Adsorption of Antimony by Birnessite and the Impact of Antimony on the Electrostatic Surface Properties of Variable-Charge Soil Minerals

Antimony (Sb) is a toxin that can be found in high concentrations in the soil due to anthropogenic sources. Antimony exists in soil as Sb(V) in the monovalent antimonate hydroxyanion. The adsorption mechanisms of Sb(V) are not well-characterized. The objective of this study was to further elucidate Sb(V) adsorption mechanisms by examining the impact of adsorption on surface charging characteristics of gibbsite, goethite, birnessite, and kaolinite. Also examined was Sb(V), SO₄ [sulfate], and PO₄ [phosphate] adsorption by birnessite. Electrophoretic mobility and potentiometric titrations were employed to examine the ζ-potential [zeta-potential] and net proton surface charge density as a function of pH and ionic strength, and in the presence or absence of adsorbed Sb(V), phosphate, or sulfate. Competitive batch adsorption studies were performed to examine Sb(V), SO₄, and PO₄ adsorption as a function of pH, ionic strength, and competitive environment. Results suggest that Sb(V) participates in inner-sphere adsorption by gibbsite, goethite, and kaolinite in acidic conditions, PO₄ participates in inner-sphere adsorption by gibbsite, goethite and kaolinite in the pH range studied, and that SO₄ participates in inner- and outer-sphere adsorption by gibbsite, goethite, and kaolinite with the former mechanisms becoming more important in acidic conditions. Adsorption of Sb(V) and PO₄ by birnessite had little impact on the surface charge characteristics indicating outer-sphere adsorption. Batch adsorption edge studies showed Sb(V) and PO₄ retention to be dependent on pH and ionic strength, supporting electrostatic adsorption mechanisms. Batch adsorption studies showed SO₄ was not adsorbed by birnessite in the pH 3 to 11 range. The adsorption data was modeled using FITEQL 4.0 and the diffuse layer model (DLM). The DLM adequately described Sb(V) and PO₄ adsorption by birnessite using electrostatic surface complexes.