engleski

57Fe 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-Fe2O3 and alpha-FeOOH were monitored by Mössbauer and FT-IR spectroscopies and thermal field emission electron microscopy. alpha-Fe2O3 crystallization in dense ferrihydrite suspension at 160 oC 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-Fe2O3 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 oC, but with decreased Fe3+ concentrations (0.1 or 0.05 M FeCl3). 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-Fe2O3 particles was observed. The geometrical shape of these alpha-Fe2O3 particles was poor compared with those crystallized in the absence of ultrasound.

alpha-FeOOH; alpha-Fe2O3; FT-IR; Mossbauer

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