A 2-mercaptobenzothiazole-functionalized ionic liquid for selective extraction of Pd(II) from a hydrochloric acid medium

Abstract

A novel 2-mercaptobenzothiazole-functionalized ionic liquid (1,10-bis(3-methylimidazolium-1-yl)decane bis(triflic)imide two 2-mercaptobenzothiazolate, {[C₁₀(mim)₂]²⁺2[MBT]⁻}) with excellent selectivity for Pd(II) extraction was fabricated. Variable factors (e.g., functional ionic liquid concentration, hydrochloric acid concentration, sodium chloride concentration, sodium nitrate concentration and hydrogen ion concentration) were also investigated for extraction experiments. Accordingly, a special complexation extraction mechanism was proposed and confirmed by further experiments. Moreover, higher selectivity towards Pd(II) extraction was realized over other metal ions especially Pt(IV). Furthermore, 0.5 M thiourea/1.0 M HCl was recognized as an efficient stripping agent for the Pd(II) loaded organic phase. Consequently, together with high efficiency and selectivity, the 2-mercaptobenzothiazolate-functionalized ionic liquid can act as a high selective extractant for Pd(II) extraction.

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Introduction

As an important noble metal, Pd(II) has received extensive research attention and has been widely used as a versatile material in automobile, pharmaceutical and petroleum industries.¹ However, the scarcity of palladium resources in nature makes it valuable to recover palladium from waste liquids and spent catalysts by solvent extraction. Lately, encouraging advances have been made to remove/recover Pd(II) from aqueous phases relying on conventional extractants (e.g., sulfur-containing,^2–4 azocalix[4]arene derivative extractant,⁵ 2-hydroxy-5-nonylacetophenone oxime⁶ and benzoylmethylenetriphenylphosphorane⁷). However, the efficiency of these extractants has been found to be limited. Hence, looking for an efficient and environmentally-friendly extractant for the separation of Pd(II) from waste liquids has become a focus point.

Ionic liquids (ILs), consisting of organic cations and either organic or inorganic anions, are characterized by the advantages of negligible volatility, non-flammability, high thermal stability, and controllable hydrophobic properties,^7–13 which make them promising alternatives to conventional extractants. In comparison with ILs, recent advances on ammonium, phosphonium, imidazolium and pyridinium-based functional ionic liquids (FILs) with superior extractability and high selectivity for the extraction of Pd(II) have also been achieved.^13–19 However, these poisonous organic solvents always involve in extraction processes acting as organic phase and betray the notion of green chemistry. On the basis of these considerations, it is favourable to bring in hydrophobic ILs as diluting agents in extraction systems. Hydrophobic ILs are often prepared by fluorinated anions, such as hexafluorophosphate (PF₆⁻) or bis(trifluoromethylsulfonyl)⁻imide (NTf₂⁻).²⁰ In this content, [NTf₂] with high solubility is a more appropriate anion to construct hydrophobic ILs as diluting agent. And introduction of a special chelating function group into the cations and/or anions of ILs to build FILs with high solubility/coordinating ability of metal ions could enhance metal affinity to the organic phase and ensure a superior extraction efficiency/yield.^21,22 Among these FILs, anionic functional groups^23–25 (e.g., dicyanamide, di(2-ethylhexyl)phosphate and (2-ethyhexyl)diglycolamate) substituted ILs in extraction of metals have been intensively studied. 2-Mercaptobenzothiazole (MBT), a bicyclic heteroatomic molecule, exhibits sulfur and nitrogen atoms acting as chelating agent. So, incorporating MBT as extractant with hydrophobic IL phase can strengthen its metal affinity and overcome its insolubility nature. In all cases, further extending the application of MBT-based FILs to the separation of noble metals and revealing the related mechanism involved²⁶ especially when it comes to Pd(II) extraction are still needed.

