Efficient Metal Extraction from Dilute Solutions: A Review of Novel Selective Separation Methods and Their Applications

Abstract

Notable increases in metal consumption and declining ore grades in recent decades have stressed the significance of dilute solutions as secondary sources of valuable metals. Moreover, environmental considerations and the imperative of sustainable development have further emphasized their treatment. Therefore, finding an efficient solution for separating metals from dilute solutions has attracted the attention of numerous researchers. This paper reviews the purification processes of dilute solutions and highlights key achievements of published research works. Although this study focuses on evaluating the efficiency of recently developed aqueous-phase purification methods, such as immobilized ligands, ionic liquids, and air-assisted solvent extraction, the application of conventional processes to treat these solutions, such as solvent extraction, ion exchange, membranes, chemical precipitation, and adsorption are also briefly outlined. To provide a comprehensive assessment, more than 200 research articles were reviewed, and their key findings are stated in this study. This research contributes to the advancement of knowledge of metal recovery from dilute solutions and sheds light on the dynamic evolution of this field.

1. Introduction

Dilute solutions, whether generated naturally in mines as acid mine drainage (AMD) [1,2] and acid rock drainage (ARD) [3,4] or produced from various industrial processes, represent a significant resource of valuable metals. The ever-growing demand for these metals has urged the extraction of low-grade ores with complex mineral compositions. Hydrometallurgical methods have emerged as an appropriate approach for recovering metals from such challenging ores. Notably, the pregnant leach solution (PLS) obtained during the leaching stage of low-grade ores, typically employing heap or bio-heap leaching methods, represents an example of dilute solutions.

Dilute solutions are characterized by metal concentrations in the range of parts per million (ppm). These low concentrations impart significant challenges for metal extraction and make the process complex and demanding from the efficiency point of view. Therefore, regarding an economic and technical perspective, conventional methods such as solvent extraction using a mixer-settler are impractical for these solutions. In fact, the low concentration of metals renders the extraction process inefficient and costly, necessitating the exploration of alternative methods or technologies to achieve viable metal recovery [5,6,7,8].

Meanwhile, AMDs that contain various potentially toxic elements cause environmental challenges by altering ambient pH levels, depositing metals in surface water streams, and disrupting marine and riverine ecosystems [9,10].

Table 1. The specifications of some AMD and ARD solutions including metal concentrations (mg/L), pH, and ORP [13,14,15].

Site	Country	pH	Eh	Cu	Ni	Zn	Al	Fe	Mn
Storwartz mine	Norway	6.5	-	0.06	-	2.13	0.03	1.6	1.35
Bullhouse	England	5.9	257	-	-	-	1.2	61	15
Wheal Jane	England	3.4	462	1.2	-	132	27	290	8
Killingdal (mine dump)	Norway	2.8	-	5.65	-	61.1	38.3	265	4.24
King’s mine stream	Norway	2.7	-	15.8	-	25.4	22.5	172	0.78
Parys mine	Wales	2.5	685	40	-	60	70	650	10
Iberian Pyrite Belt (AMD)	Spain	2.6		100	1.5	200	700	3900	60
Rio Tinto	Spain	2.2	450	109	-	225	-	-	-
Iron Mountain	USA	0.5–1	-	120–650	-	700–2600	1400–6700	13,000–19,000	17–120
Sarcheshmeh (AMD)	Iran	4.4	-	22.780	0.606	10.857	29.040	0.242	55.262
Sarcheshmeh (ARD)	Iran	3.2	-	25.000	1.140	6.230	39.300	13.700	27.900

shows the metal concentrations of some AMDs and acidic solutions in mines. For instance, the AMD from the Iron Mountain mine in the USA contains notable concentrations of Fe, Cu, and Zn, ranging from 13–19 g/L, 120–650 mg/L, and 0.7–2.6 g/L, respectively. These solutions are classified as dilute due to trace amounts of valuable metals that can be extracted through hydrometallurgical techniques. Nonetheless, it is important to highlight that the heightened concentrations of impurities, such as Fe and Al, present obstacles in extracting these valuable metals. The high volume of AMD production, with China alone generating 3.5 billion tons annually, underscores the urgency of addressing this environmental concern [11]. Moreover, the substantial financial investments made by countries like the United States and Canada to mitigate AMD-related issues witness the importance of treating these solutions [12].

Industrial streams, including the solvent extraction (SX) raffinate of hydrometallurgical processes and wastewater from metal plating, battery manufacturing, and fertilizer plants, also contain considerable valuable metals [16,17].

Table 2. The chemical properties of raffinate solutions of some copper plants [16,18,19,20,21,22,23].

Plant Name	Country	pH	Cu (mg/L)
Zijinshan copper mine	China	-	1470
Pasminco Metals-BHAS	Australia	-	1100
El Abra	Chile	-	500–1000
Sociedad Contractual mineral El Abra	Chile	1.2	690
Empresa Minera de Mantos Blancos, Mantos Mine	Chile	0.8	500
Empresa Minera de Mantos Blancos, Mantoverda Mine	Chile	1.1	500
Griilambone Copper Co.	Australia	1.5	100–500
Cerro Colorado	Chile	-	400
Phelps Dodge Morenci, Metcalf SX	USA	1.4	340
Compania Minera Cerro Colorado Ltd.	Chile	1.3	340
Phelps Dodge Morenci, Modoc SX Plant	USA	1.4	330
Mexicana de Cobre	Mexico	1.7	300
Phelps Dodge Morenci, Central SX Plant	USA	1.3	270
Zaldivar	Chile	-	250
Great Australia Mining Co.	Australia	1.5	250
Phelps Dodge Morenci, Southwest SX Plant	USA	1.7	250
Compania Minera Cameron de Andacollo	Chile	0.8	240
Sarcheshmeh	Iran	1.3	63–219
Aberfoyle Resources Ltd.	Australia	1.5	200
Zambia CCM Tailings Leach Plant	Zambia	1.2	200
Compania Minera Quebrada Balanca	Chile	1.4	200
Compania Minera Zaldivar	Chile	-	200
Burro Chief (Phelps Dodge), Santa Rita	USA	1.6	200
Southern Peru Limited	Peru	1.8	160
Zijin Minging Group Co.	China	1.4	135
Burro Chief Copper Co. (Phelps Dodge), Tyrone	USA	1.6	110
Mount Isa Mines Ltd.	Australia	1.6–1.8	100

