The Fenton Reaction in Water Assisted by Picolinic Acid: Accelerated Iron Cycling and Co-generation of a Selective Fe-Based Oxidant

Application of iron-modified biochar in the fields of adsorption and degradation of antibiotics

Antibiotics, as emerging contaminants, have led to persistent global pollution issues, prompting long-standing attention on methods for their effective removal. Among various methods, iron-modified biochar stood out for its ability to adsorb and degrade antibiotics in the environment because biochar can provide a porous structure and oxygen-rich functional groups for efficient antibiotic adsorption, while the Fe2+/Fe3+ redox cycle in the iron modification biochar enhanced electronic transmission and further increased degradation. This review systematically summarized preparation methods of different iron-modified biochar, the adsorption capacities, mechanisms, and influencing factors of pollutants. It also explored the co-activation of iron and biochar, which enhanced the release of free radicals through Fenton-like oxidation pathways and accelerated degradation through photocatalytic electron-hole pair production. Additionally, the relationship between adsorption and degradation was discussed. Notably, an environmental risk assessment of iron-modified biochar and disposal of the used iron-modified biochar were discussed, which were critical for practical applications. Finally, the review highlighted the future directions of antibiotic pollution control and the broader potential of iron-modified biochar.

Enhanced ultrasound-assisted controllable growth assembly of MoS2/Fe3O4 materials for piezoelectric Fenton reaction degradation of sildenafil

The Fenton technique is known for its high efficiency and minimal environmental impact; however, its degradation reaction process requires a sustained acidic environment. In this study, nanoflower and cookie-like morphologies of composite piezoelectric materials were synthesized for ultrasound-driven piezoelectric-Fenton degradation of sildenafil. The most effective assembly method involves growing MoS₂ on the rough-surfaced Fe₃O₄ microspheres as a substrate. Characterization and degradation results indicate that these piezoelectric materials utilize different assembly modes when operating under neutral conditions, and evaluate in depth the effect of various factors such as pH, H2O2 dose and ultrasonic (US) power on the degradation of sildenafil. Both composites exhibit the ability to initiate reactions neutrally and sustain some degradation efficacy in a neutral environment. The composite catalyst almost completely removed 50 mg L-1 of sildenafil within 60 min, and AFM characterization showed a piezoelectric response of 56 p.m./V for the core-shell nanoflower catalyst. These data suggest that the assembly of Fe3O4-based surface-grown MoS2 allows constructing of more active sites and provides favourable reaction efficacy for the Fenton reaction. The free radical experiments have shown that ·OH radicals are the main reactive species in the process. This work explored the effect of different assembly methods of composite piezoelectric material on the degradation reaction and provided new ideas for the subsequent construction of composite material systems.

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.

Surface-bound Fe(0) and Fe(II) mediated by 2-picolinic acid functionalized zero-valent iron for highly Cr(VI) removal

Electron transfer of zero-valent iron (ZVI) is significantly impeded by its oxide layer, and limiting its removal of pollutants. In this study, 2-picolinic acid (PA) and ZVI were co-ball milled to improve electron transfer in ZVI (PA-ZVIbm), and used for the removal of heavy metal Cr(VI). Characterization analysis showed that the presence of electron-rich groups on the surface of PA-ZVIbm promoted the transfer of electrons from the Fe(0) core to the surface, and the surface Fe(0) and Fe(II) contents increased from 1.1 % to 6.3 % and from 60.2 % to 72.9 %, respectively, effectively reducing Cr(VI) through an electron transfer mechanism. Theoretical calculations showed that the modification of PA enhanced the adsorption of Cr(VI) on the ZVI surface, and the adsorption energy decreased from -3.561 eV to -5.119 eV. PA-ZVIbm showed strong advantages in the removal of Cr(VI), with a reaction rate constant and adsorption capacity 17 and 13 times that of ZVIbm, respectively, and a conversion rate of 100 %. Moreover, PA-ZVIbm showed excellent performance over a wide pH range (3-10) and under different coexisting ions, while being cost-effective and having low environmental risks. This study explored the relationship between ZVI surface modification and performance, and provided new insights into the modification of ZVI using small molecule oxygen-containing organic acids.

Unexpected chloride-triggered organics removal in the zirconium oxide activated peroxymonosulfate system

Chloride ion (Cl-) is ubiquitous in diverse water bodies, yet poses a longstanding challenge in water pollution control by hindering the efficiency of pollutant degradation. Herein, we proposed a novel concept involving the direct utilization of endogenous Cl- ions in water for rapid water purification within a non-redox zirconium oxide (ZrO2)-activated peroxymonosulfate (PMS) system. In this process, PMS was complexed on the ZrO2 surface through inner-sphere coordination, and effectively activated by the partial electron cloud deviation from Zr(IV) sites to PMS, thereby forming a metastable surface complex with an elevated redox potential. Afterwards, the coexistence of Cl- could trigger the transformation of the reactive complex into free chlorine species, thus leading to a 255.0-fold enhancement in the elimination rate of micropollutants compared with the ZrO2/PMS system. Quantitative structure-activity relationship analysis revealed that the ZrO2/PMS/Cl- system displayed strong target-dependence towards electron-rich compounds, showcasing a faster oxidation rate for pollutants with higher EHOMO energy levels. Significantly, the novel system performed robust resistance to complex water matrices, achieved low oxidant consumption for pollutant removal, and demonstrated adaptation across a broad range of Cl- concentrations (1.0-100.0 mM). Overall, our findings provide new mechanistic insights into the influence of Cl- ions on PMS activation, which refresh the understanding of the role of Cl- ions on pollutant degradation, and help to guide the treatment design for chloride-containing wastewater.

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.

The profound review of Fenton process: What's the next step?

Fenton and Fenton-like processes, which could produce highly reactive species to degrade organic contaminants, have been widely used in the field of wastewater treatment. Therein, the chemistry of Fenton process including the nature of active oxidants, the complicated reactions involved, and the behind reason for its strongly pH-dependent performance, is the basis for the application of Fenton and Fenton-like processes in wastewater treatment. Nevertheless, the conflicting views still exist about the mechanism of the Fenton process. For instance, reaching a unanimous consensus on the nature of active oxidants (hydroxyl radical or tetravalent iron) in this process remains challenging. This review comprehensively examined the mechanism of the Fenton process including the debate on the nature of active oxidants, reactions involved in the Fenton process, and the behind reason for the pH-dependent degradation of contaminants in the Fenton process. Then, we summarized several strategies that promote the Fe(II)/Fe(III) cycle, reduce the competitive consumption of active oxidants by side reactions, and replace the Fenton reagent, thus improving the performance of the Fenton process. Furthermore, advances for the future were proposed including the demand for the high-accuracy identification of active oxidants and taking advantages of the characteristic of target contaminants during the degradation of contaminants by the Fenton process.

