inants which were already in the air before it arrived in the Norfolk area. Two lines of evidence lead to this conclusion. First, the source strengths cited by the authors are so small that they could generate only a very minor contribution to the ozone precursor concentration in the simulated air parcel. For NMHC, the authors estimate this contribution at 2.22 pg m-3 h-l. Since 1ppbC is equivalent to 0.67 pg m-3, this contribution amounts to 3.3 ppbC h-l or, over a 3-h period, a total of 10 ppbC. This is less than 5% of the initial NMHC concentration demanded by the model! To get the required NMHC input from the Norfolk area, the simulated air parcel would have to stagnate there for 60 h or more before starting on its trip at 8 a.m. on August 4. The second line of evidence is in a comparison of the simulation results shown in Figures 4 and 5 of the article. These figures show profiles of ozone concentration against time for simulated air parcels at two different wind speeds and several levels of initial NMHC concentration. The slower speed takes the parcel over the Norfolk urban area 2.5 h later than the faster speed. Examination shows that the same level of initial concentration gives nearly identical ozone profiles, regardless of the wind speed. A delay in the production of ozone would be expected in results for the slower parcel, if the effect of the Norfolk emissions were appreciable. Since no ozone delay can be detected, it follows that the effect of the Norfolk plume, according to the model, is negligible.
Literature Cited (1) Wakelyn, N. T.; Gregory, G. L. Enuiron. Sci. Technol. 1980,14, 1006-8.
Lowell G. Wayne Pacific Environmental Services, Inc. 1930 Fourteenth Street Santa Monica, California 90404
SIR: Having read the articles “Adsorption on Carbon: Theoretical Considerations” ( I ) and “Adsorption on Carbon: Solvent Effects on Adsorption” ( 2 ) ,and having found much to disagree with, I question why ES&T chose to give so much prominence to the research and the (at times controversial) views of a relative newcomer to the field. Professor Belfort’s published contributions to date on activated carbon appear to consist of one published paper ( 3 )and a second contribution ( 4 ) which has been presented before an ACS symposium (chaired by Professor Belfort) but which does not yet have a citation indicating acceptance by a reputable journal. One would think it appropriate to let this latter contribution go through customary review procedures, unless of course one could establish some compelling reason for immediate publicity. The cited material from this manuscript ( 4 ) does not disclose any such compelling reason; indeed, I find some features that should give pause to a knowledgeable reviewer, for example: (1) The theory at best does not aim a t estimating a broad-ranged adsorption isotherm, but only a single “adsorption capacity”. Its objective is therefore quite modest. Alternatively put, it does not take into account the energetic heterogeneity of the carbon surface. (2) The manuscript refers to linear correlation coefficients ( r > 0.918) as “excellent”, when in fact one can readily obtain such coefficients either with poor data or with strongly nonlinear dependence. For example, a linear fit of 11 equally spaced points ( x = 0, 1, . . . , 10, y = (10 - X2)1/2)on the quadrant of a circle of radius 10 gives r = 0.88; omitting the 0013-936X/81/0915-0601$01.25/0
@ 1981 American Chemical Society
last point gives r = 0.93, and omitting the last two points gives r = 0.94. Thus, the points in Figure 5 do not look as though they belong on a straight line at all, and the points on the blue line of Figure 7 show considerable scatter even for a homologous series, in which it takes some imagination to find properties that do not have some linear trend. (3) The choice of molar units to express adsorbability (Figure 6) is inappropriate for a supposed monotonic linear relationship, because the mass of adsorbate per unit mass of carbon in a homologous series approaches a limiting value with increasing molecular weight (or some variable that is correlated with it), and the number of moles must therefore go through a maximum and then decrease. We can see the approach to a limit in points 4, 5, and 6 in the plot for alcohols in Figure 6, and the attainment of a maximum with points 12, 13, and 14 in the plot for ketones, for which the extension of the line beyond the experimental points is clearly misleading. Moreover, the data from Table I of Giusti et al. ( 5 ) (from which the capacities appear to have been taken) could never have shown an “adsorbability” (in mass units) greater than 0.200 g/g of carbon because this was all that they added to their flasks. This does not, of course, mean that the hydrocarbon surface area is not necessarily a good property to look at, but its superiority as a correlating variable remains to be demonstrated. Consider now the first article ( I ) . Of a number of misstatements therein on the Polanyi model, I here pick as an example the suggestion that our work on competitive adsorption of binary and ternary solids (6, 7) was “empirical”. The fact is that, once the individual isotherms were determined, the theoretical curves for competitive adsorption of binary and ternary solids (including the prediction of the existence and the location of slope discontinuities) were calculated with no adjustable parameters at all. These very nonempirical findings, incidentally, are well outside the scope of alternative models, including that of Professor Belfort. There seems to be no need to belabor further the point that your coverage was, a t the very least, premature. As for the Polanyi model, I can state that a significant number of users (not all of whom have published their results, and not all of whom are former research associates) appear to be very happy with it. Interested readers are invited to look at our published work (8) and find out why, pending my submission of a more detailed exposition.