presents the metal concentrations of raffinate solution in some hydrometallurgical plants. The recovery of metals from such industrial solutions serves as a treatment measure and presents an opportunity to increase the productivity and profit of mining and industrial activities. This dual benefit improves sustainable development and resource conservation, aligning with global waste minimization efforts. Figure 1 displays the published papers containing the keywords AMD, dilute solution, and wastewater across chemical engineering, materials science, and earth science domains from 2012 to 2023. The rising number of publications highlights the increasing importance of these types of solutions.

For over a century, industrial hydrometallurgical plants have employed purification and concentration techniques, such as SX, ion exchange (IX), surface adsorption (SA), and chemical precipitation (CP), to recover/remove metals from aqueous phases [24]. Selecting the appropriate purification method depends on a detailed evaluation of its advantages, limitations, and compatibility with the chemical properties of the solution. Although conventional methods of SX (mixer-settler) can be applied to separate metals from relatively high metal concentrations (more than one g/L), IX and SA methods are appropriate for handling lower metal concentrations [25]. However, the higher operating costs of IX in comparison with SX is a challenge [26].

While metal extraction and removal methods from solutions share numerous similarities, their respective purposes diverge significantly. The selectivity coefficients and the intended applications of the final product determine which process is applied. The removal of metal ions from solutions has been extensively studied using CP [27,28,29], membranes [30,31,32,33,34,35], SA [36,37,38,39,40,41], and IX [42] processes.

Despite the effectiveness of conventional methods for selectively recovering metals from high concentration solutions, extracting metals from dilute solutions remains a significant challenge due to technical and economic constraints [43]. Over the past few decades, numerous studies have investigated effective approaches for extracting metals from dilute solutions. These methods have aimed to attain high selectivity and extraction efficiency levels while addressing the economic and technical challenges associated with conventional approaches for dilute solutions. Novel approaches, such as using air bubbles in SX [44,45,46], employing ionic liquids (IL) [47,48,49], and immobilized ligands [50,51,52], have shown promise in this regard.

While the existing literature extensively covers conventional methods for separating ions from solutions and treating wastewater, review articles focusing specifically on economically extracting metals from dilute solutions are needed. This study aims to bridge this gap by evaluating the advances in metal extraction methods tailored for industrial solutions. Section 3 succinctly outlines conventional techniques such as ion exchange, chemical precipitation, and surface absorption. Section 4 delves into novel approaches for metal recovery from dilute solutions, including solvent-coated bubbles, immobilized ligands, and ionic liquids, elucidating their applications in metal extraction. Section 5 of this manuscript summarizes the material described and predictions that outline potential avenues for further exploration and development. Also, this article tries to enrich the existing knowledge on dilute solution treatment based on hydrometallurgical methods and foster innovation within this field.

2. Research Methodology

Initially, a comprehensive search was conducted across various academic databases, including Google Scholar, Scopus, Science Direct, MDPI, and others. Varied combinations of keywords and phrases related to metal extraction, selective separation methods, dilute solutions, industrial wastewater, solvent extraction, ion exchange, membrane separation, and ionic liquid were employed. Articles that provided novel information on the selective separation methods for efficient metal extraction from dilute solutions were included in the paper. Both experimental studies and review articles were studied, excluding those solely focused on conventional extraction techniques or lacking attention to selective separation. No restrictions on publication date were made to ensure good coverage of recent findings.

Initial screening of search results was conducted based on titles and abstracts to identify potentially relevant articles, and full-text articles were selected for further assessment. Two independent reviewers evaluated each paper according to the inclusion criteria, and any disagreements were resolved through discussion. Data extraction from the selected articles was conducted using a standardized form. Extracted information included details on selective separation methods, such as immobilized ligands, ionic liquids, solvent-coated bubbles, target metals, operational conditions, extraction efficiencies, selectivity coefficients, and noteworthy findings regarding the applications of these methods in various industries.

Given the diverse array of the included studies, no formal quality assessment or risk of bias analysis was conducted. However, the reliability and relevance of each study were carefully considered during the selection process. The findings from the included studies were sorted using a thematic approach, and common trends in selective separation methods and their applications were identified and analyzed. The data were included in three major sections based on the type of separation method, metal species targeted, and practical applications discussed in the literature. Approximately 39% of investigated papers were published post-2020 (with ~21% from 2023 onwards).

3. Conventional Methods

Purification and concentration of the aqueous phase in modern hydrometallurgy gained significant attention and was substantially developed during World War II [53]. Conventional separation methods such as SX, IX, CP, and SA are widely employed across various industries for metal recovery or wastewater treatment [54].

Table 3. The advantages and disadvantages of conventional metal recovery processes [46,55,56].

Method	Advantages	Disadvantages
IX	Low energy requirement Effectiveness < 100 ppm	Adsorption of organics Chemical regeneration requirements Organic contamination from the resin Expensive resins Prone to fouling in mixed waste streams
SA	Wide range of commercial products and a wide variety of target contaminants Rapid kinetics Simple metal removal process Adaptable to many treatment formats Low adsorbent cost Effective for solution < 100 ppm	Requirement for several types of adsorbents Low selectivity Recurring cost of new adsorbent Disposal cost of spent adsorbent
CP	Well established Low detention time requirements	High reagent consumption Large quantities of sludge being generated Ineffective in the removal of the metal ions at low concentration Low selectivity Several unit operations
SX (mixer-settler)	Effective in extracting/removing metal ions in solutions Produces high-purity solutions and compounds Continuous mode/esed commercially Highly selective	Efficient in high-concentration solutions Capital costs Losing organic phase (emissions to raffinate) High solvent requirement

shows the pros and cons of the mentioned methods.

