Study on denitrifying biogrout to immobilize heavy metals in bottom ash in an anaerobic environment and its immobilization mechanism

Incineration bottom ash (IBA) is the residue of municipal solid waste (MSW) after incineration, which is often used for land backfilling (in the anaerobic environment). This study reports a method of anaerobic bacteria for IBA treatment, and the mechanism of heavy metals immobilization by anaerobic bacteria is discussed in depth. In this study, the IBA from three cities, namely Shenyang, Wuhan, and Guangzhou in China, were treated with anaerobic bacterial grouting, and the leaching concentrations of heavy metals were detected. Their IBA production ranks among the top ten cities in China. The results show that the anaerobic bacteria can reduce the leaching concentration of Zn, Cu, Cd, Cr, As, and Pb, and the immobilization ratios reach 50-90%. Biotreatment improves the carbonate-bound form of the noted heavy metals, and the carbonate-bound form is positively correlated with bioprecipitation of calcium carbonate. It is observed by scanning electron microscope (SEM) and optical microscope that fragmentized calcium carbonate crystals are still generated when the upper surface of the sample exposes to air. The mechanism of immobilization during the biomineralization process is revealed by the fluorescence spectrum and chemical reaction. In an anaerobic environment, the anaerobic bacteria enrich calcium ions and carbonate ions to generate biogel, which can physically envelop heavy metals. The induction of anaerobic bacteria can stabilize heavy metals by changing chemical speciation. They all reach the strength limitation of land backfill. This study provides a new idea to reduce the environmental harm from the IBA and improves the sustainable utilization of IBA.