Enhanced gold recovery from e-waste by biological thiosulfate leaching and trace gold extraction with UiO-66-NH₂

The recycling of precious metals from electronic waste (e-waste) is essential to respond to the increasing demand for strategic metal. Compared to the cyanide method, the thiosulfate bioleaching method is considered a greener and cleaner approach for gold recovery. However, gold recovery from thiosulfate bioleaching solutions remains a challenge. This study addressed these challenges by optimizing the production of biogenic thiosulfate from Acidithiobacillus thiooxidans using a combination of NaN₃ and KCN at the optimal ratio of 4:1.The thiosulfate bioleaching process achieved a 30% extraction efficiency (0.6 mg/L) of gold, from 10 g/L of spent printed circuit boards. The adsorption of gold was explored using UiO-66-NH₂, a highly porous metal–organic framework (MOF) with a Brunauer–Emmett–Teller (BET) surface area of 1840 m²/g and a pore size of 5–15 nm. Fourier Transform Infrared Spectroscopy (FTIR) analysis confirmed the presence of amino functional groups, enhance the gold-binding affinity. The synthesized metal–organic framework, UiO-66-NH₂, demonstrated a 50% gold adsorption capacity from the bioleaching solution within 2 h. The electrostatic attraction between the NH₃⁺ groups of UiO-66-NH₂ and AuS₂O₃₂^3-, combined with internal Zr-OH complexes, played a crucial role in the adsorption mechanism. Gold ions (Au³⁺) were reduced to elemental gold (Au⁰) via an oxidation–reduction reaction facilitated by amino groups, as confirmed through FTIR, field emission scanning electron microscopy (FESEM), mapping and energy-dispersive X-ray spectroscopy (EDS) analysis. Adsorption kinetics analysis indicated that the process followed a pseudo-second-order model. These results demonstratea sustainable method for recovering gold directly from e-waste bioleaching solutions.