Erivirori Sl;i Technol 1987, 2f, 1113-1114
Abiotic Transformations of Halogenated Organics. 1. Elimination Reaction of 1, I ,2,2-Tetrachloroethane and Formation of 1,I ,2-Trichloroethene William J. Cooper,**tMostafa Mehran,+David J. Riusech,+ and Jeffrey A. Joenst
Drinking Water Research Center and Chemistry Department, Florida International University, Miami, Florida 33199 ~
~
w The abiotic homogeneous aqueous-phase elimination reaction of 1,1,2,2-tetrachloroethaneto 1,1,2-trichloroethene has been studied. The reaction has been carried out in 0.100 M phosphate buffer in the pH range 5-9 and at 11temperatures. In all cases, quantitative conversion of the 1,1,2,2-tetrachloroethaneto 1,1,2-trichloroethenewas observed. The reaction is second order overall and fits the equation log k , = (15.87 i 0.54) exp([-91.1 f 3.4 kJ/ mol] / R 7').
Introduction Halogenated organic compounds are found extensively in the environment. In the subsurface environment, halogenated methanes, ethanes, ethenes, and propanes are considered to be relatively stable. However, it is possible that they may undergo changes that are biologically and/or abiologically mediated. It is also possible that they may be physically adsorbed onto particles. All of these factors, as well as others such as primary or secondary photolysis and evaporation, combine to define the "fate" of these chemicals in the aqueous environment. The focus of our investigation is the abiological reactions of halogenated organic compounds that may occur in surface or subsurface environments. The abiological reactions of interest in this study are substitution and elimination in aqueous media, sometimes combined under the general name hydrolysis. Hydrolysis may occur in the environment via either homogeneous or heterogeneous reactions. A comprehensive review of the available data on homogeneous hydrolysis has been compiled ( I ) , which gives a discussion of the effects of pH, temperature, ionic strength, and buffer composition on reaction kinetics, and serves as an excellent guide for studies in this field. In this work we have studied the abiotic homogeneous elimination of HCl from 1,1,2,24etrachloroethane(TeCA) in 0.100 M phosphate-buffered distilled water. The reaction was investigated for pH 5-9 and at 11 different temperatures ranging from 30 to 95 "C. Under all conditions, we find that the parent compound is quantitatively converted to 1,1,2-trichloroethene (TCE) by a base-promoted second-order elimination reaction. We report here the effect of temperature and pH on this reaction, compare our results to previous work on this reaction, and briefly discuss the environmental implications of the results. Experimental Methods
Materials. Phosphate buffers were made as follows: pH 5.00, an 80:l (v/v) solution of 0.100 M potassium dihydrogen phosphate and 0.100 M potassium hydrogen phosphate; pH 6.85, a 1:l (v/v) solution of 0.100 M potassium dihydrogen phosphate and 0.100 M potassium hydrogen phosphate; pH 8.85, a solution of 0.100 M potassium hydrogen phosphate. In every case reagent-grade chemicals were used (Mallinckrodt) and prepared in 'Drinking Water Research Center. Chemistry Department. 1112
Environ. Sci. Technol., Vol. 21, No. 11, 1987
deionized-distilled water. Buffer pH was determined on an Orion 811 pH meter with an accuracy of i0.03 pH units, Standard solutions of 1,1,2,2-tetrachloroethane(98%) and 1,1,2-trichloroethene (99+ % ) (both from Aldrich Chemical Co.) were prepared in methanol (Burdick and Jackson, trihalomethane grade). The internal standard was 1,1,2-trichloroethane (Chem Service). Pentane (Burdick and Jackson, trihalomethane grade) was used for the liquid-liquid extraction and was used as received. Glass ampules (Wheaton Scientific) had a nominal volume of 50 mL, with a measured volume of slightly more than 60 mL. They were cleaned prior to use with 25% 2-propanol (Burdick and Jackson, high-purity grade) and distilled water and oven-dried at 225 "C. Methods. To determine the rate of elimination, 200 p L of a standard solution of TeCA in methanol was added to 60 mL of the desired buffer to give a nominal TeCA concentration of 450 nmol/L. The ampules were then sealed and immediately placed in ice-water to minimize any reaction