Thermal Mineralization of Perfluorooctanesulfonic Acid (PFOS) to HF, CO<sub>2</sub>, and SO<sub>2</sub>
Nathan H. Weber, Cameron S. Delva, Sebastian P. Stockenhuber, Charles Grimison, John Lucas, John C. Mackie, Michael Stockenhuber, Eric M. Kennedy
Abstract
Utilizing air (O 2 ) 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), COF 2 and trace amounts of C 2 F 4 were detected. Quantum chemical calculations at the G4MP2 level of theory confirmed that CF 2 radicals can react with O 2 to form COF 2 and an O ( 3 P) atom. The inclusion of 2000 or 20 000 ppmv of water vapor (H 2 O (g) ) to the air bath gas proved to be the key step to mineralizing all PFOS into hydrogen fluoride (HF), CO 2, and SO 2 . At temperatures above 850 °C (0.95–0.84 s residence time), a feed of 20 000 ppmv of H 2 O (g) in air was observed to produce a product stream in which no gaseous fluorocarbon products were detected, with only HF, SO 2, and CO 2 being produced. A sulfur balance confirmed that 100 ± 5% of all the S in PFOS had converted into SO 2 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 CO 2 . A chemical kinetic model was developed to understand the importance of both O 2 and water vapor in the overall thermal decomposition of PFOS, leading to complete mineralization. In the presence of both O 2 and H 2 O (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( 3 P) + H 2 O → OH + OH.