Abstract
The adsorption of perfluorooctanoic acid (PFOA) and perfluorooctylsulfonic acid (PFOS) on various activated carbon felts (ACFs) was studied and related to their surface characteristics. The adsorption isotherms of the ACFs studied differ widely in terms of adsorption affinities as well as maximum loadings. Cation and anion exchange capacities (CEC and AEC) at pH 7, as well as point of zero net proton charge (PZC), were identified as good indicators for understanding of each adsorbent’s behavior towards the perfluorinated alkyl substances (PFAS) in their anionic state. Maximum loadings of the two PFAS are in all cases equal to or lower than the AEC of the ACFs. The superposition of hydrophobic interactions and electrostatic attraction between PFAS anions and positively charged sites of ACF leads to exceptionally high adsorption affinity (Kd up to 108 L/kg). In this respect, positive charges created by proton adsorption to π-electron-rich regions on the basal plane of carbon are more important than basic oxygen-containing groups. The effect of inorganic ions on adsorption of PFAS is variable and is also influenced by the surface chemistry of the ACFs. Dissolved natural organic matter did not significantly affect PFAS adsorption most likely due to size exclusion exerted by the micropores of the ACFs.
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