Leaching Characteristics of Arsenic from Aged Alkaline Coal Fly Ash Using Column and Sequential Batch Leaching

Abstract

Leaching of arsenic from ash in coal fly ash disposal facilities is a cause for concern due to possible contamination of groundwater. Sequential batch and column leaching experiments were carried out to determine the leaching of arsenic from ash found at a retired ash impoundment basin near a coal-fired power plant. This study assessed the leaching characteristics of arsenic from aged alkaline coal fly ash as a function of ash properties and ash depth. Comparable levels of arsenic leached from the ash in column leaching (10.5%) and sequential batch leaching (11.1%) after 14 days (7 stages) of leaching, while less arsenic (17.8%) leached for column leaching than for sequential leaching (22.0%) after 42 days (21 stages) of leaching. The leaching characteristics of arsenic in column leaching and sequential batch leaching were similar, where more than half of the leaching of arsenic occurred within 14 days of leaching with the maximum leaching of arsenic occurring during the second and third stages of leaching. More arsenic leached from the deeper ash samples in sequential batch leaching of ash samples from various depths. Arsenic leached out mainly as calcium hydrogen arsenate from the calcium arsenate present in ash. The short-term (2-day) leaching of arsenic from ash was limited by the solubility of calcium arsenate, while the long-term leaching of arsenic was limited by the leaching of calcium bicarbonate which controlled the leachate pH. The source of calcium bicarbonate in the leachate was the amorphous calcium carbonate in the ash.