Multiple Roles of Dissolved Organic Matter in Advanced Oxidation Processes

Nanoconfined catalytic macrostructures for advanced water remediation: From basic understanding to future application strategies

In recent years, nanoconfined catalytic macrostructures applied to advanced oxidation processes (AOPs) have been rapidly developed, effectively solving the problems of traditional heterogeneous AOPs, such as mass transfer limitation, limited diffusion of short-lived reactive oxygen species (ROS), and aggregation/leaching of catalysts. Compared with the traditional heterogeneous AOPs systems, the nanoconfined catalytic macrostructures have unique interactions between the oxidants, catalysts, ROS and micropollutants, which could significantly increase the yield and mass transfer of ROS. At present, there is a lack of comprehensive reviews on the nanoconfined catalytic macrostructures from basic theory to application performances and future development strategies. This study reviewed the preparation routines of various nanoconfined catalytic macrostructures, assessed their structural differences, catalytic properties and nanoconfined catalytic mechanisms via integrated density functional theory (DFT) and molecular dynamics (MD) stimulations. We also proposed the future strategies for nanoconfined catalytic macrostructures in combination with the machine learning, which could provide key information on the feasibility of the technology and future research directions. This review aims to enhance scholarly interest in the application of nanoconfined macrostructures in the AOPs fields, anticipating significant technical feasibilities for scale-up AOPs application of nanoconfinement.

Molecular composition difference of electron donating moieties between natural organic matter and effluent organic matter probed by chlorine dioxide

Lignin- and tannin-like phenolic compounds are shown to be the major compositions of electron donating moieties (EDM) of aquatic natural organic matter (NOM). However, little is known about the compositions of EDMs within effluent organic matter (EfOM). In the present study, chlorine dioxide (ClO2) was used as a selectively oxidative probe to investigate the difference in the molecular composition of EDM between NOM and EfOM due to its high selectivity towards electron-rich compounds. The results showed that there was a large difference in the bulk and molecular properties of ClO2-reactive moieties between EfOM and NOM. Specifically, ClO2-reactive moieties of EfOM are distributed in a narrower molecular weight range (i.e., 0.9 kDa to 3.0 kDa) compared to NOM (i.e., 1.0 kDa to 20 kDa). The molecular-level analysis demonstrated that highly aromatic, reduced formulas (O/C = 0.33 ± 0.16; H/C = 1.10 ± 0.34) referring the lignin- and tannin-like compounds within both NOM and EfOM were susceptible to oxidation by ClO2, while more saturated formulas including the peptide-like formulas (H/C = 1.59 ± 0.36) within EfOM were reactive towards ClO2. Furthermore, the nitrogen (N)-containing formulas in EfOM are suggested to be the major EDMs compared to the CHO-only formulas dominating the EDM in NOM. This study has important implications for understanding of the origin and chemical nature of EDM in DOM from various sources and provides molecular-level evidence for the selectivity of ClO2 as an oxidant towards DOM.

Electroactive bacteria-established long-distance electron transfer to oxygen facilitates bio-transformation of dissolved organic matter for sediment remediation

Electroactive bacteria (EAB) in sediment commonly establish long-distance electron transfer (LDET) to access O2, facilitating the degradation of organic contaminants, which we hypothesize is mediated by the bio-transformation of dissolved organic matter (DOM). This study confirmed that EAB-established LDET to O2 via a microbial electrochemical snorkel raised the electric potential of sediment by increasing HCl-extracted Fe(III) and NO3- concentrations while reducing DOM concentrations, which further modified microbial diversity and composition, notably reduced the relative abundance of fermentative bacteria. As a result, DOM showed the highest SUVA254 value (3.88) and SUVA280 value (1.61), preliminarily suggesting their enhanced aromaticity, humification and average molecular weight. Additionally, these DOM exhibited the highest electron transfer capacity (174.14±3.62 μmol e- /g C) and redox current. Based on these findings, we propose four possible avenues through which EAB-established LDET to O2 facilitates sediment remediation, mainly including DOM involved affinity, direct and indirect electron transfer, and induced photochemical reaction in degradation or humification process of organic contaminants. Although these proposed avenues require further verification, this work sheds light on deciphering the mechanisms underlying the augmented degradation of organic contaminants facilitated by EAB-established LDET to O2, offering fresh insights into sediment remediation.

A new insight into the straw decomposition associated with minerals: Promoting straw humification and Cd immobilization

Organic matter (OM) derived from the decomposition of crop residues plays a key role as a sorbent for cadmium (Cd) immobilization. Few studies have explored the straw decomposition processes with the presence of minerals, and the effect of newly generated organo-mineral complexes on heavy metal adsorption. In this study, we investigated the variations in structure and composition during the rice straw decomposition with or without minerals (goethite and kaolinite), as well as the adsorption behavior and mechanisms by which straw decomposition affects Cd immobilization. The degree of humification of extracted straw organic matter was assessed using excitation-emission matrix (EEM) fluorescence and Ultraviolet-visible spectroscopy (UV-vis), while employing FTIR spectroscopy and XPS to characterize the adsorption mechanisms. The spectra analysis revealed the enrichment of highly aromatic and hydrophobic components, indicating that the degree of straw decomposition and humification were further intensified during incubation. Additionally, the existence of goethite (SG) accelerated the humification of OM. Sorption experiments revealed that the straw humification increased Cd adsorption capacity. Notably, SG exhibited significantly higher adsorption performance compared to the organic matter without minerals (RS) and the existence of kaolinite (SK). Further analysis using FT-IR spectroscopy and XPS verified that the primary mechanisms involved in Cd immobilization were complexion with -OH and -COOH, as well as the formation of Cd-π binds with aromatic C=C on the surface of solid OMs. These findings will facilitate understanding the interactions of the rice straw decomposing with soil minerals and its remediation effect on Cd-contaminated farmland.

Reversible Sol-Gel Transitions Mediated Organics Selective Uptake and Release for Simultaneous Water Purification and Chemicals Recovery

The separation and recovery of useful organics from wastewater have been a promising alternative to tackling water pollution and resource shortages, while strategies that truly work have rarely been explored. Herein, a reversible CO2 triggered sol-gel state transformation mediated selective organics uptake-release system using a surface modified carbonitride (S-CN) is proposed and exhibits remarkable organics recovery performance from wastewater. Results show that CO2 can serve as a cross-linker for linking S-CN particles to form a hydrogel by electrostatic interaction and hydrogen bonding, which can be recycled to the pristine sol state simply by removing the cross-linked CO2 with Ar purging. The reversible sol-gel transformation achieves nearly complete uptake of valuable organics from wastewater with high selectivity at the first sol-to-gel stage through electrostatic interaction, hydrogen bonding, and π-π interactions together, and it recovers 90% of the organics uptaked by releasing them into a concentrated solution at the second gel-back-to-sol stage.

Carbon residue from co-pyrolysis of cartons and plastics: Characteristics, environmental behaviors and applications

The continuously growing of municipal solid waste (MSW) has posed a threat to human-being. Pyrolysis is a promising technique for MSW disposal, as it can reduce its volume and produce valuable products as well. This study evaluated the potential of carbon residue (CR) derived from waste carton as soil amendment. Additionally, considering the waste plastics, such as plastic bags, plastic tape, etc., can mix with carton in MSW, the effect of polyvinyl chloride (PVC) addition on the characteristics of CR and its environmental behavior was measured. Results showed that the CR derived from carton exhibited notable efficacy in adsorbing Cd2+ (56.111 mg/g), pesticides and PO43- (0.231 mmol/g), thereby mitigating pollutants and immobilizing nutrient in soil environment. The introduction of PVC was found to enhance the adsorption of CR for Cd2+ (65.623 mg/g) and PO43- (0.524 mmol/g), albeit exhibiting diminished performance in the removal of pesticides. Different from previous studies, this study revealed that the dissolved black carbon (DBC) released from CRs did not generate reactive oxygen species (ROS) effectively under solar irradiation, and the light screening of DBC can mitigate the photodegradation of pollutants. Furthermore, both CRs increased bacterial luminescence by approximately 70%, which indicated that no toxicity produced during the pyrolysis process. And gas chromatography-mass spectrometry (GC-MS) revealed the absence of polyaromatic hydrocarbon (PAH) in CRs. In summary, this study may provide some new insights on the disposal of MSW, and the CR made by MSW could be effective in soil amendment and reduce the leaching of pollutants and nutrient.

Initiating photocatalytic degradation of organic pollutants under ultra-low light intensity via oxygen-centered organic radicals

Photocatalysis is a promising method for in situ water pollution remediation but faces challenges due to the limited natural light intensity. Herein, we achieved highly-efficient photocatalytic removal of organic pollutants even under ultra-low light intensities of only 0.1 mW cm-2. This was accomplished by developing and effectively stabilizing novel reactive species, oxygen-centered organic radicals (OCORs), which have an impressive half-life of up to seven minutes in water. With lifetimes that are 8 to 11 orders of magnitude longer than for traditional transient radicals, OCORs can effectively wait for pollutants to diffuse, enabling them to remove organic pollutants through polymerization and degradation pathways. The mechanism behind the stability of OCORs lies in the enhanced electron-withdrawing ability of the electron acceptor and the extended conjugation of the catalyst, which effectively prevent back electron transfer. This study provides a theoretical foundation for practical applications of photochemistry in pollution remediation.

