Environ. Scl. Technol. 1993, 27, 1232-1234
COMMUNICATIONS Spontaneous Methylation of Haloacetic Acids in Methanolic Stock Solutions Yuefeng Xie, David A. Reckhow,' and R. V. Rajan
Environmental Engineering Program, Department of Civil Engineering, University of Massachusetts, Amherst, Massachusetts 01003 Introduction Since dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) were first identified in chlorinated drinking water (11,a number of studies have shown these compounds to be ubiquitous and of human health concern (2-5). Accordingly, the US. Environmental Protection Agency (USEPA) is considering these two major haloacetic acids (HAAs)for regulation in the disinfectants and disinfection byproducts (D-DBP) rule (6). The presence of monochloroacetic acid (MCAA),monobromoacetic acids (MBAA), dibromoacetic acid (DBAA), and mixed bromo/chloroacetic acids has also been reported in recent years (3, 7). The USEPA is considering the regulation of some of these compounds in the coming D-DBP rule as well (6). Two analytical methods for HAAs, USEPA method 552 and a microextraction method, are currently being used to analyze MCAA, DCAA, TCAA, MBAA, DBAA, and bromochloroacetic acid in treated drinking waters (8-10). Due to its low volatility and high aqueous solubility, methanol is the solvent of choice for the preparation of standard stock solutins of haloacetic acids prescribed by the microextraction method (9). This method recommended that primary stock solutions, kept at -10 "C, can be used for 6 months and secondary stock solutions may be kept for 2 months. In EPA method 552 (a), methyl tert-butyl ether (MtBE) is used to prepare primary and secondary stock solutions. However, this method also allows the use of methanol as an alternative to MtBE for the preparation of secondary stock solutions (10). In the present study, spontaneous methylation of haloacetic acids was tested in five seta of methanolic stocks stored at -10 "C. The rates of methylation of DCAA at 60 and 70 "C were determined and then extrapolated to ambient and subambient temperatures. The significance of spontaneous methylation of HAA methanolic stocks on water quality monitoring and process research is also discussed. Experimental Section Reagents. MBAA, DCAA, TCAA, DBAA, TBAA, methyl monobromoacetate (MMBA), methyl dichloroacetate (MDCA),and methyl trichloroacetate (MTCA) were obtained from a commercialsource (AldrichChemical Co.). Three mixed bromo/chloroacetic acids (bromochloroacetic acid, bromodichloroacetic acid, and dibromochloroacetic acid) were synthesized in the authors' laboratory (21).The stock solutions, at concentrations of 1-15 g/L in methanol, 1232
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were gravimetricallyprepared from these pure compounds. These samples were stored at -10 "C for varying periods of time. Analysis of Haloacetates. The GUMS qualitative and quantitative analyses were performed on an HP 5890 gas chromatograph (GC) coupled to an HP 5988 quadrupole mass spectrometer (MS) (Hewlett-Packard Co.). The methanolic HAAs stock solutions were analyzed directly by GUMS following dilution to about 1 g/L and without prior methylation. Two microliters of diluted sample was introduced into the GUMS via a splitless injector. Electron impact ionization and selected ion monitoring were employed in the present study. The ions selected were m/z 93 for MBAA, m/z 83 for DCAA, and m/z 117 for TCAA. The response factors were calculated from injection of duplicate sets of methyl haloacetate standard solutions. Other operating conditions are similar to those reported previously (11). GC/ECD quantitative analyses were performed on an HP5890 gas chromatograph with an electron capture detector (ECD) (Hewlett-Packard Co.). The GC column used was a 30 m X 0.32 mm, 0.25-pm film thickness DB1701 capillary column (J & W Scientific). The oven temperature was held isothermally at 45 "C for 10 min and then was ramped to 200 "C at a rate of 4 "C/min and further to 275 "C at 20 "C/min. The injector temperature and ECD temperature were 280 and 300 "C, respectively. For total acetate (free acid and ester) quantification, samples were methylated with ethereal diazomethane prior to GC analysis. For quantification of esters only, samples were injected into GC without prior methylation. Determination of Methylation Rates of MBAA, DCAA, and TCAA. MBAA, DCAA, and TCAA solutions were prepared at a concentration at 2 g/L in methanol in amber 40-mL vials and capped with PTFE-faced septa and screw caps. The vials were then immersed in 60 and 70 "C water baths. Small volumes of the methanolic stocks were removed from the vials with Pasteur pipets at varying incubation times. Following dilution (1:2000)with MtBE, the samples were analyzed by the GC/ECD method. Results and Discussion Five sets of agedmethanolic HAAs stocks were analyzed by directly quantifying the methyl ester concentration with the GC/MS or GC/ECD method. The results in Table I showed that a significant portion of the HAAs had become methylated during storage at -10 "C for 2-5 months. In general, TCAA was most completely methylated, followed 0013-936X/93/0927-1232$04.00/0
0 1993 American Chemical Society
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Table I. Spontaneous Methylation of Haloacetic Acids in Methanolic Stocks stock solutions
approximate age (months)
first seta second seta third set" fourth set' fifth setd
2
methylated haloacetic acids ( % ) MBAA DCAA TCAA 22 NDb 11 38 30
3
5 5 4
Table 11. Methylation Rate Constants and Half-Lives of MBAA and DCAA in Methanolic Solution at Different Temperatures
52 22 31 42 14
91 51 76 66 30
a Stocks from the authors'laboratory, quantified with the GC/MS method. ND, not determined. Stocks from a research laboratory, quantified with the GC/ECD method. Stocks from a commercial laboratory, quantified with the GCiECD method.
MBAA
DCAA rate constant temp ("C) 70 60 20 4 -10
9-l)
750" 450" 43b 14b 4.6b
half-life (day) 1.1
1.8 19 58 180
rate constant
(lo-* 9-l)
half-life (day)
3360" 1940" ND' ND ND
0.24 0.41 ND ND ND
Experimental data. Extrapolated data. Not determined, uncertainty at high temperature was too great to allow extrapolation.
constant (k). The best estimate of the observed rate constants at 70 "C (k70 OC) and 60 "C (k60 o c ) is 7.5 x lo4 f0.2 X 10-6 s-1 and 4.5 X 10-6 f0.2 X lo4 s-l, respectively. From the Arrhenius equation (12)
n
.C
0
c c
C 0)
u
C
0
0
a a
0 0
5 0
10
20
30
incubation Time (hours,) Flgure 1. Methylatlon of DCAA in methanolic solution at temperatures of 00 and 70 "C.
by DCAA and MBAA. Using the GUMS method, the presence of methyl esters of MCAA, DBAA, bromochloroacetic acid, bromodichloroacetic acid, dibromochloroacetic acid, and tribromoacetic acid was also observed in their respective methanolic stock solutions. A series of controlled experiments was conducted to assess the methylation rate of three more commonly reported HAAs (MBAA,DCAA, and TCAA) in methanolic solutions. Because the reaction rates are too slow at -10 "C to be measured conveniently in the laboratory, experiments were run at elevated temperatures (i.e., 60 and 70 "C), and the results were extrapolated to lower temperatures. At a reaction temperature of 70 "C for 22 h, total trichloroacetate (free acid + ester) was recovered by 38%, total monobromoacetate by 104%, and total dichloroacetate by 110%. This suggests that while the free and methylated forms of monobromoacetate and dichloroacetate are thermally stable, trichloroacetate species are not. Therefore, only the concentrations of MBAA and DCAA could be reliably obtained by subtracting MMBA and MDCA from their original acid concentrations. The loss of the free-acid form of DCAA is shown in Figure 1. One would expect that methylation of the acid is a pseudo-first-order reaction in the presence of excess methanol. For this reason, the semi-log plot of the concentration versus time data (Figure 1) gives a straight line with a slope equal to the methylation rate
