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

Enhanced ice slurry with low oxidant consumption for ultrafast in-situ removal of micropollutants sheltered in sediments of water supply pipelines

The cleaning of water supply pipelines represents an increasingly prevalent global practice with the aim of providing high-quality drinking water. Ice pigging, a widely-utilized pipe cleaning technique, can effectively remove substantial sediment sediments from pipe walls. During this process, micropollutants adsorbed to the sediments are inevitably released into the effluent, posing a potential threat to public health. Existing technologies can only address these micropollutants through various post-treatment methods. To improve pipeline cleaning efficiency and reduce costs, we have developed an enhanced ice slurry by adding a minute quantity of peroxymonosulfate (PMS) into the base ice slurry for ultrafast, in-situ removal of micropollutants within sediments. Comparative studies with the base ice slurry demonstrate a significant enhancement in the removal efficiency of the common micropollutant carbamazepine (CBZ) using the enhanced ice slurry. While the removal efficiency of CBZ ranged from 16 to 23 % with increasing NaCl content from 3 to 7 wt% over 10 min for the base ice slurry, nearly complete removal of CBZ was achieved within 1 min by introducing 10 μM PMS into the base ice slurry. The influence of operational parameters (e.g., PMS and Cl- concentrations, cleaning flow velocity) and pipeline characteristics (e.g., CBZ and Fe2+ concentrations, turbidity, pipe sediment) on CBZ removal efficiency was comprehensively examined for the enhanced ice slurry. The enhanced micropollutant removal process was mainly driven by active species such as hydroxyl and sulfate. Remarkably, chlorinated byproducts were scarcely detected during ice pigging, and the degradation products exhibited minimal ecotoxicity. With a cost of 0.16 Euro per cubic meter of cleaned pipe, the enhanced ice slurry presents a notable economic advantage over alternative methods. Overall, the enhanced ice slurry offers an environmentally friendly, cost-effective, and efficient solution for reducing micropollutants in water supply systems.

Bromide accelerates oxidation of selenite by unactivated peroxymonosulfate: PH-dependent kinetics, mechanism and pathways

Selenium (Se) is an essential trace element that is toxic to humans in a relatively small excess. In natural waters it occurs mainly in inorganic form as Se(IV) and Se(VI) oxyanions with the former being more toxic at high levels. With the increasing use of advanced oxidation processes in drinking water treatment, the oxidation of Se(IV) with unactivated peroxymonosulfate (PMS) has been investigated, but the role of bromide (Br-) on the oxidation of Se(IV) during reaction with unactivated PMS remains unknown. In the present work, several influencing factors on this reaction are reported, including PMS and Se(IV) concentrations, pH, Br-, and natural organic matter (NOM), on the oxidation of Se(IV), as well as the influence of different water matrices. Results show that the second-order rate constant for reaction of Se(IV) with PMS increases with increasing pH (5.0-10.0) from 0.02 to 0.33 M-1s-1, and that Se(IV) oxidation occurs mainly via a direct oxidation pathway. This increases with increasing initial concentrations of PMS and Se(IV), but is inhibited by the presence of NOM. However, the presence of Br- significantly enhances Se(IV) oxidation at circumneutral pH, but has negligible effect in alkaline conditions. It is proposed that Se(IV) oxidation by PMS involves formation of a hypobromous acid/hypobromite (HOBr/OBr-) intermediate in the presence of Br-, and its formation is supported by DFT calculations. Based on these results, a kinetics model for Se(VI) formation in bromide-containing water has been developed. Also, compared to the Br-/NOM/PMS system, the presence of Se(IV) inhibited the formation of brominated disinfection by-products (i.e., bromform and tribromoacetic acid). Overall, these results help improve our understanding of the behavior of selenium in water containing Br- during a common oxidative treatment process.

New insight into enhanced permanganate oxidation by lignocellulose-derived biochar: The overlooked role of persistent free radicals

Permanganate (Mn(VII)) is a traditional reagent used for water purification, but it is mild to deal with refractory organic contaminants of emerging concern. There is great interest in combination with effective and low-cost biochar to improve reaction kinetics of Mn(VII). Until recently, it still unclear how biomass composition and carbon structure of biochar influence the Mn(VII) oxidation performance. Herein, we prepared a series of biochar via pyrolyzing different sources of biomass, and their introduction enhanced 24 % of Mn(VII) oxidation of diclofenac (DCF) to 47.3 %∼100 % within 20 min. Particularly, Mn(VII)/walnut shell biochar (SBC) system achieved the highest reaction rate constant of 0.3817 min-1, 5.8 times faster than that by UVA-LED-activated Mn(VII). Physicochemical properties of biochar were found to be highly dependent on the organic compositions of biomass. According to quantitative structure-activity relationship (QSAR) studies, graphitization degree of biochar was recognized to be the decisive factor, facilitating the electron transfer from organics to Mn(VII)-biochar complexes. Lignin-abundant biomass was more conducive to producing highly-graphitized biochar with superior activity. Mn(III), identified as the sole reactive Mn intermediate in Mn(VII)/biochar systems, made the secondary contribution to contaminants removal. Impressively, Mn(III) formation was positively correlated with persistent free radicals (PFRs) intensity of biochar. Manipulation experiments and theoretical calculations corroborated that PFRs generated on pyrolyzed biomass and biopolymers (cellulose, hemicellulose and lignin), could donate electrons for Mn(VII) decomposition, regulating Mn(III) production via the synergy of PFRs' concentrations and types. Overall, this work offered new insights into the contribution of lignocellulose-derived biochar to Mn(VII) oxidation and contaminants removal.

Activation of persulfates on carbon nanotubes for water decontamination: Is the non-radical process consistently considered across different pH levels?

Carbon nanotubes-driven persulfates oxidation processes (CNTs/PS) have been extensively studied for environmental remediation. Solution pH is one of the main factors affecting wastewater treatment, but it is often overlooked. Herein, we report the effect laws of pH on the mechanism of peroxymonosulfate (PMS) or peroxydisulfate (PDS) activation by CNTs. The oxidation of organics (e.g., phenol) in the CNTs/PDS system involves an electron transfer process mediated by metastable intermediates (CNTs-PDS*), whose potential is influenced by pH, reaching the highest oxidation potential under neutral condition. In the CNTs/PMS system, the active species (i.e., SO4•-, CNTs-PMS*) shifted with pH variations. In acidic environment, phenol oxidation is governed by CNTs-PMS* . As pH increases, PMS undergoes accelerated decomposition, generating SO4•-, which plays a crucial role in pollutant oxidation. Moreover, in the CNTs/PMS system, the oxidation products of phenol were not easily accumulated on CNT surfaces, contributing to a lower total organic carbon removal in solution. Additionally, the oxidation rates of phenolic compounds in the CNTs/PMS system, which involve more complex mechanisms, exhibited a weaker correlation with their descriptors (e.g., the octanol-water partition coefficient) compared to CNTs/PDS system. This comprehensive investigation deepens our understanding of CNTs/PS systems and provides guidance for selecting superior oxidants for wastewater treatment.

Carbonaceous materials in structural dimensions for advanced oxidation processes

Carbonaceous materials have attracted extensive research and application interests in water treatment owing to their advantageous structural and physicochemical properties. Despite the significant interest and ongoing debates on the mechanisms through which carbonaceous materials facilitate advanced oxidation processes (AOPs), a systematic summary of carbon materials across all dimensions (0D-3D nanocarbon to bulk carbon) in various AOP systems remains absent. Addressing this gap, the current review presents a comprehensive analysis of various carbon/oxidant systems, exploring carbon quantum dots (0D), nanodiamonds (0D), carbon nanotubes (1D), graphene derivatives (2D), nanoporous carbon (3D), and biochar (bulk 3D), across different oxidant systems: persulfates (peroxymonosulfate/peroxydisulfate), ozone, hydrogen peroxide, and high-valent metals (Mn(VII)/Fe(VI)). Our discussion is anchored on the identification of active sites and elucidation of catalytic mechanisms, spanning both radical and nonradical pathways. By dissecting catalysis-related factors such as sp2/sp3 C, defects, and surface functional groups that include heteroatoms and oxygen groups in different carbon configurations, this review aims to provide a holistic understanding of the catalytic nature of different dimensional carbonaceous materials in AOPs. Furthermore, we address current challenges and underscore the potential for optimizing and innovating water treatment methodologies through the strategic application of carbon-based catalysts. Finally, prospects for future investigations and the associated bottlenecks are proposed.

