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DOI: 10.1039/C5CC06595J
Journal Name
COMMUNICATION
Separation of cobalt and nickel with a thermomorphic
ionic-liquid-based aqueous biphasic system
Bieke Onghena,a Tomas Opsomera and Koen Binnemansa*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The [P44414][Cl]−NaCl−H2O ionic liquid-based aqueous biphasic generally do not show thermomorphic behavior with water and
system shows promising results for the separation of cobalt(II) and hence cannot be used in HLLE.13, 20-28 Furthermore, they have a
nickel(II) with homogeneous liquid-liquid extraction. The extracting high viscosity, which makes their application to industrial
phase exists of a hydrophilic ionic liquid that is salted-out by solvent extraction less straightforward.
sodium chloride, indicating that there is no need for using In this Communication, the assets of both HLLE and fluorine-
hydrophobic ionic liquids.
free ILs for the separation of metal ions are combined by using
thermomorphic ionic liquid-salt aqueous biphasic systems (IL-
ABS) in HLLE. An IL-ABS usually consists of three components:
(1) water; (2) a hydrophilic IL, preferably with fluorine-free
cations; and (3) a salting-out agent, usually an inorganic salt.
The salting-out agent causes the IL to phase separate from the
mixture and a biphasic system is created.17-19 Phase separation
is thus a composition-induced process (CIPS, composition-
induced phase separation). At certain mass compositions, many
of the IL-ABS can also undergo temperature-induced phase
separation (TIPS), thus they can be considered thermomorphic
and applicable to HLLE.17,20,21 Despite the advantages, few
examples of the use of IL-ABS systems for the extraction of
metal ions have been reported in the literature. Furthermore,
the thermomorphic behavior has not been exploited yet.18,22-25
Here, the concept of using IL-ABS-HLLE for the separation of
metals was applied to the separation of cobalt(II) and nickel(II)
using the hydrophilic IL tributyl(tetradecyl)phosphonium
chloride, [P44414][Cl], combined with NaCl as the salting-out
agent. NaCl was selected for several reasons: (1) NaCl is cheap
and easily available with a low impact on the environment; (2)
it is a relatively good salting-out agent; and (3) the addition of
chloride ions enhances extraction of anionic metal chloro
complexes to the IL phase.26,27 Extraction with [P44414][Cl]
behaves similar to Cyphos IL 101 since both ILs are anionic
extractants, which means that they only extract metal ions in
their anionic form, such as anionic tetrachloro cobaltate(II).27,28
To examine the possibility of using [P44414][Cl] in an IL-ABS-HLLE
system, the phase behavior and phase separation were first
investigated as a function of the temperature and the NaCl
concentration. By cloud point titration at room temperature,
the concentration of NaCl required to induce phase separation
was determined for different IL concentrations (expressed in
Recent advances in sustainable hydrometallurgical processing
of metals focus on the use of hydrophobic ionic liquids (ILs) for
separations of metal ions by solvent extraction.1-4 Ionic liquids
are solvents that consist entirely of ions. Because of their low
flammability and low volatility, ILs are considered to be safer
and more environmentally friendly than the traditional diluents
that are currently used in industry, such as kerosene.4-7 The
main disadvantage of using ILs in solvent extraction is their high
viscosity, which slows down the mass transfer during extraction
and complicates the implementation of the IL solvent extraction
processes on an industrial scale.8 Recently, our group applied
the concept of homogeneous liquid-liquid extraction (HLLE) to
ionic liquids as an answer to the slow mass transfer caused by
the high viscosities of ILs. HLLE makes use of thermomorphic ILs,
i.e. ILs that show temperature-dependent phase behavior in
contact with water.9-11 Reversible conversion of the biphasic
mixture to
a homogeneous mixture by changing the
temperature, causes the extraction equilibrium to be reached
faster than by strong agitation of a biphasic mixture. Until now,
mainly fluorine-containing ILs were successfully applied to the
HLLE of metal ions. 12-14 On the other hand, ILs with fluorine-free
anions offer several advantages such as a lower price and less
issues with toxicity and persistency in natural environments.15,16
Fluorine-free
hydrophobic
ILs,
such
as
trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101), have
already been used in solvent extraction processes, but these ILs
a.KU Leuven, Department of Chemistry, Celestijnenlaan 200F PO box 2404, 3001
Heverlee (Leuven), Belgium. Email: Koen.Binnemans@chem.kuleuven.be
Electronic Supplementary Information (ESI) available: Experimental information,
orthogonal ternary phase diagram of the [P44414][Cl]−NaCl−H2O system and
quantitative 1NMR spectrum. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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