THE EFFECT OF DISSOLVED ORGANIC MATTER ON THE RELEASE OF TRACE ELEMENTS FROM COAL ASH IN NATURAL SURFACE WATERS

Major releases of coal fly ash to rivers and streams, such as the release from the Tennessee Valley Authority’s Kingston Fossil Plant into the Emory and Clinch Rivers in December, 2008, may be a concern because as coal ash makes contact with water there is potential for mercury and other trace metal to be released at elevated and potentially toxic levels. Dissolved organic matter (DOM) plays important roles in the dissolution of oxide, sulfide, and aluminosilicate minerals; oxides and aluminosilicates make up the majority of the coal ash composition. While DOM is understood to play a role in mineral dissolution, no work has been done to study the effect that DOM plays in the leaching of trace metals from coal ash.

In order to understand the effects of DOM on metal leaching from coal fly ash, a series of batch experiments were done in the presence and the absence of DOM at a neutral pH using coal ash generated at the Kingston Fossil Plant. Hydrophobic and transphilic organic acid isolates from the F1 site in the Florida Everglades and Williams Lake, MN were added in increasing concentrations. These isolates were chosen based on their contrasting SUVA₂₅₄ values, which is a proxy for DOM aromaticity and shows a positive correlation with mineral dissolution rates. Calcium was also added in increasing concentrations to the ash in the presence of ~20 mg DOM L^-1 because calcium is known to interact with DOM and may potentially affect the release of mercury. The filtered ash leachate was analyzed for major and trace metals, anions, dissolved organic carbon, UV absorbance, and pH.

Results indicate that DOM plays an important role in the leaching of mercury from fly ash. As the concentration of the F1 hydrophobic acid isolate increased from 0-40 mg L^-1, the concentration of mercury released from the ash increased from non-detectable levels to 18 ng g ash^-1. When comparing identical experiments conducted using DOM isolates of different SUVA₂₅₄ values, significantly greater mercury release was observed in the presence of DOM with higher aromatic content. Increasing the calcium concentration in the solution was found to inhibit the release of mercury from the fly ash.