Uncertainty and misinterpretation over identification, quantification and transformation of reactive species generated in catalytic oxidation processes: A review

Comparative study on CeO2 catalysts with different morphologies and exposed facets for catalytic ozonation: performance, key factor and mechanism insight

Morphology and facet effects of metal oxides in heterogeneous catalytic ozonation (HCO) are attracting increasing interests. In this paper, the different HCO performances for degradation and mineralization of phenol of seven ceria (CeO2) catalysts, including four with different morphologies (nanorod, nanocube, nanooctahedron and nanopolyhedron) and three with the same nanorod morphology but different exposed facets, are comparatively studied. CeO2 nanorods with (110) and (100) facets exposed show the best performance, much better than that of single ozonation, while CeO2 nanocubes and nanooctahedra show performances close to single ozonation. The underlying reason for their different HCO performances is revealed using various experimental and density functional theory (DFT) calculation results and the possible catalytic reaction mechanism is proposed. The oxygen vacancy (OV) is found to be pivotal for the HCO performance of the different CeO2 catalysts regardless of their morphology or exposed facet. A linear correlation is discerned between the rate of catalytic decomposition of dissolved ozone (O3) and the density of Frenkel-type OV. DFT calculations and in-situ spectroscopic studies ascertain that the existence of OV can boost O3 activation on both the hydroxyl (OH) and Ce sites of CeO2. Conversely, various facets without OV exhibit similar O3 adsorption energies. The OH group plays an important role in activating O3 to produce hydroxyl radical (∙OH) for improved mineralization. This work may offer valuable insights for designing Facet- and OV-regulated catalysts in HCO for the abatement of refractory organic pollutants.

Autocatalytic degradation of Cu-EDTA in the Calcite/PMS system: Singlet oxygen and Cu(III)

The simultaneous removal of heavy metal complexes (HMCs) and heavy metal ions presents a significant challenge in treating wastewater. To address this, we propose a Calcite/Peroxymonosulfate (Calcite/PMS) system aimed at simultaneously decomplexing Cu-EDTA and removing Cu ions. Calcite/PMS system could achieve 99.5 % Cu-EDTA decomplexation and 61.9 % Cu ions removal within 60 min under initial conditions of Cu-EDTA (10 mg/L), Calcite (3 g/L), and PMS (2 mM). Singlet oxygen (1O2) emerged as the predominant reactive species responsible for Cu-EDTA decomplexation, which selectively targeted the N-C bonds in the Cu-EDTA structure to produce intermediates with lower biotoxicity than EDTA. Interestingly, solid phase Cu(III) (≡Cu(III)) promoted the generation of superoxide radicals (O2•-) with a contribution of up to 72.8 %. Subsequently, nascent ≡Cu(III) and O2•- accelerated the degradation of intermediates. Besides, coexisting organic substances inhibited Cu-EDTA decomplexation, whereas inorganic ions had a weak impact. After five cycles of use, the Calcite/PMS system retained 99.3 % efficiency in decomplexing Cu-EDTA. This investigation provides valuable insights into using calcite to remove HMCs and enhances our comprehension of the decomplexation intermediates accelerating HMCs degradation.

Cu/Mn-mediated electron shuttle and high-valent metals enhance hydroxyl radicals production during the electrochemical oxidation on the CuMn-Sb-SnO2 electrode

In this work, a novel CuMn-Sb-SnO2 anode is developed by a simple, low-cost preparation process. The doping of Cu and Mn causes surface reconstruction, which optimizes its electronic structure, compared to the Sb-SnO2 anode. Experimental results demonstrate that the levofloxacin degradation kinetics constant in the CuMn-Sb-SnO2 system (0.188 min-1) was 8.5 times higher than that in the Sb-SnO2 system, which is surpassing most reported anodes. Moreover, electrochemical characterization also revealed that the CuMn-Sb-SnO2 anode possessed more active sites, higher OEP potential, and lower charge transfer resistance. Notably, electrochemical characterization and EPR experiments confirmed the formation of Cu (III), highlighting their crucial role in promoting the generation of •OH during the catalytic process. Additionally, theoretical calculations and XPS analysis revealed that Cu and Mn rely on self-mediated redox shuttles to act as "electron porters", significantly accelerating internal electron transfer between Sn and Sb to enhance the production of •OH. Furthermore, the CuMn-Sb-SnO2 anode exhibits great practicability due to its efficient degradation of various antibiotics. This study offers valuable new insights into developing novel electrodes for the efficient degradation of antibiotic wastewater.

Heterostructure g-C3N4/Bi2MoO6 PVDF nanofiber composite membrane for the photodegradation of steroid hormone micropollutants

Photocatalytic membrane reactors (PMRs) are a promising technology for micropollutant removal. Sunlight utilization and catalyst surface sites limit photodegradation. A poly(vinylidene fluoride) (PVDF) nanofiber composite membrane (NCM) with immobilized visible-light-responsive g-C3N4/Bi2MoO6 (BMCN) were developed. Photodegradation of steroid hormones with the PVDF-BMCN NCM was investigated with varying catalyst properties, operating conditions, and relevant solution chemistry under solar irradiation. Increasing CN ratio (0-65 %) enhanced estradiol (E2) degradation from 20 ± 10 to 75 ± 7 % due to improved sunlight utilization and photon lifetime. PVDF nanofibers reduced self-aggregation of catalysts. Hydraulic residence time and light intensity enhanced the photodegradation. With the increasing pH value, the E2 removal decreased from 84 ± 4 to 67 ± 7 % owing to electrical repulsion and thus reduced adsorption between catalysts and E2. A removal of 96 % can be attained at environmentally relevant feed concentration (100 ng.L-1) with a flux of 60 L.m-2.h-1, irradiance of 100 mW.cm-2, and 1 mg.cm-2 BMCN65 loading. This confirmed that heterojunction photocatalysts can enhance micropollutants degradation in PMRs.

Higher efficiency of vanadate iron in heterogeneous Fenton-like systems to pretreat sugarcane bagasse and its enzymatic saccharification

Pretreatment is crucial for effective enzymatic saccharification of lignocellulose such as sugarcane bagasse (SCB). In the present study, SCB was pretreated with five kinds of heterogeneous Fenton-like systems (HFSs), respectively, in which α-FeOOH, α-Fe2O3, Fe3O4, and FeS2 worked as four traditional heterogeneous Fenton-like catalysts (HFCs), while FeVO4 worked as a novel HFC. The enzymatic reducing sugar conversion rate was then compared among SCB after different heterogeneous Fenton-like pretreatments (HFPs), and the optimal HFS and pretreatment conditions were determined. The mechanism underlying the difference in saccharification efficiency was elucidated by analyzing the composition and morphology of SCB. Moreover, the ion dissolution characteristics, variation of pH and Eh values, H2O2 and hydroxyl radical (·OH) concentration of FeVO4 and α-Fe2O3 HFSs were compared. The results revealed that the sugar conversion rate of SCB pretreated with FeVO4 HFS reached up to 58.25%, which was obviously higher than that under other HFPs. In addition, the surface morphology and composition of the pretreated SCB with FeVO4 HFS were more conducive to enzymatic saccharification. Compared with α-Fe2O3, FeVO4 could utilize H2O2 more efficiently, since the dissolved Fe3+ and V5+ can both react with H2O2 to produce more ·OH, resulting in a higher hemicellulose and lignin removal rate and a higher enzymatic sugar conversion rate. It can be concluded that FeVO4 HFP is a promising approach for lignocellulose pretreatment.

Impacts of chloride ions on the electrochemical decomplexation and degradation of Cr(III)-EDTA: Reaction mechanisms of HO• and RCS

The removal of Cr(III)-organic complexes, encompassing both decomplexation and ligand degradation, presents significant challenges in industrial wastewater treatment. As one of the most common anions in wastewater, Cl- significantly improves the efficiency of electrochemically removing Cr(III)-organic complexes through generated reactive chlorine species (RCS). In the electrochemical chlorine (EC/Cl2) process, extensive experimentation revealed that ClO• plays a dominant role in the degradation of Cr(III)-EDTA, surpassing the effects of free chlorine, direct electrooxidation, HO•, and other RCS. Density functional theory calculations indicated that RCS, primarily Cl• and ClO•, preferentially oxidize the ligand in Cr(III)-EDTA via H-abstraction, whereas HO• trends to attack the Cr atom through electron transfer. The influential factors on the degradation efficiency of Cr(III)-EDTA, Cr(VI) yield, and total organic carbon removal in EC/Cl2 were also assessed, including Cl- concentration, current density, and pH. Real industrial wastewater was employed as a reaction matrix to evaluate the application of the EC/Cl2 process for treating Cr(III)-EDTA, accompanied by energy efficiency calculations. Additionally, a two-chamber reactor was established to simultaneously oxidize Cr(III)-EDTA at the anode and reduce Cr(VI) at the cathode. This study provided insight into developing RCS-dominated AOPs to effectively decomplex and decompose organic Cr(III)-complexes in Cl--containing industrial wastewater.