New insight into linear substituents influencing electrooxidation treatment of sulfonamide antibiotics: Linking kinetics, pathways, toxicity, and active species with density functional theory

Linear substituents, despite their simpler structures compared to heterocyclic ones, exhibit distinct chemical behaviors. Using sulfacetamide (SAM) and sulfaguanidine (SGD) as model compounds, we assessed the impact of these substituents on degradation efficiency, active species identification, reaction pathways, and intermediate toxicity during electrooxidation in water. Through density functional theory, we elucidated the mechanisms, focusing on electronic structural changes and interactions with active species. Notably, the acetyl group in SAM (0.1016) acquired more electrons than the guanidyl group in SGD (0.0281), resulting in SAM having a higher free energy change (ΔG=15.06kcal/mol) compared to SGD (ΔG=9.59kcal/mol). This difference makes SAM less likely to undergo direct electron transfer and less reactive towards hydroxyl radical addition, leading to slower degradation rates. The applied potential notably increased SAM's sensitivity to hydroxyl radicals. Both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were contributed by the parent fragment, facilitating electrophilic reactions mainly on the aniline part. Seventeen intermediate products and three major transformation pathways were identified, emphasizing aniline group destruction before discharge. This research enhances understanding of the degradation and environmental fate of sulfonamides, providing valuable insights for optimizing pollutant degradation and discharge reduction.

Catalytic degradation of organic pollutants in aqueous systems: A comprehensive review of peroxyacetic acid-based advanced oxidation processes

Peroxyacetic acid (PAA)-based advanced oxidation processes (AOPs) have emerged as a promising treatment method to decontaminate organic pollutants. This review thoroughly evaluated the use of PAA-based AOPs, including their synthesis techniques, physicochemical features, and reaction pathways with pollutants. It also illustrated two primary channels: free radical pathways and non-radical pathways during the PAA activation processes and introduced various methods for activating PAA, including energy radiation, transition metal catalysis, and carbon catalysis. Additionally, this review comprehensively presented the advancements in research on PAA-based AOPs for wastewater treatment. Furthermore, the influences of key parameters on system performance, such as pH, catalyst loading, PAA dosage, and interfering species, were summarized. By critically evaluating mechanisms, performance, and prospects, this review served as a valuable resource for researchers and practitioners involved in the development and implementation of PAA-based AOPs for sustainable water remediation.

Capturing differences in the release potential of dissolved organic matter from biochar and hydrochar: Insights from component characterization and molecular identification

Biochar and hydrochar have garnered widespread attention owing to their excellent performance in environmental remediation, carbon sequestration, and resource utilization from biowaste. Studies on the release potential of dissolved organic matter (DOM) have been limited, and the distinction between biochar and hydrochar remains unclear. In this study, pine sawdust was utilized as a model precursor with the aim of comparing the release quantity, components, and properties of DOM from biochar (BDOM) and hydrochar (HDOM) under various simulated conditions. The amount of DOM released by hydrochar (38.20-190.49 g/kg) was significantly greater than that released by biochar (0.57-11.96 g/kg), and more DOM was released at higher temperatures and pH values. BDOM consists of three categories of components, namely, humic-like, protein-like, and benzoic acid-like and tyrosine-like substances compounds, whereas HDOM consists of four categories of components, namely, two categories of humic-like compounds and two categories of protein-like compounds. By using ESI-FT-ICR-MS technology, 8586 compounds in BDOM and 6428 compounds in HDOM were identified. A total of 4665 unique compounds were found in BDOM, 1416 unique compounds were found in HDOM under alkaline release conditions, and HDOM contained more unique compounds than those found in other environments. CRAM/lignin-like compounds made up the majority of the released DOM and reached 31.01-65.35 % for BDOM and 54.79-73.05 % for HDOM. These findings revealed significant differences in the release potential of DOM from biochar and hydrochar, and further behavior research is needed to guide future applications of char materials in the environment and agriculture fields.

Phototransformation and photoreactivity of MPs-DOM in aqueous environment: Key role of MPs structure decoded by optical and molecular signatures

The dissolved organic matter (DOM) derived from microplastics (MPs-DOM) can be one of the photoactive components in DOM. However, information on the properties and photoreactivity of MPs-DOM during phototransformation is limited. Here, we investigated the properties and photoreactivity of MPs-DOM from polyolefins (MPs-DOM-POs), MPs-DOM derived from benzene-containing polymers (MPs-DOM-BCPs), and Suwannee River natural organic matter (SR-NOM), during a 168-hour phototransformation. After phototransformation, all examined types of DOM exhibit a decrease in concentration and molecular weight. Notably, MPs-DOM-POs display increased aromaticity and saturation, while MPs-DOM-BCPs and SR-NOM show reduced aromaticity and saturation. MPs-DOM-POs present higher steady-state concentrations of •OH but much lower steady-state concentrations of 1O2 than those of MPs-DOM-BCPs. In comparison, MPs-DOM produce more •OH but less 1O2 than SR-NOM. This study proposes that the diversification of aliphatic C─H bonds (arylation and carbonylation) by reactive intermediates (especially •OH) is the main pathway for MPs-DOM-POs phototransformation for the first time. On the other hand, the cleavage on the aromatic carboxylic acids by reactive intermediates (especially 1O2) is the main mechanism for MPs-DOM-BCPs and SR-NOM phototransformation. Our findings provide new insights into the phototransformation and photoreactivity of MPs-DOM and help to understand the potential risks of MPs in aqueous environment.

Dielectric Barrier Corona Activation of Electrical Discharge in a Cavitating Liquid

Water treatment methods based on cold plasma discharge in cavitating liquid have been actively developing in recent years. However, some conditions, such as the conductivity of the medium, can limit the possibility of plasma ignition. The authors proposed a new method for activating an electric discharge in a cavitating liquid environment based on the use of an external corona discharge electrode in the plasma reactor. It has been experimentally shown that, in such a configuration, the breakdown voltage is significantly reduced. A theoretical analysis of the process was carried out, and a modified Paschen's curve was constructed on the basis of experimental data. The following graph shows the basic diagram of the setup and plasma reactor: 1, input water tank; 2, pump; 3, reactor; 4, generator; and 5, output tank. "Gap 0" expresses the gap between the two ring electrodes, and "gap" expresses the gap between the corona electrode and the lower ring electrode.

Photo-assisted conversion of tetracycline in regulated persulfate system: Multiple roles of natural dissolved organic matter

Advanced oxidation processes (AOPs) using persulfate system can effectively remove organic pollutants. However, dissolved organic matter (DOM) has multiple effects on AOPs efficiency, and the influence of DOMs from natural sources on AOPs is still unclear. In this study, we explored the effects of soil DOM (SDOM) and fertilizer DOM (FDOM) on tetracycline (TC) removal by persulfate systems. DOMs introduction decreased light transmittance, slightly increased the pH of the systems, and destroyed original adsorption-desorption equilibrium. SDOM promoted most reactive species generation in the initial stage, thus improving the initial TC degradation rate. However, introduction of SDOM and FDOM increased the final TC residual rate. FDOM produced more obvious inhibitory effects on TC degradation. The final TC residual rates in systems containing 7.5 and 15 mg L-1 FDOM (F7.5-TC-PS and F15-TC-PS, respectively) were 25.85 % and 25.52 %, respectively. The inhibitory effects of FDOM on TC degradation were related to the combination between TC and FDOM, with humic acid-like component in FDOM being the main contributor. Besides, the main components in DOMs underwent transformation in the persulfate systems. This study sought to provide insights into the regulatory effects of DOM on TC photo-assisted conversion by AOPs.

Percarbonate-periodate system: A novel and efficient "oxidant-oxidant" strategy for selective oxidation of micropollutants in water

The development of effective and selective oxidation technology in complex water matrices is crucial for water ecological security. This study reports for the first time the synergistic use of "oxidant-oxidant" about sodium percarbonate (SPC) and periodate (PI) to selectively degrade organic micropollutants. The SPC/PI system showed degradation rates of 0.0946-0.2978 min-1 for various pollutants, which was 3.7-1787 times higher than those in the PI alone and SPC alone systems and can achieve the effect of H2O2/PI systems. Additionally, SPC/PI was a safe water treatment technology without generating reactive iodine species (e.g., HOI). The slightly reduced removal rate of bisphenol F under different ionic species and strengths is indicative of the good anti-interference of the SPC/PI system. Scavenging, probe, and electron spin resonance experiments showed that ▪OH and CO3▪- played a major role in this process, rather than O2▪- and 1O2. Finally, the oxidized products and the possible transformation pathways of three different micropollutants in the SPC/PI and H2O2/PI systems were characterized and clarified, and the toxicity of the degradation products was predicted. Generally, the study proposed a new selective oxidation strategy of SPC/PI that can effectively eliminate micropollutants in water treatment and clarified the interaction mechanisms between PI and SPC.

