Influence of pH on plutonium desorption/solubilization from sediment

Aqueous Pu concentrations and oxidation state transformations as a function of pH were quantified and compared between sorption/desorption studies and literature solubility values. When Pu(V) was added to a red subsurface sandy-clay-loam sediment collected near Aiken, South Carolina, 99% of the Pu sorbed to the sediment within 48 h. Throughout the study, > or = 94% of the Puaq remained as Pu(V), whereas < or = 6% was Pu(VI) and < or = 1% was Pu(IV). This is in stark contrast to the sorbed Pu which was almost exclusively in the +4 oxidation state. The fraction of aqueous Pu (Puaq/Pusolid) decreased by >2 orders-of-magnitude when the contact time was increased from 1- to 33-days, presumably the result of Pu(V) reduction to Pu(IV). The desorption studies were conducted with a sediment that had been in contact with Pu (originally as PuIV(NO3)4) for 24 years. At near neutral pH, a decrease of 1-pH unit resulted in almost an order-of-magnitude increase in the concentration of Puaq (7.5 x 10(-10) M at pH 7 and 3.6 x 10(-9) M at pH 6). Similar to the sorption experiment, > or = 96% of the Puaq was Pu(V/VI). The Puaq concentrations from the desorption experiment were similar to those of the Pu(V) amended sorption studies that were permitted to equilibrate for 33 days, suggesting that the latter had reached steady state. The Puaq concentrations as a function of pH followed near identical trends with literature solubility values for PuO2(am), except that the desorption values were lower by a fixed amount, suggesting either Pu sorption was occurring in this sediment system or that a more crystalline, less soluble form of Pu existed in the sediment than in the literature water-PuO2(am) system. Based on Pu sorption experiments and measured sediment surface charge properties as a function of pH, the latter explanation appears more likely. pH had a more pronounced effect on solubility and Puaq concentrations than on sediment charge density (or Puaq oxidation state distribution). Slight changes in system pH can have a large impact on Pu solubility and the tendency of Pu to sorb to sediment, thereby influencing Pu subsurface mobility.