3.1. Chemical Precipitation

In the CP process, the solubility of metals is diminished by adding counter-ions, causing them to produce solid particles [57,58,59]. CP has extensive applications in metal separation and wastewater treatment. Research indicates that the hydroxide precipitation method (Equation (1) [60]) effectively removes some metals such as Cu, Zn, and Ni from wastewater within a pH range of 9–11 [61,62,63,64]. It is noteworthy that the solubility of metal hydroxides is minimal at specific pH levels, and altering the desired pH range may result in metal re-solubilization after precipitation [65,66].

$$2{\mathrm{OH}}^{-}+{\mathrm{M}}^{2+}\to \mathrm{M}{\left(\mathrm{OH}\right)}_2 \left(\mathrm{Solid}\right)$$

(1)

Thanks to the minimal solubilization of critical elements in the form of sulfide, the precipitation method for these elements (Equation (2)) can be utilized even in acidic media [67]. Previous studies have demonstrated that the addition of sodium sulfide can precipitate nearly all Cu, Pb, Cr, Zn, and Ni ions as sulfides [68,69,70,71]. Additionally, some research has explored potentially toxic element removal using hydrogen sulfide generated during biological sulfate reduction with a carbon source (Equations (3) and (4)) [72,73]. Despite CP efficacy in removing toxic elements from solutions, its widespread application to wastewater treatment is limited by challenges such as high production volumes, operational complexities, and low selectivity [29,74].

$${\mathrm{S}}^{2-}+{\mathrm{M}}^{2+}\to \mathrm{MS} \left(\mathrm{Solid}\right)$$

(2)

$${\mathrm{H}}_2\mathrm{S}+{\mathrm{M}}^{2+}\to \mathrm{MS} \left(\mathrm{Solid}\right)+2{\mathrm{H}}^{+}$$

(3)

$$2{\mathrm{CH}}_2\mathrm{O}+{\mathrm{SO}}_4^{2-}\to {\mathrm{H}}_2\mathrm{S}+2{\mathrm{HCO}}_3^{-}$$

(4)

3.2. Surface Adsorption

The adsorption of metals onto an adsorbent surface, forming a molecular or atomic film, is a well-established method for wastewater treatment. Various adsorbents such as activated carbon, zeolite, biosorbents, and nanoparticles have been utilized for this purpose. Although both physical and chemical attachment mechanisms are employed, the strength and selectivity of the latter are superior [75]. Activated carbon has been extensively used for critical metals extraction from wastewater [76,77,78,79,80], as well as in capturing Au and Ag ions from solutions [81,82,83,84,85,86] and water purification [87,88,89].

Studies have indicated that increasing the pH of the aqueous phase can enhance the separation of valuable metals such as Hg, Pb, Cd, Ni, Cr, and Cu using activated carbon [90,91,92,93]. Despite its high specific surface area, which makes it an efficient method for separating metals from aqueous solutions, the widespread application of activated carbon is impeded by its high operational costs and low selectivity [90].

On the other hand, the biosorption process involves the separation of metals from aqueous solutions by microorganisms such as algae, bacteria, fungi, and yeast [94]. Although some investigations have confirmed the high efficiency of diverse biomasses in the separation of valuable metals from wastewater (Callinectes sapidus for Cd, Ni, and Pb [95], Spirulina platensis and Chlorella vulgaris for Al, Ni, and Cu [96], phosphate biowaste derived from fruit for Hg, Pb, Cd, Cu, Zn, and Ni [97], and Bacillus licheniformis for Cr, Fe, and Cu [98]), operational difficulties, high costs, and limited selectivity have confined this method mostly to laboratory-scale applications [99].

The unique properties of zeolite and nanoparticles have led to extensive research into their potential as alternative solutions for wastewater treatment in recent years [100,101,102]. Studies have shown that natural, modified, and synthesized zeolites can effectively adsorb valuable metals from AMD [103,104,105,106] and industrial wastewater [107,108,109].

Ok et al. [110] observed an adsorption trend for metal extraction using Zeolite, which ranked as follows: Pb²⁺ > Ni²⁺ > Cu²⁺ > Cd²⁺ > Zn²⁺ > Cr³⁺ > Co²⁺. Additionally, they determined the maximum adsorption capacities of Pb, Cu, Zn, and Cd to be 27.03, 23.25, 12.85, and 10.87 mg/g, respectively. They concluded that this sequence was influenced by ionic radius (with Pb > Cu > Cd) and the covalent index, both associated with electronegativity and ionic radii.

The separation of metals by zeolite from 560 mg/L solution occurs in two steps, with the initial part being rapid, achieving a separation efficiency of 80% in 40 min, and followed by a significant drop in kinetics in the second part [111].

Nanoparticles, including magnetite, silver, carbon nanotubes, graphene oxide, graphene-quantum dots, and metal–organic frameworks, have shown promise in adsorbing critical metals from wastewater [101,112]. Due to the magnetic properties of iron oxide nanoparticles facilitating their separation from the aqueous phase, they have attracted considerable attention for wastewater treatment [113,114,115]. While high removal efficiency using nanoparticles has been widely reported, it is essential to consider that high temperature and low media pH significantly impact this response [114,116,117,118]. Additionally, using recycled nanoparticles has been reported to reduce separation efficiency significantly.

