Aqueous photolysis of naproxen exposed to UV and natural sunlight: Formation of excited triplet and photosensitizing product.

Photochemical transformation is an important attenuation process for the non-steroidal anti-inflammatory drug naproxen (NPX) in both engineered and natural waters. Herein, we investigated the photolysis of NPX in aqueous solution exposed to both ultraviolet (UV, 254 nm) and natural sunlight irradiation. Results show that N2 purging significantly promoted NPX photolysis under UV irradiation, suggesting the formation of excited triplet state (3NPX*) as a critical transient. This inference was supported by benzophenone photosensitization and transient absorption spectra. Sunlight quantum yield of NPX was only one fourteenth of that under UV irradiation, suggesting the wavelength-dependence of NPX photochemistry. 3NPX* formed upon irradiation of NPX underwent photodecarboxylation leading to the formation of 2-(1-hydroxyethyl)-6-methoxynaphthalene (2HE6MN), 2-(1-hydroperoxyethyl)-6-methoxynaphthalene (2HPE6MN), and 2-acetyl-6-methoxynaphthalene (2A6MN). Notably, the conjugation and spin-orbit coupling effects of carbonyl make 2A6MN a potent triplet sensitizer, therefore promoting the photodegradation of the parent NPX. In hospital wastewater, the photolysis of NPX was influenced because the photoproduct 2A6MN and wastewater components could competitively absorb photons. Bioluminescence inhibition assay demonstrated that photoproducts of NPX exhibited higher toxicity than the parent compound. Results of this study provide new insights into the photochemical behaviors of NPX during UV treatment and in sunlit surface waters.

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

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

Formation of brominated and nitrated byproducts during unactivated peroxymonosulfate oxidation of phenol.

Brominated and nitrated byproducts generated from bromide (Br-) and nitrite (NO2-), respectively, by sulfate radical (SO4•-) oxidation have raised increasing concern. However, little is known about the concurrent generation of brominated and nitrated byproducts in the unactivated peroxymonosulfate (PMS) oxidation process. This study revealed that Br- can facilitate the transformation of NO2- to nitrated byproducts during unactivated PMS oxidation of phenol. In the co-existence of 0.1 mM Br- and 0.5 mM NO2-, the total yield of identified nitrated byproducts reached 2.316 µM in 20 min, while none was found with NO2- alone. Nitryl bromide (BrNO2) as the primary nitrating agent was formed via the reaction of NO2- with free bromine in situ generated through the oxidation of Br- by PMS. BrNO2 rapidly reacted with phenol or bromophenols, generating highly toxic nitrophenols or nitrated bromophenols, respectively. Increasing NO2- concentration led to more nitrated byproducts but less brominated byproducts. This study advances our understanding of the transformation of Br- and NO2- in the unactivated PMS oxidation process. It also provides important insights into the potentially underestimated environmental risks when PMS is applied to degrade organic contaminants under realistic environments, particularly when Br- and NO2- co-exist.

Formation of nitrated naphthalene in the sulfate radical oxidation process in the presence of nitrite.

Polycyclic aromatic hydrocarbons (PAHs) have become a global environmental concern due to their potential hazardous implication for human health. In this study, we found that sulfate radical (SO4•-) could effectively degrade naphthalene (NAP), a representative PAH in groundwaters, generating 1-naphthol. This intermediate underwent further degradation, yielding ring-opening products including phthalic acid and salicylic acid. However, the presence of nitrite (NO2-), a prevalent ion in subsurface environments, was observed to compete with NAP for SO4•-, thus slowing down the NAP degradation. The reaction between NO2- and SO4•- generated a nitrogen dioxide radical (NO2•). Concurrently, in-situ formed 1-naphthol underwent further oxidization to the 1-naphthoxyl radical by SO4•-. The coupling of 1-naphthoxyl radicals with NO2• gave rise to a series of nitrated NAP, namely 2-nitro-1-naphthol, 4-nitro-1-naphthol, and 2,4-dinitro-1-naphthol. In addition, the in-situ formed phthalic acid and salicylic acid also underwent nitration, generating nitrophenolic products, although this pathway appeared less prominent than the nitration of 1-naphthol. When 10 µΜ NAP was subjected to heat activated peroxydisulfate oxidation in the presence of 10 µΜ NO2-, the total yield of nitrated products reached 0.730 µΜ in 120 min. Overall, the presence of NO2- dramatically altered the behavior of NAP degradation by SO4•- oxidation and contributed to the formation of toxic nitrated products. These findings raise awareness of the potential environmental risks associated with the application of SO4•--based oxidation processes for the remediation of PAHs-polluted sites in presence of NO2-.

