Independent mode sorption of perfluoroalkyl acids by single and multiple adsorbents

Infinite dilution partition coefficients, K_p,0, of a series of unbranched perfluoralkylacids, PFAAs with 3 to 8 CF₂ units between water and commercially available weak anion exchange (WAX) and strong anion exchange (MAX) polymers, C₁₈-modified silica, hydrophilic-lipophilic balance polymer (HLB), and Al₂O₃ sorbents were determined with self-packed columns using an HPLC-MS/MS setup. The anionic WAX sorbent shows a much higher adsorption affinity (about 450 fold) for PFBA than was observed for the applied hydrophobic sorbent HLB. Since the incremental value for each CF₂ group is smaller when the electrostatic adsorption process is observed, the hydrophobic partition coefficient of HLB supersedes the electrostatic one of WAX at around PFTeDA. Adsorption of PFAAs to Al₂O₃ was weak and did not show a clear chain length dependency. A recently developed independent mode (IM) adsorption model is a more accurate model to combine the electrostatic and hydrophobic interaction terms. This model predicts the correct behaviour of especially short chain PFAAs in soil or sediment sorption experiments. Factors increasing sorption efficiency of well- and ill-defined single and multiple adsorbents towards PFAAs are discussed. The IM model provides a method to optimise sorption remediation strategies of PFAAs in contaminated waters and proposes a two-step strategy, a starting hydrophobic step followed by an electrostatic one to remove more efficiently the short chain PFAAs.