Mechanistic study of PFAS adsorption using a QPPTA linked viologen-modified covalent organic framework

Abstract

A novel viologen-based covalent organic framework (VZ-COF) featuring hydrogen-bond acceptor sites, positively charged functional groups, and hydrophobic segments was synthesized to adsorb per- and polyfluoroalkyl substances (PFAS) using a new aromatic tetraamine linker (QPPTA) and Zincke salt. Including building blocks that offer multiple types of interactions cumulatively increases the capacity of COFs to adsorb PFAS. QPPTA provides two different kinds of intermolecular interactions: hydrophobic interactions through aromatic regions and hydrogen bonding via nitrogen atoms. The Zincke salt provides electrostatic interactions through positively charged functional groups. This combination of functionalities in VZ-COF enabled competitive PFAS adsorption efficiency. VZ-COF was tested against a series of PFAS and was found to adsorb all at >90%, and adsorption of perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) was >99%. The adsorption capacity of VZ-COF was found to be 1000 mg g⁻¹ and was unaffected by salts, pH variations, and complex organic acids, such as humic and tannic acids. VZ-COF also effectively removed PFOS and PFOA, reducing 1 μg L⁻¹ concentrations to less than 1.6 ng L⁻¹ and 4.1 ng L⁻¹ respectively, even at extremely low concentrations (less than 3.8 ng L⁻¹) in tap water. A short column of VZ-COF was used for flow adsorption of tap water to preconcentrate PFAS from tap water to detect and quantify very low concentrations. VZ-COF can be regenerated and reused with little change in adsorption efficiency.