Catalytic Ozonation of Ketoprofen with In Situ N-Doped Carbon: A Novel Synergetic Mechanism of Hydroxyl Radical Oxidation and an Intra-Electron-Transfer Nonradical Reaction

Ozonolysis of ketoprofen in polluted water: Reaction pathways, kinetics, removal efficiency, and health effects

Ketoprofen (KET), as a non-steroidal anti-inflammatory drug frequently detected in aqueous environments, is a threat to human health due to its accumulation and low biodegradability, which requires the transformation and degradation of KET in aqueous environments. In this paper, the reaction process of ozone-initiated KET degradation in water was investigated using density functional theory (DFT) method at the M06-2X/6-311++g(3df,2p)//M06-2X/6-31+g(d,p) level. The detailed reaction path of KET ozonation is proposed. The thermodynamic results show that ozone-initiated KET degradation is feasible. Under ultraviolet irradiation, the reaction of ozone with water can also produce OH radicals (HO·) that can react with KET. The degradation reaction of KET caused by HO· was further studied. The kinetic calculation illustrates that the reaction rate (1.99 × 10-1 (mol/L)-1 sec-1) of KET ozonation is relatively slow, but the reaction rate of HO· reaction is relatively high, which can further improve the degradation efficiency. On this basis, the effects of pollutant concentration, ozone concentration, natural organic matter, and pH value on degradation efficiency under UV/O3 process were analyzed. The ozonolysis reaction of KET is not sensitive to pH and is basically unaffected. Finally, the toxicity prediction of oxidation compounds produced by degradation reaction indicates that most of the degradation products are harmless, and a few products containing benzene rings are still toxic and have to be concerned. This study serves as a theoretical basis for analyzing the migration and transformation process of anti-inflammatory compounds in the water environment.

One electron oxidation-induced degradation of brominated flame retardants in electroactive membrane filtration system: Vital role of dichlorine radical-mediated process

Reactive chlorine species (RCS) are inevitably generated in electrochemical oxidation process for treating high-salinity industrial wastewater, thereby resulting in the competition with coexisting hydroxyl radicals (•OH) for oxidizing recalcitrant organic compounds. Due to the low redox potentials compared to •OH, the role of RCS has been often overlooked. In this work, we developed an electroactive membrane filtration (EMF) system that had a high removal efficiency (99.1 ± 0.5 %) for tetrabromobisphenol S (TBBPS) at low energy consumption (1.45 kWh m-3). Electron spin resonance spectroscopy and molecular probing tests indicated the predominance of Cl2•-, of which steady-state concentration (2.2 ×10-10 M) was extremely higher than those of ClO• (6.7 ×10-13 M), •OH (0.95 ×10-13 M), and Cl• (2.39 ×10-15 M). The density functional theory (DFT) and intermediate product analysis highlighted that Cl2•- radicals had a higher electrophilic attack efficacy than •OH radicals for inducing changes in the electron density of the carbon atoms around phenolic hydroxyl groups, thus leading to the generation of transition state intermediates and accelerating the degradation of TBBPS. Our work demonstrates the vital role of Cl2•- radicals for pollutant degradation, highlighting the potential of this technology for cost-effective removal of recalcitrant organic compounds from water and wastewater.

Synergetic Manipulation Mechanism of Single-Atom M-N4 and M-OH (M = Mn, Fe, Co, Ni) Sites for Ozone Activation: Theoretical Prediction and Experimental Verification

Carbon-based single-atom catalysts (SACs) have been gradually introduced in heterogeneous catalytic ozonation (HCO), but the interface mechanism of O3 activation on the catalyst surface is still ambiguous, especially the effect of a surface hydroxyl group (M-OH) at metal sites. Herein, we combined theoretical calculations with experimental verifications to comprehensively investigate the O3 activation mechanisms on a series of conventional SAC structures with N-doped nanocarbon substrates (MN4-NCs, where M = Mn, Fe, Co, Ni). The synergetic manipulation effect of the metal atom and M-OH on O3 activation pathways was paid particular attention. O3 tends to directly interact with the metal atom on MnN4-NC, FeN4-NC, and NiN4-NC catalysts, among which MnN4-NC has the best catalytic activity for its relatively lower activation energy barrier of O3 (0.62 eV) and more active surface-adsorbed oxygen species (Oads). On the CoN4-NC catalyst, direct interaction of O3 with the metal site is energetically infeasible, but O3 can be activated to generate Oads or HO2 species from direct or indirect participation of M-OH sites. The experimental results showed that 90.7 and 82.3% of total organic carbon (TOC) was removed within 40 min during catalytic ozonation of p-hydroxybenzoic acid with MnN4-NC and CoN4-NC catalysts, respectively. Phosphate quenching, catalyst characterization, and EPR measurement further supported the theoretical prediction. This contribution provides fundamental insights into the O3 activation mechanism on SACs, and the methods and ideals could be helpful for future studies of environmental catalysis.

