Sustainable and feasible reagent-free electro-Fenton via sequential dual-cathode electrocatalysis

Electrochemical Hydrogen Peroxide Generation and Activation Using a Dual-Cathode Flow-Through Treatment System: Enhanced Selectivity for Contaminant Removal by Electrostatic Repulsion

To oxidize trace concentrations of organic contaminants under conditions relevant to surface- and groundwater, air-diffusion cathodes were coupled to stainless-steel cathodes that convert atmospheric O2 into hydrogen peroxide (H2O2), which then was activated to produce hydroxyl radicals (·OH). By separating H2O2 generation from its activation and employing a flow-through electrode consisting of stainless-steel fibers, the two processes could be operated efficiently in a manner that overcame mass-transfer limitations for O2, H2O2, and trace organic contaminants. The flexibility resulting from separate control of the two processes made it possible to avoid both the accumulation of excess H2O2 and the energy losses that take place after H2O2 has been depleted. The decrease in treatment efficacy occurring in the presence of natural organic matter was substantially lower than that typically observed in homogeneous advanced oxidation processes. Experiments conducted with ionized and neutral compounds indicated that electrostatic repulsion prevented negatively charged ·OH scavengers from interfering with the oxidation of neutral contaminants. Energy consumption by the dual-cathode system was lower than values reported for other technologies intended for small-scale drinking water treatment systems. The coordinated operation of these two cathodes has the potential to provide a practical, inexpensive way for point-of-use drinking water treatment.

Selective Oxidation of sp-Bonded Carbon in Graphdiyne/Carbon Nanotubes Heterostructures to Form Dominant Epoxy Groups for Two-Electron Oxygen Reduction

Two-electron oxygen reduction reaction (2e- ORR) is of great significance to H2O2 production and reversible nonalkaline Zn-air batteries (ZABs). Multiple oxygen-containing sp2-bonded nanocarbons have been developed as electrocatalysts for 2e- ORR, but they still suffer from poor activity and stability due to the limited and mixed active sites at the edges as well as hydrophilic character. Herein, graphdiyne (GDY) with rich sp-C bonds is studied for enhanced 2e- ORR. First, computational studies show that GDY has a favorable formation energy for producing five-membered epoxy ring-dominated groups, which is selective toward the 2e- ORR pathway. Then based on the difference in chemical activity of sp-C bonds in GDY and sp2-C bonds in CNTs, we experimentally achieved conductive and hydrophobic carbon nanotubes (CNTs) covering O-modified GDY (CNTs/GDY-O) through a mild oxidation treatment combined with an in situ CNTs growth approach. Consequently, the CNTs/GDY-O exhibits an average Faraday efficiency of 91.8% toward H2O2 production and record stability over 330 h in neutral media. As a cathode electrocatalyst, it greatly extends the lifetime of 2e- nonalkaline ZABs at both room and subzero temperatures.

Electronic structure modulation of iron sites with fluorine coordination enables ultra-effective H2O2 activation

Electronic structure modulation of active sites is critical important in Fenton catalysis as it offers a promising strategy for boosting H2O2 activation. However, efficient generation of hydroxyl radicals (•OH) is often limited to the unoptimized coordination environment of active sites. Herein, we report the rational design and synthesis of iron oxyfluoride (FeOF), whose iron sites strongly coordinate with the most electronegative fluorine atoms in a characteristic moiety of F-(Fe(III)O3)-F, for effective H2O2 activation with potent •OH generation. Results demonstrate that the fluorine coordination plays a pivotal role in lowering the local electron density and optimizing the electronic structures of iron sites, thus facilitating the rate-limiting H2O2 adsorption and subsequent peroxyl bond cleavage reactions. Consequently, FeOF exhibits a significant and pH-adaptive •OH yield (~450 µM) with high selectivity, which is 1 ~ 3 orders of magnitude higher than the state-of-the-art iron-based catalysts, leading to excellent degradation activities against various organic pollutants at neutral condition. This work provides fundamental insights into the function of fluorine coordination in boosting Fenton catalysis at atomic level, which may inspire the design of efficient active sites for sustainable environmental remediation.

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.

