Microscopic and spectroscopic analysis of atmospheric iron-containing single particles in Lhasa, Tibet

The Tibetan Plateau, known as the "Third Pole", is currently in a state of perturbation caused by intensified human activity. In this study, 56 samples were obtained at the five sampling sites in typical area of Lhasa city and their physical and chemical properties were investigated by TEM/EDS, STXM, and NEXAFS spectroscopy. After careful examination of 3387 single particles, the results showed that Fe should be one of the most frequent metal elements. The Fe-containing single particles in irregular shape and micrometer size was about 7.8% and might be mainly from local sources. Meanwhile, the Fe was located on the subsurface of single particles and might be existed in the form of iron oxide. Interestingly, the core-shell structure of iron-containing particles were about 38.8% and might be present as single-, dual- or triple-core shell structure and multi-core shell structure with the Fe/Si ratios of 17.5, 10.5, 2.9 and 1.2, respectively. Meanwhile, iron and manganese were found to coexist with identical distributions in the single particles, which might induce a synergistic effect between iron and manganese in catalytic oxidation. Finally, the solid spherical structure of Fe-containing particles without an external layer were about 53.4%. The elements of Fe and Mn were co-existed, and might be presented as iron oxide-manganese oxide-silica composite. Moreover, the ferrous and ferric forms of iron might be co-existed. Such information can be valuable in expanding our understanding of Fe-containing particles in the Tibetan Plateau atmosphere.

Deep-dive into iron-based co-precipitation of arsenic: A review of mechanisms derived from synchrotron techniques and implications for groundwater treatment

The co-precipitation of Fe(III) (oxyhydr)oxides with arsenic (As) is one of the most widespread approaches to treat As-contaminated groundwater in both low- and high-income settings. Fe-based co-precipitation of As occurs in a variety of conventional and decentralized treatment schemes, including aeration and sand filtration, ferric chloride addition and technologies based on controlled corrosion of Fe(0) (i.e., electrocoagulation). Despite its ease of deployment, Fe-based co-precipitation of As entails a complex series of chemical reactions that often occur simultaneously, including electron-transfer reactions, mineral nucleation, crystal growth, and As sorption. In recent years, the growing use of sophisticated synchrotron-based characterization techniques in water treatment research has generated new detailed and mechanistic insights into the reactions that govern As removal efficiency. The purpose of this critical review is to synthesize the current understanding of the molecular-scale reaction pathways of As co-precipitation with Fe(III), where the source of Fe(III) can be ferric chloride solutions or oxidized Fe(II) sourced from natural Fe(II) in groundwater, ferrous salts or controlled Fe(0) corrosion. We draw primarily on the mechanistic knowledge gained from spectroscopic and nano-scale investigations. We begin by describing the least complex reactions relevant in these conditions (Fe(II) oxidation, Fe(III) polymerization, As sorption in single-solute systems) and build to multi-solute systems containing common groundwater ions that can alter the pathways of As uptake during Fe(III) co-precipitation (Ca, Mg bivalent cations; P, Si oxyanions). We conclude the review by providing a perspective on critical knowledge gaps remaining in this field and new research directions that can further improve the understanding of As removal via Fe(III) co-precipitation.

Surface Processes Control the Fate of Reactive Oxidants Generated by Electrochemical Activation of Hydrogen Peroxide on Stainless-Steel Electrodes

Low-cost stainless-steel electrodes can activate hydrogen peroxide (H2O2) by converting it into a hydroxyl radical (•OH) and other reactive oxidants. At an applied potential of +0.020 V, the stainless-steel electrode produced •OH with a yield that was over an order of magnitude higher than that reported for other systems that employ iron oxides as catalysts under circumneutral pH conditions. Decreasing the applied potential at pH 8 and 9 enhanced the rate of H2O2 loss by shifting the process to a reaction mechanism that resulted in the formation of an Fe(IV) species. Significant metal leaching was only observed under acidic pH conditions (i.e., at pH <6), with the release of dissolved Fe and Cr occurring as the thickness of the passivation layer decreased. Despite the relatively high yield of •OH production under circumneutral pH conditions, most of the oxidants were scavenged by the electrode surface when contaminant concentrations comparable to those expected in drinking water sources were tested. The stainless-steel electrode efficiently removed trace organic contaminants from an authentic surface water sample without contaminating the water with Fe and Cr. With further development, stainless-steel electrodes could provide a cost-effective alternative to other H2O2 activation processes, such as those by ultraviolet light.

On The Nature of FeIV =Oaq in Aqueous Media: A DFT analysis

FeIV =Oaq is a key intermediate in many advanced oxidation processes and probably in biological systems. It is usually referred to as FeIV =O2+ . The pKa's of FeIV =Oaq as derived by DFT are: pKa1=2.37 M06 L/6-311++G(d,p) (SDD for Fe) and pKa2=7.79 M06 L/6-311++G(d,p) (SDD for Fe). This means that in neutral solutions, FeIV =Oaq is a mixture of (H2 O)4 (OH)FeIV =O+ and (H2 O)2 (OH)2 FeIV =O. The oxidation potential of FeIV =Oaq in an acidic solution, E0 {(H2 O)5 FeIV =O2+ /FeIII (H2 O)6 3+ , pH 0.0} is calculated with and without a second solvation sphere and the recommended value is between 2.86 V (B3LYP/Def2-TZVP, with a second solvation sphere) and 2.23 V (M06 L/Def2-TZVP without a second solvation sphere). This means that FeIV =Oaq is the strongest oxidizing agent formed in systems involving FeVI O4 2- even in neutral media.

Tailored Beta-Lapachone Nanomedicines for Cancer-Specific Therapy

Nanotechnology shows the power to improve efficacy and reduce the adverse effects of anticancer agents. As a quinone-containing compound, beta-lapachone (LAP) is widely employed for targeted anticancer therapy under hypoxia. The principal mechanism of LAP-mediated cytotoxicity is believed due to the continuous generation of reactive oxygen species with the aid of NAD(P)H: quinone oxidoreductase 1 (NQO1). The cancer selectivity of LAP relies on the difference between NQO1 expression in tumors and that in healthy organs. Despite this, the clinical translation of LAP faces the problem of narrow therapeutic window that is challenging for dose regimen design. Herein, the multifaceted anticancer mechanism of LAP is briefly introduced, the advance of nanocarriers for LAP delivery is reviewed, and the combinational delivery approaches to enhance LAP potency in recent years are summarized. The mechanisms by which nanosystems boost LAP efficacy, including tumor targeting, cellular uptake enhancement, controlled cargo release, enhanced Fenton or Fenton-like reaction, and multidrug synergism, are also presented. The problems of LAP anticancer nanomedicines and the prospective solutions are discussed. The current review may help to unlock the potential of cancer-specific LAP therapy and speed up its clinical translation.

