Identifying the Persistent Free Radicals (PFRs) Formed as Crucial Metastable Intermediates during Peroxymonosulfate (PMS) Activation by N-Doped Carbonaceous Materials

Magnetic CoFe hydrotalcite composite Co metal-organic framework material efficiently activating peroxymonosulfate to degrade sulfamethoxazole: Oxygen vacancy-mediated radical and non-radical pathways

Herein, a novel rich oxygen vacancy (Ov) cobalt-iron hydrotalcite composite cobalt metal-organic framework material (ZIF-67/CoFe-LDH) was prepared by simple urea water and heat reduction approach and utilized for the peroxymonosulfate (PMS) system to remove sulfamethoxazole (SMX). 95 ± 1.32 % SMX (20 mg/L) was able to degraded in 20 min with TOC removal of 53 ± 1.56 % in ZIF-67/CoFe-LDH/PMS system. The system maintained a fantastic catalytic capability with wide pH range (3-9) and common interfering substances (Cl-, NO3-, CO32-, PO42- and humic acid (HA)), and the degradation efficiency could even remain 80.2 ± 1.48 % at the fifth cycle. Meanwhile, the applicability and feasibility of the catalysts for practical water treatment was verified by the degradation effects of SMX in different water environments and several other typical pollutants. Co and Fe bimetallic active centers synergistically activate PMS, and density functional theory (DFT) predicted adsorption energy about Ov in ZIF-67/CoFe-LDH for PMS was 1.335 eV, and OO bond length of PMS was stretched to 1.826 Å. As a result, PMS was more easily activated and broken, which accelerated the singlet oxygen (1O2), sulfate radical (SO4•-), high-valent metals and other reactive oxygen species (ROS). Radical and non-radical jointly degrading the pollutants improved the catalytic effect. Finally, SMX degradation intermediates were analyzed to explain the degradation pathway and their biotoxicity was also evaluated. This paper provides a new research perspective of oxygen vacancy activating PMS to degrade pollutants.

Activation of peroxydisulfate by zero valent iron-carbon composites prepared by carbothermal reduction: Enhanced non-radical and radical synergies

Since its application in environmental remediation, nano zero-valent iron (nZVI) has gained wide attention for its environmental friendliness, strong reducing ability, and wide range of raw materials. However, its high preparation cost and difficulty in preservation remain the bottlenecks for their application. Carbothermal reduction is a promising method for the industrial preparation of nZVI. Micronized zero-valent iron/carbon materials (Fe0/CB) were produced in one step by co-pyrolysis of carbon and iron. The performance of the Fe0/CB is comparable to that of nZVI. In addition, Fe0/CB overcomed the disadvantages of agglomeration and oxidative deactivation of nZVI. Experiments on the Fenton-like reaction of its activated PDS showed that metronidazole (MNZ) was efficiently removed through the synergistic action of radicals and non-radicals, which were mainly superoxide radicals (·O2-), monoclinic oxygen (1O2), and high-valent iron (FeIVO). Moreover, the degradation process showed better generalization, making it suitable for a wide range of applications in the degradation of antibiotics.

A state-of-art review on the redox activity of persistent free radicals in biochar

Biochar-bound persistent free radicals (biochar-PFRs) attract much attention because they can directly or indirectly mediate the transformation of contaminants in large-scale wastewater treatment processes. Despite this, a comprehensive top-down understanding of the redox activity of biochar-PFRs, particularly consumption and regeneration mechanisms, as well as challenges in redox activity assessment, is still lacking. To tackle this challenge, this review outlines the identification and determination methods of biochar-PFRs, which serve as a prerequisite for assessing the redox activity of biochar-PFRs. Recent developments concerning biochar-PFRs are discussed, with a main emphasis on the reaction mechanisms (both non-free radical and free radical pathways) and their effectiveness in removing contaminants. Importantly, the review delves into the mechanism of biochar-PFRs regeneration, triggered by metal cations, reactive oxygen species, and ultraviolet radiations. Furthermore, this review thoroughly explores the dilemma in appraising the redox activity of biochar-PFRs. Components with unpaired electrons (particular defects and metal ions) interfere with biochar-PFRs signals in electron paramagnetic resonance spectra. Scavengers and extractants of biochar-PFRs also inevitably modify the active ingredients of biochar. Based on these analyses, a practical strategy is proposed to precisely determine the redox activity of biochar-PFRs. Finally, the review concludes by presenting current gaps in knowledge and offering suggestions for future research. This comprehensive examination aims to provide new and significant insights into the redox activity of biochar-PFRs.

