mole ratio of 4.4: 4 and 4.5:1 were obtained. The results indicate thui Li+ is solvated by four molecules of aertme.
Acknowledgment. The authors gratefully acknodedge the support of this M - ~ Cby iz graiil from the National Science Foundation.
Appllicatiorr of the Pol anyi Adsorption Potential Theory to Adsorption uieioln on Activated Carbon.
11. Adsorption of Partially
rganic Liquids from Water Solution132
y David A. Wohleber and Milton Manes*3 Chwnislry Department, Kent State University, Kent, Ohio
44240
(Received J u n e 25, 1970)
Rublwation costs assisted by Pittsburgh Actimted Carbon Dzuision, Calgon, Corporation
A. method is presented for estimating the adsorption isotherm of a partially miscible organic liquid, from water solution onto an activated carbon, from it3 solubility, molar volume, and refractive index, together with a “characterislic curve” for the carbon that may in turn be determined from either gas-phase or liquid-phase adsorption data. The method, which is based on the Polanyi adsorption potential theory, and which is applicable over a wide range of capacities, is illustrated by estimated gas-phase and experimental liquid-phase data ion 1,2-dichloroethane, diethyl ether, ethyl acetate, methylene chloride, and propionitrile.
Introduction The Polanyi adsorption potential theory has been applied by Wansen and Fackler4 to the adsorption of liquid mixtures on carbon black, and by Manes and Hofer5 to the adsorption of solids from various solvents on activated carbon. The expected relation between gas- and liquid-phase adsorption was found by Manes and I-Iofer at low loadings; however, solute adsorption at high loadings did not fit their gas-phase correlation curve, presumably 1)ecauseof packing effects. A better fit should be expected if the adsorbate is a liquid, i.e., in the adsorptioi2 of relatively strongly adsorbed solvent A good example is the adsorption of partially miscible organic liquids from water, The principal experimental problem was in the determination of solutp concentrations at low loadings on the adsorbent (down to 0.1 wt yo),where the bulk phase concentrations of most organic liquids become undetectably low. This problem was met by limiting the investigation to solutes of low molecular weight and significant solubi1it-y and by the use of a gas chromatograph with a ilanle ionization detector to analyze concentrations down to the parts-per-million range. Tb e invwtigatjon consisted of the determination of the solubilities in water solution and the adsorption isotherms at 25” from water solution of 1,2-dichloroa
ethane, diethyl ether, ethyl acetate, niethylene chloride, and propionitrile on the same activated carbon that was used by Manes and Hofer, A gas-phase correlation curve [i.e., a plot of the volumc adsorbed os. the ] caladsorption potential per unit volume, ( E / V ) was culated for each solute, using the hydrocarbon correlation curve reported for this carbon by N i n e s and Ilofer. Theoretical. The underlying theory, which is given in more detail by Manes and Eofer, may be sumrnarized as follows. For gas-phase adsorption on a given carbon, a plot of the adsorbate volume against the adsorption potential per unit volume (a = E / V )is approximately the same in a series of similar compounds (e.g., the hydrocarbon series), Compounds of different refractive index give the same plot (correlation curve) except for the scale of the abscissa. The gas-phase characteristic curve for the vapor of a liquid of given refractive index (1) Based on a thesis submitted by David A. IVohleber to Kent State University in partial fulfillment of the requirements for the Ph.D. degree. (2) Reference 5 may now be considered a6 the first article in the series. (3) T o whom inquiries should be directed. (4) R. S. Hansen and R. V. Fackler, J. P h y s . Chem , 57, 634 (1953). ( 5 ) M. Manes and L. J. E. Hofer, ibid.,73, 554 (1969). The Journal of Physical ChemistTy, “02.