Harnessing a Ti-based MOF for selective adsorption and visible-light-driven water remediation

In pursuit of universal access to clean water, photocatalytic water remediation using metal-organic frameworks (MOFs) emerges as a strong alternative to the current wastewater treatment methods. In this study, we explore a unique Ti-based MOF comprised of 2D secondary-building units (SBUs) connected via biphenyl dicarboxylic acid (H2bpdc) ligands - denoted as COK-47 - as a visible-light-driven photocatalyst for organic dye degradation. Synthesized via a recently developed microwave-assisted method, COK-47 exhibits high hydrolytic stability, demonstrates a strong dye uptake, and shows noteworthy dye-degradation performance under UV, visible, and solar light, outperforming benchmark TiO2 and MIL-125-Ti photocatalysts. Due to its nanocrystalline structure and surface termination with organic linkers, COK-47 exhibits selective degradation of cationic pollutants while remaining inert towards anionic dyes, thus highlighting its potential for selective oxidation reactions. Mechanistic studies reveal the involvement of superoxide radicals in the degradation process and emphasize the need to minimize the recombination of photogenerated electron-hole pairs to achieve optimal performance. Post-catalytic studies further confirm the high stability and reusability of COK-47, making it a promising photocatalyst for water purification, organic transformation, and water splitting reactions under visible light.

Photoexcitation-induced efficient detoxification and removal of arsenite in contaminated water by a layered double hydroxide-supported polyacrylate stabilized ferrous sulfide composite

The effective detoxification and removal of arsenite (As(III)) has been widely concerned because of its strong toxicity and migration ability. In this study, we designed a layered double hydroxide-supported polyacrylate stabilized ferrous sulfide composite (PAA/FeS@LDH) and coupled it with UV excitation to purify As(III)-polluted water. The removal efficiency of As(III) under UV irradiation reached almost 100% in 120 min, and the first-order kinetic constant was 3.12 orders of magnitude higher than under dark. UV irradiation significantly accelerated the oxidation and detoxification of As(III) at the interface of PAA/FeS@LDH and treatment solution. It is attributable to the generation of reactive oxygen species (ROS) intermediates, including .O2-, .OH, and SO4.- under UV irradiation, because of the presence of the photogenerated electron-hole pairs and iron valence states cycles. Importantly, .O2- may be rapidly captured and oxidized to 1O2 on the surface of PAA/FeS@LDH that is also an important contributor to the oxidation removal of As(III). Noticeably, As(III) concentrations in the real water were rapidly reduced to below the guideline limitation of drinking water (10 μg/L) within 20 min under UV irradiation. Our outcomes provide a novel photoexcitation treatment system for the efficient detoxification and removal of As from actual wastewater.

The critical role of electron donating rate of pyrogenic carbon in mediating the degradation of phenols in the aquatic environment

Phenols are the widely detected contaminants in the aquatic environment. Pyrogenic carbon (PyC) can mediate phenols degradation, but the specific properties of PyC or phenols influencing this reaction remain unknown. The present study investigated the kinetic process and mechanism of removal of various phenols by different PyC in aqueous phase system. To avoid the impact of the accumulated degradation byproducts on the overall reaction, we conducted a short-term experiment, quantified adsorption and degradation, and obtained reaction rate constants using a two-compartment first-order kinetics model. The adsorption rate constants (ka) of phenols by PyC were 10-220 times higher than degradation rate constants (kd), and they were positively correlated. Interestingly, no correlation was found between kd and common PyC properties, including functional groups, electron transfer capacities, and surface properties. Phenols were primarily attacked by •OH in the adsorbed phase. But neither the instantly trapped •OH, nor the accumulated •OH could explain phenol degradation. Chemical redox titration revealed that the electron transfer parameters, such as the electron donating rate constant (kED) of PyC, correlated well with kd (r>0.87, P < 0.05) of phenols. Analysis of 13 phenols showed that Egap and ELUMO negatively correlated with their kd, confirming the importance of the electronic properties of phenols to their degradation kinetics. This study highlights the importance of PyC electron transfer kinetics parameters for phenols degradation and manipulation of PyC electron transfer rate may accelerate organic pollutant removal, which contributes to a deeper understanding of the environmental behavior and application of PyC systems.

Efficient reduction-oxidation coupling degradation of nitroaromatic compounds in continuous flow processes

Nitroaromatic compounds (NACs) with electron-withdrawing nitro (-NO2) groups are typical refractory pollutants. Despite advanced oxidation processes (AOPs) being appealing degradation technologies, inefficient ring-opening oxidation of NACs and practical large-scale applications remain challenges. Here we tackle these challenges by designing a reduction-oxidation coupling (ROC) degradation process in LaFe0.95Cu0.05O3@carbon fiber cloth (LFCO@CFC)/PMS/Vis continuous flow system. Cu doping enhances the photoelectron transfer, thus triggering the -NO2 photoreduction and breaking the barriers in the ring opening. Also, it modulates surface electronic configuration to generate radicals and non-radicals for subsequent oxidation of reduction products. Based on this, the ROC process can effectively remove and mineralize NACs under the environmental background. More importantly, the LFCO catalyst outperformed most of the recently reported catalysts with lower cost (13.72 CNY/ton) and higher processing capacity (3600 t/month). Furthermore, the high scalability, material durability, and catalytic activity of LFCO@CFC under various realistic environmental conditions prove the potential ability for large-scale applications.

Simultaneous enhanced antibiotic pollutants removal and sustained permeability of the membrane involving CoFe2O4/MoS2 catalyst initiated with simple H2O2 backwashing

Membranes for wastewater treatment should ideally exhibit sustainable high permeate production, enhanced pollutant removal, and intrinsic physical rejection. In this study, CoFe2O4/MoS2 serves as a non-homogeneous phase catalyst; it is combined with polyether sulfone membranes via liquid-induced phase separation to simultaneously sustain membrane permeability and enhance antibiotic pollutant degradation. The prepared catalytic membranes have higher pure water flux (329.34 L m-2 h-1) than pristine polyethersulfone membranes (219.03 L m-2 h-1), as well as higher mean pore size, porosity, and hydrophilicity. Under a moderate transmembrane pressure (0.05 MPa), tetracycline (TC) in synthetic and real wastewater was degraded by the optimal catalytic membrane by 72.7 % and 91.2 %, respectively. Owing to the generation of the reactive oxygen species (ROS) during the Fenton-like reaction process, the catalytic membrane could exclude the natural organics during the H2O2 backwash step and selectively promote fouling degradation in the membrane channel. The irreversible fouling ratio of the catalyzed membrane was significantly reduced, and the flux recovery rate increased by up to 91.6 %. A potential catalytic mechanism and TC degradation pathways were proposed. This study offers valuable insights for designing catalytic membranes with enhanced filtration performance.

Crystal phase-driven performance of MnO2 in aqueous phase low-temperature thermal catalysis: Synergistic interactions between Mn3+ and surface lattice oxygen

Catalytic oxidation at mild conditions is crucial for mitigating the high pressure and high temperature challenges associated with current catalytic wet air oxidation (CWAO) technologies in wastewater treatment. Among potential materials for catalytic oxidation reactions, polycrystalline MnO2 existed in natural minerals holds considerable promise. However, the relationships between different crystal phases of MnO2 and their catalytic activity sources in aqueous phase remain uncertain and subject to debate. In this research, we synthesized various MnO2 crystal phases, comprising α-, β-, δ-, γ-, ε-, and λ-MnO2, and assessed their catalytic oxidation efficiency during low-temperature heating for treatment of organic pollutants. Our findings demonstrate that λ-MnO2 exhibits the highest catalytic activity, followed by δ-MnO2, γ-MnO2, α-MnO2, ε-MnO2, and β-MnO2. The variations in catalytic activity among different MnO2 are attributed to variances in their oxygen vacancy abundance and redox activity. Furthermore, we identified the primary active species, which include Mn3+ and superoxide radicals (•O2-) generated by surface lattice oxygen of MnO2. This research highlights the critical role of crystal phases in influencing oxygen vacancy content, redox activity, and overall catalytic performance, providing valuable insights for the rational design of MnO2 catalysts tailored for effective organic pollutant degradation in CWAO applications.

