18-Crown-6 and Dibenzo-18-crown-6 Assisted Extraction of Cesium from Water into Room Temperature Ionic Liquids and Its Correlation with Stability Constants for Cesium Complexes

Andrey Grigoryevich Vendilo,Igor Vladimirovich Pletnev,Dmitry Ivanovich Djigailo,Irina Ivanovna Torocheshnikova,Svetlana Valeryevna Smirnova,Vladimir Georgyevich Krasovsky,Konstantin Ivanovich Popov

doi:10.3390/molecules14125001

Abstract

The pH-profiles of the extraction of Cs+ into four conventional (1-butyl-3- methylimidazolium hexafluorophosphate and bis[trifluoromethyl)sulphonyl]imides of 1- butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, and 1-(2-ethylhexyl)-3-methylimidazolium) and two novel (trioctylmethylammonium salicylate and tetrahexyl-ammonium dihexylsulfosuccinate) room temperature ionic liquids have been determined both in the absence and in the presence of crown ether (18-crown-6 or dibenzo-18-crown-6). The pH-profiles of distribution ratio of crown ethers have been established in the same conditions. The relationship of cesium extraction efficiency both with the stability of its complexes with crown ethers and crown ethers’ distribution ratio has been clarified.

Highlights

Hydrophobic crown ethers are applied as extractants in the separation and recovery of 137Cs andSr radioactive isotopes from high-level nuclear waste solutions, containing high concentrations of acids and salts, into molecular solvents [1,2,3,4,5]
In our recent communication [21] we have reported on the thermodynamics of complex formation of cesium ions with 18-crown-6 (18C6, L) in six hydrophobic Room-temperature ionic liquids (RTILs): trioctylmethylammonium salicylate ([TOMA][Sal]), tetrahexylammonium dihexylsulfosuccinate
The main reason is that crown-ether competes with a RTIL’s anion for cesium and, being better soluble in water, provides a partial transfer of the metal ion into an aqueous phase. This suggestion is indirectly supported by the low value of [Cs(18C6)]+ stability constant in [THA][DHSS], Table 4, which appears to be even lower than in water

Summary

Introduction

Hydrophobic crown ethers are applied as extractants in the separation and recovery of 137Cs and. In our recent communication [21] we have reported on the thermodynamics of complex formation of cesium ions with 18-crown-6 (18C6, L) in six hydrophobic RTILs (see Scheme 1): trioctylmethylammonium salicylate ([TOMA][Sal]), tetrahexylammonium dihexylsulfosuccinate ([THA][DHSS]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), 1-butyl-3methylimidazolium bis[trifluoromethyl)sulphonyl]imide ([BMIM][N(Tf)2]), 1-hexyl-3-methylimidazolium bis[trifluoromethyl)sulphonyl]imide ([HMIM][N(Tf)2]), and 1-(2-ethylhexyl)-3-methylimidazolium bis[trifluoromethyl)sulphonyl]imide ([EtHMIM][N(Tf)2]) by a 133Cs-NMR technique. The present work aims to study the extraction capability of cesium by the above mentioned RTILs in the presence of 18C6 and dibenzo-18-crown-6 (DB18C6, L) and to clarify its relationship with the stability of complexes formed in aqueous and organic phases. Another objective is to study the impact of crown ether’s nature on the extraction efficiency.

Results and Discussion

Experimental

Synthesis and characterization of the RTILs

Conclusions