Department of Civil and Environmental Engineering, University of Nevada, Reno, Nevada 89557-0258, United States.
Department of Environmental Science, University of California, Riverside, California 92521, United States.
Environ Sci Technol. 2023 Apr 18;57(15):6179-6187. doi: 10.1021/acs.est.2c08210. Epub 2023 Apr 5.
The thermal decomposition products and mechanisms of per- and polyfluoroalkyl substances (PFASs) are poorly understood despite the use of thermal treatment to remediate PFAS-contaminated media. To identify the thermal decomposition products and mechanisms of perfluorocarboxylic acids (PFCAs), gaseous perfluoropropionic acid (PFPrA) and perfluorobutyric acid (PFBA) were decomposed in nitrogen and oxygen at temperatures from 200 to 780 °C. In nitrogen (i.e., pyrolysis), the primary products of PFPrA were CF═CF, CFCFH, and CFCOF. CFCF═CF was the dominant product of PFBA. These products are produced by HF elimination (detected as low as 200 °C). CF and CF were observed from both PFCAs, suggesting formation of perfluorocarbon radical intermediates. Pyrolysis products were highly thermally stable, resulting in poor defluorination. In oxygen (i.e., combustion), the primary product of both PFPrA and PFBA below 400 °C was COF, but the primary product was SiF above 600 °C due to reactions with the quartz reactor. Oxygen facilitated thermal defluorination by reacting with PFCAs and with pyrolysis products (i.e., fluoroolefins and fluorocarbon radicals). Platinum improved combustion of PFCAs to COF at temperatures as low as 200 °C, while quartz promoted the combustion of PFCAs into SiF at higher temperatures (>600 °C), highlighting the importance of surface reactions that are not typically incorporated into computational approaches.
尽管热疗法被用于修复全氟和多氟烷基物质 (PFAS) 污染的介质，但人们对其热分解产物和机制仍知之甚少。为了确定全氟羧酸 (PFCAs) 的热分解产物和机制，在氮气和氧气中于 200 至 780°C 的温度下分解气态全氟丙酸酸 (PFPrA) 和全氟丁酸 (PFBA)。在氮气中（即热解），PFPrA 的主要产物为 CF═CF、CFCFH 和 CFCOF。CFCF═CF 是 PFBA 的主要产物。这些产物是通过 HF 消除产生的（在低至 200°C 时即可检测到）。两种 PFCAs 均观察到 CF 和 CF，表明形成了全氟碳自由基中间体。热解产物具有很高的热稳定性，导致脱氟效果不佳。在氧气中（即燃烧），PFPrA 和 PFBA 的主要产物在 400°C 以下为 COF，但在 600°C 以上主要产物为 SiF，这是由于与石英反应器的反应所致。氧气通过与 PFCAs 以及与热解产物（即氟烯烃和碳氟自由基）反应来促进热脱氟。铂在低至 200°C 的温度下即可提高 PFCAs 燃烧为 COF 的效率，而石英在更高温度（>600°C）下则促进 PFCAs 燃烧为 SiF，这突出了表面反应的重要性，而这些反应通常不被纳入计算方法中。
