Formation and Occurrence of N-Chloro-2,2-dichloroacetamide, a Previously Overlooked Nitrogenous Disinfection Byproduct in Chlorinated Drinking Waters

Abstract

Haloacetamides (HAMs) are a class of newly identified nitrogenous disinfection byproducts (N-DBPs) whose occurrence in drinking waters has recently been reported in several DBP surveys. As the most prominent HAM species, it is commonly acknowledged that 2,2-dichloroacetamide (DCAM) is mainly generated from dichloroacetonitrile (DCAN) hydrolysis because the concentrations of these two compounds are often well correlated. Instead of DCAM, a previously unreported N-DBP, N-chloro-2,2-dichloroacetamide (N-Cl-DCAM), was confirmed in this study as the actual DCAN degradation product in chlorinated drinking waters. It is suspected that N-Cl-DCAM has been erroneously identified as DCAM, because its nitrogen-bound chlorine is readily reduced by most commonly used quenching agents. This hypothesis is supported by kinetic studies that indicate almost instantaneous N-chlorination of DCAM even at low chlorine residuals. Therefore, it is unlikely that DCAM can persist as a long-lived DCAN decomposition product in systems using free chlorine as a residual disinfectant. Instead, chlorination of DCAM will lead to the formation of an equal amount of N-Cl-DCAM by forming a hydrogen bond between hypochlorite oxygen and amino hydrogen. Alternatively, N-Cl-DCAM can be produced directly from DCAN chlorination via nucleophilic addition of hypochlorite on the nitrile carbon. Due to its relatively low pK value, N-Cl-DCAM tends to deprotonate under typical drinking water pH conditions, and the anionic form of N-Cl-DCAM was found to be very stable in the absence of chlorine. N-Cl-DCAM can, however, undergo acid-catalyzed decomposition to form the corresponding dichloroacetic acid (DCAA) when chlorine is present, although those acidic conditions that favor N-Cl-DCAM degradation are generally atypical for finished drinking waters. For these reasons, N-Cl-DCAM is predicted to have very long half-lives in most distribution systems that use free chlorine. Furthermore, an analytical method using ultra performance liquid chromatography (UPLC)/negative electrospray ionization (ESI)/quadrupole time-of-flight mass spectrometry (qTOF) was developed for the detection of a family of seven N-chloro-haloacetamides (N-Cl-HAMs). Combined with solid phase extraction (SPE), the occurrence of N-Cl-DCAM and its two brominated analogues (i.e., N-chloro-2,2-bromochloroacetamide and N-chloro-2,2-dibromoacetamide) was quantitatively determined for the first time in 11 real tap water samples. The discovery of N-Cl-DCAM or, more broadly speaking, N-Cl-HAMs in chlorinated drinking waters is of significance because they are organic chloramines, a family of compounds that is perceived to be more toxicologically potent than halonitriles (e.g., DCAN) and haloamides (e.g., DCAM), and therefore they may pose greater risks to drinking water consumers given their widespread occurrence and high stability.

Contribution of amide-based coagulant polyacrylamide as precursors of haloacetamides and other disinfection by-products

“…Generally, chlorination favours the formation of CF compared to chloramination because Cl2 is 224 more oxidising than NH2Cl [33]. Previous studies have shown that the reaction of NH2Cl with the 225 amide group is insignificant, and it has been proposed that Cl2 first reacts with amide to form an 226 N-haloamide, followed by enolization and subsequent halogenation at the α-carbon group during 227 the chlorination of AM and PAM [23,34]. The data indicate that N-chlorination by Cl2 consumed 228 more active Cl2 than NH2Cl in the initial reaction period because of its stronger oxidising ability 229 (Table S4).…”

“…Yu and Reckhow recently showed that that the N-chlorination reaction occurs rapidly during 303 the chlorination or chloramination of molecules that contain amide groups because the hydrogen 304 atoms of the acetamide groups are easily substituted by halogen atoms [23]. The molecular ion 305 cluster of P105 (Fig.…”

“…192As expected, the yields of DCAcAm and TCAcAm decreased with increasing pH from 5 to 9. This 193 pattern can be explained by the increased hydrolysis and chlorination of DCAcAm and TCAcAm 194 at higher pH values[23]. The decrease in DCAcAm yields were proportionally lower than those 195 in TCAcAm with increasing pH.…”

“…118Predetermined volumes of Cl2 or NH2Cl and each coagulant stock solution were injected into 10 119 mM phosphate or carbonate buffer to obtain the desired initial concentrations. After pre-120 determined time intervals, 10 mL portions of the aqueous solution were withdrawn and 121 immediately extracted by liquid−liquid extraction to analyse the HAcAm concentration to avoid 122 the interference of the quenching agent on the formation of HAcAms [23]. And another 40 mL 123 portion of the aqueous solution was withdrawn and quenched with ascorbic acid at the initial molar 124 concentration of the Cl2 or NH2Cl to analyse the other DBPs.…”

Disinfection Processes

“…Instead of 2,2-dichloroacetamide (DCAM)Cl-DCAM), was confirmed as the actual dichloroacetonitrile degradation product in chlorinated drinking waters. It was suspected that N-Cl-DCAM had been erroneously identified as DCAM, becauseits nitrogen-bound chlorine is readily reduced by most commonly used quenching agents(Yu and Reckhow, 2017).Nitrosamines. Computational chemistry methodswere used byBond et al (2017) to calculate molecular descriptors for 64 NDMA precursors containing a dimethylamine (DMA) moiety.…”