Thermal Decomposition of Perfluorooctanesulfonic Acid (PFOS) in the Presence of Water Vapor

Abstract

The thermal decomposition of perfluorooctanesulfonic acid (PFOS) was studied in an α-alumina reactor between 500 and 1000 °C using helium as a carrier gas. In addition to He carrier gas and between 150 and 300 ppm of PFOS, the effect of water vapor (H2O(g)) at two different concentrations (1700 and 10,000 ppm) on the reaction was examined. The main products, at both 1700 and 10,000 ppm of H2O(g), were HF, C2F4, CO, and SO2. Moreover, as the reaction temperature increased, an increase in the concentration of HF and CO was observed, concomitant with a decrease in the fluorocarbon concentration. Additionally, the α-alumina reactor was observed to have reduced surface activity in the presence of water vapor. Quantum chemical calculations, performed at the G4MP2 level of theory combined with a chemical kinetic model, were used to understand the influence that water vapor and OH radicals have on the CF2 and CF3 radicals that are present. It was found that at temperatures above 850 °C in 10,000 ppm of water vapor, there is a substantial decrease in the fluorocarbon concentration.

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

“…As mentioned in our previous publication, most FCO radicals will form via reaction normalC normalF 2 + normalO normalH .25em ↔ .25em normalH normalF + normalF normalC normalO …”

“…The thermal decomposition of PFOS using air as the carrier gas at atmospheric pressure was significantly different from the product distribution observed in our previous pyrolysis 19,20 and H 2 O (g) studies. 26 In the presence of air or under conditions where there is excess oxygen, the main products A reduction in C 2 F 4 concentration was evident from the Fourier-transform infrared spectroscopy (FT-IR) spectra in Figure 1. At 700 °C, COF 2 was the primary reaction product detected.…”

“…We suggest that the slight decrease in COF 2 concentration at 750 °C is caused by a small amount of H 2 O (g) (200−400 ppmv) present as an impurity in the gas supply. Our previous work 26 disclosed that even a low concentration of H 2 O (g) can reduce the concentration of COF 2 produced at elevated temperatures. CO 2 and CO were also observed at high concentrations above 750 °C, which is further evidence that H 2 O (g) is reducing the concentration of COF 2 detected at elevated temperatures.…”

“…Based on our previous work, water vapor alone, especially in a feed stream containing excess water vapor, significantly reduced the rate of COF 2 formation. 26 Experiments indicate that COF 2 is the main product in an air atmosphere. Consequently, if we combined both air and H 2 O (g) in the feed, it is plausible that at elevated temperatures, only HF, CO 2 , and SO 2 will be observable products.…”

“…As described in our previous publication, where we examined the effect of water vapor on PFOS decomposition, there were only limited chain branching reactions to form OH. This led to reactions such as reaction , ensuring that the OH concentration was relatively low. C O + O H → C O 2 + H …”