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

Magnetite (Fe3O4), 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 Fe3O4 that was associated with the production of dark hydroxyl radicals (center dot OH) and the oxidation of dissolved organic matter (DOM). This study revealed that Fe3O4 sulfidated by sulfide (S-Fe3O4) at neutral pH exhibited higher center dot OH production upon oxygenation than Fe3O4, in which the cumulative center dot 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-Fe3O4 showed that sulfidation enables electron storage in Fe3O4 by increasing both structural and surface Fe(II). Sulfide was converted into S0, acid volatile sulfur (AVS), and chromium-reducible sulfur (CRS) during Fe3O4 sulfidation. S-Fe3O4 with lower AVS/CRS ratio exhibited higher reactivity to produce center dot OH, indicating the important role of CRS in transferring electrons from Fe(II) to O2. Based on quenching experiments and electron paramagnetic resonance analysis, a one-step two-electron transfer mechanism was proposed for O2 reduction during S-Fe3O4 oxygenation, and surface-bound rather than free center dot OH were identified as the primary reactive oxygen species. The center dot OH from S-Fe3O4 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 Fe3O4 for dark center dot OH production, having an important impact on the carbon cycling in subsurface environments.

Automated summary

Sulfidation-oxidation treatment of magnetite (Fe3O4) enhances the production of dark center dot OH, which can efficiently degrade dissolved organic matter (DOM) and accelerate carbon cycling.