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Free keywords:
continuous reactive crystallization
nanoscale 6-line ferrihydrite
in-situ synchrotron
2-line ferrihydrite
iron-oxides
goethite
kinetics
transformation
mechanisms
diffraction
Chemistry
Crystallography
Materials Science
Abstract:
The transformation of ferrihydrite (5Fe(2)O(3)center dot 9H(2)O) to hematite (alpha-Fe2O3) under alkaline condition in the presence and absence of lead was for the first time investigated using in situ, time-resolved synchrotron-based energy dispersive X-ray diffraction combined with off-line chemical characterization and imaging. The results showed that the crystallization of hematite occurred via a two-stage process with goethite (alpha-FeOOH) as an intermediate phase. The presence of lead enhanced the formation of hematite and reduced the induction times (similar to 20-30%) but had little effect on the mechanism of the transformation reactions. The reaction rates for the two systems (with and without lead) ranged from 12 to 259 x 10(-4) s(-1) and 19 to 461 x 10(-5) s(-1) for the first and second stage, respectively. The activation energies of nucleation of the two systems were 16(+/- 3) and 9(+/- 2) kJ/mol, while the activation energies for crystallization ranged from 41(+/- 7) to 77(+/- 14) kJ/mol. During the hematite crystallization, the majority of the lead in the system was rapidly and irreversibly incorporated into the final hematite, while only minor amounts of lead were released back into solution.
