Dispersion stability of municipal solid waste incineration fly ash bioleached by Aspergillus niger

Municipal solid waste incineration fly ash (MSWI FA) is a hazardous byproduct from flue gas cleaning systems, whose high toxicity, poor dispersibility, and low stability severely hinder its safe utilization in construction materials. Bioleaching using fungi offers a potential detoxification route, but the influence of key process variables on the dispersion stability and pozzolanic reactivity of the treated ash remains unclear. In this study, Aspergillus niger was acclimated to 9% (w/v) raw MSWI FA (RFA), and bioleaching conditions (RFA content, temperature, and shaking speed) were optimized using a Box Behnken design. The dispersion stability mechanism of the bioleached MSWI FA (BFA) was elucidated by particle size distribution, static sedimentation, contact angle, zeta potential, and conductivity measurements. The microstructure, heavy metal leaching, and pozzolanic activity of BFA were also examined. The results showed that the optimal bioleaching conditions were found to be an RFA content of 11.8%, a temperature of 29.7℃, and a shaking speed of 155 rpm. Under these conditions, the dispersion stability and pozzolanic activity index of RFA were improved. Bioleaching refined the particle size of RFA and generated a fresher and more reactive surface. In addition, fine calcium oxalate particles were formed, while the removal of heavy metals was simultaneously promoted. This study demonstrates that tailored fungal bioleaching can effectively convert hazardous MSWI FA into a reactive secondary material with improved dispersion stability for potential use in cement-based applications.