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 hydr olysis in aqueous solutions at 90 degrees C, were investigated using X -ray powder diffraction, transmission electron microscopy, Fourier tra nsform 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, acic ular beta-FeOOH particles (monodispersed) of decreased size were obtai ned. For the same aging time and 0.25 M urotropin the particles were a morphous for X-ray diffraction, while Fe-57 Mossbauer spectroscopy ind icated that these particles were of very small dimensions (super param agnetic) 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. N o evidence of specific adsorption of organic components on the solid h ydrolytical products was detected by FTIR spectroscopy. After 7 days o f aging and for an initial urotropin concentration of up to 0.025 M, b eta-FeOOH was also obtained as a single phase, while for c greater tha n or equal to 0.1 M urotropin mixtures of goethite (alpha-FeOOH) and h aematite (alpha-Fe2O3) were formed. Phase transformations beta-FeOOH - -> alpha-Fe2O3 and beta-FeOOH --> alpha-FeOOH --> alpha-Fe2O3 were dis cussed in the sense of the dissolution/precipitation mechanism. (C) 19 98 Elsevier Science S.A.