Novel manganese and nitrogen co-doped biochar based on sodium bicarbonate activation for efficient removal of bisphenol A: Mechanism insight and role analysis of manganese and nitrogen by combination of characterizations, experiments and density functional theory calculations

A novel porous manganese and nitrogen co-doped biochar (Mn-N@SBC) was synthesized via one-step pyrolysis, utilizing loofah agricultural waste as the precursor and NaHCO3 as the activator. The behavior of bisphenol A adsorbed on Mn-N@SBC was evaluated using static batch adsorption experiments. Compared to direct manganese-nitrogen co-doping, co-doping based on NaHCO3 activation significantly increased the specific surface area (231 to 1027 m2·g-1) and adsorption capacity (15 to 351 mg·g-1). Wide pH (2-10) and good resistance to cation/anion, humic acid and actual water demonstrated the robust adaptability of Mn-N@SBC to environmental factors. The significantly reduced specific surface area after adsorption, adverse effects of ethanol and phenanthrene on the removal of bisphenol A, and theoretically predicted interaction sites indicated the primary adsorption mechanisms involved pore filling, hydrophobicity, and π-π-electron-donor-acceptor interaction. This work presented an approach to create high-efficiency adsorbents from agricultural waste, offering theoretical and practical guidance for the removal of pollutants.

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.

Recycling waste iron-rich algal flocs as cost-effective biochar activator for heterogeneous Fenton-like reaction towards tetracycline degradation: Important role of iron species and moderately defective structures

Harvesting aquatic harmful algal blooms (HABs) and reusing them is a promising way for antibiotic degradation. Herein, a novel iron-rich biochar (Fe-ABC), derived from algal biomass harvested by magnetic coagulation, was successfully designed and fabricated as activator for heterogeneous Fenton-like reaction. The modification methods and pyrolysis temperatures (400-800 °C) were optimized to enhance the formation of rich iron species and moderately defective structure, yielding Fe-ABC-600 with enhanced electron transfer and H2O2 activation capability. Thus, Fe-ABC-600 exhibited superior removal efficiency (95.33%) on tetracycline (TC), where the presence of multiple iron species (Fe3+, Fe2+ and Fe4+) and moderately defective structure accelerating the Fenton-like oxidation. The concentration of leaching Fe after each reaction was all below 0.74 mg/L in five cycles, ensuring the sustained degradation. And •OH was proved to be the major radical contributing to the degradation of TC, as well as the direct electron transfer mechanism together, in which the CO acted as electron regulator and electron donor. Fe-ABC as a cost-effective catalyst has notable application potentials in TC removal from wastewater owing to its remarkable advantages of high resource utilization, enhanced catalytic property, high ecological safe, notable TC degradation efficiency, low cost and environmental-friendliness.

Selective Removal of Organic Pollutants in Groundwater and Surface Water by Persulfate-Assisted Advanced Oxidation: The Role of Electron-Donating Capacity

The efficiency of persulfate-assisted advanced oxidation processes (PS-AOPs) in degrading organic pollutants is affected by the electron-donating capability of organic substances present in the water source. In this study, we systematically investigate the electron-donating capacity (EDC) difference between groundwater and surface water and demonstrate the dependence of removal efficiency on the EDC of target water by PS-AOPs with carbon nanotubes (CNTs) as a catalyst. Laboratory analyses and field experiments reveal that the CNT/PS system exhibits higher performance in organic pollutant removal in groundwater with a high concentration of phenols, compared to surface water, which is rich in quinones. We attribute this disparity to the selective electron transfer pathway induced by potential difference between PS-CNT and organic substance-CNT intermediates, which preferentially degrade organic substances with stronger electron-donating capability. This study provides valuable insights into the inherent selective removal mechanism and application scenarios of electron transfer process-dominated PS-AOPs for water treatment based on the electron-donating capacity of organic pollutants.

Stability enhancing of perovskite LaCoO3 by compositing with oxygen doped MoS2 in Fenton-like reactions

Perovskite materials are reported to be effective in peroxymonosulfate (PMS) based Fenton-like reactions, the leaching rates of chalcogenide materials in perovskite materials are however serious, thus leading to bad performance in long-term stability. In this study, an O-doped MoS₂ is synthesized to composite with LaCoO₃, and the high catalytic activity of LaCoO₃ is well preserved with greatly decreased Co leaching. During the BPA degradation with PMS as oxidant, ∼100% degradation can be achieved in 20 min and this degradation efficiency can be maintained for ∼45 h in a simulated fixed bed reactor, which is almost 3 times longer than the pure LaCoO₃. With the compositing of O-doped MoS₂, the leached Co was greatly decreased and the dominated reactive oxidation species (ROS) transformed from SO₄^•- into O₂^•- with longer lifespan, thus resulting in the better stability. This study could promote the application of perovskite materials in the real industrial wastewater treatment.

Catalytic ozonation of hard COD in coking wastewater with Fe2O3/Al2O3-SiC: From catalyst design to industrial application

Development of robust, reactive, and inexpensive catalyst for pollutants abatement with catalytic ozonation is of great significance. Herein, the effect of a robust and easy-recovery catalyst, Fe₂O₃/Al₂O₃-SiC, for the catalytic ozonation of hardly biodegradable COD (hard COD) in coking wastewater had been explored. Al-O-Si bond formed on modified SiC through the substitution of hydrogen in surficial Si-OH groups by Al³⁺. The Lewis acid sites improved the adsorption of ozone and facilitated the formation of ·OH and O₂·^-. For coking wastewater treatment, the removal ratio of hard COD and the generation speed of hydroxyl radical (R_ct) in the catalytic ozonation process were 71% and 253% higher than those in the ozonation group, respectively. Ozone utilization increased from 0.44 g COD removed/g O₃ in the ozonation group to 1.42 g COD removed/g O₃ in the Fe₂O₃/Al₂O₃-SiC catalytic ozonation group. In a full-scale application, Fe₂O₃/Al₂O₃-SiC catalytic ozonation decreased the consumption of O₃ to 60 mg L^-1 and decreased the operation cost by 50%. These results provided an approachable way for sharing the extraordinary capacity of ozone for contaminants remediation in industrial applications.

