Review of the Surface Chemical Heterogeneity of Bacteriogenic Iron Oxides: Proton and Cadmium Sorption

This review focuses on the detailed characterization of proton and metal reactivity on the surface of bacteriogenic iron oxides (BIOS). This substance is composed of a highly porous matrix of ferrihydrite intermixed with intact and fragmented Gallionella ferruguinea cells. Studies concerning proton and metal cation binding to the BIOS were designed to assess their surface chemical heterogeneity through the further development of two innovative chemical equilibrium models. A multisite Langmuir isotherm model coupled to a linear programming regression method (LPM) was used to assess cadmium (Cd) sorption on bacteriogenic iron oxides. The BIOS proton binding affinity was assessed using a fully optimized continuous pKa spectral model (FOCUS). LPM analysis of the results from BIOS potentiometric experiments suggested the presence of two Cd binding sites on the whole BIOS, and three on the organic (bacteria) fraction surface. LPM results were able to show the non-additive nature of the BIOS mixture. The proton speciation characteristics of the BIOS surface, and that of the organic (bacteria) fraction were probed using FOCUS. This analysis revealed a striking similarity between the surface reactivity of the BIOS mixture and that of pure iron oxyhydroxide phases such as goethite and lepidocrocite. This, in turn, was a clear indication of the extensive masking of underlying surface bacterial (organic) functional groups by ferrihydrite precipitate. FOCUS results emphasized the non-additive nature of the BIOS composite. This was further corroborated by the results for Cd sorption experiments obtained from LPM analysis.