Microemulsion Extraction of Gold(III) from Hydrochloric Acid Medium

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Microemulsion Extraction of Gold(III) from Hydrochloric Acid Medium Using Ionic Liquid as Surfactant and Extractant Yu Tong, Lu Han, and Yanzhao Yang* Key Laboratory for Special Functional Aggregated Materials of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China ABSTRACT: A microemulsion consisting of cyclohexane, n-hexanol, hydrochloric acid solution, and the ionic liquid 1-ntetradecyl-3-methylimidazolium bromide ([C14mim]Br) was investigated for Au(III) extraction. In the extraction system, [C14mim]Br bears double functions of surfactant and extractant. The anion-exchange mechanism of Au(III) extraction by [C14mim]Br was confirmed by the method of continuous variation and infrared spectrum analysis. The effects of extraction time and various material concentrations were examined for Au(III) extraction. Under optimum conditions, almost all the Au(III) in hydrochloric acid solution was extracted to the microemulsion phase, which also has high selectivity for Au(III) over some metals (Cu(II), Cd(II), Co(II), Ni(II), Sb(III), Fe(III), Al(III), and Sn(IV)). Therefore, the extraction of Au(III) by the [C14mim]Br/ cyclohexane/n-hexanol/HCl microemulsion is an efficient and effective approach with high selectivity. energy consumption.20,21 However, there is little attention paid to gold extraction by microemulsion media at present. In recent years, gold has been recoverable from secondary resources (e.g., exhausted catalysts and electronic components) via acid leaching,22,23 making the extraction process of gold from acidic media a highly promising and interesting subject. As mentioned above, a number of researchers focus on the extraction of gold by various ionic liquids, but there are little published data on the mechanism of the Au(III) extraction by ionic liquids. Furthermore, to our knowledge, it has not been previously reported that an ionic liquid is used both as extractant and surfactant to form the microemulsion medium for the Au(III) extraction. In the present work, a microemulsion containing ionic liquid as a surfactant was prepared and investigated for the extraction of gold from hydrochloric acid solution. Compared with the research mentioned above, ionic liquid bears double functions of extractant and surfactant. As surfactant, ionic liquid can promote the formation of the microemulsion phase. The extraction system can effectively accelerate the extraction and enhance the extractability in terms of the spontaneous formation of the microemulsion structure as well as the enormous microinterfacial surface area. Also, the mechanism of the Au(III) extraction by the ionic liquid based microemulsion system has been explored.

1. INTRODUCTION Ionic liquids (ILs) are organic salts having melting points near or below room temperature. They possess unique properties such as nonflammability, negligible volatility, and high thermal stability.1,2 Thus, ILs have been extensively explored as alternatives to conventional organic solvents for liquid−liquid extraction of various metallic ions.1−6 Moreover, gold is the most important metal, as well as an indispensable and nonsubstitutable strategic resource. Extractions of Au(III) using ionic liquids as solvents or carriers have been investigated in recent years. For example, gold recovery from chloride solutions has been attempted by using an ionic liquid (Cyphos IL-101) which is impregnated on Amberlite XAD-7 resin or biopolymer capsules.7,8 The application of 1-butyl-3-methylimidazolium hydrogen sulfate and analogue ionic liquids as a solvent medium to leach sulfidic copper, gold, and silver ores was tested.9 The analytical potential of room temperature ionic liquids was also examined for simultaneous extraction and preconcentration of gold and silver in various media prior to introduction to electrothermal atomic absorption spectrometry (ETAAS).10 Other ionic liquids based on some cations (pyridinium, pyrrolidinium, and piperidinium) have been also developed as carriers for extractions of noble metal ions from aqueous solution, including Au(III).11 In that research, ionic liquids were usually used only as extractants or solvents. On the other hand, long-chained ILs containing a hydrophilic headgroup and one or more hydrophobic tails are good surfactants for forming microemulsions.12 Microemulsions are formed spontaneously by two immiscible liquids, a surfactant, and occasionally a short-chained alcohol as a cosurfactant.13 Due to their special characteristics, such as homogeneity, low viscosity, and ultralow interfacial tension, microemulsions have been widely developed as solvent media for extractions of metal ions, e.g., cadmium,14 copper,15 chromium,16 lanthanides,17 gallium,18 aluminum, and zinc.19 In addition, extraction recovery of metal ions by microemulsions is obviously advantageous compared with traditional treatment processing, related to pollution with solvents and © 2012 American Chemical Society

