There is an increasing interest in developing more efficient methods for managing waste containing per- and polyfluoroalkyl substances (PFAS). While high-temperature thermal treatments (e.g., above 1000 °C) are often used for processing these materials, they require significant energy and resources. Consequently, there is a keen interest in examining lower-temperature treatments (e.g., below 600 °C). Many PFAS break down at temperatures below 700 °C; however, the by-products generated during decomposition, such as HF and volatile organic fluorine (VOF), may pose environmental and health risks. Addressing these challenges is crucial for facilitating the implementation of low-temperature treatment approaches. Gaining a deeper understanding of PFAS fate and behavior during lower-temperature thermal treatments can offer valuable insights into potential alternative waste management strategies.

The work is being conducted in two phases. The primary objectives of Phase I were to assess the impact of low-temperature thermal treatment on PFAS in simulated investigation-derived wastes (IDW) and to evaluate the potential benefits of using Ca(OH)₂ amendments. The specific technical objectives associated with this phase included:

1. Preparation of simulated IDW solids
2. Analysis of the baseline decomposition of PFOS, PFHxS, and PFOA
3. Examination of the decomposition of simulated solid IDW with Ca(OH)2

Results from Phase I can be found in the Phase I Final Report and are summarized in the Phase I results section below.

In Phase II of this project, the specific technical objectives are as follows:

1. Investigate the effect of sweep gas composition on PFAA and VOF composition
2. Develop and improve the low-temperature approaches for treating and mineralizing VOF
3. Assess the performance of the two-stage PFAS treatment

During Phase I of this project, the work involved preparing simulated solid IDW materials and conducting a series of bench-scale thermal decomposition studies on individual PFAS and simulated solid IDW. The 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 PFAS concentrations in these experiments enabled the examination of various thermal decomposition products, including fluoride minerals derived from PFAS, sulfur oxyanions from PFSAs desulfonation, and evolved VOF species. Thermal decomposition was carried out in a tube furnace at temperatures up to 575 °C, and the products remaining in solids, trapped in aqueous solutions, and collected in gas sampling bags were analyzed using a range of techniques. In addition to evaluating PFAS, modified approaches were employed to examine fluoride associated with solid species, while selected-ion monitoring gas chromatography-mass spectrometry was used to investigate the expected fluorocarbon ion fragments of collected VOF.

In Phase II of this project, we will explore the influence of sweep gas composition on the thermal decomposition of PFAAs and the evolution of VOF, particularly focusing on the presence and absence of water. We will also refine and enhance low-temperature approaches for treating and mineralizing VOF produced during the process. Lastly, utilizing the knowledge gained from the previous two tasks, we will test a two-stage treatment system for managing PFAS mixtures at low temperatures (below 700°C).

Phase I Results

During Phase I, 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 has demonstrated that low-temperature treatments can effectively remove perfluoroalkyl acids (PFAAs) from simulated investigation-derived waste (IDW) materials, and that volatile organic fluorides (VOF) evolve from this low-temperature process. Additionally, this work has shown that the use of hydrated lime (Ca(OH)₂) amendments can reduce PFAS decomposition temperatures, facilitate enhanced PFAS mineralization, and alter the composition of VOF. Further investigation into amendments such as Ca(OH)₂, which show promise for treating PFAS at low temperatures and yielding less toxic by-products, is warranted. A better understanding of these processes will support the future implementation of lower-temperature thermal treatments for PFAS management. (Anticipated Phase II Completion - 2025)