3.3. Ion Exchange

The IX process, investigated for the extraction of uranium and gold in the mid-20th century, offers advantages such as higher loading capacity, rapid kinetics, lower capacity reduction in the presence of organic components, and regeneration without thermal treatment compared to activated carbon [119,120]. In this process, both anionic and cationic resins can be applied to separate ions from the aqueous phase. The anionic type is preferred for solutions with low pollution concentration, while the cationic type is better for concentrated solutions [121]. The reaction of separating metals from the aqueous phase by carboxylic acid (–COOH) and sulfonic acid (–SO₃H) groups, as cationic exchangers, are shown in Equations (5) and (6), respectively [122].

$$\mathrm{nR}-{\mathrm{SO}}_3\mathrm{H}+{\mathrm{M}}^{\mathrm{n}+}\to {\left(\mathrm{R}-{\mathrm{SO}}_3\right)}_{\mathrm{n}}\mathrm{M}+{\mathrm{nH}}^{+}$$

(5)

$$\mathrm{nR}-\mathrm{COOH}+{\mathrm{M}}^{\mathrm{n}+}\to {\left(\mathrm{R}-\mathrm{COO}\right)}_{\mathrm{n}}\mathrm{M}+{\mathrm{nH}}^{+}$$

(6)

After the loading step, the reverse reaction, transferring ions from the resin to the aqueous phase, can also be performed by introducing appropriate reagents to the loaded resins [123]. The IX process has been evaluated for wastewater treatment [121,124,125,126] and metal extraction/removal from leach solutions, including uranium [127,128,129,130], PGMs [119,131,132,133], copper [134,135,136,137,138], and chromium [139,140,141,142].

Feng et al. [143] examined the IX process for treating AMD using commercial resins IR120H (cationic) and A375 (anionic). Their findings indicated that the removal efficiency for cations followed the order Ca²⁺ > Mg²⁺ > K⁺ > Na⁺, while for anions, it was observed as SO₄²⁻ > Br⁻ > Cl⁻ > F⁻.

Besides synthesized polymer resins, the selective separation of clinoptilolite (a natural zeolite as an ion exchanger) for valuable metals has been extensively explored. Research using clinoptilolite revealed a selectivity trend of Pb²⁺ > Fe³⁺ > Cr³⁺ ≥ Cu²⁺. Additionally, the presence of barrier cations and anions, including NH⁴⁺, K⁺, Ca²⁺, Na⁺, Mg²⁺, Li⁺, Cl⁻, and Br⁻, resulted in a reduction in metal separation efficiency [144].

The results of extracting metals by IRC718, a commercial-grade weakly acidic ion exchanger, demonstrated the ability to extract alkaline earth metals (in order of Sr²⁺ > Ba²⁺ > Ca²⁺ > Mg²⁺) and transition metals (in the order of Ni²⁺ > Cu²⁺ > Pb²⁺ > Mn²⁺ > Hg²⁺ > Fe³⁺ > Ag⁺) [145].

The loading capacity of resins varies with the functional acid group present. Weakly acidic cation exchangers exhibit increased capacity as pH rises (optimal at pH > 7), while strong acid cation exchange resins show reduced capacity as pH decreases (typically below pH 7) [146,147]. Moreover, studies have shown that increasing media temperature enhances the separation efficiency of the IX process [148,149]. However, the method’s high operational costs and limited efficiency for dilute solutions remain significant challenges.

4. Recently Developed Approaches

The importance of recovering metals from dilute solutions has led to the development of new processes in recent years. These methods aim to not only achieve high selectivity in metal recovery but also address the technical and economic challenges associated with conventional approaches for treating dilute solutions. Given the comparatively low economic value of metals in these solutions, the recovery of valuable metals presents unique challenges compared to concentrated solutions like pregnant leach solutions. The following subsections explain the application of novel methods to separating metals from dilute solutions, including immobilized ligand, solvent-coated bubble, and ionic liquid methods.

4.1. Immobilized Ligands

The concept of extracting metals using immobilized ligands, known as molecular recognition technology (MRT), emerged from supramolecular chemistry in the last decades of the 20th century [150]. Notably, the Nobel Chemistry Prize in 1987 recognized Pedersen, Cram, and Lehn for their effort in developing supermolecules with structure-specific interactions [151].

In MRT, metals (referred to as the guest) are separated from solutions by ligands (referred to as the host) designed to match the properties of the target ion, including ion size, chemistry, and geometry [152]. This approach requires designing and synthesizing macrocycles as specific ion acceptors without interfering with other ions in the solution. Various ligands have been created by modifying the dimensions, rigidity, and donor atom type of the macrocycle [153,154,155]. Rigid macrocycles can discriminate between the target ion and others in solution (smaller or larger), whereas flexible macrocycles can only differentiate target ions from smaller ions [156].

The applications of numerous parent macrocycles such as crown ethers [157,158], macrocyclic polyamines [159,160], macrocyclic polysulfides [161], cyclic peptides [162,163,164], calixarenes [165,166], cyclophanes [167,168] and cyclodextrins [169] for selectively separating metals from aqueous solutions were evaluated, and the resulting crown ether exhibited superior guest selectivity and structural diversity compared to other macrocycles [154].

Crown ethers, particularly dibenzo-18-crown-6 (C₂₀H₂₄O₆), have commonly been employed as ligands for selective separation. Crown ethers with neutral oxygen donor atoms are well-suited for alkali, alkaline earth, lanthanide, and some post-transition metal ions [153,156]. However, for specific applications, the substitution of some or all oxygen atoms with sulfur and nitrogen atoms is frequently performed [153]. The extraction of metal ions (M^m+) by crown ether components (L) and its equilibrium equation are represented in the following equations (A⁻ denotes the counter anion) [170]:

$${\mathrm{M}}^{\mathrm{m}+}+{\mathrm{L}}_{\mathrm{O}}+{\mathrm{mA}}^{-}\leftrightarrow {\left({\mathrm{MLA}}_{\mathrm{m}}\right)}_{\mathrm{O}}$$

(7)

$${\mathrm{K}}_{\mathrm{ex}}=\frac{{\left[{\mathrm{MLA}}_{\mathrm{m}}\right]}_{\mathrm{O}}}{\left[{\mathrm{M}}^{\mathrm{m}+}\right]{\left[\mathrm{L}\right]}_{\mathrm{O}}{\left[{\mathrm{A}}^{-}\right]}^{\mathrm{m}}}$$

(8)

Nitrogen donor atoms exhibited affinities for transition metal cations, while sulfur donor atoms had affinities for silver, lead, and mercury ions [156]. To enhance metal ion binding strength and selectivity, one or more side arms can be introduced into the crown ether structure [154]. Double-armed crown ethers, a common type of armed macrocycle, provide better coordination of a guest cation trapped in the crown ring through the donor group on the flexible sidearm [171].