2. EXPERIMENTS 2.1. Reagents and Materials. The ionic liquid 1-ntetradecyl-3-methylimidazolium bromide, [C 14 mim]Br (>99%), was procured from Lanzhou Greenchem ILS, LICP, CAS (Lanzhou, China), and used as received. Cyclohexane and n-hexanol (both AR) were from Damao Chemical Reagent Received: Revised: Accepted: Published: 16438

June 21, 2012 November 7, 2012 November 23, 2012 November 24, 2012 dx.doi.org/10.1021/ie301644t | Ind. Eng. Chem. Res. 2012, 51, 16438−16443

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Rw:m = 10) and then equilibrated mechanically in an orbital shaker for 6 min. Afterward, the microemulsion phase and the aqueous phase were separated quickly by use of a centrifuge at 2000 rpm for 2 min, although two-phase separation can be spontaneously accomplished under static conditions within 8 min. The separated microemulsion phase and aqueous phase were both clear and transparent.

Tianjin Corp. (Tianjin, China) and used without any further purification. Distilled water was used to prepare the aqueous solutions in all experiments. The feed solutions were prepared by dissolving metal chlorides in hydrochloric acid solutions: HAuCl4·4H2O, Sinopharm Chemical Reagent Beijing Co., Ltd. (Beijing, China); SnCl 4 ·5H 2 O, SbCl 3 , CdCl 2 ·5/2H 2 O, FeCl3·6H2O, CoCl2·6H2O, NiCl2·6H2O, CuCl2·2H2O, and AlCl3·6H2O, Kermel Chemical Reagent Tianjin Co., Ltd. (Tianjin, China). All other chemicals used in this study were of analytical or reagent grade. 2.2. Analytical Techniques. A conductivity analyzer (DDS-307, Precision & Scientific Instrument Shanghai Co., Ltd., Shanghai, China) was used to measure the electrical conductivities of the microemulsions. The concentration of [C14mim]Br in the aqueous phase was determined by UV spectrophotometry (UV759S, Precision & Scientific Instrument Shanghai Co., Ltd., Shanghai, China).24 Before and after Au(III) extraction, the Au(III) concentration in the aqueous phase was determined by an atomic absorption spectrophotometer (3150, Precision & Scientific Instrument Shanghai Co., Ltd., Shanghai, China). The operational conditions were as follows: wavelength, 243.1 nm; lamp current, 2 mA; height of the flask, 7.5 mm; slit width, 0.2 nm; flow rate of burning gas, 2.2 L min−1; flow rate of gas to help burning, 9.4 L min−1. The linear equation of Au(III) concentration was Abs = 0.001583 + 0.066535C, R = 0.9999. Linear ranges were 0−2.0 μg mL−1. Other metal ion (Cu(II), Cd(II), Co(II), Ni(II), Sb(III), Fe(III), Al(III), and Sn(IV)) concentrations in solutions were determined by an inductively coupled plasma atomic emission spectrometer (IRIS Intrepid II XSP, Thermo Electron Corp., Boston, MA,USA). The operating parameters were as follows: output power, 1.5 kW; nebulizer pressure, 193.06 kPa; sample flow rate, 3.2 mL min−1; auxiliary gas, 0.5 L min−1; peristaltic pump, 100 rev min−1. Then the concentrations of metals in the organic phase were calculated by mass balances. After the gold extraction, the gold-loaded organic phase was separated from the aqueous solutions and then was evaporated at 313 ± 1 K, 2 × 104 Pa, until the solvents volatilized completely. The residue (the IL−Au complex) was weighed, treated, and analyzed by Fourier transform infrared (FT-IR) spectoscopy. FT-IR characterization was carried out on a Fourier transform infrared spectrophotometer (Tensor27, Bruker Corp., Karlsruhe, Germany) in the 4000−500 cm−1 wavenumber region. 2.3. Microemulsion Preparation. A required amount of [C14mim]Br (0.01−0.2 g) was added into the mixture of cyclohexane and n-hexanol (total 5 mL; the volume ratio of nhexanol in the mixtures is 30%). The mixtures were ultrasonicated until all the surfactant dissolved. The mixed organic phase was gradually diluted by 1 mol L−1 HCl solution until the aqueous phase arose, and then equilibrated for 8 h. Afterward, the organic phase and the HCl solution were separated. The transparent organic phase is the [C14mim]Br/ cyclohexane/n-hexanol/HCl microemulsion. The microemulsion was centrifugated with a relative centrifugal force of 200g for 5 min, and phase separation phenomena was not observed. This method is used to preliminarily estimate the formation of microemulsion.25 Unless stated specially, all experiments were carried out at 298 ± 1 K. 2.4. Extraction of Metal Ions. For the metal extractions, the microemulsion and the aqueous solution containing Au(III) or mixed metal ions (Cu(II), Cd(II), Co(II), Ni(II), Sb(III), Fe(III), Al(III), and Sn(IV)) were added to a glass tube (the volume ratio of the aqueous phase to the microemulsion was

