Ozone is a commonly applied disinfectant and oxidant in drinking water and has more recently been implemented for enhanced municipal wastewater treatment for potable reuse and ecosystem protection. One drawback is the potential formation of bromate, a possible human carcinogen with a strict drinking water standard of 10 μg/L. The formation of bromate from bromide during ozonation is complex and involves reactions with both ozone and secondary oxidants formed from ozone decomposition, i.e., hydroxyl radical. The underlying mechanism has been elucidated over the past several decades, and the extent of many parallel reactions occurring with either ozone or hydroxyl radicals depends strongly on the concentration, type of dissolved organic matter (DOM), and carbonate. On the basis of mechanistic considerations, several approaches minimizing bromate formation during ozonation can be applied. Removal of bromate after ozonation is less feasible. We recommend that bromate control strategies be prioritized in the following order: (1) control bromide discharge at the source and ensure optimal ozone mass-transfer design to minimize bromate formation, (2) minimize bromate formation during ozonation by chemical control strategies, such as ammonium with or without chlorine addition or hydrogen peroxide addition, which interfere with specific bromate formation steps and/or mask bromide, (3) implement a pretreatment strategy to reduce bromide and/or DOM prior to ozonation, and (4) assess the suitability of ozonation altogether or utilize a downstream treatment process that may already be in place, such as reverse osmosis, for post-ozone bromate abatement. A one-size-fits-all approach to bromate control does not exist, and treatment objectives, such as disinfection and micropollutant abatement, must also be considered.

Water quality and its impacts on human and ecosystem health presents tremendous global challenges. While oxidative water treatment can solve many of these problems related to hygiene and micropollutants, identifying and predicting transformation products from a large variety of micropollutants induced by dosed chemical oxidants and in situ formed radicals is still a major challenge. To this end, a better understanding of the formed transformation products and their potential toxicity is needed. Currently, no theoretical tools alone can predict oxidatively induced transformation products in aqueous systems. Coupling experimental and theoretical studies has advanced the understanding of reaction kinetics and mechanisms significantly. This perspective article highlights the key progress made concerning experimental and computational approaches to predict transformation products. Knowledge gaps are identified, and the research required to advance the predictive capability is discussed.

The activation of peroxydisulfate (PDS) by organic compounds has attracted increasing attention. However, some inherent drawbacks including quick activator decomposition and poor anti-interference capacity limited the application of organic compound-activated PDS. It was interestingly found that 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS) could act as both activator and electron shuttle for PDS activation to enhance diclofenac (DCF) degradation over a pH range of 2.0–11.0. Multiple reactive species of ABTS^•+, ^•OH, and SO₄^•– were generated in the PDS/ABTS system, while only ABTS^•+ and ^•OH directly contributed to DCF degradation. ABTS^•+, generated via the reactions of ABTS with PDS, SO₄^•–, and ^•OH, was the dominant reactive species of DCF degradation. No significant decomposition of ABTS was observed in the PDS/ABTS system, and ABTS acted as both activator and electron shuttle. Four possible degradation pathways of DCF were proposed, and the toxicity of DCF decreased after treatment with the PDS/ABTS system. The PDS/ABTS system had good anti-interference capacity to common natural water constituents. Additionally, ABTS was encapsulated into cellulose to obtain ABTS@Ce beads, and the PDS/ABTS@Ce system possessed excellent performance on DCF degradation. This study proposes a new perspective to reconsider the mechanism of activating PDS with organic compounds and highlights the considerable contribution of organic radicals on contaminant removal.

In drinking water distribution systems, the oxidation of zerovalent chromium, Cr(0), in iron corrosion scales by chlorine residual disinfectant is the dominant reaction to form carcinogenic hexavalent chromium, Cr(VI). This study investigates inhibitive corrosion control strategies through adjustments of chemical water parameters (i.e., pH, silicate, phosphate, calcium, and alkalinity) on Cr(VI) formation through oxidation of Cr(0)_(s) by free chlorine under drinking water conditions. The results show that an increase in pH, silicate, alkalinity, and calcium suppressed Cr(VI) formation that was mainly attributed to in situ surface precipitation of new Cr(III) solids on the surface of Cr(0)_(s), including Cr(OH)_3(s), Cr₂(SiO₃)_3(s), CrPO_4(s), Cr₂(CO₃)_3(s), and Cr₁₀Ca(CO₃)_16(s). The Cr(III) surface precipitates were much less reactive with chlorine than Cr(0)_(s) and suppressed the Cr redox reactivity. The concentration of surface Cr(III) solids was inversely correlated with the rate constant of Cr(VI) formation. Adding phosphate either promoted or inhibited the Cr(VI) formation, depending on the phosphate concentration. This study provides fundamental insight into the Cr(VI) formation mechanisms via Cr(0) oxidation by chlorine and the importance of surface precipitation of Cr(III) solids with different corrosion control strategies and suggests that increasing the pH/alkalinity and addition of phosphate or silicate can be effective control strategies to minimize Cr(VI) formation.

The herbicide isoxaflutole has the potential to contaminate drinking water directly, as well as upon hydrolyzing to its active form diketonitrile. Diketonitrile also may impact water quality by acting as a precursor for dichloroacetonitrile (DCAN), which is an unregulated but highly toxic disinfection byproduct (DBP). In this study, we investigated the reaction of diketonitrile with free chlorine and chloramine to form DCAN. We found that diketonitrile reacts with free chlorine within seconds but reacts with chloramine on the time scale of hours to days. In the presence of both oxidants, DCAN was generated at yields up to 100%. Diketonitrile reacted fastest with chlorine at circumneutral pH, which was consistent with base-catalyzed halogenation involving the enolate form of diketonitrile present at alkaline pH and electrophilic hypochlorous acid, which decreases in abundance above its pK_a (7.5). In contrast, we found that diketonitrile reacts faster with chloramine as pH values decreased, consistent with an attack on the enolate by electrophilic protonated monochloramine that increases in abundance at acidic pH approaching its pK_a (1.6). Our results indicate that increasing isoxaflutole use, particularly in light of the recent release of genetically modified isoxaflutole-tolerant crops, could result in greater occurrences of a high-yield DCAN precursor during disinfection.

Ultraviolet (UV)/chlor(am)ine processes are emerging advanced oxidation processes (AOPs) for water decontamination and raising continuous attention. However, limitations appear in the UV/hypochlorite and UV/monochloramine for removing specific contaminants ascribed to the differences in the sorts and yields of free radicals. Here, this study reports UV/dichloroisocyanurate (NaDCC) as a novel source of radicals. NaDCC was demonstrated to be a well-balanced compound between hypochlorite and monochloramine, and it had significant UV absorption and a medium intrinsic quantum yield. The UV/NaDCC produced more substantial hydroxyl radicals (·OH) and reactive chlorine species (RCSs, including Cl·, ClO·, and Cl₂·^–) than conventional UV/chlor(am)ine, thereby generating a higher oxidation efficiency. The reaction mechanisms, environmental applicability, and energy requirements of the UV/NaDCC process for emerging contaminants (ECs) abatement were further investigated. The results showed that ·OH and ·NH₂ attacked ECs mostly through hydrogen atom transfer (HAT) and radical adduct formation, whereas Cl· destroyed ECs mainly through HAT and single electron transfer, with ClO· playing a certain role through HAT. Kinetic model analyses revealed that the UV/NaDCC outperformed the conventional UV/chlor(am)ine in a variety of water matrices with superior degradation efficiency, significantly saving up to 96% electrical energy per order. Overall, this study first demonstrates application prospects of a novel AOP using UV/NaDCC, which can compensate for the deficiency of the conventional UV/chlor(am)ine AOPs.

Per- and polyfluoroalkyl substances (PFASs), including perfluorohexanesulfonic acid (PFHxS), as emerging persistent organic pollutants widely detected in drinking water, have drawn increasing concern. The PFHxS contamination of drinking water always results from direct and indirect sources, especially the secondary generations through environmental transformations of precursors. However, the mechanism of the transformation of precursors to PFHXS during the drinking water treatment processes remains unclear. Herein, the potential precursors and formation mechanisms of PFHxS were explored during drinking water disinfection. Simultaneously, the factors affecting PFHxS generation were also examined. This study found PFHxS could be generated from polyfluoroalkyl sulfonamide derivatives during chlorination and chloramination. The fate and yield of PFHxS varied from different precursors and disinfection processes. In particular, monochloramine more favorably formed PFHxS. Several perfluoroalkyl oxidation products and decarboxylation intermediates were detected and identified in the chloraminated samples using Fourier-transform ion cyclotron resonance mass spectrometry. Combined with density functional theory calculations, the results indicated that the indirect oxidation via the attack of the nitrogen atom in sulfonamide groups might be the dominant pathway for generating PFHxS during chloramination, and the process could be highly affected by the monochloramine dose, pH, and temperature. This study provides important evidence of the secondary formation of PFHxS during drinking water disinfection and scientific support for chemical management of PFHxS and PFHxS-related compounds.

Permanganate (Mn(VII)) is extensively applied in water purification due to its stability and ease of handling, but it is a mild oxidant for trace organic contaminants (TrOCs). Hence, there is significant interest in strategies for enhancing reaction kinetics, especially in combination with efficient and economical carbocatalysts. This study compared the performance of four carbocatalysts (graphite, graphene oxide (GO), reduced-GO (rGO), and nitrogen-doped rGO (N-rGO)) in accelerating sulfisoxazole (SSX) oxidation by Mn(VII) and found that GO exhibited the greatest catalytic performance. Besides, the Mn(VII)/GO system shows desirable capacities to remove a broad spectrum of TrOCs. We proposed that the degradation of SSX in Mn(VII)-GO suspensions follows two routes: (i) direct oxidation of SSX by Mn species [both Mn(VII) and in situ formed MnO_2(s)] and (ii) a carbocatalyst route, where GO acts as an electron mediator, accepting electrons from SSX and transferring them to Mn(VII). We developed a mathematical model to show the contribution of each parallel pathway and found one-electron transfer is primarily responsible for accelerating SSX removal in the Mn(VII)/GO system. Findings in this study showed that GO provides a simple and effective strategy for enhancing the reactivity of Mn(VII) and provided mechanistic insights into the GO-catalyzed redox reaction between SSX and Mn(VII).

