Infrared spectroscopy of ferrihydrite: evidence for the presence of structural hydroxyl groups

IR spectroscopy has shown that adsorbed water is almost completely removed from ferrihydrite by evacuation at room temperature. Absorption bands at 3615 and 3430 cm⁻¹ appearing thereafter are interpreted as arising from OH groups located respectively at the surface and deeper in the structure. These groups are readily converted to OD on treatment with D₂O vapour and this has allowed the OH deformation vibration to be identified at 800 cm⁻¹. It is proposed that OH groups in ferrihydrite are about half as numerous as those in akaganéite (β-FeOOH) and that they may occur in environments similar to those in this mineral. The formula for ferrihydrite proposed by earlier workers, 5 Fe₂O₃.9H₂O, should thus be amended to Fe₂O₃. 2 FeOOH.2·6H₂O in order to indicate the presence of structural OH groups. A re-appraisal of the ferrihydrite structure appears desirable.