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

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

Chain Reaction of Fenton Autoxidation of Tartaric Acid: Critical Behavior at Low pH

Autoxidation of tartaric acid in air-saturated aqueous solutions in the presence of Fe(II) at low pH, 2.5, shows autocatalytic behavior with distinct initiation, propagation, and termination phases. With increasing pH, the initiation phase speeds up, while the propagation phase shortens and reduces to none. We show that the propagation phase is a chain reaction that occurs via activation of oxygen in the initiation stage with the production of hydrogen peroxide. The subsequent Fenton oxidation that regenerates hydrogen peroxide with a positive feedback is typical of a self-sustained chain reaction. The conditions for such a chain reaction are shown to be similar to those of a dynamical system with critical behavior; namely, the system becomes unstable when the kinetic matrix of pseudo-first-order reaction becomes negatively defined with a negative eigenvalue giving the rate of exponential (chain) growth of the reactive species.

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

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

Anodic oxidation of sulfamethoxazole paired to cathodic hydrogen peroxide production

A double chamber electrochemical system is developed consisting of a boron-doped diamond (BDD) anode and a graphite cathode, which not only degrades sulfamethoxazole (SMX) but also simultaneously generates hydrogen peroxide (H₂O₂). The degradation of SMX is carried out by (in)direct oxidation at the BDD anode and H₂O₂ is produced by two electron oxygen (O₂) reduction reaction (ORR) at the cathode. The effect of different parameters on the kinetics of both mechanisms was investigated. The performance of the system at the optimized conditions (pH 3, 0.05 M Na₂SO₄ as electrolyte, and 10 mA as applied current) showed that after 180 min of electrolysis, SMX was almost fully degraded (95% removal and ∼90% COD reduction) as well as about 535 μM H₂O₂ was accumulated. With the help of LC-MS, five intermediates formed during SMX electrolysis were properly identified and a degradation pathway was proposed. This study advocates methods for improving the effectiveness of energy use in advanced wastewater treatment.

Homogeneous Carbon Dot-Anchored Fe(III) Catalysts with Self-Regulated Proton Transfer for Recyclable Fenton Chemistry

Fenton chemistry has been widely studied in a broad range from geochemistry, chemical oxidation to tumor chemodynamic therapy. It was well established that Fe³⁺/H₂O₂ resulted in a sluggish initial rate or even inactivity. Herein, we report the homogeneous carbon dot-anchored Fe(III) catalysts (CD-COOFe^III) wherein CD-COOFe^III active center activates H₂O₂ to produce hydroxyl radicals (^•OH) reaching 105 times larger than that of the Fe³⁺/H₂O₂ system. The key is the ^•OH flux produced from the O-O bond reductive cleavage boosting by the high electron-transfer rate constants of CD defects and its self-regulated proton-transfer behavior probed by operando ATR-FTIR spectroscopy in D₂O and kinetic isotope effects, respectively. Organic molecules interact with CD-COOFe^III via hydrogen bonds, promoting the electron-transfer rate constants during the redox reaction of CD defects. The antibiotics removal efficiency in the CD-COOFe^III/H₂O₂ system is at least 51 times large than the Fe³⁺/H₂O₂ system under equivalent conditions. Our findings provide a new pathway for traditional Fenton chemistry.

Preparation of Cellulose Nanofibers from Corn Stalks by Fenton Reaction: A New Insight into the Mechanism by an Experimental and Theoretical Study

Agricultural biomass wastes are an abundant feedstock for biorefineries. However, most of these wastes are not treated in the right way. Here, corn stalks (CSs) were assigned as the raw material to produce cellulose nanofibers (CNFs) via in situ Fenton oxidation treatment. In order to probe the formation mechanism of an in situ Fenton reactor, the bonding interaction of hydrated Fe²⁺ ions and fiber has been systemically studied based on adsorption experiments, IR spectroscopy, density functional theory (DFT) calculations, and Raman spectroscopy. The results indicate that the coordination of the hydrated Fe²⁺ ion to the fiber generates a quasi-octahedral-coordinated sphere around the Fe center. The Jahn-Teller distortion effect of the Fe center promotes the Fe-O₂H₂ bonding interaction via reduction of the energy gap of the d_z² orbital of the Fe center and π_2py/π_2pz orbitals of the H₂O₂ molecule. The oxidation treatment of the pretreated CS by the in situ Fenton process shows the formation of a new carboxyl group on the fiber surface. The scanning electron microscopy image shows that the Fenton-treated fiber was scattered into the nanosized CNFs with a diameter of up to 50 nm. Both experimental and theoretical studies show that the pseudo-first-order kinetic reaction could describe the in situ Fenton kinetics well. Moreover, the proposed catalytic cycle shows that the large thermodynamic barrier is the cleavage of the O-O bond of H₂O₂ to generate the ^•OH radical, and the whole catalytic cycle is found to be spontaneous at room temperature.

