Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2020 Oct 14;5(42):27188–27196. doi: 10.1021/acsomega.0c03255

Recovery of Pd(II) from Hydrochloric Acid Medium by Solvent Extraction–Direct Electrodeposition Using Hydrophilic/Hydrophobic ILs

Min Shao 1, Shunling Li 1, Ci Jin 1, Muhan Chen 1, Zhangjie Huang 1,*
PMCID: PMC7594130  PMID: 33134679

Abstract

graphic file with name ao0c03255_0017.jpg

Mixed [C8bet]Br/[C4mim][NTf2] ionic liquids were used as a new extraction system to extract Pd(II) from multimetal-ion solutions. The separation factors KPd/M (M: Cu, Fe, Ni, Zn, Co) are greater than 103. Thiourea was found to be an effective stripping agent. After three cycles, the recovery efficiency was higher than 91.0%. Direct electrodeposition of palladium from the mixed ionic liquid phase was also studied. The Pd(II) complex in [C8bet]Br/[C4mim][NTf2] system was studied by cyclic voltammetry at 348 K. The results indicate the existence of three types of Pd(II) complex in the [C8bet]Br/[C4mim][NTf2] system, leading to three reductive waves. The reduction of Pd(II) to Pd(0) in this system is irreversible. A uniform black coating was obtained by constant-potential deposition at −1.7 V on a nickel foil, confirmed to be palladium metal by energy-dispersive spectrometry, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy analyses. After three cycles of continuous extraction–electrodeposition, over 90.0% of palladium was recovered.

1. Introduction

Palladium is an important element in platinum group metals (PGMs). Palladium is widely used in electronic devices, aerospace, nuclear energy, and other high-tech fields owing to its wide range of conductivity, catalysis, ductility, high-temperature resistance, and corrosion resistance.1,2 However, the application of palladium is limited because it is scarcely available in nature, and its consumption is increasing. Therefore, it is particularly essential to recover palladium from secondary resources.

In the last few decades, the methods used to recover palladium include precipitation,3,4 adsorption,5 solvent extraction,6 electrodeposition,7,8 liquid membrane separation,9 and ion exchange.10,11 Among these processes, the solvent extraction method has the advantages of simple equipment, simple process, low cost, large extraction capacity, good selectivity, and wide application, making it more suitable for large-scale industrial use.12−14 As a new generation of extractant, ionic liquids (ILs) have been widely used in the extraction and separation of palladium.15−18 Quaternary phosphonium salts ILs, that is, CYPHOSIL 101, CYPHOSIL 102, and CYPHOSIL 104, can effectively extract and separate Pd(II) from a four-component mixture solution of PGMs. The extraction efficiency of Pd(II) is all higher than 99% in an acidic chlorination medium.18 The pyridinium and ammonium ILs were also widely used for Pd(II) extraction.19 However, green extraction was not achieved using an IL dissolved in toluene or chloroform for solvent extraction; these solvents are very environmentally unfriendly. In recent years, the mixed hydrophilic/hydrophobic IL systems have been used to extract platinum,20,21 where volatile organic solvents are not required. Compared with a single hydrophobic IL, the extraction efficiency is also significantly improved. However, until now, hydrophilic/hydrophobic IL systems have not been reported for the extraction and separation of palladium.

In the last decade, electrodeposition has been widely used for the purification and separation of metal ions. Many studies have reported the recovery of metals such as Cu,22 Sr,23 U,24 Pt,25 and Ru26 through solvent extraction–direct electrodeposition. Compared with the conventional organic phase back-extraction method, direct electrodeposition can reduce the use of volatile organic solvents. The combination of solvent extraction and electrodeposition is a promising and simplest new process for effective recovery of palladium. A variety of extractants such as Aliquat 336, tri-n-octylmethylammonium nitrate, and methylimino-bis-N,N-dioctylacetamide were used to extract Pd(II) from an acidic medium. After extraction, the electrochemical characteristics of Pd(II) ions present in the organic phase were evaluated by cyclic voltammetry (CV).27−29 However, the solvent extraction–direct electrodeposition of palladium from HCl media in a hydrophilic/hydrophobic IL system has not been reported.

In this study, the precipitation of Pd(II) in HCl medium by [C8bet]Br was investigated. Based on low aqueous solubility and low viscosity (69 mPa s), hydrophobic [C4mim][NTf2] acts as the main ingredient of the organic phase. A mixed hydrophilic/hydrophobic IL extraction system was obtained by adding hydrophilic [C8bet]Br to [C4mim][NTf2]. The mixed [C8bet]Br/[C4mim][NTf2] ILs achieved significantly better extraction efficiency for Pd(II) than single IL. The extraction parameters were optimized, and the precipitation mechanism of [C8bet]Br and Pd(II) was elucidated. Direct electrodeposition of palladium from the mixed IL system was also studied. Pd(II) was recovered from acidic media by electrodeposition from the extraction system using hydrophobic IL [C4mim][NTf2] with a wide electrochemical window, high electrical conductivity, and high boiling point as a diluent.30 Experiments show that the [C8bet]Br/[C4mim][NTf2] IL system has a great significance in simplifying the recovery steps and reducing the volume of secondary waste. The mixed IL system can extract and recover palladium efficiently through solvent extraction–direct electrodeposition.

