Highlights

5.1. Melting point

Deep eutectic solvents can be identified by their depression at the melting point (Tm), Fig. 1. As noted above, DESs have melting points lower than their pure components. To date, specific data on eutectic DESs compositions and accompanying binary phase diagrams have been virtually non-existent. This further underlines the importance of obtaining phase diagrams of all DESs under investigation as they provide information on the temperature and composition range that can be expected from a liquid, which will help other researchers design DESs systems for their specific applications (Hansen et al., 2021). Most papers on DESs analyse the mixtures in their assumed eutectic compositions, but diagrams justifying the choice of composition are not provided.

5.2. Density

The density of DESs depends on the organization and molecular packaging, it is affected by the existence of gaps and vacancies within liquid DESs. That is why the density of DESs urea/choline chloride is higher than that of the urea/acetylcholine chloride system due to the presence of a large hole in acetylcholine chloride (Vigier et al., 2012)(Ul Haq et al., 2022)(Abbott et al., 2007). Another parameter that affects the density of DESs is the molar ratio HBA and HBD.

Abbot et al. (Abbott, Capper, & Gray, 2006) indicated that the addition of choline chloride to glycerol reduces the density of DESs due to increased free volume. An increase in the length of the alkyl cation chain leads to a decrease in the density of DESs, as follows: tetraethyl ammonium bromide > tetra propyl ammonium bromide > tetra butyl ammonium bromide. This indicates that the increase that occurs in the free volume is due to the elongation of alkyl chain length (García et al., 2015)(Montalbán et al., 2015).

Basaiahgari et al. (Basaiahgari et al., 2018) measured the density of ethylene, diethylene and triethylene glycol and glycerol as HBD and benzyl ammonium chloride salts as HBA. The results showed that the DESs obtained from ethylene glycol has a lower density than the DESs obtained from glycerol. According to the authors this increase in density by replacing ethylene glycol with diethylene glycol, triethylene glycol and glycerol means that the increase in the number of OH functional groups in HBD increases the formation of more H bonds, resulting in a decrease in the average available volume.

The composition and molar ratio between HBA and HBD are ways of modifying the density of eutectic mixtures. DESs based on ChCl and citric acid were studied by Shafie et al. (Shafie et al., 2019), observed that as the amount of ChCl increases in relation to citric acid, density decreases and vice versa, an increase in citric acid means an increase in viscosity.

The density of the deep eutectic solvent shows a temperature-dependent behaviour, decreasing linearly as the temperature increases, due to the thermal expansion of DES (Cui et al., 2017)(Ibrahim et al., 2019)(Florindo et al., 2014)(Shahbaz et al., 2012). According to Hole theory thermal energy can generate fluctuations in local densities, which leads to the increase of space between the HBA and HBD of the liquid DESs system (Omar & Sadeghi, 2022). The effect of the temperature on the density of DESs could be expressed in terms of isobaric thermal expansion coefficients, Eq. (1), which defines the available volume of free DES (Ijardar et al., 2022). The calculation of this expansion coefficient may be useful for understanding the compressible behaviour of DES.(1)$\alpha ={-((d\ln \left(p\right))/(dT))}_P$

So, the linear decrease in the density of the DESs is observed with an increase of the temperature, resulting in the availability more free space, the available space in DESs is related to change in α values. The value of α in DESs are minimal compared with the value of common solvents therefore the temperature showed a little effect on density and the isobaric thermal coefficient. DESS expanded or compressed less in comparison to ILs and other organic solvents.

5.3. Viscosity

Different parameters such as the chemical nature of HBA and HBD (Abbott et al., 2007)(D’Agostino et al., 2015), the temperature (Abbott, Boothby, et al., 2004); (Abbott, Capper, et al., 2004b)(Du et al., 2016)(Kareem et al., 2010), molar mass (Abbott, Harris, et al., 2011) and the molar ratio (Q. Zhang et al., 2012), in addition to the water content (Wazeer et al., 2018)(D’Agostino et al., 2015)(Florindo et al., 2014)(Shah & Mjalli, 2014) affect the viscosity of DESs. For example, the viscosity of the DESs for the different HBA/glycerol decreases according to the following order:

Benzyltrimethylammonium chloride > Diethylethanolammonium chloride > Choline chloride > Triethylmethylammonium chloride > Choline chloride > Triethylmethylammonium chloride.

And that of choline chloride DES/HBDs decreases according to the order:

Zinc chloride > Malonic acid > Triethylene glycol > Urea > Levulinic acid > P-cresol > Phenol > Ethylene glycol.

The viscosity of DESs decreases with the increase of the molar salt ratio: HBD, for example, the viscosity of ChCl:glycerol mixtures with molar ratios of 1:4, 1:3 and 1:2 decreases when the glycerol content decreases, being 503, 450 and 376 cP (at 293 K) respectively. (Wazeer et al., 2018). This is due to the breakdown of the hydrogen bonds associated with the addiction of organic salts in the DESs (Abbott, Harris, et al., 2011).

Another parameter that influences the viscosity is the temperature, which decreases with the temperature increase, this is due to the breakdown of the network of hydrogen bonds between the HBA and HBD. Water increases the solubilizing power of DESs by decreasing its viscosity (García et al., 2015).

The most interesting DESs for industrial application are those with low viscosities, a specific DESs design can be achieved through the small size of HBD and cations (Abbott, Capper, & Gray, 2006).

The DESs obtained from choline chloride:sorbitol 1:1 have a viscosity of 12,730 mPa S to 303 K, for choline chloride:glucose 1:1 a viscosity of 34,400 mPa S to 323 K. Hydrophobic deep eutectic solvents based on DL-menthol have low viscosities, such as DLmentol:octanic acid 1:3 with a viscosity for 298 K of 7.61 mPa S (Nunes et al., 2019)(Ribeiro et al., 2015).

It should be noted that there are differences in the viscosity data obtained by different authors for the same eutectic solvent, for example, 152 mPa. s vs 527.28 mPa. s for 1:2 choline chloride:urea at 303 K and 202 mPa. s vs 2142 for 1:1 choline chloride:oxalic acid at 313 K (García et al., 2015). These large differences can be attributed not only to the method of preparation, Florindo et al. (Florindo et al., 2014), but also the experimental method and the presence of impurities and water content (García et al., 2015).

5.4. Conductivity

Conductivity is other important factor to study due to the use of electrochemical techniques to recovery of metals with deep eutectic solvents. DESs have a low ionic and electrical conductivity at room temperature due to their high viscosities, there is a strong relationship between conductivity and viscosity. Most DESs tend to have poor ionic conductivities (k < 2 mS cm⁻¹ at room temperature) (El Achkar et al., 2021)(Lapeña et al., 2019)(Q. Zhang et al., 2012). The conductivity depends significantly on the temperature, so it can be predicted assuming a behaviour type Arrhenius (Wazeer et al., 2018). Abbot et al. (Abbott et al., 2003) carried out the first measurements of the conductivity of ChCl-Urea in 2003, revealing a significant increase with the increase in temperature as shown in Fig. 5. This increase in conductivity with temperature is due to the decrease in viscosity, by breaking the network of hydrogen bonds and increasing their ionic mobility (Lapeña et al., 2019)(Q. Zhang et al., 2012).

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Fig. 5

Studies have shown a dependence on conductivity with the molar ratio of HBDs/HBAs (Abbott, Boothby, et al., 2004); (Abbott, Capper, et al., 2004b), the alkyl cation chain length, temperature and water addiction (Dai et al., 2015).

Kareem et al. (Kareem et al., 2010) studied the conductivity of phosphonium-based DESs and Zhang et al. (Q. Zhang et al., 2012) showed that the conductivity of DESs increase with the increase in salt concentration (for example, for ChCl-EG). However, this behaviour is not true for all DESs (e.g., tetrabutylamonium chloride (TBAC)-EG), the change in conductivity depends on both the HBD and the nature of the salt.

