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 α-Fe2O3 and α-FeOOH were monitored by Mössbauer and FT-IR spectroscopies and thermal field emission electron microscopy. α-Fe2O3 crystallization in dense ferrihydrite suspension at 160 °C was a fast process governed by the dissolution/reprecipitation mechanism. Between 40 and 60 min of crystallization traces/small amounts of α-FeOOH precipitated and dissolved, so that between 6 and 24 h only platelet-like α-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 α-FeOOH as an end-product was detected. These α-FeOOH particles were strongly elongated along the crystallographic c-axis. α-FeOOH particles were also obtained as a single phase at 160 °C, 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−2 the crystallization of α-Fe2O3 particles was observed. The geometrical shape of these α-Fe2O3 particles was poor compared with those crystallized in the absence of ultrasound.