H4SiO4 sorption and polymerization at the magnetite - aqueous interface: The influence of interfacial redox state

Many aspects of the behaviour of iron oxide phases are influenced significantly by the interaction with H₄SiO₄ which is a ubiquitous solute in aquatic systems. This interaction can involve H₄SiO₄ sorption and interfacial oligomerization and has a dependence on the structures present on the oxide surface. In this work the sorption and oligomerization of H₄SiO₄ on the surface of magnetite (Fe₃O₄) was studied as a function of the particle surface oxidation state. Magnetite samples were prepared from FeCl₂:FeCl₃ or FeSO₄:FeCl₃ and were characterised by Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), Raman and infrared (IR) spectroscopy. Magnetite from FeCl₂:FeCl₃ was pure magnetite by XRD but Raman spectra indicated ferrihydrite was present on the “as prepared” surface. Raman also indicated that the ferrihydrite layer was decreased following reduction with hydrazine but was increased following treatment with dilute H₂O₂. In situ Attenuated Total Reflectance Infrared Spectroscopy (ATRIR) showed the propensity of H₄SiO₄ oligomerization on magnetite was enhanced following hydrazine reduction and was not influenced by H₂O₂. In all cases the proportion of oligomers to monomers was significantly greater on the surface of magnetite than on ferrihydrite. The inference is that both the as prepared and the H₂O₂ oxidised magnetite have a ferric oxide layer that has a local environment similar to ferrihydrite, as indicated by the Raman, however on the magnetite surface this phase does not have the high surface curvature of the ferrihydrite prepared directly from Fe(NO₃)₃ hydrolysis. The high surface curvature inherent with very small sized particles such as ferrihydrite (≈2 nm), has been linked with a low propensity of H₄SiO₄ oligomerization on these surfaces. The results from this study support the association between surface curvature and propensity for interfacial H₄SiO₄ oligomerization. Magnetite prepared from FeSO₄:FeCl₃ had minor amounts of goethite and was found to promote the formation of a higher degree of 3 dimensional H₄SiO₄ polymerization on this oxide which is attributed to the nature of the (021) goethite faces.