Effect of weathering transformations of coal combustion residuals on trace elements mobility in view of the environmental safety and sustainability of their disposal and use. II. Element release.

This paper is the second one of two companion papers. It presents results of a study aimed at assessing the effect of real time weathering transformations of Coal Combustion Residuals (CCRs) on trace element binding/release and its environmental implications. The study is based on the chemical composition of pore solutions extracted from primary alkaline Class F CCRs, 0 to >40 years old, sampled from the surface layer and vertical profiles at four selected typical CCRs impoundments. The long-term weathering transformations were found to lead to gradual acidification to pH < 4 of this primary alkaline material, due to internal processes of mineral formation/dissolution. Direct analysis of the pore solutions and a statistical analysis have shown different susceptibility of many trace elements to release during internal acidification processes occurring at consecutive Wash-out I (pH > 8), Dissolution II (8 ≥ pH ≥ 7) and Delayed Release III (pH < 7) stages of weathering compared to that at external sources of pH. The elements occurring in the CCRs are represented by three major groups showing the highest release to pore water: (a) within the acidic pH range (Na, K, Zn, Fe, Cd, Mo, Cr, B, Mn, Be and Ni; (b) within the near-neutral pH range (Al, V, Ba, Cu and Ag) and also Sb, Hg and Co not analyzed at pH < 7; (c) within the alkaline pH range (Ca, Mg, Pb, As, Se, Tl). Elements whose concentrations exceeded the threshold values for good chemical status of groundwater (TVs) at all weathering stages over the entire pH range studied were K, Al, B, Cr, Mo, V, As, Se, Sb and Hg, while Na, Zn, Fe and Cd showed particularly high delayed release at pH < 7, thus confirming the need of a precautionary approach to CCRs uncontrolled disposal and bulk reuse as common fill in view of long term environmental safety and sustainability.

Effect of weathering transformations of coal combustion residuals on trace element mobility in view of the environmental safety and sustainability of their disposal and use. I. Hydrogeochemical processes controlling pH and phase stability.

Coal combustion residuals (CCRs) are one of the most abundant high-volume waste materials disposed in impoundments worldwide. Some methods of CCR recycling, e.g. their use as structural fill for low lying areas or as soil amendment, also expose this material to atmospheric conditions. Combustion processes result in concentration of trace elements in CCRs at about an order of magnitude compared to coal. In order to assess an effect of long-term weathering transformations of CCRs on trace element binding/release, a study has been carried out. It is based on the chemical composition of real pore solutions extracted from the most abundant primary alkaline Class F bituminous CCRs, 0 to >40 years old, sampled from the surface layer and vertical profiles at four different impoundments. In this part of the study, results of a hydrogeochemical simulation of the saturation state of real pore solutions with respect to mineral phases of CCRs with use of the PHREEQC program, related to actual pH values reflecting the full cycle of weathering transformations, have been discussed. This study is the first geochemical proof of the general trend towards a progressive acidification up to pH < 4 of primary alkaline CCRs due to release of protons during internal processes of formation of gibbsite and aluminosilicate minerals, buffered by carbonates at the alkaline - near-neutral stages, and followed by parallel dissolution and buffering by aluminosilicates at pH < 7 after carbonate depletion, to the level up to pH∼3.5-4.0. The intrinsic geochemical changes have resulted in the different susceptibility of trace elements to release and associated changes in risk to the environment at consecutive stages of weathering.