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.