Photochemical behavior of dissolved organic matter in environmental surface waters: A review

As an important group of widespread organic substances in aquatic ecosystems, dissolved organic matter (DOM) plays an essential role in carbon recycling and transformation processes. The photochemical behavior of DOM is one of the main ways it participates in these processes, and it attracts extensive attention. However, due to a variety of sources and water conditions, including both freshwater and seawater environments, the photochemical properties of DOM exhibit great differences. Nowadays, a large number of studies have focused on the generation process of reactive species (RS) from sunlit DOM, while little effort has been made so far to provide a comprehensive summary of the photochemical behavior of DOM, especially in fresh and saline aquatic ecosystems. In this review, we analyzed the research hotspot on DOM photochemistry over the last 30 years, summarizing the generation of photoreactive species in natural water environments containing DOM (both freshwater and seawater) and listing the main factors affecting the rate, yield, and species of RS photoproduction. Compared with freshwater, seawater has unique characteristics such as high pH value, high ionic strength, and halide ions, which affect the photogeneration of RS, the photoconversion process, as well as the reaction pathways of various environmental substances. In general, DOM-induced surface water photochemistry has important impacts on the environmental transformation and toxic effects of aquatic pollutants and can even contribute significantly to the Earth's carbon cycle, which would have potential implications for both human and ecological health.

Co-catalysis strategy for low-oxidant-consumption Fenton-like chemistry: From theoretical understandings to practical applications and future guiding strategies

Recently, great effects have been made for the co-catalysis strategy to solve the bottlenecks of Fenton system. A series of co-catalysis strategies using various inorganic metal co-catalysts and organic co-catalysts have been developed in various oxidant (i.e., hydrogen peroxide (H2O2) and persulfate) systems with significantly promotion of catalytic performances and lower oxidant consumption (only 5-10 % of conventional Fenton/Fenton-like systems). However, the developments of these co-catalysis strategies from theoretical understandings to practical applications and future guiding strategies were overlooked, which was an essential problem that must be considered for the future scale-up applications of co-catalysis systems. In this paper, these co-catalysis strategies with low-oxidant-consumption characteristics have been reviewed by the comparison of their co-catalysis mechanisms, as well as their advantages and disadvantages. We also discussed the recent developments of amplifying devices based on the co-catalysis systems. The scale-up performances of co-catalysis strategies based on these amplifying devices have also been assessed. In addition, future guiding strategies for the development of co-catalysis strategy with low-oxidant-consumption characteristics have also been first time outlined by the combination of the technical-economic analysis (TEA), life cycle assessment (LCA) and machine learning (ML). Finally, the paper systematically discusses the development opportunities, technical bottlenecks and future development directions of co-catalysis strategies with the prospect of large-scale applications. Basically, this work provides a systematic review on co-catalysis strategy with low-oxidant-consumption characteristic from theoretical understandings to practical applications and future guiding strategies.

Treatment of sludge hydrothermal carbonization wastewater by ferrous/sodium percarbonate system: Effect of wastewater composition and role of coagulation and oxidation

It is crucial to explore the effect of complex wastewater compositions on the ferrous/sodium percarbonate (Fe(Ⅱ)/SPC) system and the role of oxidation-coagulation in designing water treatment processes. This study employed redundancy analysis to investigate the effects of wastewater constituents on oxidation and coagulation. Raman analysis, X-ray Photoelectron Spectroscopy, and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry were used to determine the roles of oxidation and coagulation in the system. The results showed that sulfates and phosphates formed amorphous complexes with iron species via coprecipitation, thereby promoting coagulation to remove organics. Some heavy metals can also be removed by coagulation. The co-activation of SPC by pre-existing transition metals and the added Fe(Ⅱ) facilitated the oxidative removal of organics, while chloride and arsenic were the main inhibitory inorganic substances in the system. Aromatic compounds mainly promoted coagulation, polysaccharides promoted oxidation, humic acid promoted oxidation and coagulation, and C=C/C=O inhibited the Fe(Ⅱ)/SPC system. The oxidation process removed graphitic structures and unsaturated organic matter in the region of (O/C, H/C) = (0.2-0.4, 0.9-2.0) through free radicals and generated amorphous carbon structures and saturated organic matter in the region of (O/C, H/C) = (0.3-0.7, 1.2-1.9). The coagulation process removed aromatic organics with 2-5 rings and unsaturated organics in the region of (O/C, H/C) = (0.2-0.6, 0.7-1.6) with oxygen-containing organics. The combined effects of coagulation and oxidation enhanced the removal efficiency of organic carbon by approximately 40%. This study facilitates the optimization of hydrothermal carbonization wastewater treatment and advanced oxidation processes.

Reactive Nitrogen Species Generated from Far-UVC Photolysis of Nitrate Contribute to Pesticide Degradation and Nitrogenous Byproduct Formation

Climate change has resulted in increased use of pesticides and fertilizers in agriculture, leading to elevated pesticide and nitrate levels in aquatic ecosystems that receive agricultural runoff. In this study, we demonstrate that far-UVC (UV222) photolysis of nitrate rapidly degrades four pesticides in surface water, with a degradation rate constant 37.1-144.75 times higher than that achieved by UV254 photolysis of nitrate. The improved pesticide degradation is due not only to the enhanced direct photolysis by UV222 compared to UV254 but also to the increased generation of hydroxyl radicals (HO•) and reactive nitrogen species (e.g., NO2• and ONOO-) in the UV222/nitrate process. We determined the innate quantum yields of nitrate photolysis at 222 nm and incorporated these values into a kinetic model, allowing for the accurate prediction of nitrate photodecay and reactive species generation. While reactive nitrogen species predominantly contribute to pesticide degradation in the UV222/nitrate process, they also lead to the formation of nitration byproducts. Using stable isotope-labeled nitrate (15NO3-) combined with mass spectrometry, we confirmed that the nitration byproducts are formed from the reactive nitrogen species generated from nitrate photolysis. Additionally, we demonstrate that the UV222/nitrate process increases the formation potential of highly toxic nitrogenous chlorinated products (e.g., trichloronitromethane) during postchlorination in real surface water.

Nano-confined catalysis with Co nanoparticles-encapsulated carbon nanotubes for enhanced peroxymonosulfate oxidation in secondary effluent treatment: Water quality improvement and membrane fouling alleviation

Despite widespread deployment and investigation of ultrafiltration (UF) for secondary effluent purification, the challenge of membrane fouling due to effluent organic matter (EfOM) remains formidable. This study introduced a novel pretreatment method utilizing Co nanoparticles-encapsulated carbon nanotubes activated peroxymonosulfate (Co@CNT/PMS) to degrade EfOM and mitigate membrane fouling. Characterization of Co@CNT revealed the efficient encapsulation of Co nanoparticles within nanotubes, which notably enhanced the catalytic degradation of bisphenol A and typical organics. The tube-encapsulated structure increased the concentration of reactive species within the confined nanoscopic space, thereby improving the probability of collisions with pollutants and promoting their degradation. The Co@CNT/PMS pretreatment led to substantial reductions in aromatic compounds, fluorescent components, and both high and middle molecular weight substances. These changes proved crucial in diminishing the fouling potential in subsequent UF processes, where reversible and irreversible fouling resistances decreased by 97.1 % and 72.8 %, respectively. The transition volume from pore blocking to cake filtration markedly increased, prolonging the formation of a dense fouling layer. Surface properties analysis indicated a significant reduction of pollutants on membrane surfaces after the Co@CNT/PMS pretreatment. This study underscored the efficacy of confinement-based advanced oxidization pretreatment in enhancing UF performance, presenting a viable resolution to membrane fouling.

Combination of oxidative and reductive effects of phenolic compounds on the degradation of aniline disinfection by-products by free radicals

In this study, we selected 13 phenolic compounds containing -COOH, -CHO, -OH, and -COCH3 functional groups as model compounds for dissolved organic matter (DOM), and explored the redox reactions during the co-degradation of phenolic compounds with aniline disinfection by-products (DBPs) at the molecular level. When phenolic compounds and aniline DBPs were degraded, phenoxy radicals and aniline radicals were the most important intermediates. Phenoxy radicals can degrade aniline DBPs via hydrogen atom abstraction (HAA) reactions, and the reaction rates were related to the reduction potentials of the compounds. Compounds containing electron-withdrawing groups were more likely to oxidize aniline DBPs. Aniline DBPs were more easily degraded by phenoxy radicals when they contained electron-donating groups, and the increase in the number of chlorine atoms inhibited the reaction rates of aniline DBPs degradation by phenoxy radicals. Although phenolic compounds can reduce aniline DBPs, there was no significant correlation between the reaction rates and the reduction potentials of the compounds. Considering the redox effects of phenolic compounds on aniline DBPs, co-degradation simulations showed that phenolics inhibited the degradation efficiency of aniline DBPs. This work provided new insights into the transformation mechanisms and degradation efficiencies of DOM and aniline DBPs when they were co-degraded.

