硫化零价铁(S-ZVI)协同维生素B12(VB12)催化还原全氟烷基酸(PFAAs)降解脱氟的效果及机理研究

Perfluoroalkyl acids (PFAAs) is an emerging class of persistent organic pollutants (POPs) that has been widely used and ubiquitously disseminated. Due to the high thermal and chemical stability of C-F bond, PFAAs is extremely resistant to traditional chemical and biological degradations. Vitamin B12 (VB12) is a natural cofactor that can catalyze various reductive dehalogenation processes. However, the reductive defluorination of PFAAs catalyzed by VB12 has a relatively low efficiency and relies strongly on the reaction temperature. In view of the drawbacks, a synergetic system of sulfidated zerovalent iron (S-ZVI) and VB12 was proposed as a new strategy to efficiently catalyze the reductive degradation and defluorination of PFAAs. S-ZVI not only serves as the electron donor, but also activates C-F bond, and therefore lowers the activation energy of C-F bond cleavage. Meanwhile, S-ZVI is expected to enhance the electron transfer, consequently improving the reduction efficiency of PFAAs..The main objectives are: 1) to investigate the effects of S-ZVI/VB12 synergetic system on the degradation and defluorination of PFAAs congeners, precursors and isomers and analyze the quantitative structure-activity relationship (QSAR); 2) to detect the steady-state intermediates and propose the potential degradation pathway; 3) to unravel the catalytic mechanism and electron transfer process based on the microcosmic characterization and theoretical calculation results; 4) to investigate the influential factors and explore the possible natural degradation pathway of PFAAs. The results will contribute to the development of a novel synergetic system of S-ZVI/VB12, and advance its application in the area of pollution control and environmental remediation.

全氟烷基酸(PFAAs)是一类应用和分布广泛的新型持久性有机污染物。由于C-F键键能高，PFAAs的化学稳定性强，这类化合物的降解脱氟十分困难。维生素B12(VB12)是一种能够高效催化还原脱卤的天然辅酶因子，然而以往研究表明VB12催化PFAAs脱氟的效率低，且需要较高的反应温度。因此，本项目提出利用硫化零价铁(S-ZVI)协同VB12催化还原PFAAs脱氟的新思路。S-ZVI不仅起到电子供体的作用，还能够活化C-F键，降低反应所需的活化能；同时增强电子传递，提高PFAAs还原效率。本研究将考察该体系对不同PFAAs同系物、前体物和同分异构体催化还原脱氟效果，并进行分子构效关系分析；分析稳态产物和还原降解途径；借助微观表征和理论计算探究催化机理和电子传递机制；分析环境影响因素，探索可能存在的自然降解途径。成果有助于发展S-ZVI/VB12新型协同催化还原体系在PFAAs污染修复中的应用。

全氟烷基酸(PFAAs)是一类应用和分布广泛的新型持久性有机污染物。由于C-F键键能高，这类化合物的降解脱氟十分困难。本研究以全氟辛烷磺酸(PFOS)为目标污染物，考察了硫化零价铁/维生素B12(S-ZVI/VB12)体系对PFOS去除和脱氟的效能；探明了关键影响因素和最佳工艺条件；阐明了VB12的催化过程，以及S-ZVI和VB12之间的电子传递机理和协同作用机制；并通过降解产物监测，表明了PFOS氢解脱氟、邻位消去、以及C-C键断裂的降解脱氟途径。重要结果及关键数据如下：（1）考察了S-ZVI/VB12催化还原体系对水中PFOS不同异构体的降解和脱氟效果，结果表明在30 g/L S-ZVI（S/Fe=0.025），300 μM VB12，70 ℃，初始pH不调（pH=8.8）条件下，初始浓度为300 μM的PFOS 2h吸附去除率可达99.8%，96h脱氟率可达20.2%。直链PFOS（L-PFOS）更易于吸附在S-ZVI表面，却难以被VB12催化还原降解（72h的降解率<7.5%）；支链PFOS（br-PFOS）有较高的降解速率，拟一级速率常数为0.516 h−1。（2）通过UV-Vis、XAFS、ESR、XPS等表征以及UPLC-QTOF对中间产物的监测，揭示了S-ZVI/VB12体系催化机理和电子转移机制如下：首先，S-ZVI将VB12- Co(Ⅲ)还原为Co(Ⅱ)；被还原的VB12通过Co−F结合进攻PFOS上的F原子，并发生SN2亲核取代反应；进而PFOS通过还原消去途径和氢解途径脱掉F−，生成多氟代不饱和的磺酸盐和羧酸盐。S-ZVI除作为还原剂为VB12提供电子，还能够通过对PFOS的吸附作用活化C-F键及提供限域表面。（3）系统考察了温度、pH、S/Fe比、S-ZVI浓度和VB12浓度对PFOS去除和脱氟的影响；同时考察了PFOS在Na2S/VB12和天然硫铁矿/VB12体系下的降解和脱氟，表明了该催化过程在自然条件下发生的可能性，为PFOS的自然归趋和生物降解研究奠定理论基础。