EXTERNAL AND INTERNAL CATION EXCHANGE CAPACITIES OF ZEOLITES FROM THE WESTERN UNITED STATES

Natural zeolites are characterized by high surface areas and high cation exchange capacities. Fifteen clinoptilolite-rich zeolite samples from a variety of deposits in the western United States were analyzed for their cation exchange properties. The external and internal cation exchange capacity (CEC) was determined for each clinoptilolite specimen, along with its sorption capacity for the surfactant hexadecyltrimethylammonium (HDTMA).

Typically, CECs ascertained by mineral analysis are much higher than exchange capacities determined experimentally. Data on the external CEC will be useful in sorption studies involving molecules, such as HDTMA, that are too large to enter the internal structure of the zeolite, but still sorb to the external surface.

The CEC procedure involves saturating the samples with Na⁺, then removing excess interstitial sodium ions. The external CEC is determined by washing samples with the tert-butylammonium ion. This ion exchanges with Na⁺ on the external zeolite surface, but is too bulky to exchange with Na⁺ in the internal structure. The internal CEC is determined after washing samples with ammonium acetate, which allows NH₄⁺ to exchange with Na⁺ occupying internal exchange sites. The exchanged sodium solutions are analyzed by atomic absorption spectrophotometry to determine cation exchange capacities. 

The results confirm that the internal CEC is typically much greater than the external CEC. Information derived from this research can be used to estimate the sorption of large cations to the external zeolite surface. The relationships among external CEC, total CEC, and the sorption of HDTMA will be presented.