Influence of different 3-D electrodes towards the performance of gold recovery by using an electrogenerative process

2.1 Chemicals and materials

Gold(III) chloride trihydrate, HAuCl₄·3H₂O, sodium chloride, sodium di-hydrogen phosphate and other chemicals were obtained from Sigma-Aldrich (Malaysia). The anion exchange membrane used was Neosepta ® AM-01 (Tokuyama Corp.). The anode used was pure zinc (99% purity) with dimension of 2.0 cm × 5.5 cm × 0.06 cm, since it prevents electrons transfer from zinc to the cathode. The reaction of zinc dissolution in dilute acidic media was occurred. Means that, zinc reacts with H⁺ ions and release hydrogen gas (Mostaffa et al., 1986). The cathode materials used for gold recovery were RVC 80 pores per inch (ppi) (The Electrosynthesis Co., 2.0 cm × 5.5 cm × 0.4 cm) and PG sheet (National Electrical Carbon Products, Inc., 2.0 cm × 5.5 cm × 0.4 cm). Other electrodes used in this study were platinum as counter electrode (since it is inert and stabilize the current) and Ag/ AgCl electrode as reference electrode.

2.2 Voltammetric studies

Cyclic voltammetry analysis was performed using Potentiostat (eDAQ EA161) connected to an E-corder 410 (4 channel recorder) with Echem and EChart software respectively. The potentiostat is supplied with an electrode cable comprising of three leads with terminated by an alligator clip. The electrodes used were working, reference and counter electrodes. For working electrode, RVC and PG electrodes used were obtained from the best electrodes produced in the batch cell experiment, which are detailed in Section 2.3. These electrodes were attached to copper current collector since the electrodes cannot be hold by the alligator clip. Ag/AgCl electrodes used as a reference electrode was supplied by Bioanalytical Systems, Inc., (BAS) whose potential is constant taken as a reference against which the potential of the other electrodes in the cell can be measured. All potentials were quoted against the Ag/AgCl electrode at room temperature, 25 °C. The counter electrode was required so that the complete circuit will be formed. The counter electrode from platinum Bioanalytical Systems, Inc., (BAS). The alligator clip allowed connection to a wide variety of electrodes and the leads color marked to indicate the type of electrode to which it should be attached.

A 30 mL of Pyrex cell vial was used to fill a sample solution. 500 mg L⁻¹ of Au(III) in 0.2 M sodium chloride solution was used as a solution in voltammetric studies. A Teflon cap with three openings was used to cover the cell and to place respective electrodes in the cell. Distilled water was used to prepare all electrolytes from an analytical grades reagents while catholyte solution was prepared from Au(III) chloride trihydrate with NaCl as supporting electrolyte. All analysis was carried out at room temperature, 25 °C. Cyclic voltammetric experiments were important in order to gain information of an anodic and cathodic peak (in which the peak process of an oxidation and reduction of Au(III). The experiment was carried out in triplicates unless otherwise stated to ensure reproducibility. Different scan rates 50, 100, 200 and 400 mVs⁻¹ were used in voltammetric studies.

