Microscopic insights into acid corrosion effects on chelated MSW incineration fly ash: Mechanisms of chelate destabilization

Sodium dimethyl dithiocarbamate (SDD) is commonly applied to mitigate the release of heavy metals (HMs) into the environment from municipal solid waste incineration fly ash (IFA). Despite this common application, limited research has been conducted on the impact of environmental conditions on chelate destabilization during long-term landfilling. In this study, a batch leaching test was performed on chelated IFA buried for 10 years, revealing that the leaching concentrations of Pb and Cd exceeded landfill standards by 51.7% and 26.0%, respectively, indicating the chelate destabilization occurrence. To elucidate this phenomenon, various physicochemical experiments were conducted to unveil the underlying mechanisms emphasizing changes in HMs fractions (Pb, Cd, Cr, Ni, Zn, Cu) under acidic corrosion, and exploring the microscopic perspective of the chelate destabilization process. The key findings were summarized as follows: (I) The immobilization effect of SDD on Zn and Cd was not highly effective, with most of these metals being fixed on IFA through inorganic complexes. Theses HMs destabilization showed a negative correlation with the pH levels, indicating that higher acidity facilitated the release of HMs (59.0 mg/kg for Zn and 5.8 mg/kg for Cd). (II) After exposure to acid corrosion, the stable fraction of HMs decreased, because HM-SDD complexes decomposed into inorganic salts and carbon disulfide. This process further exacerbates the transporting effect of HMs on chelated IFA, thereby increasing the long-term leaching risk. (Ⅲ) A leaching equilibrium of HMs existed at the IFA surface, primarily influenced by the types and concentrations of anions in the leachate. This study contributes to a deeper understanding of the factors influencing the stability of chelated IFA and provides valuable insights into the long-term environmental implications of such materials in landfill settings.