In industrial plants, recovering metals through MRT involves covalently attaching ligands to mesoporous silica, polyacrylate, or polystyrene for solid-phase extraction [152,172,173]. The solid-phase extraction process in MRT comprises four steps: (1) loading of ligands, where the solution passes through a column or tube containing immobilized ligands to separate the target cation while barrier ions exit the column; (2) pre-elution wash to remove any remaining feed from the column; (3) elution, where the adsorbed target ions are transferred from the ligands to an eluent solution for subsequent recovery stages; and (4) post-elution wash to collect any remaining ions (enhancing metal recovery) and prepare the column for the next cycle [174]. Due to the high selectivity of this method, the eluent solution obtained during this process was of high purity. Commercial MRT substances (Figure 2), known as SuperLigs, consist of ligands attached to porous silica and were provided by IBC Advanced Technologies, Inc. (American Fork, UT, USA).

Hojamberdiev et al. [176] evaluated the efficiency of attached 2-amino-1-methylbenzimidazole (MAB, C₈H₉N₃) and dithizone (DTZ, C₁₃H₁₂N₄S) ligands for separating Zn, Cd, and Ni from a solution, using spent Al catalyst as the immobilizer of the ligands. Their findings indicated that MAB was more efficient than DTZ in adsorbing critical metals. At the optimum contact time of 10 h, the maximum sorption values of Zn²⁺, Ni^2+, and Cd²⁺ were found to be 0.57, 0.79, and 1.06 mM/g, respectively. The results indicated that while extending the contact time led to increased extraction efficiency, this effect notably diminished when the reaction time exceeded 10 h. Altering reaction temperatures between 20 and 60 °C indicated that the highest extraction efficiency can be attained at 40 °C. It was found that the Freundlich model better fit the data than the Langmuir model.

El-Ashgar et al. [52] investigated the effectiveness of polysiloxane-immobilized iminobis (N-2-aminophenylacetamide) ligand (P-IAPA, Figure 3a) in separating Cu²⁺, Fe³⁺, Ni²⁺, Co²⁺, and Zn²⁺ from 0.02 M solution. The optimal duration for separating the mentioned metals using P-IAPA was 24 h, with diminishing returns observed upon extending the duration beyond this point. Raising the solution pH from 3.5 to 5.5 enhanced the ligand’s uptake values from ~0.2 to ~1.2 mM/g(ligand) of Cu²⁺. Also, the ligand’s capacity was reduced due to the presence of amine protons. The ligand exhibited uptake capacities of 1.15, 1.04, 1.09, 1.12, and 0.97 mM/g(ligand) for Cu, Fe, Ni, Co, and Zn, respectively. Recycling the ligand led to an approximately 5% reduction in its capacity.

Also, research on the extraction of Cu, Co, and Ni by polysiloxane-immobilized triamine ligand (P-DTA, Figure 3b) has been conducted, revealing that this ligand can achieve recoveries of approximately 100% for Cu and around 90% for Ni and Co at pH 5.5 [50].

Salimi et al. [177] studied the application of immobilized dithizone (DTZ) for Pb and Cd extraction from wastewater, using a Zn-based metal-organic framework (MOF) to immobilize DTZ. They found that increasing pH in acidic media and increasing sorbent amount enhanced Pb and Cd absorption. However, the addition of NaCl to the solution (6%) reduced the absorption efficiency to 50%. It was concluded that the extraction of metals by HNO₃ was more efficient than HCl and H₂SO₄.

Awual et al. [51] investigated the efficiency of several ligands in separating valuable metals from an aqueous phase. The extraction efficiency of Pd (II) by N,N bis (salicylidene) 1,2-bis (2-aminophenylthio) ethane (BSBAE, C₂₈H₂₄N₂O₂S₂) attached to mesoporous silica monoliths from a solution containing 2.0 mg/L was evaluated. The results showed that the solution pH significantly influenced the palladium separation efficiency. Also, ~100% Pd was adsorbed by BSBAE at pH 3.5 (the optimum level), whereas increasing pH to 12 resulted in the reduction in the response to less than 20%. Furthermore, this ligand could selectively separate Pd from a solution containing two mg/L in the presence of 10 mg/L barrier ions such as Co(II), Pb(II), Cd(II), Zn(II), Ca(II), Mg(II), Al(III), Fe(III), Ni(II), and Bi(III).

Application of immobilized (3-(3-(methoxycarbonyl) benzylidene) hydrazinyl) benzoic acid (C₁₆H₁₄N₂O₄, MBHB) for Au (III) extraction was investigated [178]. The influence of pH levels in the range of 1 to 7 on the extraction efficiency was evaluated, showing an improvement in Au extraction from pH 1 to 2 but a reduction in capacity beyond pH 2 to 7. Although individual eluents such as hydrochloric acid, sulfuric acid, and thiourea were ineffective in stripping more than 15% of Au from the loaded MBHB, a mixture of hydrochloric acid and thiourea (0.1 M) recovered more than 99% of gold, indicating a synergistic effect. Seven reuse cycles of this ligand for Au separation maintained an efficiency of over 90% in the last step. They also investigated the application of various ligands for the extraction of Cu(II), Ce(III), Eu(III), Sm(III), and Co(II) [179,180,181,182].

It can be concluded that MRT, as a rapid and selective separation method, can extract metals from very dilute solutions. By preventing waste production, utilizing safe and renewable substances, and consuming low energy, this method qualifies as a green method for extracting metals from wastewater or industrial dilute solutions [152,183,184].

