Alternative Extraction Systems for Precious Metals Recovery: Aqueous Biphasic Systems, Ionic Liquids, Deep Eutectic Solvents

Abstract

The current trend in the development of separation methodologies implies their evolution in an environmentally friendly perspective, more precisely, the transition to techniques, materials, and solvents that could be qualified as greener alternatives to conventional ones. The green extraction systems can be attributed to aqueous biphasic systems, ionic liquids, and deep eutectic solvents, which have been widely used recently for various analytical, synthetic, and industrial tasks. In this chapter, the features of the listed systems are discussed in relation to the extraction of precious metals, mainly platinum, palladium, and gold; the examples of the alternative extraction systems for separation and preconcentration of precious metals are reviewed.

1. Introduction

Platinum, palladium along with their satellites (platinum group metals, PGMs), and gold are essential and often irreplaceable in many areas of science and industry, including rapidly developing fields of electronics, automobile and engineering industry, medicine, and so on. Going forward, there is a high potential for new PGMs’ use in energy transition applications [1, 2, 3]. The limited natural reserves and the steadily growing demand for precious metals dictate an improvement of technologies for processing the primary raw materials and especially secondary resources, namely spent automotive catalysts and electronic wastes [4, 5, 6, 7]. At the same time, selective recovery of precious metals is also an important analytical problem, because it represents a crucial step in their accurate determination. This problem is relevant for searching and evaluation of new deposits and study of alternative sources, as well as for obtaining data on monitoring PGMs in the environment and biological fluids [8].

Various methods for processing platinum-containing materials and recovery of precious metals based on co-precipitation [9, 10], solvent extraction [11], sorption [12], electrodeposition [13], molecular recognition [14], and so forth are explored. The vector of progress of the mentioned methods, coinciding with that for chemical and process industries as a whole, is aimed at finding tools that align with the principles of green and white chemistry and the interests of sustainable development [15, 16, 17]. From this standpoint, utilization of alternative solvents and extraction systems of new generation is of great scientific and practical importance. As such systems, aqueous biphasic systems, supramolecular solvents, supercritical fluids, and the so-called designer compounds—ionic liquids and deep eutectic solvents—are currently considered. Some of them are addressed in this chapter relating to precious metals extraction for both technological and analytical applications.

2. Aqueous biphasic systems (aqueous two-phase systems)

Phase-forming components of aqueous biphasic systems (ABSs) are water-soluble, nontoxic, biocompatible, available, and produced in large quantities. Recently, along with traditional polymer-salt and polymer-polymer systems, ABSs produced on the basis of non-polymeric compounds, such as organic salts, in particular ionic liquids, hydrophilic solvents (acetonitrile, alcohols), and so on, have been proposed [18, 19].

Technologies for producing and purifying enzymes and other biomolecules using ABSs based on biodegradable polymers have been developed and successfully introduced into industry [20, 21, 22]. Studies on the extraction of metals, mainly nonferrous, radioactive, transplutonium, were first described in the 80s of the last century [23].

The data on extraction of precious metals in ABSs are limited to only a few publications (see Table 1). One of the first works by Bulgariu [24] described the extraction of gold(III) using an ABS prepared by mixing polyethylene glycol PEG-1500 and (NH₄)₂SO₄ aqueous solutions, including in the presence of chloride ion. It has been shown that Au(III) is almost quantitatively (> 98%) extracted into the polymer-rich phase at pH ≤ 3.0 and the concentration of chloride ion >0.08 mol L⁻¹. The developed method of gold extraction was tested on the example of electronic wastes containing Cu(II), Co(II), Ni(II), Zn(II), Fe(III), and Pb(II) ions.

Metals	ABSs composition	Extraction conditions	Reference
Au(III)	PEG-1500 − (NH4)2SO4	[Au] = 10^−2 M, pH ≤ 3, [Cl^−] > 0.08 M	[24]
Pd(II)	PEG-1500 − (NH4)2SO4	[Pd] = 0.02–0.2 mg mL^−1, [H^+] = 0.02–0.2 M, [Cl^−] = 0.25·10^−3–4 M	[25]
Pd(II)	PEG-1500 − Na2SO4	[Pd] = 0.2–1 мМ, 0.1 M HCl/(0.05 M H2SO4 + 0.1 M NaCl)	[26]
Au(III)	L64 − Li2SO4	[Au] = 0.8–21 mg kg^−1, (HCl + HNO3)	[27]
Au(III)	PEG-6000 − [C6MIm][C12SO3]	[Au] = 0.48 g L^−1, pH = 1.13–1.90	[28]
Pd(II)	PEG-2000 − K2HPO4; [CnMIm]Br, n = 4,6,8	[Pd] = 200–1200 mg L^−1, pH ≤ 1	[29]
Pd(II), Pt(IV)	PEG-1500 − (NH4)2SO4	[Pd] = 0.7–1.5 g L^−1, [Pt] = 0.1–0.7 g L^−1, (0.1 M HCl + 10–90 g L^−1 NaCl), RCC*	[30]

