Humic acid acting as a photosensitizer enhances periodate activation under visible light irradiation for efficient microcontaminant abatement.

Photoactivated periodate (PI)-based advanced oxidation processes (AOPs) are effective for removing refractory micropollutants, but PI can hardly be activated by visible light alone. This study employed ubiquitous aquatic humic acid (HA) to boost visible light-driven PI activation. 2 mgC/L HA increased naproxen (NAP) removal in the Vis460/PI system from 10% to 75.8% at pH 7.0. The kobs of NAP rose from 0.064 min-1 to 0.185 min-1 with the PI concentration increasing from 25 μM to 100 μM. Quenching and probe experiments verified hydroxyl radicals (HO•) and ozone (O3) as the dominant species, with their contributions at 77.6%-93.2% and 13.9%-20.6% across pH 4.0-9.0, respectively. The excited triplet state (3HA*) was the key photoproduced reactive intermediate for PI activation. Longer visible wavelengths inhibited NAP degradation due to lower photon energy, while acidic conditions facilitated the reaction via higher PI quantum yield. Higher HA concentration generated more 3HA* to accelerate PI activation, and aromatic ketones were more likely precursors of the triplet excited state. The system efficiently degraded seven micropollutants, and Suwannee River NOM shared a similar activation role. This work offers a reliable approach for improving the visible light-activated PI system and novel insights into HA photochemical behaviors.