Bioleaching of dewatered electroplating sludge for the extraction of base metals using an adapted microbial consortium: Process optimization and kinetics

The main objective of this study was to optimize bioleaching process parameters for high co-extraction of base metals (Cu, Ni, Zn, and Cr) from hazardous electroplating sludge using an adapted microbial consortium and to explore the relevant bioleaching mechanisms. Microbial cultivation and sludge bioleaching were separated. The effect of the relevant process parameters (i.e., bulk pH, pulp density, and Fe2+ concentration) on the extraction of four selected metals through bioleaching by an adapted microbial consortium was investigated in a 1 L stirred tank reactor. Results indicated that maximum metal solubilization (> 95.6% for each of Cu, Zn, and Ni, and 90.3% of Cr) was achieved at a bulk pH of 2.0, Fe2+ of 9.0 g/L, and pulp density of 15% (w/v). Bioleaching kinetics of the selected metals was described by a modified shrinking core model. This indicated that the interfacial transfer and diffusion across the solid film layer was the rate controlling step and controlled the dissolution kinetics. Data from bioleaching and chemical leaching systems showed that bioleaching had some advantages over simple chemical leaching. The mechanisms of improved Cu, Ni, Zn, and Cr extraction by bioleaching were demonstrated. Bioleaching improved metal release, especially from the residual fraction, as indicated by Community Bureau of Reference (BCR) three-stage sequential extraction analysis. Most of the Cu, Ni, and Zn extraction was attributed to H+ attack, as these metals were primarily distributed in the water/acid soluble and exchangeable fractions, along with Fe and Mn oxyhydroxides (> 72.3%). For the extraction of Cr, besides H+, microorganisms and Fe3+ were also responsible. They improved Cr extraction, especially from the residual fraction. These findings indicate that bioleaching with an adapted microbial consortium appears promising for recycling and reutilizing valuable heavy metals from hazardous electroplating waste.