Enhancement of per- and polyfluoroalkyl substances removal from water by pyrogenic carbons: Tailoring carbon surface chemistry and pore properties

Abstract

Several countries, including the United States, plan to set an enforceable maximum contaminant level for certain per- and polyfluoroalkyl substances (PFAS) in drinking water. Among the available treatment options, sorption by pyrogenic carbonaceous sorbents (PCS) is a practical and effective approach to remove PFAS from water in pilot- and full-scale applications. The two most important properties of PCS, surface chemistry and pore structure, were tailored in this study to understand their importance in the sorption of various anionic shorter-chain and longer-chain PFAS. Brief thermal oxidation (post-pyrolysis air oxidation, PPAO) of PCS, including biochars, at a moderate temperature (400 °C) was used to increase specific surface area and nanoporosity. The sorption distribution ratio, KD, of individual PFAS after PPAO treatment increased by as much as three orders of magnitude compared to the unmodified PCS—more effectively so for longer-chain than shorter-chain compounds. Pore reaming plays a major role in the sorption enhancement and an ion-pair (PFAS-counterions) sorption mechanism is proposed. In addition, irreversible sorption of a quaternary ammonium cationic polymer, poly(dimethyldiallylammonium) chloride (pDADMAC), was employed to reverse the surface charge of biochar. Coating with pDADMAC increased PFAS sorption by a factor of 10–3000 predominantly by an anion-exchange mechanism. Sorption enhancement was more effective for the sulfonate than the carboxylate with the same perfluoro chain length. The results of this study are expected to inform the design of carbons with greater ability to remove PFAS from water, which are urgently needed for water facilities to comply with state and federal regulations.