Extraction and Stripping of Platinum from Hydrochloric Acid Medium

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Extraction and Stripping of Platinum from Hydrochloric Acid Medium by Mixed Imidazolium Ionic Liquids Yanzhao Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie503502g • Publication Date (Web): 25 Dec 2014 Downloaded from http://pubs.acs.org on January 5, 2015

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Extraction and Stripping of Platinum from Hydrochloric Acid Medium by Mixed Imidazolium Ionic Liquids

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Industrial & Engineering Chemistry Research ie-2014-03502g.R2 Article 23-Dec-2014 Tong, Yu; Shandong University, Wang, Chen; Shandong University, Huang, Yixian; Shandong University, Yang, Yanzhao; Shandong University, School of Chemistry and Chemical Engineering

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Extraction and Stripping of Platinum from Hydrochloric Acid Medium by Mixed Imidazolium Ionic Liquids

Yu Tong, Chen Wang, Yixian Huang and Yanzhao Yang*

Key Laboratory for Special Functional Aggregated Materials of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China Corresponding Author * E-mail: [email protected]. Tel.: +86 531 88365431. Fax:+86 531 88564464.

ABSTRACT: Hydrophilic ionic liquids, 1-alkyl-3-methylimidazolium chloride ([C n mim]Cl, n = 12, 14 and 16) are tested to precipitate Pt(IV) from hydrochloric acid medium. Meanwhile the Pt(IV) extractions are performed with hydrophobic ionic liquids, including 1-alkyl-3-methylimidazolium hexafluorophosphate

([Cn mim]PF 6 ,

n

=

4,

6

and

8)

and

1-octyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([C 8 mim]NTf 2 ). Based on Job's method, infrared spectra and 1 H NMR analysis, the anion-exchange mechanism is confirmed during the Pt(IV) precipitation. The mixed hydrophilic-hydrophobic ionic liquids ([C n mim]Cl/[C 8 mim]PF 6 ) are developed for Pt(IV) extraction, which significantly increase the Pt(IV) extractability comparing to the single hydrophobic ionic liquid. Under the optimum conditions, the [C 16 mim]Cl/[C8 mim]PF6 system shows high extractability as well as outstanding selectivity for Pt(IV) over the base metals (Mn(II), Cu(II), Co(II), Ni(II), Fe(III) and Al(III)). By the reductive stripping, Pt(IV) in the organic phase can be stripped in the form of platinum powders with using hydrazine hydrate, at the same time [C 16mim]Cl is regenerated in the organic phase. The mixed [C 16 mim]Cl/[C 8 mim]PF 6 ionic liquids can be reused to extract Pt(IV). Therefore, the method is highly effective, selective and recyclable to extract Pt(IV) and recover metal platinum.

1. INTRODUCTION Platinum (Pt) is a highly valuable and strategic platinum-group element, which is widely used in jewelry, catalysts, aviation, pharmaceutical and many other applications. Based on the importance in the industry and society, platinum is not only obtained from mineral ores but also recovered from secondary sources (chemical wastes, spent catalysts and electronic devices). The recovery of platinum from ore leaching

