Gold Leaching from Printed Circuit Boards Using a Novel Synergistic Effect of Glycine and Thiosulfate

Abstract

Printed circuit boards (PCBs) are a secondary source for the extraction of precious metals, such as gold (Au), silver (Ag), platinum (Pt), palladium (Pd), etc. Thiosulfate and glycine systems have recently gained a significant amount of attention for Au leaching. However, in the thiosulfate system, the stability of leached Au tends to decrease when using lower thiosulfate concentrations. In this study, a copper–ammonia–thiosulfate system (thiosulfate system) and glycine or histidine were combined to leach Au from PCBs. The glycine–thiosulfate system resulted in a higher Au leaching than the histidine–thiosulfate system. The results revealed that the glycine–thiosulfate system exhibited a synergistic effect on Au leaching (93.7%) at pH 9.3 and 40 °C, while the Au leaching percentages were 47.1% and 50.7% for the thiosulfate and glycine systems, respectively. In the dual system, Fe leaching was insignificant, although Ag and Al leaching were 95.3% and 27.0%, respectively. Compared to the thiosulfate system, the dual system maintained the stability of the leached Au. The system required 60 mM thiosulfate and 0.5 M glycine at 40 °C and pH 9.3 in order to leach Au from PCBs. The kinetic study suggested that Au and Ag leaching from PCBs in the dual system followed the diffusion-controlled model. The Au leaching rate in the initial phase of the dual system was similar to that of the glycine–cyanide system. This novel, mild approach could be applied to hydrometallurgy to leach other precious metals from sources, such as ore and spent catalysts.

1. Introduction

Due to the depletion of reserves and increasing demand for precious metals, the extraction of precious metals from secondary resources is a timely solution [1,2,3]. One promising secondary source is printed circuit boards (PCBs). PCBs contain a considerable amount of valuable metals, such as gold (Au), silver (Ag), palladium (Pd), platinum (Pt), copper (Cu), nickel (Ni), cobalt (Co), zinc (Zn) and aluminium (Al), which occur in limited quantities in nature [4,5]. Therefore, the recycling of electronic waste generated by used electronic equipment is very important to maximise the sustainable utilisation and minimise the environmental impact [3,6,7].

Conventional methods, such as cyanidation, used for Au leaching are highly toxic, resulting in potential environmental hazards and high detoxification costs [8]. Therefore, alternative lixiviants for Au leaching have received much attention in recent years [8]. Among these lixiviants, thiosulfate, thiourea, thiocyanate and chloride have received the most attention [6,9,10,11,12]. For industrial applications, the stability of the lixiviant and the Au complex in the solution are key factors [9,13]. The development of chloride leaching is impeded mainly by its hazardous working environment, poor reaction selectivity and requirements for more corrosion protection of equipment [10]. Thiourea has been reported to be carcinogenic [9], while the thiocyanate system is highly sensitive to temperature and pH [14].

Thiosulfate leaching is widely considered to be the most promising alternative method due to its reduced environmental risk, high reaction selectivity, low corrosivity, relatively low cost, fast leaching kinetics, etc. [10,15,16,17]. The following Equation (1) shows the Au leaching reaction when the thiosulfate-only system is used [18]. However, there are some limitations associated with using the thiosulfate-only system, such as the requirement of high temperatures for the reaction conditions [10].

$$4\mathrm{A}\mathrm{u}+8{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4\mathrm{A}\mathrm{u}{{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2}^{3-}+4\mathrm{O}{\mathrm{H}}^{-}$$

(1)