Oxidation of emerging contaminants by S(IV) activated ferrate: Identification of reactive species.

Studies on the Fe(VI)/S(IV) process have focused on improving the efficiency of emerging contaminants (ECs) degradation under alkaline conditions. However, the performance and mechanisms under varying pH levels remain insufficiently investigated. This tudy delved into the efficiency and mechanism of Fe(VI)/S(IV) process using sulfamethoxazole (SMX) and ibuprofen (IBU) as model contaminants. We found that pH was crucial in governing the generation of reactive species, and both Fe(V/IV) and SO4•- were identified in the reaction system. Specifically, an increase in pH favored the formation of SO4•-, while the formation of Fe(VI) to Fe(V/IV) became more significant at lower pH. At pH 3.2, Fe(III) resulting from the Fe(VI) self-decay reactedwith HSO3-to produce SO4•-and •OH. Under near-neutral conditions, the coexistance of Fe(V/IV) and SO4•- in abundance contributed to the optimal oxidation of both pollutants in the Fe(VI)/S(IV) process, with the removal exceeding 74% in 5 min. Competitive quenching experiments showed that the contributions of Fe(V/IV) to SMX and IBU destruction dimished, while the contributions of radicals increased with an increase in pH. However, this evolution was slower during SMX degradation compared to IBU degradation. A comprehensive understnding of pH as the key factor is essential for the optimization of the sulfite-activated Fe(VI) oxidation process in water treatment.

Ketoprofen products induced photosensitization of sulfonamide antibiotics: The cocktail effects of pharmaceutical mixtures on their photodegradation.

Indirect photolysis induced by naturally occurring sensitizers constitutes an important pathway accounting for the transformation and fate of many recalcitrant micropollutants in sunlit surface waters. However, the photochemical transformation of micropollutants by photosensitizing pharmaceuticals has been less investigated. In this study, we demonstrated that the non-steroidal anti-inflammatory drug ketoprofen (KTF) and its photoproducts, 3-acetylbenzophenone (AcBP) and 3-ethylbenzophenone (EtBP), could sensitize the photodegradation of coexisting sulfonamide antibiotics, e.g., sulfamethoxazole (SMX), under artificial 365 nm ultraviolet (UV) and sunlight irradiation. Key reactive species including triplet excited state and singlet oxygen (1O2) responsible for photosensitization were identified by laser flash photolysis (LFP) and electron paramagnetic resonance (EPR) techniques, respectively. High-resolution mass spectrometry (HRMS) and structure-related reactivity analyses revealed that the sensitized photolysis of SMX occurred mainly through single electron transfer. The rate constants of sulfonamides sensitized by AcBP photolysis followed the order of sulfisoxazole (SIX)＞sulfathiazole (STZ)＞SMX＞sulfamethizole (SMT). Exposure to sunlight also enhanced the photolysis of SMX in the presence of KTF or AcBP, and water matrix had limited impact on such process. Overall, our results reveal the feasibility and mechanistic aspects of photosensitization of coexisting contaminants by pharmaceuticals (or their photoproducts) and provide new insights into the cocktail effects of pharmaceutical mixtures on their photochemical behaviors in aqueous environment.

Formation of halogenated chloroxylenols through chlorination and their photochemical activity.