76, No. 1, 1971
DAVID A. WOHLEBER AND MILTONMANES
62 may be made to coincide with the hydrocarbon correlatioii curve by the operation
where a, and a h are the respective adsorption potentials per unit volume of the solute and of the hydrocarbon line a t the mme adsorbate volume; pi is defined as
tively dilute; both assumptions are reasonable for the present case. If carbon is added to V oml of solution a t concentration co (in g/ml) of organic solute, and if V ml of neat liquid is adsorbed (ie., the adsorption volume is V ml) then, assuming volume additivity, the aqueous phase a t equilibrium, a t concentration c, occupies (V, - V )ml. If we equate the total masses of the organic component before and after adsorption we get COVO = c(V0 - V )
+ pV
(4)
or where n, is the refractive index of component i, and where s and h refer to solute and heptane; it is proportional to the polarizability per unit volume. Equation 1 is a good approximation for most organic vapors, but ii breaks down badly for water, for which E/Vis considerably lower (as we shall see, by a factor of about 3) thaii one would calculate from its refractive index. (It is this anomalously low adsorption potential per unit volume that accounts for the strong adsorption of organic solutes from water.) We cannot, therefore, use eq I to estimate the gas-phase correlation curve for water. I n the liquid-phase adsorption of a solute from a relatively weakly adsorbed solvent, a plot of the adsorbate volume against csl/l7 (where cSl is RT In c,/c and cs and c are the saiuration and equilibrium concentrations) should be the same as the hydrocarbon correlation line, except that the scalc factor ysl is now given by (3)
As noted earlier, the last equality in eq 3 does not apply to the adsorption from water. However, given the estimated correlation curve for the adsorption of the organic vapor and the experimental correlation curve for the adsorption of the organic liquid from water, one might expect to estimate y1 as the difference between y s for the solute and yst for adsorption from water. Given an empirical correlation curve for water, we should now be able to predict the adsorption from water of a wide range of organic liquids. This has been borne out by the data. An essential feature of the adsorption potential theory is that the definitions of adsorbate volume in gas-phase and in liquid-phase adsorption are quite similar, namely the volumes between the solid-adsorbate interface and either the adsorbate-gas or the adsorbatesolution interface. Just as the calculation of the adsorbate volume in gas-phase adsorption is somewhat complicated a t elevated pressuresDbut quite straightforward at low pressures, the similar calculation in adsorption from the aqueous phase becomes quite simple if one can assume that the adsorbate has the properties of the bulk liquid and if the equilibrium solution is relaThe Jozwnal o j Physical Chemistry, VoE. 76, No. I , 1971
V = Vo(c0 - c> P - c
(5)
where p is the density of the adsorbed liquid. For dilute solution and even up to saturation for liquids of low solubility we can neglect c in the denominator.
Experimental Section Isotherms for the adsorption of 1,2-dichloroethane, diethyl ether, ethyl acetate, methylene chloride, and propionitrile from water onto activated carbon were determined at 25". The solutes had a minimum purity of 99.5%, as determined by gas chromatography. As noted earlier, the adsorbent was the same as was used by Manes and Hofer, and came from a single batch of Pittsburgh Activated Carbon grade CAL activated carbon. Except for drying a t 110" for 16 hr before weighing, the carbon was used as received. The surface area (BET) was 1140 mz/g. The solute, solvent, and carbon were shaken in 125-m1 screw-capped erlenmeyer flasks, in a bath thesmostated a t 25", until equilibration was achieved. Check experiments showed that 16 hr shaking sufficed for equilibration. After equilibration, the flasks were allowed to stand in the thermostated bath until the carbon had settled out, and a portion of the supernatant was removed and analyzed. The concentration of the solute was determined by gas chromatography (gc) using an internal standard technique. Poropak Q@, a crosslinked polystyrene, was used as the stationary phase. A 6 f t X 1/4 in. 0.d. column of this material operated a t 180" and 40 cc/min carrier gas flow rate resolved the organic solute from water for sample sizes less than 25 111. By using dual flame ionization detectors, concentrations of organic solutes as low as 110 ppm could be accurately determined in this size sample. Lower concontrations could be determined by first extracting a portion of the supernatant with toluene' and chromatographing the toluene extract. Concentrations in the range of 0.05-0.1 ppm were det,ermined with a sample size of 100 111. (6) R.J. Grant, M. Manes, and 8 . €3. Smith, A.1.Ch.E. J . , 8 [3], 403 (1962). (7) More volatile impurities (not,ably benzene) were removed from the toluene by distillation a t a reflux ratio of 60: 1 or greater.
A
~
s
OF
63
~ORGANIC ~ LIQUIDS ~ ~ FROM ~ WATER o ~SOLUTION
I---
r---
- -"- -
'OO.O
-7 5
-60
-4 5
-30
-15
0
0
2
4
6
8
log c/cs
Figure 1. Adswption isotherms for 1,2-dichloroethane (e), diethyl ether ( e ) ,ethyl acetate (0),methylene chloride (a),and propiorlitrile ((5) ), plotted as volume adsorbed us. log relative concentration. Temperature is 25".
I2
IO
14
16
I€
3
t/4.6'\1
Figure 2. Volume of 1,2-dichloroethane, diethyl ether, ethyl acetate, methylene chloride, and propionitrile plotted as a function of adsorption potential. 1000 3
The solubilities of' the solutes at 25" were determined by equilibrating water with excess solute and determining the concentration of the solute in the aqueous phase by gc. The molar ~rolurneof each adsorbate in the adsorbed phase was assumed to be equai to the molar volume oi bulk: liquid a t 25".