prior to incubation at the desired temperature. The samples were incubated in a water bath, which was maintained at constant temperature within f0.1 "C. After incubation the ampules were removed from the water bath and placed in ice-water for rapid cooling and then transferred to a refrigerator and stored a t 4 O C . The samples were analyzed as soon as possible, and always within 24 h after refrigeration. The neck of the ampule was then broken, and 50 mL of the sample was transferred into a serum vial, along with 5 mL of pentane, and analyzed as described previously (2). Gas chromatographic analyses were carried out on a Hewlett-Packard Model 5890, equipped with two 'j3Ni electron capture detectors and two 3392A reporting integrators. Large-bore fused-silica capillary columns, 30 m in length and 0.53-mm i.d., coated with 1.5 pm of DB-5 (J & W Scientific, Inc.) were used. The injection port was maintained at 180 "C, while the detectors were at 250 "C. The initial temperature of the oven was held at 32 "C for 1 min and then increased by 6 deg/min to 90 "C, where it was then held for 8 min. Helium was used as a carrier gas with a flow rate of 5-8 mL/min, resulting in a velocity of about 4&60 cm/s at 30 "C. Nitrogen (30 mL/min) was used as make-up gas. The injection ports were fitted with glass liners (Kit A, J & W Scientific, Inc.) to prevent tailing of the peaks. To test for compound adsorption on glass surfaces of the test vessels, several experiments were conducted. Serum vials were prepared with distilled water only and, to increase the effective wall area, distilled water with 20 g of (8 mL) glass beads and distilled water with 40 g of (16 mL) glass beads. A standard containing chloroform, TCE, bromodichloromethane, and TeCA was then added in the same manner as in the hydrolysis experiments. The vials were allowed to stand 4-6 h and then extracted with the liquid-liquid extraction procedure. One set of vials was extracted with the aqueous phase present while another set was extracted after the aqueous phase had been decanted. The results from these experiments indicate no significant adsorption of the halogenated organic compounds on the glass ampules or the glass beads.
0013-936X/87/0921-1112$01.50/0
0 1987 American Chemical Society
Table I. Summary of Experimental Data for Homogeneous Elimination Reaction of 1,1,2,2-Tetraohloroethane temp, 'C 95 88 80 70
60
55 60 45
40 35 30
(+0,03)
no. of points
4.97 5.00 5.01 4.95 5.01 5.01 4.97 5.00 5.00 5.00 5.03 5.03 6.82 6.87 6.90 6.85 6.86 6.91 6.85 6.88 6.88 6.85 6.88 6.88 8.84 8.85 8.86 8.97 8.82 8.87 8.92 8.86 8.87 8.87 8.88 8.90 8.91 8.91 8.91 8.95
8 ,9 9 8 9 8 7 6 7 10 9 9 8 9 8 9 7 7 11 8 9 9 10 6 6 8 8 9 10 8 9 8 8 8 9 8 8 8 9 6
PH
103kObsd, h-' 146 (0.998) 153 (0.997) 132 (0.953) 58.7 (0.995) 70.5 (0.999) 61.7 (0.999) 23.0 (0.999) 21.9 (0.999) 21.2 (0.989) 5.22 (0.995) 5.54 (0.994) 5.53 (0.999) 378 (0.999) 347 (0.999) 358 (0.998) 82.8 (0.998) 78.7 (0.999) 91.5 (0.998) 27.4 (0.994) 26.0 (0.994) 25.9 (0.994) 12.3 (0.997) 16.2 (0.997) 14.1 (0.998) 612 (0.999) 917 (0.999) 948 (0.999) 960 (0,999) 294 (0.983) 465 (0.997) 392 (0.998) 192 (0.997) 184 (0.999) 174 (0.999) 76.0 (0.999) 77.7 (0.999) 78.9 (0.987) 89.4 (0.998) 60.4 (0.999) 59.1 (0.990)
key
M-1 8-1 959 941 793 535 573 490 281 249 242 92.8 93.4 93.3 103 84.5 81.5 35.0 32.6 33.7 15.2 13.5 13.4 9.06 11.2 9.70 6.24 9.15 9.24 7.26 4.29 6.06 4.54 3.56 3.34 3.15 1.90 1.86 1.85 2.09 1.43 1.26
Control experiments were conducted under sterile conditions to determine the extent of microbially mediated degradations. Ampules and buffers were autoclaved prior to the addition of TeCA. Following the addition, samples were again autoclaved. No difference in the degradation rates was observed in sterile and nonsterile ampules. For the hydrolysis experiments, 60 mL of solution was added to each ampule, leaving a void volume of 1-2 mL after sealing. It is possible that TeCA or TCE could partition into the void volume and reduce the apparent concentration of compound in the liquid phase. By use of the procedure presented by Burlinson et al. (3), the amount of halogenated organic expected to migrate from the liquid phase to the gas phase was found to be negligible for all experimental conditions.
