Comparisons of one-step and two-step bioleaching for heavy metals removed from municipal solid waste incineration fly ash

Heavy metals were bioleached from municipal solid waste incineration fly ash by Aspergillus niger (AS 3.879) using one-step and two-step bioleaching. The pH variation, organic acids, and metals concentrations, as well as the metals extraction yield of these two processes were compared. The maximum metals extraction yield was obtained in one-step bioleaching of 20 g/L fly ash, where 98.7% of Mn, 87.6% of Cd, 69.7% of Cu, 68.5% of Zn, 42.1% of Cr, 31.7% of Fe, and 36.5% of Pb were extracted; meanwhile, the pH decreased to 3.03 and the fungus secreted 36.6 mmol/L gluconic acid after 336 h. The results indicated that one-step bioleaching was preferable for the treatment of fly ash at low concentrations (10-20 g/L). Two-step bioleaching was appropriate to detoxify the fly ash at high concentrations (40-50 g/L) because A. niger grew and generated 33.9 mmol/L gluconic acid even at 50 g/L fly ash in two-step bioleaching compared to the fungal growth only observed at 10-40 g/L fly ash in one-step bioleaching. The gluconic acid was determined as the major leaching agent both in one-step and two-step bioleaching. In addition, the TCLP tests results of the bioleached fly ash showed that the leaching toxicities of the treated fly ash were far lower than the regulated levels of China.