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.

Nonheme iron catalyst mimics heme-dependent haloperoxidase for efficient bromination and oxidation

The [Fe]/H2O2 oxidation system has found wide applications in chemistry and biology. Halogenation with this [Fe]/H2O2 oxidation protocol and halide (X-) in the biological system is well established with the identification of heme-iron-dependent haloperoxidases. However, mimicking such halogenation process is rarely explored for practical use in organic synthesis. Here, we report the development of a nonheme iron catalyst that mimics the heme-iron-dependent haloperoxidases to catalyze the generation of HOBr from H2O2/Br- with high efficiency. We discovered that a tridentate terpyridine (TPY) ligand designed for Fenton chemistry was optimal for FeBr3 to form a stable nonheme iron catalyst [Fe(TPY)Br3], which catalyzed arene bromination, Hunsdiecker-type decarboxylative bromination, bromolactonization, and oxidation of sulfides and thiols. Mechanistic studies revealed that Fenton chemistry ([Fe]/H2O2) might operate to generate hydroxyl radical (HO•), which oxidize bromide ion [Br-] into reactive HOBr. This nonheme iron catalyst represents a biomimetic model for heme-iron-dependent haloperoxidases with potential applications in organic synthesis, drug discovery, and biology.

Co-catalysis strategy for low-oxidant-consumption Fenton-like chemistry: From theoretical understandings to practical applications and future guiding strategies

Recently, great effects have been made for the co-catalysis strategy to solve the bottlenecks of Fenton system. A series of co-catalysis strategies using various inorganic metal co-catalysts and organic co-catalysts have been developed in various oxidant (i.e., hydrogen peroxide (H2O2) and persulfate) systems with significantly promotion of catalytic performances and lower oxidant consumption (only 5-10 % of conventional Fenton/Fenton-like systems). However, the developments of these co-catalysis strategies from theoretical understandings to practical applications and future guiding strategies were overlooked, which was an essential problem that must be considered for the future scale-up applications of co-catalysis systems. In this paper, these co-catalysis strategies with low-oxidant-consumption characteristics have been reviewed by the comparison of their co-catalysis mechanisms, as well as their advantages and disadvantages. We also discussed the recent developments of amplifying devices based on the co-catalysis systems. The scale-up performances of co-catalysis strategies based on these amplifying devices have also been assessed. In addition, future guiding strategies for the development of co-catalysis strategy with low-oxidant-consumption characteristics have also been first time outlined by the combination of the technical-economic analysis (TEA), life cycle assessment (LCA) and machine learning (ML). Finally, the paper systematically discusses the development opportunities, technical bottlenecks and future development directions of co-catalysis strategies with the prospect of large-scale applications. Basically, this work provides a systematic review on co-catalysis strategy with low-oxidant-consumption characteristic from theoretical understandings to practical applications and future guiding strategies.

Redefining Roles: A Paradigm Shift in Tryptophan-Kynurenine Metabolism for Innovative Clinical Applications

The tryptophan-kynurenine (KYN) pathway has long been recognized for its essential role in generating metabolites that influence various physiological processes. Traditionally, these metabolites have been categorized into distinct, often opposing groups, such as pro-oxidant versus antioxidant, excitotoxic/neurotoxic versus neuroprotective. This dichotomous framework has shaped much of the research on conditions like neurodegenerative and neuropsychiatric disorders, as well as cancer, where metabolic imbalances are a key feature. The effects are significantly influenced by various factors, including the concentration of metabolites and the particular cellular milieu in which they are generated. A molecule that acts as neuroprotective at low concentrations may exhibit neurotoxic effects at elevated levels. The oxidative equilibrium of the surrounding environment can alter the function of KYN from an antioxidant to a pro-oxidant. This narrative review offers a comprehensive examination and analysis of the contemporary understanding of KYN metabolites, emphasizing their multifaceted biological functions and their relevance in numerous physiological and pathological processes. This underscores the pressing necessity for a paradigm shift in the comprehension of KYN metabolism. Understanding the context-dependent roles of KYN metabolites is vital for novel therapies in conditions like Alzheimer's disease, multiple sclerosis, and cancer. Comprehensive pathway modulation, including balancing inflammatory signals and enzyme regulation, offers promising avenues for targeted, effective treatments.

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.

Simultaneous Emerging Contaminant Removal and H2O2 Generation Through Electron Transfer Carrier Effect of Bi─O─Ce Bond Bridge Without External Energy Consumption

Conventional advanced oxidation processes (AOPs) require significant external energy consumption to eliminate emerging contaminants (ECs) with stable structures. Herein, a catalyst consisting of nanocube BiCeO particles (BCO-NCs) prepared by an impregnation-hydrothermal process is reported for the first time, which is used for removing ECs without light/electricity or any other external energy input in water and simultaneous in situ generation of H2O2. A series of characterizations and experiments reveal that dual reaction centers (DRC) which are similar to the valence band/conducting band structure are formed on the surface of BCO-NCs. Under natural conditions without any external energy consumption, the BCO-NCs self-purification system can remove more than 80% of ECs within 30 min, and complete removal of ECs within 30 min in the presence of abundant electron acceptors, the corresponding second-order kinetic constant is increased to 3.62 times. It is found that O2 can capture electrons from ECs through the Bi─O─Ce bond bridge during the reaction process, leading to the in situ production of H2O2. This work will be a key advance in reducing energy consumption for deep wastewater treatment and generating important chemical raw materials.

Oxygen-Centered Organic Radicals-Involved Unified Heterogeneous Self-Fenton Process for Stable Mineralization of Micropollutants in Water

Removing organic micropollutants from water through photocatalysis is hindered by catalyst instability and substantial residuals from incomplete mineralization. Here, a novel water treatment paradigm, the unified heterogeneous self-Fenton process (UHSFP), which achieved an impressive 32% photon utilization efficiency at 470 nm, and a significant 94% mineralization of organic micropollutants-all without the continual addition of oxidants and iron ions is presented. In UHSFP, the active species differs fundamentally from traditional photocatalytic processes. One electron acceptor unit of photocatalyst acquires only one photogenerated electron to convert into oxygen-centered organic radical (OCOR), then spontaneously completing subsequent processes, including pollutant degradation, hydrogen peroxide generation, activation, and mineralization of organic micropollutants. By bolstering electron-transfer capabilities and diminishing catalyst affinity for oxygen in the photocatalytic process, the generation of superoxide radicals is effectively suppressed, preventing detrimental attacks on the catalyst. This study introduces an innovative and cost-effective strategy for the efficient and stable mineralization of organic micropollutants, eliminating the necessity for continuous chemical inputs, providing a new perspective on water treatment technologies.