2. Results and Discussion

2.1. Mechanism Analysis between [C8bet]Br and Pd(II)

2.1.1. UV–Vis Spectrum

[C8bet]Br has no characteristic UV–Vis absorption peak (Figure 1a). The characteristic UV–Vis absorption peaks of Pd(II) aqueous solution appeared at 222 and 280 nm (Figure 1b). The peaks can be attributed to the [PdCl4]2– complex,31 similar to the characteristic UV–Vis absorption spectrum of the [C8bet]-Pd(II) complex in a methanol solution (Figure 1c). The characteristic absorption peaks shifted slightly (222 → 238 nm, 280 → 278 nm) in methanol solution. This indicates that the precipitation species is [C8bet]2+[PdCl4]2–.32

Figure 1.

Figure 1

Open in a new tab

UV–Vis spectra, (a) 0.08 mM [C8bet]Br; (b) 0.04 mM Pd(II) solution containing 0.1 M hydrochloric acid; and (c) 0.04 mM [C8bet]-Pd(II) methanol solution containing 0.1 M hydrochloric acid.

2.1.2. [C8bet]-Pd(II) Complex Analysis

The molar ratio of [C8bet]Br and Pd(II) was calculated using Job’s method.33 The total amount of [C8bet]Br and aqueous Pd(II) was maintained as 6 × 10–3 mmol, and the concentration of HCl was kept constant at 0.1 M. By measuring the concentration of Pd(II) in the aqueous phase after the reaction is completed, the amount of Pd(II) needed for the reaction to generate the precipitate was calculated by conducting a mass balance, and a continuous variation of [C8bet]Br combined with Pd(II) was obtained. The results are shown in Figure 2. The maximum equilibrium precipitation molar ratio of [C8bet]Br/Pd(II) is 2:1, and the highest precipitation amount formed in the extraction experiment is the molar ratio closest to the theoretical molar ratio of the [C8bet]-Pd(II) complex. Using the Job’s method, a chemical equation for the [C8bet]Br precipitation of Pd(II) can be written as follows

2.1.2. 1
Figure 2.

Figure 2

Open in a new tab

Continuous variation method of precipitation reaction between Pd(II) and [C8bet]Br.

2.1.3. Fourier transform infrared and 1H NMR Analyses

To confirm the interaction between Pd(II) and [C8bet]Br, [C8bet]Br and [C8bet]-Pd(II) complexes were analyzed by Fourier transform infrared (FTIR) and 1H NMR spectroscopy. Compared with the [C8bet]-Pd(II) complex, the FTIR spectra (Figure 3a) of [C8bet]Br is the same. Only the peaks shifted slightly: 2959 → 2957, 2917 → 2925, 1750 → 1748, 1475 → 1469, and 1206 → 1208. The experimental results show that the exchange between [C8bet]+ and [PdCl4]2– is mainly affected by electrostatic attraction.

Figure 3.

Figure 3

Open in a new tab

(a) FTIR spectra and (b) 1H NMR [in dimethyl sulfoxide (DMSO)] spectra of [C8bet]Br and [C8bet]-Pd(II).

Figure 3b shows the 1H NMR spectra of [C8bet]Br and [C8bet]-Pd(II) complexes. A chemical shift was observed for the 1H NMR signal near the N atom of the [C8bet]-Pd(II) complex, while the 1H NMR signals of those far away from the N atom remained unchanged. Change in the chemical shift of hydrogen nuclei close to the N atom δH: A–A′, 4.472 → 4.437, C–C′, 3.235 → 3.223; ΔδH = 0.012 – 0.035 ppm; change in the chemical shift of hydrogen nuclei away from the N atom δH: B–B′, 4.181 → 4.181, D–D′, 1.614 → 1.606, E–E′, 1.278 → 1.275, F–F′, 0.854 → 0.862; ΔδH ≤ 0.008 ppm. The experimental FTIR and 1H NMR results indicate that the precipitation reaction mechanism is anion-exchange,32 consistent with Job’s method and UV–Vis analyses.

2.2. Precipitation and Extraction Behavior

2.2.1. Effect of [C8bet]Br Concentration

To evaluate the effect of concentration of [C8bet]Br on precipitation efficiency, the effect of different contents of [C8bet]Br on the precipitation percentage of Pd(II) and the effect of its addition to hydrophobic phase [C4mim][NTf2] on the extraction efficiency were evaluated. Figure 4b shows that when the amount of [C8bet]Br is 20 times that of [PdCl4]2–, the precipitation percentage reached 100%. Based on the excellent precipitation efficiency of [C8bet]Br, hydrophobic IL [C4mim][NTf2] and a quantitative amount of [C8bet]Br were added to form the extracted organic phase. Figure 4a shows that the extraction efficiency of [C8bet]Br/[C4mim][NTf2] system for Pd(II) reached 98.3% with an aqueous-oil volume ratio (RA/O) of 5, while the extraction efficiency of [C4mim][NTf2] for Pd(II) is only 30.1%. Even if the content of [C4mim][NTf2] is 1000 times that of Pd(II), the extraction effect is still low. Therefore, a hydrophilic/hydrophobic IL [C8bet]Br/[C4mim][NTf2] system was developed to extract Pd(II). The amount of [C8bet]Br in the organic phase was selected to be 20 times that of [PdCl4]2– for subsequent experiments.

Figure 4.

Figure 4

Open in a new tab

Precipitation and extraction efficiency of Pd(II) by using (a) [C8bet]Br or (b) [C8bet]Br/[C4mim][NTf2] system. Aqueous phase: 2 mM Pd(II) solution, CHCl = 0.1 M, RA/O = 5. The mixed IL phase: n[C8bet]Br/n[C4mim][NTf2]: 0–5.85 × 10–5.