Dual heteroatom-doped porous biochar from chitosan/lignosulfonate gels for enhanced removal of tetracycline by persulfate activation: Performance and mechanism

Rational design of carbon material structures is essential for enhancing the performance of persulfate-based advanced oxidation processes (PS-AOPs) in water purification. In this study, a self-doping and self-templating strategy was devised to produce N, S co-doped biochar catalysts through pre-cryocrushing and carbonization procedures employing chitosan (N-source) and lignosulfonate (S-source) derived from biomass waste. The as-synthesized materials exhibited excellent performance in removing tetracycline (TC) through a synergistic process of adsorption and catalytic activation. Mechanistic studies confirmed that electron transfer serves as the primary pathway, while singlet oxygen plays an auxiliary role. Furthermore, the toxicity of the degradation system, the impact of the complex water matrix, and the reusability of the catalysts were thoroughly investigated. Overall, this work is devoted to the treatment and application of biomass waste, which provides a feasible method for synthesizing heteroatom-doped biochar and offers valuable insights into the critical role of heteroatom-doped carbocatalysts in non-radical activation of persulfate.

Constructing a Graphene-like Layered Carbocatalyst by the Dual Templating Effect for an Efficient Fenton-like Reaction

Two-dimensional (2D) carbon materials are receiving increasing attention due to their partly groundbreaking performance in catalysis and electrochemistry based on distinct physiochemical and textural properties. We focus on the challenge to directly achieve a well-developed layered morphology with a high doping level of heteroatoms as the active sites, a standard conflict of interests of high-temperature synthesis. Here, we report a dual-templating strategy to yield graphene-like layered carbon (GLC) by direct carbonization of a texturally prealigned zeolitic imidazolate framework-8 (ZIF-8). The recrystallization of ZIF-8 in an aqueous NaCl solution discloses a 2D packing mode that was retained after freeze-drying with recrystallized NaCl as an exotemplate and a space-confining nanoreactor. Further promoted by the chemical interaction of NaCl in promoting and stabilizing the carbonization process, the final product came with a well-separated layered morphology and high amounts of heteroatoms (16.6 wt % N and 7.5 wt % O). The structurally and catalytically special GLC functioned well in activating peroxymonosulfate-based Fenton-like reactions. It was shown that the reaction proceeded via nonfree-radical-mediated pathways, as reflected in significantly enhanced electron-transfer processes and ultrafast kinetics for pollutant removal. The proposed strategy is expected to afford a broader applicability for the bottom-up design of 2D carbon materials.

Enhanced activation of peroxide to generate singlet oxygen via boron-doped Cu single-atom catalysts for efficient water treatment

The development of advanced oxidation processes (AOPs) for environmental remediation has spurred a growing interest in catalysts that selectively generate non-radical species such as singlet oxygen (1O2). However, the precise engineering of catalytic sites to enhance targeted 1O2 production remains a formidable challenge. This study reports a B-doped graphitic carbon nitride-supported Cu single-atom catalyst (CuBCN) that significantly enhances H2O2 activation for efficient 1O2 production. Through comprehensive experimental and theoretical analysis, revealing that B doping modulates the electronic properties at the Cu active sites. This modification reduces the electron density around Cu, increasing the Cu(II) fraction essential for the formation and subsequent oxidation of ·OOH intermediates. Additionally, B influences the d-band center of Cu, optimizing the adsorption energy of ·OOH, which in turn facilitates selective 1O2 production. The CuBCN/H2O2 system exhibits exceptional Fenton-like performance, producing 1O2 as the principal active species, even in challenging water conditions characterized by high pH, high ionic strength, and high concentrations of humic substances. This work not only highlights the potential of tailored single-atom catalysts in pollution control but also offers significant insights for designing catalysts for efficient H2O2 activation.

Mo doping modulates peroxymonosulfate activation of cobalt carbon nanotube-based catalysts for efficient multi-pollutants removal: Oxygen vacancies trigger the evolution of high-valence cobalt-oxo species

Formation of defective catalysts with oxygen-rich vacancies via ion doping represents an advanced strategy for enhancing catalytic activity. Cobalt oxides supported on carbon nanotubes (CNTs) were strategically designed to enhance peroxomonosulfate (PMS) through molybdenum (Mo)-doped oxygen vacancies (Vo) management to achieve the application of high-valent cobalt oxygen (Co(IV)O) dominated degradation mechanism. The first-order rate constant for tetracycline hydrochloride degradation in the CoMo/CNTs/PMS system (0.081 min-1) was twice that of the Co/CNTs system. Additionally, the system exhibited high resistance to interference and excellent pH adaptability. The system demonstrated a high removal efficiency (91.3 %-100 %) for the emerging contaminant (sodium p-perfluorous nonenoxybenzene sulfonate), as well as other organic pollutants such as carbamazepine and imidacloprid. The prepared catalyst membranes exhibited stability and effectively degraded tetracycline hydrochloride over 10 h of continuous-flow experiments, highlighting their potential for practical applications. Theoretical calculations revealed that molybdenum doping reduced the formation energy of oxygen vacancies, while these vacancies shifted the d-band center of cobalt (Co) in CoMo/CNTs upward, bringing it closer to the Fermi energy level. Furthermore, enhanced charge transfer and stronger peroxide bond stretching were observed during PMS adsorption, thus promoting the chemical reaction of PMS adsorbed on CoMo/CNTs. More significantly, the oxygen-rich vacancies in CoMo/CNTs lowered the energy barrier for Co(IV)=O generation. This study provides insights into the mechanism of ionic doping in PMS activation by metal-based catalysts, thereby expanding the application of defective catalysts in environmental remediation.

CoSn(OH)6 nanocubes: Hydroxyl perovskite catalyst for efficient peroxymonosulfate activation in acetamiprid degradation

This study presents the synthesis of a nano-cubic metal hydroxide with a perovskite structure, CoSn(OH)6, for the efficient activation of peroxymonosulfate (PMS) towards the degradation of acetamiprid (ACE) in water treatment. The CoSn(OH)6/PMS system achieved complete degradation of ACE within only 12 min and exhibited outstanding catalytic stability. Our findings indicate that the non-radical mechanism, featuring singlet oxygen (1O2) and Co(IV)=O, is the primary contributor to the degradation process, while the role of radical species such as sulfate radical (SO4·-) and hydroxyl radicals (·OH) is subordinate. These insights were confirmed through trapping experiments, electron paramagnetic resonance (EPR), in situ Raman spectroscopy and steady-state model. This work offers novel perspectives on the application of cobalt-based hydroxide catalysts in PMS activation for the remediation of emerging contaminants in water.

Fluoride ions enhanced cobalt ferrite for peroxymonosulfate activation with efficient performance and active oxygen yield regulation

The activation of peroxymonosulfate (PMS) by cobalt-based catalysts for the degradation of organic pollutants has been widely studied, while the role of coexisting anions has received little attention. In this study, the performance of atrazine (ATZ) degradation by the addition of fluoride ions (F-) in the activation of PMS by cobalt ferrite (CoFe2O4) was investigated. The addition of F- to the CoFe2O4/PMS system increased ATZ degradation effect from 82 % to 98 % within 10 min, and the rate increased from 0.172 min-1 to 0.431 min-1. At the same time, F- could also enhance the degradation of organic substances such as sulfamethoxazole (SMX), ibuprofen, and iohexol. Based on generating SO4•-, HO• and Co(IV)=O in the CoFe2O4/PMS system, F- enhanced the generation of SO4•-. When coexisting with common substances in water (i.e., inorganic anions, humic acid, hemoglobin and dextran), F- can still increase the reaction rate and reduce their negative impacts. Ion dissolution and control tests verified Co as a valid active site. A potential reaction mechanism was proposed for the complex Co(II)F formation with Co by F-, which enhanced the activation of the PMS by CoFe2O4 and regulated the active species. Finally, it was verified that the low concentration of F- could enhance ATZ degradation within two hours and the remaining F- could be effectively removed by flocculation and precipitation. This research takes utilization of F- in wastewater to promote advanced oxidation processes based on PMS, which provides a new direction for the treatment of actual water pollution.