Virucidal efficacies of disinfectants are typically assessed by infectivity assay utilizing a single type of host cell. Enteroviruses infect multiple host cells via various entry routes, and each entry route may be impaired differently by a given disinfectant. Yet, it is unknown how the choice of host cells affects the observed inactivation kinetics. Here, we evaluated the inactivation kinetics of echovirus 11 (E11) by free chlorine, ultraviolet (UV) irradiation, and heat, using three different host cells (BGMK, RD, and A549). Inactivation rates were independent of the host cell for treatment of E11 by UV or heat. Conversely, E11 inactivation by free chlorine occurred 2-fold faster when enumerated on BGMK cells compared with RD and A549 cells. Host cell-dependent inactivation kinetics by free chlorine were also observed for echovirus 7, 9, and 13, and coxsackievirus A9. E11 inactivation by free chlorine was partly caused by a loss in host cell attachment, which was most pronounced for BGMK cells. BGMK cells lack the attachment receptor CD55 and a key subunit of the uncoating receptor β2M, which may contribute to the differential inactivation kinetics for this cell type. Consequently, inactivation kinetics of enteroviruses should be assessed using host cells with different receptor profiles.

The halogenation of quorum sensing molecules (QSMs) is known to be catalyzed by enzymes such as haloperoxidase (HPO) as well as cerium dioxide nanocrystals (NC), which mimic enzymes. Those enzymes and mimics can influence biological processes such as biofilm formation, where bacteria use QSMs for the “chemical” communication between each other and the coordination of surface colonization. However, not much is known about the degradation behavior of a broad spectrum of QSMs, especially for HPO and its mimics. Therefore, in this study, the degradation of three QSMs with different molecule moieties was elucidated. For this purpose, different batch experiments were carried out with HPOs, NCs and free active bromine (FAB). For N-β-ketocaproyl-homoserine lactone (3-Oxo-C₆-AHL), N-cis-tetradec-9Z-enoyl-homoserine lactone (C_14:1-AHL) and 2-heptyl-4-quinolone (HHQ) a fast degradation and moiety-specific transformations were observed. The HPO vanadium bromoperoxidase as well as cerium dioxide NCs catalyzed the formation of the same brominated transformation products (TPs). Since the same TPs are formed in batch experiments with FAB it is very likely that FAB is playing a major role in the catalytical reaction mechanism leading to the transformation of QSMs. In this study in total 17 TPs could be identified in different levels of confidence and the catalytic degradation processes for two QS groups (unsaturated AHLs and alkyl quinolones) with cerium dioxide NCs and vanadium bromoperoxidase were expanded.

The use of ozone/biofiltration advanced treatment has become more prevalent in recent years, with many utilities seeking an alternative to membrane/RO based treatment for water reuse. Ensuring efficient pathogen reduction while controlling disinfection byproducts and maximizing oxidation of trace organic contaminants remains a major barrier to implementing ozone in reuse applications. Navigating these challenges is imperative in order to allow for the more widespread application of ozonation. Here, we demonstrate the effectiveness of ozone for virus, coliform bacteria, and spore forming bacteria inactivation in unfiltered secondary effluent, all the while controlling the disinfection byproduct bromate. A greater than 6-log reduction of both male specific and somatic coliphages was seen at specific ozone doses as low as 0.75 O₃:TOC. This study compared monochloramine and hydrogen peroxide as chemical bromate control measures in high bromide water (Br^– = 0.35 ± 0.07 mg/L). On average, monochloramine and hydrogen peroxide resulted in an 80% and 36% decrease of bromate formation, respectively. Neither bromate control method had any appreciable impact on virus or coliform bacteria disinfection by ozone; however, the use of hydrogen peroxide would require a non-Ct disinfection framework. Maintaining ozone residual was shown to be critical for achieving disinfection of more resilient microorganisms, such as spore forming bacteria. While extremely effective at controlling bromate, monochloramine was shown to inhibit TrOC oxidation, whereas hydrogen peroxide enhanced TrOC oxidation.

Ferrate (Fe(VI)) is a novel oxidant that can be used to mitigate disinfection byproduct (DBP) precursors. However, the reaction of Fe(VI) with organic nitrogen, which is a potential precursor of potent nitrogenous DBPs, remains largely unexplored. The present work aimed to identify the kinetics and products for the reaction of Fe(VI) with primary amines, notably amino acids. A new kinetic model involving ionizable intermediates was proposed and can describe the unusual pH effect on the Fe(VI) reactivity toward primary amines and amino acids. The Fe(VI) oxidation of phenylalanine produced a mixture of nitrile, nitrite/nitrate, amide, and ammonia, while nitroalkane was an additional product in the case of glycine. The product distribution for amino acids significantly differed from that of uncarboxylated primary amines that mainly generate nitriles. A general reaction pathway for primary amines and amino acids was proposed and notably involved the formation of imines, the degradation of which was affected by the presence of a carboxylic group. In comparison, ozonation led to higher yields of nitroalkanes that could be readily converted to potent halonitroalkanes during chlor(am)ination. Based on this study, Fe(VI) can effectively mitigate primary amine-based, nitrogenous DBP precursors with little formation of toxic halonitroalkanes.

Per- and polyfluoroalkyl substances (PFAS) pose significant environmental and human health risks and thus require solutions for their removal and destruction. However, PFAS cannot be destroyed by widely used removal processes like nanofiltration (NF). A few scarcely implemented advanced oxidation processes can degrade PFAS. In this study, we apply an electric field to a membrane system by placing a nanofiltration membrane between reactive electrodes in a crossflow configuration. The performance of perfluorooctanoic acid (PFOA) rejection, water flux, and energy consumption were evaluated. The reactive and robust SnO₂–Sb porous anode was created via a sintering and sol–gel process. The characterization and analysis techniques included field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), ion chromatography, mass spectroscopy, porosimeter, and pH meter. The PFOA rejection increased from 45% (0 V) to 97% (30 V) when the electric field and filtration were in the same direction, while rejection capabilities worsened in opposite directions. With saline solutions (1 mM Na₂SO₄) present, the induced electro-oxidation process could effectively mineralize PFOA, although this led to unstable removal and water fluxes. The design achieved an exceptional performance in the nonsaline feed of 97% PFOA rejection and water flux of 68.4 L/m² hr while requiring only 7.31 × 10^–5 kWh/m³/order of electrical energy. The approach’s success is attributed to the proximity of the electrodes and membrane, which causes a stronger electric field, weakened concentration polarization, and reduced mass transfer distances of PFOA near the membrane. The proposed electric field-assisted nanofiltration design provides a practical membrane separation method for PFAS removal from water.

This study valorized scanning electrochemical microscopy (SECM) for the detection of dissolved O₃, which is increasingly in demand for water treatment. Au ultramicroelectrodes biased at 0.62 V RHE provided superior activity and selectivity for O₃ reduction, compared to Pt analogues. It allowed quantitative in situ interrogation of ozone evolution reaction (OZER) electrocatalysts with unprecedented estimations on the OZER overpotential. The difference in onset potentials between the OZER and the competing oxygen evolution reaction (OER) primarily accounted for the OZER current efficiency (CE) on boron-doped diamond (BDD, 1.4% at 10 mA cm^–2 in 0.5 M H₂SO₄), Ni–Sb-doped SnO₂ (NSS, 10.8%), and SiO_x-coated NSS (NSS/SiO_x, 34.4%). SECM areal scans in tandem with elemental mapping perspicuously visualized the improved OZER activity by the SiO_x overlayer on NSS. A shift in the charge transfer coefficient further rationalized the elevated OZER selectivity on NSS/SiO_x, in association with the weakened Sn–O bond strength confirmed by valence band X-ray photoelectron spectra. The invigorated OZER on NSS/SiO_x effectively accelerated the degradation of a model aqueous pollutant (4-chlorophenol).

Electrochemical oxidation has been demonstrated to be a useful method for removing biorefractory organic pollutants in mature landfill leachate but suffers from low efficiency in eliminating ammonium because of its resistance to being oxidized by HO^• or free chlorine (FC) at decreased pH. Here, we propose a new bipolar membrane-electrochlorination (BPM-EC) process to address this issue. We found that the BPM-EC system was significantly superior to both the undivided and divided reactors with monopolar membranes in terms of elevated rate of ammonium removal, attenuated generation of byproducts (e.g., nitrate and chloramines), increased Faradaic efficiency, and decreased energy consumption. Mechanistic studies revealed that the integration of BPM was helpful in creating alkaline environments in the vicinity of the anode, which facilitated production of surface-bound HO^• and FC and eventually promoted in situ generation of ClO^•, a crucial reactive species mainly responsible for accelerating ammonium oxidation and selective transformation to nitrogen. The efficacy of BPM-EC in treating landfill leachates with different ammonium concentrations was verified under batch and continuous-flow conditions. A kinetic model that incorporates the key parameters was developed, which can successfully predict the optimal number of BPM-EC reactors (e.g., 2 and 5 for leachates containing 589.4 ± 5.5 and 1258.1 ± 9.6 mg L^–1 NH₄⁺-N, respectively) necessary for complete removal of ammonium. These findings reveal that the BPM-EC process shows promise in treating ammonium-containing wastewater, with advantages that include effectiveness, adaptability, and flexibility.

The efficient and selective removal of refractory antibiotics from high-strength antibiotic production wastewater is crucial but remains a substantial challenge. In this study, a novel ozone micronano-bubble (MNB)-enhanced treatment system was constructed for antibiotic production wastewater treatment. Compared with conventional ozone, ozone MNBs exhibit excellent treatment efficiency for oxytetracycline (OTC) degradation and toxicity decrease. Notably, this study identifies the overlooked singlet oxygen (¹O₂) for the first time as a crucial active species in the ozone MNB system through probe and electron paramagnetic resonance methods. Subsequently, the oxidation mechanisms of OTC by ozone MNBs are systematically investigated. Owing to the high reactivity of OTC toward ¹O₂, ozone MNBs enhance the selective and anti-interference performance of OTC degradation in raw OTC production wastewater with complex matrixes. This study provides insights into the mechanism of ozone MNB-enhanced pollutant degradation and a new perspective for the efficient treatment of high-concentration industrial wastewater using ozone MNBs. In addition, this study presents a promising technology with scientific guidance for the treatment of antibiotic production wastewater.

