Extraction of Pt(IV), Pt(II), and Pd(II) from Acidic Chloride Media Using

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Applied Chemistry

Extraction of Pt(IV), Pt(II) and Pd(II) from acidic chloride media using imidazolium-based task-specific polymeric ionic liquid Zi-Xuan Xu, Yong-Lu Zhao, Pei-Yu Wang, Xin-Qi Yan, Miao-Miao Cai, and Ying Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03408 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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Industrial & Engineering Chemistry Research

Extraction of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) from acidic chloride media using imidazolium-based taskspecific polymeric ionic liquid Zixuan Xu,† Yonglu Zhao, † Peiyu Wang, Xinqi Yan, Miaomiao Cai, Ying Yang*

Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, Gansu, P. R. China. Corresponding author: Ying Yang. Email: [email protected]

† these authors made equal contributions to this work.

As an excellent separation material to recover platinum group metals (PGMs), imidazolium-based task-specific polymeric ionic liquid (CP-AMIN) was successfully synthesized and characterized by SEM, FT-IR. The effects of temperature, pH, reaction time, CP-AMIN amount, concentration of chloride ion were discussed during the process of extraction of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) from acidic chloride media. It shows high extraction efficiency under various conditions, suggesting CP-AMIN has strong adaptability of environment. CP-AMIN also presents excellent selectivity for PGMs in the solutions with coexisting metal ions (Ni2+, Mn2+, Cu2+, Cd2+, Co2+, and Cr3+), it

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indicates CP-AMIN process efficient task specificity. Notably, the acidic thiourea can desorb PGMs from CP-AMIN completely almost. After 7 cycles of adsorption-desorption experiments, the extraction efficiencies still maintain above 85%. It suggests that CP-AMIN also exhibits excellent recyclability. Meanwhile, due to its easy to handle, safe and environmentally friendly, the task-specific polymeric ionic liquid (TSIL) promises great potential to recover PGMs from aqueous phase.

Keywords: Platinum group metals, Environmentally friendly, Task-specific polymeric ionic liquid, Extraction

1. Introduction Platinum group metals (PGMs) play an essential and necessary role in various high-tech fields, such as electronic-based materials,1 catalysts,2, 3 jewelry,4 and so on. The increase in the consumption of PGMs, especially platinum (Pt) and palladium (Pd) has been observed year by year. Natural PGMs species always exist in igneous rocks with low abundances and are associated with some non-ferrous metals, such as copper, nickel, cobalt sulfide etc. Due to the limited PGMs resources and extractive technology, there has been a large gap between supply and demand. Moreover, the extraction and refining of Pt and Pd from both ores and industry wastes are profound due to their refractory chemical properties and the complex species in chloride solution, e.g., PtCl62-, PtCl42- and PdCl42-.


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Traditional approaches for extraction of PGMs include cyanidation, biohydrometallurgy, pyrometallurgy and advanced hydrometallurgy.5, 6 Yet, some inevitable defects such as high cost, poor selectivity and heavy environmental hazards limited their application. Some researchers also have synthesized the functional resins for solid-phase microextractions, which can extract PGMs via coordination interactions. In 2014, Xiong et al. reported a chelating resin based on chloromethylated polystyrene beads (CP), which showed excellent hydrophobicity and selectivity for Pt(Ⅳ). However, the maximum saturated adsorption capacity was relatively low due to the single interaction between the resin and Pt(Ⅳ).7 Ionic liquids (ILs) with a low melting point have been considered as “green” alternatives to conventional organic solvents because of their unique properties, such as high stability, nonvolatility, non-flammability, and facile manufacture.8 In recent years, ILs, based on imidazolium, pyridinium, pyrrolidinium cations, have been utilized for the extraction of PGMs.9-12 Katsuta et al. designed

mixed

ILs

of

trioctylammonium

bis

(trifluoromethanesulfonyl)amide

and

trioctylammonium nitrate to selectively extract Pt and Pd. The extraction percentages of Pd and Pt were estimated to be 99.7% and 93.6%, respectively.13 However, the back-extraction efficiency of Pd and Pt were only 57% and 91% under a harsh condition (the concentration of HNO3 was 8 mol/L), which violated the purpose of green chemistry. In 2015, Iwatsuki et al. reported a pyridinium-based task-specific ionic liquid (TSIL) to recover class b metal ions. Even though extraction efficiencies were over 99% for Pd(Ⅱ) and Pt(Ⅱ),10 still it is inevitable for the TSIL to cause environmental pollution due to the use of abundant fluorinated anions. Moreover, there are other disadvantages that hinder the application of ILs, including their high cost and high solubility in water. Recent years, Polymeric ionic liquids (PILs) and


