ABSTRACT
Field and laboratory methods were developed to preserve the chemical integrity of highly reduced utility waste leachate and to measure such important parameters as pH and redox potentials. A combination of field measurements and laboratory experiments with core samples of flue gas desulfurization (FGD) sludge and of soils underlying the sludge were used to identify geochemical reactions controlling elemental concentrations in the sludge pore waters and solute migration.
Pore waters in the sludge were found to be uniform both spatially and with depth in the sludge basin. The pore waters were highly reducing, containing sulfide at parts per million levels. It was found that precipitation/dissolution reactions with sulfate, sulfite, and sulfide solids controlled the concentrations of many elements (Ba, Ca, Cd, Cu, Fe, Ni, Pb, S, Sr, and Zn). Through a combination of techniques, (l)X-ray diffraction, (2) comparison of observed ion activity products with the thermodynamic data for given solid phases, and (3)identification of solids by spiking the pore waters or sludge suspensions with a given element, the solubility-controlling solids for these elements were hypothesized to be BaSO4/(Ba,Sr)SO4 CaSO4 H2O, CaSO3·0.5H2O, CaCO3, CdS, Cu2S/Cu1.65S, FeS, NiS, PbS, SrSO4/(Ba,Sr)SO4, and ZnS. In addition, the aqueous concentrations of Cr and Se were inferred to be controlled by Cr(OH)3 and FeSe2/FeSe, respectively. The aqueous concentrations of many elements (Cd, Cr, Cu, Ni, Pb, Se, V, and Zn) in sludge pore waters were either at or near the detection limits (parts per billion range). Therefore, these elements were neither expected nor found to migrate into underlying soils over the year disposal period. Migration into underlying soil/subsoil of those elements that were found to be present in measurable concentrations in the sludge pore waters was also minimal because of the low permeability of the soil and its high attenuation capacity. Boron was the most mobile element, and in approximately 8 years it had migrated slightly more than I m into the underlying soil, primarily through diffusion. Most other elements that were in detectable concentrations in the sludge pore waters were strongly attenuated by the underlying soil.