The reverse-reduction effect of dissolved organic matter on the degradation of micropollutants induced by halogen radicals (Cl2•- and Br2•-)

Reactive halogen radicals (e.g., Cl2•- and Br2•-) greatly impact the degradation of micropollutants in natural waters and engineered water treatment systems. The ubiquitous dissolved organic matter (DOM) in real waters is known to greatly inhibit the degradation of micropollutants by reducing micropollutant's intermediate (i.e., TC•+/TC(-H)•), however, such DOM's effects on the halogen-radical-induced system have not been understood yet. The present study focuses on investigating and quantifying such inhibitory effects of DOM during Cl2•-- and Br2•--mediated process. Guanosine (Gs) was selected as a model compound. The transient spectra show that Cl2•- and Br2•- react with Gs generating intermediates (i.e., Gs•+/Gs(-H)•) via single-electron transfer. In the presence of 1.0 mgCL-1 DOM, over 70% of this oxidized Gs was reduced back to Gs. Comparing the extent of reverse-reduction inhibitory among different reaction systems, this inhibitory in Br2•- system was slightly lower than that in Cl2•- and SO4•- system, corresponding the slightly difference of inhibition factor (IF) values as SO4•- < Cl2•- < Br2•-. The reverse-reduction effect of DOM was further quantified for 19 common micropollutants. It varied significantly with IF values of 0.21-1.26 and 0.28-1.40 in Cl2•-- and Br2•--mediated process, respectively. Purines and amines generally exhibited more pronounced inhibition than phenols in both systems. A good correlation of IF values with micropollutant's reduction potential was observed, which can be applied to predict the degradation of more unstudied micropollutants. This study highlights the important role of the reverse-reduction effect of DOM on micropollutant degradation. It can significantly improve the accuracy in predicting degradation rate in advanced oxidation processes for treating water containing halides.

Regulating the Electronic Structure of Cu Single-Atom Catalysts toward Enhanced Electro-Fenton Degradation of Organic Contaminants via 1O2 and •OH

Heterogeneous electro-Fenton degradation with 1O2 and •OH generated from O2 reduction is cost-effective for the removal of refractory organic pollutants from wastewater. As 1O2 is more tolerant to background constituents such as salt ions and a high pH value than •OH, tuning the production of 1O2 and •OH is important for efficient electro-Fenton degradation. However, it remains a great challenge to selectively produce 1O2 and improve the species yield. Herein, the electronic structure of atomically dispersed Cu-N4 sites was regulated by doping electron-deficient B into porous hollow carbon microspheres (CuBN-HCMs), which improved *O2 adsorption and significantly enhanced 1O2 selectivity in electro-Fenton degradation. Its 1O2 yield was 2.3 times higher than that of a Cu single-atom catalyst without B doping. Meanwhile, •OH was simultaneously generated as a minor species. The CuBN-HCMs were efficient for the electro-Fenton degradation of phenol, sulfamethoxazole, and bisphenol A with a high mineralization efficiency. Its kinetic constants showed insignificant changes under various anions and a wide pH range of 1-9. More importantly, it was energy-efficient for treating actual coking wastewater with a low energy consumption of 19.0 kWh kgCOD-1. The superior performance of the CuBN-HCMs was contributed from 1O2 and •OH and its high 1O2 selectivity.

The Dual Role of Natural Organic Matter in the Degradation of Organic Pollutants by Persulfate-Based Advanced Oxidation Processes: A Mini-Review

Persulfate-based advanced oxidation processes (PS-AOPs) are widely used to degrade significant amounts of organic pollutants (OPs) in water and soil matrices. The effectiveness of these processes is influenced by the presence of natural organic matter (NOM), which is ubiquitous in the environment. However, the mechanisms by which NOM affects the degradation of OPs in PS-AOPs remain poorly documented. This review systematically summarizes the dual effects of NOM in PS-AOPs, including inhibitory and promotional effects. It encompasses the entire process, detailing the interaction between PS and its activators, the fate of reactive oxygen species (ROS), and the transformation of OPs within PS-AOPs. Specifically, the inhibiting mechanisms include the prevention of PS activation, suppression of ROS fate, and conversion of intermediates to their parent compounds. In contrast, the promoting effects involve the enhancement of catalytic effectiveness, contributions to ROS generation, and improved interactions between NOM and OPs. Finally, further studies are required to elucidate the reaction mechanisms of NOM in PS-AOPs and explore the practical applications of PS-AOPs using actual NOM rather than model compounds.

Confronting the Mysteries of Oxidative Reactive Species in Advanced Oxidation Processes: An Elephant in the Room

Advanced oxidation processes (AOPs) are rapidly evolving but still lack well-established protocols for reliably identifying oxidative reactive species (ORSs). This Perspective presents both the radical and nonradical ORSs that have been identified or proposed, along with the extensive controversies surrounding oxidative mechanisms. Conventional identification tools, such as quenchers, probes, and spin trappers, might be inadequate for the analytical demands of systems in which multiple ORSs coexist, often yielding misleading results. Therefore, the challenges of identifying these complex, short-lived, and transient ORSs must be fully acknowledged. Refining analytical methods for ORSs is necessary, supported by rigorous experiments and innovative paradigms, particularly through kinetic analysis based on in situ spectroscopic techniques and multiple-probe strategies. To demystify these complex ORSs, future efforts should be made to develop advanced tools and strategies to enhance the mechanism understanding. In addition, integrating real-world conditions into experimental designs will establish a reliable framework in fundamental studies, providing more accurate insights and effectively guiding the design of AOPs.

Reactive Oxygen Species Generated in Situ During Carbamazepine Photodegradation at 222 nm Far-UVC: Unexpected Role of H2O Molecules

When 222 nm far-UVC is used to drive AOPs, photolysis emerges as a critical pathway for the degradation of numerous organic micropollutants (OMPs). However, the photodegradation mechanisms of the asymmetrically polarized OMPs at 222 nm remain unclear, potentially posing a knowledge barrier to the applications of far-UVC. This study selected carbamazepine (CBZ), a prevalent aquatic antiepileptic drug that degrades negligibly at 254 nm, to investigate its photodegradation mechanisms at 222 nm. Accelerated CBZ treatment by 222 nm far-UVC was mainly attributed to in situ ROS generation via self-sensitized photodegradation of CBZ. By quenching experiments and EPR tests, •OH radicals were identified as the major contributor to the CBZ photodegradation, whereas O2•- played a minor role. By deoxygenation and solvent exchange experiments, the H2O molecules were demonstrated to play a crucial role in deactivating the excited singlet state of CBZ (1CBZ*) at 222 nm: generating •OH radicals via electron transfer interactions with 1CBZ*. In addition, 1CBZ* could also undergo a photoionization process. The transformation products and pathways of CBZ at 222 nm were proposed, and the toxicities of CBZ's products were predicted. These findings provide valuable insights into OMPs' photolysis with 222 nm far-UVC, revealing more mechanistic details for far-UVC-driven systems.

Critical review of fluorescence and absorbance measurements as surrogates for the molecular weight and aromaticity of dissolved organic matter

Dissolved organic matter (DOM) is ubiquitous in aquatic environments and challenging to characterize due to its heterogeneity. Optical measurements (i.e., absorbance and fluorescence spectroscopy) are popular characterization tools, because they are non-destructive, require small sample volumes, and are relatively inexpensive and more accessible compared to other techniques (e.g., high resolution mass spectrometry). To make inferences about DOM chemistry, optical surrogates have been derived from absorbance and fluorescence spectra to describe differences in spectral shape (e.g., E2:E3 ratio, spectral slope, fluorescence indices) or quantify carbon-normalized optical responses (e.g., specific absorbance (SUVA) or specific fluorescence intensity (SFI)). The most common interpretations relate these optical surrogates to DOM molecular weight or aromaticity. This critical review traces the genesis of each of these interpretations and, to the extent possible, discusses additional lines of evidence that have been developed since their inception using datasets comparing diverse DOM sources or strategic endmembers. This review draws several conclusions. More caution is needed to avoid presenting surrogates as specific to either molecular weight or aromaticity, as these physicochemical characteristics are often correlated or interdependent. Many surrogates are proposed using narrow contexts, such as fractionation of a limited number of samples or dependence on isolates. Further study is needed to determine if interpretations are generalizable to whole-waters. Lastly, there is a broad opportunity to identify why endmembers with low abundance of aromatic carbon (e.g., effluent organic matter, Antarctic lakes) often do not follow systematic trends with molecular weight or aromaticity as observed in endmembers from terrestrial environments with higher plant inputs.

