Nonequilibrium Aspects of Adsorption from a Dilute Aqueous Solution

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Langmuir 1999, 15, 1307-1312

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Nonequilibrium Aspects of Adsorption from a Dilute Aqueous Solution of 1-Propanol onto Activated Carbon: Interrelation between the Sorbent “Concentration” Effect and Metastability R. Me´sza´ros,*,† M. Nagy, and I. Varga Department of Colloid Science, Lora´ nd Eo¨ tvo¨ s University, P.O. Box 32, 1518 Budapest, Hungary

K. La´szlo´ Department of Physical Chemistry, Technical University of Budapest, 1521 Budapest, Hungary Received July 8, 1998. In Final Form: November 16, 1998 The dependence of the equilibrium concentration of a dilute aqueous solution of 1-propanol on the ratio of the activated carbon mass to the mass of the solution phase was investigated at fixed initial concentrations of the solution. It was proved that the adsorbed amount depends not only on the “equilibrium” concentration but on the initial concentration of the solution and on the sorbent “concentration” (ratio of adsorbent mass to the mass of the solution) in the investigated systems. On the other hand, the equilibrium adsorption isotherms of binary liquid mixtures must be an unequivocal function of the equilibrium concentration at constant temperature and pressure. This contradiction was clarified by studying the conditions referring to the rigorous definition of equilibrium physical adsorption of binary liquid mixtures onto solids. It was shown that the investigated adsorption systems are not in equilibrium but in different metastable states and the sorbent “concentration” effect could be interpreted as a consequence of this. Finally it can be concluded that one must take special care in interpreting the surface excess isotherms of aqueous mixtures on porous adsorbents as equilibrium quantities since the time-independent supernatant concentration (at constant pressure and temperature) does not mean equilibrium state without any doubt.

Introduction The dependence of adsorption data of binary mixtures on the ratio of adsorbent mass to the mass of the solution phase has not been investigated too often, since the equilibrium adsorption isotherm of a binary mixture must be independent of this variable. There were always some special reasons if this quantity aroused the interest of the adsorption experts. In the field of natural water science the so-called particle concentration (Cp) effect has been observed in some cases. This anomalous phenomenon, namely the suspension concentration-dependent adsorption isotherms, has been attributed to various experimental artifacts.1-6 A possible source of the sorbent “concentration” effect can also be the swelling and disaggregation of the adsorbent particles in some special cases.7-10 It has been known for a long time that in the case of the adsorption of polymers from solution phase the adsorbed † Telephone: 36-1-2090555/1726. Fax: 36-1-2090602. E-mail: [email protected].

(1) Voice, T. C.; Weber, W. J. Environ. Sci. Technol. 1985, 19, 789. (2) Gschwend, P. M.; Wu, S. C. Environ. Sci. Technol. 1985, 19, 90. (3) Honeyman, B. D.; Santschi, P. H. Environ. Sci. Technol. 1988, 22, 862. (4) McKinley, J. P.; Jenne, E. A. Environ. Sci. Technol. 1991, 25, 2082. (5) Grolimund, D.; Borkovec, M.; Federer, P.; Sticher, H. Environ. Sci. Technol. 1995, 29, 2317. (6) Benoit, G. Geochim. Cosmochim. Acta 1995, 59, 2677. (7) De´ka´ny, G.; Sza´nto´, F.; Nagy, L. G. J. Colloid Polym. Sci. 1988, 266, 82. (8) De´ka´ny, I. Pure Appl. Chem. 1992, 64, 1499. (9) Marosi, T.; De´ka´ny, I.; Lagaly, G. Colloid Polym. Sci. 1992, 270, 1027. (10) Marosi, T.; De´ka´ny, I.; Lagaly, G. Colloid Polym. Sci. 1994, 272, 1136.

