Separation of lutetium using a novel bifunctional ionic liquid based on phosphonate functionalization

Abstract

By functionalizing the cation of ionic liquid (IL), a novel bifunctional ionic liquid [trioctyl(2-(diethoxyphosphoryl)ethyl)ammonium][bis(2,4,4 trimethylpentyl)phosphinate] ([N888DOPE][C272]) was developed to separate Lutetium (Lu) from mixed solution of Thulium (Tm), Ytterbium (Yb), Lutetium (Lu). Extraction behaviors of the IL in chlorinated medium were investigated by changing parameters including initial aqueous pH, equilibrium time, temperature, diluent, etc. The maximum load capacity of IL to Lu(III) reaches 2.55 g L−1 for the distribution ratio of Lu(III) and the separation coefficient of Lu/Yb were 18.56 and 1.36, respectively, which surpassed that of [A336][C272]. It can be proved that the extraction ability, maximum load capacity and selectivity of [N888DOPE][C272] for Lu(III) were better than unphosphonate-functionalized ionic liquid. The thermodynamic parameters calculated by equilibrium extractant concentration suggest the extraction process to be spontaneous and endothermic. 0.5 mol L−1 of HCl was able to almost completely strip Lu(III) from the organic phase, and IL remained stable after six cycles. The analysis of infrared spectra reveals the extraction mechanism of [N888DOPE][C272] as an ion association, i.e. the extraction of Lu(III) could be achieved by the combined action of anion and cation in chlorinated medium. To determine the stoichiometry of extraction process, a simple and effective method was used to improve the common slope analysis method. This could provide some guidance for investigations on extraction mechanism.