2002 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, 2001 through August 1, 2002
Project Amount: $334,666
RFA: Drinking Water (1999) RFA Text | Recipients Lists
Research Category: Drinking Water , Water
The main objectives of this research project are to: (1) determine the most important amino acid precursor compounds as sources of halogenated cyanogen 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.
We completed screening experiments, where we compared the relative contributions of 17 free amino acids as precursor compounds for cyanogen chloride (CNCl) during chlorination. The experiments included: (1) assessments of the relative reactivities of amino acids with hypochlorous acid; and (2) determinations of their CNCl yields.
Our measurements of CNCl yields using membrane introduction mass spectrometry (MIMS) and independent titration methods suggest that among the 17 free amino acid precursors that we tested, only glycine produces significant cyanogen chloride. This finding allows us to concentrate our efforts in developing predictive models for CNCl formation on a single amino acid system. Due to the unstable nature of CNCl, at higher ratios of chlorine to glycine, quantification of the "CNCl yield" is highly dependent on the definition of yield (i.e., the time at which CNCl is measured). A maximum "ultimate" molar CNCl yield of about 55 percent was observed at a ratio of approximately 1.8-2 NaOCl:glycine (see Figure 1). It is possible, however, that higher molar conversions are obtained at shorter timescales in solutions containing initial chlorine:glycine ratios greater than approximately 2.0, but the CNCl later decomposes to give smaller ultimate yields in these systems (see Figure 2).
Figure 1. Ultimate CNCl Formation by the Reaction of NaOCl and Glycine.
Figure 2. CNCl Formation as Determined by MIMS Due to the Chlorination of Glycine at Varying NaOCl:Glycine Initial Concentration Ratios. Initial glycine concentration was 340 µM.
The reactivity experiments indicated that methionine and cysteine react the most rapidly with chlorine, followed by histidine. A large group of amino acids in the middle of the reactivity scale were similarly reactive. Glycine and proline were the slowest to react with chlorine. Under drinking water disinfection conditions, in which there is an excess of chlorine to amino acids, competition for chlorine is of minimal importance. However, in assessing the risk of in vivo CNCl formation, for example, in human gastrointestinal (GI) tracts after ingestion of chlorinated water, amino acids are likely to be in excess relative to chlorine. Under these solution conditions, competitive reactions are of much significance. Our research suggests that glycine, which appears to be the most important free amino acid precursor of CNCl (see above), is less reactive kinetically than most of the other amino acids we considered. Therefore, competitive reactions of chlorine residual with in vivo amino acids other than glycine would serve to inhibit CNCl formation. However, little is known about the products of these reactions or their toxicity.
Kinetic studies of the chlorination of glycine and subsequent formation of CNCl also were initiated. The reaction is being studied in real time using MIMS to detect CNCl. As shown in Figure 2, the formation mechanism, particularly at chlorine:glycine ratios greater than 2, is relatively complex. Additional liquid chromatography/mass spectrometry (LC/MS) and gas chromatography/mass spectrometry (GC/MS) experiments are being conducted to identify reaction products and intermediates. Quantitative characterization of the reaction mechanism will allow us to develop predictive models for CNCl yields. Such models will improve our understanding of the role of amino acids as precursors for cyanogen halides in drinking water chlorination systems, and in understanding the health effects of chlorine reactions in vivo.
During the next reporting period, we plan to complete the kinetic study of the CNCl formation mechanism and also examine the mechanism of CNBr formation. We plan to use the findings to determine the relative importance of glycine as a CNCl or CNBr precursor in natural water samples. Chlorination studies of both Colorado River Water and Huron River samples will be conducted to achieve this objective.