In order to overcome the limitation of the thiosulfate-only system, various thiosulfate leaching systems have been developed in combination with other additives or lixiviants, such as oxygen–thiosulfate, copper–thiosulfate, copper–ammonia–thiosulfate, copper–EDA–thiosulfate, ferric–EDTA–thiosulfate and ferric–oxalate–thiosulfate leaching systems [9]. Among these, the copper–ammonia–thiosulfate system has been extensively studied. With the development of the ammonia–thiosulfate system with Cu²⁺, Au leaching can be carried out even at room temperature [10]. The presence of Cu²⁺ can accelerate the Au leaching in the ammonia–thiosulfate by the formation of a gold(I)–thiosulfate complex [19,20]. It has also been reported that Ni²⁺, Co³⁺ or Fe³⁺ can be used as a substitute for Cu²⁺ [9]. In thiosulfate leaching, the leaching chemistry was found to be very complex, and its stability is difficult to maintain [19]. The addition of ammonia stabilizes Cu²⁺ in an alkaline solution by forming a cuprammonium complex [21]. The leaching of Au in thiosulfate solutions is an electrochemical reaction with the constituent half-cell reactions being the oxidation of Au to Au–thiosulfate and the reduction of copper(lI)–amine to copper(I)–thiosulfate. These half-cell reactions are shown in Equations (2) and (3), respectively. Equation (4) shows the overall reaction related to the formation of the Au–thiosulfate complex [19].

$$\mathrm{A}\mathrm{u}+2{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}\to \mathrm{A}\mathrm{u}{{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2}^{3-}+\mathrm{e}$$

(2)

$${{\mathrm{C}\mathrm{u}\left({\mathrm{N}\mathrm{H}}_3\right)}_4}^{2+}+3{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}+\mathrm{e}\to {{\mathrm{C}\mathrm{u}\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_3}^{5-}+4{\mathrm{N}\mathrm{H}}_3$$

(3)

$$\mathrm{A}\mathrm{u}+{{\mathrm{C}\mathrm{u}\left({\mathrm{N}\mathrm{H}}_3\right)}_4}^{2+}+5{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}\to \mathrm{A}\mathrm{u}{{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2}^{3-}+{{\mathrm{C}\mathrm{u}\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_3}^{5-}+4{\mathrm{N}\mathrm{H}}_3$$

(4)

Thiosulfate forms a stable complex with Au, Au(S₂O₃)₂³⁻, at higher thiosulfate concentrations. However, it makes it difficult to recover the leached Au from the leachate. On the other hand, the Au leaching at lower thiosulfate concentrations is very slow and decreases the stability of the Au–thiosulfate complex [21]. Therefore, combining with another lixiviant could help to improve the stability of the leached Au in the system.

Amino acids have gained considerable attention as lixiviants in hydrometallurgy over conventional lixiviants due to their non-toxic, non-volatile and recyclable properties [22]. Amino acids, such as glycine, alanine, histidine cysteine, valine and aspartic acid, have been studied as alternative lixiviants to cyanide for Au leaching [23,24,25]. In systems using only amino acids, significant leaching of Au was achieved at temperatures above 60 °C [24]. Among these, glycine has been extensively studied due to its low cost, simple structure and high affinity for metals [23,26]. It is readily producible on an industrial scale or obtained as a by-product from various microorganisms [27]. It can form stable complexes with Au and Ag over a broad range of pH, Eh and temperatures [28]. The Au dissolution in the alkaline glycine solution can be described using Equation (5) [29,30]. Similarly, the possible reaction with histidine is shown in Equation (6). According to recent studies, the stability constant of the Au complex formed with glycine, Au(NH₂CH₂COO)₂⁻, is much higher than the other Au complexes, such as Au(SCN)₂⁻, AuBr₂⁻ and AuCl₂⁻ [31,32]. The presence of oxidants, such as Cu²⁺ [21,26], hydrogen peroxide [26] and permanganate [33], is favourable for Au leaching in the glycine system. In addition, it has been found that combining glycine with cyanide significantly improves Au leaching [34,35,36]. It has been reported that glycine with Cu-weak acid dissociable (WAD) cyanide significantly improves Au extraction [36]. Because of the toxicity and environmental issues associated with cyanidation systems, there is less interest in the cyanidation systems, and replacement with a milder lixiviant is required.

$$4\mathrm{A}\mathrm{u}+8{\mathrm{N}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+4\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}+{\mathrm{O}}_2\to 4\mathrm{N}\mathrm{a}{\left[{\mathrm{A}\mathrm{u}(\mathrm{N}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2]+6{\mathrm{H}}_2\mathrm{O}$$