Bromine and iodine species are precursors for forming disinfection byproducts in finished drinking waters. Our study incorporates spatial and temporal data to quantify concentrations of inorganic (bromide (Br^–), iodide (I^–), and iodate (IO₃^–)), organic, and total bromine (Br_T) and iodine (I_T) species from 286 drinking water sources and 7 wastewater effluents across the United States. Br^– ranged from <5–7800 μg/L (median of 62 μg/L in surface water (SW) and 95 μg/L in groundwater (GW)). I^– was detected in 41% of SW (1–72 μg/L, median = <1 μg/L) and 62% of GW (<1–250 μg/L, median = 3 μg/L) samples. The median Br^–/I^– ratio in SW and GW was 22 μg/μg and 16 μg/μg, respectively, in paired samples with detect Br^– and I^–. Br_T existed primarily as Br^–, while I_T was present as I^–, IO₃^–, and/or total organic iodine (TOI). Inorganic iodine species (I^– and IO₃^–) were predominant in GW samples, accounting for 60–100% of I_T; however, they contributed to only 20–50% of I_T in SW samples. The unknown fraction of I_T was attributed to TOI. In lakes, seasonal cycling of I-species was observed and was presumably due to algal productivity. Finally, Spearman Rank Correlation tests revealed a strong correlation between Br^– and I_T in SW (R_Br^–,IT = 0.83) following the log₁₀ (Br^–, μg/L) = 0.65 × log₁₀ (I_T, μg/L) – 0.17 relationship. Br^– and I^– in treated wastewater effluents (median Br^– = 234 μg/L, median I^– = 5 μg/L) were higher than drinking water sources.

A novel approach to the abatement of pollutants consisting of their conversion to separable solid polymers is explored by a heat/persulfate (PDS) process for the treatment of high-temperature wastewaters. During this process, a simultaneous decontamination and carbon recovery can be achieved with minimal use of PDS, which is significantly different from conventional degradation processes. The feasibility of this process is demonstrated by eight kinds of typical organic pollutants and by a real coking wastewater. For the treatment of the selected pollutants, 30.2–91.9% DOC abatement was achieved with 24.8–91.2% carbon recovery; meanwhile, only 5.2–47.0% of PDS was consumed compared to a conventional degradation process. For the treatment of a real coking wastewater, 71.0% DOC abatement was achieved with 66.0% carbon recovery. With phenol as a representative compound, our polymerization-based heat/PDS process is applicable in a wide pH range (3.5–9.0) with a carbon recovery of >87%. Both SO₄^•– and HO^• can be initiators for polymerization, with different contribution ratios under various conditions. Phenol monomers are semioxidized to form phenolic radicals, which are polymerized via chain transfer or chain growth processes to form separable solid phenol polymers, benzenediol polymers, and cross-linked polymers.

Fenton processes produce reactive species that can oxidize organic compounds in natural and engineered systems. While it is well-documented that Fenton reactions produce hydroxyl radical (HO^•) under acidic conditions, we demonstrated the generation of ferryl ion (Fe^IVO²⁺) in the UV/Fe(III) and UV/Fe(III)/H₂O₂ systems at pH 2.8 using methyl phenyl sulfoxide (PMSO) as the probe compound. Moreover, we clarified that Fe^IVO²⁺ is parallelly formed via the oxidation of Fe(III) by HO^• and the O–O homolysis of [Fe^III–OOH]²⁺ in the photo-Fenton process. The rate constant for the reaction between HO^• and Fe³⁺ measured by laser flash photolysis was 4.41 × 10⁷ M^–1 s^–1. The rate constant and quantum yield for thermal and photo O–O homolysis of [Fe^III–OOH]²⁺ complex were 1.4 × 10^–2 s^–1 and 0.3, respectively, which were determined by fitting PMSO₂ formation. While Fe^IVO²⁺ forms predominantly through the reaction between HO^• and Fe³⁺ in the absence of H₂O₂, the relative contribution of [Fe^III–OOH]²⁺ O–O homolysis to Fe^IVO²⁺ formation highly depends on the molar ratio of [H₂O₂]₀/[Fe(III)]₀, the level of HO^• scavenging, and incident irradiance in the UV/Fe(III)/H₂O₂ system. Accordingly, an optimized kinetic model was developed by incorporating Fe^IVO²⁺-involved reactions into the conventional photo-Fenton model, which can accurately predict Fe(II) formation and contaminant decay in the UV/Fe(III) and UV/Fe(III)/H₂O₂ systems. Our study illuminated the underlying formation mechanism of reactive oxidative species in the photo-Fenton process and highlighted the role of Fe^IVO²⁺ evolution in modulating the iron cycle and pollutant abatement therein.

Radicals in advanced oxidation processes (AOPs) degrade micropollutants during water and wastewater treatment, but the transformation of dissolved organic matter (DOM) may be equally important. Ketone moieties in DOM are known disinfection byproduct precursors, but ketones themselves are intermediates produced during AOPs. We found that aromatic alcohols in DOM underwent transformation to ketones by one-electron oxidants (using SO₄^•– as a representative), and the formed ketones significantly increased trichloromethane (CHCl₃) formation potential (FP) upon subsequent chlorination. CHCl₃-FPs from aromatic ketones (Ar–CO–CH₃, average of 22 mol/mol) were 6–24 times of CHCl₃-FPs from aromatic alcohols (Ar–CH(OH)–CH₃, average of 0.85 mol/mol). At a typical SO₄^•– exposure of 7.0 × 10^–12 M·s, CHCl₃-FPs from aromatic alcohol transformation increased by 24.8%–112% with an average increase of 53.4%. Notably, SO₄^•– oxidation of aliphatic alcohols resulted in minute changes in CHCl₃-FPs due to their low reactivities with SO₄^•– (∼10⁷ M^–1 s^–1). Other one-electron oxidants (Cl₂^•–, Br₂^•–,and CO₃^•–) are present in AOPs and also lead to aromatic alcohol–ketone transformations similar to SO₄^•–. This study highlights that subtle changes in DOM physicochemical properties due to one-electron oxidants can greatly affect the reactivity with free chlorine and the formation of chlorinated byproducts.

In this study, novel light emitting diode (LED)-activated periodate (PI) advanced oxidation process (AOP) at an irradiation wavelength in the ultraviolet A range (UVA, UVA-LED/PI AOP) was developed and investigated using naproxen (NPX) as a model micropollutant. The UVA-LED/PI AOP remarkably enhanced the degradation of NPX and seven other selected micropollutants with the observed pseudo-first-order rate constants ranging from 0.069 ± 0.001 to 4.50 ± 0.145 min^–1 at pH 7.0, demonstrating a broad-spectrum micropollutant degradation ability. Lines of evidence from experimental analysis and kinetic modeling confirmed that hydroxyl radical (^•OH) and ozone (O₃) were the dominant species generated in UVA-LED/PI AOP, and they contributed evenly to NPX degradation. Increasing the pH and irradiation wavelength negatively affected NPX degradation, and this could be well explained by the decreased quantum yield (Φ_PI) of PI. The degradation kinetics of NPX by the UVA-LED/PI AOP in the presence of water matrices (i.e., chloride, bicarbonate, and humic acid) and in real waters were examined, and the underlying mechanisms were illustrated. A total of nine transformation products were identified from NPX oxidation by the UVA-LED/PI AOP, mainly via hydroxylation, dealkylation, and oxidation pathways. The UVA-LED/PI AOP proposed might be a promising technology for the treatment of micropollutants in aqueous solutions. The pivotal role of Φ_PI during light photolysis of PI may guide the future design of light-assisted PI AOPs.

Ferryl ion ([Fe^IVO]²⁺) has often been suggested to play a role in iron-based advanced oxidation processes (AOPs) with its presence commonly determined using the unique oxidation pathway from methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO₂). However, we show here that the oxidation products of PMSO, formed on reaction with hydroxyl radical, enhance PMSO₂ formation as a result of their complexation with Fe(III) leading to the changes in the reactivity of Fe(III) species in the homogeneous Fenton reaction. As such, PMSO should be used with caution to investigate the role of [Fe^IVO]²⁺ in iron-based AOPs with these insights suggesting the need to reassess the findings of many previous studies in which this reagent was used. The other common target compounds, phthalhydrazide and hydroxybenzoic acids, were also found to modify the rate and extent of iron cycling as a result of complexation and/or redox reactions, either by the probe compound itself and/or oxidation products formed. Overall, this study highlights that these confounding effects of the aromatic probe compounds on the reactivity of iron species should be recognized if reliable mechanistic insights into iron-based AOPs are to be obtained.

Although Ag-containing photocatalysts exhibit excellent photocatalytic ability, they present great challenges owing to their photocorrosion and ease of reduction. Herein, an electron acceptor platform of Ag₂O/La(OH)₃/polyacrylonitrile (PAN) fiber was constructed using a heterojunction strategy and electrospinning technology to develop a novel photocatalytic membrane with a redesigned electron transport pathway. Computational and experimental results demonstrate that the optimized electron transport pathway included intercrystal electron transfer induced by the La–O bond between Ag₂O and La(OH)₃ as well as electron transfer between the catalyst crystal and electrophilic PAN membrane interface. In addition, the photocatalytic performance of the Ag₂O/La(OH)₃ membrane for tetracycline (TC) removal was still above 97% after five photocatalytic reaction cycles. Furthermore, the carrier life was greatly extended. Mechanistic study revealed that photogenerated holes on the Ag₂O/La(OH)₃ membrane were the main reactive species in TC degradation. Overall, this study proposes a novel electron transport pathway strategy that effectively solves the problems of photocatalyst photocorrosion and structural instability.

