Recovery of Metals from Electronic Waste-Printed Circuit Boards by Ionic Liquids, DESs and Organophosphorous-Based Acid Extraction

The extraction of metals from waste printed circuit boards (WPCBs) with ionic liquids (ILs), Deep Eutectic Solvents (DESs) and organophosphorous-based acid (Cyanex 272) has been presented. The study was undertaken to assess the effectiveness of the application of the new leaching liquids, and the new method of extraction of metals from the leachate and the solid phase with or without the leaching process. Solvent extraction from the liquid leachate phase has been studied in detail with popular ILs, such as tetraoctylphosphonium bromide, {[P8,8,8,8][Br] and tributyltetradecylphosphonium chloride, [P4,4,4,14][Cl] using Aqueous Biphasic Systems (ABS) method. Trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate, [P6,6,6,14][Cyanex272], ([P6,6,6,14][BTMPP]), trihexyltetradecylphosphonium thiocyanate, [P6,6,6,14][SCN], methyltrioctylammonium chloride (Aliquat 336), as well as bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) were also used in the extraction of metals from the leachate. Two DESs (1) {choline chloride + lactic acid, 1:2} and (2) {choline chloride + malonic acid, 1:1} were used in the extraction of metals from the solid phase. The extraction behavior of metals with DESs was compared with that performed with three new bi-functional ILs: didecyldimethylammonium salicylate, [N10,10,1,1][Sal], didecyldimethylammonium bis(2-ethylhexyl) phosphate, [N10,10,1,1][D2EHPA], and didecyldimethylammonium bis(2,4,4-trimethylpentyl) phosphinate, [N10,10,1,1][Cyanex272]. The [P6,6,6,14][Cyanex272]/toluene and (Cyanex 272 + diethyl phosphite ester) mixtures exhibited a high extraction efficiency of about 50–90% for different metal ions from the leachate. High extraction efficiency of about 90–100 wt% with the ABS method using the mixture {[P8,8,8,8][Br], or [P4,4,4,14][Cl] + NaCl + H2O2 + post-leaching liquid phase} was obtained. The DES 2 revealed the efficiency of copper extraction, ECu = 15.8 wt% and silver, EAg = 20.1 wt% at pH = 5 from the solid phase after the thermal pre-treatment and acid leaching. The solid phase extraction efficiency after thermal pre-treatment only was (ECu = 9.6 wt% and EAg = 14.2 wt%). The use of new bi-functional ILs did not improve the efficiency of the extraction of metal ions from the solid phase. Process factors such as solvent concentration, extraction additives, stripping and leaching methods, temperature, pH and liquid/solid as well as organic/water ratios were under control. For all the systems, the selectivity and distribution ratios were described. The proposed extraction processes can represent alternative paths in new technologies for recovering metals from electronic secondary waste.

