Bifunctional Fe for Induced Graphitization and Catalytic Ozonation Based on a Fe/N-Doped Carbon-Al2O3 Framework: Theoretical Calculations Guided Catalyst Design and Optimization

Mn-Fe Dual-Metal Assemblages on Carbon-Coated Al2O3 Spheres for Catalytic Ozonation Oxidation: Structure, Performance, and Reaction Mechanism

Catalysts with high catalytic activity and low production cost are important for industrial application of heterogeneous catalytic ozonation (HCO). In this study, we designed a carbon-coated aluminum oxide carrier (C-Al2O3) and reinforced it with Mn-Fe bimetal assemblages to prepare a high-performance catalyst Mn-Fe/C-Al2O3. The results showed that the carbon embedding significantly improved the abundance of surface oxygen functional groups, conductivity, and adsorption capacity of γ-Al2O3, while preserving its exceptional mechanical strength as a carrier. The prepared Mn-Fe/C-Al2O3 catalyst exhibited satisfactory catalytic ozonation activity and stability in the degradation of p-nitrophenol (PNP). Electron paramagnetic resonance (EPR) and quenching experiments reveal that radical ( ⋅ OH and ⋅ O2 ⋅ ) and nonradical oxidation (1O2) dominated the PNP degradation process. Theoretical calculations corroborated that the anchored atomic Fe and Mn sites regulated the local electronic structure of the catalyst. This modulation effectively promoted the activation of O3 molecules, resulting in the generation of atomic oxygen species (AOS) and reactive oxygen species (ROS). The economic analysis on Mn-Fe/C-Al2O3 revealed that it was a cost-competitive catalyst for HCO. This study not only deepens the understanding on the reaction mechanism of HCO with transition metal/carbon composite catalysts, also provides a high-performance and cost-competitive ozone catalyst for prospective application.

Defluorination of perfluorooctanoic acid and perfluorooctane sulfonic acid by heterogeneous catalytic system of Fe-Al2O3/O3: Synergistic oxidation effects and defluorination mechanism

In this study, catalytic ozonation by Fe-Al2O3 was used to investigate the defluorination of PFOA and PFOS, assessing the effects of different experimental conditions on the defluorination efficiency of the system. The oxidation mechanism of the Fe-Al2O3/O3 system and the specific degradation and defluorination mechanisms for PFOA and PFOS were determined. Results showed that compared to the single O3 system, the defluorination rates of PFOA and PFOS increased by 2.32- and 5.92-fold using the Fe-Al2O3/O3 system under optimal experimental conditions. Mechanistic analysis indicated that in Fe-Al2O3, the variable valence iron (Fe) and functional groups containing C and O served as important reaction sites during the catalytic process. The co-existence of 1O2, OH, O2- and high-valence Fe(IV) constituted a synergistic oxidation system consisting of free radicals and non-radicals, promoting the degradation and defluorination of PFOA and PFOS. DFT theoretical calculations and the analysis of intermediate degradation products suggested that the degradation pathways of PFOA and PFOS involved Kolbe decarboxylation, desulfonation, alcoholization and intramolecular cyclization reactions. The degradation and defluorination pathways of PFOA and PFOS consisted of the stepwise removal of -CF2-, with PFOS exhibiting a higher defluorination rate than PFOA due to its susceptibility to electrophilic attack. This study provides a theoretical basis for the development of heterogeneous catalytic ozonation systems for PFOA and PFOS treatment.

2D-Like Catalyst with a Micro-nanolinked Functional Surface for Water Purification

In water purification, the performance of heterogeneous advanced oxidation processes significantly relies upon the utilization of the catalyst's specific surface area (SSA). However, the presence of the structural "dead volume" and pore-size-induced diffusion-reaction trade-off limitation restricts the functioning of the SSA. Here, we reported an effective approach to make the best SSA by changing the traditional 3D spherule catalyst into a 2D-like form and creating an in situ micro-nanolinked structure. Thus, a 2D-like catalyst was obtained which was characterized by a mini "paddy field" surface, and it exhibited a sharply decreased dead volume, a highly available SSA and oriented flexibility. Given its paddy-field-like mass-transfer routine, the organic capture capability was 7.5-fold higher than that of the catalyst with mesopores only. Moreover, such a catalyst exhibited a record-high O3-to-·OH transition rate of 2.86 × 10-8 compared with reported millimetric catalysts (metal base), which contributed to a 6.12-fold higher total organic removal per catalyst mass than traditional 3D catalysts. The facile scale preparation, performance stability, and significant material savings with the 2D-like catalyst were also beneficial for practical applications. Our findings provide a unique and general approach for designing potential catalysts with excellent performance in water purification.

