Leaching Kinetics of Heavy Metals from Municipal Solid Waste Incineration Fly Ash: A Langmuir-Based Model

Abstract Langmuir kinetics are widely used to describe adsorption processes. In this study, a novel leaching kinetic model based on Langmuir kinetics was developed to simulate the dynamic leaching behavior of heavy metals from fly ash. The initial conditions and equilibrium concentrations of the Langmuir-based model differ fundamentally between adsorption and leaching. The proposed model can be simplified to the conventional pseudo-first-order model under two limiting conditions but cannot be reduced to the pseudo-second-order model. Leaching experiments were conducted on two fly ash samples with Zn, Pb, Cd, Cu, Ni, and Cr identified as the predominant heavy metals. The proposed Langmuir-based model, along with the pseudo-first-order and pseudo-second-order models, was used to fit the experimental data. All of the models described the data with comparable accuracy. Leaching rates were also simulated by using all models. Zn, Pb, Cd, and Cu exhibited high initial leaching rates, indicating a significant short-term leaching risk. In contrast, Ni and Cr leached at considerably lower rates but continued to release beyond 18 h without reaching equilibrium. This persistent release suggests that standard leaching protocols may underestimate the leaching toxicity of these more stable metals, implying a potential long-term environmental risk.