In our work, {[C₁₀(mim)₂]²⁺2[MBT]⁻} (Scheme 1) was synthesized and then mixed with [C₈mim][NTf₂] for Pd(II) extraction. The performance of the system for Pd(II) extraction was studied and the extraction mechanism was also discussed in detail. As a further investigation, the selectivity of this extraction system for other metal ions such as nickel, aluminium, cobalt, platinum, rhodium and ruthenium ions was assessed. According to the results, 2-mercaptobenzothiazolate-functionalized ionic liquid extractant, {[C₁₀(mim)₂]²⁺2[MBT]⁻}, showed excellent extractability and high selectivity for Pd(II).

Scheme 1 Synthesis of 2-mecaptobenzothiazolate FIL extractant, {[C₁₀(mim)₂]²⁺2[MBT]⁻}.

Experimental

Materials

PdCl₂, H₂PtCl₆, RuCl₃ and RhCl₃ were purchased from Guangfu Institute of Fine Chemical (Tianjin, China). 1-Octyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide, [C₈mim][NTf₂], (99%) were obtained from Lanzhou Greenchem ILS, LICP, CAS, China. 1,10-Dichlorodecane was supplied by Dixiai Synthetic Chemistry (Shanghai, China). CoCl₂·6H₂O, NiCl₂·6H₂O and AlCl₃·6H₂O were procured from Kermel Chemical Reagent Tianjin Co., Ltd. (Tianjin, China). All the other reagents (hydrochloric acid, sulfuric acid, sodium chloride, sodium nitrate, ethanol, acetonitrile, acetone, chloroform, etc.) were used without additional purification. Distilled water was used to prepare the aqueous solutions in all experiments.

Analysis methods

To confirm the structure and purity of FIL and FIL–Pd complexes, ¹H NMR spectra was obtained in DMSO with an AV-300 NMR spectrometer (Bruker, Fällanden, Switzerland). Thermal analysis melting point of FIL was determined by WRR melting point Apparatus (WRR, Shanghai manufacturing physical optics, Instruments Co., Ltd., Shanghai, China). In order to further investigate the mechanism of extraction Pd(II) system, UV spectrophotometry (UV-9000, Shanghai Metash, Instruments Co., Ltd., Shanghai, China) and Fourier transform infrared (FT-IR) spectrophotometry (Tensor27, Bruker corporation, Karlsruhe, Germany) with the scanning wave number ranged from 4000 to 400 cm⁻¹ were involved. The concentration of Pd(II) in the aqueous phase was determined by an atomic absorption spectrophotometer (3150, Precision & Scientific Instrument Shanghai Co., Ltd., Shanghai, China). The organic phase of Pd(II) concentration was calculated by mass balance. Other metals concentrations were tested by inductively coupled plasma atomic emission spectrometer (IRIS Intrepid II XSP, Thermo electron corporation, Boston, US).

Synthesis of 2-mecaptobenzothiazolate FIL extractant, {[C₁₀(mim)₂]²⁺2[MBT]⁻}

The synthesis of {[C₁₀(mim)₂]²⁺2[MBT]⁻} was carried out according to the literature procedure.^27–29 Briefly, C₁₀(mim)₂Cl₂ was prepared by adding equal amount of 1,10-dichlorodecane and 1-methylimdazole into 30 mL acetonitrile in a 250 mL round-bottom flask under nitrogen protection at 120 °C for 48 h. The as-formed golden viscous liquid was washed by acetone for six times and dried under vacuum. Brown viscous liquid was obtained at a yield of 90%. ¹H NMR (300 M, DMSO-d₆, δ/ppm): 9.46 (s, 2H), 7.87 (s, 2H), 7.78 (s, 2H), 4.21 (t, 4H), 3.90 (s, 6H), 1.80 (t, 4H), 1.26 (s, 12H).

