Long Term Plutonium Solubility and Speciation Studies in a Synthetic Brine

Abstract

The rate at which elements can be transported in groundwater systems is governed in part by the solubility of the element in the groundwater. This report documents plutonium solubility experiments, conducted over the past two years at the Lawrence Berkeley Laboratory, in a brine simulant relevant to the Waste Isolation Pilot Plant. Approximately 1 to 2.5 mL of five stock solutions containing single oxidation states of plutonium (Pu(IV)-polymer, Pu 3+ , Pu 4+ , Pu0 2 + , and PuOD were added to -75 mL of synthetic H-17 Brine in five reaction vessels. Initial plutonium concentrations ranged from 1.3 X10 4 to 5.1 Χ10" 4 Μ (moles per liter) total plutonium. Because these initial concentrations were far above the plutonium solubility limit in H-17 Brine, plutonium-containing solids precipitated. Aqueous plutonium concentrations were measured over time until steadystate was reached, requiring over 300 days in H-17 Brine. Steady-state plutonium concentrations ranged from 3.0 X 10~8 to 7.6 X 10~7 M, and the predominant plutonium oxidation state in solution was Pu(VI). The solid phase from the initially Pu(IV)polymer experiment remained polymeric-Pu(IV). The solids that formed in the initially Pu 3+ and Pu 4+ experiments were crystalline yet remain unidentified. The solids that formed in the initially Pu0 2 + and PuOi + experiments appear to be sodium plutonyl(V) carbonates.

Comment on ‘Plutonium chemistry: a synthesis of experimental data and a quantitative model for plutonium oxide solubility’ by J.M. Haschke and V.M. Oversby

“…[5], at pH 7.1 are difficult to relate to the other data considered here because of the high chloride concentration. This will complex the Pu(VI) and will probably affect possible alpha-induced reduction.…”

“…[1] the authors have examined the results of experiments, described in several papers [3][4][5][6] on the instability of both Pu(V) and Pu(VI) and found the data to be fit by:…”

Neptunium redox speciation at the illite surface

“…An uncertainty of ±0.3 is commonly associated with log R d determination for radionuclide sorption to clay minerals (Bradbury and Baeyens, 2009a), although for low [Np] tot and high uptake, the uncertainty on log R d might be higher. A test experiment was performed to determine the oxidation state of Np(i) for a solution in contact with illite and (ii) for the illite surface via two liquid-liquid extraction methods using either 1-phenyl-3-methyl-4-benzoylpyrazolone-5 (PMBP)or phosphoric acid bis-(2-ethyl-hexyl) ester) (HDEHP) (Bertrand and Choppin, 1982;Nitsche et al, 1988Nitsche et al, , 1994 as extracting agents. For our experimental conditions, both methods are equivalent: PMBP extracts Np(IV) whereas HDEHP extracts Np(V) in organic phase at pH = 0.…”

Plutonium(VI) accumulation and reduction by lichen biomass: correlation with U(VI)

“…Larsen and Oldham (1978) showed that approximately 70-90% of Pu was present as Pu(VI) in chlorinated drinking waters. Dissolved Pu in neutral and basic brines is present as Pu(VI) (Nitsche et al, 1994). Pu(V) is dominant and Pu(IV) is unstable in neutral waters (Choppin, 2001).…”