Avoiding Artifacts in the Determination of Per- and Polyfluoroalkyl Substance Sorbent-Water Distribution

Characterizing sorbent affinity for a target compound (described by sorbent-water distribution coefficient, Ksw) is a necessary step in the sorbent selection and performance-testing process in the process of capturing aquatic contaminants. However, no standardized procedure exists to measure Ksw, and studies display significant variations in setup and performance. For per- and polyfluoroalkyl substances (PFAS), most Ksw determinations employ batch experiments with small-scale water-sorbent mixtures, methanol-based spike of target compound(s), and analysis after assumed equilibrium, but methodological details of the above procedure differ and might cause artifacts in the determination of Ksw. We conducted several batch experiments systematically varying a general procedure to characterize the effects of suboptimal experimental design. Using a selection of PFAS (6-carbon fluorinated chain length with differing functional groups) and two sorbents, we tested variations of a solution/sorbent ratio, methanol content, and PFAS initial concentration and compared derived Ksw values. Each methodological component affected log(Ksw) usually by suppressing the value (by 0-48%) when compared with a “best design” procedure. Thus, we suggest (1) a reference procedure for PFAS and sorbents used here and (2) general guidelines for batch experiment design with different compounds and sorbents. Additionally, we report well-constrained Ksw values for 23 PFAS and two sorbents.