Per‑ and polyfluoroalkyl substances (PFAS) are persistent environmental chemicals that pose significant remediation challenges in soil matrices. Existing treatment options for PFAS‑impacted solids are limited and often rely on chemical or off‑site disposal. This proof‑of‑concept study evaluated the feasibility of supercritical water oxidation (SCWO) as a destructive treatment technology for PFAS‑impacted soil slurries. The objectives were to assess PFAS destruction efficiency across representative soil matrices, evaluate fluorine mass balance, examine the impact of process modifications, including calcium hydroxide [Ca(OH)₂] addition, and demonstrate SCWO in treating aged aqueous film‑forming foam (AFFF)‑impacted field soils.

Bench‑scale SCWO testing was conducted using a high‑pressure, custom-built batch reactor operated at 580°C and 3,500 pounds per square inch, using air as the oxidant. Testing was performed in three phases. First, PFAS destruction was evaluated using PFAS‑spiked clay, sand, and loamy soils at two residence times to assess matrix effects. Second, Ca(OH)₂ was added to selected tests to evaluate the influence of alkaline conditions on PFAS destruction, vapor‑phase byproduct formation, and fluorine recovery. Third, SCWO treatment was demonstrated on aged AFFF‑impacted field soils and soil‑wash residual fines. Untreated soils, treated solids, aqueous effluents, and vapor‑phase effluents were analyzed to quantify PFAS and total organic fluorine destruction efficiency, and evaluate fluorine mass balance.

SCWO consistently achieved greater than 99% PFAS destruction across all soil types and PFAS classes, including short‑ and long‑chain perfluoroalkyl carboxylates and sulfonates. Sandy soils exhibited near‑complete destruction at shorter residence times, while clay and loamy soils required longer residence times to overcome mass‑transfer limitations and stronger PFAS sorption. Treatment of aged AFFF‑impacted soils and soil‑wash residuals resulted in greater than 95% removal of all detected PFAS, with many compounds approaching complete destruction despite increased matrix recalcitrance.

Ca(OH)_₂ addition provided only modest improvements in PFAS removal and reduced vapor‑phase byproducts but resulted in lower apparent fluoride recovery due to fluorine retention in calcium‑rich matrices. Fluorine mass balance analysis confirmed that volatilization of organofluorine species was negligible, indicating that incomplete fluoride recovery reflects matrix sequestration rather than incomplete PFAS destruction. A moderate inverse relationship was observed between calcium concentration and percent fluorine mass balance.

This study demonstrated that SCWO is a viable and effective treatment technology for PFAS‑impacted solid matrices. Key benefits include near‑complete PFAS destruction, applicability across diverse PFAS classes and soil types, minimal air emissions, and the ability to simultaneously destroy organic co‑occurring chemicals. The results support further optimization of operating conditions and pilot‑scale evaluation to advance SCWO toward field deployment for treatment of PFAS‑impacted soils and investigation‑derived waste. (Project Completion - 2026)