Fractionating adsorption

The macroscopic proton coefficient may be determined by graphical analysis of observed system variables according to two different procedures fractional adsorption edge linearization (6) and isotherm analysis (7 ). The procedures for calculating the macroscopic proton coefficients according to these two methods are discussed in detail below, as are their relative advantages and disadvantages for use in semi-empirical descriptions of adsorption. 

Figure 3. a) Cd(II) fractional adsorption onto o-alumina as a function of pH and site concentration b) Kurbatov plots of the Cd(II)/a-alumina system. 

This combined dependency of xD on pH and surface coverage has the net effect of decreasing the slope of the pH-fractional adsorption edge as the ratio of adsorbate to adsorbent sites increases, as in Figure 3a. 

As a second example, consider the partitioning of Cd(II) between two adsorbents—a-TiC and (am)Fe20j.H20. Figure 11 shows Cd(II) fractional adsorption as a function of pH for binary mixtures of these adsorbents under experimental conditions such that Cddl) and SOUp are constant only the surface site mole fraction varies from one end-member to the next. As the site mole fraction shifts between the end-members, the fractional adsorption edges for the binary adsorbent mixtures varies between the limits defined by end-members. In the absence of particle-particle interactions, the adsorbents should act as independent ligands competing for complexa-tion of Cd(II). If this is the case, then the distribution of Cd(II) in such binary mixtures can be described by a composite mass-action expression (13) which includes a separate term for the interaction of Cd(II) with each adsorbent. 

Figure 11. Cd(Il) fractional adsorption as a function of pH in binary mixtures of a-Ti02 and (am)Fe203 -H20 at constant SOftj,.

Figure 14. Triple-layer model (1) results for Cd(II) adsorption onto a-alumina at different site/adsorbate ratios. Top, Cd(II) surface reaction best fit constants middle, Cd(II) surface species mole fractions and bottom, slopes of fractional adsorption edges used as the criteria of fit.

M. S.Tswet dehned the fractional adsorption process, with the explanation that molecules of different analytes have different affinity (interactions) with the adsorbent surface, and analytes with weaker interactions are less retained [3]. 

A much more difficult type of application is one in which a foreign odor is to be removed from honey, maple syrup, or other materials that have a bouquet or natural flavor composed of constituents present in minute quantities. Such applications require what may be termed fractionating adsorption. Unfortunately an undesired fractionation often results, and the removal of desired components of the bouquet leaves the product with an unbalanced flat flavor however, success has at times been achieved. A method occasionally found helpful is to allow the liquid to remain in contact with minute quantities of carbon the time of contact may range from a day to a month or more. Off-flavored brandies, whiskies, and other alcoholic beverages have been so treated. One explanation of the improvement is that the desired type of fractionation takes place during the longer contact another interpretation is that activated carbon catalyzes beneficial chemical changes. 

Fractional adsorption capacities in the macropore voids and the microparticles... 

This curious result is by no means a mathematical fiction. By fractional adsorption on active charcoal at the temperature of liquid hydrogen and subsequent desorption, hydrogen can in fact he separated into two gases, ortho- and para-hydrogen. The former has a specific heat (and thermal conductivity) corresponding exactly to the possession of odd numbers of rotational quanta. The properties of the latter correspond to even numbers only. The two forms are stable and interconvertible only by atomization at high temperatures, or by special catalytic methods in which molecules are taken out of the gaseous state. 

Figure 4. Fractional adsorption of Pb to hydrous Fe-oxide (HFO) as a function of pH and HFO-Pb aging time. From Ainsworth et al. (52), with permission.

It is quite important in this context that depending on the choice of parameters, the speciation of the surface may be either controlled completely by the uncomplexed surface sites (SO , SOH, SOHJ, in the case of sufficiently low ion-pair formation constants) or by the ion pairs (SO -C, SOH -A , in the case of sufficiently high ion-pair formation constants). Because electrolyte adsorption is not readily measured (i.e., high electrolyte concentrations and little fractional adsorption is usually imposed by the relevant systems), there are no high-quality data corroborating either of these two options. The interdependence between ApK and the electrolyte-binding constants does not interfere in 1-pK models. 

Partial resolutions of racemic crystalline polypropylene oxide (PPO), obtained by asymmetric-selective polymerization of PO by an organozinc initiator, have been made using fractional adsorption on poly-(-)menthylglycidyl ether or sucrose as supports, although the resolutions were not very high [97]. 

Surface tension Molar volume Pore radius Surface tension Contact angle Occupied fraction Adsorption rate Adsorption constant Desorption rate Desorption constant... 

Methods of separating diastereoisomerides other than by fractional crystallization have been developed compa,ratively recently. Fractional distillation has been successfully applied by Bailey and Hass (7), and fractional distribution between immiscible solvent pairs by Shapiro and Newton (129). Fractional adsorption on symmetrical adsorbents such as silica gel and carbon has also been achieved, chromatographic methods with solutions having been applied by Jamison and Turner (55) and Hass, de Vries, and Jaff6 (44), while Isom and Hunt (49) have noted fractional adsorption from the gaseous phase. 

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