Study On Mechanisms Of Four Halogenated Organic Compounds During KMnO₄/K₂FeO₄ Oxidation Process

Recently,there has been increasing concern about the widespread occurrence of halogenated organic compounds in the aquatic environment.The study of their degradation mechanisms in typical environmental processes and toxicity changes could be conducive to develop new strategies to deal with this concern.As two sorts of multifunctional oxidants and water treatment reagents,potassium permanganate(KMnO4)and ferrate(K2FeO4,Fe(Ⅵ))have been widely used in water treatment.Four typical halogenated organic compounds were chosen as the model halogenated organics and KMnO4/Fe(Ⅵ)as two typical oxidants in this study.Then the reactive kinetics,degradation products,pathways and toxicity changes in these processes were investigated.Detailed studies consist of the following four aspects:(1)The potential applicability of potassium permanganate for removal of triclosan(TCS)in water treatment was systematically investigated.A series of kinetic experiments were carried out to study the influence of various factors,including the pH,oxidant doses,temperature,and presence of typical anions(Cl-,SO42-,NO3-),humic acid(HA)and fulvic acid(FA)on triclosan removal.A total of eleven products were detected by liquid chromatography-quadrupole-time-of-flight-mass spectrometry(LC-Q-TOF-MS)analysis,including phenol and derivatives thereof,benzoquinone,an organic acid,and aldehyde.Two main reaction pathways involving C-0 bond cleavage(-C(8)-O(7)-)and benzene ring opening(in the less chlorinated benzene ring)were proposed,and were further confirmed based on frontier electron density calculations and point charges.Furthermore,the changes in the toxicity of the reaction solution during TCS oxidation by KMnO4 were evaluated by using both the luminescent bacteria Photobacterium phosphoreum and the water flea Daphnia magna.The toxicity of triclosan to D.magna and P.phosphoreum after 60 min at an initial concentration of 20 mg/L was reduced by 95.2%and 43.0%,respectively.Phenol and 1,4-benzoquinone,the two representative degradation products formed during permanganate oxidation,would yield low concentrations of disinfection by-products(DBPs)after chlorination and chloramination.Overall,KMnO4 can be used as an effective oxidizing agent for TCS removal in water and wastewater treatment.(2)The KMnO4 degradation of decabromodiphenyl ethane(DBDPE),an additive in brominated flame retardants(BFRs),was investigated in a sulfuric acid system.The degradation tests showed that DBDPE reacted completely with KMnO4 in sulfuric acid,and a removal rate of 99.71%seems realistic.Many products were detected by liquid chromatography-quadrupole-time-of-flight-mass spectrometry(LC-Q-TOF-MS)analysis and gas chromatography-electron ionization-mass spectrometry(GC-EI-MS)analysis.The degradation initially forms 1,2-bis(perbromophenyl)ethane-1,2-dione(Pi),pentabromophenol(P2),2-(perbromophenyl)ethanol(P3)and pentabromobenzoic acid(P28),and the subsequent transformation of these primary intermediates generates ring-opening,five-membered ring and six-membered heterocycle products over time.Two main reaction pathways involving the direct oxidation of the C-C bond of the ethyl group and the cleavage of the C-C bond between ethyl carbon and the adjacent carbon were proposed,and were further confirmed by point charges analyses and Wiberg bond order calculations.Furthermore,the toxicity of DBDPE and its degradation products were evaluated using the ECOSAR program.DBDPE was found to be a toxic substance that could cause great damage to three kinds of organisms in different trophic levels,while all the transformation products were less toxic than parent compound.This study is the first to report the degradation of DBDPE by KMn04 in acid and the obtained results can provide valuable guidance on the treatment of DBDPE leakage from point sources like storage tanks.(3)The oxidation of polychlorinated diphenyl sulfides(PCDPSs),dioxin-like compounds,by Fe(Ⅵ)was investigated.Kinetics of the reactions of Fe(Ⅵ)with seventeen PCDPSs,differ in number and positions of chlorine atoms(from 2 to 7),were investigated at pH 8.0.The second-order rate constants(k,M-1 s-1)of the reactions varied with the numbers and positions of chlorine atoms and appeared to be related with standard Gibbs free energy of formation(△fG0)of PCDPSs.Mechanism of the reaction was investigated by identifying oxidized products(OPs)of the reaction between Fe(VI)and 2,2’,3’,4,5-pentachlorodiphenyl sulfide(PeCDPS)at pH 8.0.Mechanism of oxidation involved major pathway of attack on sulfur(Ⅱ)by Fe(Ⅴ)in steps to yield sulfoxide type products,and subsequent breakage of C-S bond with the formation of sulfonic acid-containing trichloro compound.Minor pathways were hydroxylation of benzene ring and substitution of chlorine atom with hydroxyl group.A substitution step was carried out by ·OH,generated from self-decay of Fe(Ⅵ).The feasibility of the substitution reaction was supported by density functional theory(DFT)calculations.Oxidation of PeCDPS resulted in OPs of decreased toxicity.Removal experiments in the presence of ions and humic acid demonstrated degradation of PeCDPS by Fe(Ⅵ)in minutes.(4)The reaction kinetics,products,and mechanisms of the antimicrobial agent chlorophene(CP)by ferrate oxidation were investigated in aqueous solutions.Chlorophene is very readily degraded by Fe(Ⅵ),with the apparent second-order rate constant,k,being 642.4 M-1 s-1 at pH 8.0.A total of twenty-two oxidation products were identified using a liquid chromatography-quadrupole-time-of flight-mass spectrometry(LC-Q-TOF-MS)and ion chromatography,and their structures were further elucidated using MS/MS spectra.According to the extracted peak areas in mass spectrum,the coupling products(dimers,trimers,and tetramers)formed via single-electron coupling mechanism was the main reaction products of CP.Theoretical calculations demonstrated that hydrogen abstraction should easily occur at the hydroxyl group to produce reactive CP· radicals for subsequent polymerization.Cleavage of the C-C bridge bond,electrophilic substitution,hydroxylation,ring opening,and decarboxylation were also observed during Fe(Ⅵ)oxidation process.In addition,the degradation of CP by Fe(Ⅵ)was also effective in real waters,which provide a basis for potential application.