Glycine-mediated heavy metal leaching and calcium retention from municipal solid waste incineration fly ash: A mechanistic study

Traditional acid washing with agents like hydrochloric and acetic acid can effectively extract heavy metals from municipal solid waste incineration fly ash (referred to as "fly ash") but also dissolve calcium, complicating subsequent resource utilization. This study proposes the use of glycine as a green leaching agent to effectively extract heavy metals while retaining calcium under mild conditions. A systematic analysis was conducted to assess the effects of glycine concentration and temperature on the efficacy of heavy metal leaching. The concept of relative leaching efficiency was employed to evaluate the performance of various additives on fly ash. Notably, glycine improved the leaching efficiencies for heavy metals such as Cd, Cu, Ni, and Zn, bound to Fe-Mn oxides, by 79.8 %, 74.3 %, 36.3 %, and 64.5 %, respectively, compared to HCl-treated samples. Characterization studies and density functional theory (DFT) calculations demonstrate that glycine serves as proton donor and a chelating agent during the leaching process, enhancing the extraction of cationic heavy metals while minimizing calcium loss. These findings underscore glycine's potential role in advancing strategies for fly ash resource utilization.