PFAS destruction through catalyzed hydrothermal liquefaction using modified hydrochar

Per- and polyfluoroalkyl substances (PFAS) are persistent organic pollutants found in wastewater, of which a substantial portion is ultimately removed by sorption onto sewage sludge. This currently acts as a pathway of environmental release if not properly managed. To provide a sustainable and circular solution to PFAS remediation in sewage sludge, this study investigated the degradation of aqueous perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) by catalytic hydrothermal liquefaction (HTL) at 350 °C using hydrochar as catalyst. The effects of pH (acid and alkali), hydrochar dosage (0–10 wt%), and hydrochar modifications on the defluorination of PFAS were systematically investigated. Without hydrochar, HTL was found effective for the complete removal of PFAS parent compounds but showed no defluorination. Catalytic HTL of PFAS with hydrochar improved defluorination up to 25 % after 2 h. The alkali modified hydrochar did not further improve the defluorination, likely due to the collapse of the hydrochar structure caused by the thermal decomposition of amorphous carbon and subsequent structural rearrangement at high temperatures, as indicated by morphological analysis. However, alkaline conditions (2 M KOH) substantially enhanced the catalytic activity of hydrochar resulting in complete PFOA and 95 % PFOS defluorination. Post-HTL analysis linked this enhanced defluorination to structural changes and increased surface area in hydrochar, induced by KOH during HTL of PFAS. This study demonstrated that hydrochar under alkaline conditions can effectively enhance PFAS degradation, particularly for the more recalcitrant PFOS, showcasing its potential as a sustainable catalyst for environmental remediation.