One-step calcined cellulose-derived NiNx nanoclusters catalyst: Unleashing non-radical peroxymonosulfate activation for accelerated p-nitrophenol degradation

Carbon-based catalysts doped with transition metals effectively activate persulfate for the advanced oxidation of endocrine disruptors in water. However, research on nickel-doped catalysts is limited, especially regarding non-radical mechanisms, and typically focuses on synthetic organic compounds rather than natural cellulose. This study used wheat straw cellulose to synthesize NiNx nanocluster catalysts (Ni-NC) via a one-step calcination method. The Ni-NC/peroxymonosulfate system significantly enhanced the mass transfer between the catalyst, PMS, and contaminants, resulting in a 95.8 % removal efficiency of p-nitrophenol (PNP) within 30 min. Experimental results showed that non-radical pathways, particularly involving singlet oxygen and electron transfer, were key to PNP degradation. This study emphasizes the crucial role of nickel atomic sites in PNP removal through electron transfer processes between Ni(II) and Ni(III). It further confirms the formation of NiO bonds, indicating the presence of high-valent metal‑oxygen species. Fukui function analysis and high-performance liquid chromatography-mass spectrometry identified three PNP elimination pathways with minimal toxicity intermediates, as predicted by the Toxicity Estimation Software Tool. These results enhance the effective synthesis of nickel-doped biocatalysts sourced from natural cellulose and establish a theoretical framework for controlling non-radical pathways in water treatment processes.

Aeration-Free Photo-Fenton-Like Reaction Mediated by Heterojunction Photocatalyst toward Efficient Degradation of Organic Pollutants

The regulation of peroxymonosulfate (PMS) activation by photo-assisted heterogeneous catalysis is under in-depth investigation with potential as a replaceable advanced oxidation process in water purification, yet it remains a significant challenge. Herein, we demonstrate a strategy to construct polyethylene glycol (PEG) well-coupled dual-defect VO-M-Co3O4@CNx S-scheme heterojunction to degrade organic pollutants without aeration, which dramatically provides abundant active sites, excellent photo-thermal property, and distinct charge transport pathway for PMS activation. The degradation rate of VO-M-Co3O4@CNx in anaerobic conditions shows a higher efficient rate (4.58 min-1 g-2) than in aerobic conditions (1.67 min-1 g-2). Experimental evidence reveals that VO-M-Co3O4@CNx promotes more rapid redox conversion of photoexcited electrons induced by defects with PMS under anaerobic conditions compared to aerobic conditions. Additionally, in situ experiments and DFT provide mechanistic insights into the regulation pathway of PMS activation via synergistic defect-induced electron, revealing the competitive effect between O2 and PMS over VO-M-Co3O4@CNx during the reaction process. The continuous flow reactor and flow cytometry results demonstrated that the VO-M-Co3O4@CNx/PMS/Vis system has remarkably enhanced stability and purification capability for removing organic pollutants. This work provides valuable insights into regulating the heterologous catalysis oxidation process without aeration through the photoexcitation synergistic PMS activation.

Excellent bisphenol A removal performance triggered by electron-transfer regime on cobalt phosphide embedded in nitrogen, sulfur-doped carbon/MXene

The non-radical pathway dominated by the electron transfer process (ETP) has gained considerable attention for the removal of organic contaminants in persulfate-based advanced oxidation processes. Rationally designing new catalysts with optimized composition and structural merits and further elucidating the enhanced removal mechanism are of great importance. In this work, we successfully synthesized a nitrogen-sulfur co-doped carbon encapsulated cobalt phosphide (Co2P) on both sides of MXene nanosheets (MZPC) to degrade bisphenol A (BPA) from organic wastewater. The results indicated that BPA was degraded by 98.2 % in a mere 5 min using 0.1 g L-1 of peroxymonosulfate (PMS) and 0.05 g L-1 of the optimized catalyst (MZPC-9), exhibiting an excellent pseudo-first-order kinetics rate constant (k = 1.485 min-1). Uniformly dispersed Co2P nanoparticles (approximately 9.4 nm, calculated using the Scherrer equation) on both sides of MXene exhibited enhanced binding affinity with PMS, forming the MZPC-9-PMS* metastable complexes with potent oxidative capability. The resultant MZPC-9-PMS* complexes induced the polymerization reaction of BPA and achieved 81 % total organic carbon (TOC) removal. This study offers a novel perspective on the design of metal active centers to enhance the ETP-dominated non-radical pathway for pollutant degradation.

Interfacial anchoring cobalt species mediated advanced oxidation: Degradation performance and mechanism of organic pollutants

The development of highly catalytic activity, low-cost and environmentally friendly catalysts is crucial for the use of advanced oxidation processes (AOPs) to treat organic pollutants. In this study, to reduce costs, enhance catalytic activity and avoid secondary pollution form metal ions, pomelo peel was used as raw material, combined with surface crystallization, carbon layer protection and heat treatment technology to effectively construct AOPs catalyst that can efficiently activate peroxymonosulfate (PMS) to degrade harmful organic pollutants. Under the optimal conditions, the Co/BC-PMS system can degrade about 100 % of tetracycline (TC, a spectral antibiotic) within 5 min, and the degradation rate of TC can still reach 100 % even if Co/BC (cobalt anchored on biochar) was reused for 6 times. The Co/BC-PMS system can resist complex environmental conditions, including acidic solution, alkaline solution, coexisting ions, different water quality, and is universal for the degradation of most organic pollutants. The integrated purification column with Co/BC as the core realizes the continuous and complete degradation of organic pollutants and has the ability of practical application. Radical capture and monitoring combined with density-functional-theory calculations confirmed that the Co(111) and amorphous CoO sites in Co/BC are the key to driving PMS to degrade organic pollutants, Co/BC can efficiently adsorb PMS and promote the dissociation of PMS into highly active OH, SO4- and 1O2, and these reactive oxygen species jointly promote the degradation of organic pollutants. This study provides experimental support and theoretical insights for the design of efficient AOPs catalysts, and plays an important role in promoting the development of AOPs.

Enhanced FeIV═O Generation via Peroxymonosulfate Activation by an Edge-Site Engineered Single-Atom Iron Catalyst

As an oxidant, the ferryl-oxo complex (FeIV═O) offers excellent reactivity and selectivity for degrading recalcitrant organic contaminants. However, enhancing FeIV═O generation on heterogeneous surfaces remains challenging because the underlying formation mechanism is poorly understood. This study introduces edge defects onto a single-atom Fe catalyst (FeNC-edge) to promote FeIV═O generation via peroxymonosulfate (PMS) activation. In the presence of PMS, the FeNC-edge catalyst at a low dose (20 mg L-1, equivalent to 0.14 mg L-1 Fe) exhibits unprecedented activity for organic contaminant degradation. Electrochemical analysis, in situ Raman spectroscopy, and FeIV═O probe experiments confirm that FeIV═O generation is enhanced on the surface of FeNC-edge. Density functional theory calculations reveal that the introduced edge sites concentrate electron density on active Fe atoms, facilitating charge transfer from Fe to PMS. Notably, FeNC-edge immobilized on a polymeric membrane functioned as a continuous-flow oxidation system with efficient catalyst recycling and minimal Fe leaching.

Transforming Plastics to Single Atom Catalysts for Peroxymonosulfate Activation: Axial Chloride Coordination Intensified Electron Transfer Pathway

Transforming plastics into single-atom catalysts is a promising strategy for upcycling waste plastics into value-added functional materials. Herein, a graphene-based single-atom catalyst with atomically dispersed FeN4Cl sites (Fe─N/Cl─C) is produced from high-density polyethylene wastes via one-pot catalytic pyrolysis. The Fe─N/Cl─C catalyst exhibited much higher turnover frequency and surface area normalized activity (Kac) compared with the Fe─N─C catalyst without axial Cl modulation. Both experiments and density functional theory (DFT) computations demonstrated that the axial incorporation of chloride fine-tuned the coordination environment of FeN4 sites and enhanced peroxymonosulfate (PMS) activation because of improved conductivity and modulated spin state. In situ, Raman, and infrared spectroscopic techniques revealed that PMS is activated by the Fe─N/Cl─C catalyst through an electron transfer process. The formation of a key PMS* intermediate at the Fe site effectively elevated the redox capacity of the catalyst surface to realize a fast degradation of diverse pollutants. The non-radical oxidation manner secures high selectivity toward target pollutants and high chemical utilization efficiency. A continuous operation in a column reactor also demonstrated the high efficiency and stability of the (Fe─N/Cl─C + PMS) system for practical water treatment.