Ferrate (VI) oxidation of sulfamethoxazole enhanced by magnetized sludge-based biochar: Active sites regulation and degradation mechanism analysis

Developing non radical systems for antibiotic degradation is crucial for addressing the inefficiency of conventional radical systems. In this study, novel magnetic-modified sludge biochar (MASBC) was synthesized to significantly enhance the oxidative degradation of sulfamethoxazole (SMX) by ferrate (Fe (VI)). In the Fe (VI)/MASBC system, 90.46% of SMX at a concentration of 10 μM and 49.34% of the total organic carbon (TOC) could be removed under optimal conditions of 100 μM of Fe (VI) and 0.40 g/L of MASBC within 10 min. Furthermore, the Fe (VI)/MASBC system was demonstrated with broad-spectrum removal capability towards sulfonamides in single or mixture. Quenching experiments, EPR analyses, and electrochemical experiments revealed that direct electron transfer (DET) and •O2- were mainly responsible for the removal of SMX, with functional groups (e.g., -OH, C=O) and Fe-O (redox of Fe (III)/Fe (II)) acting as the active sites, while the probe experiments showed that Fe (IV)/Fe (V) made a minor contribution to the degradation of SMX. Benefiting from the DET, the Fe (VI)/MASBC system exhibited a wide pH adaptation range (e.g., from 5.0 to 10.0) and strong anti-interference ability. The N atoms and their neighboring atoms in SMX were the prior degradation sites, with the cleavage of bond and ring opening. The degradation products showed low or non-toxicity according to ECOSAR program assessment. The removal of SMX remained within a reasonable range of 71.33%-90.46% over five consecutive cycles. Also, the Fe (VI)/MASBC system was demonstrated to be effectively applied for successful SMX removal in various water matrices, including ultrapure water, tap water, lake water, Yangtze River water, and wastewater. Therefore, this study offered new insights into the mechanism of Fe (VI) oxidation and would contribute to the efficient treatment of organic pollutants.

Efficient degradation of bisphenol A and amino black 10B by magnetic composite Fe3O4@MOF-74 as catalyst

Metal-organic frameworks (MOFs) exhibit high chemical stability and porosity, and have been widely applied in various fields including selective adsorption and separation, sensors, and catalysis. When combined with Fe3O4, they effectively address issues such as aggregation of Fe3O4 particles and the difficulty in recovering MOFs as catalysts. Therefore, in this study, we used a simple solvothermal method as a catalyst to synthesize a high specific surface area magnetic composite Fe3O4@MOF-74, which was used to catalyze the degradation of bisphenol A (BPA) and amino black 10B in wastewater. We activated Na2S2O8 to generate radicals for oxidizing and degrading BPA and amino black 10B. Experimental results showed that at 35 °C, with Fe3O4@MOF-74 (Fe3O4: MOF-74=1:1) concentration of 0.2 g/L and Na2S2O8 concentration of 2 g/L, the catalytic effect is efficient and economical. Meanwhile, removal rates of BPA and amino black 10B exceeded 95.58 % over a broad pH range (pH 3-9). Furthermore, even after multiple cycles of use, Fe3O4@MOF-74 maintained catalytic degradation rates of BPA and amino black 10B above 93.24 % and 95.01 %, respectively. Additionally, in water samples, removal rates of BPA and amino black 10B exceeded 91.55 %. This study provides a new and efficient catalyst material for wastewater treatment, which is expected to play an important role in environmental remediation.

In Situ Production of Hydroxyl Radicals via Three-Electron Oxygen Reduction: Opportunities for Water Treatment

The electro-Fenton (EF) process is an advanced oxidation technology with significant potential; however, it is limited by two steps: generation and activation of H2O2. In contrast to the production of H2O2 via the electrochemical two-electron oxygen reduction reaction (ORR), the electrochemical three-electron (3e-) ORR can directly activate molecular oxygen to yield the hydroxyl radical (⋅OH), thus breaking through the conceptual and operational limitations of the traditional EF reaction. Therefore, the 3e- ORR is a vital process for efficiently producing ⋅OH in situ, thus charting a new path toward the development of green water-treatment technologies. This review summarizes the characteristics and mechanisms of the 3e- ORR, focusing on the basic principles and latest progress in the in situ generation and efficient utilization of ⋅OH through the modulation of the reaction pathway, shedding light on the rational design of 3e- ORR catalysts, mechanistic exploration, and practical applications for water treatment. Finally, the future developments and challenges of efficient, stable, and large-scale utilization of ⋅OH are discussed based on achieving optimal 3e- ORR regulation and the potential to combine it with other technologies.

Nitrite facilitated transformation of halophenols in ice

This study investigated the expediated transformation of halophenols in the presence of nitrite (NO2-) under slightly acidic conditions in ice, whereas such transformation was negligible in liquid water at 4 °C. We proposed that this phenomenon was attributed to the freeze-concentration effect, incurring a pH drop and the aggregation of NO2- and halophenols within the liquid-like grain boundary layer amid ice crystals. Within this micro-environment, NO2- underwent protonation to generate reactive nitrous acid (HNO2) and nitrosonium ions (NO+) that facilitate the nitration and oxidation of halophenols. When 10 μМ halophenol was treated by freezing in the presence of 5 μМ NO2-, the total yields of nitrated products reached 2.4 μМ and 1.4 μМ within 12 h for 2-chlorophenol (2CP) and 2-bromophenol (2BP), respectively. NO+ drove oxidative coupling reactions, generating hydroxyl polyhalogenated diphenyl ethers (OH-PBDEs) and hydroxyl polyhalogenated diphenyls via C-O or C-C coupling. These two pathways were intricately intertwined. The presence of natural organic matter (NOM) mitigated the formation of nitrated products and completely suppressed the coupling products. This study offers valuable insights into the fate of halophenols in ice and suggests potential pathways for the formation of nitrophenolic compounds and OH-PBDEs in natural cold environments. These findings also open up a new avenue in environmental chemistry research.

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.

Emerging eco-friendly technologies for remediation of Per- and poly-fluoroalkyl substances (PFAS) in water and wastewater: A pathway to environmental sustainability

Per- and polyfluoroalkyl substances (PFAS) are rampant, toxic contaminants from anthropogenic sources, called forever chemicals for their recalcitrance. Although banned in several parts of the world for public health implications, including liver, kidney, and testicular diseases, PFAS are abundant in water sources due to easy dispersion. With chemical properties resulting from strong hydrophobic bonds, they defile many physicochemical removal methods. Though adsorption processes such as granular activated carbon (GAC) are widely used, they are marred by several limitations, including cost and secondary contamination. Thus, eco-friendly methods involving a synergy of the removal principles have been preferred for ease of use, cost-effectiveness, and near-zero effect on the environment. We present novel eco-friendly methods as the solution to PFAS remediation towards environmental sustainability. Current eco-friendly methods of PFAS removal from water sources, including electrocoagulation, membrane/filtration, adsorption, and phytoremediation methods, were highlighted, although with limitations. Novel eco-friendly methods such as microbial fuel cells, photoelectrical cells, and plasma treatment offer solutions to PFAS remediation and are quite efficient in terms of cost, result, and environmental sustainability. Overall, the successful integration of eco-friendly techniques in a seamless manner ensures the desired result. We also present a balanced position on the ecosystem impact of these ecofriendly methods, noting the successes towards environmental sustainability while exposing the gaps for further research.

Overlooked Roles and Transformation of Carbon-Centered Radicals Produced from Natural Organic Matter in a Thermally Activated Persulfate System

The presence and induced secondary reactions of natural organic matter (NOM) significantly affect the remediation efficacy of in situ chemical oxidation (ISCO) systems. However, it remains unclear how this process relates to organic radicals generated from reactions between the NOM and oxidants. The study, for the first time, reported the vital roles and transformation pathways of carbon-centered radicals (CCR•) derived from NOM in activated persulfate (PS) systems. Results showed that both typical terrestrial/aquatic NOM isolates and collected NOM samples produced CCR• by scavenging activated PS and greatly enhanced the dehalogenation performance under anoxic conditions. Under oxic conditions, newly formed CCR• could be oxidized by O2 and generate organic peroxide intermediates (ROO•) to catalytically yield additional •OH without the involvement of PS. Nuclear magnetic resonance (NMR) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) results indicated that CCR• predominantly formed from carboxyl and aliphatic structures instead of aromatics within NOM through hydrogen abstraction and decarboxylation reactions by SO4•- or •OH. Specific anoxic reactions (i.e., dehalogenation and intramolecular cross-coupling reactions) further promoted the transformation of CCR• to more unsaturated and polymerized/condensed compounds. In contrast, oxic propagation of ROO• enhanced bond breakage/ring cleavage and degradation of CCR• due to the presence of additional •OH and self-decomposition. This study provides novel insights into the role of NOM and O2 in ISCO and the development of engineered strategies for creating organic radicals capable of enhancing the remediation of specific contaminants and recovering organic carbon.