Novel Insights into Sb(III) Oxidation and Immobilization during Ferrous Iron Oxygenation: The Overlooked Roles of Singlet Oxygen and Fe (oxyhydr)oxides Formation

Reactive oxygen species (ROS) produced from the oxygenation of reactive Fe(II) species significantly affect the transformation of metalloids such as Sb at anoxic-oxic redox interfaces. However, the main ROS involved in Sb(III) oxidation and Fe (oxyhydr)oxides formation during co-oxidation of Sb(III) and Fe(II) are still poorly understood. Herein, this study comprehensively investigated the Sb(III) oxidation and immobilization process and mechanism during Fe(II) oxygenation. The results indicated that Sb(III) was oxidized to Sb(V) by the ROS produced in the aqueous and solid phases and then immobilized by formed Fe (oxyhydr)oxides via adsorption and coprecipitation. In addition, chemical analysis and extended X-ray absorption fine structure (EXAFS) characterization demonstrated that Sb(V) could be incorporated into the lattice structure of Fe (oxyhydr)oxides via isomorphous substitution, which greatly inhibited the formation of lepidocrocite (γ-FeOOH) and decreased its crystallinity. Notably, goethite (α-FeOOH) formation was favored at pH 6 due to the greater amount of incorporated Sb(V). Moreover, singlet oxygen (1O2) was identified as the dominant ROS responsible for Sb(III) oxidation, followed by surface-adsorbed ·OHads, ·OH, and Fe(IV). Our findings highlight the overlooked roles of 1O2 and Fe (oxyhydr)oxide formation in Sb(III) oxidation and immobilization during Fe(II) oxygenation and shed light on understanding the geochemical cycling of Sb coupled with Fe in redox-fluctuating environments.

Efficient removal of moxifloxacin through PMS activation by CuFeS2/MXene

Due to the frequent detection and potential toxicity of moxifloxacin (MOX), its removal technology had attracted attention in recent years. In this research, CuFeS2/MXene was prepared and used to activate peroxymonosulfate (PMS) to remove MOX. The degradation efficiencies, kinetics, influences, and reaction mechanism of MOX by CuFeS2/MXene/PMS were investigated. The synergistic effect of CuFeS2 and MXene significantly enhanced PMS activation, producing SO4•-, HO•, and 1O2 as the main active species. By adding 0.12 g/L CuFeS2/MXene and 0.12 mM PMS, MOX removal efficiency reached 99.1% within 40 min, with a rate constant of 0.1073 min-1. The composite ratios of CuFeS2/MXene impacted PMS activation more significantly than catalyst dosages and PMS concentrations. Acidic conditions were favorable for the degradation of MOX, while HCO3-, HPO42-, Mn2+, and HA had the inhibitory effects. Twelve major products were detected by HPLC-MS, and DFT was used to illustrate possible degradation pathways of MOX, including the removal of nitrogen-containing heterocycle and transformations of quinolone moieties. Toxicity analysis showed that the developmental toxicity, mutagenicity, and acute toxicity of degradation products tended to decrease. CuFeS2/MXene could exhibit excellent reusability, maintaining an average MOX degradation efficiency of 90.8% in the 7-cycle experiments.

Recent advances in catalyst design, performance, and challenges of metal-heteroatom-co-doped biochar as peroxymonosulfate activator for environmental remediation

The escalation of global water pollution due to emerging pollutants has gained significant attention. To address this issue, catalytic peroxymonosulfate (PMS) activation technology has emerged as a promising treatment approach for effectively decontaminating a wide range of pollutants. Recently, modified biochar has become an increasingly attractive as PMS activator. Metal-heteroatom-co-doped biochar (MH-BC) has emerged as a promising catalyst that can provide enhanced performance over heteroatom-doped and metal-doped biochar due to the synergism between metal and heteroatom in promoting PMS activation. Therefore, this review aims to discuss the fabrication pathways (i.e., internal vs external doping and pre-vs post-modification) and key parameters (i.e., source of precursors, synthesis methods, and synthesis conditions) affecting the performance of MH-BC as PMS activator. Subsequently, an overview of all the possible PMS activation pathways by MH-BC is provided. Subsequently, Also, the detection, identification, and quantification of several reactive species (such as, •OH, SO4•-, O2•-, 1O2, and high valent oxo species) generated in the catalytic PMS system by MH-BC are also evaluated. Lastly, the underlying challenges associated with poor stability, the lack of understanding regarding the interaction between metal and heteroatom during PMS activation and quantification of radicals in multi-ROS system are also deliberated.

Resource utilization of medical waste incineration fly ash to activate peroxydisulfate for tetracycline degradation: Synergy between adsorption and PDS activation

Medical waste incineration fly ash (MWI FA) is classified as a hazardous solid waste. Therefore, the development of recycling technologies to convert MWI FA into useful products is necessary and challenging. In this study, we developed a sustainable approach for preparing a catalyst through the pyrolysis of water-washed MWI FA (WW FA-x, where x corresponds to the pyrolysis temperature). Subsequently, it was applied as a potent peroxydisulfate (PDS) activator to remove tetracycline (TC) from water. The results showed that the WW FA-800 exhibited remarkable adsorption performance as well as highly efficient catalytic activation of PDS, with a 115 mg/g maximum TC adsorption capacity and 93.5% (reaction kinetic rate = 315 μmol/g/h) TC removal within 60 min. A synergistic effect was achieved by adsorption and PDS activation. TC degradation was primarily driven by non-radical (1O2 and electron transfer) processes. WW FA-800 possesses multiple active sites, including defects, π-π*, O-CO groups, Fe0, and Cu(I). Three possible pathways for TC decomposition have been proposed, with the majority of intermediates exhibiting less toxicity than TC. Furthermore, the WW FA/PDS system exhibited an excellent anti-interference ability, and universality in the degradation of various organic contaminants. Notably, energy consumption was minimal, approximately 2.80 kWh/(g·TC), and the leachability of heavy metals in the WW FA-800 was within acceptable limits. This study provides a MWI FA recycling route for the development of highly active catalysts.

Were Persulfate-Based Advanced Oxidation Processes Really Understood? Basic Concepts, Cognitive Biases, and Experimental Details

Persulfate (PS)-based advanced oxidation processes (AOPs) for pollutant removal have attracted extensive interest, but some controversies about the identification of reactive species were usually observed. This critical review aims to comprehensively introduce basic concepts and rectify cognitive biases and appeals to pay more attention to experimental details in PS-AOPs, so as to accurately explore reaction mechanisms. The review scientifically summarizes the character, generation, and identification of different reactive species. It then highlights the complexities about the analysis of electron paramagnetic resonance, the uncertainties about the use of probes and scavengers, and the necessities about the determination of scavenger concentration. The importance of the choice of buffer solution, operating mode, terminator, and filter membrane is also emphasized. Finally, we discuss current challenges and future perspectives to alleviate the misinterpretations toward reactive species and reaction mechanisms in PS-AOPs.

Mechanisms of efficient indoor formaldehyde removal via electro-Fenton: Synergy in ·OH generation and utilization through a modified carbon cathode

Indoor formaldehyde poses a significant carcinogenic risk to human health, making its removal imperative. Electro-Fenton degradation has emerged as a promising technology for addressing this concern. In the electro-Fenton system, ·OH is identified as the primary active species responsible for formaldehyde removal. Hence, its generation and utilization are pivotal for the system's effectiveness and economy. Experimental and quantum chemical methods were employed to investigate the effects and mechanisms of nitrogen doping on various aspects influencing ·OH generation and utilization. Results indicate that nitrogen doping synergistically enhances the generation and utilization of ·OH, leading to an improved formaldehyde removal efficiency in nitrogen-doped cathodic systems. The dominant nitrogen type influencing ·OH generation and utilization varies across different stages. Pyridinic nitrogen facilitates H2O2 adsorption through hydrogen bonding, while pyrrolic and graphitic nitrogen contribute to formaldehyde adsorption and catalyze the conversion of H2O2 to ·OH. Both pyridinic nitrogen and pyrrolic nitrogen boost the degradation of formaldehyde by ·OH. In comparison to the unmodified system, the modified system with NAC-GF/700C as cathode exhibits remarkable improvements. The formaldehyde removal efficiency has increased twofold, and energy consumption reduced by 73.45%. Furthermore, the system demonstrates excellent cyclic stability. These advancements can be attributed to the activation temperature, which leads to the appropriate types and high content of nitrogen elements in NAC-GF/700C. The research represents an important step towards more economical and efficient electro-Fenton technology for indoor formaldehyde removal.

Understanding the pivotal role of ubiquitous Yellow River suspend sediment in efficiently degrading metronidazole pollutants in water environments

Sulfite-based advanced oxidation technology has received considerable attention for its application in organic pollutants elimination. However, the potential of natural sediments as effective catalysts for sulfite activation has been overlooked. This study investigates a novel process utilizing suspended sediment/sulfite (SS/S(IV)) for degradation of metronidazole (MNZ). Our results demonstrate that MNZ degradation efficiency can reach to 93.1 % within 90 min with 12.0 g SS and 2.0 mM sulfite. The influencing environmental factors, including initial pH, SS dosage, S(IV) concentration, temperature, and co-existing substances were systematically investigated. Quenching experiments and electron paramagnetic resonance analyses results indicate that SO3•- is the primary active substance responsible for MNZ degradation, with involvement of SO4•-, SO5•-, and •OH. X-ray photoelectron spectroscopy and Mössbauer spectra reveal that Fe (III)-silicates play a crucial role in activating S(IV). Furthermore, analysis of degradation intermediates and pathways of MNZ is conducted using liquid chromatography with mass spectrometry (LC -MS). The toxicity of MNZ and its intermediates were also systematically evaluated by the T.E.ST. program and wheat seeds germination test. This study offers valuable insight into the activation of sulfite by natural sediments and could contribute to the development of SS-based advanced oxidation processes (AOPs) for the in-situ remediation of antibiotics-contaminated water environments.