Degradation of phenol using Fe(II)-activated CaO2: effect of ball-milled activated carbon (ACBM) addition

In-situ chemical oxidation (ISCO) based on peroxide activation is one of the most promising technologies for removing organic contaminants from natural groundwater (NGW). However, use of the most common form of hydrogen peroxide (H₂O₂) is limited owing to its significantly rapid reaction rate and heat generation. Therefore, in the present study, the activation of calcium peroxide (CaO₂), a slow H₂O₂ releasing agent, by Fe(II) was proposed (CaO₂/Fe(II)), and the phenol degradation mechanisms and feasibility of NGW remediation were investigated. The optimum molar ratio of [phenol]/[CaO₂]/[Fe(II)] (phenol = 0.5 mM) was 1/10/10, resulting in 87.0-92.5% phenol removal within 120 min under a broad initial pH range of 3-9. HCO₃^-, PO₄^3-, and humic acid significantly inhibited degradation, whereas the effects of Cl^-, NO₃^-, and SO₄^2- were negligible. Reactive oxygen species (ROS) were identified based on the results of phenol degradation in the presence of scavengers and electron spin resonance (ESR) spectroscopy, which demonstrated that ¹O₂ played the dominant role, supported by ^•OH, in CaO₂/Fe(II). Phenol removal in NGW (67.81%) was less than that in distilled and deionized water (DIW, 92.5%) at a [phenol]/[CaO₂]/[Fe(II)] ratio of 1/10/10. However, phenol removal was significantly improved (∼100%) by increasing the CaO₂ and Fe(II) doses to 1/20/20-40. Furthermore, when 125-250 mg L^-1 of ball-milled activated carbon (AC_BM) was added (CaO₂/Fe(II)-AC_BM), phenol removal was enhanced from 67.81% to 90.94-100% in the NGW. CaO₂/Fe(II)-AC_BM exhibited higher total organic carbon (TOC) removal than CaO₂/Fe(II). In addition, no notable by-products were detected using CaO₂/Fe(II)-AC_BM, whereas the polymerisation products of hydroxylated and/or ring-cleaved compounds, that is, aconitic acid, gallocatechin, and 10-hydroxyaloin, were found in the reaction with CaO₂/Fe(II). These results strongly suggest that CaO₂/Fe(II)-AC_BM is highly promising for groundwater remediation, minimizing degradation byproducts and the adverse effects caused by the NGW components.

Mechanistic Insights into the Markedly Decreased Oxidation Capacity of the Fe(II)/S2O82- Process with Increasing pH

The poor oxidation capacity of the Fe(II)/S₂O₈^2- [Fe(II)/PDS] system at pH > 3.0 has limited its wide application in water treatment. To unravel the underlying mechanism, this study systematically evaluated the possible influencing factors over the pH range of 1.0-8.0 and developed a mathematical model to quantify these effects. Results showed that ∼82% of the generated Fe(IV) could be used for pollutant degradation at pH 1.0, whereas negligible Fe(IV) contribution was observed at pH 7.5. This dramatic decline of Fe(IV) contribution with increasing pH dominantly accounted for the pH-dependent performance of the Fe(II)/PDS process. Unexpectedly, Fe(II) could consume ∼80% of the generated SO₄^•- non-productively under both acidic and near-neutral conditions, while the larger formation of Fe(III) precipitates at high pH inhibited the SO₄^•- contribution mildly. Moreover, the strong Fe(II) scavenging effect was difficult to be compensated for by slowing down the Fe(II) dosing rate. The competition of dissolved oxygen with PDS for Fe(II) was insignificant at pH ≤ 7.5, where the second-order rate constants for reactions of Fe(II) with oxygen were much lower than or comparable to that between Fe(II) and PDS. These findings could advance our understanding of the chemistry and application of the Fe(II)/PDS process.

Anode Catalysts in Anion-Exchange-Membrane Electrolysis without Supporting Electrolyte: Conductivity, Dynamics, and Ionomer Degradation

Anion-exchange-membrane water electrolyzers (AEMWEs) in principle operate without soluble electrolyte using earth-abundant catalysts and cell materials and thus lower the cost of green H₂ . Current systems lack competitive performance and the durability needed for commercialization. One critical issue is a poor understanding of catalyst-specific degradation processes in the electrolyzer. While non-platinum-group-metal (non-PGM) oxygen-evolution catalysts show excellent performance and durability in strongly alkaline electrolyte, this has not transferred directly to pure-water AEMWEs. Here, AEMWEs with five non-PGM anode catalysts are built and the catalysts' structural stability and interactions with the alkaline ionomer are characterized during electrolyzer operation and post-mortem. The results show catalyst electrical conductivity is one key to obtaining high-performing systems and that many non-PGM catalysts restructure during operation. Dynamic Fe sites correlate with enhanced degradation rates, as does the addition of soluble Fe impurities. In contrast, electronically conductive Co₃ O₄ nanoparticles (without Fe in the crystal structure) yield AEMWEs from simple, standard preparation methods, with performance and stability comparable to IrO₂ . These results reveal the fundamental dynamic catalytic processes resulting in AEMWE device failure under relevant conditions, demonstrate a viable non-PGM catalyst for AEMWE operation, and illustrate underlying design rules for engineering anode catalyst/ionomer layers with higher performance and durability.

Nano- and micro-scale zerovalent iron-activated peroxydisulfate for methyl phenyl sulfoxide probe transformation in aerobic water: Quantifying the relative roles of SO4·-, Fe(IV), and ·OH

Multiple reactive intermediates have been proposed to be involved in peroxydisulfate (PDS) activation by zerovalent iron (ZVI), including sulfate radical (SO₄^·-) produced via iron-oxide shell mediated electron transfer, ferryl ion species (Fe(IV)) formed from Fe(II)-PDS interaction, and hydroxyl radical (^·OH) generated by ZVI aerobic oxygenation. In this study, evolution of the relative role of these intermediates in microscale and nanoscale ZVI (mZVI vs. nZVI) activated PDS processes is comparatively investigated by using a methyl phenyl sulfoxide (PMSO) probe that selectively reacts with Fe(IV) to produce methyl phenyl sulfone (PMSO₂). Interestingly, during PMSO transformation by mZVI/PDS process, yields of PMSO₂ (η(PMSO₂)) exhibit three-stage behavior that they first increase to a maximum (∼80% but lower than 100%) (Stage I) and then plateau for a period (Stage II) followed by a decrease phase (Stage III). Accordingly, the relative role of Fe(IV) in PMSO transformation is unceasingly improved in Stage I and subsequently reaches equilibrium with that of free radicals in Stage II, while it finally decreases in Stage III. Similar η(PMSO₂) evolution trends are obtained in nZVI/PDS process, except that the η(PMSO₂) increase in Stage I is negligible, possibly due to the exceptional fast nZVI dissolution. It was further clarified by tert-butyl alcohol scavenging assay that, in addition to Fe(IV), the free radical involved in Stages I and II is SO₄^·-, while ^·OH was dominant in Stage III. Moreover, studies on O₂ effect reveal that ZVI aerobic oxygenation participates in mZVI corrosion during the entire process, while it is only involved in nZVI corrosion when PDS content is reduced to a low concentration, indicating that the reactivities of PDS and O₂ are similar in mZVI corrosion, but differ greatly in nZVI corrosion. Additionally, effects of reactant dose and pH on η(PMSO₂) evolution are also explored. Dynamics of the relative role of different reactive oxidants should be taken into account in further applications of ZVI/PDS in situ chemical remediation technology considering their different chemistries.