2.3 Cell configuration and experimental approach

Batch cell consists of two electrolyte compartments of dimensions 4.7 cm × 5.3 cm × 9.3 cm separated by an anion exchange membrane Neosepta AM-1 (Tokuyama Corp.). Two types of cathodes were used; RVC 80 ppi, and PG. Zinc foil was used as an anode. The distance between anode and cathode is fixed to 4.00 cm while the membrane-electrodes gap is set at 2.00 cm. Stirring bar used in this study is Brand® magnetic stirring flattened sides bar (PTFE coated, cylindrical shape) with 10.0 mm length and 3.0 mm diameter (Merck Malaysia). The speed of the stirring bar is fixed at 400 rpm and it is placed in the middle of the catholyte compartment. Later, the electrodes were attached to copper plates (using parafilm) which were used as current collectors. The current collectors were then connected by wires to complete the circuit as shown in Fig. 1. In this experiment, Digital Multimeter (Sanwa) was used. They were connected to the cathode and an anode to measure electrical potential or voltage (volts) and flow of electric current (amperes) of the cell during experiments. The anode used as a sacrificial electrode was pure zinc foil (>99% purity, R & M chemicals). The zinc foil was cleaned with sand paper each time before use since zinc easily oxidized when exposed to air. Aliquots of Au(III) catholyte solutions were sampled at each 30 min to monitor the recovery of gold by reactor. Au(III) determinations were carried out using Atomic Absorption Spectroscopy (AAS; Perkin Elmer AAnalyst 400 Model) with an air acetylene flame at a wavelength of 242.80 nm. The volumes of samples were collected using an adjustable micropipette in 500 µL. A Quanta FEG 650 scanning electron microscopy (SEM) was used to visualize the surface morphology of the gold deposits on RVC cathode. Energy dispersive X-ray analysis (EDX) which is incorporated in the SEM was carried out to confirm the presence of Au(III) recovered on the electrodes.

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Fig. 1 Schematic diagram of batch cell.

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3.1 Voltammetric studies

The present study focused on recovering Au(III) from chloride ion media by using an electrogenerative process and monitoring the morphological changes of Au(III) deposited on RVC as the best cathode materials.

The following equations show the reactions occurred in the reactor:

Eqs. (3.1) and (3.2) show the half reaction equations while E° is the standard reduction potential. The overall positive cell potential given in Eq. (3.3) shows the reaction was spontaneous.

In cyclic voltammetry studies, the electrolyte solution is not stir and it is important that the system is at rest (steady-state) during the cyclic voltammetry experiment (Girolami et al., 1999). So under this condition, the surface concentration is regulating by diffusion of the redox active species to the electrode surface. Cyclic voltammogram of 500 mg L⁻¹ of Au(III) chloride solution in 0.2 M NaCl was carried out using RVC and PG as the working electrode within range 1200 mV to −600 mV of potential. This range is selected because the redox reaction of Au(III) appeared in this range (Yap and Mohamed, 2007). It was shown that the reaction of Au(III) was under mass-transport control through the cyclic voltammogram obtained. Fig. 2(a) and (b) show the cyclic voltammograms obtained for 500 mg L⁻¹ of Au(III) chloride solutions using RVC and PG as working electrodes, respectively. For RVC working electrode, a potential range 0.2–0.1 V shows the small peak occurred in which Au(III) reduction occurs while Au(III) oxidation peak was observed at a potential range between 0.8 to 1.0 V. For PG working electrode the voltammogram shows a small peak at a potential range of 0.2 to −0.1 V corresponding to the reduction of gold and at a potential range 0.6–0.7 V the gold oxidation peaks was occurred. As for the working electrode, the small anodic and cathodic peaks are not obvious for PG electrode compared to RVC electrode. The reductive peak currents of RVC and PG are 1.038 mA cm⁻² and 0.179 mA cm⁻², respectively. The large difference peak currents between these working electrodes because of RVC can produce a higher current compared to PG. This is due to the physical property of RVC which is consisting of an open-pore foam honeycomb material. This characteristic helps more electron to flow through the RVC electrode thus producing higher reductive peak current (Friedrich et al., 2004). The results showed that on reversing the scan direction in which at negative potential, the current produced is greater than during the initial (negative direction) scan. The reason for this is probably a reflection of the greater ease of reduction of Au(III) on the RVC and PG or due to increase the rate of hydrogen revolution. The increase of current at negative potentials is assigned to the reduction of Au(III) ions and water. At potentials more than −600 mV, reduction of water introduces and directly hydrogen evolution reaction (HER) gas is evolved on the electrode surface. At these negative potentials, water reduction usually takes place and the existence of Au(III) transfer water reduction to more positive potential (Vilchis-Carbajal et al., 2000).

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Fig. 2 Cyclic voltammograms obtained for 500 mg L⁻¹ of Au(III) chloride solutions (a) RVC and (b) PG as working electrodes.