Table 1.

The examples of ABSs application for extraction of precious metals.

^*

RCC: rotating coiled column.

Simonova and coauthors studied the extraction of palladium(II) chloride complexes in an ABS (PEG-1500, PEG-115) − NaCl − (NH₄)₂SO₄ − H₂O [25]. The experimental results have shown that palladium is extracted into the organic phase by the solvation mechanism as a compound H₂[PdCl₄]·yPEG·nH₂O. Hyphenated methods of spectrophotometric determination of palladium(II) and iridium(IV) and their separation from rhodium(III) and ruthenium(III) have been developed [31].

The effect of acid and chloride ion concentrations on phase equilibria of an ABS PEG-1500 − Na₂SO₄ − H₂O and the partition of palladium were investigated by Milevskiy et al. [26]. The maximum distribution coefficients of palladium were achieved under extraction in ABSs PEG-1500 − Na₂SO₄ − 0.1 M HCl and PEG-1500 − Na₂SO₄ − (0.05 M H₂SO₄ + 0.1 M NaCl).

A method of gold extraction from scrap central processing units with an ABS based on L64 triblock copolymer, lithium sulfate, and matrix ions of the leachate solutions has been proposed [27]. Gold is quantitatively extracted in the macromolecular-rich top phase without the use of any auxiliary extractants; copper is extracted as a by-product.

Extraction of gold(III) from acidic solutions in an ABS based on PEG-6000 and imidazolium ionic liquid was carried out [28]. For this purpose, a new ionic liquid, 1-hexyl-3-methylimidazolium dodecyl sulfonate ([C₆mim][C₁₂SO₃]) was synthesized. Under optimal conditions, the degree of gold extraction was 97%.

Tang et al. [29] studied the extraction behavior of palladium(II) from hydrochloric acid solution using a typical polymer-salt ABS with an ionic liquid as a functional additive. It was shown that extraction system based on PEG-2000 and K₂HPO₄ with imidazolium ionic liquid allowed to recover 96–99% of Pd(II) from acidic medium.

The method of extraction and separation of palladium(II) and platinum(IV) in an ABS based on PEG-1500 and ammonium sulfate from model technological solutions under dynamic conditions was developed [30]. In this case, the traditional ABS was firstly applied for extraction in a rotating coiled column (an analog of centrifugal extractor), where a polymer-rich phase is retained as a stationary phase without any solid support. A multistage extraction could be realized in the system according to the countercurrent chromatography theory [30].

3. Ionic liquids

A large number of reviews and experimental original works are devoted to ionic liquids (ILs). With ever increasing interest to fundamental and applied chemistry of ILs, the field shows no signs of slowing down. These compounds, represented as ionically bonded organic cation and organic or inorganic anion, are being defined as molten salts with melting points below 100°C. Due to their low volatility, thermal stability, and high solubility, ILs are considered to be a safer alternative to traditional molecular organic solvents [32, 33]. At the same time, compliance with the requirements for “green” solvents, namely, nontoxicity, stability, renewability, atomic efficiency, and so on, is not always feasible in the case of ILs and is determined by their composition [34, 35].

In the last two decades, studies on the use of ILs in respect to the problem of PGMs and gold separation have been actively carried out. Various ILs classified by cationic groups—ammonium, imidazolium, pyridinium, phosphonium, guanidium, and betaine—are described as exhibiting affinity and selectivity to precious metals. Examples of extraction systems based on ILs for the extraction of precious metals are quite numerous. Several comprehensive reviews on the subject have been recently published by Lee [36], Firmansyah [37], and Lanaridi [38]. The influence of the composition of cations, anions, and organic solvents on the selectivity and completeness of extraction of the target metals, as well as possible extraction mechanisms depending on the forms of metal presence in solutions, are considered in detail.