It is well known that annually over 70 million tons of e-waste from various technological processes should be recycled based on pyrometallurgy, hydrometallurgy or solvometallurgy. In hydrometallurgical processes, acid and alkaline leaching is generally employed to dissolve metals such as Cu, Au, Ag, and Pd from the spent e-waste [1][2][3]. Recent studies have proposed extraction with ionic liquids (ILs) by liquid-liquid extraction (LLE) and electrodeposition [4]. Bi-functional ILs have been used for the extraction of Mo and V from spent petroleum catalysts [5]. As an example, the IL was constructed from IL methyltrioctylammonium chloride, [N 8,8,8,1 ][Cl], (Aliquat 336) and bis (2-ethylhexyl)hydrogen phosphate, D2EHPA as a very active cationic-anionic extractant of metals in the presence of H 2 O 2 [5].
Copper extraction at the level of 72 wt% from the "black mass" of waste Li-ion batteries was obtained using trihexyltetradecylphosphonium thiocyanate, [P 6,6,6,14 ][SCN] with the addition of H 2 O 2 and didecyldimethylammonium chloride surfactant, DDACl, as well as using Cyanex 272 with the addition of diethyl phosphite ester [9]. Extraction of copper from solid e-waste with hydrogen sulphate ammonium and imidazolium-based ILs, such as [N 1,1,8,H ] [HSO 4 ] with the addition of H 2 O 2 at T = 348 K (1 h) was obtained at the level of 20-33 wt% [10]. 98% Extraction of copper from the solid phase was obtained with the use of bisulphate ILs: [ 6 ] and Cyphos 101 ([P 6,6,6,14 ][Cl]) after preliminary leaching with (H 2 SO 4 + H 2 O 2 ) and then (35% HCl + 55% HNO 3 ) was obtained at the of level 90% and 99% for Cu at the temperature T = 343 K, pH = 1 with a solid to liquid ratio of 1:1 [12]. The aqueous solution contained the following species of metals: AgCl, CuCl 2 , CuCl + and CuCl 4 2− [12]. The use of ILs in hydrometallurgy has been popular for many years [13]. The extraction of many different metals from various materials with ILs is well described [14][15][16][17]. The popular IL, used for the extraction of metals from petroleum catalysts waste and e-waste is 1-butyl-3-methylimidazolium hydrogen sulphate, [BMIM] [HSO 4 ] [18]. It is a non-toxic, easily synthesized IL with the possibility of recirculation [18].
[BMIM] [HSO 4 ] was also used instead of H 2 SO 4 in thiourea for Au extraction with the use of oxygenating agents such as Fe 2 O 3 , H 2 O 2 , or KHSO 5 [15,16]. The hydrogen sulphate anion was used with different cations such as [HMIM] + , [OMIM] + and [EMIM] + [19,20]. IL [BMIM] [HSO 4 ] was used for the extraction of Cu (82%) and Zn (99%) from brass waste with oxidizing agents such as H 2 O 2 and KHSO 5 at room temperature using the ratio IL:A = 1:1 v/v [14,21]. IL 1ethyl-3-methylimidazolium hydrogen sulphate, [EMIM] [HSO 4 ] was used for the extraction of metals Fe (80%), Sc (68%), Ti (60%) and Al (20%) from boxes at high temperature [22]. In general, many ILs were used for the extraction of metals in acidic solutions [23,24], as well as bi-functional ILs extractants used at a specified pH with the addition of Na 2 SO 4, NaCl, or NaNO 3 [24]. ILs with ammonium, or phosphonium cations and different functionalized anions such as thiol-, thioether-, hydroxyl-, carboxylate-and thiocyanate have been used for the extraction of metals from communal "wastewater" with a high extraction efficiency of 95% for Ag, Cu, Hg and Pt [25]. IL [BMIM][NTf 2 ] was found to be a good agent for the extraction of Au from chlorinated aqueous solutions [26]. Phosphonium IL, Cyphos 101 was used with good results for the extraction of many metals including Au(III) from aqueous solutions with HCl [23,27,28]. The technique of biopolymer capsules was used for the extraction of Au(III) from aqueous solutions with HCl [28]. The use of tributylmethylammonium chloride, [N 4,4,4,1 ][Cl] with the addition of trichloroizocyanic acid (TCCA) showed 100% extraction of metals: Au, Pd, Cu, and Ag at low temperature T = 298 K [29].
One of the first pieces of information about the extraction of Cu, Zn and Al from solid WPCBs material was presented with [BMIM] [HSO 4 ] and H 2 O 2 at the temperature T = 343 K for 2 h [30]. The extraction of Cu (100%) was carried out from a solution of 25 cm 3 80% IL v/v and 10 cm 3 30% H 2 O 2 in a ratio of solid:liquid = 1:25 [30].
The recycling processes of Au, Ag, Pd, and Pt from WPCBs with the use of glycine and sodium cyanide were presented in [31], as well as Cu [32]. However, a very inconvenient solid to liquid phase ratio of of 1:100 was used with the addition of 10% H 2 O 2 at the temperature T = 303 K (2 h), pH = 6-6.5 [32]. The Cu(II) extraction efficiency was 94%. Glycine is the simplest environmentally friendly amino acid and is used in heteronuclear complexes with metal ions.
In this work, the extraction of metals from WPCBs after thermal pretreatment at the temperature T = 1023 K for 7 h and different acid leaching procedures with ionic liquids (ILs), DESs and Cyanex 272 have been proposed.
Liquid The results of this research may provide a different ecological and efficient approach for Cu and Ag extraction from WPCBs at different costs. Table 1 summarizes the results of the metal content in the solid WPCBs samples of the starting material (see Figure 1) and the material after thermal pre-treatment (see Figure 2) and then the I and II leaching processes (see Figures 3 and 4). Table 1 shows that after the I leaching, the copper concentration decreased from 335 g/kg to 235 g/kg and the mass of the solid phase decreased from 0.670 kg to 0.575 kg. It is clear from this result that part of the copper has passed into the liquid phase.  9 11.5 < * < < < < * The symbol ''<" indicates the element content below the limit of quantification of used.

