N. A. Wall, M. Borkowski, L-F. Chen, G.R. Choppin

The Waste Isolation Pilot Plant (WIPP), located within a natural bedded salt formation in the South-East of New-Mexico is now open for the permanent repository of transuranic (TRU) wastes. This work presents results regarding the interaction of Am (III) with ligands relevant to the repository: humic acids (HA), fulvic acids (FA) and citric acid (Cit). The stability constants of the Am complexes were evaluated as a function of the ionic strength (NaCl), using a solvent extraction technique. The Am/Cit first stability constant log b ₁₀₁ varies between 5.9 at 0.3 m and 5.10 at 5.0 m. For a HA degree of ionization (a ) c.a. 0.6, the Am/HA binding constant log b ₁ varies from 6.18 at 1 m and 7.40 at 6.0 m. At high ionic strength (3.0 m and 6.0 m), the HA degree of ionization between 0.6 and 1.0 does not influence the Am/HA binding constant. Comparison between the binding constants of HA and FA, where they both present a degree of ionization of c.a. 1 and the stability constants with Cit shows that, at high ionic strength, log b ₁₀₁(Am/Cit) » log b ₁(Am/FA) » log b ₁(Am/HA) — 1. Literature data were review to extend this observation to other metals complexes. The values of stability constants for metal ions with citrate can serve as an estimated value for FA binding constants, leading to an approximation of HA binding constants. This work was funded at Florida State University by Sandia National Laboratory, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL8500.