4.2. Ionic Liquid

Ionic liquids have received special attention due to their exceptional properties in metal extraction. Their unique specifications, such as excellent chemical and thermal stability, negligible volatility, non-flammability, high solubility across a wide range of minerals and synthetic materials, low toxicity, and environmentally friendly nature, make them promising alternatives to conventional solvents and extractants [185,186]. Furthermore, the ability to design and synthesize ionic liquids with specific specifications and functionalize them with coordination groups (task-specified IL) [187], and the possibility of adjusting them, provides the ability to meet the requirements of metal extraction such as proper efficiency, selectivity, and recyclability. This capability optimizes performance while minimizing environmental impact [188].

Due to the ionic properties and high polarity of ionic liquids, they can interact effectively with metal complexes in the aqueous phase. This characteristic enables metal complexes to transfer into the organic phase as either neutral or charged components during extraction [189].

Ionic liquids can serve as both diluents and extractants in extraction processes. Due to their ionic nature, they can enhance extraction efficiency when used as diluents, leading to increased interactions of the extracting ligands with metal complexes. From another point of view, the interaction of extracting ligands with ionic liquids is weaker compared to their interaction with molecular solvents, which further boosts process efficiency [190].

Ionic liquids as extractants can form a complex with the metal of interest. These ionic liquids are called functionalized ionic liquids [191]. Undiluted systems have the advantage of not requiring additional extractants and having a high loading capacity. However, the industrial use of pure ionic liquid is limited due to its high viscosity (which reduces the speed of the extraction process) and its high cost [192]. To overcome these constraints, ionic liquids diluted in conventional molecular solvents have been used [193].

The mechanism of metal extraction with ionic liquids is more complicated than molecular solvents and is an area of ongoing research [194]. The mechanism generally depends on factors such as the metal complex formed and its charge, the characteristics of the anion and cation of the ionic liquid, and the surrounding environment [190]. Possible mechanisms include cationic or anionic exchange, ion pairs/neutral, or neutral co-extraction. Accurate spectroscopic techniques are employed to detect the mechanism of metal extraction with ionic liquids [195]. Figure 4 illustrates an overview of the typical mechanisms involved in the extraction of metals using ionic liquids.

Ionic liquids, with their distinct chemical properties and tunable characteristics, have exhibited remarkable effectiveness in successfully extracting and separating some metals, including Cd, Cu, Co, Ni, and Zn [48,197,198,199]. Various room-temperature ionic liquids (RTILs) derived from different bases, such as imidazolium, pyridinium, phosphonium [200], and ammonium [201], have been employed for metal extraction. Additionally, different extraction agents, such as crown ethers and dithizone (C₁₃H₁₂N₄S), have been incorporated into these ILs to enhance the extraction process [202].

In a study conducted in 2010, the RTIL [BMIm] [PF6] was used to extract one mM Ni²⁺, Cu^2+, and Pb²⁺ ions from a synthetic aqueous solution using 2-aminothiophenol ligand (C₆H₇NS) in both synthesized ionic liquid mediums, and chloroform. This research optimized the pH conditions, extraction equilibrium time, and the effect of interfering ions on the extraction process. Results indicated higher extraction efficiencies from the ionic liquid than chloroform for all mentioned metals. Also, interfering ions, including Na⁺, Ca²⁺, Mg²⁺, SO₄^2−, and Cl⁻, had a negligible effect on the extraction. Considering the presence of other ions in industrial wastewater, these findings can be interesting and practical. Metal ions in the organic phase were stripped with greater than 95% efficiency using HNO₃ for Cu²⁺ and Pb²⁺, and HNO₃/H₂O₂ for Ni²⁺. Investigation of the extraction mechanism showed that Ni²⁺ and Pb²⁺ ions were extracted through the proton transfer mechanism, while Cu²⁺ was obtained through the oxidation and reduction mechanism [203].

An issue with RTILs containing fluoride anions is their instability and tendency to be hydrolyzed. This can result in the formation of toxic and corrosive hydrofluoric acid, in addition to the already high cost of these compounds [204].

In another investigation, researchers aimed to selectively extract Co (II) from Ni (II) in an acidic chloride solution (containing each metal at a concentration of 100 ppm). They utilized Cyanex 272 as the extractant and Cyphos 101 ionic liquid as the diluent. They successfully achieved rapid and selective Co extraction (90%) from Ni using 0.5 M Cyanex 272 at pH 6 at room temperature. The study revealed that cobalt extraction was selective at acidic pH levels, while overall extraction efficiency for both metals is pH-dependent and peaks at pH = 7. The study also analyzed the effects of the extractant concentration and temperature. Further, the stripping process was carried out using 0.1 M sulfuric acid with 100% efficiency [48].

In a recent study, palladium extraction from cyanide media was investigated using microemulsion systems incorporating imidazolium-based ionic liquids ([BUIm]Br, [BOIm]Br, and [BCIm]Br) in combination with n-heptane, n-pentanol, and sodium chloride [49]. The system containing [BUIm]Br ionic liquid demonstrated a higher metal extraction. Under optimal conditions, palladium could be selectively obtained from alkaline cyanide solutions containing Co(III) and Fe(III). The study employed DFT calculations, spectroscopic methods, and Job’s method to predict the anion exchange reaction mechanism. KCl solution effectively separated Fe(III) and Co(III), while KCN solution selectively stripped Pd(II) with an efficiency exceeding 99% for both processes.

Turanov et al. [205] explored the extraction of trivalent lanthanides from nitric acid aqueous solution. They investigated the extraction of Ln(III) ions (concentrations 0.002 mM) using TODGA(N,N,N,N Tetraoctyl Diglycolamide, C₃₆H₇₂N₂O₃) and TBP (Tributyl phosphate, C₁₂H₂₇O₄P) neutral extractants, along with [N1888][Tf₂N] ionic liquid as a diluent. Their results revealed that these two extractants exhibit synergy, and the inclusion of TBP as a co-extractant enhances both the efficiency and selectivity of the extraction process. However, the results of adding TBP to molecular solvents led to a decrease in extraction efficiency. This study compares the performance of [N1888][Tf2N] IL with imidazolium-based ILs. In terms of the selectivity of trivalent lanthanides, this ionic liquid demonstrated superior efficacy. Also, the high hydrophobicity of [N1888][Tf2N] IL prevented the loss of IL into the aqueous phase.