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.

Doped Cu0 and sulfidation induced transition from R-O• to •OH in peracetic acid activation by sulfidated nano zero-valent iron-copper

Peracetic acid (PAA) has emerged as a new effective oxidant for various contaminants degradation through advanced oxidation process (AOP). In this study, sulfidated nano zero-valent iron-copper (S-nZVIC) with low Cu doping and sulfidation was synthesized for PAA activation, resulting in more efficient degradation of sulfamethoxazole (SMX, 20 μM) and other contaminants using a low dose of catalyst (0.05 g/L) and oxidant (100 μM). The characterization results suggested that S-nZVIC presented a more uniform size and distribution with fewer metal oxides, as the agglomeration and oxidation were inhibited. More significantly, doped Cu0 and sulfidation significantly enhanced the generation and contribution of •OH but decreased that of R-O• in S-nZVIC/PAA/SMX system compared with that of nZVIC and S-nZVI, accounting for the relatively high degradation efficiency of 97.7% in S-nZVIC/PAA/SMX system compared with 85.7% and 78.9% in nZVIC/PAA/SMX and S-nZVI/PAA/SMX system, respectively. The mechanisms underlying these changes were that (i) doped Cu° could promote the regeneration of Fe(Ⅱ) for strengthened PAA activation through mediating Fe(Ⅱ)/Fe(Ⅲ) cycle by Cu(Ⅰ)/Cu(Ⅱ) cycle; (ii) S species might consume part of R-O•, resulting in a decreased contribution of R-O• in SMX degradation; (iii) sulfidation increased the electrical conductivity, thus facilitating the electron transfer from S-nZVIC to PAA. Consequently, the dominant reactive oxygen species transited from R-O• to •OH to degrade SMX more efficiently. The degradation pathways, intermediate products and toxicity were further analyzed through density functional theory (DFT) calculations, liquid chromatography-mass spectrometry (LC-MS) and T.E.S.T software analysis, which proved the environmental friendliness of this process. In addition, S-nZVIC exhibited high stability, recyclability and degradation efficiency over a wide pH range (3.0∼9.0). This work provides a new insight into the rational design and modification of nano zero-valent metals for efficient wastewater treatment through adjusting the dominant reactive oxygen species (ROS) into the more active free radicals.

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

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

Bias distribution and regulation in photoelectrochemical overall water-splitting cells

The water oxidation half-reaction at anodes is always considered the rate-limiting step of overall water splitting (OWS), but the actual bias distribution between photoanodes and cathodes of photoelectrochemical (PEC) OWS cells has not been investigated systematically. In this work, we find that, for PEC cells consisting of photoanodes (nickel-modified n-Si [Ni/n-Si] and α-Fe2O3) with low photovoltage (Vph < 1 V), a large portion of applied bias is exerted on the Pt cathode for satisfying the hydrogen evolution thermodynamics, showing a thermodynamics-controlled characteristic. In contrast, for photoanodes (TiO2 and BiVO4) with Vph > 1 V, the bias required for cathode activation can be significantly reduced, exhibiting a kinetics-controlled characteristic. Further investigations show that the bias distribution can be regulated by tuning the electrolyte pH and using alternative half-reaction couplings. Accordingly, a volcano plot is presented for the rational design of the overall reactions and unbiased PEC cells. Motivated by this, an unbiased PEC cell consisting of a simple Ni/n-Si photoanode and Pt cathode is assembled, delivering a photocurrent density of 5.3 ± 0.2 mA cm-2.

Effect of interaction between dissolved organic matter and iron/manganese (hydrogen) oxides on the degradation of organic pollutants by in-situ advanced oxidation techniques

Iron and manganese (hydrogen) oxides (IMHOs) exhibit excellent redox capabilities for environmental pollutants and are commonly used in situ chemical oxidation (ISCO) technologies for the degradation of organic pollutants. However, the coexisting dissolved organic matter (DOMs) in surface environments would influence the degradation behavior and fate of organic pollutants in IMHOs-based ISCO. This review has summarized the interactions and mechanisms between DOMs and IMHOs, as well as the properties of DOM-IMHOs complexes. Importantly, the promotion or inhibition impact of DOM was discussed from three perspectives. First, the presence of DOMs may hinder the accessibility of active sites on IMHOs, thus reducing their efficiency in degrading organic pollutants. The formation of compounds between DOMs and IMHOs alters their stability and activity in the degradation process. Second, the presence of DOMs may also affect the generation and transport of active species, thereby influencing the oxidative degradation process of organic pollutants. Third, specific components within DOMs also participate and affect the degradation pathways and rates. A comprehensive understanding of the interaction between DOMs and IMHOs helps to better understand and predict the degradation process of organic pollutants mediated by IMHOs in real environmental conditions and contributes to the further development and application of IMHO-mediated ISCO technology.

Infancy of peracetic acid activation by iron, a new Fenton-based process: A review

The exacerbated global water scarcity and stricter water directives are leading to an increment in the recycled water use, requiring the development of new cost-effective advanced water treatments to provide safe water to the population. In this sense, peracetic acid (PAA, CH3C(O)OOH) is an environmentally friendly disinfectant with the potential to challenge the dominance of chlorine in large wastewater treatment plants in the near future. PAA can be used as an alternative oxidant to H2O2 to carry out the Fenton reaction, and it has recently been proven as more effective than H2O2 towards emerging pollutants degradation at circumneutral pH values and in the presence of anions. PAA activation by homogeneous and heterogeneous iron-based materials generates - besides HO• and FeO2+ - more selective CH3C(O)O• and CH3C(O)OO• radicals, slightly scavenged by typical HO• quenchers (e.g., bicarbonates), which extends PAA use to complex water matrices. This is reflected in an exponential progress of iron-PAA publications during the last few years. Although some reviews of PAA general properties and uses in water treatment were recently published, there is no account on the research and environmental applications of PAA activation by Fe-based materials, in spite of its gratifying progress. In view of these statements, here we provide a holistic review of the types of iron-based PAA activation systems and analyse the diverse iron compounds employed to date (e.g., ferrous and ferric salts, ferrate(VI), spinel ferrites), the use of external ferric reducing/chelating agents (e.g., picolinic acid, l-cysteine, boron) and of UV-visible irradiation systems, analysing the mechanisms involved in each case. Comparison of PAA activation by iron vs. other transition metals (particularly cobalt) is also discussed. This work aims at providing a thorough understanding of the Fe/PAA-based processes, facilitating useful insights into its advantages and limitations, overlooked issues, and prospects, leading to its popularisation and know-how increment.

