Formatted Title
Application of Heterogeneous Catalysts for Promoting Hydrothermal Destruction of Per- and Polyfluoroalkyl Substances
Background/Objectives
Recent work has shown that a wide range of liquid and solid matrices contaminated by per- and polyfluoroalkyl substances (PFAS) can be effectively destroyed using hydrothermal alkaline treatment (HALT) including concentrate byproduct streams derived from other separation treatment technologies (e.g., foam fractionation, activated carbon adsorption, ion exchange). Careful mass balance experiments demonstrate that the full range of PFAS, including perfluorocarboxylic acids (PFCAs) and perfluorosulfonic acids (PFSAs), can be degraded and defluorinated in subcritical water (e.g., 350 °C, 16.5 MPa) amended with strong alkali (e.g., 1 M NaOH). While the HALT process is a promising remedial strategy that is currently being commercialized, the process is reliant upon large quantities of alkali inputs and produces alkali residuals that require further management. This presentation will describe recent research aimed at identifying materials capable of catalyzing hydrothermal destruction of PFAS using much lower inputs of alkali consumables.
Approach/Activities
Batch hydrothermal reactions are conducted in sealed high-pressure reactors (6 - 25 mL). Proof-of-concept experiments were conducted in water spiked with perfluorooctane sulfonate (PFOS), representative of the more highly recalcitrant PFSAs. A series of catalyst materials were initially screened to identify materials capable of promoting PFOS degradation under subcritical hydrothermal conditions (350°C, 16.5 MPa). The most active catalyst materials were then subjected to a more in-depth study, tracking PFOS concentration, organic reaction intermediates, and inorganic fluoride. PFAS and organic intermediates were tracked using liquid chromatography-high resolution mass spectrometry (LC-HRMS) methods. Fluoride ion concentrations were measured by ion selective electrode analysis. Tests were also conducted for treatment of a concentrate byproduct stream derived from foam fractionation of PFAS-contaminated groundwater.
Results/Lessons Learned
Results of kinetics and fluorine mass balance experiments demonstrate that multiple catalyst materials, including supported ruthenium and zirconium oxide materials, promote enhanced transformation of PFOS under hydrothermal conditions. Incomplete defluorination is observed in the absence of alkali amendments, and HRMS analysis of solutions indicates formation of shorter chain partially defluorinated sulfonic acid structures. Solution phase analysis is complemented by particle induced gamma ray emission (PIGE) analysis of catalyst surfaces to identify fluorinated bound residues formed under selected conditions, and elucidate mechanisms responsible for catalytic transformation. The extent of defluorination can be enhanced by amendment with alkali amendments at concentrations much lower compared to standard HALT (only alkali amended). Future research directions and steps for deployment of catalytic hydrothermal treatment of PFAS will be discussed.