SOLID PHASE ASSOCIATIONS AND LEACHABILITY OF TRACE ELEMENTS ASSOCIATED WITH COAL FLY ASH

Coal fly ash buried in dry landfills and in ash ponds is a potential source of trace elements to the environment. Many older ash disposal facilities are unlined and have been closed for decades. At these sites, infiltrating meteoric water can react with the buried ash and leach toxic trace elements into the soil, groundwater and surface water. The trace elements As, B, Cr, Pb, Mo, Ni, Se, Sr and V are often abundant in fly ash and are of particular concern because of their low drinking water MCLs and negative human health effects. Understanding of the solid phase associations and speciation of trace elements in the fly ash is needed in order to predict the leachability of these elements from the ash and determine the potential impact of the unlined disposal facilities on the environment.

A seven step sequential chemical extraction procedure (SCEP), modified after that of Filgueiras et al. (2002), was used to determine the fractionation of trace elements in four fly ash samples. Three of the ash samples were derived from the combustion of Eastern Bituminous coal, and the fourth ash was derived from combustion of Powder River Basin coal. Although differences in trace element fractionation exist between the ash samples, some element associations can be generalized. Calcium, B, Mo, V and Ba are primarily associated with the water soluble and/or exchangeable ash fractions and would be highly mobile. Arsenic and Pb are primarily associated with the amorphous oxides of aluminum in all of the fly ash samples. Chromium and Se are largely contained in the crystalline iron oxide fraction. Arsenic, Cr, Pb and Se would therefore be expected to have low mobility in oxidizing environments, but to be potentially mobile under reducing conditions. Nickel shows different fractionation between the four ash samples; therefore, its environmental mobility would be expected to vary correspondingly. These predictions correlate with the relative mobilities of trace elements observed in column leaching experiments. This study shows that the 7-step SCEP provides valuable information about the associations of trace elements within fly ash. This data will allow better prediction of trace element mobility under leaching conditions in the environment.