Gold Extraction through Vesicles Self-Assembled by Cationic Gemini

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Gold Extraction through Vesicles Self-Assembled by Cationic Gemini Surfactant and Sodium Deoxycholate Shubin Wang, Xiaolu Yin, Yan Yan, Zeyang Xiang, Peng Liu, Yao Chen, Xia Xin, and Yanzhao Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01712 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 10, 2016

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Industrial & Engineering Chemistry Research

Gold Extraction through Vesicles Self-Assembled by Cationic Gemini Surfactant and Sodium Deoxycholate Shubin Wanga, Xiaolu Yin a, Yan Yana, Zeyang Xiang a, Peng Liu a, Yao Chen a，Xia Xinb*, Yanzhao Yanga* a

Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, PR China

b

National Engineering Technology Research Center For Colloidal Materials, Shandong University, Jinan, 250100, PR. China

*

Author to whom correspondence should be addressed, E-mail: [email protected].

Phone: +86-531-88363597. Fax: +86-531-88361008 * Author to whom correspondence should be addressed, E-mail: [email protected] Phone: +86-531-88362988. Fax: +86-531-88361008

ACS Paragon Plus Environment


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Abstract A kind of self-assembled vesicles that composed of ionic liquid-type cationic gemini surfactant (C3H6-α, ω-(Me2N+C12H25Cl-)2, 12-3-12) and anionic biological surfactant (sodium deoxycholate, SDC) were constructed for Au (III) extraction. The appearance and microstructure of vesicles system were characterized by visual and TEM images. Compared to the zeta potential of pure vesicles system (+45±1 mV), zeta potential of gold-loaded vesicles system reduced 15±1 mv (changed to +30±1 mV), which confirmed the mechanism of Au (III) was effectively extracted by vesicles through electrostatic interaction. Main influence factors including extraction equilibrium time, surfactants concentration, NaCl concentration and pH on the extraction efficiency (E%) were explored. Furthermore, effective method was devoted for gold (III) stripping. Through stepwise extraction and ligand modified vesicles system, Au (III), Cu (II) and Fe (III) were separated from mixed solution successfully. In a word, effective and potential approach for the separation of Au (III) from HCl solution was investigated in our work which can be helpful for gold recovery and environmental improvement. Keywords: Surfactant; Self-Assembled; Vesicles; Gold Extraction; Electrostatic Interaction.


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1. Introduction With the excellent physical and chemical properties, the demand of gold for jewelry, electronic, catalytic and biological applications has experienced a rapid increase recently. For this reason, gold recovery from secondary resources becomes necessary and promising1-3. However, conventional liquid−liquid extraction is limited in terms of high energy consumption4, toxicity5 and environmental pollution6. Consequently, new separation systems with the philosophy of green chemistry especially membrane separation, aqueous two phase systems, ultrafiltration have been constituted

7-9

. Recently, Micellar-enhanced ultrafiltration (MEUF) based on metal entrapping

which is a particular method in ultrafiltration has received increasing attention 10-12. The MEUF could be applied to the separation of charged substances that dissolved in water using surfactants at their critical micelle concentration (CMC). Above CMC, surfactant monomers begin to self-assemble and form micelles whose diameter is larger than the membrane pore size. The organic may dissolved at different locations within micelles such as the micelle surface, palisade layer, even hydrophobic core on account of their chemical structure13, 14. The metal ions, lacking hydrophilic properties and organic ligands can be concentrated on the micelles surface when their charges are contrary to the micelles surface. The micelles, self-assembled by cationic surfactant, positively charged surface could attract free negative charge in the water and positive charge will be constrained by anionic surfactant self-assembled micelles. Thereafter, the metal-loaded micelles with suitable hydrodynamic diameter can be withheld by ultrafiltration membranes， the solution is then passed through ultrafiltration membrane15,16. MEUF which combines ultrafiltration membrane and surfactants was firstly presented by Michaels in the 1968s 18 and employed the treatment of heavy metals of wastewater in the 1970s19.