Analysis of the Solid WPCBs Samples and Post-Leaching Solutions
1 cm    9 11.5 < * < < < < 0.328 * The symbol ''<" indicates the element content below the limit of quantification of the test method used.    9 11.5 < * < < < < 0.328 * The symbol ''<" indicates the element content below the limit of quantification of the test method used.   . Solid WPCBs material after thermal pre-treatment a g/dm 3 (NH2)2CS + 13 g/dm 3 Fe2(SO4)3). The amounts of metals leached into the liquid phas cesses are presented in Table 2. The results of the solid content after alkalization of the post-leaching solutions to phase, for the II liquid leachate phase (first step), and for ond step) are listed in Table 3. Table 4 presents the resu analysis for metal content in the liquid phase after alkali tions to pH = 3. The amounts of metals leached into the liquid phase after the I and II leaching processes are presented in Table 2. The results of the solid phase sample analysis for metal content after alkalization of the post-leaching solutions to pH = 3, for the I liquid leachate phase, for the II liquid leachate phase (first step), and for the II liquid leachate phase (second step) are listed in Table 3. Table 4 presents the results of the liquid phase samples analysis for metal content in the liquid phase after alkalization of the post-leaching solutions to pH = 3.   The concentration of Pd and Au in the solid phase after leaching was below the detection limit of the FAAS method and is not listed in Table 1. The Sn content was not measured at all by FAAS in the solid samples. This element has not been the main focus of metal recovery research. The ICP-OES analysis has shown more metals in the liquid phase samples after the leaching process (see Table 2). Table 1 shows that after the I leaching, Cu(II) passed into the liquid phase from 335 g/kg (solid material after the thermal pre-treatment) to 235 g/kg. The other metals for the most part remained in the solid phase. After the II leaching (second step), a much greater transfer of Ag(I) and other metals (except Al and Fe) to the liquid phase was observed, and the Ag(I) content in the solid material changed from 721 mg/kg to almost 0 mg/kg and Cu(II) from 335 g/kg to almost 0 g/kg. The results in Table 1 were calculated taking into account the mass changes of the samples after the thermal pre-treatment and all steps of leaching. As can be seen from Table 1, the further extraction of metals from the solid material after the II leaching (second step) does not have the essential meaning as this solid material does not contain important metals such as copper or silver. The results of the ICP-MS analysis of the liquid phase samples for the metal content after the leaching processes, revealed 29544 mg/kg of Cu(II), 6985 mg/kg of Fe(II), 6110 mg/kg of Al(III), 2205 mg/kg of Zn (II) and 1194 mg/kg of Pb(II) after the II leaching (first step) (see Table 2).

cm
After this procedure, the post-leaching solutions were treated with NaOH (solid) until pH = 3 for the precipitation of iron and aluminum hydroxides. The obtained grey and white solid phases were analyzed by the SEM/EDS method (see Table 3) and the other liquid phases were analyzed by the ICP-OES technique (see Table 4).
The SEM images and EDS spectra, taken from micro-areas of the solid samples are shown in Figures 5-9. Irregular shapes and agglomerates of grains were observed in the deposited films. As shown in Figure 5, for the grey solid phase obtained from the liquid leachate phase I, the coexistence of oxygen (O), sodium (Na), sulfur (S), iron (Fe), copper (Cu) and tin (Sn) is observed on the surface. In the EDS spectra ( Figure 6) taken from microareas of the white solid phase obtained from the liquid leachate phase I, such elements as oxygen (O), sodium (Na), sulfur (S), iron (Fe) and copper (Cu) were identified on the surface. For the brown solid phase sample, stripped from the liquid leachate phase II (first step) ( Figure 7) the SEM/EDS analysis showed the presence of oxygen (O), sodium (Na), aluminum (Al), silicon (Si), iron (Fe), copper (Cu) and calcium (Ca) on the surface.            As a result of the semi-quantitative analysis by the SEM/EDS method, it was found that the obtained solid phases contained mainly the above-mentioned elements, such as e.g., copper (7-11 wt%) or iron (9-11 wt%) in the brown solid phase precipitated out from the post-leaching solution II (first step) (see Table 3). Table 4 presents the results of the ICP-OES analysis for the metal content in the liquid phases at pH = 3 after the leaching processes I and II.