Insight into the role of reactive species on catalyst surface underlying peroxymonosulfate activation by P-Fe2MnO4 loaded on bentonite for trichloroethylene degradation

In this study, bentonite supporting phosphorus-doped Fe2MnO4 (BPF) was synthesized and applied for PMS activation to degrade TCE. Morphology and structure characterization results indicated the successfully synthesized of BPF, and the BPF/PMS system not only featured high TCE removal (97.4%) but also high reaction rate constant (kobs = 0.0554 min-1) and PMS utilization (70.4%, kobs = 0.0228 min-1). According to the results of various experiments, massive oxygen vacancies on P-Fe2MnO4 alter its charge balance and facilitate the electron transfer process named adjacent transfer (direct electron capture by adsorbed PMS from adjacent TCE). Mn(III) is the main adsorption site for PMS, and the hydroxyl groups on the catalyst (Fe sites of P-Fe2MnO4, Si and Al sites of bentonite) can also offer binding sites for PMS. The hydrogen-bonded PMS on Fe(III) and Mn(III) sites will subsequently accept the discharged electrons to generate free radicals and high-valent metal species. Meanwhile, electron loss of HSO5- that chemically bonded to hydroxyl groups on bentonite will generate SO5•-, which will further produce 1O2 through self-bonding. the active species on the catalyst surface contribute 65% of TCE degradation in the heterogeneous catalytic oxidation system.

Hydrophobicity Promoted Efficient Hydroxyl Radical Generation in Visible-Light-Driven Photocatalytic Oxidation

Hydroxyl radical (•OH) with strong oxidation capability is one of the most important reactive oxygen species. The generation of •OH from superoxide radicals (•O2 -) is an important process in visible-light-driven photocatalysis, but the conversion generally suffers from slow reaction kinetics. Here, a hydrophobicity promoted efficient •OH generation in a visible-light-driven semiconductor-mediated photodegradation reaction is reported. Hydrophobic TiO2 that is synthesized by modifying the TiO2 surface with a thin polydimethylsiloxane (PDMS) layer and rhodamine B (RhB) are used as model semiconductors and dye molecules, respectively. The surface hydrophobicity resulted in the formation of a solid-liquid-air triphase interface microenvironment, which increased the local concentration of O2. In the meanwhile, the saturated adsorption quantity of RhB on hydrophobic TiO2 is improved by five-fold than that on untreated TiO2. These advantages increased the density of the conduction band photoelectrons and •O2 - generation, and stimulated the conversion of •O2 - to •OH. This consequently not only increased the kinetics of the photocatalytic reaction by an order of magnitude, but also altered the oxidation route from conventional decolorization to mineralization. This study highlights the importance of surface wettability modulation in boosting •OH generation in visible-light-driven photocatalysis.

Phosphate-mediated degradation of organic pollutants in water with peroxymonosulfate revisited: Radical or non-radical oxidation?

Whilst it is generally recognized that phosphate enables to promote the removal of some organic pollutants with peroxymonosulfate (PMS) oxidation, however, there is an ongoing debate as to whether free radicals are involved. By integrating different methodologies, here we provide new insights into the reaction mechanism of the binary mixture of phosphates (i.e., NaH2PO4, Na2HPO3, and NaH2PO2) with peroxymonosulfate (PMS) or hydrogen peroxide (H2O2). Enhanced degradation of organic pollutants and observation of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) adducts (i.e. DMPOOH and 5,5-dimethyl-2-oxopyrroline-1-oxyl (DMPOX)) with electron paramagnetic resonance (EPR) in most phosphates/PMS system seemly support a radical-dominant mechanism. However, fluorescence probe experiments confirm that no significant amount of hydroxyl radicals (•OH) are produced in such reaction systems. PMS in the phosphate solutions (without any organics) remains relatively stable, but is only consumed while organic substrates are present, which is distinct from a typical radical-dominant Co2+/PMS system where PMS is continuously decomposed. Through density functional theory (DFT) calculation, the energy barriers of the phosphates/PMS reaction processes are greatly decreased when non-radical mechanism dominates. Complementary evidence suggests that the reactive intermediates of PMS-phosphate complex, rather than the free radicals, are capable of oxidizing electron-rich substrates such as DMPO and organic pollutants. Taking the case of phosphate/PMS system as an example, this study demonstrates the necessity of acquisition of lines of evidence for resolving paradoxes in identifying EPR adducts.

Advanced oxidation processes for water and wastewater treatment - Guidance for systematic future research

Advanced oxidation processes (AOPs) are a growing research field with a large variety of different process variants and materials being tested at laboratory scale. However, despite extensive research in recent years and decades, many variants have not been transitioned to pilot- and full-scale operation. One major concern are the inconsistent experimental approaches applied across different studies that impede identification, comparison, and upscaling of the most promising AOPs. The aim of this tutorial review is to streamline future studies on the development of new solutions and materials for advanced oxidation by providing guidance for comparable and scalable oxidation experiments. We discuss recent developments in catalytic, ozone-based, radiation-driven, and other AOPs, and outline future perspectives and research needs. Since standardized experimental procedures are not available for most AOPs, we propose basic rules and key parameters for lab-scale evaluation of new AOPs including selection of suitable probe compounds and scavengers for the measurement of (major) reactive species. A two-phase approach to assess new AOP concepts is proposed, consisting of (i) basic research and proof-of-concept (technology readiness levels (TRL) 1-3), followed by (ii) process development in the intended water matrix including a cost comparison with an established process, applying comparable and scalable parameters such as UV fluence or ozone consumption (TRL 3-5). Subsequent demonstration of the new process (TRL 6-7) is briefly discussed, too. Finally, we highlight important research tools for a thorough mechanistic process evaluation and risk assessment including screening for transformation products that should be based on chemical logic and combined with complementary tools (mass balance, chemical calculations).

Electronic structure regulation of Fe single atom coordinated nitrogen doping MoS2 catalyst enhances the Fenton-like reaction efficient for organic pollutant control

An efficient cathode for a Fenton-like reaction based on hydrogen peroxide (H2O2) has significant implications for the potential application of the advanced oxidation process. However, the low H2O2 selectivity and efficient activation remain challenging in wastewater treatment. In the present study, a single Fe atom doped, nitrogen-coordinated molybdenum disulfide (Fe1/N/MoS2) cathode that exhibited asymmetric wettability and self-absorption molecular oxygen was successfully prepared for pollutant degradation. The X-ray absorption near-edge structure and extended X-ray absorption fine structure of Fe1N3 in the Fe1/N/MoS2 catalyst were determined. The electronic structure demonstrated favorable H2O2 selectivity (75%) in a neutral solution and the cumulative hydroxyl radical concentration was 14 times higher than the pure carbon felt. After 10 consecutive reaction experiments, the removal ratio of paracetamol still reached 97%, and the catalytic performance did not decrease significantly. This work deeply understands the catalytic mechanism of Fenton-like reaction between single Fe atom and MoS2 double reaction sites, and proves that the regulation of the electronic structure of Fe single atom is an effective strategy to improve the activity of Fenton-like reaction.

Monoethanolamine enhanced iohexol degradation in the Co(II)/sulfite system: Nonnegligible role of complexation in accelerating cobalt redox cycling

Generation of sulfate radicals (SO4•-) from sulfite activation has emerged as a promising method for abatement of organic pollutants in the water and wastewater treatment. Co(II) has garnered attention due to its high catalytic activity in the sulfite activation, which is compromised by the slow Co(II)/Co(III) redox cycling. Regarding the regulation of Co(II) electronic structure via the complexation effect, monoethanolamine (MEA), a common chelator, is introduced into the Co(II)/sulfite system. MEA addition results in a significant improvement in iohexol abatement efficiency, increasing from 40% to 92%. The superior iohexol abatement relies on the involvement of SO4•-, hydroxyl radicals (HO•) and Co(IV). Hydrogen radical (•H) is unexpectedly detected, acting as a strong reducing agent, contributing to the reduction of Co(III). This enhancement of sulfite activation by MEA is due to the formation of the Co(II)-MEA complex, in which the complexation ratio of Co(II) and MEA is critical. Electrochemical characterization and theoretical calculations demonstrate that the complexation can facilitate the Co(II)/Co(III) redox cycling with the concomitant enhancement of sulfite activation. This work provides a new insight into the Co(II)/sulfite system in the presence of organic ligands.