Insights into the efficient water treatment over N-doped carbon nanosheets with layered minerals as template: The role of interfacial electron tunneling and transfer

Peroxymonosulfate (PMS) oxidation reactions have been extensively studied recently. Due to the high material cost and low catalytic capability, PMS oxidation technology cannot be effectively applied in an industrial water treatment process. In this work, we developed a modification strategy based on enhancing the neglected electron tunneling effect to optimize the intrinsic electron transport process of the catalyst. The 2D nitrogen-doped carbon-based nanosheets with small interlayer spacing were prepared by self-polymerization of dopamine hydrochloride inserted into the natural layered bentonite template. Systematic characterizations confirmed that the smaller layer spacing in the 2D nitride-doped carbon-based nanosheets reduces the depletion layer width. The weak electronic shielding effect derived by the small layer spacing on the material subsurface enhanced the bulk electron tunneling effect. More bulk electrons could be migrated to the catalyst surface to activate PMS molecules. The PMS activation system showed ultrafast oxidation capability to degrade organic pollutants and strong ability to resist interference from environmental matrixes due to the optimized electron transfer process. Furthermore, the developed membrane reactor exhibited strong catalytic stability during the continuous degradation of P-Chlorophenol (CP).

Excess sludge-based biochar loaded with manganese enhances catalytic ozonation efficiency for landfill leachate treatment

This study developed an efficient and stable landfill leachate treatment process, which was based on the combination of biochar catalytic ozonation and activated sludge technology for intensive treatment of landfill leachate, aiming to achieve the standard discharge of leachate. The focus is to investigate the effect of manganese loading on the physicochemical properties of biochar and the mechanism of its catalytic ozonation. It was found that more surface functional groups (CO, Mn-O, etc.) and defects (ID/IG = 1.27) were exposed via the change of original carbon structure by loading Mn, which is conducive to the generation of lattice oxygen. Meanwhile, generating different valence states of Mn metal can improve the redox properties and electron migration rate, and encourage the production of reactive oxygen species (ROS) during the reaction process and enhance the catalytic efficiency. The synergistic action of microorganisms, especially denitrifying bacteria, was found to play a key role in the degradation of nitrogenous pollutants during the activated sludge process. The concentration of NH+4-N was reduced from the initial 1087.03 ± 9.56 mg/L to 9.05 ± 1.91 mg/L, while COD was reduced from 2290 ± 14.14 mg/L to 86.5 ± 2.12 mg/L, with corresponding removal rates of 99.17% and 99.20%, respectively. This method offers high efficiency and stability, achieving discharge standards for leachate (GB16889-2008). The synergy between Mn-loaded biochar and microorganisms in the activated sludge is key to effective treatment. This study offers a new approach to solving the challenge of waste leachate treatment.

Robust reactive oxygen species production in interfacial reaction between organic acids and biochar: The combined effect of electron acceptance and electron conduction

Interfacial electron transport and reactive oxygen species (ROS) generation in the redox action between biochar (BC) and low-molecular-weight organic acids (LMWOAs) have been overlooked during the utilization of BC in soil amelioration/remediation. Herein, BC and N-doped BC (NBC) with various physicochemical properties were prepared at pyrolysis temperatures of 350, 550 and 750 °C (namely BC/NBC350, 550 and 750) and their interactions with LMWOAs were systematically investigated. Results of ROS quenching and electrochemical cell experiments revealed that BC/NBC could strongly interact with LMWOAs, especially ascorbic acid (AA), and acted as an electron shuttle to mediate one electron transfer from AA to oxygen to generate O2•-, followed by the robust generation of H2O2 and •OH. Interestingly, the generation rates of •OH and H2O2 in the NBC-AA system were significantly higher than that in the BC-AA system, of which the NBC550-AA system exhibited the best performance, enabling its superior ability in bisphenol A degradation. Different from conventional wisdom, the electron shuttle capacity depended on a combined aromatization degree and electron-accepting capacity of BC/NBC. These findings complement the interfacial electron transfer mechanism during the natural BC-LMWOAs interaction and provide new inspiration for the development of green and efficient organic pollutant removal technologies.

Persistent free radicals on biochar for its catalytic capability: A review

Biochar is an economical carbon material for water pollution control, which shows great promise to be applied in the up-scale wastewater remediation processes. Previous studies demonstrate that persistent free radicals (PFRs) on biochar are critical to its reactivity for wastewater remediation. A series of studies have revealed the important roles of PFRs when biochar was applied for organic pollutants degradation as well as the removal of Cr (VI) and As (III) from wastewater. Therefore, this review comprehensively concludes the significance of PFRs for the catalytic capabilities of biochar in advanced oxidation processes (AOPs)-driven organic pollutant removal, and applied in redox processes for Cr (VI) and As (III) remediation. In addition, the mechanisms for PFRs formation during biochar synthesis are discussed. The detection methods are reviewed for the quantification of PFRs on biochar. Future research directions were also proposed on underpinning the knowledge base to forward the applications of biochar in practical real wastewater treatment.