Degradation of atrazine by electro-peroxone enhanced by Fe and N co-doped carbon nanotubes with simultaneous catalysis of H2O2 and O3

Fe and N co-doped carbon nanotubes (Fe-N-CNT) was synthesized and attempted as efficient heterogeneous catalysts for simultaneous catalysis of H2O2 and O3 to improve electro-peroxone (Fe-N-CNT/EP) process efficiency for atrazine (ATZ) degradation. The removal and mineralization of ATZ was significantly enhanced, obtaining the degradation rate constant (k) by Fe-N-CNT/EP (0.23 min-1) about two times that of EP (0.12 min-1) owing to the formation of Fe0 and Fe-N coordination in Fe-N-CNT catalyst for co-catalysis of H2O2 and O3. The important factors such as applied current and ozone concentration were investigated, demonstrating that the optimized performance could be achieved at current of 30 mA and ozone concentration of 55 mg L-1. The oxidation capacity of Fe-N-CNT/EP maintained stably under wide pH range of 3∼7, obtaining the degradation rate constant 1.23-1.92 times that of EP and overcoming the defect of EP at acidic and neutral conditions. Capture experiments and electron paramagnetic resonance (EPR) experiments verified that .OH, generated by accelerating decomposition of H2O2/O3 and peroxone reaction, was the dominant active specie in Fe-N-CNT/EP. Besides, Fe-N-CNT showed high catalytic activity and good stability during six cycles. This work provides an efficient activator for enhanced EP process, exhibiting a promising prospect for water and wastewater purification.

Revisit the Role of Salinity in Heterogeneous Catalytic Ozonation: The Trade-Off between Reaction Inhibition and Mass Transfer Enhancement

Heterogeneous catalytic ozonation (HCO) is an effective technology for advanced wastewater treatment, while the influence of coexisting salts remains unclear and controversial. Here, we systematically explored the influence of NaCl salinity on the reaction and mass transfer of HCO through lab experiments, kinetic simulation, and computational fluid dynamics modeling, and proposed that the trade-off between reaction inhibition and mass transfer enhancement would affect the pollutants degradation pattern under varying salinity. The increase of NaCl salinity decreased ozone solubility and accelerated the futile consumption of ozone and hydroxyl radicals (•OH), and the maximum •OH concentration under 50 g/L salinity was only 23% of that without salinity. However, the increase of NaCl salinity also significantly reduced the ozone bubble size and enhanced the interphase and intraliquid mass transfer, with the volumetric mass transfer coefficient being 130% higher than that without salinity. The trade-off between reaction inhibition and mass transfer enhancement shifted under different pH values and aerator pore sizes, and the oxalate degradation pattern would change correspondingly. Besides, the trade-off was also identified for Na2SO4 salinity. These results emphasized the dual influence of salinity and offered a new theoretical perspective on the role of salinity in the HCO process.

Catalytic ozonation mechanism over M1-N3C1 active sites

The structure-activity relationship in catalytic ozonation remains unclear, hindering the understanding of activity origins. Here, we report activity trends in catalytic ozonation using a series of single-atom catalysts with well-defined M1-N3C1 (M: manganese, ferrum, cobalt, and nickel) active sites. The M1-N3C1 units induce locally polarized M - C bonds to capture ozone molecules onto M atoms and serve as electron shuttles for catalytic ozonation, exhibiting excellent catalytic activities (at least 527 times higher than commercial manganese dioxide). The combined in situ characterization and theoretical calculations reveal single metal atom-dependent catalytic activity, with surface atomic oxygen reactivity identified as a descriptor for the structure-activity relationship in catalytic ozonation. Additionally, the dissociation barrier of surface peroxide species is proposed as a descriptor for the structure-activity relationship in ozone decomposition. These findings provide guidelines for designing high-performance catalytic ozonation catalysts and enhance the atomic-level mechanistic understanding of the integral control of ozone and methyl mercaptan.