Trace organic contaminants usually go through multiple treatment units in a modern water treatment train. Structural modification triggered by pretreatment (e.g., prechlorination) may influence the further transformation and fate of contaminants in downstream units. However, knowledge on this aspect is still limited. In this contribution, we investigated the chlorination of chloroxylenol (PCMX), an antimicrobial agent extensively used during COVID-19 pandemic, and the photoreactivity of its halogenated derivatives. Results indicate that chlorination of PCMX mainly proceeded through electrophilic substitution to give chlorinated products, including Cl- and 2Cl-PCMX. The presence of bromide (Br-) resulted in brominated analogues. Owing to the bathochromic and "heavy atom" effects of halogen substituents, these products show increased light absorption and photoreactivity. Toxicity evaluation suggest that these halo-derivatives have higher persistence, bioaccumulation, and toxicity (PBT) than the parent PCMX. Results of this contribution advance our understanding of the transformation of PCMX during chlorination and the photochemical activity of its halogenated derivatives in subsequent UV disinfection process or sunlit surface waters.

Photo-transformation of acetaminophen sensitized by fluoroquinolones in the presence of bromide.

Fluoroquinolones (FQs) are a class of antibiotics with emerging concern. This study investigated the photochemical properties of two representative FQs, i.e., norfloxacin (NORF) and ofloxacin (OFLO). Results showed that both FQs could sensitize the photo-transformation of acetaminophen under UV-A irradiation, during which excited triplet state (3FQ*) was the main active species. In the presence of 3 mM Br‾, the photolysis rate of acetaminophen increased by 56.3% and 113.5% in the solutions with 10 µM NORF and OFLO, respectively. Such an effect was ascribed to the generation of reactive bromine species (RBS), which was verified by 3,5-dimethyl-1H-pyrazole (DMPZ) probing approach. 3FQ* reacts with acetaminophen through one-electron transfer, producing radical intermediates which then couple to each other. Presence of Br‾ did not lead to the formation of brominated products but the same coupling products, which suggests that radical bromine species, rather than free bromine, were responsible for the accelerated acetaminophen transformation. According to the identified reaction products and assisted with the theoretical computation, the transformation pathways of acetaminophen under UV-A irradiation were proposed. The results reported herein suggest that sunlight-driven reactions of FQs and Br‾ may influence the transformation of coexisting pollutants in surface water environments.

Degradation of tetracyclines by peracetic acid and UV/peracetic acid: Reactive species and theoretical computations.

As an environment-friendly oxidant and disinfectant, peracetic acid (PAA) and PAA based-advanced oxidation processes (AOPs) for the treatment of emerging micropollutants have raised increasing interest, owing to their ease of activation and less generation of harmful disinfection byproducts. Tetracyclines (TCs) antibiotics as a group of wide-spectrum antibiotics are frequently detected in sewage effluents, while the knowledge of PAA-based advanced oxidation reactions to remove the substrates is quite limited. In this work, we systematically investigated the kinetics and underlying transformation mechanisms of three TCs including tetracycline (TTC), oxytetracycline (OTC), and chlortetracycline (CTC) in the UV-activated PAA oxidation process. The results indicated that three TCs can be efficiently decayed by UV/PAA. The pseudo-first-order reaction rate constants (kobs) of TCs followed the order: kCTC (0.453 min-1) ≫ kTTC (0.164 min-1) > kOTC (0.158 min-1). Quenching experiments showed that the removal of CTC was mainly ascribed to the direct oxidation of PAA, while TTC and OTC were more susceptible to free radicals. The kobs values of the three TCs by PAA oxidation presented a fairly well correlation to the global nucleophilicity and the activation energies of the TC molecules, highlighting the structure-specific reactions of TCs to PAA. Based on product identification and theoretical calculation, N-demethylation and hydroxylation were proposed as the main pathways for TCs degradation by PAA non-radical oxidation. The combination of PAA and UV irradiation can further improve the degradation efficiency of TCs and contribute to reducing the diffusion and transmission of resistance genes in the environment.