Atomically Dispersed Fe-N4 Site as a Conductive Bridge Enables Efficient and Stable Activation of Peroxymonosulfate: Active Site Renewal, Anti-Oxidative Capacity, and Pathway Alternation Mechanism

Atomically dispersed metal sites anchored on nitrogen-doped carbonaceous substrates (M-NCs) have emerged as promising alternatives to conventional peroxymonosulfate (PMS) activators; however, the exact contribution of each site still remains elusive. Herein, isolated Fe-N4 active site-decorated three-dimensional NC substrates (FeSA-NC) via a micropore confinement strategy are fabricated to initiate PMS oxidation reaction, achieving a specific activity of 5.16 × 103 L·min-1·g-1 for the degradation of bisphenol A (BPA), which outperforms most of the state-of-the-art single-atom (SA) catalysts. Mechanism inquiry reveals enhanced chemisorption and electron transfer between PMS and FeSA-NC, enabling an inner electron shuttle mechanism in which Fe-N4 serves as a conductive bridge. The Fe-N4 sites reduce the energy barrier for the formation of SO5* and H*, thereby transforming the reaction pathway from directly adjacent electron transfer into reactive oxygen species (ROS)-dominated oxidation. Theoretical calculations and dynamic simulations reveal that the Fe-N4 sites induce facilitated desorption of reaction intermediates (PMS*/BPA*), which collectively contribute to the renewal of active sites and eventually enhance the catalytic durability. This work offers a reasonable interpretation for the important role of the Fe-N4 moiety in altering the activation mechanism and enhancing the antioxidative capacity of NC materials, which fundamentally furnishes theoretical support for SA material design.

Mercury reduction by black carbon under dark conditions

An accurate depiction of mercury (Hg) reduction is important to predict Hg biogeochemistry in both aquatic and soil systems. Although the photoreduction of Hg is well documented, reduction in the dark is poorly known and is thus the focus of this work. Black carbon (BC), an important constituent of organic matter in environments, can reduce Hg2+ in dark and oxygen-deficient conditions. Fast removal of Hg2+ in BC/Hg2+ solution was observed, with 4.99-86.88 L mg-1h-1 of the reaction rate constant, which could be ascribed to the combined actions of adsorption and reduction. Meanwhile, slow Hg reduction was obtained, compared to Hg removal, with 0.06-2.16 L mg-1h-1 of the reaction rate constant. Thus, in the initial stage, Hg2+ removal was mainly triggered by adsorption, rather than reduction. Afterward, the adsorbed Hg2+ on black carbon was converted into Hg0. Dissolved black carbon and aromatic CH on particulate black carbon were dominant triggers of Hg reduction for black carbon. During Hg reduction, the intastable intermediate, formed in the complex between aromatic CH and Hg2+, behaved as persistent free radicals, which could be detected by in situ electron paramagnetic resonance. Subsequently, the intastable intermediate was mainly converted into CO on black carbon and Hg0. Corresponding results of the present study highlight the important role of black carbon in the Hg biogeochemical cycle.

Single cobalt atoms anchored on Ti3C2Tx with dual reaction sites for efficient adsorption-degradation of antibiotic resistance genes

The assimilation of antibiotic resistance genes (ARGs) by pathogenic bacteria poses a severe threat to public health. Here, we reported a dual-reaction-site-modified CoSA/Ti3C2Tx (single cobalt atoms immobilized on Ti3C2Tx MXene) for effectively deactivating extracellular ARGs via peroxymonosulfate (PMS) activation. The enhanced removal of ARGs was attributed to the synergistic effect of adsorption (Ti sites) and degradation (Co-O3 sites). The Ti sites on CoSA/Ti3C2Tx nanosheets bound with PO43- on the phosphate skeletons of ARGs via Ti-O-P coordination interactions, achieving excellent adsorption capacity (10.21 × 1010 copies mg-1) for tetA, and the Co-O3 sites activated PMS into surface-bond hydroxyl radicals (•OHsurface), which can quickly attack the backbones and bases of the adsorbed ARGs, resulting in the efficient in situ degradation of ARGs into inactive small molecular organics and NO3. This dual-reaction-site Fenton-like system exhibited ultrahigh extracellular ARG degradation rate (k > 0.9 min-1) and showed the potential for practical wastewater treatment in a membrane filtration process, which provided insights for extracellular ARG removal via catalysts design.

Insight into Chinese medicine residue biochar combined with ultrasound for persulfate activation in atrazine degradation: Acanthopanax senticosus precursors, synergistic effects and toxicity assessment

The synergistic activation of persulfate by multiple factors could degrade pollutants more efficiently. However, the co-activation method based on metal ions has the risk of leakage. The non-metallic coupling method could achieve the same efficiency as the metal activation and meanwhile release environmental stress. In this study, the original biochar (BC) was prepared through using Chinese medicinal residue of Acanthopanax senticosus as the precursor. Compared with other biochar, the pore size structure was higher and toxicity risk was lower. The ultrasonic (US)/Acanthopanax senticosus biochar (ASBC)/persulfate oxidation system was established for Atrazine (ATZ). Results showed that 45KHz in middle and low frequency band cooperated with ASBC600 to degrade nearly 70 % of ATZ within 50 min, and US promoted the formation of SO₄^- and OH. Meanwhile, the synergy index of US and ASBC was calculated to be 1.18, which showed positive synergistic effect. Finally, the potential toxicity was examined by using Toxicity Characteristic Leaching Procedure (TCLP) and luminescent bacteria. This study provides a promising way for the activation of persulfate, which is expected to bring a new idea for the win-win situation of pollutant degradation and solid waste resource utilization.

