Physicochemical properties of novel cholinium ionic liquids for the recovery of silver from nitrate media

Abstract

Cholinium based ionic liquids (IL), i.e. N-(2-hydroxyethyl)-N,N-dimethyl-N-octylammonium bis(trifluoromethanesulfonyl)imide ([C₈linChol]⁺[NTf₂]⁻) and N-(2-hydroxyethyl)-N-(2-ethylhexyl)-N,N-dimethylammonium bis(trifluoromethanesulfonyl)imide ([C₈ramChol]⁺[NTf₂]⁻) were synthesized and characterized by ¹H NMR, ¹³C NMR, ATR-FTIR and ESI-MS. Activation energy of the viscous flow, molar volume, molar entropy, lattice potential energy and isobaric expansion coefficients were deduced from density and viscosity measurements. These calculations led to the conclusion that neat [C₈ramChol]⁺[NTf₂]⁻ exhibits a slightly more compact ion arrangement than [C₈linChol]⁺[NTf₂]⁻. After characterizing physicochemical properties of these two neat ILs, extractive properties of [C₈linChol]⁺[NTf₂]⁻ and [C₈ramChol]⁺[NTf₂]⁻ saturated with water were also evaluated for silver recovery from aqueous nitrate solutions. Investigation of Ag(I) extraction as a function of pH showed a maximum of extraction efficiency at pH 5 (98.6%) for [C₈linChol]⁺[NTf₂]⁻ and good extraction selectivity of Ag(I) and Cu(II) towards Fe(III). High stripping efficiency was achieved by using 0.44 mol L⁻¹ nitric acid.