The role of carboxylic acids in TALSQuEAK separations

• Published in: Journal of coordination chemistry 2012-08, Vol.65 (16), p.2862-2876
• Main Authors: Braley, J.C., Carter, J.C., Sinkov, S.I., Nash, K.L., Lumetta, G.J.
• Format: Article
• Language: English
• Subjects: Acronyms; ACTINIDES; CARBOXYLIC ACIDS; CHEMISTRY; ESTERS; HEDTA; HEH[EHP]; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; KINETICS; LACTATES; PHOSPHONIC ACIDS; PHOSPHORIC ACID; RARE EARTHS; Separations; SPECTROPHOTOMETRY; TALSPEAK; TALSPEAK, TALSQueak, HEH[EHP], HEDTA, Separations; TALSQuEAK; THERMODYNAMICS
• ISSN: 0095-8972; 1029-0389
• DOI: 10.1080/00958972.2012.704551

Recent reports have indicated that Trivalent Actinide-Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes (TALSPEAK)-type separations chemistry can be improved through the replacement of bis-2-ethyl(hexyl) phosphoric acid (HDEHP) and diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) with the weaker reagents 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA), respectively. This modified TALSPEAK has been provided with an adjusted acronym of TALSQuEAK (Trivalent Actinide-Lanthanide Separation using Quicker Extractants and Aqueous Komplexes). Among several benefits, TALSQuEAK chemistry provides more rapid phase transfer kinetics, is less reliant on carboxylic acids to mediate lanthanide extraction, and allows a simplified thermodynamic description of the separations process that generally requires only parameters available in the literature to describe metal transfer. This article focuses on the role of carboxylic acids in aqueous ternary (M-HEDTA-carboxylate) complexes, americium/lanthanide separations, and extraction kinetics. Spectrophotometry (UV-Vis) of the Nd 3+ hypersensitive band indicates the presence of aqueous ternary Nd-Lac-HEDTA species (Lac = lactate, K 111  = 1.83 ± 0.01 at 1.0 mol L −1 ionic strength, Nd(HEDTA) + Lac −  ⇄ Nd(HEDTA)Lac − ). While lower levels (0.1 mol L −1 vs. 1.0 mol L −1 ) of carboxylic acid will still be necessary to control pH and encourage phase transfer of the heavier lanthanides, application of different carboxylic acids does not have an overwhelming impact on Ln/Am separations or extraction kinetics relative to conventional TALSPEAK separations. TALSQuEAK separations come to equilibrium in two to five minutes depending on the system pH using only 0.1 mol L −1 total lactate or citrate.