2-Mercaptobenzothiazole was dissolved in ethyl alcohol and deprotonated with sodium hydroxide in a round-bottom flask. An equimolar amount of C₁₀(mim)₂Cl₂ was dissolved into the mixture under magnetic stirring at 50 °C for 4 h, and the precipitation was separated by filtration. After removal of ethyl alcohol by rotary evaporation, the obtained product was purified by anhydrous acetone. Yellow solid was dried under vacuum at 0.1 Pa for 24 h at 40 °C (85%, yields). The final product was characterized with ¹H NMR (300 M, DMSO-d₆, δ/ppm): 1.23 (s, 12H), 1.73 (t, 4H), 3.85 (s, 6H), 4.12 (t, 4H), 6.85 (t, 2H), 7.02 (t, 2H), 7.21 (d, 2H), 7.35 (d, 2H), 7.65 (s, 1H), 7.76 (s, 1H),9.13 (s, 1H). FT-IR (KBr, cm⁻¹) 3412(w), 3145(s), 3061(s), 2929(s), 2859(s), 1666(w), 1567(s), 1453(s), 1378(s), 1239(s), 1166(s), 1128(w), 1076(s), 969(s), 852(w), 756(s). Melting point: 79–81 °C.

Process of Pd(II) precipitation or extraction

The precipitation process of Pd(II) was performed by mixing a required amount of {[C₁₀(mim)₂]²⁺2[MBT]⁻} and Pd(II) solution, equilibrating in an orbital shaker. The vibration time was 15 minutes to reach the balance. After the precipitation reaction, an orange suspension was obtained and then centrifuged, washed by water. The FIL–Pd complexes transformed to orange powder after drying.

The extraction process was performed as follows: [C₈mim][NTf₂] mixed with a certain amount of {[C₁₀(mim)₂]²⁺2[MBT]⁻} was used as organic phase (expressed as {[C₁₀(mim)₂]²⁺2[MBT]⁻}/[C₈mim][NTf₂]) and the solution containing Pd(II) or other metal ions (Ni(II), Co(II), Al(III), Pt(IV), Ru(III) and Rh(III)) was employed as aqueous phase. A shaking time of 15 min after the mixture of two phases would be enough to reach balance. The mixture was then centrifuged (1500 rpm, 5 min) and the two phases were separated quickly (Scheme 2). All the experiments were carried out at 298 ± 1 K. Each experiment was performed three times and the standard deviations did not exceed 5%.

Scheme 2 Extraction–stripping process of Pd(II) using the FIL extractant.

The percentage of extraction (E) and distribution ratio (D) were calculated by the following relations:

where M stands for metal Pd(II), [M]_in and [M]_aq refer to the metal concentration before and after extraction, respectively.
where V_A and V_O are the volumes of aqueous phase and organic phase, the volume ratio of V_A/V_O (R_O/A) used in the extraction experiments is 10.

Crystal growth and analysis

Single crystals were grown in a mixture of n-hexane and chloroform (the volume ratio was 1.0) by solvent evaporation. The FIL–Pd complexes were dissolved in the mixed solution. Subsequently, the solution was filtered with a funnel and volatilized in a serum bottle at 298 K for about two weeks. Single crystal X-ray diffraction was performed on an area detecting system (Bruker-Nonius SMART APEX II CCD) and graphite monochromated Mo-Kα radiation (λ = 0.71000 Å). Data collection, integration and absorption corrections were conducted using SMART software and refined with SAINT on all observed reflections.³⁰ Data reduction was implemented with the SAINT software; the data were also revised by Lorentz and polarization effects. Absorption corrections were used by the program SADABS.³¹ The CCD data were integrated and scaled with the assistance of the Bruker SAINT software package, and the structure was obtained and refined using SHEXTL V6.12.2.³² Non-hydrogen atoms in structure were located from repeating examination of different F-maps following least squares refinement of earlier models. Hydrogen atoms were placed in calculated positions and allowed to ride on the carrier atoms. Crystal data for Pd₂[MBT]₄: Mr = 949.89, triclinic, space group P1̄, a = 7.7864(3) Å, b = 10.872(5) Å, c = 11.188(5) Å, T = 298 K, α = 75.96(5)°, β = 83.81(4)°, V = 868.5(7) ų, Z = 1, R = 0.0792(1983), wR₂ = 0.1750(3456).

