Adsorption behavior and surface complexation modeling of oxygen anion Sb(V) adsorption on goethite

Simulating the adsorption behavior of Sb(V) on goethite is of great significance for predicting the mobility of Sb(V) in soil. However, there is still a lack of charge distribution and multisite surface complexation (CD-MUSIC) models that conform to the actual adsorption mechanism. Therefore, our research combined the previous EXAFS results with the SCMs and established two CD-MUSIC models (Model I, which introduced bidentate binuclear and bidentate mononuclear complexes, and Model II, which introduced bidentate mononuclear and monodentate mononuclear complexes). It is also one of the rare cases that bidentate edge- and corner-sharing adsorption complexes have been distinguished by introducing different amounts of H⁺. The results showed that Model I was more suitable for predicting the adsorption behavior of Sb(V) on the goethite surface than Model II. However, at low pH and high Sb(V) loading, the presence of the monodentate complex ≡FeOHSb(OH)₅^0.5- was possible due to the fast and slow two-step mechanism. Using the optimized CD-MUSIC model parameters, the contribution of surface species to the equilibrium adsorption under different conditions was determined. In the adsorption edge and adsorption isotherm simulation process, the edge-sharing bidentate mononuclear complex ≡(FeO)₂H₂Sb(OH)₄ was always the most important product, which was consistent with the results of extended X-ray absorption fine structure (EXAFS) analysis. The applicability of the model parameters was verified by predicting the competitive adsorption between PO₄^3- and Sb(V). The CD-MUSIC model established in the present study could be a useful tool for evaluating the equilibrium distribution behavior and environmental risk of Sb(V) in different types of soils.