Molybdate adsorption

Reyes, E.D. Jurinak, J.J. (1967) A mechanism of molybdate adsorption on a-Fe203. Soil Sci. 

Zhang, P.C. Sparks, D.L. (1989) Kinetics and mechanism of molybdate adsorption/desorp-tion at the goethite/water interface using pressure-jump relaxation. Soil Sci. Soc. Am. 

Because of the millisecond time scale for these reactions, pressure-pulse perturbation (Fig. 4.1) with conductivity detection of the response can be used, as in the molybdate adsorption example. Evidently, the inner-sphere surface complexion step for sulfate occurs on time scales very much longer than those for its outer-sphere surface complexation, and therefore it was not observed experimentally with the method used. 

J.A.R. van Veen et al, Chemistry of (phospho-)molybdate adsorption on alumina surfaces, J. Phys. Chem., 94 (1990) 5275-5285. 

Fig. 3-9. Relaxation curves for molybdate adsorption on goethite (a) Typical p-jump relaxation curve showing change in conductivity vs. time for the goethite suspension and (b) semilog relaxation curves for the goethite suspension (from Zhang and Sparks, 1989).

At intermediate molybdenum loading, molybdate monomers and Anderson-type entities coexist on boehmite surface supporting the idea of a competitive adsorption mechanism. A chemisorption mechanism for molybdate monomers could lead to a limited dissolution of boehmite, necessary for the removal of A1 atoms required in the Anderson-type structure formation. This would explain the plateau reached in molybdate adsorption. 

Table 15.3. Molybdate adsorption from Mo solution (lOpgmL- ) by four soils saturated with different cations (soil solution ratio =1 5)...

Kinetics of Molybdenum Adsoiption. Zhang and Sparks 41) examined molybdate adsorption on goethite using pressure jump relaxation experiments. Molybdate adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of an inner-sphere complex of Mo04, based on optimized fits using the triple layer model. Forward rate constants were on the order of 4x10 L mol s and 40 s for the first and second reaction steps. 

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