PubMed · 4757377
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A Kyaw, U Hla-Pe. 1973. Letter: A more efficient oxidizing system for hydroxyproline assay.. https://pubmed.ncbi.nlm.nih.gov/4757377/
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Nitrosamine formation during chloramination previously has been linked to a reaction between monochloramine and organic nitrogen precursors via unsymmetrical dialkylhydrazine intermediates. Our results demonstrate the critical importance of dichloramine and dissolved oxygen. We propose a new nitrosamine formation pathway in which dichloramine reacts with secondary amine precursors to form chlorinated unsymmetrical dialkylhydrazine intermediates. Oxidation of these intermediates by dissolved oxygen to form nitrosamines competes with their oxidation by chloramines. Even when preformed monochloramine was applied, our model explained nearly all N-nitrosodimethylamine formation from the traces of dichloramine formed via monochloramine disproportionation. We suggest that, in contrast to unsymmetrical dialkylhydrazines, the weak, nonpolar nature of the N-Cl linkage in chlorinated unsymmetrical dialkylhydrazine intermediates enables incorporation of dissolved oxygen to form nitrosamines. With the improved understanding of the nitrosamine formation pathway, strategies are suggested that could significantly reduce nitrosamine formation during chloramination.
A kinetic model was developed to predict dichloroacetic acid (DCAA) formation in chloraminated systems. Equations describing DCAA formation were incorporated into an established comprehensive monochloramine-natural organic matter (NOM) reaction model. DCAA formation was theorized to be proportional to the amount of NOM oxidized by monochloramine and described by a single dimensionless DCAA formation coefficient, theta(DCAA) (M(DCAA)/M(DOC(ox)). The applicability of the model to describe DCAA formation in the presence of six different NOM sources was evaluated. DCAA formation could be described by considering a single NOM source-specific value for theta(DCAA) over a wide range of experimental conditions (i.e., pH, NOM, free ammonia, and monochloramine concentrations). DCAA formation appears to be directly proportional to the amount of active chlorine (monochloramine and free chlorine) that reacted with the NOM under these experimental conditions. Values of theta(DCAA) for all six NOM sources, determined by nonlinear regression analysis, varied from 6.51 x 10(-3) to 1.15 x 10(-2) and were linearly correlated with specific ultraviolet absorbance at 280 nm (SUVA(280)). The ability to model monochloramine loss and DCAA formation in the presence of NOM provides insight into disinfection by-product (DBP) formation pathways under chloramination conditions. The subsequent model and correlations to SUVA has the potential to aid the water treatment industry as a tool in developing strategies that minimize DBP formation while maintaining the microbial integrity of the water distribution system.
Many water districts have recently shifted from free chlorine (in the form of HOCl/OCl-) to monochloramine (NH2-Cl) as a disinfectant for drinking water to lower the concentration of chlorinated hydrocarbon byproducts in the water. There is concern that the use of NH2Cl disinfectant may lead to higher Pb levels in drinking water. In this study, the electrochemical quartz crystal microbalance is used to compare the effects of these two disinfectants on the dissolution of Pb films. A 0.5 microm thick Pb film nearly completely dissolves in a NH2Cl solution, but it is passivated in a HOCl/OCl- solution. X-ray diffraction analysis shows that the NH2Cl oxidizes Pb to Pb(II) species such as Pb3-(OH)2(CO3)2, whereas the stronger oxidant, HOCl/OCl-, oxidizes Pb to Pb(IV) as an insoluble PbO2 conversion coating. Although NH2Cl may produce less halogenated organic byproducts than HOCl/OCl- when used as a disinfectant, it may lead to increased Pb levels in drinking water.