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liquor or waste solutions shows environmental and economic significance. For recovering or refining platinum as well as most metals, solvent extraction is an excellent and popular method with obvious advantages, such as high selectivity, high metal purity and low power consumption.1-4 In the case of extraction, ionic liquids are considered as potential selected alternatives to conventional organic solvents. Ionic liquids (ILs) are a class of organic salts solely composed of relatively large organic cations and inorganic or organic anions. The common cations of ionic liquids include ammonium, phosphonium, imidazolium and pyridinium cations, anions include halide anion, PF6-, CH3COO-, CF3COO-, (CF3SO2)2N-.5,6 Based on the remarkable properties of ionic liquids, such as high thermal stability, negligible volatility and nonflammability, the extractions of noble metals using ionic liquids as extractants or media have been a focus for over 10 years.7-18 But the research on Pt extractions using ionic liquids has just been carried out in recent years. Cyphos® IL-101, tetraalkylphosphonium chloride has been immobilized in biopolymer capsules prepared by ionotropic gelation in calcium chloride solutions, which is used for the recovery of platinum from acidic solutions.19 Ionic liquids with functionalized aromatic anions were prepared and applied for the extraction of platinum from aqueous phase using liquid phase micro-extraction.20 Shoichi Katsuta et al. originally employ mixed ionic liquids of trioctylammonium bis(trifluoromethanesulfonyl)amide and trioctylammonium nitrate to selectively extract platinum and palladium.21 The maximum extraction efficiency of platinum is 92.7%, and the stripping efficiency reaches 91% by a two-stage back-extraction with 8 mol L-1 HNO3 solutions. In our opinion, there is much room for the improvement of the extraction and stripping efficiencies, and more details need to be discussed, such as the extraction temperature and the effect of the cation’s alkyl chain length. Platinum sorption from HCl solutions has been tested by using an ionic liquid (Cyphos IL101), which is impregnated on Amberlite XAD-7 resin.22 Platinum can be extracted from aqueous phase in the presence of KSCN by using 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [OMIM][NTf2].23 Precipitation and liquid-liquid extraction of PtX62- (X = Cl-, Br- and SCN-) in acidified aqueous solutions ranging from 0.1 mol L-1 to 9 mol L-1 HBr or 11 mol L-1 of HCl have been studied using water-soluble or hydrophobic ionic liquids.24 Comparing to other noble metals, the extractions of platinum using ionic liquids are rarely studied. As mentioned above, the individual extraction behaviors of some hydrophobic ionic liquids were tested for Pt(IV) extraction in the previous reports, but little attention has been paid to the mixed hydrophilic-hydrophobic ionic liquids for platinum extraction until now.

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In the present work, hydrophilic ionic liquids 1-alkyl-3-methylimidazolium chloride ([Cnmim]Cl, n = 12, 14 and 16), and hydrophobic ionic liquids, 1-alkyl-3-methylimidazolium hexafluorophosphate ([Cnmim]PF6, n = 4, 6 and 8) and 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8mim]NTf2) are tested to precipitate or extract Pt(IV) from hydrochloric acid media, and the complexes of [Cnmim]Cl and H2PtCl6 is analyzed with Job’s method, FTIR and 1H NMR. According to the separation performance of the single ionic liquid, the mixed hydrophilic-hydrophobic ionic liquids are developed for the extraction of Pt(IV), which is different from current studies. What is more, hydrazine hydrate was first tested to reduce the [C16mim]-Pt complex, by which platinum can be stripped in the form of platinum powders and the mixed ionic liquids are regenerated.

2. EXPERIMENTAL

2.1. Reagents and Materials [Cnmim]Cl

(1-alkyl-3-methylimidazolium

chloride,

n n

= =

12,

(1-alkyl-3-methylimidazolium

hexafluorophosphate,

4,

(1-octyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide)

6

14

and

and

8)

were

16), and

procured

[Cnmim]PF6 [C8mim]NTf2

from

Lanzhou

Greenchem ILS, LICP. CAS. (Lanzhou, China) and used as received. H2PtCl6·6H2O (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) was dissolved in 0.1 mol L-1 HCl, which was diluted for the feed solution. Multi-metal solutions were prepared by dissolving metal chlorides in hydrochloric acid solutions: MnCl2·4H2O, ZnCl2, SnCl4·5H2O, FeCl3·6H2O, CoCl2·6H2O, NiCl2·6H2O, CuCl2·2H2O and AlCl3·6H2O, Kermel Chemical Reagent Tianjin Co., Ltd. (Tianjin, China). Hydrazine hydrate (N2H4·H2O) was procured from Tianjin Fuyu Chemical Reagent Co., Ltd. (Tianjin, China). Distilled water was used to prepare the aqueous solutions in all experiments. All the other chemicals used in this study were of analytical or reagent grade.