Results and discussion

Influence of {[C₁₀(mim)₂]²⁺2[MBT]⁻} concentration and initial Pd(II) concentration on Pd(II) extraction

In this study, the influence of {[C₁₀(mim)₂]²⁺2[MBT]⁻} concentration on the Pd(II) extraction percentage (E) and distribution ratio (D) was investigated. As we could see from Fig. 1a, in the absence of {[C₁₀(mim)₂]²⁺2[MBT]⁻}, the partitioning of Pd(II) into [C₈mim][NTf₂] was negligible (E = 6.0%). Therefore, [C₈mim][NTf₂] just acted as diluting agent. However, as the {[C₁₀(mim)₂]²⁺2[MBT]⁻} concentration increased, Pd(II) were more inclined to transfer into the hydrophobic IL phase. When the concentration of {[C₁₀(mim)₂]²⁺2[MBT]⁻} reached 2.1 mM, the extraction percentage of Pd(II) was almost 100%. Further increase of the {[C₁₀(mim)₂]²⁺2[MBT]⁻} concentration only led to slight increase of extraction percentage. Hence, the concentration of {[C₁₀(mim)₂]²⁺2[MBT]⁻} (2.1 mM) was chosen for later experiments. Meanwhile, the initial Pd(II) concentration was also investigated. As shown in Fig. 1b, the extraction percentage of Pd(II) was almost 100% as Pd(II) concentration was less than 0.2 mM. On the contrary, the extraction percentage (E) and distribution ratio (D) of Pd(II) decreased sharply when initial Pd(II) concentration surpassed 0.2 mM. Therefore, the initial Pd(II) concentration of 0.2 mM was optimum.

Fig. 1 (a) Extraction percentage (E) and distribution ratio (D) of Pd(II) as a function of {[C₁₀(mim)₂]²⁺2[MBT]⁻} concentration. Aqueous phase: 0.2 mM Pd(II), 0.1 M HCl. Organic phase: {[C₁₀(mim)₂]²⁺2[MBT]⁻} in [C₈mim][NTf₂]. R_O/A = 10. (b) Extraction percentage (E) and distribution ratio (D) of Pd(II) as a function of initial Pd(II) concentration. Aqueous phase: Pd(II) solution, 0.1 M HCl. Organic phase: 2.1 mM {[C₁₀(mim)₂]²⁺2[MBT]⁻} in [C₈mim][NTf₂]. R_O/A = 10.

Influence of hydrochloric acid concentration, sodium chloride and sodium nitrate concentration on Pd(II) extraction

The influence of hydrochloric acid concentration varying from 0.1 M to 3.0 M on Pd(II) extraction was studied. As was shown in Fig. 2, the extraction percentage of Pd(II) decreased with the increasing concentration of hydrochloric acid. It is evident that at relatively high concentration level of {[C₁₀(mim)₂]²⁺2[MBT]⁻}, hydrochloric acid concentration between the range from 0.1 M to 1.0 M exerted a minor effect on Pd(II) extraction. However, when the concentration was above 1.0 M, extraction percentage witnessed a clear decrease. Moreover, the {[C₁₀(mim)₂]²⁺2[MBT]⁻}/[C₈mim][NTf₂] system for Pd(II) extraction are capable of resisting hydrochloric acid influence, which makes it superior to other extraction systems. However, it is hard to define whether H⁺ or Cl⁻ affects the extraction behavior of Pd(II). In order to examine the effect of H⁺ and Cl⁻ separately, the influence of H⁺ was studied at a fixed 3.0 M Cl⁻ concentration, and the H⁺ ions concentrations were varied between 0.1 M and 3.0 M using hydrochloride acid, the intermediate Cl⁻ concentrations were adjusted by sodium chloride contents. As seen from Fig. 2, the extraction percentage of palladium did not change with the increase in H⁺ concentration, suggesting that the participation of protonated extractant in Pd(II) extraction is negligible. Subsequently, the effect of Cl⁻ concentration varied from 0.1 M to 3.0 M on Pd(II) extraction was investigated (presented in Fig. 2). The result showed that the extraction percentage of Pd(II) decreased with increasing Cl⁻ concentration. It is indicated that Cl⁻ prevents Pd(II) extraction from aqueous phase to organic phase in [{[C₁₀(mim)₂]²⁺2[MBT]⁻}/[C₈mim][NTf₂] system. On the other hand, the influence of sodium nitritate was also studied on Pd(II) extraction. However, the percentage extraction of Pd(II) increased with the increasing concentration of sodium nitritate, suggesting that ionic strength may be the main factor promoting the extraction percentage of Pd(II).

