The in vitro cytotoxicities of haloacetonitriles (HANs) and halonitromethanes (HNMs) are orders of magnitude higher than that of trihalomethanes (THMs) or halo acetic acids (HAAs). Although chloramination produces less THM and HAA during drinking water treatment, the formation potential (FP) results reported that chloramination enhanced the formation of HAN and trichloronitromethane (TCNM). Proposed pathways for dichloroacetonitrile (DCAN) and TCNM formation indicate the nitrogen atom on DCAN and TCNM can be attributable to organic nitrogen moieties or chloramines. Although a great numbers of nitrogen-based compounds have been investigated for the formation of TCNM or DCAN upon chloramination, the yields from chloramination of these compounds were much lower than that obtained from chloramination of natural organic matters (NOMs). Previous studies also reported that organic fractions with high aromaticity and low concentrations of dissolved organic nitrogen gave high DCAN or TCNM yields. In addition, chloramines contributed 62%-90% nitrogen in DCAN from chloramination of NOMs. These results suggest that the presence of an unknown molecule within the NOM that contributes to the formation of DCAN or TCNM through chloramines incorporation. However, the characteristics, formation potentials, and formation kinetics of these precursors remain unclear and may raise difficulties in nitrogenous disinfection byproduct control. Therefore, the main objectives of this study are to evaluate the formation potentials and formation kinetics of TCNM and DCAN precursors from natural waters.Four NOMs, including organic-nitrogen- rich and poor waters, were investigated in this study. A series of XAD resins were used to fractionate NOM based on their hydrophobicity and functional group. Isotopic study using 15N-labeled monochloramine were conducted to explore the nitrogen source of TCNM or DCAN. The results show that 15N-Isotopic monochloramination of the organic fractions produced both 15N- and 14N-DCAN and -TCNM. 14N-TCNM dominated (>93%) in the hydrophilic and transphilic fractions. The hydrophobic neutral (HPON) fraction served as the precursor pool for 15N-TCNM formation. The percentage of 15N in TCNM from the 15N-chloramination of the HPON was 29%-95% of total TCNM formed. In contrast to 14N-TCNM, 15N-TCNM and 15N-DCAN precursors within the NOM were readily destroyed during ozonation. TCNM or DCAN formation which the nitrogen precursors originated from DON (14N-DCAN and 14N-TCNM) was relatively rapid and followed second-order reaction kinetics. In contrast, transformation of precursors which nitrogen atoms are originated from chloramines (e.g. 15N-DCAN and 15N-TCNM) was relatively slow. The concentrations of 15N-DCAN and 15N-TCNM correlated linearly with chloramine exposure. Possible kinetic equations were proposed in this study, and were supported by a model compound study. In addition, this study also characterized 4-hydroxybenzaldehyde, an oxidative product formed during the chloramination of NOM, as an important precursor in DCAN formation by chloramine incorporation during the chloramination of natural waters. This study provides a better understanding of TCNM and DCAN formation upon chloramination, and may be beneficial for water utilities on controlling their formation.

Formation kinetics of trichloronitromethane and dichloroacetonitrile from chloramination of natural organic matters

248th National Meeting (2014)

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