Heterogeneous catalytic ozonation (HCO) has gained increasing attention as an effective process to remove refractory organic pollutants from industrial effluents. However, widespread application of HCO is still limited due to the typically low efficacy of catalysts used and matrix passivation effects. To this end, we prepared an Al₂O₃-supported Fe catalyst with high reactivity via a facile urea-based heterogeneous precipitation method. Due to the nonsintering nature of the preparation method, a heterogeneous catalytic layer comprised of γ-FeOOH and α-Fe₂O₃ is formed on the Al₂O₃ support (termed NS-Fe-Al₂O₃). On treatment of a real industrial effluent by HCO, the presence of NS-Fe-Al₂O₃ increased the removal of organics by ∼100% compared to that achieved with a control catalyst (i.e., α-Fe₂O₃/Al₂O₃ or γ-FeOOH/Al₂O₃) that was prepared by a conventional impregnation and calcination method. Furthermore, our results confirmed that the novel NS-Fe-Al₂O₃ catalyst demonstrated resistance to the inhibitory effect of high concentration of chloride and sulfate ions usually present in industrial effluent. A mathematical kinetic model was developed that adequately describes the mechanism of HCO process in the presence of NS-Fe-Al₂O₃. Overall, the results presented here provide valuable guidance for the synthesis of effective and robust catalysts that will facilitate the wider industrial application of HCO.

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 (k_app, 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.

Bromine radical (Br^•) has been hypothesized to be a key intermediate of bromate formation during ozonation. Once formed, Br^• further reacts with ozone to eventually form bromate. However, this reaction competes with the reaction of Br^• with dissolved organic matter (DOM), of which reactivity and reaction mechanisms are less studied to date. To fill this gap, this study determined the second-order rate constant (k) of the reactions of selected organic model compounds, a DOM isolate, and monochloramine (NH₂Cl) with Br^• using γ-radiolysis. The k_Br• of all model compounds were high (k_Br• > 10⁸ M^–1 s^–1) and well correlated with quantum-chemically computed free energies of activation, indicating a selectivity of Br^• toward electron-rich compounds, governed by electron transfer. The reaction of phenol (a representative DOM moiety) with Br^• yielded p-benzoquinone as a major product with a yield of 59% per consumed phenol, suggesting an electron transfer mechanism. Finally, the potential of NH₂Cl to quench Br^• was tested based on the fast reaction (k_{Br•, NH2Cl} = 4.4 × 10⁹ M^–1 s^–1, this study), resulting in reduced bromate formation of up to 77% during ozonation of bromide-containing lake water. Overall, our study demonstrated that Br^• quenching by NH₂Cl can substantially suppress bromate formation, especially in waters containing low DOC concentrations (1–2 mgC/L).

Hydroxyl radical production via catalytic activation of HOCl is a new type of Fenton-like process. However, metal–chlorocomplex formation under high chloride conditions could deactivate the catalyst and reduce the process efficiency. Herein, in situ electrogenerated HOCl was activated to ^•OH via a metal-free, B/N-codoped carbon nanofiber cathode for the first time to degrade contaminant under high chloride condition. The results show 98% degradation of rhodamine B (RhB) within 120 min (k = 0.036 min^–1) under sulfate conditions, while complete degradation (k = 0.188 min^–1) was obtained in only 30 min under chloride conditions. An enhanced degradation mechanism consists of an Adsorb & Shuttle process, wherein adsorption concentrates the pollutants at the cathode surface and they are subsequently oxidized by the large amount of ^•OH produced via activation of HOCl and H₂O₂ at the cathode. Density functional theory calculations verify the pyridinic N as the active site for the activation of HOCl and H₂O₂. The process efficiency was also evaluated by treating tetracycline and bisphenol A as well as high chloride-containing real secondary effluents from a pesticide manufacturing plant. High yields of ^•OH and HOCl allow continuous regeneration of the cathode for several cycles, limiting its fast deactivation, which is promising for real application.

Low-cost stainless-steel electrodes can activate hydrogen peroxide (H₂O₂) by converting it into a hydroxyl radical (^•OH) and other reactive oxidants. At an applied potential of +0.020 V, the stainless-steel electrode produced ^•OH with a yield that was over an order of magnitude higher than that reported for other systems that employ iron oxides as catalysts under circumneutral pH conditions. Decreasing the applied potential at pH 8 and 9 enhanced the rate of H₂O₂ loss by shifting the process to a reaction mechanism that resulted in the formation of an Fe(IV) species. Significant metal leaching was only observed under acidic pH conditions (i.e., at pH <6), with the release of dissolved Fe and Cr occurring as the thickness of the passivation layer decreased. Despite the relatively high yield of ^•OH production under circumneutral pH conditions, most of the oxidants were scavenged by the electrode surface when contaminant concentrations comparable to those expected in drinking water sources were tested. The stainless-steel electrode efficiently removed trace organic contaminants from an authentic surface water sample without contaminating the water with Fe and Cr. With further development, stainless-steel electrodes could provide a cost-effective alternative to other H₂O₂ activation processes, such as those by ultraviolet light.

The kinetics of coxsackievirus serotype B5 (CVB5) inactivation with free chlorine is characterized over a range of pH and temperature relevant to drinking water treatment with the primary goal of selecting experimental conditions used for assessing inactivation mechanisms. The inactivation kinetics identified in our study is similar to or slower than experimental data reported in the literature and thus provides a conservative representation of the kinetics of CVB5 inactivation for free chlorine that could be useful in developing future regulations for waterborne viral pathogens including adequate disinfection treatment for CVB5. Untreated and free chlorine-treated viruses, and host cells synchronized-infected with these viruses, are analyzed by a reverse transcription-quantitative polymerase chain reaction (RT-qPCR) method with the goal of quantitatively investigating the effect of free chlorine exposure on viral genome integrity, attachment to host cell, and viral genome replication. The inactivation kinetics observed results from a combination of hindering virus attachment to the host cell, inhibition of one or more subsequent steps of the replication cycle, and possibly genome damage.

Difficulties arise related to the economy-of-scale and practicability in applying conventional water treatment technologies to small and remote systems. A promising oxidation technology better suited for these applications is that of electro-oxidation (EO), whereby contaminants are degraded via direct, advanced, and/or electrosynthesized oxidant-mediated reactions. One species of oxidants of particular interest includes ferrates (Fe(VI)/(V)/(IV)), where only recently has their circumneutral synthesis been demonstrated, using high oxygen overpotential (HOP) electrodes, namely boron-doped diamond (BDD). In this study, the generation of ferrates using various HOP electrodes (BDD, NAT/Ni–Sb–SnO₂, and AT/Sb-SnO₂) was investigated. Ferrate synthesis was pursued in a current density range of 5–15 mA cm^–2 and initial Fe³⁺ concentrations of 10–15 mM. Faradaic efficiencies ranged from 11–23%, depending on operating conditions, with BDD and NAT significantly outperforming AT electrodes. Speciation tests revealed that NAT synthesizes both ferrate(IV/V) and ferrate(VI), while the BDD and AT electrodes synthesized only ferrate(IV/V) species. A number of organic scavenger probes were used to test the relative reactivity, including nitrobenzene, carbamazepine, and fluconazole, whereby ferrate(IV/V) was significantly more oxidative than ferrate(VI). Finally, the ferrate(VI) synthesis mechanism by NAT electrolysis was elucidated, where coproduction of ozone was found to be a key phenomenon for Fe³⁺ oxidation to ferrate(VI).

Peroxyacids (POAs) are a promising alternative to chlorine for reducing the formation of disinfection byproducts. However, their capacity for microbial inactivation and mechanisms of action require further investigation. We evaluated the efficacy of three POAs (performic acid (PFA), peracetic acid (PAA), and perpropionic acid (PPA)) and chlor(am)ine for inactivation of four representative microorganisms (Escherichia coli (Gram-negative bacteria), Staphylococcus epidermidis (Gram-positive bacteria), MS2 bacteriophage (nonenveloped virus), and Φ6 (enveloped virus)) and for reaction rates with biomolecules (amino acids and nucleotides). Bacterial inactivation efficacy (in anaerobic membrane bioreactor (AnMBR) effluent) followed the order of PFA > chlorine > PAA ≈ PPA. Fluorescence microscopic analysis indicated that free chlorine induced surface damage and cell lysis rapidly, whereas POAs led to intracellular oxidative stress through penetrating the intact cell membrane. However, POAs (50 μM) were less effective than chlorine at inactivating viruses, achieving only ∼1-log PFU removal for MS2 and Φ6 after 30 min of reaction in phosphate buffer without genome damage. Results suggest that POAs’ unique interaction with bacteria and ineffective viral inactivation could be attributed to their selectivity toward cysteine and methionine through oxygen-transfer reactions and limited reactivity for other biomolecules. These mechanistic insights could inform the application of POAs in water and wastewater treatment.

The precise identification of predominant toxic disinfection byproducts (DBPs) from disinfected water is a longstanding challenge. We propose a new acellular analytical strategy, the ‘Thiol Reactome’, to identify thiol-reactive DBPs by employing a thiol probe and nontargeted mass spectrometry (MS) analysis. Disinfected/oxidized water samples had reduced cellular oxidative stress responses of 46 ± 23% in Nrf2 reporter cells when preincubated with glutathione (GSH). This supports thiol-reactive DBPs as the predominant drivers of oxidative stress. This method was benchmarked using seven classes of DBPs including haloacetonitriles, which preferentially reacted with GSH via substitution or addition depending on the number of halogens present. The method was then applied to chemically disinfected/oxidized waters, and 181 tentative DBP-GSH reaction products were detected. The formulas of 24 high abundance DBP-GSH adducts were predicted, among which nitrogenous-DBPs (11) and unsaturated carbonyls (4) were the predominant compound classes. Two major unsaturated carbonyl-GSH adducts, GSH-acrolein and GSH-acrylic acid, were confirmed by their authentic standards. These two adducts were unexpectedly formed from larger native DBPs when reacting with GSH. This study demonstrated the “Thiol Reactome” as an effective acellular assay to precisely identify and broadly capture toxic DBPs from water mixtures.

