Photochemical Transformation Mechanisms Of Halogenated Phenols Involving Radicals In Water

Halogenated phenols(HPs)have received extensive attention due to their large quantity existences,wide distributions and endocrine disrupting activities.Knowledge on their environmental behavior is critical for their risk assessment.It is proved that HPs can undergo photodegradation in water environment.However,data on the free radicals involvement in phototransformation is still lacking.Therefore,in this research,typical HPs were selected to investigate their phototransformation process in water environment.Electron paramagnetic resonance(EPR)and laser flash photolysis(LFP)were used to capture the transient intermediates that are generated in the reaction process.In addition,the influence mechanism of the important water environmental factors on the formation of free radicals in the transient reaction were also investigated.Based on these questions,relevant investigations were carried out and summarized as follows:(1)The distribution,discharge,and photodegradation process of typical HPs in the effluent of real water in Dalian,China along the Huang-Bo Sea area were studied.The concentrations of 6 single-ring HPs and 11 kinds of OH-PBDEs in the effluents of 12 sewage treatment plants near Huang-Bo Sea were determined by GC/MS.All the target pollutants were detected.The total concentrations of HPs(single-ring)and OH-PBDEs were 77.2-168.5 ng/L and 0.08-0.88 ng/L,respectively.Under simulated irradiation,the degradation rates of BPs were faster than those of CPs.The degradation rates of HPs in real water were influenced by fators,such as pH and the existence of dissolved organic matter(DOM).The results of EPR and the quench experiment showed that-OH produced in real water could increase the degradation rate of HPs.(2)Three monobromophenols frequently detected in the natural water were selected as target HPs to investigate the transient intermediates during the photolysis process.EPR and spin trapping techniques were employed for the online direct detecting the radial intermediates generated at once.The results indicated that carbon-centered radical(·C),H radical(-H),and OH radical(-OH)were all produced during the photolysis of three BPs solutions.The substitution of bromine atom and environmental factors affected the formation of radicals significantly.The relative amounts orders of radicals were 4-BP>3-BP>2-BP.Different origins and compositions of DOM display different photochemical behavior in the formation of radicals.Experiments with several quenchers showed that 3DCM*made the most contribution for the promotion of three kinds of radicals.And DOM with higher contents of humic-like materials and aromaticity have more promotion of radicals.(3)2,6-DBP was taken as the precursor for the exploration of polymerization process of free radicals in the phototransformation of HPs.HPLC-LTQ-Orbitrap and GC-MS were used for the products in the process and EPR was used for the detection of transient species during photolysis of 2,6-DBP solution.EPR spectrum showed that both carbon-centered radical(αN＝16.07 G,αH=23.58 G)and its resonant oxygen-centered radcial(αH(2 meta)=3.45 G,αH(1 para)=1.05 G,g=2.0046)were obtained,which have the possibility of polymerization reaction.Two tetrabrominated photoproducts were generated after irradiation for 60 min and they were 4’-OH-BDE73 and 4’4-di-OH-PBB80.Three kinds of tribrominated products(4’4-di-OH-PBB36,4-OH-BDE34 and 4’-OH-BDE27)were also detected for they could be the debromination products of the tetrabrominated products after irradiation.Based on the results,it is the polymerization of carbon-centered radical and oxygen-centered radical generated from the irradiation of 2,6-DBP solution that resulted in OH-PBDEs and di-OH-PBBs through O-para-C coupling and C-C coupling reactions.(4)LFP was employed to deduce the transformation process of eight BPs and-OH in aquatic environment and the effects of several environmental factors were also investigated.The results showed that the OH-adducts and bromophenoxyl radicals were produced in the reaction of-OH and eight BPs.And the electrophilic OH added to the ring to form OH adducts and eliminated one molecular water was the main pathway for the bromophenoxyl radicals.The production of bromophenoxyl radicals follows the order:para-position>ortho-position>meta-position.Decay rate constants(k)and lifetimes were obtained through model simulation.The k values of bromophenoxyl radicals decreased with increasing bromine atoms and all the lifetimes were longer than 100 μs.Fe(Ⅲ)and humic acid(HA)played significant roles in the photochemical reaction between BPs and-OH.More OH-adducts were generated at lower pH values and higher Fe(Ⅲ)concentration.The doubled-sided effects of HA showed promoting effects at low concentration and inhibitive effects at high concentration for the formation of two transient species.