(5)

$$4\mathrm{A}\mathrm{u}+8{\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{H}\left(\mathrm{C}4\mathrm{N}2\mathrm{H}5\right)\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+4\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}+{\mathrm{O}}_2\to 4\mathrm{N}\mathrm{a}{\left[{\mathrm{A}\mathrm{u}(\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{H}\right(\mathrm{C}4\mathrm{N}2\mathrm{H}5\left)\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2]+6{\mathrm{H}}_2\mathrm{O}$$

(6)

In this study, the synergistic effect of combined copper–ammonia–thiosulfate and glycine or histidine systems on Au leaching from PCBs was investigated. Since the copper–ammonium–thiosulfate system has a higher Au leaching kinetic than that of the cyanidation and glycine systems, in the first stage, thiosulfate could trigger the Au leaching by forming the Au–thiosulfate complex. Then, in the second stage, the glycine in the system could lead to stabilisation of the leached Au by forming an Au–glycine complex. This is the first study to demonstrate Au leaching from PCBs using combined copper–ammonia–thiosulfate and glycine systems. As copper–ammonia–thiosulfate works at pH 9 [9], and the glycine system works at pH 11 [26,34,35], the pH for the dual system was optimised. The objective of this study was to combine copper–ammonia–thiosulfate and a glycine-based lixiviant to obtain a synergistic effect on Au leaching and stabilise the leached Au in the system. At the same time, the Au leaching kinetics were also studied. Additionally, Ag leaching and its leaching kinetics together with the leaching of Al and Fe were investigated. This novel approach could be applied in hydrometallurgy to leach other precious metals from ore sources and spent catalysts.

2. Materials and Methods

2.1. Characterisation of PCBs

Crushed PCBs subjected to magnetic separation were received from a recycling company in Japan. The PCBs were ground with liquid nitrogen using a cryogenic crusher (JFC-2000; Japan Analytical Industry) and sieved to collect the particle size of 75–500 µm. The ground PCBs were observed with scanning electron microscopy (SEM; JSM-7001F, JEOL, Tokyo, Japan) with carbon sputter-coating. The ground PCBs were analysed with X-ray diffraction (XRD, Ultima IV, Rigaku, Tokyo, Japan) and X-ray fluorescence (XRF, ZSX Primus II, Rigaku, Japan). The particle size distribution of the PCB sample was analysed using a laser scattering particle size distribution analyser (LA-950, Horiba, Tokyo, Japan).

In order to analyse the elemental composition of the PCBs, 200 mg of the sample was subjected to acid digestion with 10 mL of aqua regia (HNO₃:HCl—2:1, v/v) in the microwave digestion system (Ethos Plus, Milestone, Shelton, CT, USA) (heated at 1000 W to achieve 210 °C for 30 min, kept at 210 °C for 15 min and allowed to cool to room temperature). The resultant leachate was filtered (0.45 µm membrane) and analysed for metal concentrations using inductively coupled plasma-optical emission spectrometry (ICP-OES; PerkinElmer, Optima 8300, Waltham, MA, USA). The digestion was performed in triplicate, and the mean value of each metal content was calculated.

2.2. Pre-Treatment of PCBs for Removal of Cu

Prior to the leaching tests, the PCBs were treated with 1 M H₂SO₄ at 70 °C for 7 days to remove Cu as previously reported [6]. Then, the solid residue was collected with filtration (0.45 µm), washed thoroughly with distilled water and dried in an oven at 55 °C overnight.

2.3. Glycine–Thiosulfate and Histidine–Thiosulfate Leaching Systems for Au Leaching

Au leaching from the PCBs by combining glycine (0.5 M) and the copper–ammonia–thiosulfate system (60 mM thiosulfate, 0.2 M NH₄Cl, 0.26 M NH₃ and 10 mM CuSO₄) was performed with 2% pulp density at pH 9.3 and pH 11 at 40 °C with 160 rpm. For the histidine–thiosulfate system, 0.5 M histidine was used in place of glycine at the same conditions. The pH in the systems were adjusted using a 1.0 M NaOH solution.

