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

Abstract

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− 4 onto 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 + H 2 O) 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 10 10 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.