Transformation of Flame Retardant Tetrabromobisphenol A by Aqueous Chlorine and the Effect of Humic Acid

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.

New Insights into the Role of Humic Acid in Permanganate Oxidation of Diclofenac: A Novel Electron Transfer Mechanism

Humic acid (HA) ubiquitously existing in aquatic environments has been reported to significantly impact permanganate (KMnO4) decontamination processes. However, the underlying mechanism of the KMnO4/HA system remained elusive. In this study, an enhancing effect of HA on the KMnO4 oxidation of diclofenac (DCF) was observed over a wide solution pH range of 5-9. Surprisingly, the mechanism of HA-induced enhancement varied with solution pH. Quenching and chemical probing experiments revealed that manganese intermediates (Mn(III)-HA and MnO2) were responsible for the enhancement under acidic conditions but not under neutral and alkaline conditions. By combining KMnO4 decomposition, galvanic oxidation process experiments, electrochemical tests, and FTIR and XPS analysis, it was interestingly found that HA could effectively mediate the electron transfer from DCF to KMnO4 in neutral and alkaline solutions, which was reported for the first time. The formation of an organic-catalyst complex (i.e., HA-DCF) with lower reduction potential than the parent DCF was proposed to be responsible for the accelerated electron transfer from DCF to KMnO4. This electron transfer likely occurred within the complex molecule formed through the interaction between HA-DCF and KMnO4 (i.e., HA-DCF-KMnO4). These results will help us gain a more comprehensive understanding of the role of HA in the KMnO4 oxidation processes.

Trade-off effect of dissolved organic matter on degradation and transformation of micropollutants: A review in water decontamination

The degradation of micropollutants by various treatments is commonly affected by the ubiquitous dissolved organic matter (DOM) in the water environment. To optimize the operating conditions and decomposition efficiency, it is necessary to consider the impacts of DOM. DOM exhibits varied behaviors in diverse treatments, including permanganate oxidation, solar/ultraviolet photolysis, advanced oxidation processes, advanced reduction process, and enzyme biological treatments. Besides, the different sources (i.e., terrestrial and aquatic, etc) of DOM, and operational circumstances (i.e., concentration and pH) fluctuate different transformation efficiency of micropollutants in water. However, so far, systematic explanations and summaries of relevant research and mechanism are rare. This paper reviewed the "trade-off" performances and the corresponding mechanisms of DOM in the elimination of micropollutants, and summarized the similarities and differences for the dual roles of DOM in each of the aforementioned treatments. Inhibition mechanisms typically include radical scavenging, UV attenuation, competition effect, enzyme inactivation, reaction between DOM and micropollutants, and intermediates reduction. Facilitation mechanisms include the generation of reactive species, complexation/stabilization, cross-coupling with pollutants, and electron shuttle. Moreover, electron-drawing groups (i.e., quinones, ketones functional groups) and electron-supplying groups (i.e., phenols) in the DOM are the main contributors to its trade-off effect.

The photoactivity of complexation of DOM and copper in aquatic system: Implication on the photodegradation of TBBPA

The photoactivity of dissolved organic matter (DOM) has a great impact on the photodegradation of organic pollutants in natural waters. In this study, the photodegradation of TBBPA was investigated under simulated sunlight irradiation in the presence of copper ion (Cu²⁺), dissolved organic matter (DOM) and Cu-DOM complexation (Cu-DOM) to illustrate the effect of Cu²⁺ on photoactivity of DOM. The rate of photodegradation of TBBPA in the presence of Cu-DOM complex was 3.2 times higher than that in pure water. The effects of Cu²⁺, DOM and Cu-DOM on the photodegradation of TBBPA were highly pH dependent and hydroxyl radical(·OH) responded for the acceleration effect. Spectral and radical experiments indicated that Cu²⁺ had high affinity to fluorescence components of DOM, and acted as both the cation bridge and electron shuttle, resulting the aggregation of DOM and increasing of steady-state concentration of ·OH (·OH_ss). Simultaneously, Cu²⁺ also inhibited intramolecular energy transfer leading to the decrease of steady-state concentration singlet oxygen (¹O_2ss) and triplet of DOM (³DOM^⁎_ss). The interaction between Cu²⁺ and DOM followed the order of conjugated carbonyl CO, COO^- or CO stretching in phenolic groups and carbohydrate or alcoholic CO groups. With these results, a comprehensive investigation on the photodegradation of TBBPA in the presence of Cu-DOM was conducted, and the effect of Cu²⁺ on the photoactivity of DOM was illustrated. These findings helped to understanding the potential mechanism of interaction among metal cation, DOM and organic pollutants in sunlit surface water, especially for the DOM-induced photodegradation of organic pollutants.

Comparative studies of transformation behaviors and mechanisms of halophenols in multiple chemical oxidative systems

Due to wide applications, halophenols (HPs), especially bromophenols, chlorophenols, and fluorophenols, are commonly detected but resistant to biological removal in wastewater treatment plants (WWTPs). This study investigated the overall transformation behaviors of three representative HPs (2,4-dichlorophenol: 24-DCP, 2,4-dibromophenol: 24-DBP, 2,4-difluorophenol: 24-DFP) in six chemical oxidative systems (KMnO₄, K₂FeO₄, NaClO, O₃, UV, and persulfate (PS)). The results revealed fast removal of selected HPs by O₃, PS and K₂FeO₄, while a large discrepancy in their removal efficiencies occurred under UV irradiation, KMnO₄ oxidation and particularly chlorination. Based on the analysis of the identified intermediates and products, coupling among the five routes was the general route, and dimers were the main intermediates for HP oxidation. The effect of the halogen atom on the transformation pathways of HPs was highly reaction type dependent. Among the six chemical treatments, PS could induce HPs to yield relatively low-molecular-weight polymers and obtain the highest coupling degree. Transition state (TS) calculations showed that the H atom linked to the phenoxy group of HPs was the most easily abstracted by hydroxyl radicals to form the coupling precursor, i.e., phenoxy radicals. This high coupling behavior further resulted in the increased toxicity to green algae. Characterization revealed that HP reaction solutions treated with PS had a severely negative effect on algae growth, photosynthetic pigment synthesis, and the antioxidant enzyme system. These findings can shed light on the reaction mechanisms of advanced oxidation technologies and some risk management and control of PS technique may be considered when treating phenolic pollutants.

Formation pathway of disinfection by-products of lignin monomers in raw water during disinfection

In this study, the dissolved organic matter (DOM) profiles of water samples from a water source in northeastern China were analyzed by high-resolution mass spectrometry (HRMS), and its changes after chlorination were investigated. The results showed that lignin substances accounted for a significant proportion in DOM and chlorinated products and were the main precursors of disinfection by-products (DBPs). During disinfection, macromolecular DOM was transformed into small molecules, and lignin substances have the most obvious and complex changes in reaction. Two lignin monomers 4-propylphenol (4PP) and 4-propylguaiacol (4PG) were used as model compounds to study their reaction kinetics and degradation pathways during disinfection. The degradation of both lignin monomers conformed to pseudo-first-order reaction kinetics, and the reaction rate constant of 4PG was higher than that of 4PP. The effects of chlorine dosage, pH and temperature on the degradation reaction kinetics of two lignin monomers were investigated. The degradation rates of 4PP and 4PG increased with increasing chlorine dosage, pH and temperature. The two monomers showed similar properties in the chlorination degradation process, and generated multiple intermediates, which were mainly transformed into small molecules by chlorine electrophilic substitution and nucleophilic substitution, and further generated DBPs.

