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Prediction of Ion Activities in Soil Solutions: Computer Equilibrium Modeling¹

ABSTRACT

Modeling of a multicomponent natural water system, such as the soil solution, utilizing classical ion association equilibria, allows one to estimate thermodynamic activities for many chemical species for which direct measurement is either not practical or impossible. A number of computer-based equilibrium models are presently available for use in soil chemical research. The thermodynamic formalism employed by all of the models is very nearly identical; however, small differences in the estimation of activity coefficients and temperature correction of thermodynamic data exist between models. The sophistication of the numerical routines used to obtain solutions of the problem can also vary between models. These differences are reflected in computational efficiency, but give rise to little or no difference in th epredicted activities.

The selection, extent, and flexibility of the user-supplied thermodynamic data file (TDF) is the major difference between geochemical models. After the total concentrations of metals and ligands, temperature, and pressure are specified, equilibrium constants for all complexes allowed to form are retrieved from the TDF. The selection of a TDF, therefore, specifies the equilibrium problem to be solved. The compilation of a critically reviewed TDF for use in soil chemistry is the most important component in the attempt to model soil solution geochemistry at equilibrium.

Some recent uses of geochemical modeling of the rhizosphere have been selected from the literature to demonstrate the utility of this approach.

¹ Contribution from the Dep. of Soil Science, Oregon Agric. Exp. Stn., Oregon State Univ., Corvallis, OR 97331. Technical paper no. 6918. Presented at the Div. S-2 Symposium "Chemistry in the Soil Environment: Activities," Soil Sci. Soc. Am., 29 Nov. 1982, Anaheim, CA.

² Assistant Professor of Soil Science.

Received for publication August 9, 1983. Accepted for publication November 21, 1983.