Fenton-like activity and pathway modulation via single-atom sites and pollutants comediates the electron transfer process.

The studies on the origin of versatile oxidation pathways toward targeted pollutants in the single-atom catalysts (SACs)/peroxymonosulfate (PMS) systems were always associated with the coordination structures rather than the perspective of pollutant characteristics, and the analysis of mechanism commonality is lacking. In this work, a variety of single-atom catalysts (M-SACs, M: Fe, Co, and Cu) were fabricated via a pyrolysis process using lignin as the complexation agent and substrate precursor. Sixteen kinds of commonly detected pollutants in various references were selected, and their lnkobs values in M-SACs/PMS systems correlated well (R2 = 0.832 to 0.883) with their electrophilic indexes (reflecting the electron accepting/donating ability of the pollutants) as well as the energy gap (R2 = 0.801 to 0.840) between the pollutants and M-SACs/PMS complexes. Both the electron transfer process (ETP) and radical pathways can be significantly enhanced in the M-SACs/PMS systems, while radical oxidation was overwhelmed by the ETP oxidation toward the pollutants with lower electrophilic indexes. In contrast, pollutants with higher electrophilic indexes represented the weaker electron-donating capacity to the M-SACs/PMS complexes, which resulted in the weaker ETP oxidation accompanied with noticeable radical oxidation. In addition, the ETP oxidation in different M-SACs/PMS systems can be regulated via the energy gaps between the M-SACs/PMS complexes and pollutants. As a result, the Fenton-like activities in the M-SACs/PMS systems could be well modulated by the reaction pathways, which were determined by both electrophilic indexes of pollutants and single-atom sites. This work provided a strategy to establish PMS-based AOP systems with tunable oxidation capacities and pathways for high-efficiency organic decontamination.

Effective green treatment of sewage sludge from Fenton reactions: Utilizing MoS2 for sustainable resource recovery.

Effectively managing sewage sludge from Fenton reactions in an eco-friendly way is vital for Fenton technology's viability in pollution treatment. This study focuses on sewage sludge across various treatment stages, including generation, concentration, dehydration, and landfill, and employs chemical composite MoS2 to facilitate green resource utilization of all types of sludge. MoS2, with exposed Mo4+ and low-coordination sulfur, enhances iron cycling and creates an acidic microenvironment on the sludge surface. The MoS2-modified iron sludge exhibits outstanding (>95%) phenol and pollutant degradation in hydrogen peroxide and peroxymonosulfate-based Fenton systems, unlike unmodified sludge. This modified sludge maintains excellent Fenton activity in various water conditions and with multiple anions, allowing extended phenol degradation for over 14 d. Notably, the generated chemical oxygen demand (COD) in sludge modification process can be efficiently eliminated through the Fenton reaction, ensuring effluent COD compliance and enabling eco-friendly sewage sludge resource utilization.

Local O2 concentrating boosts the electro-Fenton process for energy-efficient water remediation.

The electro-Fenton process is a state-of-the-art water treatment technology used to remove organic contaminants. However, the low O2 utilization efficiency (OUE, <1%) and high energy consumption remain the biggest obstacles to practical application. Here, we propose a local O2 concentrating (LOC) approach to increase the OUE by over 11-fold compared to the conventional simple O2 diffusion route. Due to the well-designed molecular structure, the LOC approach enables direct extraction of O2 from the bulk solution to the reaction interface; this eliminates the need to pump O2/air to overcome the sluggish O2 mass transfer and results in high Faradaic efficiencies (~50%) even under natural air diffusion conditions. Long-term operation of a flow-through pilot device indicated that the LOC approach saved more than 65% of the electric energy normally consumed in treating actual industrial wastewater, demonstrating the great potential of this system-level design to boost the electro-Fenton process for energy-efficient water remediation.

Embedding electronic perpetual motion into single-atom catalysts for persistent Fenton-like reactions.

In our quest to leverage the capabilities of the emerging single-atom catalysts (SACs) for wastewater purification, we confronted fundamental challenges related to electron scarcity and instability. Through meticulous theoretical calculations, we identified optimal placements for nitrogen vacancies (Nv) and iron (Fe) single-atom sites, uncovering a dual-site approach that significantly amplified visible-light absorption and charge transfer dynamics. Informed by these computational insights, we cleverly integrated Nv into the catalyst design to boost electron density around iron atoms, yielding a potent and flexible photoactivator for benign peracetic acid. This exceptional catalyst exhibited remarkable stability and effectively degraded various organic contaminants over 20 cycles with self-cleaning properties. Specifically, the Nv sites captured electrons, enabling their swift transfer to adjacent Fe sites under visible light irradiation. This mechanism accelerated the reduction of the formed "peracetic acid-catalyst" intermediate. Theoretical calculations were used to elucidate the synergistic interplay of dual mechanisms, illuminating increased adsorption and activation of reactive molecules. Furthermore, electron reduction pathways on the conduction band were elaborately explored, unveiling the production of reactive species that enhanced photocatalytic processes. A six-flux model and associated parameters were also applied to precisely optimize the photocatalytic process, providing invaluable insights for future photocatalyst design. Overall, this study offers a molecule-level insight into the rational design of robust SACs in a photo-Fenton-like system, with promising implications for wastewater treatment and other high-value applications.

Superoxide radicals mediated by high-spin Fe catalysis for organic wastewater treatment.

