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 (O 2 ) and N 2 O as oxidants over temperatures ranging from 400 to 1000 °C in an α-alumina reactor. In the presence of air, PFOA was found to decompose into perfluorohept-1-ene (C 7 F 14 ) and perfluoroheptanoyl fluoride (C 7 F 14 O) in addition to HF, CO, and CO 2 . At temperatures above 800 °C, both C 7 F 14 and C 7 F 14 O 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 (C 7 F 14 ). Under oxidative conditions, it was found that significant quantities of C 2 F 6 and CF 4 were formed. Further quantum chemical analysis suggests that the O atoms facilitate the formation of volatile fluorinated compounds (VFCs) such as tetrafluoromethane (CF 4 ) and hexafluoroethane (C 2 F 6 ), 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.