5.5. Refractive index

The refractive index is one of the most important physical properties of DESs, providing useful information on their composition, checking the purity of materials and in measuring concentration of solutes in solution. The refractive index is one of the most important physical properties of DESs, providing useful information on their composition. The refractive index (n_D) is a dimensionless value representing the relationship between the speed of light in a vacuum (c and the speed of light in a given material (ν) (Mjalli et al., 2023). Following the principles of Snell’s law:(2)$n_D=c/\nu$

Refractive indexes are used to calculate molar refraction using the Lorentz-Lorenz equation.(3)$R_D=\left(\frac{M}{\rho }\right)\left(\frac{n_D+1}{n_D^2+1}\right)$where R_D is the molar refraction (cm³mol⁻¹), M is the molar mass (g/mol), ρ density (g cm⁻³) y n_D refractive index.

Molar refraction shows the molecular interaction between HBA and HBD of DESs components. The refractive index depends on the interaction between HBA and HBD, a decrease in the hydrogen bond interaction between the DESs component decrease the refractive index value. Therefore, the refractive index is inversely proportional to temperature, an increase in temperature, as indicated above, decreased hydrogen bond interactions between DESs components leading to decreased DESs viscosity due to decreased refractive index (Leron et al., 2012). The size of molecules influences the refractive index; those with a larger size have a higher refractive index. Hong-Zhen et al. (SU et al., 2015) found that DESs obtained from the same HBA (tetrabutiamonium chloride, TBAC) and different HBD as phenylacetic acid (PAA) in a 1:2 M ratio had the highest refractive index compared to the 1:2 M ratio of TBAC with propionic acid (PA). Both HBDs differ in a methyl group from a phenyl group bound to the carboxylic acid structure. Another parameter that influences the refractive index is the length of the alkyl chain, the refractive index of the DESs increases according to the following series:

butyl ammonium bromide < propyl ammonium bromide < ethyl ammonium bromide < choline chloride, as shown in Table 6.

The refractive index has been used to correlate the polarizability of liquids and assess the accuracy of ab initio calculations, as well as to understand how organic molecules associate with each other in binary alcohol mixtures(Seki et al., 2012). Kucan et al. (Zagajski Kučan & Rogošić, 2019) studied the refractive index of ChCl and glycerol of different molar proportions (1:1.5; 1:2; 1:3) as a function of temperature, found that the eutectic ratio had the highest refractive index in a temperature range of 288–328 K.

5.6. Surface tension

Surface tension is an average of the energy needed to increase a material surface and is related to a material tendency to have a smaller surface (Hansen et al., 2021). DESs surface tension is considered a physical property that demonstrates the impact of molecular structure on the intensity of interaction between the hydrogen bond acceptor and the donor in the DES mixture. Studies of this physical property are more limited compared to other physical properties. Relative surface tension values generally vary between 35 and 75 mN m⁻¹ a 298 K (García et al., 2015)(Ibrahim et al., 2019). Surface tension is highly dependent on the intermolecular interaction between the HBA and the HBD forming the DESs mixture. The surface tension depends of the temperature, the molar ratio, the nature of the HBDAs/HBDs and the length of the alkyl cation chain.

Gajardo-Parra et al. (Gajardo-Parra et al., 2019) measured the surface tension of three DES based on ChCl with levulinic acid, phenol and ethylene glycol. The surface tension of ethaline measured was 45,66 mN m⁻¹ to 298 K and 101,3 kPa it was lower than that of pure ethylene glycol, 48,90 mN m⁻¹. This same trend was observed for the other ChCl:HBA combinations. The decrease in surface tension from pure HBD to DESs formation was attributed to the addition of ChCl. Surface tension measurements show that ChCl acts as a surfactant and decreases cohesive forces (which affects the H-junction) on the surface of DESs ethaline.

The increase in the alkyl chain length decreases surface tension (Omar & Sadeghi, 2020b)(Omar & Sadeghi, 2020a)(Marcus, 2019). An increase in organic salt decreases the network of hydrogen bonds decreasing the surface tension of DESs (Y. Chen, Chen, et al., 2019). Abbot et al. (Abbott, Harris, et al., 2011) concluded that surface tension in ChCl systems: Gly for different salt concentrations presents a linear correlation with temperature, as shown in Fig. 6. These same authors in another paper determined that the surface tension of DESs ChCl:U (1:2) is greater than for ChCl:EG (1:2) due to the strong interactions of hydrogen bonds (Abbott, Barron, et al., 2011). Surface tension is affected by temperature and decreases linearly with it (García et al., 2015)(Lapeña et al., 2019)(Nunes et al., 2019). The effect of surface tension is shown in Table 7.

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Fig. 6

Table 7

HBA	HBD	HBA:HBD (molar ratio)	Surface tension (Nm m−1) at 298 K	Reference
ChCl	Urea	1:2	64.14	(Lapeña et al., 2020)
ChCl	Glycerol	1:2	58	(AlOmar et al., 2016)
ChCl	EG	1:2	52	(Klein et al., 2020)
ChCl	Lactic Acid	1:2	47.4	(Y. Chen et al., 2019)
ChCl	Lactic Acid	1:4	44.4	(Y. Chen et al., 2019)
ChCl	Phenylacetic acid	1:2	57.96	(Abbott et al., 2004)
ChCl	1,4-Butanediol	1:3	47.17	(Wazeer et al., 2018)
MTPB, BTPC, TBAB	EG		51 (TBAB), 65 (BTPC), 67 (MTPB)	(Ibrahim et al., 2019)
MTPB, BTPB, ATPB, TBAB	Glycerol		37 (TBAB) − 54	(AlOmar et al., 2016)
ChCl	MEA, DEA, MDEA		32 (MDEA) − 48 (MEA	(Adeyemi et al., 2018)
Butylammonium bromide	Glycerol	1:2	44.9	(Z. Chen et al., 2017)
Propylammonium bromide	Glycerol	1:2	51.7	(Z. Chen et al., 2017)
Ethylammonium bromide	Glycerol	1:2	57.6	(Z. Chen et al., 2017)

 

Bibliography.

Abbott, A. P., Boothby, D., Capper, G., Davies, D. L., & Rasheed, R. K. (2004). Deep Eutectic Solvents Formed between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids. Journal of the American Chemical Society, 126(29), 9142–9147. https://doi.org/10.1021/ja048266j.

Adeyemi, I., Abu-Zahra, M. R. M., & AlNashef, I. M. (2018). Physicochemical properties of alkanolamine-choline chloride deep eutectic solvents: Measurements, group contribution and artificial intelligence prediction techniques. Journal of Molecular Liquids, 256, 581–590. https://doi.org/10.1016/j.molliq.2018.02.085.

AlOmar, M. K., Hayyan, M., Alsaadi, M. A., Akib, S., Hayyan, A., & Hashim, M. A. (2016). Glycerol-based deep eutectic solvents: Physical properties. Journal of Molecular Liquids, 215, 98–103. https://doi.org/10.1016/j.molliq.2015.11.032.

Chen, Y., Chen, W., Fu, L., Yang, Y., Wang, Y., Hu, X., Wang, F., & Mu, T. (2019). Surface Tension of 50 Deep Eutectic Solvents: Effect of Hydrogen-Bonding Donors, Hydrogen-Bonding Acceptors, Other Solvents, and Temperature. Industrial & Engineering Chemistry Research, 58(28), 12741–12750. https://doi.org/10.1021/acs.iecr.9b00867.

Chen, Z., Ludwig, M., Warr, G. G., & Atkin, R. (2017). Effect of cation alkyl chain length on surface forces and physical properties in deep eutectic solvents. Journal of Colloid and Interface Science, 494, 373–379. https://doi.org/10.1016/j.jcis.2017.01.109.