Novel dual-responsive hydrogel composed of polyacrylamide/Fe-MOF/zinc finger peptide for construction of electrochemical sensing platform

Responsive hydrogels have received much attention for improving the detection performance of electrochemical sensors because of their special responsiveness. However, current responsive hydrogels generally suffer from long response times, ranging from tens of minutes to several hours. This situation severely limits the detection performance and practical application of electrochemical sensors. Here, an electrochemical sensing platform was constructed by employing dual-responsive polyacrylamide/zinc finger peptide/Fe-MOF hydrogel (PZFH) as the silent layer, sodium alginate-Ni2+-graphene oxide hydrogel as the signal layer. GOx@ZIF-8, as the immunoprobe, catalyzed glucose to H2O2 and gluconic acid, resulting in the cleavage of immunoprobe as the pH decreased and subsequent release of Zn2+ ions. During the process of Fe-MOF converting from Fe3+ to Fe2+, free radicals were generated and used to destroy the structure of the PZFH. Cysteine and histidine in the zinc finger peptide can specifically bind to Zn2+ to create many pores in PZFH, exposing the signal layer. These synergistic effects rapidly decreased the impedance of PZFH and increased the electrochemical signal of Ni2+. The electrochemical sensing platform was used to detect pro-gastrin-releasing peptide with response times as short as 7 min of PZFH, a wide linear range from 100 ng mL-1 to 100 fg mL-1, and an ultra-low limit of detection of 14.24 fg mL-1 (S/N = 3). This strategy will provide a paradigm for designing electrochemical sensors.

Enhanced chalcopyrite-catalyzed heterogeneous Fenton oxidation of diclofenac by ABTS

The widely used 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) has gained growing attention in advanced oxidation processes (AOPs), whereas there was limited knowledge regarding the feasibility of ABTS in enhancing heterogeneous Fenton oxidation so far. Hereof, ABTS was introduced into the chalcopyrite (CuFeS2)- catalyzed heterogeneous Fenton oxidation process to degrade diclofenac (DCF), and the degradation efficiency was enhanced by 25.5% compared with CuFeS2/H2O2 process. The available reactive oxygen species (ROS) and the enhanced mechanism were elaborated. Experimental results uncovered that •OH was the dominant reactive species responsible for the DCF degradation in the CuFeS2/H2O2/ABTS process, and ABTS•+ was derived from both •OH and Fe(IV). The presence of ABTS contributed significantly to the redox cycle of surface Fe of CuFeS2, and the roles of reductive sulfur species and surface Cu(I) in promoting surface Fe cycling also could not be neglected. In addition, the effects of several influencing factors were considered, and the potential practicability of this oxidation process was examined. The results demonstrate that the CuFeS2/H2O2/ABTS process would be a promising approach for water purification. This study will contribute to the development of enhancing strategies using ABTS as a redox mediator for heterogeneous Fenton oxidation of pharmaceuticals.

Nanoconfinement-triggered oligomerization pathway for efficient removal of phenolic pollutants via a Fenton-like reaction

Heterogeneous Fenton reaction represents one of the most reliable technologies to ensure water safety, but is currently challenged by the sluggish Fe(III) reduction, excessive input of chemicals for organic mineralization, and undesirable carbon emission. Current endeavors to improve the catalytic performance of Fenton reaction are mostly focused on how to accelerate Fe(III) reduction, while the pollutant degradation step is habitually overlooked. Here, we report a nanoconfinement strategy by using graphene aerogel (GA) to support UiO-66-NH2-(Zr) binding atomic Fe(III), which alters the carbon transfer route during phenol removal from kinetically favored ring-opening route to thermodynamically favored oligomerization route. GA nanoconfinement favors the Fe(III) reduction by enriching the reductive intermediates and allows much faster phenol removal than the unconfined analog (by 208 times in terms of first-order rate constant) and highly efficient removal of total organic carbon, i.e., 92.2 ± 3.7% versus 3.6 ± 0.3% in 60 min. Moreover, this oligomerization route reduces the oxidant consumption for phenol removal by more than 95% and carbon emission by 77.9%, compared to the mineralization route in homogeneous Fe2++H2O2 system. Our findings may upgrade the regulatory toolkit for Fenton reactions and provide an alternative carbon transfer route for the removal of aqueous pollutants.

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

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

Unveiling the traits of dry and wet pre-magnetized zero-valent iron-activated peroxymonosulfate: Degradation of oxytetracycline

It has been previously reported that pre-magnetization could enhance the efficacy of zero-valent iron (ZVI) in removing contaminants. However, little is known about the effects and persistence of different magnetization methods on pre-magnetized ZVI (Pre-ZVI) when used in advanced oxidation processes (AOPs). Gaining a comprehensive understanding of the durability of various pre-magnetization methods in enhancing the removal efficiency of different pollutants will significantly impact the widespread utilization of Pre-ZVI in practical engineering. Herein, we investigated the efficiency of dry and wet Pre-ZVI-activated peroxymonosulfate (PMS) in eliminating oxytetracycline (OTC) and evaluated the durability of Pre-ZVI. Additionally, we examined several factors that influence the degradation process's efficiency. Our results found that the reaction constant k values corresponding to the dry Pre-ZVI/PMS system at the pH values of 3, 7, and 9 varied from approximately 0.0384, 0.0331, and 0.0349 (day 1) to roughly 0.0297, 0.0278, and 0.0314 (day 30), respectively. Meanwhile, the wet Pre-ZVI/PMS system exhibited k values ranging from approximately 0.0392, 0.0349, and 0.0374 (day 1) to roughly 0.0380, 0.0291, and 0.0322 (day 30), respectively. Moreover, we proposed four OTC degradation pathways using LC-MS/MS and density functional theory calculations. The toxicity of the degradation products was assessed using the ecological structure activity relationship and the toxicity estimation software tool. Overall, this study provides insights into the application of Pre-ZVI/PMS that can be selectively used to eliminate tetracycline antibiotics from water.

