Formation of Products of Incomplete Destruction (PID) from the Thermal Oxidative Decomposition of Perfluorooctanoic Acid (PFOA): Measurement, Modeling, and Reaction Pathways.

The thermal decomposition of perfluorooctanoic acid (PFOA) under oxidative conditions was investigated using air (O₂) and N₂O as oxidants over temperatures ranging from 400 to 1000 °C in an ?-alumina reactor. In the presence of air, PFOA was found to decompose into perfluorohept-1-ene (C₇F₁₄) and perfluoroheptanoyl fluoride (C₇F₁₄O) in addition to HF, CO, and CO₂. At temperatures above 800 °C, both C₇F₁₄ and C₇F₁₄O were no longer detected. A comprehensive analysis of the reaction mechanisms through quantum chemical analysis and kinetic modeling in combination with experimental observations was utilized to identify key reaction pathways. Quantum chemical analysis led to the conclusion that oxygen atoms are crucial in decomposing perfluoroalk-1-enes, especially the stable perfluorohept-1-ene (C₇F₁₄). Under oxidative conditions, it was found that significant quantities of C₂F₆ and CF₄ were formed. Further quantum chemical analysis suggests that the O atoms facilitate the formation of volatile fluorinated compounds (VFCs) such as tetrafluoromethane (CF₄) and hexafluoroethane (C₂F₆), particularly at higher temperatures. By elucidating these key reactions, an improved understanding of the potential formation products of incomplete combustion (PICs) or products of incomplete destruction (PIDs) is made.