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

The ecological implications of subsurface SO2−4 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 SO2−4 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 SO2−4onto several soil types with indigenous SO2−4 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 SO2−4 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 SO2−4 onto positive or neutral surface sites (XOH + H+ + SO2−4 = 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 SO2−4 adsorption is convenient for nutrient and contaminant transport modeling at the field‐scale.