Reducing the competition: A dual-purpose ionic liquid for the extraction of gallium from Iron Chloride solutions

Article abstract of DOI:10.3390/molecules25184047

The separation of gallium from iron by solvent extraction from chloride media is challenging because the anionic chloridometalates, FeCl₄^? and GaCl₄^?, display similar chemical properties. However, we report here that the selective separation of gallium from iron in HCl solution can be achieved using the dual-purpose ionic liquid methyltrioctylammonium iodide in a solvent extraction process. In this case, the reduction of Fe³⁺ to Fe²⁺ by the iodide counterion was found to inhibit Fe transport, facilitating quantitative Ga extraction by the ionic liquid with minimal Fe extraction from 2 M HCl.

Full text of DOI:10.3390/molecules25184047

molecules
Communication
Reducing the Competition: A Dual-Purpose Ionic
Liquid for the Extraction of Gallium from Iron
Chloride Solutions
Luke M. M. Kinsman 1, Carole A. Morrison 1, Bryne T. Ngwenya 2 and Jason B. Love 1,
*
1
EaStCHEM School of Chemistry, University of Edinburgh, Edinburgh EH9 3FJ, UK;
L.M.M.Kinsman@sms.ed.ac.uk (L.M.M.K.); Carole.Morrison@ed.ac.uk (C.A.M.)
School of Geosciences, University of Edinburgh, Edinburgh EH9 3FE, UK; Bryne.Ngwenya@ed.ac.uk
2
*
Correspondence: Jason.Love@ed.ac.uk; Tel.: +44-131-650-4762
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Academic Editors: Magdalena Regel-Rosocka and Isabelle Billard
Received: 10 August 2020; Accepted: 3 September 2020; Published: 4 September 2020
Abstract: The separation of gallium from iron by solvent extraction from chloride media is challenging
−
because the anionic chloridometalates, FeCl4 and GaCl4−, display similar chemical properties.
However, we report here that the selective separation of gallium from iron in HCl solution can be
achieved using the dual-purpose ionic liquid methyltrioctylammonium iodide in a solvent extraction
process. In this case, the reduction of Fe3+ to Fe2+ by the iodide counterion was found to inhibit Fe
transport, facilitating quantitative Ga extraction by the ionic liquid with minimal Fe extraction from
2 M HCl.
Keywords: ionic liquid; solvent extraction; sustainability; secondary resources; recycling; NMR
spectroscopy; UV-Vis spectrophotometry
1. Introduction
Gallium is an important component in materials used in modern electronic devices such as
light-emitting diodes (LEDs) and solar panels and is also exploited in biomedical, pharmaceutical,
and radiopharmaceutical applications, owing to the similar chemical properties of Ga3+ and Fe3+
cations [
by-product of bauxite and zinc ore processing [
a critical element, and so its recovery from alternative sources such as coal fly-ash, mine tailings,
or electronic waste is important [ ]. In these cases, however, the presence of iron poses challenging
1,
2]. There are no abundant natural sources of gallium; instead, it is primarily extracted as a
3,4]. Due to its limited supply in nature, it is considered
3,5,6
selectivity issues in its separation, for example, by solvent extraction.
Ionic liquids (ILs) are an increasingly established class of extractant that are used either neat or
diluted in a hydrophobic solvent to extract various metal ions from aqueous solutions [
trioctylammonium chloride ([TOAH][Cl]) and methyltrioctylammonium chloride ([MTOA][Cl]) have
been widely reported as reagents for the recovery of gallium and iron by solvent extraction [ 11].
7,8]. ILs such as
9
–
Phase transport is achieved through the formation of charge-neutral supramolecular assemblies such
as [MTOA][GaCl4], with GaCl4− formed under high chloride conditions in the aqueous phase. Most
