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) (UO(2)(2+), 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 UO(2)(2+) was quantified by a new approach expressed by means of a sigmoid Boltzman type equation and of a empirical parameters (pL(50)) which defines the amount of ligand necessary to sequester 50% of the total UO(2)(2+) concentration. A fairly linear correlation was obtained between pL(50) or log K(110) (log K(110) refers to the equilibrium: UO(2)(2+)+L(z-)=UO(2)L((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<or=I (mol L(-1))<or=1.0 and t=25 degrees C, by potentiometry, ISE-[H(+)] glass electrode. The formation of ML(2-), MLH(-), MLH(2)(0) and MLOH(3-) species (M=UO(2)(2+) and L=1,2,3,4-butanetetracarboxylate) was found, with log beta(110)=7.937+/-0.028, log beta(111)=13.066+/-0.027, log beta(112)=17.401+/-0.013, log beta(11-1)=2.062+/-0.040 at I=0 mol L(-1) and t=25 degrees C [beta(pqr) refer to reaction: pUO(2)(2+)+qL(4-)+rH(+)=(UO(2)(2+))(p)L(q)H(r)((2p-4q+r))]. The dependence on ionic strength of all ligand protonation constants and of the complex formation constants of UO(2)(2+)-polycarboxylate systems was modelled by the SIT (specific ion interaction theory) approach and by the Pitzer equations.