Environ. Sci. Technol. 1984, 18, 133-134
CORRESPONDENCE Comment on “Environmental Fate and Effects of Ethylene Oxlde” S I R The paper by Conway and co-workers ( I ) discusses the processes determining the fate of ethylene oxide (EO) in the environment. We would like to comment on their treatment of the kinetics of desorption of EO from water. They use the diffusion coefficient ratio raised to the 0.5 power to relate the desorption coefficient to the known coefficient for the tracer solute oxygen, and they conclude on the basis of two laboratory experiments that the desorption rate of EO from natural waters is about 0.36 times that of oxygen under the same conditions. This suggests that they have concluded that the resistance to the desorption of EO from water is predominantly in the liquid film as is true for oxygen. We believe, however, that the gas-film resistance also will be important for two reasons. First, the fact that EO is infinitely soluble in water suggests from an analysis of the two-film model that the gas-film resistance will be significant (2). Second, percentage resistances in the liquid film for desorption of EO from water at 20 OC were calculated from eq 3 of Rathbun and Tai (3) for three sets of gas-film and liquid-film coefficients considered to cover approximately the range of coefficients that might be observed on streams and rivers of the United States. These coefficients were paired to give the maximum range of percentage resistance. Henry’s law constant at 20 “Cfrom Table I of Conway and co-workers was converted to a concentration basis for use in these calculations. Percentage resistances in the liquid film were as follows: K L1
KG
% resistance
miday
miday
in liquid film
5.77 3.0 0.35
480 800 1210
33 61 95
Thus, both films are predicted to contribute to the resistance to the desorption of EO. Therefore, the statement that “EO will be desorbed from a water body with a rate dependent upon the actual oxygen-transfer rate in the system” is only partially correct. The liquid-film coefficient for EO can be estimated from the oxygen coefficient. However, to obtain the overall desorption coefficient for EO, the gas-film coefficient must be estimated independently and these two coefficients combined with Henry’s law constant by use of eq 3 of Conway and co-workers. The KG and KLvalues of 720 and 4.8 m day-’ of Liss and Slater ( 4 ) have been widely used and quoted, and Conway and co-workers assumed a ratio of these values to be constant in their development. There is no reason to expect, however, that the ratio of liquid-film and gas-film coefficients should be the same for all water bodies or even the same for a specific water body for all times. The film coefficients are determined mostly by mixing conditions within each phase. Thus, wind speed is the predominant factor in determining KG, and liquid turbulence is the predominant factor in determining KL, and these factors generally are independent. It is in this respect that the two-film model breaks down somewhat because the model predicts that virtually all resistance to absorption of oxygen should be in the liquid film. Yet it is well-known that wind 0013-936X/84/0918-0133$01.50/0
significantly affects the oxygen absorption (reaeration) coefficient (5). This presumably results from increased near surface turbulence as a result of wind shear. The results of Conway and co-workers in their Table I1 support this observation. The oxygen absorption coefficient increased 1390 for the same water mixing condition when wind was added. The EO desorption coefficient, however, increased 22%, and this larger increase supports our contention that the gas-film resistance is important for desorption of EO. Registry No. Ethylene oxide, 75-21-8.
Literature Cited (1) Conway, R. A.; Waggy, G. T.; Spiegel, M. H.; Berglund, R. L. Environ. Sci. Technol. 1983, 17, 107-112. (2) McCabe, W. L.; Smith, J. C. “Unit Operations of Chemical Engineering”; McGraw-Hill: New York, 1956; pp 655-656. (3) Rathbun, R. E.; Tai, D. Y.J. Environ. Eng. Diu. (Am. SOC. Civ. Eng.) 1982, 108, 973-989. (4) Liss, P. S.; Slater, P. G. Nature (London) 1974, 247, 181-184. (5) Rathbun, R. E. J. Hydraul. Diu., Am. SOC. Civ. Eng. 1977, 103,409-424.