amount depends not only on the equilibrium concentration but on the ratio of the surface area (adsorbent mass) to volume (mass) of the solution phase. This effect can be explained by taking into consideration the polydispersity of polymers and the fact that the high molecular mass polymer chains adsorb preferentially over the low molecular mass ones.11-13 A similar phenomenon was observed by Barton for the adsorption of KOH on activated carbons with oxidized surface.14 Namely, it turned out that the adsorption isotherms referring to those experimental systems depend on the adsorbent mass to mass of the solution ratio. These quite striking results were explained in terms of hysteresis and irreversibility effects. Very recently Pan et al. proposed the concept of the metastable-equilibrium adsorption theory (MEA) in order to interpret the Cp effect.15,16 In the framework of this theory the metastable states are more general for an adsorption system than the equilibrium state. By making use of the concept of the adsorption exchange reaction and the surface phase model a metastable-equilibrium constant (Kreal) was introduced which was arbitrarily separated into the product of the equilibrium constant (Keq) and a hypothetical metastability constant (Kme). The authors related the extent of adsorption reversibility to the particle concentration and applied an empirical (11) Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Fleer, G. J. J. Polym. Sci., Polym. Phys. 1980, 18, 559. (12) Hlady, V.; Lyklema, J.; Fleer, G. J. J. Colloid Interface Sci. 1982, 87, 395. (13) Koopal, L. K. J. Colloid Interface Sci. 1981, 83, 116. (14) Barton, S. S. Colloid. Polym. Sci. 1986, 264, 176. (15) Pan, G.; Liss, P. S. J. Colloid Interface Sci. 1998, 201, 71. (16) Pan, G.; Liss, P. S. J. Colloid Interface Sci. 1998, 201, 77.

10.1021/la980849h CCC: $18.00 © 1999 American Chemical Society Published on Web 01/20/1999

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relationship between the particle concentration Cp and the hypothetical metastability constant Kme. They made further assumptions on the activity coefficients of the surface phase. On the basis of these assumptions a Langmuir and a Freundlich-type Cp effect isotherm equation was obtained. However, the physical reality of the aforementioned crucial assumptions requires further justification. Recently the influence of the finite size of the adsorption system was investigated by Everett. He pointed out the possibility of metastable states for the adsorption from dilute solutions.17 According to our earlier results, the adsorption isotherm of 1-propanol from dilute aqueous solution onto activated carbons seemed to depend not only on the equilibrium concentration but on the initial concentration and sorbent “concentration”, too.18-20 The definition of the so-called sorbent “concentration” was introduced by Nagy.19,20 One of the possible forms of this quantity (which is an analogous form of the particle concentration) can be the quotient of the adsorbent mass to the mass of the solution phase (ms/mo). By the application of this quantity, a novel representation of the adsorption data was given, namely the equilibrium concentration-sorbent “concentration” curves at fixed initial concentrations of the solution phase.21 The adsorption of aliphatic alcohols from dilute aqueous solution onto activated carbons was quite often the matter of interest. The main aims of the earlier works were the prediction of adsorption data and testing several empirical or semiempirical expressions for the adsorption isotherms.22-25 The dependence of the adsorption data on the sorbent “concentration” or on the initial concentration of the solution was not investigated. In the present work we are going to prove that the unexpected dependence of the surface excess isotherms (of 1-propanol/water-activated carbon system) on the sorbent “concentration” and on the initial composition of the solution undoubtedly exists. We will show that the conditions of equilibrium adsorption are not satisfied in our adsorption systems at room temperature. Finally we make an attempt to interpret the sorbent “concentration” effect as a consequence of metastable states of the investigated adsorption systems. Experimental Section Two types of granulated activated carbon, Nuxit BO (produced by Muˆsze´ntermelo˜ V., Hungary) and Norit RO 0.8 (purchased from Aldrich, Steinheim, Germany), were used in the adsorption experiments. The adsorbents were subjected to a thorough purification and drying process published previously.20,21 The carbon samples were stored in a desiccator over CaCl2, and before the adsorption measurements they were degassed in a vacuum oven at 90 °C. After the purification procedure several batches of the carbons were soaked in distilled water and in pure 1-propanol, respectively. Dissolution of impurities was not observed by different (17) Everett, D. H. Proceedings of the Conference on Colloid Chemistry; Hungarian Chemical Society: Budapest, 1997 (18) Nagy, M. Magy. Kem. Foly. 1988, 94, 6. (19) Nagy, M. Langmuir 1988, 4, 93. (20) Nagy, M. Langmuir 1991, 7, 343. (21) Me´sza´ros, R.; Nagy, M.; Veress, G. Adsorpt. Sci. Technol. 1996, 13, 327. (22) Abe, I.; Hayashi, K.; Kitagawa, M.; Urahata, T. Bull. Chem. Soc. Jpn. 1979, 52, 1899. (23) Belfort, G.; Altshuler, G. L.; Thallam, K. K.; Feerick, J. R. C. P.; Woodfield, K. L. AIChE J. 1984, 30, 197. (24) Fukuchi, K.; Hamaoka, H.; Arai, Y. Mem. Fac. Eng., Kyushu Univ. 1980, 40, 107. (25) Wohleber, D. A.; Manes, M. J. Phys. Chem. 1971, 75, 3720.