industrial solvent extraction processes operate under chloride, sulfate, or nitrate conditions [7]. As such,
solvent extraction processes that feature metalate transport largely exploit chloride media to generate
chloridometalates, although processes using other aqueous halides as counterions for ILs have been
reported [12]. ILs have also been used in the direct recovery of metals from secondary sources by
selective metal dissolution [13]. In this case, a trihalide IL provided an oxidizing agent to dissolve
the metal and a cation or additional complexing agent. However, current approaches using ILs do
not address the challenges in selectivity for Fe and Ga. Under high chloride concentrations, both of
Molecules 2020, 25, 4047; doi:10.3390/molecules25184047
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these metals exist as the tetrahedral metalates, [FeCl4]− and [GaCl4]−, for which current outer-sphere,
cationic receptors do not discriminate [1,2].
Recently, the selective recovery of Ga from iron mine tailings was reported [14]. Leaching of
the metals using 8 M HCl generated a mixture of Ga and Fe chlorides which were separated by
reducing Fe3+ to Fe2+ using SnCl2. The iron was not extracted easily at this oxidation state, whereas
the monoanion [GaCl4]− was extracted with tributylphosphate (TBP, 10% in benzene), albeit with
20–30% co-extraction of iron.
In this work, we report a novel combination of the selective reduction of Fe3+ and recovery of Ga3+
by solvent extraction using the dual-purpose IL methyltrioctylammonium iodide ([MTOA][I]). Mass
spectrometry, NMR spectroscopy, and UV-Vis spectrophotometry confirm that the iodide functions as
a reducing agent for Fe3+ and that the hydrophobic quaternary ammonium group forms a stable ion
pair with [GaCl4]− in the organic phase, facilitating phase transport and separation in one step.
2. Results and Discussion
The transport of gallium into a toluene solution of the iodide IL [MTOA][I] from a binary equimolar
mixture of 0.01 M FeCl3 and GaCl3 in varying concentrations of hydrochloric acid solutions was tested
and shows excellent selectivity for gallium between 1 and 4 M HCl (Figure 1). The amount of iron
extracted increases markedly as the concentration of HCl increases above 3 M, likely due to a greater
proportion of Fe existing as [FeCl4]− in solution. In contrast, similar experiments under the same
conditions using the chloride IL [MTOA][Cl] show that both iron and gallium are efficiently extracted
at concentrations greater than 1 M HCl (Figure 1). Gallium is readily stripped from the organic phase
by a fresh aqueous phase of water, whereas <5% is stripped using 2 M HCl.
Figure 1. Percentage of gallium and iron extracted by [MTOA][I] at varying [HCl]. Interpolation
used to aid the eye only. Experiments were performed in duplicate, and the results are reported as
an average.
The nature of the gallium species extracted under these conditions was probed by 71Ga NMR
spectroscopy (Figure 2). After contact of a GaCl3 solution in 2 M HCl with either 0.1 M [MTOA][Cl]
or [MTOA][I] in toluene, a single resonance is seen at 250 ppm for both organic phases, consistent
with the formation of the [GaCl4]− anion [15]. In contrast, the 71Ga NMR spectra of aqueous solutions
of GaCl3 in 0 to 7 M HCl show a peak at 0.0 ppm assigned to the hexahydrate [Ga(H2O)6][Cl3] [15].
The metalate, [GaCl4]−, is only observed in aqueous solutions above 8 M HCl, upon which the 71Ga
resonance shifts to 250 ppm. As the metalate is not initially present at 2 M HCl, it is likely that formation
of the chlorogallate [GaCl4]− occurs at the interface between the two phases, which favors the assembly
of a stable ion pair with the quaternary ammonium cation in the organic phase.