Extraction with Ionic Liquids and Organophosphorus Based Acid from the Post-Leaching Solutions
In this study, the IL was used to leach various metals from WPCBs after the process of thermal pre-treatment at the temperature T = 1023 K for 7 h and acid leaching as described above. All ILs used in this work are listed in Table 5 and the other chemicals in Table 6. As a result of the semi-quantitative analysis by the SEM/EDS method, it was found that the obtained solid phases contained mainly the above-mentioned elements, such as e.g., copper (7-11 wt%) or iron (9-11 wt%) in the brown solid phase precipitated out from the post-leaching solution II (first step) (see Table 3). Table 4 presents the results of the ICP-OES analysis for the metal content in the liquid phases at pH = 3 after the leaching processes I and II.

Extraction with Ionic Liquids and Organophosphorus Based Acid from the Post-Leaching Solutions
In this study, the IL was used to leach various metals from WPCBs after the process of thermal pre-treatment at the temperature T = 1023 K for 7 h and acid leaching as described above. All ILs used in this work are listed in Table 5 and the other chemicals in Table 6. Table 5. Data on the ionic liquids used: structure, name, abbreviation of name, supplier, CAS number, molar mass (M), mass fraction purity (as stated by the supplier). the post-leaching solution II (first step) (see Table 3). Table 4 presents the results of the ICP-OES analysis for the metal content in the liquid phases at pH = 3 after the leaching processes I and II.

Extraction with Ionic Liquids and Organophosphorus Based Acid from the Post-Leaching Solutions
In this study, the IL was used to leach various metals from WPCBs after the process of thermal pre-treatment at the temperature T = 1023 K for 7 h and acid leaching as described above. All ILs used in this work are listed in Table 5 and the other chemicals in Table 6. Table 5. Data on the ionic liquids used: structure, name, abbreviation of name, supplier, CAS number, molar mass (M), mass fraction purity (as stated by the supplier). the post-leaching solution II (first step) (see Table 3). Table 4 presents the results of the ICP-OES analysis for the metal content in the liquid phases at pH = 3 after the leaching processes I and II.

Extraction with Ionic Liquids and Organophosphorus Based Acid from the Post-Leaching Solutions
In this study, the IL was used to leach various metals from WPCBs after the process of thermal pre-treatment at the temperature T = 1023 K for 7 h and acid leaching as described above. All ILs used in this work are listed in Table 5 and the other chemicals in Table 6. the post-leaching solution II (first step) (see Table 3). Table 4 presents the results of the ICP-OES analysis for the metal content in the liquid phases at pH = 3 after the leaching processes I and II.

Extraction with Ionic Liquids and Organophosphorus Based Acid from the Post-Leaching Solutions
In this study, the IL was used to leach various metals from WPCBs after the process of thermal pre-treatment at the temperature T = 1023 K for 7 h and acid leaching as described above. All ILs used in this work are listed in Table 5 and the other chemicals in Table 6. ICP-OES analysis for the metal content in the liquid phases at pH = 3 after the leaching processes I and II.

Extraction with Ionic Liquids and Organophosphorus Based Acid from the Post-Leaching Solutions
In this study, the IL was used to leach various metals from WPCBs after the process of thermal pre-treatment at the temperature T = 1023 K for 7 h and acid leaching as described above. All ILs used in this work are listed in Table 5 and the other chemicals in Table 6.  The extraction efficiency (E) and distribution ratio (D) were determined using the following equations:  Table 7. As shown in Table 7, all metals were not quantitatively extracted in one extraction step, meaning that there is a need for a scrubbing step with different analytical methods for different metals after the extraction process.
The mechanism for the metal extraction from the aqueous solution to the IL phase is proposed as an "ion exchange reaction". The metal cation has to replace the IL cation, which moves to the aqueous phase [33]. In general, the reaction with IL is as follows: The leaching time, IL concentration, hydrogen peroxide addition, aqueous to organic phase (1:1) and temperature were similar in all experiments. The results obtained with Aliquat 336 and [P 6,6,6,14 ][SCN] with the addition of hydrogen peroxide in the liquid phase were not successful. Table 7. Results of metals extraction with ILs from the post-leaching solution I, extraction efficiency (E/wt%), distribution ratio (D), pH of the aqueous phase after extraction at the temperature T = 303 K.  Table 8. The copper was extracted to the IL phase with the extraction efficiency of 96.4 wt%-100.0 wt% and distribution ratio, D Cu = 0.96-1, and silver with the extraction efficiency of 88.9 wt%-100.0 wt%, D Ag = 0.90-1 using two ILs with the addition of H 2 O 2 . Satisfied results with both ILs were also obtained for Fe(II), E Fe = 82.6 wt%-94.1 wt%, D Zn = 0.82-0.94 excluding the system {[P 4,4,4,14 ][Cl] + NaCl + H 2 O 2 + liquid leachate phase II (first step)}, E Fe = 45.5 wt%, D Fe = 0.45. Similar very good results were obtained for Zn(II) with the extraction efficiency of E Zn = 99.5 wt%-100.0 wt% and distribution ratio D Zn = 0.99-1. Only Al(III) after extraction was transferred mainly to the aqueous phase with the extraction efficiency of E Al = 78.2 wt%-99.7 wt%. Unfortunately, all metals were transferred together to the organic phase (excluding Al(III)). No quantitative extraction of the various metal ions to the organic, or aqueous phase was observed. The process of extraction from the post-leaching solutions using the ABS method is more attractive and the results are much better than those, obtained with ILs. The results presented in Table 8   In the ABS method, the salt-out reagent, NaCl helps to increase the effective activity of metal ions in the solution. Under the acidic conditions used, M 2+ metal ions and MeCl 2 complexes (coming from NaCl and the IL) are in the solution [34].

