Trace Elements in Water-Soluble Fractions from Bulgarian Coal Fly Ashes and Their Technological and Environmental Importance

The composition of nine Bulgarian coal fly ashes (FAs) and concentrations of 69 elements in their water-soluble leachates were studied to elucidate the presence and associations of trace elements. Each FA or element has individual leaching behavior and is strongly dependent on the mineral composition of FAs, especially the abundance of water-soluble minerals among sulfates, oxyhydroxides, and carbonates, and, to a lesser extent, certain less soluble silicates, glass, and char. The water leachates have neutral to slightly alkaline pH values (7.2-7.7) and salty to brine total mineralization (0.6-9.0%). Most elements are insoluble in water or reveal insignificant (<0.1%) to low (0.1-0.9%) water extractable proportions from FAs. However, elements such as Ca, Cl, Li, Mg, Mo, Na, S, Sb, and Sr have significant (1-10%) to high (>10%) water-leaching proportions. Fourteen minor and trace elements, namely, As, B, Cr, Ge, Hg, Li, Mo, P, Rb, Sb, Sc, Sr, Ti, and V, have high to extremely high enrichment factors (10-1776 times) for FA water leachates in comparison with the Clarke values for surface water. The leaching behavior of these 14 elements was described due to their potential industrial interests. Another assemblage of elements in some FA leachates (Al, As, B, Ca, Cr, Fe, Hg, S, Sb, and Se) reveals contents higher than those of the water-quality guideline for drinking water. However, the FAs studied normally belong to inert wastes for landfilling (excluding Sb and Se for specific FAs and total dissolved solids) according to the waste acceptance criteria. Therefore, some FA leachates could provoke some environmental and health concerns during FA storage, weathering, and water processing.