R. E. Rathbun,” D.
Y. Tai
U.S. Geological Survey, WRD Gulf Coast Hydroscience Center NSTL Station, Mississippi 39529
SIR: The interest of R. E. Rathbun and D. Y. Tai in our recent article on ethylene oxide fate is appreciated ( I ) . Their many publications on the subject of desorption of organic compounds from water demonstrate their expertise in this area. In presenting our results, we recognized that laboratory studies on volatilization have significant limitations, and therefore, we did not attempt to correlate the results of our two simple tests with the many variables encountered in our system, nor correlate our results to empirical relationships derived by others from laboratory data. Rather, we attempted to generalize our results to data actually obtained in the environment and so obtain a broader view of the desorption process. Rathbun and Tai imply that we concluded that the desorption of ethylene oxide is controlled by the liquid-film resistance only, and not also by the gas-film resistance. On the contrary, we contend (as eq 5 shows) that it is important to recognize that the desorption of ethylene oxide and most organic compounds is controlled (to varying degrees) by both the gas-film and the liquid-film resistances. The exact percentage of the resistivities can be determined from the ratio of K , / ( K f l as seen in a variation of Rathbun and Tai’s eq 3 in ref 2: % resistance in liquid film =
100 1 + K,/(HKJ
In many cases one resistance may predominate (go%+), but both resistances need to be considered when desorption data are interpreted or correlated. In our paper, we, in fact, questioned the utility of an empirical correlation developed by other researchers for the desorption of some
0 1984 American Chemical Society
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aromatic and chlorinated hydrocarbons simply because “they included a number of organics not completely controlled by the liquid film resistance.” We did, however, say that the published environmental data of Liss and Slater (3) initially applied by MacKay and Leinonen ( 4 ) and Dilling (5) suggest that the K,/Kgratio might reasonably be represented as a constant. In such a case, Kg would vary with K,, and the desorption rate of ethylene oxide or any organic compound (whether gas or liquid film controlled) could be estimated from a known liquid-film mass transfer coefficient (such as for oxygen). Although additional data we have developed and those that have been published by other researchers indicate that the K l / K g ratio may vary by a factor of 2 or more, we believe this value is a reasonable representation of typical environmental conditions. Using our cited values for the K l / K ratio (11.5) and Henry’s constant for ethylene oxide (-8.2\, one can easily calculate the resistances in the gas and liquid films (eq 3) as 59% and 41%, respectively. We were also very interested in the data cited by Rathbun and Tai covering approximately the range of coefficients that might be observed on streams and rivers in the United States. We recognize that actual environmental data are needed to fully evaluate the utility of our simple equation (the Relative Desorption Rate Constant concept), and Rathbun and Tai’s cited data would suggest that the K J K g ratio would vary by a factor of 40 under typical environmental conditions. We were disappointed that the K1 and Kgdata cited by Rathbun and Tai are not obtained concurrently in the same system but were combinations of data from several published sources selected to give the maximum possible range in the Kl/Kgratio (2, 6 ) . The cited combinations include the largest K1 (5.77 m/day) with the smallest Kg (480 m/day) and the smallest Kl (0.35 m/day) with the largest Kg (1210 m/day), as well as “approximate average values”. While we recognize that in a given system such an extreme in the K1 and K gvalues is theoretically possible, we do not believe they are reasonably representative of conditions commonly encountered in the eFvironment. Incidentally, the Rathbun and Tai Kg data appear to be for water and the K1 for benzene etc., and to compare the values as a K l / K gratio, both would need to be corrected to oxygen. Finally, we are not sure we agree with Rathbun and Tai’s belief that wind speed is the predominant factor controlling the gas-film mass transfer coefficient, that liquid turbulence is the predominant factor controlling the liquid-film
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mass transfer coefficient, and that the well-known impact of wind speed on the oxygen transfer rate implies a breakdown of the two-film model. Rather, we would tend to agree with the recently published data of MacKay and Yeun (7) which show Kg and K1 to be equally dependent upon wind speed (to the 1.5 power) or the model of Shen (8)showing Kl dependent upon the wind speed to the 0.67 power and Kgto be dependent upon the wind speed to the 0.78 power. While we have not developed experimental data to support either model, we believe that they are not inconsistent with our view that the K l / K g ratio can be reasonably represented as a constant under typical environmental conditions. In summary, we believe that assuming a constant Kl/Kg ratio will allow the relative desorption rate of organic compounds controlled by both the gas-film and the liquid-film resistances to be estimated. While a more representative Kl/Kgratio might be developed, we believe this concept can be used to classify and rank organic compounds according to desorption potential or hazard potential when present in streams, lakes, rivers, or even waste treatment facilities (impoundments and equalization basins). Registry No. Ethylene oxide, 75-21-8.
Literature Cited (1) Conway, R. A.; Waggy, G. T.; Spiegel, M. H.; Berglund, R. L. Enuiron. Sci. Technol. 1983, 17, 107-112. (2) Rathbun, R. E.; Tai, D. Y. J . Environ. Eng. Diu. (Am. SOC. Ciu. Eng.) 1982, 108, 973-989. (3) Liss, P. S.; Slater, P. G. Nature (London) 1974, 247, 181-184. (4) MacKay, D.; Leinonen, P. J. Enuiron. Sci. Technol. 1975, 9, 1180. (5) Dilling, W. L. Enuiron. Sci. Technol. 1977, 11, 405-409. (6) Rathbun, R. E.; Tai, D. Y. Water Res. 1981,15,243-250. (7) MacKay, D.; Yeun, A. T. K. Emiron. Sci. Technol. 1983, 17, 211-217. (8) Shen, T. T. paper presented a t the 75th Annual APCA Meeting, New Orleans, LA, June 20-25, 1982.
R. L. Berglund, R. A. Conway” G. T. Waggy, M. H. Spiegel
Research and Development Department Solvents and Coating Materials Division Union Carbide Corporation South Charleston, West Virginia 25303