2.2. General Procedure of Platinum Precipitation or Extraction The precipitation of Pt(IV) was conducted by mixing a required amount of [Cnmim]Cl and Pt(IV) solution (1 mL, 2 mmol L-1), then equilibrating in an orbital shaker. The vibration time was varied from 5 min to 2 h. The precipitation or extraction efficiency did not increase obviously within 40 min, so 4 hours are sufficient for the equilibration. The molar ratio of [Cnmim]Cl to Pt(IV) was 2:1 in the general precipitation

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procedure, and such amount of [Cnmim]Cl can dissolve easily in the aqueous phase without Pt(IV). After the precipitation reaction, a yellow suspension was obtained, then centrifuged to offer the transparent solution for determining Pt(IV). The precipitate would transform to yellow powders after drying. For the metal extractions, a required amount of [Cnmim]Cl was added into 0.2 mL [C8mim]PF6 to form the organic phase (expressed as [Cnmim]Cl/[C8mim]PF6), and the volume had hardly changed through measurement. The concentration of [Cnmim]Cl in the organic phase ranged from 0 to 0.3 mol L-1 in different experimental sections. The organic phase and the aqueous solution containing Pt(IV) or mixed metal ions (Mn(II), Zn(II), Cu(II), Co(II), Ni(II), Fe(III), Al(III) and Sn(IV)) were added to a tube and then equilibrated sufficiently in an orbital shaker for 4 h. The initial concentration of each metal ion was 2 mmol L-1. The volume ratio of the aqueous phase to the organic phase was Rw:o = 5. The organic phases were normally pre-saturated with the HCl solutions. The HCl concentrations are equal with the relevant aqueous phases for the extraction. Afterwards, the two phases were separated with a centrifuge to be both clear and transparent. Unless otherwise noted, all experiments were carried out at 298 K.

2.3. Stripping of Platinum and Regeneration of Ionic Liquids Based on the Pt(IV) extraction performance of the [Cnmim]Cl/[C8mim]PF6 systems, the mixed [C16mim]Cl/[C8mim]PF6 ionic liquids were tested in the stripping experimental. After

the

Pt(IV)

extraction,

the

Pt-ILs

complexes

were

extracted

to

the

organic

phase

([C8mim]PF6/[C16mim]Cl), which was an orange-yellow, clear and transparent solution. The organic phase was separated from aqueous phase and then mixed with 1 mL N2H4·H2O solutions of 0.5 mol L-1 . The reduction reaction was maintained at 333 K for 2 hours, and black platinum powders were formed in this process. After reduction reaction, the Pt precipitate was filtrated from liquid phases and the two liquid phases ([C16mim]Cl/[C8mim]PF6 phase and aqueous phase) were separated by centrifugation. The organic phase ([C16mim]Cl/[C8mim]PF6) were recovered and reused to extract Pt(IV) from new feed solutions.

2.4. Analytical Techniques The Pt(IV) concentration in aqueous phase was determined with a flame atomic absorption spectrometer (3150, Precision & Scientific Instrument Shanghai Co., Ltd., Shanghai, China). The concentrations of multi-metal ions (Mn(II), Zn(II), Cu(II), Co(II), Ni(II), Fe(III), Al(III) and Sn(IV)) in solutions were determined with an ICP-AES (IRIS Intrepid II XSP, Thermo electron corporation, Boston, US). The metal

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concentrations in organic phase or the moles of Pt in the precipitate were calculated based on mass balances, then we can work out the extraction or precipitation efficiency (E%) as well as extraction distribution ratio (D). The data were determined 3~5 times (relative standard deviation < 3%), and the mean values were taken. The concentration of [Cnmim]Cl in aqueous phase was determined by the discoloration method with bromocresol green, at pH = 5.8, λ = 614 nm.25 After the precipitation reaction of [PtCl6]2- and [Cnmim]Cl, the [Cnmim]-Pt complexes were filtrated from the aqueous solution, washed with distilled water for several times and evaporated completely at 313 K, 2.5×104 Pa. The obtained sample was pressed to a tablet with dry KBr powders, then analyzed by FT-IR (Tensor27, Bruker corporation, Karlsruhe, Germany), and the scanning regions ranged from 400 cm-1 to 4000 cm-1. With using DMSO as solvent, the samples were also analyzed by 1H NMR (AV300, Bruker corporation, Karlsruhe, Germany), and the scanning regions ranged from 0 ppm to 10 ppm. General experiments were conducted twice, if the data showed relatively large difference, the experiments were repeated.