..". C
e
\
U
$0.0 1
z 1 \
-
-
0
Data, and Results Figure 1 gives the 25" adsorption isotherms of 1,2dichloroethane, dieihyl ether, ethyl acetate, methylene chloride, and propionitrile, from water, onto CAL carbon. The solubilities of the organic solutes (in g/ 100 g of water) at 25' are: 1,Zdichloroethane, 0.99; diethgl ether, 2.50; ethyl acetate, 8.88; methylene chloride, 1 .GO; and propionitrile, 10.50. Correlation curves for the liquid-phase adsorption of the five solutes were calcu Lated by taking points at equal intervals from the corresponding isotherms. Figure 2 shows these curves, plotted as cc fiohxte adsorbed per 100 g of carbon vs. e/ 4.6 V, following the notation of earlier publications. For each solute the value of ysl was determined as noted earlier, i e . , by plotting the volume adsorbed vs. E / V
\
-
€/4.6V
Figure 3. Correlation curves for water: --, calculated from adsorption isotherm; - - - -, calculated from y1 = 0.28; , calculated from refractive index.
Figure 3. Also shown is the correlation curve calculated from the vapor-phase adsorption isotherm of water on the same carbon. The agreement between the two curves is a measure of the accuracy of the results reported here. The expected parallel between gas- and liquid-phase adsorption is better shown by partially miscible liquids than by solids. The Journal of Physical Chemistry, VoZ. 75, No. 1 , 1971
64
DAVID A. WOHLEBER AND MILTON MANES
Table 1:
y
Values Tor Solutes and Water
Compd
YE
Ysl
Yl
1,2-Dichloroethane Diethyl ether Ethyl acetate Methylene chloride Propionitrile
1.12 0.91 0.96
0.80
0.32 0.20 0.27 0.25 0.25
0.62 0.69
1.08
0.83
0.95
0.70
hv 0.28
Discussion The reported results appear to be the first application of the Polanyi adsorption potential theory to the adsorption of partially miscible liquids. For all five systems the upper liimit of adsorption was in excellent agreement with that found in gas-phase adsorption and is significantly gertter than found for adsorption of solids on the same adsorbent. This is what one would expect if one assuines with Polanyi that adsorption from the gas phase i:s enhanced liquefaction. I n addition t o confivming the Polanyi adsorption potential theory of liquid-phase adsorption, and thereby providing a method for estimating adsorption isotherms of partially rniscible liquids from water solution, the reported results provide an improved method for the determination of the characteristic (or correlation) curve for an activated carbon. The reported data may be considered, for example, as five individual determinations of the characteristic curve over the entire loading range and a t a single temperature. This may be compared with the determinations of the characteristic curve from gas-phase adsorption, which can be done with a single gas only with considerable experimental difficulty and/or a long extrapolation of vapor pressures. For these reasons, for example, the gasphase correlation ciirves reported by Grant, Manes, and Smith6 and by Grant and R/!anessare all composite
The Journal of Phwicak Chemistry, Val. 76, No. 1, 19Ti
curves comprising segments determined for different hydrocarbon gases. Although there was overlapping of points in their reported Characteristic curves, the data at low loadings were measured only for methane. The results reported here, therefore, represent an improved method both in range and in experimental convenience for determining the characteristic curve for a carbon. Gas-phase and liquid-phase data thereby become largely interchangeable. Finally, while it is possible that polar organic compounds in the vapor phase might show some specific interactions with inorganic impurities or oxygenated complexes on the carbon surface, one would expect such effects to be swamped out in water solution. Water may therefore turn out to be a surprisingly convenient solvent for adsorption on activated carbon in spite of its own rather anomalous adsorption.
Conclusions 1. The characteristic curves (and therefore the adsorption isotherms) of some partially miscible organic solutes from water solution have been correlated over a wide range of capacities from the corresponding gasphase characteristic curves, which in turn have been estimated from refractive indices. 2. An estimated characteristic curve for water is arrived at from eq 3 and is in excellent agreement with the characteristic curve calculated from the water adsorption isotherm for the same carbon. Acknowledgments. This work was supported in part by a grant from the Pittsburgh Activated Carbon Division, Calgon Corp. I n addition, D. A. Wohleber received support from an N.D.E.A. Title IT7 Fellowship and from the Goodyear Tire and Rubber Fellowship Fund. (8) R. d. Grant and M. Manes, Ind. Eng. Chem. Fundam., 3(3), 221 (1984); 5(4), 490 (1988).
