Formatted Title
Mechanistic Insight into PFAS Destruction Pathways in Supercritical Water Oxidation: Effect of Temperature and Oxidant Availability
Background/Objectives
The chemical stability of PFAS has resulted in the increasing contamination of surface water, groundwater, and soil. Incineration and landfill deposition of PFAS are not sustainable options; thus, alternative end-of-life technologies are needed. Hydrothermal processes, including supercritical water oxidation (SCWO), were shown effective in destroying recalcitrant organic molecules, including PFAS [1]. In its supercritical phase (Pc > 22.1 MPa, Tc > 374°C), water becomes a non-polar fluid, with gasses and organics becoming fully miscible. The ion product of supercritical water is orders of magnitude lower than water under ambient conditions, facilitating free radical reactions. High concentration of free radicals enables rapid oxidation of organic compounds into CO2, H2O, and the heteroatoms into their corresponding acid or salt. The high density of the SCWO environments and abundance of free radicals encourage free radical reactions to occur quickly, avoiding issues with the production of intermediate reaction products in the destruction of many hazardous wastes. However, at low concentrations of oxidizing species and at low temperatures, PFAS does not fully mineralize, yielding the formation of harmful intermediate species.
Approach/Activities
We report PFAS treatment in the continuous flow lab-scale SCWO reactor as a function of operating temperature and oxidant availability. The autogenic SCWO operation and temperature control were achieved by injection of co-fuel. The authors characterized the destruction and mineralization of multiple PFSA and PFCA in the temperature range (T = 410 – 650°C) and oxidant ratios ~0.5-2. We collected liquid effluent and gaseous samples that were analyzed for the presence of intermediates. These data were used to gain insights into chemical kinetic routes of PFAS destruction in the SCWO environment.
Results/Lessons Learned
At T = 650°C, DRE > 99.999% and complete defluorination were achieved after 30 seconds of exposure. The PFSAs are significantly more resistant to oxidization than PFCAs; thus, the presence of PFSAs in the mixture would limit the destruction and defluorination efficiencies. All PFCAs introduced into the SCWO reactor were below the detection limit at T = 510°C, while PFSAs required T > 610°C to oxidize [2]. PFSAs destruction proceeds via two parallel routes, e.g., the lower temperature PFOS experiments (T = 420 – 600 °C) yielded a pool of PFCAs (PFOA, PFHxA, PFPeA, PFBA, PFPrA, TFA) in the liquid effluents as well as the emissions of VOF such as 1H-perfluoroalkane in the gas stream [3]. These routes are discussed in the context of the available free radicals participating in the PFAS degradation.