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TECHNICAL REPORTS
Heavy Metals in the Environment

Influence of Quaternary Ammonium on Sorption of Selected Metal Cations onto Clinoptilolite Zeolite

^a Chemistry Dep. and Geology Dep., Univ. of Wisconsin-Parkside, 900 Wood Road, Kenosha, WI 53141
^b Chemistry Dep., Univ. of Wisconsin-Parkside, 900 Wood Road, Kenosha, WI 53141

* Corresponding author (li{at}uwp.edu)

Received for publication December 15, 2000. Clay minerals and zeolites have large cation exchange capacities, which enable them to be modified by cationic surfactant to enhance their sorption of organic and anionic contaminants. In this study, the influence of quaternary ammonium surfactants on sorption of five metal cations (Cs⁺, Sr²⁺, La³⁺, Pb²⁺, and Zn²⁺) onto a clinoptilolite zeolite was investigated. Generally, the metal cation sorption capacity and affinity for the zeolite decreased, indicating that presorbed cationic surfactants blocked sorption sites for metal cations, as the surfactant loading on the zeolite increased. Cesium and Pb²⁺ sorption was affected to a small extent, indicating that selective sorption for Cs⁺ and specific sorption for Pb²⁺ play an important role in addition to cation exchange. Sorption of cationic surfactants on zeolite preloaded with different metal cations showed a strong correlation with the chain length of the surfactant tail group, while the roles of the charges and types of the metal cations were minimal. As the chain length increases, the critical micelle concentration decreases and the surfactant molecules become more hydrophobic, resulting in progressive bilayer coverage. Desorption of presorbed metal cations by cationic surfactants was strongly affected by the surfactant chain length and metal type. More metal cations, particularly Sr²⁺ and Zn²⁺, desorbed with an increase in surfactant chain length. The results, in combination with those from organic and oxyanion sorption on surfactant-modified zeolite, may be used for future surfactant modification to target sorption and desorption of a specific type of contaminant or a mixture of different types of contaminants.

Abbreviations: AA, atomic absorption • CEC, cation exchange capacity • CMC, critical micelle concentration • DDTMA, dodecyltrimethylammonium • ECEC, external cation exchange capacity • HDTMA, hexadecyltrimethylammonium • HPLC, high performance liquid chromatography • OTMA, octyltrimethylammonium • SMZ, surfactant-modified zeolite