Kaolinite sorption isotherms of benzalkonium and dialkyldimethylammonium compounds under dilute conditions investigated by direct and passive sampling methods.

Cationic surfactants are widely used in various applications, making their environmental behavior a critical concern. However, soil sorption studies at environmentally relevant submicromolar concentrations remain scarce. In this study, sorption isotherms of five benzalkonium compounds (BACs) and two dialkyldimethylammonium compounds on kaolinite under dilute conditions were determined using a conventional batch-shaking and centrifugation method. In addition, the feasibility of using a polyacrylate (PA) fiber passive sampling method to measure freely dissolved surfactant concentrations in kaolinite suspensions was investigated. All kaolinite sorption isotherms were nonlinear (Freundlich exponent, 0.51-0.68), even at low surfactant concentrations (<0.2% of the cationic exchange capacity), suggesting high sorption site heterogeneity. Extensive sorption nonlinearity also indicates that sorption coefficients measured at high concentrations cannot represent those at environmentally relevant concentrations. The kaolinite-water sorption coefficients (K_kao/w) of BACs at the identical sorbed concentration on kaolinite increased with alkyl chain length; however, the incremental increase in log K_kao/w per -C₂H₄- unit varied with chain length, further indicating heterogeneous sorption sites. Kaolinite sorption isotherms measured using passive sampling were comparable to those measured by direct sampling. PA fiber fouling by kaolinite particles and the high isotherm nonlinearity of PA sorption at high concentrations were identified as major sources of error.