Soluble Mn(III) drives the oxidative polymerization of phenolics into toxic dissolved oligomers.

Phenolic compounds are widespread aromatic contaminants in aquatic environments and are of particular concern before disinfection because of their toxicity and potential contribution to disinfection byproduct formation. Although manganese-based oxidation processes are widely applied to remove phenolic contaminants, reliance on parent-compound disappearance alone may overlook toxic transformation products generated during oxidation. Herein, we demonstrated that ligand-stabilized soluble Mn(III), a reactive intermediate in Mn redox cycling, redirects phenolic transformation from apparent removal toward the formation of dissolved oligomeric products with elevated residual risks. Representative phenolic compounds were investigated in ligand-stabilized soluble Mn(III) systems using Mn(III)-pyrophosphate (Mn(III)-PP) as a model complex. Soluble Mn(III) rapidly promoted parent-compound disappearance; however, this apparent removal was accompanied by the formation of dissolved oligomeric products. Particularly in halogenated phenolic systems, these residual oligomeric mixtures exhibited higher toxicity and greater predicted ecological risks than the corresponding parent compounds. Mechanistic analyses reveal that soluble Mn(III) mediates phenolic transformation through hydrogen-bond-assisted outer-sphere pre-association, which facilitates single-electron transfer, phenoxyl radical formation, and subsequent coupling reactions. These findings uncover a previously overlooked pathway in which ligand-stabilized soluble Mn(III) converts phenolic compounds into persistent dissolved oligomers rather than simply detoxifying them. This work highlights important implications for drinking-water protection and Mn-mediated contaminant transformation in natural aquatic environments.