As controls, the thiosulfate and the glycine or histidine systems were performed separately. In the thiosulfate-only system (60 mM thiosulfate, 0.2 M NH₄Cl, 0.26 M NH₃ and 10 mM CuSO₄), leaching was performed with 2% pulp density at pH 9.3 and pH 11 at 40 °C with 160 rpm. Similarly, in the glycine-only system, leaching was carried out with 0.5 M glycine and 2% pulp density at pH 9.3 and pH 11 at 40 °C at 160 rpm.

During the leaching, pH and solution potential (Eh, platinum electrode versus Ag/AgCl reference electrode) were investigated, and samples were collected over time for ICP-OES analysis. The leaching tests were performed in triplicates, and the mean metal concentrations were calculated.

2.4. Investigation to Lower the Thiosulfate Concentration in the Glycine–Thiosulfate System

Keeping the other parameters constant (0.5 M glycine, 0.2 M NH₄Cl, 0.26 M NH₃ and 10 mM CuSO₄), Au and Ag leaching was investigated by changing the thiosulfate concentration in the dual system to 60, 40, 20 and 10 mM. During the leaching, pH and Eh were investigated, and the samples were collected over time for ICP-OES analysis.

2.5. Investigation to Lower the Glycine Concentration in the Glycine–Thiosulfate System

Similarly, keeping the other parameters constant with an optimised thiosulfate concentration (60 mM thiosulfate, 0.2 M NH₄Cl, 0.26 M NH₃ and 10 mM CuSO₄), Au and Ag leaching was investigated by varying the glycine concentration in the dual system to 0.50, 0.40, 0.20 and 0.05 M. During the leaching, pH and Eh values were monitored, and the samples were collected over time for ICP-OES analysis.

2.6. Investigation of the Presence of Cu⁺ during the Au Leaching

The Au-loaded leachate from the glycine–thiosulfate system was used to test for the presence of Cu⁺ using X-ray absorption near-edge structure (XANES) spectroscopy (Synchrotron Light Application Center, Saga, Fukuoka, Japan). As the standards, Cu, Cu⁺ (Cu₂O) and Cu²⁺ (CuSO₄) were used.

2.7. Kinetic Studies

Au and Ag leaching kinetics were studied by fitting the leaching data to the shrinking core model: diffusion reaction (diffusion-controlled model, (1 − 2/3 × X − (1 − X) ^2/3 = k_dt)) and surface chemical reaction (reaction-controlled model, (1 − (1 − X) ^1/3 = k_rt)). X in the equations represents the fraction reacted, t is the reaction time, and k is the rate constant.

3. Results and Discussion

3.1. Characterisation of PCBs

PCBs contain a variety of metals, mainly Cu, Fe and Al, attached to, covered by or mixed with different types of plastics and ceramics [6,37,38]. Therefore, size reduction is difficult using conventional grinding methods, such as planetary ball mill methods [39]. Hence, the sample was ground using a cryogenic crusher with the addition of liquid nitrogen to increase the surface area for the Au leaching studies. The crushed sample was sieved to collect a particle size of 75–500 µm. The use of a cryogenic crusher with liquid nitrogen helps to crush plastic particles in the PCBs into smaller particles, as plastic becomes harder at lower temperatures [39]. Crushing increases the surface area and is essential for producing homogeneous samples and leaching efficiency [40].

Characterisation studies were performed on the PCBs. Figure 1 shows the XRD and SEM-EDS analyses performed on the crushed PCBs. Figure 1a shows the PCBs before and after grinding. Relevant peaks for silica were observed in the XRD spectrum (Figure 1b). The elemental mapping results obtained with the SEM-EDS analysis showed a higher distribution of Si, O and Cu (Figure 1c). These results suggest that the PCBs contain a higher amount of silica. The silica in PCBs provides mechanical support to hold thin metal layers [41]. XRF is a semi-quantitative analytical method [42]. Prior to the ICP-OES analysis, the content of the ground PCBs was analysed with XRF (Figure S1, Supplementary Materials). The elemental percentages were then accurately determined with microwave-assisted acid digestion followed by ICP-OES analysis. The elemental percentages obtained from both analyses are summarised in

Table 1. Metal composition of PCBs analysed with ICP-OES.