Piezoelectric catalytic driven advanced oxidation process using two-dimensional metal dichalcogenides for wastewater pollutants remediation

The public and society have increasingly recognized numerous grave environmental issues, including water pollution, attributed to the rapid expansion of industrialization and agriculture. Renewable energy-driven catalytic advanced oxidation processes (AOPs) represent a green, sustainable, and environmentally friendly approach to meet the demands of environmental remediation. In this context, 2D transition metal dichalcogenides (TMDCs) piezoelectric materials, with their non-centrosymmetric crystal structure, exhibit unique features. They create dipole polarization, inducing a built-in electric field that generates polarized holes and electrons and triggers redox reactions, thereby facilitating the generation of reactive oxygen species for wastewater pollutant remediation. A broad spectrum of 2D TMDCs piezoelectric materials have been explored in self-integrated Fenton-like processes and persulfate activation processes. These materials offer a more simplistic and practical method than traditional approaches. Consequently, this review highlights recent advancements in 2D TMDCs piezoelectric catalysts and their roles in wastewater pollutant remediation through piezocatalytic-driven AOPs, such as Fenton-like processes and sulfate radicals-based oxidation processes.

Separation and reutilization of heavy metal ions in wastewater assisted by p-BN adsorbent

Extracting heavy metal ions from wastewater has significant implications for both environmental remediation and resource preservation. However, the conventional adsorbents still suffer from incomplete ion removal and low utilization efficiency of the recovered metals. Herein, we present an extraction and reutilization method assisted by porous boron nitride (p-BN) containing high-density N atoms for metal recovery with simultaneous catalyst formation. The p-BN exhibits stable and efficient metal adsorption performance, particularly for ultra-trace-level water purification. The distribution coefficients towards Pb2+, Cd2+, Co2+ and Fe3+ can exceed 106 mL g-1 and the residual concentrations that reduced from 1 mg L-1 to 0.8-1.3 μg L-1 are much lower than the acceptable limits in drinking water standards of World Health Organization. Meanwhile, the used p-BN after Co ion adsorption can be directly adopted as a high-efficiency catalyst for activating peroxymonosulfate (PMS) in organic pollutant degradation without additional post-treatment, avoiding the secondary metal pollution and the problems of neglected manpower and energy consumption. Moreover, a flow-through multistage utilization system assisted by p-BN/polyvinylidene fluoride (PVDF) membrane is constructed for achieving both metal ion separation and reutilization in the removal of organic pollutants, providing a new avenue for sustainable wastewater remediation.

Tailoring the Coordination Environment of Fe/Zn-BDC to Boost Peroxidase-like Activity for Highly Selective Detection of PFOS

Perfluorooctanesulfonic acid potassium salt (PFOS) residues in ecosystems over long periods are of increasing concern and require a selective and stable optical probe for monitoring. Herein, two functional groups (-F and -NH2) with opposite electronic modulation ability were introduced into Fe/Zn-BDC (denoted as Fe/Zn-BDC-F4 and Fe/Zn-BDC-NH2, respectively) to tailor the coordination environment of the Fe metal center, further regulating the nanozyme activity efficiently. Notably, the peroxidase-like activity is related to the coordination environment of the nanozymes and obeys the following order Fe/Zn-BDC-F4 > Fe/Zn-BDC > Fe/Zn-BDC-NH2. Based on the excellent peroxidase-like activity of Fe/Zn-BDC-F4 and the characteristics of being rich in F atoms, a rapid, selective, and visible colorimetric method was developed for detecting PFOS with a detection limit of 100 nM. The detection mechanism was attributed to various interaction forces between Fe/Zn-BDC-F4 and PFOS, including electrostatic interactions, Fe-S interactions, Fe-F bonds, and halogen bonds. This work not only offers new insights into the atomic-scale rational design of highly active nanozymes but also presents a novel approach to detecting PFOS in environmental samples.

Peroxymonosulfate activation by walnut shell activated carbon supported nano zero-valent iron for the degradation of tetracycline: Performance, degradation pathway and mechanism

In this study, activated carbon (WS-AC) was prepared from walnut shell. Nano-zero-valent iron (nZVI) was loaded on walnut shell activated carbon by liquid phase reduction method and used as catalyst (WS-AC/nZVI) to activate peroxymonosulfate (PMS) to efficiently degrade tetracycline (TC) in solution. The composite material with a mass ratio of WS-AC to nZVI of 1:1 has the highest catalytic performance for activating PMS to degrade TC. The results showed that under the conditions of TC concentration of 100 ppm, PMS dosage of 0.2 mM and WS-AC/nZVI dosage of 0.1 g/L, the removal efficiency of TC could reach 81%. Based on quenching experiments and electron spin resonance (EPR), it was verified that •OH, SO4•- and 1O2 bound on the catalyst surface were the main reactive oxygen species during the reaction. The intermediate products of TC were identified by liquid chromatography-mass spectrometry (HPLC-MS) and DFT calculation, and the possible degradation pathway of TC was proposed. The catalyst still maintained high removal efficiency of TC after four cycles of experiments, and the minimal iron loss on the surface of the catalyst indicated that it had good stability. The efficient and stable WS-AC/nZVI activated PMS showed great potential in the degradation of antibiotics.

One-pot solvothermal synthesis of Cu-Fe-MOF for efficiently activating peroxymonosulfate to degrade organic pollutants in water:Effect of electron shuttle

Persulfate-based advanced oxidation processes (PS-AOPs) show a bright prospect in sewage purification. The development of efficient catalysts with simple preparation process and eco-friendliness is the key for their applying in practical water treatment. Herein, a bimetallic Cu-Fe metal organic framework (MOF) was simply synthesized by using one-pot solvothermal methods and employed for activating peroxymonosulfate (PMS) to degrade organic pollutants in water. The Cu-Fe-MOF/PMS exhibited excellent degradation efficiencies (over 95% in 30 min) for a variety of pollutants, including phenol, bisphenol A, 2,4-dichlorophenol, methyl blue, rhodamine B, tetracycline and sulfamethoxazole. The degradation efficiency was impacted by dosages of Cu-Fe-MOF, PMS concentrations, reaction temperature, solution pH and anionic species. Phenol could be efficiently decomposed in a wide pH range of 5-9, with the highest degradation and mineralization efficiency of nearly 100% and 70%, respectively. Free radicals and non-free radicals participated in degrading of phenol at the same time, with dominantly free radical process, because sulfate radicals (SO4·-) and hydroxyl radicals (·OH) were the primary active substances by contribution calculation. Cu-Fe-MOF was acted as electron shuttle between molecules of phenol and PMS, and the cooperation effect of Fe and Cu on the Cu-Fe-MOF promoted the electron transfer, achieving the high degradation efficiency of phenol. Thus, Cu-Fe-MOF is an ideal catalyst for activating PMS, which is conducive to promote the applying of catalyst-activated PMS processes for practical wastewater treatments.

Vitamin C-regulated CoAl- layered double hydroxide with oxygen vacancies to efficiently activate peroxydisulfate for sulfamethoxazole removal triggered via reactive oxygen and high-valent cobalt species

In this study, a vitamin C-regulated CoAl-layered double hydroxide with abundant oxygen vacancies was synthesized via a simple hydrothermal process. The resulting CoAl-layered double hydroxide was employed to activate peroxydisulfate for removal of sulfamethoxazole. The effect of the experimental parameters such as pH, catalyst dose and peroxydisulfate concentration on sulfamethoxazole removal was investigated. The current system exhibited excellent catalytic performance for sulfamethoxazole removal in a broad pH range (i.e., pH 3.0-11.0). Under the optimized condition, 94.2% of sulfamethoxazole was degraded within 15 min, accompanied by a 67.6% reduction in chemical oxygen demand. The effective sulfamethoxazole degradation could be attributed to four pathways. Firstly, the ≡ Co2+ in catalyst reacted with peroxydisulfate to generate reactive species, including SO4•-, •OH, O2•- and 1O2, which could degrade sulfamethoxazole. Secondly, the oxygen vacancies could modulate intrinsic electrons, resulted in the surface activation of catalyst and accelerated charge transfer, which was favorable for the degradation of sulfamethoxazole. Thirdly, the presence of vitamin C not only promoted the formation of oxygen vacancies but also expanded the interlayer spacing of layered double hydroxide. A large interlayer spacing facilitated the diffusion of peroxydisulfate and pollutants in the interlayer and improved the utilization efficiency of the active site. Lastly, the high-valent cobalt species exhibited excellent oxidation ability and enhanced the catalyst performance through continuously being employed as an electron acceptor. This study provided a valuable insight for the design and application of Co-based catalysts in peroxydisulfate-based advanced oxidation processes.

