Effect of Formation pH on the Structure of Ferrihydrite and Its Photochemical Activity

Acid mine drainage (AMD) is a widespread environmental pollution that releases millions of tons of acidic, metal-laden mine effluents into surface waters, where steep pH gradients (from strongly acidic (pH < 3) to near-neutral) promote the continuous formation of metastable ferrihydrite (Fh). Under light irradiation, Fh can undergo photochemical reactions that influence iron redox cycling and contaminant behavior; however, how Fh formation pH controls its subsequent photochemical reactivity and environmental function remains poorly understood. Herein, Fh was synthesized via the hydrolysis of Fe3+ under pH conditions ranging from 3 to 7 to simulate its formation, and its structural and physicochemical properties (including morphology, crystallinity, and particle size) as well as photochemical activities were then systematically characterized. The results revealed that Fh synthesized under lower pH conditions (e.g., pH ∼3 in AMD upstream) exhibits low crystallinity and pronounced photoreduction, leading to substantial Fe2+ release, continuous surface renewal, and potential dissolution-precipitation-driven iron redistribution; whereas in neutral downstream environments (pH ∼7), elevated pH drives the formation of Fh with higher crystallinity, resulting in weakened photoreduction activity, reduced Fe2+ release, and mineral deposition. Importantly, structure-activity fitting revealed that abundant surface Fe-OH govern the photolysis and ligand-to-metal charge transfer (LMCT) activity of Fh, which in turn influences •OH generation. These results indicate that Fh in AMD systems functions not as a passively aging mineral particle, but as a dynamically renewed photoreactive phase whose redox activity, aging behavior, and phase transformation are preconditioned by Fh formation pH. This study provides novel insights into the spatial variability of iron mobility and contaminant fate in AMD systems, with implications for predictive modeling and remediation strategies.