2.2.2. Effect of Vibration Time

Vibration time is one of the factors that affect the extraction efficiency. To evaluate the effect of vibration time on extraction efficiency and optimize the experimental parameters, RA/O was set as 5, and 2 mM Pd(II) solution containing 0.1 M acid chloride media was added. The vibration time was set as 1–40 min (Figure 5). The extraction efficiency shows a stable trend after 20 min. Thus, all the experiments were conducted at a vibration time of 30 min, following centrifugation at 9000 rpm for 5 min.

Figure 5.

Figure 5

Open in a new tab

Effect of vibration time on the extraction efficiency. Aqueous phase: 2 mM Pd(II) solution, CHCl = 0.1 M, RA/O = 5. The mixed IL phase: 0.5 mL [C4mim][NTf2], C[C8bet]Br = 0.04 mol L–1.

2.2.3. Effect of HCl Concentration

Figure 6 shows the extraction efficiency of Pd(II) when the concentration of hydrochloric acid varied from 0.01 to 5 M. When the concentration of HCl in the aqueous phase is higher than 0.2 M, both [PdCl4]2– and Cl– act as anions in a competitive system and combine with [C8bet]+ cations, consistent with other studies of similar palladium extraction systems in hydrochloric acid media.34,35 When the concentration of HCl in the aqueous phase is lower than 0.1 M, the morphology of Pd(II) under low acidity was dominated by [PdCl3(H2O)−] and [PdCl2(H2O)2].36 Thus, when CHCl = 0.1 M, the highest extraction efficiency was obtained.

Figure 6.

Figure 6

Open in a new tab

Acidity affects the extraction efficiency of Pd(II). Aqueous phase: 2 mM Pd(II) solution, RA/O = 5. The mixed IL phase: 0.5 mL [C4mim][NTf2], C[C8bet]Br = 0.04 mol L–1.

2.2.4. Effect of NaCl and NaBr Concentration

The concentration of Cl– or Br– ions can significantly affect the extraction of Pd(II). With increasing concentration of NaCl or NaBr from 0.25 to 5 M, the extraction efficiency of Pd(II) decreased (Figure 7). This experiment further confirmed that the extraction involves the anion-exchange mechanism.

Figure 7.

Figure 7

Open in a new tab

Concentration of NaCl (a) or NaBr (b) affect extraction on Pd(II). Aqueous phase: 2 mM Pd(II) solution, CHCl = 0.1 M, RA/O = 5. The mixed IL phase: 0.5 mL [C4mim][NTf2], C[C8bet]Br = 0.04 mol L–1.

2.3. Selective Extraction of Pd(II) from Multimetal Mixture Solution

Natural palladium ores are usually obtained from basic volcanic rocks, usually accompanied by sulfide ores of copper, nickel, iron, and other metals. If palladium is enriched or refined by extraction, these multimetal components might affect the extraction of palladium. The selectivity of Pd(II) extraction in the [C8bet]Br/[C4mim][NTf2] system was evaluated. First, a variety of chloride salts of metals were added to 0.1 M HCl solution to obtain a multimetal system. The concentration of each metal ion is 2 mM. As shown in Figure 8, the extraction efficiency of [C8bet]Br/[C4mim][NTf2] for Pd (II) is as high as 98.1%, and the distribution ratio is as high as 261. The extraction efficiencies, distribution ratios, and separation factors of other metal ions are as follows: Cu(II): 3.1%, 0.16, 1.6 × 103; Fe(III): 5.2%, 0.27, 1.0 × 103; Ni(II): 4.7%, 0.25, 1.0 × 103; Zn(II): 1.8%, 0.094, 2.8 × 103; Co(II): 4.2%, 0.22, 1.2 × 103. The separation factors KPd/M (M: Cu, Fe, Ni, Zn, Co) are greater than 1.0 × 103. The experiment results show that the mixed [C8bet]Br/[C4mim][NTf2] ILs can selectively extract Pd(II) from multimetal-ion solutions in hydrochloric acid media.

Figure 8.

Figure 8

Open in a new tab

Extraction of various metals from a mixed metal solution (2 mM each metal in 0.1 M HCl), RA/O = 5. The mixed IL phase: [C8bet]Br in 0.5 mL [C4mim][NTf2], C[C8bet]Br = 0.04 mol L–1.

2.4. Stripping of Pd(II) and Regeneration of ILs

After Pd(II) was extracted using the [C8bet]Br/[C4mim][NTf2] system, thiourea was selected as the stripping agent for Pd(II). The concentration of thiourea was 1.2 M in 0.5 M HCl; the vibration time was 30 min. When the RA/O was 5, the stripping efficiency was higher than 99.0%.

Table 1 shows the reuse performance of the [C8bet]Br/[C4mim][NTf2] system. After the stripping experiment, the upper aqueous phase was separated by centrifuging for 5 min. The regenerated organic phase was obtained by mixing with saturated KBr solutions. The extraction–stripping process was repeated three times; the recovery was always more than 91.0%.

