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.