Water Flow-Driven Coupling Process of Anodic Oxygen Evolution and Cathodic Oxygen Activation for Water Decontamination and Prevention of Chlorinated Byproducts

Electrochemical advanced oxidation process (EAOP) is a promising technology for decentralized water decontamination but is subject to parasitic anodic oxygen evolution and formation of toxic chlorinated byproducts in the presence of Cl-. To address this issue, we developed a novel electrolytic process by water flow-driven coupling of anodic oxygen evolution reaction (OER) and cathodic molecular oxygen activation (MOA). When water flows from anode to cathode, O2 produced from OER is carried by water through convection, followed by being activated by atomic hydrogen (H*) on Pd cathode to produce •OH. The water flow-driven OER/MOA process enables the anode to be polarized at low potential (1.7 V vs SHE) that is lower than that of conventional EAOP whose •OH is produced from direct water oxidation (>2.3 V vs SHE). At a flow rate of 30 mL min-1, the process could achieve 94.8% removal of 2,4-dichlorophenol (2,4-DCP) and 71.5% removal of chemical oxygen demand (COD) within 45 min at an anode potential of 1.7 V vs SHE and cathode potential of -0.5 V vs SHE. To achieve the comparable 2,4-DCP removal performance, 4.3-fold higher energy consumption was needed for the conventional EAOP with titanium suboxide anode (anode potential of 2.9 V vs SHE), but current efficiency declined by 3.5 folds. Unlike conventional EAOP, chlorate and perchlorate were not detected in the OER/MOA process, because low anode potential <2.0 V vs SHE was thermodynamically unfavorable for the formation of chlorinated byproducts by anodic oxidation, indicated by theoretical calculations and experimental data. This study provides a proof-in-concept demonstration of water flow-driven OER/MOA process, representing a paradigm shift of electrochemical technology for water decontamination and prevention of chlorinated byproducts, making electrochemical water decontamination more efficient, more economic, and more sustainable.

Recent Advances of Electrocatalyst and Cell Design for Hydrogen Peroxide Production

Electrochemical synthesis of H2O2 via a selective two-electron oxygen reduction reaction has emerged as an attractive alternative to the current energy-consuming anthraquinone process. Herein, the progress on electrocatalysts for H2O2 generation, including noble metal, transition metal-based, and carbon-based materials, is summarized. At first, the design strategies employed to obtain electrocatalysts with high electroactivity and high selectivity are highlighted. Then, the critical roles of the geometry of the electrodes and the type of reactor in striking a balance to boost the H2O2 selectivity and reaction rate are systematically discussed. After that, a potential strategy to combine the complementary properties of the catalysts and the reactor for optimal selectivity and overall yield is illustrated. Finally, the remaining challenges and promising opportunities for high-efficient H2O2 electrochemical production are highlighted for future studies.

Tuning Fe3O4 for sustainable cathodic heterogeneous electro-Fenton catalysis by acetylated chitosan

Peroxidase-like catalysts are safe and low-cost candidates to tackle the dilemma in constructing sustainable cathodic heterogeneous electro-Fenton (CHEF) catalysts for water purification, but the elusive structure-property relationship of enzyme-like catalysts constitutes a pressing challenge for the advancement of CHEF processes in practically relevant water and wastewater treatment. Herein, we probe the origins of catalytic efficiency in the CHEF process by artificially tailoring the peroxidase-like activity of Fe3O4 through a series of acetylated chitosan-based hydrogels, which serve as ecofriendly alternatives to traditional carbon shells. The optimized acetylated chitosan wrapping Fe3O4 hydrogel on the cathode shows an impressive activity and stability in CHEF process, overcoming the complicated and environmentally unfavored procedures in the electro-Fenton-related processes. Structural characterizations and theoretical calculations reveal that the amide group in chitosan can modulate the intrinsic redox capacity of surficial Fe sites on Fe3O4 toward CHEF catalysis via the neutral hydrogen bond. This work provides a sustainable path and molecule-level insight for the rational design of high-efficiency CHEF catalysts and beyond.

Resolving the "health vs environment" dilemma with sustainable disinfection during the COVID-19 pandemic

The overuse of disinfection during the COVID-19 pandemic leads to an emerging "health versus environment" dilemma that humans have to face. Irresponsible and unnecessary disinfection should be avoided, while comprehensive evaluation of the health and environmental impacts of different disinfectants is urgently needed. From this discussion, we reach a tentative conclusion that hydrogen peroxide is a green disinfectant. Its on-demand production enables a circular economy model to solve the storage issues. Water, oxygen, and electrons are the only feedstock to generate H₂O₂. Upon completion of disinfection, H₂O₂ is rapidly converted back into water and oxygen. This model adopts several principles of green chemistry to ensure overall sustainability along the three stages of its whole life cycle, i.e., production, disinfection, and decomposition. Physical methods, particularly UV irradiation, also provide sustainable disinfection with minimal health and environmental impacts.

