There is intense interest in developing better approaches for managing wastes laden with per- and polyfluoroalkyl substances (PFAS). High temperature thermal treatment (e.g., above 1000°C) of these materials is frequently performed, but lower temperature treatment (e.g., below 600°C) may be appropriate to remove PFAS from materials as well. The focus of this proof-of-concept project was to help develop a better understanding of PFAS fate associated with lower temperature thermal treatment approaches. Specifically, the objectives of this project were to evaluate effects of low temperature thermal treatment on PFAS in simulated investigation derived wastes (IDW) and to assess the potential benefit of Ca(OH)2 amendments for lowering PFAS decomposition temperatures and release of volatile organic fluorine (VOF) species.
Simulated solid IDW materials were prepared with high concentrations of perfluorosulfonic acids (PFSAs) and perfluorocarboxylic acids (PFCAs) with Ca(OH)2 added to a subset of these materials. The high concentrations of PFAS used in these experiments facilitated examination of a variety of thermal decomposition products, including fluoride mineralized from the PFAS, sulfur oxyanions from desulfonation of PFSAs, and VOF species that evolved. Thermal decomposition was performed in a tube furnace at temperatures up to 575°C and products remaining in solids, trapped in aqueous solutions, and collected in gas sampling bags were examined through a variety of techniques. In addition to evaluation of PFAS, modified approaches were used to examine fluoride associated with solid species, and selected-ion monitoring gas chromatography-mass spectrometry was used to examine expected fluorocarbon ion fragments of collected VOF.
Removal of PFSAs and perfluorooctanoic acid (PFOA) from the solids was essentially complete (>99.9%) when final temperatures reached 575 °C and 450 °C, respectively, with representative decomposition temperatures for PFSAs occurring near 360°C, and that for PFOA occurring below 300°C. With the amendment of Ca(OH)2 to solids, decomposition of PFSAs appeared to occur below 300°C, while PFOA was less affected. Without Ca(OH)2, no more than 30% of initial fluorine in the PFAS was observed as fluoride, consistent with long perfluoroalkyl chains associated with 1H-perfluoroalkane and perfluoroalkene VOF observed. Fluorine mineralization was particularly low for PFOA. Ca(OH)2 amendments increased fluorine mineralization in all cases, but this remained below 50% of initial fluorine content under all conditions evaluated. While up to 5% of PFSAs added to solids were observed as PFCAs in aqueous traps without Ca(OH)2 amendments, these PFCAs were not observed when Ca(OH)2 was present. The inclusion of Ca(OH)2 appeared to cause a shift in the composition of VOF species, possibly suppressing the evolution of perfluoroalkene species.
This project demonstrated that low temperature treatments can remove perfluoroalkyl acids (PFAAs) from simulated IDW materials, and that VOF evolve from this low temperature process. Further, this work demonstrated the concept that hydrated lime (Ca(OH)2) amendments can lower PFAS decomposition temperatures, facilitate greater PFAS mineralization, and change the composition of VOF. The use of amendments such as Ca(OH)2 that offer promise for treating PFAS at low temperatures to products with lower toxicity should be investigated further. Better understanding of these processes will aid future implementation of lower temperature thermal treatments for PFAS.