Enhancing water purification through F and Zn-modified Fe-MCM-41 catalytic ozonation

Due to its low interfacial electron migration ability and highly hydrophilic, Fe-MCM-41 (FeM) had poor activity and stability during catalytic ozonation. To this end, the secondary metal Zn and Si-F group were introduced into the framework of FeM to create surface potential difference and hydrophobic sites. Comparative characterizations showed that there existed rich acid sites with great potential difference on F-Fe-Zn-MCM-41 (FFeZnM). Additionally, because of the existence of hydrophobic and electron-withdrawing Si-F unit, the electron migration ability, hydrophobicity and acidity of FFeZnM were enhanced. The greater O3 mass transfer was induced by Si-F group and O3 was directly activated at Fe and Zn Lewis acid sites into •OH, •O2- and 1O2. With •OH acting as main species, FFeZnM/O3 achieved the superior IBP removal (93.4%, 30 min) and TOC removal (46.6%, 120 min) over those of sole O3 and F-FeM/O3 processes, respectively. HCO3-, Cl-, NO3- and SO42- hindered IBP degradation by FFeZnM/O3, but high concentration humic acid (HA) exhibited promotion by forming HA-IBP complex. IBP degradation by FFeZnM/O3 was enhanced with tap water, river water, and effluent from the secondary sedimentation tank of the sewage plant acting as medium. This study proposed an innovative approach to catalyst design for catalytic ozonation.

Mn-doped carbon-Al2SiO5 fibers enable catalytic ozonation for wastewater treatment: Interface modulation and mass transfer enhancement

Heterogeneous catalytic ozonation is an efficient approach to remove hazardous and refractory organic contaminants in wastewater. It is crucial to design an ozone catalyst with high catalytic activity, high mass transfer and facile separation properties. Herein, easily separable aluminosilicate (Al2SiO5) fibers were developed as carriers and after interface modulation, Mn-doped carbon-Al2SiO5 (Mn-CAS) fibrous catalysts were proposed for catalytic ozonation. The growth of carbon shells on Al2SiO5 fiber surface and the introduction of metal Mn provided abundant Lewis acid sites to catalyze ozone. The Mn-CAS fiber/O3 system exhibited superior reactivity to degrade oxalic acid with a rate constant of 0.034 min-1, which was about 19 times as high as Al2SiO5/O3. For coal gasification wastewater treatment, Mn-CAS fibers also demonstrated high catalytic activity and stability and the COD removal was over 56%. Computational fluid dynamic simulations proved the high mass transfer properties of fibrous catalysts. Hydroxyl radicals (•OH) were identified as the predominant active species for organic degradation. Particularly, the catalytic pathways of O3 to •OH on Mn-O4 sites were revealed by theoretical calculations. This work provides a novel fibrous catalyst with high reactivity and mass transfer as well as easy separation characteristics for catalytic ozonation and wastewater purification.

Accelerated Catalytic Ozonation in a Mesoporous Carbon-Supported Atomic Fe-N4 Sites Nanoreactor: Confinement Effect and Resistance to Poisoning

