) species were observed for Fe-MoS₂ . The quenching experiment was implemented to disclose the predominant role of oxoiron(IV) species in the Fe-MoS₂ /H₂ O₂ Fenton-like system. Furthermore, oxoiron(IV) species could transform into the ⋅O₂ ^- and ¹ O₂ , which further expedites the Fenton-like reaction.

In-situ production of iron flocculation and reactive oxygen species by electrochemically decomposing siderite: An innovative Fe-EC route to remove trivalent arsenic

The removal of trivalent arsenic (As (III)) from water has received extensive attention from researchers. Iron electrocoagulation (Fe-EC) is an efficient technology for arsenic removal. However, electrode passivation hinders the development and application of Fe-EC. In this work, an innovative Fe-EC route was developed to remove As (III) through an electrochemical-siderite packed column (ESC). Ferrous ions were produced from siderite near the anode, and hydroxide was generated near the cathode during the electrochemical decomposition of siderite. As a result, an effect of Fe-EC-like was obtained. The results showed that an excellent removal performance of As (III) (>99%) was obtained by adjusting the parameters (As (III) concentration at 10 mg/L, pH at 7, Na₂SO₄ at 10 mM and the hydraulic retention time at 30 min) and the oxidation rate of As (III) reached 84.12%. The mechanism analysis indicated that As (III) was oxidized to As (Ⅴ) by the produced active oxide species and electrode, and then was removed by capturing on the iron oxide precipitates. As (III) was likely to be oxidized in two ways, one by the reactive oxygen species (possibly •OH, Fe(IV) and •O₂^- species), and another directly by the anode. The long-term effectiveness of arsenic removal demonstrated that ESC process based on the electrochemical-siderite packed column was an appropriate candidate for treating As (III) pollution.

Carbon supported "core-shell structure" of Fe nanoparticles for enhanced Fenton reaction activity and magnetic separation

Effectively facilitating Fe³⁺/Fe²⁺ cycles and expanding its operating pH range are keys to optimizing the traditional Fenton reaction. In this exploration, we used chitosan and ferrous sulfate as precursors to prepare a multicomponent magnetic Fe/C Fenton-like catalyst, which exhibited extraordinary catalytic properties and excellent stability performance in a pH range of 4~8. Besides, it could be easily separated from the solution by a magnet. The characterization showed that the supported Fe species include troilite-2H (FeS), lepidocrocite (FeOOH), and pyrrhotite-6T (Fe_{1 - x}S) with a unique "core-shell structure." The presence of reductive iron sulfide core in the system can accelerate the reduction of Fe(III). Meanwhile, the graphite-like structure formed after calcination can adsorb and enrich priority pollutants near the active site through π-π coupling and strengthen electron transfer, which endows its high catalytic performance and enables it invulnerable to dissolved organic compounds.

Roles of soil active constituents in the degradation of sulfamethoxazole by biochar/persulfate: Contrasting effects of iron minerals and organic matter

The biochar/persulfate (BC/PS) has been extensively applied in the degradation of organic contaminants in the aqueous solutions. However, much less work has been done on the degradation of organic contaminants in soil by BC/PS, especially on the unclear roles of soil active constituents in the degradation. This study was conducted to investigate the degradation of sulfamethoxazole (SMX) in two soils through PS oxidation activated by biochar. Biochar was produced via the pyrolysis of peanut shell at 400 °C and 700 °C, which was denoted as BC400 and BC700, respectively. Two soils used were red soil and paddy soil, mainly differing in iron minerals and organic matter. Both biochar promoted SMX degradation (42.6 %-90.7 %) in two soils, compared to PS alone (20.9 %-41.7 %). In BC400/PS system, the free radicals were the dominant reactive species for SMX degradation, while the electron transfer pathway played a vital role in the SMX degradation by BC700/PS. Higher SMX degradation was observed in red soil (41.7 %-97.8 %) than that in paddy soil (20.3 %-94.8 %), which was ascribed to the promotion of iron minerals in red soil yet the inhibition of organic matter in paddy soil. Specifically, the reaction between ≡Fe(III)/≡Fe(II) and PS on the surface of iron minerals in red soil generated more SO₄^•- and ^•OH, resulting in the enhanced SMX degradation. However, the consumption of free radicals and suppression of electron transfer pathway by organic matter in paddy soil inhibited SMX degradation. As the comparative carbonaceous materials to biochar, graphite exerted no obvious degradation effect, whereas activated carbon exhibited the comparable promoting efficacy to BC700. Both biochar, especially BC700, significantly (p < 0.05) alleviated the adverse effects of PS treatment on wheat (Triticum aestivum L.) growth. Overall, this study demonstrates that biochar/persulfate was effective in SMX degradation in soil and the degradation was affected by soil iron minerals and organic matter, which should be paid more attention in the persulfate remediation of organic contaminated soils at a specific site.