Extraction with DESs and bi-Functional ILs from the Solid WPCB Samples
In recent years DESs have been used as primary extracting solvents for the extraction of metal ions from the solid material of spent batteries [9]. The advantage of using DESs in place of the ILs is their lower cost (especially those based on choline chloride) and less corrosive effects on the equipment.
In this work, the solid material after thermal pre-treatment at the temperature T = 1023 K for 7 h and acid leaching is used directly in the DES solution with the addition of DDACl and H 2 O 2 . The solid WPCBs sample was stirred with the solution of (DES + H 2 O + DDACl + H 2 O 2 ) for 2 h at the temperature T = 333 K. Two popular DESs were used for the extraction of metal ions: DES 1 (choline chloride:lactic acid, 1:2) and DES 2 (choline chloride:malonic acid, 1:1). After phase separation in the mixtures, the content of metal ions were analyzed. The results of the extraction of metals from the solid material after the I leaching with {4M H 2 SO 4 + 100 g/dm 3 (NH 2 ) 2 CS + 13 g/dm 3 Fe 2 (SO 4 ) 3 } are presented in Table 9. Unfortunately in the case of DES 1, the high extraction efficiency was observed at pH = 2. Three new bi-functional ILs were used for the extraction of metal ions for comparison with DESs. The results of the extraction of metals from the solid material after thermal pre-treatment (T = 1023 K, 7 h) and after the I leaching with {4M H 2 SO 4 + 100 g/dm 3 (NH 2 ) 2 CS + 13 g/dm 3 Fe 2 (SO 4 ) 3 } are presented in Table 9, and for the solid WPCBs sample after thermal pre-treatment (T = 1023 K, 7 h) without leaching in Table 10. High efficiency of metal extraction from the solid material after the I leaching with {4M H 2 SO 4 + 100 g/dm 3 (NH 2 ) 2 CS + 13 g/dm 3 Table 9. Results of metals extraction from the solid WPCBs material after thermal pre-treatment (T = 1023 K, 7 h) and the I leaching {4M H 2 SO 4 + 100 g/dm 3 (NH 2 ) 2 CS + 13 g/dm 3 Fe 2 (SO 4 ) 3 }, with DESs and bi-functional ILs, extraction efficiency (E/wt%), distribution ratio (D) and pH of the aqueous phase after extraction. DES 2 and DES 2 with the addition of Na 2 SO 4 were used for the extraction at pH = 5 from the solid material after the process of thermal pre-treatment at the temperature T = 1023 K for 7 h. The addition of Na 2 SO 4 did not improve the extraction. The results for DES 2 (without Na 2 SO 4 ) were as follows: for Al(III), E Al = 67.3 wt%, D Al = 0.67, for Cu(II), E Cu = 9.6 wt%, D Cu = 0.09, and for Ag(I), E Ag = 14.2 wt%, D Ag = 0.14 (see Table 10). The results of extraction with DES 2 at pH = 5 are slightly worse than those obtained for the solid material after leaching (E Cu = 15.8 wt% and E Ag = 20.1 wt%).

