Adsorption of Multicomponent Liquids from Water onto Activated

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Downloaded by CHINESE UNIV OF HONG KONG on March 18, 2016 | http://pubs.acs.org Publication Date: March 15, 1983 | doi: 10.1021/ba-1983-0202.ch002

Adsorption of Multicomponent Liquids from Water onto Activated Carbon: Convenient Estimation Methods MILTON MANES and MICK G R E E N B A N K Kent State University, Chemistry Department, Kent, O H 44242

The conceptual simplicity of the Polanyi model is demonstrated by using a macroscopic gravitational analogy. Its application to the adsorption of multicomponent mixtures of organic liquids partially miscible in water and completely miscible in each other is considered. The use of the Polanyi model is compared to various aspects of the ideal adsorbate solution (IAS) model; whereas in many systems both models give similar predictions, the Polanyi model works for a number of systems that are outside the scope of the IAS model. It also may be used to supplement the IAS model and to improve convenience where it applies.

P

(1), we considered the Polanyi adsorption potential theory and its application to the adsorption, from water solution onto activated carbon, of single organic liquids and single and multiple organic solids. We now consider its application to the adsorption of multicomponent mixtures of organic liquids partially miscible in water and completely miscible in each other. An extensive treatment of the theory and a considerable body of data on binary and ternary solutes is presented elsewhere (2); a representative sampling of the experimental results will be discussed. The conceptual simplicity of the Polanyi model is demonstrated by showing how it is directly analogous to a macroscopic gravitational model. The Polanyi model is of interest as a means toward understanding a wide diversity of adsorption phenomena on activated carbon (I). The practical applications of interest may be illustrated by the fact that we have a computer program to incorporate, as a data base, appropriate data REVIOUSLY

0065-2393/83/0202-0009/$06.00/0 © 1983 American Chemical Society

McGuire and Suffet; Treatment of Water by Granular Activated Carbon Advances in Chemistry; American Chemical Society: Washington, DC, 1983.


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TREATMENT OF WATER BY GRANULAR ACTIVATED CARBON

(molar volume, refractive index, and water solubility) for a number of adsorbates, together with the characteristic curve for the carbon. This program practically immediately yields estimates of the adsorbate mole numbers of a multicomponent mixture (up to 25 components) from the individual equilibrium concentrations. Except for some rather exotic systems (which can also be handled), the calculations are simple enough to be carried out for simple systems on a hand calculator. Whereas we continue to maintain that the Polanyi-based model is deserving of more widespread use, we have earlier noted some of its limitations (i). The model can be used both to predict individual isotherms (3) and to predict multicomponent adsorption (2, 4) from either estimated or experimental isotherms; therefore, any errors in the estimation of individual isotherms [as might come about, for example, from steric effects (5)] may be corrected by incorporating more accurate experimental isotherms into the model. For a wide variety of systems, however, the isotherms estimated by the Wohleber-Manes (3) method should suffice. For others, one can use the model to extrapolate isotherms. Of the alternative approaches to adsorption from water solution, the most popular approach in recent years has been the ideal adsorbate solution (IAS) model of Radke and Prausnitz (6), which has recently been applied, e.g., by DiGiano et al. (7) and Jossens et al. (8) and which is considered here only in its application to liquid adsorbates. For many systems in which one has the individual adsorption isotherms over the required range, this model leads to practically the same calculations as the Polanyi-based model and the choice between the two turns out to be one of computational convenience; given the improved power of modern computers, the superiority of the Polanyi-based model in this respect may not be important We found some systems for which the two models give quite different results; some of these systems are handled quite routinely by the Polanyi model but are completely outside the scope of the IAS model. Although these systems are important for comparing the physical validity of alternative models, the distinction may not necessarily be significant for many practical systems. We have found (2) that the Polanyi approach may be used to supplement the IAS model to estimate some awkward integrals that appear in the IAS model and therefore to improve its convenience and applicability for those who prefer to use it. Two criticisms have been, made of at least some applications of the Polanyi model (in addition to the limitations already noted): excessive mathematical complexity in multicomponent systems (7) and thermodynamic consistency (9). For multiple liquids, the first was met some 10 years ago in Wohleber's dissertation (4), which gives a straightforward Polanyi-based model for multicomponent liquid solutes. The second applies, not to the full Polanyi-based model, but to a simplifying assump-



