Spin state-dependent in-situ photo-Fenton-like transformation from oxygen molecule towards singlet oxygen for selective water decontamination

Synergy of F-Fe dual sites in KFeF3 promoting Fenton-like cycle and refractory organics degradation through direct electron transfer

The electron cycling of single active site on catalyst was commonly restrict in Fenton-like reactions, thus limits the degradation of refractory organics in water treatment. In this work, a novel strategy for inducing efficient Fenton-like reaction through F-Fe dual sites on the surface of perovskite fluoride KFeF3 was practiced. The KFeF3 and a series of perovskite fluoride material were synthesized by simple hydrothermal method. The KFeF3 exhibited very high performance for remove of multiple refractory organics and chemical oxygen demand in lignin wastewater. Rhodamine B and phenol could be completely removed within 2 s and 16 s respectively and the mineralization rate of phenol reached ∼90 % within 5 min. Detection of ROS and in situ analysis combining density functional theory calculation of the interface proved that the rapid degradation of phenol was attribute to the synergistic effect of F-Fe dual sites and degradation pathway through direct electron transfer. F site serves as the activation site of H2O2 and reduce H2O2 to form OH, whereas, OH attacking was not the dominating pathway for degradation. The redox reaction between F site and H2O2 triggered the direct electron transfer from the Fe-phenol (or hydroxylation intermediates of phenol) complex to F site thus avoided the passivation of Fe site and accelerated Fenton-like cycle. This work revealed a fire-new mechanism of Fenton-like reaction and was expected to impulse treatment of refractory wastewater.

Enhanced activation of peroxide to generate singlet oxygen via boron-doped Cu single-atom catalysts for efficient water treatment

The development of advanced oxidation processes (AOPs) for environmental remediation has spurred a growing interest in catalysts that selectively generate non-radical species such as singlet oxygen (1O2). However, the precise engineering of catalytic sites to enhance targeted 1O2 production remains a formidable challenge. This study reports a B-doped graphitic carbon nitride-supported Cu single-atom catalyst (CuBCN) that significantly enhances H2O2 activation for efficient 1O2 production. Through comprehensive experimental and theoretical analysis, revealing that B doping modulates the electronic properties at the Cu active sites. This modification reduces the electron density around Cu, increasing the Cu(II) fraction essential for the formation and subsequent oxidation of ·OOH intermediates. Additionally, B influences the d-band center of Cu, optimizing the adsorption energy of ·OOH, which in turn facilitates selective 1O2 production. The CuBCN/H2O2 system exhibits exceptional Fenton-like performance, producing 1O2 as the principal active species, even in challenging water conditions characterized by high pH, high ionic strength, and high concentrations of humic substances. This work not only highlights the potential of tailored single-atom catalysts in pollution control but also offers significant insights for designing catalysts for efficient H2O2 activation.

Praseodymium-regulated microregion electron structures of Bi2WO6 nanosheets to construct non-radical platforms for the photocatalytic decontamination of water by hydrogen peroxide activation

H2O2-based photo-Fenton-like systems are promising technologies for the generation of singlet oxygen (1O2) aimed at water decontamination. However, in conventional systems, the yield of 1O2 is relatively low owing to competition among free radicals. The oriented construction of a non-radical system could address this issue but challenging. To address this issue, a non-radical platform (i.e., OV-Pr-BinWOx) was developed through praseodymium regulation to facilitate the efficient and directional generation of 1O2. Experimental and theoretical analyses demonstrated that the regulation of praseodymium enhanced both the adsorption and dissociation of H2O2, while simultaneously increasing the O-H bond length and reducing the O-O bond length. This modification results in a more favorable pathway for ∗OOH formation rather than ∗OH formation. Consequently, this special non-radical mechanism exhibits clear differences from conventional 1O2 production via radical pathways. The BWPr-5/H2O2/light system, dominated by 1O2 and electron transfer, achieved a remarkable 93.8 % methylene blue (MB) degradation within 60 min. The MB degradation pathway was elucidated via intermediate identification and theoretical calculations. Furthermore, phytotoxicity assessments indicated a reduction in potential environmental risks associated with this process. Overall, this system demonstrates significant potential for application in the photocatalytic water decontamination due to its robust anti-interference capabilities and excellent stability.