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

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

Picolinic Acid-Mediated Catalysis of Mn(II) for Peracetic Acid Oxidation Processes: Formation of High-Valent Mn Species

Metal-based advanced oxidation processes (AOPs) with peracetic acid (PAA) have been extensively studied to degrade micropollutants (MPs) in wastewater. Mn(II) is a commonly used homogeneous metal catalyst for oxidant activation, but it performs poorly with PAA. This study identifies that the biodegradable chelating ligand picolinic acid (PICA) can significantly mediate Mn(II) activation of PAA for accelerated MP degradation. Results show that, while Mn(II) alone has minimal reactivity toward PAA, the presence of PICA accelerates PAA loss by Mn(II). The PAA-Mn(II)-PICA system removes various MPs (methylene blue, bisphenol A, naproxen, sulfamethoxazole, carbamazepine, and trimethoprim) rapidly at neutral pH, achieving >60% removal within 10 min in clean and wastewater matrices. Coexistent H2O2 and acetic acid in PAA play a negligible role in rapid MP degradation. In-depth evaluation with scavengers and probe compounds (tert-butyl alcohol, methanol, methyl phenyl sulfoxide, and methyl phenyl sulfone) suggested that high-valent Mn species (Mn(V)) is a likely main reactive species leading to rapid MP degradation, whereas soluble Mn(III)-PICA and radicals (CH3C(O)O• and CH3C(O)OO•) are minor reactive species. This study broadens the mechanistic understanding of metal-based AOPs using PAA in combination with chelating agents and indicates the PAA-Mn(II)-PICA system as a novel AOP for wastewater treatment.

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

Improving the reactivity of Fe(III) for activating peroxymonosulfate (PMS) at circumneutral pH is critical to propel the iron-activated PMS processes toward practical wastewater treatment but is yet challenging. Here we employed the complexes of Fe(III) with the biodegradable picolinic acid (PICA) to activate PMS for degradation of selected chlorinated phenols, antibiotics, pharmaceuticals, herbicides, and industrial compounds at pH 4.0-6.0. The FeIII-PICA complexes greatly outperformed the ligand-free Fe(III) and other Fe(III) complexes of common aminopolycarboxylate ligands. In the main activation pathway, the key intermediate is a peroxymonosulfate complex, tentatively identified as PICA-FeIII-OOSO3-, which undergoes O-O homolysis or reacts with FeIII-PICA and PMS to yield FeIV=O and SO4•- without the involvement of commonly invoked Fe(II). PICA-FeIII-OOSO3- can also react directly with certain compounds (chlorophenols and sulfamethoxazole). The relative contributions of PICA-FeIII-OOSO3-, FeIV=O, and SO4•- depend on the structure of target compounds. This work sets an eligible example to enhance the reactivity of Fe(III) toward PMS activation by ligands and sheds light on the previously unrecognized role of the metal-PMS complexes in directing the catalytic cycle and decontamination as well.

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

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

Self-carbon-thermal-reduction strategy for boosting the Fenton-like activity of single Fe-N4 sites by carbon-defect engineering

Carbon-defect engineering in metal single-atom catalysts by simple and robust strategy, boosting their catalytic activity, and revealing the carbon defect-catalytic activity relationship are meaningful but challenging. Herein, we report a facile self-carbon-thermal-reduction strategy for carbon-defect engineering of single Fe-N4 sites in ZnO-Carbon nano-reactor, as efficient catalyst in Fenton-like reaction for degradation of phenol. The carbon vacancies are easily constructed adjacent to single Fe-N4 sites during synthesis, facilitating the formation of C-O bonding and lowering the energy barrier of rate-determining-step during degradation of phenol. Consequently, the catalyst Fe-NCv-900 with carbon vacancies exhibits a much improved activity than the Fe-NC-900 without abundant carbon vacancies, with 13.5 times improvement in the first-order rate constant of phenol degradation. The Fe-NCv-900 shows high activity (97% removal ratio of phenol in only 5 min), good recyclability and the wide-ranging pH universality (pH range 3-9). This work not only provides a rational strategy for improving the Fenton-like activity of metal single-atom catalysts, but also deepens the fundamental understanding on how periphery carbon environment affects the property and performance of metal-N4 sites.

New Insights into Selective Singlet Oxygen Production via the Typical Electroactivation of Oxygen for Water Decontamination

Selective production of singlet oxygen (1O2) as an electrophilic oxidant is crucial for the precise control of chemical targets in environmental fields. Herein, we proposed a strategy to construct a redox interface on electrodes, which can in situ produce inorganic metal hydroperoxides with appropriate oxidative ability during oxygen activation. Benefiting from atomic Cu sites (CuN4) in a copper-carbon aerogel electrode, almost complete production of 1O2 was achieved, thereby refraining the competitive formation of other reactive oxygen species. The fast electron transfer rate between CuN4 and electrogenerated H2O2 promoted the in situ formation of copper hydroperoxide (N4-Cu-OOH), thereby selectively and efficiently oxidizing intermediate O2•- to 1O2. The optimized production of 1O2 was up to 2583 μmol L-1 without additional chemical reagents. We further considered the high production of 1O2 for efficiently removing electron-rich organic pollutants from a complex water matrix. Fast kinetics was achieved and considered for removing various pollutants with electron-donating substituents in a nonradical oxidation pathway. The BPA degradation efficiency is less susceptible to the coexisting natural organic matter (NOM) and inorganic ions. Specifically, the kinetic constant for BPA removal is 34 times higher than that for a nanoparticle of a copper-carbon electrode while producing a hydroxyl radical. Our findings highlight the innovative interfacial surface engineering of an electrocatalytic O2 activation system to selectively generate 1O2 for future potential applications.

Enhanced Oxidation of Organic Compounds by the Ferrihydrite-Ferrate System: The Role of Intramolecular Electron Transfer and Intermediate Iron Species

Previous studies mostly held that the oxidation capacity of ferrate depends on the involvement of intermediate iron species (i.e., FeIV/FeV), however, the potential role of the metastable complex was disregarded in ferrate-based heterogeneous catalytic oxidation processes. Herein, we reported a complexation-mediated electron transfer mechanism in the ferrihydrite-ferrate system toward sulfamethoxazole (SMX) degradation. A synergy between intermediate FeIV/FeV oxidation and the intramolecular electron transfer step was proposed. Specifically, the conversion of phenyl methyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2) suggested that FeIV/FeV was involved in the oxidation of SMX. Moreover, based on the in situ Raman test and chronopotentiometry analysis, the formation of the metastable complex of ferrihydrite/ferrate was found, which possesses higher oxidation potential than free ferrate and could achieve the preliminary oxidation of organics via the electron transfer step. In addition, the amino group of SMX could complex with ferrate, and the resulting metastable complex of ferrihydrite/ferrate would combine further with SMX molecules, leading to intramolecular electron transfer and SMX degradation. The ferrate loss experiments suggested that ferrihydrite could accelerate the decomposition of ferrate. Finally, the effects of pH value, anions, humic acid, and actual water on the degradation of SMX by ferrihydrite-ferrate were also revealed. Overall, ferrihydrite demonstrated high catalytic capacity, good reusability, and nontoxic performance for ferrate activation. The ferrihydrite-ferrate process may be a green and promising method for organic removal in wastewater treatment.

