Characterization of the Molecular Weight and Reactivity of Natural Organic Matter in Surface Waters

Abstract

Natural organic matter (NOM) can impact on all aspects of water treatments processes. Understanding the physical and chemical characteristics of NOM is essential to improving drinkingwater treatment processes. The size of NOM has important implications for drinking water treatment and the formation of DBPs, where the high molecular weight, hydrophobic components of NOM have been found to be effectively removed by conventional drinking water treatment processes, while the lower molecular weight and certain hydrophilic components of NOM are more difficult to remove using these processes. In our study, we collected raw (untreated) watersfrom three different drinking water reservoirs, characterised the molecular weight (MW) distribution of the NOM in these waters using analytical scale high performance size exclusion chromatography (HPSEC), and isolated apparent MW (AMW) fractions of the NOM using preparative scale HPSEC. We also investigated the reactivity of the AMW fractions of NOM interms of disinfection by-products (DBP) formation potential from chlorination and chloramination. We focused on the formation potential of halogen-specific adsorbable organic halogen (AOX) and nitrogen-containing DBPs (N-DBPs), since brominated and iodinated DBPs and N-DBPs have beenreported to be significantly more cytotoxic, genotoxic, and carcinogenic than the regulated DBPs. Our study found that the AMW fractions of NOM with higher SUVA254 values generally produced higher concentrations of halogenated DBPs, measured as halogen-specific AOX. Halogenated N-DBPs formed only a small fraction of AOX in both chlorination and chloramination, with higher relative contributions from halogenated N-DBPs in chloraminated samples. The propensity of the formation of N-DBPs, especially N-nitrosamines and haloacetamides, was higher in chloramination. Since these DBPs are more toxic than the regulated DBPs, further evaluation of the health risk trade-offs when selecting chlorine or chloramine as a disinfectant is essential. The size of NOM had little influence on the formation of halogenated N-DBPs, but the low to medium AMW fractions of NOM tended to form higher concentrations of N-nitrosamines. Chlorine tended to be incorporated into the higher AMW fractions of NOM, while bromine and iodine seemed to be preferentially incorporated into the lower AMW fractions of NOM. Since conventional water treatment processes are ineffective for the removal of the low to medium MW fractions of NOM, improved water treatment processes may be needed to minimise the formation of brominated andiodinated DBPs, as well as N-nitrosamines, especially for source waters that contain significant amounts of organic matter of low to medium MW.