title: Np(V) and Pu(v) ion exchange and surface-mediated reduction mechanisms on montmorillonite.
abstract: 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 NpO(2)(+) → 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.
authors.affiliation: Glenn T Seaborg Institute, Physical and Life Science Directorate, Lawrence Livermore National Laboratory, Livermore California 94550, United States. Zavarin1@LLNL.gov
authors.forename: Mavrik
authors.initials: M
authors.lastname: Zavarin
authors.forename: Brian A
authors.initials: BA
authors.lastname: Powell
authors.forename: Mathilde
authors.initials: M
authors.lastname: Bourbin
authors.forename: Pihong
authors.initials: P
authors.lastname: Zhao
authors.forename: Annie B
authors.initials: AB
authors.lastname: Kersting
mesh_terms: D000327:Adsorption; D001546:Bentonite / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007474:Ion Exchange; D007700:Kinetics; D009405:Neptunium / Q000737:chemistry*; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D011005:Plutonium / Q000737:chemistry*; D012965:Sodium Chloride / Q000737:chemistry; D013499:Surface Properties; D013997:Time Factors
chemical_list: D001546:Bentonite; D012965:Sodium Chloride; D011005:Plutonium; D009405:Neptunium