Determination of stability constants of U(VI)-Fe(III)-citrate complexes.

Ion-exchange experiments were performed to evaluate the formation of the uranium-citrate and uranium-iron-citrate complexes over a wide concentration range; i.e., environmentally relevant concentrations (e.g., 10-6 M in metal and ligand) and concentrations useful for spectroscopic investigations (e.g., 10-4 M in metal and ligand). The stability of the well-known uranium-citrate complex was determined to validate the computational and experimental methods applied to the more complex system. Values of the conditional stability constants for these species were obtained using a chemical equilibrium model in FITEQL. At a pH of 4.0, the stability constant for uranium-citrate complex (log β1,1) was determined to be 8.71 ± 0.6 at I = 0. Finally, analysis of the results of ion-exchange experiments for the U-Fe-citric acid system indicates the formation of the 1:1:1 and 1:1:2 ternary species with stability constants (log β) of 17.10 ± 0.41 and 20.47 ± 0.31, respectively, at I = 0.