Effects of water-washing pretreatment on bioleaching of heavy metals from municipal solid waste incinerator fly ash

Previous studies demonstrated that the bioleaching of municipal solid waste incinerator fly ash by Aspergillus niger was an efficient "green technology" for heavy metals removal, however, it demanded a long operational period. In this study, water-washing was used as a fly ash pretreatment before the bioleaching process (one-step and two-step). This pretreatment extracted 50.6% of K, 41.1% of Na, 5.2% of Ca and 1% of Cr from the fly ash. Due to the dissolution of alkali chlorides which hold particles together, fly ash particles were smashed into smaller granules by the hydraulic flushing action caused by vibration. After the pretreatment, the lag phase and bioleaching period were reduced by 45 and 30%, respectively, in one-step bioleaching of 1% (w/v) fly ash. Meanwhile, the metals extraction yield both in one-step and two-step bioleaching was increased markedly, e.g. in two-step bioleaching, 96% Cd, 91% Mn, 73% Pb, 68% Zn, 35% Cr and 30% Fe was extracted from 1% water-washed fly ash, respectively. The reduction of the bioleaching period and improvement of metals extraction yield will likely allow the practical application of the bioleaching technology for heavy metals removal from fly ash.