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.

Novel solar-driven ferrate(VI) activation system for micropollutant degradation: Elucidating the role of Fe(IV) and Fe(V)

This paper presents a novel process of solar-ferrate(VI) [Fe(VI)] for micropollutant degradation. The solar-Fe(VI) process promoted micropollutant degradation compared with Fe(VI) alone and solar. The radical scavenging and probing experiment results suggested that Fe(V) and Fe(IV) but not reactive oxygen species were most likely involved in the solar-Fe(VI) process. Through building a kinetic model, Fe(IV) and Fe(V) were observed to play an equally significant role in the solar-Fe(VI) process. Afterward, the reaction mechanism of the photochemistry of Fe(VI) was elaborated. Fe(IV) formed from Fe(VI) photolysis and then decomposed to Fe(II) which reacted with Fe(VI) to form Fe(V). Furthermore, the effect of pH on carbamazepine (CBZ) degradation was studied and the quantum yields of Fe(VI) were determined, with (1.98 ± 0.16)× 10^-3 mol∙einstein^-1, (5.90 ± 0.27)× 10^-4 mol∙einstein^-1, and (1.66 ± 0.14)× 10^-4 mol∙einstein^-1 at pH 7.0, 8.0, and 9.0, respectively. Inorganic ions, including Cl^-, HCO₃^-, and Br^- displayed negligible influence on the CBZ degradation, whereas humic acid inhibited the CBZ degradation. Finally, the solar-Fe(VI) process exhibited good applicability in authentic waters and under different irradiation (natural sunlight, ultraviolet light, and visible light from solar cut-off emission). Overall, this study provides a new routine for efficient micropollutant elimination and reveals the photochemistry of Fe(VI).

Formation and transformation of reactive species in the Fe2+/peroxydisulfate/Cl- system

The influence of Cl^- on the formation mechanism of active components is often neglected in the Fe²⁺/peroxydisulfate (PDS) system containing a large amount of ferryl ion reactive specie (Fe(Ⅳ)). In the current investigation, the effects of Cl^- concentration on the removal of methyl phenyl sulfoxide (PMSO), the formation of methyl phenyl sulfone (PMSO₂), the transformation of reactive species and oxidation products were investigated under different reaction conditions that included Fe²⁺ dosage, PDS dosage, and pH₀. The results showed that Cl^- complexing Fe²⁺ increased the formation path of sulfate radical (SO₄^·-) in the Fe²⁺/PDS system. Fe²⁺ dosage and pH₀ value affected the content and morphology of Fe²⁺-Cl^- complex, thus affecting the composition of reactive species. According to the experiment of free radical steady-state concentration, it was found that low concentration of Cl^- reacted with SO₄^·- and increased the steady-state concentration of chlorine radicals (8.09 × 10^-13 M [·Cl]_ss at 1.41 mM Cl^-), while at high concentration of Cl^-, the contents of SO₄^·-, hydroxyl radical (·OH) and dichloride anion radicals (Cl₂^·-) increased and the contents of Fe(Ⅳ) and ·Cl decreased. ·Cl had strong reactivity with PMSO, and PMSO and its oxidation products were chlorinated under the combined action of ·Cl and Cl₂^·-. This work reveals the reaction mechanism and environmental application risks of Fe²⁺/PDS technology and lays the groundwork for subsequent industrial application of Fe²⁺/PDS system.

