Development of an integrated bio-based cyanide production system for recovery of Au from waste printed circuit boards

Electronic waste (E-waste) contains valuable metals that can be recovered, but current industrial recycling often results in significant losses of precious metals like Au, which should be minimized. Biocyanidation, which uses cyanogenic bacteria to produce cyanide, offers advantages such as cost-effectiveness and high recovery rates for Au leaching. However, its application is hindered by low recovery efficiency and cyanide volatility at the typical cultivation pH (∼9). Exploiting this characteristic, this study proposes an integrated system to separate biologically produced cyanide from microbial cultures and subsequently utilize it to leach Au from waste printed circuit boards (WPCBs). The integrated system includes a fermenter for cultivating Chromobacterium violaceum and a collection system where 1 M NaOH is used to absorb cyanide from the fermenter. In the collection system, cyanide concentrations reached 163.2 ± 13.8 ppm after eight days of cultivation—significantly higher than the cyanide concentration typically found in C. violaceum culture broth. The Au leaching efficiency of the biologically produced cyanide reached 90.5 ± 1.4 %, comparable to that of chemical cyanide. After leaching, 97.7 ± 0.0 % of the Au was recovered within one hour using an ion-exchange resin. Afterward, the residual cyanide in the bioleachate was also treated by mixing it with fermenter broth, achieving a 96.8 ± 1.5 % removal rate. Overall, the integrated system proposed in this study achieves high Au leaching efficiency using biologically produced cyanide, comparable to that of chemically synthesized cyanide, while preserving the eco-friendly and sustainable nature of bio-cyanidation.