A FAIR comparison of activated carbon, biochar, cyclodextrins, polymers, resins, and metal organic frameworks for the adsorption of per- and polyfluorinated substances

Summary

Per- and polyfluorinated substances (PFAS) have complex sorption behaviors, complicating removal from water and selection of suitable adsorbents. We evaluated adsorption of 44 PFAS across four adsorbent groups: activated carbon and biochar (AC and BC), cyclodextrin-based adsorbents (cyclodextrins), polymer-based adsorbents and resins, and inorganic adsorbents and metal organic frameworks (MOFs). We analyzed over 500 adsorption coefficients (K_d) from literature, calculated at aqueous equilibrium concentration of 1 ± 0.3 µg/L under comparable experimental conditions. On average, K_d of AC and BC exceeded 10⁷ L/kg for PFAS with C-F bonds > 7, unlike other adsorbents with K_d < 10⁷. This trend holds for C-F bonds ≥ 4. Cyclodextrins, polymer-based adsorbents and resins outperform AC and BC for C-F bonds < 4. For AC and BC, K_d follows the order PFOS>PFOA>PFBS>PFBA, with adsorption increasing with increasing point of zero charge of the adsorbent. For AC and BC, as well as cyclodextrins, K_d values were related to PFAS hydrophobicity and steric properties. Additionally, adsorption was influenced by head group type, non-fluorinated carbon atoms, and the presence of oxygen and/or chlorine in the PFAS. No clear relationship was found for the other adsorbents. Adsorption prediction using a Random Forest Regressor and literature data was feasible for AC and BC, but not for other adsorbents. Cyclodextrins outperform AC and BC for removing PFAS across varying mobilities from water, whereas AC and BC are superior for low mobility PFAS. To support further data use all data and code used are freely available, following FAIR data principles.