Ozonation is a process widely applied in drinking or wastewater treatment where it can be used as a cost-effective disinfectant or for micropollutant removals. Additionally to these uses, ozone reacts with dissolved organic matter (DOM) present in the water matrix affecting both DOM optical properties and the generation potential of reactive intermediates such as singlet oxygen (¹O₂) and hydroxyl radicals (^●OH). Our experiments show that ozonation induced a large decrease (factor ≤ 50) in the absorption properties of DOM while an important increase in ¹O₂ and ^●OH quantum yields (Φ_¹O2 and Φ_^●OH respectively) was observed (factor ≤ 8 and 28 respectively). The decrease was attributed to the preferential reaction of ozone with the phenolic and aromatic moieties of DOM while the increase in Φ_¹O2 to the production of quinone moieties from the ozonation of phenolic compounds. Several pathways that lead to the formation of ^●OH from DOM irradiation were proposed. They include H₂O₂ dependent pathways such as the Fenton reaction and H₂O₂ independent pathways that include the irradiation of hydroxy-substituted benzoic acid and the oxidation of water or hydroxyl by ³DOM*. The observed increase in Φ_^●OH upon ozonation indicates that the later process is more plausible as ozonation of DOM should lead to the disappearance of hydroxy-substituted benzoic acids and to the formation of quinones that have high triplet quantum yield under photo-irradiation.
This study is a step forward in understanding the effects of ozone on DOM and hope to give some insights in DOM photochemistry.

Jeremy Torres Ruiz

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