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Kinetics of Metal Dissolution from Forest Soils by Soluble Organic Acids¹

ABSTRACT

Two kinetic models are presented that describe the dissolution of Al, Fe, Mn, and Mg from two forest soils by soluble organic acids. Dissolution of Al, Fe, and Mn occurred as a first-order kinetic reaction. The rate was in the order Mn >> Al> Fe in di- and tricarboxylic acids, salicylic, p-hydroxybenzoic, and tannic acids, and the order Mn >> Fe > Al in gallic acid and catechol. Complexing acids removed these metals faster than noncomplexing acids. The dissolution of Mn followed first-order kinetics from Holland Variant soil and parabolic-type kinetics from Shaver soils. Dissolution of Mn by parabolic-type kinetics followed a similar ion exchange process as in NH₄OAc leaching of Al, Fe, Mn, and Mg from both soils. The chemical structure, and the type and position of functional groups within a given organic chemical series, were important factors affecting the dissolution rates of Al, Fe, and Mn, but not for Mg. The di- and tricarboxylic acids containing β-hydroxy functional groups and phenolic acids containing orthohydroxy groups dissolved Al and Fe about 3 to 6 times faster than similar organic compounds with other functional-group combinations. The interaction between -carboxy and β-hydroxy groups and these metals is thought to favor the formation of a stable chelated six-members ring structure. Gallic acid was effective in dissolving Mn, and the mode of chelation appeared to be the two adjacent phenolic-OH groups capable of forming a stable five-membered ring structure. The rate of dissolution of Mn in gallic acid was about 3 to 6 times faster than in salicyclic or p-hydroxybenzoic acids. Dissolution of Mg was assumed to be by H⁺ ion exchange and not by chelating action.

Key Words: leaching • chelating agents • chemical structure • aluminum • iron • manganese

¹ Contribution form the Dep. of Plant and Soil Biology, Univ. of California, Berkeley, CA 94720. The research was supported by the Agric. Exp. Stn., Univ. of California, Project CA-B-3664-H, and the National Science Foundation, Grant BSR 84-16661.

² Associate Specialist and Professor, respectively.

Received for publication November 5, 1984.

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