Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi

Microbiological processes were applied to mobilize metals from electronic waste materials. Bacteria (Thiobacillus thiooxidans, T. ferrooxidans) and fungi (Aspergillus niger, Penicillium simplicissimum) were grown in the presence of electronic scrap. The formation of inorganic and organic acids caused the mobilization of metals. Initial experiments showed that microbial growth was inhibited when the concentration of scrap in the medium exceeded 10 g L⁻¹. However, after a prolonged adaptation time, fungi as well as bacteria grew also at concentrations of 100 g L⁻¹. Both fungal strains were able to mobilize Cu and Sn by 65%, and Al, Ni, Pb, and Zn by more than 95%. At scrap concentrations of 5–10 g L⁻¹, Thiobacilli were able to leach more than 90% of the available Cu, Zn, Ni, and Al. Pb precipitated as PbSO₄ while Sn precipitated probably as SnO. For a more efficient metal mobilization, a two-step leaching process is proposed where biomass growth is separated from metal leaching.