3. RESULTS AND DISCUSSION

3.1. Extraction or Precipitation of Pt(IV) with a Single Ionic Liquid The hydrophilic ionic liquids, [Cnmim]Cl (n = 12, 14 and 16) were tested for the precipitation of Pt(IV), and the hydrophobic ionic liquids, [Cnmim]PF6 (n = 4, 6 and 8) and [C8mim]NTf2 were examined to extract Pt(IV) from aqueous phase. With using the aqueous phases of 2 mmol L-1 Pt(IV) and 5 mmol L-1 HCl, various hydrophobic ionic liquids (0.2 mL) were examined for the extraction of Pt(IV). For the precipitation reaction, the moles of hydrophilic ionic liquids added into the aqueous phases were twice of Pt(IV). The extraction and precipitation performances are shown in Figure 1. [C8mim]PF6 and [C16mim]Cl show the highest extraction and precipitation efficiency respectively. The extraction or precipitation ability enhances with the increase of the alkyl side chain length, but [Cnmim]Cl performed significantly better on the separation of Pt(IV) than the hydrophobic ionic liquids. Comparing the behaviors of [C8mim]PF6 and [C8mim]NTf2, PF6- anion is more conducive to the Pt(IV) extraction than NTf2-.

3.2. [Cnmim]-Pt(IV) Complexes Analysis

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The precipitation reactions of [Cnmim]Cl and [PtCl6]2- were studied and the [Cnmim]-Pt complexes were analyzed, by which we can also understand the extraction mechanism indirectly. The method of continuous variation (Job's method) is widely used to investigate the coordination number of complexes.26, 27

A modified method of continuous variation was first employed to study the precipitation reactions of

[Cnmim]Cl and [PtCl6]2-. The total amount of [Cnmim]Cl and Pt(IV) was maintained at 6×10-6 mol, and the initial concentration of Pt(IV) in aqueous phase, CPt, initial ranged from 0.5 to 3.5 mmol L-1. The results are shown in Figure 2 by plotting CPt, initial as a function of Pt(IV) moles in the precipitate (nPt, precipitation). When the mole ratio of [Cnmim]Cl to Pt(IV) is 2 : 1 (CPt, initial = 2 mmol L-1), the maximum of nPt, precipitation appears. At this point, the largest amount of [Cnmim]-Pt complexes is formed, which indicates that the [Cnmim]-Pt complexes are in the mole ratio of 2:1. To obtain direct evidence of the interaction between [Cnmim]Cl and [PtCl6]2-, IR was used to analyze the [Cnmim]-Pt complexes. The IR spectra of [C16mim]Cl and the [Cnmim]-Pt complexes are shown in Figure 3. After [Cnmim]Cl was combined with Pt(IV), many changes appeared between the spectra of [Cnmim]Cl and the complexes. Take the case of [C16mim]Cl and the [C16mim]-Pt complexes, the vibration bands of [C16mim]Cl are shown: ring N-H and O−H stretch, 3474; ring C−H stretch, 3056; aliphatic C−H stretch, 2918, 2850; ring C−C stretch, 1636; ring C−N stretch, 1573; MeC−H deformation, 1471; ring C−H deformation in plane, 1178. Firstly, the O−H stretch of [C16mim]Cl (at 3474 cm-1) is caused by trace adsorbed water because of the hydrophilicity of [C16mim]Cl. With [C16mim]Cl combining to Pt(IV), the [C16mim]-Pt complexes show a strong hydrophobicity unlike [C16mim]Cl, which will adsorb less water or no water, and the stretch strength at 3474 cm-1 decreases obviously. Secondly, all the absorption bands related to imidazole ring have shifted largely: 3056→3101; 1636→1628; 1573→1564; 1178→1162. Finally, the aliphatic C-H stretch peaks have hardly shifted because of the weaker interaction between [PtCl6]2- and the alkyl side chains. The similar phenomenon also takes place in the [C14mim]-Pt and [C12mim]-Pt complexes. The results show a stronger interaction between [PtCl6]2- and [Cn mim]+ cationic head group, but the alkyl side chain is slightly influenced by the combination with [PtCl6]2-. The [Cnmim]-Pt complexes were also analyzed by 1H NMR. The 1H NMR spectra of [C16mim]Cl and the [Cnmim]-Pt complexes are shown in Figure 4. Take the case of [C16mim]Cl and [C16mim]-Pt complex, the chemical shifts of hydrogen nuclei (δH, ppm) changed as follows:

