CATIONIC SURFACTANT SORPTION TO A VERMICULITIC SUBSOIL VIA HYDROPHOBIC BONDING

Organoclays formed in soil from the addition of the cationic surfactant hexadecyftrimethylammonium (HDTMA) can effectively immobilize organic contaminants dissolved in water. The adsorption and desorption of HDTMA in a subsoil was studied to determine the stability of surfactant-soil clay complexes as affected by surfactant retention mechanism. We found that HDTMA was initially adsorbed by cation exchange in the interlayer, causing extensive clay aggregation. As the loading increased, HDTMA adsorbed to the external surfaces of aggregates via both cation exchange and hydrophobic bonding, the latter causing positive charge development on surfaces and ultimately clay dispersion. When the equilibrium concentration of HDTMA reached the critical micelle concentration, no further HDTMA adsorption occurred. Desorption of HDTMA was more significant when the HDTMA retention mechanism was hydrophobic bonding. HDTMA adsorption can be affected by cation type as well as the type and concentration of electrolytes, and these factors can be used to minimize the undesirable effects of hydrophobic bonding such as clay dispersion and HDTMA desorption.