Geochemical charaterization and reactive transport modeling of coal-combustion byproducts utilized as capping materials in mine reclation and implications for long-term weathering in the ambient environment.

摘要

Fixed scrubber sludge (FSS), is a coal-combustion byproduct composed of flue gas desulphurization sludge, fly ash, and quick lime. FSS has potential usages as structural fill and capping materials in mine land reclamation because of its low permeability and acid neutralization capacity. The Midwest Abandoned Mine Land Site, located in Pike County, Indiana, underwent reclamation in 1996 using FSS as a hydrologic barrier to isolate relict acid mine water. Sediment cores extracted from the reclamation site were analyzed in an effort to quantify the chemical phases (both primary and secondary) that constitute the FSS, and to evaluate any evidence of weathering that has occurred during the 18 years since emplacement. The primary solid phase is amorphous spheroids. Four different types were identified and their quantitative compositions were obtained, with SiO2, Al2O3, and FeO being the primary components. Sequential extractions also revealed the concentrations of As, B, Ba, Cd, Cr, Cu, Ni, Pb, and Zn in different fractions as indications of mobility. The highest concentration of the trace elements was contained in the residual fraction, which is consistent with the amorphous spheroids being very resistant to chemical weathering. A reactive transport model was developed based on the previously obtained experimental data. The model was calibrated and optimized using data from a column leachate experiment, and further validated by comparing a 17-yr long simulation to historical site chemistry data. The results of numerical simulations demonstrated the capacity of FSS to buffer acidic mine drainage by generating moderate alkalinity and also demonstrate that there are significant aqueous and mineral alterations occurring along the surface of the FSS. Moreover, the fate of several trace elements, including As, B, Ba, and Zn were investigated. Model results indicate there was a substantial initial increase in aqueous concentrations, which resulted from dissolution of host minerals or leaching from loosely adhered trace elements. However, the trace elements were subjected to subsequent sequestration by re-precipitation or surface adsorption onto adsorbents, and thus the likelihood to exceed maximum contaminant level (MCL) is low.