Task-specific ionic liquids (TSILs) are designed to address the efficiency limitations of conventional ionic liquids. They are functionalized by incorporating a chelating agent onto an organic cation, significantly enhancing the metal affinity to the ionic liquid phase [206,207]. Another method to improve extraction efficiency is using a strong coordinating anion [208].

Pirkwiser et al. [209] synthesized three 3-hydroxy-2-naphthoate-based task-specified ionic liquids to extract Ag, Cd, Cu, and Pb from natural and synthetic feed solutions. These ILs showed high extraction efficiency, especially for Cu and Pb. Adding NaCl salt (seawater-like concentration) increased Ag and Cd extraction. They found that ammonium-based cation ILs had higher extraction rates and stability compared to phosphonium ones but were more sensitive to different matrices. One significant advantage of the synthesized ionic liquids is their low loss of 0.07% during leaching, attributed to the hydrophobic nature of the anion.

Research on the extraction of metals using ionic liquids highlights their notable features, such as high efficiency, selectivity, easy stripping process, and recyclability. However, their costly nature prevents widespread use.

To solve economic concerns and enhance performance, it is necessary to implement measures to reduce consumption and increase extraction capacity and speed. These strategies may involve enhancing the hydrophobicity of the ionic liquid by extending the alkyl chain length to prevent organic phase migration into the aqueous phase [47]. Additionally, increasing the contact surface area of the ionic liquid by immobilizing them on substrates such as graphene oxide [210] and membranes [211,212] can increase their absorption capacity and reduce their consumption [213]. Figure 5 displays the chemical structures of cations and anions discussed in this section.

4.3. Solvent-Coated Bubbles

The concept of utilizing coated bubbles in flotation was first proposed by Taggart in 1927 [214], suggesting their potential to enhance the collection of hydrophobic minerals compared to conventional methods. In 1987, a study demonstrated a significant reduction in induction time from 88 to 5 ms using this approach [215]. Peng and Li [216] investigated the coal flotation using collector-coated bubbles and highlighted the possibility of reducing the dosage of the collector without compromising coal recovery.

In 1998, extracting Cu by injecting compressed air into a mixing tank was investigated [217]. This method eliminated moving parts of conventional SX equipment, such as mixer settlers and sieves/packed pulsed columns. The Cu extraction rate constant using LIX-860 varied between 0.18 and 0.40 min⁻¹. It was observed that increasing gas flow and decreasing the diameter of the air injection tube raised the extraction rate constant by enhancing the velocity of air. However, exceeding the optimum velocity led to a drop-off in the extraction rate constant by expelling the aqueous phase and losing kinetic energy.

In 2003, Chen et al. [218] introduced air-assisted solvent extraction (AASX) for recovering metals from dilute solutions. This method involves coating air bubbles with the organic phase to increase the surface area, thereby improving the specific surface area and facilitating rapid separation of aqueous and organic phases (Figure 6). The results of this research showed that ~38% of Cu extraction from a dilute solution containing 62 mg/L of Cu was achieved within a 3 h extraction time. Additionally, they reported positive values of thermodynamic coefficients such as entering, bridging, and spreading, which play an important role in producing covered air bubbles [219,220].

Tarkan and Finch [46] made modifications to the previous AASX setup by altering the method of introducing air into the system (Figure 7a). In this modified setup, both the organic phase and the air were introduced into a glass container, resulting in the formation of coated bubbles. These coated bubbles were then injected into the aqueous phase through an orifice connecting the bubble-making section to the aqueous phase section. As the coated bubbles ascended in the reaction part, specific ions transferred from the aqueous to the organic phase. Observations revealed that the thickness of the organic phase on the surface of a 0.44 cm bubble was 3 µm, corresponding to a specific surface area of 3000 cm²/cm³.

Additional research was conducted to explore various aspects of the AASX process using a similar setup. It was found that (1) the addition of 4 ppm of silicone oil increased foam stability, and (2) the organic phase thickness reduced to 500 nm after 10 min [223,224,225].

The application of a modified AASX setup underscored several advantages of the AASX method, including the reduced organic phase consumption, minimized loss of organic phase remaining in the raffinate solution, and decreased retention time required for phases separation [226]. In these research studies, a 25% extraction of Cu from a solution containing 500 mg/L was achieved using an A/O phase ratio of 150. However, a significant challenge of this method was the extended duration of 122 min required for producing the coated bubbles, coupled with low Cu extraction efficiency.

In 2012, an attempt was made to address this issue by integrating an ultrasonic nozzle into the setup to generate a solvent aerosol (Figure 7b). They found that this modification enabled the extraction of 57% of Cu from a 3.5 mg/L Cu solution. However, despite incorporating ultrasonics into the modified setup, the required time for extraction did not make a significant difference [221].

In 2008, the compressed air solvent extraction method (CASX) was investigated for metal removal (Figure 7c) [222]. Compressed air was utilized to create micro-sized solvent-coated air bubbles (MSAB) to remove Cr and Cd from aqueous solutions [227,228]. Although ~99% of Cr was extracted from a synthesized solution containing 645 mg/L of Cr and 50 mg/L of Cd, both the extraction time and the separation of the aqueous/organic phases were prolonged.

In 2013, Finch et al. [45] evaluated AASX efficiency using the Jameson downcomer (Figure 7d). They directed the aqueous phase into the downcomer through pumping, while the organic phase was introduced in three ways: directly into the aqueous solution, into the bubble bed within the downcomer, and into the air inlet as an aerosol solvent. The results showed no significant differences among these injection techniques [229]. Despite achieving an extraction rate of up to 80% for copper by adding 50 ppm methyl isobutyl carbinol (MIBC) as a frother reagent to the aqueous phase using the Jameson cell in the AASX process, challenges (such as bubble bed collapse and organic phase loss) persisted. Coated bubbles in the column floextraction (CFE) method are produced by a downcomer, like the AASX method using the Jameson cell [230,231,232].

