Degree

Doctor of Philosophy (PhD)

Department

The School of Plant, Environmental, and Soil Sciences

Document Type

Dissertation

Abstract

Much uncertainty exists in mechanisms and kinetics controlling the adsorption-desorption and transport of molybdenum (Mo) in the soil environment. Results from kinetic batch experiments indicated that adsorption isotherms for Mo were strongly nonlinear for all soils. Strong kinetic adsorption of Mo was also observed, where the rate of retention was rapid initially and was followed by slow retention behavior with time. Desorption or release results indicated there were significant fractions of Mo that appeared to be irreversible or slowly reversible sorbed in soils. Column experiments indicated that Mo breakthrough curves (BTCs) were asymmetrical and exhibited extensive tailing with incomplete recovery in the effluent: these features demonstrate that rate-limited or non-equilibrium process influenced Mo transport behavior. Results of competitive experiments indicated kinetic rates and amounts of Mo retention by soils were significantly reduced by increasing addition of P and the presence of P resulted in increased mobility of Mo on soil columns. A range of transport models (multi-reaction model, second order model, and competitive model) for describing and predicting Mo fate and transport behavior in the environment have been evaluated and they concluded that an additional consecutive irreversible reaction phase was necessary to describe Mo retention and transport for soils with strong affinity and low mobility of Mo. In addition, our work provide a general stirred-flow model which is capable of describing reactive and non-reactive solutes. The strong retardation and slow release behaviors of Mo from stirred-flow experiments were successfully described using the proposed stirred-flow MRM where retardation and irreversible reactions were necessary.

Furthermore, based on extended Freundlich equation, we proposed a modified multi-reaction model including a pH term, which was utilized to describe pH-dependent sorption and transport process. An advantage of modified Freundlich approaches is that they are relatively less complex and require fewer parameters. On the other hand, geochemical models, which are based on surface complexation concepts, were incorporated into kinetic models to accurately describe the reactive transport of Mo in soil components under a wide range of environmental conditions. X-ray absorption near edge structure (XANES) analysis provided evidences of surface irreversible reactions at long residence times (1 year).

Date

10-30-2019

Committee Chair

Selim, H. Magdi

Available for download on Monday, October 26, 2026

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