k = Ae-EdRT (1) the activation energy (E,) is estimated at 11.5 kcal/mol. The reaction rate constants at 20, 4, and -10 "C were calculated from eq 1, as shown in Table 11. By a similar procedure, the methylation rate constants of MBAA at 60 and 70 "C were determined to be 1.9 X f 0.8 X s-l and 3.4 X f 0.5 X s-l, respectively. Since the uncertainty in these rate constants is higher than with DCAA, accurate determination of the activation energy (E,) is not possible. Based on the estimated reaction rate constant of DCAA at -10 "C (Table 111,one would expect a loss of 50% for DCAA in the methanolic stocks stored at -10 "C for 6 months. In the microextraction method, however, the methanolic stocks may be kept at -10 "C for 6-8 months. Obviously, allowing the stocks to sit at room temperature during standard solution preparation would accelerate this loss. Unwanted spontaneous methylation in HAA stocks can be problematic for a number of reasons. First, the formation of esters in methanolic stocks will affect the apparent methylation efficiency for the HAAs during quantitative analysis. Second, due to differences in polarity, hydrogen-bonding, etc. between acids and esters, methylation of HAAs will affect the estimates of extraction efficiency, and possibly change method detection limit, precision, and calibration curves. Finally, when methanolic stocks of HAAs are used as calibrating standards for other types of analytical procedures (e.g., TOX) or as spiking solutions for laboratory-scale treatability studies (e.g., advanced oxidation processes, chemical reduction, biological degradation, adsorption), partial or complete methylation of the HAAs would obviously affect the results of such work. Therefore, the authors recommend that HAA stocks be freshly prepared or alternative solvents should be used. Acknowledgments This work was funded by the U.S. National Science Foundation and Dr. Edward H. Bryan under Grant BCS8958392. The authors acknowledge the support of the Hewlett-Packard Co. and its university equipment grant program. Registry numbers supplied by the authors: dichloroacetic acid, 79-43-6; trichloroacetic acid, 76-03-9; monoEnviron. Scl. Technol., Vol. 27, No. 6, 1993
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bromoacetic acid, 79-08-3; methyl dichloroacetate, 11654-1; m e t h y l trichloroacetate, 598-99-2; methyl monobromoacetate, 96-32-2; methanol, 67-56-1.
Literature Cited Uden, P. C.; Miller, J. W. J.-Am. Water Works Assoc. 1983, 75 (lo), 524-527. Reckhow, A. D.; Singer, P. C. J.-Am. Water Works Assoc. 1990, 82 (4), 173-180. Nonvood,D. L.;Thompson, G. P.; Johnson, J. D.; Christman, R. F. In Water Chlorination: Chemistry, Environmental Impact and Health Effects;Jolley, R. L., et. al., Eds.; Lewis Publishers Inc.: Chelsea, MI, 1985; Vol. 5, pp 1115-1122. Krasner, S. W.; McGuire, M. J.; Jacangelo, J. G.; Patania, N. L.; Reagan, K. M.; Aieta, E. M. J.-Am. Water Works 1989,81 (8), 41-53. ASSOC. Bull, R. J.; Sanchez, I. M.; Nelson, M. A,; Larson, J. K.; Lansing, A. J. Toxicology 1990, 63, 341-359.
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(6) US.EPA. Status Report on Development of Regulations for Disinfectants and Disinfection By-products, 1991. (7) Pourmoghaddas, H. Ph.D. Dissertation, University of Cincinnati, 1990. (8) Fair, P. S. Measurement of Disinfection By-products in Chlorinated Drinking Water. Proceedings of the A WWA Water Quality Technol.Conference;AWWA Denver, 1987. (9) Chinn, R.; Krasner, S. W. A Simplified Technique for the Measurement of Halogenated Organic Acids in Drinking Water by Electron Capture Gas Chromatography. Presentation at the 28th Pacific conference on Chemistry and Spectroscopy, Pasadena, CA, 1989. (10) Barth, R. C.; Fair, P. S. J.-Am. Water Works Assoc. 1992, 84 (ll),94-98. (11) Xie, Y.; Rajan, R. V.; Reckhow, D. A. Org. Mass Spectrom. 1992,27,807-810. (12) Skinner, G. B. Introductionto ChemicalKinetics;Academic Press, Inc.: New York, 1974; p 26. Received for review August 31,1992. Accepted February 19,1993.
