Thermodynamic modeling of actinide complexation with oxalate at high ionic strength

Abstract

A geochemical model describing the solubility of actinides in underground water at the Waste Isolation Pilot Plant (WIPP) Project is under development. The database for this model consists of standard chemical potentials and Pitzer model parameters for hundreds of species that may be present in the WIPP disposal room. Organic ligands used in separation and decontamination processes may be present in the nuclear wastes placed in the WIPP site and could have a significant impact on mobile actinide concentrations. In this work the β₁ and β₂ stability constants of NpO₂ ⁺, UO₂ ²⁺, Am³⁺ and Th⁴⁺ with the oxalate anion have been measured in 0.3–5.0M NaCl media at 25 °C by a solvent extraction technique. For the 1:1 complexation, the values of the stability constants increased in the order: NpO₂ ⁺<Am³⁺<UO₂ ²⁺<Th⁴⁺, in accordance with the actinide charge density and reflecting the strongly ionic bonding of the complexes. The Pitzer ionic interaction model was used to model the data. Because the data were collected mainly in the high ionic strength region, values of β⁽¹⁾ were estimated from these plus literature values. The Pitzer model gives a good representation of the data using three interaction parameters β⁽⁰⁾, β⁽¹⁾, and c^φ.