Amorphous zirconium oxide activates peroxymonosulfate for selective degradation of organic compounds: Performance, mechanisms and structure-activity relationship.

Application of heterogeneous advanced oxidation processes (AOPs) for wastewater treatment suffers from the low oxidant utilization efficiency, slow catalytic cycling and severe matrix interference. Herein, we report that amorphous zirconium dioxide (aZrO2), a redox-inert metal oxide, can efficiently activate peroxymonosulfate (PMS) to degrade organic micropollutants under very low oxidant doses and complex coexisting matrices. Distinct from conventional AOPs where radicals are formed, the surface Zr(IV)-PMS* complex was identified as the principal reactive species, and primarily conducted oxygen-atom-transfer route with selected molecules. Quantitative structure-activity relationship analysis indicated that the formation of Zr(IV)-PMS* complex was governed by the density of the surface hydroxyl groups. The strong interaction between the Zr atom and PMS caused the deviation of the negative charge from Zr(IV) metal sites to the oxidant. As a result, the O-O bond of the adsorbed PMS was prolonged and its oxidation potential elevated, which enabled it to directly react with contaminants. This study indicates the potential of aZrO2 as a novel and eco-friendly catalyst that activates PMS to selectively tackle organic contaminants, and sheds light on the designing of Fenton-like catalysts using redox-inert metals.

Photodegradation of phenylurea herbicides sensitized by norfloxacin and the influence of natural organic matter.

The photochemical activity of fluoroquinolone antibiotics (FQs) has gained attention due to the discovery of their phototoxicity and photocarcinogenicity in clinics. This study reveals that norfloxacin (NOR) can sensitize the photodegradation of phenylurea (PU) herbicides. This is attributed to the formation of an excited triplet of norfloxacin (3NOR*) by UV-A irradiation of its quinolone chromophore, which can further react with O2 to form singlet oxygen (1O2). The second-order rate of 3NOR* with PU ranges from 1.54 × 1010 to 2.76 × 1010 M-1s-1. The steady-state concentrations of 3NOR* were calculated as (4.29-31.2)× 10-16 M at 10 µM NOR under UV365nm irradiation. Natural organic matter (NOM) inhibited the degradation of PU induced by 3NOR*. In the presence of 10 mg L-1 NOM, the pseudo-first-order rate constants (kobs,NOM) of the degradation of diuron (DIU), isoproturon (IPU), monuron (MOU), and chlorotoluron (CLU) decreased by 65%, 19%, 36%, and 62%, respectively. NOM mainly acts as a reductant which reacted with the radical intermediates of the PU generated by 3NOR*oxidation, thus reversing the oxidation. The inhibitory effect increases with increasing NOM concentration. Results of this study underscore the role of NOR as a photosensitizer in accelerating the abatement of PU pesticides in sunlit surface waters. This study significantly advances the understandings of the behavior of NOR in aquatic environments.

New Insights into the Role of Nitrite in the Degradation of Tetrabromobisphenol S by Sulfate Radical Oxidation.

Tetrabromobisphenol S (TBBPS) is a brominated flame retardant and a contaminant of emerging concern. Several studies found that sulfate radical (SO4•-) oxidation is effective to degrade TBBPS. Here, we demonstrate that the presence of nitrite (NO2-) at environmentally relevant levels causes dramatic changes in the kinetics and pathways of TBBPS degradation by SO4•-. Initially, NO2- suppresses the reaction by competing with TBBPS for SO4•-. At the same time, SO4•- oxidizes NO2- to form nitrogen dioxide radicals (NO2•), which actively react with some key TBBPS degradation intermediates, thus greatly altering the transformation pathway. As a result, 2,6-dibromo-4-nitrophenol (DBNP) becomes the primary TBBPS product. As TBBPS undergoes degradation, the released bromide (Br-) is oxidized by SO4•- to form bromine radicals and free bromine. These reactive bromine species immediately combine with NO2• or NO2- to form nitryl bromide (BrNO2) that in turn attacks the parent TBBPS, resulting in its accelerated degradation and increased formation of toxic nitrophenolic byproducts. These results show that nitryl halides (e.g., BrNO2 or ClNO2) are likely formed yet inadequately recognized when SO4•- is applied to remediate halogenated pollutants in the subsurface environment where NO2- is ubiquitously found. These insights further underscore the potential risks of the application of SO4•- oxidation for the remediation of halogenated compounds in realistic environmental conditions.