In situ turning defects of exfoliated Ti3C2 MXene into Fenton-like catalytic active sites

Controllable in situ formation of nanoclusters with discrete active sites is highly desirable in heterogeneous catalysis. Herein, a titanium oxide-based Fenton-like catalyst is constructed using exfoliated Ti₃C₂ MXene as a template. Theoretical calculations reveal that a redox reaction between the surface Ti-deficit vacancies of the exfoliated Ti₃C₂ MXene and H₂O₂ molecules facilitates the in situ conversion of surface defects into titanium oxide nanoclusters anchoring on amorphous carbon (TiO_x@C). The presence of mixed-valence Ti^δ+ (δ = 0, 2, 3, and 4) within TiO_x@C is confirmed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) characterizations. The abundant surface defects within TiO_x@C effectively promote the generation of reactive oxygen species (ROS) leading to superior and stable Fenton-like catalytic degradation of atrazine, a typical agricultural herbicide. Such an in situ construction of Fenton-like catalysts through defect engineering also applies to other MXene family materials, such as V₂C and Nb₂C.

Manipulating Selectivity of Hydroxyl Radical Generation by Single-Atom Catalysts in Catalytic Ozonation: Surface or Solution

Hydroxyl radical-dominated oxidation in catalytic ozonation is, in particular, important in water treatment scenarios for removing organic contaminants, but the mechanism about ozone-based radical oxidation processes is still unclear. Here, we prepared a series of transitional metal (Co, Mn, Ni) single-atom catalysts (SACs) anchored on graphitic carbon nitride to accelerate ozone decomposition and produce highly reactive ·OH for oxidative destruction of a water pollutant, oxalic acid (OA). We experimentally observed that, depending on the metal type, OA oxidation occurred dominantly either in the bulk phase, which was the case for the Mn catalyst, or via a combination of the bulk phase and surface reaction, which was the case for the Co catalyst. We further performed density functional theory simulations and in situ X-ray absorption spectroscopy to propose that the ozone activation pathway differs depending on the oxygen binding energy of metal, primarily due to differential adsorption of O₃ onto metal sites and differential coordination configuration of a key intermediate species, *OO, which is collectively responsible for the observed differences in oxidation mechanisms and kinetics.

Ferromagnetic Biochar Prepared from Hydrothermally Modified Calcined Mango Seeds for Fenton-like Degradation of Indigo Carmine

Biochar and ferromagnetic biochar obtained from the pyrolysis of dried mango seeds and modified using a hydrothermal method were used as catalyst for the heterogeneous degradation of indigo carmine in an aqueous medium. These prepared biochars were characterized using different techniques: Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The analyses of the results revealed the presence of iron oxide in the form of magnetite (Fe3O4) in the catalyst. The catalytic tests carried out with this composite material showed a significant degradation of indigo carmine. The maximum degradation of indigo carmine in the aqueous solution was reached after 240 min of agitation. The Fenton degradation process using irradiation with a 100 W electric lamp and hydrogen peroxide (concentration 4 mol/L) showed the best results at pH = 3. From this study, it emerged that the second-order kinetic model better described the degradation process, and it gave lower half-lives compared to those obtained with the first-order kinetic law. The study also showed that ferromagnetic biochar could be prepared from mango seeds and used for the degradation of indigo carmine in an aqueous solution.

Taguchi L16 (44) orthogonal array-based study and thermodynamics analysis for electro-Fenton process treatment of textile industrial dye

Reactive orange 16 (RO16) is the most widely used azo dye in Textile industry. Complex aromatic structures and resistivity to biological decay caused the dye pollutants incompletely treated by the conventional oxidative methods. The current study presents the electro-Fenton-based advanced oxidation treatment of RO16 dye and the process optimization by Taguchi-based design of experiment (DOE). Using a 500 mL volume lab-scale experimental setup, the process was first studied for the principal operational parameters (initial dye concentration (q); [H2O2]/[Fe+2] (R); current density (ρ); and temperature (T)) effect on decolourization (D R ) and COD removal (C R ). Then, by means of the L16 (44) orthogonal array (OA) formation, standard mean and signal-to-noise (S/N) ratio, the process was optimized for the response variables. The result showed the optimized result at q = 100 mg/L, R = 100, ρ = 8 mA/cm2, and T = 32 °C; with D R and C R as 90.023 and 84.344%, respectively. It was found that the current density affects the process most, followed by [H2O2]/[Fe+2] ratio, initial dye concentration, and temperature i.e., ρ > R > q > T. Also, with the analysis of variance (ANOVA), model equations for D R and C R were developed and its accuracy was verified for experimental results. At optimized conditions, the first order removal rate constants (k a ) were found from batch results. Additionally, the thermodynamic constants (ΔH e , ΔS e , and ΔG b ) were also calculated for the nature of heat-energy involved and temperature effect study on dye degradation. The results showed that the process was thermodynamically feasible, endothermic, and non-spontaneous with a lower energy barrier (E A  = 46.7 kJ mol−1).