Method	Metal (%)
	Cu	Al	Fe	Ag	Au
ICP-OES	8.10	3.69	1.49	0.28	0.026

. The results revealed that Cu (8.10%) is dominant with a significant amount of Al (3.69%) and Fe (1.49%). Au and Ag were detected at 0.026% and 0.28%, respectively. In some studies, Cu has been removed from the PCBs prior to Au and Ag leaching [43,44]. The removal of Cu from the PCBs would be beneficial because of the consumption of a higher amount of thiosulfate in the presence of more Cu [9]. In addition, a higher Cu content in the PCBs could negatively affect Au leaching [45]. As a pre-treatment to remove Cu from the PCBs, the sample was subjected to acid leaching (1 M H₂SO₄ at 70 °C) as previously reported [6]. This process removed 99.3% of the Cu from the sample. Therefore, after the acid pre-treatment, the percentages of Au, Ag, Al, Fe and Cu were 0.057%, 0.28%, 0.17%, 0.24% and 0.06%, respectively.

The particle size distribution of the ground PCBs was measured using the laser scattering particle size distribution analyser, as shown in Figure 2. According to the results, the mean and mode diameters were 143 and 322 μm, respectively. The cumulative frequency revealed that 10% of the particles were within 0–6 μm, 50% of the particles were within 0–78 μm, and 90% of the particles were within 0–380 μm. In addition, there were insignificant amounts of particles that were larger than 500 μm and smaller than 75 μm in particle size in the PCB sample. Particles larger than 500 μm could pass through the sieve mesh, depending on the particle geometry, but those smaller than 75 μm could be retained due to contact with larger particles.

According to previous studies, the temperature of the system is a key factor in Au leaching [26]. Indeed, temperatures of approximately 30 and 40 ˚C in the thiosulfate system are required to achieve the best Au dissolution and to minimise ammonia losses and Cu²⁺ reduction [46]. By performing some preliminary screening, the optimum temperature for the thiosulfate and glycine systems was selected as 40 ˚C.

3.2. Glycine–Thiosulfate and Histidine–Thiosulfate Systems for Au Leaching

Thiosulfate leaching has been studied at pH 8 to 10 [47], while for amino acids such as glycine and histidine, leaching has been mainly focused on pH 9.5 to 11.5 [33]. To select the best system and the pH for the glycine–thiosulfate and histidine–thiosulfate systems, Au leaching was investigated at pH 9.3 and 11 in both systems. As controls, Au leaching in glycine-only, histidine-only and thiosulfate systems was investigated separately at both pH 9.3 and 11. The percentages of Au leaching in the glycine–thiosulfate and histidine–thiosulfate systems are shown in Figure 3. As shown in Figure 3a, in the glycine-only system, the Au leaching was 50.7% after 48 h at pH 11, while there was no observable Au leaching at pH 9.3. However, Au leaching was relatively lower in the glycine–thiosulfate system at pH 11 than in the glycine-only system at pH 11. A similar trend was observed in the histidine-only system (Figure 3b). Au leaching at pH 9.3 in the histidine–thiosulfate system (72.9%) was comparatively lower than that in the glycine–thiosulfate system (93.7%), suggesting that the glycine–thiosulfate system at pH 9.3 is better for Au leaching than the histidine–thiosulfate system at pH 9.3. In the thiosulfate system, there was a considerable leaching (46.0%) in the initial phase at pH 9.3, and after 10 h, dissolution dropped significantly with time (Figure 3a). This could be due to the instability of the Au–thiosulfate complex that leads to precipitation. For the thiosulfate system, it is recommended to maintain the concentration in the range of 1–25 mM Cu²⁺, 0.1–0.8 M thiosulfate and 0.25–1.7 M ammonia for Au leaching [21,48]. In the current system, 60 mM of thiosulfate was used. According to a previous study, a significantly low Au leaching from PCBs (<20% within 2 h) was reported when 60 mM thiosulfate, 0.2 M NH₃ and 10 mM CuSO₄ were used [48]. Therefore, the instability of leached Au may be due to the insufficient amounts of thiosulfate. Some studies have reported that insufficient thiosulfate ions may precipitate Au [9,21]. Similarly, in the thiosulfate system, the Au leaching at pH 11 was significantly lower when compared to that at pH 9.3. In the glycine–thiosulfate system, the synergistic effect of glycine and thiosulfate in Au leaching was observed at both pH 9.3 and 11. The results suggested that the system at pH 9.3 leached 93.6% of Au within 48 h, while it was 47.9% at pH 11. Interestingly, the glycine–thiosulfate system showed extra stability of the leached Au in the leachate. This could be due to the formation of the Au–glycine complex ([Au(NH₂C₂H₂COO)₂]^-), although Au initially forms a Au–thiosulfate complex. Figure 4 illustrates the anodic and cathodic reaction mechanism that explains Au leaching in the glycine–thiosulfate system and formation of the Au–glycine complex. Equations (7) and (8) show the possible reaction of forming the Au–glycine and Au–histidine complexes from the Au–thiosulfate complex, respectively.

