AFFF on fire: new insights into the thermal fate of the AFFF compound Capstone B

The widespread use and uncontrolled release of per- and polyfluoroalkyl substances (PFAS) have led to numerous environmental contamination hotspots in the past. Frequently, these contaminations result from fire-fighting measures employing PFAS-containing aqueous film-forming foams (AFFF). Remediation of these sites is often expensive and generates large amounts of waste. Thermal remediation methods have been discussed as an economically viable option for PFAS-contaminated materials and soils. During thermal treatment, depending on the applied temperature and original PFAS, the formation of products of incomplete destruction (PIDs) occurs. These PIDs are problematic as they have not been characterized well in the past for AFFF PFAS and can accumulate during thermal treatment. We studied the thermal fate of AFFF PFAS 6:2 FTSAm-Pr-B, also known as Capstone B, under pyrolytic conditions over a temperature range from 100 to 800 °C. PIDs were characterized by liquid and gas chromatography coupled to (high-resolution) mass spectrometry (LC-(HR)MS and GC-HRMS). The degree of mineralization was determined by fluoride production and ranged between 5.3% at 200 °C and 61% at 600 °C. In total, we identified 55 PIDs over a temperature range from 100 to 400 °C (levels 1-4). At 600 and 800 °C, only fluoride was detected. The majority of the PIDs (47) was detected by nontarget screening (NTS) and PIDs were classified in five substance groups: fluorotelomer sulfonic acids (FTSAs), fluorotelomer sulfonamides (FTSAms), hydroxy-carboxylic acids (OH-CAs), unknown polyfluoro-hydroxy alkyls (PFOHs), and others. A mass balance (MB) was established by molar fluorine equivalents of the (semi)quantified PIDs. The MB showed that transformation processes of 6:2 FTSAm-Pr-B already started at temperatures above 150 °C. Between 200 and 400 °C, PIDs play a significant role and accounted for 8.5% of the fluorine MB at maximum.