Study of detailed geochemistry of hazardous elements in weathered coal ashes

Abstract

The geochemical changes and the behaviors of the hazardous elements during the weathering of coal ash are not well known despite the potential toxicity of the hazardous elements leached from the ashes. In this work, ashes disposed in ash ponds were collected from different depths to investigate the effects of weathering on the geochemical changes and the mobilities of hazardous elements. The mineral phases identified in ash samples are quartz, mullite, K-feldspar, magnetite, illite, and glass phase. The contents of most major ions such as Si, Al, and K are strongly correlated with the changes in the mineral compositions of the ashes. However, the decrease in Ca and increase in loss on ignition (LOI) values with depth at both sites are observed owing possibly to the weathering of ashes. Our results show that leaching of hazardous elements caused by saturation with sea water is very important for the mobility of hazardous elements. Sequential extraction experiments show that high contents of hazardous elements are in the residual fraction, particularly in the raw coal ashes. For weathered ashes, a higher fraction of hazardous elements, especially As is bound to iron and manganese oxides, which increases significantly with depth. This indicates that As can be easily leached owing to the concentrated As on the surfaces of ash grains. Moreover, it can be strongly sorbed on iron oxide and hydroxide or co-precipitated with them. Therefore, magnetite and weathering products such as iron hydroxide can reduce the toxicity of As and other hazardous elements leached out during weathering.