FLUORINE AND MOLYBDENUM SOLUBILITY RELATIONSHIPS IN COMBUSTED OIL-SHALE

Fluorine (F) and molybdenum (Mo) may adversely affect vegetation, grazing animals, groundwater, and surface waters that are proximate to spent oil shale disposal sites. The chemistry of F and Mo in spent oil shale leachates may be controlled by the precipitation of fluorite (CaF2) and powellite (CaMoO4), respectively. Thus, the fate and behavior of these elements may be predicted using equilibrium solubility methods. Recent findings and the examination of methods cast doubt on the interpretations of many spent oil shale solubility studies. Recent research shows that the formation of soluble molybdate species in spent oil shale leachates can be significant. Further, reaction times necessary for equilibrium conditions were not attained in many solubility experiments. Moreover, solubility experiments were only performed on fresh solid waste, yielding little information on element behavior in a disposal site. The objectives of this study were to reexamine spent oil shale literature data and perform laboratory equilibrium weathering studies to assess the solubility relationships of F and Mo in spent Green River Formation oil shale. Ion association model examination of literature data shows that F is present as fluorite in spent oil shale. However, spent oil shale leachates are undersaturated with powellite. During the initial cycles of the equilibrium weathering study, the solubility relationships for F and Mo were similar to those of the literature equilibrations. As weathering progressed, the spent oil shale leachates became supersaturated with fluorite and saturated with powellite. These results indicate that in a weathering spent oil shale environment, subject to the slow infiltration of water, the behavior of Mo may be predicted on the basis of powellite solubility. However, F behavior can not be predicted.