J. Phys. Chem. 1982, 86, 4221-4223
voids in the solid adsorbate are impervious to the liquid adsorbate, since it is this postulate that results in excessively low prediction for the adsorption of the liquid. Since the counter postulate of complete voids accessibility gives predictions that are much too high, one can postulate that voids accessibility varies over the adsorption space, and one can always find a voids accessibility function that would account for the data. However, the use of such an ad hoc function would not be economical of hypothesis; moreover, since it would have to be different for each solid (and possibly for each solid-liquid combination as well), it would in all likelihood be of limited utility. Furthermore, the ubiquitous occurrence of Henry’s law isotherms probably represents some physical situation that cannot be,ignored. Finally, one cannot readily use a voids accessibility function to account for the mixed solids systems (e.g., of Jossens et al.)12that follow the IAS model at least reasonably well. (12) Jossens, L.; Prauenitz, J. M.; Fritz, W.; Schlunder, E. U.; Myers, A. L. Chem. Eng. Sci. 1978, 33, 1097.
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As if in compensation for our difficulties in finding a truly general model, we note again that the appearance of Henry’s law isotherms, in what appear to be many systems with one dominant component, introduces some new experimental simplicities, on which we report in a companion arti~1e.l~We note also in closing that systems with trace impurities in a dominant component are precisely the purification systems for which activated carbon has been long used. Acknowledgment. We thank the Calgon Corp. and the National Science Foundation (Grant No. CME-7909247) for supporting this work. Supplementary Material Available: Tables I-V, listing adsorption data for phthalic acid (Table I) and for the binary systems in Figures 1-7 (Tables 11-V) (13 pages). Ordering information is available on any current masthead page. (13) Gu, Tiren; Manes, M. J . Phys. Chem., follow paper in this issue.
Multicomponent Adsorption from Water onto Activated Carbon. Trace Liquids and a Dominant Component Tlren Gu and Mllton Manes’ Chemistry Department, Kent State University, Kent
Ohio 44242 (Received: February 16, 1982; I n Final Form: Ju/y 6, f982)
A Polanyi-based treatment predicts that multiple trace liquids, in the presence of a more concentrated (liquid or solid) solute of at least comparable displacing power, and in equilibriumwith activated carbon in water solution, should exhibit adsorption isotherms that are both linear and independent of each other, with slopes that depend on the concentration of the dominant component. These expectations are borne out by studies on solutions of 1-pentanolcontaining propionitrile, diethyl ether, and 1,2-dichloroethane,both singly and in combinations, and similar solutions of p-nitrophenol containing propionitrile, 1,2-dichloroethane,pentanol, and chloroform.
Whereas the earlier studies of Greenbank and Manes’ have resulted in theoretical treatments for the adsorption of many-component solutes from water onto activated carbon, their reported experimental data have been largely limited to binary solutes and have emphasized the adsorption of one trace (minor) component in the presence of a dominant (major) component. We here extend the experimental study to multiple trace liquids, again in the presence of a dominant (liquid or solid) adsorbate, and with the dominant component of comparable or greater displacing power. For all such systems, and for any number of minor components, the predicted adsorption isotherms are remarkably simple; each is linear and all are independent. Moreover, the model estimates the slope of each linear isotherm as a function of the concentration of the major component. The data to be reported are in agreement with the expected linearity and independence of the individual isotherms, and in reasonable agreement with the predicted slopes. However, observed differences between predicted and observed slopes are comparable with earlier results and (1) Greenbank, M.; Manes, M. J . Phys. Chem. 1981, 85, 3050. 0022-3654/82/2086-4221$01.25/0
represent deficiencies in the model. The model used here was the Polanyi-based miscible (multiple liquids) model of Greenbank and Manes112which incorporates the assumption that the adsorbate may be treated as a mixture of components that keep their bulk solubilities in water but that are ideally miscible in each other, whether they are liquid or solid in bulk. The model was used in two versions. The general model (the “nonuniform adsorbate” model) takes into account the presumed variation of adsorbate composition with adsorption potential, which is quite analogous to the composition variation that results when a solution of components of different densities is placed in a powerful gravitational field; the approximate model (“uniformadsorbate” model) makes the simplifying approximation of adsorbate uniformity. Both versions predict linear and independent isotherms for multiple minor components, provided that the major component is not a very weakly displacing adsorbate, and both predict similar slopes if the major and minor components are of similar displacing power. For (2) Greenbank, M. Ph.D. Dissertation, Kent State University, Kent, OH, May 1980.
