Degradation of humic acid by UV/PMS: process comparison, influencing factors, and degradation mechanism

Abstract

In natural water bodies, humic acid (HA), generated during the chlorination disinfection process at water treatment plants, can produce halogenated disinfection by-products, increasing the risk to drinking water safety and posing a threat to human health. Effectively removing HA from natural waters is a critical focus of environmental research. This study established a synergistic ultraviolet/peroxymonosulfate (UV/PMS) system to remove HA from water. It compared the efficacy of various UV/advanced oxidation processes (AOPs) on HA degradation, and assessed the influence of different water sources, initial pH, oxidant concentration, and anions (HCO₃⁻, Cl⁻, NO₃⁻) on HA degradation. The degradation mechanism of HA by the UV/PMS process was also investigated. Results showed that under the conditions of 3 mmol L⁻¹ PMS concentration, 10 mg L⁻¹ HA concentration, initial solution pH of 7, and a reaction time of 240 minutes, the mineralization rate of HA by UV/PMS reached 94.15%. The pseudo-first-order kinetic constant (k_obs) was 0.01034 and the single-electric energy (EE/O) was 0.0157 kW h m⁻³, indicating superior HA removal efficiency compared to other systems. Common anions (HCO₃⁻, Cl⁻, NO₃⁻) in water were found to inhibit the degradation of HA, and acidic conditions were more conducive to HA removal, with the optimal pH being 3. Free radical quenching experiments showed that both sulfate radical (SO₄⁻˙) and hydroxyl radical (˙OH) radicals were involved in HA degradation, with SO₄⁻˙ being the primary oxidant and ˙OH as the auxiliary species. Analyses using 3D-excitation-emission matrix (EEM), parallel factor analysis (PARAFAC), specific fluorescence index, and absorbance demonstrated that UV/PMS technology could effectively degrade HA in water. This study provides theoretical references for further research on the removal of HA and other organic substances using UV/PMS technology.