Peroxymonosulfate Activation by Fe(III)-Picolinate Complexes for Efficient Water Treatment at Circumneutral pH: Fe(III)/Fe(IV) Cycle and Generation of Oxyl Radicals

High-valent ferryl intermediates generation, reactivity and kinetic characterization with contaminants of emerging concern via a facile photo-Fenton competition kinetic methodology

High-valent ferryl (FeIV) intermediates are important reactive species in biological oxidation and Fe-catalyzed advanced oxidation processes (Fe-AOPs). Notwithstanding notable progress has been made on FeIV identification, the second-order reaction rate constants of FeIV with contaminants of emerging concern (CECs) were rarely reported in literature, severely hindering understanding its reactivity and kinetics toward various CECs. To this end, we discovered a novel system, i.e., photo-Fenton reaction of peracetic acid with Fe3+-nitrilotriacetate complex, which enabled stable generation of FeIV with steady-state concentrations at ∼10-8-10-7 M at neutral pH, as evidenced by electron spin resonance (ESR) trapping detection, quenching experiments and probe testing. Notably, a facile competition kinetic methodology was developed by using methyl phenyl sulfoxide (PMSO) as a probe, which enabled to characterize the reactivity and kinetics of FeIV with 12 target CECs (e.g., phenolic compounds, endocrine disruptor, herbicide, and pharmaceuticals). The measured second-order rate constants were in a range of 3.99 × 103-4.75 × 105 M-1 s-1, which were correlated to the ionization potential of the target CECs, owing to electrophilic attack by FeIV. This achievement can fill a critical gap in uncovering the reactivity and kinetics of FeIV toward CECs for promising environmental application.

Modulation of C=O groups concentration in carbon-based materials to enhance peroxymonosulfate activation towards degradation of organic contaminant: Mechanism of the non-radical oxidation pathway

Current research has found that C=O groups hold a significant position in the oxidation process of peroxymonosulfate (PMS) in the activated carbon system, influencing both the generation of reactive species and the degradation of organic contaminants. Quinone organic compounds, which are rich in C=O groups, exhibit high reactivity but also inherent toxicity. Additionally, these compounds decompose easily at high temperatures, complicating their use in carbon-based materials. This study introduces a novel approach to enhance the content of C=O groups in carbon materials by controlling the synthesis of sulfur heterocyclic organic quinone (SHQ) precursors. For the first time, we established a sulfur heterocyclic organic quinone-derived carbon (SHQC)/PMS system, where SHQC catalysts demonstrate remarkable catalytic efficiency in activating PMS for the degradation of organic pollutants. In different natural water matrices, the removal rate of tetracycline hydrochloride (TC) exceeds 90 % under optimized conditions (SHQC-9: 0.2 g L-1, PMS: 1 mM, TC: 4.45 mg L-1). The C=O group was identified as the active site for PMS activation through a combination of quantitative structure-activity relationships, specific site blocking, and antithesis methods. The activation mechanism was elucidated through scavenger experiments and electron paramagnetic resonance (EPR) analysis. The results indicate that the superior catalytic performance of the SHQC-9/PMS system is due to a non-radical pathway, with singlet oxygen (1O2) being the primary species responsible for TC degradation. These findings offer a novel pathway for designing highly efficient, safe, and environmentally friendly carbon material catalysts.

Performance and mechanism of CuFe2O4/2D-V to activate persulfate for levofloxacin removal

Supported catalysts have gradually become a research hotspot in the field of removing antibiotics from wastewater. In this study, CuFe2O4@2D-V composite catalyst was prepared by basic hydrothermal synthesis. Two-dimensional expanded vermiculite (2D-V) was prepared by high temperature and ultrasonic stripping to provide a loading platform for CuFe2O4 metal ions. The characterization of CuFe2O4@2D-V was analysis by SEM, XRD and EDS. Then, levofloxacin (LVX) was chosen as representative to investigate the catalytic performance of CuFe2O4@2D-V by persulfate (PMS) activation. The optimal preparation conditions of CuFe2O4@2D-V and operating parameters of activating PMS to remove LVX were also studied. The result showed that under the condition of CuFe2O4 loading 50 %, initial pH 9.2, LVX concentration 10 mg/L, catalyst concentration 0.5 g/L and PMS concentration of 0.3 mM, the removal efficiency and reaction rate of LVX could reach 89 % and 0.071 min-1, respectively. Meanwhile, the catalyst had a good stability and high reusability. Based on the analysis of intermediate products by LC-MS, three possible degradation pathways were proposed. Quenching experiments found that non-radical O21 dominated LVX degradation process. Also, the redox reaction between Cu (I)/Cu (II) and Fe (II)/Fe (III) played an important role in LVX removal process. This work might fill the gap in the application of vermiculite as a two-dimensional carrier, and provide further understanding for antibiotic wastewater treatment by metal based two-dimensional composite materials.

