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

TLDR: In this paper, the spectrum of a synthetic ferrihydrite is re-examined using a film technique in conjunction with D20 exchange, and it is suggested that this may not have been the most appropriate technique.

Abstract: 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 D20 vapour and this has allowed the OH deformation vibration to be identified at 800 cm- I. It is proposed that OH groups in ferrihydrite are about half as numerous as those in akagan+ite (fl-FeOOH) and that they may occur in environments similar to those in this mineral. The formula for ferrihydrite proposed by earlier workers, 5 Fe203.9H20, should thus be amended to Fe203.2 FeOOH.2.6H20 in order to indicate the presence of structural OH groups. A re-appraisal of the ferrihydrite structure appears desirable. Naturally occurring ferric hydroxide gel has been studied by Chukhrov et al. (1971, 1972, 1973) using X-ray diffraction (XRD) and infrared (IR) spectroscopy. They proposed the name ferrihydrite and concluded that it had a defect-hematite structure, as proposed earlier by Towe & Bradley (1967) for a synthetic specimen. This structure was sustained by the failure of the latter authors to detect absorption bands assignable to deformation vibrations of structural OH groups in IR spectra. Schwertmann & Fischer (1973) arrived at a similar conclusion from IR spectra of naturally occurring ferric hydroxides. Chukh- rov et al. (1971) further stated that the presence of OH groups was precluded by the typical bonding between Fe octahedra in this type of structure. Thus on the basis of characteriza- tion by XRD, DTA and IR spectroscopy, ferrihydrite was accepted as a distinct mineral species of formula 5Fe203.9H20, containing no structural OH groups (Fleischer et al., 1975). The IR results quoted by these authors are central to the elucidation of the structure of ferrihydrite, but the procedures employed all involved the use of K Br pressed disks. It is suggested that this may not have been the most appropriate technique, and in this study, the spectrum of a synthetic ferrihydrite is re-examined using a film technique in conjunction with D20 exchange.