Understanding the Importance of Labile Fe(III) during Fe(II)-Catalyzed Transformation of Metastable Iron Oxyhydroxides

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)(aq)). Recent evidence points to the solution mass transfer of labile Fe(III) (Fe(III)(labile)) 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)(labile )throughout the Fe(II)-catalyzed trans- formation of lepidocrocite (Lp) versus ferrihydrite (Fh) at equal Fe(III) mass loadings with 0.2-1.0 mM Fe(II)(aq) at pH = 7.0. Similar to Fh, the conversion of Lp to product phases occurs by a dissolution-reprecipitation mechanism mediated by Fe(III)(labile) 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)(labile) 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.

Automated summary

The transformation of metastable Fe(III) oxyhydroxides is an important process in natural environments, which can be accelerated by the presence of Fe(II). Recent evidence suggests that the solution mass transfer of labile Fe(III) is a key intermediate species. However, the dependency of phase outcomes on the identity of the metastable Fe(III) oxyhydroxide precursor is unclear. In this study, we compared the transformation of lepidocrocite and ferrihydrite and found that different precursors lead to different products. These findings enhance our understanding of factors that affect iron bioavailability, system redox potential, and the fate and transport of coupled elements.