Thermal Mineralization of Perfluorooctanesulfonic Acid (PFOS) to HF, CO2, and SO2

Abstract

Utilizing air (O2) as the bath gas at reaction temperatures between 500 and 1000 °C, the thermal decomposition of perfluorooctanesulfonic acid (PFOS) in an α-alumina reactor was studied. It was found that in an air bath gas (and in the absence of water vapor), COF2 and trace amounts of C2F4 were detected. Quantum chemical calculations at the G4MP2 level of theory confirmed that CF2 radicals can react with O2 to form COF2 and an O (3P) atom. The inclusion of 2000 or 20 000 ppmv of water vapor (H2O(g)) to the air bath gas proved to be the key step to mineralizing all PFOS into hydrogen fluoride (HF), CO2, and SO2. At temperatures above 850 °C (0.95–0.84 s residence time), a feed of 20 000 ppmv of H2O(g) in air was observed to produce a product stream in which no gaseous fluorocarbon products were detected, with only HF, SO2, and CO2 being produced. A sulfur balance confirmed that 100 ± 5% of all the S in PFOS had converted into SO2 with a chemical kinetic model predicting in excess of 99.99999% destruction removal efficiency of PFOS at temperatures above 700 °C. Furthermore, from an elementary balance of F and C atoms, it was determined that at 1000 °C, approximately 99 ± 5% of F atoms present in PFOS have been converted into HF, and approximately 100 ± 5% of C atoms had been converted into CO2. A chemical kinetic model was developed to understand the importance of both O2 and water vapor in the overall thermal decomposition of PFOS, leading to complete mineralization. In the presence of both O2 and H2O(g), it was found that relatively high concentrations of OH radicals were produced, with significant contribution to OH formation attributed to the well-known chain branching reaction O(3P) + H2O → OH + OH.