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.

Application of Heterogeneous Catalytic Ozonation in Wastewater Treatment: An Overview

Catalytic ozonation is a non-selective mineralization technology of organic matter in water by using active free radicals generated by ozone degradation. Catalytic ozonation technology can be divided into homogeneous catalytic reactions using metal ions as catalysts and heterogeneous catalytic reactions using solid catalysts. Homogeneous catalytic ozonation technology has many problems, such as low mineralization rate, secondary pollution caused by the introduction of metal ions and low utilization efficiency of oxidants, which limit its practical application. Compared with homogeneous catalytic ozonation technology, heterogeneous catalytic ozonation technology has the advantages of easy recovery, lower cost of water treatment, higher activity and improved mineralization rate of organic matter. This overview classifies and describes catalysts for heterogeneous catalytic ozonation technology, including the different types of metal oxides, metal-free catalysts, and substrates used to immobilize catalysts. In addition, the heterogeneous catalytic ozonation process involved in the multiphase complex reaction process is discussed. The effects of different parameters on the performance of heterogeneous catalytic ozonation are also discussed.

Degradation of organic pollutants from water by biochar-assisted advanced oxidation processes: Mechanisms and applications

Biochar has shown large potential in environmental remediation because of its low cost, large specific surface area, porosity, and high conductivity. Biochar-assisted advanced oxidation processes (BC-AOPs) have recently attracted increasing attention to the remediation of organic pollutants from water. However, the effects of biochar properties on catalytic performance need to be further explored. There are still controversial and knowledge gaps in the reaction mechanisms of BC-AOPs, and regeneration methods of biochar catalysts are lacking. Therefore, it is necessary to systematically review the latest research progress of BC-AOPs in the treatment of organic pollutants in water. In this review, first of all, the effects of biochar properties on catalytic activity are summarized. The biochar properties can be optimized by changing the feedstocks, preparation conditions, and modification methods. Secondly, the catalytic active sites and degradation mechanisms are explored in different BC-AOPs. Different influencing factors on the degradation process are analyzed. Then, the applications of BC-AOPs in environmental remediation and regeneration methods of different biochar catalysts are summarized. Finally, the development prospects and challenges of biochar catalysts in environmental remediation are put forward, and some suggestions for future development are proposed.

Advanced treatment of high-salinity wastewater by catalytic ozonation with pilot- and full-scale systems and the effects of Cu2+ in original wastewater on catalyst activity

In this work, heterogeneous catalytic ozonation for the treatment of bio-treated saccharin sodium production wastewater (BSSW) was comprehensively investigated with pilot- and full-scale systems, with special emphasis on the effects of Cu²⁺ in the original wastewater on catalyst activity. The results of semi-batch and continuous experiments show that heterogeneous catalytic ozonation was effective in removing organic compounds from high-salinity wastewater and that Cu²⁺ in the original wastewater had a substantial effect on the performance of the process. The retention of 0.15 mM Cu²⁺ in BSSW increased the chemical oxygen demand (COD) removal by 31% in semi-batch reactor with heterogeneous catalytic ozonation. The stable COD removal efficiencies ranged from 74% to 66.4% for a 9-month operation, indicating that Cu²⁺ with an appropriate concentration in the original BSSW not only improved the COD removal efficiencies but also inhibited catalyst deactivation; catalyst deactivation was mainly caused by the deposition of inorganic salts on the catalyst surface. Cu²⁺ combined with some anions to inhibit the formation and deposition of inorganic salts that could easily cause deactivation. The deposited copper salts were readily eliminated, especially during backflushing operations. Moreover, in a full-scale study, heterogeneous catalytic ozonation with 0.15 mM Cu²⁺ in BSSW exhibited stable COD removal efficiencies (51%-83%) after over 3 years of operation. This study offers a new idea for using the inherent properties of wastewater to perform advanced treatments on high-salinity industrial wastewater through heterogeneous catalytic ozonation.

Catalytic ozonation of phenol by ZnFe2O4/ZnNCN: performance and mechanism

A novel magnetic catalyst was synthesized and applied in heterogeneous catalytic ozonation process. The ZnFe₂O₄/ZnNCN material was synthesized by hydrothermal and high-temperature calcination method and characterized by XPS, XRD, FTIR, VSM, and SEM techniques. In the system of O₃/ZnFe₂O₄/ZnNCN, the removal rates of phenol and chemical oxygen demand (COD) reached 93% and 43% at 60 min. Further analysis shows that ZnFe₂O₄/ZnNCN has a significant catalytic effect on O₃, which is demonstrated by the first-order kinetic constant being 1.93 times than O₃ alone. The catalyst exhibits excellent cycling stability during repeated catalytic ozonation process and can be fully recycled under an applied magnetic field. The role of hydrogen peroxide (H₂O₂) and surface hydroxyl groups was investigated, and a mechanism for catalytic ozonation was proposed. This work not only builds an efficient catalytic ozonation system, but also provides a potential modification strategy for spinel oxides.

