Immobilization of multiple hazardous metals in fly ash waste by enzyme-induced carbonate precipitation

Abstract

The global generation of fly ash waste is projected to increase, with substantial quantities ultimately disposed of in landfills. However, due to the abundant hazardous metals in fly ash, the recycling necessitates a “green” and effective treatment to immobilize hazardous metals and avoid leaching. Enzyme-induced carbonate precipitation (EICP) has been demonstrated to be effective in treating soil contaminated with hazardous metals. However, limited research focused on the interaction between EICP and fly ash, as well as the migration of hazardous metals, which will be beneficial in guiding EICP treatment of fly ash from a core perspective. Therefore, the EICP technique was applied for fly ash treatment. To determine the immobilization effects on multiple hazardous metals, key measurements were made, including calcium carbonate content, speciation changes, zeta potential, and hazardous metal leaching concentrations, coupled with microstructural analyses. Results showed that with the EICP treatment, the speciation of multiple hazardous metals tended to shift towards stable organic and residual states. At lower EICP solution percentages, carbonates played a significant role in the adsorption of hazardous metals (e.g., Cd, Co, Cu, Mn, Ni, Pb, and Zn). The release of most metals was observed in the Fe/Mn oxide state, while Pb showed the most significant reduction. Copper exhibited a significant increase in organic states due to its strong affinity with the introduced organic substances, whereas other metals remained at low levels. Silicates became the dominant adsorbent at higher EICP solution percentages, as was presented in the residual state. Overall, the EICP treatment effectively immobilized multiple hazardous metals in fly ash waste. This research provides a robust foundation for the innovative application of EICP, contributing to the mitigation of environmental contamination and advancing the recycling development of fly ash waste.