Bioleaching of Metals from Electronic Waste by Heterotrophic Bacteria In Fluidized - Bed Bioreactor

Abstract

The technological novelty has led to a decline in the lifespan of electronic gadgets, consequently discarding these as electronic wastes (e-wastes). Printed Circuit Boards (PCBs) which form the major proportion of the e-wastes, can serve as secondary sources of metals. The escalating global consumption and pressing demand for base and precious metals due to exhaustion from their natural resources has necessitated recycling the wastes for metal recovery. Bioleaching is the microbe-mediated mobilization of metals, and it can treat e-wastes to recycle it into the circular economy. The present study aims to recover metals from PCBs using heterotrophic bacteria in a Fluidized-bed bioreactor (FBR). Alcaligenes aquatilis and Chromobacterium violaceum were chosen for Cu, Ag, and Au bioleaching, respectively, through screening studies. The performance of FBR for Cu bioleaching revealed 0.175 mm particle size, 5% inoculum (v/v), and 2% e-waste load (w/v) as the optimum conditions. The mechanism of Cu bioleaching by A. aquatilis through contact and non-contact modes with redoxolysis and complexolysis mechanisms was proposed. Both bacteria performed better in bioleaching of metals as individual cultures than as co-cultures. Comparison of the three bioleaching methods in the FBR proved that the two-step method using C. violaceum is efficient for precious metal bioleaching with 9% Au and 23.7% Ag bioleaching. In contrast, the spent medium of A. aquatilis could bioleach 52.2% Cu. The recovery could be improved through three sequential batch runs of one-step bioleaching in FBR with A. aquatilis, yielding cumulative recovery of 80% Cu and 28.6% Ag. The recovery was also improved through a two-stage sequential batch operation involving a first stage of three sequential batches using the spent medium of A. aquatilis, followed by a second stage involving three sequential batches of a two-step method with C. violaceum, accomplishing a cumulative recovery of 99% Cu, 36.8% Ag, and 21.9% Au. The process can potentially serve as a greener and more economical method for bioleaching.