ous compounds a t 25OC in the same form as Mackay and Wolkoff ( I ) . The values of k,G and klL used are those of Liss and Slater (2) corrected, as they suggest, for the molecular weights of the compound diffusing. The relative magnitudes of the liquid and vapor resistances are l/k,L and RT/H,k,G respectively. It is interesting to consider which resistance dominates by examining the percentage of the resistance lying in the liquid phase. In all cases except the pesticides, which have very low vapor pressures, the liquid phase resistance dominates. For substances such as alkanes with high Henry’s law constants-Le. the vapor pressure to solubility ratio is high, the rate of evaporation is controlled by the small concentration difference driving forces for diffusion attainable in the liquid phase, and the rate of evaporation is controlled by the liquid phase coefficient h l ~ .Conversely, for substances with low values of HI the evaporation rate is controlled by concentration gradient in the vapor. The phase resistances are approximately equal for a H, of 1.6 X atm m3/mol. The half-lives for a water depth of 1 m show that most of these compounds evaporate rapidly from solution. In situations where the water body is turbulent with frequent exchange between the surface water layer and the bulk, for example in a fast-flowing shallow river, or during whitecapping on a lake or ocean, the liquid phase mass transfer coefficient may be considerably increased and the evaporation rate increased correspondingly. For depths greater
than 1 m the half-life is correspondingly increased, assuming that the rate of eddy diffusion is substantial. The vapor phase mass transfer coefficient used here is higher than’the value used by Mackay and Wolkoff that was based on a mean water evaporation rate of 1000 mm/ year. At 10°C and 30% relative humidity, the k, value of 3000 cm/hr yields an evaporation rate of 1730 mm/year. Since the object of the present note is to give only approximate rates and indicate the factors controlling these rates, the difference is relatively small and is attributable to uncertainty in evaporation rates and conditions. Average environmental conditions are usually closer than 10°C than 25OC; thus it is interesting to consider the effect of temperature on the rates. The mass transfer coefficients and aqueous solubilities are relatively temperature insensitive, the principal effect being on the vapor pressure, PL,\. This only affects the rate significantly if the system is vapor phase controlled; thus for most of the compounds in Table I the rates and half-lives are insensitive to temperature as illustrated by the data for benzene. Literature Cited
Vi’.,Enuiron. Sci. Technol 7, 611-14 (1973). ( 2 ) Liss. P. S.. Slater. P . G.. Nature. 247. 181-4 (1974). (3) Treybal, R. E., “Mass Transfer Operations,” 2nd ed., McGrawHill, New York, N.Y., 1968.
(1) Mackay, D., Wolkoff, A.
Receioed for revieti November 1 , 1974 Accepted August 18, 197,5
CORRESPONDENCE SIR: In their article dealing with organic matter in New Orleans drinking water, Dowty et al. [Enuiron. Sci. Technol., 9, 762 (197511 presented data on the content of organics in Mississippi River water before and after treatment. One of the treatment procedures they used involved a commercial activated carbon-ion-exchange system for removal of organic matter and dissolved salts. While the data on the organic content of the water from this system were presented only as “preliminary data”, I question whether any significance can be given to them based on what is known about the activated carbon in the treatment unit and the process operating parameters. As is well known, removal of organic compounds by carbon beds depends upon the detention time of the water within the bed as well as on the previous operation history of the bed. No information on the depth of carbon or the rate of application of water is given, let alone information on the types and amounts of organic compounds previously adsorbed by the carbon, if any. If carbon beds are designed and operated properly they can do a good job of removing many organic compounds, but careful monitoring of the process and regeneration of the carbon a t the proper time are required. Because adsorption in many cases is reversible, it is possible for previously adsorbed compounds to appear in the effluent from the carbon bed thus illustrating the need for careful process control. I agree that further studies are needed to determine optimum procedures for operating carbon beds, but it is important to note that much is already known which should be taken into account, even when collecting preliminary data on the efficiency of removal of certain compounds.
SIR: We are in agreement with Dr. Snoeyink in that the efficient use of carbon in filtering devices is a complex and delicate process requiring careful monitoring. Since the release of recent findings about dissolved organics in tap water ( I , 2), many consumers have started purchasing bottled water and commercial filtering devices for home use. Many types of filtering devices are available to consumers of which most have not received as much testing or monitoring of performance as needed. The device we used in our study is manufactured by a well-established company which services a large portion of the scientific community and is available for home use and therefore possibly representative of such devices. Further, we share with Dr. Snoeyink his concern about the importance of the pretreatment of the charcoal. Thus our statement, “These compounds could have originated from the plastics used somewhere in the preparation or storage of the ion-exchange resins or charcoal.” Because of the inherent problems associated with charcoal filtering and the need to evaluate such devices, it seems that some federal or state agency should assume the responsibility of seeing that the claims made to the consumer are indeed met by the devices on the market. Literature Cited (1) Draft Analytical Report New Orleans Area Water Supply, prepared and submitted by the Lower Mississippi River Facility, Slidell, La., Surveillance and Analysis Division, Region VI, U S . Environmental Protection Agency, Dallas, Tex., November 1974. (2) Dowty, B., Carlisle, D., Laseter, J. L., Science, 187, 75-7 (1975).
Vernon L. Snoeyink Associate Professor of Sanitary Engineering University of Illinois at Urbana-Champaign Department of Civil Engineering Urbana, 111. 61801 1180
Environmental Science 8, Technology
John L. Laseter Chairman and Professor Department of Biological Sciences University of New Orleans New Orleans, La. 70122