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Later, metal ions including Cd2+, Ni2+, Zn2+, Fe3+, Cu2+, Pb2+, CrO43− and Fe(CN)63− were reported high removal efficiency20-22. Au (III) separation was also achieved with SDS micelles23，but recent studies undertaken regarding gold removal from aqueous solution via MEUF process were primarily based on the use of single surfactant. Studies were demonstrated that free energy, CMC and consumption of mixed surfactants are lower than that of single, and consequently the mixed system between new types of surfactants appears distinct importance. Ionic liquid-type gemini surfactants have been extensively studied owing to the tunable performance of the obtained aggregation nanostructures recently. Ionic liquids (ILs) are solvents constituted by organic cations and organic or inorganic anions with melting points near or below room temperature and have received great attention in recent years because of the unique properties such as good thermal stability, negligible steam pressure and incombustibility24-28. Therefore, ionic liquids have been developed as substitute to traditional organic solvents and extractants, especially in noble metal separation and extraction field. In comparison with conventional surfactants, gemini surfactants have two hydrophobic chains and two hydrophilic head groups connected with a spacer29, 30. They have unusual characteristics such as low CMC, excellent foaming and wetting properties, high efficiency in reducing the oil–water interfacial tension and interacting with counterions31. Yanzhao Yang and co-workers32-34 have been devoted considerable efforts in gold extraction by ionic liquids-type gemini surfactants. Thus, it can be prospected that ionic liquid-type gemini surfactants involving in metal extraction provides a prospective approach. It is worth noting that, most of MEUF are limited to micelles35, 36 and seldom researches have focused on the surfactant based vesicles ultrafiltration membrane which is attractive and promising. In this study, a new kind of ionic liquid-type gemini surfactant (12-3-12) and sodium deoxycholate


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were used for the construction of vesicles, and then developed as micelles substitute in MEUF. The vesicles extraction mechanism was explored systematically and proved to be a viable technique for the purification of Au (III) in HCl solution. 2. Experimental 2.1. Reagents and Materials The sodium deoxycholate (SDC,A. R.) was from Sinopharm Chemicals Reagent Co., and the cationic symmetry quaternary ammonium gemini surfactants (12-3-12) was from simoinstitute of organic chemistry (Zhejiang, China). The structure of SDC and cationic gemini surfactants (ionic liquid-type gemini surfactants, melting point: 65 ℃) were shown in Scheme 1. The Kraft temperature for the12-3-12 and SDC is -24.5 ℃ and -16.0 ℃. Gold solution was prepared by dissolving metal chlorides in hydrochloric acid solutions: HAuCl4.4H2O, Sinopharm Chemical Reagent Beijing Co., Ltd. (Beijing, China). CuCl2.6H2O and FeCl3.6H2O were procured from Kermel Chemical Reagent Tianjin Co., Ltd. (Tianjin, China). These metal chloride salts were added to the hydrochloric acid to prepare mixed solution. Sodium hydroxide and hydrochloric acid (36%) were used to adjust the pH of the aqueous solutions or for membrane cleaning. Sodium chloride (99.5%, A. R.) was used for chloride ion influence experiment. All of the reagents and materials were used as received. Water used in the experiments was triply distilled by a quartz water purification system. Its conductivity was lower than 1.8 µS·cm-1 as measured by a DDSJ-308A type conductivity instrument in our laboratory. The temperature that used for vesicles preparation and extraction process was 25 ℃.



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Scheme 1. Chemical structures of SDC and 12-3-12.

2.2. Analytical Techniques For transmission electron microscopy (TEM) observations, 5 mL of solution was placed on a copper grid and the copper grid was checked by a JEOL JEM-100 CXII (Japan) at 100 kV. Negative staining agent was uranyl acetate. A micro-electrophoresis meter (JS94H, Zhongchen Ltd. Co., Shanghai) was employed to determine the Zeta potential. The particle size was measured by micro-electrophoresis meter (JS94H, Zhongchen Ltd. Co., Shanghai) and the concentration of Au (III), Cu (II) and Fe (III) before and after extraction was measured by an atomic absorption spectrophotometer (3150, Precision & Scientific Instrument Shang hai Co., Ltd., Shanghai, China). An ultrafiltration membrane (25 mm * 0.22 um) from Shanghai Xinya purification device Company in China was used for all the MEUF experiments. Each sample data was measured three times, and the values were averaged. 2.3. Vesicles Preparation and Extraction Process A certain concentration of 12-3-12 and SDC were added into distilled water severally. The two systems were ultrasonicated until all the surfactants dissolved. The self-assembled vesicles were facilely obtained via mixing 12-3-12 with SDC solutions under stirring conditions. Then the color of the solutions changed to white (blue glow in white background) after the sample equilibrated for


