Loading...
Hydrothermal alkaline treatment of perfluoroalkyl acids: influence of alkali identity
Donaldson, Seth
Donaldson, Seth
Citations
Altmetric:
Advisor
Editor
Date
Date Issued
2024
Date Submitted
Keywords
Collections
Research Projects
Organizational Units
Journal Issue
Embargo Expires
2026-04-04
Abstract
Hydrothermal Alkaline Treatment (HALT) has proven to be successful in the degradation and defluorination of per- and polyfluoroalkyl substances (PFAS), using NaOH as the alkali additive. This thesis project will expand on previous work by assessing the effectiveness of alternative alkali amendments for treatment of perfluoroalkyl sulfonic acids (PFSAs) and perfluoroalkyl carboxylic acids (PFCAs). Alkalis tested include Group I hydroxides (LiOH, NaOH, KOH, RbOH, and CsOH), Group II hydroxides (Ca(OH)2 and Sr(OH)2), and weak bases (Na2CO3 and NH4OH). HALT reactions amended with KOH, CsOH, and RbOH resulted in similar degradation and defluorination of trifluoromethanesulfonic acid (TFMS), a model PFSA, as reactions amended with NaOH (1 N alkali, 350°C, 60 min reaction). Reactions amended with LiOH, group 2 hydroxides, and weak bases showed diminished TFMS degradation and defluorination for the same reaction conditions. The observed differences in TFMS reactivity are attributed to differences in concentration of free OH- available for reaction at hydrothermal reaction conditions. Experiments with perfluorooctanoic acid (PFOA), a representative PFCA, showed rapid degradation under all treatment conditions, including alkali-free controls, but defluorination required the addition of an alkali. Unlike TFMS, complete defluorination of PFOA was also observed at 350°C using either 1 M Na2CO3 or a reduced concentration of NaOH (0.01 N). Further, tests conducted at 200°C showed similar degradation profiles for reactions with no alkali, 0.01 N NaOH, and 1 M Na2CO3 conditions, but a slightly faster rate of degradation was observed when 1 N NaOH was added, suggesting both alkali-independent and alkali-dependent reaction pathways for PFCAs. However, measurable PFOA defluorination was only observed for reactions amended with 1 N NaOH and 1 M Na2CO3 at the lower reaction temperature. Lastly, HALT reactions conducted with a foam fractionation-derived concentrate showed similar reactivity patterns observed for PFOA and TFMS, where PFCAs degraded in the absence or presence of strong alkali amendments, whereas PFSAs degradation were dependent on the amount of free OH- the alkali contributed to solution. These findings inform the design of hydrothermal treatment processes where alkali amendments may be altered depending on the PFAS makeup of the target waste stream and available options at the time of treatment operations.
Associated Publications
Rights
Copyright of the original work is retained by the author.