The design of a micro-/nanoreactor is of great significance for catalytic ozonation, which can achieve effective mass transfer and expose powerful reaction species. Herein, the mesoporous carbon with atomic Fe-N4 sites embedded in the ordered carbon nanochannels (Fe-N4/CMK-3) was synthesized by the hard-template method. Fe-N4/CMK-3 can be employed as nanoreactors with preferred electronic and geometric catalytic microenvironments for the internal catalytic ozonation of CH3SH. During the CH3SH oxidation process, the mass transfer coefficient of the Fe-N4/CMK-3 confined system with sufficient O3 transfer featured a level of at least 1.87 × 10-5, which is 34.6 times that of the Fe-N4/C-Si unconfined system. Detailed experimental studies and theoretical calculations demonstrated that the anchored atomic Fe-N4 sites and nanoconfinement effects regulated the local electronic structure of the catalyst and promoted the activation of O3 molecules to produce atomic oxygen species (AOS) and reactive oxygen species (ROS), eventually achieving efficient oxidation of CH3SH into CO2/SO42-. Benefiting from the high diffusion rate and the augmentation of AOS/ROS, Fe-N4/CMK-3 exhibited an excellent poisoning tolerance, along with high catalytic durability. This contribution provides the proof-of-concept strategy for accelerating catalytic ozonation of sulfur-containing volatile organic compounds (VOCs) by combining confined catalysis and atomic catalysts and can be extended to the purification of other gaseous pollutants.

An extra-chelator-free fenton process assisted by electrocatalytic-induced in-situ pollutant carboxylation for target refractory organic efficient treatment in chemical-industrial wastewater

For traditional Fenton processes, the quenching behavior of radical contenders (e.g., most aliphatic hydrocarbons) on hydroxyl radicals (·OH) usually hinders the removal of target refractory pollutants (aromatic/heterocyclic hydrocarbons) in chemical industrial wastewater, leading to excess energy consumption. Herein, we proposed an electrocatalytic-assisted chelation-Fenton (EACF) process, with no extra-chelator addition, to significantly enhance target refractory pollutant (pyrazole as a representative) removal under high ·OH contender (glyoxal) levels. Experiments and theoretical calculations proved that superoxide radical (·O₂^-) and anodic direct electron transfer (DET) effectively converted the strong ·OH-quenching substance (glyoxal) to a weak radical competitor (oxalate) during the electrocatalytic oxidation process, promoting Fe²⁺ chelation and therefore increasing radical utilization for pyrazole degradation (reached maximum of ∼43-fold value upon traditional Fenton), which appeared more obviously in neutral/alkaline Fenton conditions. For actual pharmaceutical tailwater treatment, the EACF achieved 2-folds higher oriented-oxidation capability and ∼78% lower operation cost per pyrazole removal than the traditional Fenton process, demonstrating promising potential for future practical applications.

New strategy of drinking water sludge as conditioner to enhance waste activated sludge dewaterability: Collaborative disposal

Drinking water sludge (DWS) and waste activated sludge (WAS) are usually treated separately. With the continuous deepening understanding of the characteristics of two types sludge, the research and application of the collaborative disposal is worth considering. The heated modification DWS (HDWS) rich in inorganic matter and aluminum (Al₂O₃) can be used as a conditioner to enhance WAS dewaterability using its properties with physical skeleton and chemically catalyzed ozone (O₃). The results showed that the minimum values of capillary water time (CST) and specific resistance filtration (SRF) for WAS were 20.9±2.40 s and 1.07±0.19×10¹³ m/kg at pH=4, O₃ dosage=60 mg/g VS and HDWS dosage=700 mg/g VS, corresponding to the reduction of sludge cake water content (Wc) to 60.37±0.97 %. The mechanism of HDWS+O₃ enhanced WAS dewaterability was systematically elucidated through pyridine-infrared analysis and density functional theory (DFT) calculations. The surface of Al₂O₃ in HDWS had more Lewis acidic sites, and the oxygen atoms of O₃ combined with Al atoms to form Al-O bonds and undergo electron transfer, while O₃ molecules dissociated to produce more hydroxyl radicals (·OH). With the oxidation of ·OH, the extra-microcolony/cellular polymers (EMPS/ECPS) structure were destroyed and became looser, promoting the conversion of internal moisture to free moisture. Zeta potential tended to zero, particle size increased, and the surface was more hydrophobic. Correlation analysis revealed that the component content, protein (PN) secondary structure and molecular weight (MW) in ECPS were positively and more strongly correlated with the sludge dewaterability compared to EMPS. The discovery of HDWS+O₃ applied to effectively enhance WAS dewaterability provided an inspiring perspective on the emerging DWS and WAS co-processing disposition.

