Characterization of heavy metals solidification during hydrothermal synthesis of tobermorite from incineration fly ash

The environmentally safe utilization and disposal of municipal solid waste incineration fly ash is an urgent problem that needs to be addressed by environmental management and industry departments. To mitigate the potential risk of heavy metals in incineration fly ash to the environment, the effects of Ca/(Si+Al) (molar ratio) on the crystalline phase composition, microscopic morphology, and surface functional groups of hydrothermally synthesized tobermorite using washed fly ash were investigated. Additionally, the changes in leaching concentration, leaching rate, liquid-phase mobility, morphological distribution, and environmental risk of heavy metals (Hg, Ni, Pb, Zn, and Cr) during the hydrothermal process were examined. The results demonstrated that Ca/(Si+Al) had a significant impact on the type of hydrothermal products, higher Ca/(Si+Al) molar ratio was favorable for inhibiting the formation of zeolite-like structures and promoting the formation of tobermorite. As tobermorite formed, the toxic leaching concentrations and leaching rates of the five heavy metals in the hydrothermal products gradually decreased. Compared to washed fly ash, the leaching concentrations of heavy metals in the hydrothermal products under the optimal ratio (1.1) condition decreased by 99.5%, 99.0%, 99.4%, 88.9%, and 63.7%, respectively, while the leaching rates reached as low as 0.25%, 0.08%, 0.01%, 0.01%, and 2.73%. Moreover, the migration of heavy metals to the liquid phase was limited to 1.41%, 4.28%, 0.29%, 0.05%, and 0%, indicating that most heavy metals were stably present in solid-phase products rather than migrating to the hydrothermal liquid. This was attributed to the formation of tobermorite, which increased the proportion of the five heavy metal residue states and decreased heavy metal mobility. The risk assessment code (RAC) results revealed that RAC of the five heavy metals in the hydrothermal products under the optimal ratio conditions was less than 10%, reaching an environmentally low-risk level. In summary, hydrothermal synthesis of tobermorite was a promising method for stabilizing heavy metals in incineration fly ash, providing a viable alternative for the safe disposal and recycling of hazardous wastes rich in heavy metals.