Current Use of Fenton Reaction in Drugs and Food

Iron is the most abundant mineral in the human body and plays essential roles in sustaining life, such as the transport of oxygen to systemic organs. The Fenton reaction is the reaction between iron and hydrogen peroxide, generating hydroxyl radical, which is highly reactive and highly toxic to living cells. “Ferroptosis”, a programmed cell death in which the Fenton reaction is closely involved, has recently received much attention. Furthermore, various applications of the Fenton reaction have been reported in the medical and nutritional fields, such as cancer treatment or sterilization. Here, this review summarizes the recent growing interest in the usefulness of iron and its biological relevance through basic and practical information of the Fenton reaction and recent reports.

Aqueous ·OH Oxidation of Highly Substituted Phenols as a Source of Secondary Organic Aerosol

Biomass burning (BB) releases large quantities of phenols (ArOH), which can partition into cloud/fog drops and aerosol liquid water (ALW), react, and form aqueous secondary organic aerosol (aqSOA). While simple phenols are too volatile to significantly partition into particle water, highly substituted ArOH partition more strongly and might be important sources of aqSOA in ALW. To investigate this, we measured the ^·OH oxidation kinetics and aqSOA yields for six highly substituted ArOH from BB. Second-order rate constants are high, in the range (1.9-14) × 10⁹ M^-1 s^-1 at pH 2 and (14-25) × 10⁹ M^-1 s^-1 at pH 5 and 6. Mass yields of aqSOA are also high, with an average (±1σ) value of 82 (±12)%. ALW solutes have a range of impacts on phenol oxidation by ^·OH: a BB sugar and some inorganic salts suppress oxidation, while a nitrate salt and transition metals enhance oxidation. Finally, we estimated rates of aqueous- and gas-phase formation of SOA from a single highly substituted phenol as a function of liquid water content (LWC), from conditions of cloud/fog (0.1 g-H₂O m^-3) to ALW (10 μg-H₂O m^-3). Formation of aqSOA is significant across the LWC range, although gas-phase ^·OH becomes dominant under ALW conditions. We also see a generally large discrepancy between measured and modeled aqueous ^·OH concentrations across the LWC range.

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

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

Recovery of Graphite and Cathode Active Materials from Spent Lithium-Ion Batteries by Applying Two Pretreatment Methods and Flotation Combined with a Rapid Analysis Technique

This work investigates the comprehensive recycling of graphite and cathode active materials (LiNi0.6Mn0.2Co0.2O2, abbreviated as NMC) from spent lithium-ion batteries via pretreatment and flotation. Specific analytical methods (SPME-GC-MS and Py-GC-MS) were utilized to identify and trace the relevant influencing factors. Two different pretreatment methods, which are Fenton oxidation and roasting, were investigated with respect to their influence on the flotation effectiveness. As a result, for NMC cathode active materials, a recovery of 90% and a maximum grade of 83% were obtained by the optimized roasting and flotation. Meanwhile, a graphite grade of 77% in the froth product was achieved, with a graphite recovery of 75%. By using SPME-GC-MS and Py-GC-MS analyses, it could be shown that, in an optimized process, an effective destruction/removal of the electrolyte and binder residues can be reached. The applied analytical tools could be integrated into the workflow, which enabled process control in terms of the pretreatment sufficiency and achievable separation in the subsequent flotation.

Generation of Reactive Oxygen Species and Degradation of Pollutants in the Fe2+/O2/Tripolyphosphate System: Regulated by the Concentration Ratio of Fe2+ and Tripolyphosphate

Tripolyphosphate (TPP) has many advantages as a ligand for the optimization of the Fe²⁺/O₂ system in environmental remediation applications. However, the relationship between remediation performance and the Fe²⁺/TPP ratio in the system has not been previously described. In this study, we report that the degradation mechanism of p-nitrophenol (PNP) in Fe²⁺/O₂ systems is regulated by the Fe²⁺/TPP ratio under neutral conditions. The results showed that although PNP was effectively degraded at different Fe²⁺/TPP ratios, the results of specific reactive oxygen species (ROS) scavenging experiments and the determination of PNP degradation products showed that the mechanism of PNP degradation varies with the Fe²⁺/TPP ratio. When C_Fe²⁺ ≥ C_TPP, the initially formed O₂^•- is converted to •OH and the •OH degrades PNP by oxidation. However, when C_Fe²⁺ < C_TPP, the O₂^•- persists long enough to degrade PNP by reduction. Density functional theory (DFT) calculations revealed that the main reactive species of Fe²⁺ in the system include [Fe(TPP)(H₂O)₃]^- and [Fe(TPP)₂]^4-, whose content in the solution is the key to achieve system regulation. Consequently, by controlling the Fe²⁺/TPP ratio in the solution, the degradation pathways of PNP can be selected. Our study proposed a new strategy to regulate the oxidation/reduction removal of pollutants by simply varying the Fe²⁺/TPP ratio of the Fe²⁺/O₂ system.

Removal of organotin compounds and metals from Swedish marine sediment using Fenton's reagent and electrochemical treatment

Metal and tributyltin (TBT) contaminated sediments are problematic for sediment managers and the environment. This study is the first to compare Fenton's reagent and electrochemical treatment as remediation methods for the removal of TBT and metals using laboratory-scale experiments on contaminated dredged sediment. The costs and the applicability of the developed methods were also compared and discussed. Both methods removed > 98% TBT from TBT-spiked sediment samples, while Fenton's reagent removed 64% of the TBT and electrolysis 58% of the TBT from non-spiked samples. TBT in water phase was effectively degraded in both experiments on spiked water and in leachates during the treatment of the sediment. Positive correlations were observed between TBT removal and the added amount of hydrogen peroxide and current density. Both methods removed metals from the sediment, but Fenton's reagent was identified as the most potent option for effective removal of both metals and TBT, especially from highly metal-contaminated sediment. However, due to risks associated with the required chemicals and low pH level in the sediment residue following the Fenton treatment, electrochemical treatment could be a more sustainable option for treating larger quantities of contaminated sediment.