Enhanced removal of organoarsenic by chlorination: Kinetics, effect of humic acid, and adsorbable chlorinated organoarsenic

In this study, the effective removal of organoarsenic by the combined process of "chlorination + Fe(II)" was achieved. Chlorine could effectively degrade roxarsone (ROX) over pH from 5 to 10. The fitting results of acid-base protonation model proved that the degradation of ROX was mainly attributed to the reaction of HOCl and deprotonated ROX. The transformation of arsenic species conformed to the fitting results of two-channel kinetic model, in which 32.4% of ROX was oxidized to As(V) via electron transfer pathway (ii) and the rest was converted into monochloro-ROX via electrophilic substitution pathway (i). Humic acid inhibited the degradation of ROX due to the competitive consumption of chlorine and the restraint on the pathway ii. Subsequently, an enhanced removal of total arsenic achieved after chlorination, due to that the generating As(V) and monochloro-ROX were easier adsorbed compared with ROX, over 97.8% of total arsenic was removed by ferric (oxyhydr)oxides which in-situ formed from the oxidation of Fe(II). Additionally, toxicity studies indicated that the acute toxicity was significantly eliminated by adding Fe(II) after chlorination, likely due to the removal of As(V) and chlorinated products. Furthermore, organoarsenic was also effectively removed by the combined process of "chlorination + Fe(II)" in real water.

Transformation of bisphenol AF by chlorination: kinetic study and product identification

Bisphenol AF (BPAF), commonly used as a substitute for bisphenol A (BPA), is also an endocrine disruptor with cytotoxicity, neurotoxicity, genotoxicity, and biotoxicity. In this study, we found that BPAF could be effectively degraded by free chlorine. The second order rate constant of the reaction ranged from 1.67 to 126.67 M^-1·s^-1 in the pH range of 5.0-11.0. Nineteen products were detected by LC-Q-TOF-MS analysis, including chlorinated BPAF (i.e., mono/di/tri/tetrachloro-BPAF), 8 dimers, and 6 trimers. According to the identified products, two transformation pathways of electrophilic substitution and electron transfer are proposed. Humic acid (HA) could inhibit the degradation rate of BPAF due to its ability to reduce the reactive BPAF radical intermediates to the parent compound. The addition of low concentrations of Br^- and I^- accelerates the reaction rate of BPAF, due to the formation of HOBr and HOI with a higher oxidizing capacity. In seawater, BPAF degraded rapidly, and 16 new halogenated products were formed. Theoretical calculation shows that electrophilic substitution is more prone to occur at the ortho position of the hydroxyl group to form chlorinated products, while electron transfer tends to occur at the hydroxyl oxygen, resulting in the formation of BPAF radical and its subsequent coupling products.

Accelerated oxidation of the emerging brominated flame retardant tetrabromobisphenol S by unactivated peroxymonosulfate: The role of bromine catalysis and formation of disinfection byproducts

Tetrabromobisphenol S (TBBPS) is an emerging brominated flame retardant (BFR) that can cause endocrinological abnormalities in aquatic species and is neurotoxic and cytotoxic to humans. Herein, we investigated the oxidation of TBBPS by unactivated peroxymonosulfate (PMS) in aqueous solution. Results show that PMS was capable of oxidizing TBBPS without activation, and the transformation of TBBPS was pH-dependent. Interestingly, the unactivated PMS oxidation of TBBPS exhibited an autocatalytic behavior. Radical quenching experiments and electron paramagnetic resonance (EPR) analyzes ruled out the involvement of hydroxyl radical (HO•) and sulfate radical (SO₄^•‑) as reactive species. While the generation of singlet oxygen (¹O₂₎ was confirmed in PMS solution, it was also not responsible for TBBPS oxidation. The bromine substituents are believed to be responsible for the autocatalysis observed during PMS oxidation. We propose that the initial oxidation of TBBPS by PMS resulted in the release of bromide ions (Br^-) via debromination, which could be rapidly oxidized to hypobromous acid (HOBr). 3,5-Dimethyl-1H-pyrazole (DMPZ) trapping coupled with liquid chromatography-mass spectrometry (LC-MS) analysis evidenced the formation of HOBr in PMS/TBBPS system. Therefore, the presence of Br^-, albeit at trace level, could significantly accelerate the oxidation of TBBPS in PMS solution via HOBr formation. The intermediate products of TBBPS were identified by solid phase extraction (SPE) coupled with high resolution-mass spectrometry (HR-MS). The oxidation of TBBPS by unactivated PMS was likely initiated through a single electron transfer mechanism, and the transformation pathways included β-scission, debromination, and cross-coupling reactions. Further oxidation and ring-opening of the intermediates yielded three brominated disinfection byproducts (Br-DBPs), including bromoform (CHBr₃), mono-, and di-bromoacetic acids (MBAA and DBAA), as quantified by gas chromatography (GC). The presence of natural organic matter (NOM) inhibited the oxidation of TBBPS and reduced the yields of Br-DBPs. Our results indicate that unactivated PMS was efficient in the abatement of TBBPS in aqueous solution due to the accelerated oxidation by bromine catalysis; however, the formation of brominated intermediate products and Br-DBPs should be scrutinized due to their potential carcinogenicity and mutagenicity.

Oxidative transformation of emerging organic contaminants by aqueous permanganate: Kinetics, products, toxicity changes, and effects of manganese products

Permanganate (Mn(VII)) has been widely studied for removal of emerging organic contaminants (EOCs) in water treatment and in situ chemical oxidation process. Studies on the reactive intermediate manganese products (e.g., Mn(III) and manganese dioxide (MnO₂)) generated from Mn(VII) reduction by EOCs in recent decades shed new light on Mn(VII) oxidation process. The present work summarizes the latest research findings on Mn(VII) reactions with a wide range of EOCs (including phenols, olefins, and amines) in detailed aspects of reaction kinetics, oxidation products, and toxicity changes, along with special emphasis on the impacts of intermediate manganese products (mainly Mn(III) and MnO₂) in-situ formed. Mn(VII) shows appreciable reactivities towards EOCs with apparent second-order rate constants (k_app) generally decrease in the order of olefins (k_app = 0.3 - 2.1 × 10⁴ M^-1s^-1) > phenols (k_app = 0.03 - 460 M^-1s^-1) > amines (k_app = 3.5 × 10^-3 - 305.3 M^-1s^-1) at neutral pH. Phenolic benzene ring (for phenols), (conjugated) double bond (for olefins), primary amine group and the N-containing heterocyclic ring (for amines) are the most reactive sites towards Mn(VII) oxidation, leading to the formation of products with different structures (e.g., hydroxylated, aldehyde, carbonyl, quinone-like, polymeric, ring-opening, nitroso/nitro and C-N cleavage products). Destruction of functional groups of EOCs (e.g., benzene ring, (conjugated) double bond, and N-containing heterocyclic) by Mn(VII) tends to decrease solution toxicity, while oxidation products with higher toxicity than parent EOCs (e.g., quinone-like products in the case of phenolic EOCs) are sometimes formed. Mn(III) stabilized by model or unknown ligands remarkably accelerates phenolic EOCs oxidation by Mn(VII) under acidic to neutral conditions, while MnO₂ enhances the oxidation efficiency of phenolic and amine EOCs by Mn(VII) at acidic pH. The intermediate manganese products participate in Mn(VII) oxidation process most likely as both oxidants and catalysts with their generation/stability/reactivity affecting by the presence of NOM, ligand, cations, and anions in water matrices. This work presents the state-of-the-art findings on Mn(VII) oxidation of EOCs, especially highlights the significant roles of manganese products, which advances our understanding on Mn(VII) oxidation and its application in future water treatment processes.