$$4\mathrm{A}\mathrm{u}{{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2}^{3-}+8{\mathrm{N}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+4\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\to 4\mathrm{N}\mathrm{a}{\left[{\mathrm{A}\mathrm{u}(\mathrm{N}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2]+8{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}+4{\mathrm{H}}_2\mathrm{O}+{4\mathrm{H}}^{+}$$

(7)

$$4\mathrm{A}\mathrm{u}{{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2}^{3-}+8{\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{H}\left(\mathrm{C}4\mathrm{N}2\mathrm{H}5\right)\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+4\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\to 4\mathrm{N}\mathrm{a}{\left[{\mathrm{A}\mathrm{u}(\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{H}\right(\mathrm{C}4\mathrm{N}2\mathrm{H}5\left)\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2]+8{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}+4{\mathrm{H}}_2\mathrm{O}+{4\mathrm{H}}^{+}$$

(8)

Since the Au–thiosulfate complex is less stable, the formation of the Au–glycine complex could last longer in the lixiviant. The glycine–thiosulfate system at pH 9.3 showed a significantly higher amount of Au leaching than the other tested systems, and the leached Au remained in the lixiviant longer than the thiosulfate system. The stability constant of the Au–glycine complex Au(NH₂C₂H₂COO)₂⁻ is much higher than the other Au complexes, such as Au(SCN)₂⁻, AuBr₂^- and AuCl₂⁻ [31,32]. No significant pH changes were observed in any of the leaching systems (Figure S2a,b, Supplementary Materials). The thiosulfate system showed a buffering effect due to NH₃ and NH₄⁺ [49]. Similarly, in the glycine system, the buffering effect came from the glycine itself because of the zwitterions [50]. Therefore, the combination of thiosulfate and glycine systems results in the buffering effect coming from NH₃/NH₄⁺ and glycine. Figure S2c,d, Supplementary Materials, show the Eh changes in the leaching systems. According to the results, in all the systems, Eh varied between 200 and 350 mV except for the glycine leaching system at pH 11, which showed the Eh at approximately 150 mV. Furthermore, there were no significant changes in the initial Eh during the leaching process except for the thiosulfate system at pH 9.3. In the thiosulfate system at pH 9.3, Eh increased from 200 to 300 mV during the leaching process. This could be due to the reduction of unstable Au⁺ in the system. In the glycine–thiosulfate system at pH 9.3, the Eh varied at approximately 250 mV. For further optimisation of the system for Au leaching, pH and temperature were selected as pH 9.3 and 40 °C, respectively.

