氯代及溴代二苯并二噁英的水环境光化学生成途径及机理

Source identification is of great importance for the pollution prevention and control of dioxins. Previous studies showed that chlorinated and brominated dibenzo-p-dioxins could be formed via phototransformation of halogenated phenols and hydroxylated diphenyl ethers (HO-PXDEs) in waters. However, the detailed photochemical formation pathways, mechanisms, as well as the influence of photosensitizers existing in waters are unclear at the moment. As a result, 2,4-dichlorophenol, 2,4-dibromophenol, triclosan and some frequently detected hydroxylated polybrominated diphenyl ethers are selected as the model compounds in this study, the yields of dioxins during the direct photolysis and the photooxidation via singlet oxygen and hydroxyl radical will be clarified. The photochemical formation mechanisms of the dominant pathways will be also disclosed. Further more, the influence of the protonation states and the substituents (i.e., Cl- and Br-), as well as the important photosensitizers in the sea water and fresh water (e.g. chlorine ions and the dissolved organic matters) on the photochemical formation of dioxins are going to be unveiled. A combined photochemical experiment, transient species analysis and quantum chemical calculation method will be employed to better elucidate theses questions. The study is of great helpful for the understanding of dioxin occurrence in waters, and also for the pollution prevention of dioxins.

二噁英的来源研究对于其污染控制至关重要。已有的研究表明，水体中卤代酚及羟基卤代联苯醚(HO-PXDEs)的光化学过程中能够产生二噁英。但二噁英的主要光化学产生途径、机理以及重要水环境因素的影响还有待揭示。本项目采用光化学实验测试、瞬态物种表征与量子化学计算模拟相结合的研究方法，选取2,4-二氯酚、2,4-二溴酚、三氯生及常见的羟基多溴联苯醚为模型化合物，研究其直接光解以及单线态氧和羟基自由基引发的光氧化反应过程的二噁英产率，阐明重要反应途径的微观机制；阐释pH值、海水/淡水中的重要光活性物质(氯离子和可溶性有机质)以及母体取代基种类和取代模式对二噁英产率的影响。这些问题的揭示，对于理解水环境中二噁英的来源及二噁英的污染防治具有重要意义。

羟基卤代联苯醚(HO-PXDEs)是环境中广泛存在的新兴污染物，其光化学转化可产生二噁英类物质，并对环境中的二噁英水平有明显贡献，但该过程二噁英的产生途径及对产率的影响因素还有待阐释。鉴于此，本项目采用光化学实验和量子化学计算结合的方法，围绕二噁英的产生途径及母体解离形态、取代基种类、共存物质对其产率的影响进行研究。(1)测定了不同解离形态2'-HO-BDE-68直接光解及1O2和·OH参与的光氧化过程中1,3,8-TBDD的产率，发现其主要产生于离子态直接光解过程，pH值通过影响解离形态影响二噁英的生成速率及产率。本研究首次报道了2'-HO-BDE-68光解过程中1,3,8-TBDD的生成和降解速率，同时揭示了2'-HO-BDE-68不同光化学途径的主要产物。(2)三氯生(TCS)和2'-HO-BDE-28的直接光解过程始于激发单线态，进而生成二噁英；碱性溶液中TCS具有比2'-HO-BDE-28更高的二噁英生成速率和产率；TCS和2'-HO-BDE-28的激发三线态通过敏化溶解氧参与反应，但对二噁英的生成没有贡献。(3)草酸通过与激发单线态2'-HO-BDE-28, 68分子的电子转移反应促进其光化学转化，但对离子形态的2'-HO-BDE-28, 68的光化学转化没有影响；(4)离子强度的增大抑制HO-PXDEs光解；(5)Suwannee River DOM (SRDOM)通过光屏蔽、静态淬灭和动态淬灭抑制HO-PXDEs的光解，模拟淡水中，SRDOM增大了TCS和2-HO-BDE-28光解中二噁英的表观产率，同时也加速了二噁英的降解。模拟河口水中，SRDOM增大了TCS光解时二噁英产率，但Cl-和SRDOM的共存显著降低了2,8-DBDD的产率；(6)甲醇中5-MeO-BDE-47可发生光致环化反应生成呋喃，其他MeO-PBDEs以及其他溶剂中的对比研究发现，MeO-PBDEs的光致成环反应取决于溴原子取代方式和溶剂环境。水中MeO-PBDEs直接和自敏化光解及·OH反应均无二噁英/呋喃产物检出，主要直接光解途径为脱溴加氢。(7)五种三溴苯氧基溴代阻燃剂在甲醇和正己烷的主要光解途径为脱溴和醚键断裂，无呋喃/二噁英类产物检出。本项目的研究成果可用于分析环境中二噁英的的光致产生过程，同时也丰富了对于MeO-及HO-取代PXDEs类化合物的环境光化学过程的理解。