Therefore, this chapter will cover just some recent examples of the following applications of ILs:

• as extractants;

• as solvents;

• synergetic mixtures of ILs;

• task-specific ionic liquids;

• as a phase-forming component or as an additive in aqueous biphasic systems.

3.1 ILs as extractants

A piperazine-based IL 1-(2-(dimethylamino) ethyl)-4-methyl-piperazin bis(trifluoromethylsulfonyl)imide ([C₆-Et-TMEDA-PIP][Tf₂N]₂) with two functional groups was synthesized to construct an extraction system for Au(III) and Pt(IV) recovery [39]. The anion exchange mechanism was verified through the combination of methods. The stripping procedure is carried out using H₂C₂O₄ and CS(NH₂)₂-HCl solutions for gold and platinum, respectively.

A commercially available quaternary ammonium salt Aliquate 336 has been used in a series of studies by Binnemans and coauthors dealing with different extraction approaches for precious metals separation [40, 41, 42]. Undiluted IL in the original chloride form [A336][Cl] or its substituted bromide form [A336][Br] enables separation of gold and palladium from base metals under nonequilibrium conditions in a milliflow set-up operating in the slug flow regime [41]. Another technique is a split-anion extraction with [A336][X] (X− = Br − and I−) developed for the separation of precious metals from aqueous chloride media. Ammonia solution, sodium thiosulfate, and thiourea were used for the selective stripping of Pd(II), Au(III), and Pt(IV), respectively, from loaded [A336][I] phase [42].

Deng et al. proposed a microemulsion extraction technique for palladium recovery from alkaline cyanide solutions using imidazolium ILs [43]. 1-butyl-3-undecyl imidazolium bromide ([BUIm]Br) was used as an extractant in a mixture of n-pentanol and n-heptane. Under optimal conditions, Pd(II) quantitatively transfers to the organic phase along with Fe(III) and Co(III) ions. The metals are separated through a two-step stripping procedure.

Quaternary ammonium chloride pseudo-protic ILs (PPILs) generated from the reaction of a primary, secondary, and tertiary amines and dissolved in toluene were used for gold(III) extraction from HCl media [44]. The metal could be conveniently precipitated as zero-valent gold nanoparticles after its stripping by sodium thiocyanate solution.

IL-based thermomorphic systems have been studied by Wang and coauthors for precious metals separation [45, 46]. UCST (upper critical solution temperature)-type IL 1,4,7-trimethyltriazonane bis-(trifuloromethanesulfonyl) amide ([1,4,7-TMTA][Tf₂N]) saturated with water was used for homogenous liquid-liquid extraction of Au(III), Pd(II), and Pt(IV) at elevated temperature (40–65°C) [45]. Ethyl chloroacetate N,N,N′,N′-tetramethyl-ethylenediamine-based IL [EA-TMEDA][Tf₂N]₂ with temperature-responsive behavior revealed the high selectivity toward gold ions in the hydrochloric acid multicomponent solutions [46].

It should be noted that phosphonium-based ILs remain the most widely explored ILs in solvent extraction of platinum metals. Cyphos IL 101 has remarkable extraction ability to Pt(IV) and Pd(II) ions in a wide range of HCl concentrations and can be used for their recovery from various chloride-based leach solutions [47, 48, 49]. Other Cyphos ILs are full described in the above-afforded reviews [4, 5, 11, 36, 37, 38].

3.2 ILs as solvents

To separate PGMs from simulated high-level liquid waste (HLLW), a novel system containing N,N′-dimethyl-N,N′-di-(2-phenylethyl)-thiodiglycolamide (MPE-TDGA) as extractant and 1-butyl-3-methyl-imidazolium nonafluorobutansulfonate ([Bmim][NfO]) as a solvent was proposed [50]. The system allowed the rapid and selective extraction of Pd(II), as well as Ru(III) and Rh(III). These extractions were accelerated by increasing the temperature (50°C).

3.3 Synergetic mixtures of ILs

Under carefully selected experimental conditions, the distribution coefficients of some metals in systems containing a mixture of two extractants are much higher than the additive distribution coefficient of individual extractants. This phenomenon, called synergism, is due to, as it is supposed, different types of interactions between two extractants and/or the formation of metal-containing compounds of a special composition that differs from the composition of compounds in systems with one type extractant.

A synergistic effect in the extraction of precious metals is manifested when using the mixtures of hydrophilic and hydrophobic ILs.