Peracetic acid activation by mechanochemically sulfidated zero valent iron for micropollutants degradation: Enhancement mechanism and strategy for extending applicability

In this study, mechanically sulfidated microscale zero valent iron (S-ZVI) was found to effectively activate the peracetic acid (PAA) with a result of almost complete degradation of six micropollutants within 10 min under neutral conditions, and > 95% sulfamethoxazole (SMX) removal after six cycles. Reactive oxidized species (ROS) including HO•, carbon-centered radicals, and Fe(IV) were generated in the S-ZVI/PAA system, while HO• was the main contributor towards micropollutants degradation. This study clearly revealed that enhancement of the electron donating ability of ZVI by the formed conductive iron sulfides was crucial for promoted Fe(II) generation and subsequent PAA activation over several cycles, rather than the ability of sulfides to reduce Fe(III) for Fe(II) regeneration as reported previously. Interestingly, it's discovered that co-existence of Fe(III) would dramatically improve the contaminants removal efficiency of the S-ZVI/PAA system; transform the surfaced Fe(II) dominated ROS generation process to aqueous Fe(II) one; enhance the tolerance of the proposed system to water matrix. The promoting effect of predosed Fe(III) on PAA activation by S-ZVI should be mainly associated with: the greater ability of Fe(III) than H₂O to accept electron from Fe⁰ for obtaining more active sites; slower Fe⁰ consumption and solid sulfur species release for elevated electron utilization efficiency and PAA activation. Considering the convenient and cost-effective access of Fe(III), the decrease of acute toxicity of treated SMX, excellent stability and good removal of various micropollutants fully demonstrate the superiority of S-ZVI/PAA system for practical application.

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.

Oxytetracycline induced the redox of iron and promoted the oxidation of As(III)

Antibiotics and arsenic are two frequently detected contaminants in soils and waters, both of which have potential threats to human health. There are few studies focusing on the interaction between these two groups of contaminants in the environment. In this study, we found that the presence of oxytetracycline could significantly promote the oxidation of inorganic As(III) to As(V) with trace Fe(III) (10 μM) and H₂O₂ (100 μM) at near natural pH, and OTC was degraded simultaneously. The most possible mechanism was that OTC could complex with Fe(III) and reduce Fe(III) to Fe(II), which further induced the Fenton-like reaction. Furthermore, structural Fe(III) of α-FeOOH and adsorbed Fe(III) of montmorillonite could also induce these reactions, and the oxidation rate of As(III) was higher with Fe(III)-montmorillonite than aqueous Fe(III). Based on this study, the transformation of As(III) and OTC could occur in four natural water samples, including river water, groundwater and livestock wastewaters. The results of this study revealed the overlooked effect of residual tetracyclines antibiotics on the transformation of co-existing As(III) in natural waters and soils, which might greatly reduce the toxicity of As(III) in the environment.

Reducing agents enhanced Fenton-like oxidation (Fe(III)/Peroxydisulfate): Substrate specific reactivity of reactive oxygen species

Reduction of Fe(III) is the rate-limiting step of iron induced Fenton-like systems, such as the iron/peroxydisulfate system, reducing agents (RAs) were frequently employed as electron donors to directly reduce Fe(III) to further promote the formation of reactive oxygen species (ROS), mainly including hydroxyl radical (^•OH), sulfate radical (SO₄^•-), and ferryl ion (Fe(IV)). However, the intrinsic distinctions among these ROS cause the substrate specific reactivity towards oxidation of diverse organic contaminants. In this study, various RAs (representative solid amorphous boron (A-Boron) and dissolved hydroxylamine (HA)) were added to enhance the Fe(III)/PDS system for investigating the substrate specific reactivity of ROS. It is demonstrated that RAs remarkably boost the Fe(III)/Fe(II) cycles to produce ^•OH, SO₄^•-, and Fe(IV) in the RAs/Fe(III)/PDS systems, based on the results of EPR analysis, quenching tests, and chemical probe analysis. Furthermore, the different yields of methyl phenyl sulfone (PMSO₂) indicate that the distribution of multiple oxidizing species changed with various factors (i.e., type and dosage of RAs added, solution pH, Fe(III) and PDS dosage). This work provides the possibility for the adjustment of oxidation selectivity of RAs/Fe(III)/PDS systems by regulating contribution of radicals and non-radical for oxidizing organic contaminants due to the substrate specific reactivity of ^•OH, SO₄^•-, and Fe(IV), moreover, the comparison of homogeneous and heterogeneous RAs provides assistance in the application of RAs for environmental remediation.

