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Kinetics of Sulfate Adsorption and Desorption by Cecil Soil Using Miscible Displacement¹

ABSTRACT

The kinetics of SO^2-₄ adsorption and desorption at 4, 22, and 40°C by untreated and CaCl₂-treated samples of a Cecil Bt2 horizon were studied using miscible displacement and a rapidly stirred batch procedure. Adsorption data was plotted according to the linear forms of the first order, Elovich, parabolic diffusion, and shell progressive particle and film diffusion equations. The latter two equations consider the effects of strong interaction between SO^2-₄ and the soil surface on diffusion. The shell progressive particle diffusion, the Elovich, and the first-order equations provided the best fits of the adsorption data (R² values > 0.95). The desorption data was best fit by the shell progressive particle diffusion, the Elovich, and the parabolic diffusion equations. Desorption of SO^2-₄ by KCl was not reversible within a period of two times that of the adsorption period. Decreases in the apparent reaction rate coefficients (slopes of the regression equations) were unexpectedly observed with increasing temperature for the untreated samples, indicating that soil-bound Al may affect the rate of SO^2-₄ reactions as well as the quantities of SO^2-₄ adsorbed. Under the assumption that the miscible displacement procedure more accurately models the field situation, a batch equilibrium isotherm procedure overestimated the SO^2-₄ adsorption capacity, and the rapidly stirred batch procedure underestimated the time required for SO^2-₄ to react with soil surfaces. Dependency of the reaction rate coefficients on the mixing rate and flow rate, a limited temperature dependency, and the good fit of the data by the shell progressive particle diffusion model indicate that the apparent reaction rate in the miscible displacement procedure is controlled by diffusion of SO^2-₄ through a reacted soil particle or aggregate rather than an elementary chemical reaction or diffusion through the film surrounding soil particles.

¹ Contribution from the Dep. of Agronomy and Soils, South Carolina Agric. Exp. Stn. as Journal Series no. 2623, Clemson, SC 29634. Although the research described in this article has been funded in part by the USEPA, it has not been subjected to EPA's policy review and therefore does not necessarily reflect the views of the Agency.

² Assistant Professor, Dep. of Agronomy and Soils, and former Graduate Assistant, Dep. of Horticulture, Clemson Univ., Clemson, SC 29634-0359.

Received for publication January 24, 1986.

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