Characterizing the precursors of byproducts formed by chlorine and chlorine dioxide disinfection using unknown screening analysis with Orbitrap mass spectrometry.

Chlorine (Cl) and chlorine dioxide (ClO) are commonly used to disinfect water but unfavorable interactions with dissolved organic matter (DOM) result in the formation of disinfection byproducts (DBPs). This study investigated the formation of organic DBPs arising from Cl and ClO disinfections under different contact times in two surface waters in Thailand and Suwannee River natural organic matter with/without bromide using unknown screening analysis with Orbitrap mass spectrometry. Many CHOCl-DBPs and CHOBr-DBPs intermediates were rapidly formed during the initial period of contact (5-30 min). Subsequently, the number of DBPs either decreased or increased (60-1440 min) due to the ongoing formation and decomposition of intermediate DBPs reacting with disinfectants. Over one hundred newly formed chlorinated DBPs were produced by Cl and ClO disinfections (CHOCl Cl-DBPs and CHOCl ClO-DBP, respectively). At least 40 % of the chlorinated DBPs were commonly found in the presence of both disinfectants, probably due to HOCl impurity formed by ClO. In addition, CHO features with high degree of unsaturation ([DBE-O]/C) and moderate degree of carbon oxidation state (C) were found to be statistically correlated with several CHOCl-DBP and CHOBr-DBP features in Cl and ClO disinfections, and are therefore considered as putative precursors. Furthermore, the putative CHOBr-DBP precursors showed a more highly oxidized character than the putative CHOCl-DBP precursors. By tracking precursors from reactions using mass difference analysis, Cl preferentially reacts with saturated precursors via electrophilic substitution reaction, where the Cl addition reaction occurs more favorably in the presence of unsaturated precursors.