Low ultraviolet dose with high efficiency: Synergistic coupling of far-UVC and ferrate(VI) for ultrafast and selective degradation of micropollutants

Enhancing the reactivity and yield of reactive species to reduce the ultraviolet (UV) fluence requirement for activating ferrate (Fe(VI)) is critical for advancing UV-based Fe(VI) processes toward practical wastewater treatment applications, yet it remains challenging. Herein, we developed a far-UVC-driven Fe(VI) activation system for the efficient degradation of micropollutants. The results demonstrated that switching from conventional low-pressure UV lamps (LPUV, UV254) and UVA365 to 222 nm far-UVC achieved ultrafast degradation of carbamazepine (CBZ) at an extremely low UV dose of 29.76 mJ/cm2 under pH 8.0 conditions. The fluence-based degradation rate constants were 15.8 and 142.0 times higher than those achieved by UV254 and UVA365 photolysis of Fe(VI), respectively. This improved degradation can be attributed to the increased generation of high-valent iron intermediates [(Fe(V)/Fe(IV)] in the system. Notably, the presence of complex matrixes barely influenced CBZ degradation, and the UV222/Fe(VI) system maintained significantly enhanced performance in various real waters compared to Fe(VI) alone. Additionally, 10 structurally diverse pollutants were selected for evaluation the selectivity of the UV222/Fe(VI) system, finding that their lnkobs values correlated well with their EHOMO and vertical IP (R2 = 0.86). Overall, this study proposes a promising oxidation technology that was efficient, energy-saving, cost-effective, and selective for the rapid elimination of micropollutants.

MnxCo3-xO4 spinel activates peroxymonosulfate for highly effective bisphenol A degradation with ultralow catalyst and persulfate usage

Persulfates-based advanced oxidation processes are highly efficient in degrading refractory organic contaminants in wastewater. However, their practical application is often limited by the extensive consumption of catalysts and oxidants. Therefore, constructing catalysts with abundant and efficient reaction interfaces is essential for improving the efficiency of persulfate activation. In this work, we develop a novel MnxCo3-xO4 spinel with highly exposed surface active sites by etching Mn-based precursors with Co ions. This process forms sufficient interface Co-O-Mn bonds, which effectively activate peroxymonosulfate (PMS) for bisphenol A (BPA) degradation. A clear structure-activity relationship is observed between the Co/Mn content ratio and the BPA degradation rate in the MnxCo3-xO4/PMS system. Notably, Mn0.1Co2.9O4 demonstrates superior PMS activation efficiency, achieving 100 % degradation of 10 mg/L BPA within 2 minutes with 0.05 g/L catalyst and 0.05 g/L persulfate usage. Experimental analyses combined with theoretical calculations identify the interface Co-O-Mn as the active site, which plays a crucial role in accelerating PMS molecule adsorption and O-O bond activation. Additionally, the spatially adjacent Co-O-Mn sites promote redox cycling for efficient interface electron transfer during the PMS activation process. Furthermore, Zebrafish toxicity studies revealed a considerable reduction in the toxicity of the BPA treatment residue in the MnxCo3-xO4/PMS system. Overall, this work presents a novel strategy for constructing spatially adjacent redox sites in dual-metal spinel materials, offering valuable insights into reducing chemical input and advancing persulfate-based environmental remediation technology.

Coprecipitates between iron oxides and biodegradable organic acids for boosting Fenton-like catalysis under neutral pH conditions

While the role of dissociated and complexed forms of organic acids (OAs) on Fenton-like reactions has been widely investigated, the catalytic ability and detailed mechanisms of coprecipitates between iron oxides and OAs were still obscure. Herein, coprecipitates of iron oxides with eco-friendly organic acids including maleic acid (MAA), tetrasodium-N,N-bis(carboxymethyl)-L-glutamate (GLDA), and protocatechuic acid (PCA) possessing different C/Fe molar ratios (OA-FeOx, x: C/Fe molar ratios) were respectively prepared under ambient conditions. The coprecipitates exhibited eminent Fenton-like performance in removing various pollutants (e.g., rhodamine B, atrazine, sulfanilamide, bisphenol A, and phenol) under neutral pH conditions. Higher •OH accumulation, Fe(III)/Fe(II) cycling, and pollutant degradation rates were observed in the systems mediated by PCA-FeOx and GLDA-FeOx as compared with that by MAA-FeOx counterparts possessing the same C/Fe molar ratios. Higher level of dissolved and surface-bound Fe(III)/Fe(II) was identified as the main Fe species for improving the Fenton-like reactions in the PCA-FeOx and GLDA-FeOx mediated systems, respectively. Spectroscopic analysis demonstrated that OAs could promote the homolysis cleavage of Fe-OOH, thus contributing to improved Fe(III)/Fe(II) cycling and •OH production. This work provided a facile method for tailoring Fe-C catalyst in removing recalcitrant contaminants under neutral conditions, and provided insights into the underlying mechanisms of Fenton-like reactions catalyzed by Fe-C composites.

