Kinetics of the Adsorption of Radionuclides on Tuff From Yucca Mountain

Abstract

ABSTRACTThe sorption of simple cations in tuff is dominated by adsorption on aluminosilicates that have charged surfaces, suc as zeolites and clays. The most significant sorbing minerals present in Nevada tuff are clinoptilolite, heulandite, mordenite, and montmorillonite. The kinetics of sorption on tuffs containing the minerals clinoptilolite and montmorillonite has been determined by studying the uptake of strontium, cesium, and barium on thin tuff wafers. The rate constants for uptake of these elements on tuff are consistent with a model of sorption that is diffusion limited but where diffusion occurs in two stages. First the cations diffuse into the rock through the water-filled pore space. Next, the cations must diffuse into the much narrower channels within the aluminosilicate crystals. After they are within the zeolite framework o between the clay planes, the cations may rapidly sorb on the negatively charged surfaces. The actinide elements have a time constant for the apparent sorption that is inconsistent with this model and may have a radically different mechanism o removal from solution.