Chen et al. developed extraction-electrodeposition method for platinum recovery from the multicomponent solutions using the mixture of ILs [C₁₄PIm][Br]/[C₈MIm][PF₆] (1-tetradecyl-3-propylimidazolium bromide/1-octyl-3-methylimidazolium hexafluorophosphate) [51]. Pt(IV) ions were selectively separated from metal solutions containing Rh³⁺, Fe³⁺, Ni²⁺, Cu²⁺, and Zn²⁺ metal ions, followed by direct electrodeposition as Pt(0) on the copper cathode.

Betaine-based IL [C₆Bet]Br was firstly applied to the separation of platinum metals [52]. [C₆Bet]Br showed remarkable extractability for Pt(IV) and Ir(IV) in the presence of 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C₆MIm][NTf₂] as the hydrophobic phase. Ir(IV) was separated from Pt(IV) by reduction process with the use of hydroxylamine hydrochloride (NH₂OH·HCl).

The synergetic mixture of the imidazolium-based ILs [C₆MIm]Cl, [C₆MIm][NTf₂], and [C₆MIm][DDTC] (1-octyl-3-methylimidazolium chloride, bis[(trifluoromethyl)sulfonyl]imide and diethyldithiocarbamate, respectively) was offered for the extraction of Pt(IV), Pd(II), Ru(III), and Rh(III) [53]. Separation of these four PGMs can be realized by changing the composition of the synergetic mixture and concentrations of ILs.

3.4 Task-specific ionic liquids (TSILs)

These have functional group(s) involving donor atom(s) to selectively extract the targeted metal ions through metal complex formation.

A pyridinium-based TSIL with a monothioether group, 3-thiapentylpyridinium bis(trifluoromethylsulfonyl)imide [3-TPPy][NTf₂], was prepared for the extraction of typical class b metal ions, including precious metal ions in high selectivity [54].

A novel extraction-photocatalysis method was developed to recover Pd from HCl leaching solutions using [C₅MIm][DDTC] (1-pentyl-3-methylimidazolium diethyldithiocarbamate) as extractant [55]. The extraction was considered due to Pd-S coordination. Direct stripping of the metal can be accomplished through photocatalytic reduction process.

The TSILs bearing one or two tetrahydropyran-2H-yl(THP)-protected thiols were designed as palladium extractants from an aqueous phase to [C₄MIm][NTf₂] [56]. Such a kind of synergetic mixture allows to selectively bind Pd(II) ions from 4 M HCl in the presence of Pt(IV).

2-Mercaptobenzothiazole-functionalized IL ([C₆mim][2MBT]) in combination with the nonionic surfactant (TX-114) was used for palladium separation via cloud point extraction [57]. The obtained results showed a strong coordination of palladium with anion of the IL.

An urea-based imidazolium IL, 1-butyl-3-{3-(3-methyl-2H-imidazol-1-yl)propyl}urea bis(trifluoromethylsulfonyl)imide ([C₄UC₃mim][Tf₂N]), was synthesized and studied for Pt(IV), Pd(II) and Rh(III) separation [58]. Based on experimental data, the anion exchange mechanism was supposed for the extraction of Pt(IV) at pH = 1.13 and the inner-sphere coordination for Pd(II) extraction at pH = 5.45.

3.5 ILs as a phase-forming component or as an additive in aqueous biphasic systems

An IL-based ABSs can overcome the limitations of the use of ILs associated with their high viscosity and hydrophobicity [59]. A novel type of acidic aqueous biphasic systems (AcABS) based on an inorganic acid and an IL has been recently gaining interest for the extraction of metal ions as a promising alternative to conventional extraction systems [60]. The AcABS composed from hydrochloric acid with high concentrations and phosphonium-based IL ([P₄₄₄₁₄][Cl]) is characterized by thermotropic behavior and can be successfully applied to the separation of some strategic metals, including Pt(IV) [60].

3.6 Other application of ILs

Solid-phase extraction and membrane separation have such advantages as lower consumption of IL and its improved stability. Both technologies are considered as promising alternatives to traditional solvent extraction and deserve a separate chapter. Here are just a few references on the use of ILs as functionalizing modifiers of various solid carriers: resins, membranes, and nanoparticles [61, 62, 63, 64, 65, 66].

