An electrostatic approach for the prediction of actinide complexation constants with inorganic ligands-application to carbonate complexes

Abstract

A new semi-empirical approach is presented for the prediction of actinide complexation constants with inorganic ligands and applied to mononuclear carbonate complexes. The approach is based on an energy term describing the inter-ligand electrostatic repulsion. For a given metal ion M and ligand L, the formation constants log β°nof the complexes MLnare related by:log β°n= n log β°1-repEL(MLn) / RT ln 10The electrostatic ligand repulsion termrepELis derived from the charge and inter-distance of ligands involved in a given complex and their angular distribution. Semi-empirical parameters are required to describe the effective electrostatic shielding between complexing ligand ions with the metal ion and hydration water molecules between them. These shielding coefficients are specific for the ligands, but independent of the actinide ion and its oxidation state. The shielding coefficients for carbonate ligands are determined from the known formation constants of Am(III) and Cm(III) carbonate complexes, and then used to calculate carbonate complexation constants for other actinides of different oxidation states. Excellent agreement is found between the calculated and experimental values for Np(V), U(VI), Pu(VI), and even for the transition metals Fe(II) and Cu(II). Based on known constants log β°4and log β°5for U(IV) and Pu(IV), the unknown constants log β°1, log β°2and log β°3are estimated.