Formation of Perfluorocarboxylic Acids (PFCAs) from the Incomplete Thermal Decomposition of Perfluorooctanesulfonic Acid (PFOS) and Perfluorooctanoic Acid (PFOA)

Two solutions containing ~500 mg/L of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) were atomized at two post-flame locations within a natural gas-fired pilot-scale combustor with peak temperatures of ~860 °C and ~750 °C. The thermal destruction of PFOS and PFOA and the formation of fluorocarbon products of incomplete combustion (PICs) were analyzed using EPA Other Test Method (OTM)-45, a prepublication version of EPA OTM-50, an activated carbon-based thermal desorption (TD) tube gas chromatography/mass spectrometry (GC/MS) method, and two real-time methods including iodine-adduct chemical ionization mass spectrometry (CIMS) and Fourier transform infrared spectrometry (FTIR). Destruction efficiencies (DEs) determined for PFOS by OTM-45 and for PFOA by both OTM-45 and CIMS, were >99.99% for both peak injection temperatures. OTM-45 and CIMS confirmed the presence of several perfluorocarboxylic acids (PFCAs) as PICs. These PFCAs included trifluoroacetic acid (TFA) and perfluoropropionic acid (PFPrA) for both solutions and included the formation of PFOA from PFOS and similarly perfluoroheptanoic acid (PFHpA) from PFOA. PFCA concentrations and the relative amounts of PFOA and PFHpA increased at the lower temperature. OTM-50 and the TD tube analysis identified numerous fluorocarbon PICs including 1H-pefluoroalkanes, and perfluoroalkanes such as trifluoromethane (CHF₃) and hexafluoroethane (C₂F₆).