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H nuclei in imidazole ring, A-A', 9.160→9.105; B-B', 7.775→7.763; C-C', 7.708→7.696; ∆δH = 0.012 ~ 0.055. H nuclei in the aliphatic chain near imidazole ring, D-D', 4.148→4.143; E-E', 3.848→3.844; F-F', 1.770→1.768; ∆δH = 0.002 ~ 0.005. H nuclei in the aliphatic chain away from imidazole ring, G-G', 1.236→1.236; H-H', 0.854→0.854; ∆δH had hardly changed. The results clearly show that δH related to the aliphatic chain are not or slightly influenced by the combination with [PtCl6]2-, but δH of imidazole ring have changed more. The results imply that [PtCl6]2primarily affects the positively charged imidazole ring, and the alkyl side chain is less affected. The conclusion is consistent with that of the IR analysis. Based on the analysis above, 1:2 associates are formed between Pt(IV) and [Cnmim]Cl, and a stronger interaction exists between [PtCl6]2- and cationic head group of [Cnmim]+. Furthermore, [Cnmim]Cl is composed of [Cnmim]+ cation and Cl- anion, so the anion-exchange mechanism was supposed. The Pt(IV) extraction or precipitation by [Cnmim]Cl is deduced as the formation of [Cnmim]+2 [PtCl6]2- ion-pairs.

3.3. Extraction of Pt(IV) by [Cnmim]Cl/[C8mim]PF6 Ionic Liquids According to the precipitation or extraction performance of the single ionic liquid, we can suppose that if [Cnmim]Cl were added into hydrophobic ionic liquids to form a new extraction system, the extractability of Pt(IV) would be enhanced comparing with the single hydrophobic ionic liquid. So the mixed ionic liquids ([Cnmim]Cl/[C8mim]PF 6) were prepared to extract Pt(IV) from HCl media. With 1 mL aqueous phase of 2 mmol L-1 Pt(IV) in 5 mmol L-1 HCl, and 0.2 mL [C8mim]PF6 as the main body of organic phase, [Cnmim]Cl concentrations in the organic phase (C[Cnmim]Cl, org) were varied to study the effect on the extraction efficiency (E). The [Cnmim]Cl concentrations in the aqueous phase were determined with the method mentioned in Section 2.4, which showed only about 2% [Cnmim]Cl distributed into the aqueous phase. As shown in Figure 5, the extraction efficiency increases with [Cnmim]Cl concentration. Take the case of [C16mim]Cl/[C8mim]PF6 , E improves up to 93.6% from 60.9%, when the mole concentration reaches 0.3 mol L-1, i.e., the mass fraction of [C16mim]Cl in the organic phase is 7.66%. The results verified our assumption that the mixed ionic liquids have better extraction performance than the single hydrophobic ionic liquid.

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3.4. Extraction of Pt(IV) at Various Temperatures To investigate the effect of temperature on the extraction, the extraction experiments were carried out under the temperatures ranging from 25 o C to 45 o C. By using 1 mL aqueous phase of 2 mmol L-1 Pt(IV) in 5 mmol L-1 HCl, and 0.2 mL organic phase of 0.3 mol L-1 [Cnmim]Cl, the results are shown in Figure 6. Higher temperatures show a negative effect on the Pt(IV) extraction. This is a classic phenomenon that exists in the extraction of platinum group metals using amine derivatives or similar extractants.18, 28, 29 The imidazolium ionic liquids used in the work are similar with quaternary ammonium extractants. This type of extraction reaction is exothermic, the efficiency decreases slightly with the increasing extraction temperature.

