Electron-Driven Processes for Sustained Dark Hydroxyl Radical Generation under Redox Fluctuations in Mangrove Soils
Hydroxyl radicals (·OH) generated via dark reactions under fluctuating redox conditions significantly influence pollutant degradation and elemental cycling, yet the key electron supply mechanisms driving their sustained production remain unclear. This study aimed to elucidate the electron-driven processes underlying sustained dark ·OH generation in mangrove soils under redox fluctuations. Simulated tidal redox cycles were conducted, and electron donating capacity (EDC), three-dimensional fluorescence spectroscopy, nuclear magnetic resonance, and high-throughput sequencing were employed to analyze the dynamics of different Fe(II) species, key organic matter components, and microbial communities. Results demonstrated that, without exogenous electron donors, mangrove soils exhibited stable ·OH generation potential and EDC. During early redox cycles, total EDC of soil suspensions was dominated by reactive Fe(II), while the contribution of reduced organic matter increased over time, with solid-phase components accounting for 94.5%–97.6% of total EDC. Humic acids in soil organic matter facilitated reversible electron transfer via quinone functional groups, maintaining redox activity. Geothermobacter and Desulfobulbus were identified as dominant iron-reducing bacteria, likely key microorganisms for regenerating the "iron-organic matter" electron sources. This study reveals the mechanisms of endogenous electron donor regeneration and sustained dark ·OH generation mediated by iron and organic matter cycling in mangrove soils, providing theoretical support for understanding the long-term environmental effects of ·OH in tidal environments and its impact on biogeochemical cycles.