Transformation of Two-Line Ferrihydrite to Goethite and Hematite as a Function of pH and Temperature

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• 作者：Soumya Das ; M. Jim Hendry ; Joseph Essilfie-Dughan
• 刊名：Environmental Science & Technology
• 出版年：2011
• 出版时间：January 1, 2011
• 年：2011
• 卷：45
• 期：1
• 页码：268-275
• 全文大小：341K
• 年卷期：v.45,no.1(January 1, 2011)
• ISSN：1520-5851

文摘

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 掳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 掳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.