3. RESULTS AND DISCUSSION 3.1. Conductivity Properties of the [C14mim]Br/Cyclohexane/n-Hexanol/HCl System. Electrical conductivity is a structure sensitive characteristic and is widely used to measure structural changes in microemulsions. The electrical percolation of microemulsions can reflect the interactions between the droplets and structural changes in microemulsions.26−28 In this study, the conductivities of the microemulsions composed of [C14mim]Br as surfactant were investigated. The cyclohexane/ n-hexanol organic phases (total 25 mL; the volume ratio of nhexanol in the mixtures is 30%) with different [C14mim]Br contents were gradually diluted by 1 mol L−1 HCl solution until the aqueous phase arose obviously. The electrical conductivity values (κ) of the microemulsions were determined in this process and plotted versus aqueous weight (waq), as shown in Figure 1a. Taking a sample, tetradecyltrimethylammonium bromide, TTAB, is a typical surfactant and contains the same aliphatic moiety as [C14mim]Br, and the κ vs waq curves of them

Figure 1. Electric conductivity κ of microemulsion as a function of aqueous weight waq. (a) The initial concentration of [C14mim]Br ranges from 0.03 to 0.12 mol L−1; (b) the initial concentrations of [C14mim]Br and TTAB are both 0.09 mol L−1 . 16439