Laterite Integrated Persulfate Based Advanced Oxidation and Biological Treatment for Textile Industrial Effluent Remediation: Optimization and Field Application

This study investigated a combined approach of a persulfate-based advanced oxidation process (AOP) followed by biological treatment of a textile industrial effluent. The effluent from the textile industry is primarily composed of various dyes in varying concentrations, resulting in high chemical oxygen demand (COD) and biological oxygen demand (BOD). The model pollutant rhodamine B (RhB) was used in the optimization studies. During the persulfate oxidation process (PSO), persulfate activation is required to generate sulfate radicals (SO4 •-). Raw laterite soil was used as a catalyst for the treatment of RhB in batch studies, and it was able to reduce the dye concentration by about 20% in 60 min of operation, with initial RhB concentrations of 150 mg L-1 and persulfate concentrations of 200 mg L-1. Furthermore, alkali-treated laterite soil (ATLS) was used as a catalyst, achieving 57%-60% removal in 60 min at pH 3 and complete removal after 72 h of biological treatment. Furthermore, the optimized conditions were tested on real field waters to determine efficiency, and it was observed that the PSO removed approximately 45% of COD, with further biological treatment for 72 h increasing the removal efficiency to 64%. All other parameters of water quality were reduced by more than 60%.

Peroxide Directing the Iron Cycling for Tailored Generation of Active Oxidants in Aqueous Fenton Reactions

The diverse electron transfer between iron and peroxides endows Fenton and related reactions with versatility in generating multiple active oxidants, underpinning their important contribution to environmental remediation. The type of active oxidant generated can be tailored by the structure of peroxides, yet the underlying mechanism remains to be uncovered. Herein, taking the reaction of Fe(III)-picolinate (FeIII-PICA) with peroxides as an examplary case, we provide a detailed structure-activity analysis to clarify this issue. Experimental results show that the reaction of FeIII-PICA with H2O2, tert-butyl hydroperoxide, and isopropyl hydroperoxide initiates the Haber-Weiss cycle to generate radicals exclusively, whereas the reaction with peroxymonosulfate, peracetic acid, and m-chloroperoxybenzoic acid generates Fe(IV). Theoretical calculations reveal that the peroxide-dependent generation of active oxidants is attributed to the selectivity in the lysis of the PICA-FeIII-OOR intermediate, which serves as a rate-limiting step in Fenton reactions. The inductive effect of R dynamically modulates the strength of Fe-O/O-O bonding and the stability of cleavage products to favor Fe-O homolysis of PICA-FeIII-OOR toward Fe(III)/Fe(II) cycling. Conversely, the coordination of R to Fe(III) stabilizes transition states to favor O-O homolysis for Fe(III)/Fe(IV) cycling. These findings are believed to shed new light on the pathway selectivity of iron cycling in aqueous Fenton reactions.

Internal electric field triggered charge redistribution in CuO/Fe2O3 composite to regulate the peroxymonosulfate activation for enhancing the degradation of organic pollutants

Herein, we adopt a feasible method to synthesize the CuO/Fe2O3 composite with heterostructure. Owing to the significant differences in work functions, an internal electric field is built at the interface of heterojunction after the combination of CuO with Fe2O3, which can reduce interface resistance and accelerate charge transfer. Interestingly, under the induction of electrostatic interaction provided by internal electric field, the CuO/Fe2O3 composite will form electron-rich and electron-deficient active zones. More importantly, the peroxymonosulfate (PMS) can be oxidized by the CuO with electron-deficient active zone to generate SO5•‒, subsequently converting into 1O2. Meanwhile, the Fe2O3 component with electron-rich active zone can provide electrons for PMS to achieve the heterolysis of Fe-O-O, thereby producing the high-valent metal complex (namely ≡ Fe5+=O). Consequently, the CuO/Fe2O3-2-mediated PMS system with good anti-interference ability displays excellent performance in wastewater treatment. Benefiting from the electrophilic reaction of 1O2 and ≡ Fe5+=O, various typical organic pollutants can be ultimately mineralized into CO2, H2O and other nontoxic by-products by the CuO/Fe2O3-2-mediated PMS system. In short, current work shares some novel insights into the effect of internal electric field on PMS activation, which can provide valuable references for future research.

