Fluctuating redox conditions accelerate the electron storage and transfer in magnetite and production of dark hydroxyl radicals.

Magnetite (FeO), known as a geo-battery that can store and transfer electrons, often co-occurs with sulfide in subsurface environments with fluctuating redox conditions. However, little is known about how fluctuating redox conditions (e.g., sulfidation-oxidation) affect the electron storage and transfer in FeO that was associated with the production of dark hydroxyl radicals (⋅OH) and the oxidation of dissolved organic matter (DOM). This study revealed that FeO sulfidated by sulfide (S-FeO) at neutral pH exhibited higher ⋅OH production upon oxygenation than FeO, in which the cumulative ⋅OH concentration increased with increasing initial S/Fe ratio (≤ 0.50), sulfidation duration and number of sulfidation-oxidation cycle. X-ray photoelectron spectroscopy and wet-chemical analyses of Fe and S species of S-FeO showed that sulfidation enables electron storage in FeO by increasing both structural and surface Fe(II). Sulfide was converted into S, acid volatile sulfur (AVS), and chromium-reducible sulfur (CRS) during FeO sulfidation. S-FeO with lower AVS/CRS ratio exhibited higher reactivity to produce ⋅OH, indicating the important role of CRS in transferring electrons from Fe(II) to O. Based on quenching experiments and electron paramagnetic resonance analysis, a one-step two-electron transfer mechanism was proposed for O reduction during S-FeO oxygenation, and surface-bound rather than free ⋅OH were identified as the primary reactive oxygen species. The ⋅OH from S-FeO oxygenation was shown to be efficient in degradation of DOM. Overall, these results suggested that sulfidation-oxidation can accelerate the electron storage and transfer in FeO for dark ⋅OH production, having an important impact on the carbon cycling in subsurface environments.