dx.doi.org/10.1021/ie301644t | Ind. Eng. Chem. Res. 2012, 51, 16438−16443


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at the same concentration of 0.09 mol L−1 were analyzed and are compared in Figure 1b. As shown in Figure 1b, the κ vs waq curve could be divided into three parts. At low aqueous content, the microstructure is perceivably water-in-oil (W/O), and the low conductivities are characteristic of W/O discrete droplets.29 With the aqueous content increasing, W/O droplets begin to contact with each other and form interconnected channels, resulting in the steep increase of the κ value, which indicates the microstructure transformation from a W/O to a bicontinuous microemulsion.30 Then a two-phase system is emerging; however, the oil-in-water (O/W) microstructure is not observed, and the result is different from some of the published information.26,27 This can be explained that the surfactants employed under these experimental conditions are not sufficient for the formation of an O/W microstructure, because it is unnecessary and uneconomical to employ higher concentrations of [C14mim]Br for gold extraction. The results also show that the water solubilization capacity increased with surfactant concentration and the water solubilization capacity of the [C14mim]Br system is larger than the TTAB system. The broad imidazolium head shows a higher capacity for solutes than the tert-ammonium cationic system. The strong attraction between the imidazolium ring and cosurfactants could facilitate the immobilization of the latter at the fluid/fluid interface, which prevents its leakage into the conjugated water solution.31 3.2. Extraction Mechanism Analysis. The method of continuous variation, also called Job’s method, was originally employed to measure the composition of metal complexes in spectrochemistry.32 This method is widely used for the investigation and analysis of complex formation, which involves measurement of complex formation at various mole ratios of the reactants while maintaining constant the total amount of reactants.33−35 A modified method of continuous variation was employed to investigate the complexation of the extracted Au(III) by the [C14mim]Br/cyclohexane/n-hexanol/HCl microemulsion. The total amount of [C14mim]Br and Au(III) was maintained at 0.005 mmol, while the volume ratio of the aqueous phase to the microemulsion phase was 10, and the initial Au(III) concentration in the aqueous phase, CAu,aq, ranged from 0.1 to 0.8 mmol L−1. By plotting the initial Au(III) concentration in the aqueous phase as a function of Au(III) equilibrium concentration in the organic phase (CAu,or), the results are shown in Figure 2. In Figure 2, the maximum Au(III) equilibrium concentration in the organic phase can be deemed as CAu,aq = 0.5 mmol L−1. At this point, the concentrations of [C14mim]Br and Au(III) in such a system are equal and the largest amount of Au(III)− [C14mim]Br complex is formed. The data are indicative of the notion that 1:1 associates are formed between Au(III) and [C14mim]Br. Furthermore, [C14mim]Br is composed of [C14mim]+ cation and Br− anion, the structure of which is similar to quaternary ammonium extractants, e.g., trioctylmethylammonium chloride (Aliquat 336), containing CH3(C8H17)3N+ cation and Cl− anion. The anion-exchange mechanism of Au(III) extraction using quaternary ammonium is universally accepted, which is formed in a neutral complex as R4N+AuCl4−.36,37 According to Job plot analysis of the experiment results, the Au(III) extraction from hydrochloric acid solution is deduced as the formation of the neutral complex as [C14mim]+AuCl4−. The

Figure 2. Job’s plot for Au(III)−IL system. CAu,aq ranged from 0.1 to 0.8 mmol L−1; Rw:m = 10.

extraction reaction of Au(III) by the [C14mim]Br/cyclohexane/ n-hexanol/HCl microemulsion can be given as the following equations: [C14mim]+ Br −or ⇔ [C14mim]+aq + Br −aq

(1)

[AuCl4]−aq + [C14mim]+aq ⇔ [C14mim]+ [AuCl4]−or

(2)

To obtain direct evidence of the interaction between [C14mim]+ and AuCl4−, FT-IR measurements were employed to analyze the IL−Au complex. The [C14mim] to Au(III) molar ratio in the IL−Au complex was controlled by fixing the amount of ionic liquid in the microemulsion phase and varying the Au(III) amount in the aqueous phase. Then, the sample preparation was followed as in section 2.2. The FT-IR spectra of pure IL and the IL−Au complex with [C14mim] to Au(III) molar ratio 1:1 are shown in Figure 3. The vibration bonds are

Figure 3. Infrared spectra of IL and Au(III)−IL complex.

shown in the IR spectra of pure IL: ring O−H stretch, 3431.31; ring C−H stretch, 3083.75, 3062.40; aliphatic C−H stretch, 2915.77, 2850.52; ring CC stretch, 1630.81; ring CN stretch, 1572.74; MeC−H deformation, 1473.65; ring C−H deformation in plane, 1177.98. The ring O−H stretch may be caused by a small amount of water in the sample. With Au(III) loaded in the IL, the aliphatic C−H stretch peaks shift to higher wavenumbers about only 2 cm−1 because of the weaker interaction between AuCl4− and the alkyl side chain. However, 16440