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.

Reduction of refractory Maillard reaction products by Fe3+ during thermal hydrolysis pretreatment and enhanced sludge biodegradability

Refractory Maillard reaction products (MRPs) produced during thermal hydrolysis pretreatment (THP) of waste activated sludge (WAS) may negatively impact the performance of downstream anaerobic digestion (AD) and nitrogen removal processes. Operating THP at lower temperature can mitigate the production of MRPs and improve biodegradability of WAS, while solubilization of WAS is reduced. This study intends to develop a method to reduce the refractory MRPs of WAS without compromising on the solubilization. Fe³⁺ was introduced into THP process (165 °C, 30 min) to mitigate Maillard reaction. Effects of Fe³⁺ on solubilization of WAS, reduction of refractory residuals, accumulative methane production, and microbial community shift were studied. Results confirm that solubilization of WAS was improved and refractory residuals were reduced with the amendment of 10 mg-Fe/L FeCl₃. MRPs mitigation mechanisms were investigated and mainly attributed to Fe³⁺-triggered Fenton-like reactions. Methane production was enhanced by 10.4 ± 0.8% and attributed to the improved biodegradability of THP liquor, as well as to the enrichment of protein degradation and methane production related microbial community. This work provides a simple, economical, and safe strategy to reduce refractory residuals discharged from THP-AD system and to enhance methane production for more energy recovery.

Green rust (GR) and glucose oxidase (GOX) based Fenton-like reaction: Capacity of sustainable release, promoted conversion of glucose through GOX-iron and pH self-adjustment

The Fenton reaction is regarded as highly efficient for the degradation of organic contaminants. However, the traditional Fenton reaction is still flawed in a narrow pH working range and low utilization efficiency of the reagents. Based on two striking features, a sustained release of H₂O₂ in-situ under the catalysis of glucose oxidase (GOX) and the rapid electron donation & transferability from green rust (GR), an adaptable biological Fenton-like system (GGGMFs) was established. The coupling roles of glucose, GOX and GR in the degradation of 3,4-dimethylaniline (3,4-DMA) and the types of reactive species were deduced by electron spin resonance (ESR), etc.. Results demonstrated that the suitable pH range of the system was optimized from acidic to circumneutral, which was favorable for practical application, owing to the heterogeneous formation of GR and the pH self-adjustable capacity of GOX-Glucose. Meanwhile, hydroxyl radical (·OH), superoxide radical (·O₂^-) and Fe (IV) were identified to be the main oxidizing reactive species. Taking different selectivity of the reactive species to certain pollutant functional groups into consideration, the degradation pathways of 3,4-DMA were proposed. Moreover, it was shown that GR not only acted as the activating substance of the Fenton-like reaction, but also enhanced the activity of GOX, resulting in the promotion of glucose conversion in GGGMFs. This study shed light on the enhancement mechanism consisting of two aspects: (i) the elimination of product inhibition (ii) the formation of a 2Fe(III)-FAD complex with FAD, the active center of GOX, which prompted the electronic transfer in the enzyme catalytic reaction.

Destruction of 4-chlorophenol by the hydrogen-accelerated catalytic Fenton system enhanced by Pd/NH2-MIL-101(Cr)