Application of biochar-based photocatalysts for adsorption-(photo)degradation/reduction of environmental contaminants: mechanism, challenges and perspective

The fast increase of population results in the quick development of industry and agriculture. Large amounts of contaminants such as metal ions and organic contaminants are released into the natural environment, posing a risk to human health and causing environment ecosystem problems. The efficient elimination of contaminants from aqueous solutions, photocatalytic degradation of organic pollutants or the in-situ solidification/immobilization of heavy metal ions in solid phases are the most suitable strategies to decontaminate the pollution. Biochar and biochar-based composites have attracted multidisciplinary interests especially in environmental pollution management because of their porous structures, large amounts of functional groups, high adsorption capacities and photocatalysis performance. In this review, the application of biochar and biochar-based composites as adsorbents and/or catalysts for the adsorption of different contaminants, adsorption-photodegradation of organic pollutants, and adsorption-(photo)reduction of metal ions are summarized, and the mechanism was discussed from advanced spectroscopy analysis and DFT calculation in detail. The doping of metal or metal oxides is the main strategy to narrow the band gap, to increase the generation and separation of photogenerated e−-h+ pairs, to produce more superoxide radicals (·O2−) and hydroxyl radicals (·OH), to enhance the visible light absorption and to increase photocatalysis performance, which dominate the photocatalytic degradation of organic pollutants and (photo)reduction of high valent metals to low valent metals. The biochar-based composites are environmentally friendly materials, which are promising candidates in environmental pollution cleanup. The challenge and perspective for biochar-based catalysts are provided in the end.

Graphical Abstract

Production of biochar from rice straw and its application for wastewater remediation - An overview

The valorization of biochar as a green and low-cost adsorbent provides a sustainable alternative to commercial wastewater treatment technologies that are usually chemical intensive and expensive. This review presents an in-depth analysis focusing on the rice straw-derived biochar (RSB) for removal of various types of contaminants in wastewater remediation. Pyrolysis is to date the most established technology to produce biochar. Subsequently, biochar is upgraded via physical, chemical or hybrid activation/modification techniques to enhance its adsorption capacity and robustness. Thus far, acid-modified RSB is able to remove metal ions and organic compounds, while magnetic biochar and electrochemical deposition have emerged as potential biochar modification techniques. Besides, temperature and pH are the two main parameters that affect the efficiency of contaminants removal by RSB. Lastly, the limitations of RSB in wastewater remediation are elucidated based on the current advancements of the field, and future research directions are proposed.

Challenges and pitfalls in the investigation of the catalytic ozonation mechanism: A critical review

Catalytic ozonation is a promising technology for pollutant abatement in water and wastewater treatment. However, there are many controversies and contradictions regarding the mechanisms of catalytic ozonation in literature, which has seriously confounded the development of the technology towards industrial applications. Herein, a critical review of literature is conducted to reveal possible underlying causes of the controversies and contradictions, and several common pitfalls in the experimental design and data interpretation are identified, e.g., the fundamentally flawed quenching method popularly used for evaluating the role of reactive oxygen species for pollutant abatement in catalytic ozonation and the neglect of monitoring ozone transfer doses in lab-scale experiments. Based on the identified pitfalls, several measures are suggested to improve the experimental design and data interpretation of catalytic ozonation studies. In addition, recent advances in mechanistic understanding of catalytic ozonation by principle-based modelling approaches are described. Finally, additional works that are needed to shrink the gap between academic research and practical applications and the prospect of catalytic ozonation in future water and wastewater treatment systems are analyzed.

Insights into the Mechanism of Ozone Activation and Singlet Oxygen Generation on N-Doped Defective Nanocarbons: A DFT and Machine Learning Study

N-doped defective nanocarbon (N-DNC) catalysts have been widely studied due to their exceptional catalytic activity in many applications, but the O₃ activation mechanism in catalytic ozonation of N-DNCs has yet to be established. In this study, we systematically mapped out the detailed reaction pathways of O₃ activation on 10 potential active sites of 8 representative configurations of N-DNCs, including the pyridinic N, pyrrolic N, N on edge, and porphyrinic N, based on the results of density functional theory (DFT) calculations. The DFT results indicate that O₃ decomposes into an adsorbed atomic oxygen species (O_ads) and an ³O₂ on the active sites. The atomic charge and spin population on the O_ads species indicate that it may not only act as an initiator for generating reactive oxygen species (ROS) but also directly attack the organics on the pyrrolic N. On the N site and C site of the N₄V₂ system (quadri-pyridinic N with two vacancies) and the pyridinic N site at edge, O₃ could be activated into ¹O₂ in addition to ³O₂. The N₄V₂ system was predicted to have the best activity among the N-DNCs studied. Based on the DFT results, machine learning models were utilized to correlate the O₃ activation activity with the local and global properties of the catalyst surfaces. Among the models, XGBoost performed the best, with the condensed dual descriptor being the most important feature.

Preparation of novel N-doped biochar and its high adsorption capacity for atrazine based on π-π electron donor-acceptor interaction

Novel nitrogen (N)-doped cellulose biochar (NC1000-10) with large adsorption capacity (103.59 mg g^-1) for atrazine (ATZ) was synthesized through the one-pot method. It has the best adsorption efficiency than N-doped biochars prepared from hemicellulose and lignin. The adsorption behaviors of ATZ by N-doped biochars with different N doping ratios (NC1000-5, NC1000-10, NC1000-20 and NC1000-30) were significantly different, which was attributed to the difference of sp² conjugate C (I_D/I_G = 0.99-1.18) and doped heteroatom N (pyridinic N, pyrrolic N and graphitic N). Adsorption performance of ATZ on NC1000-10 conformed to the pseudo-second-order kinetic and Langmuir adsorption isotherm model. Thermodynamic calculations showed that adsorption performance was favorable. Besides, wide pH adaptability (pH = 2-10), good resistance to ionic strength and excellent recycling efficiency make it have extensive practical application potential. Further material characterizations and the density functional theory (DFT) calculations indicated that good adsorption performance of NC1000-10 for ATZ mainly depended on chemisorption, and π-π electron donor-acceptor (EDA) interaction contributed the most due to high graphitization degree. Specifically, pyridinic N and graphitic N further promoted adsorption performance by hydrophobic effect and π-π EDA interaction between ATZ and NC1000-10, respectively. Pyrrolic N and other surface functional groups (-COOH, -OH) facilitated the hydrogen bond effect.

