Langmuir 1991, 7, 344-349
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Experimental Study of Adsorption from Dilute Aqueous Solutions of a Nonelectrolyte: 1-Propanol on Activated Carbon M. Nagy Department of Colloid Science, Lorhnd Eotvos University, P.O. Box 32, 1518 Budapest, Hungary Received March 12, 1990 The dependence of equilibrium concentration of 1-propanol from dilute aqueous solutions on mass of sorbent was investigated in order to test the meaning and applicability of the widely used equation for calculation of excess amount of mixture components. On the basis of the experimental data, it was found that the adsorption isotherm was not a single-valued function of the 1-propanolequilibrium concentration. By making use of the exact mass balance of the adsorption system, it was possible to calculate the specific capacity of the sorbent and the absolute composition of the adsorption layer. The partitioning diagram of the system has indicated the strong preferential binding of 1-propanol to the activated carbon applied in our experiments.
Introduction
Experimental Section
In a previous paper' i t was pointed out that under appropriate conditions there is a close analogy between the selective sorption of macromolecules in mixed solvents and the adsorption from solutions a t solid/liquid (S/L) interfaces. Furthermore, a hypothesis was put forward that stated the dependence of equilibrium concentration of one of the mixture components, ~ i , on ~ , the mass of sorbent, m,,at fixed mixturevolume (or mass) may exhibit a more or less extended linear initial portion with all its consequences. In previous communications some experimental data were published that seemed to support our idea; i.e., linear initial parts on the ~ i = f(m,) , ~ curves were f~und.~,~ However, it has to be mention here that the adsorption interaction prevailing at S/L and other types of interfaces can be experimentally characterized for any values of m,, but as shown, only the so-called characteristic isotherm of the system' can be additive for a multisorbent system. Also, in one of our previous works the well-known equation recommended for calculation of surface excesses has been analyzed and it has turned out to be inconsistent with the accepted and sensible convention used for a long time in the field of chemistry and physic^.^ According to this, any changes that can take place in a system must be given as a difference between the final and initial states. In contrast to this, in the case of adsorption from solutions, the excess is calculated from the difference between the initial and final concentrations. Furthermore, if a finite volume is attributed to the adsorption layer, which is the physical reality, even the application of the mass balance for such an adsorption system is erronous. In order to resolve all these contradictions, several suggestions were made,1-3 among others the extrapolation of the traditionally calculated excess amounts to zero values of the sorbent mass or sorbent concentration. The aim of the present article is to analyze the results of our recent and detailed experimental investigation on the adsorption from dilute aqueous solutions of a nonelectrolyte, 1-propanol, onto a n activated carbon having high specific surface area in order to check statements and suggestions made in our earlier works.
Accordingto some preliminary experimental testa a granulated activated carbon, Nuxit AL-IV. (MuszBntermel6V., Hungary), was chosen as a sorbent. The BET specific surface area of the sorbent was estimated to be 950 m2/g, *30'%. Prior to the adsorption measurements, the sorbent was extracted 3 times with a 10% by mass 1-propanol-water mixture at room temperature, then the mixture was removed by soaking repeatedly the sorbent in distilled water. The purity of the supernatant water was frequently checked and the sorbent was considered pure when the presence of solute could not be detected by interferometry. The granules of the activated carbon were separated from the supernatant by filtration and dried in a vacuum oven at 102 O C until constant mass. The dried sample was stored in adesiccator that contained PzO5. For determination of density of the highly porous sorbent, an exactly known mass of activated carbon (about 2.0 g) was weighted in a picnometer of 10 cm3volume and a wetting liquid, l-propanol, was added to just cover completely the sorbent. In this state the system was carefully degassed and the picnometer was filled at 25 h 0.05 "C. The density was calculated in the usual manner and was found to be 1.736 0.003 kg/dm3. The liquid (1-propanol)uptake by the porous granules was determined from the same sample; i.e. from the surface of the filtered granules the excess of 1-propanol was removed by filter paper and, in a closed vessel, the mass of wet granules was measured. After drying under vacuum at 60 "C, the dry mass was also determined. The amount of 1-propanol that completely fills the pores was found to be 0.94 i 0.01 g/g of activated carbon. This value corresponds approximately to a volumetric porosity of 50-607; and to an apparent density of 0.5-0.6 kg/dm3. The sorption of pure components, water and n-propanol, from vapor phase was also measured. For this purpose a known mass of sorbent (2.000 g), kept previously in a desiccator, was placed in a closed vessel above a small amount of the liquid in question, thus ensuring saturated vapor around the sorbent at 25 1 "C. After 1week of contact time, the increasein mass of the sorbent was measured. From the vapor phase 0.54 f 0.02 g of 1-propanol and 0.64 0.02 g of water were taken up by 1 g of activated carbon. A series of stock solutions from 1-propanol (Analyticalgrade, Reanal, Hungary) and distilled water was prepared in the concentration range 0-7.00 g/100 g measuring the mass of each component by an analytical balance into conical flasks of about 50 cm3 volume provided with plastic stoppers. The total mass of each mixture was 50 g. From time to time the mass of each mixture was carefully checked in order to detect the possible occurrence of evaporational loss. However, neither decrease in mass nor change in the interferometer reading was observed. This series of dilute aqueous solutions of 1-propanol served to set up the calibration curve for the interferometer. The
(1) Nagy, M. Langmuir 1988, 4, 93. (2) Nagy, M. Magy. K6m.Foly. 1987, 93, 42. (3) Nagy, M. Magy. K&m.Foly. 1988, 94, 6.