Dichloramine (NHCl₂) often coexists with monochloramine (NH₂Cl) in reverse osmosis (RO) permeate in potable reuse scenarios when NH₂Cl is added upstream of RO for membrane fouling control such that UV photolysis of NHCl₂ occurs during the downstream UV/chloramine process. However, the formation of reactive nitrogen species (RNS) and their incorporation into byproducts during the UV/NHCl₂ process are largely unknown. This study quantitatively evaluated the generation of RNS in the UV/NHCl₂ process and investigated the role of RNS in micropollutant transformation. UV photolysis of NHCl₂ produced comparable RNS concentration to that of NH₂Cl at the same oxidant dosage (100 μM) at pH 5.5. Under the experimental conditions, the RNS contributed greatly (40.6%) to N,N-diethyl-3-methylbenzamide (DEET) degradation. By using ¹⁵N-labeling and mass spectrometry methods, seven nitrogenous byproducts of DEET degradation with the incorporation of nitrogen originating from the RNS were detected. Among these seven byproducts, six were identified to contain a nitro group (−NO₂). While the UV/NHCl₂ process formed comparable intensities of −NO-containing products to those in the UV/NH₂Cl process, the later process formed 3–91% higher intensities of −NO₂-containing products. These findings are essential in furthering our understanding of the contribution of the UV/NHCl₂ process in potable reuse scenarios.

Conventional water treatment practices utilizing chemical disinfection, especially chlorination, are considered generally effective in producing microbiologically safe drinking water. However, protozoan pathogens such as oocysts of Cryptosporidium parvum are very resistant to chlorine, which has led to consideration of alternative disinfectants for their control. Free bromine, HOBr, has not been evaluated extensively as an alternative halogen disinfectant for inactivation of Cryptosporidium parvum in drinking water or reclaimed water for non-potable uses. Bromine is a versatile disinfectant consisting of different chemical forms with persistent microbicidal efficacy under varied water quality conditions and is effective against a range of waterborne microbes of health concern. The objectives of this study are to (1) compare the efficacy of free bromine to free chlorine at similar concentrations (as milligrams per liter) for disinfection of Cryptosporidium parvum oocysts, Bacillus atrophaeus spores, and MS2 coliphage in a model buffered water and (2) evaluate the kinetics of inactivation of these microorganisms using appropriate disinfection models. Overall, at a target concentration of ∼5 mg/L, bromine averaged 0.6 log (73.8%) reductions of C. parvum oocyst infectivity after 300 min (CT: 1166 min·mg/L) and produced up to a 0.8 log reduction disinfectant activity. An ∼5.0 mg/L chlorine dose increased oocyst infectivity by only 0.4 log (64%) after 300 min (CT: 895 min·mg/L). Bacillus atrophaeus spores and MS2 coliphage treated with bromine and chlorine were reduced by 4 log₁₀ (99.99%) for both disinfectants over the duration of the experiments.

Formation of nitrogenous disinfection byproducts from aliphatic amines is a widespread concern owing to the serious health risks associated with them. However, the mechanisms of transforming aliphatic amines and forming nitro products in the UV/chlorine process have rarely been discussed, which are investigated in this work. Initially, secondary amines (R₁R₂NH) are transformed into secondary organic chloramines (R₁R₂NCl) via chlorination. Subsequently, radicals, such as HO• and Cl•, are found to contribute predominantly to such transformations. The rate constants at which HO•, Cl•, and Cl₂^•– react with R₁R₂NCl are (2.4–5.1) × 10⁹, (1.5–3.8) × 10⁹, and (1.2–6.1) × 10⁷ M^–1 s^–1, respectively. Consequently, R₁R₂NCl are transformed into primary amines (R₁NH₂/R₂NH₂) and chlorinated primary amines (R₁NHCl/R₂NHCl and R₁NCl₂/R₂NCl₂) by excess chlorine. Furthermore, primarily driven by UV photolysis, chlorinated primary amines can be transformed into nitroalkanes with conversion rates of ∼10%. Dissolved oxygen and free chlorine play crucial roles in forming nitroalkanes, and post-chlorination can further form chloronitroalkanes, such as trichloronitromethane (TCNM). Radicals are involved in forming TCNM in the UV/chlorine process. This study provides new insights into the mechanisms of transforming aliphatic amines and forming nitro products using the UV/chlorine process.

The onset of spring runoff in northern climates and tap water odor events are difficult to predict because common water quality parameters cannot fully explain the intermittent odor events that occurred over past decades. Studies have shown that small polar water-soluble compounds, such as amino acids (AAs), leach first from ice/snowmelt. AAs are known to produce odorous compounds, such as aldehydes and chloroaldimines, upon chlorination. Therefore, we proposed that AAs may serve as markers for small and soluble organics that contribute to the odor of chlorinated tap water. Here, we studied the occurrence of AAs in source water collected at two water treatment plants and the odor profiles of tap water at >300 homes during the 2021 and 2022 spring runoff events. AA concentrations were at baseline levels (<100 ng/L) during the 2021 runoff but much higher (up to 5500 ng/L) in 2022 and associated with an escalation in odor complaints. AA concentrations peaked at the onset of the 2022 spring runoff and corresponded with the strongest reported odor intensities in tap water. We obtained high resolution MS and MS/MS spectra of chloroaldimines and confirmed the formation of chloroaldimines under chlorination of the six AAs detected in source water. The results indicate that AAs signal the onset of spring runoff and represent small polar water-soluble compounds that may contribute to tap water odor problems.

Pharmaceuticals have been considered a priority group of emerging micropollutants in source waters in recent years, while their role in the formation and toxicity of disinfection byproducts (DBPs) during chlorine disinfection remains largely unclear. In this study, the contributions of natural organic matter (NOM) and pharmaceuticals (a mixture of ten representative pharmaceuticals) to the overall DBP formation and toxicity during drinking water chlorination were investigated. By innovatively “normalizing” chlorine exposure and constructing a kinetic model, we were able to differentiate and evaluate the contributions of NOM and pharmaceuticals to the total organic halogen (TOX) formation for source waters that contained different levels of pharmaceuticals. It was found that at a chlorine contact time of 1.0 h, NOM (2 mg/L as C) and pharmaceuticals (total 0.0062–0.31 mg/L as C) contributed 79.8–99.5% and 0.5–20.2%, respectively, of TOX. The toxicity test results showed that the chlorination remarkably increased the toxicity of the pharmaceutical mixture by converting the parent compounds into more toxic pharmaceutical-derived DBPs, and these DBPs might contribute significantly to the overall developmental toxicity of chlorinated waters. This study highlights the non-negligible role of pharmaceuticals in the formation and toxicity of overall DBPs in chlorinated drinking water.

As disinfection byproducts (DBPs) are ubiquitous sources of chemical exposure in disinfected drinking water, identifying unknown DBPs, especially unknown drivers of toxicity, is one of the major challenges in the safe supply of drinking water. While >700 low-molecular-weight DBPs have been identified, the molecular composition of high-molecular-weight DBPs remains poorly understood. Moreover, due to the absence of chemical standards for most DBPs, it is difficult to assess toxicity contributions for new DBPs identified. Based on effect-directed analysis, this study combined predictive cytotoxicity and quantitative genotoxicity analyses and Fourier transform ion cyclotron resonance mass spectrometry (21 T FT-ICR-MS) identification to resolve molecular weight fractions that induce toxicity in chloraminated and chlorinated drinking waters, along with the molecular composition of these DBP drivers. Fractionation using ultrafiltration membranes allowed the investigation of <1 kD, 1–3 kD, 3–5 kD, and >5 kD molecular weight fractions. Thiol reactivity based predictive cytotoxicity and single-cell gel electrophoresis based genotoxicity assays revealed that the <1 kD fraction for both chloraminated and chlorinated waters exhibited the highest levels of predictive cytotoxicity and direct genotoxicity. The <1 kD target fraction was used for subsequent molecular composition identification. Ultrahigh-resolution MS identified singly charged species (as evidenced by the 1 Da spacing in ¹³C isotopologues), including 3599 chlorine-containing DBPs in the <1 kD fraction with the empirical formulas CHOCl, CHOCl₂, and CHOCl₃, with a relative abundance order of CHOCl > CHOCl₂ ≫ CHOCl₃. Interestingly, more high-molecular-weight CHOCl_1–3 DBPs were identified in the chloraminated vs chlorinated waters. This may be due to slower reactions of NH₂Cl. Most of the DBPs formed in chloraminated waters were composed of high-molecular-weight Cl-DBPs (up to 1 kD) rather than known low-molecular-weight DBPs. Moreover, with the increase of chlorine number in the high-molecular-weight DBPs detected, the O/C ratio exhibited an increasing trend, while the modified aromaticity index (AI_mod) showed an opposite trend. In drinking water treatment processes, the removal of natural organic matter fractions with high O/C ratio and high AI_mod value should be strengthened to minimize the formation of known and unknown DBPs.

Parabens and salicylates were examined as disinfection byproduct (DBP) precursors to explore the possible influence of ipso substitution (i.e., halogen exchange) on the yield and speciation of trihalomethanes (THMs) formed during water chlorination. Substoichiometric conversion of C–Br bonds into C–Cl bonds was confirmed for several parabens and salicylates. The co-occurrence of (mono)brominated and nonhalogenated precursors in the presence of free chlorine (but in the absence of added Br^–) generated polybrominated THMs, implicating ipso substitution. The THM molar yield, bromine incorporation, and bromine recovery from brominated and nonhalogenated precursor mixtures were commensurate with those observed from equimolar additions of NaBr, indicating efficient displacement of aromatic bromine by free chlorine followed by reincorporation of liberated HOBr into DBP precursors. The THM molar yield from brominated precursors was enhanced by a factor of ≤20 relative to that from nonhalogenated precursors. Trends in THM molar yields and bromine incorporation differed between brominated parabens and brominated salicylates, suggesting that the influence of ipso substitution on THM formation varies with the structure of the organic precursor. Collectively, these results provide new evidence of the often-overlooked role ipso substitution can play in promoting halogen exchange and bromine enrichment among DBPs in chlorinated waters.

