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Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
David A. Reckhow
The fate of HANs in drinking waters from their precursors in natural waters to their degradation products in consumers’ tap were systematically investigated in this study.
Combined amino acids were proved reactive with chlorine to form DCAN under typical drinking water conditions. However, the rate of DCAN formation from bound aspartyl residues was much slower compared to free aspartic acid. The key to DCAN formation from combined amino acids was a chlorine-induced peptide degradation process, which slowly degraded the peptide backbone to continuously produce reactive amine functional groups at the N-terminal end. Particularly, when an N-terminal aspartyl residue is chlorinated, it will form an N-chloroimine, which can undergo C-C cleavage to remove a cyanoacetic acid from the peptide structure. This cyanoacetic acid will then transform to DCAN as an essential intermediate precursor.
Simultaneous to their continuous formation, HANs were found to be chemically unstable and can undergo considerable decomposition via several types of degradation reactions. The rate of HAN loss generally increased with increasing pH but varied among different HAN analogues depending on the nature of their halogenated substituents. Additionally, free chlorine was shown to be an important facilitator and HAN degradation was accelerated in its presence. Perhaps most importantly, a mathematical kinetic model was established for seven chlorinated and brominated HAN species and their second-order hydrolysis and chlorination reaction rate constants were estimated using a Bayesian modeling framework, so that their lifetimes under typical sets of drinking water conditions can be quantitatively predicted
As HANs degrade, they leave other reaction products in their place. In the absence of chlorine, HANs decomposed to form the corresponding HAMs as reaction intermediates and HAAs as endpoint products. When chlorine was present, a group of previously unreported compounds, the N-Cl-HAMs were proved to be the HAN chlorination intermediates. However, N-Cl-HAMs are often misidentified in chlorinated drinking waters in the form of HAMs because the nitrogen-bound chlorine in N-Cl-HAMs is highly labile and thus can be readily dechlorinated by common reducing agents during sample preservation. N-Cl-HAMs are weakly acidic and they exhibited very high stability in water under a wide range of pH conditions without the presence of chlorine. On the other hand, it can undergo acid-catalyzed chlorination by hypochlorous acid to form the corresponding DCAA. Lastly, an analytical method using ultra performance liquid chromatography (UPLC)-quadrupole time-of-flight mass spectrometry (qTOF) was developed for a family of seven N-Cl-HAMs. Combined with solid phase extraction, the occurrence of N-Cl-DCAM and its two brominated analogues (i.e., N-Cl-BCAM and N-Cl-DBAM) in real tap waters was quantitatively determined for the first time.
Yu, Yun, "The Fate of Haloacetonitriles in Drinking Waters" (2016). Doctoral Dissertations. 823.