Long-term operation of cathode-enhanced ecological floating bed coupled with microbial electrochemical system for urban surface water remediation: From lab-scale research to engineering application

Ecological floating bed coupled with microbial electrochemical system (ECOFB-MES) has great application potential in micro-polluted water remediation yet limited by low electron transfer efficiency on the microbial/electrode interface. Here, an innovative cathode-enhanced EOCFB-MES was constructed with nano-Fe₃O₄ modification and applied for in-situ remediation both at lab scale (6 L, 62-day operation) and demonstration scale (2300 m², 1-year operation). The cathode-enhanced ECOFB-MES exhibited superior removal in TOC (81.43 ± 2.05%), TN (85.12% ± 1.46%) and TP (59.80 ± 2.27%), much better than those of original ECOFB-MES and anode-enhanced ECOFB-MES in the laboratory test. Meanwhile, cathode-enhanced ECOFB-MES boosted current output by 33% than that of original ECOFB-MES, which made a great contribution to the improvement of ectopic electronic compensation for pollutant decontamination. Notably, cathode-enhanced ECOFB-MES presented high efficiency, stability and durability in the demonstration test, and fulfilled the average concentration of COD (9.5 ± 2.81 mg/L), TN (1.00 ± 0.21 mg/L) and TP (0.10 ± 0.04 mg/L) of effluent water to meet the Grade III (GB 3838-2002) with stable operation stage. Based on the KOSIM calculation, the removal loads of cathode-enhanced ECOFB-MES in carbon, nitrogen and phosphorus could reach 37.14 g COD/(d·m²), 2.62 g TN/(d·m²) and 0.55 g TP/(d·m²), respectively. According to the analysis of microbial communities and functional genes, the cathode modified by Fe₃O₄ made a sensible enrichment in electroactive bacteria (EAB) and nitrogen-converting bacteria (NCB) as well as facilitated the functional genes expression in electron transfer and nitrogen metabolism, resulting in the synergistic removal of carbon in sediment and nitrite in water. This study provided a brandnew technique reference for in-situ remediation of surface water in practical application.

Selective activation of peroxymonosulfate govern by B-site metal in delafossite for efficient pollutants degradation: Pivotal role of d orbital electronic configuration

Radical and non-radical oxidation pathways have been universally validated in transition metals (TMs) oxides activated peroxymonosulfate (PMS) processes. However, achieving high efficiency and selectivity of PMS activation remains challenging due to the ambiguous tuning mechanism of TMs sites on PMS activation in thermodynamic scope. Herein, we demonstrated that the exclusive PMS oxidation pathways were regulated by d orbital electronic configuration of B-sites in delafossites (CuBO₂) for Orange I degradation (Co^III 3d⁶ for reactive oxygen species (ROSs) vs. Cr^III 3d³ for electron transfer pathway). The d orbital electronic configuration was identified to affect the orbital overlap extent between 3d of B-sites and O 2p of PMS, which induced B-sites offering different types of hybrid orbital to coordinate with O 2p of PMS, thereby forming the high-spin complex (CuCoO₂@PMS) or the low-spin complex (CuCrO₂@PMS), on which basis PMS was selectively dissociated to form ROSs or achieve electron transfer pathway. As indicated by thermodynamic analysis, a general rule was proposed that B-sites of less than half-filled 3d orbital tended to act as electron shuttle, i.e., Cr^III (3d³), Mn^III (3d⁴), interacting with PMS to execute an electron transfer pathway for degrading Orange I, while B-sites of between half-filled and full-filled 3d orbital preferred to be electron donator, i.e., Co^III (3d⁶), Fe^III (3d⁵), activating PMS to generate ROSs. These findings lay a foundation for the oriented design of TMs-based catalysts from the atomic level according to d orbital electronic configuration optimization, as so to facilitate the achievement of PMS-AOPs with highly selective and efficient remediation of contaminants in water purification practice.

Effective degradation of 2,4,4'-trichlorodiphenyl by Fe3C@Fe-800 activated peroxymonosulfate: Superoxide radical and singlet oxygen-dominated advanced oxidation process

Polychlorinated biphenyls (PCBs) degradation by peroxymonosulfate (PMS) activation through •OH and SO₄^•- radical oxidation process was the effective technology in the last decades; however, there were few research focusing on removing PCBs by O₂^•- and ¹O₂ induced by PMS activation. In this work, 90.86% of 2,4,4-trichlorodiphenyl (PCB 28) was degraded by 0.3 g/L Fe₃C@Fe-800 activated 0.5 mM PMS system under the synergistic action of O₂^•- and ¹O₂. The structures of Fe₃C@Fe-800 were identified by Scanning electron microscope (SEM), High resolution-transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), Raman spectra and Fourier transform infrared (FT-IR) spectra. Electron paramagnetic resonance (EPR) measurements and quenching tests verified that O₂^•- and ¹O₂ were the primary reactive species in Fe₃C@Fe-800/PMS/PCB 28 ternary reaction system. Density functional theory (DFT), Linear sweep voltammetry (LSV), and chronoamperometry test revealed that electron-deficient Fe atoms on Fe₃C were the main active sites in Fe₃C@Fe-800 for PMS activation to generate ¹O₂. Unlike the reported •OH and SO₄^•- mediated degradation induced by the iron-based catalyst, both O₂^•- and ¹O₂ contributed to PCB 28 degradation： nucleophilic dichlorination reaction by O₂^•- and then ring-open oxidation process by ¹O₂. Fe₃C@Fe-800/PMS system had excellent catalytic performance under different reaction conditions and possessed desirable inorganic salt and natural organic matter resistance. This work elucidated the important role of Fe₃C in PMS activation to generate O₂^•- and ¹O₂ for PCB 28 decontamination by nonradical way and provided a clue to design rationally catalysts in polychlorinated biphenyl pollution remediation.

