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) (UO2 2+, uranyl). Ligands taken into account are mono- (acetate), di- (oxalate, malonate, succinate and azelate), tri- (1,2,3-propanetricarboxylate) and hexacarboxylate (1,2,3,4,5,6-benzenehexacarboxylate). The sequestering ability of polycarboxylic ligands towards UO2 2+ 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 UO2 2+ concentration. A fairly linear correlation was obtained between pL50 or log K110 (logK110 refers to the equilibrium: UO2 2+ +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−, MLH2 0 and MLOH3− species (M =UO2 2+ 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: pUO2 2+ + qL4− + rH+ = (UO2 2+)pLqHr (2p−4q+r)]. The dependence on ionic strength of all ligand protonation constants and of the complex formation constants of UO2 2+–polycarboxylate systems was modelled by the SIT (specific ion interaction theory) approach and by the Pitzer equations.