Microplastic-derived dissolved organic matter: Generation, characterization, and environmental behaviors

Microplastics (MPs) represent a substantial and emerging class of pollutants distributed widely in various environments, sparking growing concerns about their environmental impact. In environmental systems, dissolved organic matter (DOM) is crucial in shaping the physical, chemical, and biological processes of pollutants while significantly contributing to the global carbon budget. Recent findings have revealed that microplastic-derived dissolved organic matter (MP-DOM) constitutes approximately 10 % of the DOM present on the ocean surface, drawing considerable attention. Hence, this study's primary objective is to explore, the generation, characterization, and environmental behaviors of MP-DOM. The formation and characteristics of MP-DOM are profoundly influenced by leaching conditions and types of MPs. This review delves into the mechanisms of the generation of MP-DOM and provides an overview of a wide array of analytical techniques, including ultraviolet-visible (UV-Vis) spectroscopy, fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR), and mass spectroscopy, used to assess the MP-DOM characteristics. Furthermore, this review investigates the environmental behaviors of MP-DOM, including its impacts on organisms, photochemical processes, the formation of disinfection by-products (DBPs), adsorption behavior, and its interaction with natural DOM. Finally, the review outlines research challenges, perspectives for future MP-DOM research, and the associated environmental implications.

Inhibit or promote? Trade-off effect of dissolved organic matter on the laccase-mediator system

A biocatalytic system comprising fungal laccase and mediators can generate phenol radicals and efficiently eliminate various triarylmethane dyes. This study systematically explores the kinetic impact of dissolved organic matter (DOM), represented by humic substance (HS consisting of 90% fulvic acid, from lignite), on the decolorization of seven typical triarylmethane dyes by Trametes versicolor laccase and twenty natural mediators. Among these, 4-hydroxybenzyl alcohol (4-HA) and methyl violet (MV) undergo in-depth investigation regarding degradation products, pathways, and reaction mechanisms. In instances where HS hampers laccase-alone decolorization, such as malachite green, Coomassie brilliant blue, bromophenol blue, and acid magenta, this inhibition may persist despite mediator introduction. Conversely, in cases where HS facilitates decolorization, such as crystalline violet and ethyl violet, most laccase-mediator systems (LMSs) can still benefit. For MV decolorization by laccase and 4-HA, HS's kinetic effect is controlled by concentration and reaction time. A 5 mg/L HS increased the decolorization rate from 50% to 67% within the first hour, whereas 10 mg/L HS only achieved 45%. After 16 h of reaction, HS's impact on decolorization rate diminishes. Furthermore, the addition of HS enhances precipitation production, probably due to its involvement in polymerization with MV and mediator. Computational simulations and spectral monitoring reveal that low HS concentrations accelerate laccase-mediated demethylation by disrupting the chromophores bound to MV, thus promoting the decolorization of MV. Conversely, inhibition by high HS concentrations stems from the competitive binding of the enzyme pocket to the mediator, and the reduction of phenol free radicals in the system. Molecular docking and kinetic simulations revealed that laccase forms complexes with both the mediator and MV. Interestingly, the decolorization of MV occurred through a non-radical mechanism in the presence of HS. This work provided a reference for screening of high catalytic performance mediators to remove triarylmethane dyes in the actual water environment.

Photo-transformation of isoproturon under UV-A irradiation: The synergy of nitrite and natural organic matter

Isoproturon (IPU), a widely utilized phenylurea herbicide, is recognized as an emerging contaminant. Previous studies have predominantly attributed the degradation of IPU in natural waters to indirect photolysis by natural organic matter (NOM). Here, we demonstrate that nitrite (NO2-) also serves as an important photosensitizer that induces the photo-degradation of IPU. Through radical quenching tests, we identify hydroxyl radicals (•OH) and nitrogen dioxide radicals (NO2•) originating from NO2- photolysis as key players in IPU degradation, resulting in the generation of a series of hydroxylated and nitrated byproducts. Moreover, we demonstrate a synergistic effect on the photo-transformation of IPU when both NOM and NO2- are present in the reaction mixture. The observed rate constant (kobs) for IPU removal increases to 0.0179 ± 0.0002 min-1 in the co-presence of NO2- (50 μM) and NOM (2.5 mgC/L), surpassing the sum of those in the presence of each alone (0.0135 ± 0.0004 min-1). NOM exhibits multifaceted roles in the indirect photolysis of IPU. It can be excited by UV and transformed to excited triplet states (3NOM*) which oxidize IPU to IPU•+ that undergoes further degradation. Simultaneously, NOM can mitigate the reaction by reducing the IPU•+ intermediate back to the parent IPU. However, the presence of NO2- alters this dynamic, as IPU•+ rapidly couples with NO2•, accelerating IPU degradation and augmenting the formation of mono-nitrated IPU. These findings provide in-depth understandings on the photochemical transformation of environmental contaminants, especially phenylurea herbicides, in natural waters where NOM and NO2- coexist.

Mechanisms and Strategies of Advanced Oxidation Processes for Membrane Fouling Control in MBRs: Membrane-Foulant Removal versus Mixed-Liquor Improvement

Membrane bioreactors (MBRs) are well-established and widely utilized technologies with substantial large-scale plants around the world for municipal and industrial wastewater treatment. Despite their widespread adoption, membrane fouling presents a significant impediment to the broader application of MBRs, necessitating ongoing research and development of effective antifouling strategies. As highly promising, efficient, and environmentally friendly chemical methods for water and wastewater treatment, advanced oxidation processes (AOPs) have demonstrated exceptional competence in the degradation of pollutants and inactivation of bacteria in aqueous environments, exhibiting considerable potential in controlling membrane fouling in MBRs through direct membrane foulant removal (MFR) and indirect mixed-liquor improvement (MLI). Recent proliferation of research on AOPs-based antifouling technologies has catalyzed revolutionary advancements in traditional antifouling methods in MBRs, shedding new light on antifouling mechanisms. To keep pace with the rapid evolution of MBRs, there is an urgent need for a comprehensive summary and discussion of the antifouling advances of AOPs in MBRs, particularly with a focus on understanding the realizing pathways of MFR and MLI. In this critical review, we emphasize the superiority and feasibility of implementing AOPs-based antifouling technologies in MBRs. Moreover, we systematically overview antifouling mechanisms and strategies, such as membrane modification and cleaning for MFR, as well as pretreatment and in-situ treatment for MLI, based on specific AOPs including electrochemical oxidation, photocatalysis, Fenton, and ozonation. Furthermore, we provide recommendations for selecting antifouling strategies (MFR or MLI) in MBRs, along with proposed regulatory measures for specific AOPs-based technologies according to the operational conditions and energy consumption of MBRs. Finally, we highlight future research prospects rooted in the existing application challenges of AOPs in MBRs, including low antifouling efficiency, elevated additional costs, production of metal sludge, and potential damage to polymeric membranes. The fundamental insights presented in this review aim to elevate research interest and ignite innovative thinking regarding the design, improvement, and deployment of AOPs-based antifouling approaches in MBRs, thereby advancing the extensive utilization of membrane-separation technology in the field of wastewater treatment.

New insight into wastewater treatment by activation of sulfite with humic acid under visible light irradiation

Sulfite (S(IV)), as an alternative to persulfate, has demonstrated its cost-effectiveness and environmentally friendly nature, garnering increasing attention in Advanced Oxidation Processes (AOPs). Dissolved organic matter (DOM) commonly occurred in diverse environments and was often regarded as an interfering factor in sulfite-based AOPs. However, less attention has been paid to the promotion of the activation of sulfite by excited DOM, which could produce various reactive intermediates. The study focused on the activation of sulfite using visible light (VL) - excited humic acid (HA) to efficiently degrade many common organic pollutants, which was better than peroxydisulfate (PDS) and peroxymonosulfate (PMS) systems. Quenching experiments and electron paramagnetic resonance (EPR) analysis revealed that the triplet states of HA (3HA*) activated sulfite through energy transfer, resulting in the production of SO4·-, O2·-, and 1O2. The most significant active species found in the degradation of roxarsone (ROX) was 1O2, which was a non-radical pathway and exhibits high selectivity for pollutant degradation. This non-radical pathway was not commonly observed in traditional sulfite-based AOPs. Additionally, the coexistence of various inorganic anions, such as NO3-, Cl-, SO42-, CO32-, and PO43-, had little effect on the degradation of ROX. Furthermore, DOM from different natural water demonstrated efficient activation of S(IV) under light conditions, opening up new possibilities for applying sulfite-based advanced oxidation to the remediation of organic pollution in diverse sites and water bodies. In summary, this research offered promising insights into the potential application of sulfite-based AOPs, facilitated by photo-excited HA, as a new strategy for efficiently degrading organic pollutants in various environmental settings.

pH-modulated oxidation of organic pollutants for water decontamination: A deep insight into reactivity and oxidation pathway

Solution pH is one of the primary factors affecting the efficiency of water decontamination. Although the influence of pH on oxidants activation, catalyst activity, and reactive oxygen species have been widely explored, there is still a scarcity of systemic studies on the changes in the oxidation behavior of organic pollutants at different pH levels. Herein, we report the influence laws of pH on the forms, reactivities, active sites, degradation pathways, and products toxicities of organic pollutants. Changes in pH cause the protonation or deprotonation of organic pollutants and further affect their forms and chemistry (e.g., electrostatic force, hydrophobicity, and oxidation potential). The oxidation potential of organic pollutants follows the order: protonated form > pristine form > deprotonated form. Moreover, protonation or deprotonation can modify the active sites and degradation pathways of organic pollutants, wherein deprotonation renders them more susceptible to electrophilic attack, while protonation reduces their activity against electrophilic and nucleophilic attacks. Additionally, pH adjustments can modify the degradation pathway and the toxicity of transformation products. Overall, pH changes can affect the oxidation fate of organic pollutants by altering their structure, which distinguishes it from the effect of pH on oxidants or oxidant activation processes.