Degradation of Tetrabromobisphenol S by Thermo-activated Persulfate Oxidation: Reaction Kinetics, Transformation Mechanisms, and Brominated By-products.

Brominated flame retardants (BFRs) are a group of contaminants of emerging environmental concern. In this study, systematic exploration was carried out to investigate the degradation of tetrabromobisphenol S (TBBPS), a typical emerging BFRs, by thermally activated persulfate (PDS) oxidation. The removal of 5.0 µM TBBPS was 100% after 60 min oxidation treatment under 60 °C. Increasing the temperature or initial PDS concentration facilitated the degradation efficiency of TBBPS. The quenching test indicated that TBBPS degradation occurred via the attack of both sulfate radicals and hydroxyl radicals. Natural organic matter (NOM) decreased the removal rate, however, complete disappearance of TBBPS could still be obtained. Six intermediate products were formed during reactions between TBBPS and radicals. Transformation pathways including debromination, ß-Scission, and cross-coupling were proposed. Brominated disinfection byproducts (DBPs) in situ formed during the degradation of TBBPS were also investigated, such as bromoform and dibromoacetic acid. The presence of NOM reduced the formation rates of brominated DBPs. Results reveal that although thermo-activated PDS is a promising method for TBBPS-contaminated water, it can lead to potential brominated DBPs risks, which should be paid more attention when SO4•--based oxidation technology is applied.

Aquatic photolysis of the pharmaceutical ambroxol: The role of 2,4-dibromoaniline chromophore and heavy atom effect of bromine.

As one of the most effective expectorant class drugs, ambroxol (AMB) has been frequently used to treat acute and chronic bronchitis. Extensive use and human excretion result in the widespread occurrence of AMB in wastewater. Herein, we reported the photolysis of AMB in aqueous solution upon 254 nm ultraviolet radiation (UV254). Spectroscopic characterization showed that 2,4-dibromoaniline (DBA) moiety is the core chromophore of AMB. Quantum yield of DBA changed little at pH 4.0 - 9.0; however, AMB showed higher quantum yield at pH > 8.0. Both DBA and AMB have high photoreactivity, which can be attributed to the "heavy atom" effect of bromine substituents. The photolysis of AMB occurred through photoreduction, photoionization, photonucleophilic substitution, side-chain cleavage, and coupling reactions. Both AMB and DBA underwent debromination with yields reaching 80% under 3200 mJ cm-2 UV fluence. Photo-debromination occurred preferentially at the para-position. The presence of natural organic matter inhibited the photodegradation, mainly due to the light-screening effect. The photolysis of AMB was slightly enhanced in the presence of NO3- likely due to radical-induced oxidation. Bioluminescence inhibition assay revealed that photoproducts were not toxic. The results show that UV254 radiation with fluences relevant to advanced oxidation processes was effective for the abatement of AMB in wastewater. However, UV254 treatment of wastewater containing higher concentrations (˃ µg L-1) of AMB should be done with caution because the released Br- can be converted to toxic brominated disinfection byproducts and bromate in subsequent oxidation process.

Electrochemical Reduction and Oxidation of Chlorinated Aromatic Compounds Enhanced by the Fe-ZSM-5 Catalyst: Kinetics and Mechanisms.