Hansen, B. B., Spittle, S., Chen, B., Poe, D., Zhang, Y., Klein, J. M., Horton, A., Adhikari, L., Zelovich, T., Doherty, B. W., Gurkan, B., Maginn, E. J., Ragauskas, A., Dadmun, M., Zawodzinski, T. A., Baker, G. A., Tuckerman, M. E., Savinell, R. F., & Sangoro, J. R. (2021). Deep Eutectic Solvents: A Review of Fundamentals and Applications. Chemical Reviews, 121(3), 1232–1285. https://doi.org/10.1021/acs.chemrev.0c00385.

Ibrahim, R. K., Hayyan, M., AlSaadi, M. A., Ibrahim, S., Hayyan, A., & Hashim, M. A. (2019). Physical properties of ethylene glycol-based deep eutectic solvents. Journal of Molecular Liquids, 276, 794–800. https://doi.org/10.1016/j.molliq.2018.12.032.

Klein, J. M., Squire, H., Dean, W., & Gurkan, B. E. (2020). From Salt in Solution to Solely Ions: Solvation of Methyl Viologen in Deep Eutectic Solvents and Ionic Liquids. The Journal of Physical Chemistry B, 124(29), 6348–6357. https://doi.org/10.1021/acs.jpcb.0c03296.

Lapeña, D., Bergua, F., Lomba, L., Giner, B., & Lafuente, C. (2020). A comprehensive study of the thermophysical properties of reline and hydrated reline. Journal of Molecular Liquids, 303, 112679. https://doi.org/10.1016/j.molliq.2020.112679.

Omar, K. A., & Sadeghi, R. (2022). Physicochemical properties of deep eutectic solvents: A review. Journal of Molecular Liquids, 360, 119524. https://doi.org/10.1016/j.molliq.2022.119524.

Wazeer, I., Hayyan, M., & Hadj-Kali, M. K. (2018). Deep eutectic solvents: designer fluids for chemical processes. Journal of Chemical Technology & Biotechnology, 93(4), 945–958. https://doi.org/10.1002/jctb.5491.

 

DESs with high viscosity also have high surface tension. Sugar-based DESs such as choline chloride:D-glucose and choline chloride: D-fructose have higher surface tension values, reflecting their extensive network of hydrogen bonds (Hayyan et al., 2013)(Hayyan et al., 2012).

5.7. Polarity

Polarity or polarizability reflects the overall solvation capacity of solvents it can affect the solubility of metals in hydrometallurgy methods. DESs can be used as substitutes for conventional organic solvents for organic reactions, extraction of natural products and metallurgy, therefore the determination of DESs polarity is an essential parameter. However, despite their importance, the number of publications based on the polarity of DESs are limited. Different parameters can be used to express polarity, the most frequent are permittivity (dielectric constant) and spectral parameters (Shaibuna et al., 2022).

Most DESs are polar and their polarity can be calculated by solvatochromic parameters, where hypochromic (blue) or bathochromic (red) displacement of UV–Vis bands is considered for solvatochromic negative dyes, such as Reichardt dye, or positive solvatochromic dyes, such as Nile red, depending on the polarity of the solvent (El Achkar et al., 2021) (Dimroth et al., 1963)(Reichardt, 1994). The most used scales are the polarity of Dimroth Reichardt 30 on a scale of ET (30) (Reichardt, 1994) which is defined as the electronic transition energy of a probe dye in kcal/mol at normal temperature and pressure according to the expression:(4)$ET\left(30\right)=\frac{28591,5}{{\lambda }_{\mathit{max}}}$

The polarity of DES from different proportions of ChCl:Gly was determined by Abbott et al. (Abbott, Harris, et al., 2011), results showed a linear increase of polarity with ChCl concentration. Pandey et al. (Pandey et al., 2014) studied common DESs such as reline, ethanol, glyceline, and maline (a combination of ChCl (HBA) and urea, 1,2-ethanol, glycerol, and malonic acid (HBD) in ratio 1:2, respectively) using several optical spectroscopic probes, They concluded that high polarity was significantly influenced by the nature of HBD, the ChCl:Gly(1:2) having the highest polarity value.

DESs are highly soluble in water, methanol, ethanol, etc., and insoluble in aprotic solvents (e.g., toluene, hexane, ethyl acetate and acetonitrile). This indicates that solvents capable of forming strong hydrogen bonds with the chloride ion tend to be miscible (Naser et al., 2013). In 2003, Abbott et al. (Abbott et al., 2003) studied the solvation properties of ChCl-urea DESs at 50 C. Compounds such as amino acids, aromatic acids, inorganic salts and poorly soluble salts in water (e.g., AgCl) are very soluble in DESs. DESs can dissolve several metal oxides due to the high anion concentration of these liquids. Compared to most molecular solvents, DESs has unusual solvent properties. They are similar to IL and solvation properties are strongly influenced by the strength of hydrogen bonds. HBDs have a significant effect on the determination of the physical properties of DESs (Shaibuna et al., 2022).

5.8. Acidity and basicity

The acidity and basicity of DESs are considered one of the most important properties that will give them applications in various industrial fields, in addition this parameter can influence the selection of pipe materials, Reaction tanks etc. in an industrial process. This is another critical parameter in the selection of DES for application in hydrometallurgy processes, mainly in leaching methods (W. Chen, Jiang, et al., 2019).

Hydrogen ions and the pH have an important role in hydrometallurgy processes. The term pH refers to the concentration of hydrogen ions in aqueous solution. This is calculated to the negative log of the hydrogen ion concentration. The term “aqueous solution” means pure water or water with a small quantity of substances dissolved in it. The values of pH in different media are related through the Gibbs free energy of the proton exchange between solvents. However, even at the theoretical level, a valid comparability of pH values in different media has been impossible. The pH is indicative of the acidic or basic conditions of water. However, pH is not equivalent to acidity or alkalinity. The alkalinity and acidity are defined as the capacity of an aqueous solution to resist a change in the pH. Alkalinity and acidity are measured by determining the amount of a solution of acid or base, as appropriate, of known concentration that is required to completely neutralize the acidity or alkalinity of the aqueous solution (Jančíková et al., 2022).

DESs can be defined as a system formed from a mixture of Lewis or BrØnsted acids and bases. Therefore, the acidity and basicity of the HBA and HBD govern the pH of the DESs system. This acidity and basicity depend on the nature of pure constituents (HBA and HBD) and their molar ratio.

The BrØnsted acidity/basicity of nonaqueous solvent is evaluated by Hammett acidity function or by the pH measurement (Shaibuna et al., 2022). For a basic solution, the Hammett function measures the tendency of a solution to capture protons (Q. Zhang et al., 2012). The Hammett function is defined:(5)${H\_}_{=pK\left(HI\right)}+\mathrm{l}\mathrm{o}\mathrm{g}(\frac{\left[I^{-}\right]}{\left[HI\right]})$where pK(HI) is the thermodynamic ionization constant of the indicator in water, [I^-] and [HI] represent the molar concentrations of anionic and neutral forms of the indicator, respectively. Higher values of H_indicates strong basicity medium. The H_value for the DES ChCl: Urea (1:2) was 10.86 using as indicator 4-nitrobenziylcyanide, this parameter indicate that this DES has weakly basic nature (Li et al., 2008). It is also important to indicate that the content in water affect the H_value, when the system contain 1–3% of water, the H_value decrease from 10.77 to 10.65 due to a partial solvation of basic sites (Q. Zhang et al., 2012).

The Hammett acidity function is restricted to BrØnsted acidic DES and it is not used to DESs containing both BrØnsted and Lewis acid sites. The acidity measurements of this types of mixtures have done by FT-IR spectroscopy using as basic molecule pyridine. The methods are based in the vibration bands in the range of 1400–1700 cm⁻¹, the vibration band at 1450 cm⁻¹ indicates the Py-Lewis complex (band formed by the interaction of pyridine with Lewis acid sites) and the band at 540 cm⁻¹ indicate the interaction between pyridine with BrØnsted acid sites (Zhou et al., 2022) (Shaibuna et al., 2022).