Photochemical Transformation of Monochloramine Induced by Triplet State Dissolved Organic Matter

The photoexcited dissolved organic matter (DOM) could produce reactive intermediates, affecting chemical oxidant transformation in UV based advanced oxidation processes (AOPs). This study confirmed the critical role of triplet state DOM (3DOM*), generated from DOM photoexcitation, in the transformation of monochloramine (NH2Cl), a commonly used chemical oxidant and disinfectant in water treatment. NH2Cl (42.25 μM, as Cl2) was decayed by 17.4-73.4 % within 60 min, primarily due to 3DOM* , in DOM (2-30 mgC L-1) solutions irradiated by 365 nm, where NH2Cl has no absorption. The second-order quenching rate constants of triplet state model photosensitizers by NH2Cl were determined to be 0.95(± 0.04)-4.49(± 0.04)× 108 M-1 s-1 by using laser flash photolysis. As a reductant, 3DOM* reacted with NH2Cl through one-transfer mechanism, leading to amino radical (NH2•) generation, which then transferred to ammonia (NH4+, pKa 9.25) through H-abstraction by the phenolic moieties in DOM. Additionally, the intermediate product of 3DOM* oxidized by NH2Cl or those triplet state quinones can hydrolyze to form phenolic moieties, elevating NH4+ yield to higher than 99% upon 365 nm irradiation. These findings suggest that the widespread DOM can be applied to convert NH2Cl via 3DOM* with minimal toxic risks.

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.

The determination of thallium in the environment: A review of conventional and advanced techniques and applications

Thallium (Tl) is a potential toxicity element that poses significant ecological and environmental risks. Recently, a substantial amount of Tl has been released into the environment through natural and human activities, which attracts increasing attention. The determination of this hazardous and trace element is crucial for controlling its pollution. This article summarizes the advancement and progress in optimizing Tl detection techniques, including atomic absorption spectroscopy (AAS), voltammetry, inductively coupled plasma (ICP)-based methods, spectrophotometry, and X-ray-based methods. Additionally, it introduces sampling and pretreatment methods such as diffusive gradients in thin films (DGT), liquid-liquid extraction, solid phase extraction, and cloud point extraction. Among these techniques, ICP-mass spectrometry (MS) is the preferred choice for Tl detection due to its high precision in determining Tl as well as its species and isotopic composition. Meanwhile, some new materials and agents are employed in detection. The application of novel work electrode materials and chromogenic agents is discussed. Emphasis is placed on reducing solvent consumption and utilizing pretreatment techniques such as ultrasound-assisted processes and functionalized magnetic particles. Most detection is performed in aqueous matrices, while X-ray-based methods applied to solid phases are summarized which provide non-destructive analysis. This work improves the understanding of Tl determination technology while serving as a valuable resource for researchers seeking appropriate analytical techniques.

Positive profile of natural small molecule organic matters on emerging antivirus pharmaceutical elimination in advance reduction process: A deep dive into the photosensitive mechanism of triplet excited state compounds

Natural small molecular organic matter (NSOM), ubiquitous in natural waters and distinct from humic acid or fulvic acid, is a special type of dissolved organic matter (DOM) which is characterized as strong photosensitivity and simple molecular structure. However, little study had been directed on the role of NSOM in eliminating emerging contaminants in advanced reduction process (ARP). This study took three small molecular isomeric organic acids (p-hydroxybenzoic acid, pHBA; salicylic acid, SA; m-hydroxybenzoic acid, mHBA) as the representative substances of NSOM to explore these mechanisms on promoting Ribavirin (RBV, an anti COVID-19 medicine) degradation in ultraviolet activated sulfite (UV/Sulfite) process. The results demonstrated that the observed degradation rate constant of RBV (kobs-RBV) was 7.56 × 10-6 s-1 in UV/Sulfite process, indicating that hydrated electron (eaq-) from UV/Sulfite process could not effectively degrade RBV, while it increased by 178 and 38 times when pHBA and SA were introduced into UV/Sulfite process respectively, suggesting that pHBA and SA strongly promoted RBV degradation while mHBA had no promotion on RBV abatement in UV/Sulfite process. Transient absorption spectra and reactive intermediates scavenging experiment indicated that the triplet excited state pHBA and SA (3pHBA* and 3SA*) contributed to the degradation of RBV through non-radical process. Notably, eaq- played the role of key initiator in transforming pHBA and SA into their triplet states. The difference of kobs-RBV in UV/Sulfite/pHBA and UV/Sulfite/SA process was attributed to different generation pathways of 3pHBA* and 3SA* (high molar absorptivity at the wavelength of 254 nm and photosensitive cycle, respectively) and their second order rate constants towards RBV (kRBV-3pHBA* = 8.60 × 108 M-1 s-1 and kRBV-3SA* = 6.81 × 107 M-1 s-1). mHBA could not degrade RBV for its lack of intramolecular hydrogen bond and low molar absorptivity at 254 nm to abundantly transform into its triplet state. kobs-RBV increased as pH increased from 5.0 to 11.0 in UV/Sulfite/SA process, due to the high yield of eaq- in alkaline condition which promoted the generation of 3SA* and the stable of the absorbance of SA at 254 nm. By contrast, kobs-RBV underwent a process of first increasing and then decreasing in UV/Sulfite/pHBA process as the increase of pH, and its highest value achieved in a neutral condition. This lied in the exposure of eaq- increased as the increase of pH which promoted the generation of 3pHBA*, while the molar absorptivity of pHBA at 254 nm decreased as the increase of pH in an alkaline condition which inhibited the yield of 3pHBA*. The RBV degradation pathways and products toxicity assessment indicated that UV/Sulfite/pHBA had better detoxification performance on RBV than UV/Sulfite/SA process. This study disclosed a novel mechanism of emerging contaminants abatement through non-radical process in NSOM mediated ARP, and provide a wide insight into positive profile of DOM in water treatment process, instead of only taking DOM as a quencher of reactive intermediates.

Dissolved Organic Matter-Mediated Photosensitized Activation of Monochloramine for Micropollutant Abatement in Wastewater Effluent

Utilizing solar light and water matrix components in situ to reduce the chemical and energy demands would make treatment technologies more sustainable for micropollutant abatement in wastewater effluents. We herein propose a new strategy for micropollutant abatement through dissolved organic matter (DOM)-mediated photosensitized activation of monochloramine (NH2Cl). Exposing the chlorinated wastewater effluent with residual NH2Cl to solar irradiation (solar/DOM/NH2Cl process) degrades six structurally diverse micropollutants at rate constants 1.26-34.2 times of those by the solar photolysis of the dechlorinated effluent (solar/DOM process). Notably, among the six micropollutants, the degradation rate constants of estradiol, acetaminophen, bisphenol A, and atenolol by the solar/DOM/NH2Cl process are 1.13-4.32 times the summation of those by the solar/DOM and solar/NH2Cl processes. The synergism in micropollutant degradation is attributed to the generation of reactive nitrogen species (RNS) and hydroxyl radicals (HO·) from the photosensitized activation of NH2Cl. Triplet state-excited DOM (3DOM*) dominates the activation of NH2Cl, leading to the generation of RNS, while HO· is produced from the interactions between RNS and other photochemically produced reactive intermediates (e.g., O2·- and DOM·+/·-). The findings advance the knowledge of DOM-mediated photosensitization and offer a sustainable method for micropollutant abatement in wastewater effluents containing residual NH2Cl.

