Formation Of Brominated Organic Compounds Via Chloroperoxidase-Mediated Oxidative Process

Halogenated organic compounds such as hydroxylated polybrominated biphenyls(OH-PBBs),hydroxylated polybrominated diphenyl ether(OH-PBDEs)and polybrominated dibenzo-p-dioxins/dibenzofurans(PBDD/Fs)are a class of emerging environmental contaminants.In the recent years,increasing attentions have been paid to the origin,transport and fate of these compounds.Studies to date have shown that OH-PBB/PBDEs and PBDD/Fs may be fromed from phenol,bromophenols,and hydroxybenzoic acid via either abiotic oxidation processess(e.g.,photocatalysis,manganese/iron oxides)or some biotic oxidation processess(e.g.,vanadium-bromoperoxidase).However,it is unclear whether OH-PBB/PBDEs and PBDD/Fs can be formed from phenol and bromophenols via chloroperoxidase(CPO)-mediated oxidative process.This work was conducted to verify this hypothesis and the main results obtained include:(1)A new analytical method based on LC-MS/MS precursor ion scan(PIS)mode was developed for the selective screening of halogenated compounds in water samples.Eight halogenated carbazoles and six estrogens(non-halogenated)were chosen as model compounds to evaluate the effects of PIS analysis.Unlike full scan,PIS analysis showed specific selectivity to halogenated compounds by filtering out numerous instrumental and matrix interferences.To simplify the estimation of halogen atom numbers in target compounds,Bernoulli experiment models were used to calculate theoretical isotopic abundances.Using the established analytical method,eight halogenated carbazoles were detected in laboratory reaction samples and two unknown brominated compounds were detected in seawater samples.This novel method is simple,specific,and suitable for preliminary screening of halogenated compounds in various environmental media.(2)The oxidative transformation of phenol and bromophenols by CPO in the presence of Br-and H2O2 was investigated.The main reaction products included five bromophenols,twelve OH-PBB/PBDEs and two PBDD/Fs.A possible reaction pathway was proposed based on the detected reaction products.Briefly,the catalytic reaction was induced by CPO,and the subsequent reactions involved bromination,debromination,radical coupling(e.g.,C-C coupling,C-O coupling),cyclization,and decomposition.Effects of environmental parameters(e.g.,pH value,enzyme concentration,halide ion concentration)on the CPO-oxidation of phenol were evaluated.In the range of pH 3.0-7.0,halogenation ability of CPO decreased as the pH increased,while coupling ablity increased in the first and then declined gradually.Higher CPO enzyme or Br-concentrations facilitated the formation of brominated products.When the initial concentration of Br-was close to seawater level(about 1.0 mmol/L),the reaction could reach equilibrium within 5 min.Although the addition of Cl-into the reaction system significantly slowed down the bromination rates,brominated organic compounds were still the dominant products.In summary,a new analytical method for the preliminary screening of halogenated organic compounds was successfully established.In addition,this work provides a possible interpretation for the natural formation of halogenated organic compounds in the environment.