Identifying the Role of Surface Hydroxyl on FeOCl in Bridging Electron Transfer toward Efficient Persulfate Activation

Regulation of Surface Terminal Hydroxyl Coverage of FeOCl Catalyst via Crystal Facet Protection for Enhanced H2O2 Activation

The unique Fe coordination environment in the FeOCl catalyst confers superior reducible electronic properties, rendering them attractive Fenton-like active sites. DFT calculations reveal that the U-shaped coordinated Fe sites formed with 50% terminal hydroxyl coverage exhibit the best H2O2 activation performance, which allows the adsorbed H2O2 to form a ·OH directly with much lower activation energy. Herein, a crystal facet protection strategy induced by rapid high-temperature annealing is developed to synthesize FeOCl with high exposure of Fe atoms while regulating the surface hydroxyl coverage. The dominant expression of the (021) facet resulted in an optimized surface terminal hydroxyl coverage of 58.3%, increasing the intrinsic activity of FeOCl by 4.3 times. The d-band center of FeOCl with optimized terminal hydroxyl coverage is closer to the Fermi level, thus exhibiting higher affinity for H2O2, and the increased amount of U-shaped coordinated Fe sites enables sufficient ·OH generation for enhanced decontamination performance. Since the terminal hydroxyl groups can be consumed by Ca2+ and Mg2+ through coprecipitation, preremoving the hardness of actual wastewater is indispensable in the application of the FeOCl/H2O2 system. Our finding provides a new way to improve the intrinsic activity of FeOCl catalyst, which is helpful for its application in other environmental remediations.

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.

Humic acid inhibits hydroxyl radical generation during oxygenation of Fe(II) on goethite surface

The dark formation of hydroxyl radicals (•OH) by the oxidation of Fe(II) has been increasingly recognized at anoxic-oxic interface. Minerals play significant roles on oxidation of Fe(II) owing to the adsorption changed the reactivity of Fe(II). However, the impact of dissolved organic matter (DOM) on the oxidation of mineral adsorbed Fe(II) and •OH generation remains unknown. Herein, we examined the impact of humic acid (HA) on •OH accumulation during oxygenation of goethite surface-adsorbed Fe(II). We found the addition of 100-500 mg C•L-1 HA inhibited 3.7 % - 24.8 % •OH accumulation, compared to •OH generation facilitated by goethite-adsorbed Fe(II), and the electron utilization efficiency for •OH generation reduced from 15.4 % to 11.8 %. The adsorption experiment under anoxic condition showed that HA compete absorption with Fe(II) on goethite surface. XPS spectroscopy revealed that HA reduces the surface Fe-OH functional group by 4.8 %, thereby reducing the active sites on goethite. Voltammetric cycling curves demonstrated that HA decreased the reducing capacity and inhibited the electrical conductivity of the goethite-adsorbed Fe(II). This study elucidates the effects and mechanisms of HA adsorption on •OH generation during the oxygenation of Fe(II) on goethite surface.

Insights into excitonic behavior in single-atom covalent organic frameworks for efficient photo-Fenton-like pollutant degradation

The generation of radicals through photo-Fenton-like reactions demonstrates significant potential for remediating emerging organic contaminants (EOCs) in complex aqueous environments. However, the excitonic effect, induced by Coulomb interactions between photoexcited electrons and holes, reduces carrier utilization efficiency in these systems. In this study, we develop Cu single-atom-loaded covalent organic frameworks (CuSA/COFs) as models to modulate excitonic effects. Temperature-dependent photoluminescence and ultrafast transient absorption spectra reveal that incorporating acenaphthene units into the linker (CuSA/Ace-COF) significantly reduces exciton binding energy (Eb). This modification not only enhances peroxymonosulfate adsorption at Cu active sites but also facilitates rapid electron transfer and promotes selective hydroxyl radical generation. Compared to CuSA/Obq-COF (Eb = 25.6 meV), CuSA/Ace-COF (Eb = 12.2 meV) shows a 39.5-fold increase in the pseudo-first-order rate constant for sulfamethoxazole degradation (0.434 min-1). This work provides insights into modulating excitonic behavior in single-atom catalysts via linker engineering for EOCs degradation.

