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Isolation of complex dissolved organic matter (DOM) from environmental water is a major challenge for unknown screening analysis by high-resolution mass spectrometry. In this study, DOM in process water during advanced drinking water treatment was fractionated sequentially by three solid-phase extraction (SPE) cartridges based on the polarity and charge of DOM molecules. By sequential SPE with unknown screening analysis, over 3000 DOM features were found in raw water, whereas around 2000 were obtained by a single SPE. The hydrophobic neutral (HPON) fraction contained CHO features with highest averaged molecular weight followed by hydrophobic acid (HPOA) and then hydrophilic acid (HPIA). The average degree of carbon double bond equivalents and carbon oxidation states indicated that the HPON fraction contained molecules that were more unsaturated and less oxidized than those of the HPOA and HPIA fractions. Ozone selectively decomposed (1) more unsaturated and less oxidized HPON features, (2) more unsaturated HPOA compounds, and (3) less oxidized HPIA molecules. Oxidation by-products were mostly HPON and HPIA compounds that were more oxidized than the decomposed molecules. During biological activated carbon (BAC) filtration, less oxidized HPON were preferentially removed, whereas HPOA were removed without selectivity. HPON and HPIA molecules with more oxidized character were found to be refractory to BAC treatment. HPON with more unsaturated and HPIA with more oxidized characters were decomposed by chlorine. Many types of HPIA decomposed during chlorination were the oxidation by-products of ozonation that were refractory to BAC treatment. Sequential SPE with unknown screening analysis provided previously unknown details of the molecular characteristics of DOM and its changes during advanced water treatment. • Sequential solid-phase extraction effectively recovers dissolved organic matter (DOM). • Ozone preferentially decomposes more unsaturated and less oxidized DOM. • Ozone oxidation by-products are found as highly oxidized hydrophobic neutral (HPON) and hydrophilic acid (HPIA). • Biological activated carbon filtration removes less oxidized HPON and hydrophobic acid (HPOA). • Chlorine selectively reacts with HPON and HPIA species. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) have been reported as precursors of disinfection byproducts (DBPs) and its molecular characteristics are rarely investigated due to its complexity. In this study, changes in the characteristics of DOM were investigated in the lower Phong River in Thailand in dry season and after the first rain in rainy season, using a non-targeted analysis with Orbitrap mass spectrometry. The river was rich with CHO features dominated by lignin-like molecules, while lipid-like molecules increase after domestic wastewater discharges. Wastewater discharge released DOM with higher molecular weight (MW) that was less oxygenated (low O/C) and less oxidized (low carbon oxidation state [C os ]). A lake affected by anthropogenic activities contributed more oxidized DOM into the river, while surface runoff carried DOM that is more oxygenated (high O/C), less hydrogenated (low H/C), and more oxidized (high C os) to the stream. Water treatment further modified DOM to be lower MW. Approximately three hundred Cl-containing features (CHOCl) detected upstream were also found downstream. Disinfection by chlorine (Cl 2) or chlorine dioxide (ClO 2) formed both CHO and CHOCl DBPs. Low chlorine dosage applied to upstream and downstream samples resulted in many common unknown DBPs while increasing chlorine dosage resulted in more unique DBPs. At the same dosage, Cl 2 reacted with DOM more than ClO 2 , including more oxidized molecules that are refractory to ClO 2. Both Cl 2 and ClO 2 produced chlorinated and non-chlorinated DBPs, and some DBPs were commonly found by both disinfections. Cl 2 -produced DBPs were more unsaturated (higher [DBE–O]/C) and oxidized (higher C os) than ClO 2 -DBPs. Image 1 • Molecular changes in dissolved organic matter (DOM) along the river were tracked. • Domestic discharges release DOM with high molecular weight (MW) and less oxidized. • A natural lake contributed more oxidized DOM into the river. • Water treatment further modified DOM to be lower MW. • Unique and shared CHO and CHOCl disinfection byproducts (DBPs) were found by chlorine or chlorine dioxide. [ABSTRACT FROM AUTHOR]