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In addition, 500 mg L⁻¹ of Au(III) and 250 mg L⁻¹ concentrations were chosen in voltammetric studies because the Au(III) redox peaks are clearly visible compared to 50 mg L⁻¹ and 100 mg L⁻¹ of concentrations. Fig. 3 shows the cyclic voltammograms obtained for 250 mg L⁻¹ of Au(III) chloride using RVC and PG as working electrodes. The results show that there are small peaks on the curve at a potential range of −0.10 to −0.20 V associated with Au(III) reduction and at a potential range of 0.20–0.30 V associated with Au(III) oxidation. Meanwhile, for PG working electrode peaks obtained at potential range of −0.08 to −0.11 V and 0.60–0.70 V which are representing the reduction and oxidation processes, respectively. On the other hand, less concentrated gold solutions (50 mg L⁻¹, 100 mg L⁻¹) could not produce any peaks for the redox process. The controlling step of the gold redox reaction changes with concentration at a carbon surface, while low concentration of Au(III) could not showed it (Yap and Mohamed, 2007). In this cyclic voltammetric study, the different scan rates were carried out on the Au(III) chloride solution. As shown in Fig. 3, the scan rate of 50, 100, 200 and 400 mV s⁻¹ was used in this study. The optimum scan rate for this study is 100 mV s⁻¹, as shown in Figs. 2 and 3. The reason is the redox reaction which is reduction and oxidation processes were complete occurred. The influence of the scan rates on the redox reaction can be clearly seen in the voltammograms. It can be observed that slow scan rate produces slow reaction, which it takes longer time to complete. The complete redox reaction was obtained when the starting point of voltammogram was back to the original point (complete cycle).

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Fig. 3 Cyclic voltammograms obtained for 250 mg L⁻¹ of Au(III) chloride using (a) RVC and (b) PG as working electrode.

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3.2 Galvanic cell

A galvanic cell transforms the energy released by a spontaneous redox reaction into electrical energy that can be used to perform work. The oxidative and reductive-half reactions usually occur in separate compartments that are connected by an external electrical circuit. Here, the anode is negative and cathode is the positive electrode. The reaction at the anode is oxidation and that at the cathode is reduction. The electrons are supplied by the species getting oxidized. They move from anode to the cathode in the external circuit. This reaction opposed to the electrolytic cell (Roslan et al., 2017).

3.3 Mass transfer studies

A model of concentration-time relationship for three-dimensional electrodes can be expressed by Eq. (1) under mass control (Bertazzoli et al., 1997):

Based on the surface area changes that involved during the recovery process, the ‘volumetric’ mass transport coefficient value will lead to a clear understanding of the comparative performance of the different cathode system as the effective electrode area is quite difficult to be determined (Walsh and Reade, 1994). Here k_m A_e exists as the figures merit for three-dimensional electrode systems (Walsh and Reade, 1994). The values k_m A_e of can be obtained from the slope of the graph in which representing the rate constant that involved on recovery of Au(III) (Fig. 4).

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Fig. 4 (a) Percent of Au(III) recovery vs. time with different cathode from 500 mg L⁻¹ of Au(III) solution within 4 h of the experiment and (b) linearization of normalized Au(III) recovery from 500 mg L⁻¹ Au(III) chloride media at 90 min.

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In the batch cell studies, the effect of cathodes materials on the recovery of Au(III) from chloride media was investigated. Two types of cathode materials were used. Fig. 4 shows the comparison of Au(III) recovery using different types of cathodes. From the results obtained, RVC shows the full Au(III) recovery in the shortest time in 1 h of operation, followed by PG but requires more than 2 h of operation. The Au(III) recovery was more than 90% at longer operation time of 4 h. All of these cathodes were conducted in a fixed of time regarding an optimum time for Au(III) recovery. For metal recovery with dilute solutions, 3-D RVC porous material serve as a perfect electrode material owing to the large specific area, high porosity, chemical inertness, good fluid permeability, good electrical conductivity, and mechanical resistance. An open-foam material of honeycomb structure of RVC allows the operations with a low current density at the electrode–electrolyte interface, but with a relatively high current per unit of cell volume (Walsh et al., 2016; Doherty et al., 1996). When the surface area of the cathode was increased, the mass transfer also increased. Besides, the deposition of the Au(III) increased when the current was increased.