Me´ sza´ ros et al.

Figure 1. Adsorption and desorption isotherms of nitrogen at 77 K on Nuxit BO and Norit RO 0.8. The adsorbed amounts are expressed in the standard STP units. Table 1. Most Important Parameters of the Activated Carbons Which Were Derived from the Nitrogen Adsorption/Desorption Isotherm specific surface area (multipoint BET, m2/g) mesopore surface area (t-method,27 m2/g) micropore surface area (t-method,27 m2/g) micropore surface area (DR-method,28 m2/g) pore vol (cm3/g)

Nuxit BO

Norit RO 0.8

1300

920

739

317

551

603

1654

1460

0.76

0.67

analytical techniques (interferometry, electrical conductance, and UV spectroscopy). N2 adsorption/desorption isotherms were used for the characterization of the pore and surface structure of our adsorbents. The nitrogen adsorption/desorption isotherms were measured at the boiling point of liquid nitrogen (77 K) from p/po ) 0.004 by the AUTOSORB (Quantachrome, Syosset, NY) computercontrolled surface analyzer. Samples were outgassed at 400 °C in high-vacuum p < 5 × 10-7 Torr). In Figure 1 the N2 adsorption/desorption isotherms can be seen. According to the IUPAC classification, the adsorption isotherms of Nuxit BO and Norit RO 0.8 belong to types I and II and both of them possess H4 type hysteresis.26 In Table 1 the most important parameters are listed, which were determined from the N2 sorption experiments according to the refs 26-28. According to Figure 1 and Table 1, the activated carbons have significant pore volume in both the meso- and micropore range. The 1-propanol was an analytical grade product. It was purified by refluxing and distillation in the presence of fresh metal Ca granules. In all experiments fresh, double-distilled water was used. The applied concentration range of the aqueous 1-propanol solution was approximately 0-7% by mass (0-0.025 by mole fraction) of 1-propanol. The adsorption experiments were done in a static manner at 25 ( 1 °C by making use of conical flasks with Teflon stoppers. Control experiments were carried out in order to test the applicability of these flasks to our purposes, considering the possible loss of evaporation. A small part of our experiments was completed at 65.0 ( 0.1 °C. In this case suitable ampules were used for the storage of our adsorption systems. The concentrations (26) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (27) deBoer, J. H.; Linsen, B. G.; van der Plas, T.; Zondervan, G. J. J. Catal. 1965, 4, 649. (28) Dubinin, M. M.; Radushkevich, L. V. Dokl. Akad. Nauk. SSSR 1947, 55, 331.

