Biological leaching of fly ash from municipal waste incineration by Aspergillus niger was examined in batch cultures and compared with chemical leaching. A. niger grew in the presence of up to 10% (w/v) of fly ash in the medium. In the presence of fly ash A. niger produced gluconate, whereas in its absence citrate was produced. Variation of the fly ash concentration in the growth medium (one-step process) resulted in different amounts of solubilized metals. A total of 3% (w/v) fly ash generally gave maximum extraction yields (in percent of the amount applied). In a two-step process A. niger first was cultivated in the growth medium, and subsequently the microbiologically produced citric acid was used as the leaching agent. At 6% (w/v) fly ash, the amounts of leached metals (leaching for 1 day) were 81% of Cd, 66% of Zn, 57% of Cu, 52% of Pb, 32% of Mn, 27% of Al, and less than 10% of Cr, Fe, and Ni, respectively. Chemical leaching with commercial citric acid of equal molarity was only slightly higher than microbial leaching. The environmental quality of the residues can be improved with respect to a re-use of these materials for construction purposes.
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Bosshard et al. (1996) studied this question.