SARS- CoV-2 infection and oxidative stress in early-onset preeclampsia

SARS-CoV-2 causes coronavirus disease 2019 (COVID-19) also in pregnant women. Infection in pregnancy leads to maternal and placental functional alterations. Pregnant women with vascular defects such as preeclampsia show high susceptibility to SARS-CoV-2 infection by undefined mechanisms. Pregnant women infected with SARS-CoV-2 show higher rates of preterm birth and caesarean delivery, and their placentas show signs of vasculopathy and inflammation. It is still unclear whether the foetus is affected by the maternal infection with this virus and whether maternal infection associates with postnatal affections. The SARS-CoV-2 infection causes oxidative stress and activation of the immune system leading to cytokine storm and next tissue damage as seen in the lung. The angiotensin-converting-enzyme 2 expression is determinant for these alterations in the lung. Since this enzyme is expressed in the human placenta, SARS-CoV-2 could infect the placenta tissue, although reported to be of low frequency compared with maternal lung tissue. Early-onset preeclampsia (eoPE) shows higher expression of ADAM17 (a disintegrin and metalloproteinase 17) causing an imbalanced renin-angiotensin system and endothelial dysfunction. A similar mechanism seems to potentially account for SARS-CoV-2 infection. This review highlights the potentially common characteristics of pregnant women with eoPE with those with COVID-19. A better understanding of the mechanisms of SARS-CoV-2 infection and its impact on the placenta function is determinant since eoPE/COVID-19 association may result in maternal metabolic alterations that might lead to a potential worsening of the foetal programming of diseases in the neonate, young, and adult.

Aqueous Iron(IV)-Oxo Complex: An Emerging Powerful Reactive Oxidant Formed by Iron(II)-Based Advanced Oxidation Processes for Oxidative Water Treatment

High-valent iron(IV)-oxo complexes are of great significance as reactive intermediates implicated in diverse chemical and biological systems. The aqueous iron(IV)-oxo complex (Fe_aq^IVO²⁺) is the simplest but one of the most powerful ferryl ion species, which possesses a high-spin state, high reduction potential, and long lifetime. It has been well documented that Fe_aq^IVO²⁺ reacts with organic compounds through various pathways (hydrogen-atom, hydride, oxygen-atom, and electron transfer as well as electrophilic addition) at moderate reaction rates and show selective reactivity toward inorganic ions prevailing in natural water, which single out Fe_aq^IVO²⁺ as a superior candidate for oxidative water treatment. This review provides state-of-the-art knowledge on the chemical properties and oxidation mechanism and kinetics of Fe_aq^IVO²⁺, with special attention to the similarities and differences to two representative free radicals (hydroxyl radical and sulfate radical). Moreover, the prospective role of Fe_aq^IVO²⁺ in Fe_aq²⁺ activation-initiated advanced oxidation processes (AOPs) has been intensively investigated over the past 20 years, which has significantly challenged the conventional recognition that free radicals dominated in these AOPs. The latest progress in identifying the contribution of Fe_aq^IVO²⁺ in Fe_aq²⁺-based AOPs is thereby reviewed, highlighting controversies on the nature of the reactive oxidants formed in several Fe_aq²⁺ activated peroxide and oxyacid processes. Finally, future perspectives for advancing the evaluation of Fe_aq^IVO²⁺ reactivity from an engineering viewpoint are proposed.

A Ferrofluid with Surface Modified Nanoparticles for Magnetic Hyperthermia and High ROS Production

A ferrofluid with 1,2-Benzenediol-coated iron oxide nanoparticles was synthesized and physicochemically analyzed. This colloidal system was prepared following the typical co-precipitation method, and superparamagnetic nanoparticles of 13.5 nm average diameter, 34 emu/g of magnetic saturation, and 285 K of blocking temperature were obtained. Additionally, the zeta potential showed a suitable colloidal stability for cancer therapy assays and the magneto-calorimetric trails determined a high power absorption density. In addition, the oxidative capability of the ferrofluid was corroborated by performing the Fenton reaction with methylene blue (MB) dissolved in water, where the ferrofluid was suitable for producing reactive oxygen species (ROS), and surprisingly a strong degradation of MB was also observed when it was combined with H2O2. The intracellular ROS production was qualitatively corroborated using the HT-29 human cell line, by detecting the fluorescent rise induced in 2,7-dichlorofluorescein diacetate. In other experiments, cell metabolic activity was measured, and no toxicity was observed, even with concentrations of up to 4 mg/mL of magnetic nanoparticles (MNPs). When the cells were treated with magnetic hyperthermia, 80% of cells were dead at 43 °C using 3 mg/mL of MNPs and applying a magnetic field of 530 kHz with 20 kA/m amplitude.

Photo-Fenton Process over an Fe-Free 3%-CuO/Sr0.76Ce0.16WO4 Photocatalyst under Simulated Sunlight

Photo-Fenton is a promising photocatalytic technology that utilizes sunlight. Herein, an Fe-free 3%-CuO/Sr0.76Ce0.16WO4 photocatalyst was synthesized to apply simulated wastewater degradation via a photo-Fenton process under simulated sunlight. The photodegradation efficiency of RhB solution over the 3%-CuO/Sr0.76Ce0.16WO4 photocatalyst is 93.2% in the first 3 h; its photocatalytic efficiency remains at 91.6% even after three cycle experiments. The kinetic constant of the 3%-CuO/Sr0.76Ce0.16WO4 photocatalyst is 0.0127 min-1, which is 2.8-fold that of an intrinsic Sr0.76Ce0.16WO4 sample. The experiment of radical quenching revealed that the photogenerated electrons and holes are transferred to CuO to form hydroxyl radicals. Besides, the photocatalyst was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), diffused reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) measurements. It has some reference significance for the design of iron-free photocatalysts.

