For each test, 30 polypropylene (PP) microtubes (1.5 mL) were used, 14 of which received 1100 μL of kaolinite suspension, 100 μL of one of the aqueous stocks of the target cationic surfactant, and 1-2 pieces of 3 cm PA fiber. The kaolinite concentration was 4.6-18.3 mg/mL. Seven concentration levels of cationic surfactant were prepared in duplicate. In parallel, 14 PP tubes containing 15 mM NaCl solution and PA fiber, but without kaolinite, were also prepared to investigate the sorption of the cationic surfactant by the PA fiber. These no-kaolinite tubes also serve as controls to check the sorption of cationic surfactants to the tube walls. The remaining two tubes served as blanks, one with and one without kaolinite. All tubes were roller-shaken overnight, or up to 24 h, in an incubator at 25°C. Preliminary experiments with C8-BAC showed that equilibrium was reached within 3 h (Figure ). Tubes were centrifuged for 15-30 min at 10,000 rpm (~4000g) using a Chibitan-II Microcentrifuge (Millipore, Darmstadt, Germany). The supernatant was pipetted into a glass vial.