Table 1. Reuse Performance of the [C8bet]Br/[C4mim][NTf2] System.

cycle index extraction efficiency (%) recovery efficiency (%)
1 97.4 96.4
2 95.1 94.2
3 92.6 91.7
Open in a new tab

2.5. Electrochemical Analysis

Figure 9a shows that [C4mim][NTf2] IL is relatively stable in the range from 1.8 to −2.5 V. The electrochemical window exceeds 4.3 V (the blue line). At −2.6 V, [C4mim]+ begins to decompose. Figure 9b shows the CV of Pd(II) complex in the [C8bet]Br/[C4mim][NTf2] system at 348 K (the red line). Three peaks (e.g., −0.05, −1.04, and −1.58 V) were observed at the cathode; the peaks at −0.05, −1.04, and −1.58 V can be attributed to Pd(0), indicating that Pd(II) was reduced

2.5. 2

Figure 9.

Figure 9

Open in a new tab

Cyclic voltammograms of the GC electrode in (a) [C4mim][NTf2] and (b) [C8bet]Br/[C4mim][NTf2] systems containing 2250 mg L–1 Pd(II) (concentration of [C8bet]Br: 0.42 mol L–1, deposition temperature: 348 K, scan rate: 50 mV/s).

Three types of Pd complex ions ([C8bet]-Pd(II), [C4min]-Pd(II),20 and [PdCl4]2–) were assumed to be present in the mixed IL system, exhibiting three reductive waves.37−40 Three oxidation peaks were also observed in the anode at 0.045, −0.18, and 0.95 V; they can be attributed to Pd(II), indicating that a small amount of electrodeposited Pd was reoxidized on the electrode surface during cathode scanning.37,39

Figure 10 shows the CV curves of the Pd(II) complex in the [C8bet]Br/[C4mim][NTf2] system for different scanning rates. With the increase in scanning speed, the reduction peak current density JP increased gradually, while the peak potential φP shifted negatively (Figure 10). The electrode reaction of Pd(II) reduction to Pd(0) is irreversible. Figure 11 shows the change in potential φP from Pd(II) to Pd(0) with the scanning rate. The results show a good linear relationship between φP and the natural logarithm of ln ν, indicating that it is feasible to promote palladium separation by controlling potential deposition.

Figure 10.

Figure 10

Open in a new tab

Cyclic voltammograms of the GC electrode at different scan rates in the [C8bet]Br/[C4mim][NTf2] system containing 2250 mg L–1 Pd(II) at 348 K, C[C8bet]Br = 0.42 mol L–1.

Figure 11.

Figure 11

Open in a new tab

Relationship between the Pd(II) peak potential and scan rate in the [C8bet]Br/[C4mim][NTf2] system containing 2250 mg L–1 Pd(II) at 348 K, C[C8bet]Br = 0.42 mol L–1.

To obtain the palladium metal, a constant-potential deposition was carried out on nickel foil at −1.6 and −1.7 V. After 16 h, a black coating was obtained at −1.6 V; it was uneven and easily fell off. At −1.7 V, a uniform black solid was observed on the coating surface. Then, the samples at −1.7 V were analyzed by scanning electron microscopy (SEM) and HRTEM. The SEM images show nanoparticles with a spherical structure and even distribution (Figure 12a). Figure 12b shows that the size of nanoparticles is not uniform, probably because of different current densities during deposition.38 According to the HRTEM atomic lattice fringe analysis (Figure 12c), the lattice spacing is 0.23 nm, conforming to the crystal surface size of Pd(111). It is very close to the crystal surface of Pd(111) reported in the literature.41,42 Other crystal faces of Pd(0) can be obtained from the electron diffraction pattern of the selected region (Figure 12d). HRTEM analysis did not show other crystal phases except Pd(0). It was verified that the electrodeposited nanoparticles are palladium metal. After 16 h electrolysis, 92.8% of palladium metal was obtained.

Figure 12.

Figure 12

Open in a new tab

SEM and transmission electron microscopy (TEM) of palladium on the nickel foil after 16 h deposition at a constant potential of −1.7 V; (a) SEM; (b) SEM; (c) TEM; (d) SAED pattern.

Figure 13 shows a composition analysis. Energy-dispersive spectrometry (EDS) (Figure 13a) analysis shows that the peaks of Pd particles also include some small signals from ILs (O, S), nickel foil (Ni), and other elements on the deposition layer. Figure 13b shows the XPS spectrum. The double peaks at 335.7 and 341.0 eV belong to Pd 3d5/2 and Pd 3d3/2, respectively, consistent with the energy spectrum of Pd(0).43 Any valence of Pd other than Pd(0) was not observed in XPS analysis.

Figure 13.

Figure 13

Open in a new tab

EDS (a) and XPS (b) of palladium on nickel foil after 16 h deposition at a constant potential of −1.7 V.

Finally, continuous extraction–electrodeposition of Pd(II) in the [C8bet]Br/[C4mim][NTf2] system was evaluated to determine the reusability of the [C8bet]Br/[C4mim][NTf2] IL system. After each electrodeposition, the mixed ILs of palladium were mixed with a saturated KBr solution and regenerated after washing. Figure 14 shows the corresponding extraction efficiency and recovery efficiency for three extraction–electrodeposition cycles. After three cycles, the recovery is still over 90.0%. The mixed [C8bet]Br/[C4mim][NTf2] system is very stable and provides a basis for solvent extraction–electrodeposition of Pd(II) in future industrial applications.

Figure 14.

Figure 14

Open in a new tab

Reuse experiments of extraction–electrodeposition in the [C8bet]Br/[C4mim][NTf2] system.

