Submitted to: American Chemical Society Symposium Series
Publication Type: Proceedings
Publication Acceptance Date: 7/8/1996
Publication Date: N/A
Citation: Interpretive Summary: Molybdate is a specifically adsorbing anion that can be detrimental to animals at elevated levels. Detrimental levels can occur from ingestion of forage plants grown on soils irrigated with waters containing high concentrations of molybdate. A better understanding of the adsorption behavior of molybdate is necessary. Adsorption of molybdate by aluminum and iron oxides, clay minerals, and soils was investigated under changing conditions of solution pH and ionic strength. The adsorption behavior of molybdate on these surfaces can be describe using a computer model. Our results will benefit scientists who are developing models of molybdate movement in arid zone soils. The results can be used to improve predictions of molybdate behavior in soils and thus aid action and regulatory agencies in the management of soils and waters which contain elevated concentrations of molybdate.
Technical Abstract: Molybdate adsorption on all materials exhibited a maximum at low pH (3 to 5). With increasing solution pH, adsorption decreased rapidly with little adsorption occurring above pH 7 to 8. Molybdate adsorption was lowest for the highest solution ionic strength. Ionic strength dependence of molybdate adsorption was slight on goethite,montmorillonite, and soils suggesting an inner-sphere adsorption mechanism. Ionic strength dependence of molybdate adsorption was obvious on gibbsite, Aluminium Oxid C, and kaolinite suggesting an outer-sphere adsorption mechanism. The constant capacitance model was able to describe molybdate adsorption on the oxides, clay minerals, and soils as a function of solution pH. The triple layer model was able to describe molybdate adsorption on goethite, gibbsite, Aluminium Oxid C, kaolinite, montmorillonite, and two soils as a function of solution pH and ionic strength using a universal site density value of 2.31 sitesùnm2,recommended for natural materials. Good fits of the model to the data were obtained for all materials using both inner-sphere and outer-sphere adsorption mechanisms except montmorillonite where an acceptable fit was obtained only with an outer-sphere mechanism. Results from the FTIR spectroscopy indicate that ligand exchange is a mechanism for Mo adsorption on amorphous iron hydroxide. Hydroxyl release suggest a mixture of monodentate and bidentate Mo surface complexes.