In recent years, researchers have been seeking an environmentally viable alternative to traditional cyanidation processes, which are deemed harmful to the environment and public safety. Thiosulfate has emerged as the most promising cyanide substitute due to its non-toxicity, relatively low cost, and high leaching efficiency. Extensive research has been conducted on using copper-ammonia-thiosulfate solutions to leach gold from ores or concentrates. To address the environmental challenges posed by ammonia, studies on recovering gold from thiosulfate leach solutions are limited. Reports suggest that gold from cyanide or thiosulfate complexes can be reduced to metallic gold using zinc, iron, and copper as precipitating agents. However, the feasibility of using activated carbon as an adsorbent for gold recovery from thiosulfate solutions remains uncertain. Nonetheless, studies indicate that modified activated carbon can adsorb gold through a ligand exchange mechanism. For untreated activated carbon, gold recovery from pregnant thiosulfate solutions using thiourea (TSC) as an additive has been explored. After five adsorption cycles under optimal conditions, a gold loading capacity of 42.6 kg/t can be achieved, and for carbon containing 5 kg/t gold, the desorption efficiency using NN-dimethylformamide (DMF) desorbent reaches 95.3% after 24 hours.
However, due to the extensive use of copper as a catalyst in thiosulfate gold leaching systems, the copper content in the pregnant leach solution is inevitably much higher than the gold content. Therefore, studying the adsorption behavior of thiosulfate copper and elucidating its potential impact on gold recovery from thiosulfate solutions is meaningful. This paper comparatively investigates the adsorption behavior of thiosulfate copper and gold complexes on activated carbon in aqueous solutions. Various adsorption kinetics models were used to fit the adsorption processes, clarifying the adsorption mechanisms of thiosulfate copper and gold on activated carbon. The potential challenges of using activated carbon to recover gold from thiosulfate solutions are also discussed.
Adsorption Test Materials
Analytical grade reagents, including sodium thiosulfate, sodium hydroxide, sulfuric acid, copper sulfate, and gold standard solutions, were obtained from Sinopharm Chemical Reagent Co., Ltd. (SCRC) in Shanghai, China. The coconut shell activated carbon used in these adsorption tests is of analytical grade. Scanning electron microscope (SEM) images show the microstructure of the activated carbon, which has a large specific surface area (1000−1500 m²/g) and clear granules.
Adsorption Kinetics
Copper adsorption kinetics experiments were conducted with 100 mL of thiosulfate copper solution with an initial copper concentration of 3 g/L. The molar ratio of thiosulfate to copper was 8:1. Unless otherwise specified, 0.5 g of activated carbon was used for the adsorption tests. The adsorption kinetics of Cu(S₂O₃)₂³⁻ on activated carbon are shown in Figure 2. The adsorption capacity of activated carbon for copper increased sharply within the first hour, then slightly decreased and stabilized.
Gold adsorption experiments were conducted with 100 mL of thiosulfate gold solution with an initial gold concentration of 50 mg/L. The molar ratio of thiosulfate to gold was 8:1. Unless otherwise specified, 1.0 g of activated carbon was used for the adsorption tests. The adsorption kinetics of gold on activated carbon are shown in Figure 3. The adsorption capacity of activated carbon for gold increased rapidly during the initial stage. An adsorption peak was observed at approximately 3 hours, after which the adsorption slightly decreased, indicating that equilibrium adsorption of gold on activated carbon was essentially established.
Potential Adsorption Mechanisms
Figure 4 shows the FTIR spectra of raw activated carbon and activated carbon loaded with copper and gold. Peaks around 3430, 1600, and 1100 cm⁻¹ are associated with O-H, C=O, and C-O stretching vibrations, respectively. After adsorption, the carbonyl peak (C=O) increased significantly, particularly for copper adsorption. For copper-loaded carbon, the peak at 1100 cm⁻¹ almost disappeared, while two distinct peaks appeared at 1211 and 1016 cm⁻¹, attributed to the stretching vibrations of ether (-C-O-C-). A new peak at 2922 cm⁻¹ appeared on gold-loaded carbon, likely due to the stretching of alkyl (-CH₂) groups. Weak adsorption peaks around 500-600 cm⁻¹, corresponding to S=O or S-O bonds, were observed for both copper- and gold-loaded carbons. FTIR analysis indicates that functional groups present in raw activated carbon include hydroxyl (-OH), carbonyl (C=O), and oxygen bonds (-C-O-). After adsorption, the intensities of hydroxyl (-OH) and carbonyl (C=O) bonds increased, while the carbon-oxygen bond (-C-O-) almost disappeared for copper-loaded carbon. This suggests that the adsorption mechanism of gold on activated carbon differs from that of copper.
Conclusion
- In thiosulfate solutions, activated carbon has good adsorption performance for copper but poor selectivity for gold. Under optimal conditions, the adsorption capacity of activated carbon for copper is significantly higher than that for gold.
- The adsorption kinetics of copper and gold thiosulfate complexes on activated carbon conform well to the pseudo-second-order model. The adsorption isotherm of copper fits the Freundlich model, while that of gold fits the Langmuir model, indicating that copper adsorption involves multilayer chemisorption, while gold adsorption involves monolayer adsorption.
- FTIR and XPS analyses suggest potential interactions between Cu and S and the formation of covalent bonds with active groups on the activated carbon surface. The adsorption mechanisms of gold and copper on activated carbon differ significantly.
Article Keywords: Activated carbon adsorption, thiosulfate gold recovery, copper adsorption, gold recovery, gold leaching, adsorption kinetics, environmental technology, adsorption mechanisms
