Formation-pH-Dependent Structural Difference as a Significant Control on Ferrihydrite Transformation Behaviors

Ferrihydrite (Fhy) is a metastable iron (oxyhydr)oxide whose transformation into more stable crystalline phases is an important manifestation of its reactivity. However, whether nominally identical 2-line Fhy samples retain formation-dependent differences in short-range structure and how these differences control subsequent phase transformation behavior remain poorly understood. Here, Fhy was synthesized at different pH levels to establish formation-pH-dependent variations in the structure and phase transformation. Although all products were identified as 2-line Fhy by conventional X-ray diffraction, low-pH Fhy contained more hydroxyl-related components and exhibited lower structural order, while the local Fe coordination environments varied with the formation pH. These structural differences strongly affected the subsequent transformation. Under aqueous conditions, Fhy formed at pH 3.0 showed a greater tendency toward dissolution-mediated transformation to goethite, whereas Fhy formed at higher pH values generally yielded a greater relative proportion of hematite. Under nonaqueous conditions, the greater hydroxyl content of low-pH Fhy facilitated dehydroxylation and promoted hematite formation at a lower temperature and shorter reaction time. These results demonstrate that the formation pH leaves a persistent structural imprint on Fhy that governs hydroxylation degrees, Fe coordination environments, and phase transformation pathways. This work establishes formation-controlled short-range structures as a vital determinant of the transformation behavior of metastable materials.