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hydrophobic ionic liquids show more potentials than traditional ILs due to their superior hydrophobicity.14, 15 Compared with hydrophobic ionic liquids, PILs have more advantages due to their remarkable chemical and physical stability, facile synthesis, and excellent recyclability. On the other hand, PILs can be synthesized with industrial materials, effectively reducing the production cost. Thus, PILs have been widely used as novel materials in many fields,16-19 such as catalysis, gas separation, and electrolytes, however few studies have explored the solid-phase microextraction of metal ions.20 Recently, our group had synthesized a series of PILs as solid-phase microextraction agents to enrich metal ions from the aqueous solution14,

21-23

which show high adsorption capacities,

desirable selectivity, and excellent recyclability. The enrichment mechanism has been confirmed as a synergistic effect of electrostatic attractions and multiple supramolecular interactions12. Through our further research, we synthesized a novel PILs (CP-AMIN) via modifying the chloromethylated polystyrene beads (CP) with 5-Aminoimidazole-4-carboxamide and used it as a solid-phase microextraction to recover Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) ions from aqueous solution with coexisting metal ions for simulating hydrometallurgical complex system. The factors of temperature, concentration of HCl, concentration of chloride ion, coexisting metal ions, extraction time and reusability have been investigated to evaluate the adsorbent. It exhibited the high extraction efficiencies and high selectivity of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ). Meanwhile, its excellent recyclability makes it possess a tremendous prospect in industrial green separation process. 2. Experimental 2.1 Reagents and materials


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Chloromethylated polystyrene beads (CP) and 5-Aminoimidazole-4-carboxamide, acetonitrile were purchased from Energy Chemical Reagent Co., Ltd., Pt powders and Pd powders were purchased from Longyingda Trading Co., Ltd. NiSO4·6H2O, MnSO4·H2O, CoSO4·7H2O, CrSO4·6H2O, CuSO4·5H2O, CdSO4·8H2O, were purchased from Shanghai Reagent Factory Co., Ltd. Acetonitrile was purchased from Rionlon Bo Hua (Tianjin) Pharmaceutical & Chemical Co., Ltd. Thiourea (99%), HCl (36.5%), HNO3 (68%) were provided by Nanjing Chemical Reagent Co., Ltd. All other chemicals were analytical grade reagent without any further purification. 2.2 Synthesis of PILs (CP-AMIN) The synthetic route of the polymer imidazole ionic liquids is presented in Scheme 1. Chloromethylated polystyrene beads (CP, 11.3310 g) in acetonitrile (350 mL) were added to a three-neck flask equipped with an electric stirrer. The mixture was heated to 70 ℃. 5Aminoimidazole-4-carboxamide (10 g) and NaOH (5 g) were added slowly into the flask. The reaction was maintained for 4 hours. After the reaction mixture was cooled down, the unreacted reactants were washed with distilled water over three times until the pH of the residual water became neutral and dried under vacuum. The PILs presented black resin pellets. The size of the pellets slightly increased compared to CP and the PILs were insoluble in both water and ordinary organic solvents (acetonitrile, chloroform, methylbenzene, tetrahydrofuran et al.)

Scheme 1. Synthesis of the imidazolium-based task-specific polymeric ionic liquid