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Figure 2. Top: 71Ga NMR spectrum of 0.1 M [MTOA][I] or [MTOA][Cl] in toluene after contact with
0.01 M GaCl3 dissolved in 2 M HCl. Middle: 71Ga NMR spectrum of 0.01 M GaCl3 dissolved in 8 M
HCl. Bottom: 71Ga NMR spectrum of 0.01 M GaCl3 dissolved in 2 M HCl.
Electrospray ionization mass spectrometry (ESI-MS) was also used to probe the organic phase
speciation of extracted Ga and Fe solutions (Supplementary Materials Figures S1 and S2). Organic
phases resulting from extractions from 2 M HCl show no evidence of mixed halometalates such as
[GaCl3I]− or [FeCl3I]−, and instead the anions [FeCl4]– and [GaCl4]− are seen in the negative-ion
spectrum, while ([MTOA]2[GaCl4])+ is one of the dominant molecular ions in the positive-ion spectrum.
The absence of anions such as [GaCl3I]− in the negative-ion mass spectrum suggests that [GaCl4]− is
initially formed in the aqueous phase (or at the interface) prior to transport across to the organic phase,
as opposed to the transport of the neutral complex GaCl3 with subsequent metalate formation in the
organic phase due to the presence of I− ions.
Slope analysis (Log D vs. Log [L]) of the extracted Ga species analyzed for extractant concentrations
of 0.001 to 0.25 M at 2 M HCl gives an L:Ga ratio of approximately 1 (Figure 3) and suggests that the
simple ion pair [MTOA][GaCl4] is present in the organic phase. In addition, there is no correlation
between Ga transport and [H+] concentration from slope analysis (Log D vs. Log [H+]) of varying
[H+] concentrations of 0.01 to 1 M at 2 M NaCl (Figure S3), ruling out transport of neutral species such
as HGaCl4 and therefore confirming an ion-exchange extraction mechanism between I− and [GaCl4]−.
Evidence for reduction of Fe3+ by I− during extractions is apparent, as the color of the organic
phase changes from bright yellow to deep red, and the initially yellow aqueous phase turns colorless.
The deep red color of the organic phase is consistent with the presence of the triiodide anion, I3−, and is
supported by the appearance of the absorption at 375 nm in the UV-Vis spectrum of the metal-loaded
organic phase (Figure 4a) [16]. The UV-Vis spectrum of the Fe3+ aqueous phase at 2 M HCl prior to
contact with [MTOA][I] shows two absorption maxima at 220 and 336 nm which are consistent with
the presence of FeCl2+ (Figure 4b) [17
,18]. After contact with [MTOA][I], the UV-Vis spectrum of the
aqueous phase bleaches and shows only one absorption maximum at 225 nm due to the presence
of aqueous Fe2+ (Figure 4b); Fe(II) chloridometalates are unlikely to be present, as high chloride
concentrations are needed for their formation, and therefore no extraction of Fe(II) complexes is seen
using the anion extractant [MTOA][I] [19,20].

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Figure 3. Slope analysis for the transport of Ga by [MTOA][I]. Conditions: GaCl3 (0.01 M) in HCl (2.0 M,
2 mL), contacted with [MTOA][I] (0.001 to 0.25 M) in toluene (2 mL) for 1 h at RT with magnetic stirring.
Figure 4. UV-Vis spectra. (a) Toluene solutions of [MTOA][I] before (black) and after (red) contact with
FeCl3. Each solution diluted 500
after (red) contact with 0.1 M [MTOA][I] in toluene. Each solution diluted 100× in 2 M HCl.
×
in toluene. ( ) The 2 M HCl solutions of FeCl3 before (black) and
b
3. Materials and Methods
Unless otherwise stated, all solvents and reagents were purchased from Sigma-Aldrich, Fisher
Scientific UK, Alfa Aesar (Heysham, UK), Acros Organics (Geel, Belgium), or VWR International
(Lutterworth, UK) and used without further purification. Deionized water was produced using a
1
Milli-Q purification system. H and 13C NMR Spectra were recorded on a Bruker AVA500 spectrometer
operating at 500.12 and 125.76 MHz for 1H and 13C, respectively. 71Ga NMR spectra were recorded on
a Bruker PRO500 spectrometer at 152.55 MHz.
3.1. General Solvent Extraction Procedure
Aqueous solutions of FeCl3 and GaCl3 (0.01 M) in 0–12 M HCl (2 mL) were contacted with a toluene
organic phase (2 mL) containing either methyltrioc◦tylammonium iodide or methyltrioctylammonium
chloride (0.1 M) and stirred (1 h, 1000 rpm, 25 C). The phases were then separated physically,
and samples were from each taken and diluted with 1-methoxy-2-propanol for ICP-OES analysis.
Samples from relevant phases were also taken for UV-VIS, NMR, and ESI-MS analysis as required.

