115-1 Transformation of Hydroxycarbonate Green Rust (GR1(CO32-)) into Ferric (oxyhydr)Oxides: The Role of Silicate.

See more from this Division: SSSA Division: Soil Mineralogy
See more from this Session: Soil Mineralogy: I (Includes Student Competition)

Monday, November 16, 2015: 2:45 PM
Minneapolis Convention Center, 103 A

Xionghan Feng1, Xiaoming Wang2, Mengqiang Zhu3, Luuk Koopal4, Wenfeng Tan5 and Fan Liu5, (1)Huazhong Agricultural University (HAU), Wuhan, Hubei, CHINA
(2)Ecosystem Science and Management, University of Wyoming, Laramie, WY
(3)Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY
(4)Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Wageningen, Netherlands
(5)Huazhong Agricultural University, Wuhan, China
Abstract:
Green rusts (GRs) commonly occur in the environment and affect the redox processes in suboxic environments, such as paddy soils, ground-water, sediments and iron corrosion products. The transformation of GR1(CO32-) was conducted in the presence of silicate (Si) and followed by combining solution analysis and product characterizations by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). Results indicate that Si significantly affects the transformation of GR1(CO32-) through adsorption on the mineral surfaces. The transformation time and the type of transformation products and their crystallinity and morphology all depend on the Fe/Si molar ratio. The transformation can be promoted in the presence of low Si concentration as compared to the control without Si. The products are goethite for Fe/Si ratios of 48 - 12, but the crystallinity and particle size of goethite decrease and its morphology changes from acicular (absence of Si) to plate-like or isodimensional particles with decreasing Fe/Si, comparable to those of natural goethite samples commonly found in soils. At Fe/Si = 3, the products are platelets of EX-GR1 with some goethite and ferrihydrite. The possible pathway of oxidative GR1(CO32−) transformation in the presence low concentrations of Si (Fe/Si ≥ 12) and in the absence of Si could be denoted as GR1(CO32−) → amorphous γ-FeIIIOOH-like phase → α-FeIIIOOH via a dissolution-oxidation-precipitation mechanism. In addition, the transformation of amorphous γ-FeIIIOOH-like phase to α-FeIIIOOH is rapidly catalyzed by Fe2+ released during oxidative transformation of GR1(CO32−) into amorphous FeIIIOOH. Clearly, the oxidative transformation of green rust to various crystalline iron (oxyhydr)oxides depends on the type and concentration (Fe/anion molar ratio) of co-existing silicate and other anions. Therefore, it is suggested that the prominent occurrence of natural goethite in soils in the form of plate-like or isodimensional particles will be, at least partly, related to the ubiquitous presence of silicates in soil environments.

See more from this Division: SSSA Division: Soil Mineralogy
See more from this Session: Soil Mineralogy: I (Includes Student Competition)

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