蒙脱石表面生成二噁英机理的理论研究

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are ubiquitous persistent organic pollutants (POPs) in the environment. Recently much more attention has been paid on the characteristic pollution profile observed in various soil and freshwater sediment samples that octachlorodibenzo-p-dioxin (OCDD) exists as the most prevalent congener with very low level PCDFs contamination. Unknown sources may contribute to such congener feature since neither known natural sources nor revealed anthropogenic formation pathways can lead to the same profile. Fortunately, the spontaneous in situ formation of OCDD from pentachlorophenol on Fe(III)-montmorillonite clay at room temperature has been documented, and surface mediated formation of dioxins from chlorophenols (CPs) has been regarded as one of the potential sources in the environment. However, great efforts have been made to explore the homogeneous gas-phase formation pathways of PCDD/Fs while little information about the mechanism of interface formation of PCDD/Fs from CPs is available. Therefore, the project will focus on the theoretical study of the mechanism of surface-mediated formation of PCDD/Fs from CPs on various montmorillonite (MMT) clays. Common MMT clays such as Fe(III)-, Ca-, and Na-MMT will be adopted as the model surface, and periodic first-principle calculations will be used to elucidate both interaction modes between MMT and CPs and PCDD/F formation pathways. Specifically, key structural feature for CP sorption on MMT will be extracted on the basis of determination of stable CP-MMT complexes and corresponding sorption sites. The structural basis of phenoxy radical formation will be discussed as well as the relevant effect of surface micro-structure. Subsequently, density functional theory method will be used to investigate possible pathways such as radical-radical combinations and radical-molecule reactions, and to determine the favorable reaction channels. The understanding in the essential role of typical clays for PCDD/F formation from CPs will then be revealed. We expect that the project may provide new insight into PCDD/F sources in the environment and facilitate the effective control of PCDD/F contamination.

土壤及水体沉积物中高浓度八氯代二苯并二噁英(OCDD)污染引起极大关注，有实验表明氯酚经环境界面转化生成二噁英是环境OCDD重要来源之一。遗憾地是目前关于环境二噁英生成机理的研究多集中于气相化学反应的探索，环境界面二噁英生成机制尚不清晰。本项目拟以Fe(III)-、Ca-及Na-蒙脱石等为典型环境界面，开展氯酚在蒙脱石表面生成二噁英的理论研究。项目运用基于密度泛函理论的第一性原理方法，在探知蒙脱石表面氯酚吸附方式基础上，阐明其表面生成酚氧自由基的结构基础；并进一步探索酚氧自由基通过自由基/自由基或自由基/分子耦合形成二噁英的反应机理，建立反应势能面，确定反应进行的通道，揭示蒙脱石表面微结构对于氯酚转化为二噁英类物质的影响和其催化反应的关键分子历程。项目成果不仅对阐明OCDD环境生成的化学本质具有重要理论意义，而且面向我国履行国际公约这一重大需求，其结论有助于从源头减少和控制二噁英的生成。

环境中未知二噁英生成途径的确立对于污染控制和人群污染毒害防护具有重要意义。典型界面二噁英原位生成是环境中二噁英重要来源之一，鉴于实验分析方法的制约，计算化学方法在一定程度上可以很好地弥补实验上的困难，能够描绘出反应过程中各种可能的反应路径，明确关键中间体和过渡态的结构和能量。本项目以计算化学的手段研究了五氯酚分子在典型Na-、Ca-和Fe(III)-蒙脱石界面的吸附构型，确立的最稳定的吸附构型。酚氧自由基是氯酚生成二噁英的重要中间体，本项目中探讨了氯酚在表面反应生成表面负载的自由基中间体的分子机制，进一步研究了吸附态自由基与游离分子耦合生成二噁英和呋喃前体的反应机理。计算了热力学和动力学参数，建立了势能面剖面图，并合理解释了环境现象。所得结果为揭示二噁英新的生成途径提供了理论依据。