Fast transformation of iron oxyhydroxides by the catalytic action of aqueous Fe(II)
Fast transformation of iron oxyhydroxides by the catalytic action of aqueous Fe(II)
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DOI：
10.1016/j.gca.2005.03.016
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发表时间：
2005-08
影响因子：
5
通讯作者：
H. Pedersen;D. Postma;R. Jakobsen;O. Larsen
中科院分区：
文献类型：
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作者：
H. Pedersen;D. Postma;R. Jakobsen;O. Larsen
当进入缺氧环境时，氧化铁可能会发生结构变化。利用水中Fe(II)和氧化铁之间的同位素交换研究了这些转变。55Fe被一致地结合到一个亚铁水合物中，两个轻铁镁石(#1和#2)分别在10℃和25℃下合成，一个针铁矿和一个赤铁矿。然后将氧化铁浸入pH为6.5的Fe2+溶液(0-1.0 mm)中。在Fe~(2+)存在的条件下，水合铁酸铁、两种轻铁铁矿和针铁矿都能迅速释放~(55)Fe，而在无Fe~(2+)存在的情况下，没有观察到~(55)Fe的释放，赤铁矿即使在较高的Fe~(2+)浓度下也不能释放~(55)Fe。其释放速率主要受氧化铁的性质控制，而Fe2+浓度的影响较小。亚铁水合物和5纳米的锂镁石晶体在几天内就与水中的Fe(II)达到了完全的同位素平衡。在这段时间内，亚铁水合物完全转变成新的、更稳定的相，如锂镁铁矿和针铁矿。2号锂铁石和针铁矿具有较大的颗粒，在实验的时间范围内没有达到同位素平衡；然而，55Fe的持续缓慢释放表明最终将达到同位素平衡。我们的结果表明，Fe(II)水溶液的催化作用导致了固体Fe(III)相的再结晶。因此，氧化铁应该被适当地视为当暴露在可变的氧化还原条件下时会改变组成的动态相。这些结果需要重新评估还原条件下痕量金属的释放、铁氧化物对重金属的隔离以及稳定的铁同位素特征的重要性的现有模型。
Iron oxides may undergo structural transformations when entering an anoxic environment. These transformations were investigated using the isotopic exchange between aqueous Fe(II) and iron oxides in experiments with55Fe-labelled iron oxides.55Fe was incorporated congruently into a ferrihydrite, two lepidocrocites (#1 and #2), synthesised at 10°C and 25°C, respectively, a goethite and a hematite. The iron oxides were then submerged in Fe2+solutions (0–1.0 mM) with a pH of 6.5. In the presence of aqueous Fe2+, an immediate and very rapid release of55Fe was observed from ferrihydrite, the two lepidocrocites and goethite, whereas in the absence of Fe2+no release was observed.55Fe was not released from hematite, even at the higher Fe2+concentration. The release rate is mainly controlled by characteristics of the iron oxides, whereas the concentration of Fe2+only has minor influence. Ferrihydrite and 5-nm-sized lepidocrocite crystals attained complete isotopic equilibration with aqueous Fe(II) within days. Within this timeframe ferrihydrite transformed completely into new and more stable phases such as lepidocrocite and goethite. Lepidocrocite #2 and goethite, having larger particles, did not reach isotopic equilibrium within the timeframe of the experiment; however, the continuous slow release of55Fe suggests that isotopic equilibrium will ultimately be attained. Our results imply a recrystallization of solid Fe(III) phases induced by the catalytic action of aqueous Fe(II). Accordingly, iron oxides should properly be considered as dynamic phases that change composition when exposed to variable redox conditions. These results necessitate a reevaluation of current models for the release of trace metals under reducing conditions, the sequestration of heavy metals by iron oxides, and the significance of stable iron isotope signatures.