Results and Discussion Kinetics of Elimination Reaction. At each temperature, three or more independent sets of experimental data were obtained. Each data set consisted of 6-11 measurements of the concentration of both TeCA and the TCE reaction product, over a range of up to five half-lives for the reaction. A summary of the experimental results is presented in Table I. Because the concentrations of both the reactant and the product of the elimination reaction were measured experimentally, it is possible to determine whether other reactions of TeCA were in competition with the elimination
0
40
20
0
Time (hrs)
Figure 1. Disappearance of 1,1,2,2-tetrachloroethane (H) and appearance of 1,1,2-trichIoroethene ( 0 )in 0.100 M phosphate buffer, pH 6.88, T = 55 'C. The solid lines are based on calculations using the overall equation from this research.
reaction. In all cases, the disappearance of TeCA is balanced by a corresponding appearance of TCE, with the total concentration of reactant plus product remaining constant during the course of the reaction. A typical data set is given in Figure 1. On the basis of the experimental data, reactions removing TeCA to form products other than TCE can account for at most about 5% of the total reaction. Other reactions, such as the substitution reaction to form dichloroacetaldehyde ( 4 , 5 ) ,must therefore be a minor reaction pathway in this system. The elimination reaction for TeCA is also found to be base promoted for values of pH in the range 5-9. This conclusion is based on two sets of observations. First, in experiments where pH was varied while temperature was held constant, the observed rate constant is found to be directly proportional to the OH- concentration. Second, the Arrhenius plot of all of the experimental data indicates that only a single, base-promoted, reaction is responsible for the disappearance of TeCA, with no evidence for the occurrence of neutral- or acid-catalyzed processes. Data Analysis. Pseudo-first-order rate constants (kobsd) were obtained by fitting each experimental data set to an equation of the form In [TeCA], = -hob&
+C
(1)
which is appropriate for reactions that follow pseudofirst-order kinetics. A weighted linear least-squares procedure was used to obtain values for the observed rate constants (6). Values for the observed rate constants were independently obtained by fitting the data for the appearance of TCE to [TCElt/[TCEl, = 1 - exp(-k,b,dt)
(2)
by a nonlinear least-squares procedure. In general, the rate constants obtained by the two methods were in good agreement. Because the values for the observed rate constants obtained from the appearance of TCE showed more scatter (due to sensitivity of the fitting procedure to data taken at short times and in the value selected for [TCE],), the observed rate constants reported in Table I are based on the rate of disappearance of TeCA. Also reported are the correlation coefficients for each data set. In 90% of the data sets, the correlation coefficient was better than 0.99. Environ. Sci. Technol., Vol. 21, No. 11, 1987
1113
4
:
1\
-1
O I -2
/
2.4
,
, 2.8
,
, 2.8
,
,
,
3
,
,
3.2
, 3.4
,
, 3.6
1/T x 1000
Flgure 2. Arrhenius plot of the elimination of 1,I ,2,24etrachIoroethane in 0.100 M phosphate buffers, pH 5-9: (0)data from this study; (e) data from ref 4 and 8.
