Characterization and modeling of toxic fly ash constituents in the environment

Coal fly ash is a by-product of coal combustion that has drawn renewed public scrutiny due to the negative environmental impacts from accidental release of this waste material from storage facilities. Historically, the leaching of toxic elements from coal fly ash into the environment has always been a major environmental concern. Despite extensive efforts into the characterization of coal fly ash, effective models for the fate and transport of toxic fly ash constituents have remained lacking, making it difficult to perform accurate environmental impact assessment for coal fly ash. To close this critical knowledge gap, the overall objective of this study was to develop a predictive model for the leaching of toxic elements from fly ash particles. First, physical properties of coal fly ash were characterized to evaluate their contribution to elemental transport. Unburned carbon was shown to contribute to the sorption of arsenic to fly ash, which slowed the release of arsenic from fly ash. In parallel, leaching properties of various elements were determined to differentiate species of varying leaching capacities, demonstrating that the majority of toxic elements were not mobile under environmentally relevant conditions. Subsequently, a mechanistic model for the dissolution of fly ash elements was developed and validated with batch kinetics studies. Furthermore, elemental dissolution was integrated with hydrodynamic modeling to describe the leaching of toxic elements from fly ash in dry disposal facilities, which was validated by column studies. The mechanistic model developed and validated in this research represents the first such model that successfully characterized the complex processes underlying the release and transport of toxic elements in coal fly ash, providing a valuable tool to predict the environment impact of coal fly ash and develop more effective management practices for both the industry and regulators.