The Influence of Organic Complexing Agents on Iron Mobility in a Simulated Rhizosphere

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Abstract
The effect of complexing agents on Fe mobility was investigated using a simulated root‐soil system that consisted of 0.24‐mm o.d. cellulose fibers embedded in a clay‐agar matrix. Complexing agents diffused into the clay‐agar matrix from a solution flowing through the fibers and their effect on Fe flux to the fiber surface was determined by following Fe concentration in the effluent solution from the fibers. Experiments with the chelating agents ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), ethylenediamine di‐(o‐hydroxyphenylacetic acid) (EDDHA), and ethylene glycol‐bis‐(2‐amino‐ethylether) tetraacetic acid (EGTA) demonstrated that solubilization of Fe qualitatively followed patterns predicted from known thermodynamic stability constant‐pH relationships. The concentration of Fe in the fibers 24 h after introduction of either EDTA, DTPA, or EDDHA was increased from close to 0 to 30 to 100 g Fe L−1, whereas EGTA had no measurable effect on Fe solubility. The measured fluxes of Fe were compared with calculations based on diffusion and reaction. An equilibrium model, where the complexing agent was assumed to equilibrate instantaneously with the soil (reaction rate constant Kr = 1), predicted concentrations of Fe 103 times greater than those observed. A first‐order kinetic model, which was solved by numerical methods, gave results similar to those found experimentally when Kr was on the order of 10−7. The partial success of the thermodynamic stability constant approach was reconciled with the kinetic model by considering Kr to combine both kinetic and equilibrium factors.