Solubility and sorption of redox-sensitive radionuclides (Np, Pu) in J-13 water from the Yucca Mountain site: comparison between experiment and theory

This study presents the characterization of Pu-bearing precipitates and the results from uptake studies of Np and Pit on inorganic colloidal particulates in J-13 water from the Yucca Mountain site. Plutonium solubilities determined experimentally at pH values of 6, 7. and 8.5 are about two orders of magnitude higher than those calculated using the existing thermodynamic database indicating the influence of colloidal Pu(IV) species. Solid phase characterization using X-ray diffraction revealed primarily Pu(IV) in all precipitates formed at PH 6 7, and 8.5. The solubility controlling Pu-bearing solids precipitated at ambient temperature consisted of amorphous Pu(OH)4(S) with several Pu-O distances between 2.3 and 2.7 Angstrom that are characteristic for Pu(IV) colloids. High temperature (90 degreesC) increased solid phase crystallinity and produced Pu(IV) solids that contained Pu oxidation state impurities, X-ray absorption spectroscopic studies revealed diminished Pu-O and Pu-Pu distances that were slightly different from those in crystalline PuO2(s). A Pu-O bond of 1.86 Angstrom was identified that is consistent with the plutonyl(V) distance of 1.81 Angstrom in PuO2+ (aq). Hematite, montmorillonite, and silica colloids were used for uptake experiments with Pu-239(V) Lind Np-237(V). The capacity of hematite to sorb Pu significantly exceeded that of montmorillonite and silica. A low desorption rate as indicative of highly stable Pu-hematite colloids, which may facilitate Put transport to the accessible environment. Neptunium uptake on all mineral phases was far less than Pu(V) uptake suggesting that a potential Pu(V)-Pu(IV) reductive sorption process was involved. The temperature effect on Pu solubility and pseudocolloid formation is also discussed. (C) 2002 Published by Elsevier Science Ltd.