Mixture
The mechanism of "ion-pairing" extraction, or two of them with "ion exchange" extraction may be the explanation of the mechanism of extraction with DES [33]. The "ion-pairing" mechanism of DES extraction of metal ions from the liquid phase is, for example, depicted by the following equation Thus, with DES, it is possible to obtain better extraction of metal ions with two possible mechanisms. However, the volume of the organic phase after the extraction was always much lower than the volume of the aqueous phase.
The use of new bi-functional ILs did not improve the metal ions extraction efficiency. The results are slightly worse than those obtained for DES 2. The best results were obtained with [N 10,10,1,1 ][Sal], where the extraction efficiency for Al(III) was E Al = 34.0 wt%, D Al = 0.34, and E Cu = 6.0 wt%, D Cu = 0.06 for Cu(II) and E Zn = 6.4 wt%, D Zn = 0.06 for Zn(II). Unfortunately, the extraction efficiency for silver ions was very low, E Ag = 0.4 wt% with the distribution ratio equal to zero. The organic phase of [N 10,10,1,1 ][Sal] was stripped twice and the results for both phases are shown. This is obvious that the method described above for DES 2 is much better. However, the separation of copper and silver from the other metals was unsuccessful. The separation of copper, zinc, and aluminum after the precipitation of silver with NaCl has to be solved.
The extraction from the solid material in the presence of bi-functional IL and the possible interaction with acidic anion may be interpreted as follows [38]:  Table 10). The extraction of all metal ions was not successful.
Nevertheless, the extraction of metal ions from solid material without leaching is a less-cost evolution of the recycling process for WPCB materials. However, extraction from the post-leaching solutions and from the solid material after leaching can result in almost 80-100 wt% extraction of metal ions from WPCBs materials. Unfortunately, in most of the processes, the distribution ratio is too low for technological use.

Materials
Samples of WPCBs came from Elemental H2Tech waste management in Poland. The WPCB blend was cut into small pieces and then crushed in a hydraulic press (see Figure 1). The final particles were put to the trial of thermal pre-treatment at the temperature T = 1023 K for 7 h in a Resistance Chamber Furnace (IZO), 16.1 kW. The mass of the sample after the thermal pre-treatment changed from 1.025 kg to 0.691 kg, a difference of 32.5%. In this way, undesirable coarse particles including plastic and different organic substances were eliminated (see Figure 2). Next, the solid material was additionally under pulverization process to obtain as small particles as possible.
Due to the heterogeneity of the solid WPCBs sample (see Figure 1) and the related difficulty in taking a representative sample for analysis, the metal content in the starting material used for ILs extraction was calculated on the basis of quantitative analysis (mass balance) of the residue and the solution obtained after leaching with aqua regia of the solid WPCBs sample (1 kg) after thermal pre-treatment (as a sum of metal content in the residue and solution after leaching). The metal ions contained in the aqueous solution were determined by the ICP-MS method using the Agilent 7900 inductively coupled plasma mass spectrometer. For determination of metal content in the solid phase (pre-analyzed for elemental composition by the SEM/EDS method), the Milestone UltraWAVE digestion system combined with the PerkinElmer AAnalyst 800 atomic absorption spectrometer (FAAS technique) was used. Metals were determined by the FAAS method after microwaveassisted digestion of the sample in an UltraWAVE mineralizer using concentrated nitric acid as solvent (acid-insoluble parts of the sample were leached in aqua regia and then fused with K 2 S 2 O 7 and Na 2 CO 3 to ensure complete dissolution of the sample prior to quantitative analysis).
The post-leaching liquid phase (leaching I and II) of pH = 1 was alkalized with solid NaOH to pH = 3. As a result, two different solid phases were obtained: grey one and white one, which were separated from the post-leaching liquid phases. Next, the analyses by the SEM/EDS method (solid phases) and the ICP-OES technique (liquid phases) were carried out. The Jeol JSM-6490 LV scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS), and the Thermo IRIS Advantage inductively coupled plasma optical emission spectrometer were used.