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tion that nevertheless works well in many systems (2). The latter criticism is considered in the Discussion. The following components have been studied in binary and ternary mixtures at 25°C (2): diethyl ether (EE), ethyl acetate (EA), propionitrile (PN), dichloromethane (DCM), 1,2-dichloroethane (DCE), and 1-pentanol (PEN). With the exception of PEN, all had been studied as single components (on the same carbon) by Wohleber and Manes (3). The PEN was incorporated into this study because it was well suited for liquidsolid studies (to be reported later). The systems studied were: E E - E A , PN-EA, E E - D C E , D C M - D C E , D C E - P E N , E E - E A - P N , E E - E A - P E N , and D C M - D C E - P E N . The study also included coumarin (COU), phthalide (PHL), and pnitrophenol (PNP) as liquid components above their (underwater) melting points. Each of these molten adsorbates was studied in binary mixtures with E A and PEN. Each molten liquid adsorbate had been studied previously by Chiou and Manes (10). They were chosen for study in the expectation that they would exhibit unequivocal adsorbate nonuniformity because of their high refractive indices; an additional factor was their convenient solubilities in water. They provided an interesting challenge to adsorption models.

Theoretical The details of the theoretical approach are given elsewhere (1, 2). A brief outline is presented together with a gravitational analogy that illustrates the essential simplicity of the model. Following the basic model of Polanyi (11, 12), a carbon surface is postulated in which practically all of the volume active in adsorption is in the form of crevices or pores of varying size and unspecified shape. Polanyi showed the model schematically as one pore of varied cross-section (I); it may be represented equally well as a distribution of pores of varying cross-section and shape. Rather than characterizing an element of the adsorption space by an assumed pore shape or size (as in the various approaches to pore size distribution that use the Kelvin equation), the Polanyi model characterizes it by the negative of the adsorption energy (which Polanyi calls the adsorption potential, e) of some specified adsorbate. The relative adsorption energies of any single adsorbate at different locations depend on relative proximity to carbon and may be expected to be higher in finer pores because of proximity to more carbon. A plot of the cumulative volume of pores (or in Polanyi language, "adsorption space") with adsorption energy to equal or exceed some given value, against that value as abscissa, is referred to as a "characteristic curve." If one assumes pores of



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specified shape, the characteristic curve of Polanyi becomes a cumulative pore size distribution with a somewhat remapped abscissa In vapor phase adsorption, the Polanyi model postulates that the (location-dependent) adsorption potential results in a corresponding location-dependent concentration of the vapor, to an extent described by the Boltzmann equation; condensation to a liquid results wherever the adsorption potential (or energy loss due to the adsorptive forces) suffices to concentrate the vapor to its saturation concentration. In effect, the attractive forces of the carbon for the adsorbate molecules reinforce their normal attractive forces for each other. The model is therefore very specific about the interactions of adsorbate molecules, setting them as equal to normal bulk interactions. With some exceptions (e.g., for very dilute fixed gases at elevated temperatures), the condensation accounts for practically all of the observed adsorption. When a carbon sample, initially in a vacuum, is exposed to increasing pressures of a single vapor, condensation begins in the finer pores and progresses to the coarser ones until the adsorption volume is filled at saturation pressure. A plot of adsorbate volume against the equilibrium adsorption potential reproduces the characteristic curve for the carbon with that adsorbate. (Characteristic curves for other adsorbates differ only by an abscissa scale factor.) To relate this characteristic curve to the variables in an adsorption isotherm (mass adsorbed versus pressure or relative pressure), one uses the adsorbate liquid density [usually approximated by the bulk liquid density, with some modifications (13)] to relate the adsorbate mass to adsorbate volume. In the abscissa scale, the equilibrium adsorption potential is related to the inverse relative pressure, pjp, by the Polanyi (or Boltzmann) equation: e = RT In (pjp)

(1)

For adsorption of partially miscible organic liquids from water, the model is essentially the same (3, 12), except that the effective or net adsorption potential of the adsorbate is its (gas phase) adsorption potential corrected for the adsorption potential of an equal volume of the water it must displace. The correction is quite analogous to Archimedes' principle. A gravitational analogy is now considered. Consider (Figure 1) a vessel in a very powerful gravitational field, which for simplicity is assumed to be uniform. The vessel may be of any shape or volume; for this illustration, we arbitrarily pick a vessel of the approximate size and shape of a centrifuge tube of 10 cm in length. If the top of the tube is designated as the level of zero potential energy and distances are measured downward from this level, then the "gravitational potential," e\ of a molecule in the tube ("adsorption space") may be defined as:


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Adsorption of Multicomponent Liquids from Water onto Activated

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