Efficient degradation of sulfadiazine via facilitated electron transfer by iron-carbon catalyst with highly exposed active sites

The inaccessible active sites, excessive metal leaching and radical mediated degradation pathway greatly hinder the performances of Fe-C composite catalyst oxidation process in the advanced oxidation water treatment. Herein, a facile method was developed to in situ growth of MIL-53 (Fe) on the powder active carbon (PAC) surface by a mild condition, which finally yields PAC supported Fe3O4@C particles (PAC@MOFs-2T) after heat treatment. The detailed characterizations indicate that the fine Fe3O4 particles encapsulated with carbon layers were evenly anchored on the PAC as active sites, which made the catalytic centers highly accessible for the peroxydisulfate activation and sulfadiazine degradation. In addition, the carbon layers, coated on the active sites could prevent the metal leaching during the catalytic process resulting in the high stability in a wide pH range. More attractively, the density functional theory (DFT) simulations and emperimental evidences further proved that the oxidation was dominated by a electron transfer process (ETP), during which, the peroxydisulfate (PDS) was adsorbed on Fe3O4 to form PDS∗ with high oxidation potential to initiate the ETP. Meanwhile, it was also demonstrated that the optimized sample PAC@MOFs-2T enriched with electron donating groups could selectively degrade the sulfadiazine, which avoid the negative impacts from the co-existed foreign ions and organic matters during the oxidation process. In addition, the toxicity analysis of intermediate products revealed that the sulfadiazine can be degradated into low-toxic or non-toxic products, which further permits viability of this ETP mediated advanced oxidation processes.

Mechanistic insights into activation of peracetic acid by sludge biogas residue biochar for efficient sulfamethoxazole degradation in aqueous solution

The application of peracetic acid (PAA) in the advanced oxidation process has been demonstrated to be an effective approach for treating aqueous organic pollutants. In this study, it is the first time that biogas residue biochar (BRBC) derived from sludge anaerobic digestion plants was prepared and used as a PAA activator for sulfamethoxazole (SMX) degradation. The optimal SMX removal could achieve 92 % within 120 min under acidic conditions. The SMX degradation was slightly enhanced in the presence of Cl-, while it could be inhibited by HCO3-. Quenching experiment and EPR analysis demonstrated that both radical and non-radical processes contributed to SMX degradation. ECOSAR analysis showed a significant reduction in intermediate toxicity. Meanwhile, BRBC700 exhibited excellent reusability and stability even in real water matrices. The study presented an innovative approach for biogas residue application and provided a novel pretreatment for SMX-containing wastewater for further biological treatment method after simple acid-base regulation.

Selective oxidation of nitrogenous heterocyclic compounds by heat/peroxymonosulfate in phenol-rich wastewater

In phenol-rich wastewater, such as coking wastewater, due to the high reactivity of phenol to various reactive oxygen species, it is difficult to selectively oxidize pollutants having lower biodegradability and higher toxicity than phenol. As one kind of such pollutants in coking wastewater, some nitrogenous heterocyclic compounds (NHCs) are more difficult to be removed by SO4•- or HO• than phenol, but this study found that NHCs (quinoline, isoquinoline, and pyridine) can be selectively removed by peroxymonosulfate (PMS) direct oxidation in the presence of 10 mM phenol under thermal condition. The selective oxidation of NHCs needs a suitable pH range (4 < constant pH < 9) because protonated state of NHCs (pH < 4) is unfavorable to their oxidation and high pH would improve the extra PMS consumption by phenol. Under the conditions benefiting the removal of NHCs in heat/PMS system, there was no generation of SO4•- and HO•. Being treated by 60 °C/PMS for 60 min, the biodegradability (BOD5/COD) of real coking wastewater (RCW) was improved from 0.21 to 0.44 with low removal rate of phenols (about 10 %). Quinoline and indole, as the two typical NHCs in the studied RCW, their removal rates can be up to 45 % and 85 %, respectively. Thus, heat/PMS pretreatment is a potential good way to selectively remove high toxic pollutants in phenol-rich wastewater.

Reversible Sol-Gel Transitions Mediated Organics Selective Uptake and Release for Simultaneous Water Purification and Chemicals Recovery

The separation and recovery of useful organics from wastewater have been a promising alternative to tackling water pollution and resource shortages, while strategies that truly work have rarely been explored. Herein, a reversible CO2 triggered sol-gel state transformation mediated selective organics uptake-release system using a surface modified carbonitride (S-CN) is proposed and exhibits remarkable organics recovery performance from wastewater. Results show that CO2 can serve as a cross-linker for linking S-CN particles to form a hydrogel by electrostatic interaction and hydrogen bonding, which can be recycled to the pristine sol state simply by removing the cross-linked CO2 with Ar purging. The reversible sol-gel transformation achieves nearly complete uptake of valuable organics from wastewater with high selectivity at the first sol-to-gel stage through electrostatic interaction, hydrogen bonding, and π-π interactions together, and it recovers 90% of the organics uptaked by releasing them into a concentrated solution at the second gel-back-to-sol stage.

Combating Antibiotic Resistance in Persulfate-Based Advanced Oxidation Processes: Activation Methods and Energy Consumption

Antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs) have become increasing concerning issues, threatening human health. Persulfate-based advanced oxidation processes (PS-AOPs), due to their remarkable potential in combating antibiotic resistance, have garnered significant attention in the field of disinfection in recent years. In this review, we systematically evaluated the efficacy and underlying mechanism of PS integration with various activation methods for the elimination of ARB/ARGs. These approaches encompass physical methods, catalyst activation, and hybrid techniques with photocatalysis, ozonation, and electrochemistry. Additionally, we employed Chick's model and electrical energy per log order (EE/O) to assess the performance and energy efficiency, respectively. This review aims at providing a guide for future investigation on PS-AOPs for antibiotic resistance control.

Electronic coupling of iron-cobalt in Prussian blue towards improved peroxydisulfate activation

Prussian blue analogs (PBAs) have attracted extensive attention in the field of aqueous organic degradation due to the tremendous potential for peroxydisulfate (PDS) activation. However, the relationship between the d-band center of the catalyst and the activation behavior of PDS remained largely unexplored. Herein, a series of Fe-Co PBAs-based catalysts with different Fe/Co ratios (Fe-Co PBAs-1 = 1: 0.52; Fe-Co PBAs-2 = 1: 1.21, and Fe-Co PBAs-3 = 1: 1.48) have been prepared by a facile hydrothermal procedure and subsequent acid treatment (Fe-Co PBAs-xH). The as-prepared Fe-Co PBAs-xH exhibited superior PDS activation performance and excellent recyclability in the degradation of methylene blue (MB). Density functional theory calculations revealed that the electron-occupied state of the Fe-Co PBAs was shifted to the Fermi level, indicating a strong interaction and easier electron transfer. Moreover, the d-band center of Fe-Co PBAs was upshifted relative to that of Fe PBAs, suggesting easier adsorption of MB and PDS, which was beneficial to enhancing catalytic activation and subsequent dissociation. Radicals such as •OH, 1O2, O2•-, and SO4•- were determined by the radical quenching experiment and electron paramagnetic resonance (EPR) testing in the Fe-Co PBAs-3H/PDS system, and the order of MB degradation by the free active radical is •OH > 1O2 > O2•- > SO4•-. The degradation pathway and potential ecotoxicity of MB and its intermediates were also studied. This work can provide new insights to construct the efficient catalysts for the activation of PDS and the degradation of organic pollutants.