Fig. 2 Extraction percentage (E) of Pd(II) as a function of HCl, NaCl, NaNO₃ and H⁺ concentration under different {[C₁₀(mim)₂]²⁺2[MBT]⁻} concentration. Aqueous phase: 0.2 mM Pd(II). Organic phase: 1.3 mM and 2.1 mM {[C₁₀(mim)₂]²⁺2[MBT]⁻} in [C₈mim][NTf₂]. R_O/A = 10.

Extraction mechanism

Spectrum analysis. The crystal and extractant were dissolved in chloroform respectively, with pure chloroform as blank comparison. Organic phase after extraction was dissolved in chloroform with [C₈mim][NTf₂] chloroform solution as comparison. The three samples were tested by UV-vis spectrum. The peaks of the crystal were consistent with the absorption peaks of the extracted complexes in the ionic liquids phase after extraction, signifying that they were in fact the same species (Fig. 3a). The absorption peaks at 325 nm and 249 nm can be attributed to the ultraviolet absorption [MBT]⁻ of the extractant. There was a shifting of absorption peaks from 325 nm to 309 nm, 249 nm to 254 nm when Pd(II) combined with the extractant after extraction. As the absorption peak at ∼309 nm and at ∼254 nm belonged to [MBT]⁻, therefore, [MBT]⁻ was successfully incorporated into the FIL–Pd complexes. It is indicated that Pd(II) was coordinated with the anion of FIL ([MBT]⁻). Simultaneously, the aqueous phase was detected after extracting by the UV-vis spectrum. The pure C₁₀(mim)₂Cl₂ had an absorption peak at 212 nm. As was shown in Fig. 3b, there also existed a peak at 212 nm of the aqueous solution was found after extraction, demonstrating that C₁₀(mim)₂Cl₂ could transfer to the aqueous phase.

Fig. 3 (a) UV-vis spectra of {[C₁₀(mim)₂]²⁺2[MBT]⁻}, crystal and IL-phase after extraction. (b) UV-vis spectra of the aqueous phase after extraction and C₁₀(mim)₂Cl₂ solution.

To further investigate the mechanism of Pd(II) extraction by the {[C₁₀(mim)₂]²⁺2[MBT]⁻}/[C₈mim][NTf₂] system, the FT-IR spectra and ¹H NMR spectra of Pd–MBT complexes were recorded in order to confirm any structural change in the FIL. The results were shown in Fig. 4a and b. The characteristic peaks of the chemical bonds which belong to {[C₁₀(mim)₂]²⁺2[MBT]⁻} were listed as follows. The peaks observed at ∼3145, 2929, 2859, 1666 and 1567 cm⁻¹ could be assigned to imazole cation. The peaks at ∼1453, 1378 cm⁻¹ were associated with the vibrations of the C–N–H group, while the peaks at ∼1239, 1166 and 1076 cm⁻¹ were typical of the N–C=S group vibrations.³³ However, after extraction, imidazole ring vibration peaks disappeared and many peaks which belong to [MBT]⁻ have changed clearly. In comparison with the pure {[C₁₀(mim)₂]²⁺2[MBT]⁻}, the peaks of N–C–H group shifted from 1453 cm⁻¹ and 1378 cm⁻¹ to 1449 cm⁻¹ and 1395 cm⁻¹, respectively; the peaks of N–C=S also shifted from 1239 cm⁻¹ and 1166 cm⁻¹ to 1245 cm⁻¹ and 1082 cm⁻¹, suggesting that Pd(II) has been coordinated with nitrogen and sulfur of the [MBT]⁻ anion.

