Changes in molecular dissolved organic matter during coagulation/sedimentation and chlorine and chlorine dioxide disinfection by non-target (or unknown) screening analysis

River water is a major water supply source, and its quality is strongly affected by seasonal changes in dissolved organic matter (DOM) and anthropogenic discharge. This study uses unknown screening analysis with Orbitrap mass spectrometry to investigate differences in molecular DOM in upstream and downstream areas of Phong River, Thailand, in addition to changes caused by coagulation/sedimentation, disinfection byproducts (DBPs), and formation by chlorine (Cl2) and chlorine dioxide (ClO2) in the dry season and after the first rainfall in the wet season. Among 2000 DOM features in the river, DOM with carbon, hydrogen, and oxygen (CHO features) was the most abundant class followed by CHO features with other heteroatoms such as N, NS, and S. CHO features with a relatively higher degree of oxidation (high carbon oxidation state, Cos) and highly oxygenated (high oxygen to carbon ratio, O/C) characteristics decreased preferentially via coagulation and sedimentation by polyaluminium chloride. Approximately 130 Cl-containing (CHOCl) features increased or were newly formed after Cl2 and ClO2 disinfection, respectively. Only 14 Cl2-CHOCl and four ClO2-CHOCl features were commonly formed in all water samples, while many unique DBPs were identified in each disinfection; therefore, reactions of unique DBP precursors in each water sample can produce unique DBPs. Additionally, around 100 CHO DBPs were also found after both Cl2 and ClO2 disinfection. Both Cl2 and ClO2 selectively reacted with CHO features with a relatively higher degree of oxidation (high Cos). Cl2 disinfection produced more oxygenated (high O/C) and oxidized (Cos) DBPs than ClO2 disinfection. Several hundred DBP precursors of Cl2 electrophilic substitution were found not only from natural sources but also from wastewater discharge or runoff.

Automated summary

This study investigates the differences in molecular dissolved organic matter (DOM) in upstream and downstream areas of Phong River, Thailand, as well as the effects of coagulation/sedimentation and disinfection on DOM composition. The results show that the most abundant class of DOM in the river is carbon, hydrogen, and oxygen-containing compounds (CHO features). Disinfection by chlorine (Cl2) and chlorine dioxide (ClO2) leads to the formation of chlorinated CHO compounds (CHOCl) and other disinfection byproducts (DBPs). The study also identifies unique DBPs formed in each disinfection process, indicating the presence of unique DBP precursors in different water samples.