COUNTERION EFFECTS ON THE SORPTION OF CATIONIC SURFACTANT AND CHROMATE ON NATURAL CLINOPTILOLITE

We determined the effect of selected counterions (Cl-, Br-, and HSO4-) on the sorption of the cationic surfactant hexadecyltrimethylammonuiu m (HDTMA) on clinoptilolite zeolite and on the subsequent sorption of chromate by HDTMA-zeolite. The HDTMA sorption on the zeolite, as chara cterized by the Langmuir sorption maximum, followed the trend HDTMA-Br > HDTMA-Cl > HDTMA-HSO4 (208, 151, and 132 mmol/kg, respectively). Th e same counterion trend was observed for HDTMA sorption on KGA-1 kaoli nite. Measurement of counterion sorption indicated that HDTMA-Br and H DTMA-Cl formed complete bilayers on the zeolite, whereas HDTMA-HSO4 sh owed less than full bilayer formation. Competitive sorption between HD TMA-Br and HDTMA-Cl on the zeolite also showed a preference for the Br - counterion. The counterion stabilization of HDTMA admicelles on the zeolite surface follows the same trends as the counterion stabilizatio n of micelles in solution. Chromate sorption was also strongly influen ced by the HDTMA-zeolite counterion, with chromate sorption maxima dec reasing in the order HDTMA-HSO4 > HDTMA-Cl > HDTMA-Br (28, 16, and 11 mmol/kg, respectively). The sorption of chromate and other divalent an ions on HDTMA-zeolite results from a combination of entropic, Coulombi c, and hydrophobic effects, all of which are functions of the initial HDTMA counterion. In the design of surfactant-modified clays and zeoli tes for environmental applications, the strong influence of the surfac tant counterion must be considered.