Np(V) and Pu(V) Ion Exchange and Surface-Mediated Reduction Mechanisms on Montmorillonite

Due to their ubiquity and chemical reactivity, aluminosilicate clays play an important role in actinide retardation and colloid-facilitated transport in the environment. In this work, Pu­(V) and Np­(V) sorption to Na-montmorillonite was examined as a function of ionic strength, pH, and time. Np­(V) sorption equilibrium was reached within 2 h. Sorption was relatively weak and showed a pH and ionic strength dependence. An approximate NpO₂⁺ → Na⁺ Vanselow ion exchange coefficient (Kv) was determined on the basis of Np­(V) sorption in 0.01 and 1.0 M NaCl solutions at pH < 5 (Kv ∼ 0.3). In contrast to Np­(V), Pu­(V) sorption equilibrium was not achieved on the time-scale of weeks. Pu­(V) sorption was much stronger than Np­(V), and sorption rates exhibited both a pH and ionic strength dependence. Differences in Np­(V) and Pu­(V) sorption behavior are indicative of surface-mediated transformation of Pu­(V) to Pu­(IV) which has been reported for a number of redox-active and redox-inactive minerals. A model of the pH and ionic strength dependence of Pu­(V) sorption rates suggests that H⁺ exchangeable cations facilitate Pu­(V) reduction. While surface complexation may play a dominant role in Pu sorption and colloid-facilitated transport under alkaline conditions, results from this study suggest that Pu­(V) ion exchange and surface-mediated reduction to Pu­(IV) can immobilize Pu or enhance its colloid-facilitated transport in the environment at neutral to mildly acidic pHs