Chemical Partitioning and Mobility of Trace Elements in Dry Disposed Weathered Ash Conditioned with High-Saline Effluents

Authors

S.A. Akinyemi, Fossil Fuel and Environmental Geochemistry Group, Department of Earth Sciences, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
A. Akinlua, Fossil Fuel and Environmental Geochemistry Group, Department of Earth Sciences, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
W.M. Gitari, Environmental Remediation and Water Pollution Chemistry Group, Department of Ecology and Resources Management, School of Environmental Studies, University of Venda, Private Bag, X5050, Thohoyandou 0950, South Africa
R.O. Akinyeye, Environmental and Nano Sciences Group, Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
L.F. Petrik, Environmental and Nano Sciences Group, Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa

Document Type

Article

Abstract

Weinvestigated the mobility of inorganic elements in the ash dump due to chemical interaction of weathered dry disposed ash conditioned with high-saline effluents and ingressed CO2 from atmosphere and percolating rainwater. Drilled ash core samples collected from dry disposed ash dump at a South African coal-burning power station were characterized using X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses. A modified sequential extraction procedure was selected to determine the mineralogical association of the investigated elements in the 1-year-old drilled ash cores. Major mineral phases are quartz and mullite and other minor phases included hematite, lime, calcite, anorthite, mica, and enstatite. Mica and calcite in the ash cores is attributed to carbonation process, which led to long-term reduction in pore water pH. The 2-week-old ash cores are sialic but the 1-year-old ash cores were both sialic and ferrocalsialic in chemical composition. The ferrocalsialic nature of 1-year-old ash cores could be attributed to drastic changes in feed coal in the power station. The chemical index of alteration (CIA) and chemical index of weathering (CIW) values suggest a relatively high degree of weathering. The changes in CIA and CIW values in the ash cores depend on the pore water pH, leaching rate, carbonation process, and possibly the conversion of the alkali and alkali earth metals into carbonates. Multivariate analysis results suggest that the major oxides and carbon percent show differences and have a greater contribution to the differentiation in the 1-year-old ash cores. Trace elements such as Mo, Cr, and Se show high mobility but As, B, and Pb exhibit fairly low mobility in the water-soluble fraction. Aconsiderable mobility of trace elements in the exchangeable and carbonate fractions is attributed to the alkaline nature and pore water pH. A decreasing response of As, Mo, Cr, and Pb in the reducible fraction with depth suggests immobility attributed to coprecipitation with the Fe- and Mn-oxide phases. On the contrary, the increasing response of Pb, Mo, and B in the carbonate fraction with depth implies mobility due to dissolution and flushing of soluble major chemical phases, as evident in the pore water pH. A considerable amount of B and Pb are concentrated in the residual fraction of 1-year-old ash cores. This suggests that, under natural settings, these elements will not be released in solution over a long time; therefore, they are not considered to be an environmental risk.

First Page

57

Last Page

72

DOI

10.4177/CCGP-D-13-00002.1

Publication Date

1-1-2013

Recommended Citation

Akinyemi, S.A., Akinlua, A., Gitari, W.M., Akinyeye, R.O., Petrik, L.F., 2013, Chemical Partitioning and Mobility of Trace Elements in Dry Disposed Weathered Ash Conditioned with high-Saline Effluents. Coal Combustion and Gasification Products 5, 57-72, doi: 10.4177/CCGP-D-13-00002.1