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 hexadecyltrimethylammonuium (HDTMA) on clinoptilolite zeolite and on the subsequent sorption of chromate by HDTMA-zeolite. The HDTMA sorption on the zeolite, as characterized by the Langmuir sorption maximum, followed the trend HDTMA-Br > HDTMA-Cl > HDTMA-HSO4 (208, 151, and 132 mmol/kg, respectively). The same counterion trend was observed for HDTMA sorption on KGA-1 kaolinite. Measurement of counterion sorption indicated that HDTMA-Br and HDTMA-Cl formed complete bilayers on the zeolite, whereas HDTMA-HSO4 showed less than full bilayer formation. Competitive sorption between HDTMA-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 stabilization of micelles in solution. Chromate sorption was also strongly influenced by the HDTMA-zeolite counterion, with chromate sorption maxima decreasing in the order HDTMA-HSO4 > HDTMA-Cl > HDTMA-Br (28, 16, and 11 mmol/kg, respectively). The sorption of chromate and other divalent anions on HDTMA-zeolite results from a combination of entropic, Coulombic, and hydrophobic effects, all of which are functions of the initial HDTMA counterion. In the design of surfactant-modified clays and zeolites for environmental applications, the strong influence of the surfactant counterion must be considered.