pH effects on iron-catalyzed oxidation using Fenton's reagent

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.

Catalyst durability in electrocatalytic H2O2 production: key factors and challenges

On-demand electrocatalytic hydrogen peroxide (H2O2) production is a significant technological advancement that offers a promising alternative to the traditional anthraquinone process. This approach leverages electrocatalysts for the selective reduction of oxygen through a two-electron transfer mechanism (ORR-2e-), holding great promise for delivering a sustainable and economically efficient means of H2O2 production. However, the harsh operating conditions during the electrochemical H2O2 production lead to the degradation of both structural integrity and catalytic efficacy in these materials. Here, we systematically examine the design strategies and materials typically utilized in the electroproduction of H2O2 in acidic environments. We delve into the prevalent reactor conditions and scrutinize the factors contributing to catalyst deactivation. Additionally, we propose standardised benchmarking protocols aimed at evaluating catalyst stability under such rigorous conditions. To this end, we advocate for the adoption of three distinct accelerated stress tests to comprehensively assess catalyst performance and durability.

Nanocatalysts for modulating antitumor immunity: fabrication, mechanisms and applications

Immunotherapy harnesses the inherent immune system in the body to generate systemic antitumor immunity, offering a promising modality for defending against cancer. However, tumor immunosuppression and evasion seriously restrict the immune response rates in clinical settings. Catalytic nanomedicines can transform tumoral substances/metabolites into therapeutic products in situ, offering unique advantages in antitumor immunotherapy. Through catalytic reactions, both tumor eradication and immune regulation can be simultaneously achieved, favoring the development of systemic antitumor immunity. In recent years, with advancements in catalytic chemistry and nanotechnology, catalytic nanomedicines based on nanozymes, photocatalysts, sonocatalysts, Fenton catalysts, electrocatalysts, piezocatalysts, thermocatalysts and radiocatalysts have been rapidly developed with vast applications in cancer immunotherapy. This review provides an introduction to the fabrication of catalytic nanomedicines with an emphasis on their structures and engineering strategies. Furthermore, the catalytic substrates and state-of-the-art applications of nanocatalysts in cancer immunotherapy have also been outlined and discussed. The relationships between nanostructures and immune regulating performance of catalytic nanomedicines are highlighted to provide a deep understanding of their working mechanisms in the tumor microenvironment. Finally, the challenges and development trends are revealed, aiming to provide new insights for the future development of nanocatalysts in catalytic immunotherapy.

An electrochlorination process integrating enhanced oxidation of phosphonate to orthophosphate and elimination: Verification of matrix chloridion-induced oxidation mechanism

Phosphonate used as scale inhibitor is a non-negligible eutrophic contaminant in corresponding polluted waters. Besides, its conversion to orthophosphate (ortho-P) is a precondition for realizing bioavailable phosphorus recovery. Due to the feeble degradation efficiency with less than 30 % from classical Fenton commonly used in industrial wastewater treatment and itself vulnerable to strong inhibition interference of matrix chloride ions, we proposed an electrochemical approach to transform the native salt in the solution into oxidizing substances, sort of achieving beneficial utilization of matrix waste, and enhanced the ortho-P conversion rate of 1-Hydroxyethane-1,1-diphosphonic acid (HEDP) to 89.2 % (± 3.6 %). In electrochlorination system, it was found that HEDP rapidly complexed with Fe(II) and then coordinated in-situ Fe(III) to release free HEDP via intramolecular metal-ligand electron transfer reaction. The subsequent degradation mainly rooted in the oxidation of pivotal reactive species HClO, FeIVO2+ and 1O2, causing C-P and CC bonds to fracture in sequence. Eventually the organically bound phosphorus of HEDP was recovered as ortho-P. This study acquainted the audiences with the rare mechanism of chloridion-triggered HEDP degradation under electrochemical way, as well as offered a feasible technology for synchronous transformation of organically bound phosphorus to ortho-P and elimination from phosphonates.

Cu2O/Fe3O4/UiO-66 nanocomposite as an efficient fenton-like catalyst: Performance in organic pollutant degradation and influencing factors based machinelearning

The persistent presence of organic pollutants like dyes in water environment necessitates innovative approaches for efficient degradation. In this research, we developed an advanced hybrid catalyst by combining metal oxides (Cu2O, Fe3O4) with UiO-66, serving as a heterogeneous Fenton catalyst for for efficient RB19 breakdown in water with H2O2. The control factors to the catalytic behavior were also quantified by machine learning. Experimental results show that the catalytic performance was much better than its individual components (P < 0.05 & non-zero 95% C.I). The improved catalytic efficiency was linked to the occurrence of active metal centers (Fe, Cu, and Zr), with Cu(I) from Cu2O playing a crucial role in promoting increased production of HO•. Also, UiO-66 served as a catalyst support, attracting pollutants to the reaction center, while magnetic Fe3O4 aids catalyst recovery. The optimal experimental parameters for best performance were pH at 7, catalyst loading of 1.6 g/L, H2O2 strength of 0.16 M, and reaction temperature of 25 °C. The catalyst can be magnetically separated and regenerated after five recycling times without significantly reducing catalytic activity. The reaction time and pH were ranked as the most influencing factors on catalytic efficiency via Random Forest and SHapley Additive exPlanations models. The findings show that developed catalyst is a suitable candidate to remove dyes in water by Fenton heterogeneous reaction.