Bioengineered iron-based heterojunction orientation in optimizing activation pathways for superoxide radical-mediated photoreduction of Cr(VI) from water

Photocatalytic technology has been widely employed for Cr(VI) remediation. However, the inadequate generation of reactive oxygen species associated with the Cr(VI) reduction, caused by the uncontrollable photo-Fenton reaction, significantly restricts the reduction efficiency. Herein, a bioengineered iron-based heterojunction (Bio-Fe2O3/Fe2(WO4)3) was fabricated via a two-step process of biomineralization and calcination, where tungstate was doped into the precursor during iron metabolism in acidophilic bacteria to optimize the heterojunction structure. Bio-Fe2O3/Fe2(WO4)3 exhibited a short-range ordered structure and superior photocatalytic performance, achieving 100 % reduction of 20 mg/L Cr(VI) within 60 min by photocatalytic oxalic acid (OA) under simulated light conditions. The system provided robust operation in complex environments, notably, operating effectively under mild solar radiation as an alternative to the simulated light. The heterojunction structure intensified the H2O2 activation and selectively boosted the yield of superoxide radical (O2·-), the primary Cr(VI)-reducing species, from 48.02 % to 72.96 %. The high oxidation state of Fe in Bio-Fe2O3/Fe2(WO4)3 contributed to stronger adsorption performance towards OA and H2O2, accompanied with the tendency to take the O2·--generated activation pathway. This work provides a broader perspective on the rational design of photocatalysts to modulate the OA photocatalysis and the H2O2 activation pathway, selectively elevating the yield of O2·- for Cr(VI) reduction.

Oxygen vacancies-mediated the peracetic acid activation to selectively generate 1O2 for water decontamination

As a pre-oxidation unit, developing non-radical pathway-dominant advanced oxidation processes (AOPs) with remarkably-efficient oxidation, superior environmental robustness, and ecological safety is essential in actual water pollution control. Herein, using Co3O4 as an example, we present an oxygen vacancies (OVs)-mediated peracetic acid (PAA) activation process, thereby predominantly generating singlet oxygen (1O2) for degrading contaminants. In-situ monitoring of PAA activation by OVs-rich Co3O4 (Co3O4-OVs) reveals that surface oxygen-containing intermediates (e.g., *OH and *O) are the precursors of 1O2. Theoretical calculations show that the selective adsorption of terminal oxygen atoms (ATO) in PAA serves as an activity descriptor for 1O2 generation. OVs can induce electron redistribution, triggering the ATO-dominated PAA adsorption to form the Co3O4-OVs-PAA* complex, followed by O-O bond breakage to yield *OH. Concurrently, OVs modulate the Co d-band center, lowering the energy barrier for 1O2 formation. The system enables ultra-fast catalytic performance (kobs = 1.17 min-1) for degrading sulfamethoxazole, outperforming pristine Co3O4 by 11.64-fold. The high-selectivity towards non-radical pathway endows the Co3O4-OVs/PAA system with remarkable stability in complex environment backgrounds and continuous-flow microreactor. This work not only provides a broad perspective on the modulation of non-radical pathways via defect engineering, but also advances the development of PAA-based AOPs for water decontamination.

