Benzene Ring-Driven Metal Hydrolysis and Floc Formation in Coagulation: Mechanistic Insights from Organic Structural Units.

The chemical structure of natural organic matter (NOM) is known to influence its removal during coagulation, yet the underlying molecular-scale mechanisms remain elusive. This has limited the rational design of advanced water treatment processes. Here, we reveal a previously overlooked mechanism, the cation-π interaction, as a key driver for the efficient removal of aromatic organic matter. Using model compounds with and without a benzene ring (benzoate/phthalate vs acetate), we demonstrate that the benzene ring is not merely a passive scaffold but an active participant in coagulation. It acts as an initial anchor, attracting trivalent metal ions via strong cation-π interactions. This initial attraction then facilitates a more stable, secondary binding as functional groups (e.g., -COOH and -OH) on the ring chelate with the metal ions, ultimately promoting floc growth. This dual-binding mechanism, supported by spectroscopic and microscopic evidence, explains how the presence of a benzene ring participates in metal ion hydrolysis and floc formation, leading to significantly improved coagulation performance. This finding highlights the critical role of aromatic structures in the coagulation process and provides a new theoretical foundation for optimizing coagulants to achieve the selective and efficient removal of specific aromatic pollutants.