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24 h. The prepared vesicles solution was initially loaded with 10ml of the investigated gold solution and then subject to ultrafiltration under an applied nitrogen pressure of 1 bar. The percent of metal retained by the surfactant, E, has been calculated using equation 1： E = (1-Mp / Mi) ×100% (1) Where Mp is the metal amount in the permeated and Mi is the metal amount in the initial solution. 3. Results and Discussion 3.1 The Formation of Vesicles The aqueous solutions of 12-3-12 and SDC at certain concentrations were prepared by directly dissolving quantitative amounts of corresponding compounds in water and fixed together. In all samples, the concentration of SDC was fixed at 2 mmol L-1.The CMC values for pure and mixed 12-3-12/SDC systems at different molar ratios were determined from surface tension and showed in Table 1. To confirm the interaction between 12-3-12 and SDC, the appearance and microstructure of 12-3-12/SDC mixed system by visual and TEM observations were carried out (Figure 1). When the 12-3-12/SDC molar ratio increased above 1:1, the self-assembled systems were transparent and clear. Increasing the anionic surfactant amount, leaded the self-assembled systems to exhibit opalescence with bluish tinge, suggesting that self-assembled vesicles were formed (Figure 1, a1-a7). It could be seen that pure 12-3-12 and SDC microstructure characterized by TEM at different molar ratios confirmed the presence of spherical and irregular assemblies (Figure 1, b1-b7). In addition, closed spherical vesicles with a single core revealed excellent uniformity and dispersity, and the vesicles amount decreased when the 12-3-12/SDC molar ratio decreased (Figure 1, b2-b6).



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Stability of the vesicles systems were also investigated by TEM image since the vesicle microstructure was also observed even 3 months (Figure 1, b8). Table 1 The CMC values for pure and mixed 12-3-12/SDC systems at different molar ratios 12-3-12/SDC Pure 12-3-12 1:1 1:2 1:4 1:10 1:20 Pure SDC

CMC value/mmol L-1 0.300 0.007 0.010 0.012 0.030 0.035 0.700

Figure 1 Visual images (a1-a7) and TEM images (b2-b6) of 12-3-12/SDC systems at different molar ratios, TEM images (b1, b7) of pure 12-3-12 and SDC, TEM images (b8) of 12-3-12/SDC systems after three months. n12-3-12/nSDC: (a2, b2) 1:1, (a3, b3) 1:2, (a4, b4) 1:4, (a5, b5) 1:10, (a6, b6) 1:20, (b8) 1:2.

Diameter distribution and zeta potential of vesicles for pure and mixed 12-3-12/SDC system were monitored to investigate the size and surface character. Firstly, the diameter size of vesicles depends on various factors, such as energy input, adsorption kinetics and the proportion of anionic-cationic surfactants. In our systems, when other conditions are fixed, the droplet size solely relies on the


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proportion of anionic-cationic surfactant. Figure 2A showed the variation of the diameter size as a function of 12-3-12/SDC molar ratio. It can be seen that there is a great gap in diameter size for pure 12-3-12 and SDC system while the diameters ranged around 180 nm and 280 nm when the mole ratio of 12-3-12 to SDC is in the range of 1:2 to 1:20, which consistent with the result of TEM images. Secondly, as depicted in Figure 2B, the Zeta potential displayed positive values at high 12-3-12/SDC ratios (10.55 mV for pure 12-3-12, 51.4 mV for 1:1 and 49.8 mV for 1:2) while negative values at low 12-3-12/SDC ratios (-31.7 mV for 1:4, -32.4 mV for 1:10, -35.7 mV for 1:20, -52.3 mV for pure SDC), which indicated that the vesicles systems(except pure 12-3-12) were stable (the values of zeta potential higher than +30 mV or lower than -30 mV 37).

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Figure 2 (A) Diameter size (B) Zeta potential of vesicles prepared using 12-3-12 and SDC at different 12-3-12/SDC molar ratios.

3.2 Extraction Mechanism Analysis Extraction efficiency (E%) was employed to evaluate the pros and cons of each system. Due to the vesicles size, positive and negative charges, charge density38-40, gold (III) was mainly extracted with the extraction efficiency of 99.85% (12-3-12:SDC=1:2) while extraction efficiency less than 8% for other 12-3-12/SDC ratios (Figure 3). Therefore, the n12-3-12/nSDC ratio was set at 1:2 in the follow



