Influence of pH on Plutonium Desorption/Solubilization from Sediment

Abstract

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.

Sorption coefficients and molecular mechanisms of Pu, U, Np, Am and Tc to Fe (hydr)oxides: A review

“…With extended contact time, the sorption edge for Pu(V) shifted to lower pH values and this appeared to be due to the reduction of Pu(V) to Pu(IV) in the presence of the goethite surface. These results suggest that redox transformation is likely an important aspect of Pu In addition, Kaplan et al [43] investigated the influence of pH on Pu desorption/solubilization from SRS sediments in which sand grains and clays are coated with goethite. They found that >99% of the Pu sorbed onto the sediment within 48 h, >94% of the aqueous Pu remained as Pu(V), <6% as Pu(VI) and <1% as Pu(IV); in contrast, the sorbed Pu was exclusively Pu(III/IV).…”

Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

“…This trend in dissolved plutonium concentrations as a function of pH was nearly identical to the trend for the solubility for PuO 2 (am) (amorphous PuO 2 ) given in the literature, except that the desorption concentrations were lower by a fixed amount. Although they could not rule the possibility of plutonium sorption occurring in their desorption experiments, Kaplan et al (2006a) proposed that the measured desorption behavior was likely due to dissolution of a solid form of plutonium that was more crystalline and less soluble than the solubility data in the literature for the PuO 2 (am)-water system. Dissolved plutonium in the environment is typically present at less than 10 -15 mol/L (EPA 1999).…”

“…Laboratory studies conducted by , calculates to be oversaturated at 25°C for a total activity of 1×10 -16 M dissolved plutonium based on the available thermodynamic data. Kaplan et al (2006a) measured the dissolved concentrations and oxidation state transformations of plutonium as a function of pH in sorption and desorption experiments with a sandy-clay-loam sediment taken from the field lysimeter study described in Kaplan et al (2004Kaplan et al ( , 2006b. The sediment used for the field lysimeter study was collected from the vadose zone at the Savannah River Site (SRS) near Aiken, South Carolina.…”

“…When Pu(V) was added to a suspension of this uncontaminated sediment, the resulting sorbed plutonium was found to have been reduced completely to the Pu(IV) oxidation state, whereas the dissolved plutonium was present as ≥94% Pu(V), ≤6% Pu(VI), and ≤1% Pu(IV). Kaplan Kaplan et al (2006a) used a sediment sample that was previously contaminated from contact with a Pu IV (NO 3 ) 4 source disk for 24 years in the lysimeter study. In a series of batch desorption tests with the plutonium-contaminated sediment at different suspension pH values from 2.51 to 7.95, they measured approximately an order-of-magnitude increase in the concentration of dissolved plutonium with a decrease of one pH unit at near neutral pH conditions.…”

Literature Review on the Sorption of Plutonium, Uranium, Neptunium, Americium and Technetium to Corrosion Products on Waste Tank Liners

“…In addition, Kaplan et al (2006) investigated the influence of pH on Pu desorption / solubilization from SRS sediments in which sand grains and clays are coated with goethite. They found that >99% of the Pu adsorbs onto the sediment within 48 hours, >94% of the aqueous Pu remains as Pu(V), <6% as Pu(VI) and <1% as Pu(IV); in contrast, the adsorbed Pu is exclusively Pu(IV).…”