One of the recent areas of research should also be noted. It is related to the leaching of platinum-containing materials using ILs. This approach is intended as an alternative to the refining processes, where strong acids with oxidants (aqua regia, concentrated HCl in the presence of chlorine gas or H₂O₂) are usually used to dissolve chemically inert platinum metals. Thus, a nonaqueous direct leaching process based on trihexyl(tetradecyl)phosphonium chloride (P₆₆₆₁₄Cl) in combination with methanesulfonic acid and trichloroisocyanuric acid was proposed to solubilize metallic platinum [67].

4. Deep eutectic solvents

Since the first publication about 20 years ago by Abbott [68], deep eutectic solvents (DESs) have evolved from a subclass of ionic liquids to a wide range of liquids that have applications in many fields, including analytical chemistry and separation techniques. The original description of DES as a mixture of a salt of a quaternary ammonium base (a hydrogen bond acceptor, HBA) and a hydrogen bond donor (HBD) has been recently extended to a combination of Lewis and Bronsted acids and bases with much lower melting temperatures of the resulting mixture compared to the that of the original components [69].

DESs have a number of advantages: easy and convenient preparation, non-toxicity, non-volatility, non-combustibility, and biodegradability, which allow these compounds to be classified as environmentally benign [19, 70]. As in the case of ILs, due to the variability of constituent components, DESs are referred to designer solvents. In the emerging era of the transfer to active use of artificial intelligence, the selection of a certain composition of DESs using neural networks seems to be an extremely interesting and promising area of research [71, 72].

Publications on precious metal extraction by DESs are summarized in Table 2, focusing on the selected DES composition and extraction conditions. One of the pioneering works on DES application in the separation of precious metals appeared in 2019 and was devoted to the extraction of gold. Geng et al. studied the DESs based on quaternary ammonium salts for this purpose [73]. The hydrophobic DESs with [N₃₃₃₃]Br, [N₄₄₄₄]Br and [N₈₈₈₁]Br as HBAs and N-hexanoic acid as HBD (1:1 molar ratio) were screened out for gold recovery from hydrochloric acid solutions with various acidity and salinity. Based on combination of UV–Vis and FT-IR spectroscopy data, the anion exchange mechanism was proposed: anions [C₅H₁₃-COOH⋯Br]⁻ are exchanged with AuCl₄⁻ during the extraction process. The outcomes under different conditions showed that N₈₈₈₁Br has the best gold extraction ability and tolerance to higher salinity. NaBH₄ solution (0.1 mg/L) was used as a stripping agent. The gold extraction ability of DESs was preserved during 5 cycles of extraction.

Metal	DES composition	Extraction conditions	Stripping/determination conditions	Reference
Au(III)	HBA: N8881Br, HBD: N-hexanoic acid	pH = 2, 1 mL of 1 mM DES, t = 30 min, Sample V = 1 mL, C0 = 0.2–0.6 mM (39–118 μg·mL^−1)	0.1 g·mL^−1 NaBF4, V = 1 mL	[73]
Au(III)	HBA: choline chloride, HBD: phenol	pH = 6, 1 mL of 0.05% sodium diethyl dithiocarbamate, 500 μL of DES, t = 15 s, V = 10 mL, C0 = 500 μg·L^−1; 0.5 mL of THF as emulsifier	FAAS determination in 100 μL of DES, LOD = 5.1 μg·L^−1	[74]
Pd(II)	HBA: choline chloride, HBD: phenol	pH = 5, 400 μL of 0.1% HMBATSC*, 500 μL of DES, V = 35 mL, C0 = 10 μg·L^−1; 0.8 mL of THF as emulsifier	FAAS determination in 100 μL of DES, LOD = 4.0 μg·L^−1	[75]
Pd(II)	HBA: hydroxyl ammonium chloride, HBD: phenol, and disodium 4,5-dihydroxy-1,3-benzenedisulfonate; FeCl3	pH = 6, 100 μL of DES, t = 1 min, V = 10–35 mL	FAAS determination in DES diluted by 0.3 mL of conc. HNO3, LOD = 1.18 μg·L^−1	[76]
Pd(II)	HBA: DL-menthol, HBD: phenyl salicylate (salol)	pH = 4, 1.5·10^−5 M PAN, 60 μL of DES, T = 65°C, t = 1 min, V = 10 mL	ETAAS determination in 20 μL of DES, LOD = 0.03 μg·L^−1	[77]
Pd(II)	HBA: N8881Cl, HBD: ethylene glycol, glycerol, 1-hexanol, C2H5COOH, C5H11OOH	0.1–0.3 M HCl, m = 0.05 g, V = 5 mL, t = 20 min, C0 = 60–350 μg·mL^−1	80 wt% N2H4·H2O, V = 0.5 mL	[78]
Pd(II), Pt(IV)	HBA: N8888Br, HBD: octanoic acid	0.01–6 M HCl, m = 0.05–1.5 g, V = 1.5–10 mL, t = 10 min, C0 = 5–10 μg·mL^−1	0.25 M NH4OH (Pd), 2 M HNO3 (Pt)	[79]
Pt(IV)	HBA: TOPO, HBD: 1-butanol	1–4 M HCl, m = 0.042 g, V = 3 mL, t = 60 min, C0 = 682–1092 μg·mL^−1	0.5 M NaOH, V = 3 mL	[80]
Pt(IV)	HBA: TOPO, HBD: thymol	2 M HCl, O/A = 1 (vol/vol, total volume 2.0 mL), t = 1 min, C0 = 390 μg·mL^−1	0.1 M thiourea in 0.5 M HCl, V = 1 mL	[81]

