Trace Element Release from Coal Fly Ash: Quantitative Geochemical Modeling Using Leaching Tests.

Thermal process residues such as coal fly ash are produces world-wide in ever increase quantities. Thus, the impact of their storage must be considered. Quantifying effective mass fluxes of trace elements such as As, Zn and Cr from waste material into groundwater systems is an important prerequisite to assess the potential for the contamination of drinking water resources. In that context leaching tests play an important role for the assessment of long-term impact of contaminated materials on groundwater and particularly for estimating long term solute fluxes.

Two scale of leaching tests (batch and upward percolation column) were performed in order to identify and quantify the main transport phenomena and chemical processes. A coupled chemistry-transport modeling (PHREEQC, iPHT3D) was developed and applied to study the leaching of coal fly ash in order to assess the trace element (As, Zn and Cr) long-term behavior, and compared to the experimental data.

At each scale of experiment, dynamic processes have been identified and quantified. In batch experiment we found that dissolution minerals were controlled by equilibrium or fast dissolution. However, we noticed in column experiment that this fast dissolution was not sufficient to explain the long-term leaching. Slow dissolution kinetics has been identified for phases like Albite, Ettringite and Portlandite. The geochemical model helped to highlight factors such as increase of pH, redox potential and precipitation of certain minerals. Those factors are essential as they play an important role on long-term leaching of trace element. Direct long-term extrapolation from batch to large scale or time would lead to erroneous results if these factors are not taken into account.