Optimisation on bioleaching of incinerator fly ash by Aspergillus niger – use of central composite design

Previous work has shown that heavy metals may be bioleached from municipal solid waste (MSW) incinerator fly ash. The bioleaching of heavy metals from the ash depends on physical, chemical and biological factors. Our research objective is to establish the factors that exert a greater impact on the bioleaching process, and to determine the optimal bioleaching conditions in order to achieve the maximum metal leaching efficiency for selected metals. Four factors were investigated: sucrose concentration, inoculum spore concentration, fly ash pulp density, and the time of addition of fly ash to the fungus Aspergillus niger. Using the central composite design (CCD), 31 batch bioleaching tests were run under high and low values of these four factors. Empirical models obtained through second-order (Taylor series approximation) regression provided the optimal bioleaching conditions. Results showed that sucrose concentration and pulp density were more important factors than spore concentration and the time of addition of the fly ash. The concentration of Al, Fe, Zn, citric acid, and gluconic acid attained in the leached liquor was 12.3, 12.3, 77.6 ppm, 51 and 281 mM, respectively.