A TIME-RESOLVED SYNCHROTRON X-RAY DIFFRACTION STUDY OF THE IN SITU, HYDROTHERMAL SYNTHESIS OF GOETHITE FROM 2-LINE FERRIHYDRITE

The hydrothermal transformation of 2-line ferrihydrite to goethite at pH 13.6 was studied at 80, 90, and 100 °C using time-resolved, angle-dispersive X-ray diffraction at the Advanced Photon Source, Argonne National Lab. We found that these in situ experiments could be successfully performed by injecting freshly made ferrihydrite gels into polyimide capillaries that were then sealed with fast curing epoxy. The integrity of these reaction cells held steady when heated to 100 °C for up to 6 hr, and the low background scattering from the polyimide allowed for an extremely high sensitivity for the detection of weak X-ray diffraction peaks from the first goethite nanocrystals to nucleate.

Rietveld analysis of the time-resolved series of X-ray diffraction patterns as goethite crystallized enabled a high-resolution extraction of crystallographic and kinetic data. Particle sizes for goethite increased with time at similar rates for all three temperatures, from particle diameters of ≈135 Å for the first crystals detected to ≈290 Å after 6 hr. With increasing particle size, unit-cell volume decreased, primarily as a result of a contraction along the c-axis, the direction of closest-packing (S.G. Pnma). At 100 °C cdecreased from 4.631 ± 0.001 to 4.613 ± 0.001 Å. We found no evidence for high vacancy concentrations in incipient goethite nanocrystals, in contrast to our prior study of hematite crystallization (Peterson, et al. 2015).

Two different kinetic models were used to calculate the activation energy of the reaction: a pseudo first-order model based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, and a linear Arrhenius analysis based on only the initial reaction rates. Both approaches yielded excellent fits to the data (R² > 0.95), but the calculated activation energies using the JMAK and linear models were 72.74 and 100.1 kJ/mol, respectively. This disparity reveals that the transformation of ferrihydrite to goethite was most temperature-dependent during the initial stages of crystallization.