The chemical nature of the HBDs has a strong effect on the acid or basic strength of the DESs. Different studies have been carried out evaluating the effect of HBDs on the pH of DESs, showing that an increase in the molar ratio of fructose caused an increase in the pH of DESs (Guo et al., 2013). Abbot et al. (Abbott et al., 2018) verify that the addition of chloride ions to the ChCl: Glycerol mixture reduces the acidity of the DESs and a change in the pH of the mixture to basic is observed.

Other factors affecting the pH of DESs are temperature, an increase in temperature decreases linearly the pH of DES(Skulcova et al., 2018), this decrease depends on donors of hydrogen bonds, such as alcohol-based DESs, where the pH slowly decreases with increasing temperature. However, the DES obtained from carboxylic acids the pH decreases abruptly with increasing temperature.

Hayyan et al. (Hayyan et al., 2013) studied the pH for different molar compositions of ChCl with D glucose DESs, the result indicated a pH value of 7 between 298 and 318 K, these authors observed a slight linear dependence of the pH with the temperature increase at 358 K. A similar study conducted by Skulcova et al. (Skulcova et al., 2018) analysed this behaviour and observed a similar behaviour, a decrease in pH to increase in temperature.

5.9. Hydrophilicity and hydrophobicity of DESs

Most DESs are hydrophilic, although there are DESs that are hydrophobic, immiscible in water. Hydrophobic DESs are widely used for the extraction of solutes by creating biphasic systems with hydrophilic compounds such as water. The hydrophobicity of the DESs depend on the chemical nature of the eutectic mixture, namely the HBAs and HBDs. For example, long hydrophobic alkyl chains of HBAs lead to hydrophobic DES due to steric impediment that prevents the salt of the core from being charged with water.

The size of the anion influences the physical properties of hydrophobic DESs a larger size of the anion leads to a greater viscosity of DES, the same effect is observed with longer HBA alchemical chains. Increased temperature reduces density and viscosity of hydrophobic DESs (Rocha et al., 2013).

According to the definition of hydrophobic DES is all DES insoluble in water and can be formed by both hydrophilic and hydrophobic compounds, such as a DES obtained from a water-soluble quaternary ammonium salt such as HBA and an apolar compound such as fatty acids or alcohols such as HBD. However, studies carried out by Shishov et al. (Shishov et al., 2020) considered that this type of DESS would fall into the category of quasi-hydrophobes or pseudo-hydrophobes, since the soluble parts of DESS, such as quaternary ammonium or short-chain carboxylic acids, Upon contact with water, these water-soluble components are dissolved and therefore the mixture of DESs is disintegrated.

It should be noted that the synthesis of hydrophobic DES can be limited by the lack of availability of its constituents in an easy and inexpensive way.

5.10. Effect of water on DESs physicochemical properties

DES are obtained from the formation of hydrogen bonds with their constituents, making most DES hygroscopic mixtures, so the presence of water makes their uptake inevitable. (Florindo et al., 2014)(Du et al., 2016) and makes difficult to dry out completely.

Water presents duality properties as acceptor and donor of hydrogen bonds. Being able to interact with the HBD or HBA of the DES and break the hydrogen bonding interactions between the organic salt and the hydrogen bond donor by forming multiple hydrogen bonds (Shah & Mjalli, 2014).

Therefore, knowing the effect of water on DES before and after hydrometallurgical processes is fundamental to know their reuse capacity due to the effect that water has on their properties. The traces of water can be considered as impurities in the DES, there is an infinity of works in which water is added intentionally in order to adjust the properties of the DES. However, it must be taken into account that the presence of water not only affects the physicochemical properties, but can also endanger the integrity of the DES. (El Achkar et al., 2019).

The adsorption capacity of water from the air during 8 h of DES obtained from Choline Chloride as HBA and urea, methyl urea, ethylene glycol, glycerol, glucose, xylitol and oxalic acid as HBD was studied by Chen et al. (Y. Chen, Yu, et al., 2019). Research has shown that glutamic acid-based DESs have a higher water adsorption capacity than other DESs. They described the effect of water adsorption according to the hydroxyl, methyl, and chain length groups. The results indicated that the presence of methyl group has a lower absorption capacity for water molecules and that HBD with a high number of hydroxyl groups have a higher water absorption capacity. Furthermore, an increase in the chain length of carboxylic acid-DES increases water absorption. HBAs and HBDs for DESs obtained from choline chloride as HBA and from diethylene glycol triethylene glycol and poly(ethylene glycol) 200 show strong hydrogen interactions, which gradually decrease as water is added, preserving to a certain extent the super molecular structure (Gabriele et al., 2019).

Hydrophobic DESs, such as that obtained from tetrabutylammonium chloride and decanoic acid, contain minor amounts of water, however, these amounts significantly influence the physical properties of DESs by switching from binary to tertiary DES systems. (Kivelä et al., 2022).

The effect of water on the physicochemical properties of DESs, melting point, density, viscosity, etc., is described.

An increase in the water content of DESs mixtures lowers the melting point. (Omar & Sadeghi, 2022). Choline Chloride: Urea absorbs up to 20% by weight of water, Meng et al. (Meng et al., 2016) studied the effect on the melting point of adding up to 10% by weight, the results showed a linear decrease in the melting point, falling from 303 K for the dry DESs to 288 K when the DESs contains 5% by weight. of water. The research of Smith et al. (P. J. Smith et al., 2019) are in agreement with previous results, these authors followed the variation of the melting point of DESs over a full range of water content, reaching a minimum melting point of 225 K for a mole fraction of water of 0.67, from which a linear increase in the melting point is observed. Due to the behaviour of DESs the authors propose the formation of a ternary DES ChCl:Urea:water 1:2:6.

Therefore, the observed differences in melting points may be due to the amounts of water absorbed by the DESs.

An increase in the water content in the DESs mixture decreases the density and viscosity, due to the decrease in hydrogen bonds between the HBA and the HBD of the DESs, as well as an increase in ionic mobility. Van Osch et al. (Gutiérrez et al., 2009) studied the effect of water on the viscosity of various hydrophobic DESs, the water content varied between 200 and 1000 ppm and observed a slight decrease in viscosity with increasing water content. The inverse relationship between viscosity and ionic conductivity leads to an increase in the conductivity of DESs. Viscosity and conductivity are 13 times lower and 10 times higher for reline (ChCl:Urea 1:2) hydrated (Du et al., 2016).

Agieienko et al. (Agieienko & Buchner, 2019) observed a slight decrease in density for the DES ChCl:urea of 0.14% for a 0.008 water mass fraction and 22% for a 0.005 fraction. Shah et al. (Shah & Mjalli, 2014) studied the effect of water on the properties of the reline, a change in the melting point, density and viscosity of the DES was observed. A 10% by weight of water decreases the viscosity by 80% and increases 3 times the conductivity compared to dry DESs.

The linear decrease in viscosity and density in the presence of water is also observed for natural DESs. The conductivity of five natural ternary DESs from choline chloride, organic acids, sugars and water increases to peak values 10 to 100 times higher than pure DES upon addition of 60–80 wt% in water, from these values the conductivity decreases (Dai et al., 2015). For DESs obtained from choline chloride and different glycols such as HBD the viscosity value is reduced by half with the addition of 7–10% by weight of water, and the conductivity increases by 6–15 times when incorporating 60% by weight of water, and then decreases. Ionic dissociation of deep eutectic solvents causes the initial increase in conductivity and the effect of dilution of electrolytes to higher water contents causes the conductivity to decrease. Polarity studies carried out by Gabriele et al. (Gabriele et al., 2019) for these DES showed a linear increase in polarity with increasing water content.

The study of the surface tension of DL-methanol:octanoic acid showed a decrease in surface tension with increasing water content up to a value of 4000 ppm of water, a value above which the surface tension increaseS (Nunes et al., 2019). The same behaviour was observed for choline chloride:malonic acid DESs (Sanchez-Fernandez et al., 2017).