Low-cost materials for swine wastewater treatment using adsorption and Fenton's process

Untreated swine wastewater (SW) discharge leads to serious consequences such as water quality decreasing related to eutrophication and proliferation of harmful algae containing cyanotoxins, which can cause acute intoxication in humans. The use of untreated pig farming effluent as fertilizer can lead to the accumulation of polluting compounds. Biological treatments can degrade organic matter but have the disadvantage of requiring large areas and high retention times and demonstrating low efficiencies in the degradation of refractory compounds such as pharmaceutical compounds. In this ambit, the performance of four low-cost materials was evaluated for treatment of a swine wastewater using physical-chemical processes such as adsorption and Fenton's process. The tested materials are two natural resources, red volcanic rock from Canary (RVR) Islands and black volcanic rock (BVR) from Azores, and two industry residues, red mud (RM) and iron filings (IF). Among the tested materials, only IFs are catalytically active for Fenton's peroxidation. Still, RVR, BVR, and RM were efficient adsorbents removing up to 67% of COD. The combination between adsorption followed by Fenton's process using IF as catalyst showed interesting results. When RM is applied as adsorbent in the diluted effluent, it was able to remove 67% and 90% of COD for adsorption and adsorption followed by IF Fenton, respectively. At those conditions, the resultant treated effluent accomplishes the requirements for direct discharge in the natural water courses as well as the parameters for water reusing.

Degradation of the Selected Antibiotic in an Aqueous Solution by the Fenton Process: Kinetics, Products and Ecotoxicity

Sulfonamides used in veterinary medicine can be degraded via the Fenton processes. In the premise, the process should also remove the antimicrobial activity of wastewater containing antibiotics. The kinetics of sulfathiazole degradation and identification of the degradation products were investigated in the experiments. In addition, their toxicity against Vibrio fischeri, the MARA^® assay, and unselected microorganisms from a wastewater treatment plant and the river was evaluated. It was found that in the Fenton process, the sulfathiazole degradation was described by the following kinetic equation: r₀ = k C_STZ^{-1 or 0} C_Fe(II)³ C_H2O2^{0 or 1} C_TOC^-2, where r₀ is the initial reaction rate, k is the reaction rate constant, C is the concentration of sulfathiazole, Fe(II) ions, hydrogen peroxide and total organic carbon, respectively. The reaction efficiency and the useful pH range (up to pH 5) could be increased by UVa irradiation of the reaction mixture. Eighteen organic degradation products of sulfathiazole were detected and identified, and a possible degradation mechanism was proposed. An increase in the H₂O₂ dose, to obtain a high degree of mineralization of sulfonamide, resulted in an increase in the ecotoxicity of the post-reaction mixture.

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.

Multifunctional Magnetic CuS/Gd2O3 Nanoparticles for Fluorescence/Magnetic Resonance Bimodal Imaging-Guided Photothermal-Intensified Chemodynamic Synergetic Therapy of Targeted Tumors

Chemodynamic therapy (CDT), which consumes endogenous hydrogen peroxide (H₂O₂) to generate reactive oxygen species (ROS) and causes oxidative damage to tumor cells, shows tremendous promise for advanced cancer treatment. However, the rate of ROS generation based on the Fenton reaction is prone to being restricted by inadequate H₂O₂ and unattainable acidity in the hypoxic tumor microenvironment. We herein report a multifunctional nanoprobe (BCGCR) integrating bimodal imaging and photothermal-enhanced CDT of the targeted tumor, which is produced by covalent conjugation of bovine serum albumin-stabilized CuS/Gd₂O₃ nanoparticles (NPs) with the Cy5.5 fluorophore and the tumor-targeting ligand RGD. BCGCR exhibits intense near-infrared (NIR) fluorescence and acceptable r₁ relaxivity (∼15.3 mM^-1 s^-1) for both sensitive fluorescence imaging and high-spatial-resolution magnetic resonance imaging of tumors in living mice. Moreover, owing to the strong NIR absorbance from the internal CuS NPs, BCGCR can generate localized heat and displays a high photothermal conversion efficiency (30.3%) under 980 nm laser irradiation, which enables photothermal therapy and further intensifies ROS generation arising from the Cu-induced Fenton-like reaction for enhanced CDT. This synergetic effect shows such an excellent therapeutic efficacy that it can ablate xenografted tumors in vivo. We believe that this strategy will be beneficial to exploring other advanced nanomaterials for the clinical application of multimodal imaging-guided synergetic cancer therapies.