3.5. Extraction of Pt(IV) at Various HCl Concentrations The extraction behaviors of Pt(IV) by the [Cnmim]Cl/[C8mim]PF6 systems with various concentrations of hydrochloric acid in aqueous phase were studied. The HCl concentrations varied from 0.005 mol L-1 to 5.00 mol L-1. The results are shown in Figure 7. The extraction efficiency increases rapidly with HCl concentration, then follows a slow decline. The maximum of E is about 97.8%, and appears around CHCl = 0.8 mol L-1. The extraction results are better than the preceding study with the maximum E = 92.7%.21 The variation tendency of E depended on the HCl concentration is a classic phenomenon that exists in the Pt(IV) extraction.19, 21, 22 High ionic strengths are against the distribution of organics in aqueous phase, but higher concentration Cl- will compete with [PtCl6]2- to associate with [Cnmim]+ based on the anion-exchange reaction.

3.6. Extraction of Pt(IV) from Multi-metal-ion Solutions To investigate the selectivity of the [C16mim]Cl/[C8mim]PF 6 system for Pt(IV), an aqueous multimetal solution was prepared by adding other chloride metal salts into 0.8 mol L-1 HCl solution. The concentration of each metal ion (Mn(II), Zn(II), Cu(II), Co(II), Ni(II), Fe(III), Al(III) and Sn(IV)) was 2 mmol L-1. The extraction behaviors are shown in Figure 8. Pt(IV) is almost extracted from the aqueous phase, E = 96.5% and D = 138. The extraction efficiencies of Zn(II) and Sn(IV) are slightly high, 9.67% and 9.51% respectively. Other metal ions are hardly extracted, distribution ratios and extraction efficiencies are as follows: 0.187 and 3.61% for Al(III), 0.352 and 6.58% for Co(II), 0.295 and 5.56% for Cu(II), 0.334 and 6.25% for Mn(II), 0.294 and 5.55% for Ni(II), 0.204 and 3.93% for Fe(III). Additionally,

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Pd(II) often accompanies Pt(IV) and is the most problematic to be separated from Pt(IV). We also tested the [C16mim]Cl/[C8mim]PF6 system for Pd(II), the extraction efficiency of Pd(II) is about 85%. The result is not better than previous studies.11, 14, 19, 21 The method is more suitable to be used in a none-palladium system, such as waste platinum catalysts and medical waste waters.

3.7. Regeneration of [C16mim]Cl and Recovery of Platinum In the present study, various reagents had been tested for back extraction of Pt(IV), such as formaldehyde, oxalic acid, sodium sulfite, sodium borohydride, ascorbic acid and hydrazine. Most reagents do not strip Pt or have some disadvantages for stripping Pt(IV). For example, sodium borohydride breaks down too rapidly in the process. Finally, hydrazine was selected for the stripping of Pt from organic phase, it can reduce Pt(IV) to platinum powders from the [C16mim]-Pt complexes in the organic phase. The reduction reaction may be as the following: [C16mim]2[PtCl6] + N2H4→Pt↓ + 2[C16mim]Cl + 4HCl + N2↑ When Pt(IV) in the complexes were reduced to platinum metal, Cl- in [PtCl6]- will dissociate, and then combine to [C16mim]+. In this way, [C16mim]Cl will regenerate and remain in the organic phase, while platinum is stripped from organic phase. After the stripping process, the liquid phases were digested by aqua regia. The Pt(IV) concentration was determined by FAAS, then the stripping efficiency was calculated based on mass balances. The results show that trace amounts of Pt(IV) remained in liquid phases, which means Pt(IV) was almost entirely reduced to platinum powders, all the stripping efficiencies were over 98.2%. As mentioned above, the stripping efficiencies are higher than the preceding study.21 The remained Pt(IV) may not be reduced, and left over in the organic phase. By plotting the extraction efficiency or the distribution ratio as a function of the number of stripping cycles, the results were shown in Figure 9. After multiple cycles of extraction-stripping process, the extraction of Pt(IV) is still effective. The decline of extraction efficiencies should be caused by the slight loss of ionic liquids in the laboratory operating, such as the filtration of platinum powders and the separation of ionic liquid phase and aqueous phase. Nevertheless, the optimization conditions of the reductive stripping still need to be further explored. The general extraction-stripping-regeneration process is shown in Figure 10.

