Hydrogen Peroxide Formation during Ozonation of Olefins and Phenol: Mechanistic Insights from Oxygen Isotope Signatures.

Mitigation of undesired byproducts from ozonation of dissolved organic matter (DOM) such as aldehydes and ketones is currently hampered by limited knowledge of their precursors and formation pathways. Here, the stable oxygen isotope composition of HO formed simultaneously with these byproducts was studied to determine if it can reveal this missing information. A newly developed procedure, which quantitatively transforms HO to O for subsequent O/O ratio analysis, was used to determine the δO of HO generated from ozonated model compounds (olefins and phenol, pH 3-8). A constant enrichment of O in HO with a δO value of ∼59‰ implies that O-O bonds are cleaved preferentially in the intermediate Criegee ozonide, which is commonly formed from olefins. HO from the ozonation of acrylic acid and phenol at pH 7 resulted in lower O enrichment (δO = 47-49‰). For acrylic acid, enhancement of one of the two pathways followed by a carbonyl-HO equilibrium was responsible for the smaller δO of HO. During phenol ozonation at pH 7, various competing reactions leading to HO via an intermediate ozone adduct are hypothesized to cause lower δO in HO. These insights provide a first step toward supporting pH-dependent HO precursor elucidation in DOM.