Reagents and Chemicals
The ILs used in this work were obtained from different firms such as Alfa Aesar, IoLiTec, Sigma Aldrich, or Heavy Water, or were synthesized in our laboratory. The chemical structure, name, abbreviation of the name, molar mass and mass fraction purity of the ILs are listed in Table 5. The synthesis and 1 H NMR and 13 C NMR spectra of the new three bi-functional ILs are presented in the Supplementary Material (SM). The list of solvents and other chemicals used is presented in Table 6. All other reagents employed in this work were of analytical grade. The water used was deionized by a Millipore purification system.
The samples of ILs were dried for 72 h at T = 340 K under reduced pressure, p = 6 kPa to remove volatile impurities and trace amounts of water. The water content in IL was analyzed by the Karl-Fischer titration technique (Metrohm, 716 DMS Titrino). The mass fraction of water in a sample was less than 800 × 10 −6 g with an uncertainty of u(w.c.) = 10 × 10 −6 g. The uncertainty of the temperature measurements was ±0.1 K. All weighing involved in the experimental work was carried out using a Mettler Toledo AB 204-S balance, with an accuracy of ±1 × 10 −4 g.

Synthesis of DESs
DESs were proposed as primary solvents for the extraction of metal ions from the solid phase. The IL used for the preparation of DESs was dried under reduced pressure (10 hPa) at the temperature T = 323K for 8 h.  [40]. The synthesis of these DESs was presented in our earlier work [41].

Extraction Procedure
In all extraction experiments, 1.5 g of the solid phase was added to 15 cm 3 of the aqueous phase and 15 cm 3 of the organic phase (in some cases with the addition of acids, H 2 O 2 , DDACl). The mixture was shaken on a magnetic stirrer for 30 min or 2 h. The organic phase consisted of ILs immiscible with water, DES, or Cyanex 272 extractant with different additives. The mixtures were placed into a 100 cm 3 jacketed glass cell with coated magnetic stirring bars for the stirring procedure, 5000 rpm. The vessels were sealed to avoid evaporation losses or the unwanted appearance of moisture from the atmosphere. The jacketed vessel was connected to a thermostatic water bath (PolyScience temperature controller) to maintain a constant temperature. The organic phase was stripped at different concentrations of sulphuric acid and under different conditions (time, temperature). After phase separation, samples of approximately 0.1-0.3 cm 3 were taken for analysis from the aqueous phase after extraction and after stripping with H 2 SO 4 using disposable plastic syringes with coupled stainless steel needles. The concentration of metal ions in the aqueous phase and stripped organic phase was determined with the IRIS Advantage inductively coupled plasma optical emission spectrometer (ICP-OES). The Litmus bromothymol blue (or other indicator papers sensitive to pH 1 to 10) were used to measure the pH of the solution after extraction.

Extraction with ILs or Organophosphorous Based Acid from the Post-Leaching Solutions
The results of the extraction of metal ions from the aqueous phase presented in our previous work [33], showed high efficiency of metal ions extraction with [P 6,6,6,14 ][SCN] and [P 6,6,6,14 ][Cyanex272] ILs.
In this work, the extraction was performed by magnetically stirring the of mixture containing 4 cm 3 of the liquid leachate phase of pH = 3-7, 5 cm 3 IL and 1 cm 3 H 2 O 2 (30 wt%). The [P 6,6,6,14 ][Cyanex272] was used with toluene as a solvent due to its high viscosity (6.4 g of IL + 2.07 g of toluene). The mixture was placed into a 100 cm 3 jacketed glass cell with a coated magnetic stirring bar and was stirred for 30 min, 5000 rpm at the temperature T = 303 K. Both phases after the separation were analyzed for the metal ions concentration. The organic phase was stripped with 1.2 M H 2 SO 4 . The volume ratio of acid to organic phase was 1:2 (2.5 cm 3 :5.0 cm 3 ).
The best results were obtained with [P 6,6,6,14 ][Cyanex272]/toluene and with the mixture (Cyanex 272 + diethyl phosphite ester). The mixture contained 6 cm 3 of the liquid leachate phase of pH = 3, 4 cm 3 of Cyanex 272, 1 cm 3 of ester, 2 cm 3 of naphtha (kerosene) and 1 cm 3 H 2 O 2 . (30 wt%). The mixture was placed into a 100 cm 3 jacketed glass cell with a coated magnetic stirring bar and was stirred for 30 min, 5000 rpm at the temperature T = 303 K. After the separation, both phases were analyzed for the metal ions concentration. The organic phase was stripped with 1.2 M H 2 SO 4 . The volume ratio of acid to organic phase was 1:2 (3.5 cm 3 :7.0 cm 3 ). Results are presented in Table 7. The efficiency of metal ion extraction in wt% was calculated taking into account the determined density of the post-leaching solutions at pH = 3:1.217 g/cm 3 (I)1.229 g/cm 3 (II/first step), 1.212 g/cm 3 (II/second step).