Comparative Experimental and Computational Studies of Hydroxyl and Sulfate Radical-Mediated Degradation of Simple and Complex Organic Substrates

Persulfate (PS)-based advanced oxidation processes (AOPs) have been promoted as alternatives to H₂O₂-based AOPs. To gauge the potential of this technology, the PS/Fe(II) and Fenton (H₂O₂/Fe(II)) processes were comparatively evaluated using formate as a simple target compound and nanofiltration concentrate from a municipal wastewater treatment plant as a complex suite of contaminants with the aid of kinetic modeling. In terms of the short-term rate and extent of mineralization of formate and the nanofiltration concentrate, PS/Fe(II) is less effective due to slow Fe(II)/Fe(III) cycling attributable to the scavenging of superoxide by PS. However, in the concentrate treatment, PS/Fe(II) provided a sustained removal of total organic carbon (TOC), with ∼81% removed after 7 days with SO₄^•- consistently produced via homolysis of the long-life PS. In comparison, H₂O₂/Fe(II) exhibited limited TOC removal over ∼57% after 10 h due to the futile consumption of H₂O₂ by HO^•. PS/Fe(II) also offers better performance at transforming humic-like moieties to more biodegradable compounds as a result of chlorine radicals formed by the reaction of SO₄^•- with the matrix constituents present in the concentrate. The application of PS/Fe(II) is, however, subject to the limitations of slow oxidation of organic contaminants, release of sulfate, and formation of chlorinated byproducts.

Fe-Based Theranostic Agents Respond to the Tumor Microenvironment for MRI-Guided Ferroptosis-/Apoptosis-Inducing Anticancer Therapy

Tumor microenvironment-specific magnetic resonance imaging (MRI) contrast agents are conducive to accurate diagnoses by visualization of biochemical and pathological changes for suitable treatment. Herein, we reported a pH-responsive contrast agent DFeZd NP with MRI diagnosis and tumor treatment capabilities. DFeZd NPs can map the pH change by modulating the MR signal in different acid-base environments. Moreover, T1 signals are stronger in the tumor site, which proves efficient in distinguishing malignant tumors from normal tissues, as well as demarcating the tumor boundary. Subsequently, sustained supply of Fe through the Fe-based contrast agent leads to Fe redox cycling and lipid peroxides, inducing ferroptosis in tumor cells. Furthermore, under an acidic tumor microenvironment, in the presence of ascorbic acid, increased Fe²⁺ is generated, which serves as a stronger inducer of ferroptosis. Moreover, due to the different relaxivity of Fe³⁺ and Fe²⁺, redox cycling and ferroptosis in tumors can be monitored by MRI. Therefore, we propose DFeZd NPs as accessible and promising Fe-based dopamine-derived contrast agents for specific MRI imaging and ferroptosis induction for anticancer therapy.

The principle of detailed balancing, the iron-catalyzed disproportionation of hydrogen peroxide, and the Fenton reaction

The iron-catalyzed disproportionation of H₂O₂ has been investigated for over a century, as has been its ability to induce the oxidation of other species present in the system (Fenton reaction). The mechanisms of these reactions have been under consideration at least since 1932. Unfortunately, little or no attention has been paid to ensuring the conformity of the proposed mechanisms and rate constants with the constraints of the principle of detailed balancing. Here we identify more than 200 publications having mechanisms that violate the principle of detailed balancing. These violations occur through the use of incorrect values for certain rate constants, the use of incorrect forms of the rate laws for certain steps in the mechanisms, and the inclusion of illegal loops. A core mechanism for the iron-catalyzed decomposition of H₂O₂ is proposed that is consistent with the principle of detailed balancing and includes both the one-electron oxidation of H₂O₂ by Fe(III) and the Fe(II) reduction of HO₂˙.

Degradation of sulfonated polyethylene by a bio-photo-fenton approach using glucose oxidase immobilized on titanium dioxide

Polyethylene (PE) plastics are highly recalcitrant and resistant to photo-oxidative degradation due to its chemically inert backbone structure. We applied two novel reactions such as, Bio-Fenton reaction using glucose oxidase (GOx) enzyme alone and Bio-Photo-Fenton reaction using GOx immobilized on TiO₂ nanoparticles (TiO₂-GOx) under UV radiation, for (bio)degradation of pre-activated PE with sulfonation (SPE). From both the reactions, GC-MS analyses identified small organic acids such as, acetic acid and butanoic acid as a major metabolites released from SPE. In the presence of UV radiation, 21 fold and 17 fold higher amounts of acetic acid (4.78 mM) and butanoic acid (0.17 mM) were released from SPE after 6 h of reaction using TiO₂-GOx than free GOx, which released 0.22 mM and 0.01 mM of acetic acid and butanoic acid, respectively. Our results suggest that (bio)degradation and valorization of naturally weathered and oxidized PE using combined reactions of biochemistry, photochemistry and Fenton chemistry could be possible.

