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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 on the chemical and microstructural properties of Fe-oxide powders, prepared by FeCl3 hydrolysis in aqueous solutions at 90oC, were investigated using X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and 57Fe Mössbauer spectroscopy. In the absence of urotropin, akaganéite (b-FeOOH) particles showed different morphologies, such as spindles, stars, X- and Y-shapes. After 5 hours of aging and for an initial 0.025 M urotropin concentration, acicular b-FeOOH particles (monodispersed) of decreased size were obtained. For the same time of aging and 0.25 M urotropin the particles were amorphous for X-ray diffraction, while 57Fe Mössbauer spectroscopy indicated that these particles were of very small dimensions (superparamagnetic) and probably of the FeOOH structure. The b-FeOOH formation at pH values significantly higher than pH~2 was explained by incomplete substitution of Cl– ions in Fe(III)-hydroxy polymers with OH– ions. The presence of specific adsorption of organic components on the solid hydrolytical products was not confirmed by FT-IR spectroscopy. After 7 days of aging and for an initial urotropin concentration up to 0.025 M, b-FeOOH was also obtained as a single phase, while for c ł 0.1 M urotropin mixtures of goethite (a-FeOOH) and hematite (a-Fe2O3) were formed. Phase transformations b-FeOOHŽ a-Fe2O3 and b-FeOOHŽ a- FeOOHŽ a-Fe2O3 were discussed in the sense of the dissolution/precipitation mechanism.

urotropin ; iron oxide powders ; hydrolysis ; chemical properties ; microstructure

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