0743-7463/91/2407-0344$02.50/0
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*
0 1991 American Chemical Society
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Langmuir, Vol. 7, No. 2, 1991 345
w,,;102
3.00 0 9913tOl% 4 19 198 10.1%
e )e.608to.i%
I
-A-
2.0
4.0
6.0
m, ,g
Figure 1. Sorbent mass (m,) dependence of equilibrium l-propanol concentration at two different initial concentrations(wl,i). The total mass of the mixture was 19.798 g. The dotted lines show the initial linear sections of the curves. (0 and X denote parallel measurements.)
composition of the supernatant solutions in equilibrium with the sorbent was determined by making use of an interferometer type ITR-2 (made in the USSR). The sensitivity in the actual concentration range was on average f0.003 g/100 g of 1-propanol referred to one scale division when cuvette of 0.50 cm were used. During measurements the temperature of the two-comparment cuvette was kept constant at 25 f 0.1 "C. On the basis of our earlier experiencess~~ as well as the repeated experiments carried out now with this series of solutions, it can be stated that the reproducibility of the reading was f l scale division and the overall accuracy of determination of 1-propanolconcentration was estimated to be iO.01 g/100 g or a little less. The sorption experiments were carried out in the same type of conical flasks as that were used for storage of solutions for calibration of the interferometer. The exactly known amounts of activated carbon (ranging from 0.2 to 10.0 g) were quickly transferred from the desiccator to the dry conical flasks and the masses of the closed systems were measured by an analytical balance. After this a given volume (10.00,20.00, or 40.00 i 0.02 cm2) of stock solution with known 1-propanolconcentrationwas added to the sorbent and the mass of the system was again measured. In this manner in all cases both the total volumes and total masses of the solutions have been accurately known, which were in conctact with the activated carbon. Very good wettability of the porous carbon by the aqueoussolutionswas indicated by the vigorous bubbling after addition of solutions to the dry but air-covered sorbent. The initial 1-propanol concentrations at which the mass dependence was studied were the following: 1.500,2.493,3.798, and 5.788 g/100 g. According to some spot experimentsthe time needed to attain the equilibrium was to a good approximation 24 h. In the actual experiments48 h of contact time was applied and the closed systems were gently shaken 3 times per day. The temperature during the sorption experiments was kept at 25 i 1 O C . The composition of the supernatant solutions was then determined with the aid of the calibrated interferometer.
Results and Discussion Dependence of Equilibrium Concentration on Sorbent Mass and Sorbent Concentration. In Figure 1 the experimentally obtained dependence of 1-propanol equilibrium concentration on the mass of activated carbon is shown for the two smallest initial concentrations a t constant mixture volume (20 f 0.02 cm3). (The average mass of the supernatant solutions was 19.798 g f 0.1 5% .) In both cases two series of measurements were performed in order to test the reproducibility of sorption experiments. As can be seen from the figure, a good reproducibility was found in the entire range of the mass of sorbent. T h e average deviation of the experimental value of w ~between , ~ the parallel measurements does not exceeds f0.01-0.02 5% (4)Nagy, M.; Wolfram, E.; GyBrfi-Szemerei, A. C Polym. S y m p . 1972, 39, 169.
J. Polym. Sci., Part
I
2.0
4.0
6.0 m,,g
*
Figure 2. Sorbent mass (m,)dependence of equilibrium l-propanol concentration at one initial 1-propanol concentration and at three values of the total mass, mt, of the mixture. The dotted lines show the linear portion of the curves.
