Mineralogy and Mobility Pattern of Trace Metals in Brine Irrigated Dry Disposed Weathered Coal Fly Ash

Abstract

This study determines mineralogical association of trace elements in the ash core samples and provides insight into various factors that promote their mobility and eventual release into surface and groundwater systems. Fly ash from a power station ash dump in Mpumalanga Province (South Africa) was characterized using X-ray diffraction and X-ray fluorescence. A five-step sequential extraction procedure was used to determined mineralogical association of elements as a function of depth in the ash dump. The dry disposed ash cores are sialic in chemical composition. The mineral peak height as revealed by X-ray diffraction analysis showed variations due to heterogeneity in the ash dump and in homogenous continuous brine irrigation. The sequential extraction results showed that some trace elements are moving in a leaching pathway through the fly ash in the water soluble fraction. Toxic analytes are present in water soluble, exchangeable, and carbonate fractions. Observed Pb concentration maxima in the carbonate fraction of ash cores suggests mobility due to reduction in the pore water pH. A relatively high concentration of As and Se in the exchangeable fraction indicates their possible release when fly ash comes in contact with slightly acidic rain water. This is of major environmental concern for possible contamination of surface and groundwater systems. The relative enrichment of As and Se in the residual fraction suggests that the dust from 8-year-old brine irrigated ash dump would be toxic to human health. This investigative study demonstrates that reduction in pore water pH, inhomogeneity in the ash dump, continuous brine irrigation, leachate from topsoil, moisture content level, ingressed CO2 from atmosphere, and percolating rain water are factors that promote trace metal mobility and leaching in the brine irrigated dry disposed ash dump.