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2.3 Analytical techniques Stock solution of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) 1 g/L were prepared by dissolving the analytical grade platinum powders and palladium powders in aqua regia solution respectively. The superfluous nitric acid can be decomposed by hydrochloric acid at high temperature. The concentration of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) and other multi-metal ions (nickel, manganese, cadmium, chromium, copper and cobalt) were determined by a flame atomic absorption spectrometer (AA240, Edinburgh Instruments). The concentration of metals which were extracted by solid polymeric imidazole ionic liquids was calculated by mass balance. To characterize the structure of polymeric imidazole ionic liquids, Pt-loaded on CP-AMIN and Pd-loaded on CP-AMIN were analyzed by Fourier transform infrared spectrophotometer (FTIR: Nexus Nicolet, US)， scanning electron microscope (SEM: JSM-6701F, Japan electron optical laboratory, Japan)， energy dispersive spectroscopy (EDS: JSM-6701F, Japan electron optical laboratory, Japan) and X-ray photoelectron spectroscopy (XPS Kratos Analytical-A Shimadzu Group Company, Japan) 2.4 Extraction of PGMs ions The concentration of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) in hydrochloric acid solution were 50 mg/L respectively by diluting the stock solution. Suitable content (150mg) of CP-AMIN was added into the aqueous solution containing Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) (50 mg/L, 10mL) and other multi-metal ions (nickel, manganese, cadmium, chromium, copper and cobalt which are 1 g/L, 10 mL). The mixture was shaken by orbital shaker from 5 minutes to 12 hours at different temperature. In the extraction experiments, the concentration of HCl and Cl- were adjusted by 3 M HCl aqueous


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solution and solid NaCl. Afterwards, the aqueous phase and the PILs were separated by filtrating process. The metal-loaded (platinum and palladium) solid phase were obtained after washing with distilled water 5 times and drying completely under dynamic vacuum. We worked out the extraction efficiency (E%), distribution radio (D) and separation factor (β) We calculated extraction capacity (qe, mg g-1) and recovery percentage (E%) according to Eq (1) and Eq (2):

𝑞𝑒 =

(𝐶0 ― 𝐶𝑒)𝑉

(1)

𝑚

𝐸% =

𝐶0 ― 𝐶𝑒 𝐶0

× 100%

(2)

where C0 and Ce (mg L-1) represent the initial and equilibrium concentration of PGMs respectively. V (L) is the volume of the solution and m (g) is the amount of CP-AMIN. The distribution ratio (D) and separation factor (β) can be calculated by following equations:

𝐷𝑝 =

𝐶𝑠,𝑝𝑉𝑠

𝐶𝑆,𝑃

𝛽 𝑃 = 𝐶𝑆,𝑀 × 𝑀

(3)

𝐶𝑒𝑉

𝐶aq,𝑚 𝐶𝑒

(4)

where Cs,p (mg L-1) and Cs,m (mg L-1) represent the concentration of PGMs and other metals in the metal-loaded CP-AMIN respectively. Vs(mL) represents the volume of dissolved solution. Caq,m is the residue concentration of other metal ions in the aqueous phase. 2.5 Back- extraction of PGMs and polymer ionic liquids regeneration


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After extraction, metal-loaded (platinum and palladium) CP-AMINs were mixed with 20ml thiourea solution of different concentration from 0.01 mol/L to 1 mol/L. The desorption reaction was maintained at 333K for 24 hours. Afterwards, the Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) ions were backextracted into aqueous phase, and the polymer imidazole ionic liquids were recovered and reused to extract platinum(Ⅳ), platinum(Ⅱ)and palladium(Ⅱ) from the aqueous solutions. Scheme 2. shows adsorption-desorption processes of PGMs by CP-AMIN.

Scheme 2. Adsorption-desorption processes and mechanisms of PGMs from aqueous solution by CP-AMIN

3. Results and discussion 3.1 Characterization of polymer imidazole ionic liquids


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The chemical compositions of CP-AMIN have been determined. The elemental analysis of polymer imidazole ionic liquids is shown in Table 1. Due to the existence of nitrogen element from CP-AMIN, it’s indicated that the 5-Aminoimidazole-4-carboxamide is grafted onto the chain of the polymer (Scheme 1.). The grafting rate of the polymer imidazole ionic liquids was calculated by Eq (5): 54𝑊𝑁%

𝑔𝑟𝑎𝑓𝑡𝑖𝑛𝑔 𝑟𝑎𝑡𝑒 = 14𝑊𝐶% ― 12𝑁𝑊𝑁%

(5)

where WC is the mass percentage of carbon atoms and WN is the mass percentage of nitrogen atoms in the polymer imidazole ionic liquids. The experimental results show that the grafting rate is 81.88%, suggesting the effectiveness of the polymer imidazole ionic liquids on platinum(Ⅳ), platinum(Ⅱ)and palladium(Ⅱ) extraction. Table 1. Elemental analysis of the polymer imidazole ionic liquids.

