Uranium(VI) adsorption and surface complexation modeling onto vadose sediments from the Savannah River Site

Uranium U(VI) adsorption was measured as function of pH (3–10) on goethite, kaolinite, quartz, two binary mixtures of goethite and kaolinite, and a vadose zone sediment collected on The Department of Energy’s Savannah River Site (SRS), the clay mineral fraction of which is composed largely of kaolinite and goethite. Diffuse-layer surface complexation models were parameterized using the code PEST together with PHREEQC to fit U(VI) sorption data for the pure goethite, kaolinite, and quartz. U(VI) adsorption on kaolinite and goethite was modeled as the formation of two bidentate U(VI) complexes at mineral edge sites on a variable charge site. U(VI) adsorption on quartz was described using a one-site diffuse-layer with the formation of bidentate complex on a variable charge site. These models were used to predict U(VI) adsorption on the binary sorbent mixtures and the SRS sediment using a simple component-additivity approach. In general, the predicted adsorption edges were in good agreement with measured data, with statistically similar goodness of fit compared to that obtained for the pure mineral systems.