Ciprofloxacin removal by ultrasound-enhanced carbon nanotubes/permanganate process: In situ generation of free reactive manganese species via electron transfer

Recently, free reactive manganese species (RMnS) generated via permanganate catalytic oxidation technology has been applied to contaminants abatement and sludge dewatering. This study proposed a novel free RMnS generation method in ultrasound enhanced carbon nanotube (CNTs)/permanganate process (UCP) for organics removal. Taking ciprofloxacin as a target contaminant, the removal efficiency in the UCP process (9.78 s^-1) was remarkably higher than that of the permanganate (0.71 s^-1) and CNTs/permanganate (2.57 s^-1) processes. CNTs could enrich manganese compounds and ciprofloxacin, and act as an electronic platform for the electronic transfer from ciprofloxacin to manganese compounds for free RMnS generation, which was revealed by DFT calculation and spectrum analysis. Meanwhile, ultrasound further regulated the generation of RMnS as it could transform the inactive solid Mn(IV) into free RMnS. In the UCP process, non-free radical modes including RMnS oxidation (49.8%) and electron transfer (23.5%) were the dominant processes for ciprofloxacin removal in the UCP process, and hydroxyl radical oxidation (13.2%), CNTs adsorption (5.5%), and PM oxidation (8.0%) also contributed to ciprofloxacin removal. Interestingly, CNTs could be well reused in the UCP process as more than 88.75% of ciprofloxacin was removed after five times reuse of CNTs. The UCP process provides a novel strategy for rapid contaminants removal in water treatment via continuous generation of free RMnS.

Bromine and iodine species in drinking water supply system along the Changjiang River in China: Occurrence and transformation

Bromine (Br) and iodine (I) in source water can form highly toxic brominated or iodinated disinfection byproducts in treatment plants. For the first time, the occurrence of Br and I speciation and their proportion, transformation in the drinking water supply system along the Changjiang River were investigated. 96 water samples were collected from eight drinking water treatment plants under conditions of low, normal, and flood water regimes. Total Br (TBr) and total I (TI) concentrations were quantified by inductively coupled plasma mass spectrometry (ICPMS) and inorganic Br/I forms (bromide, bromate, iodide, and iodate) were determined by high-performance liquid chromatography coupled with ICPMS. Concentrations of organic Br/I were calculated as the difference between total Br/I and inorganic Br/I. Water regimes had different effect on Br and I species, and there were different rules in untreated and treated water samples. Apparent increase of TBr and TI concentrations after water treatment were observed, which indicated the possibility of Br/I introduction by chlorine-containing disinfectant. The occurrence of TBr, TI, bromide, and total organic I in the river were investigated to increase with the direction of flow. In addition, TBr and TI concentrations correlated with the concentrations of artificial sweeteners (e.g., acesulfame and sucralose, a kind of wastewater indicator), suggesting the influence of domestic sewage on Br and I in the river. In untreated water, bromide was the main Br species, and after treatment more than 50% was transformed into organic Br. Iodoorganics were the majority of I species in raw water and were partly transformed into iodate after treatment. Overall, the Br/I species have accumulation potential in the Changjiang River and organic forms occupy high proportion in treated water samples, which should be paid more attention.

Degradation of tetrabromobisphenol A by ferrate(VI)-CaSO3 process: Kinetics, products, and impacts on following disinfection by-products formation

Tetrabromobisphenol A (TBBPA) is one of the most widely applied brominated flame retardants and has been widely detected in water environment, which might pose risks of brominated disinfection by-products formation in water treatment system. Ferrate(VI)-CaSO₃ (Fe(VI)-CaSO₃) system could effectively degrade TBBPA at pH 7.0-9.0 but the decomposition rate of TBBPA dropped with increasing pH. The presence of 0.5 mg C/L humic acid (HA) had negligible impact on TBBPA removal, but the removal of TBBPA decreased to ~87% and 80% at pH 7.0 and 8.0, respectively, in the presence of 5.0 mg C/L HA. The transformation products of TBBPA detected in Fe(VI)-CaSO₃ process revealed that TBBPA degradation mainly proceeded via electron abstraction, debromination, and ring-opening pathways and Br^- was released. In the presence of TBBPA, Fe(VI)-CaSO₃ pre-oxidation decreased the generation of all determined DBPs during chlorination at pH 8.0 but it lessened the generation of some DBPs and slightly increased the formation of the other DBPs at pH 7.0. The toxic risk analysis showed that Fe(VI)-CaSO₃ pre-oxidation of TBBPA could reduce the toxic risk of DBPs in both synthetic water and natural water at pH 8.0, indicating that Fe(VI)-CaSO₃ process has the potential to be applied in practical water treatment.

Particle electrode materials dependent tetrabromobisphenol A degradation in three-dimensional biofilm electrode reactors

The completely biological degradation of Tetrabromobisphenol A (TBBPA) contaminant is challenging. Bio-electrochemical systems are efficient to promote electrons transfer between microbes and pollutants to improve the degradation of refractory contaminants. In particular, three-dimensional biofilm electrode reactors (3DBERs), integrating the biofilm with particle electrodes, represent a novel bio-electrochemical technology with superior treatment performances. In this study, the electroactive biofilm is cultured and acclimated on two types of particle electrodes, granular activated carbon (GAC) and granular zeolite (GZ), to degrade the target pollutant TBBPA in 3DBERs. Compared to GZ, GAC materials are more favorable for biofilm formation in terms of high specific surface area and good conductivity. The genus of Thauera is efficiently enriched on both GAC and GZ particles, whose growth is promoted by the electricity. By applying 5 V voltage, TBBPA can be removed by over 95% in 120 min whether packing GAC or GZ particle electrodes in 3DBERs. The synergy of electricity and biofilm in TBBPA degradation was more significant in GAC packed 3DBER, because the improved microbial activity by electrical stimulation accelerates debromination rate and hence the decomposition of TBBPA. Applying electricity also promotes TBBPA degradation in GZ packed 3DBER mainly due to the enhanced electrochemical effects. Roles of particle electrode materials in TBBPA removal are distinguished in this work, bringing new insights into refractory wastewater treatment by 3DBERs.

Aqueous photodecomposition of the emerging brominated flame retardant tetrabromobisphenol S (TBBPS)

As an emerging brominated flame retardant (BFR), tetrabromobisphenol S (TBBPS) has been frequently detected in the environmental media and organisms. Knowledges on the transformation and fate of TBBPS in both environment and engineering systems are essential to its ecological risk assessment. Herein, we reported the photochemical decomposition of TBBPS in aqueous solution upon 254 nm ultraviolet irradiation (UV₂₅₄). Results show that TBBPS was highly photoreactive, most likely due to the presence of four ortho-bromine substituents. The molar absorption coefficient and quantum yield of TBBPS were found to be pH-dependent, with the monoanionic form being most photoreactive. A series of photoproducts were identified by solid phase extraction (SPE) combined with liquid chromatography-electrospray ionization-triple quadrupole mass spectrometry (LC-ESI(+)-MS/MS. The photolysis of TBBPS likely proceeded through photonucleophilic substitution, photoreductive debromination, and β-scission reactions. A ketocarbene, possibly derived from the lower lying excited triplet state, was proposed to be involved in the photolysis of TBBPS. Ion chromatography analysis revealed that debromination occurred quickly, and the yield of bromide (Br^-) approached 100% after 90 min irradiation. The presence of SRNOM and MRNOM inhibited the photodegradation rate of TBBPS, which is likely due to the light-screening and physical quenching effects of natural organic matter (NOM). Our results reveal that photolysis is an important process for the attenuation of TBBPS in aquatic system; however, naturally occurring species such as NOM can appreciably retard the decay of TBBPS.

