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

Abstract

Characterizing sorbent affinity for a target compound (described by sorbent–water distribution coefficient, K sw) 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 K sw, and studies display significant variations in setup and performance. For per- and polyfluoroalkyl substances (PFAS), most K sw 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 K sw. 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 K sw values. Each methodological component affected log(K sw) 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 K sw values for 23 PFAS and two sorbents.