Unraveling timescale-dependent Fe-MOFs crystal evolution for catalytic ozonation reactivity modulation

Iron-based metal-organic frameworks (Fe-MOFs) have been considered competitive catalyst candidates for the effective degradation of organic pollutants via advanced oxidation processes (AOPs) due to their unique porous architecture and tunable active site structure. However, little is known about the role of synergetic relationship between porous architecture and active site exposure of Fe-MOFs on catalysis for AOPs yet. Here, we demonstrated an overlooked compromise over these two features on modulating the catalytic ozonation reactivity of MIL-53(Fe) through a timescale-dependent crystal evolution. Enabled by intramolecular hydrogen bonds, the MIL-53(Fe) was subjected to six evolution steps in terms of crystal morphology, leading to a volcano plot of catalytic ozonation reactivity for Rhodamine B (RhB) degradation versus the crystallization time. Evidence suggested that the surface area of MIL-53(Fe) decreased dramatically, while the density of accessible active site increased when prolonging crystallization time, allowing for the facile modulation of catalytic ozonation reactivity of MIL-53(Fe). Electron paramagnetic resonance and fluorescence quantification tests verified that the screened MIL-53(Fe)s had a much better capacity for ∙OH generation than benchmark ozonation catalyst α-MnO₂ and α-FeOOH. Moreover, the MIL-53(Fe) with the highest reactivity (i.e., MIL-53(Fe)-18H) could effectively destruct a broad spectrum of emerging and refractory organic pollutants and allow the thorough purification of secondary effluents discharged from textile dyeing & finishing industry for in situ reuse. Therefore, our study advances the understanding of the compromise effect between porous architecture and active site on catalysis reactivity of Fe-MOFs and promotes the rational design of more effective Fe-MOFs as well as their derivatives for environmental applications.

Recycling chestnut shell for superior peroxymonosulfate activation in contaminants degradation via the synergistic radical/non-radical mechanisms

The efficient recycling of agricultural chestnut shell waste is of considerable interest due to its large availability and economic feasibility. Herein, an alkaline-activated biochar was thermally prepared using chestnut shell by finely regulating main conditions; its morphological, structural and physic-chemical properties were well characterized. Fenton-like capacity to trigger peroxymonosulfate activation for superior pollutant degradation with high efficiency and good selectivity was validated in different water matrix. Both radical formation and electron transfer were identified as reaction pathways, while the selective non-radical mechanism played the major role in pollutant degradation. Surface ketonic groups were identified as the main reactive sites for non-selective radical production, while crystal edges and structural defects on sp²/sp³ carbon network could smoothly mediate the selective electron transfer from pollutant to oxidant in the non-radical Fenton-like catalysis. The two-mixed radical/non-radical pathways exhibited important advantages for environmental decontamination, in comparison with the one-single radical or non-radical mechanism. Our study provided a promising recycling strategy for agricultural chestnut shell, as well as an environment-friendly catalyst for heterogeneous Fenton-like catalysis in green water purification rendered by the synergistic radical/non-radical reaction pathways.

Enhanced electrochemical removal of sulfadiazine using stainless steel electrode coated with activated algal biochar

With the increasingly discharging and inappropriately disposing of antibiotics from human disease treatment and breeding industry, extensive development of antibiotic resistance in bacteria raised serious public health concern. In this work, algal biochar was coated onto the stainless steel mesh, and was employed as cathodic electrode for the degradation of sulfadiazine (SDZ) in an electro-Fenton (EF) system. It was found that algal biochar pyrolyzed at 600 °C with 1:1 KOH achieved best catalytic performance to generate H₂O₂ via oxygen reduction. Moreover, removal efficiency of SDZ reached 96.11% in 4 h with an initial concentration of 25 μg/mL, under the optimized condition as: initial pH at 3, 50 mM of Na₂SO₄ as electrolyte and an applied current of 20 mA/cm². In addition, it was found that the SDZ removal kept at about 96.99% even after four repeated degradation process. Moreover, four possible SDZ degradative pathways during the EF process were proposed according to determined intermediates, model optimization and density functional theory calculation. Finally, acute and chronic biotoxicity of the degradative products against fish and green algae was evaluated, to further elaborate the environmental impact of SDZ after electrochemical degradation.

Superior degradation of phenolic contaminants in different water matrices via non-radical Fenton-like mechanism mediated by surface-disordered WO3

Heterogeneous Fenton-like catalysis mediated by solid catalyst is a promising oxidation technology for water purification. The redox reactivity, cost-effectiveness, and environmental compatibility of solid catalyst play governing roles in oxidant activation, radical generation, and pollutant degradation. Herein, the surface-disordered WO₃ (D-WO₃) functionally engineered by the unique crystalline-amorphous core-shell structure is proven to be a superior solid catalyst of heterogeneous Fenton-like catalysis for peroxymonosulfate (PMS) activation and pollutant degradation in various water matrices. Six typical phenolic and dye pollutants are effectively and selectively degraded in the D-WO₃/PMS system with much reduced matrix effects. Both radical identifying and scavenging tests elucidate the important role of non-radical ¹O₂ and mediated electron transfer during PMS activation on the D-WO₃ surface. The superior Fenton-like activity of D-WO₃ can be mainly attributed to the surface and sub-surface distorted lattice sites with finely tailored atomic and electronic structures and surface chemistry. These distorted lattice sites can thermodynamically serve as the key reactive centers of dissociative adsorption and catalytic activation for both PMS and pollutant, with high adsorption energy, strong structural activation, and smooth electron transfer. Our findings provide a new chance for heterogeneous Fenton-like catalysis mediated by transition metal oxides with high capacity, low cost, and no toxicity for promising water purification.