Boron Bifunctional Catalysts for Rapid Degradation of Persistent Organic Pollutants in a Metal-Free Electro-Fenton Process: O2 and H2O2 Activation Process

Metals usually served as the active sites of the heterogeneous bifunctional electro-Fenton reaction, which faced the challenge of poor stability under acidic or even neutral conditions. Exploring a metal-free heterogeneous bifunctional electro-Fenton catalyst can effectively solve the above problems. In this work, a stable metal-free heterogeneous bifunctional boron-modified porous carbon catalyst (BTA-1000) was synthesized. For the BTA-1000 catalyst, the yield of H2O2 (294 mg/L) significantly increased. The degradation rate of phenol by BTA-1000 (0.242 min-1) increased by an order of magnitude, compared with the porous carbon catalyst (0.0105 min-1). The BTA catalyst could rapidly degrade industrial dye wastewater, and its specific energy consumption was 5.52 kW h kg-1 COD-1, lower than that in previous reports (6.38-7.4 kW h kg-1 COD-1). DFT and XPS revealed that C═O and -BC2O groups jointly promoted the generation of H2O2, and the -BCO2 group played dominant roles in the generation of •OH because the oxygen atom near the electron-giving groups (-BCO2 group) facilitated the formation of hydrogen bond and H2O2 adsorption. This work gained deep insights into the reaction mechanism of the boron-modified porous carbon catalyst, which helped to guide the development of metal-free heterogeneous bifunctional electro-Fenton catalysts.

Quantitative probing of reactive oxygen species and their selective degradation on contaminants in peroxymonosulfate-based process enhanced by picolinic acid

The processes of Fe(III) activated peroxymonosulfate (PMS) in degrading contaminants have been extensively studied. Herein, a biodegradable chelating agent, picolinic acid (PICA), was introduced to the PMS/Fe(III) process to improve the reaction efficiency. The emphases of this study were placed on the quantification of steady-state concentrations of reactive oxygen species (ROS). Experiments presented that five types of ROS, including Fe(IV), SO4•-, HO•, 1O2 and O2•- coexisted in this system. Four typical probe compounds were used to quantify the steady-state concentration of ROS under different variables. The steady-state concentration of Fe(IV) ([Fe(IV)]ss) was 3-5 orders of magnitude higher than that of other ROS, followed by 1O2 and SO4•-, whereas HO• had the lowest concentration. The reaction between PMS and PICA was first explored in our study and results showed that 1O2 and O2•- would form in this reaction. Owing to the hybrid oxidation by multiple ROS, this system showed high oxidation capacity, and could effectively degrade a variety of pollutants. The contributions of ROS to the alleviation of pollutants varied depending on their concentrations and specific reactivity of substrates. Generally, organic contaminants with phenol structures were prone to react with Fe(IV). Overall, this study compared the steady-state concentrations of different ROS and revealed the intrinsic ROS formation mechanisms.

Metal Borides as Excellent Co-Catalysts for Boosted and Long-Lasting Fenton-like Reaction: Dual Co-Catalytic Centers of Metal and Boron

The continuous electron supply for oxidant decomposition-induced reactive oxygen species (ROS) generation is the main contributor for the long-standing micropollutant oxidation in the iron-based advanced oxidation processes (AOPs). Herein, as a new class of co-catalysts, metal borides with dual active sites and preeminent conductive performance can effectively overcome the inherent drawback of Fenton-like reactions by steadily donating electrons to inactive Fe(III). Among the metal borides, tungsten boride (WB) exhibits a significant co-catalytic performance run ahead of common heterogeneous co-catalysts and exceptionally high stability. Based on qualitative and semi-quantitative tests, the hydroxyl radical, sulfate radical, and iron(IV)-oxo complex are all produced in the WB/Fe(III)/PDS system and Fe(IV)-induced methyl phenyl sulfoxide decomposition is up to 72%. Moreover, the production efficiency of ROS and relative proportions of radical and nonradical pathways change with various experimental conditions (dosages of PDS, WB, and solution pH) and water matrices. The rate-determining step of Fe(II) regeneration is greatly accelerated resulting from the synergetic effect between exposed metallic reactive sites and nonmetallic boron with reductive properties of WB. In addition, the self-dissolution of surface tungsten oxide and boron oxide leads to a renovated surface for sustainable Fe(III) reduction in long-term operations. Our discovery provides an efficient and sustainable strategy in the field of enhanced AOPs for water remediation.

Dithionite accelerated copper slag heterogeneous-homogeneous coupled Fenton degradation of organic pollutants

The heterogeneous-homogeneous coupled Fenton (HHCF) processes combine the advantages of rapid reaction and the catalyst reuse, which makes them attractive for wastewater treatment. Nevertheless, the lack of both, cost-effective catalysts and the desirable Fe³⁺/Fe²⁺ conversion mediators limit the development of HHCF processes. This study investigates a prospective HHCF process, in which solid waste copper slag (CS) and dithionite (DNT) act as catalyst and mediator of Fe³⁺/Fe²⁺ transformation, respectively. DNT enables controlled leaching of iron and a highly efficient homogeneous Fe³⁺/Fe²⁺ cycle by dissociating to SO₂^{- •} under acidic conditions, leading to the enhanced H₂O₂ decomposition and ^•OH generation (from 48 μmol/L to 399 μmol/L) for p-chloroaniline (p-CA) degradation. The removal rate of p-CA in the CS/DNT/H₂O₂ system increased by 30 times in comparison with the CS/H₂O₂ system (increased from 1.21 × 10^-3 min^-1 to 3.61 × 10^-2 min^-1). Moreover, batch dosing of H₂O₂ can greatly promote the yield of ^•OH (from 399 μmol/L to 627 μmol/L), by mitigating the side reactions between H₂O₂ and SO₂^{- •}. This study highlights the importance of the iron cycle regulation for improvement of the Fenton efficiency and develops a cost-effective Fenton system for organic contaminants elimination in wastewater.