Transformation of bromophenols by aqueous chlorination and exploration of main reaction mechanisms

Bromophenols (BPs) are ubiquitous phenolic contaminants and typical halogenated disinfection byproducts (DBPs) that are commonly detected in aquatic environments. The transformation of 2,4-dibromophenol (2,4-DBP) during chlorination process was fully explored in this research. It was found that active chlorine can react with 2,4-DBP effectively in a wide pH range of 5.0-11.0, with an apparent second-order rate constant (k_app) varying from 0.8 M^-1 s^-1 to 110.3 M^-1 s^-1. The addition of 5 mM ammonium ions almost completely suppressed the reaction via competitive consumption of free chlorine. With the concentration of HA increasing from 1.0 to 10.0 mg L^-1, the inhibition on the degradation of 2,4-DBP increased from 8.7% to 63.4%. By contrast, bromide ions at a concentration of 5 mM accelerated the process by about 4 times, due to the formation of hypobromous acid. On the basis of the eleven products (with eight nominal masses) identified by LC-TOF-MS, electrophilic substitution reactions and single-electron transfer reactions were mainly involved in the chlorination process. The concentration of primary chlorine-substituted products was about 4 times that of the dimer products, demonstrating that electrophilic substitution reaction was predominant during chlorination of 2,4-DBP. Density functional theory (DFT) based calculations revealed that HOCl is the dominant active oxidizing species for elimination of 2,4-DBP and coupling reaction occurs more easily at para and ortho position of hydroxyl group in the phenolic moiety. These findings could provide some new insights into the environmental fate of bromophenols during chlorine disinfection of water and wastewaters.

Natural polyphenols enhanced the Cu(II)/peroxymonosulfate (PMS) oxidation: The contribution of Cu(III) and HO•

Copper ion (Cu(II)) in water or wastewater has been reported to trigger peroxymonosulfate (PMS) oxidation of organic contaminants (OCs). However, this process can only work in alkaline condition, which limits its potential application. In this study, we found that the introduction of natural polyphenols in the Cu(II)/PMS process can significantly promote the degradation of tetrabromobisphenol A (TBBPA), one of the most widely used brominated flame retardants, in the pH range of 4.3-9.0. With gallic acid (GA) as a representative natural polyphenol, the degradation of TBBPA by GA/Cu(II)/PMS process reached 84.6% in 10 min at initial pH of 4.3 (without pH adjustment), which was 2.2 times higher than that by Cu(II)/PMS process. Multiple reactive oxidants, including Cu(III), hydroxyl radical (HO^•) and singlet oxygen, were generated in this process among which Cu(III) and HO^• contributed to TBBPA degradation with Cu(III) playing the dominant role. GA accelerated the reduction of Cu(II) to Cu(I) due to the strong chelation and electron-donating capacity of ortho-hydroxyl groups in GA, and then Cu(I) was quickly oxidized by PMS to Cu(III) which can be further acid-catalyzed to produce HO^•. TBBPA transformation mainly proceeded through electron abstraction, oxidative debromination and ring-opening reaction pathways. The feasibility of in-situ utilizing natural organic matter (NOM, enriched with polyphenol moieties) to accelerate the degradation of TBBPA by Cu(II)/PMS process in surface water and wastewater was confirmed. The findings of this study indicate that the coupling of NOM and Cu(II), which are present in contaminated water or wastewater, can potentially improve PMS oxidation of OCs in a wide range of pH.

Formation of free radicals by direct photolysis of halogenated phenols (HPs) and effects of DOM: A case study on monobromophenols

The free radicals play an important role to understand direct/indirect transformation mechanisms of organic pollutants. However, very few efforts have been made to elucidate the radicals produced by direct photolysis. In this study, the short-lived radicals generated under simulated sunlight irradiation from representative halogenated phenols (HPs), monobromophenols, were investigated by electron paramagnetic resonance (EPR). The results showed that three radicals, carbon-centered radical (C), hydrogen radical (H) and hydroxyl radical (OH), were generated from the direct irradiation of HPs. Compared to other substitutions, halogenated atom at para-position led to the highest production of these radicals which is in accordance with the energies calculated by density functional theory. Based on the analyses of the reactive species and corresponding intermediate adducts, the possible reaction pathways for these radicals were tentatively proposed. Dissolved organic matters (DOM) could enhance the photodegradation of HPs by directly affecting the radicals' formation, mainly due to generation of excited triplet DOM (³DOM*). A positive correlation was found between the concentrations of hydrated electron and the steady state ³DOM* from different DOM. Our findings provided insights into environmental photochemical fate of HPs through their direct photolysis and will help more accurately understand their phototransformation mechanisms in the environment.

Oxidation of tetrabromobisphenol A (TBBPA) by peroxymonosulfate: The role of in-situ formed HOBr

The degradation of tetrabromobisphenol A (TBBPA), one of the most widely used brominated flame retardant, was evaluated during peroxymonosulfate (PMS) oxidation. TBBPA degradation was pH-dependent, with peak degradation rate constants occurring at pH 8.0-9.0, which was distinct from some other phenolic compounds. Singlet oxygen and radicals were found to play negligible roles in TBBPA degradation. TBBPA oxidation by PMS mainly proceeded via a direct oxidation pathway and the in-situ formed HOBr was found to greatly accelerate its degradation rates. The values of species-specific second-order rate constants for the reactions of PMS with the TBBPA k_HSO5-+TBBPA, k_HSO5-+TBBPA- and k_{HSO5-+TBBPA2-} were determined to be (1.11 ± 0.84) × 10^-2, (8.05 ± 2.31) × 10^-2, and (1.34 ± 0.25) × 10^-1 M^-1 s^-1, respectively, while the reaction rate constants for HOBr/OBr^- with TBBPA k_HOBr+TBBPA, k_HOBr+TBBPA-, k_HOBr+TBBPA2-and k_OBr-+TBBPA2- were determined to be (9.38 ± 2.10) × 10³, (1.59 ± 0.56) × 10⁵, (8.22 ± 0.41) × 10⁶, and (1.81 ± 0.12) × 10⁶ M^-1 s^-1, respectively. The bromine mass balance analysis showed that bromide ion and HOBr/OBr^- occupied 19.5% of total Br and brominated organic compounds accounted for the remaining percentages at pH 7.0. No formation of bromate was observed. Based on the identified products, a reaction pathway was proposed, which included oxidation, β-scission, hydroxylation, and dimerization reaction pathways. The results indicate that unactivated PMS is useful for the remediation of TBBPA contaminated water.