Insights on the electron transfer pathway of phenolic pollutant degradation by endogenous N-doped carbonaceous materials and peroxymonosulfate system

In this study, chitosan, a low-price and easily obtainable natural polymerized sugar containing abundant nitrogen element, was employed as a precursor for preparing hierarchically porous carbon (PC) to activate peroxymonosulfate (PMS). The PC fabricated at 800 °C obtained the optimum catalytic performance with complete removal of p-hydroxybenzoic acid (HBA) in 30 min. The selective degradation toward phenolic pollutants with different substituent groups and the resistance over the interference of typical anions and natural organic matter implied a non-radical pathway contributed most for HBA degradation. The investigation of structure-activity relationship suggested a positive linear correlation between graphitic N content and HBA removal. The chemical quenching experiment and electron paramagnetic resonance (EPR) excluded the crucial role of radicals and ¹O₂. Solid evidence based on electrochemical techniques demonstrated the essential contribution of electron transfer pathway achieved by three successive processes including the first close adsorption of PMS by PC800 to form metastable intermediates, then an internal electron transfer from active graphitic N to PMS within metastable intermediates and finally external electron transfer from HBA to metastable intermediates. This study provided insightful mechanism understanding of a promising organics elimination strategy by PMS activation through N-doped carbonaceous materials utilizing chitosan as a simultaneous carbon and nitrogen precursor.

Application of Biochar as Functional Material for Remediation of Organic Pollutants in Water: An Overview

In recent years, numerous studies have focused on the use of biochar as a biological material for environmental remediation due to its low-cost precursor (waste), low toxicity, and diversity of active sites, along with their facile tailoring techniques. Due to its versatility, biochar has been employed as an adsorbent, catalyst (for activating hydrogen peroxide, ozone, persulfate), and photocatalyst. This review aims to provide a comprehensive overview and compare the application of biochar in water remediation. First, the biochar active sites with their functions are presented. Secondly, an overview and summary of biochar performance in treating organic pollutants in different systems is depicted. Thereafter, an evaluation on performance, removal mechanism, active sites involvement, tolerance to different pH values, stability, and reusability, and an economic analysis of implementing biochar for organic pollutants decontamination in each application is presented. Finally, potential prospects to overcome the drawbacks of each application are provided.

Nonprecious bimetallic Fe, Mo-embedded N-enriched porous biochar for efficient oxidation of aqueous organic contaminants

Bimetallic Fe- and Mo-embedded N-enriched porous biochar (Fe-Mo@N-BC) is developed and serves as a cost-effective and highly efficient catalyst for mineralization of non-biodegradation organic contaminants. Fe-Mo@N-BC was prepared by pyrolysis of complex Fe/Mo -containing precursors. Transmission electron microscopy and elemental mapping suggested that Fe and Mo are uniformly dispersed in nitrogen-doped biochar with hierarchical mesopores. In comparison to Fe@N-BC and Mo@N-BC, Fe-Mo@N-BC exhibited a superior activity for activating peroxymonosulfate (PMS). The stable activity was ascribed to N-doping and synergistic effect of Fe and Mo species, where both Fe-N_x and Mo-N_x can simultaneously serve as the active sites and N-BC can act as a carrier and an activator as well as an electron mediator. Electron paramagnetic resonance and quenching experiments indicated that HO^•, O₂^•- and ¹O₂ were responsible for organic degradation. The effects of PMS dosage, initial Orange II concentration, temperature, solution pH, coexisting anions and humic acids on organic degradation were also investigated. With the assistance of an external magnet, Fe-Mo@N-BC can be easily separated after reaction and remains stable in the reusability tests. This work demonstrates a feasible strategy towards the fabrication of Fe, Mo-embedded N-enriched porous biochar catalysts for the detoxification of organic contaminants.

Enhanced degradation of dimethyl phthalate in wastewater via heterogeneous catalytic ozonation process: performances and mechanisms

Ozonation process is a promising yet challenging method for the removal of refractory organic matter due to the sluggish reaction for generating hydroxyl radical (˙OH) at a neutral pH condition. Herein, an efficient heterogeneous catalytic ozonation system using CeO₂/Al₂O₃ catalyst was developed to remove dimethyl phthalate (DMP) from wastewater. Under a neutral condition of pH = 6, this system achieved almost 100% DMP removal within 15 min at an optimized catalyst dosage of 30 g L⁻¹ and the ozone flow rate of 22.5 mg min⁻¹. Moreover, the catalytic ozonation system exhibited a stable degradation performance of DMP in a wider pH range (pH = 5–10). The results of electron paramagnetic resonance (EPR) and quantitative tests confirmed the ultrafast conversion of O₃ to ˙OH (0.774 μM min⁻¹) on the surface of CeO₂ based ceramic catalyst. The quenching experiments further supported the predominant role of ˙OH in the mineralization of DMP. These results highlight the potential of using the heterogeneous catalytic ozonation system for the efficient removal of refractory organic matter from wastewater.