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the ring C−H stretch peaks shift to higher wavenumbers about 50 cm−1, and the vibrational frequency increases. The results show that the aliphatic C−H stretch is less influenced by the loading of Au(III), and there is a stronger interaction between the tetrachloroaurate anion and the IL cationic headgroup. To further confirm the validity of the anion-exchange mechanism, the dependence of extractability of Au(III) on the bromide anion concentration in the aqueous phase was investigated (Figure 4). The bromide anion concentrations

Figure 5. Extraction yield (E%) and distribution ratio (D) of Au(III) as a function of [C14mim]Br concentration. Feed solution, 0.05 g L−1 Au(III) in 1 mol L−1 HCl; Rw:m = 10.

concentrations of HCl were used. The results are shown in Figure 6. The variation of Au(III) concentration does not

Figure 4. Dependence of extractability of Au(III) on NaBr concentration.

were adjusted by NaBr. As shown in Figure 4, the degree of Au(III) extraction by the [C14mim]Br/cyclohexane/n-hexanol/ HCl microemulsion is gradually reduced with increasing Br− concentration in terms of the equilibrium shift (eq 1). These data suggest that the extraction of Au(III) into the microemulsion phase proceeds by an anion-exchange mechanism, as represented by eqs 1 and 2. 3.3. Effect of Extraction Time. The extraction rate was preliminarily studied in order to establish the optimum extraction time. With the use of the aqueous phase of 0.05 g L−1 gold(III) in 1 mol L−1 HCl solution and the microemulsion of 10 g L−1 [C14mim]Br, Rw:m = 10, the influence of vibration time on the Au(III) extraction by the [C14mim]Br/cyclohexane/n-hexanol/HCl microemulsion was studied. The vibration time was varied from 1 to 21 min. The equilibrium was soon achieved; the gold(III) extraction yield (E%) reaches 98% within 1 min, and then the extraction is independent of the vibration time. 3.4. Effect of [C14mim]Br Concentration. The effect of the [C14mim]Br concentration in the microemulsion on the Au(III) extraction yield (E%) and distribution ratio (D) was studied. The results are shown in Figure 5. Initially, the gold extraction yield, E%, increases with [C14mim]Br concentration. When the concentration reaches 11 g L−1, the extraction yield remains. It should be noted that higher IL ([C14mim]Br) concentration can promote the formation of microemulsion which could accelerate the extraction and improve the extractability. 3.5. Effect of Au(III) and Hydrochloric Acid Concentrations. The extraction behaviors of Au(III) by the [C14mim]Br/cyclohexane/n-hexanol/HCl microemulsion at various concentrations of hydrochloric acid and Au(III) in the aqueous phase were studied. The [C14mim]Br concentration in the organic phase was maintained at 10 g L−1, while the Au(III) concentration was varying in solutions and different

Figure 6. Extraction yield (E%) versus Au(III) and hydrochloric acid concentrations. C[C14mim]Br = 10 g L−1; Rw:m = 10.

significantly affect the extraction yield under the experimental conditions. It is reliable that the lowest concentration of Au(III) in the aqueous phase that can effectively extract with the method could reach 0.02 g L−1 at least. However, the study of gold extraction using the microemulsion showed a negative effect with increasing hydrochloric acid concentration; i.e., the extraction of gold tends to decrease as hydrochloric acid concentration is increased. As mentioned in section 2.3, the structure of [C14mim]Br is similar to that of quaternary ammonium extractants. The decrease in Au(III) extraction at higher HCl concentrations may be also caused by competition between AuCl4− and Cl− to associate with [C14mim]+. Cl− + [C14mim]+ ⇔ [C14mim]+ Cl−

(3)

Therefore and based on the anion-exchange reactions expressed by eq 2, it is clear that the combination between Cl− and [C14mim]+ will decrease the effective concentration of [C14mim]+ on the basis of interionic-attraction theory, which causes the equilibrium of eq 2 to left shift and the decrease of Au(III) extraction. This behavior has been shown in similar gold-extractant systems.37−39 3.6. Gold Extraction from Multimetal Ion Solutions. This method may be used for the gold recovery from secondary resources, for example, printed circuit boards (PCBs). The electronic waste may contain other metals, particularly copper, cobalt, nickel, iron, and aluminum.22,40 To investigate the 16441



