| Abstract
| - We determined the effect of selected counterions(Cl-,Br-, and HSO4-) on the sorptionof the cationic surfactanthexadecyltrimethylammonuium (HDTMA) on clinoptilolitezeolite and on the subsequent sorption of chromate byHDTMA−zeolite. The HDTMA sorption on the zeolite,ascharacterized by the Langmuir sorption maximum, followedthe trend HDTMA-Br > HDTMA-Cl > HDTMA-HSO4(208,151, and 132 mmol/kg, respectively). The samecounteriontrend was observed for HDTMA sorption on KGA-1 kaolinite.Measurement of counterion sorption indicated thatHDTMA-Br and HDTMA-Cl formed complete bilayers on the zeolite,whereas HDTMA-HSO4 showed less than fullbilayerformation. Competitive sorption between HDTMA-Br andHDTMA-Cl on the zeolite also showed a preference forthe Br- counterion. The counterion stabilization ofHDTMAadmicelles on the zeolite surface follows the same trendsas the counterion stabilization of micelles in solution.Chromate sorption was also strongly influenced by theHDTMA−zeolite counterion, with chromate sorption maximadecreasing in the order HDTMA-HSO4> HDTMA-Cl>HDTMA-Br (28, 16, and 11 mmol/kg, respectively). Thesorptionof chromate and other divalent anions on HDTMA−zeoliteresults from a combination of entropic, Coulombic, andhydrophobic effects, all of which are functions of theinitialHDTMA counterion. In the design ofsurfactant-modifiedclays and zeolites for environmental applications, thestrong influence of the surfactant counterion must beconsidered.
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