4.00.
3.w 2 00.
1.00.
2.0
4.0
6.0
m,,g
Figure 3. Sorbent mass (mJdependence of equilibrium I-propanol concentrationat one initial 1-propanolconcentration and at three different values of the total mass of the mixture.
by mass 1-propanol, Also, the average scattering of experimental points around the fitted curves was estimated to be not greater than &0.02-0.03% by mass 1-propanol. In Figure 2 and Figure 3 the mass dependence of equilibrium concentration a t two higher initial 1-propanol concentrations and three different volumes of the mixtures are shown. Here, also, the average deviation of the experimental points was not greater than f0.03 % by mass independently of the total mass of mixture applied in the actual experiments. The common feature of all experimentally determined wl,*= f ( m J functions is that they have linear, initial sections (this problem will be discussed below) which are then followed by curved parts. The extension of the linear parts depends mainly on the initial 1-propanol concentration, w1,i)and, especially toward the greater mixture volumes, on the volume of the mixture. It is interesting to mention here that relatively long linear section was also obtained in some preliminary experiments performed on a powdered, i.e. on only a slightly porous, sorbent. The relatively high values of the negative initial slopes are related to the strong preferential binding of the l-propanol by the activated carbon of this kind and the transition from the linear t o the curved zone shows that the amount adsorbed, at fixed initial concentration and mixture mass, depends on the mass of sorbent. It is important t o mention here that a very similar experimental finding was published by Barton for KOH uptake by oxidized porous carbon from aqueous
[email protected] 346 Langmuir, Vol. 7, No, 2, 1991
c
B
A mt
0 9,913 t 0.1%
A19.798?0.1% 39.608tO.lYo
5.0C
w,
4.00
w;
no supernatant
w;’
in equilibrium with vapour of n-propanol
3.0C
c
1.oc I
0120 0140 0.’60 w, Figure 4. Dependence of equilibrium concentration of I-propanol on the concentration of sorbent (w,)at four different initial concentrations. (For explanation of regime A, B, and C, see the text and Figure 5.) The dotted lines indicate the extrapolation to UJ, = 1.00 and to w,”. it was suggested In some of our earlier that instead of the mass of sorbent, the concentration of it can also be a relevant quantity that can also control the equilibrium concentration of one of the components in the supernatant mixture. Similarly to the equilibrium dialysis experiments: here in the calculation of sorbent concentration, the mass of the system, that is the total mass of the mixture, mt, plus the mass of the sorbent, m,, is used as a reference state. Accordingly, the mass fraction of the sorbent can be expressed as follows
m, w, = m, + mt while the same quantity for the mixture is
m1 w1 = m1+ m2 where ml is the mass fraction of the component 1 in the solution phase, ml and m2 are the masses of component 1 and 2, respectively, and ml + m2 = mt. It can also be pointed out that the relationship between w1 and w1‘ (=ml. (ml + m2 + m,)-l) is the following W1‘
w1 = 1- ws
w;
w,
n
w;
w,
s
w;’
Figure 5. Schematic representation of the physical situation in three different ranges, A, B, and C, of sorbent concentration showing a possible relation between adsorption from a binary mixture and adsorption from a two-component saturated vapor. The dotted lines mark the boundary between the sorption layer and bulk phase.
ut 298K
2.0c
G
(3)
Of course, the same holds for any other components of the system. In Figure 4 the dependence of equilibrium 1-propanol concentration a t four different initial concentrations is shown as a function of sorbent concentration. As can be seen, the basic feature of these functions is very similar to that obtained for the sorbent mass dependence of W I , ~ , and here it is unambiquously proven that the concentration of the sorbent is indeed a relevant variable determining ( 5 ) Barton, S. S. Colloid Polym. Sci. 1986, 264, 176. (6) %vny, A.; Pouchlp, J.; solc, K . Collect. Czech. Chem. Commun. 1967,32, 2753.
