Unveiling the Promoting Mechanism of Humic Acid in Photocatalysis via Bismuth Heavy-Atom Effect.

Humic acid (HA) can contribute to photochemical transformation of organic pollutants in natural waters via triplet-excited states (3HA*). However, in HA-containing water matrices for practical photocatalytic treatment (e.g., surface waters or wastewater effluents), HA is conventionally regarded as a persistent inhibitor; its rapid quenching of reactive species dominates the process, while the beneficial 3HA* pathway remains restricted by spin-forbidden singlet-to-triplet conversion. Herein, a heavy-atom-effect-driven molecular assembly with a bismuth catalyst was designed to overcome the spin-forbidden barrier limiting 3HA* formation. This strategy converted HA into an in situ promoter and increased antibiotic degradation by 4.24-fold. The catalyst formed a close-contact electron donor-acceptor (EDA) complex with HA, through which the bismuth heavy-atom effect enhanced spin-orbit coupling in attached HA, accelerated intersystem crossing, and raised 3HA* steady-state concentration by 12.1-fold. Explainable machine learning and multidimensional spectroscopy further revealed the molecular selectivity of this enhanced pathway, showing that higher-molecular-weight HA components rich in oxygenated groups and conjugated aromatic skeletons preferentially participated in the Bi3-P-enhanced photochemical process and were transformed into smaller products. The mechanism was validated in real water matrices, indicating that heavy-atom-enabled intersystem crossing can be leveraged to improve photocatalytic tolerance to organic backgrounds and guide photochemical design for water treatment.