Leaching behaviour and chemical speciation changes of heavy metals in mixed electroplating sludge during ferrous oxidation‑assisted two‑stage bioleaching

Reutilization of electroplating sludge attracts growing interests due to the high concentration of heavy metals. In this study, a suitable microbial consortium was screened for better extraction of Cu, Ni, Zn and Cr from mixed electroplating sludge at a high pulp density of 12 % (w/v) within a short time (24 h) and the influence of microbe–sludge interactions on the bioleaching behaviors and chemical speciation transformation of heavy metals was explored. The results demonstrated that the two-step bioleaching in conjoint with ferrous energy‑enriched microbial consortium achieved superior metal solubilization (>96.1 % for each of Cu, Zn and Ni, and 75.4 % of Cr) in comparison to the sulfur-enriched microbial consortium and ferrous-sulfur mixed energy-enriched microbial consortium. Bioleaching performed better than chemical leaching and involved more leaching mechanisms rather than merely the contributions of H⁺ and Fe³⁺ to the dissolution of different metal species. Based on analysis of bioleaching dynamics and changes in the speciation and distribution of heavy metals and succession of microbial community during the ferrous oxidation‑assisted two-stage bioleaching process, it was speculated that both active microorganisms and their biogenic active substrates (H⁺ or Fe³⁺) directly or indirectly reacted with the sludge; more H⁺ or Fe³⁺ generated by microbial oxidation passed through the solid shell and entered inside the reacted core, disrupted metal compounds in four chemical forms and simultaneously enhanced the transformation of the specific metal chemical speciation from the passive fraction to the unstable exchangeable/acid soluble fraction and reducible fraction. Through these complex processes and alterations, it eventually facilitated more metal release from the sludge. These findings are expected to have significant implications for achieving rapid and deep metal extraction from electroplating sludge through bioleaching. Moreover, they have the potential to create new approaches for the sustainable and environmentally friendly treatment and management of electroplating sludge.