Electron-transfer pathways insights into contaminants oxidized by Cu-OOSO3- intermediate: Effects of oxidation states of Cu and solution pH values

The copper-peroxy complex (Cu-OOSO₃^-) metastable intermediate has been confirmed to oxidize contaminants via a single-electron-transfer pathway or an oxygen-atom-transfer pathway. And the effects of Cu oxidation states and reaction pH conditions on the intermediate properties have not been explored in depth. Here, copper oxide (CuO_x) catalysts with different Cu oxidation states were synthesized by a simple precipitation method by controlling the reaction temperature from 0 to 45 °C. CuO_x displayed a strong catalytic dependence on the Cu oxidation state, and CuO_x-30 with Cu average valence on the catalyst surface of 1.61 was more reactive for catalytic degradation of bisphenol A with peroxymonosulfate (PMS). Notably, CuO_x-30, with the best electron-accepting ability, was easier to bonding with PMS to form the Cu-OOSO₃^- reactive complex, and the generated intermediate exhibited the strongest capacity to obtain electrons from contaminants. Moreover, the electron-transfer pathways were closely related to the average valence of Cu, and the contribution of the oxygen-atom-transfer pathway changed volcanic with increasing Cu valence. Meanwhile, the reaction predominantly involved the oxygen-atom-transfer pathway under acidic conditions (pH=3), while the contribution of the single-electron-transfer pathway raised with increasing pH values. Hence, this work was devoted to providing new insights into the CuO_x-inducing PMS activation and vital supplementary to the properties of the Cu-OOSO₃^- intermediate.

Advanced oxidative degradation of sulfamethoxazole by using bowl-like FeCuS@Cu2S@Fe0 catalyst to efficiently activate peroxymonosulfate

A novel hierarchical bowl-like FeCuS@Cu₂S@Fe⁰ nanohybrid catalyst (B-FeCuS@Cu₂S@Fe⁰) was synthesized for removing sulfamethoxazole (SMX) through catalytic activation of peroxymonosulfate (PMS). It was found that this catalyst exhibited excellently high catalytic activity. Under optimized reaction conditions, all the added SMX (12 mg/L) could be completely degraded within 5 min. The SMX degradation followed pseudo first order kinetics with a rate constant k of 0.89 min^-1, being 1.38, 4.51, 8.99 and 35.6 times greater than that of other catalysts including Fe⁰ (0.644 min^-1 in the very initial stage), bowl-like iron-doped CuS (B-FeCuS, 0.197 min^-1), bowl-like CuS (B-CuS, 0.099 min^-1) and Cu₂O (0.025 min^-1), respectively. During the degradation, several reactive oxygen species (·OH, SO₄^·- and ¹O₂) were generated with ·OH as the main one as confirmed by electron paramagnetic resonance analysis. The SMX degradation in the present system included both radical and non-radical mediated processes. A possible mechanistic insight of the PMS activation by bowl Fe⁰ decorated CuS@Cu₂S-based catalyst was proposed according to X-ray photoelectron spectroscopic (XPS) analysis, and the degradation pathway of SMX was speculated by monitoring the degradation intermediates with liquid chromatography coupled with mass spectrometry (LC-MS).

Nitrogen-doped biochar (N-doped BC) and iron/nitrogen co-doped biochar (Fe/N co-doped BC) for removal of refractory organic pollutants

The presence of refractory organic pollutants (ROPs) in the ecosystem is a serious concern because of their impact on environmental constituents as well as their known or suspected ecotoxicity and adverse health effects. According to previous studies, carbonaceous materials, such as biochar (BC), have been widely used to remove pollutants from ecosystems owing to their desirable features, such as relative stability, tunable porosity, and abundant functionalities. Nitrogen (N)-doping and iron/nitrogen (Fe/N) co-doping can tailor BC properties and provide supplementary functional groups as well as extensive active sites on the N-doped and Fe/N co-doped BC surface, which is advantageous for interaction with and removal of ROPs. This review investigates the impact of N-doped and Fe/N co-doped BC on the removal of ROPs through adsorption, activation oxidation, and catalytic reduction due to the synergistic Fe, N, and BC features that modify the physicochemical properties, surface functional groups, and persistent free radicals of BC to aid in the degradation of ROPs. Owing to the attractive properties of N-doped and Fe/N co-doped BCs for the removal of ROPs, this review focuses and evaluates previous experimental investigations on the manufacturing (including precursors and influencing parameters during manufacturing) and characterizations of N-doped and Fe/N co-doped BCs. Additionally, the effective applications and mechanisms of N-doped and Fe/N co-doped BCs in adsorption, activation oxidation, and reductive remediation of ROPs are investigated herein. Moreover, the application of N-doped and Fe/N co-doped BC for progressive environmental remediation based on their effectiveness against co-pollutants, regeneration, stability, affordability, and future research prospects are discussed.