CP-AMIN

N (%)

C (%)

H (%)

grafting rate (%)

1-1

11.69

65.09

5.404

81.88

1-2

11.73

65.10

5.397

82.18

To confirm the structure of the polymer imidazole ionic liquids, FT-IR spectroscopic study was carried out. The FT-IR spectrum of the TSIL, chloromethylated polystyrene beads (CP) and 5-Aminoimidazole-4-carboxamide are demonstrated in Fig. 1. CP-AMIN displays a characteristic peak corresponding to stretching vibrations of C-H bonds in alkane at about 2924cm-1. The peaks at 1450cm-1 and 1423cm-1 are assigned to C-H bending vibrations in alkane. The peak at 1644cm-1 is related to the carboxyl groups. The typical peaks of the imidazole ring and the benzene ring appeared at 1585cm-1 and 1511cm-1. In the imidazole ring, C-H bending vibrations were observed at 1167cm-1, 1116cm-1 and 1018cm-1. The peak at 750cm-1 can be attributed to the out-of-plane bending vibrations of monosubstituted benzene derivatives. To compare with the


9


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chloromethylated polystyrene (CP) and 5-Aminoimidazole-4-carboxamide, many changes appeared between the spectra of the reactants and the polymer imidazole ionic liquids. All of the peaks above indicate that the 5-Aminoimidazole-4-carboxamide has been grafted to CP successfully. In order to investigate the stability of CP-AMIN, we also confirmed the structure by FT-IR after heating at 60 ℃. (ESI Fig. S1). According to the spectrum, there are no signal peak changes compared with CP-AMIN. It’s indicated that CP-AMIN have strong stability which can be used in industry.

Figure. 1. FT-IR spectra of CP, 5-Aminoimidazole-4-carboxamide and CP-AMIN The scanning electron micrographs (SEM) of the CP and CP-AMIN (Fig. 2.) show that there are significant differences. The morphology of CP is smooth and flat and some cracks can be found. However, for CP-AMIN, its surface structure has become rough, irregular with many holes, suggesting that the 5-Aminoimidazole-4-carboxamide has been grafted onto the chain of polymer. Meanwhile, the specific surface area of CP-AMIN is enhanced, which is helpful to extract PGM ions. The results above are closely related with the FT-IR spectra analysis.


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Figure. 2. SEM images of CP (a) and CP-AMIN (b) 3.2 Effect of temperature Fig. 3 shows the extraction behaviors of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) by CP-AMIN in different temperatures. The temperature varies over the range of 20 to 70 ℃. With the increasing of the temperature, the extraction efficiency of Pt(IV), Pt(II) and Pd(II) by the PILs were also increased. Interestingly, we are able to observe relatively low extraction efficiency of Pt(Ⅳ) compared to Pt(Ⅱ) and Pd(Ⅱ) at low temperature. When the temperature reached 60 ℃, the extraction efficiency of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) are maximized, reaching 95.72%, 98.37% and 97.38%. According to the result, we believe that CP-AMIN has excellent adsorption capacity to recover PGMs. Moreover, CP-AMIN is easier to adsorb Pd(Ⅱ) and Pt(Ⅱ) than Pt(Ⅳ) at different temperatures. This result can be explained by the differences in structure among PtCl62-, PtCl42and PdCl42-. Palladium(II) and platinum(II) are d8 metal center with planar square structure and higher substitution activity than platinum(IV) (d6 center).24 In our previous work, we also found multiple supramolecular interactions between aromatic rings and metal ion with d8 metal center. 14


11


100 95

E(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90 85

Pt(IV) Pt(II) Pd(II)

80 75

20

30

40

50

60

70

Temperature (C)

Figure. 3. Effect of temperature on the PGMs extraction. (Aqueous phase: 10mL, 50mg/L Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ), 50mg CP-AMIN, 0.1 mol/L HCl, 5h.) 3.3 Effect of Cl- concentration The effect of Cl- concentration on extraction efficiencies of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) by CPAMIN were described in Fig. 4. It is clear that the extraction efficiencies of PGMs by CP-AMIN slightly decreases with the increase of Cl- concentration. While, CP-AMIN still processes excellent efficiencies over 90% for PGMs even in high Cl- concentration (3 mol/L). Since the volume of PtCl62-, PtCl42- and PdCl42- are larger than Cl-, they are easily to be adsorbed by imidazole cations of CP-AMIN.