An efficient heterogeneous catalytic ozonation system using CeO₂/Al₂O₃ catalyst was developed to remove dimethyl phthalate (DMP) from wastewater.

Synthesis and characterization of iron-nitrogen-doped biochar catalysts for organic pollutant removal and hexavalent chromium reduction

Fe, N atoms deposited on porous biochar (Fe-N@BC) composites were synthesized and employed as an efficient catalyst for organic pollutant removal and Cr^VI reduction. Naturally abundant, renewable and N-rich pomelo peel as a carbon and nitrogen source and unsubstituted phthalocyanine/iron phthalocyanine complexes as a Fe and nitrogen resource were used to develop the Fe-N@BC via a carbonization process. It is found that Fe-N@BC hybrids have homogeneous dispersion of Fe and N atoms on 3D hierarchically porous biochar, which significantly improves the performance toward the detoxification of organic pollutants using peroxymonosulfate as an oxidant, as well as the reduction of hexavalent chromium by formic acid as a reductant. Furthermore, the effects of Fe loading and pyrolytic temperature on catalysis were comprehensively analyzed and optimized. The excellent activity of Fe-N@BC in acid media can be attributed to the high dispersion of Fe species, high content of doped nitrogen as well as hierarchical micro-mesopores, which induce to expose more active sites for catalysis. Owing to the structure-enabled acidic stability, Fe-N@BC efficiently retains its activity as well as its structural stability after several cycles of reactions. This work provides a new approach to construct Fe, N-doped biochar as an effective catalyst for the detoxification of organic and inorganic pollutants.

Defect-Engineered Graphene Films as Ozonation Catalysts for the Devastation of Sulfamethoxazole: Insights into the Active Sites and Oxidation Mechanism

Graphene-based catalysts have been widely applied for catalytic ozonation. However, as it is difficult to obtain graphene with high structural precision, it is currently unfeasible to comprehend the relationships between the intrinsic structure of the layered carbon catalysts with its catalytic activities. Here, an advanced plasma-assisted etch strategy was used to fine tune the ozonation activity of monolayered graphene films by tailoring the defect types. Raman mapping indicated that the defects of the as-prepared monolayered graphene films were predominantly sp3, vacancy, and boundary-type defects, respectively. The roles and contributions of these active defects in manipulating the oxidative potential of monolayered graphene films were revealed by quenching experiments, electron paramagnetic resonance results, and density functional theory calculations. The catalytic results showed that the monolayered graphene films with boundary-like defects exhibited the best catalytic performance toward the degradation of sulfamethoxazole. This work contributes new insights into the design of high-efficiency carbonaceous catalysts by structuring additional defective sites.

Toward Selective Oxidation of Contaminants in Aqueous Systems

The presence of diverse pollutants in water has been threating human health and aquatic ecosystems on a global scale. For more than a century, chemical oxidation using strongly oxidizing species was one of the most effective technologies to destruct pollutants and to ensure a safe and clean water supply. However, the removal of increasing amount of pollutants with higher structural complexity, especially the emerging micropollutants with trace concentrations in the complicated water matrix, requires excessive dosage of oxidant and/or energy input, resulting in a low cost-effectiveness and possible secondary pollution. Consequently, it is of practical significance but scientifically challenging to achieve selective oxidation of pollutants of interest for water decontamination. Currently, there are a variety of examples concerning selective oxidation of pollutants in aqueous systems. However, a systematic understanding of the relationship between the origin of selectivity and its applicable water treatment scenarios, as well as the rational design of catalyst for selective catalytic oxidation, is still lacking. In this critical review, we summarize the state-of-the-art selective oxidation strategies in water decontamination and probe the origins of selectivity, that is, the selectivity resulting from the reactivity of either oxidants or target pollutants, the selectivity arising from the accessibility of pollutants to oxidants via adsorption and size exclusion, as well as the selectivity due to the interfacial electron transfer process and enzymatic oxidation. Finally, the challenges and perspectives are briefly outlined to stimulate future discussion and interest on selective oxidation for water decontamination, particularly toward application in real scenarios.

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

Heterogeneous catalytic ozonation is regarded as a feasible technology in advanced wastewater treatment. Catalytic performance, mass transfer, and mechanical strength are the key elements for large-scale applications of catalysts. To optimize those elements, Fe was selected for its dual role in graphitization and catalytic ozonation. A Fe/N-doped micron-scale carbon-Al2O3 framework (CAF) was designed and applied to a fluidized catalytic process for the treatment of secondary effluent from coal gasification. The chemical oxygen demand removal rate constant and the hydroxyl radical generation efficiency (Rct) of the Fe/N-doped CAF were 190% and 429% higher than those of pure ozone, respectively. Theoretical calculations revealed that higher Fe valence promoted ozone decomposition, which implied increasing FeIII content for further catalyst optimization. The rate constant and Rct with a higher FeIII-proportion catalyst were increased by 13% and 16%, respectively, compared to those with the lower one. Molecular dynamics and density functional theory calculations were performed to analyze the reaction kinetics qualitatively and quantitatively. The energy barrier corresponding to FeIII configuration was 1.32 kcal mol-1, 27% lower than that for FeII configuration. These theoretical calculations guided the catalyst optimization and provided a novel solution for designing ozonation catalysts. The Fe/N-doped CAF demonstrated a great potential for practical applications.