Oxidation kinetics of anilines by aqueous permanganate and effects of manganese products: Comparison to phenols

In this study, the potential applicability of potassium permanganate (Mn(VII)) for anilines elimination was systematically investigated firstly, with a focus on the effect of manganese intermediates on the kinetics of anilines versus phenols. It was found that Mn(VII) could fairly oxidize anilines, where the second-order rate constants (k_Mn(VII)) values for anilines always decreased as pH increased from 5 to 9. This interesting pH-dependency was successfully described by the kinetic models proposed in literature to account for the unusual pH-rate profiles for phenols, where the formation of intermediates between Mn(VII) and phenols or anilines was likely involved. The effect of manganese products such as MnO₂ and Mn(III) on the oxidation of anilines by Mn(VII) was demonstrated. Under slightly acidic conditions, the reactions of Mn(VII) with anilines displayed autocatalysis, suggesting a similar catalytic role of MnO₂ formed in situ as compared to phenols. Several ligands (e.g., pyrophosphate) inhibited the formation of MnO₂ colloids and lowered the oxidation rates of anilines by Mn(VII) at acidic pH, while these ligands greatly accelerated the kinetics of phenols under similar conditions. The contrasting effects of ligands might be mainly attributed to the different reactivity of ligand-stabilized Mn(III) formed in situ toward anilines vs phenols. The complex effect of humic acid was highly dependent on solution pH, possible due to the dual role of humic acid that it could act as a reductant (competitively consuming Mn(VII) and phenoxy or aniline radical) as well as a ligand (stabilizing manganese intermediates such as Mn(III) species) to affect Mn(VII) reactions.

Coupling mesoporous imprinted polymer based DGT passive samplers and HPLC: A new tool for in-situ selective measurement of low concentration tetrabromobisphenol A in freshwaters

Accurate measurement of tetrabromobisphenol A (TBBPA) is very important because of its widespread environmental pollution. Diffusive gradients in thin films technique (DGT), an in-situ passive sampling method, is regarded as a reliable and robust measurement technique. A new DGT technique based on mesoporous imprinted polymer was combined with high-pressure liquid chromatography (HPLC) method for sampling, preconcentration and monitoring low concentration TBBPA in natural waters. The diffusion coefficient of TBBPA through the diffusive gel was measured by diffusion cell test and simulated using mathematical expression. The effects of different ambient conditions were tested under laboratory conditions and the performance of DGT sampler was validated in natural waters. The diffusion coefficient of TBBPA in the diffusive gel was measured as 2.18 × 10^-6 cm² s^-1 and simulated as in the range 1.41-3.48 × 10^-6 cm² s^-1 by Amsden model. Comparison of experimental and theoretical data, the validity of the experimental method can be verified by the mathematical model. The binding agent with mesoporous imprinted polymer showed selective affinity to TBBPA and its adsorption rate met the requirement of DGT device. The DGT method detection limit was at the level of ng L^-1 for 7 days deployment. DGT sampler was suitable for application in aquatic environment with a range of pH (4.5-7.6), ionic strength (1 × 10^-4 - 0.5 mol L^-1), and dissolved organic matter (DOM) concentration (0-10 mg C L^-1). The coupling method of DGT and HPLC was a promising technique for in situ sampling, preconcentration and monitoring low concentration TBBPA in most typical natural waters.

Aerobic cometabolism of tetrabromobisphenol A by marine bacterial consortia

The coastal environments worldwide are subjected to increasing TBBPA contamination, but current knowledge on aerobic biodegradability of this compound by marine microbes is lacking. The aerobic removal of TBBPA using marine consortia under eight different cometabolic conditions was investigated here. Results showed that the composition and diversity of the TBBPA-degrading consortia had diverged after 120-day incubation. Pseudoalteromonas, Alteromonas, Glaciecola, Thalassomonas, and Limnobacter were the dominant genera in enrichment cultures. Furthermore, a combination of beef extract- and peptone-enriched marine consortia exhibited higher TBBPA removal efficiency (approximately 60%) than the other substrate amendments. Additionally, Alteromonas macleodii strain GCW was isolated from a culture of TBBPA-degrading consortium. This strain exhibited about 90% of degradation efficiency toward TBBPA (10 mg L^-1) after 10 days of incubation under aerobic cometabolic conditions. The intermediates in the degradation of TBBPA by A. macleodii strain GCW were analyzed and the degradation pathways were proposed, involving β-scission, debromination, and nitration routes.

Further insights into the combination of permanganate and peroxymonosulfate as an advanced oxidation process for destruction of aqueous organic contaminants

Recent studies have reported a novel advanced oxidation process (AOP) by combining permanganate (KMnO₄) and peroxymonosulfate (PMS) for destruction of organic contaminants (i.e., acid orange 7, trichloroethylene, and benzene), where hydroxyl (^•OH) and sulfate radicals (SO₄^•-) are proposed to be generated from PMS activation by amorphous manganese dioxide (MnO₂) formed in situ from KMnO₄ reduction. In this work, appreciable degradation of p-chlorobenzoic acid (p-CBA) was confirmed in KMnO₄/PMS system, while KMnO₄ or PMS alone showed inert reactivity toward p-CBA. Moreover, it was found that pre-synthesized amorphous MnO₂ showed invalid PMS activation for p-CBA degradation, and pre-addition of inorganic or organic reducing agents to promote the formation of amorphous MnO₂ showed negligible influence on p-CBA degradation as well. In these regards, a tentative mechanism for PMS activation by KMnO₄ rather than its product MnO₂ was proposed, involving the substitution of oxo atoms of KMnO₄ by peroxo groups, subsequent reductive generation of peroxomanganese (VI) complexes, and intramolecular disproportionation of these complexes to generate radicals. Efficient degradation of p-CBA was achieved at acid or basic conditions with a maximum rate occurring at pH 3. The coexisting chloride anions showed suppressive effect on p-CBA degradation for scavenging SO₄^•- and ^•OH, while metal ions accelerated the degradation of p-CBA, possibly due to the cation bridging function between negatively-charged MnO₄^- and HSO₅^-. Hydroxylated intermediates of p-CBA were identified in KMnO₄/PMS system. This work improved the fundamental understanding of a new class of AOPs by combining KMnO₄ and PMS for environmental decontamination.

Application of (LC/)MS/MS precursor ion scan for evaluating the occurrence, formation and control of polar halogenated DBPs in disinfected waters: A review

Water disinfection can result in the unintended formation of halogenated disinfection byproducts (DBPs), which have been the subject of intensive investigation over the past 40 years. Robust methods for evaluating and characterizing the formation of halogenated DBPs are prerequisites for ultimately controlling the formation of DBPs and ensuring quality and safe disinfected water. Only a fraction of the total organic halogen (TOX) formed during disinfection has been chemically identified or even well characterized by the classical (derivatization-)gas chromatography/mass spectrometry (GC/MS) method. Such a method may not be amenable to the detection of polar halogenated DBPs, which constitute a major portion of the TOX that is still unaccounted for. Accordingly, a novel precursor ion scan (PIS) method using (liquid chromatography/) electrospray ionization-triple quadrupole mass spectrometry was developed for the rapid selective detection of all polar halogenated DBPs-no matter whether the DBPs are known or unknown-in water. This article reviews recent literature on the application of the PIS method for evaluating the occurrence, formation and control of polar halogenated DBPs in disinfected waters. The challenges in developing the PIS method were briefly summarized. Application of the powerful method pinpointed >150 previously unknown DBPs and revealed the formation, speciation and transformation of halogenated DBPs in disinfected drinking water, wastewater effluents, and swimming pool water. For the same source water, positive correlations were found between the total ion intensity (TII) levels in the PIS spectra of m/z 35/79/126.9 and the total organic chlorine/bromine/iodine levels in the disinfected water sample, and a disinfected sample with a higher TII level generally showed a higher toxic potency. Accordingly, the TII value can be used as a surrogate to comparatively reflect the water quality and assess the efficiency of a DBP control approach. To achieve a more comprehensive and systematic understanding of the DBP compositions in different waters and thus better control the DBP formation and reduce their overall toxicity, topics for future work were discussed.

