Thermodynamic Insights into Anoxic Arsenic(III) Oxidation: The Critical Role of Oxygen Vacancies in Regulating Redox Reactivity of Fe(II) Bound to Fe(III) (Oxyhydr)oxide Redox Couples

Ferrous iron (Fe(II)) bound to Fe(III) minerals redox couples are ubiquitous in sediments and groundwater, playing a crucial role in arsenic migration, transformation, and associated risks. Although Fe(III) oxides–Fe(II) couples are well known for their reductive transformation of pollutants, the thermodynamic mechanisms driving anoxic As(III) oxidation observed in these systems remain poorly understood. In this study, we combined high-resolution transmission electron microscopy–selected area electron diffraction mapping, Mössbauer spectroscopy, and density functional theory calculations to investigate whether anoxic As(III) oxidation in hematite–Fe(II) couples is influenced by oxygen vacancies (Ov), an intrinsic characteristic of iron (oxyhydr)oxide minerals. Batch experiments demonstrated that Ov in hematite significantly enhanced the As(III) oxidation efficiency (62.7%) compared to defect-free hematite (45.9%) at pH 6 with 0.2 mM Fe(II). Spectroscopic and microscopic analyses revealed recrystallization processes that generated active metastable iron phases after Fe(II) adsorption, acting as electron acceptors for As(III) oxidation. Notably, the defective hematite–Fe(II) couples generated a higher content of active iron phase compared to their defect-free counterparts, which was attributed to thermodynamically favorable electron transfer between Fe(II) and the defective hematite due to the relatively higher redox potential of defective hematite ( E H 0 = +1297 mV vs +670 mV). These findings provide evidence of the thermodynamically driven anoxic transformation of As(III), emphasizing the critical role of oxygen vacancies in regulating redox reactivity in oxide-Fe(II) systems.