Enhanced and selective gold recovery from phone waste: Use of thiosulfate, dissolved oxygen, and an agriculture-based, low-cost adsorbent

Efficient methods were proposed to improve the gold recovery from printed circuit boards (PCBs) in mobile phones. Gold was selectively extracted in the form of Au(S2O3)23- using Na2S2O3-CuSO4-NH3-O2/N2 system. Under optimal conditions (temperature: 25 & DEG;C; stirring: 200 rpm; O2/N2: 3.0/0.5 L/min; time: 3 h; Na2S2O3: 160 mmol/L; CuSO4: 120 mmol/L; NH3: 0.7 mol/L (pH 9.9); solid/liquid: 4 g/L), 98% of gold was extracted. The extracted gold was adsorbed by corncob-based activated carbon (CACw), which was prepared using pyrolyzed wood vinegar. The modification of CACw with mercaptobenzothiazole (MBT), forming MBT-CACw, which displayed maximum adsorption efficiency of 49.9 mg/g. The optimal pH for adsorption was found to be 7.5 at 40 & DEG;C with 0.2 mol/L (NH4)2S2O3. The adsorption was described by the pseudo-second-order model and the Freundlich isotherm, owing to the high affinity of MBT-CACw for the sulfur donor. Copper was eluted from the loaded MBTCACw using 0.7 mol/L (NH4)2S2O3 followed by the gold elution with 40% v/v CH3OH. The two-step elution recovered 98.7% of copper and 97.6% of gold. These findings suggest that the oxygen-assisted extraction, combined with the adsorption conducted using low-cost, agriculture-based adsorbent and selective elution, is an eco-friendly approach to efficiently recovering gold from e-waste.