Fig. 4 FT-IR spectra (a) and ¹H NMR spectra (b) of {[C₁₀(mim)₂]²⁺2[MBT]⁻} and Pd–MBT complexes.

¹H NMR spectra also supplied ESI† of FIL and Pd–MBT complexes. As seen in Fig. 4b, the peaks of cation of FIL have vanished which were consistent with the FT-IR spectra, the chemical shifts of hydrogen atoms related to benzene ring (E and H) altered as follows: F–F′, 6.854 → 7.272; G–G′, 7.019 → 7.474; H–H′, 7.204 → 7.709; E–E′, 7.351 → 8.948. It is estimated that the coordination of Pd(II) with nitrogen atom or sulfur atom resulted in the hydrogen atoms change in their chemical shifts. Taken together, the results suggested that Pd(II) entered into the IL phase via the formation of Pd–MBT complexes.

X-ray crystallographic analysis. It is evident that there exist two bonding sites as the ligand of FLI anion: the thiocarbonyl sulfur atom and the nitrogen atom. Pd(II) could coordinate with MBT in two different ways called monodentate or bidentate. We assume that there remain only two bonding sites on the ligand of MBT and take another measure to verify our hypothesis.

In order to get accurate information about the mode of coordination and calculate the complexing number, we employed the method of single crystal X-ray diffraction to analyse the structure of FIL–Pd complexes (shown in Fig. 5). The result showed that the stoichiometric number of FIL and Pd was 1 : 1 during the extraction procedure. Two Pd(II) atoms each shares two bonding nitrogens and two bonding sulfurs which were respectively provided by four isolated MBT molecules, constructing a lantern-typed in the triclinic space group P1̄. It is obviously revealed that Pd(II) simultaneously coordinated with nitrogen atom and sulfur atom. The lantern-typed complexes had an inversion center at the mid-point of the Pd(II)⋯Pd(II) bond, the Pd(II)⋯Pd(II) bond length (2.759 Å) completely agreed well with the value of 2.75 Å observed in metallic Pd.³⁴ The four MBT ligands were in a syn–anti–syn–anti conformation. In addition, N1–C7 bond length (1.3266 Å) was appreciably shorter than the C–N single bond length (1.47 Å),³⁵ yet longer than C=N double bond length (1.29 Å). The difference in bond length might resulted from charge delocalization of two sulphur atoms to nitrogen atom, presenting the resonating structure of 2-mercaptobenzothiazole. The S1–C7 single bond length (1.7660 Å) agreed well with those correlative compound, which is slightly longer than S2–C7 (1.7202 Å) double bond length, as the long pairs on the sulfur atom present in the skeleton of the ring which have very weak coordinating ability. Therefore, palladium was coordinated with the double bond sulfur instead of the single bond sulfur. As seen from the structure of the molecular, the N2–Pd1–S3 angle (179.255°) and the N1–Pd1–S2a angle (178.613°) suggested that the Pd1 atom and the N1, N2, S2a, S3a atoms were coplanar. All the aforementioned data was determined by X-ray diffraction (displayed in Table 1). According to the discussion, the extraction mechanism of Pd(II) using {[C₁₀(mim)₂]²⁺2[MBT]⁻} system can be represented by the following equation:

2PdCl₄²⁻_(aq) + 2{[C₁₀(mim)₂]²⁺2[MBT]⁻}_(IL) ⇔ [Pd(MBT)₂]_2(IL) + 2[C₁₀(mim)₂Cl₂]_(aq) + 2Cl⁻_(aq) (1)

Fig. 5 Molecular structure of Pd₂[MBT]₄. ORTEP diagram shows 50% probability ellipsoids.

