Effect of Clay Mineralogy on the Partition Coefficients of Perfluoroalkyl Substances

The sorption behavior of 20 per-polyfluoroalkyl substances (PFASs) of varying chain lengths and functional groups was studied with kaolinite, illite, and montmorillonite (as pure clay minerals) at pH 7. The six classes of compounds studied were perfluoroalkyl carboxyl acids, perfluoroalkyl sulfonic acids, alkyl-sulfonamidoacetic acids, fluorotelomer sulfonic acids, alkane sulfonamides, and ether carboxylic acids. The sorption isotherms were obtained using a 0.01 M calcium chloride solution spiked with a multi-solute target PFAS solution, yielding a solute concentration varying from 100 to 400 mu g/L. The partition coefficients (Kd) of all PFASs varied by molecular weight, degree of fluorine substitution, functional group, and cation-exchange capacity. The measured Kd value was larger for longer chain lengths and perfluoroalkyl compounds compared with shorter chain lengths and polyfluoroalkyl compounds. PFASs with sulfonamide functional groups showed the strongest sorption, followed by sulfonic and carboxylic acid functional groups. The Kd values for all PFASs were found to correlate with increasing cation-exchange capacity of the clay minerals, with the strongest sorption observed by montmorillonite, followed by illite and kaolinite. A parametric model has been established allowing the prediction of PFAS sorption Kd in three different common clay minerals.