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

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.