Pre- and Postapplication Thermal Treatment Strategies for Sorption Enhancement and Reactivation of Biochars for Removal of Per- and Polyfluoroalkyl Substances from Water

Exposure to per- and polyfluoroalkyl substances (PFAS) in drinking water poses a major public health threat. Commercial granular activated carbon (GAC) has been used for the sorptive removal of PFAS in practical applications. Biochar is a possible cheaper alternative to GAC for small-scale water treatment systems. Here, we report a strategy for employing biochar for PFAS removal that combines post-pyrolysis modification, which greatly improves performance, with a reactivation step that enables its reuse. Modification entails brief postpyrolysis air oxidation at 400 °C, which considerably enlarges pore size and specific surface area and thereby increases the solid-to-water distribution ratio, KD, of individual PFAS by as much as 3 orders of magnitude. In some cases (e.g., perfluorooctanoic acid) the KD was comparable to that of commercial GAC. The sorbed PFAS could be decomposed by brief thermal reactivation of the spent biochar at 500 °C in N2 or air. After thermal reactivation in air, the biochars exhibited even greater PFAS KD values in a second cycle. While thermal reactivation of a GAC in air could be achieved, as well, sorption affinity for the shorter-chain PFAS was noticeably reduced. Overall, this study points to a new strategy of using biochars for PFAS removal.