ABS Method of Extraction from the Post-Leaching Solutions
The 10 cm 3 of the liquid leachate phase I of pH = 3, or the liquid leachate phase II (first/second step) was placed in a thermostated vessel. Then, 2.200 g of sodium chloride NaCl was, added in two portions. Next, 8 (30 wt%). The mixture was stirred for 2 h at the temperature T = 303 K. During the addition of NaCl, the IL separated as the upper layer of pH = 7. The extraction was carried out for 2 h at the temperature T = 318 K. The aqueous phase (approx. 10-11 cm 3 ) was analyzed for the content of metal ions. The metal content in the upper organic phase was calculated from the difference between the aqueous phase and the liquid leachate phase. The results are presented in Table 8.

Extraction with DESs from the Solid Material
The solvent extraction and stripping were carried out in a 100 cm 3 thermostated vessel with a coated magnetic stirrer bar under the water reflux at the temperature T = 333 K for 2 h. To 1.5 g of the solid material after thermal pre-treatment, 15 cm 3 of DES 1 or DES 2 was added, 8 cm 3 of DDACl (50 wt%) surfactant, 4.0 cm 3 H 2 O 2 (30 wt%), 3 cm 3 of water and 2 M H 2 SO 4 to pH = 3 or 5. For the solid phase, obtained after thermal pre-treatment at the temperature T = 1023 K for 7 h, the 0.4 g of Na 2 SO 4 was added to DES 2 to get a higher transfer of metals from the solid to the liquid phase. The ratio (O:A = 1:1 v/v). The liquid phases after the separation were analyzed for the metal ions content. The results obtained at different modifications of extraction conditions are shown in Tables 9 and 10.

Extraction with bi-Functional Ionic Liquids from the Solid Material
The solvent extraction from the solid material after thermal pre-treatment and stripping was carried out in a 100 cm 3 thermostatic vessel with coated magnetic stirrer bar under the water reflux at the temperature T = 333 K for 2 h at pH = 6.
The extraction efficiency was calculated from the sum of the metal content in the aqueous phase and the stripped organic phase in relation to the metal content in the solid phase sample in the starting material, obtained after thermal pre-treatment at the temperature T = 1023 K for 7 h.
The extraction from the solid material after thermal pre-treatment was presented with two DESs, DES 1 (choline chloride + lactic acid) and DES 2 (choline chloride + malonic acid). Better metal extraction results were obtained with DES 2 at pH = 5 at the temperature T = 333 K. The efficiency of extraction of various metals from the solid material after thermal pre-treatment and I leaching was: E Cu = 15.8 wt%, E Ag = 20.1 wt%, E Al = 48.9 wt%, E Fe = 24.7 wt%. Factors influencing the metal extraction efficiency were investigated in detail, and the addition of H 2 O 2 and DDACL to the extraction effects was also examined. The results showed that the extraction with DES 2 from the solid material without leaching is slightly worse than those after leaching: for Al(III), E Al = 67.3 wt%, E Cu = 9.6 wt% and E Ag = 14.2 wt% at pH = 5. Only one bi-functional IL, [N 10,10,1,1 ][Sal] could successfully leach metals out from the solid material only after thermal pre-treatment, where the extraction efficiency for Al(III) was E Al = 34.0 wt%, for Cu(II) E Cu = 6.0 wt%, for Zn(II) E Zn = 6.4 wt% and for Ag(I) E Ag = 0.4 wt%. The extraction efficiency (E) and distribution ratio (D) of many Cu(II), Ag(I), Al(III) and Zn(II) extraction processes were examined. The [P 6,6,6,14 ][SCN] and Aliquat 336, ILs proposed for this extraction was not effective.
The WPCBs particle size and the leaching solvent had a similar effect on the metals leaching performance, while the amount of DDACl addition showed no influence. This innovation provides a new option for recovering valuable metals such as copper or silver from WPCBs materials. The processes presented in this work may be used in a new technology of the recovery of metal ions from WPCBs in the form of a temperature pretreated solid material. The leaching process is time-consuming, very costly and toxic. The absence of acid leaching results in a lower metal extraction efficiency. All these results show that the solid phase extraction after leaching plus leachate extraction can yield 90-100% metal extraction, or only about 10-15% major metals such as Cu and Ag with solid material extraction without leaching. These results ensure a feasible and future-proof industrial application.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27154984/s1-The synthesis of three bi-functional ILs with NMR is presented.