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up experiment research. Based on the above results, the n12-3-12/nSDC=1:2 vesicles system was further investigated. TEM images were employed to analyze the modification of surface morphology between pure vesicles and goad-loaded vesicles. The color of vesicle solution changed from white to blue when the cationic surfactant concentration was decreased from 1.0 mmol L-1 to 0.0325 mmol L-1(Figure 4A, a1-a6), and the zone of milky appearance could be correlated to the increasing in the number of visuals (Figure 4A, b1-b6). Before extraction, pure vesicles showed a clear grey outline, and inner pool of vesicles were the same as background color (Figure 4A, b1-b6) while gold-loaded vesicleshad higher contrast in comparison to surrounding（Figure 4A, c1-c5）. This phenomenon might be related to the crowded AuCl4- on the surface of the vesicles after extraction, which leaded to locally high brightness. From the DLS data (Figure 5A), the effect of surfactant concentration on the mean diameter size of vesicles became negligible after fixing a certain ratio and the diameter size of gold-loaded vesicles were varied only slightly compare with the pure sample. The zeta potentials of 12-3-12 /SDC vesicles and goad-loaded vesicles were measured, and the results were given in Figure 5B. Different from their diameter size variation, zeta potential of the vesicles experienced a notable variation, decreasing from +45±1 mV to around +30 ±1 mV, which might be related to the formation of charge neutralization on the vesicles surface. This charge neutralization could be explained that positively charged vesicles surface attractsfree negative charge (AuCl4-) in the water and formed 12-3-12/SDC/AuCl4- complexes.


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12-3-12/SDC molar ratios Figure 3 Effect of 12-3-12/SDCmolar ratios on gold (III) extraction efficiency.

Figure 4 Visual images (a1-a6) and TEM images (b1-b6) of different concentration vesicles before extraction, TEM images (c1-c6) of different concentration vesicles after extraction. The concentration of 12-3-12 from a1to a6: 1 mmol L-1, 0.5 mmol L-1, 0.25 mmol L-1, 0.125 mmol L-1, 0.0625 mmol L-1, 0.03125 mmol L-1.



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Figure 5 (A) The diameter size distribution and (B) Zeta potential of vesicles at different SDC concentration.

The mechanism of extraction can be speculated according to the results mentioned above. Firstly, the TEM images indicate that the addition of AuCl4- lead to a high brightness on the surface of vesicles. Secondly, zeta potential indicate that the surface of vesicles possessed positive charge and can attract AuCl4- which is negative charge. Thirdly, charge neutralization reacts on vesicles surface in the extraction process. Therefore, it can be concluded that the negative charge AuCl4- is immobilized onto the surface of vesicles by electrostatic interaction. Complementary, extraction of Au (III) can be regarded as a transfer process of Au (III) from the aqueous to the surface of vesicles, Combines with the above conclusions, the electrostatic attraction mechanism is represented in Scheme 2.


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Scheme 2 Schematic illustration of the formation of self-assembled vesicles and the process of gold extraction by the vesicles

3.3 Influence Factors of Extraction To make clear the optimum extraction conditions, the influence factors of extraction (equilibrium time, concentration of cationic surfactant, NaCl concentration and pH) were further investigated. The experimental results indicated that five minutes was sufficient for the extraction system as the extraction equilibrium was achieved quickly, as shown in Figure 6A. Extraction efficiency (E%) decreased with the decrease of 12-3-12 concentrations, which might be related to the vesicle population decline. Under optimum conditions, the extraction efficiency (E%) could approach 100% (Figure 6B). Compared to the previous literature23, the initial concentration of micelles that used in their work was 0.02 mol L-1. The initial concentration of vesicles in our work was used from 1.0 mmol L-1 to 0.0325 mmol L-1 which was much lower than the value of the previous literature reported and showed a great consumption reducing. As seen in Figure 6C, the addition of NaCl leaded to a linear decline of the extraction efficiency which could be associated



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with the reduction of potential on the surface of vesicles36. This process was similar to anion-exchange which Cl- replaced the position of AuCl4- on the vesicles surface. The pH of the gold (III) solution was a key parameter for the performance assessment of vesicles extraction. Figure 6D showed the influence of HCl concentration in gold (III) solution on the extraction efficiency (E%) of Au (III) by the vesicles system. The HAuCl4, source of gold (III), totally dissociated in aqueous solutions generating AuCl4- complex ions of square planar geometry. The addition of NaOH into the gold (III) solution neutralized the acidity, and the rapid formation of hydroxyl containing substituted the Cl- of gold complexes stepwise41, the reaction equations can be written as: AuCl4-+nOH- ⇋ AuCl4-n（OH）n(2) The E% vs pH curve could be divided into three regions obviously. In the range of low pH, an increase in extraction efficiency (E%) could be registered until a maximum (pH=6) was reached. This behavior was related to the absorption of Cl- on the vesicles surface. It also must be noted that, precipitate appeared in the case of pH value was less than 1, which resulted in the decrease of permeation flux. This phenomenon could be explained that sodium deoxycholate was rapidly hydrolyzed into deoxycholic acid (a kind of white deposit). After this point (pH=6), a second zone of terrace could be correlated to the neutral environment. The subsequent addition of NaOH solution leaded to a nonlinearity of decreasing of extraction efficiency (E%) which could be associated with the high OH- content.


