Thermodynamic Stability and Solubility of Molybdenum Minerals in Soils
1977
Vlek, P. L. G. | Lindsay, W. L.
The chemical reactions of molybdenum in soils were studied in the laboratory. Solubility diagrams were constructed from thermodynamic data obtained from the literature and from measurements made on pure minerals. Included in these diagrams were solubility relationships for MoS₂ (molybdenite), Mo₃O₈ (ilsemannite), Fe₂(MoO₄)₃ (ferrimolybdite), CaMoO₄ (powellite), and PbMoO₄ (wulfenite). From these diagrams it appears that wulfenite is one of the most stable Mo minerals likely to form in soils. Lack of thermodynamic data on other Mo minerals prevents their evaluation as possible reaction products. The solubility of Mo in selected Colorado soils in the pH range of 5.5 to 7.7 was plotted on the wulfenite solubility diagram. Of the 13 soils used in this study, only one showed equilibrium with respect to wulfenite. When wulfenite was added to several other soils, the molybdenum solubility increased 10- to 100-fold, confirming that wulfenite was not initially present. These wulfenite-amended soils showed a close correspondence between soluble Mo and the wulfenite solubility isotherm. In the absence of solid phase Mo compounds, the solubility of Mo in soils is controlled by specific adsorption.
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