Improved Sequential Approach for Hybrid Bioleaching of Metals from E-Waste

The continual and unprecedented consumer demand for the electrical and electronic equipment (EEE) coupled with their accelerated product obsolescence rate due to the rapid advances in technology has led to an increased generation of waste electrical and electronic equipment (WEEE) or electronic waste (e-waste) worldwide. Printed circuit board (PCB), the core component of e-waste, is considered as secondary metal reservoir owing to its rich metallic content including base, toxic, and precious metals. E-waste recycling is an important aspect not only from the point of waste treatment for environmental protection but also from the recovery of metals for economic development. Being an emerging technique, bioleaching employing mostly acidophilic microorganisms for metal extraction from e-waste is time consuming despite higher efficiency. A hybrid process comprising of both chemical and biological leaching involving combination of safer, non-toxic ligands, and bioleaching microbes has shown enhanced metal extraction from waste PCBs albeit time consuming. In this context, a sequential strategy consisting of maximum biological production of ferric iron in the first step followed by hybrid bioleaching of metals from high-grade PCB of obsolete desktop has been devised in the present study to overcome the impediment of higher time requirement. The ferric iron (Fe³⁺) production was maximized via biological oxidation of ferrous iron (Fe²⁺) by Acidithiobacillus ferrooxidans in 250 mL of shake flasks containing 100 mL of optimized 4.5 K media with initially adjusted pH of 2 at 170 rpm and 30 °C. As corroborated from the maximum Fe³⁺ concentration and oxidation-reduction potential (ORP), complete biological oxidation of Fe²⁺ was observed at 60 h using initial concentration of around 4 g Fe²⁺/L. Subsequently, the comminuted PCB in the particle size range of 0.038–1 mm was added to the same shake flasks at an e-waste pulp density of 10 g/L along with 0.2 M citric acid to imitate hybrid condition to assess the extraction efficiency of the targeted metals, i.e., Cu, Zn, and Ni. Results revealed that a maximum of 85% Cu, 71% Zn, and 54% Ni extraction was achieved at an excellent time duration of 96 h of PCB addition. Results from the present study, thus, highlight the practical feasibility of improving the hybrid bioleaching technique for metal recovery from e-waste following the sequential approach.