Dual Role of Natural Organic Matter as an Inhibitor or Stimulator of Hydroxyl Radical Formation in the Ozonation Process

The degradation of ozone (O3)-recalcitrant organic micropollutants (OMPs) is limited by the selectivity of O3 toward electron-rich moieties. This study investigates the dual role of natural organic matter (NOM) in ozonation, focusing on its ability to inhibit or promote hydroxyl radical (HO•) formation, thereby influencing the degradation of O3-resistant OMPs. Tannic acid (TA), gallic acid (GA), catechin (CAT), and tryptophan (Trp) were selected as NOM surrogates, representing phenolic, carboxylic, and amine functional groups. Atrazine (ATZ) served as a model OMP to evaluate the effects of NOM moieties, molecular weight, and concentration on degradation kinetics. During an initial phase (<20 s), aromatic, phenolic, and amine groups enhanced HO• formation, achieving 50% ATZ degradation, with over 85% achieved within 200 s. However, higher NOM concentrations (>5 mg/L) exhibited inhibitory effects due to increased O3 depletion and radical scavenging. Phenolic moieties strongly enhanced ATZ degradation, with their effect increasing proportionally with concentration, while carboxylic groups, especially GA, scavenged HO•, resulting in low HO• yields (0.08–0.12) at 3–10 mg/L. These findings provide critical insights into the role of NOM functional groups in ozonation, advancing the understanding of O3 demand, OMP fate, and process optimization in water treatment.