Suitable polar solvents enhance metal separations in liquid-liquid extraction

Liquid-liquid extraction is a popular technique for separation and purification of metals. The less polar phase of liquid-liquid extraction systems has been studied extensively for improving metal separations, however, the role of the more polar phase has been overlooked for far too long. Zheng Li (SOLVOMET KU Leuven) and colleagues investigated the extraction of metals from a variety of polar solvents and discovered that, the influence of polar solvents on metal extraction is so significant that extraction of many metals can be largely tuned, and the metal separations can be significantly enhanced by selecting suitable polar solvents. The work has been published in Chem. Eur. J.

Tuning metal extraction using polar solvents

Extraction of Co(II), Ni(II) and Mn(II) by the same extractant (Alqiaut 336) from different polar solvents was found to vary significantly. The effect of polar solvents on the metal extraction is even larger than changing extractants from Aliquat 336 to Cyphos IL 101. Furhermore, polar solvents affect the extraction of different metals to differnet extents, which forms the basis for tuning metal separations by selecting suitable polar solvents. To validate this hypothesis, La(III)/Ni(II) (as chloride salts) were extracted by A336 from five selected polar solvents and the extraction equilibrium curves varied dramatically. Interestingly, Ni(II) was quantitatively extracted from N-methylformamide, while extraction of La(III) was negligible. The preferential extraction of Ni(II) over La(III) by Aliquat 336 is not possible by conventional liquid-liquid extraction using water as a polar solvent.

The effects of polar solvents

The mechanism of how polar solvents affect the extraction of metals was investigated comprehensively by both experimental characterisations and theoretical computations. Polar solvents affect metals extraction in two ways: (1) enhance the formation of chlorometallate complexes; (2) affect the speciation and relative solubility of species in the two phases. In principle, the extraction of metals by any extractant is subject to the influence of polar solvents because the effects of polar solvents take place in the more polar phase, independently of the extractants.   

Full reference paper
Zheng Li, Zidan Zhang, Simon Smolders, Xiaohua Li, Stijn Raiguel, Erik Nies, Dirk E. De Vos, and Koen Binnemans, Enhancing Metal Separations by Liquid–Liquid Extraction Using Polar Solvents, Chem. Eur. J. 2019, 25, 9197 – 9201, DOI: 10.1002/chem.201901800

Acknowledgements
The research was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme: Grant Agreement 694078–Solvometallurgy for critical metals (SOLCRIMET). S.S. and S.R. acknowledge FWO for an Aspirant Grant. The authors thank Mr. K. Duerinckx for recording 35Cl and 139La NMR spectra.

Find out more about Zheng Li in this interview.