3.3. Investigation to Lower Thiosulfate Concentration in the Glycine–Thiosulfate System

Next, the possibility of reducing the thiosulfate concentration was investigated by keeping the other parameters the same. Figure 5a–d show the Au, Ag, Al and Fe leaching at different concentrations of thiosulfate in the glycine–thiosulfate system, respectively. According to the results, it is suggested that a higher thiosulfate concentration improves Au leaching. An increasing thiosulfate concentration increases the Au leaching rate in the initial phase. A similar trend was observed for Ag and Al leaching with 95.3% and 27.0%, respectively, in the system with a higher thiosulfate concentration. Fe leaching was negligible in all the systems. Therefore, the 60 mM thiosulfate concentration was selected as the best thiosulfate concentration. Furthermore, pH in all systems was stable (Figure S3a, Supplementary Materials). In the 60 mM thiosulfate system, the Eh was approximately 250 mV. When the thiosulfate concentration decreased, the Eh changed from 200 to 275 mV (Figure S3b, Supplementary Materials). In combined systems having a thiosulfate concentration lower than 60 mM, the trend in the Eh variation was similar to the thiosulfate system at pH 9.3. Jeon et al. have used 31.6 and 8.8 g of Na₂S₂O₃ per 1 mg of Au for leaching of Au from PCBs using a thiosulfate system [51,52]. However, in the current study, 0.9 g of Na₂S₂O₃ per 1 mg of Au was used after combining the thiosulfate system with the glycine system.

3.4. Investigation to Lower Glycine Concentration in the Glycine–Thiosulfate System

The possibility of lowering the glycine concentration in the glycine–thiosulfate system for Au and Ag leaching from PCBs was investigated using 60 mM of thiosulfate in the glycine–thiosulfate system. The results for Au, Ag, Al and Fe leaching are shown in Figure 6a, Figure 6b, Figure 6c and Figure 6d, respectively. The results indicated that high glycine levels enhanced the Au and Ag leaching. A similar trend was observed for Al leaching, although the Al amount (27.0%) was relatively low. Fe leaching was negligible in all the systems. Therefore, the 0.5 M glycine concentration was selected as the best glycine concentration for the glycine–thiosulfate system. It has been reported that Au leaching increases with increasing amino acid concentration [24]. Glycine is recommended to be used in the range of pH 9.5 to 11.5 due to its pKa₂ = 9.6 at 25 °C [33]. However, in the current system, glycine was used at pH 9.3 at 40 °C. As the temperature increases, the pKa value decreases; at 40 °C, glycine could form a glycinate anion for the complexation [53]. In some studies, glycine has been used for the pre–treatment of Au-bearing ores. It has been reported that the use of ammonia leaching, alkaline leaching and pre–treatment with glycine helps to leach 17, 94 and 97% Au from the thiosulfate system, respectively [54]. A previous study found that the addition of amino acids, such as valine, glycine, alanine and histidine, reduced thiosulphate consumption [55]. According to the studies by Eksteen et al., 98% of Au leaching from Au–Ag concentrate using a glycine–peroxide lixiviant was observed at an elevated temperature (~60 °C) [26]. Based on the results in Figure S4a,b, Supplementary Materials, pH and Eh were stable 5 h after starting the leaching test.

Similarly, the leaching of Al and Fe from the PCBs was also studied. According to the results, 27% of Al and a negligible amount of Fe were observed (Figure 6c,d). In alkaline pH, leaching of base metals is lower compared to that of the acidic leaching systems [56]. It has been reported that Ag leaching from PCBs is significantly low in alkaline glycine systems even at 60 °C [29]. However, the thiosulfate system has the ability to leach both Au and Ag. Therefore, the combination of both systems resulted in the leaching of Au and Ag from PCBs in the current study. The determination of the mass balance of a process is important for the consideration of technical and economic feasibility [57]. The mass balance for the glycine–thiosulfate system is shown in Figure S5, Supplementary Materials.

