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

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.