0 1982 American Chemical Society
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minor Components in the presence of a much more strongly displacing major component the nonuniform adsorbate model predicts slopes that are lower by not more than about fivefold, the exact difference depending on the components and the concentration of the major component. Given the individual adsorption isotherms as plots of adsorbate volume against concentration, the calculation of linear isotherms for the minor components from the uniform adsorbate model is quite simple. The total adsorbate volume, V, is estimated as essentially equal to the adsorbate volume of the major component corresponding to its fixed concentration. The individual values of ci* are from the individual isotherms of the minor components, all at volume V. The equilibrium concentrations, ci, are then given by ci = xici*
Gu and Manes -.-
1.5-
3
(1)
where the xi are the mole fractions of the minor components in the organic adsorbate (which again consists essentially of the major component). For a near-saturated major component the calculation becomes equivalent to partition of the minor component between water and bulk major component, which is assumed to be an ideal solvent; for other fixed concentrations of major component the calculation is again equivalent to partition, except that the ci* are substituted for the bulk solubilities in water. The details of this and the nonuniform model are given by Greenbank and Manes1 and Greenbank.2 The assumption of ideal adsorbate miscibility is found in the model of Radke and Prausnitz3 (also known as the “ideal adsorbed solute” or IAS model); however, the overall Polanyi-based treatment differs from the IAS model in that the individual components are compared at equal adsorbate volumes rather than at equal “spreading pressure”. Whereas both models give quite similar predictions for many systems, the IAS model (for which the derivation is limited to dilute solutions) goes out of bounds for many systems containing a dominant component at high concentrations; the Polanyi-based model never goes out of bounds, and we have here used it exclusively. Details of the calculations and a discussion of the strengths and weaknesses of alternative models are given by Greenbank2 and by Greenbank and Manes.lp4 The systems that were studied comprise the following: propionitrile (PN), diethyl ether (EE), and 1,2-dichloroethane (DCE) as minor components (individual, pairs, and triplets), with (liquid) 1-pentanol (PEN) as the major component; and PN, DCE, PEN, and chloroform (CHL) as the minor components with (solid) p-nitrophenol (PNF’) as the major component. The components were chosen for experimental convenience and because single-component isotherms were available from earlier work. The minor components were all liquids because preliminary studies (now under way) suggest that the prediction of adsorption isotherm slopes may be less accurate for solid-solid systems. Finally, we continue to use the same carbon that was used in earlier studies.’
Experimental Section The carbon, organic components, and shaker-bath method were essentially the same as in earlier studies. The major component (pentanol or PNP) was added as a measured pure component to the flask, and its volume was estimated in calculations of the adsorbate solution volume. (3) Radke, C. M.; Prausnitz, J. M. AZChE J. 1972, 18, 761. (4)Greenbank, M.; Manes, M. J. Phys. Chem., preceding paper in this issue.
CONC. (mg/L)
Flgure 1. Adsorption of 1,2dichloroethane (DCE), diethyl ether (EE), and propionbile (PN) from a solution containing 16 g/L pentanol. The open circles (0)represent runs with a single minor component. The half-shaded circles (@,e) represent runs with two minor components; each such symbol is plotted twice, unless it has an adjoining letter, in which case the concentration of the other component is given below. The fully shaded circles all represent runs with three minor components and are each plotted on three curves; individual runs may be identified by the coding of the fully shadsd points, again in the absence of adjoining letters. For the lettered points, the other components and concentrations in mg/L are as follows: (A) 11.5 PN, (B) 32.7 PN, (C) 37.5 DCE, (D) 25.9 DCE, (E) 49.1 PN, 44.0 DCE,(F) 15.2 PN, 20.5 DCE.
The (125 mL) flasks were shaken at 25 “C for at least 24 h, usually 48 h or longer; preliminary check experiments on comparison of the effect of preaddition of either the major or minor components showed that equilibrium was achieved. The equilibrium concentrations of the major components were essentially constant in successive runs and were estimated from the previously determined adsorption isotherms. Following equilibration, the centrifuged carbon-free solutions were analyzed by gas chromatography on a Tenax column. The data were plotted as loading (mg/g of carbon) of each minor component against its equilibrium concentration (mg/L), with no reference to the concentration of other minor components, except that the runs with single, double, and triple minor components are distinguishable in the resulting plots. The slopes were determined from the single-component runs, which were more numerous, and were compared with the slopes calculated from the alternative Polanyi-based models.
