engleski

57^Fe Mössbauer, FT-IR and FE SEM investigation of the formation of hematite and goethite at high pH values

The ferrihydrite suspensions were precipitated at high pH values, and crystallizations of alpha-Fe_2O_3 and alpha-FeOOH were monitored by Mössbauer and FT-IR spectroscopies and thermal field emission electron microscopy. alpha-Fe_2O_3 crystallization in dense ferrihydrite suspension at 160 o^C was a fast process governed by the dissolution/reprecipitation mechanism. Between 40 and 60 min of crystallization traces/small amounts of alpha-FeOOH precipitated and dissolved, so that between 6 and 24 h only platelet-like alpha-Fe_2O_3 particles were present. On the other hand, the kinetics of crystallization in a dense ferrihydrite suspension at RT was slow and upon 90 days of ageing alpha-FeOOH an end-product was detected. These alpha-FeOOH particles were strongly elongated along the crystallographic c-axis. alpha-FeOOH particles were also obtained as a single phase at 160 o^C, but with decreased Fe^3+ concentrations (0.1 or 0.05 M FeCl_3). These particles also showed a strong lateral aggregation and the formation of a dendritic microstructure. The crystallization process in a dense ferrihydrite suspension was strongly influenced by ultrasound energy. The application of ultrasound energy of 141 W cm^-2 for 2 h induced a coalescence-like effect in the ferrihydrite suspension, as observed by FE SEM. With ultrasound energy increased up to 424 W cm^-1 the crystallization of alpha-Fe_2O_3 particles was observed. The geometrical shape of these alpha-Fe_2O_3 particles was poor compared with those crystallized in the absence of ultrasound.

ferrihydrite; alpha-Fe2O3; alpha-FeOOH; spectroscopy; FE SEM

Journal of Molecular Structure, vol. 834-836, pages 141-149 (27 May 2007. Molecular Spectroscopy and Molecular Structure 2006 - A Collection of Papers Presented at the XXVIIIth European Congress on Molecular Spectroscopy, Istanbul, Turkey - September 3-8, 2006