Fly ash from a municipal solid waste (MSW) incinerator in Singapore was physically and chemically characterized. The very fine amorphous ash particle was found to have a non-porous structure with a low specific surface area and a tendency to agglomerate. Chemical analysis indicated that a significant amount (2000-20,000 mg/kg) of toxic heavy metals, such as Al, Ph and Zn were present in the fly ash. Bioleaching using the fungus Aspergillus niger was carried out at different pulp densities of fly ash (1-8%, w/v), and results were compared with chemical leaching using various organic and inorganic acids. Although the fungus grew only in the presence of 1-2% (w/v) fly ash when it was incubated together with the fly ash (one-step bioleaching), growth was observed even at 4% (w/v) fly ash when the ash was added after 2 days of fungal incubation (two-step bioleaching). Both one-step and two-step bioleaching experiments showed similar metal extraction yield for 1% (w/v) of fly ash pulp density (80-100% for Al, Mn and Zn; 60-70% for Cu and Ph, and about 30% for Fe). Optimum pulp density for bioleaching was observed at 1% (w/v), and leaching efficiency decreased with increasing pulp density in fungal bioleaching as well as in spent medium leaching. The main lixiviant in bioleaching was shown to be gluconic acid, which was produced only in the presence of fly ash (in one-step and two-step bioleaching). Compared with chemical leaching at 1% pulp density, the fungus was more efficient in the leaching of Mn and Zn, and showed similar extraction yield for A]. However, Cu extraction yield was lower in bioleaching than chemical leaching. These results suggest that bioleaching by A. niger may be an alternative or adjunct to conventional physicochemical treatment processes of MSW fly ash to remove hazardous heavy metals. Bioleaching mechanisms are also discussed. (c) 2005 Elsevier Inc. All rights reserved.