Effect of temperature on the protonation of N-(2-hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid in aqueous solutions: Potentiometric and calorimetric studies

Journal Article · Mon Jun 01 00:00:00 EDT 2015 · Journal of Chemical Thermodynamics

DOI:https://doi.org/10.1016/j.jct.2014.12.026· OSTI ID:1177628

Li, Xingliang ^[1]; Zhang, Zhicheng ^[2]; Endrizzi, Francesco ^[2]; Martin, Leigh R. ^[3]; Luo, Shunzhong ^[4]; Rao, Linfeng ^[2]

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1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Div.; China Academy of Engineering Physics, Mianyang (China). Inst. of Nuclear Physics and Chemistry
2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Sciences Div.
3. Idaho National Lab. (INL), Idaho Falls, ID (United States)
4. China Academy of Engineering Physics, Mianyang (China). Inst. of Nuclear Physics and Chemistry

The TALSPEAK process (Trivalent Actinide Lanthanide Separations by Phosphorus-reagent Extraction from Aqueous Komplexes) has been demonstrated in several pilot-scale operations to be effective at separating trivalent actinides (An³⁺) from trivalent lanthanides (Ln³⁺). However, fundamental studies have revealed undesired aspects of TALSPEAK, such as the significant partitioning of Na⁺, lactic acid, and water into the organic phase, thermodynamically unpredictable pH dependence, and the slow extraction kinetics. In the modified TALSPEAK process, the combination of the aqueous holdback complexant HEDTA (N-(2-hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid) with the extractant HEH[EHP] (2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester) in the organic phase has been found to exhibit a nearly flat pH dependence between 2.5 and 4.5 and more rapid phase transfer kinetics for the heavier lanthanides. To help understand the speciation of Ln³⁺ and An³⁺ in the modified TALSPEAK, systematic studies are underway on the thermodynamics of major reactions in the HEDTA system under conditions relevant to the process (e.g., higher temperatures). Thermodynamics of the protonation and complexation of HEDTA with Ln³⁺ were studied at variable temperatures. Equilibrium constants and enthalpies were determined by a combination of techniques including potentiometry and calorimetry. This paper presents the protonation constants of HEDTA at T = (25 to 70) °C. The potentiometric titrations have demonstrated that, stepwise, the first two protonation constants decrease and the third one slightly increases with the increase of temperature. This trend is in good agreement with the enthalpy of protonation directly determined by calorimetry. The results of NMR analysis further confirm that the first two protonation reactions occur on the diamine nitrogen atoms, while the third protonation reaction occurs on the oxygen of a carboxylate group. These data, in conjunction with the thermodynamic parameters of Ln³⁺/An³⁺ complexes with HEDTA at different temperatures, will help to predict the speciation and temperature-dependent behavior of Ln³⁺/An³⁺ in the modified TALSPEAK process.

• Research Organization: Idaho National Lab. (INL), Idaho Falls, ID (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC07-05ID14517
• OSTI ID: 1177628
• Journal Information: Journal of Chemical Thermodynamics, Vol. 85, Issue C; ISSN 0021-9614
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
TALSPEAK PROCESS
HEDTA
Lanthanide Complexes
Actinide Complexes
TEMPERATURE DEPENDENCE
Temperature Range 0273-0400 K
PROTONS
TALSPEAK