Selective recovery of gold from discarded cell phones by silk fibroin from Bombyx mori

Selectivity is a crucial factor for gold recovery from discarded cell phones. This is because discarded cell phones contain a large variety of metals. The use of biomaterials as gold adsorbents is attracting much attention because it is in accordance with sustainability goals. Here, we reported selective adsorption of gold on silk fibroin and its application to gold recovery from the actual leachate of discarded cell phones in a chloride medium. The experimental maximum gold-adsorption capacity of silk fibroin was 5.800 mg/g. Based on a kinetic study, the gold-adsorption mechanism followed the pseudo-second-order model with a rate constant (k₂) of 0.077 g mg⁻¹min⁻¹. The adsorption fitted well with the Langmuir model, suggesting a monolayer adsorption mechanism. Despite the small adsorption capacity, silk fibroin showed remarkable selectivity toward gold in synthetic and actual leachate containing Pt⁴⁺, Pd²⁺, Al³⁺, Cu²⁺, Ni²⁺, Zn²⁺, Pb²⁺, and Fe³⁺ metal ions in 0.5 mol/L HCl. The gold uptake from the actual leachate of discarded cell phones was approximately 95 %. The gold loaded on the silk fibroin was completely desorbed by 0.1 mol/L thiourea.