Green synthesis of akaganéite (β-FeOOH) nanocomposites as peroxidase-mimics and application for discoloration of methylene blue

This work reports an environmentally benign and readily scalable process for production of akaganéite (β-FeOOH) nanocomposites by using abundant gallic acid or grape seed tannins and urea. Influences from those phytochemicals on the properties of β-FeOOH nanocomposites were investigated by X-ray powder diffraction, Fourier transform infrared spectroscopy, Thermogravimetric analysis, Scanning electron microscopy, Transmission electron microscopy, UV-Vis spectroscopy and Photoluminescence. The addition of 0.1% (w/v) grape seed tannins or gallic acid (640 mg L^-1) solution yielded single-crystalline β-FeOOH nanocomposites with reduced dimensions, increased porosities and BET surface area, and no oxidized impurities such as hematite (Fe₂O₃) were formed. The added grape seed tannins (S_0.8) or gallic acid together with less urea (0.8 M) produced β-FeOOH nanocomposites with higher activities as peroxidase mimics compared to those prepared with only urea (C_0.8). Moreover, S_0.8 was more efficient in methylene blue (MB) discoloration compared to C_0.8 at all three pH values of 4, 7 and 11, and the S_0.8-mediated MB degradation pathways at pH 4 and 7 were different from those at pH 11 due to the generation of different predominant oxidants. The overall MB discoloration efficacies by S_0.8 at pH 4, 7 and 11 were combinative effects of both physical adsorption and chemical reactions. These β-FeOOH nanocomposites possess great potential as peroxidase mimics for facile monitoring of excess hydrogen peroxide and applications in environmental remediation.

Toward enhanced catalytic activity of magnetic nanoparticles integrated into 3D reduced graphene oxide for heterogeneous Fenton organic dye degradation

Composite Fenton nanocatalyst was prepared by water-based in situ creation of Fe3O4 nanoparticles integrated within the self-assembly 3D reduced graphene oxide (rGO) aerogel. The hybrid applied for the degradation of Acid Green 25 (AG-25) organic dye in an aqueous solution, in the presence of H2O2. By investigating the conditions that maximize the dye adsorption by the 3D composite, it was found that the pH of the solution should be adjusted between the pKa of the functional groups present on the rGO surface (carboxylic acid) and that of the dye (sulfonic acid) to promote electrostatic interactions dye-3D structure. Performed under these conditions, Fenton degradation of AG-25 in presence of H2O2 was completed in less than 30 min, including all the intermediate products, as demonstrated by MALDI-TOF-MS analysis of the aqueous solution after discoloration. Moreover, this was achieved in a solution with as high a dye concentration of 0.5 mg/mL, with only 10 mg of 3D composite catalyst, at room temperature and without additional energy input. The high performance was attributed to the creation of charge-transfer complex between rGO and Fe3O4 nanoparticles throughout covalent bond C-O-Fe, the formation of which was promoted by the in situ synthesis procedure. For the first time, up to the authors' knowledge, AG-25 degradation mechanism was proposed.

A novel diagnostic method for distinguishing between Fe(IV) and •OH by using atrazine as a probe: Clarifying the nature of reactive intermediates formed by nitrilotriacetic acid assisted Fenton-like reaction

In this work, we found that the distribution of two specific atrazine (ATZ) oxidation products (desethyl-atrazine (DEA) and desisopropyl-atrazine (DIA)) was different in oxidation processes involving aqueous ferryl ion (Fe(IV)) species and ^•OH. Specifically, the molar ratio of produced DEA to DIA (i.e., [DEA]/[DIA]) increased from 7.5 to 13 with increasing pH from 3 to 6 when ATZ was oxidized by Fe(IV), while the treatment of ATZ by ^•OH led to the [DEA]/[DIA] value of 2 which was independent of pH. Moreover, ATZ showed high reactivity towards Fe(IV) over a wide pH range, especially at near-neutral pH, at which ATZ oxidation in Fe(II)/peroxydisulfate system was even much faster than another well-defined Fe(IV) scavenger, the sulfoxides. By using this approach, it was obtained that the [DEA]/[DIA] value remained at 2 during ATZ transformation by the nitrilotriacetic acid (NTA) assisted Fenton-like (Fe(III)/H₂O₂) system, which was independent of solution pH and reactants dosage. This result clarified that ^•OH was the primary reactive intermediate formed in the NTA assisted Fe(III)/H₂O₂ system. This study not only developed a novel sensitive diagnostic tool for distinguishing Fe(IV) from ^•OH, but also provided more credible evidence to the nature of reactive intermediate in a commonly controversial system.

An Upper Limit to O2 Evolution as Test for Radical and Nonradical Mechanisms for the Fenton Reaction

The origin of an upper limit to the amount of O2 evolved in the rapid reaction between Fe2+ and H2O2 was investigated at a high concentration of H2O2. Using a nonradical model, including the formation of a primary Fe2+–biperoxy complex with a diminished rate of formation of the active intermediate FeO2+, agreement has been reached for the first time with the experimental data obtained by Barb et al. A limited formation of O2 requires that a finite concentration of H2O2 should be present in the reaction mixture when [Fe2+] falls to zero. It has been shown that in Barb et al.’s model the condition for such a state ([Fe2+] = 0, [H2O2] > 0) does not exist. Free radical based models fail as mechanisms for the Fenton reaction.

DFT mechanistic study on the formation of 8-oxoguanine and spiroiminodihydantoin mediated by iron Fenton reactions

Fenton reactions unavoidably take place in the human body and have been demonstrated to cause oxidative DNA damage. However, the molecular-level understanding of DNA damage mediated by Fenton reactions is limited. Herein, density functional theory (DFT) calculations were made to investigate the counterion effects on aqueous Fenton reactions and the detailed mechanisms of chemical modifications to guanine induced by Fenton reactions. Our calculations show that the activation energy of the Fenton reaction catalyzed by a pure aquo complex [FeII(H2O)6]2+ is too high to agree with experiments, whereas complexation with counteranions reduces the activation energy to a reasonable range. This result suggests that FeII-counteranion complexes are the real catalyst for fast aqueous Fenton reactions. In addition, we found that the Fenton oxidation mediated by FeII bonded to the N7 atom of guanine can result in the formation of 8-oxoguanine and spiroiminodihydantoin through multiple reaction pathways, including the electrophilic addition of ˙OH, H-abstraction by ˙OH, and oxygen atom transfer of oxoiron(iv) species. The activation of hydrogen peroxide by ferrous iron is the rate-determining step. The guanine N7-bound iron ion and the coordinated counteranion were found to play an important role in the Fenton oxidation of guanine.

