A kinetic analysis of the transformation from akaganeite to hematite: An in situ time-resolved X-ray diffraction study

• Activation energies (Ea) for nucleation and growth of akaganeite in FeCl3 aqueous solutions are presented for the first time.
• Akaganeite is the first phase to form and transforms directly into hematite without the formation of an intermediate phase.
• Akaganeite nucleation in 0.45 M FeCl3 solutions is instantaneous above 123 °C.
• Hematite induction times decrease with increasing temperature.
• Similar k and Ea for akaganeite dissolution and hematite crystallization suggest that these processes are closely coupled.

The nucleation and growth of akaganeite and its transformation to hematite under hydrothermal conditions were monitored over a temperature range of 80 to 200 °C using time-resolved synchrotron X-ray diffraction. In each experiment, akaganeite was the first phase to form and hematite was the final phase. No intermediate phases were identified. The induction time to akaganeite nucleation was ~ 5525 s and 537 s at 80 °C and 100 °C, respectively, yielding an activation energy of 129 ± 15 kJ/mol. However, akaganeite nucleated at a constant temperature of 123 ± 5 °C when the heater set point was 150 °C or higher, suggesting an activation energy for akaganeite nucleation of 0 kJ/mol between 150 and 200 °C. Hematite nucleation induction times decreased with increasing temperature from 1723 s to 110 s between 150 and 200 °C. Based on a JMAK analysis, the activation energies for the crystal growth and dissolution of akaganeite were 74 ± 8 kJ/mol and 125 ± 7 kJ/mol, respectively. Our calculated activation energies for hematite nucleation and crystal growth were 80 ± 13 kJ/mol and 110 ± 21 kJ/mol, respectively.

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