Enhanced peroxymonosulfate activation for emerging contaminant degradation via defect-engineered interfacial electric field in FeNC

Peroxymonosulfate (PMS) activation technology has important application value in treating emerging contaminant (ECs), but it still faces challenges in achieving efficient electron transfer and metal valence cycling. In this study, the interfacial electric field characteristics of FeNC catalysts were adjusted by introducing NC defects to affect the electron transfer process, thereby enhancing the catalytic performance of PMS. It is found that in the FeNC structure, the shift of the charge generates an interfacial electric field, which can promote the directional transfer of electrons. Through quantitative structure-activity relationship (QSAR) analysis, it was confirmed that the defect played a decisive role in regulating the interfacial electric field and improving the catalytic reaction efficiency. The interfacial electric field-mediated superexchange interaction realizes the electron donor effect of organic pollutants and the effective electron transfer between the Fe site, accelerates the electron cycling of the Fe site, and realizes the rapid and stable catalysis of PMS. The increase of the occupancy state distribution of d orbitals near the Fermi level provides favorable conditions for electron transitions and catalytic activation of PMS. ECs can be converted into environmentally friendly, non-toxic and harmless substances through. This defect-controlled interface electric field strategy realizes rapid electron directional transfer, which provides a new solution for improving the catalytic efficiency of PMS and the safe treatment of ECs in water.

Magnesium Oxide-Supported Single Atoms with Fine-Modulated Steric Location for Polymerization Transfer Removal of Water Pollutants

Organic pollutants removal via a polymerization transfer (PT) pathway based on the use of single-atom catalysts (SACs) promises efficient water purification with minimal energy/chemical inputs. However, the precise engineering of such catalytic systems toward PT decontamination is still challenging, and the conventional SACs are plagued by low structural stability of carbon material support. Here, we adopted magnesium oxide (MgO) as a structurally stable alternative for loading single copper (Cu) atoms to drive peroxymonosulfate-based Fenton-like reactions. Through fine-tuning the Cu atom steric location from lattice-embedding to surface-loading, the system exhibited a fundamental transition in the catalytic pathways toward the PT process and drastically improved decontamination efficiency. The catalytic pathway change was mainly ascribed to a downshifted d-band center of the Cu atoms. The optimized catalyst achieved complete, rapid removal of phenolic compounds from water via nearly 100% PT pathway, accompanied by high oxidant utilization efficiency surpassing most state-of-the-art SACs. Moreover, it showed excellent structural stability and environmental robustness and was successfully used for the treatment of lake water and industrial coking wastewater. The adaptability of the spatial engineering strategy to other MgO-supported single atoms, including Fe, Co, and Ni SACs, was also demonstrated. Our work lays a foundation for further advancing SACs-based advanced oxidation technologies toward sustainable water purification applications.

High-entropy alloys catalyzing polymeric transformation of water pollutants with remarkably improved electron utilization efficiency

High-entropy alloy nanoparticles (HEA-NPs) exhibit favorable properties in catalytic processes, as their multi-metallic sites ensure both high intrinsic activity and atomic efficiency. However, controlled synthesis of uniform multi-metallic ensembles at the atomic level remains challenging. This study successfully loads HEA-NPs onto a nitrogen-doped carbon carrier (HEAs) and pioneers the application in peroxymonosulfate (PMS) activation to drive Fenton-like oxidation. The HEAs-PMS system achieves ultrafast pollutant removal across a wide pH range with strong resistance to real-world water interferences. Furthermore, the nonradical HEAs-PMS system selectively transforms phenolics into high-molecular-weight products via a polymerization pathway. The unique non-mineralization regime remarkably reduces PMS consumption and achieves a high electron utilization efficiency of up to 213.4%. Further DFT calculations and experimental analysis reveal that Fe and Co in HEA-NPs act as the primary catalytic sites to complex with PMS for activation, while Ni, Cu, and Pd serve as charge mediators to facilitate electron transfer. The resulting PMS* complexes on HEAs possess a high redox potential, which drives spatially separated phenol oxidation on nitrogen-doped graphene support to form phenoxyl radicals, subsequently triggering the formation of high-molecule polymeric products via polymerization reactions. This study offers engineered HEAs catalysts for water treatment with low oxidant consumption and emissions.

Low-peroxide-consumption fenton-like systems: The future of advanced oxidation processes

Conventional heterogeneous Fenton-like systems employing different peroxides have been developed for water/wastewater remediation. However, a large population of peroxides consumed during various Fenton-like systems with low utilization efficiency and associated secondary contamination have become the bottlenecks for their actual applications. Recent strategies for lowering the peroxide consumptions to develop economic Fenton-like systems are primarily devoted to the effective radical generation and subsequent high-efficiency radical utilization through catalysts/systems engineering, leveraging emerging nonradical oxidation pathways with higher selectivity and longer life of the reactive intermediate, as well as reactor designs for promoting the mass transfer and peroxides decomposition to improve the yield of radicals/nonradicals. However, a comparative review summarizing the mechanisms and pathways of these strategies has not yet been published. In this review, we endeavor to showcase the designated systems achieving the reduction of peroxides while ensuring high catalytic activity from the perspective of the above strategic mechanisms. An in-depth understanding of these aspects will help elucidate the key mechanisms for achieving economic peroxide consumption. Finally, the existing problems of these strategies are put forward, and new ideas and research directions for lowering peroxide consumption are proposed to promote the application of various Fenton-like systems in actual wastewater purification.

Novel catalytic behavior of defective nanozymes with catalase-mimicking characteristics for the degradation of tetracycline

Although nanozymes have shown significant potential in wastewater treatment, enhancing their degradation performance remains challenging. Herein, a novel catalytic behavior was revealed for defective nanozymes with catalase-mimicking characteristics that efficiently degraded tetracycline (TC) in wastewater. Hydroxyl groups adsorbed on defect sites facilitated the in-situ formation of vacancies during catalysis, thereby replenishing active sites. Additionally, electron transfer considerably enhanced the catalytic reaction. Consequently, numerous reactive oxygen species (ROS) were generated through these processes and subsequent radical reactions. The defective nanozymes, with their unique catalytic behavior, proved effective for the catalytic degradation of TC. Experimental results demonstrate that •OH, •O2-, 1O2 and e- were the primary contributors to the degradation process. In real wastewater samples, the normalized degradation rate constant for defective nanozymes reached 26.0 min-1 g-1 L, exceeding those of other catalysts. This study reveals the new catalytic behavior of defective nanozymes and provides an effective advanced oxidation process for the degradation of organic pollutants.

Electron transfer tuning for persulfate activation via the radical and non-radical pathways with biochar mediator

Electron mediator-based in-situ chemical oxidation (ISCO) offers a novel strategy for groundwater remediation due to diverse reaction pathways. However, distinguishing and further tuning the reaction pathway remains challenging. Herein, biochar as an electron mediator targeted active peroxysulphate (PDS) via the radical or non-radical pathway. Exemplified by the triazin pesticides removal, the complex radical (•OH and SO4•-) and non-radical active species (electron transfer oxidation) were generated and identified in different biochar/PDS systems. The electron transfer process between biochar and PDS was significantly distinguished via an innovatively in-situ visualization of radical pathway, and the electron transfer oxidation non-radical pathway is directly unveiled via a galvanic cell experiment combined with LC-MS analyses. The electron transfer mechanism was revealed via establishing the quantitative structure-activity relationships between biochar and ln kobs. The redox capacity of biochar was assessed as a key for tuning the atrazine degradation by non-radical pathway, and the surface carbon-centered persistent free radicals (PFRs) were identified as key electron donors for triggering the radical pathway. This study gives new insights into the electron transfer mechanism during tuning radical and non-radical activation pathways and the enhanced utilization of oxidants in ISCO technology.

Revealing the Overlooked Catalytic Ability of γ-Al2O3: Efficient Activation of Peroxymonosulfate for Enhanced Water Treatment

Activated alumina (γ-Al2O3) is one of the few nanomaterials manufactured at a ton-scale and successfully implemented in large-scale water treatment. Yet its role in advanced oxidation processes (AOPs) has primarily been limited to functioning as an inert carrier due to its inherently nonredox nature. This study, for the first time, presents the highly efficient capability of γ-Al2O3 to activate peroxymonosulfate (PMS) for selectively eliminating electron-rich organic pollutants in the presence of Cl-. Through experimental and theoretical analysis, we revealed that γ-Al2O3, characterized by uniquely strong Lewis acid sites, enabled robust inner-sphere complexation between PMS and Al(III) sites, triggering the oxidation of Cl- to free chlorine through a distinctive, low-energy-barrier Eley-Rideal pathway. Such a unique pathway resulted in a 42.7-fold increase in free chlorine generation, culminating in a remarkable 145.9-fold enhancement in the degradation of carbamazepine (CBZ) compared with the case without γ-Al2O3. Furthermore, this catalyst exhibited high oxidant utilization efficiency, stable performance in real-world environmental matrices, and sustained long-term activation for over 1206 bed volumes (BV) with a CBZ removal rate exceeding 90% in fixed-bed experiments. These favorable features render γ-Al2O3 an extremely promising nanomaterial for sustainable water treatment initiatives.

