Development of a Composite Non-Electrostatic Surface Complexation Model Describing Plutonium Sorption to Aluminosilicates

Due to their ubiquity in nature and chemical reactivity, aluminosilicate minerals play an important role in retarding actinide subsurface migration. However, very few studies have examined Pu interaction with clay minerals in sufficient detail to produce a credible mechanistic model of its behavior. In this work, Pu(IV) and Pu(V) interactions with silica, gibbsite (Aloxide), and Na-montmorillonite (smectite clay) were examined as a function of time and pH. Sorption of Pu(IV) and Pu(V) to gibbsite and silica increased with pH (4 to 10). The Pu(V) sorption edge shifted to lower pH values over time and approached that of Pu(IV). This behavior is apparently due to surface mediated reduction of Pu(V) to Pu(IV). Surface complexation constants describing Pu(IV)/Pu(V) sorption to aluminol and silanol groups were developed from the silica and gibbsite sorption experiments and applied to the montmorillonite dataset. The model provided an acceptable fit to the montmorillonite sorption data for Pu(V). In order to accurately predict Pu(IV) sorption to montmorillonite, the model required inclusion of ion exchange. The objective of this work is to measure the sorption of Pu(IV) and Pu(V) to silica, gibbsite, and smectite (montmorillonite). Aluminosilicate minerals are ubiquitous at the Nevada National Security Site and improving our understanding of Pu sorption to aluminosilicates (smectite clays in particular) is essential to the accurate prediction of Pu transport rates. These data will improve the mechanistic approach for modeling the hydrologic source term (HST) and provide sorption Kd parameters for use in CAU models. In both alluvium and tuff, aluminosilicates have been found to play a dominant role in the radionuclide retardation because their abundance is typically more than an order of magnitude greater than other potential sorbing minerals such as iron and manganese oxides (e.g. Vaniman et al., 1996). The sorption database used in recent HST models (Carle et al., 2006) and upscaled for use in CAU models (Stoller-Navarro, 2008) includes surface complexation constants for U, Am, Eu, Np and Pu (Zavarin and Bruton, 2004). Generally, between 15 to 30 datasets were used to develop the constants for each radionuclide. However, the constants that describe Pu sorption to aluminosilicates were developed using only 10 datasets, most of which did not specify the oxidation state of Pu in the experiment. Without knowledge or control of the Pu oxidation state, a high degree of uncertainty is introduced into the model. The existing Pu surface complexation model (e.g. Zavarin and Bruton, 2004) drastically underestimates Pu sorption and, thus, will overestimate Pu migration rates (Turner, 1995). Recent HST simulations at Cambric (Carle et al., 2006) suggest that the existing surface complexation model may underpredict Pu K{sub d}s by as much as 3 orders of magnitude. In order to improve HST and CAU-scale transport models (and, as a result, reduce the conservative nature Pu migration estimates), sorption experiments were performed over a range of solution conditions that brackets the groundwater chemistry of the Nevada National Security Site. The aluminosilicates examined were gibbsite, silica, and montmorillonite