Photochemical Transformation Of Bromophenols And Hydroxylated Polybrominated Diphenyl Ethers In Seawater

Bromophenols(BPs)and its derivatives are not only common chemical intermediates,but also have a wide range of natural sources.They are widely distributed in the environment and have endocrine disrupting properties,threatening human health and ecological safety.BPs is also considered to be precursors of brominated dioxins.Therefore,the natural source and transformation process of BPs have always been hot issues in scientific research.Photochemical process is a common transformation and degradation process of halogenated phenolic compounds in water environment.At present,researches on the phototransformation of BPs in water are mostly carried out in freshwater systems,but its phototransformation in seawater is still lacking.In this thesis,several bromophenols and their derivatives hydro xylated poly brominated diphenyl ethers(OH-PBDEs)frequently detected in the environment were selected as model compounds to investigate phototransformation of BPs in seawater.Electron paramagnetic resonance(EPR),laser flash photolysis technology(LFP)and quenching experiment were respectively performed to explain the free radical reaction mechanism involved in the phototransformation processes of BPs in seawater.Besides,the effects of C?,Fe(?)and other environmental factors on the photochemical process of BPs were investigated.The main research contents and results are summarized as follows:(1)2,4,6-Tribromophenol(2,4,6-TBP)was selected as target BPs to explore the transformation of BPs in seawater under simulated sunlight conditions(xenon lamp,?>290 nm).The results indicated that photochlorination,photo hydro lysis,photodebromination,photobromination and photopolymerization occurred during the phototransformation of 2,4,6-TBP(10 pg/L)in seawater,and photochlorination was the characteristic and most important phototransformation pathway.Three bromine atoms in 2,4,6-TBP were successively replaced by chlorine atoms to form 2,4,6-trichlorophenol(2,4,6-TCP).The phototransformation rate of 2,4,6-TBP and the yield of 2,4-DBr-4-CP could be enhanced at higher Cl-concentration or higher Fe(?)concentration;the phototransformation rate of 2,4,6-TBP increased but the yield of 2,4-DBr-4-CP decreased with increasing pH of the solution.In addition,the toxicity test results showed that the acute toxicity of the system to Photobacterium luminescens would increase in a certain light period during the phototransformation of 2,4,6-TBP in seawater,and the increased toxicity was related to yields of the products formed by photochlorination and photohydrolysis.(2)Several BPs with different positions of bromine substitution(2-BP?4-BP?2,4-DBP?2,6-DBP and 2,4,6-TBP)were selected as model compounds to explore the free radical reaction mechanisms involved in the phototransfromation of BPs in seawater.Results of LFP and EPR experiments showed that ·OH,C-centered bromophenol-phenyl radical and X2·-(X=Cl,Br)were the main active intermediates involved in the phototransformation of BPs in seawater.LC-MS-MS combining with spin trap technology was used to analyze adducts formed by trapping agent and radicals.The results indicated that the C-centered bromophenol-aryl radicals were detected,and they were formed through the preferential loss of para-Br in bromophenols under irradiation.Based on the radicals,the reaction mechamism were proposed for the photochlorination,photohydrolysis and photodimerization of BPs in seawater.Specially,OH-PBDEs should be formed by the coupling of the C-centered bromophenol-aryl radical and the O-centered bromophenol-phenoxy radical.For example,2'-OH-BDE68 was detected as a photoproduct of 2,4-DBP in seawater,and the presence of Fe(?)could inhibit the formation of 2'-OH-BDE68.(3)2'-OH-BDE68 was selected as model compound to study the phototransformation of OH-PBDEs in seawater under simulated sunlight(xenon lamp,?>290 nm).The results showed that 12 main products were recongnized during the phototransformation of 2'-OH-BDE68 in seawater,involving five reaction types:photochlorination,photocyclizatio n,photohydrolysis,photodebromination and ether bond cleavage.The quenching experiment results showed that direct photolysis and indirect photolysis were involved in the photoconversion of 2'-OH-BDE68 in seawater.Specifically,the photochlorination reaction was related to the excited triplet state and the oxidation of ·OH and 1O2;the photocyclization did not involve the excited triplet state or the oxidation of ·OH or 1O2;the photo hydro lysis involved the excited triplet state and the oxidation of 1O2;the photodebromination only involved the oxidation of 1O2,and ether bond cleavage involved excited triplet states and the oxidation of ·OH.In addition,Cl-could promote the photochlorination and photo hydro lysis 2'-OH-BDE68,but inhibit its photodebromination,ether bond cleavage and photocyclization.As the coexisting photosensitive substances,SRFA,Fe(?)and NO3-could inhibit the ether bond breakage of 2'-OH-BDE68,but promoted its photocyclization,photo hydro lysis and photo chlorination,and had no effect on the debromination process.For the photo hydro lysis of 2'-OH-BDE68,SRFA had a promoting effect while Fe(?)and NO3-basically had no effect.The above results indicated that the phototransformation of BPs and OH-PBDEs in seawater involved a variety of reaction pathways,and photochlorination was the main reaction pathway.Therefore,in the marine environment,the phototransfromation of BPs might be an important natural source of its homologous organochlorinated phenolic pollutants.The results provided a scientific basis for understanding the natural sources of halogenated phenolic compounds and for correct assessment for the environmental risks of BPs.