Oxidative Phosphorus Chemistry Perturbed by Minerals

Life is a complex, open chemical system that must be supported with energy inputs. If one fathoms how simple early life must have been, the complexity of modern-day life is staggering by comparison. A minimally complex system that could plausibly provide pyrophosphates for early life could be the oxidation of reduced phosphorus sources such as hypophosphite and phosphite. Like all plausible prebiotic chemistries, this system would have been altered by minerals and rocks in close contact with the evolving solutions. This study addresses the different types of perturbations that minerals might have on this chemical system. This study finds that minerals may inhibit the total production of oxidized phosphorus from reduced phosphorus species, they may facilitate the production of phosphate, or they may facilitate the production of pyrophosphate. This study concludes with the idea that mineral perturbations from the environment increase the chemical complexity of this system.

In-house-prepared carbon-based Fe-doped catalysts for electro-Fenton degradation of azo dyes

Compounds used in the fashion industry effect the water bodies in the vicinity of textile factories, resulting in the visible coloration of surface water. Fe-doped graphite-based in house prepared electrodes were used in the Fenton- -like degradation of Reactive Blue 52 (RB52). The electrodes consisting of high-density graphite in three granulation sizes and three levels of Fe content were characterized using scanning electron microscopy (SEM). The amount of Fe in the electrodes and H2O2 concentration in synthetic textile wastewater were optimized. Additionally, the size of graphite grains was varied to investigate whether it effects the degradation rate. Under only 10 min of electro-Fenton degradation, a system with 10 mmol dm-3 of H2O2 and an electrode made of 7 % of Fe and 70 ?m of granulation size of graphite, degraded over 75 % of RB52, and over 99 % after 40 min of treatment. The obtained results indicate that the proposed approach could be beneficial in the field of novel materials for environmental application and that in house prepared carbon could be an excellent replacement for commercially available supports.

Iron-containing poly(ionic liquid) membranes: a heterogeneous Fenton reaction and enhanced anti-fouling ability

Iron-containing poly(ionic liquid) membranes were prepared by Cu(0)-mediated reversible deactivation radical polymerization, which was achieved to catalyze a heterogeneous Fenton reaction and realize self-cleaning of the membrane surface.

Remediation and mineralization processes for per- and polyfluoroalkyl substances (PFAS) in water: A review

Per- and polyfluoroalkyl substances (PFAS) are synthetic organic molecules used to manufacture various consumer and industrials products. In PFAS, the CF bond is stable, which renders these compounds chemically stable and prevents their breakdown. Several PFAS treatment processes such as adsorption, photolysis and photocatalysis, bioremediation, sonolysis, electrochemical oxidation, etc., have been explored and are being developed. The present review article has critically summarized degradative technologies and provides in-depth knowledge of photodegradation, electrochemical degradation, chemical oxidation, and reduction mineralization mechanism. Also, novel non-degradative technologies, including nano-adsorbents, natural and surface-modified clay minerals/zeolites, calixarene-based polymers, and molecularly imprinted polymers and adsorbents derived from biomaterials are discussed in detail. Of these novel approaches photocatalysis combined with membrane filtration or electrochemical oxidation via a treatment train approach shows promising results in removing PFAS in natural waters. The photocatalytic mineralization mechanism of PFOA is discussed, leading to recommendations for future research on novel remediation strategies for removing PFAS from water.

Heterogeneous Fenton Chemistry Revisited: Mechanistic Insights from Ferrihydrite-Mediated Oxidation of Formate and Oxalate

The heterogeneous Fenton process has been widely applied though some aspects of this process are still poorly understood. In this study, we simultaneously quantify the adsorption and decomposition of formate and H₂O₂ at pH 4.0 in the presence of freshly formed ferrihydrite and provide new insights into the ferrihydrite-induced heterogeneous Fenton mechanism with the aid of kinetic and reactive-transport modeling. Our results show that the decomposition of H₂O₂ and formate is controlled by surface-initiated reactions. Adsorbed formate occupies the surface sites otherwise available for reaction with H₂O₂ and therefore hampers the surface Fenton reactions despite the negligible accumulation of H₂O₂ on the surface. The minimal impact of methanol (an effective HO^• scavenger) on formate oxidation as well as the poor oxidation of fully adsorbed oxalate compared with the ready oxidation of partially adsorbed formate demonstrates that oxidation mainly occurs in the solid-liquid boundary layer, rather than in bulk or on the surface. This is suggested to be due to the diffusion of surface-generated HO^•, rather than surface Fe(II), to the boundary layer with the results of kinetic and reactive-transport modeling supporting this conclusion. The new findings are critical to our understanding of the removal behavior of more complex organic target species and to the design of more effective heterogeneous Fenton technologies.

"CO in a pill": Towards oral delivery of carbon monoxide for therapeutic applications

Along with the impressive achievements in understanding the endogenous signaling roles and mechanism(s) of action of carbon monoxide (CO), much research has demonstrated the potential of using CO as a therapeutic agent for treating various diseases. Because of CO's toxicity at high concentrations and the observed difference in toxicity profiles of CO depending on the route of administration, this review analyzes and presents the benefits of developing orally active CO donors. Such compounds have the potential for improved safety profiles, enhancing the chance for developing CO-based therapeutics. In this review, the difference between inhalation and oral administration in terms of toxicity, CO delivery efficiency, and the potential mechanism(s) of action is analyzed. The evolution from CO gas inhalation to oral administration is also extensively analyzed by summarizing published studies up to date. The concept of "CO in a pill" can be achieved by oral administration of novel formulations of CO gas or appropriate CO donors.

