Sunlight-Driven Transformation of Ferrihydrite via Ligand-to-Metal Charge Transfer: The Critical Factors and Arsenic Repartitioning

Ferrihydrite, a poorly ordered metastable iron oxide, is closely associated with dissolved organic matter (DOM) in soils and sediments. Although sunlight-induced photoreductive dissolution of ferrihydrite via ligand-to-metal charge transfer (LMCT) has been extensively studied, its potential impacts on mineralogical transformation and environmental behaviors of coexisting contaminants remain largely unknown. Here, we systematically investigated the effects of environmental parameters (e.g., solution pH, pO2 level, arsenic speciation, and content) on ferrihydrite transformation with the representative DOM-oxalate under simulated solar irradiation. Results showed that the oxalate-mediated LMCT process synchronously initiated Fe(II) production and proton consumption, the latter of which facilitated interfacial electron transfer and atom exchange (IET-AEFh-Fe2+) processes among ferrihydrite and newly formed Fe(II). At pH 5.0–8.0, ferrihydrite was prone to transform into goethite due to sufficient Fe(II) (approximately 80–2700 μM) from LMCToxa and enough affinity of Fe(II) with mineral to trigger IET-AEFh-Fe2+, while it only underwent reductive dissolution at pH 3.0–5.0 or kept a quasi-steady state over pH 8.0. Increasing the pO2 level and arsenic content hampered the recrystallization of ferrihydrite by reducing Fe(II) duration or altering the surface property of ferrihydrite, whereas the presence of As(III/V) also led to the formation of lepidocrocite with As(V) being more prominent. Additionally, chemical extraction and As K-edge EXAFS spectroscopy revealed that As was consecutively incorporated into the structures of goethite and lepidocrocite in the form of As(V) regardless of primary As speciation. These findings shed novel insights into low-crystalline iron oxide transformation and element migration driven by sunlight in natural environments.