Bioleaching of Metals From Waste Computer Printed Circuit Boards using Indigenous Bacterial Isolates From Metal Contaminated Soil

Abstract

The industrial economy and technology have rapidly released electronic products, accompanied by significant advancements. This surge has resulted in a substantial increase in outdated and discarded electronic equipment, contributing to the global issue of e-waste. The unsustainable consumption of natural resources for the continuous advancement of electric and electronic equipment designates e-waste as the core of 'urban mining.' Thus, this metal reservoir has prompted our focus on addressing this solid waste stream, explicitly directing our efforts toward bioleaching e-waste. In the current study, the shredded dust from waste computer printed circuit boards (CPCBs) generated during mechanical dismantling is subjected to metal content analysis and recovery via bioleaching. The presence of various base metals, including copper, aluminum, nickel, iron, and precious metals such as silver, gold, and platinum in the shredded dust of CPCBs designates this waste as a viable 'secondary source' for a variety of metals within the 'urban mining' strategy. Despite the numerous cyanogenic microorganisms previously documented for their metal solubilization potential in biohydrometallurgical techniques, constraints such as low toxicity tolerance and limited efficiency persist. These limitations necessitate ongoing research efforts to identify novel microbial strains acclimated to metal-rich environments, intending to enhance the efficacy of the bioleaching process for industrial-scale e-waste management. Consequently, we aimed to investigate the bacterial community that thrives within the metal-acclimated environment for their application in bioleaching. The present study identified Bacillus sporothermodurans ISO1 as a novel Bacillus strain exhibiting higher toxicity tolerance (EC50 = 425 g/L) and leaching potential than earlier reported bacterial strains. It is imperative to optimize various parameters for maximum recovery of metals, as these significantly influence bacterial growth and the efficiency of the bioleaching process. Response Surface Methodology (RSM), a statistical technique, was employed to optimize various culture variables, including temperature, pH, glycine concentration, pulp density, and time. The interactive effects of critical factors in the RSM approach resulted in up to 95% Cu and 44% Ag recovery. Furthermore, a chemo-biohydrometallurgy approach with suitable chemical lixiviant (FeCl3) was utilized for Cu recovery before bioleaching by Bacillussporothermodurans ISO1 and Pseudomonasbalearica SAE1. This strategy improved the leaching of precious metals, such as Ag (57%) and Au (67%). Furthermore, to reduce the dependence on chemical lixiviants and to enhance process economy, we endeavored to stimulate the availability of biogenic cyanide for efficient metal recovery using a suitable inducer. A concentration of 1 mg/L of inducer (i.e., methionine) efficiently promoted glycine-utilized cyanide production, resulting in an 86% solubilization of Cu and a 75% solubilization of Au. Additionally, the metal solubilization and operating conditions were explored at an increased volume (i.e., 3 L working volume) of the bioleaching medium to assess the industrial-scale potential of this potent bacterial strain. The substantial recovery of Cu (˃60%) and other metals at this increased volume suggests the feasibility of implementing the bioleaching process with this potent bacterial strain at a large-scale operation. Thus, the present study has provided a potential bacterial strain and a suitable chemo-biohydrometallurgy approach, which could be a proficient strategy for metals recovery from e-waste.