The leachability of major elements at different stages of weathering in dry disposed coal fly ash

Large quantities of solid residue are generated by coal-fired power stations in many parts of the World. The disposal and management of the unused fly ash remains a major problem to the environment. The weathered dry disposed ash cores comprise of major element constituents such as Al, Si, Ca, Mg, Fe, Mn, Na+ and K+. The mobility patterns and mineralogical associations of major elements in weathered dry disposed ash dumps aged 1-year-old, 8-year-old and 20-year-old from a coalfired power station in South Africa were investigated using a modified sequential extraction scheme. The extraction sequence was as follows: (1) water soluble, (2) exchangeable, (3) carbonate, (4) Fe and Mn and (5) residual. A total acid digestion was carried out on the original sample prior to extraction to validate the extraction procedure. The distribution of Si, Fe, Mn, Ca, Mg, Na+, and K+ in 59 drilled ash core samples was determined by inductively coupled plasma mass spectrometry. The leachability of the seven elements from different fractions proved to be different; so various distribution patterns have been achieved. The highest concentration of analytes is recorded in the water soluble, exchangeable, and carbonates of 1-year-old ash cores hence it is the least leached. The concentration of each element in each fraction was calculated as a percentage of the total metal content for the 1-year-old ash cores. The average amount of the major elements in the easily soluble fractions of 1-year-old ash core samples are: water soluble: Na (21%) . Ca (10.2%) . Mn (8.4%) . Si (4.0%) . K (2.58%) . Mg (0.05%) . Al (0.003%) . Fe (0.001%), exchangeable: Ca (37.04%) . Mg (12.6%) . Na (11.26%) . Mn (10.3%) . K (3.17%) . Si (1.6%) .Al(0.27%) . Fe (0.33%), carbonate: Mn (41.21%) . Ca (37.9%) . Mg (32.9%) . Al (29.25%) . Si (25.39%) . Fe (21.39%) .Na(2.6%) . K (2.23%). The mobility of major elements in the weathered ash dumps are influenced by heterogeneity in the ash dump, inhomogeneous continuous brine irrigation and chemical interaction of ash cores with ingressed CO2 from atmosphere and percolating rain water.