Adsorption of naphthalene by HDTMA modified kaolinite and halloysite

The adsorption of the cationic surfactant hexadecyltrimethylammonium (HDTMA) by kaolinite and halloysite was studied. Fourier transformed infrared (FTIR) spectroscopic studies revealed a change of the surfactant arrangement with solution conditions. Naphthalene was partitioned into the organic phase created by the surfactant tails of the HDTMA modified kaolinite and halloysite. The adsorption isotherms for naphthalene were nearly linear, suggesting that adsorption could be described by a distribution process. The distribution coefficients were primarily affected by the amount of surfactant adsorbed. Adsorption of naphthalene, however, was particularly dependent on the arrangement of the surfactant cations. At high surface coverage (e.g., >60% cation exchange capacity, CEC), the bilayer surfactant structure formed on kaolinite adsorbed large amounts of naphthalene. On halloysite, HDTMA formed surfactant clusters, and no prominent increase of naphthalene adsorption even at high HDTMA loadings was observed. Thus, the clay mineral structure and morphology had a considerable influence on the surfactant arrangement responsible for partitioning hydrophobic organic contaminants (HOCs) such as naphthalene.