Literature Cited (1) Miller, S. Enuiron. Scz. Technol 1980,14,910. (2) Miller, S. Enuzron Sci. Technol. 1980,14, 1037. (3) Belfort. G. Enuzron Sci. Technol 1979.13.939. (4) Belfort, G., cited in ref 2. ( 5 ) Giusti, D. M.: R. A. Conwav: Lawson. C. T. J . Water Pollut. Control Fed. 1974,46, 947. (6) Rosene, M. R.; Manes, M. J . Phys Chem 1976,80,953. (7) Rosene, M. R.; Manes, M. J Phys Chem. 1977,81,1646. (8) Manes, M. In “Activated Carbon Adsorption of Organics from the Aqueous Phase”; Suffet, I. H., McGuire, M. J., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1980; Vol. I, Chapter 2. This article gives citations to earlier work. I
,
Milton Manes Chemistry Department Kent State University Kent, Ohio 44242
SIR: In answer to Professor Manes’ correspondence regarding the two recent interviews ( I , 2 ) , several comments seem appropriate. (1) A seminal objective of our research is to present a comprehensive formalism of aqueous-phase adsorption including fundamental formulations of all known interactions between solute, solvent, and sorbent. Another objective is to use this formalism to predict a priori a ranking order of adVolume 15, Number 5,
May 1981 601
sorption capacity without previously measured single-solute adsorption data. The first results of this approach are both exciting and encouraging especially since none of the current theories satisfies these objectives. (2) An important result, both theoretically (see eq 25, third term in Table 7 ( 2 ) )and experimentally (Figure 7 ( 2 ) )shows explicitly how the surface tension of an aqueous solution directly affects the expected adsorption capacity of a solute. Although Traub expected this, it took nearly 100 yr to formally predict this effect. T h i s was the important point presented in Figure 7. That there is some scatter ofthe data is less relevant to the overall import of the argument. In fact, Professor Manes omitted to point out the excellent fit for the mixed aromatics in Figure 7 , which is probably somewhat fortuitous anyway. (3) Different reasons exist that favor the use of molar or mass units for liquid-phase adsorption. In general, when the equilibrium adsorption is expressed in terms of classical thermodynamic properties such as free energy, entropy, and enthalpy, it has been most convenient to use molar units ( 3 , 4). With respect to the solvophobic interaction approach for the association reaction a molar basis is more useful than a conservative mass basis ( 5 ) . The choice of cavity surface area as a correlating variable falls directly out of the theory from the cavity formation term (see eq 21a, Table 5 ( 2 ) ) .I t has been successfully used in high-performance LC (6) and now in adsorption ( 2 , 5 ) .I t has also proved useful in understanding protein structure (7). Intuitively it is attractive because “areas” play a central role in the adsorption process and because branching and position of isomer are directly accounted for. Adsorption capacity correlations are consistently better with cavity surface area than, for example, with molecular weight. The major gist of my comments in the first interview ( I ) on the Polanyi adsorption potential theory has to do with the difficulties in specifying the ill-defined pressure (potential) gradient across the sorption region, and other problems con-
602
Environmental Science & Technology
cerning the validity of the (physical) model. Since indiuidual experimental isotherms first need to be measured before others can be predicted (by using the equation of the state) and since “scaling factors” are used to compare single-solute isotherms, the so-called theoretical curves are based upon an experimental rather than purely theoretical foundation. I t would have been particularly interesting if Professor Manes had commented on the substantive criticisms of the model discussed in the interview rather than defending the empirical nature of the model. As an aside, this author has recently shown that the ideal adsorbed solution and potential theories are mathematically equivalent when expressed through a simple relation that equates “adsorption potential” with the work needed to change the spreading pressure or interfacial tension by a specific amount a t constant molar area (8).
Literature Cited (1) Miller, S. Enuiron. Sci. Technol. 1980,14, 910-4. (2) Miller, S. Environ. Sci. Technol. 1980,14, 1037-49. (3) Radke, C. J.; Prausnitz, J. M. AIChE J. 1972,18,761-8. (4) Jossens, L.; Prausnitz, J. M.; Fritz, W.; Schlunder, E. U.; Myers,
A. L. Chem. Eng. Sei. 1978,33,1097-106. (5) Belfort, G. “Selective Adsorption of Organic Homologues onto Activated Carbon from Dilute Aqueous Solutions. Solvophobic Interaction Approach-II”, presented before the Division of Environmental Chemistry Symposium, 179th National Meeting of the American Chemical Society, Houston, TX, March 23-28,1980, and accepted for publication in “Chemistry and Water Reuse-11”; Cooper, W. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981. (6) Horvath, C.; Melander, W.; Molnar, I. J. Chromatogr. 1976,125, 127-56. (7) Richards, F. M. Ann. Reu. Biophys. Bioeng. 1977,6,151-76. (8) Belfort, G. AIChe J.,in press.
Georges Belfort Department of Chemical and Environmental Engineering Rensselaer Polytechnic Institute Troy, New York 12181