Enhanced electro-fenton degradation of tetracycline pharmaceutical wastewater by N-doped carbon modified titanium membrane aeration: Formation of highly selective singlet oxygen

Singlet oxygen (1O2), the lowest excited electronic state of molecular oxygen, plays an important role in advanced oxidation, but how to directionally enhance the generation of 1O2 is a challenge. In this study,we use membrane aeration electrode modified by carbon-nitrogen for the first time to enhance the generation of 1O2 in the EF (Electro-Fenton) system. The carbon-nitrogen supported tubular titanium membrane (TTM@CN) aeration electrode was prepared by a simple dopamine-loaded one-step sintering method. A membrane aeration EF system was designed with TTM@CN as cathode and netted ruthenium-iridium titanium electrode as anode, and the output of 1O2 was greatly improved. The results of quenching experiments show that the main way of singlet oxygen production is 3O2 → H2O2 → 1O2. In addition, the results of density functional theory (DFT) show that the empty orbital of C above Fermi level in heterojunction is obviously filled, and the density of states tends to shift to the depth of valence band. The system with metal Ti as carrier can quickly transfer electrons to the layer of C, which makes the states density of C increase significantly near Fermi level. It can reduce 3O2 to H2O2 more quickly, and H2O2 can be further converted to 1O2. The system showed excellent degradation performance in a wide pH range (1-12) and excellent stability in 20 cycle experiments, which provided a reference significance for promoting the development of sustainable EF technology.

Spin State Regulation for Peroxide Activation: Fundamental Insights and Regulation Mechanisms

Peroxides are widely used in environmental applications due to their strong oxidizing properties, however, traditional activation methods often face challenges such as uncontrolled reactive oxygen species (ROS) generation and high energy barriers. Recent advancements in spin state regulation provide a promising alternative to enhance the efficiency of peroxide activation. This review provides an overview of spin fundamentals and discusses the key factors affecting spin state in catalytic materials, including crystal field configuration, ligand environment, and valence changes. Subsequently, the role of electron spin state in peroxide activation is comprehensively analyzed, with a focus on how spin state regulation can tune adsorption energy, lower energy barriers, facilitate electron transfer between transition metals and peroxides, and promote selective ROS generation. Finally, this review briefly outlines the practical applications of peroxide activation in water treatment and concludes with a summary and perspectives on future research directions. This review aims to provide a comprehensive perspective on the role of spin state regulation in advancing peroxide activation strategies.

Optimizing 3d electronic structure of LaCoO3 based on spin state tuning for enhancing photo-Fenton activity on tetracycline degradation

The water pollution caused by the abuse of antibiotics has significant harmful effects on the environment and human health. The photo-Fenton process is currently the most effective method for removing antibiotics from water, but it encounters challenges such as inadequate response to visible light, low yield and utilization of photogenerated electrons, and slow electron transport. In this study, spin state regulation was introduced into the photo-Fenton process, and the spin state of Co3+ was regulated through Ce displacement doping. The intermediate-spin state Ce-LaCoO3 could degrade 91.6 % of tetracycline within 120 min in the photo-Fenton system, which is 15.2 % higher than that of low-spin state LaCoO3. The improved degradation effect is attributed to the reasons that Ce-LaCoO3 in the intermediate-spin state have lower band gap, better charge transfer ability, and stronger adsorption capacity of H2O2, which can accelerate the redox cycle of Co2+/Co3+ and promote the generation of ·OH. This study presents a unique strategy for synthesizing efficient photo-Fenton materials to treat antibiotic wastewater effectively.

Upcycling Waste Biomass to Biochar: Feedstocks, Catalytic Mechanisms, and Applications in Advanced Oxidation for Wastewater Decontamination

Advanced oxidation technology plays an important role in wastewater treatment due to active substances with high redox potential. Biochar is a versatile and functional biomass material. It can be used for resource management of various waste biomasses. In addition, carbonaceous materials are commonly used to enhance the synergistic mechanisms of advanced oxidation processes, because of their good electrical conductivity and metal-free leaching. Biochar produced from waste biomass through pyrolysis has catalytic potential, is cost-effective, and is environmentally friendly. It is commonly used to activate hydrogen peroxide, persulfate, ozone, photocatalysis, and other systems for degrading organic pollutants in water. This review provides a summary of the feedstocks, pyrolysis conditions, and modification methods used in biochar production. It also described the effects of these factors on the yield, structure, and active sites of the biochar. The review summarized the mechanisms of various catalytic systems and their applications in wastewater decontamination, as well as their potential for practical application. Eventually, the limitations of this current technique and the outlook for future research were noted.

