Influence of Charge Compensating Anions on the Adsorption of Perfluorobutanesulfonate in MOF-808

Per- and poly fluoroalkyl substances (aka PFAS) are a class of anthropogenic compounds that have come under scrutiny given their ability to bioaccumulate in the environment and negatively impact health outcomes. Metal-organic frameworks (MOFs) are an emerging category of sorbents that are attractive for PFAS remediation given their readily modifiable nature. Here we show that the charge compensating anions (cca's) formate (FA), acetate (AA), trifluoroacetate (TFA), and chloride have a significant impact on the adsorption of perfluorobutanesulfonate (PFBS) within MOF-808. Kinetic measurements indicate that MOF-808 rapidly adsorbs PFBS with equilibrium reached within 50 min or less. The kinetic data is well-fit to the pseudo first-order Langmuir model and the resultant pseudo first-order rate constants vary by a factor of 4 (0.16-0.61 min^-1) based on the cca's identity. The adsorption capacity of PFBS also varies by a factor of 4 (95-372 mg/g) when challenged with 500 mg/L solutions and that PFBS adsorption correlates with the quantity of cca's (monocarboxylate plus chloride) removed during PFBS adsorption. PFBS adsorption isotherms indicate that MOF-808 exhibits excellent maximum adsorption capacities up to 837 mg PFBS/g MOF but binds PFBS relatively weakly (K _L values no larger than 7.72 × 10^-3 L/mg). Kinetic, IR spectroscopic, and cca-dependent adsorption data are consistent with PFBS adsorption occurring via ion-exchange of cca's. Our data demonstrate the importance of controlling and understanding the composition of cca's when studying PFAS adsorption within MOFs.