A New Mechanism of the Selective Photodegradation of Antibiotics in the Catalytic System Containing TiO2 and the Inorganic Cations

The mechanism of sulfisoxazole (SFF) selective removal by photocatalysis in the presence of titanium (IV) oxide (TiO2) and iron (III) chloride (FeCl3) was explained and the kinetics and degradation pathways of SFF and other antibiotics were compared. The effects of selected inorganic ions, oxygen conditions, pH, sorption processes and formation of coordination compounds on the photocatalytic process in the presence of TiO2 were also determined. The Fe3+ compounds added to the irradiated sulfonamide (SN) solution underwent surface sorption on TiO2 particles and act as acceptors of excited electrons. Most likely, the SFF degradation is also intensified by organic radicals or cation organic radicals. These radicals can be initially generated by reaction with electron holes, hydroxyl radicals and as a result of electron transfer mediated by iron ions and then participate in propagation processes. The high sensitivity of SFF to decomposition caused by organic radicals is associated with the steric effect and the high bond polarity of the amide substituent.

Kinetic and thermodynamic studies of the degradation of methylene blue by photo-Fenton reaction

Syrian natural magnetite has been utilized for the removal of methylene blue from aqueous solutions by photo-Fenton reaction. Experiments were carried out to evaluate the kinetic and thermodynamic parameters.

Pseudo-first order, pseudo-second-order models were used to analyze the kinetic data obtained at different initial MB concentrations and temperatures.

The photo-Fenton degradation process of MB is better described by the pseudo-first-order model. The activation energy Ea = 16.89 and 18.02 kJ/mol for MB degradation at concentrations 40 and 80 mg/l respectively, and that suggesting the degradation reaction proceeded with a low energy barrier, the values obtained (ΔG∗, ΔS∗, and ΔH∗) indicate that the process is endothermic and spontaneous.

Photothermal-Enhanced Fenton-like Catalytic Activity of Oxygen-Deficient Nanotitania for Efficient and Safe Tooth Whitening

The growing demand for charming smiles has led to the popularization of tooth bleaching procedures. Current tooth bleaching products with high-concentration hydrogen peroxide (HP, 30-40%) are effective but detrimental due to the increased risk of enamel destruction, tooth sensitivity, and gingival irritation. Herein, we reported a less-destructive and efficient tooth whitening strategy with a low-concentration HP, which was realized by the remarkably enhanced Fenton-like catalytic activity of oxygen-deficient TiO₂ (TiO_2-x). TiO_2-x nanoparticles were synthesized with a modified solid-state chemical reduction approach with NaBH₄. The Fenton-like activity of TiO_2-x was optimized by manipulating oxygen vacancy (OV) concentration and further promoted by the near-infrared (NIR)-induced photothermal effect of TiO_2-x. The TiO_2-x sample named BT45 was chosen due to the highest methylene blue (MB) adsorption ability and Fenton-like activity among acquired samples. The photothermal property of BT45 under 808 nm NIR irradiation was verified and its enhancement on Fenton-like activity was also studied. The BT45/HP + NIR group performed significantly better in tooth whitening than the HP + NIR group on various discolored teeth (stained by Orange II, tea, or rhodamine B). Excitingly, the same tooth whitening performance as the Opalescence Boost, a tooth bleaching product containing 40% HP, was obtained by a self-produced bleaching gel based on this novel system containing 12% HP. Besides, negligible enamel destruction, safe temperature range, and good cytocompatibility of TiO_2-x nanoparticles also demonstrated the safety of this tooth bleaching strategy. This work indicated that the photothermal-enhanced Fenton-like performance of the TiO_2-x-based system is highly promising in tooth bleaching application and can also be extended to other biomedical applications.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Reactive Oxygen Species-Regulating Strategies Based on Nanomaterials for Disease Treatment

Reactive oxygen species (ROS) play an essential role in physiological and pathological processes. Studies on the regulation of ROS for disease treatments have caused wide concern, mainly involving the topics in ROS-regulating therapy such as antioxidant therapy triggered by ROS scavengers and ROS-induced toxic therapy mediated by ROS-elevation agents. Benefiting from the remarkable advances of nanotechnology, a large number of nanomaterials with the ROS-regulating ability are developed to seek new and effective ROS-related nanotherapeutic modalities or nanomedicines. Although considerable achievements have been made in ROS-based nanomedicines for disease treatments, some fundamental but key questions such as the rational design principle for ROS-related nanomaterials are held in low regard. Here, the design principle can serve as the initial framework for scientists and technicians to design and optimize the ROS-regulating nanomedicines, thereby minimizing the gap of nanomedicines for biomedical application during the design stage. Herein, an overview of the current progress of ROS-associated nanomedicines in disease treatments is summarized. And then, by particularly addressing these known strategies in ROS-associated therapy, several fundamental and key principles for the design of ROS-associated nanomedicines are presented. Finally, future perspectives are also discussed in depth for the development of ROS-associated nanomedicines.