Oxidative Polymerization in Water Treatment: Chemical Fundamentals and Future Perspectives

For several decades, the methodology of complete destruction of organic pollutants via oxidation, i.e., mineralization, has been rooted in real water treatment applications. Nevertheless, this industrially accepted protocol is far from sustainable because of the excessive input of chemicals and/or energy as well as the unregulated carbon emission. Recently, there have been emerging studies on the removal of organic pollutants via a completely different pathway, i.e., polymerization, meaning that the target pollutants undergo oxidative polymerization reactions to generate polymeric products. These studies have collectively shown that compared to the conventional mineralization pathway, the polymerization pathway allows more efficient removal of target pollutants, largely reduced input of chemicals, and suppressed carbon emission. In this review, we aim to provide a comprehensive examination of the fundamentals of the oxidative polymerization process, current state-of-the-art strategies for regulation of the polymerization pathway from both kinetic and thermodynamic perspectives, and resource recovery of the formed polymeric products. In the end, the limitations of the polymerization process for pollutant removal are discussed, with perspectives for future studies. Hopefully, this review could not only provide critical insight for the advancement of polymerization-oriented technologies for removal of more organic pollutants in a greener manner but also stimulate more paradigm innovations for low-carbon water treatment.

Incidental iron oxide nanoclusters drive confined Fenton-like detoxification of solid wastes towards sustainable resource recovery

The unique properties of nanomaterials offer vast opportunities to advance sustainable processes. Incidental nanoparticles (INPs) represent a significant part of nanomaterials, yet their potential for sustainable applications remains largely untapped. Herein, we developed a simple strategy to harness INPs to upgrade the waste-to-resource paradigm, significantly reducing the energy consumption and greenhouse gas emissions. Using the recycling of fly ash from municipal solid waste incineration (MSWI) as a proof of concept, we reveal that incidental iron oxide nanoclusters confined inside the residual carbon trigger Fenton-like catalysis by contacting H2O2 at circumneutral pH (5.0-7.0). This approach efficiently detoxifies the adsorbed dioxins under ambient conditions, which otherwise relies on energy-intensive thermal methods in the developed recovery paradigms. Collective evidence underlines that the uniform distribution of iron oxide nanoclusters within dioxin-enriched nanopores enhances the collision between the generated active oxidants and dioxins, resulting in a substantially higher detoxification efficiency than the Fe(II)-induced bulk Fenton reaction. Efficient and cost-effective detoxification of MSWI fly ash at 278‒288 K at pilot scale, combined with the satisfactory removal of adsorbed chemicals in other solid wastes unlocks the great potential of incidental nanoparticles in upgrading the process of solid waste utilization and other sustainable applications.

pH dependence of reactive oxygen species generation and pollutant degradation in Fe(II)/O2/tripolyphosphate system

It has been reported that tripolyphosphate (TPP) can effectively enhance the activation of O2 by Fe(II) to remove organic pollutants in the environment. However, the influence of solution pH on the generation and conversion of reactive oxygen species (ROS) and their degradation of pollutants in the Fe(II)/O2/TPP system needs further investigation. In this study, we demonstrated that O2•- and •OH were the main ROS responsible for degradation in the system at different pH conditions, and their formation rates were calculated using a steady-state model. Experiments combined with density functional theory (DFT) calculations showed that the p-nitrophenol (PNP) degradation pathway in the Fe(II)/O2/TPP system is regulated by solution pH. Specifically, at pH = 3, the existence of Fe(II) in the solution is dominated by [Fe(II)(HTPP)2]2-, which leads to a rapid conversion from O2 and HO2• to generate •OH, and PNP is primarily oxidatively degraded. However, at pH = 5/7, [Fe(II)(TPP)2]4- is taking the lead with which O2•- is accumulated in the solution due to the slow conversion to •OH in this condition, and the PNP is mainly reductively degraded. This study proposes a new strategy to achieve the targeted oxidative/reductive removal of different types of pollutants by simply varying the solution pH in the Fe(II)/O2/TPP system.