The addition of water increases the dipolarity/polarizability and decreases the basicity of hydrogen bonds for choline chloride:urea, choline chloride:glycerol, and choline chloride:ethylene glycol DES. The fluorescent probes show more intense hydrogen bonding interactions between added water and choline chloride:glycerol DES and choline chloride:ethylene glycol DES compared to urea DES. These differences may be due to structural differences between the HBDs, as well as the interstitial arrangement of water molecules within the choline chloride:urea (Pandey & Pandey, 2014).

Water can be used as an environmentally friendly co-solvent to improve the physical properties of DES for use in various applications (Shaibuna et al., 2022).

Therefore, a fundamental parameter that needs to be controlled is the water content of the DES, given that the proportion of water can significantly modify its properties and this have an influence in the metal recovery methods (Omar & Sadeghi, 2023). So, when the DESs are used the content of water have to be mentioned to the aim to understand different in the DES properties.

5.11. Solubility

DES and their mixture are getting increasing attentions as green solvents alternatives to conventional aqueous and organic solvent because their unique properties, such as high solubilization ability. Due to the flexibility in their numerous combinations of constituents (HBA-HBD) DESs can exhibit high selectivity towards metals of interest, which makes them suitable for use in metal extraction processes.

The ability to accept or donate electrons or protons to form hydrogen bonds gives them these excellent solvation properties (Abbott, Frisch, et al., 2011). They are a suitable medium for the solubilisation of a wide variety of substances, including metal salts. Currently, DESs are among the non-aqueous solvents that have advanced various metal leaching processes by addressing major challenges (Tran et al., 2019) (Dupont & Binnemans, 2015).

In 2003 Abbot et al. demonstrated the ability of DESs for the solution of metal oxides, the solubility of CuO in ChCl:urea at 323 K was 0.12 mol/L (E. L. Smith et al., 2014). he nature of the DESs influences the solubility of the different compounds, Fe₃O₄ is more soluble in ChCl:oxalic acid (1:1) while it is 20 times less soluble in the DES ChCl:phenylpropionic acid (1:2). On the contrary, CuO is more soluble in DES ChCl:phenylpropionic acid (1:2) than in ChCl:Oxalic acid (1:1). These differences in the solubility of the different metal oxides in DESs allow for the selective recovery of metals (Q. Zhang et al., 2012).

Despite the advantages and characteristics of DES, the application of these solvents in metal leaching processes on a pilot or industrial scale is still very limited. (Han et al., 2023). This literature review covers the use of DES as leaching and extraction agents in metallurgical processes.

6.1. Leaching of metals with deep eutectic solvents

The electric arc furnace process of steel production generates large amounts of dust loaded with toxic metals (Zn, Pb, Cd, etc.) present as oxides. Abbot et al. (Abbott et al., 2009) demonstrated the possibility to leach these metals from the dust with DESs of 1 ChCl: 0.5 Urea + 2 Ethylene glycol. The extraction of Zn (60–70%) was efficient and its precipitation was as zinc chloride it was feasible by adding ammonia to the DESs. The extraction of Pb was lower.

Zürner et al. (Zürner & Frisch, 2019) recovered valuable metals (Fe, Zn, Pb, Cu, In and Sn) from zinc flue dust using an choline chloride and oxalic acid based deep eutectics solvent. The leaching rates obtained were higher than 80% for all metals studied after 48 h of reaction at 323 K.

Jenkin et al. (Jenkin et al., 2016) used DESs for the leaching of metals contained in minerals (sulfides and tellurides) as Au, Ag, Bi and Te. The authors showed that selective dissolution could be obtained by electrolysis of the DES. Dissolution rates were similar (t = 5–15 min, T = 318–323 K) to the cyanide processes currently applied.

Riaño et al. (Riaño et al., 2017) studied choline chloride and lactic acid based DESs (1 ChCl:2 Lactic acid) for the leaching of rare earths (Nd, Dy, Pr and Gd) and other metals (Fe, B and Co) from NdFeB magnets. High leaching rates (80%) were obtained after 24 h of leaching at 343 K. The high solubilities of the oxides in the DES can be explained by the reaction of the protons in the lactic acid with the oxides to form water and the coordinating abilities of lactic acid and choline chloride helping dissolution, according to the following equation:

Nd₂O₃ + 6CH₃OHCOOH ↔ 2(NdCH₃OHCOO)₃ + 3H₂O Equation 6.

The formation of a complex between Nd and deprotonated lactic acid allow the solubilization of the metal from the oxide. Thus, the use of acidic DES is favorable for metal leaching (Cui et al., 2017).

Entezari-Zarandi et al.(Entezari-Zarandi & Larachi, 2019) used DESs of choline chloride and urea or malonic acid as hydrogen bond donors for separation of heavy and light rare earths (REs) due to differences in solubility of each metal in the DESs used. Heavy REs are more soluble in DES than light REs.

DESs also have application for leaching of metals from discarded electrical and electronic equipment. Tran et al. (Tran et al., 2019) published that a ChCl:Ethylene glycol DES can extract metals (Li and Co) from spent lithium-ion batteries. High leaching rates were obtained (99.3%) for both metals (t ≥ 24 h, T > 323 K). The DES colour turned blue after leaching, which was attributed to the formation of the CoCl₄^- anion. The leaching of metals seems to occur by coordination with the chloride from ChCl as hydrogen bond acceptor, and this seems to promote the leaching. Despite the high percentage of extraction obtained, the leaching kinetics is slow (t ≥ 24 h) and high temperatures (T > 323 K) are needed. Leaching the same material with nitric acid only takes 2 h and lower temperatures (338 K) obtaining similar leaching yields (Lee & Rhee, 2002).

Chen et al. (Y. Chen et al., 2020) studied different DESs (PEG/thiourea (2:1), p-toluenesulfonic acid/water/ChCl (1:1:1) and ChCl/ethylene glycol (1:2)) to dissolve lithium-ion batteries cathode LiCoO₂ at different temperatures. The authors also use slow leaching kinetics (t = 24 h) and high temperatures (298–433 K). Results showed that the Co concentrations in PEG200/thiourea (2:1) at 393 and 433 K for 24 h were 3480.02 and 8440.14 ppm, respectively, which is much higher than the Co concentration at 353 K for 24 h (2121.14 ppm), showing the increase in mass transfer with increasing temperature. This solubility of LiCoO₂in PEG200/thiourea (2:1) at 353 K is about 2 (1283.11 ppm) and 35 times (61.03 ppm) that in p-toluenesulfonic acid/water/ChCl (1:1:1) and ChCl/ethylene glycol (1:2), respectively, while keeping other conditions identical. At room temperature (298 K), the Co concentration in PEG200/thiourea (2:1) could reach 33.73 ppm at 353 K for 24 h. The recovery of Co from the DESs-LiCoO₂ mixture could be realized by electrodepositing or extraction. The same authors Chen et al. (Y. Chen et al., 2021) later used Polyethylene Glycol-Based Deep Eutectic Solvents (PEG:PSA (1:1)) at low temperatures to dissolve lithium-ion batteries cathode LiCoO₂ with leaching efficiency of 99.5% at 373 K and 70.4% at 353 K, which is much higher than previous reports by DESs in the same condition. This work provides novel DESs for the green and efficient dissolution of lithium cobalt oxide at a relatively mild condition.

Topçu et al. (Topçu et al., 2021) studied a treatment of copper converter slag with a deep eutectic solvent (1 ChCl: 2 Urea) as a leaching agent for recovery copper and zinc. The leaching conditions were T = 298–368 K and long reaction times (t = 2–72 h). The optimized conditions (T = 368 K and t = 48 h) for the leaching process gave a extraction of 89.9% Cu and 65.3% Zn. The total copper recovery was determined as 63% after the iron cementation as metallic copper form. The Zn is recovered by electrowining.

