Bioleaching of heavy metals from municipal solid waste incineration fly ash by Penicillium citrinum and analysis of associated metabolites.

With the continuous expansion of municipal solid waste (MSW) incineration, MSW incineration (MSWI) fly ash has garnered significant attention due to its heavy metal leaching toxicity. Bioleaching offers a sustainable, cost-effective, and energy-efficient remediation strategy. Heavy metals were extracted from MSWI fly ash via Penicillium citrinum using one-step, two-step, and spent-medium bioleaching. Metabolite variations, including organic acids, siderophores, and extracellular polymeric substances (EPS), were compared. Interactions between the fungal strain, metabolites, and fly ash particles were investigated using Fourier transform infrared (FT-IR) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). Results indicated the two-step method achieved optimal performance, extracting 48.3% Cr, 82% Cu, 80.3% Zn, 89.6% Cd, and 78% Pb. Gluconic acid emerged as the dominant organic metabolite, peaking at 29.7 mg/mL. Siderophores and EPS also contribute to heavy metal mobilization from the fly ash matrix. Leaching efficiency positively correlated with organic acid and siderophore concentrations, with the carboxyl group serving as the main functional group responsible. Mechanistically, Cu, Zn, Cd, and Pb extraction was mainly driven by complexation, followed by acidolysis, whereas Cr extraction was primarily attributed to acidolysis, followed by redox reactions. Additionally, fly ash chloride similarly complexed with both Cd and Pb. Significant morphological changes were observed in both the fly ash and the fungal strain, which exhibited a notable biosorption capacity for heavy metal ions. This research provides a theoretical foundation for subsequent experimental optimization and offers a novel strategic framework for fly ash treatment technologies.