Secondary mineral formation in coal combustion byproduct disposal facilities: implications for trace element sequestration

Abstract

Abstract Coal combustion byproducts (CCBs) are high-volume wastes produced by the electrical power industry and typically disposed of in landfills and lagoon impoundments. In the disposal environment, meteoric water or groundwater may percolate through and interact with the ash materials, producing leachate that contains elevated levels of many trace elements. Various geochemical reactions control the release of solutes and the formation of secondary minerals in CCB disposal facilities during weathering. Concern about the potential release of trace elements into the environment has motivated a large number of studies aimed at predicting the maximum concentrations of elements in leachate solutions. Secondary minerals formed during weathering of CCBs have the potential to limit the mobility of trace elements in an ash disposal facility. Geochemical modelling has been used by many investigators to predict the equilibrium concentrations of solutes in CCB leachate solutions and the stable secondary minerals that will form in weathered ash. Unfortunately, basic kinetic, thermodynamic and adsorption data are lacking for many solid phases, particularly those that may contain trace elements. In addition, secondary solid phases are very difficult to identify by direct analytical methods due to their low abundances and/or amorphous character, so it is often not possible to directly determine the identity and compositions of secondary phases in ash disposal environments. Despite these difficulties, numerous secondary solid phases have been directly observed and/or predicted to form via weathering reactions in CCBs. A tabulation of all secondary phases that have formed or have the potential to form in the CCB disposal environment is given, along with the relevant references. The potential for secondary phases to sequester trace elements via precipitation, adsorption and co-precipitation is discussed. Secondary phases with the greatest potential to limit the mobility of trace elements are amorphous Fe-oxyhydroxide and amorphous aluminosilicate phases, which are metastable precursors to Fe-oxide and clay minerals, respectively.