New insights into engineering the core size and carbon shell thickness of Co@C core-shell catalysts for efficient and stable Fenton-like catalysis

Herein, we engineered the cobalt core size and carbon shell thickness of Co@C by molten salt electrolysis (MSE) to investigate the enhanced essence of decreasing core size as well as the shell thickness dependence-mediated transition of catalytic mechanisms. We found that the reaction activation energy (RAE) of Co@C/peroxymonosulfate (PMS) systems was intimately dependent on the core sizes for sulfamethoxazole (SMX) degradation. The smaller core size of 26 nm provided a lower RAE of 13.39 kJ mol^-1. In addition, increasing carbon shell thicknesses of Co@C altered the catalytic mechanisms from a radical pathway of SO₄^•- and •OH to to a non-radical pathway of ¹O₂ and electron-transfer process (ETP), which were verified by experimental results and density functional theory (DFT) calculations. Interestingly, increasing carbon shell thicknesses promoted the charge transfer between Co metal slab and carbon shell, increased the adsorption energy of PMS molecule on the Co@C slab, and decreased the length of OO, which favoured the occurrence of non-free radical processes.

Single-Atom Fe-N4 Sites for Catalytic Ozonation to Selectively Induce a Nonradical Pathway toward Wastewater Purification

Nonradical oxidation has been determined to be a promising pathway for the degradation of organic pollutants in heterogeneous catalytic ozonation (HCO). However, the bottlenecks are the rational design of catalysts to selectively induce nonradicals and the interpretation of detailed nonradical generation mechanisms. Herein, we propose a new HCO process based on single-atom iron catalysts, in which Fe-N₄ sites anchored on the carbon skeleton exhibited outstanding catalytic ozonation activity and stability for the degradation of oxalic acid (OA) and p-hydroxybenzoic acid (pHBA) as well as the advanced treatment of a landfill leachate secondary effluent. Unlike traditional radical oxidation, nonradical pathways based on surface-adsorbed atomic oxygen (*O_ad) and singlet oxygen (¹O₂) were identified. A substrate-dependent behavior was also observed. OA was adsorbed on the catalyst surface and mainly degraded by *O_ad, while pHBA was mostly removed by O₃ and ¹O₂ in the bulk solution. Density functional theory calculations and molecular dynamics simulations revealed that one terminal oxygen atom of ozone preferred bonding with the central iron atom of Fe-N₄, subsequently inducing the cleavage of the O-O bond near the catalyst surface to produce *O_ad and ¹O₂. These findings highlight the structural design of an ozone catalyst and an atomic-level understanding of the nonradical HCO process.

A comprehensive review on metal based active sites and their interaction with O3 during heterogeneous catalytic ozonation process: Types, regulation and authentication

Heterogeneous catalytic ozonation (HCO) was a promising water purification technology. Designing novel metal-based catalysts and exploring their structural-activity relationship continued to be a hot topic in HCO. Herein, we reviewed the recent development of metal-based catalysts (including monometallic and polymetallic catalysts) in HCO. Regulation of metal based active sites (surface hydroxyl groups, Lewis acid sites, metal redox cycle and surface defect) and their key roles in activating O₃ were explored. Advantage and disadvantage of conventional characterization techniques on monitoring metal active sites were claimed. In situ electrochemical characterization and DFT simulation were recommended as supplement to reveal the metal active species. Though the ambiguous interfacial behaviors of O₃ at these active sites, the existence of interfacial electron migration was beyond doubt. The reported metal-based catalysts mainly served as electron donator for O₃, which resulted in the accumulation of oxidized metal and reduced their activity. Design of polymetallic catalysts could accelerate the interfacial electron migration, but they still faced with the dilemma of sluggish Me^(n+m)+/Meⁿ⁺ redox cycle. Alternative strategies like coupling active metal species with mesoporous silicon materials, regulating surface hydrophobic/hydrophilic properties, polaring surface electron distribution, coupling HCO process with photocatalysis and H₂O₂ were proposed for future research.