Table 1 Selected bond length (Å) and angles (deg) for Pd₂[MBT]₄

A1	A2	Bond A1–A2	A1 A2 A3	Angle A2 A1 A3
Pd1	–N1	2.0830(7)	Pd1–N1–N2	90.994(16)
Pd1	–N2	2.0878(6)	Pd1a–N1a–S3	89.614(14)
Pd1	–S3	2.2963(7)	Pd1–N1–S2a	178.613(22)
Pd1	–S2	2.3002(8)	Pd1–N2–S2a	90.359(14)
S1	–C6	1.7323(6)	Pd1–N2–S3a	179.255(14)
S1	–C7	1.7660(6)	Pd1–S3–S2	89.031(14)
S2	–C7	1.7202(4)	S1–C6–C7	90.080(19)
N1	–C7	1.3266(5)	N1–C7–C5	112.220(20)

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The selectivity of functional ionic liquid with liquid–liquid extraction of single-metals. In the present study, single-metal extraction with six metals (Ni(II), Al(III), Co(II), Pt(IV), Ru(III) and Rh(III)) was investigated using 2.1 mM extractant in 0.1 M HCl media. The result of the extraction was shown in Fig. 6. It revealed that the {[C₁₀(mim)₂]²⁺2[MBT]⁻} extractant has a remarkable affinity for Pd(II) ions with the extraction percentage of 99.4%. On the other hand, the extraction percentage of other metal ions was less than 10.0%. Usually the separation of palladium and platinum constitutes a difficult problem owing to their similar structures and chemical behaviour. However, this {[C₁₀(mim)₂]²⁺2[MBT]⁻}/[C₈mim][NTf₂] extraction system offers an excellent method for separation of palladium and platinum.

Fig. 6 The aqueous stock: 0.2 mM Pd(II), Ni(II), Al(III), Co(II), Pt(IV), Ru(III) and Rh(III), 0.1 M HCl; the organic phase: 2.1 mM {[C₁₀(mim)₂]²⁺2[MBT]⁻} in [C₈mim][NTf₂]. R_O/A = 10.

Back-extraction of Pd(II) from loaded organic phase. In the present study, the Pd(II) complexes in the organic phase was back-extracted by 0.5 M thiourea/1.0 M HCl mixtures with ten times the amount of the loaded ionic liquid phase. The organic phase faded from orange to transparent quickly after adding the stripping regent. Thiourea in the back-extracted phase containing Pd(II) was decomposed by aqua regia before ICP-AES was applied to test the concentration of Pd(II). The result showed that the stripping percentage of Pd(II) was almost 99%. Pd(II) was nearly completely back-extracted to the aqueous phase as thiourea has more tendency to combine with Pd(II) than the extractant ({[C₁₀(mim)₂]²⁺2[MBT]⁻}).

Conclusions

The extraction behavior of Pd(II) was investigated using {[C₁₀(mim)₂]²⁺2[MBT]⁻} for the first time. The extraction efficiency of palladium was found to be nearly 100%. Low concentration of hydrochloric acid was in favor of palladium extraction to the organic phase. The palladium extraction is independent of H⁺ concentration, while chloride ion concentration exerts a negative effect on palladium extraction, nitrate ion produce a positive effect on palladium extraction. The mechanism of the palladium extraction system is a special complexation mechanism in which two palladium atoms are coordinated with four anions of the FLI. Under the optimum condition, the {[C₁₀(mim)₂]²⁺2[MBT]⁻}/[C₈mim][NTf₂] system shows higher selectivity for Pd(II) extraction than other metals. Stripping of Pd(II) from loaded organic phase was successful using the combination of 0.5 M thiourea in 1.0 M hydrochloride acid with recovery percentage about 99%. Hence, the 2-mercaptobenzothiazolate functional ionic liquid has excellent extraction efficiency and high selectivity for Pd(II) extraction.

Acknowledgements

This work was supported by the Natural Science Foundation of China (21276142, 21476129), the Science & Technology Development Projects of Shandong Province (grant no. 2014GSF117024) and Special Fund for “Taishan Scholar” construction engineering “agricultural nonpoint source pollution prevention and control” position.

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Footnote

† CCDC 1450377. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra12002d

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