Because of its sorption properties, transformation of the nanomineral ferrihydrite (Fh) into more stable lepidocrocite (Lp) or goethite (Gt) has important impacts on the fate of metals, nutrients, and contaminants in soils/sediments. Although it is well known that the transformation rate is greatly accelerated under suboxic conditions by aqueous Fe(II), the enabling mass transfer process remains an ongoing debate among various mechanisms including dissolution/reprecipitation, solid-state recrystallization, and particle-mediated growth. Here, using electron microscopy, we examine the mineralogical evolution of 2-line Fh to Lp/Gt catalyzed by Fe(II) under strict anoxic conditions, including evaluation of Cl–SO₄–HCO₃ anion effects. Emergence of Lp/Gt crystallites at the nanoscale was observed at ∼20 min of reaction, earlier than previously reported. Lp is the first phase to nucleate in Cl-rich solutions without HCO₃⁻; whereas Lp and Gt concomitantly nucleate in SO₄²⁻-rich solutions, and also when co-solute HCO₃⁻ is added. Lp crystallites nucleate as quasi-2D nanosheets one-unit-cell thick that contour the Fh surface; in contrast, rod-shaped (in Cl/SO₄) or acicular needle-shaped (in HCO₃) Gt crystals nucleate and grow radially outward from the Fh aggregates. Stages of transformation monitored by in situ μ-XRD coupled with aqueous Fe(II) uptake/release measurements are correlated with a short initial sorption stage followed by the onset of Lp/Gt growth that then progresses to Lp loss in favor of Gt. Microscopy data overwhelmingly support dissolution/reprecipitation as the underlying mechanism, including direct evidence for classical ion-by-ion Lp/Gt growth and Lp dissolution. The collective findings imply that the iron mass transfer through solution to distal Lp/Gt growth fronts is a critical enabling process facilitating rapid transformation.

中文翻译：

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Fe（II）引起的水铁矿转变的纳米尺度观察

由于其吸附特性，纳米矿物三水铁矿（Fh）转变为更稳定的纤铁矿（Lp）或针铁矿（Gt）会对土壤/沉积物中金属，养分和污染物的命运产生重要影响。尽管众所周知，Fe（II）水溶液在亚氧条件下可大大加快转化速率，但在包括溶解/再沉淀，固态重结晶和颗粒介导的生长在内的各种机制之间，是否能够进行传质过程仍是一个有争议的问题。在这里，我们使用电子显微镜检查了在严格的缺氧条件下，Fe（II）催化2线Fh到Lp / Gt的矿物学演变，包括对Cl–SO ₄ –HCO _3的评估。阴离子效应。在约20分钟的反应时间内，观察到了Lp / Gt微晶的出现，比以前报道的要早。LP是丰富的氯的解决方案，而无需核HCO第一阶段₃ ^- ; 而脂蛋白和GT伴随核在SO ₄ ^2-富解决方案，以及还共同溶质HCO时₃ ^-溶液。Lp微晶成核为准2D纳米片，厚度为1个单元格，可形成Fh表面轮廓。相反，棒状（Cl / SO ₄）或针状针状（HCO ₃）Gt晶体成核并从Fh聚集体径向向外生长。原位μ-XRD结合Fe（II）监测转化阶段）吸收/释放的测量值与较短的初始吸附阶段，随后的Lp / Gt生长开始，然后逐渐发展为Lp丢失，有利于Gt相关。显微镜数据绝大多数支持溶出/再沉淀作为潜在机制，包括经典的逐个离子Lp / Gt生长和Lp溶出的直接证据。集体发现表明，铁质通过溶液转移至远端Lp / Gt生长前沿是促进快速转变的关键使能过程。