The Key Laboratory of Water and Sediment Sciences, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China.
Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, 1102 South Goodwin Avenue, Urbana, Illinois 61801, United States.
Environ Sci Technol. 2022 Mar 15;56(6):3801-3811. doi: 10.1021/acs.est.1c08044. Epub 2022 Feb 21.
Transformation of metastable Fe(III) oxyhydroxides is a prominent process in natural environments and can be significantly accelerated by the coexisting aqueous Fe(II) (Fe(II)). Recent evidence points to the solution mass transfer of labile Fe(III) (Fe(III)) as the primary intermediate species of general importance. However, a mechanistic aspect that remains unclear is the dependence of phase outcomes on the identity of the metastable Fe(III) oxyhydroxide precursor. Here, we compared the coupled evolution of Fe(II) species, solid phases, and Fe(III) throughout the Fe(II)-catalyzed transformation of lepidocrocite (Lp) versus ferrihydrite (Fh) at equal Fe(III) mass loadings with 0.2-1.0 mM Fe(II) at pH = 7.0. Similar to Fh, the conversion of Lp to product phases occurs by a dissolution-reprecipitation mechanism mediated by Fe(III) that seeds the nucleation of products. Though for Fh we observed a transformation to goethite (Gt), accompanied by the transient emergence and decline of Lp, for initial Lp we observed magnetite (Mt) as the main product. A linear correlation between the formation rate of Mt and the effective supersaturation in terms of Fe(III) concentration shows that Fe(II)-induced transformation of Lp into Mt is governed by the classical nucleation theory. When Lp is replaced by equimolar Gt, Mt formation is suppressed by opening a lower barrier pathway to Gt by heterogeneous nucleation and growth on the added Gt seeds. The collective findings add to the mechanistic understanding of factors governing phase selections that impact iron bioavailability, system redox potential, and the fate and transport of coupled elements.
不稳定态 Fe(III) 氢氧化物的转化是自然环境中的一个重要过程，共存的水合态 Fe(II)（Fe(II)）可显著加速这一过程。最近的证据表明，不稳定态 Fe(III)（Fe(III)）的溶液传质是普遍重要的主要中间物种。然而，一个尚未阐明的机制方面是，相态结果对不稳定态 Fe(III) 氢氧化物前体的身份的依赖性。在这里，我们比较了在 pH = 7.0 时，等 Fe(III) 质量负载下，在 0.2-1.0 mM Fe(II) 存在下，针铁矿（Lp）与水铁矿（Fh）的 Fe(II) 催化转化过程中，Fe(II) 物种、固体相和 Fe(III) 的耦合演化。与 Fh 相似，Lp 向产物相的转化是通过溶解-再沉淀机制发生的，该机制由引发产物成核的 Fe(III) 介导。尽管对于 Fh，我们观察到向赤铁矿（Gt）的转化伴随着 Lp 的短暂出现和消失，但对于初始 Lp，我们观察到磁铁矿（Mt）是主要产物。Mt 的形成速率与 Fe(III) 浓度有效过饱和度之间的线性相关性表明，Fe(II) 诱导的 Lp 向 Mt 的转化受经典成核理论控制。当 Lp 被等摩尔 Gt 取代时，由于在添加的 Gt 种子上异质成核和生长开辟了通向 Gt 的较低势垒途径，Mt 的形成受到抑制。总的来说，这些发现增加了对控制相选择的因素的机制理解，这些因素影响铁的生物利用度、系统氧化还原电位以及耦合元素的命运和迁移。
