Influence of DOM and redox potential on the leaching of As and Cr from coal fly ash

Coal ash, a byproduct of coal-fired power plants, contains toxic trace elements such as arsenic (As) and chromium (Cr). Coal ash has recently been scrutinized as a potential source of toxic trace elements to aquatic systems and potable water sources, and the legislation pertinent to coal ash management is currently under revision. Coal ash is currently stored in surface impoundments and landfills that are poorly regulated and at risk of failure. Impoundment failure can result in the mobilization of coal ash and leachates into aquatic systems and potable water-sources. The current understanding of the environmental fate (i.e., transformation, toxicity and mobility) of As and Cr in coal ash is largely limited to leaching protocols that are not environmentally relevant, as they exclude parameters such as redox potential and dissolved organic matter (DOM) that are prevalent in aquatic systems. Furthermore, the relationship between coal-ash particle size and the speciation and leaching behavior of As and Cr has not been well investigated. The size of host particles may influence the speciation and coordination environment of trace elements, and may be a critical factor in the leaching/dissolution behavior of As and Cr from coal ash into solution. In this study, coal ash samples from three different coal-fired power plants using different coal sources and different combustion processes were segregated into size fractions (< 1 mm to ≥ 100 μm, < 100 μm to ≥ 10 μm, < 10 μm to ≥ 1 μm, and < 1 μm) using a combination of dry sieving and particle impaction. Coal ash size fractions were examined using synchrotron x-ray absorption spectroscopy (XANES/ EXAFS) to determine whether there were any differences in As and Cr concentration and speciation/coordination environment as a function of particle size. Coal-ash size fractions were also exposed to a buffered solution (pH ~7) with varying DOM concentration (1 to 30 mg/L) and redox potential (reducing, oxic). Dissolved concentrations of As and Cr were quantified over time to determine how DOM and redox potential influence the leaching of As and Cr from coal ash into the aqueous phase. Aliquots of leached coal ash were also obtained for synchrotron XANES/ EXAFS to determine: (1) how the speciation and coordination environment of arsenic associated with coal ash alter under the DOM and redox conditions tested; and (2) how these changes in the speciation and coordination chemistry of As and Cr associated with coal ash control the leaching of these elements. Preliminary XANES and EXAFS data suggested that As speciation and coordination environment do not vary as a function of coal ash particle size but rather as a function of coal source. As and Cr also appeared to be concentrated in the smaller size-fractions (e.g., < 100 μm to ≥ 10 μm). Data obtained from preliminary leaching experiments (t = 4 days, pH ~7, no DOM, and oxic conditions) indicate that both As and Cr leach from coal ash with concentrations of 200 μg/L dissolved total As and 150 μg/L dissolved total Cr obtained in the leachate. This study will help in the development of new coal ash regulations and policies regarding the management of coal ash by providing data on the environmental fate and behavior of As and Cr associated with coal ash produced from different coal sources and using different combustion processes.