Gallic acid-mediated semiquinone radicals drive a self-sustaining fenton-like system for PAH remediation in contaminated soil.

The in-situ remediation of recalcitrant polycyclic aromatic hydrocarbons (PAHs) in heavily contaminated coking plant soils is severely constrained by the classic Fenton process, which fails to generate hydroxyl radicals (•OH) efficiently at circumneutral pH due to catalyst deactivation. Here, we demonstrate that this critical bottleneck can be overcome by harnessing gallic acid (GA), a natural polyphenol, to engineer a self-sustaining catalytic cycle. Our optimized Fe(III)/GA/H2O2 system achieved a remarkable 99.4% degradation of high-concentration PAHs (2149.69 mg/kg) in authentic aged soil within 120 min at its natural pH. Mechanistic investigations revealed that the system's high reactivity across a broad pH range (3.0-8.0) is rooted in the pH-dependent formation of highly stable and catalytically potent 1:2 and 1:3 Fe(III)-GA complexes. More critically, we identified a radical-mediated feedback loop as the core of the catalytic engine: while •OH was confirmed as the primary oxidant for PAH destruction, its continuous generation is sustained by the GA-to-semiquinone radical (SQ•) transformation, which in turn rapidly regenerates Fe(II) from Fe(III). This work unveils a previously overlooked mechanism wherein a natural polyphenol actively participates in and accelerates the catalytic cycle via radical intermediates. This strategy of engineering a self-sustaining •OH generation engine offers a robust and green solution for remediating heavily contaminated industrial sites.