Highly Efficient Extraction Separation of Lanthanides Using a Diglycolamic Acid Extractant

Abstract

Liquid-liquid extraction of lanthanide ions (Ln 3+ ) using N,N-dioctyldiglycolamic acid (DODGAA) was comprehensively investigated, together with fluorescence spectroscopic characterization of the resulting extracted complexes in the organic phase. DODGAA enables the quantitative partitioning of all Ln 3+ ions from moderately acidic solutions, while showing selectivity for heavier lanthanides, and provides remarkably high extraction separation performance for Ln 3+ compared with typical carboxylic acid extractants. Furthermore, the mutual separation abilities of DODGAA for light lanthanides are higher than those of organophosphorus extractants. Slope analysis, loading tests, and electrospray ionization mass spectrometry measurements demonstrated that the transfer of Ln 3+ with DODGAA proceeded through a proton-exchange reaction, forming a 1:3 complex, Ln(DODGAA)3. The stripping of Ln 3+ from the extracting phase was successfully achieved under acidic conditions. Time-resolved laser-induced fluorescence spectroscopy revealed that the extracted Eu 3+ ions were completely dehydrated by complexation with DODGAA.

Comprehensive Extraction Study Using N,N-Dioctyldiglycolamic Acid

“…In a previous study, 12 we determined the pKa of DODGAA in H2O-C2H5OH (1:1, v/v) by potentiometric titration because DODGAA is insoluble in water. In this study, water-soluble DEDGAA was used to determine the pKa of the DGAA framework in water.…”

“…7,8,12 The analogous ligand DEDGAA was synthesized as follows: diglycolic anhydride (5.0 g, 41 mmol) was dispersed in acetonitrile (15 mL) and diethylamine (3.5 g, 47 mmol) dissolved in acetonitrile (10 mL) was slowly added dropwise to the solution in an ice-bath. The mixture was stirred in the ice-bath for 2 h, followed by stirring at room temperature overnight.…”

“…12,18 The obtained titration curves were analyzed using a Bjerrum plot, which provides an excellent means for obtaining the conditional acid dissociation constants (Ka) from titration data. The ordinate, nH, was the average number of H + ions bound to a weak acid, and is defined as (moles of bound H + )/ (total moles of weak acid).…”

“…This is an advantage compared to neutral extractants because metal transfer can be simply controlled by the pH of the aqueous solution, and the process involves no anionic species. In previous studies, we reported that DODGAA provided better extraction performance and excellent separation ability for lanthanides 7,8,12 and lead ions 11 compared with conventional commercial extractants. DODGAA is a novel extractant, and therefore it is important to evaluate its chemical properties and extraction performance for other metal ions in an effort to expand its application range.…”

“…1), which is an acidic tridentate ligand with a carbamoyl group and a carboxy group connected by an ether chain. [7][8][9][10][11][12] DODGAA can be synthesized simply and readily in a single step, which helps to reduce its production cost. Furthermore, this extractant only contains C, H, O and N atoms, and can potentially be used in residual-waste-free extraction processes because of complete decomposition by incineration to gaseous products (CHON principle).…”