Application of Hydrothermal Alkaline Treatment to Spent Granular Activated Carbon: Destruction of Adsorbed PFASs and Adsorbent Regeneration

Granular activated carbon (GAC) adsorption is the most common technology applied to treat water contaminated with per- and polyfluoroalkyl substances (PFASs), but rapid exhaustion of the media necessitates frequent replacement and costly off-site thermal regeneration. Here, we extend the application of hydrothermal alkaline treatment (HALT), which uses strong alkali and near-critical temperatures and pressures (e.g., 350 °C, 16.5 MPa, and 1 M NaOH) to degrade and mineralize PFASs, to the regeneration of spent GAC. Mass balance experiments wherein a known mass of perfluorooctanesulfonate (PFOS) was adsorbed onto GAC prior to treatment showed that HALT achieved >99% destruction of PFOS and 96 ± 4% defluorination with no observed fluoro-organic intermediates [167 g L–1 GAC, 350 °C, 1 M NaOH, trxn = 400 min, and 10–15 mg of PFOS (g of GAC)−1]. Treatment of GAC collected from a field pilot study also showed effective destruction of the range of adsorbed PFASs. Moreover, repeated HALT cycles did not significantly affect the GAC specific surface area, and similar adsorption isotherms for perfluoropentanoic acid and PFOS were recorded for virgin and HALT-treated GAC. These findings suggest a promising strategy for on-site regeneration of PFAS-contaminated GAC and other adsorbent media that may be an alternative to off-site thermal regeneration practices.