The temperature and time required are lower using leaching agents such citric or oxalic acid. Peeters et al. (Peeters et al., 2020) used DESs made of ChCl and citric acid (2 ChCl: 1 citric acid) diluted with 35 wt% water to leach spent lithium-ion batteries. When 35% water is added, leaching may accelerate due to lower viscosity of the lixiviant. The optimized leaching conditions were 1 h of leaching at 313 K with a solid-to-liquid ratio of 20, which ensured to leach>98% Co (II) of the LiCoO₂. In this study the copper was the most effective reducing agent for cobalt (III), thus requiring no additional reducing reagents. Cobalt in the DES phase was found to exist predominantly in the form of chloro-cobalt complexes due to the interaction with the chloride anion from coline chloride. The use of DESs allowed avoid the release of chlorine gas, which is a major issue when using hydrochloric acid as leaching reactive in the traditional process. The authors also studied the extraction and stripping stages.

This process consisted of a Cu(I/II) extraction step with the extractant LIX 984, followed by selective extraction of Co(II) with the extractant Aliquat 336. Both metals were stripped from the loaded organic phases by oxalic acid. The recovery yield of cobalt was 81%, as a 99.9% pure oxalate precipitate.

Thompson et al. (Thompson et al., 2022) studied choline chloride oxalic acid and based DESs (ChCl:Oxalic acid) to extract Co (II), Mn (II), Li (II) and Ni (II). The authors were able to extract with a stirring time of 2 h, temperature 353 K and a solid–liquid ratio of 15 g/L, achieving the separation of nickel (<10% extraction) under the same experimental conditions (Fig. 7). In the Fig. 8 shows a diagram of the process.

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Fig. 7

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Fig. 8

Tian et al. (Tian et al., 2022) studied a multi-functional DES based on lactic acid (LA) and guanidine hydrochloride (GHC) to extract cobalt and lithium ions from LiCoO₂. Due to the strong acidity (protons) and abundant chlorine coordinating ions of LA/GHC, the solubility of LiCoO₂ in LA/GHC could reach as high as 19.9 mg/g (stirred at 353 K for 24 h). Cl- ions in guanidine hydrochloride have a stronger coordination ability than those in choline chloride (the typical hydrogen bond acceptor in DESs), which could be attributed to the delocalized π bond in guanidine, making Cl^- ions easier to coordinate with metals.

Schiavi et al. (Schiavi et al., 2021) used a novel solvometallurgical process (Fig. 9) with a DES of choline chloride and ethylene glycol (1 ChCl: 2 Ethylene glycol) for the selective recovery of Co from the electrode powder of end-of-life lithium ion batteries (LIBs). Solvometallurgy involves process which uses non-aqueous solvents. These non-aqueous solvents can be molecular organic solvents, ionic liquids, deep-eutectic solvents (DESs), but also the inorganic solvents such as liquefied ammonia, concentrated sulfuric acid, or super-critical carbon dioxide. However, the use of non-aqueous solvents do not imply anhydrous conditions, but rather a low water content, no >50 vol% (Binnemans & Jones, 2017). In the Fig. 10. can be observed this DES reached around 90% Co and 10% Ni extraction at 453 K and 24 h. Co was recovered as cobalt oxalate which was employed to produce lithium cobalt oxide cathode material. The deep eutectic solvent was recycled and reused as feed to the process, with similar results to unused DES.

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Fig. 9

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Fig. 10

Peeters et al. (Peeters et al., 2022) studied choline chloride and ethylene glycol/urea based DESs (1 ChCl:2 EG and 1 ChCl:2 U) to for cobalt recovery from lithium-ion battery cathode materials. 1 ChCl:2 EG was used as a lixiviant at 453 K, for up to 24 h to leach cobalt. The authors showed that the DES 1 ChCl:2 EG is not thermally stable at these elevated temperatures (453 K) and that toxic decomposition products are even formed, such as trimethylamine or 2-chloroethanol. They also utilized el DES 1 ChCl:2 U to leach lithium cobalt oxide (LiCoO₂) at 453 K during 18 h. Unfortunately, an extremely viscous DES leachate was obtained after leaching. The high viscosity of the 1 ChCl:2 U leachate maked it difficult to upscale the recovery process.

Roldán-Ruiz et al. (Roldán-Ruiz et al., 2020) utilized choline chloride and p–Toluene sulfonic acid (PTSA) based DESs (ChCl:PTSA) to cathode recycling of Li batteries. The authors studied the Co and Li recovery from spent LIBs using DESs (PTSA:H₂O:ChCl; 1 PTSA: 2 H₂O: 1ChCl and 1 PTSA: 3 H₂O: 1 ChCl) at temperatures as low as 363 K, at times of reaction of 15 min. The leaching efficient of Co was 97/100/88% with DESs of molar ratio 1:1:1/1:2:1/1:3:1 respectively. The results for leaching of Li were 91/100/85% with DESs of molar ratio 1:1:1/1:2:1/1:3:1 respectively. Co recovery from the PTSA:ChCl-based DESs solutions was accomplished using either Na₂CO₃ or (NH₄)₂CO₃ to precipitate a salt suitable for the preparation of new LIBs (Co(CO₃)0.5(OH)·0·.11H₂O) that, after calcination, resulted in its complete transformation into Co₃O₄. Co recovery efficiencies from spent LIBs was of up to 94% when considering the whole process. In this work, it is worth noting the fast kinetics that leaching presents, since the authors used 15 min for the process.

Luo et al. (Luo et al., 2022) carried out the efficient dissolution of the cathode materials of spent Ni-Co-Mn lithium batteries using deep eutectic solvents. DES was formed from betaine hydrochloride and ethylene glycol (1 Betaine hydrochloride:5 Ethylene glycol), The leaching rates of Ni, Co, Mn, and Li all reached 99% under the conditions of T = 413 K, t = 10 min. Compared with previous work, this study was able to accomplish efficient leaching in an very short time. The ionized hydrogen in the solvent reacted with the oxygen of lithium batteries, thereby destroying the crystal structure. Subsequently, the Cl⁻in B-DES complexed with transition metal ions, which led to lithium batteries dissolution. Further extraction of metal ions from the leachate can be achieved by electrodeposition or extraction.

Recently, other authors (Suriyanarayanan et al., 2023) have also extracted Co from spent lithium-ion batteries using a nonionic deep eutectic solvent (ni-DES) composed of N-methylurea and acetamide under relatively mild conditions (heating at 453 K for 24 h). Cobalt could be recovered with an extraction efficiency of > 97%. Recovered extracted cobalt was used to fabricate new fully functional new lithium-ion batteries from which the cobalt could again be retrieved for subsequent reuse.

Ma et al. (Ma et al., 2023) studied the cobalt and lithium recovery from spent lithium-ion batteries using a microwave-assisted deep eutectic solvents to shorten the extraction time and decrease the leaching temperatureFig. 10. It was found that 99.05% of Li could be selectively leached and 99.21% of Co could be precipitated as cobalt oxalate at 373 K in 10 min simultaneously. The purity of cobalt oxalate precipitated was 95.36% without any purification procedures. It formed a new polymer compound (Co²⁺[(Ch⁺)_x(Cl⁻)_y(HC₂O₄⁻)_z]) during the microwave-assisted leaching process. Fin ally, the authors showed that the DES for the recovery of ithium-ion batteries could be cycled four times with favorable selectivity.

Zhu et al. (Zhu et al., 2023) studied a microwave-enhanced approach to shorten the leaching time in the urea/lactic acid: choline chloride: ethylene glycol DES system for recovery of valuable metals from spent lithium-ion batteries. The synthesized DESs were ChCl: Ethylene glycol, ChCl: Ethylene glycol:Lactic acid and ChCl: Ethylene glycol:Urea. 90% of Li and Co could be fast leached at 4 min and 160 W in the lactic acid/urea: choline chloride: ethylene glycol DES system. The sequences of Li and Co leaching ability at the same conditions were ChCl: Ethylene glycol:Lactic acid DES > ChCl: Ethylene glycol:Urea DES > ChCl:Ethylene glycol DES. That means that acidity and reducibility are helpful in this leaching process. The ultra-fast leaching in this process was achieved because of the electric field of the microwave-induced dipole momentof active components in the DESs system.