Devising cost-effective electrochemical catalyst system for the efficient degradation of chlorinated aromatic compounds is urgently needed for environmental pollution control. Herein, a Fe-ZSM-5 zeolite was used as a suspended catalyst to facilitate the degradation of lindane as a model chlorinated pesticide in an electrochemical system consisting of the commercial DSA (Ti/RuO2-IrO2) anode and graphite cathode. It was found that the Fe-ZSM-5 zeolite greatly accelerated the degradation of lindane, with the degradation rate constant more than 8 times higher than that without Fe-ZSM-5. In addition, the Fe-ZSM-5 zeolite widened the working pH range from 3 to 11, while efficient degradation of lindane in the absence of Fe-ZSM-5 was only obtained at pH ≤ 5. The degradation of lindane was primarily due to reductive dechlorination mediated by atomic H* followed by •OH oxidation. Fe-ZSM-5 zeolite could enrich lindane, H*, and •OH on its surface, thus provided a suitable local environment for lindane degradation. The Fe-ZSM-5 zeolite exhibited high stability and reusability, and reduced the energy consumption. This research provides a potential reduction-oxidation strategy for removing organochlorine compounds through a cost-efficient Fe-ZSM-5 catalytic electrochemical system.

Formation of chlorate and perchlorate during electrochemical oxidation by Magnéli phase Ti4O7 anode: inhibitory effects of coexisting constituents.

Formation of chlorate (ClO3-) and perchlorate (ClO4-) as by-products in electrooxidation process has raised concern. In the present study, the formation of ClO3- and ClO4- in the presence of 1.0 mM Cl- on boron doped diamond (BDD) and Magneli phase titanium suboxide (Ti4O7) anodes were evaluated. The Cl- was transformed to ClO3- (temporal maximum 276.2 µM) in the first 0.5 h on BDD anodes with a constant current density of 10 mA cm2, while approximately 1000 µM ClO4- was formed after 4.0 h. The formation of ClO3- on the Ti4O7 anode was slower, reaching a temporary maximum of approximately 350.6 µM in 4.0 h, and the formation of ClO4- was also slower on the Ti4O7 anode, taking 8.0 h to reach 780.0 µM. Compared with the BDD anode, the rate of ClO3- and ClO4- formation on the Ti4O7 anode were always slower, regardless of the supporting electrolytes used in the experiments, including Na2SO4, NaNO3, Na2B4O7, and Na2HPO4. It is interesting that the formation of ClO4- during electrooxidation was largely mitigated or even eliminated, when methanol, KI, and H2O2 were included in the reaction solutions. The mechanism of the inhibition on Cl- transformation by electrooxidation was explored.

Nitrophenolic byproducts formation during sulfate radical oxidation and their fate in simulated drinking water treatment processes.

Nitrite can be transformed to nitrophenolic byproducts in sulfate radical oxidation processes (SR-AOPs). These nitrophenols are highly mobile in subsurface and can potentially contaminate drinking water sources. However, their fate in a drinking water treatment remains ambiguous. Herein, the removal and transformation of four nitrophenolic byproducts formed during a heat activated peroxydisulfate oxidation process, i.e., 4-nitrophenol, 2,4-dinitrophenol, 5-nitrosalicylic acid, and 3,5-dinitrosalicylic acid, in a simulated drinking water treatment train were comprehensively examined. The removal of these nitrophenolic compounds in coagulation by either aluminum sulfate or ferric chloride ranged from 3.8% to 13.4%. In the chlorination process, 4-nitrophenol was removed only by 45.4% in 24 h at a chlorine dose of 5.0 mg/L. The removal of the other three nitrophenolic byproducts were less than 20%. Reaction between nitrophenolic byproducts and chlorine via electrophilic substitution gave rise to their chlorinated derivatives. Chlorinated nitrophenolic byproducts were more recalcitrant and toxic than their parent compounds, but still a tiny fraction of them could undergo further oxidation to form trichloronitromethane. This work implied that once nitrophenolic byproducts enter water source, they can penetrate the drinking water treatment train and react with the residual chlorine in distribution pipelines to form more hazardous byproducts. The findings raised additional concerns to the potential risk of the nitrophenolic byproducts formed in SR-AOPs.

Experimental and theoretical study on Fe(VI) oxidative degradation of dichlorophen in water: Kinetics and reaction mechanisms.

