Effects of urotropin on the chemical and microstructural properties of Fe-oxide powders prepared by the hydrolysis of aqueous FeCl3 solutions

The effects of urotropin (hexamethylenetetramine) on the chemical and microstructural properties of Fe-oxide powders, prepared by FeCl3 hydrolysis in aqueous solutions at 90 degrees C, were investigated using X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy (FTIR) and Fe-57 Mossbauer spectroscopy. In the absence of urotropin, akaganeite (beta-FeOOH) particles showed different morphologies, such as spindles, stars, X-and Y-shapes. After 5 h of aging and for an initial 0.025 M urotropin concentration, acicular beta-FeOOH particles (monodispersed) of decreased size were obtained. For the same aging time and 0.25 M urotropin the particles were amorphous for X-ray diffraction, while Fe-57 Mossbauer spectroscopy indicated that these particles were of very small dimensions (super paramagnetic) and probably of the FeOOH structure. The beta-FeOOH formation at pH values significantly higher than pH was explained by incomplete substitution of Cl- ions in Fe(III)-hydroxy polymers with OH- ions. No evidence of specific adsorption of organic components on the solid hydrolytical products was detected by FTIR spectroscopy. After 7 days of aging and for an initial urotropin concentration of up to 0.025 M, beta-FeOOH was also obtained as a single phase, while for c greater than or equal to 0.1 M urotropin mixtures of goethite (alpha-FeOOH) and haematite (alpha-Fe2O3) were formed. Phase transformations beta-FeOOH --> alpha-Fe2O3 and beta-FeOOH --> alpha-FeOOH --> alpha-Fe2O3 were discussed in the sense of the dissolution/precipitation mechanism. (C) 1998 Elsevier Science S.A.