Utilization of size-fractionated pozzolanic bottom ash to immobilize heavy metals in MSW incinerator fly ash

Abstract

Fly ash (FA) from Municipal solid waste incineration (MSWI) is disposed of in landfills after the immobilization of hazardous heavy metals. Pozzolans found in the fine-fraction of MSWI bottom ash (BA) have the potential to immobilize heavy metals. The aim of this study was to examine the effect of size-fractionated BA in immobilizing heavy metals in FA, in order to enhance the effective utilization of resources. BA was applied at two distinct fractions (d<0.25 and d<2.0 mm) when considerable amorphous reactive pozzolans (i.e., siliceous and aluminous) were present. In order to conduct the experiments, FA, BA, and water were mixed in a ratio of 10:1:10, dried at room temperature and in an oven at 120 °C for 4 h, and settled for 4, 8, 16, 30, and 60 days. The results showed that under the air-dried conditions, both BA fractions significantly immobilized Pb and Zn. Pb and Zn leaching stabilized at about 99.9% and 95.6%, respectively, over 16 days with a BA fraction of d< 0.25 mm in an air-dry environment. The pH reduction, the calcite, C-S-H gel, and several new secondary minerals formation demonstrated that the carbonation and pozzolanic reactions were the immobilization mechanisms. Despite the method's simplicity, the findings suggested a potential approach of utilizing residual BA fractions that can be reused directly in the incineration facility. Therefore, this technique can minimize the amount of raw materials required as a metal stabilizer, reduce the amount of waste sent to landfills, and create a new material that can be reused as fillers.