Water resources are indispensable basic resources and important environmental carriers; the presence of organic contaminants in wastewater poses considerable risks to the health of both humans and ecosystems. Although the Fenton-like reactions using H2O2 as the oxidant to destroy organic pollutants are attractive, there are still challenges in improving reaction activity under neutral or even alkaline conditions. Herein, we designed a H2O2 activation pathway with O2•- as the main active species and elucidated that the spin interaction between Fe sites and coordinated O atoms effectively promotes the generation of the key intermediate Fe-*OOH. Furthermore, we successfully captured and analyzed the Fe-*OOH intermediate by in situ Raman spectroscopy. When applying FBOB to a continuous-flow reactor, CIP removal efficiency remained at around 90% within 600 min of continuous operation, achieving excellent efficiency, stability, and pH tolerance in removing pollutants.

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.

Dual-functional single-atomic Mo/Fe clusters-decorated C3N5 via three electron-pathway in oxygen reduction reaction for tandemly removing contaminants from water.

Inspired by the development of single-atom catalysts (SACs), the fabrication of multimetallic SACs can be a promising technical approach for the in situ electro-Fenton (EF) process. Herein, dual-functional atomically dispersed Mo-Fe sites embedded in carbon nitride (C3N5) (i.e., MoFe/C3N5) were synthesized via a facile SiO2 template method. The atomically isolated bimetallic configuration in MoFe/C3N5 was identified by combining the microscopic and spectroscopic techniques. The MoFe/C3N5 catalyst on the cathode exhibited a remarkable catalytic activity toward the three electron-dominated oxygen reduction reaction in sodium sulfate, leading to a highly effective EF reaction with a low overpotential for the removal of organic contaminants from wastewater. The new catalyst showed a superior performance over its conventional counterparts, owing to the dual functions of the dual-metal active sites. Density functional theory (DFT) analysis revealed that the dual-functional 50-MoFe/C3N5 catalyst enabled a synergistic action of the Mo-Fe dual single atomic centers, which can alter the adsorption/dissociation behavior and decrease the overall reaction barriers for effective organic oxidation during the EF process. This study not only sheds light on the controlled synthesis of atomically isolated catalyst materials but also provides deeper understanding of the structure-performance relationship of the nanocatalysts with dual active sites for the catalytic EF process. Additionally, the findings will promote the advanced catalysis for the treatment of emerging organic contaminants in water and wastewater.

A green edge-hosted zinc single-site heterogeneous catalyst for superior Fenton-like activity.

Developing green heterogeneous catalysts with excellent Fenton-like activity is critical for water remediation technologies. However, current catalysts often rely on toxic transitional metals, and their catalytic performance is far from satisfactory as alternatives of homogeneous Fenton-like catalysts. In this study, a green catalyst based on Zn single-atom was prepared in an ammonium atmosphere using ZIF-8 as a precursor. Multiple characterization analyses provided evidence that abundant intrinsic defects due to the edge sites were created, leading to the formation of a thermally stable edge-hosted Zn-N4 single-atom catalyst (ZnN4-Edge). Density functional theory calculations revealed that the edge sites equipped the single-atom Zn with a super catalytic performance, which not only promoted decomposition of peroxide molecule (HSO5-) but also greatly lowered the activation barrier for •OH generation. Consequently, the as-prepared ZnN4-Edge exhibited extremely high Fenton-like performance in oxidation and mineralization of phenol as a representative organic contaminant in a wide range of pH, realizing its quick detoxification. The atom-utilization efficiency of the ZnN4-Edge was ~104 higher than an equivalent amount of the control sample without edge sites (ZnN4), and the turnover frequency was ~103 times of the typical benchmark of homogeneous catalyst (Co2+). This study opens up a revolutionary way to rationally design and optimize heterogeneous catalysts to homogeneous catalytic performance for Fenton-like application.

Distinguishing homogeneous advanced oxidation processes in bulk water from heterogeneous surface reactions in organic oxidation.

Clarifying the reaction pathways at the solid-water interface and in bulk water solution is of great significance for the design of heterogeneous catalysts for selective oxidation of organic pollutants. However, achieving this goal is daunting because of the intricate interfacial reactions at the catalyst surface. Herein, we unravel the origin of the organic oxidation reactions with metal oxide catalysts, revealing that the radical-based advanced oxidation processes (AOPs) prevail in bulk water but not on the solid catalyst surfaces. We show that such differing reaction pathways widely exist in various chemical oxidation (e.g., high-valent Mn3+ and MnOX) and Fenton and Fenton-like catalytic oxidation (e.g., Fe2+ and FeOCl catalyzing H2O2, Co2+ and Co3O4 catalyzing persulfate) systems. Compared with the radical-based degradation and polymerization pathways of one-electron indirect AOP in homogeneous reactions, the heterogeneous catalysts provide unique surface properties to trigger surface-dependent coupling and polymerization pathways of a two-electron direct oxidative transfer process. These findings provide a fundamental understanding of catalytic organic oxidation processes at the solid-water interface, which could guide the design of heterogeneous nanocatalysts.

An ultrafast and facile nondestructive strategy to convert various inefficient commercial nanocarbons to highly active Fenton-like catalysts.

The Fenton-like process catalyzed by metal-free materials presents one of the most promising strategies to deal with the ever-growing environmental pollution. However, to develop improved catalysts with adequate activity, complicated preparation/modification processes and harsh conditions are always needed. Herein, we proposed an ultrafast and facile strategy to convert various inefficient commercial nanocarbons into highly active catalysts by noncovalent functionalization with polyethylenimine (PEI). The modified catalysts could be in situ fabricated by direct addition of PEI aqueous solution into the nanocarbon suspensions within 30 s and without any tedious treatment. The unexpectedly high catalytic activity is even superior to that of the single-atom catalyst and could reach as high as 400 times higher than the pristine carbon material. Theoretical and experimental results reveal that PEI creates net negative charge via intermolecular charge transfer, rendering the catalyst higher persulfate activation efficiency.