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100

90

E (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80


70

60

0.0

0.5

1.0

1.5

CNaCl (mol/L)

2.0

2.5

3.0

Figure. 4. Effect of Cl- concentration on the PGMs extraction. (Aqueous phase: 10mL, 50mg/L Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) respectively, 50mg CP-AMIN, 0.1 mol/L HCl, 60 ℃, 5h.)

3.4 Effect of HCl concentration

To investigate the influence of HCl concentration on PGMs extraction efficiencies by CPAMIN, we adjusted the HCl concentration in the aqueous phase ranging from low concentration (0.1 mol/L) to high concentration (1 mol/L). The results are shown in Fig. 5. The maximum of the extraction efficiencies of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) can be found in 0.1 mol/L HCl concentration which are 95.5%, 97.1% and 96.2%. However, while HCl concentration increased, extraction efficiency of PGMs ions declined. We suggest two explanations for the result. Firstly, higher HCl concentration led to higher concentration of H+ and Cl-, higher Cl- will be conducive to form PtCl62, PtCl42- and PdCl42- which can be easily adsorbed by CP-AMIN based on the anion-exchange


13


reaction.25, 26. However, the excessive level of Cl- will also compete with PtCl62-, PtCl42- and PdCl42-, which is not helpful for the anion-exchange reaction. Secondly, the structure of CP-AMIN was destroyed in the high H+ concentration. CP-AMIN can be easily hydrolyzed in strong acid condition due to the amide groups. Thus, the extraction efficiency of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) declined.


100 90 80

E (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70 60 50 0.2

0.4

0.6

0.8

1.0

CHCl(mol/L)

Figure. 5. Effect of HCl concentration on the PGMs extraction. (Aqueous phase: 10mL, 50mg/L Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) respectively, 50mg CP-AMIN, 60 ℃, 5h.)

3.5 Effect of extraction time


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The extraction efficiencies of PGMs by CP-AMIN were investigated at different time intervals which varied from 5 minutes to 12 hours. As shown in Fig.6a. The solution of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) were adjusted to pH=1. For Pt(Ⅳ) and Pd(Ⅱ), they have similar adsorption equilibrium time. E% quickly reached 84.7% and 89.8% respectively within 2 hours and reached 96.71% and 97.82% after 5 hours, which achieved adsorption equilibrium. Interestingly, as a solid phase microextraction to recover Pt(Ⅱ), CP-AMIN reached adsorption equilibrium in a relatively short amount of time. E% quickly reached 94.32% within 1 hour. In addition, CP-AMIN achieved adsorption equilibrium after 2 hours and E% was 97.81%. The extraction time was set to 5 hours as the optimal value. Thus, CP-AMIN has great potential to be used in industrial applications. In kinetic study, two kinetics models were applied to analyze obtained data (Fig. S2 and Fig. 6b). Pseudo-first-order (Eq (6)) and pseudo-second-order (Eq (7)) are as follows: where qe (mg/g) qt (mg/g) and t (min) are adsorbing quantity at equilibrium time, the adsorbing quantity at any time and adsorbing time respectively. k1 is the first-order rate constant at equilibrium (h−1). k2 is the second-order rate constant at equilibrium (g (mg h)−1). According to the R2 value, the extraction kinetics of PGMs can be more favorably fitted by pseudo-second-order rate model. The pseudo-second-order rate model indicates that the ratecontrolling step was a chemical reaction.

ln (qe-qt)=lnqe-k1t

(6)

t/qt=1/k2qe2+t/qe

(7)


15


(b)

(a) 100

Pt(IV) t/qt=0.10021t+0.02362 R2=0.9996 Pt(II) t/qt=0.10059t+0.01127 R2=0.9998 Pd(II) t/qt=0.09905t+0.02772 R2=0.9998

1.4 1.2

80

1.0

t/qt (g·h/mg)

E (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

60

0.6

40

0.4


20

0.2 0.0

0

2

4

6

t (h)

8

10

12

0

2

4

6

t (h)

8

10

12

Figure. 6. Effect of extraction time. (a). linearized kinetics models for PGMs adsorption on CPAMIN (b)pseudo-second-order model (Aqueous phase: 10mL, 50mg/L Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) respectively, 0.1 mol/L HCl, 50mg CP-AMIN, 60 ℃.) 3.6 Effect of CP-AMIN content To investigate the influence of CP-AMIN contents on extraction efficiency of PGMs, another experiment was carried out at different initial CP-AMIN contents in 20mL 50mg/L PGMs aqueous solutions (Fig. 7). With the increasing of CP-AMIN contents, extraction efficiencies of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) were also increased and reached the maximum values by using 100 mg CPAMIN. The maximum extraction efficiencies of PGM were 95% above. It’s indicated that CPAMIN have great potential to recover PGMs in the hydrometallurgy field and metal manufacturing industry.