N-doped porous bowl-like carbon with superhigh external surface area for ultrafast degradation of bisphenol A: Key role of site exposure degree

High-temperature nitrogen (N) doping boosts the activity of biochars for peroxymonosulfate (PMS) activation, but the N heat loss causes the unsatisfactory catalytic efficiency. Improving the surface area for obtaining the high exposure of N sites is a promising solution. Herein, a soft template-KHCO₃ etching strategy is used to synthesize the N-doped porous bowl-like carbon (NPBC) with ultrahigh external surface area (1610.8 m² g^-1). The bowl-like structure eliminates inert bulk interior and allows unobstructed mass transfer of reactants onto both outer and inner surfaces, while the large pore channels by KHCO₃ etching further improves the exposure degree of limited N sites. Although NPBC has only 0.43% N content, 93.1% of bisphenol A (BPA) is removed within 1 min through the electron-transfer pathway by fully utilizing the N active centers, and the kinetic rate constant (k) reaches 5.29 min^-1, exceeding reported values by 2-270 times. Moreover, the NPBC/PMS system possesses excellent applicability for various organics and conditions, effectively mineralizes BPA and reduces effluent biotoxicity. A quantitative index W representing N exposure degree is first proposed and shows high linearity with the k values of BPA degradation (R²=0.992, 0 <W<3750 m² g^-1%^-1), proving the critical role of W in determining catalytic efficiency.

Rational design of cobalt sulfide anchored on nitrogen-doped carbon derived from cyanobacteria waste enables efficient activation of peroxymonosulfate for organic pollutants oxidation

With the increasing of eutrophication in water body, algae blooms have become one of the global environmental problems. The cyanobacteria waste has placed a severe burden on the environment and transforming cyanobacteria into functional materials may be a wise approach. Herein, cobaltous sulfide/nitrogen-doped biochar (N-BC/CoS_x) composite was synthesized by pyrolysis of cyanobacteria waste. The N-BC/CoS_x showed excellent performance in peroxymonosulfate (PMS) activation for enrofloxacin (ENR) degradation, which could remove more than 90% ENR within 60 min. The influencing factors of pH and catalyst dosage on ENR removal efficiency were studied. The N-BC/CoS_x showed good recyclability in the cycle runs. The radicals (O₂^•-, OH andSO₄^•-) and the non-radical species (charge transfer and ¹O₂) were generated in the ENR degradation. The cycle of Co(II)/Co(III) m ay contribute to the radical generation process. This work proved that metal sulfide modified cyanobacteria biochar has a specific application value in water pollution control and provides a new method for resource utilization of cyanobacteria.

Oxidation of tetracycline hydrochloride with a photoenhanced MIL-101(Fe)/g-C3N4/PMS system: Synergetic effects and radical/nonradical pathways

MIL-101(Fe)-based catalysts have been widely used for degradation of organic pollutants based on peroxymonosulfate (PMS) activation. Hence, a facile calcination and hydrothermal method was used in this study to prepare a MIL-101(Fe)/g-C₃N₄ composite catalyst with high activity and high stability for PMS activation to degrade tetracycline hydrochloride (TC) under visible-light irradiation. We clearly elucidated the mechanism involved in the MIL-101(Fe)/g-C₃N₄ photo Fenton-catalyzed PMS activation process by separating the PMS activation and pollutant oxidation processes. The synergetic effects of MIL-101(Fe) and g-C₃N₄ involved MIL-101(Fe) acting as an electron shuttle mediating electron transfer from the organic substrate to PMS, accompanied by redox cycling of the surface Fe(II)/Fe(III). Multiple experimental results indicated that PMS was bound to the surface of MIL-101(Fe)/g-C₃N₄ during visible irradiation and generation of sulfate radicals (SO₄^•-), hydroxyl radicals (•OH) and superoxide anion free radicals (•O₂^-) for the radical pathway and singlet oxygen (¹O₂) and holes (h⁺) for the nonradical pathway. The major degradation pathways for TC can be described as demethylation, deamination, deamidation and carbonylation. This work provides valuable information and advances the fundamental understanding needed for design and syntheses of metal-free conjugated polymers modified by metal-organic frameworks for heterogeneous photo-Fenton reactions.

