An innovative biotechnology for metal recovery from printed circuit boards

The increase of waste from electric and electronic equipment has pushed the research towards the development of high sustainability treatments for their exploitation. The end-of-life printed circuit boards (PCBs) represent one of the most significant waste in this class. The interest for these scraps is due to the high Cu and Zn content, with concentrations around 25% and 2% respectively, combined with further precious metals (e.g. Au, Ag, Pd). Currently, the most common approaches developed for PCBs recycling include pyrometallurgical and hydrometallurgical treatments. On the other hand, biohydrometallurgical strategies are gaining increasing prominence, for the possibility to decrease both the environmental and the economic costs. Nevertheless, these techniques show the main limit due to the possibility to treat low quantities of waste, which makes unsustainable the further scale-up. To overcome this criticality, the present paper introduces an innovative bioleaching process carried out by Acidithiobacillus ferrooxidans (At. ferrooxidans) and Leptospirillum ferrooxidans (L. ferrooxidans). The developed technology allows to reach high PCB concentration, up to 5% (w/v), thanks to a high efficiency two-step design, able to reduce the metal toxicity on the bacteria metabolism. The treatment uses the ferric iron generated by bacterial oxidation, as oxidant, to leach Cu and Zn from PCBs. The possibility to overcome the solid concentration criticality is combined with high yield of 94% and 70% for Cu and Zn, respectively. The best selected conditions involve the At. ferrooxidans bacteria use at: 30 °C, solid concentration of 5% (w/v), 10 g/L of Fe2+, time of treatment 9 days. The experimental results are further enhanced by the carbon footprint assessment which proved the environmental advantage, compared to both the reference chemical treatment through ferric iron and literature processes (hydrometallurgical and bioleaching approaches). The analysis explained as the PCBs concentration in the solution allows to decrease the bioreactor size with the consequent reduction of energy and raw material demand. This benefit can be translated into a 4 times reduction of the CO2-eq./kg treated PCB emissions, compared to the best bioleaching processes, reported in the literature.