3. Conclusions

Hydrophilic betaine-based IL ([C8bet]Br) forms a precipitate with Pd(II) in acidic chloride media. The [C8bet]-Pd(II) precipitate was identified by UV–Vis, FTIR, 1H NMR spectroscopy, and Job’s method as a form of ion pair ([C8bet]2+[PdCl4]2–). The mixed hydrophilic/hydrophobic IL ([C8bet]Br/[C4mim][Nf2T]) system shows a higher extraction efficiency for Pd(II) than single hydrophobic IL ([C4mim][NTf2]). At a low hydrochloric acid concentration, the [C8bet]Br/[C4mim][NTf2] system shows excellent selectivity for Pd(II) over mixed metal-ion solutions. Moreover, the recovery of palladium from the organic phase can be achieved by thiourea stripping or direct electrodeposition.

4. Experimental Section

4.1. Instruments and Reagents

Palladium powder (>99.99%) was purchased from the Institute of Precious Metals (Kunming, China). Betaine, 1-bromooctane, and [C4mim][NTf2] were purchased from Alfa Aesar company (Beijing, China). Other reagents were purchased from Shanghai Titan Technology Co., Ltd. (Shanghai, China). All the reagents were of commercially available AR grade unless specified otherwise.

The chemical composition of betaine-based functionalized IL was analyzed by HR-MS (Agilent 1100 HPLC/TOF, Agilent Technologies Inc, Beijing, China). The concentration of single palladium ions in the solution was determined using an atomic absorption spectroscopy (AAS) (HITACHI Z-2000, Hitachi, Tokyo, Japanese). The concentrations of multimetal-ion mixture solutions were determined by ICP-AES (Thermo Fisher ICAP 6300 USA). The form of the Pd(II) complex was analyzed by UV–Vis spectroscopy (Lambda 365, PerkinElmer, Waltham, USA). The change in characteristic structural groups of [C8bet]Br before and after precipitation was determined by 1H NMR (Ascend Aeon 400, Bruker, Germany) and FTIR (NicoletiS10, Waltham, MA, USA) analyses. The melting point of compound [C8bet]Br was determined using a microscopic melting point meter (Beijing Taike Instrument Co., Ltd., Beijing, China). The Pd(II) complex in the mixed IL system was electrochemically analyzed and electrodeposited using an electrochemical workstation (CHI760E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The morphology and EDS analyses of palladium after deposition were carried out by SEM (Nova NanoSEM, Thermo Scientific, Waltham, USA), and the valence of palladium on the surface of coating was analyzed by XPS (K-Alpha+, Thermo USA) and HRTEM (JEM-210, JEOL, Tokyo, Japanese).

4.2. Methods

4.2.1. Metal-Ion Extraction

A stock solution of 10.00 g·L–1 [PdCl4]2– was prepared following the literature.44 Concentration of working of palladium solution can be obtained by diluting the standard stock solution.

The precipitation experiment was conducted by mixing different contents of [C8bet]Br with 2 mM solution of Pd(II) (HCl: 0.1 M) in a constant-temperature oscillator. The amount of [C8bet]Br was 1.5–20 times that of Pd(II), and the vibration time was 30 min. Precipitation was obtained by centrifuging the mixture for 5 min. The aqueous phase was analyzed with AAS, and the precipitate was analyzed by UV–Vis, FTIR, and 1H NMR spectroscopies.

In the extraction study, an organic phase was obtained by adding a specified amount of [C8bet]Br required for the experiment to the hydrophobic IL [C4mim][NTf2], and an extraction system was obtained using Pd(II) or aqueous solutions of mixed metal ions. The concentration of metal ions in the aqueous phase was 2 mM, and that of HCl was 0.1 M. The value of RA/O was 5. The mixture was vibrated for 30 min to achieve equilibrium. After separating at 9,000 rpm for 5 min, the metal concentration of aqueous phase was analyzed. All the extraction experiments were performed three times with a standard deviation of less than ±3%.

The precipitation percentage (P %) or extraction efficiency (E %), distribution ratio (D), separation factor (K), stripping efficiency (S %), and recovery efficiency (R %) were calculated as follows

4.2.1. 3
4.2.1. 4
4.2.1. 5
4.2.1. 6
4.2.1. 7

where [C]in and [C]eq are the concentrations of Pd(II) in the aqueous phase before and after precipitation or extraction; [C]or is the concentrations of Pd(II) in the mixed IL phase.

4.2.2. Synthesis of [C8bet]Br

Scheme 1 shows the synthesis of target hydrophilic IL [C8bet]Br. First, 1-bromooctane (0.06 mol, 11.52 g) was slowly added to acetonitrile (100 mL), and the mixture was stirred for 10 min. A certain amount of betaine (0.05 mol, 5.86 g) was added to the mixture and stirred at 90 °C and refluxed for 24 h. The solvent was removed by rotary evaporation in vacuum at 50 °C. Thus, a solid white product [C8bet]Br was obtained. Yield: 14.27 g (92%); melting point: 57.5 ± 0.5 °C.

Scheme 1. Synthesis Hydrophilic IL of [C8bet]Br.

Scheme 1

Open in a new tab

[C8bet]Br was characterized by HR-MS and 1H NMR analyses. 1H NMR (400 MHz, DMSO) δ 4.47 (s, 2H), 4.18 (t, J = 8.0 Hz, 2H), 3.24 (s, 9H), 1.61 (m, 2H), 1.45–1.23 (m, 10H), 0.88–0.85 (m, 3H). HR-MS (ESI+): [M – Br]+ calcd: (C13H28NO2) 230.2114; found, 230.2115.