During the ozonation of wastewater, hydroxyl radicals (^•OH) induced by the reactions of ozone (O₃) with effluent organic matters (EfOMs) play an essential role in degrading ozone-refractory micropollutants. The ^•OH yield provides the absolute ^•OH formation during ozonation. However, the conventional “tert-Butanol (t-BuOH) assay” cannot accurately determine the ^•OH yield since the propagation reactions are inhibited, and there have been few studies on ^•OH production induced by EfOM fractions during ozonation. Alternatively, a “competitive method”, which added trace amounts of the ^•OH probe compound to compete with the water matrix and took initiation reactions and propagation reactions into account, was used to determine the actual ^•OH yields (Φ) compared with that obtained by the “t-BuOH assay” (φ). The Φ were significantly higher than φ, indicating that the propagation reactions played important roles in ^•OH formation. The chain propagation reactions facilitation of EfOMs and fractions can be expressed by the chain length (n). The study found significant differences in Φ for EfOMs and fractions, precisely because they have different n. The actual ^•OH yield can be calculated by n and φ as Φ = φ (1 + n)/(nφ + 1), which can be used to accurately predict the removal of micropollutants during ozonation of wastewater.

Aliphatic amines are abundant micropollutants in wastewater treatment plant effluents. In order to mitigate such micropollutants, ozonation is one of the most commonly employed advanced treatment processes. Current research regarding ozone efficiency is heavily focusing on reaction mechanisms of different contaminant groups, including structures with amine moieties as reactive sites. This study analyzes pH-dependent reaction kinetics and pathways of gabapentin (GBP), an aliphatic primary amine with an additional carboxylic acid group. The transformation pathway was elucidated applying a novel approach using isotopically labeled ozone (¹⁸O) and quantum chemistry calculations. While the direct reaction of GBP with ozone is highly pH-dependent and slow at pH 7 (13.7 M^–1 s^–1), the rate constant of the deprotonated species (1.76 × 10⁵ M^–1 s^–1) is comparable to those of other amine compounds. Pathway analysis based on LC-MS/MS measurements revealed that ozonation of GBP leads to the formation of a carboxylic acid group and simultaneous nitrate formation, which was also observed in the case of the aliphatic amino acid glycine. Nitrate was formed with a yield of approximately 100%. Experiments with ¹⁸O-labeled ozone demonstrated that the intermediate aldehyde does most likely not include any oxygen originating from ozone. Furthermore, quantum chemistry calculations did not provide an explanation for the C–N scission during GBP ozonation without ozone involvement, although this reaction was slightly more favorable than for respective glycine and ethylamine reactions. Overall, this study contributes to a deeper understanding of reaction mechanisms of aliphatic primary amines during wastewater ozonation.

Dichloroacetonitrile (DCAN) as one of the potentially prioritized regulated DBPs has drawn great attention; however, understanding its formation, especially the C–C bond cleavage mechanisms, is limited. In this study, DCAN formation mechanisms from long-chain primary amines, amino acids, and dipeptides during chlorination were investigated by a combined computational and experimental approach. The results indicate that nitriles initially generate for all of the above precursors, then they undergo β-C-hydroxylation or/and α-C-chlorination processes, and finally, DCAN is produced through the C_α–C_β bond cleavage. For the first time, the underlying mechanism of the C–C bond cleavage was unraveled to be electron transfer from the O^– anion into its attached C atom in the chlorinated nitriles, leading to the strongly polarized C_α–C_β bond heterocleavage and DCAN^– formation. Moreover, DCAN molar yields of precursors studied in the present work were found to be determined by their groups at the γ-site of the amino group, where the carbonyl group including −CO₂^–, −COR, and −CONHR, the aromatic group, and the −OH group can all dramatically facilitate DCAN formation by skipping over or promoting the time-consuming β-C-hydroxylation process and featuring relatively lower activation free energies in the C–C bond cleavage. Importantly, 4-amino-2-hydroxybutyric acid was revealed to possess the highest DCAN yield among all the known aliphatic long-chain precursors to date during chlorination. Additionally, enonitriles, (chloro-)isocyanates, and nitriles can be generated during DCAN formation and should be of concern due to their high toxicities.

Chlorine dioxide (ClO₂) is an oxidant applied in water treatment processes that is very effective for disinfection and abatement of inorganic and organic pollutants. Thereby phenol is the most important reaction partner of ClO₂ in reactions of natural organic matter (NOM) and in pollutant degradation. It was previously reported that with specific reaction partners (e.g., phenol), free available chlorine (FAC) could form as another byproduct next to chlorite (ClO₂^–). This study investigates the impact of different functional groups attached to the aromatic ring of phenol on the formation of inorganic byproducts (i.e., FAC, ClO₂^–, chloride, and chlorate) and the overall reaction mechanism. The majority of the investigated compounds reacted with a 2:1 stoichiometry and formed 50% ClO₂^– and 50% FAC, regardless of the position and kind of the groups attached to the aromatic ring. The only functional groups strongly influencing the FAC formation in the ClO₂ reaction with phenols were hydroxyl- and amino-substituents in ortho- and para-positions, causing 100% ClO₂^– and 0% FAC formation. Additionally, this class of compounds showed a pH-dependent stoichiometric ratio due to pH-dependent autoxidation. Overall, FAC is an important secondary oxidant in ClO₂ based treatment processes. Synergetic effects in pollutant control and disinfection might be observable; however, the formation of halogenated byproducts needs to be considered as well.

Ozone is commonly used as a predisinfectant in potable water reuse treatment trains. Nitromethane was recently found as a ubiquitous ozone byproduct in wastewater, and the key intermediate toward chloropicrin during subsequent secondary disinfection of ozonated wastewater effluent with chlorine. However, many utilities have switched from free chlorine to chloramines as a secondary disinfectant. The reaction mechanism and kinetics of nitromethane transformation by chloramines, unlike those for free chlorine, are unknown. In this work, the kinetics, mechanism, and products of nitromethane chloramination were studied. The expected principal product was chloropicrin, because chloramines are commonly assumed to react similarly to, although more slowly than, free chlorine. Different molar yields of chloropicrin were observed under acidic, neutral, and basic conditions, and surprisingly, transformation products other than chloropicrin were found. Monochloronitromethane and dichloronitromethane were detected at basic pH, and the mass balance was initially poor at neutral pH. Much of the missing mass was later attributed to nitrate formation, from a newly identified pathway involving monochloramine reacting as a nucleophile rather than a halogenating agent, through a presumed S_N2 mechanism. The study indicates that nitromethane chloramination, unlike chlorination, is likely to produce a range of products, whose speciation is a function of pH and reaction time.

Increasing the radical yield and reducing energy consumption would enhance the sustainability and competitiveness of advanced oxidation processes (AOPs) for micropollutant degradation in water. We herein report a novel AOP coupling far-UVC radiation at 222 nm with chlorinated cyanurates (termed the UV₂₂₂/Cl-cyanurates AOP) for radical generation and micropollutant abatement in water. We experimentally determined the concentrations of HO^•, Cl^•, and ClO^• in the UV₂₂₂/Cl-cyanurates AOP in deionized water and swimming pool water. The radical concentrations are 10–27 times and 4–13 times, respectively, higher than those in the UV₂₅₄/Cl-cyanurates AOP and the well-documented UV₂₅₄/chlorine AOP under comparable conditions (e.g., same UV fluence and oxidant dosing). We determined the molar absorption coefficients and innate quantum yields of two chlorine species and two Cl-cyanurates at 222 nm and incorporated these parameters into a kinetic model. The model enables accurate prediction of oxidant photodecay rates as well as the pH impact on radical generation in the UV₂₂₂/Cl-cyanurates AOP. We predicted the pseudo-first-order degradation rate constants of 25 micropollutants in the UV₂₂₂/Cl-cyanurates AOP and demonstrated that many micropollutants can be degraded by >80% with a low UV fluence of 25 mJ cm^–2. This work advances the fundamental photochemistry of chlorine and Cl-cyanurates at 222 nm and offers a highly effective engineering tool in combating micropollutants in water where Cl-cyanurates are suitable to use.

Chlorine photolysis is an advanced oxidation process that relies on the combination of direct chlorination by free available chlorine, direct photolysis, and reactive oxidants to transform contaminants. In waters that contain bromide, free available bromine and reactive bromine species can also form. However, little is known about the underlying mechanisms or formation potential of disinfection byproducts (DBPs) under these conditions. We investigated reactive oxidant generation and DBP formation under dark conditions, chlorine photolysis, and radical-quenched chorine photolysis with variable chlorine (0–10 mg-Cl₂/L) and bromide (0–2,000 μg/L) concentrations, as well as with free available bromine. Probe loss rates and ozone concentrations increase with chlorine concentration and are minimally impacted by bromide. Radical-mediated processes partially contribute to the formation targeted DBPs (i.e., trihalomethanes, haloacetic acids, haloacetonitriles, chlorate, and bromate), which increase with increasing chlorine concentration. Chlorinated novel DBPs detected by high-resolution mass spectrometry are attributable to a combination of dark chlorination, direct halogenation by reactive chlorine species, and transformation of precursors, whereas novel brominated DBPs are primarily attributable to dark bromination of electron-rich formulas. The formation of targeted and novel DBPs during chlorine photolysis in waters with elevated bromide may limit treatment applications.

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

Peracetic acid (PAA) and performic acid (PFA) are two major peroxyacid (POA) oxidants of growing usage. This study reports the first systematic evaluation of PAA, PFA, and chlorine for their disinfection byproduct (DBP) formation potential in wastewater with or without high halide (i.e., bromide or iodide) concentrations. Compared with chlorine, DBP formation by PAA and PFA was minimal in regular wastewater. However, during 24 h disinfection of saline wastewater, PAA surprisingly produced more brominated and iodinated DBPs than chlorine, while PFA effectively kept all tested DBPs at bay. To understand these phenomena, a kinetic model was developed based on the literature and an additional kinetic investigation of POA decay and DBP (e.g., bromate, iodate, and iodophenol) generation in the POA/halide systems. The results show that PFA not only oxidizes halides 4–5 times faster than PAA to the corresponding HOBr or HOI but also efficiently oxidizes HOI/IO^– to IO₃^–, thereby mitigating iodinated DBP formation. Additionally, PFA’s rapid self-decay and slow release of H₂O₂ limit the HOBr level over the long-term oxidation in bromide-containing water. For saline water, this paper reveals the DBP formation potential of PAA and identifies PFA as an alternative to minimize DBPs. The new kinetic model is useful to optimize oxidant selection and elucidate involved DBP chemistry.