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Figure 6 (A) Effect of extraction equilibrium time on gold (III) extraction efficiency, (B) effect of the concentration of cationic surfactant on gold (III) extraction efficiency, (C) effect of NaCl concentration on gold (III) extraction efficiency, (D) effect of pH of gold (III) solution on gold (III) extraction efficiency.

3.4 Recovery of Gold by Reductive Stripping Further experiments had been performed to research convenient and effective methods that allowing the recovery of the gold adsorbed on the vesicles. In this work, two methods had been employed to the stripping of gold. The first method was that gold vesicles -loaded filter membrane mixed with mixed solutions (NaCl + NH3.H2O). NaCl could reduce the surface potential of the vesicles, and NH3.H2O to convert the negatively charged AuCl4− into the positively charged Au(NH3)43+ complex which gold (III) was expelled from the vesicles to the aqueous. Next, A



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required amount of NaBH4 (0.01 g) was added into the mixture, which was gradually turned violet in the agitating process.Then the mixture was centrifuged. The supernatant liquid was determined by atomic absorption spectrometry (AAS) and there was hardly any gold (III) left in the supernatant liquid, which means that gold (III) was completely reduced to metallic gold. Then, the morphology of the sediment (metallic gold) was investigated by the TEM images, as shown in Figure 7. It can be clearly identified that the gold nanoparticles with reticular structure were obtained and this kind of gold nanoparticles can be applied in SERS, biomedicine, sensing, and catalysis42, 43. In short, the experimental results indicated that the gold stripping by the two methods were convenient and effective.

Figure 7 TEM image of the gold nanoparticles that were synthesized using the goad-loaded vesicles and NaBH4.

3.5 Multimetal Seperation Gold waseasily coexisted with iron and copper whether in nature minerals or secondary resource, however, micellar enhanced ultrafiltration lacks of selectivity on account of its extraction mechanism41. To enhance the selectivity of vesicles extraction, a method that involves the addition of suitable metal complexing ligands (metal extractors) had been developed. This method, denoted as ligand modified micellar enhanced ultrafiltration (LM-MEUF) was based on the ability of micellar could constrain the charged coordination complexes byelectrostatic interaction44. In our work, considerable efforts have been devoted to the gold (III) selective extraction. Scheme 3


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showed the process flowsheet of Au (III), Cu (II) and Fe (III) extraction separation from multi-metal solution. In this first separation process, negatively charged gold was mainly extracted with the extraction efficiency of 99.92% while positively charged copper and iron ions were almost not adsorbed (5.98% and 4.39%) and pass through the ultrafiltration membrane in the aqueous. Afterwards, sulfosalicylic acid (SSal) was added into the residual solution after extraction gold, pH was adjusted to 4， and SSal reacted with Fe (III) giving a chelated species according to the equation (3): 2SSal + Fe3+⇋ [Fe(SSal)2]-

(3)

Note that, while metal cation was repelled by the vesicles, the complex [Fe(SSal)2]- could be retained on the vesicles surface in principle owing to the positive charge. Scheme2 showed the extraction efficiency of Cu (7.88%) and Fe (95.44%) which were separated easily. Thus，it can be concluded in our experiment that the selective extraction of Au (III), Cu (II) and Fe (III) could be achieved by the ligand modified vesicles enhanced ultrafiltration. It is also expected that this method can be expanded to extraction other different metal ions.



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Scheme 3 Process flowsheet of extraction separation of Au (III), Cu (II), Fe (III) from multimetal solution

4. Conclusions Vesicles were fabricated by self-assembly complexes of cationic gemini surfactants (12-3-12) and anionic surfactantsin aqueous solution. Visual vesicles solution with bluish tinge exhibit opalescence appearance and TEM images confirmed that the spherical microstructure was enclosed by amphiphiles membrane. Then, zeta potential confirmed the positive charge of vesicles surface at n12-3-12/nSDC=1:2 and the electrostatic attraction mechanism of Au (III) extraction. In appropriate extraction condition, vesicles system showed excellent extractability of Au (III). Next, reducing agent was employed to the stripping of gold and satisfactory result was achieved. Ligand modified vesicles system separated Au (III), Cu (II), and Fe (III) from HCl solution stepwise. We can rationally prospect that green and low-consumption gold (III) extraction will be carried forward if the surfactants self-assembled vesicles are developed systematically.

Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21476129, 21203109) and Ji’nan Youth Science and Technology Star Program (2013040).

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Gold Extraction through Vesicles Self-Assembled by Cationic Gemini

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