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Hydrothermal technologies for destruction of per- and polyfluoroalkyl substances (PFASS) in aqueous film-forming foam (AFFF) and AFFF-impacted wastes
HAO, SHILAI
HAO, SHILAI
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2022
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2023-11-04
Abstract
Per- and polyfluoroalkyl substances (PFASs) are a family of chemicals with at least one perfluoroalkyl moiety (CnF2n+1-) used in a variety of industries and consumer products since the 1940s. The ubiquity of PFASs in the environment, wildlife, and humans has raised significant concerns and calls for action globally. One of the major sources of PFAS contamination is the use of aqueous film-forming foam (AFFF), water-based mixtures of fluorinated and hydrocarbon surfactants that are applied to rapidly extinguish hydrocarbon and solvent-based fires. Groundwater and soil at many sites in the U.S. have been reported to be impacted by the historic use of AFFF. Current treatment technologies mostly focus on separation (e.g., activated carbon, membrane separation), and there remain few options for achieving destruction and defluorination of the full range of PFASs in AFFF-impacted matrices (e.g., groundwater and soil). Thus, there remains a critical need to develop a rapid, effective, and robust treatment method for the destruction of PFASs. This thesis advanced the understanding and application of hydrothermal alkaline treatment (HALT) technologies for the destruction of PFASs in contaminated environmental and concentrate matrices, and key findings provide guidance on the integration of HALT within treatment trains that will be applied for real-world site remediation.
Initially, I evaluated the effectiveness of HALT for destruction and defluorination of PFASs identified in AFFFs produced by suppliers via electrochemical fluorination- (ECF) and fluorotelomerization (FT) processes. Quantitative and semi-quantitative high-resolution mass spectrometry was used to track a wide range of PFAS structures during treatment with HALT. Results demonstrated rapid degradation of all 109 PFASs identified in two AFFFs when the solutions were amended with alkali (e.g., 1-5 M NaOH) at near-critical temperatures and pressures (350 °C, 16.5 MPa). This includes perfluoroalkyl acids (PFAAs) and a range of polyfluoroalkyl precursors. Most PFASs were degraded to non-detectable levels within 15 min. Perfluoroalkyl sulfonates (PFSAs) were the most recalcitrant class of PFASs, but these were degraded to non-detectable levels within 30 min when treated with 5 M NaOH. 19F-NMR spectroscopic analysis and fluoride ion analysis confirmed that destruction was accompanied by near-complete defluorination of PFASs in both dilute and concentrated AFFF mixtures (total fluorine up to 0.36 M), and no stable volatile organofluorine species were detected in reactor headspace gases analyzed by gas chromatography with mass spectrometry (GC-MS) detection.
The application of the HALT was then extended to AFFF-impacted groundwater and soil matrices. Results showed that 148 PFASs identified in the collected field samples (2 groundwater samples, 3 soils), including 10 cationic, 98 anionic, and 40 zwitterionic PFASs, were mostly degraded to non-detectable levels within 90 min when treated with 5 M NaOH at 350 °C. The near-complete defluorination, as evidenced by fluoride release measurements, confirmed the complete destruction of PFASs. Rates of PFSA destruction in groundwater samples were similar to those measured in laboratory water solutions, but reactions in soil were slowed, attributed to base-neutralizing properties of the soil (e.g., reaction with silicate minerals). Further, the degradation of PFASs in groundwaters and soils was found to be a function of reaction temperature, NaOH concentration, and reaction times. The dissolution of soil minerals during HALT presents a challenge to direct soil treatment applications and suggests the need for future research to optimize PFAS destruction while minimizing soil matrix reactions.
To reduce the overall energy requirements for treatment and destruction of PFASs, I then examined application of HALT for treatment of PFAS concentrates produced by foam fractionation (FF) processes being developed for groundwater remediation. Results showed that all 62 PFASs identified in two FF-derived concentrates were degraded by >90% within 90 min when treated with 1 M NaOH at 350 °C; concentrations were reduced below the detection limit when treated with 5 M NaOH for the same reaction time. The foam concentrate matrix, including elevated dissolved organic carbon (DOC; up to 4.5 g/L) did not significantly affect reaction kinetics for the most recalcitrant PFSAs. Efforts were included to characterize and track organic constituents during treatment, with results showing partial reduction of bulk DOC, but complete degradation of 43 hydrocarbon surfactants identified in an ECF-derived AFFF concentrate. An initial analysis of energy requirements for an integrated process coupling FF with HALT was estimated to be ~0.7 kWh/m3 groundwater, with the HALT step being a negligible contributor to the overall treatment process due to the small volume of concentrate requiring treatment.
Studies were also conducted using ultra-short chain perfluoroalkyl acids (PFAAs) to better understand the mechanisms responsible for PFAA destruction and defluorination during HALT. Reactions were conducted with trifluoroacetic acetate (TFA) and trifluoromethanesulfonate (TFMS; triflate). Results confirmed the destruction and defluorination through HRMS, nuclear magnetic resonance (NMR), and fluoride ion measurement. TFMS showed much lower reactivity (~95 fold lower) than TFA, consistent with measurements of longer chain analogues. The carbon atoms of TFA and TFMS were converted to a mixture of formate and dissolved carbonate species, and the sulfonate group of TFMS was converted stoichiometrically to sulfate. Experiments also show TFMS defluorination could be mediated by the addition of alternative nucleophiles (iodide, bisulfide) in place of hydroxide. Results support a proposed stepwise nucleophilic substitution mechanism that destabilizes the alkyl chain within PFASs, leading to complete defluorination and partial carbon mineralization.
Overall, this thesis demonstrated that HALT is a rapid, effective, and robust treatment method for the destruction of the full range of PFASs identified in water-containing environmental matrices and concentrates. A proposed nucleophile substitution mechanism produces inorganic fluoride ion as the sole product with no evidence for formation of undesirable volatile fluorinated products that can be produced by other thermal treatment processes, including incineration. Therefore, results support a conclusion that HALT has significant potential for addressing remediation and industrial treatment needs at a growing number of sites.
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