Dark Evolution of Oxalate-Derived Radicals Unlocks Sustainable Decontamination via Soil Component Interactions

We report a previously unrecognized dark aerobic pathway for radical formation from low-molecular-weight organic acids (LMWOAs)─distinct from well-documented photogenerated routes─that drives efficient soil decontamination. In ten ofloxacin-spiked soils with varying properties, oxalic acid (OA) boosted 24 h ofloxacin degradation from 20% (without LMWOAs) to 79% (7.5 mM OA), outperforming equimolar malic and citric acids. The degradation kinetics are governed by LMWOA molecular configurations─specifically, fewer carboxyl groups and lower pKa1 values that enhance reactivity─and correlate with soil organic matter (SOM) and iron contents. By combining a newly developed in situ fluorescence detector with quenching experiments, we demonstrated that in four representative soils with contrasting SOM-Fe combinations, higher Fe promotes the yields of •OH, O2•–, 1O2, and COO•–, but the coexisting SOM acts as the primary quencher. Validation experiments using artificial sand-based model soils and theoretical calculations revealed that crystalline Fe minerals (e.g., hematite), rather than dissolved Fe ions, drive radical evolution: oxalate coordinates to surface hydroxyls as a bifunctional ligand bearing electron-withdrawing carboxylates and transferable α-H, forming unstable ≡Fe(III)-oxalate complexes that undergo heterolysis and subsequent radical chain reactions, efficiently generating reactive species. Our study expands the understanding of LMWOA-derived radical geochemistry and provides a basis for nature-based, low-cost soil remediation using naturally occurring organic acids.