Detailed characterization and preliminary adsorption model for materials for an intermediate-scale reactive-transport experiment

An intermediate-scale transport-model experiment, involving the migration of fluid and tracers (Li, Br, Ni) through a 6-m-high x 3-m-diameter caisson filled with Wedron 510 sand, is being carried out for the Yucca Mountain Site Characterization Project. The surface chemistry of the sand has been studied and a preliminary surface-complexation model of Ni adsorption has been formulated for use in transport calculations. XPS and leaching studies suggest that the surface of the quartz sand is partially covered by thin layers of Fe-oxyhydroxide and Ca-Mg carbonate and by flakes of kaolinite. Ni adsorption by the sand is strongly pH-dependent, showing no adsorption at pH 5 and near-total adsorption at pH 7. Ni adsorption edges for goethite and quartz, two components of the sand were also measured. Ni adsorption on pure quartz is only moderately pH-dependent and differs in shape and location from that of the sand, whereas Ni adsorption by goethite is strongly pH-dependent. A triple-layer surface-complexation model developed for goethite provides a good fit to the Ni-adsorption curve of the sand. Based on this model, the apparent surface area of the Fe-oxyhydroxide coating is estimated to be ∼560 m2/g, compatible with its occurrence as amorphous Fe-oxyhydroxide. Potentiometric titrations on the sand also differ from those of pure quartz and suggest that the effective surface area of the sand may be significantly greater than that measured by N2-BET gas adsorption. Attempts to model the adsorption characteristics of the bulk sand in terms of the properties of pure end member components suggest that much of the sand surface is inert. Although the exact mechanisms of Ni adsorption remain ambiguous, this preliminary adsorption model provides an initial set of parameters that can be used in transport calculations