Phytotoxicity and genotoxicity evaluation of 2,4,6-tribromophenol solution treated by UV-based oxidation processes

The environmental detriment due to the presence of emerging contaminants has encouraged the development of advanced oxidation processes. Such methods deal with non-selective chemical reactions. Therefore, toxic byproducts can be generated and distinct post treatment toxicity levels can be expected. The present study investigates the phytotoxicity of 2,4,6-tribromophenol (TBP) to L. sativa seeds and A. cepa bulbs, as well as the TBP phytotoxicity and genotoxicity to A. cepa root. L. sativa seeds and A. cepa bulbs were germinated by being exposed to solutions containing TBP before and after treatment by UV-based processes: direct photolysis (DP), heterogeneous photocatalysis (HP) and photoelectrolysis (PEC). Subsequent analysis of the root length, to determine phytotoxicity, as well as evaluation of chromosomal abnormalities, revealed that the samples treated by DP presented higher phytotoxicity than the untreated ones. On the other hand, samples treated by HP and PEC did not present phytotoxicity. In fact, for the A. cepa assays, phytotoxicity was not observed, including the initial sample. However, genotoxicity assays showed a high frequency of chromosomal aberrations in the initial sample, before the UV-based process treatment. After 140 min of treatment by HP, there was a reduction in genotoxicity, while PEC treatment resulted in a sample with no genotoxicity. In contrast, DP presented high levels of phytotoxicity and genotoxicity. Additionally, DP shows similar degradation and debromination values, when compared to the HP and PEC processes, but less mineralization. Therefore, considering that the DP process did not deals with the HO^• radical, the oxidation pathway can generate byproducts with higher toxicity, which lead to higher levels of phytotoxicity and genotoxicity. These results show that different UV-based oxidation processes are associated to distinct byproducts and toxicity levels. In addition, a toxicity assessment with different organisms should be performed to ensure a safe outcome.

Transformation/degradation of tetrabromobisphenol A and its derivatives: A review of the metabolism and metabolites

Although the abiotic and biotic transformation/degradation (T/D) processes of tetrabromobisphenol A (TBBPA) have been widely investigated in model experiments, few reviews have focused on these processes along with their metabolites or degradation products. In this paper, we summarize the current knowledge on the T/D of TBBPA and its derivatives, including abiotic and biotic T/D strategies/conditions, mechanisms, metabolites and environmental occurrences. Various treatments, such as pyrolysis, photolysis, chemical reactions and biotransformation, have been employed to study the metabolic mechanism of TBBPA and its derivatives and to remediate associated contaminated environments. To date, more than 100 degradation products and metabolites have been identified, dominated by less brominated compounds such as bisphenol A, 2,6-dibromo-4-isopropylphenol, 2,6-dibromo-4-hydroxyl-phenol, 2,6-dibromophenol, isopropylene-2,6-dibromophenol, 4-(2-hydroxyisopropyl)-2,6-dibromophenol, etc. It can be concluded that the T/D of TBBPA mainly takes place through debromination and β-scission. In some environmental media and human and animal tissues, brominated metabolites, glucoside and sulfate derivatives are also important T/D products. Here, the T/D products of TBBPA and its derivatives have been most comprehensively presented from the literature in recent 20 years. This review will enhance the understanding of the environmental behaviors of TBBPA-associated brominated flame retardants along with their ecological and health risks.

Oxidation of fluoroquinolone antibiotics by peroxymonosulfate without activation: Kinetics, products, and antibacterial deactivation

While fluoroquinolone (FQ) antibiotics are susceptible to degradation by sulfate and/or hydroxyl radicals formed in peroxymonosulfate (PMS) based advanced oxidation processes, here we report that unactivated PMS itself exhibits a specific high reactivity toward FQs for the first time. Reaction kinetics of PMS with two model FQs, ciprofloxacin (CF) and enrofloxacin (EF), showed a strong pH dependency with apparent second-order rate constants of 0.10-13.05 M^-1s^-1 for CF and 0.51-33.17 M^-1s^-1 for EF at pH 5-10. This pH dependency was well described by species-specific parallel reactions. On the basis of reaction kinetics and structure-activity assessment, the tertiary and secondary aliphatic N4 amines on the FQs' piperazine ring were proposed to be the main reaction sites. High performance liquid chromatography/electrospray ionization tandem mass analysis showed the formation of hydroxylated, N-oxide, and dealkylated products. Bacterial growth inhibition bioassays using Escherichia coli showed that oxidation products of FQs by PMS retained negligible antibacterial potency in comparison to parent FQs. Kinetic modeling using the rate constants estimated from pure water well predicted the oxidation kinetics of low levels of CF and EF by PMS in surface water. The degradation efficiency of FQs by PMS in surface water was slightly lower than that by ozone, comparable to that by ferrate, and much higher than that by permanganate. These results suggest that PMS is a promising oxidant for the treatment of FQs in water.

Transformation of bisphenol AF and bisphenol S by manganese dioxide and effect of iodide

In this work, transformation of bisphenol A (BPA) alternatives bisphenol AF (BPAF) and bisphenol S (BPS) by manganese dioxide (MnO₂) and the effect of iodide (I^-) during these processes were investigated in comparison with BPA for the first time. These three bisphenols showed appreciable reactivity towards MnO₂ with the half-lives of their loss following the order of BPA < BPAF < BPS under similar conditions, and a higher transformation efficiency was generally obtained at a lower pH. The presence of I^- apparently accelerated the transformation of BPAF and BPS by MnO₂ at pH ≤ 7 but negligibly affected BPA transformation over the pH range of 5-9. This discrepancy could be well explained by the relative contribution of hypoiodous acid (HOI) in situ formed from I^- oxidation by MnO₂. Polymers, hydroxylated derivatives, and bond-cleavage products were detected from BPAF and BPS treated by MnO₂, where a series of reactions of BPAF/BPS radicals formed from one-electron oxidation of BPAF/BPS were likely involved, similar to the case of BPA reported in literature. A group of iodinated aromatic products were additionally identified from BPAF/BPS treated by MnO₂ in the presence of I^- (e.g., iodinated BPAF/BPS and iodinated BPAF/BPS dimers), and they could be further transformed. This study suggests that naturally occurring manganese oxides play a significant role in the attenuation of bisphenols released into the environment and the presence of I^- can display a great effect on their transformation.

17β-estradiol as precursors of Cl/Br-DBPs in the disinfection process of different water samples

During chlorine disinfection process, reactions between the disinfectant and 17β-estradiol (E2) lead to the formation of halogenated disinfection byproducts (DBPs) which can be a risk to both ecosystem and human health. The degradation and transformation products of E2 in sodium hypochlorite (NaClO) disinfection processes of different water samples were investigated. The reaction kinetics research showed that the degradation rates of E2 were considerably dependent on the initial pH value and the types of water samples. In fresh water, synthetic marine aquaculture water and seawater, the reaction rate constant was 0.133 min^-1, 2.067 min^-1 and 2.592 min^-1, respectively. The reasons for the above phenomena may be due to the different concentrations of bromide ions (Br^-) in these three water samples which could promote the reaction between NaClO and E2. Furthermore, Br^- could also cause the formation of brominated DBPs (Br-DBPs). The main DBPs, reaction centers and conceivable reaction pathways were explored. Seven halogenated DBPs have been observed including three chlorinated DBPs (Cl-DBPs) and four Br-DBPs. The active sites of E2 were found to be the pentabasic cyclic ring and the ortho position of the phenol moiety as well as C9-C10 position. The identified Cl/Br-DBPs were also confirmed in actual marine aquaculture water from a shrimp pond. The comparison of bio-concentration factors (BCF) values based on calculation of EPI-suite showed that the toxicities of the Br-DBPs were stronger than that of their chloride analogues. The absorbable organic halogens (AOX) analysis also suggested that the DBPs produced in the marine aquaculture water were more toxic than that in the fresh water system.

