Unveiling the unrecognized role of phytic acid in promoting hydroxyl radical generation during FeS colloids...

Interactions between iron (Fe) mineral colloids and phytic acid (myo-inositol hexaphosphate, IHP)-a widely distributed organic phosphorus compound-can strongly influence cycling of carbon and coupled elements in aquatic systems. While mechanisms of IHP interacting with Fe(III) minerals are understood, its role in modulating Fe(II)-sulfide (FeS) oxygenation and hydroxyl radical (•OH) generation at anoxic-oxic interfaces remains unresolved. Here, we demonstrate that IHP significantly enhances •OH production during FeS oxygenation at neutral pH in a concentration-dependent manner. Density functional theory calculations reveal that IHP preferentially coordinates with Top-Fe compared to Top-S sites on FeS surfaces, forming Fe(II)-IHP complexes that suppress structural Fe(II) oxidation. This shifts the oxygen reduction pathway from a two-electron to sequential single-electron pathway, increasing •OH yield by 2.0-8.9 fold. Spectroscopic and microscopic analyses show that IHP favors the formation of nanosized ferrihydrite over crystalline lepidocrocite, further enhancing •OH production. Additionally, IHP facilitates the release of surface-associated •OH into the aqueous phase, increasing availability to react with bulk constituents. Mineralization experiments show that 250 μM IHP enables FeS oxygenation to mineralize 7.4-41.9 % of dissolved organic matter (DOM, 1-15 mg C/L) to CO₂, while no mineralization occurs in IHP-free systems. Taken together, our results unveil previously unrecognized mechanisms by which IHP enhances •OH production during FeS oxygenation and provide new insights into organic carbon dynamics in IHP(-FeS) rich, redox-dynamic aquatic environments.