COMPETITIVE SURFACE COMPLEXATION REACTIONS OF SULFATE AND NATURAL ORGANIC-CARBON ON SOIL

The ecological implications of subsurface SO42- loading on nutrient ca tion leaching, acidification, and the destruction of concrete containe rs used to store low-level radioactive waste, has been thoroughly addr essed. Processes favoring SO42- adsorption by the subsurface matrix te nd to alleviate these adverse ecological conditions and this has been investigated to a lesser extent. In this study, the adsorption of SO42 - onto several soil types with indigenous SO42- and organic carbon rem oved, 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 consisten t decrease in sulfate adsorption over the pH range 4.5 to 8. The tende ncy of these soils to adsorb SO42- was related to their large quantity of Fe-oxides and the presence of kaolinite in the <2-mu m clay fracti on. A surface complexation model based on electrical double layer theo ry was used to model the adsorption behavior of sulfate. A single reac tion involving the adsorption of SO42- onto positive or neutral surfac e sites (XOH + H+ + SO42- = XSO(4)(-) + H2O) as an inner-sphere comple x proved successful in describing the adsorption of sulfate under the experimental conditions. The estimated value of the intrinsic equilibr ium constant (K) for the above reaction was of the order 10(10) sugges ting strong sulfate adsorption. Estimated K values were found to be un affected by the presence of added NOM. The spatial consistency and lac k of NOM effects on tbe intrinsic equilibrium constants for SO42- adso rption is convenient for nutrient and contaminant transport modeling a t the field-scale.