Fate of Metals in Fly Ash During Aging in Laboratory-Scale Ash Impoundments

Release of metals from fly ash stored in ash impoundments can pose environmental risks. Laboratory-scale impoundments loaded with Class C and Class F fly ashes were studied to investigate the release of metals during aging, changes in the mineralogy of the ash, and associated changes in the leachability of the metals. The water column above the settled ash was sampled over 9-12 months, and cores of ash were regularly collected from one of the Class C ash impoundments and subjected to a sequential extraction procedure that targeted metals bound by specific mechanisms. The long-term nature of experiments and integration of aqueous- and solid-phase analyses are unique aspects of the study that yielded insights into the fate of metals during fly ash storage and disposal. For Class C ashes, high concentration of alkaline solids led to pH values as high as 11.6 that gradually declined as the impoundment took up carbon dioxide from the atmosphere. A considerable fraction of the selenium in the ash (similar to 50%) was rapidly released to solution, and measurable release of chromium (>0.7%) to the water column also occurred quickly. Slower release of arsenic was observed. For the Class C ash impoundment with sampled cores, the total concentrations of arsenic and chromium did not change substantially, but these elements became more resistant to chemical extraction with time. The high pH maintained in the ash resulted in the formation of solid phases that may reduce metals release