Effect of iodide on transformation of phenolic compounds by nonradical activation of peroxydisulfate in the presence of carbon nanotube: Kinetics, impacting factors, and formation of iodinated aromatic products

Our recent study has demonstrated that iodide (I^-) can be easily and almost entirely oxidized to hypoiodous acid (HOI) but not to iodate by nonradical activation of peroxydisulfate (PDS) in the presence of a commercial carbon nanotube (CNT). In this work, the oxidation kinetics of phenolic compounds by the PDS/CNT system in the presence of I^- were examined and potential formation of iodinated aromatic products was explored. Experimental results suggested that I^- enhanced the transformation of six selected substituted phenols, primarily attributed to the generation of HOI that was considerably reactive toward these phenolic compounds. More significant enhancement was obtained at higher I^- concentrations or lower pH values, while the change of PDS or CNT dosages exhibited a slight impact on the enhancing effect of I^-. Product analyses with liquid chromatography tandem mass spectrometry clearly revealed the production of iodinated aromatic products when p-hydroxybenzoic acid (p-HBA, a model phenol) was treated by the PDS/CNT/I^- system in both synthetic and real waters. Their formation pathways probably involved the substitution of HOI on aromatic ring of p-HBA, as well as the generation of iodinated p-HBA phenoxyl radicals and subsequent coupling of these radicals. Given the considerable toxicity and harmful effects of these iodinated aromatic products, particular attention should be paid when the novel PDS/CNT oxidation technology is applied for treatment of phenolic contaminants in iodide-containing waters.

Extracellular degradation of tetrabromobisphenol A via biogenic reactive oxygen species by a marine Pseudoalteromonas sp

Tetrabromobisphenol A (TBBPA) has attracted considerable attention due to its ubiquitous presence in different environmental compartments worldwide. However, information on its aerobic biodegradability in coastal environments remains unknown. Here, the aerobic biodegradation of TBBPA using a Pseudoalteromonas species commonly found in the marine environment was investigated. We found that extracellular biogenic siderophore, superoxide anion radical (O₂^•-), hydrogen peroxide (H₂O₂), and hydroxyl radical (^•OH) were involved in TBBPA degradation. Upregulation of genes (nqrA and lodA) encoding Na⁺-translocating NADH-quinone oxidoreductase and l-lysine-ε-oxidase supported the extracellular O₂^•- and H₂O₂ production. The underlying mechanism of TBBPA biodegradation presumably involves both O₂^•- reduction and ^•OH-based advanced oxidation process (AOP). Furthermore, TBBPA intermediates of tribromobisphenol A, 4-isopropylene-2,6-dibromophenol, 4-(2-hydroxyisopropyl)-2,6-dibromophenol, 2,4,6-tribromophenol (TBP), 4-hydroxybenzoic acid, and 2-bromobenzoic acid were detected in the culture medium. Debromination and β-scission pathways of TBBPA biodegradation were proposed. Additionally, membrane integrity assays revealed that the increase of intracellular catalase (CAT) activity and the extracellular polymeric substances (EPS) might account for the alleviation of oxidative damage. These findings could deepen understanding of the biodegradation mechanism of TBBPA and other related organic pollutants in coastal and artificial bioremediation systems.

Transformation of methylparaben during water chlorination: Effects of bromide and dissolved organic matter on reaction kinetics and transformation pathways

The reaction kinetics, products, and pathways of methylparaben (MeP) during water chlorination with and without bromide (Br^-) were investigated to better understand the fate of parabens in chlorinated waters. During the chlorination of MeP-spiked waters without Br^-, MeP was transformed into mono-Cl-MeP and di-Cl-MeP with apparent second-order rate constants (k_app) of 64M^-1s^-1 and 243M^-1s^-1 at pH7, respectively, while further chlorination of di-Cl-MeP was relatively slower (k_app=1.3M^-1s^-1 at pH7). With increasing Br^- concentration, brominated MePs, such as mono-Br-MeP, Br-Cl-MeP, and di-Br-MeP, became major transformation products. The di-halogenated MePs (di-Cl-MeP, Br,Cl-MeP, and di-Br-MeP) showed relatively low reactivity to chlorine at pH7 (k_app=1.3-4.6M^-1s^-1) and bromine (k_app=32-71M^-1s^-1), which explains the observed high stability of di-halogenated MePs in chlorinated waters. With increasing pH from 7 to 8.5, the transformation of di-halogenated MePs was further slowed due to the decreasing reactivity of di-MePs to chlorine. The formation of the di-halogenated MePs and their further transformation become considerably faster at Br^- concentrations higher than 0.5μM (40μg/L). Nonetheless, the accelerating effect of Br^- diminishes in the presence of dissolved organic matter (DOM) extract (Suwannee River humic acid (SRHA)) due to a more rapid consumption of bromine by DOM than chlorine. The effect of Br^- on the fate of MeP was less in the tested real water matrices, possibly due to a more rapid bromine consumption by the real water DOM compared to SRHA. A kinetic model was developed based on the determined species-specific second-order rate constants for chlorination/bromination of MeP and its chlorinated and brominated MePs and the transformation pathway information, which could reasonably simulate the transformation of MePs during the chlorination of water in the presence of Br^- and selected DOM.

Transformation of Methylparaben by aqueous permanganate in the presence of iodide: Kinetics, modeling, and formation of iodinated aromatic products

This work investigated impacts of iodide (I^-) on the transformation of the widely used phenolic preservative methylparaben (MeP) as well as 11 other phenolic compounds by potassium permanganate (KMnO₄). It was found that KMnO₄ showed a low reactivity towards MeP in the absence of I^- with apparent second-order rate constants (k_app) ranging from 0.065 ± 0.0071 to 1.0 ± 0.1 M^-1s^-1 over the pH range of 5-9. The presence of I^- remarkably enhanced the transformation rates of MeP by KMnO₄ via the contribution of hypoiodous acid (HOI) in situ formed, which displayed several orders of magnitude higher reactivity towards MeP than KMnO₄. This enhancing effect of I^- was greatly influenced by solution conditions (e.g., I^- or KMnO₄ concentration or pH), which could be well simulated by a kinetic model involving competition reactions (i.e., KMnO₄ with I^-, KMnO₄ with MeP, HOI with KMnO₄, and HOI with MeP). Similar enhancing effect of I^- on the transformation kinetics of 5 other selected phenols (i.e., p-hydroxybenzoic acid, phenol, and bromophenols) at pH 7 was also observed, but not in the cases of bisphenol A, triclosan, 4-n-nonylphenol, and cresols. This discrepancy could be well explained by the relative reactivity of KMnO₄ towards phenols vs I^-. Liquid chromatography-tandem mass spectrometry analysis showed that iodinated aromatic products and/or iodinated quinone-like product were generated in the cases where I^- enhancing effect was observed. Evolution of iodinated aromatic products generated from MeP (10 μM) treated by KMnO₄ (50-150 μM) in the presence of I^- (5-15 μM) suggested that higher I^- or moderate KMnO₄ concentration or neutral pH promoted their formation. A similar enhancing effect of I^- (1 μM) on the transformation of MeP (1 μM) by KMnO₄ (12.6 μM) and formation of iodinated aromatic products were also observed in natural water. This work demonstrates an important role of I^- in the transformation kinetics and product formation of phenolic compounds by KMnO₄, which has great implications for future applications of KMnO₄ in treatment of I^--containing water.

