Reducing the Competition: A Dual-Purpose Ionic Liquid for the Extraction of Gallium from Iron Chloride Solutions

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.


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 Ga 3+ and Fe 3+ cations [1,2]. There are no abundant natural sources of gallium; instead, it is primarily extracted as a by-product of bauxite and zinc ore processing [3,4]. Due to its limited supply in nature, it is considered a critical element, and so its recovery from alternative sources such as coal fly-ash, mine tailings, or electronic waste is important [3,5,6]. In these cases, however, the presence of iron poses challenging 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 [7,8]. ILs such as 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 [9][10][11]. Phase transport is achieved through the formation of charge-neutral supramolecular assemblies such as [MTOA][GaCl 4 ], with GaCl 4 − 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  [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 Fe 3+ to Fe 2+ using SnCl 2 . The iron was not extracted easily at this oxidation state, whereas the monoanion [GaCl 4 ] − 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 Fe 3+ and recovery of Ga 3+ 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 Fe 3+ and that the hydrophobic quaternary ammonium group forms a stable ion pair with [GaCl 4 ] − in the organic phase, facilitating phase transport and separation in one step.

Results and Discussion
The  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.
Molecules 2020, 25, x FOR PEER REVIEW 2 of 7 both of these metals exist as the tetrahedral metalates, [FeCl4] − and [GaCl4] − , for which current outersphere, 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 Fe 3+ to Fe 2+ 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 Fe 3+ and recovery of Ga 3+ 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 Fe 3+ 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.

Results and Discussion
The  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.  [15]. In contrast, the 71 Ga 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 71 Ga 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.   Evidence for reduction of Fe 3+ 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 metalloaded organic phase (Figure 4a) [16].    Evidence for reduction of Fe 3+ 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 metalloaded organic phase (Figure 4a) [16]. The UV-Vis spectrum of the Fe 3+ 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 FeCl 2+ (Figure 4b) [17,18].  [19,20].

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 Milli-Q purification system. 1 H and 13 C NMR Spectra were recorded on a Bruker AVA500 spectrometer operating at 500.12 and 125.76 MHz for 1 H and 13 C, respectively. 71 Ga NMR spectra were recorded on a Bruker PRO500 spectrometer at 152.55 MHz.

General Solvent Extraction Procedure
Aqueous solutions of FeCl 3 and GaCl 3 (0.01 M) in 0-12 M HCl (2 mL) were contacted with a toluene organic phase (2 mL) containing either methyltrioctylammonium 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.

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 N 2 . 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. 1

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 Ga 3+ from Fe 3+ in a single step between 1-4 M HCl by reduction of Fe 3+ to Fe 2+ and transport of Ga as its metalate [GaCl 4 ] − 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 SnCl 2 . 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].