Aqueous Film-Forming Foam Treatment under Alkaline Hydrothermal Conditions

Abstract

Historical use of aqueous film-forming foam (AFFF) has led to widespread contamination of water and soil, due to the high per- and polyfluoroalkyl substance (PFAS) content in AFFF. Due to the high toxicity and environmental persistence of PFAS, legacy AFFF is being phased out, and end-of-life disposal options are needed. While incineration has historically been the preferred disposal option for AFFF, the practice is being phased out due to mounting concerns around the emission of harmful byproducts. Hydrothermal alkaline treatment (HALT) is an effective PFAS destruction process, which has been previously shown to generate no toxic PFAS degradation byproducts. In this case study, HALT is evaluated as an end-of-life destruction option for AFFF stockpiles. A legacy AFFF sample containing ∼3.2 wt% total PFAS is treated at 350°C, >20.7 MPa, 5 M-NaOH loading, and residence times of 2 to 6 h in a batch reactor setup. In two cases, a proprietary catalytic surface is introduced, which is demonstrated to increase the rate and extent of PFAS destruction. Forty PFAS analytes are measured before and after HALT treatment to quantify destruction efficacy and to demonstrate that all PFAS are degraded during processing. Ion chromatography is used to quantify fluoride yield at one condition, where 120.5% theoretical defluorination efficiency is measured. This indicates that HALT is effective at degrading detected and undetected PFAS, through cleavage of the strong carbon-fluorine bonds. Overall, HALT appears to be a technically viable option for safe disposal of legacy AFFF stockpiles.