Does Soluble Mn(III) Oxidant Formed in Situ Account for Enhanced Transformation of Triclosan by Mn(VII) in the Presence of Ligands?

In previous studies, we interestingly found that several ligands (e.g., pyrophosphate, nitrilotriacetate, and humic acid) could significantly accelerate the oxidation rates of triclosan (TCS; the most widely used antimicrobial) by aqueous permanganate (Mn(VII)) especially at acid pH, which was ascribed to the contribution of ligand-stabilized Mn(III) (defined Mn(III)_L) formed in situ as a potent oxidant. In this work, it was found that the oxidation of TCS by Mn(III)_L resulted in the formation of dimers, as well as hydroxylated and quinone-like products, where TCS phenoxy radical was likely involved. This transformation pathway distinctly differed from that involved in Mn(VII) oxidation of TCS, where 2,4-dichlorophenol (DCP) was the major product with a high yield of ∼80%. Surprisingly, we found that the presence of various complexing ligands including pyrophosphate, nitrilotriacetate, and humic acid, as well as bisulfite slightly affected the yields of DCP, although they greatly enhanced the oxidation kinetics of TCS by Mn(VII). This result could not be reasonably explained by taking the contribution of Mn(III)_L into account. Comparatively, the degradation of TCS by manganese dioxide (MnO₂) was also greatly enhanced in the presence of these ligands with negligible formation of DCP, which could be rationalized by the contribution of Mn(III)_L. In addition, it was demonstrated that DCP could not be generated from Mn(VII) oxidation of unstable phenoxy radical intermediates and stable oxidation products formed from TCS by Mn(III)_L. These findings indicate that manganese intermediates other than Mn(III) are likely involved in the Mn(VII)/TCS/ligand systems responsible for the high yields of DCP product.

Chlorination of bisphenol S: Kinetics, products, and effect of humic acid

Bisphenol S (BPS), as a main alternative of bisphenol A for the production of industrial and consumer products, is now frequently detected in aquatic environments. In this work, it was found that free chlorine could effectively degrade BPS over a wide pH range from 5 to 10 with apparent second-order rate constants of 7.6-435.3 M^-1s^-1. A total of eleven products including chlorinated BPS (i.e., mono/di/tri/tetrachloro-BPS), 4-hydroxybenzenesulfonic acid (BSA), chlorinated BSA (mono/dichloro-BSA), 4-chlorophenol (4CP), and two polymeric products were detected by high performance liquid chromatography and electrospray ionization-tandem quadrupole time-of-flight mass spectrometry. Two parallel transformation pathways were tentatively proposed: (i) BPS was attacked by stepwise chlorine electrophilic substitution with the formation of chlorinated BPS. (ii) BPS was oxidized by chlorine via electron transfer leading to the formation of BSA, 4CP and polymeric products. Humic acid (HA) significantly suppressed the degradation rates of BPS even taking chlorine consumption into account, while negligibly affected the products species. The inhibitory effect of HA was reasonably explained by a two-channel kinetic model. It was proposed that HA negligibly influenced pathway i while appreciably inhibited the degradation of BPS through pathway ii, where HA reversed BPS phenoxyl radical (formed via pathway ii) back to parent BPS.

Oxidation Kinetics of Bromophenols by Nonradical Activation of Peroxydisulfate in the Presence of Carbon Nanotube and Formation of Brominated Polymeric Products

This work demonstrated that bromophenols (BrPs) could be readily oxidized by peroxydisulfate (PDS) activated by a commercial carbon nanotube (CNT), while furfuryl alcohol (a chemical probe for singlet oxygen (¹O₂)) was quite refractory. Results obtained by radical quenching experiments, electron paramagnetic resonance spectroscopy, and Fourier transform infrared spectroscopy further confirmed the involvement of nonradical PDS-CNT complexes rather than ¹O₂. Bicarbonate and chloride ion exhibited negligible impacts on BrPs degradation by the PDS/CNT system, while a significant inhibitory effect was observed for natural organic matter. The oxidation of BrPs was influenced by solution pH with maximum rates occurring at neutral pH. Linear free energy relationships (LFERs) were established between the observed pseudo-first-order oxidation rates of various substituted phenols and the classical descriptor variables (i.e., Hammett constant σ⁺, and half-wave oxidation potential E_1/2). Products analyses by liquid chromatography tandem mass spectrometry clearly showed the formation of hydroxylated polybrominated diphenyl ethers and hydroxylated polybrominated biphenyls on CNT surface. Their formation pathway possibly involved the generation of bromophenoxyl radicals from BrPs one-electron oxidation and their subsequent coupling reactions. These results suggest that the novel nonradical PDS/CNT oxidation technology is a good alternative for selectively eliminating BrPs with alleviating toxic byproducts in treated water effluent.

Azo compound degradation kinetics and halonitromethane formation kinetics during chlorination

The chlorination of azo compounds can produce halonitromethanes (HNMs), which have attracted increasing concern due to their high genotoxicity. By impacting the speciation of chlorine and azo compounds, pH impacts apparent second-order rate constants of Methyl Orange (MO, 27.5-1.4 × 10³ M^-1 s^-1), Acid Orange II (AO, 16.7-99.3 M^-1 s^-1), and Acid Red 1 (AR 1, 3.7-72.5 M^-1 s^-1) (pH range 6.3-9.0). The two-compartment first-order model successfully described the chloropicrin (TCNM) formation kinetics, suggesting that both fast- and slow-reacting precursors of TCNM are generated from the chlorination of azo compounds. The ratios between fast and slow formation rate constants for MO and AO were 15.6-5.4 × 10², while that of AR 1 was 9.8-19.4 (pH range 6.5-9.0). The fraction of the fast-reacting TCNM precursors decreased with increasing pH for MO and AO; while that for AR 1 decreased when pH increased from 6.5 to 8.0, and then increased when pH increased from 8.0 to 9.0. The impact of pH on TCNM formation was also precursor-specific. The highest molar yields of TCNM predicted from the model in this study were 2.4%, 2.5%, and 1.5% for MO, AO, and AR 1, respectively. The study demonstrates that azo compounds are important HNM precursors, and pose a potential threat to drinking water safety.

Chlorination Revisited: Does Cl- Serve as a Catalyst in the Chlorination of Phenols?

The aqueous chlorination of (chloro)phenols is one of the best-studied reactions in the environmental literature. Previous researchers have attributed these reactions to two chlorine species: HOCl (at circum-neutral and high pH) and H₂OCl⁺ (at low pH). In this study, we seek to examine the roles that two largely overlooked chlorine species, Cl₂ and Cl₂O, may play in the chlorination of (chloro)phenols. Solution pH, chloride concentration, and chlorine dose were systematically varied in order to assess the importance of different chlorine species as chlorinating agents. Our findings indicate that chlorination rates at pH < 6 increase substantially when chloride is present, attributed to the formation of Cl₂. At pH 6.0 and a chlorine dose representative of drinking water treatment, Cl₂O is predicted to have at best a minor impact on chlorination reactions, whereas Cl₂ may contribute more than 80% to the overall chlorination rate depending on the (chloro)phenol identity and chloride concentration. While it is not possible to preclude H₂OCl⁺ as a chlorinating agent, we were able to model our low-pH data by considering Cl₂ only. Even traces of chloride can generate sufficient Cl₂ to influence chlorination kinetics, highlighting the role of chloride as a catalyst in chlorination reactions.