Occurrence and Mobilization Potential of Trace Elements from Disposed Coal Combustion Fly Ash

Abstract

Despite many beneficial properties, high amounts of fly ash (FA) from coal-fired power plants have been disposed in landfills. These amounts are continuously growing in consequence of the limited possibility to utilize all generated FA due to the lack of market for it, or too high shipping/handling costs compared to the natural competing materials. Besides landfilling, the increasing utilization of FA as agricultural soil amendment that give an opportunity to dispose this material in a big scale also leads to its spreading on the vast land surface where it is exposed to the atmospheric conditions. To assess the effect of weathering processes on the mobilization potential of trace elements in disposed FA, the unique studies were carried out that comprised sampling FA along the vertical profile of 12 years’ old FA pond in the post-closure period, extracting pore solution from the material by a pressure method and direct analysis of its chemical composition using ICP-OES technique. Studied FA represented alkaline aluminum silicate material of a composition and trace element enrichment within the range typical for FA from the majority of other hard coal-fired power plants. The chemical composition of pore solutions along the three vertical profiles in the fly ash disposal pond in the post-closure period after 12 years’ operation was found to reflect both the altered water flow (vertical downward redistribution of ions) and the changed equilibria conditions. While the pore solution in FA in H6 profile reflects the Dissolution stage (II) (pH 7–10), in the looser FA profiles H-2 and H-3 it indicates alteration of buffering properties of the system that can be defined as Delayed Release (III) stage. The character of pore solutions along these profiles suggests that the major buffering mechanisms controlling pH after depletion of carbonates comprise reactions involving hydrolysis of aluminum ions from amorphous phase exposed to the direct contact with percolating water due to the devitrification of glaze, with further formation of the secondary minerals. These processes in simplified form can be described as reactions between dissolved silica, water, as well as kaolinite and gibbsite at the stage of their formation. This caused high non-linear release of trace elements from FA and significant qualitative/quantitative increase of its contamination potential with respect to the ground water and soils in adjacent area (decrease of pH to min. 4.3–4.5, and delayed extensive release of Zn, Fe, Mn, Mo, Cd, Cr, Be, B, Vin high concentrations). In conformity with pH-Eh-stability fields metals can be grouped according to the similar release-dissolution response to controlling parameters, e.g.: I: (Zn-Cd-W-Be); lI:(FeMn) (reverse pH-dependent increase). Several metals (mainly oxyanions with broad fields of aqueous species) show weak influence of pH-Eh parameters (Li, Mo, Se, Sr, B), while Al, Cu and V are immobilized at pH 4.3–5.0. The screening study proved (i) possibility of FA acidification and discontinuous non-linear time-delayed increase of its pollution potential to the hazardous level due to weathering transformations (ii) necessity of life-cycle screening/monitoring of FA disposal sites for trace element release as a function of controlling factors along the vertical profile of anthropogenic or natural vadose zone. The results suggest also caution in use of FA as acidic soils improver.