Adsorption of 1-Propanol onto Activated Carbon

Figure 2. Dependence of the equilibrium concentration of 1-propanol on the sorbent “concentration” (ms/mo f adsorbent mass to the mass of the solution ratio) for two different samples of Norit RO 0.8 (x01 ) 1.325 × 10-2, t ) 25 °C). The open circles denote the experimental points referring to the measurements with the activated carbon sample A purified by the method given in the Experimental Section and in refs 16 and 17. The solid squares refer to carbon sample B that first (after the same purification procedure) had been soaked in dilute aqueous 1-propanol solution for 1 week. This sample was used for the adsorption experiments after regeneration. The heights of symbols are commensurable with the standard error of concentration. of the dilute aqueous 1-propanol solutions were determined by an interferometer (ITR-2) and by a differential-refractometer (Brice-Phoenix). The surface excess isotherms were calculated according to the following formula:

ne1 )

n0(x01 - x11) ms

Here ms is the adsorbent mass in grams, x01 and x11 denote the initial and equilibrium mole fractions of the first component in the solution, respectively, n0 denotes the total amount of the components in the solution given in moles, and ne1 is the specific excess amount of component 1 (mol/g). To test the regenerating ability of the activated carbons two samples of Norit RO 0.8 were compared. One of these samples (sample A) was used in an adsorption experiment immediately after the purification and drying process. The other carbon sample (sample B)safter the same purification and drying procedures was first soaked for 1 week in a dilute aqueous 1-propanol solution. This sample was used in the actual adsorption experiment after regeneration. In Figure 2 the experimental points of the equilibrium concentration-sorbent “concentration” curves are shown at one fixed, initial concentration for the two kinds of carbon samples. As it can be seen, the two series of points belong to the same curve within the standard error. So the carbon samples are completely regenerable after the adsorption experiments and the irreversible sticking of one of the solution components can be excluded. The time needed to attain equilibrium was determined by independent experiments. Several adsorption systems were prepared which were characteristic of the applied range of the solution concentration and the sorbent “concentration”. Although the concentration values of the supernatants seemed not to change after a few hours, our adsorption systems were equilibrated for 48 h.20,21

Results and Discussion One of our purposes was to prove the existence of the dependence of the adsorption isotherms of our experi-

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Figure 3. Adsorption isotherm of 1-propanol from dilute aqueous solution on Nuxit BO at five initial concentrations (at 25 °C). The various symbols refer to different values of the initial composition of the solution phase. The estimated confidence intervals (referring to the values of specific surface excess amount at R ) 0.05 significance level) are plotted by gray bands. The applied range of sorbent “concentration” (ms/mo) at different initial compositions of the solution was between 0.01 and 0.310.

mental systems on the initial composition of the solution phase. Because of this the quantity of the activated carbon was varied at fixed masses and initial concentrations of the 1-propanol solution. To estimate the confidence intervals of the surface excess isotherms referring to fixed initial concentrations of the solution a statistical treatment was worked out on the basis of the description of the equilibrium concentration-sorbent “concentration” curves (x11-ms/mo) at fixed initial concentrations.21 For the description of these curves the solution function of a differential equation was proposed. By making use of the results of a nonlinear fitting procedure referring to the analytical expression of the x11-(ms/mo) functions and the definition of the specific surface excess amount, one can give a good estimation for the smoothed adsorption isotherms and their confidence intervals belonging to the different initial concentrations of the solution phase.21 The smoothed surface excess isotherms of 1-propanol (component 1) from dilute aqueous solution and their confidence intervals at five initial concentrations were plotted in Figures 3 and 4 (for the two types of activated carbons at 25 °C). (As the equilibrium concentration of the 1-propanol solution approaches the initial concentration, the confidence interval of the surface excess isotherm referring to a fixed initial concentration increases because of statistical reasons and not because of wrong measurements.) As it can be seen the adsorption isotherms belonging to different initial concentrations are split up into separate curves in a quite wide range of the equilibrium concentration. This means that the surface excess isotherms (in addition to the equilibrium supernatant concentration) significantly depend on the initial concentration, too! However, it is necessary to test the existence of this strange phenomenon by at least one alternative statistical method. Let us suppose that instead of the initial concentration the ratio of the adsorbent mass to the mass of the solution phase is fixed. If the adsorption isotherm actually depends on the initial concentration, it must also depend on the sorbent “concentration”!