Heterogeneous Photodegradation Behavior of Liquid Crystal Monomers in Dust: Quantitative Structure-Activity Relationship and Product Identification

The heterogeneous photodegradation behavior of liquid crystal monomers (LCMs) in standard dust (standard reference material, SRM 2583) and environmental dust was investigated. The measured photodegradation ratios for 23 LCMs in SRM and environmental dust in 12 h were 11.1 ± 1.8 to 23.2 ± 1.1% and 8.7 ± 0.5 to 24.0 ± 2.8%, respectively. The degradation behavior of different LCM compounds varied depending on their structural properties. A quantitative structure-activity relationship model for predicting the degradation ratio of LCMs in SRM dust was established, which revealed that the molecular descriptors related to molecular polarizability, electronegativity, and molecular mass were closely associated with LCMs' photodegradation. The photodegradation products of the LCM compound 4'-propoxy-4-biphenylcarbonitrile (PBIPHCN) in dust, including •OH oxidation, C-O bond cleavage, and ring-opening products, were identified by nontarget analysis, and the corresponding degradation pathways were suggested. Some of the identified products, such as 4'-hydroxyethoxy-4-biphenylcarbonitrile, showed predicted toxicity (with an oral rat lethal dose of 50%) comparable to that of PBIPHCN. The half-lives of the studied LCMs in SRM dust were estimated at 32.2-82.5 h by fitting an exponential decay curve to the observed photodegradation data. The photodegradation mechanisms of LCMs in dust were revealed for the first time, enhancing the understanding of LCMs' environmental behavior and risks.

Enhancing dye degradation using a novel cobalt metal-organic framework as a peroxymonosulfate activator

Among transition metals, cobalt ions exhibit superior catalytic activity in the peroxymonosulfate (PMS) degradation of pollutants. However, practical application is hindered by their high rate of ion leaching and the propensity for particle reunion issues. In this study, a novel cobalt metal-organic framework catalyst, denoted as CUST-565 ([Co3(BTB)2(BIPY)2]·4.5H2O·DMA), was synthesized via a one-step solvothermal method. The obtained crystal was employed as a catalyst to activate PMS for degrading two pollutants, methyl orange (MO) and rhodamine B (RhB), in wastewater. The catalyst demonstrated efficacy in PMS, achieving 97% degradation of MO and 98% degradation of RhB within 30 min at an initial concentration of 20.0 mg L-1. Additionally, various factors affecting dye degradation, including PMS dosage, catalyst dosage, temperature, initial pH, and coexisting anions, were investigated. Radical quenching experiments confirmed the presence of sulfate radicals (SO4˙-), hydroxyl radicals (HO˙), superoxide radicals (O2˙-), and singlet oxygen (1O2) in the system. After four cycles, CUST-565 retained its ability to catalytically degrade approximately 80% of the pollutants. These observed stability and reusability properties, corroborated by a series of characterization analyses before and after use, suggest that CUST-565 exhibits reliable performance. This work contributes to the development of cobalt-PMS catalysts for efficiently degrading dyes in wastewater.

Efficient degradation of chlorpyrifos and intermediate in soil by a novel microwave induced advanced oxidation process: A two-stage reaction

The application of microwave/peroxymonosulfate (MW/PMS) in soil remediation has been limited by some shortages including low utilization efficiency of oxidants, low MW absorption capacity of soil particles and incomplete degradation of intermediate. In this study, heating pad waste (HPW) was added in the MW/PMS system to increase the ability of absorbing MW and degradation efficiency of toxic intermediate. A two-stage method for degradation of chlorpyrifos (CPF) and its intermediate 3,5,6-trichloro-2-pyridinol (TCP) by MW/PMS assisted with HPW was proposed. In the first stage, more than 90% of CPF was degraded within 15 min before the addition of HPW, and most of the CPF was converted into TCP through direct or indirect pathways under the action of 1O2. In the second stage, more than 70% of the generated TCP was rapidly degraded through SO4•- oxidation and electron transfer. The TCP was further degraded with the assistance of HPW through methylation, hydroxylation and dechlorination etc., and the toxicity of degradation products was decreased significantly. pH and soil organic matter had little influences on CPF and TCP degradation. Therefore, a new strategy for remediation of CPF contaminated-soil was provided based on MW/PMS technology and the concept of "treating waste with waste".

Novel Magnetic MnFe2O4-Decorated Graphite-Like Porous Biochar as a Heterogeneous Catalyst for Activation of Peroxydisulfate Toward Degradation of Rhodamine B

A magnetic MnFe2O4-modified graphite-like porous biochar composite (MnFe2O4/KFS800) was synthesized by the hydrothermal method, and its catalytic activity was evaluated in the activation of peroxydisulfate toward degradation of Rhodamine B. After characterization by SEM, XRD, and the BET method, the specific surface area and total pore volume of the MnFe2O4/KFS800 catalyst reached 121 m2/g and 0.263 m3/g, and exhibited plate-like morphology with good crystallinity. The degradation rate of Rhodamine B by the obtained composite was more than 91.1% when the initial concentration of RhB was 10 mg/L, the dosage of MnFe2O4/KFS800 was 0.2 g/L, and the initial pH was 6.7. Then the anti-interference ability of the obtained composite was studied, and it was found that there was a little effect on the degradation of Rhodamine B with the presence of humic acid. Finally, quenching test, EPR research, and XPS analysis were conducted to reveal the catalytic mechanism, and possible mechanism was a synergistic behavior of free radicals (SO4•-, •OH, O2•-) and nonfree radicals (1O2), and trace amounts of uncarbonized bagasse was also involved in the formation of free radicals.

Activation of periodate by ABTS as an electron shuttle for degradation of aqueous organic pollutants and enhancement effect of phosphate

Periodate (PI) based advanced oxidation processes (AOPs) have recently attracted much attention due to their high application potential in water purification through production of reactive species. In the study, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) was used as a representative electron shuttle, and its reaction with PI was investigated in detail. It was found that PI can be activated by ABTS via one-electron transfer to produce ABTS•+ and IO3•, cooperatively promoting oxidation of organic contaminants such as bisphenol A (BPA). Their contribution in BPA oxidation at pH 7 was estimated as 81.9% and 18.1%, respectively. With phosphate, BPA oxidation rate in the PI/ABTS process increased linearly with raised phosphate concentrations from 0 to 10 mM. The enhancement effect of phosphate is attributed to formation of PI-phosphate complexes, which facilitate PI activation by ABTS, and production of more ABTS•+ and IO3•, and additional phosphate radicals. Accordingly, the contribution of IO3• and phosphate radicals in BPA oxidation raised to 57.7% in the process with 4 mM phosphate, while that of ABTS•+ decreased to 42.3%. The reaction stoichiometry ratio of ABTS to PI was measured as 1.1 at pH 7, suggesting the little involvement of IO3• and phosphate radicals in production of ABTS•+ due to their high self-quenching. The PI/ABTS process exhibited excellent anti-interference capacity towards water matrix components (e.g. Cl-, HCO3- and natural organic matters). Moreover, an immobilized ABTS (ABTS/ZnAl-LDH) was successfully developed as a heterogeneous electron shuttle for PI oxidation, which resultantly exhibited the good catalytic activity and stability in degradation of BPA, further improving feasibility of the process in treatment of actual water. This work advances understanding on reaction of PI with ABTS from stoichiometric and kinetic aspects, and provides a high performance AOP for selective oxidation of trace organic contaminants.

Efficient activation of peroxymonosulfate by Fe single-atom: The key role of Fe-pyrrolic nitrogen coordination in generating singlet oxygen and high-valent Fe species

Nitrogen-doped carbon matrix single-atom catalysts (SACs) for the efficient removal of organic pollutants have attracted widespread attention. However, the ligand structure and the origin of the high activity between nitrogen species and single-atoms remain elusive. Herein, nitrogen-doped carbon matrix iron single-atom catalysts (Fe/NC-SACs) that exhibit high catalytic reactivity (98.2% SMX degradation in 5 min), broad pH resistance (pH 3.0-11.1), high stability, and sustainable water treatment capacity are reported. High-valent iron (Fe IV=O) and singlet oxygen (1O2) were the reactive oxygen species observed. The electrochemical results demonstrated the generation of catalyst-PMS complexes. The DFT calculations revealed that Fe-pyrrolic N4 was the best ligand for PMS, exhibiting the highest adsorption energy, bond length variation and electron transfer capacity. The central Fe single atom and the carbon electrons adjacent to the pyrrolic N were the reactive sites of the PMS. The main source of 1O2 was the oxidation of PMS. This work provides guidance for the discovery of high-performance catalysts and provides a single-atom catalyst that can be used for practical environmental purification.

