Separation and recovery of rare earth elements using novel ammonium-based task-specific ionic liquids with bidentate and tridentate O-donor functional groups

Abstract

In the present work, two novel ammonium-based functional ionic liquids (FILs) with oxygen donating groups: trioctyl(2-ethoxy-2-oxoethyl)ammonium dihexyl diglycolamate, [OcGBOEt][DHDGA], and tricaprylmethylammonium dihexyl diglycolamate, [A336][DHDGA] were synthesized and tested for the recovery and separation of rare earth elements from aqueous solutions. The synthesized FILs were characterized using nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), high-resolution mass spectrometry (HRMS), thermal gravimetric analysis (TGA), disc scanning calorimetry (DSC) in addition to density and viscosity analysis. The extraction behavior of europium ions (Eu3+) in HNO3 solution was investigated in detail by changing key process parameters, including solution acidity, concentration of Eu3+ ions, extraction temperature, extraction time, and the type of organic diluent. Kinetic studies indicated that the extraction process was relatively fast with 97% of Eu3+ions recovered after 5 min using [OcGBOEt][DHDGA], whereas it took 15 min for [A336][DHDGA] system to reach 80% recovery. Extraction thermodynamics was evaluated by analyzing the effect of temperature on the extractability of Eu3+ ions in nitrate solution. Results indicated that the extraction reactions were favorable for both FILs. Back extraction studies indicated that ~99% of Eu3+ can be stripped off [OcGBOEt][DHDGA] and [A336][DHDGA] using 0.1 and 0.5 molL−1 HNO3, respectively.Separation efficiencies of rare earth ions, including La3+, Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Y3+, Er3+, and Lu3+ were also investigated to examine the selectivity of the synthesized FILs. Results showed that both FILs have significant affinity to heavy rare earth elements, however, the separation efficiency of [A336][DHDGA] was superior to that of [OcGBOEt][DHDGA].