Sorption kinetics and impact of temperature, ionic strength and colloid concentration on the adsorption of plutonium-239 by inorganic colloids

Abstract

Plutonium (Pu) is one of the primary actinides of concern for long-term disposal and storage of nuclear waste. Strong sorption of Pu onto colloids of iron oxide, clay, and silica could result in colloid-facilitated transport of this actinide in groundwater systems. However, fundamental data on Pu sorption to colloids is sparse, resulting in large uncertainties in long-term predictions of colloid-facilitated Pu transport. This sparseness of data and the potential to significantly reduce uncertainties in predictive models served as a motivation for this study. The authors investigated the sorption and desorption behaviors of ²³⁹Pu(V) on three types of inorganic colloids (hematite, montmorillonite and silica), with adsorption being measured as a function of temperature, ionic strength and colloid concentration for each colloid. Natural ground water collected from Well-13 near Yucca Mountain, Nevada, and synthetic ground water (SYN.J-13) were used in the experiments. The results indicated that the adsorption of ²³⁹Pu(V) onto hematite colloids was faster and had a higher partition coefficient (K_p, ml g^-1) than onto montmorillonite or silica colloids in both J-13 and SYN.J-13 waters. Temperature did not significantly influence the adsorption of ²³⁹Pu(V) onto hematite and silica colloids, but the adsorption of ²³⁹Pu(V) onto montmorillonite colloids increased significantly with increasing temperature. While ionic strength did not significantly influence the adsorption of ²³⁹Pu(V) onto hematite colloids, an increase in ionic strength decreased the adsorption of ²³⁹Pu onto montmorillonite and silica colloids. Adsorption of ²³⁹Pu(V) per unit mass of colloid was much higher at low colloid concentrations than at high colloid concentrations. Desorption of ²³⁹Pu from ²³⁹Pu-loaded colloids was considerably slower than the adsorption process.