Chloride corrosion destabilizes chelation of fresh and aged MSWI fly ash: Mechanism and long-term behavior

Chloride ion (Cl^-) contributes to the chelated incineration fly ash (CIFA) destabilization, yet there is limited research available on the effect of exogenous Cl^- corrosion. This study conducted 60-day column leaching experiments on fresh and aged CIFA (CIFA-F and CIFA-A), utilizing NaCl solutions at concentrations of 0 wt%, 1 wt%, and 3 wt%. It investigated the leaching behaviors of typical heavy metals (HMs) including lead, chromium, and nickel, associated with the leaching features like contents of calcium and dissolved organic matter (DOM), electrical conductivity, and pH. These leaching features were influenced by Cl^- level through buffering and salting-out effects, indirectly affecting HM leaching. HM leaching followed a multi-step mechanism: Initially, HM leaching was primarily controlled by outer-sphere ion exchange and diffusion. As the process transitioned, the presence of Cl^- hindered the incorporation of OH^-, affecting the formation of secondary minerals like Ca₂Al(OH)₆(H₂O)₂Cl. This decreased the net charge and specific surface area, reducing CIFA adsorption capacities towards HMs and DOM. Eventually, large quantities of DOM reacted with HM forming non-adsorptive complexes or colloids. Compared to CIFA-F, the more porous structure in CIFA-A that resulted from carbonation may enhanced Cl^- interaction with the internal composition, escalating HM long-term leaching risks. To predict future HM leaching behavior, five machine learning models based on the experimental results were constructed, moving beyond traditional decay models. The multi-output long short-term memory model showed best performance (R²> 0.85, MAE < 5.00 %), confirming its superiority. This study offers microscopic insights into the mechanisms of Cl^- corrosion causing CIFA destabilization and advances predictive approaches for HM leaching behaviors.