A mixed solvent system of amino crown ether and ionic liquids for a high-efficient extraction of cesium

Abstract

Efficient extraction of Cs⁺ from aqueous solution was investigated by liquid–liquid extraction based on ionic liquids (ILs). In this study, Di(aminobenzo)-18-crown-6 (DAB18C6) was used as the extractant and three ILs (1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim][PF₆]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([C₆mim][PF₆]), and 1-butyl-3-methylimidazolium bis-trifluoromethyl sulfonamide ([C₄mim][NTf₂])) were employed as co-extractants in the construction of a liquid–liquid extraction system for efficient and selective extraction of Cs⁺ from aqueous solutions. It was demonstrated that the DAB18C6-[C₄mim][NTf₂] mixed solvent system at 0.12 mol/L DAB18C6, a mass ratio of ILs/CHCl₃ of 1:4 and a volumetric ratio of O/A of 1:1 exhibited a high % extraction of Cs⁺ up to 99.94 %, with a higher selectivity for Cs⁺ relative to coexisting ions such as K⁺ (β_Cs/K = 1216.7) and Rb⁺ (β_Cs/Rb = 139.03). The high selectivity of the extraction system for Cs⁺ was attributed to the lower hydration binding energy of Cs⁺ (−366.51 kJ/mol) and the stronger interaction between DAB18C6 and Cs⁺, g(r) = 12.7. Further, the hydrogen bonding interaction between the amino crown ether and ionic liquid increases the viscosity and surface tension of the extraction system, thus enhancing the stability of the system. Noteworthy, the most stable structure formed by DAB18C6-Cs⁺-[NTF₂]⁻ is attributed to cation exchange. In summary, this study demonstrates the great potential for efficient separation and extraction of cesium from aqueous solutions.