| 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. |
