Sequestering Ability of Dicarboxylic Ligands Towards Dioxouranium(VI) in NaCl and KNO3 Aqueous Solutions at T=298.15 K

摘要

The formation constants of dioxouranium(VI)-2,2'-oxydiacetic acid (diglycolic acid, ODA) and 3,6,9-trioxaundecanedioic acid (diethylenetrioxydiacetic acid, TODA) complexes were determined in NaCl (0.1 <= I <= 1.0 mol.L-1) and KNO3 (I = 0.1 mol.L-1) aqueous solutions at T = 298.15 K by ISE-[H+] glass electrode potentiometry and visible spectrophotometry. Quite different speciation models were obtained for the systems investigated, namely: ML0, MLOH-, ML22-, M2L2(OH)(-), and M2L2(OH)(2)(2-), for the dioxouranium(VI)-ODA system, and ML0, MLH+, and MLOH- for the dioxouranium(VI)TODA system (M = UO22+ and L = ODA or TODA), respectively. The dependence on ionic strength of the protonation constants of ODA and TODA and of both metal-ligand complexes was investigated using the SIT (Specific Ion Interaction Theory) approach. Formation constants at infinite dilution are [for the generic equilibrium pUO(2)(2+) + q(L2-)+ rH(+) reversible arrow (UO22+)(p)(L)(q) H-r((2p-2q+ r)); beta(pqr)]: log(10) beta(110) = 6.146, log(10) beta(11-1) = 0.196, log(10) beta(120) = 8.360, log(10) beta(22-1) = 8.966, log(10) beta(22-2) = 3.529, for the dioxouranium(VI)-ODA system and log beta(110) = 3.636, log(10) beta(111) = 6.650, log(10) beta(11-1) = -1.242 for dioxouranium(VI)-TODA system. The influence of etheric oxygen(s) on the interaction towards the metal ion was discussed, and this effect was quantified by means of a sigmoid Boltzman type equation that allows definition of a quantitative parameter (pL(50)) that expresses the sequestering capacity of ODA and TODA towards UO22+; a comparison with other dicarboxylates was made. A visible absorption spectrum for each complex reaching a significant percentage of formation in solution (KNO3 medium) has been calculated to better characterize the compounds found by pH-metric refinement.