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Figure 4. Adsorption isotherm of 1-propanol from dilute aqueous solution on Norit RO 0.8 at five initial concentration (at 25 °C). The various symbols refer to different values of the initial composition of the solution phase. The estimated confidence intervals (referring to the values of specific surface excess amount at R ) 0.05 significance level) are plotted by gray bands. The applied range of sorbent “concentration” (ms/mo) at different initial compositions of the solution was between 0.01 and 0.310.

Figure 5. Adsorption isotherm of 1-propanol from dilute aqueous solution on Norit RO 0.8 at two fixed values of sorbent “concentration” (at 25 °C). The averages and confidence intervals of the specific surface excess amount (at R ) 0.05) were estimated on the basis of repeated experiments (9-10). The gray band belongs to the confidence interval of the adsorption isotherm at (ms/mo) ) 0.0669. In the case of (ms/mo) ) 0.202, the confidence intervals were not plotted, because they are commensurable with the size of the open circles.

In the next series of experiments we fixed two values of ms/mo (0.0669 and 0.202). In Figure 5 several points of the adsorption isotherms belonging to the two sorbent “concentration” were plotted for the adsorption of 1-propanol on Norit RO 0.8. In this case the averages and confidence intervals of the adsorbed amount were estimated on the basis of repeated adsorption measurements. The reproducibility of these measurements was extremely good, and the standard errors of the supernatant concentration estimated from the repeated experiments were completely negligible compared with the standard errors of adsorbed amount. The gray band belongs to the

Me´ sza´ ros et al.

confidence interval of the adsorption isotherm at (ms/mo) ) 0.0669. In case of larger sorbent concentration the diameters of the open circles are commensurable to the confidence intervals. It is clearly seen that the adsorption isotherms belonging to the two different sorbent “concentrations” are significantly different. An interesting additional feature of Figure 5 is the local maximum at the smaller sorbent “concentration” (at x11 = 0.0175). In light of these results it can be stated that in the investigated adsorption systems the surface excess isotherms depend not only on the equilibrium concentration but on the sorbent “concentration” or alternatively on the initial composition of the solution phase! According to the laws of classical thermodynamics the number of independent intensive variables (which are needed for the complete description of the equilibrium state) can be unambiguously determined.29 For the adsorption of a binary liquid mixture onto a homogeneous and inert adsorbent with plane surface (neglecting other interfaces than the solid/liquid interface) the total number of intensive variables is 6, namely the pressure (p), the temperature (T), the chemical potentials of the two components of the solution (µ1, µ2), the chemical potential of the solid (µ3), and the solid/liquid interfacial tension (σ). In equilibrium one can write three Gibbs-Duhem equations (one for the solid phase, one for the liquid phase, and one for the adsorption layer by making use of the concept of the surface excesses.30) So the number of degrees of freedom is equal to 6 - 3 ) 3 in this type of adsorption system. It can be proved that the number of degrees of freedom remains the same in the case of structurally or energetically heterogeneous adsorbents and curved surfaces.31,32 Under the usual circumstances the pressure and the temperature are constant, so the chemical potential of component 1 determines unambiguously the equilibrium state of the adsorption system. If µ1 (or x11) is fixed, than the adsorbed amount (of component 1) could be unequivocally determined as a function of the equilibrium concentration (of component 1). This means that the observed dependence of the adsorbed amount on the initial composition of the solution and the sorbent “concentration” is in contradiction with the principles of equilibrium thermodynamics! To understand these observations first of all one should examine whether the conditions referring to the phenomenon of equilibrium physical adsorption from binary liquid mixtures onto solids are rigorously satisfied or not. These requirements are the following:33,34 (1) The solution phase has strictly two components. (2) The mutual solubility of the solution and solid phase can be neglected. (3) The chemical reactions between the solution components and between the adsorbent and adsorbate molecules are excluded. (4) Both components of the solution can access the same interfacial region. (5) The structure of the adsorbent does not depend on the composition of the solution phase. (6) The adsorption system is in the equilibrium state. On the basis of our purification procedure, dissolution experiments, and regeneration test, it can be stated that the first three conditions are satisfied. The possibility of (29) Callen, H. B. Thermodynamics and an Introduction to Thermostatistics, 2nd ed.; John Wiley & Sons: New York, 1985; Chapter 1-8. (30) Defay, R.; Prigogine, I.; Bellemans, A.; Everett D. H. Surface Tension and Adsorption; Longmans: London, 1966. (31) Jaroniec, M.; Madey, R. Physical adsorption on heterogeneous solids; Elsevier: Amsterdam, 1988. (32) Li, D.; Neumann, A. W. Adv. Colloid Interface Sci. 1994, 49, 147.