Results and Discussion The experimental data for the pentanol-dominant and PNP-dominant systems are presented respectively in Figures 1and 2. The points are experimental points, and the lines are least-squares plots that are fitted to the single-componentexperimental points and to the origin. The points for multiple minor components are multiply plotted, so that each individual loading may be compared with the corresponding loading for a single (minor) component at the same concentration; the points are coded so that all of the individual loadings and concentration are available for each run. In several runs in which the loadings were not all determined because of analytical problems, the points are so identified, and data otherwise not available from the plots are provided in the captions. Table I, which we first consider, gives the experimental Henry coefficients for the individual minor components in both pentanol and PNP, together with the predicted coefficients by the alternative models. For dominant
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4223
Adsorption from Water onto Activated Carbon
h
M M
3E
0.6
-
zn a
0-
PN
0
40
20
60
80
CONC. (mg/L)
F@re 2. Adsorption of chloroform, 1,2dichloroethane (DCE), pentanol (PEN), and proplonitrile (PN) from 12 g/L p-nitrophenol. Legend as in Figure 1. A: 54 CHCl3 (off scale).
TABLE I: Experimental and Predicted Henry Coefficients Henry coeff, L/g
component
exptl
uniform nonuniform adsorbate adsorbate model model
(a) In 16 g/L 1-Pentanol propionitrile 0.0015 * 0.0002 0.0028 diethyl ether 0.0123 * 0.0005 0.0085 dichloro0.050 i 0.0007 0.053 ethane ( b ) In 12 g/L p-Nitrophenol propionitrile 0.0029 f 0.0004 0.0026 1,2-dichloro- 0.020 k 0.0008 0.050 ethane 1-pentanol 0.0078 * 0.0003 0.023 chloroform 0.0405 i 0.0010 0.12
0.0032 0.011 0.063
0.00094 0.018 0.0053 0.046
pentanol both models give the expected similar predictions, and the maximum deviation from experimental results (by a factor of about 2) occurs with propionitrile. For dominant PNP the nonuniform model is closer to the experimental data, except again for propionitrile, where the
uniform model gives a good estimate and the nonuniform model underestimates the Henry coefficient by a factor of about 3; overall, however, the nonuniform model does somewhat better. Since the PNP increases the equilibrium concentrations by at least 100-fold over the single-component concentrations at the same loading (i.e., with no PNP), the approximation is not bad, although there is obviously room for improvement. These data may be considered as an extension of the earlier work of Greenbank and Manes' on similar systems, and the overall accuracy of the prediction of the Henry coefficients is essentially similar. We now consider the independence of the Henry coefficients in multicomponent mixtures, which is the principal subject of this study. Although the precision of our experiments does not exclude interaction effects of as high as 107'0,we have thus far found no significant effect of the presence of one minor component on the adsorption of any other. This is not especially surprising, since one would usually expect the principal interaction of each minor component to be with the major component, except perhaps for components that react with each other in bulk. However, the expected and observed simplicity of these multicomponent systems should have considerable applicability. For example, the systems studied here are quite directly related to the use of activated carbon to remove single or multiple impurities from a crude product or to remove multiple impurities from a wastewater where one component predominates. We now consider the expected generality of our rather preliminary results on the independence of linear isotherms in multicomponent systems. We already know from the work of Greenbank and Manes' that the adsorption isotherm is nonlinear for a much more strongly displacing (single) minor component, and one would expect nonlinearity in the case of chemisorption or molecular sieving. Given these exceptions, and excluding strong chemical affinities between individual components, we would expect the linearity and independence of adsorption isotherms for minor components to be quite ubiquitous for both liquid and solid adsorbates, and to hold for an unlimited number of components, as long as the total loadings are some small fraction of the loading of the major component. We expect to report on all-solid systems in due course. Acknowledgment. This article is based upon work supported by the National Science Foundation, under Grant No. CME 7909247, and by Calgon Corporation.