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3.2. Synthesis of Methyltrioctylammonium Iodide ([MTOA][I])
Following a standard preparation [21], iodomethane (4.44 g, 31 mmol) was added dropwise to
a stirred solution of trioctylamine (8.85 g, 25 mmol) in THF (100 mL), and the mixture was stirred
at 40 ◦C for 12 h under a flow of N2. The crude mixture was concentrated under vacuum to yield a
viscous orange oil (100%) and diluted with toluene to form a 1.0 M stock solution.
1H NMR (500 MHz, CDCl3): δH 3.41–3.35 (m, 6H, NC
H
2), 3.24 (s, 3H, NC
H3), 1.74–1.64 (m, 6H,
C
H
2), 1.43–1.23 (m, 30H, C 2), 0.89 (t, J = 6.8 Hz, 9H, CH2CH
H
3). 13C{1H} NMR (126 MHz, CDCl3): δC
61.70, 48.83, 31.62, 29.10, 29.01, 26.30, 22.57, 22.43, 14.04.
4. Conclusions
It is clear from this work that the quaternary ammonium salt, [MTOA][I], functions as a novel
dual-purpose reductant and extractant, efficiently and selectively separating Ga3+ from Fe3+ in a single
step between 1–4 M HCl by reduction of Fe3+ to Fe2+ and transport of Ga as its metalate [GaCl4]− by
anion exchange. This process is operationally simple, featuring low-cost, readily available reagents.
Excellent separation of Ga from Fe occurs under low to moderate HCl concentrations and eliminates
the need for external reducing agents such as Fe powder or SnCl2. Back extraction (stripping) of
gallium from the organic phase occurs readily with water. In principle, the organic phase could be
regenerated by contact with aqueous potassium iodide and a mild reducing agent, such as sodium
thiosulfate [22].
Supplementary Materials: The following supplementary materials are available online. Figure S1: Negative ion
ESI-MS of [MTOA][I] in toluene after contact with FeCl3 and GaCl3 in 2 M HCl. Figure S2: Positive ion ESI-MS of
[MTOA][I] in toluene after contact with FeCl3 and GaCl3 in 2 M HCl. Figure S3: Slope analysis for the transport of
Ga by [MTOA][I] with varying [H+].
Author Contributions: Conceptualization, L.M.M.K. and J.B.L.; methodology and investigation, L.M.M.K.; data
analysis, L.M.M.K., C.A.M., B.T.N., and J.B.L.; resources, C.A.M., B.T.N., and J.B.L.; writing—original draft
preparation, L.M.M.K.; writing—review and editing, L.M.M.K., C.A.M., B.T.N., and J.B.L.; supervision, C.A.M.,
B.T.N., and J.B.L.; project administration, J.B.L.; funding acquisition, C.A.M., B.T.N., and J.B.L. All authors have
read and agreed to the published version of the manuscript.
Funding: This research was funded by the University of Edinburgh and the Natural Environment Research
Council E3-DTP, grant number NE/L002558/1 (PhD studentship, L.M.M.K.).
Acknowledgments: We thank Jamie P. Hunter for providing initial samples of methyltrioctylammonium iodide
and his valuable comments.
Conflicts of Interest: There are no conflict to declare.
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