A critical review on metal complexes removal from water using methods based on Fenton-like reactions: Analysis and comparison of methods and mechanisms

Metals mainly exist in the form of complexes in urban wastewater, fresh water and even drinking water, which are difficult to remove and further harm human health. Fenton-like reaction has been used for the removal of metal complexes. Effective removal of metal complexes using Fenton-like reaction requires the removal of both metals and organic ligands, meanwhile, the fate of metals and organic pollutions must be clearly understood. Thus, this review summarizes the relevant research on metal complex removal from using Fenton-like reactions in the past ten years, with the detailed removal approaches and mechanisms analyzed. Electro-, photo-, microwave/ultrasound-Fenton reactions or the synergistic Fenton reaction have been shown to exhibit excellent metal complex treatment capabilities. Furthermore, various catalysts, such as transition metals, bimetals and metal-free catalytic systems can expand the potential applications of Fenton-like reactions. Novel Fenton reaction methods without the addition of metals or H₂O₂, with construction of a dual active center catalyst, or with the introduction of other free radicals, are all worthy of further investigation. Due to increasing levels of environmental metal and organic pollutions remediation requirements, more research is required for the development of economical and efficient novel Fenton-like processes.

Overlooked enhancement of chloride ion on the transformation of reactive species in peroxymonosulfate/Fe(II)/NH2OH system

Though hydroxylamine (NH₂OH) is effective for accelerating pollutants degradation in Fenton and Fenton-like systems, the effect of anions simultaneously introduced by the hydroxylamine salts have always been ignored. Herein, effect of two commonly used hydroxylamine salts, hydroxylamine hydrochloride (NH₂OH·HCl) and hydroxylamine sulfate [(NH₂OH)₂·H₂SO₄], for the degradation of dimethyl phthalate (DMP) in peroxymonosulfate (PMS)/Fe(II) system was comparatively investigated. Degradation efficiency of DMP with NH₂OH·HCl was 1.6 times of that with same dosages of (NH₂OH)₂·H₂SO₄. SO₄^·-, Fe(IV) and ·OH formed in the PMS/Fe(II)/NH₂OH system, but ·OH was the major species for DMP degradation. Addition of Cl^- significantly improved the production of ·OH and Cl·, and the exposure dose of ·OH (CT_·OH) was more than 10 times that of CT_Cl· as the concentration of Cl^- increased to 1 mM. Calculations based on branching ratios of Cl· and ·OH indicated that the reactions of Cl^- with SO₄^·- and Cl· with H₂O were not the only production sources of ·OH in the system. Further experiments with methyl phenyl sulfoxide (PMSO) as the probe indicated that Cl^- would facilitate the shift of reactive species from Fe(IV) to radicals (SO₄^·- or ·OH) in the system. Both hydroxylation and nitration intermediate products were detected in the oxidation of DMP. Cl^- promoted the formation of hydroxylation intermediates and reduced the formation of nitration intermediates. This study revealed for the first time that Cl^- could shift reactive species from Fe(IV) to radicals in PMS/Fe(II) system, raising attention to the influence of the coexisting anions (especially Cl^-) for pollutants oxidation in iron-related oxidation processes.

Kinetics of autoxidation of tartaric acid in presence of iron

The kinetics of the autoxidation reaction of tartaric acid in an air-saturated solution in the presence of Fe(II) show autocatalytic behavior with distinct initiation, propagation, and termination phases. The initiation phase, which involves activation of dissolved oxygen, decreases with increasing pH, over the test range of pH of 2.5-4.5, indicating that activation of oxygen is catalyzed by an Fe(II)-tartrate complex. The autocatalytic nature of this reaction indicates the presence of a catalytic intermediate that is produced during the initiation phase and regenerated during the propagation phase. The addition of catalase, as well as direct measurements, provided evidence of the presence and kinetic action of hydrogen peroxide as one of the intermediates. Direct addition of hydrogen peroxide resulted in shortening of the initiation stage and the propagation phase with similar rates as in the autoxidation reaction at low pH. The propagation is approximately a zero order reaction with respect to oxygen and iron. The kinetic analysis suggests that an intermediate catalytic complex(s) involving a ferryl ion (FeO²⁺) controls the rate of the propagation reaction. The Fe(III) formation shows autocatalytic behavior that mirrors the dissolved oxygen consumption patterns under all pH conditions studied. At pH values of 2.5 and 3.0, Fe(III) accumulated to a maximum, before it was partially consumed. This maximum coincided with the depletion of dissolved oxygen. The consumption of Fe(III), or the reduction of Fe(III) back to Fe(II), reflects the catalytic nature of Fe(II) and the essential role of tartaric acid in the initiation phase of Fenton's original reaction.

Rapid and Efficient Arsenic Removal by Iron Electrocoagulation Enabled with in Situ Generation of Hydrogen Peroxide

Millions of people are exposed to toxic levels of dissolved arsenic in groundwater used for drinking. Iron electrocoagulation (FeEC) has been demonstrated as an effective technology to remove arsenic at an affordable price. However, FeEC requires long operating times (∼hours) to remove dissolved arsenic due to inherent kinetics limitations. Air cathode Assisted Iron Electrocoagulation (ACAIE) overcomes this limitation by cathodically generating H₂O₂ in situ. In ACAIE operation, rapid oxidation of Fe(II) and complete oxidation and removal of As(III) are achieved. We compare FeEC and ACAIE for removing As(III) from an initial concentration of 1464 μg/L, aiming for a final concentration of less than 4 μg/L. We demonstrate that at short electrolysis times (0.5 min), i.e., high charge dosage rates (1200 C/L/min), ACAIE consistently outperformed FeEC in bringing arsenic levels to less than WHO-MCL of 10 μg/L. Using XRD and XAS data, we conclusively show that poor arsenic removal in FeEC arises from incomplete As(III) oxidation, ineffective Fe(II) oxidation and the formation of Fe(II-III) (hydr)oxides at short electrolysis times (<20 min). Finally, we report successful ACAIE performance (retention time 19 s) in removing dissolved arsenic from contaminated groundwater in rural California.