An alternative method utilized was dissolved nitrogen predispersed solvent extraction (DNPSE) [233,234,235,236,237], involving the blending of the aqueous and organic phases using colloidal gas aphrons (CGAs). In this process, the organic phase transforms into colloid liquid aphron (CLA), which is then converted to CLA using compressed air spargers. The inclusion of silicone oil as an anionic surfactant was found to improve foaming properties in this approach.

Rahmati et al. [238] introduced an AASX apparatus designed to treat dilute solutions continuously. In this method, the coated organic phase is added to the aqueous phase through a venturi tube (creating vacuum pressure), after which the mixture of phases enters a column to extract metals and separate the loaded organic phase from the raffinate solution. The passage of air bubbles through the organic coating cylinder leads to the generation of organic-coated bubbles.

It was observed that the production of narrow-sized bubbles led to improving the AASX process efficiency. Moreover, the addition of silicone oil to the organic phase enhanced extraction efficiency by increasing thermodynamic coefficients. However, the transfer time to the top output was longer for small coated bubbles, while large coated bubbles with low specific surface area exhibited a shorter retention time [239]. Despite achieving approximately 80% copper extraction efficiency from a solution containing 150 mg/L Cu with an A/O phase ratio of ~100 under optimal conditions, the value of the organic phase recycling was found to be low [240]. This setup was modified by incorporating an additional stage and introducing aeration to the columns (Figure 7e), resulting in an increased organic phase recycling (84%) and copper extraction (98%) [44].

Applying solvent-coated bubbles to the selective extraction of metals from dilute solutions shows potential, especially in terms of efficiency, selectivity, and economic considerations. Nevertheless, all the investigations carried out are limited to synthesized solutions and in the absence of barrier ions.

5. Summary and Future Research Perspective

Technological advancements in metal extraction from dilute solutions will focus on new approaches such as immobilized ligands, ionic liquids, and solvent-coated bubbles. These new techniques promise to be transformative, impactful, and environmentally sustainable.

Considering immobilized ligands, researchers are investigating various structures to optimize the selectivity, affinity, and stability of ligands. This approach will enable the development of ligands capable of efficiently extracting metals from complex media while minimizing interference from other ions present in the solution. Immobilized ligands offer efficient and cost-effective solutions for metal recovery in mining, recycling, and environmental remediation industries. In addition, immobilized ligands can be combined with other innovative technologies due to the synergistic influence on the efficiency of the metal extraction system. For example, the integration of ligand-functionalized nanoparticles or porous materials into the process will enhance mass transfer and metal uptake.

Similarly, pairing immobilized ligands with advanced separation methods such as membrane use or chromatography will enable selective metal recovery from more complicated solutions. Sustainability and environmental protection are future concerns in the applications of immobilized ligands, and researchers will focus on greener synthesis routes by utilizing renewable raw materials and minimizing the use of hazardous chemicals. Additionally, recyclable ligand systems will be developed to enable the regeneration of ligands for multiple extraction cycles, thus reducing waste generation and the consequent environmental impacts.

The refinement and diversification of ionic liquid formulations are the most significant advances in the future of ionic liquid-based extractants. A vast array of chemical structures, cations, and anions will be explored to manufacture ionic liquids with optimized solubility, selectivity, and stability for specific metal extraction functions. These optimized agents will develop highly efficient and selective systems capable of extracting metals from industrial effluents, electronic waste, and ores. Developments in synthetic chemistry and process engineering will help to scale up and commercialize ionic liquid-based techniques.

Efforts will be concentrated on the synthesis of ionic liquids from renewable resources and minimizing their environmental impact. Continuous and low-cost metal recovery processes will be developed for industrial-scale operations by designing innovative reactors and methodologies. Also, hybrid extraction systems will emerge by combining ionic liquids with other separation techniques such as SX, membrane filtration, or adsorption to exploit the synergistic effects that enhance selective metal recovery, increase sustainability, and decrease energy consumption. Research works will center on developing green and recyclable ionic liquid formulations, minimizing toxicity and waste generation, and further enhancing the sustainability of these metal recovery technologies.

Regarding solvent-coated bubbles, future efforts will be concentrated on the optimization of bubble generation technologies, extractant formulations, and operation parameters. Some details, such as bubble size, surface tension, and coating composition, will be researched to engineer bubble stability, buoyancy, and carrying capacity. By optimizing these methods, target metals will be effectively captured from dilute solutions, while energy consumption and waste generation will be reduced.

Moreover, continuous operation, integration into current industrial processes, scale-up, and commercialization of solvent-coated bubble extraction technology will be facilitated by innovation and advancement in bubble generation strategies, reactor designs, and bubble delivery techniques. Automation and control systems will be employed to optimize bubble generation and metal recovery, further enhancing the reliability and performance of these extraction systems. Furthermore, the integration of solvent-coated bubbles into hybrid extraction platforms will emerge.

By coupling solvent-coated bubbles with other separation techniques, such as membrane filtration, adsorption, or electrochemical deposition, the synergistic effects of the mentioned processes will be employed to enhance efficiency and selectivity. These hybrid approaches will enable the extraction of metals from complex solutions with higher selectivity and flexibility. Efforts will be made to develop eco-friendly extractants and coatings, minimize hazardous chemical use, and reduce environmental impact.

Overall, the future of immobilized ligands, ionic liquids, and solvent-coated bubbles for the extraction of metals from diluted solutions holds great promise for addressing global challenges related to resource scarcity, pollution, and waste management. Through continued innovation and collaboration across disciplines, these advanced ligand systems will play a crucial role in shaping our planet’s more sustainable and resilient future.