Probing the coordination of europium(III) in a functionalized ionic liquid using luminescence spectroscopy

Abstract

In view of the ever increasing attention on the rare earth recovery by functionalized ionic liquids (FILs), an attempt was made using luminescence spectroscopy to understand the detailed extraction and complexation behavior of europium(III) using a renowned phosphonium based functionalized ionic liquid, trihexyltetradecylphoshonium bis(2,4,4-trimethylpentyl)phosphinate ([P66614][C16H34PO2]) dissolved in 1,3-diisopropyl benzene (DIPB). Europium(III) complexation with [P66614][C16H34PO2] was investigated by luminescence spectroscopy by changing various extraction parameters such as aqueous phase acidity, concentration of the ionic liquid extractant, initial Eu(III) concentration and initial nitrate ion concentration. The peak intensity decreases gradually with increasing the acidity of the aqueous phase from pH 3 to 0.5 M nitric acid and the peak shape for 5D0 → 7F2 transition changes completely at 0.5 M nitric acid. Increase in the concentration of the ionic liquid and the initial metal ion in the aqueous phase increases the peak intensity of the hypersensitive transition and hence the asymmetry ratio (AR). The advantages of [P66614][C16H34PO2] were explored by comparing the emission intensity of the extracted Eu(III) in [P66614][C16H34PO2] phase with that of the organic phase containing the mixture of the precursors used for the synthesis of FIL and organic phase containing Cyanex 272 dissolved in DIPB phase. The emission profiles of Eu(III) extracted in different organic phases follows the order: [P66614][C16H34PO2] > [P66614][NO3] + 0.05 M Cyanex 272 > Cyanex 272 irrespective of the aqueous phase nitric acid concentration. The nature of Eu(III) coordination during the complex formation was ascertained from the decay profiles of Eu(III) emission. The luminescence spectra of Eu(III) were recorded in chloride medium and the results were compared with the nitrate medium.