Effects of Surfactant Modification on the Sorption Behavior of Natural Zeolites for Strontium(2+) and Uranium(6+)

Natural zeolite minerals have strong ion exchange selectivity for alkali and alkaline earth radionuclides, such as strontium and cesium, but only limited sorption affinity for actinides such as uranium. In a previous study, surfactant-modification of the zeolite surface was shown to enhance uranium(6+) sorption at solution pHs greater than about 6. In this study, experiments were conducted to evaluate the ability of surfactant-modified zeolite (SMZ) to sorb both strontium(2+) and uranium(6+). A surfactant-modified zeolite was prepared by treating specimens composed mainly of clinoptilolite with the cationic surfactant hexadecyltrimethylammonium-bromide (HDTMA). The surfactant-modified clinoptilolite was reacted with strontium solutions having a range of strontium concentration, and with a mixed solution of strontium and uranium. Experiments were conducted as a function of pH and in equilibrium with atmospheric CO 2 (g). Surfactant-modification of clinoptilolite decreases the ion exchange selectivity of clinoptilolite for strontium at acidic to near-neutral pH. Strontium exchange data were compared to previously developed model predictions of strontium ion exchange behavior on clinoptilolite. The presence of both uranium and strontium in solution had little effect on either the ion exchange selectivity of the surfactant-modified clinoptilolite for strontium or on the enhanced sorption of uranium by SMZ at pH greater than 6.