Fundamental Insights into the Direct Electron Transfer Mechanism on Ag Atomic Cluster

The nonradical oxidation pathway for pollutant degradation in Fenton-like catalysis is favorable for water treatment due to the high reaction rate and superior environmental robustness. However, precise regulation of such reactions is still restricted by our poor knowledge of underlying mechanisms, especially the correlation between metal site conformation of metal atom clusters and pollutant degradation behaviors. Herein, we investigated the electron transfer and pollutant oxidation mechanisms of atomic-level exposed Ag atom clusters (AgAC) loaded on specifically crafted nitrogen-doped porous carbon (NPC). The AgAC triggered a direct electron transfer (DET) between the terminal oxygen (Oα) of surface-activated peroxodisulfate and the electron-donating substituents-containing contaminants (EDTO-DET), rendering it 11-38 times higher degradation rate than the reported carbon-supported metal catalysts system with various single-atom active centers. Heterocyclic substituents and electron-donating groups were more conducive to degradation via the EDTO-DET system, while contaminants with high electron-absorbing capacity preferred the radical pathway. Notably, the system achieved 79.5% chemical oxygen demand (COD) removal for the treatment of actual pharmaceutical wastewater containing 1053 mg/L COD within 30 min. Our study provides valuable new insights into the Fenton-like reactions of metal atom cluster catalysts and lays an important basis for revolutionizing advanced oxidation water purification technologies.

Picolinic acid-mediated Mn(II) activated periodate for ultrafast and selective degradation of emerging contaminants: Key role of high-valent Mn-oxo species

The utilization of periodate (PI, IO4-) in metal-based advanced oxidation processes (AOPs) for the elimination of emerging contaminants (ECs) have garnered significant attention. However, the commonly used homogeneous metal catalyst Mn(II) performs inadequately in activating PI. Herein, we exploited a novel AOP technology by employing the complex of Mn(II) with the biodegradable picolinic acid (PICA) to activate PI for the degradation of electron-rich pollutants. The performance of the Mn(II)-PICA complex surpassed that of ligand-free Mn(II) and other Mn(II) complexes with common aminopolycarboxylate ligands. Through scavenger, sulfoxide-probe transformation, and 18O isotope-labeling experiments, we confirmed that the dominant reactive oxidant generated in the Mn(II)-PICA/PI system was high-valent manganese-oxo species (Mn(V)=O). Due to its reliance on Mn(V)=O, the Mn(II)-PICA/PI process exhibited remarkable selectivity and strong anti-interference during EC oxidation in complex water matrices. Nine structurally diverse pollutants were selected for evaluation, and their lnkobs values in the Mn(II)-PICA/PI system correlated well with their electrophilic/nucleophilic indexes, EHOMO, and vertical IP (R2 = 0.79-0.94). Additionally, IO4- was converted into non-toxic iodate (IO3-) without producing undesired iodine species such as HOI, I2, and I3-. This study provides a novel protocol for metal-based AOPs using PI in combination with chelating agents and high-valent metal-oxo species formation during water purification.

Rapid Size Determination of Quasispherical Gold Nanoparticles by Electrocatalysis Efficiency-Regulated Electrochemiluminescence

The size of gold nanoparticles (AuNPs) largely decides their properties and applications, making the rapid screening of AuNP size important. Despite the fact that AuNP-amplified electrochemiluminescence (ECL) is widely used in various ECL sensing applications, the mechanism of ECL enhancement remains elusive, especially the quantitative relationship between the enhanced ECL intensity and the size of AuNPs. In this work, taking quasispherical and citrate-stabilized AuNPs as model nanoparticles, we have reported that the ECL intensity of the S2O82--O2 system enhanced significantly with the increasing AuNP size. AuNPs acted as bielectrocatalysts for reducing the S2O82- and O2. The further study of enhancement mechanism demonstrates that AuNPs with increasing size facilitate the electron transfer and promote the generation of radicals required for the ECL emission, which produces more emitters-singlet oxygen. Meanwhile, the high surface density of citrate on small AuNPs suppresses the ECL signal by forming an electrostatic barrier. On the basis of the above phenomena, an ECL-based rapid AuNP size screening approach has been established. The accuracy of this platform is verified by the consistent results in comparison to transmission electron microscopy (TEM) measurements. This work not only provides deep insight into the correlation between the AuNP size and the ECL enhancement but also contributes an alternative to the TEM technique for the rapid AuNP size screening. Additionally, this study also extends the exploration of ECL-based structure analysis techniques toward nanomaterials through clarifying the structure-electrocatalytic activity correlation.

Activation of peroxymonosulfate by novel magnetically recyclable CoFe2O4/MXene quantum dots composites for rapid degradation of tetracycline: Synergistic performance and mechanisms

