Complexation of Uranium(VI) and Samarium(III) with Oxydiacetic Acid:  Temperature Effect and Coordination Modes

Abstract

The complexation of uranium(VI) and samarium(III) with oxydiacetate (ODA) in 1.05 mol kg(-1) NaClO(4) is studied at variable temperatures (25-70 degrees C). Three U(VI)/ODA complexes (UO(2)L, UO(2)L(2)(2-), and UO(2)HL(2)(-)) and three Sm(III)/ODA complexes (SmL(j)((3-2)(j)+) with j = 1, 2, 3) are identified in this temperature range. The formation constants and the molar enthalpies of complexation are determined by potentiometry and calorimetry. The complexation of uranium(VI) and samarium(III) with oxydiacetate becomes more endothermic at higher temperatures. However, the complexes become stronger due to increasingly more positive entropy of complexation at higher temperatures that exceeds the increase in the enthalpy of complexation. The values of the heat capacity of complexation (Delta C(p) degrees in J K(-1) mol(-1)) are 95 +/- 6, 297 +/- 14, and 162 +/- 19 for UO(2)L, UO(2)L(2)(2-), and UO(2)HL(2)(-), and 142 +/- 6, 198 +/- 14, and 157 +/- 19 for SmL(+), SmL(2)(-), and SmL(3)(3-), respectively. The thermodynamic parameters, in conjunction with the structural information from spectroscopy, help to identify the coordination modes in the uranium oxydiacetate complexes. The effect of temperature on the thermodynamics of the complexation is discussed in terms of the electrostatic model and the change in the solvent structure.

Complexation of Uranium(VI) with Thiodiacetic Acid in Solution at 10–85 °C

“…The enhancement of complexation at elevated temperatures is consistent with the predictions by a simple electrostatic model that was developed to interpret the effect of temperature on the formation of lanthanide and actinide carboxylate complexes. [6][7][8][9]14] …”

“…Many ligands could affect the behavior of actinides in liquid nuclear wastes by forming complexes. We have studied a series of mono-and dicarboxylic acids at elevated temperatures, including acetic, [4][5][6][7] malonic, [8] and oxydiacetic [9] acids. Some of the ligands exist in the nuclear wastes as degradation products of more complex organic compounds whose complexation with actinides has a direct impact on the behavior of actinides in waste storage and processing.…”

Complexation of thorium(IV) with malonate at variable temperatures

“…As the temperature is elevated, the complexation becomes more endothermic, but stronger due to the increasingly more positive entropy at higher temperatures. These results are discussed in terms of the temperature effect on the structure of water and its solvation of actinide ions [1,2]. …”

Separation of actinides and lanthanides: crystal and molecular structures of N,N′-bis(3,5-di-t-butylsalicylidene)-4,5-dimethyl-1,2-phenylenediamine and its uranium complex

“…There has been great interest in the separation of actinides (Ac) and lanthanides (Ln) in solution to aid the safe management of nuclear waste [1][2][3]. Many Schiff-base ligands have been designed and prepared with the aim of discriminating between Ln and Ac [4][5][6][7].…”