The Role of Natural Organic Matter in the Degradation of Phenolic Pollutants by Sulfate Radical Oxidation: Radical Scavenging vs Reduction

Dissolved natural organic matter (NOM) significantly influences the performance of water treatment processes. It is generally recognized that NOM acts as a radical scavenger, thus inhibiting the degradation of organic pollutants in advanced oxidation processes (AOPs). This study examined the impacts of 8 different NOM isolates on the degradation of 4-chlorophenol (CP), a representative phenolic pollutant, in sulfate radical (SO₄^•-)-based AOPs. We developed an improved probe method to measure the steady-state concentration of SO₄^•- ([SO₄^•-]_ss) in both the absence and presence of NOM. Results show that adding 1.00 mg_C L^-1 NOM resulted in only a 1.3-3.4% decrease in [SO₄^•-]_ss. However, the apparent rate constants of CP degradation decreased by 76-88%. This discrepancy indicates that radical scavenging cannot be the primary mechanism for observed inhibition. We proposed NOM primarily acts as a reducing agent, reacting with the phenoxy radical intermediates generated from the single-electron oxidation of CP by SO₄^•-. Based on this hypothesis, we developed and validated a kinetic model using experimental data. The reductive capacity of NOM, as determined by the kinetic model, correlates positively with its electron-donating capacity. These findings enhance the understanding of NOM's role in SO₄^•--based AOPs and provide a foundation for developing strategies to mitigate its adverse effects.