Identifying mechanisms for the mobilization and attenuation of elemental constituents from weathered alkaline coal ash using geochemical modeling

Nearly 70% of coal combustion products (CCPs) produced annually in the United States (110 million tons in 2001) are disposed of in saturated ash impoundments or ash landfills. The large volume of disposed CCPs and their potential to leach toxic trace metals (e.g., As, Se, Hg, Pb, and Cr) at concentrations above established groundwater quality standards is a major concern. A large amount of empirical data is available pertaining to the effects of CCPs on water quality; however, only relatively few studies have attempted to develop a mechanistic approach to predicting the chemical properties of leachate derived from weathered coal ash. Emphasis of this work was on identifying the mechanisms of attenuation of major and trace elements in weathered alkaline coal ash residues under a range of environmental conditions representative of those found in alkaline ash impoundments. Experimental and theoretical analysis focused on utilizing (1) existing data on the chemical and mineralogical properties of coal ash residues; (2) extensive characterization of the chemical and mineralogical properties of weathered coal ash collected from multiple surface impoundments; and (3) geochemical equilibrium modeling of leachate derived from these ashes under a range of pH and redox conditions characteristic of those found in alkaline ash impoundments. Synthesis of the literature suggested that although process variables of coal combustion are often highly variable, the thermal transformation reactions that occur during coal combustion are predictable and lead to the formation of ash residues with a predictable and similar mineralogy. For most major elements and several environmentally relevant trace elements (e.g., Ca, Si, Al, Fe, K, Mn, Cu, Se, and Cr), leachate concentrations derived from ash residues from multiple alkaline impoundments under conditions likely to exist in alkaline ash disposal facilities were shown to be controlled by the precipitation/dissolution of discrete mineral phases present in fresh ash residues or that form during weathering. With only a few exceptions, the solubility behavior of these elements was similar for ashes from multiple alkaline impoundments independent of age and to data previously reported for freshly combusted and moderately weathered ashes. These observations strongly suggest that the solubility behavior of alkaline ash residues may be consistent throughout the operational life of a typical ash impoundment. Therefore, a mechanistic approach using geochemical equilibrium modeling of the minerals identified as solubility controls may be used in predicting the composition of ash leachate in field settings, regardless of the age of an ash impoundment or the degree of weathering of the ash residues contained within the impoundment