Low-peroxide-consumption fenton-like systems: The future of advanced oxidation processes

Conventional heterogeneous Fenton-like systems employing different peroxides have been developed for water/wastewater remediation. However, a large population of peroxides consumed during various Fenton-like systems with low utilization efficiency and associated secondary contamination have become the bottlenecks for their actual applications. Recent strategies for lowering the peroxide consumptions to develop economic Fenton-like systems are primarily devoted to the effective radical generation and subsequent high-efficiency radical utilization through catalysts/systems engineering, leveraging emerging nonradical oxidation pathways with higher selectivity and longer life of the reactive intermediate, as well as reactor designs for promoting the mass transfer and peroxides decomposition to improve the yield of radicals/nonradicals. However, a comparative review summarizing the mechanisms and pathways of these strategies has not yet been published. In this review, we endeavor to showcase the designated systems achieving the reduction of peroxides while ensuring high catalytic activity from the perspective of the above strategic mechanisms. An in-depth understanding of these aspects will help elucidate the key mechanisms for achieving economic peroxide consumption. Finally, the existing problems of these strategies are put forward, and new ideas and research directions for lowering peroxide consumption are proposed to promote the application of various Fenton-like systems in actual wastewater purification.

Pilot-scale and large-scale Fenton-like applications with nano-metal catalysts: From catalytic modules to scale-up applications

Recently, great efforts have been made to advance the pilot-scale and engineering-scale applications of Fenton-like processes using various nano-metal catalysts (including nanosized metal-based catalysts, smaller nanocluster catalysts, and single-atom catalysts, etc.). This step is essential to facilitate the practical applications of advanced oxidation processes (AOPs) for these highly active nano-metal catalysts. Before large-scale implementation, these nano-metal catalysts must be converted into the effective catalyst modules (such as catalytic membranes, fluidized beds, or polypropylene sphere suspension systems), as it is not feasible to use suspended powder catalysts for large-scale treatment. Therefore, the pilot-scale and engineering applications of nano-metal catalysts in Fenton-like systems in recent years is exciting. In addition, the combination of life cycle assessment (LCA) and techno-economic analysis (TEA) can provide a useful support tool for engineering scale Fenton-like applications. This paper summarizes the designs and fabrications of various advanced modules based on nano-metal catalysts, analyzes the advantages and disadvantages of these catalytic modules, and further discusses their Fenton-like pilot scale or engineering applications. Concepts of future Fenton-like engineering applications of nano-metal catalysts were also discussed. In addition, current challenges and future expectations in pilot-scale or engineering applications are assessed in conjunction with LCA and TEA. These challenges require further technological advances to enable larger scale engineering applications in the future. The aim of these efforts is to increase the potential of nanoscale AOPs for practical wastewater treatment.

Reconstructing the electron and spin structures of nanoscale iron sulfide through a biosurfactant layer towards radical-nonradical co-dominant regime

Radical-nonradical co-dominant pathways have become a hot topic in advanced oxidation, but achieving this on transition metal sulfides (TMS) remains challenging because their inherently higher electron and spin densities always induce radicals rather than nonradicals. Herein, a biosurfactant layer (BLR) was introduced to redistribute the electron and spin structure of nanoscale iron sulfide (FeS), which allowed both radical and nonradical to co-dominate the catalytic reaction. The resulting BLR-encased FeS hybrid (BLR@FeS) exhibited satisfactory removal efficiency (98.5 %) for hydrogen peroxide (H2O2) activation, outperforming both the constituent components [FeS (70.9 %) and BLR (86.2 %)]. Advanced characterizations showed that C, O, N-related sites (-CO and -NC) in BLR attracted electrons in FeS due to their strong electronegativity and electron-withdrawing capacity, which not only decreased electron density in FeS, but also resulted in a shift of the Fe/S sites from the high-spin to the medium-spin state. The reaction routes established by the BLR@FeS/H2O2 system maintained desirable stability against environmental interferences such as common inorganic anions, humic acid and changes in pH. Our study provides a state-of-the-art, molecule-level understanding of tunable co-dominant pathways and expands the targeted applications in the field of advanced oxidation.