Wavelength dependency and photosensitizer effects in UV-LED photodegradation of iohexol

Iodinated X-ray contrast media (ICM) are ubiquitously present in water sources and challenging to eliminate using conventional processes, posing a significant risk to aquatic ecosystems. Ultraviolet light-emitting diodes (UV-LED) emerge as a promising technology for transforming micropollutants in water, boasting advantages such as diverse wavelengths, elimination of chemical additives, and no induction of microorganisms' resistance to disinfectants. The research reveals that iohexol (IOX) degradation escalates as UV wavelength decreases, attributed to enhanced photon utilization efficiency. Pseudo-first-order rate constants (kobs) were determined as 3.70, 2.60, 1.31 and 0.65 cm2 J-1 at UV-LED wavelengths of 255, 265, 275 and 285 nm, respectively. The optical properties of dissolved organic matter (DOM) and anions undeniably influence the UV-LED photolysis process through photon competition and the generation of reactive substances. The influence of Cl- on IOX degradation was insignificant at UV-LED 255, but it promoted IOX degradation at 265, 275 and 285 nm. IOX degradation was accelerated by ClO2-, NO3-and HA due to the formation of various reactive species. In the presence of NO3-, the kobs of IOX followed the order: 265 > 255 > 275 > 285 nm. Photosensitizers altered the spectral dependence of IOX, and the intermediate photoactivity products were detected using electron spin resonance. The transformation pathways of IOX were determined through density functional theory calculations and experiments. Disinfection by-products (DBPs) yields of IOX during UV-LED irradiation decreased as the wavelength increased: 255 > 265 > 275 > 285 nm. The cytotoxicity index value decreased as the UV-LED wavelength increased from 255 to 285 nm. These findings are crucial for selecting the most efficient wavelength for UV-LED degradation of ICM and will benefit future water purification design.

The synergistic effect of periodate/ferrate (VI) system on disinfection of antibiotic resistant bacteria and removal of antibiotic resistant genes: The dominance of Fe (IV)/Fe (V)

The proliferation of antibiotic resistant genes (ARGs) and antibiotic resistant bacteria (ARB) caused by antibiotic abuse has raised concerns about the global infectious-disease crisis. This study employed periodate (PI)/ferrate (VI) (Fe (VI)) system to disinfect Gram-negative ARB (Escherichia coli DH5α) and Gram-positive bacteria (Bacillus subtilis ATCC6633). The PI/Fe (VI) system could inactivate 1 × 108 CFU/mL of Gram-negative ARB and Gram-positive bacteria by 4.0 and 2.8 log in 30 min. Neutral and acidic pH, increase of PI dosage and Fe (VI) dosage had positive impacts on the inactivation efficiency of ARB, while alkaline solution and the coexistence of 10 mM Cl-, NO3-, SO42- and 20 mg/L humic acid had slightly negative impacts. The reactive species generated by PI/Fe (VI) system could disrupt the integrity of cell membrane and wall, leading to oxidative stress and lipid peroxidation. Intracellular hereditary substance, including DNA and ARGs (tetA), would leak into the external environment through damaged cells and be degraded. The electron spin resonance analysis and quenching experiments indicated that Fe (IV)/Fe (V) played a leading role in disinfection. Meanwhile, PI/Fe (VI) system also had an efficient removal effect on sulfadiazine, which was expected to inhibit the ARGs transmission from the source.

A novel remediation strategy of mixed calcium peroxide and degrading bacteria for polycyclic aromatic hydrocarbon contaminated water

Polycyclic aromatic hydrocarbons (PAHs) are a class of toxic organic pollutants commonly detected in the aqueous phase. Traditional biodegradation is inefficient and advanced oxidation technologies are expensive. In the current study, a novel strategy was developed using calcium peroxide (CP) and PAH-degrading bacteria (PDB) to effectively augment PAH degradation by 28.62-59.22%. The PDB consisted of the genera Acinetobacter, Stenotrophomonas, and Comamonas. Applying the response surface model (RSM), the most appropriate parameters were identified, and the predictive degradation rates of phenanthrene (Phe), pyrene (Pyr), and ΣPAHs were 98%, 76%, and 84%, respectively. The constructed mixed system could reduce 90% of Phe and more than 60% of ΣPAHs and will perform better at pH 5-7 and lower salinity. Because PAHs tend to bind to dissolved organic matter (DOM) with larger molecular weights, humic acid (HA) had a larger negative effect on the PAH-degradation efficiency of the CP-PDB mixed system than fulvic acid (FA). The proposed PAH-degradation pathways in the mixed system were based on the detection of intermediates at different times. The investigation constructed and optimized a novel environmental PAH-degradation strategy. The synergistic application of PDB and oxidation was extended for organic contaminant degradation in aqueous environments.

Prediction of Nanoparticle Photoreactivity in Mixtures of Surface Foulants Requires Kinetic (Non-equilibrium) Adsorption Considerations

The adsorption of foulants on photocatalytic nanoparticles can suppress their reactivity in water treatment applications by scavenging reactive species at the photocatalyst surface, screening light, or competing for surface sites. These inhibitory effects are commonly modeled using the Langmuir-Hinshelwood model, assuming that adsorbed layer compositions follow Langmuirian (equilibrium) competitive adsorption. However, this assumption has not been evaluated in complex mixtures of foulants. This study evaluates the photoreactivity of titanium dioxide (TiO2) nanoparticles toward a target compound, phenol, in the presence of two classes of foulants ─ natural organic matter (NOM) and a protein, bovine serum albumin (BSA) ─ and mixtures of the two. Langmuir adsorption models predict that BSA should strongly influence the nanoparticle photoreactivity because of its higher adsorption affinity relative to phenol and NOM. However, model evaluation of the experimental phenol decay rates suggested that neither the phenol nor foulant surface coverages are governed by Langmuirian competitive adsorption. Rather, a reactivity model incorporating kinetic predictions of adsorbed layer compositions (favoring NOM adsorption) outperformed Langmuirian models in providing accurate, unbiased predictions of phenol degradation rates. This research emphasizes the importance of using first-principles models that account for adsorption kinetics when assumptions of equilibrium adsorption do not apply.

Applying the modified UV-activated TOP assay to complex matrices impacted by aqueous film-forming foams

Per- and polyfluoroalkyl substances (PFAS) are a large chemical family, and numerous chemical species can co-exist in environmental samples, especially those impacted by aqueous film-forming foams (AFFFs). Given the limited availability of chemical standards, capturing the total amount of PFAS is challenging. Thus, the total oxidizable precursor (TOP) assay has been developed to estimate the total amount of PFAS via the oxidative conversion of precursors into perfluorocarboxylic acids (PFCAs). This study aims to enhance the robustness of the TOP assay by replacing heat activation with UV activation. We evaluated the molar yields of known precursors in water in the presence of varying levels of Suwannee River natural organic matter (SRNOM) and in two soils. The impact of UV activation was also evaluated in two soils spiked with three well-characterized AFFFs, six AFFF-impacted field soils, and nine rinse samples of AFFF-impacted stainless-steel pipe. In the presence of 100 mg/L SNROM, 6:2 fluorotelomer sulfonate (FTS), 8:2 FTS, and N-ethyl perfluorooctane sulfonamidoacetic acid (N-EtFOSAA) in deionized water had good molar recovery as PFCAs (average of 102 ± 9.8 %); at 500 mg/L SNROM, the recovery significantly dropped to an average of 51 ± 19 %. In two soils (with 4 % and 8.8 % organic matter) with individual precursor spikes, the average molar recovery was 101 ± 9.4 %, except N-EtFOSAA, which had a reduced recovery in the soil with 8.8 % organic matter (OM). UV-activated assays outperformed heat-activated ones, especially in AFFF-impacted soils and pipe extract samples, with an average of 1.4-1.5× higher PFCA recovery. In almost all test samples, UV activation resulted in a notable shift towards longer PFCA chain lengths, particularly for samples with high OM content. The study confirmed the advantages of UV activation, including a significantly shortened exposure time (1 h vs. 6 h) and reduced matrix effects from OM due to the dual functions of UV in activating persulfate and photodegrading OM.

Peroxydisulfate-Driven Reductive Dechlorination as Affected by Soil Constituents: Free Radical Formation and Conversion

We report a previously unrecognized but efficient reductive degradation pathway in peroxydisulfate (PDS)-driven soil remediation. With supplements of naturally occurring low-molecular-weight organic acids (LMWOAs) in anaerobic biochar-activated PDS systems, degradation rates of 12 γ-hexachlorocyclohexanes (HCH)-spiked soils boosted from 40% without LMWOAs to a maximum of 99% with 1 mM malic acid. Structural analysis revealed that an increase in α-hydroxyl groups and a diminution in pKa1 values of LMWOAs facilitated the formation of reductive carboxyl anion radicals (COO•-) via electrophilic attack by SO4•-/•OH. Furthermore, degradation kinetics were strongly correlated with soil organic matter (SOM) contents than iron minerals. Combining a newly developed in situ fluorescence detector of reductive radicals with quenching experiments, we showed that for soils with high, medium, and low SOM contents, dominant reactive species switched from singlet oxygen/semiquinone radicals to SO4•-/•OH and then to COO•- (contribution increased from 30.8 to 66.7%), yielding superior HCH degradation. Validation experiments using SOM model compounds highlighted critical roles of redox-active moieties, such as phenolic - OH and quinones, in radical formation and conversion. Our study provides insights into environmental behaviors related to radical activation of persulfate in a broader soil horizon and inspiration for more advanced reduction technologies.