The presence of kaolinite in water had negligible or only small effects on the PA-water sorption isotherms, as shown in Figure , where the data points in the presence of kaolinite (open symbols) are mostly within the 95% prediction interval of the Langmuir-Freundlich model fitted to the data in the absence of kaolinite. Exceptions are C6 and C8-BACs. For these short-chain BACs, the presence of kaolinite increased the sorption of BACs by PA by up to a factor of 2. Since the passive sampling method assumes no influence of suspended material on the PA sorption parameters, this variation of PA sorption will cause an error in Cw. The increase in PA sorption for C6 and C8-BACs can be explained by the kaolinite particles adhering to the PA surface, which were co-extracted and increased the apparent (measured) CPA. This increase in CPA is only significant for C6-and C8-BACs because PA sorption is weak relative to kaolinite sorption for these short-chain BACs. Figure directly compares Ckao and CPA in the kaolinite suspension at equilibrium and shows that PA sorption is generally stronger than kaolinite sorption, but that the difference is small or absent for C6-and C8-BACs. Kaolinite fouling on the PA surface has a negligible effect on CPA for the other cationic surfactants because the adherent kaolinite particles hold only a small amount of these surfactants compared to the PA fiber.