Iron-based advanced oxidation processes for enhancing sludge dewaterability: State of the art, challenges, and sludge reuse

The increasing amount of sewage sludge produced in wastewater treatment plants (WWTPs) poses a great challenge to both environment and economy globally. As a requisite process during sludge treatment, sludge dewatering can significantly minimize the sludge volume and lower the operational cost for downstream transportation and disposal. Iron-based advanced oxidation process (AOP), a robust and cost-effective technique with relatively low technical barriers for high-level sludge dewatering, has been widely explored in the past 20 years. The development was mainly driven by the demands of efficient and sustainable sludge conditioning technology and the flexible sludge management approaches. The application of iron-based AOPs in sludge dewatering process attracts more and more attention. In this work, we discussed the current application of iron-based AOPs technology in the sludge dewatering processes in a holistic manner, summarized the factors affecting the sludge dewaterability in the treatment processes, and analyzed the mechanisms of iron-based AOPs to improve dewatering processes. Furthermore, we elaborated potential advantages, limitations, and challenges associated with implementing iron-based AOPs in the full-scale plants and shared the opportunities for sludge reutilization. This review aims to contribute to the development of highly efficient iron-based AOPs for sludge dewatering and offer perspectives and directions towards the new-generation of WWTPs with the sustainable and eco-friendly benefits.

Coupling Co-N-C with MXenes Yields Highly Efficient Catalysts for H2O2 Production in Acidic Media

The electrochemical oxygen reduction reaction (ORR) offers a promising method to replace the anthraquinone process for hydrogen peroxide (H₂O₂) production. However, the efficiency of this process suffers from sluggish kinetics, particularly in an acidic environment. Therefore, employing catalysts with high electroactivity is highly desirable for H₂O₂ synthesis. Here, an effective strategy for preparing Co-N-C/Ti₃C₂T_x with high H₂O₂ selectivity and ORR reactivity is proposed. The acquired Co-N-C/Ti₃C₂T_x shows excellent H₂O₂ electrosynthesis performance in acidic media with H₂O₂ productivity of up to 3200 ppm h^-1, superior to state-of-the-art catalysts. Interestingly, a H₂O₂ concentration of 6.0 wt % was obtained after the stability test, and the Co-N-C/Ti₃C₂T_x catalyst was found to effectively catalyze organic dye degradation. Further analysis reveals that the enhanced H₂O₂ electrosynthesis performance originates from the layered structure and the oxygen functional groups of Ti₃C₂T_x. The layered structure can effectively promote increased exposure of active sites, while the oxygen functional groups will fine-tune the electronic structure of Co atoms, allowing a selective ORR pathway to produce H₂O₂. This work provides a strategy to design and fabricate highly efficient catalysts for H₂O₂ production and degradation of organic pollutants.

Identification of Fenton-like active Cu sites by heteroatom modulation of electronic density

The Fenton-like process based on peroxymonosulfate (PMS) has been widely investigated and recognized as a promising alternative in recent years for the degradation of persistent organic pollutants. However, the sluggish kinetics of PMS activation results in prohibitive costs and substantial chemical inputs, impeding its practical applications in water purification. This work demonstrates that tuning the electronic structure of single-atom sites at the atomic level is a powerful approach to achieve superior PMS activation kinetics. The Cu-based catalyst with the optimized electronic structure exhibits superior performance over most of the state-of-the-art heterogeneous Fenton-like catalysts, while homogeneous Cu(II) shows very poor activity. This work provides insights into the electronic structure regulation of metal centers and structure–activity relationship at the atomic level.

Developing heterogeneous catalysts with atomically dispersed active sites is vital to boost peroxymonosulfate (PMS) activation for Fenton-like activity, but how to controllably adjust the electronic configuration of metal centers to further improve the activation kinetics still remains a great challenge. Herein, we report a systematic investigation into heteroatom-doped engineering for tuning the electronic structure of Cu-N₄ sites by integrating electron-deficient boron (B) or electron-rich phosphorus (P) heteroatoms into carbon substrate for PMS activation. The electron-depleted Cu-N₄/C-B is found to exhibit the most active oxidation capacity among the prepared Cu-N₄ single-atom catalysts, which is at the top rankings of the Cu-based catalysts and is superior to most of the state-of-the-art heterogeneous Fenton-like catalysts. Conversely, the electron-enriched Cu-N₄/C-P induces a decrease in PMS activation. Both experimental results and theoretical simulations unravel that the long-range interaction with B atoms decreases the electronic density of Cu active sites and down-shifts the d-band center, and thereby optimizes the adsorption energy for PMS activation. This study provides an approach to finely control the electronic structure of Cu-N₄ sites at the atomic level and is expected to guide the design of smart Fenton-like catalysts.