Unveiling the critical roles of nascent MnO2 in accelerating permanganate carbocatalysis

To probe the underlying mechanisms of carbocatalysis in enhanced permanganate (PM) oxidation and identify the exact roles of nascent MnO2, graphene aerogels (GA) were fabricated to activate PM for naproxen (NPX) degradation. All the three GA samples could accelerate NPX oxidation by PM, the rate constants and reaction stoichiometric efficiency (RSE) followed the order of GA900 > GA600 > GA300. Mechanistic studies revealed that Mn(VI), Mn(V) and Mn(III) were not the major reactive species involved in NPX oxidation, but highlighted the essential contribution of electron transfer pathway (ETP) mediated directly by GA and indirectly by nascent MnO2. For GA300 with strong electron-donating capability, it mainly served as the electron donor for PM decomposition, and indirectly oxidized NPX via nascent MnO2 mediated ETP, thereby exhibiting inferior RSE as well as mediocre recycling performance. GA600 and GA900 could serve as the electron shuttle to directly mediate the ETP for NPX degradation, the nascent MnO2 accumulated on GA framework during the reaction would also mediate the ETP from NPX to PM, thus displaying an obvious accelerating recycling performance. This work provides novel insights into the structure-dominated PM carbocatalysis, which contributes better to development of promising carbocatalysts and utilization of nascent MnO2.

Efficient activation of peracetic acid by defect-engineered MoO2-x: Oxygen vacancies and surface Mo(Ⅴ)-mediated electron transfer processes

The role of defect regulation of transition metal catalysts in peracetic acid (PAA) activation is equivocal. To reveal the corresponding mechanism, this work provides a high-efficiency and eco-friendly catalyst (MoO2-x) for PAA activation by introducing various degrees of oxygen vacancies on the MoO2 surface. Interestingly, 95.83 % of tetracycline (TC) is rapidly degraded by MoO2-x with rich oxygen vacancies within 20 min via PAA activation, which is superior over that of MoO2-x with poor oxygen vacancies and other typical oxidants (H2O2, SO32-, S2O82-, HSO5-, IO4-). In addition, the defect-regulated MoO2-x exhibits good de-biotoxicity towards TC. Moreover, MoO2-x shows satisfactory purification of various contaminants and actual pharma wastewater. Active species identification suggests that the electron transfer process triggered by the active complex (MoO2-x -PAA*) of PAA bonded on the MoO2-x surface plays the dominant role in TC degradation, while •OH plays a minor role. Mechanism analysis reveals that oxygen vacancies play an indispensable role in accelerating the adsorption and complexation of PAA as well as improving electrical conductivity. Active site analysis demonstrates that Mo(Ⅴ) on the MoO2-x surface acts as an electron shuttle and is the main PAA activation site. This work provides a new approach into the application of MoO2 in hospital wastewater purification via defect engineering.

Unveiling electron transfer and radical transformation pathways in coupled electrocatalysis and persulfate oxidation reactions for complex pollutant removal

The degradation of multiple organic pollutants in wastewater via advanced oxidation processes might involve different radicals, of which the types and concentrations vary upon interacting with different pollutants. In this study, electrochemical activation of peroxymonosulfate (E/PMS) using advanced activated carbon cloth (ACC) as electrode was applied for simultaneous degradation of mixed pollutants, e.g., metronidazole (MNZ) and p-chloroaniline (PCA). 92.5 % of MNZ and 91.4 % of PCA can be degraded at the cathode and anode at a low current density and PMS concentration, respectively. The rate constants for the simultaneous removal of MNZ and PCA in the E/PMS/MNZ(PCA) system were 118 times and 6 times higher than those in the sole PMS system, and 2.5 times and 1.6 times higher than those in the E/Na2SO4/MNZ(PCA) system, respectively. Different electrochemical characteristics, EPR spectra and radical quenching tests verified that the degradation of MNZ and PCA in the optimal system proceeded primarily through non-radical-dominated oxidation, involving electron transfer and 1O2 effect. The system also exhibited low energy consumption (0.215 kWh/m-3·order-1), broad operational pH range, excellent removal efficiency for water matrix, and low by-products toxicity, indicating its strong potential for practical applications. The ACC, with its super stable, low cost, and electrochemical activity, make it as a promising materials applicable in the E/PMS system for degradation of multiple pollutants. The study further elucidated the mechanism of pollutant interaction with electrode materials in terms of radical and non-radical transformation, providing fundamental insight into the application of this system for treatment of complex wastewater.

Task decomposition strategy based on machine learning for boosting performance and identifying mechanisms in heterogeneous activation of peracetic acid process

Heterogeneous activation of peracetic acid (PAA) process is a promising method for removing organic pollutants from water. Nevertheless, this process is constrained by several complex factors, such as the selection of catalysts, optimization of reaction conditions, and identification of mechanism. In this study, a task decomposition strategy was adopted by combining a catalyst and reaction condition optimization machine learning (CRCO-ML) model and a mechanism identification machine learning (MI-ML) model to address these issues. The Categorical Boosting (CatBoost) model was identified as the best-performing model for the dataset (1024 sets and 7122 data points) in this study, achieving an R2 of 0.92 and an RMSE of 1.28. Catalyst composition, PAA dosage, and catalyst dosage were identified as the three most important features through SHAP analysis in the CRCO-ML model. The HCO3- is considered the most influential water matrix affecting the k value. The errors between all reverse experiment results and the predictions of the CRCO-ML and MI-ML models were <10 % and 15 %, respectively. This interdisciplinary work provides novel insights into the design and application of the heterogeneous activation of PAA process, significantly contributing to the rapid development of this technology.

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.

Coordination engineering of heterogeneous high-valent Fe(IV)-oxo for safe removal of pollutants via powerful Fenton-like reactions

Coordination engineering of high-valent Fe(IV)-oxo (FeIV=O) is expected to break the activity-selectivity trade-off of traditional reactive oxygen species, while attempts to regulate the oxidation behaviors of heterogeneous FeIV=O remain unexplored. Here, by coordination engineering of Fe-Nx single-atom catalysts (Fe-Nx SACs), we propose a feasible approach to regulate the oxidation behaviors of heterogeneous FeIV=O. The developed Fe-N2 SACs/peroxymonosulfate (PMS) system delivers boosted performance for FeIV=O generation, and thereby can selectively remove a range of pollutants within tens of seconds. In-situ spectra and theoretical simulations suggest that low-coordination Fe-Nx SACs favor the generation of FeIV=O via PMS activation as providing more electrons to facilitate the desorption of the key *SO4H intermediate. Due to their disparate attacking sites to sulfamethoxazole (SMX) molecules, Fe-N2 SACs mediated FeIV=O (FeIVN2=O) oxidize SMX to small molecules with less toxicity, while FeIVN4=O produces series of more toxic azo compounds through N-N coupling with more complex oxidation pathways.

Pilot-scale and large-scale Fenton-like applications with nano-metal catalysts: From catalytic modules to scale-up applications

Recently, great efforts have been made to advance the pilot-scale and engineering-scale applications of Fenton-like processes using various nano-metal catalysts (including nanosized metal-based catalysts, smaller nanocluster catalysts, and single-atom catalysts, etc.). This step is essential to facilitate the practical applications of advanced oxidation processes (AOPs) for these highly active nano-metal catalysts. Before large-scale implementation, these nano-metal catalysts must be converted into the effective catalyst modules (such as catalytic membranes, fluidized beds, or polypropylene sphere suspension systems), as it is not feasible to use suspended powder catalysts for large-scale treatment. Therefore, the pilot-scale and engineering applications of nano-metal catalysts in Fenton-like systems in recent years is exciting. In addition, the combination of life cycle assessment (LCA) and techno-economic analysis (TEA) can provide a useful support tool for engineering scale Fenton-like applications. This paper summarizes the designs and fabrications of various advanced modules based on nano-metal catalysts, analyzes the advantages and disadvantages of these catalytic modules, and further discusses their Fenton-like pilot scale or engineering applications. Concepts of future Fenton-like engineering applications of nano-metal catalysts were also discussed. In addition, current challenges and future expectations in pilot-scale or engineering applications are assessed in conjunction with LCA and TEA. These challenges require further technological advances to enable larger scale engineering applications in the future. The aim of these efforts is to increase the potential of nanoscale AOPs for practical wastewater treatment.