Tetracycline (TC), a commonly used antibiotic in wastewater, poses environmental and health risks, thus demanding advanced catalysts for its effective removal. In this work, for the first time, we integrated cobalt ferrite (CoFe2O4) and MXene quantum dots (MQDs) to form magnetic heterojunctions for rapid degradation of TC in the presence of peroxymonosulfate (PMS). Anchoring MQDs on the CoFe2O4 nanoparticles remarkably promoted the overall degradation rate of TC to 98.2% within 20 min via both radical and non-radical pathways. The first-order kinetic constant was 0.170 min-1, 3.5 and 15.5 times higher than that of CoFe2O4 and MQDs alone, respectively. Quenching experiments revealed that the addition of p-benzoquinone (p-BQ) and furfuryl alcohol (FFA) reduced the degradation of TC within 20 min to 56.2% and 28.4%, respectively, indicating that the primary reactive oxygen species for TC degradation in the CoFe2O4/MQDs + PMS system are •O2- and 1O2. CoFe2O4/MQDs also exhibited superparamagnetic property, which enabled their effective recovery by external magnetic field. Their reusability was verified by retaining 81.4% of catalytic efficacy in the consecutive 8th cycle. The CoFe2O4/MQDs + PMS system also exhibited excellent practicability in natural water samples as the degradation rates in both tap water and lake water environments exceeded 90%. Three potential pathways for TC degradation were proposed based on the liquid chromatography-mass spectrometry (LC-MS) characterizations and TC progressively transformed into 13 intermediates. This work may contribute to the ongoing efforts to develop advanced catalysts and strategies for mitigating the environmental impact of antibiotic pollution, offering a pathway toward sustainable and efficient water treatment technologies.

Picolinic acid promotes organic pollutants removal in Fe(III)/periodate process: Mechanism and relationship between removal efficiency and pollutant structure

The application of Fe-catalyzed periodate (PI) processes is often limited by both the narrow applicable pH range and weak reaction between Fe(III) and oxidant. Here, the biodegradable picolinic acid (PICA) was used as one kind of chelating ligands (CLs) to enhance the removal of organic pollutants (OPs) at initial pH 3.0-8.0, which displayed superior properties than the other CLs in Fe(III)/PI process. The dominant reactive species produced in the Fe(III)-PICA/PI process turned out to be high-valent iron-oxo (FeIV=O) species and hydroxyl radical (•OH) by quenching, sulfoxide probe transformation, and 18O isotope-labeling tests. The relative contribution of FeIV=O and •OH was dependent on OPs ionization potential (IP) and energy gap (ΔE). The degradation of OPs was also directly associated with their structure, the apparent rate constants correlated well with the highest occupied molecular orbital energy (EHOMO), IP, and ΔE, and among them ΔE had a greater effect. Furthermore, Fe(III)-PICA complexes displayed excellent long-term effectiveness for OPs removal in actual water matrixes, along with the non-toxic conversion of PI, indicating a broad application perspective of Fe(III)-PICA/PI process. This study provides an efficient method to improve the performance of Fe(III)/PI process and reveals the mechanism and relationship between removal efficiency and pollutant structure.

Revisiting the Iron(II)/Cobalt(II)-Based Homogenous Fenton-like Processes from the Standpoint of Diverse Metal-Oxygen Complexes

The aqueous FeIV-oxo complex and FeIII-peroxy complex (e.g., ligand-assisted or interfacial FeIII-hydroperoxo intermediates) have been recognized as crucial reactive intermediates for decontamination in iron-based Fenton-like processes. Intermediates with terminal oxo ligands can undergo the oxygen atom exchange process with water molecules, whereas peroxides are unable to induce such exchanges. Therefore, these distinct metal-oxygen complexes can be distinguished based on the above feature. In this study, we identified previously unknown intermediates with a peroxy moiety and cobalt center that were generated during peroxymonosulfate (PMS) activation via aqueous CoII ions under acidic conditions. Results of theoretical calculations and tip-enhanced Raman spectroscopy revealed that the CoII ion tended to coordinate with the PMS anion to form a bidentate complex with a tetrahedral structure. These reactive cobalt intermediates were collectively named the CoII-PMS* complex. Depending on the inherent characteristics of the target contaminants, the CoII-PMS* complex can directly oxidize organic compounds or trigger PMS disproportionation to release hydroxyl radicals and sulfate radicals for collaborative decontamination. This work provides a comparative study between iron- and cobalt-based Fenton-like processes and proposes novel insights from the standpoint of diverse metal-oxygen complexes.

Deciphering the Novel Picolinate-Mn(II)/peroxymonosulfate System for Sustainable Fenton-like Oxidation: Dominance of the Picolinate-Mn(IV)-peroxymonosulfate Complex