4-chlorophenol (4-CP) could be rapidly mineralized by using Fenton reaction. However, massive iron sludge will be generated because of the excessive consumption of iron salt and poor recycling of Fe^III back to Fe^II. In this paper, by introducing hydrogen gas and solid catalyst Pd/NH₂-MIL-101(Cr) to classic Fenton reactor, the novel system named MHACF-NH₂-MIL-101(Cr) was constructed. Much less Fe^II was needed in this system because the hydrogen could significantly accelerate the regeneration of Fe^II. The catalyst improved the utilization of H₂. The degradation reaction of 4-CP could be driven by using only trace amount of Fe^II. It could be rapidly degraded by the hydroxyl radical detected by the 4-Hydroxy-benzoicacid which is the oxidative product of benzoic acid and hydroxyl radical. The effects of dosage of ferrous salt, H₂O₂ and catalyst, H₂ flow, Pd content, and initial pH of and concentration of 4-CP aqueous solution were investigated. The robustness and morphology changes of this catalytic material were also systematically analysed. By clarifying the role of this solid MOFs material in this hydrogen-mediated Fenton reaction system, it will provide a new direction for the research and development of advanced oxidation processes with high efficiency and low sludge generation in future.

Taping into the super power and magic appeal of ultrasound coupled with EDTA on degradation of 2,4,6-TCP by Fe0 based advanced oxidation processes

Chlorophenol is a widely used organic compound, and the environmental and health problems caused by it have being worsened in recent years. This study used 2,4,6-trichlorophenol (2,4,6-TCP) as the target pollutant, and employed ultrasound (US) enhanced zero-valent iron (Fe⁰)/EDTA/air system (FEA), namely US/FEA, to remove 2,4,6-TCP. The influence of single factor experimental conditions such as EDTA concentration, Fe⁰ dosage, US power, pH and pollutant concentration on the removal efficiency of 2,4,6-TCP was investigated, and the optimal reaction conditions were determined. The mechanism of reactive oxygen species (ROS) produced by US/FEA was explored. The degradation process and removal mechanism of 2,4,6-TCP in the US/FEA were discussed through the determination and analysis of intermediate products. The results showed that US could continuously activate and renew the Fe⁰ surface, accelerate its oxidation and corrosion process, and then continuously and stably produce sufficient amounts of Fe²⁺ and Fe³⁺. Ultrasonic cavitation effect could reduce the difficulty of O₂ activation reaction, and promote the production of sufficient H₂O₂. The addition of EDTA made the system have a wide range of pH applications, and its performance under neutral and alkaline conditions was also superior. The ROS of US/FEA included ·OH, O₂^·- and Fe(IV), where Fe(IV) was the main contributor to the removal of 2,4,6-TCP. In addition, the degradation of 2,4,6-TCP had two processes including dechlorination and benzene ring opening. First, 2,4,6-TCP was dechlorinated and degraded into phenol. And then, phenol was degraded into small molecular acids by ring-opening, and finally it was mineralized into CO₂ and H₂O completely. US/FEA is a promising technology for high-efficiency degradation of organic matter and deep environmental purification.

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.

pH Dependence of Hydroxyl Radical, Ferryl, and/or Ferric Peroxo Species Generation in the Heterogeneous Fenton Process

The heterogeneous Fenton process in the presence of Fe-containing minerals is ubiquitous in nature and widely deployed in wastewater treatment. While there have been extensive relevant studies, the dependence on pH of the nature and extent of oxidant generation and key reaction pathways remain unclear. Herein, the adsorption and decomposition of formate and H₂O₂ were quantified in the presence of ferrihydrite within the pH range of 3.0-6.0, and experiments with methyl phenyl sulfoxide were conducted to distinguish between HO^• and weaker oxidant(s) which react via oxygen atom transfer including ferryl ion ([Fe^IVO]²⁺) and/or ferric hydroperoxo intermediates (≡Fe^III(O₂H)). Both HO^• and [Fe^IVO]²⁺/≡Fe^III(O₂H) are concurrently produced on the surface over the acidic to near-neutral pH range. Despite the simultaneous formation of both oxidants, HO^• is the major oxidant responsible for substrate oxidation in the interfacial boundary layer with [Fe^IVO]²⁺/≡Fe^III(O₂H) exhibiting limited exposure to substrates. With an increase of pH, the yield of both oxidants is inhibited by the decreasing availability of surface sites due to ferrihydrite particle aggregation. Increasing pH also favors the nonradical decay of H₂O₂ as evident from the consistent oxidant production rate relative to the surface area (SSA) despite an accelerated H₂O₂ decay rate relative to SSA with pH increase.