MOSSBAUER STUDY OF FERRIC IONS ADSORBED AT ALPHA-FERRIC OXIDE AQUEOUS-SOLUTION INTERFACE

In situ and ex situ Mossbauer observations were made on 57Fe3+ ions adsorbed on a-56Fe203 from a 0.1 mol dm-3 NaCl solution of pH 2.5, 4.0, and 6. The in situ spectrum at pH 2.5 consisted of a sextet with a reduced magnetic splitting compared with bulk a-Fe203. The result is interpreted to show that the adsorbed 67Fe ions form a new surface layer as an extension of the corundum structure of the substrate. Filtration and drying of the in situ sample brought about no appreciable change in the spectrum, while heating at and above 200°C resulted in an increase of the splitting due to rearrangement of surface fine structures and diffusion of the surface 67Fe3+ ions into the bulk. The relative absorption areas of dried and heated samples were found to be the same as that of the in situ sample within the experimental uncertainty of about 10%. At pH 6, the in situ sample gave no observable absorption, while an intense paramagnetic doublet and a weak sextet were observed in the frozen and dried states. On the basis of these results, physisorption of ferric hydroxide gel is concluded to be prevailing at pH 6. Measurement at pH 4.0 gave spectra indicating that both the mechanisms observed at 2.5 and 6 operate in parallel. © 1990, American Chemical Society. All rights reserved.