An extra-chelator-free fenton process assisted by electrocatalytic-induced in-situ pollutant carboxylation for target refractory organic efficient treatment in chemical-industrial wastewater

For traditional Fenton processes, the quenching behavior of radical contenders (e.g., most aliphatic hydrocarbons) on hydroxyl radicals (·OH) usually hinders the removal of target refractory pollutants (aromatic/heterocyclic hydrocarbons) in chemical industrial wastewater, leading to excess energy consumption. Herein, we proposed an electrocatalytic-assisted chelation-Fenton (EACF) process, with no extra-chelator addition, to significantly enhance target refractory pollutant (pyrazole as a representative) removal under high ·OH contender (glyoxal) levels. Experiments and theoretical calculations proved that superoxide radical (·O₂^-) and anodic direct electron transfer (DET) effectively converted the strong ·OH-quenching substance (glyoxal) to a weak radical competitor (oxalate) during the electrocatalytic oxidation process, promoting Fe²⁺ chelation and therefore increasing radical utilization for pyrazole degradation (reached maximum of ∼43-fold value upon traditional Fenton), which appeared more obviously in neutral/alkaline Fenton conditions. For actual pharmaceutical tailwater treatment, the EACF achieved 2-folds higher oriented-oxidation capability and ∼78% lower operation cost per pyrazole removal than the traditional Fenton process, demonstrating promising potential for future practical applications.

Accelerated electron transfer process via MOF-derived FeCo/C for enhanced degradation of antibiotic contaminants towards heterogeneous electro-Fenton system

The Fe(III) to Fe(II) process limits the rate of the electro-Fenton system. In this study, MIL-101(Fe) derived porous carbon skeleton-coated FeCo bimetallic catalyst Fe₄/Co@PC-700 was prepared as a heterogeneous electro-Fenton (EF) catalytic process. The experimental results showed its good performance in catalytic removal of antibiotic contaminants, the rate constant of tetracycline (TC) degradation catalyzed by Fe₄/Co@PC-700 was 8.93 times higher than that of Fe@PC-700 under the pH conditions of raw water (pH = 5.86), exhibited good removal of TC, oxytetracycline (OTC), hygromycin (CTC), chloramphenicol (CAP) and ciprofloxacin (CIP). It was shown that the introduction of Co promoted more Fe⁰ production, allowing the material to exhibit faster Fe(III)/Fe(II) cycling rates. ¹O₂ and high-priced metal oxygen species were identified as the main active species of the system, in addition to the analysis of possible degradation pathways and toxicity of intermediates of TC. Finally, the stability and adaptability of Fe₄/Co@PC-700 and EF systems to different water matrices were evaluated, showing that Fe₄/Co@PC-700 was easy to recover and could be applied to different water matrices. This study provides a reference for the design and system application of heterogeneous EF catalysts.

FeMoS2 micoroparticles as an excellent catalyst for the activation of peroxymonosulfate toward organic contaminant degradation

The FeMoS₂ catalyst for activating peroxymonosulfate (PMS) is a promising pathway for removing organic pollutants in wastewater, however, the dominant FeS₂ phases and sulfur (S) vacancies in it are little involved. Herein, for the first time, novel bimetallic FeMoS₂ microparticles were synthesized by a simple method and then applied for PMS activation for degrading organic pollutants. The catalysts were characterized by several techniques, including X-ray diffraction and X-ray photoelectron spectroscopies. The results revealed that new FeMoS₂ microparticles containing S vacancies in the main FeS₂ phases were obtained. FeS₂ and S vacancies were found to play important roles for activating PMS by radical and nonradical pathways. More Fe²⁺ and Mo⁴⁺ were formed in the presence of S vacancies, which offered a new strategy for exploring novel heterogeneous catalysts in the activation of PMS for environmental remediation.

Redox-Active Polymers as Robust Electron-Shuttle Co-Catalysts for Fast Fe3+/Fe2+ Circulation and Green Fenton Oxidation

Accelerating the rate-limiting Fe³⁺/Fe²⁺ circulation in Fenton reactions through the addition of reducing agents (or co-catalysts) stands out as one of the most promising technologies for rapid water decontamination. However, conventional reducing agents such as hydroxylamine and metal sulfides are greatly restricted by three intractable challenges: (1) self-quenching effects, (2) heavy metal dissolution, and (3) irreversible capacity decline. To this end, we, for the first time, introduced redox-active polymers as electron shuttles to expedite the Fe³⁺/Fe²⁺ cycle and promote H₂O₂ activation. The reduction of Fe³⁺ mainly took place at active N-H or O-H bonds through a proton-coupled electron transfer process. As electron carriers, H atoms at the solid phase could effectively inhibit radical quenching, avoid metal dissolution, and maintain long-term reducing capacity via facile regeneration. Experimental and density functional theory (DFT) calculation results indicated that the activity of different polymers shows a volcano curve trend as a function of the energy barrier, highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap, and vertical ionization potential. Thanks to the appropriate redox ability, polyaniline outperforms other redox-active polymers (e.g., poypyrrole, hydroquinone resin, poly(2,6-diaminopyridine), and hexaazatrinaphthalene framework) with a highest iron reduction capacity up to 5.5 mmol/g, which corresponds to the state transformation from leucoemeraldine to emeraldine. Moreover, the proposed system exhibited high pollutant removal efficiency in a flow-through reactor for 8000 bed volumes without an obvious decline in performance. Overall, this work established a green and sustainable oxidation system, which offers great potential for practical organic wastewater remediation.

Accelerating Fe2+/Fe3+ cycle via biochar to improve catalytic degradation efficiency of the Fe3+/persulfate oxidation

The sluggish Fe³⁺/Fe²⁺ cycle was the rate-limiting step in the Fenton-like reaction, and metal-free carbonaceous materials are considered as emerging alternatives to solve this problem. However, the effect of carbon material properties on the distribution of reactive species remains poorly understood. This study investigated the possibility and mechanism of using biochar to accelerate the Fe³⁺/Fe²⁺ cycle to overcome the low efficiency of Fe³⁺/persulfate (PS) catalytic oxidation of phenanthrene. More importantly, the contribution of reactive species in the reaction systems with the variation of biochar pyrolysis temperatures was quantitatively studied. The results showed that medium-temperature derived biochar (BC500) had the greatest ability to enhance the Fenton-like system compared to the low- and high-temperature (BC350/700), and the first-order rate constant achieved 5.2 and 35.7-fold increase against the biochar/PS and Fe³⁺/PS systems, respectively. Using electrochemical evidence, sulfoxide probe tests, and steady-state concentration calculations, radicals yields were found to rise and then reduce with decreasing pyrolysis temperature, while the nonradical contribution of Fe(IV) increased to 56.3%. Electron paramagnetic resonance, Boehm titration, and Raman spectroscopy unraveled that the enhanced effect of biochar resulted from itself persistent free radicals, phenolic-OH, and edge defects, which enabled electron transfer between Fe³⁺ and biochar. Fe²⁺ was thus continuously generated and effectively activated the PS. This work enables a better understanding of the Fe³⁺-mediated Fenton-like reaction in the presence of biochar and provides a sustainable green strategy for Fenton chemistry with potential applications.