the equilibrium concentration of one of the components a t a given initial concentration. All curves, according to our previous suggestion,l have initial linear sections and at the lower sorbent concentrations and a t the two higher initial concentrations first they have a slight downward curvature and then a curvature in the same sense as shown in the previous figures. This latter observation means that there is a very flat inflection on these two curves a t sorbent concentration about 0.10. There are two characteristic sorbent concentrations, w8/ and w / , which were calculated from data given in the Experimental Section. w,’means the average sorbent concentration when all pores are filled with liquid and there is no supernatant liquid present in the system (this concentration is very similar to a volume fraction of a polymer in a polymer gel swollen to equilibrium in a pure solvent). It means also the upper limit until adsorption experiments in solutions can be performed applying the traditional methods. The w;’ value for this sorbent was calculated from the experimentally determined uptake of 1-propanol from vapor phase. In this case the liquid mixture spreads over the surface of sorbent (layer-position) and fills the (small) pores for which the conditions of capillary condensation is fulfilled and it corresponds also to a sorbent capacity when the adsorption takes place from a single or from multicomponent saturated vapors. The simplified physical situation is shown schematically in Figure 5 . In connection with this it is interesting t o note that there must be a close relationship between adsorption from solutions and that from mixed vapors. Namely, starting from a given initial concentration and amount of asolution it would be a t least in principle possible to conduct experiments through vapor phase to measure the W1,e = f(m,) or ~ 1 = f‘(w,) , ~ curve up to the value of w/ or even beyond it. From this kind of experiment it would be feasible to determine the extent and absolute composition of the adsorption layer. Such experiments, however, have not yet been performed in our laboratory. The other possibility would be to extrapolate the W1,e = f’(w,) functions starting from different initial concentrations to w/. This kind of extrapolation would give equilibrium concentrations and would mean physically the transfer of the total amount of the mixture to the adsorption layer of the sorbent with exactly known composition. Although this kind of extrapolation for lack of any rigorous theoretical ground is not easy to perform, it may be possible to approximate the shape of functions unaccessible for measurements from solution phase. The basis to do so is the fact that the equilibrium concentration of 1-propanol should go down undoubtedly to zero if w, reaches the value 1.0. This is shown in Figure 4 by dotted lines, and from the vertical line drawn a t w,” one can read off the equilibrium
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Langmuir, Vol. 7, No. 2, 1991 347
V 0.01
0.02
0.03
0.04
0.05
wl,(
-
Figure 6. Plot of experimental data using the traditional way of calculation of excesses of 1-propanol as well as the values of excesses when both m,and w, tend to zero. (For explanation of the dotted straight lines, see text.)
concentrations to which the four layer concentrations (which correspond now to the four initial concentrations 0.058,0.038, 0.025, and 0.015) belong. These in turn are 0.0039, 0.0025, 0.0012, and 0.0008. Of course, the same procedure can be applied to w l , too. Only one thing, namely the presence of any discontinuus or rather sudden change in w ~ =, f’(w,) ~ function in the zone between w; and u ) / , would disturb this simple picture. However, any jumplike downward curvature appearing in the Wl,e =f’(w,) function in this region would give an unreasonably high partitioning coefficient for the system studied.
The Fine Structure of Isotherms. A Simplified Layer Model. In Figure 6 the specific excess amounts of 1-propanol, mlU,expressed in mass/mass unit and calculated in the conventional way are plotted as a function of equilibrium concentration of 1-propanol. Starting from different initial concentrations, essentially maximum type curves were obtained which in a certain range have approached linearly or nearly in a linear way the specific excess extrapolated to the limit m, 0. This is indicated by the dotted lines with arrows a t their ends. (In this respect further explanation will be given below.) In the Figure 6 excesses calculated from the initial slopes of W1,e = f’(w,) functions are also given and there is an excellent agreement between the values obtained by the two methods (see also Table I). It also means very clear evidence for the equivalency of different equations used for calculation of mln-Ovalues, as was discussed in detail in our previous works.’J Nevertheless, the most striking feature of the experimentally obtained curves is that the adsorption isotherms cannot be considered in general as single-valued functions of the equilibrium concentration, that is, these kinds of excesses are not directly governed by the bulk chemical potentials of mixture components. This is understandable, because, for instance, in the case of the azeotropic adsorption a t a given equilibrium concentration the excess can be zero but the bulk chemical potentials remain finite. The observed multiplicity of the adsorption isotherms means that between the lower (which is the characteristic isotherm of the system1) and the upper (experimental upper limit) curves there are an infinite number of isotherms, the positions and shape of which depend on the mt/m,ratio or on ws. In practical terms, the difference between the characteristic isotherm and the upper limit can be as much as 30-40 90 or more depending on the shape of the ~ 1= f(m,) , ~ functions. In order to make the survey easier, in Figure 7 adsorption isotherms are shown a t different mt/ms ratios. The new information that this figure offers is that even the shape of an isotherm may depend on this ratio. As shown, the characteristic isotherm exhibits a stepwise change at equilibrium concentration of about 0.030 while the upper limit is a steadily increasing
-
.