Fig. 4(b) shows the linearization of normalized concentration of Au(III) as a function of time with different cathodes at 90 min. The system behaves as simple batch reactor under mass control. A model of the concentration-time relationship for three-dimensional electrodes is shown in Eq. (1). The coefficients of determination (R²) values for each best trend line are given in Table 1. The deviation of data points from the trend line due to the increased surface area of Au(III) deposition as well as the increased movement in the fluid flow on the cathode. The R² value for RVC was low and it showed that the system deviated from undergoing a simple pseudo-first order reaction because Cl^- is in excess, so it does not appear in first order kinetic equation (Matos and Guirardello, 2000). Table 1 shows the performance for the gold recovery from 500 mg L⁻¹ Au(III) chloride solutions using different cathode materials with t_90% is the time in which 90% of Au(III) in the catholyte was recovered. The values of k_m A_e is shown with the slope values representing the rate constant for Au(III) deposition on the cathode. The possibility to calculate the mass transport coefficient by considering that is constant for the cell configuration used in this study which is 0.5. The RVC electrode shows the best performance with the highest value. So, RVC is the most suitable cathode material in this electrogenerative system. RVC served as the best cathode for recovery of Au(III) because of the speciality of this cathode due to its excellent physical properties. RVC has a high value of electrode area per unit electrode volume, which leads to a high capacity of metal packing and contribute to the faster deposition of Au(III) (Walsh et al., 2016; Walsh et al., 1994). In addition, RVC has other excellent properties such as large specific area, high porosity, chemical inertness, good electrical conductivity, good fluid permeability and mechanical resistance.

Fig. 5 shows the effect of cathode potential versus SCE (saturated calomel electrode) on the Au(III) recovery. Results showed the higher Au(III) recovered, the potential shifted to the negative values. While, when the resistance loaded was high, the slow of performance of the system was produced as the cathode potential shifted to the positive values. A slow electron-transfer electrode reaction occurs at high resistance due to the limitation in kinetics and transport at catalytic sites (Yap and Mohamed, 2007). As a conclusion, the optimum conditions for this effect could be achieved by operating this system in a short-circuit manner (Walsh and Reade, 1994).

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Fig. 5 Effect of varying the cathode potential vs SCE at a constant Au(III) with initial concentration of 100 mg L⁻¹.

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3.4 Surface analysis

As observed, the deposited Au(III) on RVC can be seen clearly in which is spherical, granular and homogeneous and smaller in size (Fig. 6). By increasing the recovery time, the particles tend to form sponge-like deposits. Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX) analysis was carried out on Au(III) deposited from 100 mg L⁻¹ Au(III) solution on RVC electrode for 2 h in order to study the morphology and to confirm the formation of metallic Au(III). From Fig. 7(a), it is confirmed the initial RVC contains only carbon (27.3%) and oxygen (72.7%). This is attributed to the fact that RVC is made from carbon and contains oxygen too. In contrast, Fig. 7(b) shows the presence of Au(III) atom after the recovery process of Au(III) in initial solution of 100 mg L⁻¹ at 2 h of operation. The atomic weight of Au(III) observed is more than 85% while carbon and oxygen were reduced to 13.8% and 62.8% respectively which confirms that Au(III) was deposited on the RVC.

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Fig. 6 SEM micrograph of (a) an original RVC (before operation) and (b) a deposited Au(III) on RVC (after operation) after 2 h of operation at 100 × maginification.

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Fig. 7 EDX analysis of RVC as a cathode (a) before (b) after 2 h of operation.

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