MODELING THE BINDING OF EUROPIUM AND THE ACTINIDES BY HUMIC SUBSTANCES

Humic Ion-Binding Model V is used to describe europium and actinide bi nding by fulvic acid (FA) and humic acid (HA) under different solution conditions. The interactions are characterized by a single parameter, an intrinsic equilibrium constant for metal-proton exchange (pK(MHA)) . Values of pK(MHA) are estimated firstly from published experimental data for the interactions of Eu3+, UO22+ and Am3+ with FA, and for tho se of Eu3+, Th4+, UO22+, Am3+ and Cm3+ with HA, and secondly from line ar free energy relationships involving the metal-proton exchange react ions of simple organic acids, and hydrolysis reactions. Values of pK(M HA) vary over three orders of magnitude, decreasing in the approximate order; Pa4+ = U4+ = Pu4+ > Np4+ > Th4+ much greater than Eu3+ = Am3> NpO22+ > PuO22+ > UO22+ > Pu3+ > Np3+ > Bk3+ = Cf3+ > U3+ > NpO2+ = Cm3+ > puO2+ > UO2+ The extent of actinide binding under a given set o f conditions depends not only upon pK(MHA), but also on the charges of the metal species and the humic substances, ionic strength, pH, hydro lysis reactions, complexation reactions with other ligands, and compet ition effects, notably with alkaline earths.