Krypton chloride (KrCl*) excimer ultraviolet (UV) light may provide advantages for contaminant degradation compared to conventional low-pressure (LP) UV. Direct and indirect photolysis as well as UV/hydrogen peroxide-driven advanced oxidation (AOP) of two chemical contaminants were investigated in laboratory grade water (LGW) and treated secondary effluent (SE) for LPUV and filtered KrCl* excimer lamps emitting at 254 and 222 nm, respectively. Carbamazepine (CBZ) and N-nitrosodimethylamine (NDMA) were chosen because of their unique molar absorption coefficient profiles, quantum yields (QYs) at 254 nm, and reaction rate constants with hydroxyl radical. Quantum yields and molar absorption coefficients at 222 nm for both CBZ and NDMA were determined, with measured molar absorption coefficients of 26 422 and 8170 M^–1 cm^–1, respectively, and QYs of 1.95 × 10^–2 and 6.68 × 10^–1 mol Einstein^–1, respectively. The 222 nm irradiation of CBZ in SE improved degradation compared to that in LGW, likely through promotion of in situ radical formation. AOP conditions improved degradation of CBZ in LGW for both UV LP and KrCl* sources but did not improve NDMA decay. In SE, photolysis of CBZ resulted in decay similar to that of AOP, likely due to the in situ generation of radicals. Overall, the KrCl* 222 nm source significantly improves contaminant degradation compared to that of 254 nm LPUV.

Improving the reactivity of Fe(III) for activating peroxymonosulfate (PMS) at circumneutral pH is critical to propel the iron-activated PMS processes toward practical wastewater treatment but is yet challenging. Here we employed the complexes of Fe(III) with the biodegradable picolinic acid (PICA) to activate PMS for degradation of selected chlorinated phenols, antibiotics, pharmaceuticals, herbicides, and industrial compounds at pH 4.0–6.0. The Fe^III–PICA complexes greatly outperformed the ligand-free Fe(III) and other Fe(III) complexes of common aminopolycarboxylate ligands. In the main activation pathway, the key intermediate is a peroxymonosulfate complex, tentatively identified as PICA–Fe^III–OOSO₃^–, which undergoes O–O homolysis or reacts with Fe^III–PICA and PMS to yield Fe^IV=O and SO₄^•– without the involvement of commonly invoked Fe(II). PICA–Fe^III–OOSO₃^– can also react directly with certain compounds (chlorophenols and sulfamethoxazole). The relative contributions of PICA–Fe^III–OOSO₃^–, Fe^IV=O, and SO₄^•– depend on the structure of target compounds. This work sets an eligible example to enhance the reactivity of Fe(III) toward PMS activation by ligands and sheds light on the previously unrecognized role of the metal–PMS complexes in directing the catalytic cycle and decontamination as well.

Metal-based advanced oxidation processes (AOPs) with peracetic acid (PAA) have been extensively studied to degrade micropollutants (MPs) in wastewater. Mn(II) is a commonly used homogeneous metal catalyst for oxidant activation, but it performs poorly with PAA. This study identifies that the biodegradable chelating ligand picolinic acid (PICA) can significantly mediate Mn(II) activation of PAA for accelerated MP degradation. Results show that, while Mn(II) alone has minimal reactivity toward PAA, the presence of PICA accelerates PAA loss by Mn(II). The PAA-Mn(II)-PICA system removes various MPs (methylene blue, bisphenol A, naproxen, sulfamethoxazole, carbamazepine, and trimethoprim) rapidly at neutral pH, achieving >60% removal within 10 min in clean and wastewater matrices. Coexistent H₂O₂ and acetic acid in PAA play a negligible role in rapid MP degradation. In-depth evaluation with scavengers and probe compounds (tert-butyl alcohol, methanol, methyl phenyl sulfoxide, and methyl phenyl sulfone) suggested that high-valent Mn species (Mn(V)) is a likely main reactive species leading to rapid MP degradation, whereas soluble Mn(III)-PICA and radicals (CH₃C(O)O^• and CH₃C(O)OO^•) are minor reactive species. This study broadens the mechanistic understanding of metal-based AOPs using PAA in combination with chelating agents and indicates the PAA-Mn(II)-PICA system as a novel AOP for wastewater treatment.

Peracetic acid (PAA) is an emerging alternative disinfectant for saline waters; HOBr or HOCl is known as the sole species contributing to halogenation reactions during PAA oxidation and disinfection. However, new results herein strongly indicated that the brominating agents (e.g., BrCl, Br₂, BrOCl, and Br₂O) are generated at concentrations typically lower than HOCl and HOBr but played significant roles in micropollutants transformation. The presence of Cl^– and Br^– at environmentally relevant levels could greatly accelerate the micropollutants (e.g., 17α-ethinylestraiol (EE2)) transformation by PAA. The kinetic model and quantum chemical calculations collectively indicated that the reactivities of bromine species toward EE2 follow the order of BrCl > Br₂ > BrOCl > Br₂O > HOBr. In saline waters with elevated Cl^– and Br^– levels, these overlooked brominating agents influence bromination rates of more nucleophilic constituents of natural organic matter and increase the total organic bromine. Overall, this work refines our knowledge regarding the species-specific reactivity of brominating agents and highlights the critical roles of these agents in micropollutant abatement and disinfection byproduct formation during PAA oxidation and disinfection.

Mitigation of undesired byproducts from ozonation of dissolved organic matter (DOM) such as aldehydes and ketones is currently hampered by limited knowledge of their precursors and formation pathways. Here, the stable oxygen isotope composition of H₂O₂ formed simultaneously with these byproducts was studied to determine if it can reveal this missing information. A newly developed procedure, which quantitatively transforms H₂O₂ to O₂ for subsequent ¹⁸O/¹⁶O ratio analysis, was used to determine the δ¹⁸O of H₂O₂ generated from ozonated model compounds (olefins and phenol, pH 3–8). A constant enrichment of ¹⁸O in H₂O₂ with a δ¹⁸O value of ∼59‰ implies that ¹⁶O–¹⁶O bonds are cleaved preferentially in the intermediate Criegee ozonide, which is commonly formed from olefins. H₂O₂ from the ozonation of acrylic acid and phenol at pH 7 resulted in lower ¹⁸O enrichment (δ¹⁸O = 47–49‰). For acrylic acid, enhancement of one of the two pathways followed by a carbonyl–H₂O₂ equilibrium was responsible for the smaller δ¹⁸O of H₂O₂. During phenol ozonation at pH 7, various competing reactions leading to H₂O₂ via an intermediate ozone adduct are hypothesized to cause lower δ¹⁸O in H₂O₂. These insights provide a first step toward supporting pH-dependent H₂O₂ precursor elucidation in DOM.

Electrical energy consumption per order (E_EO) is an important figure-of-merit for the selection and optimization of ultraviolet (UV)-based advanced oxidation processes (UV-AOPs). However, E_EO applications are limited by the lack of an accurate and facilitative evaluation method because E_EO presents reactor property dependence. In this study, we developed an E_EO prediction method for multiscale UV-AOP reactors for micropollutant removal in water. The method utilized the reaction rate constants determined in a reference reactor (e.g., mini-fluidic photoreaction system), complemented by a scale-up method that clarified the dependence of E_EO on reactor properties. The predicted results of various UV-AOPs were verified experimentally in four bench/pilot-scale reactors in laboratory and a full-scale flow-through reactor (FFR) in field using sulfamethazine as a model micropollutant. For example, E_EO values of 0.105 and 0.058 kWh m^–3 order^–1 were predicted in the FFR at H₂O₂ doses of 5 and 10 mg L^–1, respectively, which generally agreed with verification results. Additionally, the developed method could assist the identification of appropriate reactors in the laboratory for E_EO measurements, providing a valuable supplement for the E_EO prediction in practice. The developed method presents acceptable accuracy, convenience, and low cost, which would significantly facilitate E_EO evaluations for practical UV-AOP applications.

Per- and polyfluoroalkyl substances (PFAS) are widely used anthropogenic chemicals. Because of the strength of the carbon–fluorine bond, PFAS are not destroyed in typical water treatment processes. Sulfate (SO₄^•–) and hydroxyl (^•OH) radicals can oxidize some PFAS, but the behavior of per- and polyfluoroalkyl ether acids (PFEAs) in processes involving SO₄^•– and ^•OH is poorly understood. In this study, we determined second-order rate constants (k) describing the oxidation of 18 PFAS, including 15 novel PFEAs, by SO₄^•– and ^•OH. Among the studied PFAS, 6:2 fluorotelomer sulfonate reacted most readily with ^•OH [k_•OH = (1.1–1.2) × 10⁷ M^–1 s^–1], while polyfluoroalkyl ether acids containing an -O-CFH- moiety reacted more slowly [k_•OH = (0.5–1.0) × 10⁶ M^–1 s^–1]. In the presence of SO₄^•–, polyfluoroalkyl ether acids with an -O-CFH- moiety reacted more rapidly [k_SO4^•– = (0.89–4.6) × 10⁶ M^–1 s^–1] than perfluoroalkyl ether carboxylic acids (PFECAs) and a chloro-perfluoro-polyether carboxylic acid (ClPFPECA) [k_SO4^•– = (0.85–9.5) × 10⁴ M^–1 s^–1]. For homologous series of perfluoroalkyl carboxylic acids, linear and branched monoether PFECAs, and multiether PFECAs, PFAS chain length had little impact on second-order rate constants. SO₄^•– reacted with the carboxylic acid headgroup of perfluoroalkyl carboxylic acids and PFECAs. In contrast, for polyfluoroalkyl ether carboxylic and sulfonic acids with an -O-CFH- moiety, the site of SO₄^•– attack was the -O-CFH- moiety. Perfluoroalkyl ether sulfonic acids were not oxidized by SO₄^•– and ^•OH under the conditions evaluated in this study.