3.5. Formation of Cu⁺ during the Au Leaching

As illustrated in Figure 4, thiosulfate initially attacks Au and forms the Au–thiosulphate complex while reducing Cu²⁺ to Cu⁺. In the second stage, glycine forms the Au–glycine complex by grabbing the Au from the Au–thiosulfate complex. To confirm the mechanism, the presence of Cu⁺ was investigated. In previous studies, it was found that cupric ion (Cu²⁺) plays an important role in Au leaching in the thiosulfate system, acting as an oxidant for Au and as a catalyst [19,34]. In the thiosulfate–glycine system, thiosulfate contributes to Au leaching by forming Au(S₂O₃)₂³⁻ while releasing an electron. The released electron is accepted by Cu²⁺ and reduced to Cu⁺. The presence of Cu⁺ in the system was investigated by analysing the leachates obtained from the Au leaching from PCBs using XANES spectroscopy (Figure 7). According to the results, as the valency state of Cu increases from 0 to 2+, the spectra have shifted to higher energy. The resulting spectrum for the leachate was in between the Cu⁺ and Cu²⁺, suggesting that the leachate contained Cu⁺. The linear combination fitting results of the XANES analysis revealed that it was 20% of Cu⁺ and 80% of Cu²⁺ (ATHENA software). It has been reported that the presence of Cu in the lixiviant can speed up the dissolution of Au by 20 times [58,59]. However, higher concentrations of Cu²⁺ also have a negative effect on thiosulfate oxidation [58]. In a previous study, the best Au leaching rate was achieved when the Cu²⁺ concentration was 10 mM [46]. Therefore, in the current study, a concentration of 10 mM Cu²⁺ was used. It was found that, in the glycine-only system, Cu was present as Cu²⁺, whereas in the glycine–cyanide system, both Cu²⁺ and Cu⁺ were excited [60]. These results suggest that both Cu²⁺ and Cu⁺ are present in the thiosulfate–glycine system.

3.6. Kinetic Studies Related to Au and Ag Leaching

In hydrometallurgy, metal leaching kinetics are usually described with the shrinking core model. In this study, Au and Ag leaching kinetics were observed using the diffusion-controlled model (1 − 2/3 × X − (1 − X) ^2/3 = k_dt) and reaction-controlled model (1 − (1 − X) ^1/3 = k_rt) [61,62]. Figure 8 shows the data fitted in both kinetic models. The correlation coefficients (R²) were 0.9839 and 0.9856 for the diffusion-controlled model for Au and Ag, respectively. For the reaction-controlled model, the R² values were 0.8785 and 0.9062 for Au and Ag, respectively. The results suggested that the leaching of Au and Ag in the glycine–thiosulfate system followed the diffusion-controlled model. During the leaching process, thiosulfate can self-decompose or reduce to S⁰, S²⁻ or S₂O₃²⁻_, and the resulting derivatives can deposit on the Au to form a passivation layer [10]. A kinetic study related to Cu leaching from PCBs with thiosulfate has also been found to be the diffusion-controlled model [63].

The glycine system is unsuitable for agitated leaching systems due to having a low kinetic in Au leaching compared to that in cyanide [18]. According to the study by Eksteen et al., Au leaching was achieved within 6 h in the glycine–cyanide system [60]. However, in the current glycine–thiosulfate system, 80% of the Au was leached within 5 h, and the leaching of Au was completed within 48 h. Thus, Au leaching in the initial phase is much faster in the glycine–thiosulfate system, similar to the glycine–cyanide system. Therefore, this dual system could also be applied in agitated systems. Further studies can be done to improve the purity of Au and Ag by suppressing base metals in the leachate by utilizing the selective leaching property of amino acids [64].

4. Conclusions

In the thiosulfate system, Au leached faster in the initial stage and showed less stability of the leached Au in the system. The Au leaching efficiency was higher in the glycine–thiosulfate system than that in the histidine–thiosulfate system. The Au leaching in the glycine–thiosulfate system was 93.6% within 48 h at pH 9.3. This Au leaching was two times higher than that of the systems working individually. The suitable conditions for the system for the Au leaching from PCBs were 60 mM thiosulfate and 0.5 M glycine at 40 °C and pH 9.3. This dual system showed a significant Ag leaching from PCBs, while 27% of Al and a negligible amount of Fe were also leached. Furthermore, the pH remained constant in the system by the buffering effect of glycine. The kinetic study suggested that Au and Ag leaching from PCBs in the glycine–thiosulfate system followed the diffusion-controlled model. Overall, the glycine–thiosulfate system showed a synergistic effect on Au leaching from PCBs and stabilised the leached Au in the leachate. The Au leaching rate in the initial phase was faster in the glycine–thiosulfate system, similar to the glycine–cyanide system. This novel approach could be applied in hydrometallurgy to leach other precious metals from ore sources and spent catalysts.