Bioleaching of copper from waste printed circuit boards by bacteria-free cultural supernatant of iron-sulfur-oxidizing bacteria

Background: The toxicity of waste printed circuit boards (PCBs) to bacteria was considered as the major limitation in bioleaching of copper from PCBs. To reduce the toxicity of PCBs, copper extraction from PCBs was investigated using bacteria-free cultural supernatant from some metallurgical microbial consortium, whose predominant organisms were Leptospirillum ferriphilum and Sulfobacillus thermosulfidooxidans. Results: About 100% copper was recovered in 2 h from 5 g/L PCBs by bacteria-free cultural supernatant. The result indicated that the indirect non-contact mechanism was the predominant mechanism in bioleaching of copper from PCBs. It was not necessary for bacteria to exist in copper extraction. In addition, the role of bacteria was most likely to regenerate Fe3+ as an oxidant. Furthermore, the biooxidation of Fe2+ to Fe3+ was determined as the rate-limited step in bioleaching of copper from PCBs. In addition, the biooxidation activity of bacteria would be strongly inhibited by the toxicity of PCBs. Conclusion: The separation of bacteria from the PCBs probably was the optimum way to avoid the negative effect of PCBs. Accordingly, a biooxidation-leaching-separation cycle was designed to avoid the toxicity of PCBs. Eventually, 93.4% of copper was recovered in total from 100 g/L PCB concentrates in 9 days.