Dichlorophenol (DCP), a commonly used fungicide and insecticide, is widely found in waters and wastewaters. Herein, the degradation of DCP by Ferrate (Fe(VI)) in different matrices was comprehensively investigated. In pure water, a complete removal of DCP was achieved in 300 s at [Fe(VI)]:[DCP] molar ratio of 2:1. The presence of HA (10 mg L-1) inhibited DCP degradation to a certain extent. A total of twenty degradation products were identified by HPLC/MS analysis. Based on these products, reaction pathways including the cleavage of C-C bridge bond, hydroxylation, and radical coupling were proposed. These reaction mechanisms were further rationalized by theoretical calculations. The analyses of Wiberg bond orders and transition state indicated that C7-C8 bond was the most vulnerable site for cleavage, and C12 site was the most likely site for hydroxyl addition. Mulliken atomic spin densities distribution suggested that self-coupling products was easily generated via C-O-C coupling ways. Finally, the feasibility of applying Fe(VI) to degrade DCP (20 µM) in a municipal wastewater effluent and a lake water was evaluated and verified. The findings in this study are of relevance in designing Fe(VI)-based treatment strategy for chlorine-containing persistent pesticides.

Differentiation of Pathways of Nitrated Byproduct Formation from Ammonium and Nitrite During Sulfate Radical Oxidation.

Recent studies found that both nitrite (NO2-) and ammonium (NH4+) lead to nitrophenolic byproducts in SO4•- oxidation processes, during which NO2• generated through the oxidation of the inorganic nitrogen by SO4•- is the key nitrating agent. This study demonstrates that the formation of phenoxy radicals to which NO2• can be incorporated immediately is another governing factor. Two types of sites having distinct reactivities in natural organic matter (NOM) molecules can be transformed to phenoxy radicals upon SO4•- oxidation. Fast sites associated with phenolic functionalities are primarily targeted in the reaction sequence involving NO2-, because both are preferentially oxidized. Following the depletion of NO2-, NH4+ becomes the main precursor of NO2• that interacts with slow sites associated with the carboxylic functionalities. Experimental data show that the formation of total organic nitrogen in 24 h reached 6.28 µM during SO4•- oxidation of NOM (4.96 mg/L organic carbon) in the presence of both NO2- (0.1 mM) and NH4+ (1.0 mM), while the sum of those formed in the presence of each alone was only 3.52 µM. Results of this study provide further insights into the mechanisms of nitrated byproduct formation when SO4•- is applied for environmental remediation.

Formation of brominated by-products during the degradation of tetrabromobisphenol S by Co2+/peroxymonosulfate oxidation.

Tetrabromobisphenol S (TBBPS), an emerging brominated flame retardant, can cause neurotoxic and cytotoxic effects to human physiology. In this study, the degradation of TBBPS in Co2+ activated peroxymonosulfate (PMS) oxidation process was explored. In particular, brominated by-products formed during the degradation of the TBBPS were examined. It was found that TBBPS could be effectively removed in the Co2+/PMS oxidation process. The pseudo-first-order rate constants were 0.13 min-1 at 0.2 mM PMS and 0.5 µM Co2+ initially. It appeared that TBBPS degradation occurred via and HO attacks, but played a dominant role. The presence of natural organic matter (NOM) greatly inhibited the transformation of the TBBPS, which can be explained by the scavenging of the radical species. ß-Scission, debromination, and cross-coupling were identified as the main reaction pathways of TBBPS degradation in the Co2+/PMS system. Further oxidation and ring-opening of the intermediates generated brominated by-products including bromoform, monobromoacetic acid, and dibromoacetic acid. The formation of the brominated by-products increased gradually in approximately 48 h. But, the presence of NOM reduced the yields of the brominated -by-products. The findings of this study indicate that organic bromine contaminants can be effectively removed but lead to brominated by-products in the activated PMS oxidation process, which should be taken into consideration when -based oxidation technology is applied.