Hydrophobic neutral dissolved organic matter: First insights into compositional changes and formation of disinfection by-products induced by rainfall and degradation

This study examined the structural characteristics of dissolved organic matter (DOM) in bulk river samples and their fractionations (isolated using Supelite™ DAX-8 resin) under rainfall conditions, as well as two primary degradation processes: biodegradation and photodegradation. Compared to bulk samples, DAX-8 resin fractionations provided more detailed insights into structural changes in DOM influenced by rainfall and subsequent degradation. Rainfall was found to increase the proportions of hydrophilic (Hi) and hydrophobic-neutral (HoN) fractions. Notably, this study is the first to confirm that HoN consistently exhibits lower specific ultraviolet absorbance, humification index, fulvic- and humic-like compounds, and smaller molecular sizes of humic substances (1–20 kDa) across all conditions. Disinfection by-product (DBP) experiments showed that HoN primarily contained precursors for trihalomethane formation, while the formation of haloacetic acids and haloacetonitrile was closely related to Hi and hydrophobic acid fractions. Principal component analysis revealed that HoN was associated with elevated levels of tyrosine and tryptophan, as well as organic compounds in the 300–500 Da range, which contributed more significantly to trihalomethane formation than to haloacetic acids and haloacetonitriles. Although the characteristics of the DAX fractions were distinct, there were minimal differences between rainfall and subsequent biodegradation or photodegradation. This study is the first to characterize HoN and its role in the formation of DBP. As HoN became the dominant fraction after degradation processes, these findings provide important insights into water management and supply practices.