A novel multi-components hierarchical porous composite prepared from solid wastes for benzohydroxamic acid degradation

In this study, a novel carbon-wrapped-iron hierarchical porous catalyst (Fe/C-Mn₈₀₀) was prepared from electrolytic manganese residue (EMR) and sewage sludge (SS), which showed outstanding degradation ability toward benzohydroxamic acid (BHA, nearly 90 % was removed within 60 min) with low metal leaching rate. Mechanism exploration found transition metal ions (Fe and Mn) can serve as electron acceptors and facilitate the generation of persistent free radicals (PFRs). These transition metal ions and PFRs mainly participated in the single-electron pathway via activating PMS to generate a large amount of reactive oxygen species (ROS). While the electron negative graphitic N and CO groups not only improve the electronegatively of catalyst, but also acted as the electron sacrificers to favor the electron transfer and directly oxidized the absorbed BHA through the ternary activated outer-sphere complexes. Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) analysis further demonstrated the crucial role of pre-adsorption during the degradation process. This work provided a deep insight into the degradation mechanism of metal/carbon composite and promising opportunity widened the horizon of the high-value utilization of EMR and SS.

Regulation of defects and nitrogen species on carbon nanotube by plasma-etching for peroxymonosulfate activation: Inducing non-radical/radical oxidation of organic contaminants

The structural defects and heteroatom dopants of carbonaceous materials play critical roles in their activation of peroxymonosulfate (PMS) for organic pollutants' removal. This study uses plasma-etching technology to control the levels of structural defects and nitrogen species in nitrogen-doped carbon nanotubes (N-CNTs) for excellent PMS activation. The vacancy defects, CO, pyrrolic N and graphitic N could be rationally designed by controlling the plasma-etching time. Obviously, the I_D/I_G (from 0.56 to 0.94) and CO contents (from 0.07 to 0.44 at%) of N-CNTs increase with rising etching time, exhibiting good linear positive correlations with phenol oxidation rates. Furthermore, through active species identification, quantitative structure-activity relationships analysis and theoretical calculations, vacancy defects (adsorbing PMS O1 site) and CO are confirmed to be the active sites for the generation of ¹O₂, which is major pathway (82%) for phenol degradation. While radicals induced by pyrrolic N and graphitic N adsorbing PMS O2 site are the minor pathway (18%). Overall, this study sheds new light on the crucial roles of defects and N species in inducing PMS non-radical/radical activation by carbocatalyst via efficiently controlled plasma-etching technology.

Covalent organic frameworks@ZIF-67 derived novel nanocomposite catalyst effectively activated peroxymonosulfate to degrade organic pollutants

Metal organic frameworks-Covalent organic frameworks (MOFs-COFs) nanocomposites could improve the catalytic performance. Herein, a novel nanocomposite catalyst (CC@Co₃O₄) derived from MOFs-COFs (COF@ZIF-67) was prepared on peroxymonosulfate (PMS) activation for bisphenol A (BPA) and rhodamine B (RhB) degradation. Owing to the Co species, oxygen vacancy (O_V), surface hydroxyl (-OH), graphite N and ketone groups (C=O), the CC@Co₃O₄ exhibited higher catalytic degradation performance and total organic carbon (TOC) for BPA (93.8% and 22.3%) and RhB (98.2% and 82.5%) with a small quantity of catalyst (0.10 g/L) and low concentration of PMS (0.20 g/L) even without pH adjustment. Sulfate radicals (•SO₄^-), hydroxyl radicals (•OH), single oxygen (¹O₂), superoxide radicals (•O₂^-) and electron transfer process were all involved in the degradation of BPA and RhB. Among them, the degradation of BPA and RhB mainly depended on •O₂^- and ¹O₂, respectively. Meanwhile, the degradation pathways of BPA and RhB were proposed, and the biotoxicity of the degradation products was evaluated by freshwater chlorella. The results illustrated that the degradation products were environmentally friendly to organisms. In addition, the role of COF in the nanocomposites was also studied. The addition of COF remarkably improved the catalytic performance of CC@Co₃O₄ due to the faster electron transfer, more graphite N and C=O. Overall, this work may open the door to the development of COF-based catalysts in the field of water pollutant remediation.

Enhanced degradation of sulfamethoxazole by non-radical-dominated peroxymonosulfate activation with Co/Zn co-doped carbonaceous catalyst: Synergy between Co and Zn

Bimetallic catalysts are often used for peroxymonosulfate (PMS) activation in recent years due to the synergistic effects between two different metal species. However, the synergy between Zn and other transition metal in PMS activation are rarely studied because of the ease of evaporation of Zn species at high temperature. In this work, a Co/Zn co-doped carbonaceous catalyst derived from ZIF-67@ZIF-8 (Z67@8D) was prepared successfully by the core-shell replacement strategy, and used to activate PMS for sulfamethoxazole (SMX) degradation. Due to the co-existence of Co/Zn species (e.g., Co/Zn-N site), Z67@8D showed a much higher catalytic activity than that of Z8D, Z67D, and several commercial oxides. Importantly, the CoZn synergy was deeply revealed by combining experiments and density functional theory (DFT) calculations, in which Zn could adjust the electron distribution of Co, reducing the PMS adsorption energy and thus enhancing PMS decomposition and singlet oxygen (¹O₂) formation. Moreover, formed ZnO and graphitic structure of Z67@8D could also promote the catalytic activity. In addition, the good stability and reusability, universal applicability, and high environmental robustness of Z67@8D were demonstrated. Our findings may provide a new insight into the Zn-based bimetallic catalysts for PMS activation and pollutant degradation.

