Solubility and release of fluorine and molybdenum from oil shale solid wastes

Abstract

Surface disposal of oil shale solid wastes may contaminate surface water and ground-water resources with toxic levels of fluorine (F) and molybdenum (Mo). Three different oil shale solid wastes produced from processing raw oil shale, at different temperatures, were subjected to equilibrium solubility studies to examine the solid phases responsible for the release of F and Mo from oil shale solid wastes into the infiltrating water. The results suggested that increased retorting temperatures increased pH from 10.75 to 12.35, decreased dissolved F concentrations from 48.8 to 3.5 mg I −1, and increased dissolved Mo concentrations from 1.9 to 3.9 mg I −1 in oil shale solid waste equilibrium waters. Chemical speciation of total soluble F and Mo concentrations in oil shale solid waste equilibrium waters indicated that F was present mainly as F −, NaF°, and CaF +. Molybdenum was present as MoO 4 2− and HMoO 4 −. Further results of this study suggested that dissolved F and Mo concentrations in oil shale solid waste equilibrium waters appear to be controlled by CaF 2 (fluorite) and CaMoO 4 (powellite), respectively.