g-C3N4 nanosheet supported NiCo2O4 nanoparticles for boosting degradation of tetracycline under visible light and ultrasonic irradiation

The doping of semiconductor materials through some facile and appropriate methods holds significant promise in enhancing the catalytic performance of catalysts. Herein, NiCo2O4/g-C3N4 composite catalysts were synthesized via a high-energy ball milling method. The microstructure and physicochemical characterization of the as-prepared composites confirmed the successful loading of NiCo2O4 nanoparticles onto the g-C3N4 nanosheets. The NiCo2O4/g-C3N4 composites showed excellent catalytic effect under visible light/ultrasonic irradiation, and the efficiency of tetracycline hydrochloride (TCH) degradation reached 90% within 15 min. The optical properties of g-C3N4 nanosheets were improved by doping, and the diffusion of active materials and carrier migration rate were improved by ultrasonic assistance. Possible catalytic mechanisms and potential pathways of the NiCo2O4/g-C3N4 composites for the degradation of TCH triggered by visible light/ultrasonic irradiation were proposed. This study provides a new strategy for energy-assisted photocatalytic degradation of organic pollutants.

Investigation of singlet oxygen and superoxide radical produced from vortex-based hydrodynamic cavitation: Mechanism and its relation to cavitation intensity

Presently, the hydroxyl radical oxidation mechanism is widely acknowledged for the degradation of organic pollutants based on hydrodynamic cavitation technology. The presence and production mechanism of other potential reactive oxygen species (ROS) in the cavitation systems are still unclear. In this paper, singlet oxygen (1O2) and superoxide radical (·O2-) were selected as the target ROS, and their generation rules and mechanism in vortex-based hydrodynamic cavitation (VBHC) were analyzed. Computational fluid dynamics (CFD) were used to simulate and analyze the intensity characteristics of VBHC, and the relationship between the generation of ROS and cavitation intensity was thoroughly revealed. The results show that the operating conditions of the device have a significant and complicated influence on the generation of 1O2 and ·O2-. When the inlet pressure reaches to 4.5 bar, it is more favorable for the generation of 1O2 and ·O2- comparing with those lower pressure. However, higher temperature (45 °C) and aeration rate (15 (L/min)/L) do not always have positive effect on the 1O2 and ·O2- productions, and their optimal parameters need to be analyzed in combination with the inlet pressure. Through quenching experiments, it is found that 1O2 is completely transformed from ·O2-, and ·O2- comes from the transformation of hydroxyl radicals and dissolved oxygen. Higher cavitation intensity is captured and shown more disperse in the vortex cavitation region, which is consistent with the larger production and stronger diffusion of 1O2 and ·O2-. This paper shed light to the generation mechanism of 1O2 and ·O2- in VBHC reactors and the relationship with cavitation intensity. The conclusion provides new ideas for the research of effective ROS in hydrodynamic cavitation process.

Pyrite-activated persulfate to degrade 3,5,6-trichloro-2-pyridyl in water: Degradation and Fe release mechanism