Mechanisms and pathways of ethidium bromide Fenton-like degradation by reusable magnetic nanocatalysts

Ethidium bromide (3,8-diamino-6-phenyl-5-ethylphenanthridinium bromide, EtBr) is a carcinogenic compound widely used for staining nucleic acids that is difficult to treat. In this study, magnetic nanocatalysts (MNCs) were synthesized for the heterogeneous Fenton-like degradation of EtBr. The initial pH, MNC content, and H₂O₂ concentration were the key factors affecting the EtBr degradation performance and dynamics. An EtBr removal efficiency of 98.97% was achieved within 4 h under optimal conditions (initial pH, 3.0; MNC content, 1 g/L; H₂O₂ concentration, 50 mM), and the degradation followed the ring-open pathway with (2E,4Z,8E)-3-amino-N-ethyl-7,9-dihydroxynona-2,4,8-trienamide as an intermediate, as determined by liquid chromatography and mass spectrometry (LC/MS). Unexpected and satisfactory Fenton-like oxidation of EtBr occurred under basic conditions, which was explained by a novel denitration pathway with 2-[nitro(phenyl)methyl]-(1,1'-biphenyl)-4,4'-diamine as an intermediate. The MNCs retained 62.17% of their degradation efficiency after five consecutive reaction and harvest cycles. Our work elucidated the mechanisms and pathways of EtBr removal in a Fenton-like reaction using MNCs, and comprehensively discussed the optimal reaction conditions and its potential for re-use.

Graphitic Carbon Nitride-Based Composite in Advanced Oxidation Processes for Aqueous Organic Pollutants Removal: A Review

In recent decades, a growing number of organic pollutants released have raised worldwide concern. Graphitic carbon nitride (g-C3N4) has drawn increasing attention in environmental pollutants removal thanks to its unique electronic band structure and excellent physicochemical stability. This paper reviews the recent progress of g-C3N4-based composites as catalysts in various advanced oxidation processes (AOPs), including chemical, photochemical, and electrochemical AOPs. Strategies for enhancing catalytic performance such as element-doping, nanostructure design, and heterojunction construction are summarized in detail. The catalytic degradation mechanisms are also discussed briefly.

A novel Electro-Fenton process characterized by aeration from inside a graphite felt electrode with enhanced electrogeneration of H2O2 and cycle of Fe3+/Fe2

A novel Electro-Fenton process characterized by aeration from inside a graphite felt electrode with enhanced generation of H₂O₂ and cycle of Fe³⁺/Fe²⁺ was proposed. The new type of Electro-Fenton process was used to degrade organic pollutants via graphite felt electrode aeration (GF-EA). The H₂O₂ concentration by GF-EA could reach 152-169 mg/L in a wide pH range (3-10), which was much higher than that achieved by graphite felt using solution aeration (GF-SA, 37-113 mg/L). For the degradation of nitrobenzene (NB), benzoic acid (BA), bisphenol A (BPA), and sulfamethoxazole (SMX) at pH 5.5, the percentage degradation by GF-EA could reach 55%, 56%, 80%, and 60% higher than those obtained by GF-SA, respectively. The solution TOC removal by GF-EA were enhanced by 29-51% relative to GF-SA. Mechanism analysis showed both OH and ferryl species were involved in the reaction system, and the amounts of OH and dissolved iron species in GF-EA group were 7.7 times and 4-8 times higher than those in GF-SA group, respectively. Besides, the mass transfer rate of GF-EA system was 5.4 times higher than that of GF-SA system. High amounts of H₂O₂, dissolved iron species and OH were attributed to the enhanced mass transfer of O₂ and the solution.

Magnetic field boosted ferroptosis-like cell death and responsive MRI using hybrid vesicles for cancer immunotherapy

We report a strategy to boost Fenton reaction triggered by an exogenous circularly polarized magnetic field (MF) to enhance ferroptosis-like cell-death mediated immune response, as well as endow a responsive MRI capability by using a hybrid core-shell vesicles (HCSVs). HCSVs are prepared by loading ascorbic acid (AA) in the core and poly(lactic-co-glycolic acid) shell incorporating iron oxide nanocubes (IONCs). MF triggers the release of AA, resulting in the increase of ferrous ions through the redox reaction between AA and IONCs. A significant tumor suppression is achieved by Fenton reaction-mediated ferroptosis-like cell-death. The oxidative stress induced by the Fenton reaction leads to the exposure of calreticulin on tumor cells, which leads to dendritic cells maturation and the infiltration of cytotoxic T lymphocytes in tumor. Furthermore, the depletion of ferric ions during treatment enables monitoring of the Fe reaction in MRI-R2* signal change. This strategy provides a perspective on ferroptosis-based immunotherapy.

