Role of the propagation reactions on the hydroxyl radical formation in ozonation and peroxone (ozone/hydrogen peroxide) processes.

To better predict the elimination of highly ozone-refractory organic micro-pollutants from wastewater in ozonation and peroxone (O₃/H₂O₂) processes, it is important to understand the OH• formation therein. Nevertheless, the contribution of the propagation reactions (in brief, OH• + DOM (Dissolved Organic Matter) + O₂ → O₂•⁻, O₃ + O₂•⁻ → O₃•⁻ → OH•) to the OH• yields (Ф) in these two processes has not received great attention so far. In this study, >25% of O₃ was estimated to be consumed via the propagation reactions in ozonation of wastewater effluents. The competition method (taking the OH• exposure and scavenging capacity of water matrix into account) was recommended to determine the Ф values, and thus the relatively higher values (i.e., 33–58% vs. 6–24%) in ozonation were obtained as compared with the "tert-Butanol (tBuOH) assay" (with excess tBuOH to scavenge OH• producing stoichiometric formaldehyde), where the contribution of the propagation reactions was otherwise neglected when excess tBuOH completely scavenged OH. In peroxone of wastewater effluents, the rate constant of O₃ consumption increased significantly with the increase of H₂O₂ concentration ([H₂O₂]:[O₃] = 0.1–0.35). However, compared to ozonation alone, the improvement of the Ф values was negligible over a wide range of [H₂O₂]:[O₃] = 0.1–2.0. This discrepancy was mainly ascribed to the fact that substantial O₃ consumption via the propagation reactions resulted in comparable Ф values in peroxone vs. ozonation processes.