GEOCHEMICAL MODELING FOR PREDICTING POTENTIAL SOLID-PHASES CONTROLLING THE DISSOLVED MOLYBDENUM IN COAL OVERBURDEN, POWDER-RIVER BASIN, WY,USA

Surface coal mining processes may increase the dissolved Mo concentrat ions in overburden (mine spoils) and in nearby soils. Spoil and soil s amples were subjected to solubility studies for determining potential solid phases controlling the dissolved Mo. Additionally, native soil s amples were included in the study for comparison. Ion activity product s (IAPs) were calculated from the total elemental concentrations of aq ueous extracts and compared with solubility products (K(sp)s) of Mo so lid phases. Effects of dissolved organic carbon (DOC) on metal ion com plexation were also evaluated. The pH of aqueous extracts ranged from 7.14 to 8.31. DOC in aqueous extracts varied from 17.3 to 58.7 mg/l. M olybdenum concentrations in aqueous extracts ranged between 11.5 and 3 5.6 mug/l for spoils, 13.5 and 20.0 mug/l for soils near mine site and 14.5 and 29.6 mug/l for native soils. Initially, IAPs suggested varyi ng degrees of supersaturation with respect to wulfenite (PbMoO4). Howe ver, when DOC-Pb2+ complexes were considered, the IAPs indicated a clo se approach to saturation with respect to PbMoO4. Furthermore, the IAP s suggested a high degree of undersaturation with respect to powellite (CaMoO4) and ferrimolybdite (FeMoO4). Results in this study suggest t he dissolved Mo concentrations in mine spoils and soils that are near coal mine may be controlled by PbMoO4 solid phase.