.
.
.
~-
,
0.05
%e
Figure 7. A comparative plot of adsorption isotherms obtained by different methods of evaluation of experimental data. (For further information, see text.)
function of the equilibrium concentration. The dotted line shows the isotherm at an intermediate value of mt/ m.9. Considering all the experimental findings discussed so far the following questions arise: Is it possible to find a realistic model that is able a t least as a first approximation to describe the linear or closely linear dependence of ~ 1 on m, or w,? Are all these results in some way related to the properties of adsorption layers, namely, to its extent and absolute composition? The starting point of such a modeling might be based on our earlier suggestion2 that in calculation of the exact mass balance of the adsorption system one has to take into account the finite volume of the sorbed layer, that is, in the calculation of the excess of one of the components the product wl,emtcanbe applied correctly if the volume of the sorbed layer is zero. However, on the basis of a large amount of evidence and also taking into account the finite size of any kind of molecules, the extent of the adsorption layer cannot be considered as a negligible one. For instance, in the case of adsorption of polymers from solutions on colloidal particles, its thickness can be commensurable with the size of the particles itself. The simplest way to get a deeper insight of the problem is to apply the exact mass balance for such a system (4) mtw1,i = meW1,e + m l w ~ , ~ where mt is the total mass of the mixture, w1,i is the initial mass fraction of component 1, me is the mass of the equilibrium (bulk) mixture, ml is the mass of the adsorption layer, and W ~ isJ the average mass fraction of the component 1 in the adsorption layer. Introducing me = m, - ml in eq 4 and rearranging, one may write mP1i - mP‘1,l = (5) Wl,e mt - ml or substituting the specific capacity of the sorbent defined as k = ml/m,
From eq 6 one can express easily the traditional relationship for calculation of the excess of component 1, mla (7) Thus, if we keep constant the layer composition and the specific capacity, eq 7 gives a linear relationship between mlu and the equilibrium concentration. In Figure 6 we attempted to approximate the upper part of each curve started from different initial concentrations with straight lines with negative slopes and to determine the slope and
,
~
348 Langnuir, Vol. 7, No. 2, 1991
NagY Table 1. Characteristic Parameters of the Adsorption System k
mid
0 605
i
w1.i
010
"I
n
0.015 0 0.024 93 0.037 98 0.057 88
,
k.0520
Figure 8. Two examples of determination of the specific sorbent capacity and absolute surface concentration. The straight lines were calculated by making use of eq 7 and data obtained from Figure 6. The curves show the experimental upper limit of the adsorption isotherms of the system.
h.0605
(ms+O)
- ( a ~ l l a w s ) ~ , ~ , ,k, , ( ~ ~ - 0 ) g/g 0.095 0.110 0.134 0.165
0.103 0.134 0.162
0.520 0.605 0.570
-
w1.1
d," nm av layer thickness
0.2006 0.223 0.256 0.344
0.55 0.64 0.60
a Calculation based on the specific surface area 950 m2/g and the density of the dilute solutions ( p 1.00 kg/dm3). The w1,l value was estimated by assuming k = 0.50.
1 I
"1,l
*
ow,-0 0 w,,-w;
Figure 10. Dependence of layer composition on equilibrium l-propanol concentration. The dotted lines refer to the 1:l
v,,:O 256
partitioning between the bulk phase and adsorption layer.
0'20
040
w5
Figure 9. A comparative plot of curves obtained from eq 8using experimentally determined values of the specific capacity and layer composition (curve A) and the measured usdependence of the l-propanol equilibrium concentration (curve B). intercept of them. The average scattering of the experimentally determined points from the straight lines did not exced f2-3 5%. From the intercept and slopes the wl,l and k values were then calculated. In order to check the self-consistency of the procedure given above, for two cases by making use of the w1,1and k values obtained previously, mlbwere calculated from eq 7 as a function of wive and are plotted together with the curves that correspond to the experimental upper limit (Figure 8). As can be seen, the calculated straight lines, as expected, smoothly converge to the linear part of the experimental upper limit curves, indicating in one way the correctness of the calculations. On the other hand, that means that in a certain range of mixture mass/sorbent mass ratio or sorbent concentration, the absolute composition of the adsorption layer and the specific capacity of the sorbent are indeed constant. This statement is valid when m,
