RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The prediction of radionuclide retardation and transport in geologic media requires a quantitative knowledge of the sorption behavior of radioactive species, including the effects of groundwater chemistry and the water/rock ratio. In this study, experiments were conducted to determine the effects of solution pH and the sorbent-surface area on the sorption behavior of uranium (+6). The sorbent phase used was α-alumina (National Institute of Standards and Technology Reference Materials 8005, 8006, and 8007) with reported surface areas of 2.09, 0.229, and 0.0686 m²/g, respectively. Experimental results indicate that uranium (+6) sorption on α-alumina occurs at near-neutral pH where UO₂(OH)₂⁰ is the predominant aqueous species, and that the total uranium sorbed increases as the sorbent-surface area/solution-volume ratio increases. Results also show that as the sorbent-surface area/solution-volume ratio increases the "adsorption edge" shifts to more acidic pH, whereas the "desorption edge" shifts to more alkaline pH. Surface complexation modeling using the diffuse-layer model reproduces the sorptive behavior of uranium (+6) on the α-alumina relatively well.