A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate

A multisite surface complexation (MUSIC) model for ferrihydrite has been developed. The surface structure and composition are described for ferrihydrite in terms of site densities, molar mass, bulk density and reactive surface area, all linked to ion binding and surface charge development. Singly coordinated surface groups dominate the surface of ferrihydrite. These groups are present in two structural configurations, each linked to different surface patches. The configurations either form bidentate complexes by edge sharing coordination or form double-corner complexes. Both types of configurations react differently in terms of binding of ions such as uranyl and carbonate.
The adsorption of uranyl (UO22+) on ferrihydrite has been evaluated with the Charge Distribution (CD) model. Modeling shows that uranyl forms bidentate innersphere complexes at binding sites that do not react with carbonate ions. This observation is used to develop a surface structural model for ferrihydrite. Uranyl is bound by singly coordinated surface groups present at particular edges of Fe-octahedrons of ferrihydrite while another set of singly coordinated surface groups may form double-corner bidentate complexes with for instance carbonate ions. The uranyl surface speciation strongly changes in the presence of carbonate due to the specific adsorption of carbonate ions as well as the formation of ternary uranyl-carbonate surface complexes. The CD model reveals the formation of a uranyl-triscarbonato surface complex, i.e. ≡ (UO2)(CO3)34-, particularly present in systems with a high pH and carbonate concentration. This finding remarkably differs from results from previous interpretations made in literature. The presence of this surface species agrees quantitatively with a recent interpretation of EXAFS data (Rossberg et al. 2008). In the specific case of a high pH value, but at low carbonate level, as can be prepared in CO2-closed systems, the model suggests the presence of a ternary uranyl-monocarbonato complex. The binding mode (type A or type B complex) is uncertain. At high uranyl concentrations, uranyl will polymerize at the surface of ferrihydrite. The differences and similarities in U(VI) adsorption behavior of goethite and ferrihydrite is discussed from a structural point of view.