Chemical Weathering and Mobility of Inorganic Species in Dry Disposed Ash

The burning of low-grade coal produces vast amounts of ash and other solid by-products. In South Africa, coal combustion by-products undergo disposal on land as dry heaps or slurried to dams. In this study, the geochemical partitioning, transport, and mobility of elements in dry disposed ash dumps were investigated using a modified sequential extraction scheme. The chemical and mineralogical compositions of 50 drilled core samples were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), and inductively coupled plasma mass spectrometry. Ternary plots of major elements as determined by XRF showed that the 15-year-old and 2-week-old dry disposed ashes are sialic, and the 4-year-old ash cores are sialic and ferrocalsialic. The relationship between SiO2 and the chemical index of alteration showed a moderate to high degree of weathering. The variation in chemical compositions and degree of chemical weathering are ascribed to differences in the chemistry of feedstock coals and ash/water/CO2 interaction chemistry. Multivariate statistical analysis revealed the subtle chemical alteration differences and disparity in dissolution of major components of fly ash. The pH profile of the cores indicates that contact with atmosphere and consequent ingress of CO2, leaching by rainwater, and point of saturation have a great effect on the chemical weathering of the dry disposed fly ash. XRD analysis of two weathered drilled core samples taken from 4-m depth intervals showed the presence of calcite. The chemical interaction of fly ash with ingressed CO2 from the atmosphere will ultimately lead to a reaction with buffering components such as CaO and the subsequent conversion into calcite. The modified geochemical partitioning scheme revealed that relative enrichment and depletion in the inorganic elements at various depth sections of the ash dump are governed by chemical interaction of fly ash with ingressed CO2 from the atmosphere, porewater pH, leaching by percolating rainwater, fluctuation in water level (i.e., weathering over time), heterogeneity in fly ash, continuous irrigation of fly ash by high saline effluents, and fresh water.