Abstract Ferrihydrite (Fh) is a major Fe(III)-(oxyhydr)oxide nanomineral distinguished by its poor crystallinity and thermodynamic metastability. While it is well known that in suboxic conditions aqueous Fe(II) rapidly catalyzes Fh transformation to more stable crystalline Fe(III) phases such as lepidocrocite (Lp) and goethite (Gt), because of the low solubility of Fe(III) the mass transfer pathways enabling these rapid transformations have remained unclear for decades. Here, using a selective extractant, we isolated and quantified a critical labile Fe(III) species, one that is more reactive than Fe(III) in Fh, formed by the oxidation of aqueous Fe(II) on the Fh surface. Experiments that compared time-dependent concentrations of solid-associated Fe(II) and this labile Fe(III) against the kinetics of phase transformation showed that its accumulation is directly related to Lp/Gt formation in a manner consistent with the classical nucleation theory. 57Fe isotope tracer experiments confirm the oxidized Fe(II) origin of labile Fe(III). The transformation pathway as well as the accelerating effect of Fe(II) can now all be explained on a unified basis of the kinetics of Fe(III) olation and oxolation reactions necessary to nucleate and sustain growth of Lp/Gt products, rates of which are greatly accelerated by labile Fe(III).

中文翻译：

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来自吸附的 Fe(II) 氧化的不稳定 Fe(III) 是 Fe(II) 催化的水铁矿转化的关键中间体

摘要 水铁矿 (Fh) 是一种主要的 Fe(III)-(羟基) 氧化物纳米矿物，其特点是结晶度和热力学亚稳定性差。众所周知，在低氧条件下，Fe(II) 水溶液会迅速催化 Fh 转化为更稳定的结晶 Fe(III) 相，例如纤铁矿 (Lp) 和针铁矿 (Gt)，因为 Fe(III) 的溶解度低几十年来，实现这些快速转变的传质途径一直不清楚。在这里，我们使用选择性萃取剂分离并量化了一种关键的不稳定 Fe(III) 物质，该物质比 Fh 中的 Fe(III) 更具反应性，由 Fh 表面上的 Fe(II) 水溶液氧化形成。将固体相关 Fe(II) 和这种不稳定 Fe(III) 的时间依赖性浓度与相变动力学进行比较的实验表明，其积累与 Lp/Gt 形成直接相关，其方式与经典成核理论一致。57Fe 同位素示踪实验证实了不稳定 Fe(III) 的氧化 Fe(II) 来源。Fe(II) 的转化途径和加速作用现在都可以在 Fe(III) 化和氧化反应动力学的统一基础上进行解释，这些动力学是 Lp/Gt 产物成核和维持生长所必需的，其速率不稳定的 Fe(III) 大大加速。