Adsorption of 1-Propanol onto Activated Carbon

a molecular sieve effect (fourth condition) could not be completely excluded in the case of microporous adsorbents. The fifth requirement is also not satisfied for activated carbon in aqueous medium, since the sorbent slightly swells during the adsorption process.35,36 On the other hand, at the present state it is extremely difficult to take these effects into account. For the sake of simplicity, we assume that the influence of these effects can be neglected. The attainment of equilibrium may seem a trivial question, because the equilibration time was established by independent experiments. However, the spatial and temporal homogeneity of the intensive parameters are satisfied not only in equilibrium but also (during a proper time period) in metastable states. Since the equilibrium state must not depend on the route of attaining the equilibrium state, one can decide if a system reached an equilibrium or metastable state by checking whether the same state can be reached along different routes.29 In our case that means that if the sorbent “concentration” is a real independent variable of the adsorbed amount, then the adsorption system should reach the same state either in a single step (assembling the adsorption system with a given sorbent “concentration”) or in two steps (changing the initial sorbent “concentration” after “equilibration”). However our experiments showed that the adsorbed amounts and supernatant concentrations belonging to the two sorbent “concentrations” in Figure 5 did not change when we changed the sorbent “concentration” by adding or removing supernatant with the same “equilibrium” composition. So the dependence of the adsorbed amount on the sorbent “concentration” is an apparent dependence and probably due to the influence of some initial parameters which may “freeze” the adsorption system in a metastable state. According to the dependence of the adsorbed amount on the initial concentration (Figures 3 and 4), the initial composition of the solution may play such a role in the adsorption process. Because of this it is worth testing the influence of the initial solution composition on the adsorbed amount in those cases when the final amounts of the solution components are attained along different routes. In Figure 6 the experimental points and confidence intervals of the adsorption isotherm of the dilute aqueous 1-propanol solutionstaken up in three different waysswere plotted at (ms/mo) ) 0.0669. The confidence intervals were estimated from repeated experiments. The gray band belongs to the confidence interval of the traditional adsorption isotherm measured in one step. In the second case (open squares), first a precise amount of pure water was poured on the activated carbon, and then after passage of the necessary “equilibration” time, a calculated amount of 1-propanol was added to the adsorption system. In the third case (solid circles), first a calculated amount of concentrated 1-propanol-water mixture (19-20% by mass) was added to the activated carbon, and then after “equilibration” a precise quantity of water was poured on the system. (After passage of the same time, the concentration was determined.) As it can be seen in Figure 6, the values of the adsorbed amount significantly depend on (33) Dabrowsky, A.; Jaroniec, M.; Oscik, J. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1987; Vol. 14, Chapter 2. (34) Parfitt G. D.; Rochester, C. H. Adsorption from Solution at Solid/ Liquid Interface; Academic Press: New York, London, 1983; Chapters 1 and 2. (35) Kaneko, K.; Fujiwara, Y.; Nishikawa, K. J. Colloid Interface. Sci. 1989, 127, 298. (36) Marinin, D. V.; Avramenko, V. A.; Glushchenko, V. Yu. Zh. Fiz. Khim. 1989, 63, 166.

