Leaching of Inorganic Selenium Species from Coal-Combustion by-Products

Selenium found in coal utilization by-products, such as fly ash, can be released into the environment. Analysis of the species present is crucial because Se toxicity and mobility are species dependent. The leaching of Se from four fly ash samples was measured in column experiments using acetic acid, nitric acid and sodium carbonate solutions. The extractants were analyzed for total Se as well as oxyanions selenite (SeO₃^-2) and selenate (SeO₄^-2). Speciation was conducted using a reverse-phase column coupled to a Perkin Elmer ICP-MS with DRC system. These oxyanions have been reported as the most likely Se species associated with fly ash samples (Mattigod and Quinn, 2003). Total selenium was also analyzed using the ICP-MS.

Initial results produced an increased leachability of the inorganic species with sodium carbonate compared to the other solutions. There is mass balance deficit between the total Se and the sum of the inorganic Se-species concentrations for nearly all supernants analyzed. This indicates that there is a loss of species during the extraction or speciation, or there are other species present that are not accounted for using this method such as elemental selenium or organic phases, or instrument interferences. The presence of a deficit has been reported by other researchers (EPRI, 2006) although many studies have been focused on these species alone (EPRI, 2006; NSMP, 2006). Recovery and analysis of the wash solution, extension of analysis time, and analytical adjustments will help in determining if the loss of species is due to precipitation or lack of identification.