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

A nonradical mechanism involved in peroxymonosulfate (PMS) activation in carbonaceous materials (CMs) is still controversial. In this study, we prepared N-doped CMs, including hollow carbon spheres (NHCSs) and carbon nanotubes (N-CNTs), to probe the crucial intermediates during PMS activation. The results suggested that the higher efficiency and lower activation energy (13.72 kJ mol^-1) toward phenol (PN) degradation in an NHCS/PMS system than PMS alone (∼24.07 kJ mol^-1) depended on a typical nonradical reaction. Persistent free radicals (PFRs) with a g factor of 2.0033-2.0045, formed as crucial metastable intermediates on NHCS or N-CNT in the presence of PMS, contribute largely to the organic degradation (∼73.4%). Solid evidence suggested that the formation of PFRs relied on the attack of surface-bonded ^•OH and SO₄^•- or peroxides in PMS, among which surface-bonded SO₄^•- was most thermodynamically favorable based on theoretical calculations. Electron holes within PFRs on NHCSs shifted the Fermi level to the positive energy with the valance band increasing from 1.18 to 1.98 eV, promoting the reactivity toward nucleophilic substances. The degradation intermediates of aromatic compounds (e.g., PN) and electron rearrangement triggered the evolution of PFRs from oxygen-centered to carbon-centered radicals. Moreover, due to the specific electron configuration, graphitic N on NHCS was critical for stabilizing the PFRs. This study provides insightful understanding of the fate of organic contaminants and the structure-activity relationship of reactivity of CMs toward PMS activation.

In-situ pyrolysis of Taihu blue algae biomass as appealing porous carbon adsorbent for CO2 capture: Role of the intrinsic N

An environment-friendly, cost-effective, and facile N self-doping porous carbon (NC) were prepared through in-situ pyrolysis of nitrogen abundant Taihu blue algae biomass for CO₂ uptake. It was found that the CO₂ sorption capacity of porous carbon prepared through carbonization at 800 °C with KOH activation (N-C-800) exhibit higher CO₂ uptake capacity of 4.88 (1 bar and 0 °C) and 2.76 mmol/g (1 bar and 25 °C) respectively, with the CO₂/N₂ selectivity of N-C-800 attaining 39.3. Besides, the adsorption capacity of N-C-800 remained stable even after 7 repeated cycles, with a slight loss of nearly 6%. Moreover, total graphitic N (N_tg) sources from the intrinsic N in N-C-800 is not only higher than other agro-sourced porous carbon materials, but the graphitic N performed a sound correlation with the CO₂ uptake capacity. Combining experiments with Density Functional Theory (DFT) calculations, higher adsorption energy of N-C-800 (-13.6 kJ/mol, comparing with -6.9 kJ/mol of N-free carbon framework) would render the efficient adsorption of CO₂ molecular onto the graphitic N site. The current study not only provides a new option for the reclamation of Taihu blue algae biomass as N self-doping material, but a proof-of-concept investigation employing NC materials as an appealing candidate for CO₂ capture.

Efficient persulfate non-radical activation of electron-rich copper active sites induced by oxygen on graphitic carbon nitride

Peroxymonosulfate (PMS) non-radical reactions possess high catalytic activity for specific pollutants under complex water environments. However, the synthesis of high-performance catalysts and the discussion of non-radical reaction mechanisms are still unsatisfactory. Here, a novel and efficient non-radical catalyst (O-CuCN) was successfully assembled using the scheme of Copper (Cu) and oxygen (O) co-doping. The O element with great electronegativity induces graphite carbon nitride (g-C₃N₄) to act as a medium to change the phase properties and electron density distribution of g-C₃N_4, and provides a support for the targeting of Cu. Cu is introduced into g-C₃N₄ as an active site in the phase structure, and an electron-rich center with the Cu site is formed, which forms a metastable intermediate after the adsorption of PMS by Cu as the active site. The new catalyst O-CuCN has outstanding activity in the PMS system, and its degradation rate for bisphenol A (BPA) is increased by more than 20 times compared to that of g-C₃N₄, and it has excellent environmental tolerance and stability. This work demonstrates that the formation of metastable intermediates and the initiation of effective non-radical reactions can be achieved by constructing differentiated electron density structures.

Rational design of efficient metal-free catalysts for peroxymonosulfate activation: Selective degradation of organic contaminants via a dual nonradical reaction pathway

Designing efficient and low-cost catalysts to activate peroxymonosulfate (PMS) to rapidly degrade organic contaminants is important for the practical applications of the advanced oxidation process. Herein, inspired by the water absorption process of the baby diaper, we design nitrogen-doped porous carbon network catalysts (N-PCNs) for peroxymonosulfate activate to degrade recalcitrant organic pollutants. The resulting product called nitrogen-doped porous carbon networks carbonized at 800 °C (N-PCN8) exhibits enhanced adsorption and catalytic activity due to its large specific surface area (1137.7 m² g^-1), highly graphitic degree, and high graphite N content (50.3%).4-CP (0.02 g/L) was completely degraded in 30 min by using N-PCN8 (0.2 g/L) and PMS (0.2 g/L). The catalytic system is efficient over a wide pH range (3-9) and shows strong resistance to interference with inorganic anions (Cl^-, HCO₃^-, CO₃^2-). Several aromatic pollutants, including 4-CP, BA, NB, HBA, CBZ, and BPA, are used as target pollutants to further evaluate the oxidative capacity of the system, and the degradation rate were 100%, 19.5%, 3.5%,5 5.7%, 79% and 100%, respectively. Results suggest that the system is selective for pollutants, and singlet oxygen oxidation and mediated electron transfer effects are the main causes of 4-CP degradation.