A highly efficient and sustainable water treatment system was developed herein by combining Mn(II), peroxymonosulfate (PMS), and biodegradable picolinic acid (PICA). The micropollutant elimination process underwent two phases: an initial slow degradation phase (0-10 min) followed by a rapid phase (10-20 min). Multiple evidence demonstrated that a PICA-Mn(IV) complex (PICA-Mn(IV)*) was generated, acting as a conductive bridge facilitating the electron transfer between PMS and micropollutants. Quantum chemical calculations revealed that PMS readily oxidized the PICA-Mn(II)* to PICA-Mn(IV)*. This intermediate then complexed with PMS to produce PICA-Mn(IV)-PMS*, elongating the O-O bond of PMS and increasing its oxidation capacity. The primary transformation mechanisms of typical micropollutants mediated by PICA-Mn(IV)-PMS* include oxidation, ring-opening, bond cleavage, and epoxidation reactions. The toxicity assessment results showed that most products were less toxic than the parent compounds. Moreover, the Mn(II)/PICA/PMS system showed resilience to water matrices and high efficiency in real water environments. Notably, PICA-Mn(IV)* exhibited greater stability and a longer lifespan than traditional reactive oxygen species, enabling repeated utilization. Overall, this study developed an innovative, sustainable, and selective oxidation system, i.e., Mn(II)/PICA/PMS, for rapid water decontamination, highlighting the critical role of in situ generated Mn(IV).

Protocatechuic acid enhanced the selective degradation of sulfonamide antibiotics in Fe(III)/peracetic acid process under actually neutral pH conditions

The practical application of the Fe-catalyzed peracetic acid (PAA) processes is seriously restricted due to the need for narrow pH working range and poor anti-interference capacity. This study demonstrates that protocatechuic acid (PCA), a natural and eco-environmental phenolic acid, significantly enhanced the removal of sulfonamide antibiotics in Fe(III)/PAA process under actually neutral pH conditions (6.0-8.0) by complexing Fe(III). With sulfamethoxazole (SMX) as the model contaminant, the pseudo-first-order rate constant of SMX elimination in PCA/Fe(III)/PAA process was 63.5 times higher than that in Fe(III)/PAA process at pH 7.0, surpassing most of the previously reported strategies-enhanced Fe-catalyzed PAA processes (i.e., picolinic acid and hydroxylamine etc.). Excluding the primary contribution of reactive species commonly found in Fe-catalyzed PAA processes (e.g., •OH, R-O•, Fe(IV)/Fe(V) and 1O2) to SMX removal, the Fe(III)-peroxy complex intermediate (CH3C(O)OO-Fe(III)-PCA) was proposed as the primary reactive species in PCA/Fe(III)/PAA process. DFT theoretical calculations indicate that CH3C(O)OO-Fe(III)-PCA exhibited stronger oxidation potential than CH3C(O)OO-Fe(III), thereby enhancing SMX removal. Four potential removal pathways of SMX were proposed and the toxicity of reaction solution decreased with the removal of SMX. Furthermore, PCA/Fe(III)/PAA process exhibited strong anti-interference capacity to common natural anions (HCO3-, Cl-and NO3-) and humic acid. More importantly, the PCA/Fe(III)/PAA process demonstrated high efficiency for SMX elimination in actual samples, even at a trace Fe(III) dosage (i.e., 5 μM). Overall, this study provided a highly-efficient and eco-environmental strategy to remove sulfonamide antibiotics in Fe(III)/PAA process under actually neutral pH conditions and to strengthen its anti-interference capacity, underscoring its potential application in water treatment.

Enhanced Degradation of Micropollutants in a Peracetic Acid/Mn(II) System with EDDS: An Investigation of the Role of Mn Species

Extensive research has been conducted on the utilization of a metal-based catalyst to activate peracetic acid (PAA) for the degradation of micropollutants (MPs) in water. Mn(II) is a commonly employed catalyst for homogeneous advanced oxidation processes (AOPs), but its catalytic performance with PAA is poor. This study showed that the environmentally friendly chelator ethylenediamine-N,N'-disuccinic acid (EDDS) could greatly facilitate the activation of Mn(II) in PAA for complete atrazine (ATZ) degradation. In this process, the EDDS enhanced the catalytic activity of manganese (Mn) and prevented disproportionation of transient Mn species, thus facilitating the decay of PAA and mineralization of ATZ. By employing electron spin resonance detection, quenching and probe tests, and 18O isotope-tracing experiments, the significance of high-valent Mn-oxo species (Mn(V)) in the Mn(II)-EDDS/PAA system was revealed. In particular, the involvement of the Mn(III) species was essential for the formation of Mn(V). Mn(III) species, along with singlet oxygen (1O2) and acetyl(per)oxyl radicals (CH3C(O)O•/CH3C(O)OO•), also contributed partially to ATZ degradation. Mass spectrometry and density functional theory methods were used to study the transformation pathway and mechanism of ATZ. The toxicity assessment of the oxidative products indicated that the toxicity of ATZ decreased after the degradation reaction. Moreover, the system exhibited excellent interference resistance toward various anions and humid acid (HA), and it could selectively degrade multiple MPs.