4.2.3. Electrochemical Test

Constant-potential deposition and electrochemical analysis were carried out using an electrochemical workstation. A three-electrode system was prepared as follows: a platinum wire with an inner diameter of 0.5 mm was used as the reference electrode. A platinum plate (1.2 mm × 1.2 mm) was used as the counter electrode. A glassy carbon (GC) electrode was used as the working electrode. Before measurement, the GC electrode was polished with alumina, successively washed with acetone and distilled water, and dried. CV was conducted using an electrochemical workstation at 348 K. For CV analysis, an electrolyte containing 2250 mg·L–1 Pd(II) dissolved in the [C8bet]Br/[C4mim][NTf2] system was used. The scanning rate was 50 mV/s. All ILs were vacuum-dried at 385 K for 12 h.

To separate Pd(0) from the Pd(II) complex, a three-electrode system with a platinum wire as the reference electrode, a platinum sheet as the counter electrode, and a nickel foil (1 cm2) as the working electrode was used. Pd(II) (C = 2250 mg·L–1) was extracted by electrodepositing with controlled potentiostatic deposition (deposition temperature: 348 K, potential: −1.7 V, deposition time: 16 h). The amount of Pd(II) in the mixed IL system after electrodeposition was determined following the literature method.8

Acknowledgments

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (51764051 and 51264038), Free exploration fund for academician (202005AA160002), the Key Natural Science Foundation of Kunming (2017-1-S-12305), and the Yunnan University’s Research Innovation Fund for Graduate Students (2019z048).