Novel quantification of formation trend and reaction efficiency of hydroxyl radicals for investigating photocatalytic mechanism of Fe-doped TiO2 during UV and visible light-induced degradation of acid orange 7

A detailed mechanistic investigation of the hydroxyl radical (•OH) formation and organic pollutant degradation over transition metal-doped and undoped TiO₂ photocatalysts was performed by the quantitative measurement of •OH and the identification of intermediate products under various experimental conditions. The Fe-doped TiO₂ as a typical subject was prepared, characterized and used to degrade an azo dye Acid Orange 7 (AO7). It is indicated that the enhanced photocatalytic activity of Fe-doped TiO₂ for AO7 degradation was attributed to the increase in surface area, the facilitated charge transfer via Fe-dopant, and a red shift of absorbable wavelength, maintaining a great formation of •OH under visible irradiation. The oxidation of H₂O by holes was estimated as the major pathway of •OH formation rather than the reduction of dissolved O₂ by electrons, and their formation trends reached to approximately 75% and 25%, respectively. Meanwhile the synergistic effect of Fe-dopant produced nearly 10% of extra •OH by visible light photoactivation. The intermediate products and pathways of AO7 degradation varied greatly with different photocatalysts and conditions of the process, involving several reaction mechanisms such as the azo bond cleaving, naphthalene oxidation, desulfonation, and hydroxylated products generation. Through the quantification of •OH-reacted efficiency we proposed, a stoichiometry of •OH affecting overall reaction mechanisms in the TiO₂-assisted photodegradation of AO7 was further established. This study can provide new insights on how to better clarify the variation regularity of organic pollutant degradation from different treatments of the •OH-based advanced oxidation processes.

Fabrication of iron-dipicolinamide catalyst with Fe-N bonds for enhancing non-radical reactive species under alkaline Fenton process

Iron dipicolinamide (Fedpa), as an efficient Fenton-like catalyst, was fabricated to excite hydrogen peroxide (H₂O₂) for the removal of 2,4-dichlorophenol (2,4-DCP). The unique structures and the electronic properties of Fedpa were contributed to its excellent catalytic performance in alkaline Fenton process. Fe was chelated with dpa by four Fe-N bonds leaved two labile sites, which reduced the oxidation potential of dpa[Fe^III/Fe^II], dpa[Fe^V/Fe^III] or dpa[Fe^IV/Fe^II] to 0.316 V and 1.189 V respectively, and made it easily be bound with H₂O₂ to initiate the reaction. The results showed that 99.5% removal rate of 2,4-DCP (0.58 mM) was achieved by using 0.027 g/L Fedpa and 5.8 mM H₂O₂ in 60 min at pH 9.9. The coordination between Fe and dpa enhanced the catalytic efficiency of Fe^II. The active species generated in Fedpa/H₂O₂ system contained the iron-oxo species (dpaFe^V = O or dpa^IV = O), O₂^- and HO. The iron-oxo species was the main non-radical reactive species for the degradation of 2,4-DCP and some degradation intermediates were detected by GC-QTOF. Furthermore, the influence of factors, such as Fedpa loading, solution pH, temperature and anions (F^-, Cl^-, SO₄^2-, NO₃^- and PO₄^3-) on the catalytic performance of Fedpa were also discussed. This process of complexation between Fe and dpa combined with a green oxidant H₂O₂ presents a new insight for the use of Fenton-like system in the degradation of refractory organics.

Rapid Removal of Azophloxine via Catalytic Degradation by a Novel Heterogeneous Catalyst under Visible Light

Azo dyes are the most widely used synthetic dyes in the printing and dyeing process. However, the discharge of untreated azo dyes poses a potential threat to aqueous ecosystems and human health. Herein, we fabricated a novel heterogeneous catalyst: activated-carbon-fiber-supported ferric alginate (FeAlg-ACF). Together with peroxymonosulfate (PMS) and visible light, this photocatalytic oxidation system was used to remove an azo dye—azophloxine. The results indicated that the proposed catalytic oxidation system can remove 100% of azophloxine within 24 min, while under the same system, the removal rates were only 92% and 84% when ferric alginate was replaced with ferric citrate and ferric oxalate, respectively, which showed the superiority of FeAlg-ACF. The degradation of azophloxine is achieved by the active radicals (SO4•− and •OH) released from PMS and persistent free radicals from activated carbon fiber. Moreover, due to ferric alginate’s highly intrinsic photosensitivity, visible radiation can further enhance the ligand-to-metal charge transfer (LMCT) processes. After 24 min of treatment, the total organic carbon of the azophloxine solution (50 μmol/L) decreased from 1.82 mg/L to 79.3 μg/L and the concentration of nitrate ions increased from 0.3 mg/L to 8.6 mg/L. That is, up to 93.5% of azophloxine molecules were completely degraded into inorganic compounds. Consequently, potential secondary contamination by intermediate organic products during catalytic degradation was prohibited. The azophloxine removal ratio was kept almost constant after seven cycles, indicating the recyclability and longevity of this system. Furthermore, the azophloxine removal was still promising at high concentrations of Cl−, HCO3−, and CO32−. Therefore, our proposed system is potentially effective at removing dye pollutants from seawater. It provides a feasible method for the development of efficient and environmentally friendly PMS activation technology combined with FeAlg-ACF, which has significant academic and application value.