Monobromamine (NH₂Br) and dibromamine (NHBr₂) produced from reactions of hypobromous acid (HOBr) with ammonia can react with phenolic structures of natural organic matter (NOM) to produce disinfection byproducts such as bromoform (CHBr₃). The reactivity of NH₂Br was controlled by the reaction of the bromoammonium ion (NH₃Br⁺) with phenolate species, with specific rate constants ranging from 6.32 × 10² for 2,4,6-tribromophenol to 1.22 × 10⁸ M^–1 s^–1 for phenol. Reactions of NHBr₂ with phenol and bromophenols were negligible compared to its self-decomposition; rate constants could be determined only with resorcinol for pH > 7. At pH 8.1–8.2, no formation of CHBr₃ was observed from the reaction of NH₂Br with phenol while the reaction of NH₂Br with resorcinol produced a significant concentration of CHBr₃. In contrast to NH₂Br, a significant amount of CHBr₃ produced with an excess of NHBr₂ over phenol was explained by the reactions of HOBr produced from NHBr₂ decomposition. A comprehensive kinetic model including the formation and decomposition of bromamines and the reactivity of HOBr and NH₂Br with phenolic compounds was developed at pH 8.0–8.3. Furthermore, the kinetic model was used to evaluate the significance of the NH₂Br and NHBr₂ reactions with the phenolic structures of two NOM isolates.

This study explores the formation of bromate (BrO₃^–) in the copresence of Fe(VI) and bromide (Br^–). It challenges previous beliefs about the role of Fe(VI) as a green oxidant and highlights the crucial role of intermediates Fe(V) and Fe(IV) in the conversion of Br^– to BrO₃^–. The results show that the maximum concentration of BrO₃^– of 48.3 μg/L was obtained at 16 mg/L Br^– and that the contribution of Fe(V)/Fe(IV) to the conversion was positively related to pH. The study suggests that a single-electron transfer from Br^– to Fe(V)/Fe(IV) along with the generation of reactive bromine radicals is the first step of Br^– conversion, followed by the formation of OBr^– which was then oxidized to BrO₃^– by Fe(VI) and Fe(V)/Fe(IV). Some common background water constituents (e.g., DOM, HCO₃^–, and Cl^–) significantly inhibited BrO₃^– formation by consuming Fe(V)/Fe(IV) and/or scavenging the reactive bromine species. While investigations proposing to promote Fe(V)/Fe(IV) formation in Fe(VI)-based oxidation to enhance its oxidation capacity have been rapidly accumulated recently, this work called attention to the considerable formation of BrO₃^– in this process.

Regulation of the fast electron transport process for the generation and utilization of reactive oxygen species (ROS) by achieving fortified electron “nanofluidics” is effective for electrocatalytic oxidation of organic microcontaminants. However, limited available active sites and sluggish mass transfer impede oxidation efficiency. Herein, we fabricated a conductive electrocatalytic membrane decorated with hierarchical porous vertically aligned Fe(II)-modulated FeCo layered double hydroxide nanosheets (Fe(II)-FeCo LDHs) in an electro-Fenton system to maximize exposure of active sites and expedite mass transfer. The nanospaced interlayers of Fe(II)-FeCo LDHs within the microconfined porous structure formed by its vertical nanosheets highly boost the micro/nanofluidic distribution of target pollutants to active centers/species, achieving accelerated mass transferability. Aliovalent substitution by Fe(II) activates in-plane metallics to maximize the available active sites and makes each Fe(II)-FeCo LDH nanosheet a geometrical nanocarrier for constructing a fast electron “nanofluidic” to accelerate Fe(II) regeneration in Fe(III)/Fe(II) cycles. As a result, the Fe(II)-FeCo LDHs exhibited improved reactivity in catalyzing H₂O₂ to •OH and ¹O₂. Accordingly, the membrane exhibited a higher atrazine degradation kinetic (0.0441 min^–1) and degradation rate (93.2%), which were 4.7 and 2.1 times more than those of the bare carbon nanotube membrane, respectively. Additionally, the enhanced hydrophilic and strongly oxidized reactivity synergistically mitigated the organic fouling occurring in the pores and surface of the membrane. These findings clarify the activation mechanism of ROS over an innovative electrocatalytic membrane reactor design for organic microcontaminant treatment.

A carbon-based advanced oxidation process is featured for the nonradical electron-transfer pathway (ETP) from electron-donating organic compounds to activated persulfate complexes, enabling it as a green technology for the selective oxidation of organic pollutants in complex water environments. However, the thermodynamic and kinetic behaviors of the nonradical electron-transfer regime had been ambiguous due to a neglect of the influence of pH on the mechanisms. In this study, three kinds of organic pollutants were divided in the carbon-based ETP regime: (i) physio-adsorption, (ii) adsorption-dominated ETP (oxidation rate slightly surpasses adsorption rate), and (iii) oxidation-dominated ETP (oxidation rate outpaces the adsorption rate). The differential kinetic behaviors were attributed to the physicochemical properties of the organic pollutants. For example, the hydrophobicity, molecular radius, and positive electrostatic potential controlled the mass-transfer process of the adsorption stage of the reactants (peroxydisulfate (PDS) and organics). Meanwhile, other descriptors, including the Fukui index, oxidation potential, and electron cloud density regulated the electron-transfer processes and thus the kinetics of oxidation. Most importantly, the oxidation pathways of these organic pollutants could be altered by adjusting the water chemistry. This study reveals the principles for developing efficient nonradical systems to selectively remove and recycle organic pollutants in wastewater.

The industrial effluent from glyphosate production has high salinity and refractory organic contaminants. The removal of organics and the recycling of inorganic salts from this kind of water are challenging issues. In this study, electro-Fenton (EF) and membrane distillation (MD) were coupled in a single reactor utilizing a membrane-based electrode (Mem-GDE) with the ability to bidirectionally transfer vapor and oxygen and electrochemically synthesize H₂O₂. The operating thermal conditions for MD significantly promoted Fenton reactions and, thus, the removal of glyphosate. During operation, Fe species deposited on the Mem-GDE and enhanced its catalytic activity and adsorptive capacity, which markedly increased the apparent reaction rate constant of glyphosate by 6 times. This novel EF-MD process simultaneously removed organics and concentrated the inorganics, which is very meaningful for decreasing the costs for subsequent crystallization and achieving high-quality crystal salts. This study provides an efficient method for the treatment of organic–inorganic hybrid wastewater.

The increasing presence of antibiotics in water sources threatens public health and ecosystems. Various treatments have been previously applied to degrade antibiotics, yet their efficiency is commonly hindered by the presence of natural organic matter (NOM) in water. On the contrary, we show here that nine types of NOM and NOM model compounds improved the removal of trimethoprim and sulfamethoxazole by ferrate(VI) (Fe^VIO₄^2–, Fe(VI)) under mild alkaline conditions. This is probably associated with the presence of phenolic moieties in NOMs, as suggested by first-order kinetics using NOM, phenol, and hydroquinone. Electron paramagnetic resonance reveals that NOM radicals are generated within milliseconds in the Fe(VI)–NOM system via single-electron transfer from NOM to Fe(VI) with the formation of Fe(V). The dominance of the Fe(V) reaction with antibiotics resulted in their enhanced removal despite concurrent reactions between Fe(V) and NOM moieties, the radicals, and water. Kinetic modeling considering Fe(V) explains the enhanced kinetics of antibiotics abatement at low phenol concentrations. Experiments with humic and fulvic acids of lake and river waters show similar results, thus supporting the enhanced abatement of antibiotics in real water situations.

Previous studies showed that cupric oxide (CuO) can enhance the formation of trihalomethanes (THMs), haloacetic acids, and bromate during chlorination of bromide-containing waters. In this study, the impact of CuO on the formation kinetics and mechanisms of halogenated disinfection byproducts (DBPs) during chlorination was investigated. CuO does not enhance the formation of DBPs (i.e., 1,1,1-trichloropropanone, chloroform, and trichloroacetaldehyde (TCAL) /dichloroacetonitrile) during chlorination of acetone, 3-oxopentanedioic acid (3-OPA), and aspartic acid, respectively. This indicates that the halogen substitution pathway cannot be enhanced by CuO. Instead, CuO (0.1 g L^–1) accelerates the second-order rate constants for reactions of chlorine (HOCl) with TCAL, citric acid, and oxalic acid at pH 8.0 and 21 °C from <0.1 to 29.4, 7.2, and 15.8 M^–1 s^–1, respectively. Oxidation pathway predominates based on the quantification of oxidation products (e.g., a trichloroacetic acid yield of ∼100% from TCAL) and kinetic modeling. CuO can enhance the formation of DBPs (e.g., THMs, haloacetaldehydes, and haloacetonitriles) during chlorination of model compounds and dissolved organic matter, of which both halogen substitution and oxidation pathways are required. Reaction rate constants of rate-limiting steps (e.g., citric acid to 3-OPA, aromatic ring cleavage) could be enhanced by CuO via an oxidation pathway since CuO-HOCl complex is more oxidative toward a range of substrates than HOCl in water. These findings provide novel insights into the DBP formation pathway in copper-containing distribution systems.

Peroxymonosulfate (PMS)-based electrochemical advanced oxidation processes (EAOPs) have received widespread attention in recent years, but the precise nature of PMS activation and its impact on the overall process performance remain poorly understood. This study presents the first demonstration of the critical role played by the oxygen reduction reaction in the effective utilization of PMS and the subsequent enhancement of overall pollutant remediation. We observed the concurrent generation of H₂O₂ via oxygen reduction during the cathodic PMS activation by a model nitrogen-doped carbon nanotube catalyst. A complex interplay between H₂O₂ generation and PMS activation, as well as a locally increased pH near the electrode due to the oxygen reduction reaction, resulted in a SO₄^•–/^•OH-mixed oxidation environment that facilitated pollutant degradation. The findings of this study highlight a unique dependency between PMS-driven and H₂O₂-driven EAOPs and a new perspective on a previously unexplored route for further enhancing PMS-based treatment processes.