Author Contributions

† M.S. and S.L. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

References

  1. Lei J.-F. Palladium-chromium strain gauges for high temperatures. Mater. Des. 1991, 12, 213–216. 10.1016/0261-3069(91)90166-2. [DOI] [Google Scholar]
  2. Thiele J. A.; Da Cunha M. P. Platinum and palladium high-temperature transducers on langasite. IEEE Trans. Ultrason. Eng. 2005, 52, 545–549. 10.1109/tuffc.2005.1428035. [DOI] [PubMed] [Google Scholar]
  3. Soylak M.; Tuzen M. Coprecipitation of gold(III), palladium(II) and lead(II) for their flame atomic absorption spectrometric determinations. J. Hazard. Mater. 2008, 152, 656–661. 10.1016/j.jhazmat.2007.07.027. [DOI] [PubMed] [Google Scholar]
  4. Ozturk N.; Bulut V. N.; Duran C.; Soylak M. Coprecipitation of palladium(II) with 1,5-diphenylcarbazite–copper(II) and determination by flame atomic absorption spectrometry. Desalination 2011, 270, 130–134. 10.1016/j.desal.2010.11.034. [DOI] [Google Scholar]
  5. Godlewska-Żyłkiewicz B. Biosorption of platinum and palladium for their separation/preconcentration prior to graphite furnace atomic absorption spectrometric determination. Spectrochim. Acta, Part B 2003, 58, 1531–1540. 10.1016/s0584-8547(03)00076-4. [DOI] [Google Scholar]
  6. Senthil K.; Akiba U.; Fujiwara K.; Hamada F.; Kondo Y. New Heterocyclic Dithioether Ligands for Highly Selective Separation and Recovery of Pd(II) from Acidic Leach Liquors of Spent Automobile Catalyst. Ind. Eng. Chem. Res. 2017, 56, 1036–1047. 10.1021/acs.iecr.6b03874. [DOI] [Google Scholar]
  7. Qian J.-g.; Yin Y.; Li X.; Li T.-j. Electrodeposition of iridium from composite ionic liquid. Trans. Nonferrous Met. Soc. China 2015, 25, 1685–1691. 10.1016/s1003-6326(15)63773-6. [DOI] [Google Scholar]
  8. Yang X.; Yang R.; Shi D.; Wang S.; Chen J.; Guo H. Hydrophobic ionic liquids as novel extractants for gold(I) recovery from alkaline cyanide solutions. J. Chem. Technol. Biotechnol. 2015, 90, 1102–1109. 10.1002/jctb.4418. [DOI] [Google Scholar]
  9. Uheida A.; Zhang Y.; Muhammed M. Transport of palladium(II) through hollow fiber supported liquid membrane facilitated by nonylthiourea. J. Membr. Sci. 2004, 241, 289–295. 10.1016/j.memsci.2004.05.020. [DOI] [Google Scholar]
  10. Hubicki Z.; Leszczyńska M.; Łodyga B.; Łodyga A. Recovery of palladium(II) from chloride and chloride–nitrate solutions using ion-exchange resins with S-donor atoms. Desalination 2007, 207, 80–86. 10.1016/j.desal.2006.04.068. [DOI] [Google Scholar]
  11. Shams K.; Goodarzi F. Improved and selective platinum recovery from spent α-alumina supported catalysts using pretreated anionic ion exchange resin. J. Hazard. Mater. 2006, 131, 229–237. 10.1016/j.jhazmat.2005.09.044. [DOI] [PubMed] [Google Scholar]
  12. Truong H. T.; Lee M. S. Separation of Pd(II) and Pt(IV) from hydrochloric acid solutions by solvent extraction with Cyanex 301 and LIX 63. Miner. Eng. 2018, 115, 13–20. 10.1016/j.mineng.2017.10.001. [DOI] [Google Scholar]
  13. Pan L.; Zhang Z.-d. Solvent extraction and separation of palladium(II) and platinum(IV) from hydrochloric acid medium with dibutyl sulfoxide. Miner. Eng. 2009, 22, 1271–1276. 10.1016/j.mineng.2009.07.006. [DOI] [Google Scholar]
  14. Nguyen T. H.; Sonu C. H.; Lee M. S. Separation of Pt(IV), Pd(II), Rh(III) and Ir(IV) from concentrated hydrochloric acid solutions by solvent extraction. Hydrometallurgy 2016, 164, 71–77. 10.1016/j.hydromet.2016.05.014. [DOI] [Google Scholar]
  15. Firmansyah M. L.; Kubota F.; Goto M. Solvent extraction of Pt(IV), Pd(II), and Rh(III) with the ionic liquid trioctyl(dodecyl) phosphonium chloride. J. Chem. Technol. Biotechnol. 2018, 93, 1714–1721. 10.1002/jctb.5544. [DOI] [Google Scholar]
  16. Tang N.; Wang Y.; Liu M.; Liu L.; Yin C.; Yang X.; Wang S. Ionic liquid as adjuvant in an aqueous biphasic system composed of polyethylene glycol for green separation of Pd(II) from hydrochloric solution. Sep. Purif. Technol. 2020, 246, 116898. 10.1016/j.seppur.2020.116898. [DOI] [Google Scholar]
  17. Li S.; Chu N.; Li X.; Dong F.; Shen Y. Recovery of palladium from acidic nitrate media with triazole type extractants in ionic liquid. Hydrometallurgy 2019, 189, 105148. 10.1016/j.hydromet.2019.105148. [DOI] [Google Scholar]
  18. Rzelewska-Piekut M.; Regel-Rosocka M. Separation of Pt(IV), Pd(II), Ru(III) and Rh(III) from model chloride solutions by liquid-liquid extraction with phosphonium ionic liquids. Sep. Purif. Technol. 2019, 212, 791–801. 10.1016/j.seppur.2018.11.091. [DOI] [Google Scholar]
  19. Tong Y.; Wang C.; Li J.; Yang Y. Extraction mechanism, behavior and stripping of Pd(II) by pyridinium-based ionic liquid from hydrochloric acid medium. Hydrometallurgy 2014, 147-148, 164–169. 10.1016/j.hydromet.2014.05.016. [DOI] [Google Scholar]
  20. Tong Y.; Wang C.; Huang Y.; Yang Y. Extraction and Stripping of Platinum from Hydrochloric Acid Medium by Mixed Imidazolium Ionic Liquids. Ind. Eng. Chem. Res. 2015, 54, 705–711. 10.1021/ie503502g. [DOI] [Google Scholar]
  21. Papaiconomou N.; Billard I.; Chainet E. Extraction of iridium(IV) from aqueous solutions using hydrophilic/hydrophobic ionic liquids. RSC Adv. 2014, 4, 48260–48266. 10.1039/c4ra06991a. [DOI] [Google Scholar]
  22. Hsu S. C. N.; Su C.-J.; Yu F.-L.; Chen W.-J.; Zhuang D.-X.; Deng M.-J.; Sun I.-W.; Chen P.-Y. Extracting Cu(II) from aqueous solutions with hydrophobic room-temperature ionic liquid in the presence of a pyridine-based ionophore to attempt Cu recovery: A laboratory study. Electrochim. Acta 2009, 54, 1744–1751. 10.1016/j.electacta.2008.09.068. [DOI] [Google Scholar]