Efficient activation of peracetic acid by mixed sludge derived biochar: Critical role of persistent free radicals

Peracetic acid (PAA)-based advanced oxidation processes (AOPs) were increasingly identified as the alternative scheme in wastewater treatment. Cost-effective and easily available catalyst for activation of PAA was in urgent demand for promoting engineering application process. In this study, a new type of biochar catalyst derived from pyrolysis of mixture of primary sludge (PSD) and secondary sludge (SSD) was prepared and showed effective PAA activation ability. The degradation of p-chlorophenol (4-CP) improved with PAA activation by mixed sludge derived biochar (PS-SDBC) than secondary sludge derived biochar (S-SDBC) and primary sludge derived biochar (P-SDBC), and the highest removal efficiency achieved by PS-SDBC with the PSD/SSD ratio of 5/5 (k_obs=0.057 1/(M·min), pH 9). Correlation analysis firstly indicated that persistent free radicals (PFRs) rather than chemical composition and material structure dominated PAA activation and organic radicals (RO•) was proved to be the major reactive species through electron paramagnetic resonance (EPR) detection. The mixture of PSD and SSD caused the synergy of inorganic metals and organic matters through pyrolysis processes, resulting in larger specific surface area (SSA) (110.71 m²/g), more abundant electron-donating groups (e.g., C = O, -OH) and massive defects (I_D/I_G = 1.519) of PS-SDBC than P-SDBC and S-SDBC, which eventually promoted PFRs formation. A fascinating phenomenon was observed in PS-SDBC/PAA system that the active sites of PFRs could be regenerated by RO• attacking onto PS-SDBC, which contributed to the wide pH applicability and continuous stability of PS-SDBC/PAA system in practical wastewater treatment. This study not only significantly deepened the understanding of the reaction mechanism between PAA and biochar, but also provided a potential PAA-based AOPs for micropollutants removal in wastewater.

Carbon materials in persulfate-based advanced oxidation processes: The roles and construction of active sites

Many researchers have paid more attention to the progress of carbon materials owing to their advantages, such as high activity, low cost, large surface area, high conductivity and high stability. Carbon materials have been widely used in persulfate-based advanced oxidation processes (PS-AOPs), especially for graphene (G), carbon nanotubes (CNTs) and biochar (BC). Various strategies are applied to promote their activity, however, up to now, the relationship between the structures of carbon materials and their activities in PS-AOPs has not been specifically reviewed. The methods to switch reaction pathway (radical and nonradical pathways) in carbon-persulfate-based AOPs have not been systematically explored. Hereon, this review illustrated the active sites of G, CNTs, BC and other carbon materials, and generalized the modification methods to promote the activity of carbon materials and to switch reaction pathway in PS-AOPs. The roles of carbon materials in PS-AOPs were discussed around reactive oxygen species (ROS) and the structures. ROS are frequently complex in AOPs, but main ROS generation is related to the active sites on carbon materials. The structures of carbon materials (e.g., metal-carbon bonds, the electron-deficient C atoms, unbalanced electron distribution and graphitized structures) play a decisive role in the nonradical pathway. Finally, future breakthroughs of carbon materials were proposed for practical engineering and multi-field application.

Elucidating the Mechanistic Origin of a Spin State-Dependent FeNx-C Catalyst toward Organic Contaminant Oxidation via Peroxymonosulfate Activation

Atomically dispersed metals on nitrogen-doped carbon matrices have attracted extensive interest in the removal of refractory organic pollutants. However, a thorough exploration of the particular structure for each active site and specific effects of these sites still remains elusive. Herein, an Fe-pyridinic N₄ structure in a single-atom catalyst (FeN_x-C) was constructed using a facile pyrolysis strategy, and it exhibited superior catalytic activity in peroxymonosulfate (PMS) activation toward organic contaminant oxidation. The various Fe species and relative amounts of each Fe site in the FeN_x-C catalyst were validated using X-ray absorption spectroscopy and ⁵⁷Fe Mössbauer spectroscopy, which showed critical dependencies on the precursor ratio and calcination temperature. The positive correlations between relative content of high-spin state species (Fe^II and Fe^III) and catalytic performance were found to determine the reactive species generation and electron transfer pathway in the FeN_x-C/PMS system. Moreover, catalytic performance and theoretical calculation results revealed that Fe^II-N₄ in the high-spin state (S = 2) tends to activate PMS to form sulfate and hydroxyl radicals via a one-electron transfer process, while the Fe^III-N₄ moiety (S = 5/2) is prone to high-valent iron species generation with lower free energy. Benefiting from finely tuned active sites, a single-atom FeN_x-C catalyst achieved favorable applicability in actual wastewater treatment with efficient resistance of the common water matrix. The present work advances the mechanistic understanding of spin state-dependent persulfate activation in single-atom catalysts and provides guidance to design a superior catalyst based on spin state descriptions.