溶解性有机质引发2,3-二溴丙基-2,4,6-三溴苯基醚光转化机理

Phototransformation mechanism of 2,3-dibromopropyl-2,4,6-tribromophenyl ether induced by dissolved organic matter

光转化是水体中有机污染物的重要转化途径,决定污染物的环境暴露和风险.激发三线态溶解性有机质(^(3)DOM^(*))能够引发酚类、胺类和共轭二烯类污染物的间接光降解,反应机理涉及电子转移、质子转移和能量转移.然而,3DOM^(*)与新型溴代阻燃剂(NBFRs)及具有类似结构的污染物的反应机理尚不清楚.本研究选取2,3-二溴丙基-2,4,6-三溴苯基醚(DPTE)为NBFRs模型化合物,以苯乙酮模拟DOM,通过光化学实验和密度泛函理论(DFT)计算,探究3DOM^(*)与NBFRs的反应活性、反应位点和反应途径.结果表明,激发三线态苯乙酮(^(3)AP^(*))对DPTE表观光降解的贡献为41.7%±2.2%,二者的二级反应速率常数为(1.49±0.24)×10^(8)L·mol^(-1)·s^(-1).DPTE的子结构模型化合物与苯乙酮的光化学实验表明,3AP^(*)与DPTE的反应位点为DPTE苯基上的溴.DFT计算表明,3AP^(*)通过向激发态DPTE转移电子使DPTE生成脱溴中间体,脱溴中间体与基态苯乙酮发生质子转移反应生成脱溴产物.所揭示的3AP^(*)引发DPTE光转化机理,有助于评价和预测水体中其他有溴代苯基醚结构的污染物光转化产物和动力学.

Phototransformation is an important transformation pathway determining environmental exposure and risks of organic pollutants in natural waters.Excited triplet state of dissolved organic matter(^(3)DOM^(*))can react with phenolics,amines and conjugated dienes through electron transfer,proton transfer and energy transfer.However,reaction mechanism of 3DOM^(*)and novel brominated flame retardants(NBFRs)or other pollutants with similar structures remains unclear.In this study,reactivity,reaction sites and reaction pathways of 3DOM^(*)and NBFRs were investigated through simulating photochemical experiments and density functional theory(DFT)calculation,adopting 2,3-dibromopropyl-2,4,6-tribromophenyl ether(DPTE)as a representative of NBFRs and acetophenone(AP)as a model of DOM.Results show that contribution of excited triplet state of acetophenone(^(3)AP^(*))to apparent photodegradation of DPTE is 41.7%±2.2%.The second-order reaction rate constant between 3AP^(*)and DPTE was determined to be(1.49±0.24)×108 L·mol−1·s−1.Photochemical experiments on sub-structural moieties of DPTE and acetophenone indicates that the reaction site of DPTE and 3AP^(*)is the bromine on the phenyl.According to the DFT calculation,electron transfer from 3AP^(*)to excited state of DPTE leads to the generation of debromination intermediates,which further react with ground state of acetophenone via proton transfer reaction and generate debromination product.The phototransformation mechanism of DPTE induced by 3AP^(*)unveiled in this study contributes to evaluating and predicting phototransfomation products and kinetics of pollutants with bromophenyl ether structures in water body.