Kinetics of Molybdenum Adsorption-Desorption in Soils.

Much uncertainty exists in mechanisms and kinetics controlling the adsorption-desorption of molybdenum (Mo) in the soil environment. To investigate the characteristics of Mo adsorption-desorption and predict Mo behavior in the vadose zone, kinetic batch experiments were conducted in Webster loam, Windsor sand and Mahan sand with different properties, followed by Mo release using successive dilutions. Results showed that the Freundlich reaction order (N) indicated adsorption isotherms of Mo were strongly nonlinear all three soils with range of 0.457 to 0.758. Strong kinetic adsorption of Mo by all soils was observed, where the rate retention was rapid initially and was followed by slow retention behavior with reaction time. Desorption results indicated there were significant irreversible or slowly reversible processes for Windsor and Mahan. A multireaction model (MRM) with nonlinear equilibrium and kinetic sorptions was capable of describing the adsorption and desorption kinetics of Mo for Webster loam and Mahan sand. Several MRM model versions with different rates of reactions equally described the adsorption-desorption data. Comparison with Webster loam, Windsor sand exhibited strongest affinity for Mo. A sequential extraction procedure provided evidence that a significant amount of Mo was irreversibly sorbed. This study highlighted a significantly negative correlation between solution pH (x) and Freundlich coefficient K_F (y) (y=297.49-44.47x). We concluded that solution pH was a main factor controlling for Mo adsorption-desorption mechanisms and that pH should be incorporated into kinetic approaches to improve predication of Mo kinetic behavior in the vadose zone.