Sustainable gold leaching from waste printed circuit boards using biogenic thiosulphate produced by Acidithiobacillus...

This study investigated biogenic thiosulphate as an environmentally friendly alternative to cyanide for gold leaching from waste printed circuit boards. Biogenic thiosulphate, an unstable intermediate formed during sulphur oxidation by Acidithiobacillus thiooxidans, was stabilised by pH adjustment and use of inhibitors; the effects of sodium fluoride and sodium diethyldithiocarbamate were evaluated. The inhibitory effects of sodium diethyldithiocarbamate and sodium fluoride at concentrations up to 10^-2 M on thiosulphate oxidation were not sufficiently strong, which was attributed to differences in metabolic pathways during bacterial growth. The highest thiosulphate concentration, 219 mg/L, was achieved without inhibitors at pH 6. The maximum gold leaching rate reached 10% after 3 days under conditions of 30 °C, 120 mg/L biogenic thiosulphate, 0.02 M Cu²⁺, 1 M ammonia, and 1% pulp density. Ammonia and copper were added as stabiliser and catalyst, respectively. Kinetic and thermodynamic analyses supported these findings and guided the routes for biogenic thiosulphate production as a sustainable leaching agent for gold leaching from electronic waste. Overall, this study demonstrates that biogenic thiosulphate produced by Acidithiobacillus thiooxidans is a promising alternative to chemical thiosulphate for sustainable gold recovery from electronic waste, offering a viable route toward efficient and green microbial-based processing.