Ferrous Oxidation-Driven Adsorptive and Oxidative Transformation Boosts Electron Exchange Capacity of Dissolved Organic Matter: Environmental Implications

Dissolved organic matter (DOM) plays an important role in microbial electron transfer, influencing elemental biogeochemical cycles and pollutant fate. However, its electron exchange capacity (EEC) can be strongly altered by ferrous {Fe(II)} oxidation-driven adsorptive fractionation and chemical transformations under long-term periodic hydrological fluctuations, which remain largely unexplored. Here, we demonstrate that periodic Fe(II) oxidation progressively increases DOM's EEC by up to 6.2-fold. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis revealed that Fe(II) oxidation increased the low-molecular-weight polyphenolic and highly unsaturated phenolic fractions of DOM by 26.3 and 24.4%, respectively, and enriched quinone and phenolic groups that can mediate electron transfer. Meanwhile, integration of FT-ICR MS data with machine learning identified the changes in molecular weight and O/C ratio as the critical characteristics determining the enhanced EEC of DOM, with newly produced sulfur-containing groups contributing thereafter. Both adsorptive fractionation and reactive oxygen species-triggered oxidative transformation during Fe(II) oxidation determined the increased EEC of DOM, with adsorptive fractionation playing the dominant role. These dynamic DOM changes significantly promoted microbial electron transfer, continuously stimulating iron reduction and concurrent arsenic release in paddy soils, thereby potentially exacerbating toxic metal bioavailability and posing risks to food safety.