Enhanced mineralization of nitrophenols by a novel C@ZVAl-PS based sequential reduction-oxidation process

Widely employed nitrophenols (NPs) are refractory and antioxidant due to their strong electron-withdrawing group (-NO2). Actually, NPs are readily reduced to aminophenols (APs). However, APs remain toxic and necessitate further treatment. Herein, we utilized a novel sequential reduction-oxidation system of carbon-modified zero-valent aluminum (C@ZVAl) combined with persulfate (PS) for the thorough removal of both NPs and APs. The results demonstrated that p-nitrophenol (PNP, up to 1000 mg/L) exhibited complete reduction to p-aminophenol (PAP), and then over 98.0 % of PAP could be effectively oxidized, in the meantime the removal rate of chemical oxygen demand (COD) was as high as 95.9 %. Based on the SEM and XPS characterizations, we found that C@ZVAl has exceptionally high reactivity that generates massive electrons and reduces PNP to PAP through accelerated electron transfer. In the subsequent oxidation step, PS can be rapidly activated by C@ZVAl to generate SO4- radicals for PAP oxidization. Meanwhile, the mineralization of COD proceeds. The temporal binding of reduction and oxidation can be regulated by varying the PS dosing time. Namely, the appropriate delay in PS dosing facilitates sufficient reduction to provide enough reactants for oxidation, favoring the mineralization of PNP and COD. More crucially, dinitrodiazophenol (DDNP) in an actual explosive wastewater without any pretreatment can be effectively mineralized by this sequential reduction-oxidation system, affirming the excellent performance of this process in practical applications. In conclusion, the C@ZVAl-PS based sequential reduction-oxidation looks very promising for enhanced mineralization of nitro-substituted organic contaminants.

Accelerating Small Electron Polaron Dissociation and Hole Transfer at Solid-Liquid Interface for Enhanced Heterogeneous Photoreaction

In a photocatalysis process, quick charge recombination induced by small electron polarons in a photocatalyst and sluggish kinetics of hole transfer at the solid-liquid interface have greatly limited photocatalytic efficiency. Herein, we demonstrate hydrated transition metal ions as mediators that can simultaneously accelerate small electron polaron dissociation (via metal ion reduction) and hole transfer (through high-valence metal production) at the solid-liquid interface for improved photocatalytic pollutant degradation. Fe3+, by virtue of its excellent redox ability as a homogeneous mediator, enables the BiVO4 photocatalyst to achieve drastically increased photocatalytic degradation performance, up to 684 times that without Fe3+. The enhanced performance results from Fe(IV) species production (via Fe3+ oxidation) induced by dissociation of small electron polarons (via Fe3+ reduction), featuring an extremely low kinetic barrier (5.4 kJ mol-1) for oxygen atom transfer thanks to the donor-acceptor orbital interaction between Fe(IV) and organic pollutants. This work constructs a high-efficiency artificial photosynthetic system through synergistically eliminating electron localization and breaking hole transfer limitation at the solid-liquid interface for constructing high-efficiency artificial photosynthetic systems.

Sludge-derived hydrochar modulates complete nonradical electron transfer in peroxydisulfate activation via pyrrolic-N and carbon defect: Implication for degrading electron-rich ionizable anilines compounds

Nonradical electron transfer process (ETP) is a promising pathway for pollutant degradation in peroxydisulfate-based advanced oxidation processes (PDS-AOPs). However, there is a critical bottleneck to trigger ETP by sludge-derived hydrochar due to its negatively charged surface, inferior porosity and electrical conductivity. Herein, pyrrolic-N doped and carbon defected sludge-derived hydrochar (SDHC-N) was constructed for PDS activation to degrade anilines ionizable organic compounds (IOC) through complete nonradical ETP oxidation. Degradation of anilines IOC was not only affected by the electron-donating capacity but also proton concentration in solution because of the ionizable amino group (-NH2). Diverse effects including proton favor, insusceptible and inhibition were observed. Impressively, addition of HCO3 with strong proton binding capacity boosted aniline degradation nearly 10 times. Moreover, characterizations and theoretical calculations demonstrated that pyrrolic-N increased electron density and created positively charged surface, profoundly promoting generation of SDHC-N-S2O82-* complexes. More delocalized electrons around carbon defect could enhance electron mobility. This work guides a rational design of sludge-derived hydrochar to mediate nonradical ETP oxidation, and provides insights into the impacts of proton on anilines IOC degradation.

Electron transfer mediated activation of periodate by contaminants to generate 1O2 by charge-confined single-atom catalyst

The electron transfer process (ETP) is able to avoid the redox cycling of catalysts by capturing electrons from contaminants directly. However, the ETP usually leads to the formation of oligomers and the reduction of oxidants to anions. Herein, the charge-confined Fe single-atom catalyst (Fe/SCN) with Fe-N3S1 configuration was designed to achieve ETP-mediated contaminant activation of the oxidant by limiting the number of electrons gained by the oxidant to generate 1O2. The Fe/SCN-activate periodate (PI) system shows excellent contaminant degradation performance due to the combination of ETP and 1O2. Experiments and DFT calculations show that the Fe/SCN-PI* complex with strong oxidizing ability triggers the ETP, while the charge-confined effect allows the single-electronic activation of PI to generate 1O2. In the Fe/SCN + PI system, the 100% selectivity dechlorination of ETP and the ring-opening of 1O2 avoid the generation of oligomers and realize the transformation of large-molecule contaminants into small-molecule biodegradable products. Furthermore, the Fe/SCN + PI system shows excellent anti-interference ability and application potential. This work pioneers the generation of active species using ETP's electron to activate oxidants, which provides a perspective on the design of single-atom catalysts via the charge-confined effect.

Efficient oxidative remediation of polycyclic aromatic hydrocarbons (PAHs)-contaminated soil: A thorough comprehension of Fe-loaded biochar activated persulfate

The porous and defective structure of biochar (BC) can accelerate surface electron transfer, promote the generation of more reactive oxygen species (ROS) by persulfate (PS), and effectively degrade organic pollutants in the soil. Electron transfer is a crucial link in this process, directly determining its oxidative degradation efficiency. In this study, using a novel strategy of enhancing electron transfer on the surface of BC by loading iron, three Fe-loaded BC activators (Fe-FeOx@BC, Fe2O3@BC and Fe3O4@BC) were synthesized to support the oxidative remediation of benzo(a)pyrene (BaP, Model compound of PAHs)-contaminated soil by PS. The results showed that Fe3O4@BC supported PS oxidation and remediation of BaP-contaminated soil had the best effect among the three BC-based activators, and the reuse effect was stable. Under the conditions of Fe3O4@BC addition of 1.00 wt%, PS addition of 0.75 wt%, reaction temperature of 35 °C, and solid-liquid ratio of 1:2.5, the removal rate of BaP in the soil reached the maximum of 93.84% at 120 min, and the soil toxicity was significantly reduced after remediation. The defect structure, conductive magnetic particles, and active functional groups on the surface of Fe3O4@BC were the key factors for activating PS to degrade BaP. With the combined action of the free radical pathway caused by ROS and the non-free radical pathway caused by 1O2, electron transfer, and active functional groups, BaP was degraded to small molecules such as CO2 and H2O, achieving rapid and efficient remediation of organic contaminated soil.