Facet-Specific Photocatalytic Degradation of Extracellular Antibiotic Resistance Genes by Hematite Nanoparticles in Aquatic Environments

The persistence of extracellular antibiotic resistance genes (ARGs) in aquatic environments has attracted increasing attention due to their potential threat to public health and the environment. However, the fate of extracellular ARGs in receiving water remains largely unknown. This study investigated the influence of hematite nanoparticles, a widespread natural mineral, on the photodegradation of extracellular ARGs in river water. Results showed that under exposure to visible light, hematite nanoparticles, at environmental concentrations, resulted in a 3-5 orders of magnitude reduction in extracellular ARGs. This photodegradation of extracellular ARGs is shown to be facet-dependent; the (001) facet of hematite demonstrates a higher removal rate than that of the (100) facet, which is ascribed to its enhanced adsorption capability and higher hydroxyl radical (•OH) production. Density functional theory (DFT) calculations corroborate this finding, indicating elevated iron density, larger adsorption energy, and lower energy barrier of •OH formation on the (001) facet, providing more active sites and •OH generation for extracellular ARG interaction. Gel electrophoresis and atomic force microscopy analyses further confirm that the (001) facet causes more substantial damage to extracellular ARGs than the (100) facet. These findings pave the way for predicting the photodegradation efficiency of hematite nanoparticles with varied facets, thereby shedding light on the inherent self-purification capacity for extracellular ARGs in both natural and engineered aquatic environments.

Efficient degradation and detoxification of antibiotic Fosfomycin by UV irradiation in the presence of persulfate

Fosfomycin (FOS) as a widely used antibiotic has been found in abundance throughout the environment, but little effort has been devoted to its treatment. In this study, we systemically looked into the degradation of FOS by ultraviolet-activated persulfate (UV/PS) in aqueous solutions. Our findings demonstrated that FOS can be degraded efficiently under the UV/PS, e.g., >90 % of FOS was degraded with 19,200 mJ cm-2 of UV irradiance and 20 μM of PS. HO was the dominant radical responsible for FOS degradation. FOS degradation increased as PS dosage increased, and higher degradation efficiency was observed at neutral pH. Natural water constitutes either promoted (e.g., Cu2+, Fe3+, and SO42-) or inhibited (e.g., humic acid, HCO3-, and CO32-) FOS degradation to varying degrees. Hydroxyl substitution, CP bond cleavage, and coupling reactions were the major degradation pathways for FOS degradation. Finally, the toxicity evaluation revealed that FOS was toxic to E. coli and S. aureus, but the toxicity of the intermediate products of FOS to E. coli and S. aureus rapidly decreased over time after UV/PS treatment. Therefore, these findings provided a fundamental understanding of the transformation process of FOS and supplied useful information for the environmental elimination of FOS contamination and its toxicity.

N, S, and Cl tri-doped carbon boost the switching of radical to non-radical pathway in Fenton-like reactions: Synergism of N species and defects

Heteroatom doping represents a promising strategy for enhancing the generation of singlet oxygen (1O2) during the activation of peroxymonosulfate (PMS) using carbon-based catalysts; however, it remains a formidable challenge. In this study, we systematically controlled the structure of metal-free carbon-based materials by introducing different heteroatoms to investigate their efficacy in degrading organic pollutants in water via PMS activation. The results of reactive oxygen species detection showed that the dominant free radical in the four samples was different: CN (•SO4- and •OH), CNS (•O2-), CNCl (1O2), and CNClS (1O2). This led to the transformation of active species from free radicals to non-free radicals. The tri-doped carbons with nitrogen, sulfur, and chlorine (CNClS) exhibited exceptional performance in PMS activation and achieved a remarkable degradation efficiency of 95% within just 6 min for tetracycline. Moreover, a strong linear correlation was observed between the ratio of pyridine-N/graphite-N and ID/IG with the yield of 1O2, indicating that N species and defects play a crucial role in CNClS as they facilitate the transition from radical to non-radical pathways during PMS activation. These findings highlight the possibility that adjustable tri-heteroatom doping will expand the Fenton-like reaction for the treatment of actual wastewater.

Self-assembly construction of 1D carbon nitride nanotubes and cobalt-modified for superior photocatalytic degradation of sulfonamide antibiotics

In the present work, a cobalt-doped carbon nitride nanotubes (Co-CNt) was synthesized via self-assembly process. Contributed to the narrow band gap, enlarged specific surface area and abundant active sites, Co-CNt has excellent photoelectric properties and superior performance than pristine CN in sulfisoxazole (SIZ) degradation under blue light irradiation, which achieved 100% removal within 40 min. Meanwhile, the system not only exhibited practical applicability by efficiently degrading SIZ, but also generating high levels of H2O2. Moreover, the Co-CNt/visible light system shows superior operability over a wide pH range, micro-concentration contaminants, various anions, water matrices and other sulfonamides with promising catalytic stability and applicability. The contribution of RSs in the degradation process were elucidated based on radical scavenging and spin-trapped tests, clarifying that O2·- and h+ majorly dominated the process. In addition, 4 probable degradation pathways of SIZ were provided and the generated intermediates' toxicity were evaluated. Overall, this study successfully synthesized a self-assembled 1D tubular photocatalyst with Co-doped and demonstrated the potential Co-CNt/visible light system for environmental remediation, providing a promising approach for the development of photocatalysis.

Enhanced Mass Transfer of Ozone and Emerging Pollutants through a Gas-Solid-Liquid Reaction Interface for Efficient Water Decontamination

Ozone (O3), as an environmentally friendly oxidant, is widely used to remove emerging pollutants and ensure the safety of the water supply, whereas the restricted accessibility of O3 and limited collision frequency between pollutants and O3 will inevitably reduce the ozonation efficiency. To promote the chemical reactions between O3 and target pollutants, here we developed a novel gas-solid-liquid reaction interface dominated triphase ozonation system using a functional hydrophobic membrane with an adsorption layer as the O3 distributor and place where chemical reactions occurred. In the triphase system, the functional hydrophobic membrane simultaneously improved the interface adsorption performance of emerging pollutants and the access pathway of O3, leading to a marked enhancement of interfacial pollutant concentration and O3 levels. These synergistic qualities result in high ciprofloxacin (CIP) removal efficiency (94.39%) and fast apparent reaction rate constant (kapp, 2.75 × 10-2 min-1) versus a traditional O3 process (41.82% and 0.48 × 10-2 min-1, respectively). In addition, this triphase system was an advanced oxidation process involving radical participation and showed excellent degradation performance of multiple emerging pollutants. Our findings highlight the importance of gas-solid-liquid triphase reaction interface design and provide new insight into the efficient removal of emerging pollutants by the ozonation process.

Occurrence and removal of antibiotics from aquaculture wastewater by solar-driven Fe(VI)/oxone process

In this study, the occurrence and removal of ten selected antibiotics from aquaculture wastewater by the process solar + Fe(VI)+oxone were investigated. The detection levels of the antibiotics in the aquaculture wastewater samples were at ng/L. The degradation of the selected antibiotics under the process solar + Fe(VI)+oxone followed pseudo-first-order kinetics. As the most abundant antibiotic in the studied aquaculture wastewater, norfloxacin (NFX) was used as the model compound to study the reaction mechanism and detoxification ability of the treatment system, as well as the effects of reaction parameters and environmental factors. The active species including O2•-, O21, and Fe(V)/Fe(IV) contributed to NFX degradation in the process solar + Fe(VI)+oxone. Decarboxylation, the piprazine ring opening, defluorination of the benzene ring, oxygen addition and the cleavage of the quinolone/benzene ring were main degradation pathways of NFX. Around 20% mineralization was reached and the inhibition rate of the bacteria (Escherichia Coli) growth was reduced from 95.5% to 47.1% after the NFX degradation for 60 min. Despite the suppression of NFX degradation by NO2-, PO43- and humic acid, the NFX degradation in three aquaculture wastewater samples was faster than that in ultrapure water due to the positive effect of Br-and other factors. The above results demonstrate the treatment process solar-driven Fe(VI)/oxone has a good potential in antibiotics removal from the aquaculture wastewater.

Polar electric field-modulated peroxymonosulfate selective activation for removal of organic contaminants via non-radical electron transfer process

Nonradical electron transfer process (ETP) in peroxomonosulfate (PMS) based advanced oxidation processes (AOPs) is regarded promising for selective degradation of organic contaminants in water, however, the subjective modulation strategy and the definitive mechanistic elucidation of ETP are still lacking. Herein, we proposed a heretofore unreported yet efficient ETP indution approach by construction of polar electrical field on biochar via nonmetallic elements co-doping. Physicochemical characterizations and density functional theory (DFT) calculations verified the electronegativity difference among boron, nitrogen, and sulfur elements bestowed robust local electric fields on biochar surface (BC-BNS), which effectively enhanced the adsorption complexation and charge transfer between biochar and PMS. Compared to the other single-doped or co-doped biochar, BC-BNS exhibited superior catalytic performance of PMS activation for degradation of atrazine (ATZ) (kobs=0.036 min-1), as well as various kinds of electron-rich organics. The remarkable catalytic degradation capacity was further verified in various aqueous matrices and background factors, representing the excellent selectivity. Analysis of contribution from reactive oxygen species and electrochemical testing together substantiated the role of polar electric fields in facilitating the modulation from singlet oxygen (1O2) to ETP as a prevailing mechanism. DFT calculations and apparent interactions revealed the dissociation of S-O bond was thermodynamically favored within this potent localized electric field, which further induced the cleavage of OO bond and ultimately promoted the dual electron transfer between ATZ and PMS. The superiority of BC-BNS/PMS system was further validated with the low ecotoxicity caused by enhanced dechlorination, the low energy consumption, and the long-term effectiveness. The novel modulation principle and atomic-level mechanism exploration gave suggestions for advancing ETP-dominated AOP to remove recalcitrant contaminants during water treatment and restoration.