Wang et al. (M. Wang et al., 2023) utilized choline chloride and formic acidbased DESs (ChCl:FA) for extraction of critical metals (Li, Ni, Co, and Mn) from cathode materials (LiFePO₄:Li(NiCoMn)_1/3O₂ mass ratio of 1:1) under mild conditions (353 K, 2 h) with a solid − liquid mass ratio of 1:200. The leaching performance of critical metals could be further enhanced by mechanochemical processing (600 rpm, 30 min) because of particle size reduction, grain refinement, and internal energy storage. The system can simultaneously solve the recycling challenges of spent Ni−Co−Mn and LiFePO₄ batteries. Compared with the high temperature (973 K) of pyrometallurgy, the energy consumption of mechanochemical processing combined with DESs would be significantly lower. Compared with hydrometallurgy, the leaching time of critical metals by ChCl–FA could be shortened and rendered applicable for mixed cathode materials. Although the environmental impact of DES system needs to be further quantified, it has undoubtedly suggested considerable potential for future applications.

The analysis of these works shows that time and temperature of leaching with DESs are critical parameters for comparison with the values used in the leaching with mineral acids. The use of DESs formed by citric, oxalic or lactic acid could be an alternative to reduce the temperature and time of leaching con DESs. In these processes considering the physicochemical properties of the mixture is also important since diffusion mechanisms usually govern the mass transfer. In view of the two works done by Ma et al.(Ma et al., 2023) and Zhu et al. (Zhu et al., 2023) it can be considered that using microwaves for the leaching process, the process time is considerably reduced, obtaining similar results in the percentage of extraction of metals contained in Li-ion batteries. Another challenge is the extraction or recovery of the metals from the DES after the leaching. This is a subject currently rarely addressed by authors who have studied metal leaching processes with DESs, so it would be important to optimize the process with these solvents. Another point of interest is to study the corrosivity and long-term behaviour of DESs to determined to allow their application.

6.2. Liquid-liquid extraction of metals with hydrophobic deep eutectic solvents

Most of the above DESs are hydrophilic, which limits their use for metal extraction using L-L extraction. Some authors have studied the use of hydrophobic DESs (HDESs) that can replace conventional solvents used in dissolvent extraction. The first authors who described the ability of HDESs to extract metals from aqueous solutions were Tereshatov et al.(Tereshatov et al., 2016) in 2016. These authors extracted In from hydrochloric media into tetraheptylammonium cloride based HDESs containing carboxylic acids (decanoic acid, oleic acid and ibuprofen) as hydrogen bond donor. The metal extraction was similar to what was obtained with quaternary amines extracting molecules.

The metal extraction with these HDESs is likely to occur thanks to carboxylic acid, according to the follow equation:(7)(Mⁿ⁺)_aq + n(RCOO^-)_aq ↔ (M(RCOO)_n)_DES

The carboxylic acids allowed the extraction of cationic species. Since this mechanism requires the deprotonation of the carboxylic acid, the pH of the aqueous phase should be lowly acidic.

The authors of this work shown a lauric acid y DL-menthol based DES that allowed extract indium in both hydrochloric and oxalic acid media.

Ruggeri et al.(Ruggeri et al., 2019) used decanoic acid and tetrabutylammonium chloride (TBACL) HDESs (1 TBACL: 2 Decanoic acid) with 2.4% w/w H₂O to extract Cr (VI) from aqueous media. Extraction of Cr was higher than 99%. Other metals as Cu (II) and Ni (II) led to the jellification of the HDES.

Ola et al. (Ola & Matsumoto, 2019) used the same HDES (lidocaine:decanoic acid in molar ratio 1:2) for the separation of Fe and Mn from aqueous solutions. Depending on the pH, the extraction could occur by exchange cationic with lidocaine or ion pair formation with decanoic acid. Fe (III) was extracted to pH = 1.0–2.0 by formation of ion pair formation with decanoic acid. At > pH the extraction mechanism cannot be evaluated due to the precipitation of the aqueous phase. Mn (II) was extracted to pH < 2.2 and > 3.5 by formation of ion pair formation with decanoic acid and to pH = 2.2–3.5 by exchange del Mn (II) with lidocaine.

Fig. 11 shows that the concentration of DES needed to reach the completely separation of Fe (III) was lower than that for Mn (II). In the case of Fe (III), the separation was complete at the DES concentration of about 25 g/L, whereas the complete separation of Mn (II) was reached at the DES concentration of about 300 g/L. The authors obtained a high extractability of iron (100% for 10 mmol/L of iron) and a high separability from manganese (separation factors > 100).

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Fig. 11

This method was very selective when was applied to the separation of Fe (III) from Mn (II), because the separation of Fe (III) was complete at the DES concentration of about 25 g/L whereas the complete separation of Mn (II) was reached at the DES concentration of about 300 g/L. Extraction of Fe was 100% (for an iron concentration = 100 mmol/L) and the separation factor Fe-Mn was high (SF_Fe/Mn). h (SF_Fe/Mn).

Three new DHESs based on thymol + TOPO (trioctylphosphine oxide), TOPO + capric acid and hydrocinnamic acid + capric acid were investigated as extracting media for the recovery and separation of platinum group and transition metals in HCl media (Schaeffer et al., 2020). Systems based on TOPO showed good selectivity for Pt⁴⁺, Pd²⁺ and Fe³⁺ ions in the range of conditions studied. The same authors (Schaeffer et al., 2018) used natural HDESs made from terpenes for the extraction of Cu²⁺, Fe³⁺ and Zn²⁺, using decanoic acid combined with thymol or menthol. Its extraction highly depends on the initial concentration of the metal: [Cu]_initial < 0.01 mol/L - E (Cu) > 90%, [Cu]_initial = 0.02 mol/L - E (Cu) < 50%.

Yueyue et al. (Shi et al., 2020) used trioctylmethylammonium chloride and alkyl chain parabens HDESs to separate Cr (VI) from water samples. The extraction efficiency of HDES for Cr (VI) was>90%. After being placed in air for 3 months, the chemical stability, hydro-phobicity and extraction performance of the HDES did not change.

DHESs based on tetra butyl ammonium chloride (TBAC) and oleic acid (TBAC: oleic acid) (W. Chen et al., 2021) were investigated for selective lithium recovery from the mother liquor obtained during the process of Li₂CO₃ production. The authors used different molar ratios to carry out the tests (TBAC:Oleic acid, 1 TBAC:2 Oleic acid and 1 TBAC:3 Oleic acid). In the Fig. 12a it can be observed that when the organic phase is pure HDES, the extraction efficiency of Li⁺ is 19.1% without ammonia in aqueous phase. It increased to 23.2% when ammonia is added into the aqueous phase. When TBP is used as organic phase, the extraction efficiency is relatively low and almost unchanged, regardless of whether ammonia is added to the aqueous phase or not. However, when the mixture phase of HDES and TBP is used as the organic phase, the extraction efficiency increased significantly from 11.8% to 45% with the addition of ammonia in the aqueous phase. The results show that ammonia and HDES played a crucial role in Li⁺ extraction, suggesting the synergism of HDES, TBP and ammonia.

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Fig. 12

Fig. 12b shows the influence of the DES concentration on the extraction performance. As the concentration of HDES increases, the extraction efficiency increases. Li recovery was 45.0%, 52.4% and 63.1% (TBAC/OA, TBAC/2OA and TBAC/3OA respectively). With the improvement of extraction performance, the content of TBAC cation (TBA⁺) in aqueous phase also increases. The Li extraction process occurred by a cation exchange mechanism between Li⁺ and TBA⁺, which improves the extraction efficiency of metal ions. Stripping of lithium was carried out with solutions of hydrochloric acid 0.5–2.5 mol/L, being the optimal concentration used 1 mol/L HCl. Stripping was 96.8% Li. Herein, HDESs have great potential to replace traditional organic solvents and ionic liquid in liquid–liquid extraction.