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Figure 6. Adsorption isotherm of 1-propanol from dilute aqueous solution taken up in three different way on Norit RO 0.8 at 25 °C and at (ms/mo) ) 0.0669. The averages and confidence intervals of the adsorbed amount were estimated at R ) 0.05 significance level on the basis of repeated experiments. Key: (i) “equilibration” in distilled water and then addition of 1-propanol to the adsorption system (open squares); (ii) “equilibration” in concentrated 1-propanol/water mixtures (19-20% by mass) and then addition of distilled water to the system (solid circles); (iii) adsorption isotherm refers to the traditional, one-step experiment. The gray band denotes the confidence interval of the adsorption isotherm.

the way of performing the experiments at the same total values of the input parameters (ms, mo, and x01). (The local maximum at x11 = 0.0175 can be detected in each case.) So it appears that the adsorption of 1-propanol from dilute aqueous solution onto activated carbon could not be considered as a reversible equilibrium phenomenon. Recently reversibility problems were observed for the adsorption of some aromatic compounds from dilute aqueous solution onto activated carbons.37,38 Namely, an extremely slow adsorption process and adsorption/desorption hysteresis were found in the investigated systems. The interpretation was based on the irreversible adsorption of the aromatic compounds. The irreversible sticking of one of the components in our adsorption system can be excluded on the basis of the regenerating ability of our adsorbents (Figure 2). One of the reasonable treatments could be the assumption of the existence of metastable states in our adsorption systems supposing that these metastable states depend on the initial concentration. By increase of the temperature, there is a real chance to convert the system from a metastable state to the equilibrium state because of the increasing average kinetic energy of the molecules. In Figure 7 the adsorption isotherm of 1-propanol was plotted at 65.0 °C at the same two values of sorbent “concentration” that were used at 25 °C. The confidence intervals were estimated from repeated experiments. The gray band belongs to the confidence interval of the adsorption isotherm at (ms/mo) ) 0.0669. In case of the larger sorbent “concentration” the heights of the open circles are commensurable to the confidence intervals. It is clearly seen that the adsorption isotherms belonging to the two sorbent “concentration” (37) Tamon, H.; Atsushi, M.; Okazaki, M. J. Colloid Interface Sci. 1996, 177, 384. (38) Tamon, H.; Okazaki, M. J. Colloid Interface Sci. 1996, 179, 11.

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Conclusion

Figure 7. Adsorption isotherm of 1-propanol from dilute aqueous solution on Norit RO 0.8 at 65.0 °C and at (ms/mo) ) 0.0669 (solid squares) and at (ms/mo) ) 0.202 (open circles). The averages and confidence intervals of the adsorbed amount were estimated at R ) 0.05 significance level on the basis of repeated experiments. The gray band belongs to the confidence interval of the adsorption isotherm at (ms/mo) ) 0.0669. In the case of (ms/mo) ) 0.202, the confidence intervals are commensurable with the height of the symbols.

are the same apart from the neighborhood of the local maximum at x11 = 0.0175. So, at this higher temperature the adsorption system seems to be closer to the equilibrium state.

It was proved that the unusual dependence of surface excess isotherms on the sorbent “concentration” and on the initial concentration of the solution phase undoubtedly exists in the investigated adsorption system. The spatial and temporal homogeneity of the supernatant concentration (at constant pressure and temperature) together with the sorbent concentration “dependence” of the surface excess isotherms clearly indicate the existence of metastable states. The dependence of the adsorbed amount on the initial concentration suggests that depending on the initial solution composition different metastable states develop in the adsorption system. The smaller extent of the sorbent “concentration” effect at 65.0 °C also proves the existence of metastable states. The origin of the initial concentration dependent metastable states may be in connection with the complex interaction of the porous carbon and the dilute aqueous 1-propanol solution and with the special transport properties in the actual porous environment. However, the interpretation of these observations requires further investigations. It can be concluded that in the case of porous carbonaceous adsorbents and aqueous medium special care must be taken when one tries to apply the equilibrium formalism of adsorption. Although the preliminary equilibration experiments may indicate no further change in the supernatant concentration (within the experimental error), additional experiments are needed to exclude the lack of reversibility or the existence of the sorbent “concentration” effect. If this effect exists, the meaning of surface excess isotherm will be different compared with the classical equilibrium isotherm. LA980849H