GEOCHEMICAL AND STRONTIUM ISOTOPE INVESTIGATION OF COAL UTILIZATION BY-PRODUCTS INTERACTING WITH AQUEOUS FLUIDS: LABORATORY LEACHING EXPERIMENTS

Coal combustion generates by-products such as fly ash, bottom ash, boiler slag, fluidized bed combustion ash and other solid fine particles, collectively referred to as coal utilization by-products (CUB). The majority of this material is disposed of in landfills or settling lagoons where it has the potential to interact with precipitation, runoff and groundwater. To understand the effects of CUB disposal on surface and groundwater geochemistry, we carried out a geochemical and strontium isotope investigation of coal fly ash and its effects on environmental fluids using both flow-through column leaching experiments (parallel columns with sodium carbonate, acetic acid and nitric acid) and sequential batch leaching experiments (water, acetic acid and hydrochloric acid). During column leaching, most major elements (e.g., Na, Ca, Mg, K, S) were released from fly ash samples early in the leaching procedure, indicating that these elements are associated with soluble phases or bound on surfaces. The delayed release observed for certain elements (e.g., Al, Fe, Si) suggests gradual alteration or dissolution of more resistant silicate or glass phases in the later stages of the leaching process. In both column and batch experiments, the ⁸⁷Sr/⁸⁶Sr ratios started out at relatively low values, then increased with continued leaching, yielding a total range from 0.7107 to 0.7138. The experimental data suggest the presence of a more resistant, highly radiogenic silicate phase in fly ash that survives the high temperatures of combustion, and is leached after the more soluble minerals are removed. For comparison, the isotopic composition of water collected throughout a fly ash impoundment in West Virginia (including slurry inflow and water outflow) falls within a narrow range of 0.7127±0.0003. In any given fly ash impoundment, the bulk composition of the fly ash will be controlled by the source and chemistry of the coal that fed the generator. However, early-released Sr tends to be isotopically uniform, making the Sr isotopic composition of fly ash potentially distinguishable from other sources of solutes in waterways affected by multiple discharge types.