Hydrogen peroxide and peroxymonosulfate intensifying Fe-doped NiC-Al2O3-framework-based catalytic ozonation for advanced treatment of landfill leachate: Performance and mechanisms

The biotreated effluent of landfill leachate still contains numerous refractory organic contaminants, which poses potential threats to human health and ecosystems. Influenced by landfill ages and other factors, the concentration of organic matter varies. Heterogeneous catalytic ozonation (HCO) is a promising technology for advanced wastewater treatment. Aiming to achieve the up-to-standard discharge of low-concentration landfill leachate (COD ≈ 108 mg·L^-1) and improve the biodegradability of high-concentration landfill leachate (COD ≈ 1720 mg·L^-1), the active component Fe was incorporated into a firm Ni-induced C-Al₂O₃-framework (_NiCAF) composite support to synthesize a Fe-_NiCAF catalyst for efficient catalytic ozonation. When the Fe-_NiCAF dosage was 4 g·L^-1, the gas flow rate was 0.5 L·min^-1, and the ozone concentration was 20.0 mg·L^-1, the COD of low-concentration landfill leachate effluent decreased to 43 mg·L^-1, and the COD removal rate constant of low-concentration landfill leachate was 154% higher than that of pure ozone. For high-concentration landfill leachate with the BOD₅/COD of 0.058, the COD removal efficiency in Fe-_NiCAF/O₃ increased from 39% to 57% compared with ozonation, and the effluent BOD₅/COD increased to 0.282. Furthermore, the addition of hydrogen peroxide (H₂O₂) and peroxymonosulfate (PMS) can further enhance the treatment performance of Fe-_NiCAF/O₃ process and different strengthening mechanisms were revealed. The results indicated that surface hydroxyls on the Fe-_NiCAF catalyst surface were the main catalytic sites for ozone, and hydroxyl radical (•OH) and singlet oxygen (¹O₂) were identified as the main reactive oxygen species for the removal of organics in landfill leachate. Adding H₂O₂ can promote the generation of •OH for nonselective degradation of various organics, while PMS mainly enhanced the production of ¹O₂ to decompose macromolecular humus. This work highlighted an efficient Fe-_NiCAF ozone catalyst and an innovative peroxide intensified HCO strategy for the advanced treatment of landfill leachate.

Self-Accelerating Interfacial Catalytic Elimination of Gaseous Sulfur-Containing Volatile Organic Compounds as Microbubbles in a Facet-Engineered Three-Dimensional BiOCl Sponge Fenton-Like Process

The elimination of gaseous sulfur-containing volatile organic compounds (S-VOCs) by a microbubble-assisted Fenton-like process is an innovative strategy. Herein, we established a microbubble-assisted Fenton-like process to eliminate malodorous microbubble CH₃SH as representative gaseous S-VOCs, in which BiOCl nanosheets loaded on a three-dimensional sponge were exposed to (001) or (010) facets and induced Fenton-like interface reactions. Intriguingly, the microbubble-assisted Fenton-like process significantly removed 99.9% of CH₃SH, higher than that of the macrobubble-assisted Fenton-like process (39.0%). The self-accelerating interfacial catalytic mechanism was in-depth identified by in situ ATR-FTIR, PTR-TOF-MS, EPR, and DFT computational study. The extraordinary elimination performance of microbubble-assisted Fenton-like process lies in the enhancing dissolution/mass transfer of gaseous CH₃SH in the gas/liquid phase and the tight contact between CH₃SH-microbubbles and 3D-BiOCl sponge due to the low rising velocity (0.13 mm s^-1) and negative charge (-45.53 mV) of CH₃SH-microbubbles, as well as the effective generation of ¹O₂ by activating the enriched dissolved oxygen in CH₃SH-microbubble via effective electron-polarized sites on 3D-BiOCl sponge. Furthermore, CH₃SH-microbubbles transferred electrons to H₂O₂ through electron-rich oxygen vacancy centers of the 3D-BiOCl sponge to generate more •OH, thus achieving excellent elimination performance. Overall, this study demonstrates the enhanced self-accelerating interfacial catalytic elimination by S-VOC microbubble and provides the underlying mechanisms.