Heptafluoropropylene oxide dimer acid (HFPO-DA), as an alternative to perfluorooctanoic acid (PFOA), has been shown to pose similar environmental and health risks as other perfluorinated compounds. The electrochemical-based advanced oxidation processes are promising techniques for the treatment of perfluorinated compounds, and the boron-doped diamond (BDD) anode could degrade HFPO-DA under mild conditions. However, the roles of radicals in the degradation and how to overcome the steric hindrance of the –CF₃ branch on the carboxyl group were not yet clear. In this study, we investigated the degradation mechanism of HFPO-DA on the BDD anode. Instead of other non-active anodes (PbO₂ and SnO₂ electrodes), HFPO-DA can be degradable on the BDD electrode with a rate constant logarithmic correlation to the applied current density. The hydroxyl radical (•OH) was one of the key factors in the degradation of HFPO-DA, accounting for almost 89% of the significant effect, and the direct electron transfer was the rate-limiting step in the degradation reaction. Physicochemical characterization including field emission scanning electron microscope (FE-SEM), X-ray photo-electron spectroscopy (XPS), water contact angle, and electrochemical property indicated that the BDD electrode was fluorinated after electrolysis, the electrode surface became more hydrophobic due to the bonding of –C_xF_y, leading to a decrease in the electrochemically active area. Moreover, degradation products (pentafluoropropionic acid, trifluoroacetic acid, and fluorine ion) were detected and the mass balance of carbon and fluorine was calculated during the degradation. Therefore, a degradation mechanism for HFPO-DA was proposed, which involved direct electron transfer, decarboxylation, radical reaction, decarboxylation, and decarboxylation. The de–CF₃ step initiated the fluorination of the BDD electrode, which was initiated by the defluorination process. This study contributes to the understanding of the electro-oxidative degradation of perfluoroalkyl ether carboxylic acids and provides guidance for the application of electrochemical advanced oxidation processes.

中文翻译：

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表面氟化介导了 HFPO-DA 在掺硼金刚石电极上的电氧化降解


七氟环氧丙烷二聚酸 （HFPO-DA） 作为全氟辛酸 （PFOA） 的替代品，已被证明会带来与其他全氟化合物类似的环境和健康风险。基于电化学的高级氧化过程是处理全氟化合物的有前途的技术，硼掺杂金刚石 （BDD） 阳极可以在温和条件下降解 HFPO-DA。然而，自由基在降解中的作用以及如何克服 –CF₃ 分支对羧基的空间位阻尚不清楚。在这项研究中，我们研究了 HFPO-DA 在 BDD 阳极上的降解机制。HFPO-DA 可以在BDD电极上降解，而不是其他非活性阳极（PbO₂ 和 SnO₂ 电极），其速率常数与施加的电流密度呈对数相关。羟基自由基 （•OH） 是 HFPO-DA 降解的关键因素之一，占显著效应的近 89%，直接电子转移是降解反应中的限速步骤。包括场发射扫描电子显微镜 （FE-SEM）、X 射线光电子能谱 （XPS）、水接触角和电化学性质在内的物理化学表征表明，BDD 电极在电解后被氟化，由于 –C_xF_y 的键合，电极表面变得更加疏水，导致电化学活性面积减小。此外，还检测了降解产物（五氟丙酸、三氟乙酸和氟离子），并计算了降解过程中碳和氟的质量平衡。 因此，提出了一种 HFPO-DA 的降解机制，涉及直接电子转移、脱羧、自由基反应、脱羧和脱羧。de-CF₃ 步骤启动了 BDD 电极的氟化，这是由脱氟过程启动的。本研究有助于理解全氟烷基醚羧酸的电氧化降解，并为电化学高级氧化过程的应用提供指导。