Sequestering ability of polycarboxylic ligands towards dioxouranium(VI)

In this paper we report a comparison on the sequestering ability of some polycarboxylic ligands towards dioxouranium(VI) (UO22+, uranyl). Ligands taken into account are mono- (acetate), di- (oxalate, malonate, succinate and azelate), tri- (1,2,3-propanetricarboxylate) and hexa-carboxylate (1,2,3,4,5,6-benzenehexacarboxylate). The sequestering ability of polycarboxylic ligands towards UO22+ was quantified by a new approach expressed by means of a sigmoid Boltzman type equation and of a empirical parameters (pL50) which defines the amount of ligand necessary to sequester 50% of the total UO22+ concentration. A fairly linear correlation was obtained between pL50 or log K110 (log K110 refers to the equilibrium: UO22+ + Lz− = UO2L(2−z); L = generic ligand) and the polyanion charges. In order to complete the picture, a tetra-carboxylate ligand (1,2,3,4-butanetetracarboxylate) was studied in NaCl aqueous solutions at 0 ≤ I (mol L−1) ≤ 1.0 and t = 25 °C, by potentiometry, ISE-[H+] glass electrode. The formation of ML2−, MLH−, MLH20 and MLOH3− species (M = UO22+ and L = 1,2,3,4-butanetetracarboxylate) was found, with log β110 = 7.937 ± 0.028, log β111 = 13.066 ± 0.027, log β112 = 17.401 ± 0.013, log β11−1 = 2.062 ± 0.040 at I = 0 mol L−1 and t = 25 °C [βpqr refer to reaction: pUO22+ + qL4− + rH+ = (UO22+)pLqHr(2p−4q+r)]. The dependence on ionic strength of all ligand protonation constants and of the complex formation constants of UO22+–polycarboxylate systems was modelled by the SIT (specific ion interaction theory) approach and by the Pitzer equations.