Strategies for promoting the degradation of phenol by electro-Fenton: Simultaneously promoting the generation and utilization of H2O2

The use of the electro-Fenton process to continuously generate H2O2 and efficiently degrade organic pollutants is considered a promising technology. The ratio of generation of H2O2 is usually regarded as the critical step; however, how the H2O2 is utilized is also of particular importance. Herein, activated carbon was activated at different temperatures and used to explore the effect of nitrogen doping on the production and utilization of H2O2 in the electro-Fenton-based degradation of organic pollutants. The experimental results indicate that nitrogen-doped activated carbon simultaneously promotes the generation and utilization of H2O2, which is attributed to the regulation of the competition between phenol and O2 adsorption by the doped nitrogen. Nitrogen doping not only improves 2e-ORR selectivity but also aggregates phenol near the cathode to balance the concentrations of phenol and ·OH. Density functional theory (DFT) calculations further confirmed that pyrrole-N as a dopant promoted the adsorption of phenol, while pyridine-N was more favorable for O2 adsorption. The unique balance of nitrogen types possessed by modified activated carbon NAC-750 permits the efficient synergistic generation and utilization of H2O2 in a balanced manner during the degradation of phenol. This work provides a new direction for the rational nitrogen-doping modification of activated carbon for the electro-Fenton-based degradation of organic pollutants.

Influence of organic matter on the photocatalytic degradation of steroid hormones by TiO2-coated polyethersulfone microfiltration membrane

Water treatment in photocatalytic membrane reactors (PMR) holds great promise for removing micropollutants from aquatic environments. Organic matter (OM) that is present in any water matrix may significantly interfere with the degradation of steroid hormone (SH) micropollutants in PMRs. In this study, the interference of various OM types, humic acid (HA), Australian natural organic matter (AUS), worm farm extract (WF), tannic acid (TA), and gallic acid (GA) with the SH degradation at its environmentally relevant concentration (100 ng/L) in a flow-through PMR equipped with a polyethersulphone-titanium dioxide (PES-TiO2) membrane operated under UV light (365 nm) was investigated. Results of this study showed that OM effects are complex and depend on OM type and concentration. The removal of β-estradiol (E2) was enhanced by HA at its levels below 5 mgC/L while the enhancement was abated at higher HA concentrations. The E2 removal was inhibited by TA, and GA, while no significant interference observed for AUS, and WF. The data demonstrated diverse roles of OM that acts in PMRs as a light screening agent, a photoreactive species scavenger, an adsorption alteration trigger, and a photosensitizer. The time-resolved fluorescence measurement showed that HA, acting as a photosensitizer, promoted the sensitization of TiO2 by absorbing light energy and transferring energy/electron to the TiO2 substrate. This pathway dominated the mechanism of the enhanced E2 degradation by HA. The favorable effect of HA was augmented as increasing the light intensity from 0.5 to 10 mW/cm2 and was weakened at higher light intensities due to the increased scavenging reactions and the limited amount of HA. This work clarifies the underlying mechanism of the OM interference on photocatalytic degradation of E2 by the PES-TiO2 PMR.

Generation and identification of 1O2 in catalysts/peroxymonosulfate systems for water purification

Catalysts for peroxymonosulfate (PMS) activation are appealing in the purification of organic wastewater. Singlet oxygen (1O2) is widely recognized as a crucial reactive species for degrading organic contaminants in catalysts/PMS systems due to its adamant resistance to inorganic anions, high selectivity, and broad pH applicability. With the rapid growth of studies on 1O2 in catalysts/PMS systems, it becomes necessary to provide a comprehensive review of its current state. This review highlights recent advancements concerning 1O2 in catalysts/PMS systems, with a primary focus on generation pathways and identification methods. The generation pathways of 1O2 are summarized based on whether (distinguished by the geometric structures of metal species) or not (distinguished by the active sites) the metal element is included in the catalysts. Furthermore, this review thoroughly discusses the influence of metal valence states and metal species with different geometric structures on 1O2 generation. Various potential strategies are explored to regulate the generation of 1O2 from the perspective of catalyst design. Identification methods of 1O2 primarily include electron paramagnetic resonance (EPR), quenching experiments, reaction in D2O solution, and chemical probe tests in catalysts/PMS systems. The principles and applications of these methods are presented comprehensively along with their applicability, possible disagreements, and corresponding solutions. Besides, an identifying procedure on the combination of main identification methods is provided to evaluate the role of 1O2 in catalysts/PMS systems. Lastly, several perspectives for further studies are proposed to facilitate developments of 1O2 in catalysts/PMS systems.

pH adjustable MgAl@LDH-coated MOFs-derived Co2.25Mn0.75O4 for SMX degradation in PMS activated system

Sulfate radical-based advanced oxidation processes (SR-AOPs) is considered as one of the most promising technologies for antibiotic pollution. In this study, a core-shell catalyst of cobalt-manganese oxide derived from CoMn-MOFs coating by MgAl-LDH (Co/Mn@LDH) was synthesized for peroxymonosulfate (PMS) activation to degrade sulfamethoxazole (SMX). Degradation efficiency of nearly 100% and a mineralization efficiency of 68.3% for SMX were achieved in Co/Mn@LDH/PMS system. Mn species and out shell MgAl-LDH greatly suppressed the cobalt ions leaching, which only 23 μg/L Co ions were detected by ICP after the reaction. SO4·- was identified as dominant reactive species in the system. Furthermore, the possible reactive sites of SMX were predicted by the density functional theory (DFT) calculations. And the intermediates of SMX were detected by LC-MS and the degradation pathway was proposed based on the results above. The ECOSAR results suggested the intermediates of SMX showed a relatively low toxicity compared to SMX, indicating huge potential of utilization of Co/Mn@LDH in SR-AOPs system.

A review of factors affecting the formation and roles of primary and secondary reactive species in UV254-based advanced treatment processes

The presence of organic micropollutants (OMPs) in water has been threatening human health and aquatic ecosystems worldwide. Ultraviolet-based advanced treatment processes (UV-ATPs) are one of the most effective and promising technologies to transform OMPs in water; therefore, an increasing number of emerging UV-ATPs are proposed. However, appropriate selection of UV-ATPs for practical applications is challenging because each UV-ATP generates different types and concentrations of reactive species (RSs) that may not be sufficient to degrade specific types of OMPs. Furthermore, the concentrations and types of RSs are highly influenced by anions and dissolved organic matter (DOM) coexisting in real waters, making systematic understandings of their interfering mechanisms difficult. To identify and address the knowledge gaps, this review provides a comparison of the generations and variations of various types of RSs in different UV-ATPs. These analyses not only prove the importance of water matrices on formation and consumption of primary and secondary RSs under different conditions, but also highlight the non-negligible roles of optical properties and reactivities of DOM and anions. For example, different UV-ATPs may be applicable to different target OMPs under different conditions; and the concentrations and roles of secondary RSs may outperform those of primary RSs in OMP degradation for real applications. With continuous progress and outstanding achievements in the UV-ATPs, it is hoped that the findings and conclusions of this review could facilitate further research and application of UV-ATPs.

Coating CoFe2O4 shell on Fe particles to increase the utilization efficiencies of Fe and peroxymonosulfate for low-cost Fenton-like reactions

Bimetallic composites (Fe@CoFe2O4) with zero-valent Fe as the core encapsulated by CoFe2O4 layers are synthesized by a coprecipitation-calcination method, which are applied to activate PMS for the degradation of bisphenol A (BPA). Enhanced activity of Fe@CoFe2O4 is achieved with very fast degradation rate (kobs = 0.5737 min-1). In the fixed-bed reactor, the catalyst lifetime (tul) of Fe@CoFe2O4 is determined to be 22 h compared to 11 h of Fe, and the deactivation rate constant (kd) for Fe@CoFe2O4 is 0.0083 mg·L-1·h-1, only ∼1/10 of Fe (0.0731). The XPS results indicate that the core-shell structure of Fe@CoFe2O4 could promote the redox cycles of Co3+/Co2+ and Fe3+/Fe2+. It is proved that the coating of CoFe2O4 shell on Fe0 can protect the Fe0 core from being oxidized by PMS to form passivation layer. The electrons of Fe0 can therefore be used effectively for activating PMS to produce ROSs via the CoFe2O4 shell. This modification method of Fe0 would decrease the cost of PMS based wastewater remediation greatly, thus should have great potential on an industrial scale.