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100 90

E (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70


60 20

40

60

80

100 120 140 160 180 200

m (mg)

Figure. 7. Effect of the contents of CP-AMIN on PGMs extraction (Aqueous phase: 10mL, 50mg/L Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) respectively, 0.1 mol/L HCl; 60 ℃, 5h.). 3.7 The maximum extraction capacity In order to obtain the maximum extraction capacity of PGMs, the impact of the original PGMs concentration has been researched (Fig. 8). PGMs adsorbing quantity rapidly improved while the augment of the initial concentration from 50 mg/L to 1400 mg/L. Pt(Ⅱ) reached a stable state at 700 mg/L. Furthermore, Pt(Ⅳ) and Pd(Ⅱ) reached a stable state at 1400 mg/L. The maximum extraction capacity of CP-AMIN to recover Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) were 282.80 mg g-1, 292.98 mg g-1, and 285.45 mg g-1 respectively. Under lower PGMs concentration, active sites on CPAMIN are enough to bind PGMs ions. With the increasing of the concentration, the active sites of CP-AMIN were saturated at the equilibrium concentration. Therefore, the absorption capacity of PGMs would not change anymore.


17


300 250

qe (mg g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200 150 100


50 0

0

200

400

600

Ce (mg L

-1

800

)

1000

1200

Figure. 8. Effect of original PGMs concentration on adsorption.

3.8 Extraction of PGMs from simulating hydrometallurgical solutions To investigate the selectivity of CP-AMIN for PGMs, aqueous multi-metal M solutions were prepared by adding other sulfate metal salts in 0.1 mol/L hydrochloric acid medium. The concentrations of the M (nickel, manganese, cadmium, chromium, copper and cobalt) were 1g/L. The date E, D, β are summarized in Table 2 and the extraction behavior for each metal ion was shown in Fig. 9. Almost all of the PGMs are extracted from the aqueous phase. The extraction abilities of M were inefficient relatively. Recovery efficiencies (Re%) of PGMs were more than 95% even though the interference ion concentrations were twenty times higher than the PGMs. The result indicates that interfering ions have no competitive effect and CP-AMIN has high selectivity for PGMs. High Re% of PGMs is deduced as the strong static interactions between PtCl62- , PtCl42-, PdCl42- and CP-AMIN. Yet, metal ions (Ni, Mn, Cd, Cr, Cu and Co) always exist


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as Ni2+, Mn2+, Cd2+, Cr3+, Cu2+, and Co2+ which cannot be combined with imidazolium cations of CP-AMIN. Thus, the novel task-specific PILs has great potential to be used in separation and recovery of PGMs.

Table 2. Extraction efficiencies (E), distribution ratios (D) and coefficient of specificity (β) of metals with the polymer imidazole ionic liquids from aqueous phase at 60℃. Ni2+

Mn2+

Cu2+

E(%)

D

β

E(%)

D

β

E(%)

D

β

Pt(IV)

97.80

44.45

737.4

94.66

17.73

351.8

96.20

25.33

373.1

Pd(II)

98.35

59.61

421.3

98.23

55.49

7394

98.58

69.42

4654

Pt(II)

96.10

21.54

2310

95.51

9.25

20500

97.73

23.91

1110

Cd2+

Co2+

Cr3+

E(%)

D

β

E(%)

D

β

E(%)

D

β

Pt(IV)

95.52

21.32

356.7

96.63

28.67

658.5

95.93

23.57

498.6

Pd(II)

97.75

43.44

2440

97.82

44.87

2487

98.12

52.19

1843

Pt(II)

91.92

14.61

2112

96.42

19.63

10201

97.71

23.93

11100


19


10

1.0

0.4

0.2

Ni

Mn

Pt(II) Adsorption (%)

50

M Adsorption (%)

0.6

0

100

0.8

6 50

Cd

Co

4

2

0

0.0 Cu

8

M Adsorption (%)

100

Pt(IV) Adsorption (%)

0 Ni

Cr

Mn

Cu

Cd

Co

Cr

1.0 100

0.8

0.6 50

0.4

0.2

M Adsorption (%)

Pd(II) Adsorption (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0

0 Ni

Mn

Cu

Cd

Co

Cr

Figure. 9. Extraction behavior of various metals by CP-AMIN. (Aqueous phase: 10mL, 50mg/L Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) with various metals 1g/L, 0.1 mol/L HCl, 50mg CP-AMIN, 60 ℃, 5h.)