Gaseous toluene abatement by the heterogeneous Fenton-like process using iron/carbon-coated monolith as catalyst: Proof of concept

The degradation of toluene from a gas stream by the heterogeneous Fenton process was evaluated over a carbon-coated monolith impregnated or not with iron as catalyst in a bubble column reactor (BCR). The carbon-coated monolith support (CM) was prepared by chemical vapor deposition and the catalyst (CM impregnated with iron - herein called CM-Fe) by adsorption. In the screening of processes (absorption, adsorption and reaction), it was shown that the heterogeneous Fenton process catalyzed by CM-Fe presents the best efficiency (toluene transfer (η) = 10 × 10^-3 mol, for 300 mL of liquid solution and 0.69 g of catalyst). Finally, the stability of CM and CM-Fe was evaluated, wherein ten consecutive runs were carried out, the results showing a considerable deactivation of CM during the first five cycles. In contrast, the CM-Fe sample only slightly decreases its activity from the 1st to 2nd cycle (due to a small amount of iron leached from the monolith, 0.7%), remaining stable after that, which is important for applying this technology at the industrial level. This work showed for the first time that the treatment of gaseous effluents containing organic compounds by the Fenton process (which takes place in the liquid phase) using a carbon-coated monolith impregnated with iron is plausible, so the proof of concept was successfully accomplished.

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

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

Supplementation of tea polyphenols in sludge Fenton oxidation improves sludge dewaterability and reduces chemicals consumption

The Fenton oxidation improves sludge dewatering but faces notable technical and economic challenges, including a narrow acidic pH range, slow reduction of Fe(III), and the use of high doses of chemicals. Herein, we used a natural polyhydroxyphenol tea polyphenols (TP), as an iron redox conversion enhancer, to mitigate these issues. Compared with the classical Fenton process at pH 3.0, the process with TP (33.8 mg/g dry solids (DS)) improved sludge dewaterability at pH 7.5 in a Fenton-like system with faster Fe(II)/Fe(III) cycling and two times lower consumption of the Fenton reagent. Sludge capillary suction time and specific resistance to filtration decreased from 70 s to 22 s and from 2.7 × 10¹³ m/kg to 5.2 × 10¹¹ m/kg, respectively, while the required doses of Fe(II) and H₂O₂ were cut to 25 mg/g DS and 31.2 mg/g DS. Mechanistically, TP could bond readily with Fe(II)/Fe(III) at neutral pH to form stable complexes with complexation constants of 34 ± 161 M^-1 and 52 ± 70 M^-1, respectively, and reduce part of the Fe(III) to Fe(II) simultaneously. This maintained sufficient soluble Fe in the sludge and boosted efficient conversion of Fe(II)/Fe(III) to yield more hydroxyl radicals (•OH). Subsequently, •OH oxidation resulted in the decomposition of biopolymers with a molecular weight of 10⁸ Da (e.g., 58.2% of polysaccharides and 31.6% of proteins in tightly bound extracellular polymeric substances) into small molecules and disintegration of bioflocs into smaller particles with increased porosity, contact angle, and cell lysis; these changes helped reduce bound water content and improved sludge dewaterability. In addition, the TP-mediated Fenton process disinfected fecal coliforms in the sludge and preserved the sludge organic matters. This work proposes a new paradigm for developing cost-effective sludge dewatering technologies that relies on the synergistic effects of plant polyphenols and advanced oxidation processes.

Mn(II) Acceleration of the Picolinic Acid-Assisted Fenton Reaction: New Insight into the Role of Manganese in Homogeneous Fenton AOPs

The homogeneous Fe-catalyzed Fenton reaction remains an attractive advanced oxidation process for wastewater treatment, but sustaining the Fe(III)/Fe(II) redox cycle at a convenient pH without the costly input of energy or reductants remains a challenge. Mn(II) is known to accelerate the Fenton reaction, yet the mechanism has never been confidently established. We report a systematic kinetic and spectroscopic investigation into Mn(II) acceleration of atrazine or 2,4,6-trichlorophenol degradation by the picolinic acid (PICA)-assisted Fenton reaction at pH 4.5-6.0. Mn(II) accelerates Fe(III) reduction, superoxide radical (HO₂^•/O₂^•-) formation, and hydroxyl radical (HO^•) formation. A Mn(II/III)-H₂O₂ redox cycle as an independent source of reactive oxygen species, as proposed in the literature, is shown to be insignificant. Rather, Mn(II) assists by participating directly and catalytically in the Fe(III)/Fe(II) redox cycle. Initially, Mn(II) (as Mn^II(PICA)⁺) complexes with a ferric hydroperoxo species, PICA-Fe^III-OOH. The resulting binuclear complex undergoes intramolecular electron transfer to give Fe(II), which later generates HO^• from H₂O₂, plus MnO₂⁺, which later decomposes to HO₂^•/O₂^•- (an Fe(III) reductant) and Mn(II), completing the catalytic cycle. This scheme may apply to other Fenton-type systems that go through an Fe^III-OOH intermediate. The findings here will inform the design of practical and sustainable Fenton-based AOPs employing Mn(II) in combination with chelating agents.

Enhanced Degradation of Micropollutants in a Peracetic Acid-Fe(III) System with Picolinic Acid

Activation of peracetic acid (PAA) with iron species is an emerging advanced oxidation process (AOP). This study investigates the use of the chelating agent picolinic acid (PICA) to extend the pH range and enhance the performance of the PAA-Fe(III) AOP. Compared to the PAA-Fe(III) system, the PAA-Fe(III)-PICA system degrades various micropollutants (MPs: methylene blue, naproxen, sulfamethoxazole, carbamazepine, trimethoprim, diclofenac, and bisphenol-A) much more rapidly at higher pH, achieving almost complete removal of parent compounds within 10 min. PAA significantly outperforms the coexistent H₂O₂ and is the key oxidant for rapid compound degradation. Other chelating agents, EDTA, NTA, citric acid, proline, and nicotinic acid, could not enhance MP degradation in the PAA-Fe(III) system, while 2,6-pyridinedicarboxylic acid with a structure similar to PICA moderately enhanced MP degradation. Experiments with scavengers (tert-butyl alcohol and methyl phenyl sulfoxide) and a probe compound (benzoic acid) confirmed that high-valent iron species [Fe(IV) and/or Fe(V)], rather than radicals, are the major reactive species contributing to MP degradation. The oxidation products of methylene blue, naproxen, and sulfamethoxazole by PAA-Fe(III)-PICA were characterized and supported the proposed mechanism. This work demonstrates that PICA is an effective complexing ligand to assist the Fenton reaction of PAA by extending the applicable pH range and accelerating the catalytic ability of Fe(III).