Metabisulfite as an Unconventional Reagent for Green Oxidation of Emerging Contaminants Using an Iron-Based Catalyst

In this work, contaminants of emerging concern were catalytically degraded in the homogeneous phase with the use of unconventional green reagents. Three reagents, namely, sulfite, metabisulfite, and persulfate, were tested and compared with conventional hydrogen peroxide in the degradation process activated by Fe-TAML. The latter is a biodegradable, homogeneous tetra-amido macrocyclic ligand catalyst containing iron(III). Metabisulfite showed the highest efficiency among the three tested reagents, and its reactivity was similar to that of H₂O₂. However, metabisulfite is a safer and cleaner reagent compared to H₂O₂. A comprehensive study of the activity of metabisulfite with Fe-TAML was carried out toward the oxidative degradation of eight contaminants of emerging concern. The catalytic process was tested at different pH values (7, 9, and 11). Metabisulfite showed the highest activity at pH 11, completely degrading some of the tested micropollutants, but in several cases, the system was active at pH 9 as well. In particular, metabisulfite showed the best efficiency toward phenolic compounds. A preliminary study on the reaction mechanism and the nature of the active species in the Fe-TAML/metabisulfite system was also conducted, highlighting that a high-valent iron-oxo species might be involved in the degradation pathways.

Decreasing COD in Sugarcane Vinasse Using the Fenton Reaction: The Effect of Processing Parameters

An experiment on Fenton degradation of sugarcane vinasse was carried out to determine its effect on the wastewater characteristics. Vinasse, a by-product of distillation in the bioethanol industry, contains high organic matter, as the value of chemical oxygen demand (COD) is >100,000 mg/L and BOD5 is 31,250 mg/L. The Fenton reaction is one of the advanced oxidation process (AOP) methods which has been widely applied for the treatment of wastewater containing organic pollutants and contaminants. This method utilizes hydroxyl radical (•OH) produced from the catalyzing reaction between Fe2+ or Fe3+ and hydrogen peroxide. The effect of pH, the ratio of [H2O2] and [COD], and the ratio of [H2O2] to [Fe3+] were studied in this research to evaluate the Fenton reaction. Results from this experiment showed that treatment of vinasse using the Fenton reaction decreased the COD value to 48.10%, and its biodegradability enhanced almost two times at a pH value of 3.8, a ratio of [H2O2] to [COD] of 0.62, and a ratio of [H2O2] to [Fe3+] of 50 (g/g), which demonstrated that the Fenton treatment was effective to reduce organic matter of sugarcane vinasse. Three kinetic models (first order, second order, and Behnajad–Modirshahla–Ghanbery (BMG) kinetic model) were used to evaluate the degradation of the COD value. On the basis of the value of R2 (coefficient of determination), we suggested that BMG represented the best kinetic model. This study finds that the Fenton treatment is able to mitigate the environmental impacts of sugarcane vinasse.

Deciphering Biosignatures in Planetary Contexts

Microbial life permeates Earth's critical zone and has likely inhabited nearly all our planet's surface and near subsurface since before the beginning of the sedimentary rock record. Given the vast time that Earth has been teeming with life, do astrobiologists truly understand what geological features untouched by biological processes would look like? In the search for extraterrestrial life in the Universe, it is critical to determine what constitutes a biosignature across multiple scales, and how this compares with "abiosignatures" formed by nonliving processes. Developing standards for abiotic and biotic characteristics would provide quantitative metrics for comparison across different data types and observational time frames. The evidence for life detection falls into three categories of biosignatures: (1) substances, such as elemental abundances, isotopes, molecules, allotropes, enantiomers, minerals, and their associated properties; (2) objects that are physical features such as mats, fossils including trace-fossils and microbialites (stromatolites), and concretions; and (3) patterns, such as physical three-dimensional or conceptual n-dimensional relationships of physical or chemical phenomena, including patterns of intermolecular abundances of organic homologues, and patterns of stable isotopic abundances between and within compounds. Five key challenges that warrant future exploration by the astrobiology community include the following: (1) examining phenomena at the "right" spatial scales because biosignatures may elude us if not examined with the appropriate instrumentation or modeling approach at that specific scale; (2) identifying the precise context across multiple spatial and temporal scales to understand how tangible biosignatures may or may not be preserved; (3) increasing capability to mine big data sets to reveal relationships, for example, how Earth's mineral diversity may have evolved in conjunction with life; (4) leveraging cyberinfrastructure for data management of biosignature types, characteristics, and classifications; and (5) using three-dimensional to n-D representations of biotic and abiotic models overlain on multiple overlapping spatial and temporal relationships to provide new insights.