Table 2.

The examples of DES application for the extraction of precious metals.

^*

HMBATSC – 2-hydroxy-3-methoxy benzaldehyde thiosemicarbazone.

Choline chloride: phenol mixture in 1:2 molar ratio was introduced by Yilmaz et al. for gold extraction [74]. The goal of the work was to develop a sensitive method for determination of gold traces in plating bath solutions. The combined approach was used to achieve this objective: liquid phase microextraction by DES in the presence of a complexing agent and detection by flame atomic absorption spectrometry with a slotted quartz tube (SQT-FAAS). Additionally, the conventional parameters of the extraction efficiency were studied as a function of the mixing mode. Out of hand mixing, mechanical shaking, ultrasonic bath, and vortexing, the latter one was selected as the most efficient. For better phase separation, tetrahydrofuran was used for the emulsification of DES.

A similar approach and a DES of the same type were used by Panhwar et al. for palladium extraction coupled with FAAS determination in environmental samples [75]. Choline chloride:phenol mixture in 1:4 molar ratio, 2-hydroxy-3-methoxy benzaldehyde thiosemicarbazone as a complexing agent, and tetrahydrofuran as an emulsifier were dispersed in sample solution by 8 repeated cycles of quick uptaking and discharging with the use of a syringe. Selectivity of Pd(II) extraction was evaluated in the presence of Co(II), Cu(II), Cd(II), Ni(II), Mn(II), Zn(II), Pb(II), and Cr(III) ions.

Four-component FeCl₃-based DES was applied by ALOthman et al. for dispersive microextraction of palladium followed by FAAS determination [76]. To prepare the DES disodium 4,5-dihydroxy-1,3-benzenedisulfonate, hydroxylammonium chloride, iron(III) chloride and phenol were mixed in the ratio of 1:1:2:1. In addition to typical hydrogen-bonding interactions, the presence of FeCl₃ in the formulation leads to coordination interactions with oxygen atoms of donor ligands ensuring the formation of a liquid. The developed procedure does not require a complexing agent; palladium recovery is quantitative and tolerant to the presence of common matrix ions.

Another kind of DES was proposed for palladium extraction by Abdi. et al. [77]. DL-menthol was mixed with phenyl salicylate (1:1), and the prepared DES was dispersed in the sample solution together with 1-(2-pyridylazo)-2-naphtol complexing agent. The extraction procedure was carried out at an elevated temperature (65°C), when the homogeneous solution was formed. The extremely low limit of detection could be achieved using the ETAAS determination method.

Abovementioned long-chain quaternary ammonium salt was investigated by Tang. et al. for the extraction of palladium from acid solutions [78]. DES was constructed by combining N₈₈₈₁Cl with saturated fatty acids or fatty alcohols in molar ratio 1:1 and 1:2. An anion exchange mechanism of extraction was confirmed by FTIR, UV-Vis, and 1H NMR analysis. Authors proposed stripping of palladium from the DES phase via the hydrazine hydrate reduction method; the regenerated DES was used repeatedly for Pd(II) extraction for 5 cycles.