Values for k,, the second-order rate constant for the elimination reaction, were obtained from kobsdfrom the relationship (3) The hydroxide ion concentrations were found from the experimental pH and the equilibrium constant for the ionization of water at various temperatures, calculated with an equation given by Marshall (7). The values for k , are presented in Table I. The dependence of the elimination rate constant on temperature was found by fitting the rate constants to the Arrhenius equation
k , = A exp(-E,/RT)
(4)
by plotting log k, vs 1/T, as shown in Figure 2. On the basis of this plot, the Arrhenius preexponential factor was found to be log A = 15.87 f 0.54, and the activation energy for the reaction was found to be E, = 91.1 f 3.4 kJ/mol, where the uncertainties are given for 95% confidence limits. Previous Studies, The reaction of TeCA has previously been investigated by Walraevens et al. (8). In their experiments the reaction of TeCA was studied in unbuffered aqueous solutions of TeCA and NaOH at 11,20,30, and 40 "C. The disappearance of the parent compound was followed by titration of the C1- produced from the elimination of HC1 to form TCE. Their experimental data are shown in the Arrhenius plot in Figure 2 [the solid ( 0 ) data points]. The data are in good agreement with the results found in this study. The values for the Arrhenius parameters found by Walraevens et al. (8) (log A = 16.4 f 2.3; E, = 94 f 13 kJ/mol; uncertainties given at the 95% confidence limit and calculated from the data in Table I1 of ref 8) also agree with the present results to within the error limits of the two experiments. Mill and co-workers ( 4 , 5 )have also made some preliminary measurements of the rate of disappearance of TeCA under slightly acidic conditions. They find that at pH 5 the rate of disappearance of TeCA is a factor 2-3 times greater than expected on the basis of the Arrhenius ex-
1114 Environ. Sci. Technol., Vol. 21, No. 11, 1987
pression of Walraevens et al. (8). The enhanced rate is attributed to a neutral hydrolysis process that might form dichloroacetaldehdye as a substitution product. However, in this experiment measurements of both the TeCA and the TCE elimination product indicate quantitative conversion of TeCA to TCE for pH in the range 5-9. In addition, there is no evidence of an enhanced rate of reaction in the pH 5 data, as would be expected if a neutral-catalyzed process was taking place in the system. Therefore, it does not appear that a neutral-catalyzed reaction is a significant reaction pathway for values of pH greater than 5. Environmental Implications. From the results presented here, the half-life of TeCA can be calculated at 25 "C for any pH in the range 5-9. A t pH 7, typical of natural waters, the half-life is 102 days, and for pH 9, sometimes encountered in water treatment, the half-life is 1.02 days. One limitation of our data set is the fact that all of the experiments were conducted in 0.100 M phosphate buffer solutions. Further studies are in progress to determine the effect of ionic strength and various buffer anions that may affect the reaction rate. It is clear that TeCA will undergo abiotic transformation a t rates significant in most aquatic environments.
Acknowledgments The technical assistance of Pat Russell, Mehrzad F. Mehran, Rose Ann Slifker, Cindy Dwyer, and Nahid Golkar is gratefully appreciated. Howard E. Moore and Ronald D. Jones provided especially helpful discussions during this work. Registry No. Cl2CHCHCl2,79-34-5; Cl&=CHCl, 79-01-6.
Literature Cited (1) Mabey, W.; Mill, T. J. Phys. Chem. Ref. Data 1978, 7, 383-415. (2) Mehran, M. F.; Slifker, R. A.; Cooper, W. J. J . Chromatogr. Sci. 1984, 22, 241-243. (3) Burlinson, N. E.; Lee, L. A.; Rosenblatt, D. H. Enuiron. Sci. Technol. 1982,16,627-632. (4) Mabey, W. R.; Barich, V.; Mill, T. Preprints of the Division of Environmental Chemistry, American Chemical Society National Meeting, Washington, DC, Sept 1983; American Chemical Society: Washington, DC, 1983; pp 359-361. (5) Haag, W. R.; Mill, T.; Richardson, A. Preprints of the Division of Environmental Chemistry, American Chemical Society National Meeting, Anaheim, CA, Sept 1986; American Chemical Society: Washington, DC, 1986; pp 248-253. (6) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969. (7) Marshall, W. K.; Franck, E. U. Water Steam: Their Prop. Curr. Znd. Appl., Proc. Znt. Conf. Prop. S t e a m , 9 t h 1979, 506-512. (8) Walraevens, R.; Trouillet, P.; Devos, A. Int. J . Chem. Kinet. 1974,6, 777-186.
Received for review December 23,1986. Accepted J u n e 23, 1987. W e thank the U.S. Environmental Protection Agency, Grant R-811473-01-0, and the Drinking Water Research Center, Florida International University, for support of this research. T h i s research has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