  23. Chen P.-Y. The assessment of removing strontium and cesium cations from aqueous solutions based on the combined methods of ionic liquid extraction and electrodeposition. Electrochim. Acta 2007, 52, 5484–5492. 10.1016/j.electacta.2007.03.010. [DOI] [Google Scholar]
  24. Giridhar P.; Venkatesan K. A.; Subramaniam S.; Srinivasan T. G.; Vasudeva Rao P. R. Extraction of uranium (VI) by 1.1M tri-n-butylphosphate/ionic liquid and the feasibility of recovery by direct electrodeposition from organic phase. J. Alloys Compd. 2008, 448, 104–108. 10.1016/j.jallcom.2007.03.115. [DOI] [Google Scholar]
  25. Matsumiya M.; Song Y.; Tsuchida Y.; Ota H.; Tsunashima K. Recovery of platinum by solvent extraction and direct electrodeposition using ionic liquid. Sep. Purif. Technol. 2019, 214, 162–167. 10.1016/j.seppur.2018.06.018. [DOI] [Google Scholar]
  26. Song Y.; Tsuchida Y.; Matsumiya M.; Tsunashima K. Recovery of ruthenium by solvent extraction and direct electrodeposition using ionic liquid solution. Hydrometallurgy 2018, 181, 164–168. 10.1016/j.hydromet.2018.09.011. [DOI] [Google Scholar]
  27. Giridhar P.; Venkatesan K. A.; Srinivasan T. G.; Vasudeva Rao P. R. Extraction of fission palladium by Aliquat 336 and electrochemical studies on direct recovery from ionic liquid phase. Hydrometallurgy 2006, 81, 30–39. 10.1016/j.hydromet.2005.10.001. [DOI] [Google Scholar]
  28. Jayakumar M.; Venkatesan K. A.; Srinivasan T. G.; Rao P. R. V. Extraction–electrodeposition (EX–EL) process for the recovery of palladium from high-level liquid waste. J. Appl. Electrochem. 2009, 39, 1955–1962. 10.1007/s10800-009-9905-3. [DOI] [Google Scholar]
  29. Matsumiya M.; Song Y.; Tsuchida Y.; Sasaki Y. Separation of palladium by solvent extraction with methylamino-bis-N,N-dioctylacetamide and direct electrodeposition from loaded organic phase. Sep. Purif. Technol. 2020, 234, 115841. 10.1016/j.seppur.2019.115841. [DOI] [Google Scholar]
  30. Ota H.; Matsumiya M.; Sasaya N.; Nishihata K.; Tsunashima K. Investigation of electrodeposition behavior for Nd(III) in [P2225][TFSA] ionic liquid by EQCM methods with elevated temperatures. Electrochim. Acta 2016, 222, 20–26. 10.1016/j.electacta.2016.11.038. [DOI] [Google Scholar]
  31. Tait C. D.; Janecky D. R.; Rogers P. S. Z. Speciation of aqueous palladium(II) chloride solutions using optical spectroscopies. Geochim. Cosmochim. Acta 1991, 55, 1253–1264. 10.1016/0016-7037(91)90304-N. [DOI] [Google Scholar]
  32. Yan Y.; Wang Q.; Xiang Z.; Zheng Y.; Wang S.; Yang Y. Behavior and mechanism investigation of separating Pt and Ir by liquid–liquid extraction using a mixed [C6bet]Br/[C6mim][NTF2] system. New J. Chem. 2017, 41, 8985–8992. 10.1039/c7nj02361h. [DOI] [Google Scholar]
  33. Katami T.; Hayakawa T.; Furukawa M.; Shibata S.; Hara T. Spectrophotometric study of some metal complexes with 2-(2-(3,5-dibromopyridyl)azo)-5-dimethylaminobenzoic acid and 2-(2-benzothiazolylazo)-5-dimethylaminobenzoic acid. Anal. Sci. 1986, 2, 169–174. 10.2116/analsci.2.169. [DOI] [Google Scholar]
  34. Cieszynska A.; Wisniewski M. Extraction of palladium(II) from chloride solutions with CyphosIL 101/toluene mixtures as novel extractant. Sep. Purif. Technol. 2010, 73, 202–207. 10.1016/j.seppur.2010.04.001. [DOI] [Google Scholar]
  35. Navarro R.; Saucedo I.; Gonzalez C.; Guibal E. Amberlite XAD-7 impregnated with Cyphos IL-101 (tetraalkylphosphonium ionic liquid) for Pd(II) recovery from HCl solutions. Chem. Eng. J. 2012, 185–186, 226–235. 10.1016/j.cej.2012.01.090. [DOI] [Google Scholar]
  36. Kim H. Y.; Nakano Y. Adsorption mechanism of palladium by redox within condensed-tannin gel. Water Res. 2005, 39, 1324–1330. [DOI] [PubMed] [Google Scholar]
  37. Huang H.-Y.; Chen P.-Y. PdNi- and Pd-coated electrodes prepared by electrodeposition from ionic liquid for nonenzymatic electrochemical determination of ethanol and glucose in alkaline media. Talanta 2010, 83, 379–385. 10.1016/j.talanta.2010.09.032. [DOI] [PubMed] [Google Scholar]
  38. Bando Y.; Katayama Y.; Miura T. Electrodeposition of palladium in a hydrophobic 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide room-temperature ionic liquid. Electrochim. Acta 2007, 53, 87–91. 10.1016/j.electacta.2007.02.074. [DOI] [Google Scholar]
  39. Hsiu S.-I.; Tai C.-C.; Sun I.-W. Electrodeposition of palladium–indium from 1-ethyl-3-methylimidazolium chloride tetrafluoroborate ionic liquid. Electrochim. Acta 2006, 51, 2607–2613. 10.1016/j.electacta.2005.07.042. [DOI] [Google Scholar]
  40. Jayakumar M.; Venkatesan K. A.; Srinivasan T. G.; Rao P. R. V. Electrochemical behavior of ruthenium (III), rhodium (III) and palladium (II) in 1-butyl-3-methylimidazolium chloride ionic liquid. Electrochim. Acta 2009, 54, 6747–6755. 10.1016/j.electacta.2009.06.043. [DOI] [Google Scholar]
  41. Mazumder V.; Sun S. Oleylamine-Mediated Synthesis of Pd Nanoparticles for Catalytic Formic Acid Oxidation. J. Am. Chem. Soc. 2009, 131, 4588–4589. 10.1021/ja9004915. [DOI] [PubMed] [Google Scholar]
  42. Yoshii K.; Oshino Y.; Tachikawa N.; Toshima K.; Katayama Y. Electrodeposition of palladium from palladium(II) acetylacetonate in an amide-type ionic liquid. Electrochem. Commun. 2015, 52, 21–24. 10.1016/j.elecom.2015.01.003. [DOI] [Google Scholar]
  43. He C.; Li P.; Cheng J.; Wang H.; Li J.; Li Q.; Hao Z. Synthesis and characterization of Pd/ZSM-5/MCM-48 biporous catalysts with superior activity for benzene oxidation. Appl. Catal., A 2010, 382, 167–175. 10.1016/j.apcata.2010.04.033. [DOI] [Google Scholar]
  44. Zhong L.; Zhang J.; Zhang Q.; Chen M.; Huang Z. Novel poly(aniline-co-3-amino-4-methoxybenzoic acid) copolymer for the separation and recovery of Pd(II) from the leaching liquor of automotive catalysts. RSC Adv. 2017, 7, 39244–39257. 10.1039/c7ra06404g. [DOI] [Google Scholar]

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES