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

Abstract

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.

Slow-release formulations of the herbicide picloram by using Fe–Al pillared montmorillonite

“…On soils, it was 398 described as inner sphere complexation which was correlated with the soil iron oxides content 399 (Kooner et al 1995), which was in agreement with hematite oxides results (Eggleston et al 1998). …”

Kinetic analysis and modeling of oleate and ethanol stimulated uranium (VI) bio-reduction in contaminated sediments under sulfate reduction conditions

“…Previously, we determined the values for log K e1 , log K e2 , log K e3 , and log K e4 to be −2.64, −6.84, −13.68, −17.28, respectively, and strong and weak sorbing site densities to be 0.0018 mole sites/mole Fe(III) and 0.8732 mole sites/mole Fe(III), respectively, for the sediment from the studied site [41]. In addition, a surface complexation model describing sulfate sorption [42] was introduced to model.…”

Enhancement of Carbon Sequestration in US Soils

“…Soil anions such as sulfate and, in particular, phosphate can effectively compete for dissolved organic carbon (DOC) sorption sites, releasing DOC into the pore water (Jardine et al 1989, 1990, Kooner et al 1995. In soils with deep profiles and limited lateral flow, this process could actually serve to enhance organic carbon sequestration, since the DOC would have ample opportunity to readsorb on mineral particles deeper in the soil.…”