Sludge-derived hydrochar modulates complete nonradical electron transfer in peroxydisulfate activation via pyrrolic-N and carbon defect: Implication for degrading electron-rich ionizable anilines compounds

Nonradical electron transfer process (ETP) is a promising pathway for pollutant degradation in peroxydisulfate-based advanced oxidation processes (PDS-AOPs). However, there is a critical bottleneck to trigger ETP by sludge-derived hydrochar due to its negatively charged surface, inferior porosity and electrical conductivity. Herein, pyrrolic-N doped and carbon defected sludge-derived hydrochar (SDHC-N) was constructed for PDS activation to degrade anilines ionizable organic compounds (IOC) through complete nonradical ETP oxidation. Degradation of anilines IOC was not only affected by the electron-donating capacity but also proton concentration in solution because of the ionizable amino group (-NH2). Diverse effects including proton favor, insusceptible and inhibition were observed. Impressively, addition of HCO3 with strong proton binding capacity boosted aniline degradation nearly 10 times. Moreover, characterizations and theoretical calculations demonstrated that pyrrolic-N increased electron density and created positively charged surface, profoundly promoting generation of SDHC-N-S2O82-* complexes. More delocalized electrons around carbon defect could enhance electron mobility. This work guides a rational design of sludge-derived hydrochar to mediate nonradical ETP oxidation, and provides insights into the impacts of proton on anilines IOC degradation.

Efficient Photocatalytic Activation of Peroxymonosulfate by Cobalt-Doped Oxygen-Vacancies-Rich BiVO4 for Rapid Tetracycline Degradation

In this work, cobalt-doped oxygen-vacancies-rich BiVO4 (Co/BiVO4-Vo) was successfully synthesized for the degradation of tetracycline (TC) by activated peroxymonosulfate (PMS) under visible light. The morphologies, microstructures, and optical properties of the photocatalysts were analyzed in detail. Co/BiVO4-Vo exhibited significantly enhanced degradation, removing 92.3% of TC within 10 min, which was greater than those of pure BiVO4 (62.2%) and oxygen-vacancies-rich BiVO4 (BiVO4-Vo) (72.0%), respectively. The photogenerated charge separation and transport properties were explored through surface photovoltage (SPV), photoluminescence spectrum (PL), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS) measurements. Additionally, an in-depth investigation was conducted on the photocatalytically assisted advanced oxidation processes based on SO4•- (SR-AOPs) for the degradation of organic pollutants. The experimental results showed that the introduction of oxygen vacancies and Co doping achieved an effective separation of photogenerated carriers, which could accelerate the cycling between Co3+ and Co2+ and further activate PMS. The results of free radical capture experiments and electron spin resonance (ESR) experiments showed that reactive oxygen species (ROSs) such as 1O2, •O2-, and SO4•- played a dominant role in the removal of pollutants. This work provides a novel insight into the further development of efficient and rapid PMS photoactivators for environmental remediation of water bodies.

Oxygen-containing functional groups in Fe3O4@three-dimensional graphene nanocomposites for enhancing H2O2 production and orientation to 1O2 in electro-Fenton

In electro-Fenton (EF), development of a bifunctional electrocatalyst to realize simultaneous H2O2 generation and activation efficiently for generating reactive species remains a challenge. In particular, a nonradical-mediated EF is more favorable for actual wastewater remediation, and deserves more attention. In this study, three-dimensional graphene loaded with Fe3O4 nanoparticles (Fe3O4@3D-GNs) with abundant oxygen-containing functional groups (OFGs) was synchronously synthesized using a NaCl-template method and served as a cathode to establish a highly efficient and selective EF process for contaminant degradation. The amounts of OFGs can be effectively modulated via the pyrolysis temperature to regulate the 2e- oxygen reduction reaction activity and reactive oxygen species (ROS) production. The optimized Fe3O4@3D-GNs synthesized at 750 °C (Fe3O4@3D-GNs-750) with the highest -C-O-C and -C꞊O group ratios exhibited the maximum H2O2 and 1O2 yields during electrocatalysis, thus showing remarkable versatility for eliminating organic contaminants from surface water bodies. Experiments and theoretical calculations have demonstrated the dominant role of -C-O-C in generating H2O2 and the positive influence of -C꞊O sites on the production of 1O2. Moreover, the surface-bound Fe(II) favors the generation of surface-bound •OH, which steers a more favorable oxidative conversion of H2O2 to 1O2. Fe3O4@3D-GNs were proven to be less pH-dependent, low-energy, stable, and recyclable for practical applications in wastewater purification. This study provides an innovative strategy to engineer active sites to achieve the selective electrocatalysis for eliminating pollution and reveals a novel perspective for 1O2-generation mechanism in the Fenton reaction.

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.