阴极电Fenton协同阳极高效氧化降解全氟化合物的方法及机制研究

Perfluorinated compounds (PFCs) possess high toxicity, environmental persistence and chemical stability. The extensive use of PFCs has resulted in their release into natural environmental matrices. Therefore, it is urgent to develop effective methods for removing PFCs. In this project, we propose a novel electrochemical oxidation method which was assisted with electro-Fenton technology. The “FeCu bimetal-carbon” aerogel was firstly fabricated as cathode through a simple one-step process from metal-resin precursors, and then activated by CO2 and N2 sequentially at a given temperature. The CO2 activation enhanced the accessibility of the aerogel’s pores, and the secondary N2 activation enlarged the porosity and regenerated the ultradispersed bimetallic iron and copper with reductive carbon. After the activation, the BET surface area of carbon aerogel was greatly increased, beneficial to the 2e reduction of oxygen to form H2O2, while the regenerated iron and copper with ultradispersity can efficiently decomposed H2O2 to form ∙OH. During this novel electrochemical oxidation process, at the onset, PFCs transfer an electron to the anode and form PFCs radicals. The highly unstable PFCs radicals undergo Kolbe decarboxylation or desulfonated reaction to form perfluoroalkyl radicals. Subsequently, the perfluoroalkyl radicals react with large amount of ∙OH, yielding progressively shorter chained PFCs, until complete mineralization occurs. The synergistic effect between anodic electrochemical (EC) oxidation and cathodic electro-Fenton (EF) oxidation greatly improve the electron utilization and reduce the energy consumption. Moreover, main emphasis is also placed on the understanding the synergistic effect mechanism between anodic EC and cathodic EF reaction. Results concerning the characterization of generated radicals during the degradation process are studied in detail with in-situ EC-EPR combined technology. Finally, the reasonable catalytic reaction mechanism would be also investigated through the determination and analysis of intermediate pollutant products.

全氟化合物(PFCs)是一类毒性高、难生化降解且分子结构稳定的污染物。本项目针对这一难题，提出一种阴极电Fenton协同阳极氧化PFCs的电化学新方法。采用简便的一步合成法制得“FeCu双金属-碳”气凝胶，并经CO2–N2二步活化，构筑出高活性的电Fenton阴极：超高比表面“碳”，有利于O2发生2电子还原产生H2O2；活化时原位再生的超分散、小尺寸“FeCu双金属”，有利于H2O2迅速催化分解产生∙OH。在该新方法处理过程中，PFCs 首先在具有高析氧电位的阳极失去1电子，易诱导形成全氟烃基自由基，继而在阴极产生的大量∙OH作用下，快速发生后续的氧化和矿化。阴阳电极并行协同氧化，充分体现电子的经济利用和处理的低能耗。利用电化学–EPR联用等原位技术，揭示阴极电Fenton产生∙OH的效率，掌握污染物中间体自由基的形成和变化规律，阐明阴阳电极协同氧化的作用机制。解析中间产物，阐释反应机理。

全氟化合物（PFCs）是一类毒性高、难生物降解且分子结构稳定的污染物。本项目提出阴阳极协同高效降解PFCs的电化学新方法，PFCs（CnF2n+1COO-）首先在阳极失去一个电子形成PFCs自由基（CnF2n+1COO-），继而脱羧形成全氟烃基自由基（CnF2n+1∙），随后该自由基与∙OH形成不稳定中间体CnF2n+1OH，再进一步反应形成新的短链PFCs或者PFCs自由基，循环氧化直至被完全矿化，因此电子转移能力和·OH浓度是决定PFCs降解效率的关键因子。电子转移能力由阳极的电化学活性决定，而∙OH的产生效率则由阴极氧还原的性能决定。研究结果表明，掺硼金刚石BDD阳极具有强的PFCs失电子能力，而双金属碳气凝胶阴极（FeCuCA，FeMnCA）则能实现电化学还原分子氧原位产生·OH。通过一步合成法将铁、铜或者铁、锰双金属原位生长到具有3D网络结构的碳气凝胶矩阵结构中，不仅克服传统负载型异相催化剂易脱落、易凝聚等难题，提高活性中心的稳定性。同时利用CO2–N2的二步高温活化方法，通过CO2与CA反应产生气体CO实现扩孔作用，增加比表面积。N2活化则利用高温条件下基底CA的还原性，原位还原在CO2活化过程产生的金属氧化物，原位再生具有高分散性的纳米金属颗粒，又能降低金属颗粒的尺寸，从而增加了其反应位点的芬顿活性。以BDD为阳极、FeMnCA为阴极的协同电催化体系中降解全氟辛酸（PFOA），4小时后PFOA的去除率为97%，总有机碳（TOC）去除率达到93%，体现了阴极电芬顿和阳极氧化的高效协同作用，随着降解时间的增长，中间产物C6F13COO-的量逐渐降低，而C5F13COO-、C4F9COO-、C3F7COO- C2F5COO-以及CF3COO-的浓度逐渐增加，结合溶液和电极表面中氟原子的NEXAFS图谱分析，进一步证实了阳极氧化和阴极电芬顿之间持续的协同电催化作用。