WWRC 94-04
Geochemical Modeling for Predicting Potential Solid Phases Controlling the Dissolved Molybdenum in Coal Overburden, Powder River Basin, WY, USA
Abstract
Surface coal mining processes may increase the dissolved Mo concentrations in overburden
(mine spoils) and in nearby soils. Spoil and soil samples were subjected to solubility studies for
determining potential solid phases controlling the dissolved Mo. Additionally, native soil samples were
included in the study for comparison. Ion activity products (IAPs) were calculated from the total
elemental concentrations of aqueous extracts and compared with solubility products (KSPS) of Mo solid
phases. Effects of dissolved organic carbon (DOC) on metal ion complexation 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.
Molybdenum concentrations in aqueous extracts ranged between 11.5 and 35.6 ug/l for spoils, 13.5 and
20.0 ug/l for soils near mine site and 14.5 and 29.6 ug/l for native soils. Initially, IAPs suggested varying
degrees of supersaturation with respect to wulfenite (PbMoO4). However, when DOC-Pb2+ complexes
were considered, the IAPs indicated a close approach to saturation with respect to PbMoO4. Furthermore,
the IAPs suggested a high degree of undersaturation with respect to powellite (CaMoO4 and
femmolybdite (FeMoO4). Results in this study suggest the dissolved Mo concentrations in mine spoils and
soils that are near coal mine may be controlled by PbMoO4 solid phase.
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