GEOCHEMICAL CHARACTERIZATION OF COAL-COMBUSTION BYPRODUCTS UTILIZED AS CAPPING MATERIALS IN MINE RECLAMATION AND IMPLICATIONS FOR LONG-TERM WEATHERING IN THE AMBIENT ENVIRONMENT

Fixed scrubber sludge (FSS), a coal-combustion byproduct (CCB), is composed of flue gas desulphurization sludge, fly ash, and lime. FSS has potential usage as structural fill and capping material in abandoned mine land site because of its low permeability and acid-neutralization capacity. However, actual CCB usage has been limited due to concerns about toxic elements that are leached out in lab experiments. Although leaching experiments are useful for determining worst case situations, little is known about the how FSS weathers in natural environment and how rate might change over time.

Such long-term issues of how applying engineered CCBs might affect groundwater quality could be addressed using coupled physical-chemical models, but that requires detailed mineralogical composition and more explicit understanding of how trace elements occur and are distributed within FSS. In this study, FSS samples obtained from a mine reclamation site where FSS has been deposited as a barrier to oxygen ingress and groundwater recharge. The chemical phases, and the morphology were analyzed using scanning electron microscopy. The analyses show that the FSS is comprised of aluminum/calcium rich silicate amorphous spheroids, iron-rich ferroshperes, and other minerals. Four different types of amorphous spheroids were identified based on their morphology, including vesicular, solid, dendritic iron-coated, and crystalized iron-coated spheroids. The quantitative chemical compositions of the four types of spheroids were analyzed using electron microprobe, providing the requisite information for future modeling. Al₂O₃, SiO₂, Fe₂O₃ were the major oxides observed with other minor components.

Total digestion and sequential extraction was conducted to analyze trace element concentrations and their distribution in different phases. Total digestion results indicate that depletion of trace elements has occurred mostly along the upper surface of the FSS where it is in contact with oxygenated soil water, while the bottom of the FSS has experienced minimal weathering even though it is in contact with acid mine water. The highest concentration of the trace elements was contained in the residual fraction. The ICP-MS analysis suggest the mobility of the trace elements decrease in the following order: Bo, Cu>Cd, Zn, Pb, Cr>Ba>Ni>As.