Study On Photochemical Transformation Mechanism Of Brominated Flame Retardants Induced By Dissolved Organic Matter

Photochemical transformation is one of the important transformation pathways of organic pollutants in natural waters.Dissolved organic matter（DOM）,which exists widely in water,could promote phototransformation of pollutants by generating photochemically produced reactive intermediates（PPRIs）.Excited triplet state of DOM（³DOM^*）,an important category of PPRIs,could promote photodegradation of phenolics and amines.However,reactivity of ³DOM^*with other pollutants is still limited.Recent studies indicates that brominated flame retardants（BFRs）might react with ³DOM^*,but reaction mechanism is still unclear.In this study,2,3-dibromopropyl-2,4,6-tribromophenyl ether（DPTE）was introduced as representative of BFRs,and acetophenone was adopted as DOM model compound to study reactivity,reaction sites and pathways of ³DOM^*and BFRs.Major contents and results are shown as follows:（1）Effects of acetophenone on photochemical transformation kinetics of DPTE were investigated through quenching experiments and probing experiments.Results show that addition of acetophenone promotes apparent photodegradation rate constant of DPTE from（4.1±0.3）×10^-2 h^-1 to（9.4±1.0）×10^-2 h^-1.Excited triplet state of acetophenone is the major contributor to photodegradation of DPTE,and the contribution of ³AP^*is 41.7%±2.2%.Ground state of acetophenone contributes 28.4%±1.5%to rate constant of DPTE.Steady concentration of ³AP^*was measured to be 7.31×10^-14mol?L^-1 by 2,4,6-trimethylphenol in the experiment condition.Second-order rate constant of ³AP^*and DPTE was calculated to be（1.49±0.24）×10⁸L?mol^-1?s^-1.（2）Allyl-2,4,6-tribromophenoxy ether（ATE）,2,4,6-tribromoanisole（TBA）,3-phenoxypropyl bromide（3BPE）and propoxybenzene（PB）were selected as sub-structural model compounds（SSMCs）of DPTE.Steady irradiation experiments of SSMCs were conducted to determine reactivity of SSMCs,which further used to conjecture reaction sites on DPTE.Results show that ATE and TBA,representatives of substituted bromines on phenyl,own reactivity to ³AP^*.3BPE and PB,representative of alkyl substituted bromines and aromatic ether,have no reactivity to ³AP^*,which indicates substituted bromines on phenyl are reaction sites of DPTE and ³AP^*.（3）Reaction mechanisms of DPTE and ³AP^*were revealed by density functional theory（DFT）calculations.Results show that electron transfer reactions between excited state of DPTE and ³AP^*are thermodynamically spontaneous,which indicates electron transfer from³AP^*to excited state of DPTE promotes photodegradation of DPTE.Reaction energy barriers（ΔG^?）of hydrogen transfer from acetophenone to DPTE debromination intermediates were calculated to be 18.60 kcal?mol^-1（alkyl hydrogen of acetophenone）,21.12 kcal?mol^-1（benzene hydrogen of acetophenone）,21.77 kcal mol^-1（alkyl hydrogen of ³AP^*）and 21.16 kcal?mol^-1（benzene hydrogen of ³AP^*）,respectively.Among the four reactions,only the transfer reaction involved alkyl hydrogen of acetophenone is thermodynamically spontaneous.DFT calculation results indicate hydrogen transfer from alkyl hydrogen of acetophenone to DPTE debromination intermediates is the reason of promotion effect of DPTE photodegradation caused by ground state of acetophenone.Electron transfer mechanism from ³DOM^*to excited state of DPTE,and hydrogen transfer mechanism from ground state of DOM to debromination intermediates of DPTE unveiled in this study contributes to evaluating and predicting phototransfomation products and kinetics of BFRs and other pollutants with bromophenyl ether structures in water body,which is of great significance for a comprehensive understanding of the photochemical transformation behaviors of BFRs.