Molecular characteristics of dissolved organic matter transformed by O3 and O3/H2O2 treatments and the effects on formation of unknown disinfection by-products

We investigated semiquantitative changes in almost 1000 dissolved organic matter (DOM) features during oxidation with 1 mg of O-3 per liter (mg OA), 4 mg O-3/L, or 4 mg O-3/L + 2.5 mg of H2O2 per liter (advanced oxidation process, AOP) by unknown screening analysis with Orbitrap mass spectrometry. The consequential effects on formation of unknown disinfection by-products (DBPs) by chlorination were evaluated in laboratory-scale experiments. Several hundred unsaturated DOM features with positive oxygen-subtracted double bond equivalents per carbon ((DBE-O)/C) were decomposed by the ozone only treatment and AOP. The AOP decomposed some saturated (negative (DBE-O)/C)) and reduced molecules, which had negative carbon oxidation states (C-os). Several hundred saturated oxidation by-products were detected after ozonation and the AOP. After chlorination, the samples pre-treated with ozone alone resulted in higher formation of unknown DBPs than the AOP pre-treated sample or the sample without oxidation. Over half of the DBP precursors, estimated by electrophilic substitution, were not totally decomposed by any oxidation process, but they were increased after the ozone-only process and AOR DBP precursors produced by the ozone-only process or AOP formed unique unknown DBPs. Therefore, post-treatment processes after oxidation and before chlorination are important to minimize formation of unknown DBPs. (C) 2019 Elsevier Ltd. All rights reserved.