Microbial leaching of heavy metals using Escherichia coli and evaluation of bioleaching mechanism

Abstract

Bioleaching is recognized as a green process for the extraction of metals from various solid wastes. In the recovery of metals from spent catalyst using bioleaching with acidophilic bacteria, low Mo extraction is often reported. To address this problem, bioleaching of spent hydrodesulphurization catalyst with Escherichia coli and its mechanism was examined. Two-step bioleaching of ground catalyst (75–100 μm) at 0.5% w/v pulp density resulted in about 72% Mo and 68% Ni extraction in 30 days. The increase in pH of the media during bioleaching promoted the dissolution of Mo trioxide, i.e. Mo (VI). X-ray Photoelectron Spectroscopy also confirmed a higher concentration of Mo (IV) in the spent catalyst after bioleaching. Higher solubility under bioleaching conditions and complex formation with various metabolites led to increased Mo extraction. Thermal pre-treatment reduced resistance due to carbon deposits, and oxidized Mo (IV) to Mo (VI), leading to increased Mo bioleaching at 96%.