TCP (3,5,6-trichloro-2-pyridinol), the main recalcitrant degradation product of chlorpyrifos, poses a high risk to human health and ecological systems. This study provided a comprehensive exploration of the pyrite-activated persulfate (PS) system for the removal of TCP in water and placed particular emphasis on the pyrite oxidation process that releases Fe. The results showed that the pyrite-activated PS system can completely degrade TCP within 300 min at 5.0 mmol/L PS and 1000 mg/L pyrite at 25 °C, wherein small amounts of PS (1 mmol/L) can effectively facilitate TCP removal and the oxidation of pyrite elements, while excessive PS (>20 mmol/L) can lead to competitive inhibitory effects, especially in the Fe release process. Aimed at the dual effects, the evident positive correlation (R2 > 0.90) between TCP degradation (kTCP) and Fe element release (kFe), but the value of k (0.00237) in the pyrite addition variable experiment was less than that in the PS experiment (k = 0.00729), further indicating that the inhibition effect of excessive addition consists of PS but not notably pyrite. Moreover, the predominant free radicals and non-free radicals produced in the pyrite/PS system were tested, with the order of significance being •OH < Fe (Ⅳ) < SO4•- < •O2- < 1O2, wherein 1O2 emerged as the principal player in both TCP degradation and Fe release from the pyrite oxidation process. Additionally, CO32- can finitely activate PS but generally slows TCP degradation and inhibit pyrite oxidation releasing Fe process. This study provides a theoretical basis for the degradation of TCP using pyrite-activated PS.

Sulfur Vacancies in Pyrite Trigger the Path to Nonradical Singlet Oxygen and Spontaneous Sulfamethoxazole Degradation: Unveiling the Hidden Potential in Sediments

Pharmaceutical residues in sediments are concerning as ubiquitous emerging contaminants. Pyrite is the most abundant sulfide minerals in the estuarine and coastal sediments, making it a major sink for pharmaceutical pollutants such as sulfamethoxazole (SMX). However, research on the adsorption and redox behaviors of SMX on the pyrite surface is limited. Here, we investigated the impact of the nonphotochemical process of pyrite on the fate of coexisting SMX. Remarkably, sulfur vacancies (SVs) on pyrite promoted the generation of nonradical species (hydrogen peroxide, H2O2 and singlet oxygen, 1O2), thereby exhibiting prominent SMX degradation performance under darkness. Nonradical 1O2 contributed approximately 73.1% of the total SMX degradation. The SVs with high surrounding electron density showed an advanced affinity for adsorbing O2 and then initiated redox reactions in the sediment electron-storing geobattery pyrite, resulting in the extensive generation of H2O2 through a two-electron oxygen reduction pathway. Surface Fe(III) (hydro)oxides on pyrite facilitated the decomposition of H2O2 to 1O2 generation. Distinct nonradical products were observed in all investigated estuarine and coastal samples with the concentrations of H2O2 ranging from 1.96 to 2.94 μM, while the concentrations of 1O2 ranged from 4.63 × 10-15 to 8.93 × 10-15 M. This dark-redox pathway outperformed traditional photochemical routes for pollutant degradation, broadening the possibilities for nonradical species use in estuarine and coastal sediments. Our study highlighted the SV-triggered process as a ubiquitous yet previously overlooked source of nonradical species, which offered fresh insights into geochemical processes and the dynamics of pollutants in regions of frequent redox oscillations and sulfur-rich sediments.

Modulating electron density enable efficient cascade conversion from peroxymonosulfate to superoxide radical driven by electron-rich/poor dual sites

The superoxide radical (•O2-)-mediated peroxymonosulfate (PMS)-based photo-Fenton-like reaction enables highly selective water decontamination. Nevertheless, the targeted construction of •O2--mediated photo-Fenton-like system has been challenging. Herein, we developed an electron-rich/-poor dual sites driven •O2--mediated cascade photo-Fenton-like system by modulating electron density. Experimental and theoretical results demonstrated that PMS was preferentially adsorbed on electron-poor Co site. This adsorption promoted O-O bond cleavage of PMS to generate hydrogen peroxide (H2O2), which then migrated to electron-rich O site to extract eg electrons for O-H bond cleavage, rather than competing with PMS for Co site. The developed versatile cascade reaction system could selectively eliminate contaminants with low n-octanol/water partition constants (KOW) and dissociation constants (pKa) and remarkably resist inorganics (Cl-, H2PO4- and NO3-), humic acid (HA) and even real water matrices (tap water and secondary effluent). This finding provided a novel and plausible strategy to accurately and efficiently generate •O2- for the selective water decontamination.