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

Abstract

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 ~5525s and 537s at 80°C and 100°C, respectively, yielding an activation energy of 129±15kJ/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 0kJ/mol between 150 and 200°C. Hematite nucleation induction times decreased with increasing temperature from 1723s to 110s between 150 and 200°C. Based on a JMAK analysis, the activation energies for the crystal growth and dissolution of akaganeite were 74±8kJ/mol and 125±7kJ/mol, respectively. Our calculated activation energies for hematite nucleation and crystal growth were 80±13kJ/mol and 110±21kJ/mol, respectively.