The measured sorption isotherms of cationic surfactants on kaolinite using the conventional method are shown in Figure . To our knowledge, such an extensive set of sorption isotherms on kaolinite at low surfactant loading levels (Ckao < 1000 µmol/kg) has not been reported in the literature. All isotherms obtained were nonlinear even at Ckao < 100 µmol/kg (i.e., < 0.2% of CEC), with nFr,kao = 0.51-0.68 (Table ). The Freundlich equation fits the data well, and there was no clear indication of surface saturation or cooperative sorption, which is consistent with the fact that the concentrations studied (< 10 mmol/kg) were below the CEC of the kaolinite sample (i.e., 62 meq/kg). The sorption of cationic chemicals by kaolinite at low solute loading is considered cation exchange, which is characterized by a selectivity coefficient for a given cation and follows a linear isotherm. In fact, models used to fit measured isotherms assume linearity at low surfactant concentrations. However, closer examination of the literature data shows that linearity at low concentrations does not always hold true. For example, Xu and Boyd reported that the sorption isotherm of hexadecyltrimethylammonium by Na-kaolinite in the presence of 42 mM NaCl was linear up to the sample CEC, but nonlinear at 2 mM NaCl. Kaolinite sorption isotherms of denatonium reported by Crosson et al. were linear when K + was the background cation, whereas they were nonlinear with Ca 2+ and Na + . Droge and Goss 29 conducted column retention experiments with various organic cations and concluded that the sorption coefficient was generally concentrationindependent, i.e., the sorption isotherm was linear (n ~ 1) on clay minerals including kaolinite. However, their individual data show that the sorption nonlinearity varies for individual compounds (e.g., nFr = 0.67-1.00 for quaternary ammoniums). Brownawell et al. reported that the sorption of dodecylpyridinium on Borden Sand was extensively nonlinear (n = 0.58) even in the low concentration range. Overall, it cannot be generally concluded that the sorption of cationic surfactant by kaolinite is linear even at low concentrations. Therefore, it is sensible not to extrapolate sorption characteristics to concentrations outside the tested range.