2003 Progress Report: Membrane Introduction Mass Spectrometry Studies of Halogenated Cyano Byproduct Formation in Drinking WaterEPA Grant Number: R828231
Title: Membrane Introduction Mass Spectrometry Studies of Halogenated Cyano Byproduct Formation in Drinking Water
Investigators: Olson, Terese M.
Institution: University of Michigan
EPA Project Officer: Nolt-Helms, Cynthia
Project Period: August 1, 2000 through August 1, 2003 (Extended to August 31, 2004)
Project Period Covered by this Report: August 1, 2002 through August 1, 2003
Project Amount: $334,666
RFA: Drinking Water (1999) RFA Text | Recipients Lists
Research Category: Drinking Water , Water
The objectives of this research project are to: (1) determine the most important amino acid precursor compounds as sources of halogenated cyanogen disinfection byproducts (DBPs); (2) characterize the kinetics and formation mechanism of chlorinated and brominated cyanogen compounds; and (3) model their formation in a natural water sample containing a significant fraction of nonhumic organic matter.
During the reporting period, a mechanistic investigation of the formation and stability of cyanogen chloride (CNCl) was conducted using glycine as a model system precursor. Based on our previous screening experiments, in which 17 different amino acids were chlorinated, only glycine was observed to yield significant CNCl. In the case of glycine, these CNCl yields also were sensitive to the reaction conditions. The mechanistic reasons for the importance of glycine as a CNCl precursor and the variability of yields were investigated since the last reporting period. Some important findings and conclusions from these studies are summarized in this report.
Stability of Key Intermediates and Relevance to CNCl Formation
Close examination of the mass spectroscopic evidence collected during the screening experiments suggested that chloroaldimine intermediates form rapidly when amino acids are chlorinated. This finding is consistent with two reported investigations of valine and phenylalanine chlorination from the literature (McCormick, et al., 1993; Conyers and Scully, 1993). Among the 17 amino acids we studied, the chloroaldimine intermediates generally were stable except in the case of glycine. The chlorination of glycine leads to the formation of chloromethylimine, which was identified using mass spectrometry, as illustrated in Figure 1a.
Figure 1. Spectrum and Time-Profile of CNCl and Its Precursor CH2=NCl During the Chlorination of Glycine. The experiment was conducted with 34 µM initial glycine and a chlorine:glycine ratio of 2.5 at pH 7 and 25°C. Figure 1a shows spectral evidence of CNCl at m/z 62 and 64 and CH2=NCl at m/z 63 and 65. Time profiles of CNCl and CH2=NCl concentrations are represented by the membrane introduction mass spectrometry (MIMS) abundances at m/z 62 and 63, respectively (Figure 1b).
This intermediate, however, decomposes to give CNCl (see Figure 1b), and a possible pathway is as follows:
To evaluate the kinetic rate constants for CNCl formation in the mechanism above, however, we found that it was first necessary to understand the decomposition kinetics of CNCl because the rates of CNCl decomposition were comparable to chloromethylimine decay rates at higher free chlorine:glycine ratios. Although previous studies of the decomposition kinetics of CNCl in the presence of chlorine were available in the literature, there was considerable disagreement between the published rate constants as well as the mechanisms proposed (Price, et al., 1947; Xie and Reckhow, 1992). We performed a side study to characterize the kinetics and mechanism of CNCl decay (in the presence of free chlorine), so that we could develop a model for CNCl formation. This effort is described in the next section. As an additional incentive to add this to our scope, we hypothesized that CNCl decomposition was likely to be an important process in chlorination systems, and that it could partially explain the phenomenological observed association of CNCl with chloramination systems.
After characterizing the CNCl decay kinetics, we returned to developing a quantitative model to predict CNCl yields resulting from glycine chlorination, and currently are preparing a manuscript that describes both the screening and the kinetic modeling studies.
Stability of CNCl in the Presence of Free Chlorine and Monochloramine
Because the decomposition rates of CNCl in the presence of free chlorine were not well characterized and its stability in chloramine-containing solutions had not been studied, we examined the decomposition of CNCl with both disinfectants using Membrane Introduction Mass Spectrometry (MIMS). The mechanistic findings of these experiments are described in a paper that has been submitted to Environmental Science & Technology and currently is in review.
In the presence of free chlorine, the mechanism for CNCl decay was shown to be consistent with a hypochlorite-catalyzed hydrolysis reaction, as illustrated here.
Cyanate, a known hydrolysis product of CNCl, also was identified in these experiments using ion chromatography. At a pH less than 9, the decomposition kinetics can be described by
where kOCl is 121 (± 2) M-1s-1.
In the presence of monochloramine, little decomposition of CNCl was detected. Thus, our results indicate that the observed associations of higher CNCl concentrations among water utilities using chloramines is explained partly by the much greater stability of CNCl in chloramine solutions relative to free chlorine. This association, for example, was acknowledged in the monitoring requirements of the U.S. Environmental Protection Agency's recent Information Collection Rule. These findings provide a scientific basis for this association. Our results, however, also help to predict the treatment conditions in chlorination systems (i.e., colder temperatures, smaller chlorine residuals, lower pH) under which the half life of CNCl could be more problematic. These summary simulations are depicted in Figure 2.
Figure 2. Simulation of CNCl Stability for Conditions Relevant to Drinking Water Disinfection. The left figure was estimated at 15°C at various free chlorine concentrations. The right figure was obtained with 0.5 mg/L as Cl2 at various temperatures. Free chlorine is in excess in both simulations.
McCormick EF, Conyers B, Scully FE. N-chloroaldimines: chlorination of valine in model solutions and wastewater. Environmental Science and Technology 1993;27:255-261.
Conyers B, Scully FE. N-chloroaldimines: chlorination of phenylalanine in model solutions and in a wastewater. Environmental Science and Technology 1993;27:261-266.
Price CC, Larson TE, Beck KM, Harrington FC, Smith LC, Stephanoff I. Hydrolysis and chlorinolysis of cyanogen chloride. Journal of the American Chemical Society 1947;69:1640-1644.
Xie YF, Reckhow DA. Stability of cyanogen chloride in the presence of sulfite and chlorine. In: Proceedings of the American Water Works Association Water Quality Technology Conference, 1992.
We will complete a manuscript that outlines the important amino acid precursors of CNCl and the formation mechanism during the no-cost extension period. We also will complete experimental studies of the formation kinetics of cyanogen bromide from hypobromous acid and glycine. Subsequently, we will compare the relative importance of glycine as a CNCl precursor as suggested from our model system studies to CNCl yields of natural water samples. Chlorination studies of both Colorado River water and Huron River, MI, samples that have been characterized in terms of their amino acid content will be undertaken to achieve this objective.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
|Other project views:||All 14 publications||4 publications in selected types||All 4 journal articles|
||Na C, Olson TM. Stability of cyanogen chloride in the presence of free chlorine and monochloramine. Environmental Science & Technology 2004;38(22):6037-6043.||