Pyrazine-based iron metal organic frameworks (Fe-MOFs) with modulated O-Fe-N coordination for enhanced hydroxyl radical generation in Fenton-like process

Rational design of coordination environment of Fe-based metal-organic frameworks (Fe-MOFs) is still a challenge in achieving enhanced catalytic activity for Fenten-like advanced oxidation process. Here in, novel porous Fe-MOFs with modulated O-Fe-N coordination was developed by configurating amino terephthalic acid (H2ATA) and pyrazine-dicarboxylic acid (PzDC) (Fe-ATA/PzDC-7:3). PzDC ligands introduce pyridine-N sites to form O-Fe-N coordination with lower binding energy, which affect the local electronic environment of Fe-clusters in Fe-ATA, thus decreased its interfacial H2O2 activation barrier. O-Fe-N coordination also accelerate Fe(II)/Fe(III) cycling of Fe-clusters by triggering the reactive oxidant species mediated Fe(III) reduction. As such, Fe-ATA/PzDC-7:3/H2O2 system exhibited excellent degradation performance for typical antibiotic sulfamethoxazole (SMX), in which the steady-state concentration of hydroxyl radical (OH) was 1.6 times higher than that of unregulated Fe-ATA. Overall, this study highlights the role of O-Fe-N coordination and the electronic environment of Fe-clusters on regulating Fenton-like catalytic performance, and provides a platform for precise engineering of Fe-MOFs.

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.

Recovering palladium and gold by peroxydisulfate-based advanced oxidation process

Palladium (Pd) and gold (Au) are the most often used precious metals (PMs) in industrial catalysis and electronics. Green recycling of Pd and Au is crucial and difficult. Here, we report a peroxydisulfate (PDS)-based advanced oxidation process (AOPs) for selectively recovering Pd and Au from spent catalysts. The PDS/NaCl photochemical system achieves complete dissolution of Pd and Au. By introducing Fe(II), the PDS/FeCl2·4H2O solution functioned as Fenton-like system, enhancing the leaching efficiency without xenon (Xe) lamp irradiation. Electron paramagnetic resonance (EPR), 18O isotope tracing experiments, and density functional theory calculations revealed that the reactive oxidation species of SO4·-, ·OH, and Fe(IV)═O were responsible for the oxidative dissolution process. Lixiviant leaching and one-step electrodeposition recovered high-purity Pd and Au. Strong acids, poisonous cyanide, and volatile organic solvents were not used during the whole recovery, which enables an efficient and sustainable precious metal recovery approach and encourage AOP technology for secondary resource recycling.

A manganese-based catalyst system for general oxidation of unactivated olefins, alkanes, and alcohols

Non-noble metal-based catalyst systems consisting of inexpensive manganese salts, picolinic acid and various heterocycles enable epoxidation of the challenging (terminal) unactivated olefins, selective C-H oxidation of unactivated alkanes, and O-H oxidation of secondary alcohols with aqueous hydrogen peroxide. In the presence of the in situ generated optimal manganese catalyst, epoxides are generated with up to 81% yield from alkenes and ketone products with up to 51% yield from unactivated alkanes. This convenient protocol allows the formation of the desired products under ambient conditions (room temperature, 1 bar) by employing only a slight excess of hydrogen peroxide with 2,3-butadione as a sub-stoichiometric additive.

Selective formation of high-valent iron in Fenton-like system for emerging contaminants degradation under near-neutral and high-salt conditions

In view of the near-neutral and high-salt conditions, the Fenton technology with hydroxyl radicals (HO•) as the main reactive species is difficult to satisfy the removal of trace emerging contaminants (ECs) in pharmaceutical sewage. Here, a layered double hydroxide FeZn-LDH was prepared, and the selective formation of ≡Fe(IV)=O in Fenton-like system was accomplished by the chemical environment regulation of the iron sites and the pH control of the microregion. The introduced zinc can increase the length of Fe-O bond in the FeZn-LDH shell layer by 0.22 Å compared to that in Fe2O3, which was conducive to the oxygen transfer process between ≡Fe(III) and H2O2, resulting in the ≡Fe(IV)=O formation. Besides, the amphoteric hydroxide Zn(OH)2 can regulate the pH of the FeZn-LDH surface microregion, maintaining reaction pH at around 6.5-7.5, which could avoid the quenching of ≡Fe(IV)=O by H+. On the other hand, owing to the anti-interference of ≡Fe(IV)=O and the near-zero Zeta potential on the FeZn-LDH surface, the trace ECs can also be effectively degraded under high-salt conditions. Consequently, the process of ≡Fe(IV)=O generation in FeZn-LDH system can satisfy the efficient removal of ECs under near-neutral and high-salt conditions.