Reactive extraction of europium(III) and neodymium(III) by carboxylic acid modified calixarene derivatives: Equilibrium, thermodynamics and kinetics

In this paper, extraction behaviors of different carboxylic acid modified calixarenes towards Eu(III) and Nd(III) were performed. These extractants were extensively evaluated for the extraction abilities for extraction of Eu(III) and Nd(III) under different conditions, including pH, concentration of extractants and temperature. Carboxylic acid modified calix[6]arene (extractant-2) showed remarkably high extraction ability for Eu(III) and Nd(III). The thermodynamic parameters (enthalpy, entropy and Gibbs free energy) were determined. The extraction of Eu(III) with extractant-2 occurred with a cation exchange reaction and this extraction reaction is an exothermic process. Kinetic studies of reactive extraction of Eu(III) with extractant-2 were performed in a stirred cell. The value of mass-transfer coefficient of Eu(III) in chloroform was evaluated to be 3.35 × 10⁻⁶ m s⁻¹. The Hatta number was calculated and observed to vary in the range of 3.80 × 10⁻⁴–9.10 × 10⁻⁴. The effects of stirring speed, the phase volume ratio and species concentration on the initial extraction rate and Hatta number, demonstrate that the reaction is a very slow chemical reaction process occurring at the bulk of the organic phase. The rate constants of forward and backward reactions were determined to be 3.05 × 10⁻² (kmol m⁻³)^−0.64 s⁻¹ and 1.55 × 10⁻³ (kmol m⁻³)^−2.64 s⁻¹, respectively.