Hahada et al.(Hanada & Goto, 2021) prepared HDESs for Li recovery from a model brine solution containing a high concentration of alkali metals. The recovery of Li with the HDES (1 trioctylphosphine oxide (TOPO):2 thenoyltrifluoroacetone (HTTA)) was 95.7% (T = 298 K, t = 1 h, A/O ratio = 2/1). Furthermore, stripping of Li from the metal-loaded HDES phase was achieved using 1 M HCl and 89% of the Li could be recovered in a two-stage stripping procedure.

Milevskii et al.(Milevskii et al., 2022) studied a process based on Aliquat 336 and L-menthol to form a HDES with for the extraction of Li (I), Co (II), Ni (II), Mn (II) and Fe (III), from aqueous solutions in hydrochloric acid medium. The authors studied different molar ratios for the experimental tests (3 Aliquat 336:1 L-menthol, 3 Aliquat 336:2 L-menthol, 3 Aliquat 336: 3 L-menthol and 3 Aliquat 336: 7 L-menthol), being 3 Aliquat 336: 7 L-menthol the most convenient molar ratio. The extraction process carried out in different stages. In a first stage the authors recovered 99% of Fe (III) (with 1 mol/L HCl in solution and O/A ratio = 1/5). In a second stage they recovered 99% of Mn (II) (with 3 mol/L HCl in solution and O/A ratio = 1/5) using 20 extraction stages. Co (II) recovery was 99% (with 5 mol/L LiCl in solution and O/A ratio = 1/5) using 3 extraction stages. The stripping of each metal was studied until two cycles, 1 mol/L NaH₂PO₄ and 0.5 mol/L H₃PO₄ were used as stripping agents for Fe (III), 0.01 mol/L HCl was used for Co (II) and H₂O for Mn (II). In all cases, 99% stripping efficiency was obtained. The Ni (II) was separated via precipitation, leading to a concentrated hydrochloric solution of Li (I).

Lastly, Kozhevnikova et al. (Kozhevnikova et al., 2022) used Aliquat 336, Di-(2-ethylhexyl)phosphoric acid (D2EHPA) and menthol based HDESs for the extraction of metal ions (Co (II), Cu (II), Ni (II), Fe (III), Al (III) and Li (I)) from a real hydrochloric acid solution after leaching the cathodes of three different types of Li-ion batteries (LIB 1, 2 and 3). The optimal HCl leaching conditions chosen were 2 M HCl, T = 353 K, t = 6 h and S/L ratio = 1/25. The eutectic mixtures used were Aliquat 336: menthol (HDES 1) and D2EHPA: menthol (HDES 2). In LIB 1 the authors extracted Cu (II) and Co (II) with HDES 1 and Al (III) with HDES 2, obtaining a rafinate of Li (I). In LIB 2 they extracted Fe (III) and Co (II) with HDES 1 and Al (III) with HDES 2 leading to a raffinate of Ni (II) and Li (I). The Ni (II) was separated via precipitation. In LIB 3 they extracted Fe (III) and Al (III) with HDES 2 obtaining a raffinate of Li (I). The extraction rate of valuable individual elements was above 98%.

The metals (Cu, Co, Al and Fe) stripping of LIB 1 and 2 was realized with water from the HDES phase. Al (III) and Fe (III) of LIB 3 were re-extracted with a HCl solution.

The studies of liquid–liquid extraction de metals applying DESs as solvent media are recent in the literature, and still lacking information about the process. The structure and the nature of the HBA and the HBD have impact on the ability to extract the metals using liquid–liquid extraction (Tereshatov et al. 2016), more recently, different authors have used TOPO-based DESs, and carboxylics acids as HBDs to produce DESs and achieve this separation (Hanada et al, 2021) (Shaeffer at al., 2018) (Phelps et al., 2018) (Gilmore et al., 2018). The properties of DESs for liquid–liquid extraction of metals are still limited by their intrinsic properties, particularly their high viscosity. In these processes extraction of metals the cation exchange mechanism leads to the contamination of the aqueous phase and should be avoided. The pH is related to precipitation and metal complex formation, which depends on the nature of the metal studied; therefore, the impact of this parameter must be addressed in the extraction processes. Another challenge is the recycling and reuse of DESs after extraction.

6.3. Electrochemical recovery of metals with deep eutectic solvents

In 2013, Mendes et al. (Mendes et al., 2013) used 1 ChCl:1.5 EG:0.5 Malonic acid DES to show that solvents made with complex agents as ethylene glycol/malonic acid increase the electrical conductivity of the solvents when compared to the standard DES (pure hydrogen bond donor). The metals were dissolved in the DES solution for 48 h at 353 K. It was demonstrated the use of DES to dissolve large amount of minerals (Ag, Al, Co, Cr, Cu, Ni, Pb, Zn, Au, etc).

Poll et al. (Poll et al., 2016) used etilenglycol and choline chloride based DESs (1 ChCl:2 Ethylene glycol) to recycling of Pb from hybrid perovskites of solar panels using the electrodeposition technique. 99.8% of Pb was removed from the DES with 120 h of electrodeposition, using a potential of −0.9 V vs. Ag. The same technique was applied to the extraction of Zn from electric arc furnace dust (Bakkar & Neubert, 2019). The DES used was 1.5 ChCl:1.5 Urea:0.5 EG. The arc furnace dust was dissolved in the DES, then the electrochemical behaviour of Zn was established using cyclic voltammetry. 38% of Zn was removed from the DES with 48 h of electrodeposition, using a potential of −1.2 a −1.5 V vs. Ag. The authors showed that the process is limited by the diffusion of Zn ions in the DES. Bakkar et al. (Bakkar, 2014) show that about 60% of Zn and 39% of Pb could be dissolved from the dust when stirred for 48 h in the 1 ChCl:2 Urea DES at 60 °C.

Almeida et al. (Almeida et al., 2020) studied the recovery of W and As from secondary mine resources with a malonic acid + oxalic acid and choline chloride based DES (1 ChCl:2 Malonic acid:1 Oxalic acid). 82% of As and 77% of W were recovered by electrodialysis using a current of 100 mA for 4 days.

Wang et al. (S. Wang et al., 2020) used DESs made of ChCl and urea (1 ChCl:2 urea) for recycling of spent Li-ion batteries (LIBs), using the cyclic voltammetry technique to determine the reducing power of DESs. The Li and Co extraction efficiency was around 95% at a temperature of 453 K and duration of 12 h. Kinetic essays kinetic tests showed that the solution diffusion and electron diffusion through the DES controlled the metals extraction. The time needed for leaching was long and the temperature high compared to leaching using minerals acid. The cobalt was recycled as a cubic cobalt oxide spinel (Co₃O₄) using H₂C₂O₄ and NaOH precipitants using a dilution-precipitation-calcination process.

Perera et al. (Perera et al., 2023) studied the cobalt electrochemical recovery using DESs compounded of ethylene glycol (EG) (67 M %, 82 M % or 100 M %) and choline chloride (ChCl). The electrochemical results show that increasing the amount of EG together with a small concentration of sulfate anions, in conjunction with Cl^- anions (two different cobalt sources, CoCl₂·6H₂O and CoSO₄·7H₂O, were studied), in the solution mixture favours the reduction of Co²⁺. This improved electrochemistry seems to be related to changes of Co²⁺speciation easing the reduction process. The nature of the Co salt had a significant impact on the recovery efficiency, morphology, and purity of the Co electrodeposit.

The recovery of metals through of electrochemical techniques with DESs could be a via promising, but in some cases the efficiency was not good (Abbott et al., 2009) and sometimes the reaction times requires to recover the metals are long. The water content and decomposition of the DEs during the application of a current lead to a decrease in current efficiency, which appears to be a clear limitation and deserves further investigation. The use of low viscosity DESs will also be a priority to favor mass transfer kinetics.