Adsorption of Antimonate, Phosphate, and Sulfate by Manganese Dioxide: Competitive Effects and Surface Complexation Modeling

Abstract

Antimony is a co‐contaminant with Pb in shooting range soils. The in situ immobilization of Pb in these soils may be accomplished through the application of PO4. However, the impact of this treatment on the mobility and bioaccessibility of Sb is unknown. Further, the ability to predict Sb fate and behavior in contaminated soils, or as influenced by treatment technologies, has not been suitably developed. The objectives of this research were to characterize the adsorption behavior of Sb(V) by birnessite (δ‐MnO2) as a function of ionic strength, pH, and in the presence of a competing ligand (PO4 or SO4) and to develop and evaluate the capability of the triple‐layer surface complexation model to predict Sb(V) adsorption. The adsorption of Sb(V) by birnessite increased with decreasing pH and ionic strength. Antimonate adsorption did not influence the proton adsorption characteristics of birnessite relative to that in NO3 suspensions; however, the zeta potential of the mineral was decreased in the presence of Sb(V), relative to NO3, when the pH was <5. Antimonate adsorption by birnessite was not impacted by either PO4 or SO4; however, the presence of Sb(V) resulted in reduced PO4 retention. Antimonate adsorption as a function of pH and ionic strength was successfully predicted by using the triple‐layer surface complexation model that considers both outer sphere [≡MnOH2+–Sb(OH)6−] and inner sphere [≡MnOSb(OH)5−] adsorption mechanisms. In general, however, the models generated for single‐ligand systems required reoptimization to successfully predict adsorption in the competitive [Sb(V) and PO4] systems.