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analysis. When evaluating the Au(III) extraction behaviors in HCl solution, the results show that higher hydrochloric acid concentrations show negative effect for the Au(III) extraction because of the competition between AuCl4− and Cl− to associate with [C14mim]+. Under the optimum conditions, the gold extraction yield (E%) is almost 100%, when the [C14mim]Br concentration reaches 28 mmol L−1 in the microemulsion phase. The initial Au(III) concentration in the aqueous phase has little influence on the extraction under the experimental conditions employed. The extraction behaviors of Au(III) from other metal ions show that the [C14mim]Br/ cyclohexane/n-hexanol/HCl microemulsion also has high selectivity for Au(III) over base metals (Cu(II), Cd(II), Co(II), Ni(II), Sb(III), Fe(III), Al(III), and Sn(IV)). Therefore, the extraction of Au(III) by the [C14mim]Br/cyclohexane/nhexanol/HCl microemulsion is considered to be a efficient, effective, and highly selective approach for the extraction of Au(III).

ability of the [C14mim]Br/cyclohexane/n-hexanol/HCl microemulsion to extract gold(III) selectively, an aqueous multimetal solution was prepared by adding other chloride metal salts in 1 mol L−1 HCl solution. The concentration of each metal ion (Cu(II), Cd(II), Co(II), Ni(II), Sb(III), Fe(III), Al(III), and Sn(IV)) in hydrochloric acid media was 0.05g L−1. The extraction behavior for each metal ion is plotted in Figure 7.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 531 88365431. Fax: +86 531 88564464.

Figure 7. Extraction behavior of gold from multimetal solution. C[C14mim]Br = 10 g L−1; feed solutions, 0.05 g L−1 metals.

Notes

The authors declare no competing financial interest.

Gold is mainly removed from the aqueous phase: 98.23% extraction. The microemulsion has very low distribution coefficients and extraction percentages for the other metal ions: 0.32 and 3.12% for Cu(II), 0.69 and 6.48% for Cd(II), 0.13 and 1.31% for Co(II), 0.32 and 3.07% for Ni(II), 0.73 and 6.84% for Sb(III), 0.51 and 4.88% for Fe(III), 0.15 and 1.45% for Al(III), and 0.45 and 4.29% for Sn(IV). High selectivity for the extraction of Au(III) can be achieved by the [C14mim]Br/ cyclohexane/n-hexanol/HCl microemulsion. The results clearly show that the [C14mim]Br/cyclohexane/ n-hexanol/HCl microemulsion is found to be highly selective for the extraction of Au(III). 3.7. Recovery of Au by Reductive Stripping. The complex associate of Au(III)−[C14mim]Br in the organic phase was stripped by the reduction with oxalic acid in the present study. The gold-loaded organic phase was separated from the aqueous phase and then mixed with an equal volume of H2C2O4 aqueous solution (1.5 mol L−1). The reduction reaction was maintained at 80 °C, for 4 h. Afterward, the organic phase was centrifuged and digested by nitrohydrochloric acid. The gold content was determined by atomic absorption spectrometry (AAS). The results show that there is hardly any gold in the organic phase after reduction, which means that Au(III) is completely reduced to Au in the form of gold precipitation. Nevertheless, the optimization conditions need to be further explored.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Natural Science Foundation of China (Grant 21076115).

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4. CONCLUSIONS A 1-tetradecyl-3-methylimidazolium bromide/cyclohexane/nhexanol/HCl microemulsion was investigated for Au(III) extraction from hydrochloric acid solutions and the ionic liquid [C14mim]Br was used as surfactant and extractant. The results show that the extraction equilibrium can be rapidly achieved and the two phases are easily separated. The extracted complex is likely to be [C14mim]+AuCl4−, which is confirmed by the method of continuous variation and the infrared spectrum 16442



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Microemulsion Extraction of Gold(III) from Hydrochloric Acid Medium

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