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Competitive Surface Complexation Reactions of Sulfate and Natural Organic Carbon on Soil

Environmental Sciences Div., Oak Ridge Natl. Lab., P.O. Box 2008, Oak Ridge, TN 37831-6038;

S. Feldman

Dep. of Crop, and Soil Environmental Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061.

^* Corresponding author (ipj{at}ornl.gov).

ABSTRACT

The ecological implications of subsurface SO^2–₄ loading on nutrient cation leaching, acidification, and the destruction of concrete containers used to store low-level radioactive waste, has been thoroughly addressed. Processes favoring SO^2–₄ adsorption by the subsurface matrix tend to alleviate these adverse ecological conditions and this has been investigated to a lesser extent. In this study, the adsorption of SO^2–₄onto several soil types with indigenous SO^2–₄ and organic carbon removed, was measured as a function of pH in the presence and absence of added natural organic matter (NOM). Sulfate adsorption was strongly pH dependent and the presence of >2 mg L^–1 NOM resulted in a consistent decrease in sulfate adsorption over the pH range 4.5 to 8. The tendency of these soils to adsorb SO^2–₄ was related to their large quantity of Fe-oxides and the presence of kaolinite in the <2-µm clay fraction. A surface complexation model based on electrical double layer theory was used to model the adsorption behavior of sulfate. A single reaction involving the adsorption of SO^2–₄ onto positive or neutral surface sites (XOH + H⁺ + SO^2–₄ = XSO^–₄ + H₂O) as an inner-sphere complex proved successful in describing the adsorption of sulfate under the experimental conditions. The estimated value of the intrinsic equilibrium constant (K) for the above reaction was of the order 10¹⁰ suggesting strong sulfate adsorption. Estimated K values were found to be unaffected by the presence of added NOM. The spatial consistency and lack of NOM effects on the intrinsic equilibrium constants for SO^2–₄ adsorption is convenient for nutrient and contaminant transport modeling at the field-scale.

Joint contribution from Oak Ridge National Laboratory and Virginia Tech.

This research was supported by the Subsurface Science Program of the Ecological Research Division, Office of Health and Environmental Research, U.S. Department of Energy, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. Publication no. 4400, Environmental Sciences Division, ORNL.

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