Highly stable carbon-coated nZVI composite Fe0@RF-C for efficient degradation of emerging contaminants

Nanoscale zerovalent iron (nZVI) has garnered significant attention as an efficient advanced oxidation activator, but its practical application is hindered by aggregation and oxidation. Coating nZVI with carbon can effectively addresses these issues. A simple and scalable production method for carbon-coated nZVI composite is highly desirable. The anti-oxidation and catalytic performance of carbon-coated nZVI composite merit in-depth research. In this study, a highly stable carbon-coated core-shell nZVI composite (Fe0@RF-C) was successfully prepared using a simple method combining phenolic resin embedding and carbothermal reduction. Fe0@RF-C was employed as a heterogeneous persulfate (PS) activator for degrading 2,4-dihydroxybenzophenone (BP-1), an emerging contaminant. Compared to commercial nZVI, Fe0@RF-C exhibited superior PS activation performance and oxidation resistance. Nearly 95% of BP-1 was removed within 10 min in the Fe0@RF-C/PS system. The carbon layer promotes the enrichment of BP-1 and accelerates its degradation through singlet oxygen oxidation and direct electron transfer processes. This study provides a straightforward approach for designing highly stable carbon-coated nZVI composite and elucidates the enhanced catalytic performance mechanism by carbon layers.

Remarkable Active Site Utilization in Edge-Hosted-N Doped Carbocatalysts for Fenton-Like Reaction

Improving the utilization of active sites in carbon catalysts is significant for various catalytic reactions, but still challenging, mainly due to the lack of strategies for controllable introduction of active dopants. Herein, a novel "Ar plasma etching-NH3 annealing" strategy is developed to regulate the position of active N sites, while maintaining the same nitrogen species and contents. Theoretical and experimental results reveal that the edge-hosted-N doped carbon nanotubes (E-N-CNT), with only 0.29 at.% N content, show great affinity to peroxymonosulfate (PMS), and exhibit excellent Fenton-like activity by generating singlet oxygen (1O2), which can reach as high as 410 times higher than the pristine CNT. The remarkable utilization of edge-hosted nitrogen atom is further verified by the edge-hosted-N enriched carbocatalyst, which shows superior capability for 4-chlorophenol degradation with a turnover frequency (TOF) value as high as 3.82 min-1, and the impressive TOF value can even surpass those of single-atom catalysts. This work proposes a controllable position regulation of active sites to improve atom utilization, which provides a new insight into the design of excellent Fenton-like catalysts with remarkable atom utilization efficiency.

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

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

The Dual Role of Natural Organic Matter in the Degradation of Organic Pollutants by Persulfate-Based Advanced Oxidation Processes: A Mini-Review

Persulfate-based advanced oxidation processes (PS-AOPs) are widely used to degrade significant amounts of organic pollutants (OPs) in water and soil matrices. The effectiveness of these processes is influenced by the presence of natural organic matter (NOM), which is ubiquitous in the environment. However, the mechanisms by which NOM affects the degradation of OPs in PS-AOPs remain poorly documented. This review systematically summarizes the dual effects of NOM in PS-AOPs, including inhibitory and promotional effects. It encompasses the entire process, detailing the interaction between PS and its activators, the fate of reactive oxygen species (ROS), and the transformation of OPs within PS-AOPs. Specifically, the inhibiting mechanisms include the prevention of PS activation, suppression of ROS fate, and conversion of intermediates to their parent compounds. In contrast, the promoting effects involve the enhancement of catalytic effectiveness, contributions to ROS generation, and improved interactions between NOM and OPs. Finally, further studies are required to elucidate the reaction mechanisms of NOM in PS-AOPs and explore the practical applications of PS-AOPs using actual NOM rather than model compounds.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Overlooked Complexation and Competition Effects of Phenolic Contaminants in a Mn(II)/Nitrilotriacetic Acid/Peroxymonosulfate System: Inhibited Generation of Primary and Secondary High-Valent Manganese Species

Organic contaminants with lower Hammett constants are typically more prone to being attacked by reactive oxygen species (ROS) in advanced oxidation processes (AOPs). However, the interactions of an organic contaminant with catalytic centers and participating ROS are complex and lack an in-depth understanding. In this work, we observed an abnormal phenomenon in AOPs that the degradation of electron-rich phenolics, such as 4-methoxyphenol, acetaminophen, and 4-presol, was unexpectedly slower than electron-deficient phenolics in a Mn(II)/nitrilotriacetic acid/peroxymonosulfate (Mn(II)/NTA/PMS) system. The established quantitative structure-activity relationship revealed a volcano-type dependence of the degradation rates on the Hammett constants of pollutants. Leveraging substantial analytical techniques and modeling analysis, we concluded that the electron-rich phenolics would inhibit the generation of both primary (Mn(III)NTA) and secondary (Mn(V)NTA) high-valent manganese species through complexation and competition effects. Specifically, the electron-rich phenolics would form a hydrogen bond with Mn(II)/NTA/PMS through outer-sphere interactions, thereby reducing the electrophilic reactivity of PMS to accept the electron transfer from Mn(II)NTA, and slowing down the generation of reactive Mn(III)NTA. Furthermore, the generated Mn(III)NTA is more inclined to react with electron-rich phenolics than PMS due to their higher reaction rate constants (8314 ± 440, 6372 ± 146, and 6919 ± 31 M-1 s-1 for 4-methoxyphenol, acetaminophen, and 4-presol, respectively, as compared with 671 M-1 s-1 for PMS). Consequently, the two-stage inhibition impeded the generation of Mn(V)NTA. In contrast, the complexation and competition effects are insignificant for electron-deficient phenolics, leading to declined reaction rates when the Hammett constants of pollutants increase. For practical applications, such complexation and competition effects would cause the degradation of electron-rich phenolics to be more susceptible to water matrixes, whereas the degradation of electron-deficient phenolics remains largely unaffected. Overall, this study elucidated the intricate interaction mechanisms between contaminants and reactive metal species at both the electronic and kinetic levels, further illuminating their implications for practical treatment.

Confronting the Mysteries of Oxidative Reactive Species in Advanced Oxidation Processes: An Elephant in the Room

Advanced oxidation processes (AOPs) are rapidly evolving but still lack well-established protocols for reliably identifying oxidative reactive species (ORSs). This Perspective presents both the radical and nonradical ORSs that have been identified or proposed, along with the extensive controversies surrounding oxidative mechanisms. Conventional identification tools, such as quenchers, probes, and spin trappers, might be inadequate for the analytical demands of systems in which multiple ORSs coexist, often yielding misleading results. Therefore, the challenges of identifying these complex, short-lived, and transient ORSs must be fully acknowledged. Refining analytical methods for ORSs is necessary, supported by rigorous experiments and innovative paradigms, particularly through kinetic analysis based on in situ spectroscopic techniques and multiple-probe strategies. To demystify these complex ORSs, future efforts should be made to develop advanced tools and strategies to enhance the mechanism understanding. In addition, integrating real-world conditions into experimental designs will establish a reliable framework in fundamental studies, providing more accurate insights and effectively guiding the design of AOPs.

Catalytic oxidation of Mn(II) in the co-presence of Fe(II) by free chlorine: significance of in situ formed Mn(II)-doped Fe(III) oxides

Fe(II) and Mn(II) are abundant in groundwater and require operationally simple and efficient method to remove in drinking water treatment. The rapid oxidation of Mn(II) is essential in water treatment. This study investigates the efficiency of Mn(II) oxidation by free chlorine in the presence of Fe(II). The results demonstrate that the presence of Fe(II) significantly accelerates the oxidation rate of Mn(II) by free chlorine under neutral and alkaline conditions. The rapid oxidation of Fe(II) by free chlorine and the presence of Mn(II) promote the formation of in situ Mn(II)-doped ferrihydrite. Kinetic modeling and characterization of Fe(III) oxides confirm that the heterogeneous catalytic effect of the Mn(II)-doped ferrihydrite, rather than manganese oxides or their coupled catalytic effect, is responsible for the enhanced oxidation rates. The doped Mn(II) substitutes the tetrahedral Fe(III) ions in the ferrihydrite, introducing additional negative charges at the doped sites. The increased charge enhances Mn(II) adsorption and lowers its redox potential, thereby accelerating Mn(II) oxidation rate through direct electron transfer with adjacent free chlorine. Additionally, the lepidocrocite formed by the reaction between Fe(II) and dissolved oxygen significantly impedes the catalytic performance. These findings provide new insights into the catalytic co-oxidation mechanism of Fe(II) and Mn(II), and help the optimization of water treatment engineering practices.