Oxygen vacancy mediated surface charge redistribution of Cu-substituted LaFeO3 for degradation of bisphenol A by efficient decomposition of H2O2

The novel catalyst LaCu_0.5Fe_0.5O_3-δ with oxygen vacancies (OVs) was prepared and demonstrated excellent stability and activity for the degradation of bisphenol A. The removal rate of 92.1 % and H₂O₂ utilization efficiency of 70.4 % were obtained due to the efficient hydroxyl radical generation mediated by OVs. The density functional theory calculation showed that the substitution of Cu and formation of OVs significantly increases the charge density near the active sites. Bader charge analysis revealed that the charge offset accelerated the reduction of Fe. The elevation of electron transfer efficiency also promotes the valence transition of copper and iron atoms. The reversible electronic transition between Fe²⁺ ⇆ Fe³⁺, Cu⁺ ⇆ Cu²⁺ and Cu²⁺ ⇆ Fe²⁺ involved in this reaction were considered to be enhanced and the homolytic bond clearage of H₂O₂ was simultaneously promoted, facilitated by the electron-rich region combined with OVs on the surface of LaCu_0.5Fe_0.5O_3-δ.

A Microbial Siderophore-Inspired Self-Gelling Hydrogel for Noninvasive Anticancer Phototherapy

Microbial carboxyl and catechol siderophores have been shown to have natural iron-chelating abilities, suggesting that hyaluronic acid (HA) and the catechol compound, gallic acid (GA), may have iron-coordinating activities. Here, a photoresponsive self-gelling hydrogel that was both injectable and could be applied to the skin was developed on the basis of the abilities of HA and GA to form coordination bonds with ferric ions (Fe³⁺). The conjugate of HA and GA (HA-GA) instantly formed hydrogels in the presence of ferric ions and showed near-infrared (NIR)-responsive photothermal properties. Following their subcutaneous injection into mice, HA-GA and ferric ion formed a hydrogel, which remained at the injection site for at least 8 days. Intratumoral injection of HA-GA/Fe hydrogel into mice allowed repeated exposure of the tumor to NIR irradiation. This repeated NIR irradiation resulted in complete tumor ablation in KB carcinoma cell-xenografted mice and suppressed lung metastasis of 4T1-Luc orthotopic breast tumors. Application of HA-GA/Fe hydrogel to the skin of A375 melanoma-xenografted tumor sites, followed by NIR irradiation, also resulted in complete tumor ablation. These findings demonstrate that single applications of HA-GA/Fe hydrogel have photothermal anticancer effects against both solid tumors and skin cancers. SIGNIFICANCE: These findings provide new insights into noninvasive anticancer phototherapy using self-gelling hydrogels. Application of these hydrogels in preclinical models reduces the sizes of solid tumors and skin cancers without surgery, radiation, or chemotherapy.

On the Inhibition of Hydroxyl Radical Formation by Hydroxycinnamic Acids: The Case of Caffeic Acid as a Promising Chelating Ligand of a Ferrous Ion

Density functional theory has been employed here to explore the ability of caffeic acid (CA) to trap Fe(II) to prevent the Fenton reaction thus limiting the hydroxyl radical formation. Electronic and structural features of complexes for metal-to-ligand different ratios were fully elucidated. Results confirm that the anionic forms of CA are able to form very stable complexes and show that all the possible coordination modes lead to formation of complexes that are thermochemically accessible. In addition, the change in free energies for the oxidation reaction, according to which hydrogen peroxide directly interacts with the metal center to produce the hydroxyl radical, confirms that Fe(II) complexed by CA is less active toward H₂O₂ than the purely solvated one. Even the energy required for the ligand exchange (H₂O₂ in place of water), supposed to be the first step involved in the Fenton reaction in a physiological environment, supports the propensity of CA to deactivate the hydroxyl radical formation by sequestering the ferrous ion. The rationalization of absorption spectra for various Fe(II)-CA complexes shows neutral and monoanionic species as conceivable ligands of the ferrous ion and the carboxylic group as the most probable site of coordination.

Why the Reactive Oxygen Species of the Fenton Reaction Switches from Oxoiron(IV) Species to Hydroxyl Radical in Phosphate Buffer Solutions? A Computational Rationale

It has been shown that the major reactive oxygen species (ROS) generated by the aqueous reaction of Fe(II) and H2O2 (i.e., the Fenton reaction) are high-valent oxoiron(IV) species, whereas the hydroxyl radical plays a role only in very acidic conditions. Nevertheless, when the Fenton reaction is conducted in phosphate buffer solutions, the resulting ROS turns into hydroxyl radical even in neutral pH conditions. The present density functional theory (DFT) study discloses the underlying principle for this phenomenon. Static and dynamic DFT calculations indicate that in phosphate buffer solutions, the iron ion is highly coordinated by phosphoric acid anions. Such a coordination environment substantially raises the pK a of coordinated water on Fe(III). As a consequence, the Fe(III)-OH intermediate, resulting from the reductive decomposition of H2O2 by ferrous ion is relatively unstable and will be readily protonated by phosphoric acid ligand or by free proton in solution. These proton-transfer reactions, which become energetically favorable when the number of phosphate coordination goes up to three, prevent the Fe(III)-OH from hydrogen abstraction by nascent •OH to form Fe(IV)=O species. On the basis of this finding, a ligand design strategy toward controlling the nature of ROS produced in the Fenton reaction is put forth. In addition, it is found that while phosphate buffers facilitate •OH radical generation in the Fenton reaction, phosphoric acid anions can act as •OH radical scavengers through hydrogen atom transfer reactions.

The construction of accelerated catalytic Fenton reaction based on Pd/MIL-101(Cr) and H2

A novel catalytic Fenton system based on H₂ and the solid catalyst Pd/MIL-101(Cr) (MHACF-MIL-101(Cr)) was developed at normal temperature and pressure. In this system, the reduction process of Fe^III back to Fe^II was accelerated significantly.