Ion adsorption modeling as a tool to characterize metal (hydr)oxide behavior in soil

This study aims to provide a better basis for application of adsorption models for metal (hydr)oxides to natural multicomponent systems. Adsorption of any ion in the environment will be potentially influenced by the effect of other ions present like calcium, phosphate, carbonate etc. The study starts with a detailed study of the binding of ions as outersphere complexes. The CD model has been extended to use the charge distribution for ions that bind as outersphere surface complex. This indicates that neither innersphere nor outersphere surface complexes are treated as point charges anymore. The new approach was applied to describe the adsorption of various electrolyte ions, phosphate, and carbonate. Batch experiments were performed using goethite as an adsorbent to determine the adsorption behavior of electrolyte ions (Li ⁺¹ , Na ⁺¹ , K ⁺¹ , Cs ⁺¹ , Ca ⁺² , Mg ⁺² , Cl ^-1 , NO ₃^-1 ), phosphate, and carbonate. The adsorption of phosphate and carbonate ions was studied in a 'single ion' system and their interaction in a competition system. The charge distribution value of innersphere surface complexes of phosphate and carbonate was calculated using the new approach. New is also the use of quantum chemical calculations to derive the CD value based on a calculated geometry of the surface complexes. The calculated geometries were interpreted with the Brown bond-valence model, resulting in a calculated CD of the surface complex. The calculated CD values were used as a constraint in the surface complexation modeling. The CD model for inner- and outersphere surface complexation successfully described the adsorption data of electrolyte ions, phosphate, and carbonate. For accommodation of adsorbed ions within the Stern layer, a Three Plane (TP) model was used as a framework. For outersphere surface complexes, it was shown that the minimum distance of approach of adsorbed ions depends on the finite size of ions and their degree of hydration, which determine their relative distances to the surface of minerals. It has been shown that the capacitance of the inner Stern layer is determined by the minimum distance of approach of the ion closest to the surface, while the capacitance of the outer layer is determined by the minimum distance of approach of the ion furthest away from the surface. Modeling of phosphate adsorption data revealed that phosphate adsorbed mainly as a bidentate surface complex. At low pH, protonated species of phosphate are a combination of monodentate and bidentate surface complexes. The new CD approach shows that phosphate interacts with sodium at the mineral surface, which could not be detected using previous approaches. Carbonate adsorption data were successfully described using a bidentate surface complex. This complex interacts with sodium at high pH and high salt level. The new approach not only predicts the shift in the isoelectric point as a function of phosphate loading, but also the measured zeta potential is in quite good agreement with predictions based on the assumption that the zeta potential coincides with the potential of the head end of the DDL. Furthermore, it has been shown that the parameterized CD model can be used to determine the effective reactive surface area of metal (hydr)oxides and the total reversibly adsorbed phosphate fraction in soils.