Selenium and Weathering of Wet-Stored Coal Fly Ash: Analyses of Data From Two Coal Fly Ash Spills.

Two recent coal fly ash spills have provided data suggesting a chemical and mineralogical basis for observed releases of selenium (Se) from disturbed wet-stored coal fly ash. Following a 2005 spill and emergency responses ground water showed two Se peaks. The first was due to emergency pumping of fresh ash slurry into a secondary ash basin. The second peak appeared about one month later, lasting several months. Ash and groundwater pH were near neutral. Water and TCLP extracts of selected ash samples did not show troubling selenium levels, but 0.1 N HCl-extractable Se ranged widely. Elevated HCl-extractable Se occurred in ash samples with slightly lower redox potentials. Microscopic examination showed substantial amounts of apparent oxyhydroxide gels. Prior to the spill related emergency use of the secondary basin, the ash had locally stable redox conditions, 230-280 mV in wet-saturated volumes, >280 mV in better aerated areas. At near neutral pH, Se(IV) undergoes oxidation to Se(VI) at around 280 mV. Se(IV) is strongly adsorbed by oxyhydroxide minerals, Se(VI) about ten times less so. Before the spill, in the lower-redox zones, oxyhydroxide gels had selectively adsorbed and accumulated the Se(IV) weathering out of the ash. In the more aerated zones the Se(IV) had oxidized to Se(VI) and leached out, not accumulating in the oxyhydroxide phase. It was concluded that the observed second ground water Se peak was due to an increase in redox potential caused by the introduction and infiltration of large amounts of well aerated water during the emergency response. Scanning electron microscopic examinations of ash from a 2008 spill confirm the presence of oxyhydroxide gels and heavy cumulative loading of those gels with, at least, arsenic. Direct verification of elevated selenium in the oxyhydroxide gels is expected as SEM and chemical characterization efforts continue.