Transformation of two-line ferrihydrite to goethite and hematite as a function of pH and temperature.
Under oxic aqueous conditions, two-line ferrihydrite gradually transforms to more thermodynamically stable and more crystalline phases, such as goethite and hematite. This temperature- and pH-dependent transformation can play an important role in the sequestration of metals and metalloids adsorbed onto ferrihydrite. A comprehensive assessment of the crystallization of two-line ferrihydrite with respect to temperature (25, 50, 75, and 100 °C) and pH (2, 7, and 10) as a function of reaction time (minutes to months) was conducted via batch experiments. Pure and transformed phases were characterized by X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The rate of transformation of two-line ferrihydrite to hematite increased with increasing temperature at all pHs studied and followed first-order reaction kinetics. XRD and XANES showed simultaneous formation of goethite and hematite at 50 and 75 °C at pH 10, with hematite being the dominant product at all pHs and temperatures. With extended reaction time, hematite increased while goethite decreased, and goethite reaches a minimum after 7 days. Observations suggest two-line ferrihydrite transforms to hematite via a two-stage crystallization process, with goethite being intermediary. The findings of this study can be used to estimate rates of crystallization of pure two-line ferrihydrite over the broad range of temperatures and pH found in nature.
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Canada. sod671@mail.usask.ca
Soumya
S
Das
M Jim
MJ
Hendry
Joseph
J
Essilfie-Dughan
D000327:Adsorption; D005290:Ferric Compounds / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000737:chemistry; D058085:Iron Compounds / Q000737:chemistry*; D007700:Kinetics; D018625:Microscopy, Atomic Force; D008855:Microscopy, Electron, Scanning; D008903:Minerals / Q000737:chemistry*; D013499:Surface Properties; D013696:Temperature; D013995:Time; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
D005290:Ferric Compounds; D006878:Hydroxides; D058085:Iron Compounds; D008903:Minerals; C094886:goethite; C000499:ferric oxide; C092844:ferric oxyhydroxide