Sequestering ability of polyaminopolycarboxylic ligands towards dioxouranium(VI) cation

Abstract

In the present paper, some results of an investigation (at t = 25 °C by potentiometry, ISE-H + glass electrode) on the sequestering ability of five different polyaminopolycarboxylic ligands [Nitrilotriacetate (NTA), ethylenediamine- N,N,N′ ,N′-tetraacetate (EDTA), ethylene glycol-bis(2-aminoethylether)- N,N,N′ ,N′-tetraacetate (EGTA), diethylenetriamine- N,N,N′ ,N″ ,N″-pentaacetate (DTPA), triethylenetetraamine- N,N,N′ ,N″ ,N′′′ ,N′′′-hexaacetate (TTHA)] towards dioxouranium(VI) cation in sodium chloride aqueous solutions, at I = 0.7 mol L −1 are reported. Calculations performed on potentiometric data gave evidence of the formation of the following species (log β in parenthesis): UO 2(NTA)H 0 (12.27); UO 2(NTA) − (8.21); UO 2(NTA)OH 2− (2.39); UO 2(EDTA)H − (15.19); UO 2(EDTA) 2− (9.81); UO 2(EDTA)OH 3− (3.58); UO 2(EGTA)H − (17.35); UO 2(EGTA) 2− (11.60); UO 2(EGTA)OH 3− (2.77); UO 2(DTPA)H 2 − (22.14); UO 2(DTPA)H 2− (17.86); UO 2(DTPA) 3− (11.79); UO 2(DTPA)OH 4− (3.09); UO 2(TTHA)H 3 − (28.41); UO 2(TTHA)H 2 2− (24.73); UO 2(TTHA)H 3− (19.14); UO 2(TTHA) 4− (12.12). In presence of complexones, the formation of scarcely soluble species occurs at higher pH values than in simple UO 2 2+ aqueous solutions. The stability of dioxouranium(VI)–polyaminopolycarboxylate species follows the trend: TTHA > DTPA > EGTA > EDTA > NTA, it is dependent on the ligand structure and this dependence was modeled. Polyaminopolycarboxylate ligands are good sequestering agents towards UO 2 2+, indicating the possibility of being employed in the UO 2 2+ polluted sites remediation, and in the chelating therapy in medicine. A literature data critical evaluation was made, evidencing the high discrepancies occurring between different authors in both the stability constant values and the proposed speciation models.