Extraction of the whole group of platinum group metals from acid solutions with high amount of chloride-ions and matrix components was studied using DESs based on tetraoctylammonium bromide (N₈₈₈₈Br) and carboxylic acids [79]. The scheme of separation of Pd(II) and Pt(IV) as well as rare platinum metals was proposed for processing technological solutions to obtain individual fractions of precious metals with purity of >99.9%.

Trioctylphosphine oxide (TOPO) as HBA was mixed with various HBD reagents by Liu et al. to construct the extraction systems for Pt(IV) recovery from secondary resources [80]. TOPO-1-butanol, TOPO-L-menthol, and TOPO-1-hexanol were recommended for Pt(IV) extraction under conditions of high acidity and salinity. The ion-association mechanism of extraction was elucidated: each of the two protonated P=O groups in the TOPO molecule combined with PtCl₆²⁻ during the process of extraction. The TOPO-1-butanol possessed the best Pt(IV) extraction ability, selectivity, and cycling extraction (using NaOH as a stripping agent).

One of the most critical and comprehensive study was carried out by Vargas, Schaeffer, and their colleagues [81, 82]. The work provides the features of Pt(IV) and Pd(II) extraction using TOPO-based DES in comparison with an equivalent extractant system in organic diluent. The conclusions of this work are quite important and deserve to be summarized here [81, 82]:

• on a molecular level, in the case of TOPO and diluent organic system, less molecules of extractant are required to form metal complexes in comparing with TOPO-decanoic acid DES;

• there is the competitive intermolecular hydrogen bonding between the DES components and TOPO⋯H⁺ adduct formation, which precedes the metal extraction; the higher HBD concentration in the DES, the higher HCl concentration, at which metal extraction occurs;

• due to extensively developed hydrogen bonded network, the DES extraction system does not form the third phase even at 8 M HCl in contrast to extractant in diluents;

• Pt(IV)/Pd(II) separation factor could be tuned by HBD selection, for example, at lower HCl concentrations in the TOPO-thymol system Pd(II) extraction is inhibited, while Pt(IV) extraction remains unaffected.

The reviewed publications demonstrate mainly the possibility of using DESs in analytical methods for the determination of three metals: gold, platinum, and palladium. The components of the studied DESs used as HBAs, namely the quaternary ammonium salts and TOPO, have been well studied in the traditional extraction of platinum metals. If the use of DESs is to be positioned as a transition to more environmentally friendly technologies, then DESs based on nontoxic compounds like choline chloride and menthol should be given a preference. However, in the case of such “green” components of the DESs, a complex-forming agent needs to be added to the extraction system [74, 75]. In order not to complicate the system and the subsequent detection stage, the design of DESs using components, which are able to complex-forming or specific interaction with platinum metals for their selective recovery, is a subject of a scientific interest. The recent work, authored by Liu, has been devoted to Au(III), Pd(II), and Pt(IV) extraction using the eutectic mixture of natural components: lidocaine and thymol [83]. Thus, the search for safe and efficient eutectic extraction systems for precious metals can be expected to continue.

5. Conclusion

The imperative need to improve the existing and create new schemes for separation of PGMs and gold from complex matrices concerns both technological processes and the analytical control of the metals contents at all production stages. In many cases, selective separation of precious metals is necessary, and this is the most difficult stage of processing platinum-containing materials owing to their multicomponent composition, specific chemistry of PGMs, and their extremely low concentrations. Conventional leaching and extraction methods for the separation of precious metals involve the use of aggressive reagents and toxic solvents and are characterized by high energy consumption. The presented review confirms the relevance of fundamental and applied research in the use of alternative solvents for separation of these critical elements. At the moment, the processing of technological wastes, spent catalysts, and other secondary resources is of priority importance because of their large quantities and depletion of natural resources. In this regard, the problem of precious metals production, on the one hand, becomes a global challenge and, on the other hand, requires innovative technological solutions for countries that have primary raw materials and operate outdated approaches and materials.

Applications of ABSs, ILs, and DESs in precious metals’ recycling benefit from their excellent solubility properties, extraction and electrochemical activities, and eco-friendly portfolio. Further studies in this field are likely to focus on the search for green selective components with high affinity to precious metals, especially to rare platinum metals (rhodium, iridium, and ruthenium), including the computational research and design of extraction systems, combining extraction and leaching procedures with subsequent determination, or recovery of precious metals from leachate using simple and sustainable approaches.

Acknowledgments

This work was supported by the Ministry of Science and High Education of the Russian Federation [GEOKHI RAS].