A surface structural model for ferrihydrite I: Sites related to primary charge, molar mass, and mass density

A multisite Surface complexation (MUSIC) model for ferrihydrite (Fh) has been developed. The surface structure and composition of Fh nanoparticles are described in relation to ion binding and surface charge development. The site densities of the various reactive surface groups, the molar mass, the mass density, the specific surface area, and the particle size are quantified. As derived theoretically, molecular mass and mass density of nanoparticles will depend on the types of surface groups and the corresponding site densities and will vary with particle size and surface area because of a relatively large contribution of the surface groups in comparison to the mineral core of nanoparticles. The nano-sized (similar to 2.6 nm) particles of freshly prepared 2-line Fh as a whole have an increased molar mass of M similar to 101 +/- 2 g/mol Fe, a reduced mass density of similar to 3.5 +/- 0.1 g/cm(3), both relatively to the mineral core. The specific surface area is similar to 650 m(2)/g. Six-line Fh (5-6 nm) has a molar mass of M similar to 94 +/- 2 g/mol, a mass density of similar to 3.9 +/- 0.1 g/cm(3), and a surface area of similar to 280 +/- 30 m(2)/g. Data analysis shows that the mineral core of Fh has an average chemical composition very close to FeOOH with M similar to 89 g/mol. The mineral core has a mass density around similar to 4.15 +/- 0.1 g/cm(3), which is between that of feroxyhyte, goethite, and lepidocrocite. These results can be used to constrain structural models for Fh. Singly-coordinated surface groups dominate the surface of ferrihydrite (similar to 6.0 +/- 0.5 nm(-2)). These groups can be present in two structural configurations. In pairs, the groups either form the edge of a single Fe-octahedron (similar to 2.5 nm(-2)) or are present at a single corner (similar to 3.5 nm(-2)) of two adjacent Fe octahedra. These configurations can form bidentate surface complexes by edge- and double-corner sharing, respectively, and may therefore respond differently to the binding of ions such as uranyl, carbonate, arsenite, phosphate, and others. The relatively low PZC of ferrihydrite can be rationalized based on the estimated proton affinity constant for singly-coordinated surface groups. Nanoparticles have an enhanced surface charge. The charging behavior of Fh nanoparticles can be described satisfactory using the capacitance of a spherical Stern layer condenser in combination with a diffuse double layer for flat plates. (C) 2009 Elsevier Ltd. All rights reserved.