Sunlight-driven activation of periodate by excited triplet humic acid for efficient microcontaminant abatement

Humic acid (HA) may inhibit the degradation of contaminants by quenching reactive species in some traditional advanced oxidation processes (AOPs). However, our study found that HA can remarkably enhance the degradation of contaminants by the sunlight-activated periodate (PI) system in practical applications. Herein, a HA-boosted sunlight-activated PI system for contaminant degradation was systematically investigated. The results showed that 2 mgC/L HA increased the degradation efficiency of naproxen (NAP) by about 2.4-fold in the sunlight/PI system. Increased HA or PI concentration enhanced NAP degradation by the sunlight/PI/HA system. Quencher, fluorescence and nitrogen experiments proved that hydroxyl radical (HO^·) and ozone (O₃) were the main reactive species for the degradation of NAP. Radical probe and O₃ experiments confirmed that the increase of pH decreased the quantum yield of PI photolysis, resulting in the decrease of average O₃ concentration and the steady-state concentration of HO^·, thereby lowering the degradation of NAP. The contributions of HO^· and O₃ to NAP degradation were determined to be from 65.4% to 81.1% and from 19.3% to 28.1% at pH 4.0-9.0, respectively. The situation of PI decline in different systems proved that the triplet excited state of HA (³HA*) rather than HA^·- was the key intermediate species for the accelerated activation of PI, and PI was eventually converted into iodate. The accelerated activation of PI with increasing HA concentration was attributed to the elevated steady-state concentration of ³HA*. Additionally, the applicability of the sunlight/PI/HA system was verified by the degradation of seven different micropollutants. Suwannee River natural organic matter (SRNOM) was also demonstrated to exhibit similar enhancing effects on PI activation as HA under sunlight exposure. Cl^- and HCO₃^- inhibited the degradation of NAP in view of the conversion of HO^· into chlorine-containing and carbonate radicals. This work provides a novel insight into improving the solar-photoactivated PI process for water treatment.