3.9 Back- extraction of PGMs and CP-AMIN regeneration

In this study, various reagents have been tested for back- extraction of PGMs such as NaCl solution, hydrochloric acid, and oxalic acid. Finally, thiourea was selected for the stripping of PGMs from CP-AMIN due to the high back-extraction efficiency. During the process of backextraction, CP-AMIN was regenerated again for recycling. The flame atomic absorption spectrometer was used to determine the metal concentration. With the increasing of the thiourea concentration, the back-extraction efficiencies of Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) were increased. (Fig.


20

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10). In 1 mol/L thiourea solution, they are 97.96%,95.18%, and 99.41% respectively. Therefore, it was confirmed that CP-AMIN was regenerated in the back-extraction process with thiourea solution and it can be reused for metal extraction.

100

80

R (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40


20 0.0

0.2

0.4

0.6

C (mol L

-1

)

0.8

1.0

Figure. 10. Back-extraction efficiencies of PGMs from the polymer imidazole ionic liquids by thiourea solution.

3.10 Recycling of CP-AMIN

The results of 7 adsorption-desorption cycles are shown in Fig. 11. After 7 cycles, the adsorption capacity for Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) with CP-AMIN almost did not decay, and the PILs almost remained at the initial adsorption capacity. It’s indicated that CP-AMIN possesses good recyclability and great potential to be a novel recyclable material to recover PGMs.


21


(a)

(b) Pt(IV)

100

Pt(II)

100

R (%)

80

R (%)

60 40

50

20 0

1

2

3

4

cycles

5

6

0

7

1

2

3

4

cycles

5

6

7

(c) Pd(II)

100

R (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

50

0

1

2

3

4

5

6

7

cycles

Figure. 11 Recycling of CP-AMIN in the recovery of PGMs

SEM was also used to study morphology after extraction of Pt(Ⅳ)and Pd(Ⅱ). The results were shown in Fig. 12. The microstructure of the CP-AMIN- Pt(Ⅳ) and CP-AMIN- Pd(Ⅱ) process structures that are rough, irregular and with many holes. The morphology of CP-AMINPt and CP-AMIN-Pd have no obvious differences compared with CP-AMIN.


22

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Figure. 12 SEM images of the CP-AMIN- Pt(Ⅳ) (a, b) and CP-AMIN- Pd(Ⅱ) (c, d).

The chemical compositions of CP-AMIN-Pt(Ⅳ), CP-AMIN-Pt(Ⅱ) and CP-AMIN-Pd(Ⅱ) have been determined by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The EDS spectrum (Fig. S3) and XPS spectrum (Fig. S4) show existence of platinum and palladium, illustrating that the PILs can successfully extract Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) from hydrochloric acid medium.

4. Conclusion

In this study, a novel task-specific PILs (CP-AMIN) was successfully prepared by grafting 5Aminoimidazole-4-carboxamide on CP, which was characterized by FT-IR, SEM. CP-AMIN was used as the solid-phase microextractions for extracting trace PGMs from aqueous solution with high efficiency and selectivity. PGMs could be effectively enriched from coexisting metals of Ni2+, Mn2+, Cu2+, Cd2+, Co2+ and Cr3+. The extraction equilibrium was achieved after 5 hours without


23


Page 24 of 25

harmful organic solvents or additional extracting agent, while the maximum extraction capacities for Pt(Ⅳ), Pt(Ⅱ) and Pd(Ⅱ) were 282.80 mg g-1, 292.98 mg g-1, 285.45 mg g-1 respectively. The maximum extraction efficiencies were 97.86%, 97.51% and 98.95%. In addition, PGMs can be

desorbed from CP-AMIN by acidic thiourea. The excellent recyclability makes CP-AMIN promising in industrial green separation processes.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant 51474118).


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Extraction of Pt(IV), Pt(II), and Pd(II) from Acidic Chloride Media Using

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