Chromium Reaction Mechanisms for Speciation using Synchrotron in-Situ High-Temperature X-ray Diffraction

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• 作者：Fiona Low ; Justin Kimpton ; Siobhan A. Wilson ; Lian Zhang
• 刊名：Environmental Science & Technology
• 出版年：2015
• 出版时间：July 7, 2015
• 年：2015
• 卷：49
• 期：13
• 页码：8246-8253
• 全文大小：356K
• ISSN：1520-5851

文摘

We use in situ high-temperature X-ray diffraction (HT-XRD), ex-situ XRD and synchrotron X-ray absorption near edge structure spectroscopy (XANES) to derive fundamental insights into mechanisms of chromium oxidation during combustion of solid fuels. To mimic the real combustion environment, mixtures of pure eskolaite (Cr³⁺₂O₃), lime (CaO) and/or kaolinite [Al₂Si₂O₅(OH)₄] have been annealed at 600鈥?200 掳C in air versus 1% O₂ diluted by N₂. Our results confirm for the first time that (1) the optimum temperature for Cr⁶⁺ formation is 800 掳C for the coexistence of lime and eskolaite; (2) upon addition of kaolinite into oxide mixture, the temperature required to produce chromatite shifts to 1000 掳C with a remarkable reduction in the fraction of Cr⁶⁺. Beyond 1000 掳C, transient phases are formed that bear Cr in intermediate valence states, which convert to different species other than Cr⁶⁺ in the cooling stage; (3) of significance to Cr mobility from the waste products generated by combustion, chromatite formed at >1000 掳C has a glassy disposition that prevents its water-based leaching; and (4) Increasing temperature facilitates the migration of eskolaite particles into bulk lime and enhances the extent to which Cr³⁺ is oxidized, thereby completing the oxidation of Cr³⁺ to Cr⁶⁺ within 10 min.