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4. CONCLUSIONS A series of hydrophilic and hydrophobic ionic liquids were tested to extract or precipitate Pt(IV) from hydrochloric acid media, and the ILs-Pt complexes were deemed as [Cnmim]+2 [PtCl6]2-, according to the analysis of continuous variation method, infrared and 1H NMR spectra. The work reveals that room temperature and the HCl solutions of 0.8 mol L-1 show positive effects on Pt(IV) extraction by the mixed hydrophilic-hydrophobic ionic liquids ([C16mim]Cl/[C8mim]PF6). Under the optimum conditions, the [C16mim]Cl/[C8mim]PF6 system shows high extractability and selectivity for Pt(IV) over base metals (Mn(II), Cu(II), Co(II), Ni(II), Fe(III) and Al(III)). Moreover, the Pt stripping and the recovery of the mixed ionic liquids can be achieved by the reduction using hydrazine. Therefore the extraction of Pt(IV) by the mixed hydrophilic-hydrophobic ionic liquids ([C16mim]Cl/[C8mim]PF6) is an effective, highly selective and recyclable approach.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the Natural Science Foundation of China (Grant 21276142 and 21476129)

REFERENCES (1) Jha, M. K.; Gupta, D.; Lee, J. C.; Kumar, V.; Jeong, J. Solvent Extraction of Platinum Using Amine Based Extractants in Different Solutions: A Review. Hydrometallurgy 2014, 142, 60. (2) Parajuli, D.; Hirota, K.; Inoue, K. Trimethylamine-Modified Lignophenol for the Recovery of Precious Metals. Ind. Eng. Chem. Res. 2009, 48, 10163. (3) Parajuli, D.; Kawakita, H.; Inoue, K.; Funaoka, M. Recovery of Gold(III), Palladium(II), and Platinum(IV) by Aminated Lignin Derivatives. Ind. Eng. Chem. Res. 2006, 45, 6405. (4) Marinho, R. S.; Afonso, J. C.; da Cunha, J. W. S. D. Recovery of Platinum from Spent Catalysts by Liquid-Liquid Extraction in Chloride Medium. J. Hazard. Mater. 2010, 179, 488.

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(5) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Characterization and Comparison of Hydrophilic and Hydrophobic Room Temperature Ionic Liquids Incorporating the Imidazolium Cation. Green Chem. 2001, 3, 156. (6) Freemantle, M. Designer Solvents - Ionic Liquids May Boost Clean Technology Development. Chem. Eng. News 1998, 76, 32. (7) Shimojo, K.; Goto, M. Solvent Extraction and Stripping of Silver Ions in Room-Temperature Ionic Liquids Containing Calixarenes. Anal. Chem. 2004, 76, 5039. (8) Reyna-González, J. M.; Torriero, A. A. J.; Siriwardana, A. I.; Burgar, I. M.; Bond, A. M. Extraction of Silver(I) from

