Separation and Recycling for Rare Earth Elements by Homogeneous Liquid-Liquid Extraction (HoLLE) Using a pH-Responsive Fluorine-Based Surfactant
A selective separation and recycling system for metal ions was developed by homogeneous liquid-liquid extraction (HoLLE) using a fluorosurfactant. Sixty-two different elemental ions (e.g., Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Hg, Ho, In, Ir, La, Lu, Mg, Mn, Mo, Nb, Nd, Ni, Os, P, Pb, Pd, Pr, Pt, Re, Rh, Ru, Sb, Sc, Se, Si, Sm, Sn, Sr, Ta, Tb, Te, Ti, Tl, Tm, V, W, Y, Yb, Zn, and Zr) were examined. By changing pH from a neutral or alkaline solution (pH ≥ 6.5) to that of an acidic solution (pH 90% efficiency into a sedimented Zonyl FSA® (CF3(CF2)n(CH2)2S(CH2)2COOH, n = 6–8) liquid phase. Moreover, all rare earth elements were obtained with superior extraction and stripping percentages. In the recycling of rare earth elements, the sedimented phase was maintained using a filter along with a mixed solution of THF and 1 M sodium hydroxide aqueous solution. The Zonyl FSA® was filtrated and the rare earth elements were recovered on the filter as a hydroxide. Furthermore, the filtrated Zonyl FSA was reusable by conditioning the subject pH.
Recovery of rare earth elements with ionic liquids
Rare earth elements (REEs) are in high demand as critical materials in modern green and other advanced technologies. This review focuses on the recovery of REEs from waste with ionic liquids (ILs) as green dilutants and extractants to tackle the REE supply challenge. Extraction mechanisms in different extraction systems, including pure ILs, non-functional ILs with extractants, and functional ILs, are discussed in depth. The IL-based extraction process is different from the traditional molecular solvent-based extraction process. In addition, stripping is crucial to recovering ILs, and supercritical CO2 (Sc-CO2) is introduced to recover REE complexes from the IL phase. Also, Sc-CO2 can accelerate the whole extraction process by enhancing mass transfer. The challenge is to understand the extraction mechanism of the IL/Sc-CO2 system to advance the green recovery of REEs from waste.
Homogeneous liquid-liquid extraction followed by X-ray fluorescence spectrometry of a microdroplet on filter-paper for the simultaneous determination of small amounts of metals
Homogeneous liquid–liquid extraction with 
perfluorooctanate ion (PFOA–) was utilized for the 
preconcentration of certain types of metal chelates, such as 
diethyldithiocarbamate, 1,10-phenanthroline and desferrioxamine B. Under 
the experimental conditions {i.e., 
[PFOA−]T = 6.0 × 10−3 
mol l−1, [acetone]T = 2.0 vol%, 
[HCl]T=0.72 mol l−1, room temperature}, 
the maximum concentration factor was 500-fold. A method was then developed 
to determine those metal chelates deposited in a microdroplet on 
filter-paper using X-ray fluorescence spectrometry. As a result, when three 
types of chelating agent were used together in the process, 15 metal ions 
[Ti(IV), Zr(IV), V(V), Nb(V), 
Ta(V), W(VI), Fe(II), Co(II), 
Ni(II), Pd(II), Cu(II), 
Au(III), Hg(II), Bi(III) and 
Se(IV)] were simultaneously determined. The recovery of each 
metal was ca. 83–100%, except for W(VI) which was 
60.2%. The calibration curves for each metal ion were linear over the range 
from 5.0 × 10−7 to 1.0 × 10−5 
mol l−1. The detection limits were at the 
10−8 mol l−1 level and the relative 
standard deviations were below 5%.