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Copper(II) with

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Figure 1. The extraction or precipitation efficiencies of Pt(IV) with hydrophobic or hydrophilic ionic liquids. Figure 2. Method of continuous variation for the precipitation reaction of Pt(IV) and [Cnmim]Cl. Figure 3. Infrared spectra of [C16mim]Cl and [Cnmim]-Pt(IV) complexes. Figure 4. 1H NMR spectra of [C16mim]Cl and [Cnmim]-Pt(IV) complexes. Figure 5. Extraction of Pt(IV) as a function of [Cnmim]Cl concentration in the organic phase. Aqueous phase: 1 mL, 2 mmol L-1 Pt(IV), 5 mmol L-1 HCl. Organic phase: 0.2 mL [C8mim]PF6. Figure 6. The effect of temperature on the Pt(IV) extraction. Aqueous phase: 1 mL, 2 mmol L-1 Pt(IV), 5 mmol L-1 HCl. Organic phase: 0.2 mL [C8mim]PF6, 0.3 mol L-1 [Cnmim]Cl. Figure 7. Extraction of Pt(IV) as a function of HCl concentration. Aqueous phase: 1 mL, 2 mmol L-1 Pt(IV). Organic phase: 0.2 mL [C8mim]PF6, 0.3 mol L-1 [Cnmim]Cl. Figure 8. The extraction of various metals by the [C16mim]Cl/[C8mim]PF6 system. Aqueous phase: 1 mL, 2 mmol L-1 each metal, 0.8 mol L-1 HCl. Organic phase: 0.2 mL [C8mim]PF6, 0.3 mol L-1 [C16mim]Cl. Figure 9. The effect of regeneration cycles on the Pt(IV) extraction. Aqueous phase: 1 mL, 2 mmol L-1 Pt(IV), 0.8 mol L-1 HCl. Organic phase: 0.2 mL [C8mim]PF6, 0.3 mol L-1 [C16mim]Cl. Figure

10.

The

extraction-stripping-regeneration

process

of

hydrophilic-hydrophobic ionic liquids.

13


Pt(IV)

using

the

mixed


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Figure 1 The extraction or precipitation efficiencies of Pt(IV) with hydrophobic or hydrophilic ionic liquids 287x201mm (300 x 300 DPI)


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Figure 2 Method of continuous variation for the precipitation reaction of Pt(IV) and [Cnmin]Cl 287x219mm (300 x 300 DPI)



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Figure 3 Infrared spectra of [C16min]Cl and [Cnmin]-Pt(IV) complexes 287x238mm (300 x 300 DPI)


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Figure 4 1H NMR spectra of [C16min]Cl and [Cnmin]-Pt(IV) complexes 289x232mm (300 x 300 DPI)



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Figure 5 Extraction of Pt(IV) as a function of [Cnmim]Cl concentration in the organic phase. Aqueous phase: 1 mL, 2 mmol L-1 Pt(IV), 5 mmol L-1 HCl. Organic phase: 0.2 mL [C8mim]PF6. 289x213mm (300 x 300 DPI)


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Figure 6 The effect of temperature on the Pt(IV) extraction. Aqueous phase: 1 mL, 2 mmol L-1 Pt(IV), 5 mmol L-1 HCl. Organic phase: 0.2 mL [C8mim]PF6, 0.3 mol L-1 [Cnmim]Cl 287x224mm (300 x 300 DPI)



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Figure 7 Extraction of Pt(IV) as a function of HCl concentration.. Aqueous phase: 1 mL, 2 mmol L-1 Pt(IV). Organic phase: 0.2 mL [C8mim]PF6, 0.3 mol L-1 [Cnmim]Cl. 287x229mm (300 x 300 DPI)


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Figure 8 The extraction behaviors of various metals by the [C16mim]Cl/[C8mim]PF6 system. Aqueous phase: 1 mL, 2 mmol L-1 each metal, 0.8 mol L-1 HCl. Organic phase: 0.2 mL [C8mim]PF6, 0.3 mol L-1 [C16mim]Cl. 289x180mm (300 x 300 DPI)



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Figure 9 The effect of regeneration cycles on the Pt(IV) extraction. Aqueous phase: 1 mL, 2 mmol L-1 Pt(IV), 0.8 mol L-1 HCl. Organic phase: 0.2 mL [C8mim]PF6, 0.3 mol L-1 [C16mim]Cl 289x198mm (300 x 300 DPI)


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Figure 10 The extraction-stripping-regeneration process of Pt(IV) using the mixed hydrophilic-hydrophobic ionic liquids 294x145mm (120 x 120 DPI)


Extraction and Stripping of Platinum from Hydrochloric Acid Medium

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