Specific adsorbents

Adsorption, which utilizes the ability of a solid adsorbent to adsorb specific components from a gaseous or a liquid solution onto its surface. Examples of adsorption include the use of granular activated carbon for the removal of ben-zene/toluene/xylene mixtures from underground water, the separation of ketones from aqueous wastes of an oil refinery, aad the recovery of organic solvents from the exhaust gases of polymer manufacturing facilities. Other examples include the use of activated alumina to adsorb fluorides and arsenic from metal-finishing emissions. 

In particular, the coupling between the ion transfer and ion adsorption process has serious consequences for the evaluation of the differential capacity or the kinetic parameters from the impedance data [55]. This is the case, e.g., of the interface between two immiscible electrolyte solutions each containing a transferable ion, which adsorbs specifically on both sides of the interface. In general, the separation of the real and the imaginary terms in the complex impedance of such an ITIES is not straightforward, and the interpretation of the impedance in terms of the Randles-type equivalent circuit is not appropriate [54]. More transparent expressions are obtained when the effect of either the potential difference or the ion concentration on the specific ion adsorption is negli-... 

In several cases application of various additives to the surface of a semiconductor adsorbent, specifically adsorbing or reacting with particles to be detected enables one to improve selectivity. As an example we can mention the use of hygroscopic salts to bind water in humidity sensors, the application of particles of sulfanilic acid to the surface of hhO to detect NO2 [10]. However, the high operational temperature in majority of semiconductor sensors deprives the method of specific surface additives of its general character. 

The charge density of specifically adsorbed ions (assuming that anions are adsorbed specifically while cations are present only in the diffuse layer) for a valence-symmetrical electrolyte (z+ = — z = z) is... 

No "Jilt has so far been assumed that the semiconductor-electrolyte interphase does not contain either ions adsorbed specifically from the electrolyte or electrons corresponding to an additional system of electron levels. These surface states of electrons are formed either through adsorption (the Shockley levels) or through defects in the crystal lattice of the semiconductor (the Tamm levels). In this case—analogously as for specific adsorption on metal electrodes—three capacitors in series cannot be used to characterize the semiconductor-electrolyte interphase system and Eq. (4.5.6) must include a term describing the potential difference for surface states. 

In Ganges Delta, groundwater contaminated with arsenic causes health risk of 40 million people.1 Since arsenic exists as both arsenite (As(III)) and arsenate (As(V)), removal methods for both arsenic species are desired to resolve the arsenic calamity in Ganges Delta.2 Granular adsorbents specific... 

We will first describe a relatively simple scenario for the enhancement of the dissolution of Al203 by a (complex-forming) ligand. As we have seen ligands tend to become adsorbed specifically and to form surface complexes with the AI(III) Lewis acid centers of the hydrous oxide surface. They also usually form complexes with AI(III) in solution. Complex formation in solution increases the solubility. This has no direct effect on the dissolution rate, however, since the dissolution is surface-controlled. 

Such species adsorb specifically and modify the surface charge of the colloids. As shown in Fig. 7.7 these hydrolysis products have a different effect than Al3+ (at low... 

The improvement of enzyme like MIP is currently another area of intense research. Beside the use of the MIP themselves as catalysts, they may also be applied as enhancer of product yield in bio-transformation processes. In an exemplary condensation of Z-L-aspartic acid with L-phenylalanine methyl ester to Z-aspartame, the enzyme thermolysin was used as catalyst. In order to shift the equilibrium towards product formation, a product imprinted MIP was added. By adsorbing specifically the freshly generated product from the reaction mixture, the MIP helped to increase product formation by 40% [130]. MIP can also be used to support a physical process. Copolymers of 6-methacrylamidohexanoic acid and DVB generated in the presence of calcite were investigated with respect to promotion of the nucleation of calcite. Figure 19 (left) shows the polymer surface with imprints from the calcite crystals. When employing these polymers in an aqueous solution of Ca2+ and CO2 the enhanced formation of rhombohedral calcite crystals was observed see Fig. 19 (right) [131]. 

The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent While all the adsorbates have access to the adsorbent sites, the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction. This in turn results in higher adsorbent selectivity for one component than the others. One important parameter that affects the equilibrium-selective adsorption mechanism is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate. Specific physical properties of zeolites, such as framework structure, choice of exchanged metal cations, Si02/Al203 ratio and water content can be... 

Laurie acid (C11H22COOH) and oleic acid (C17H33COO) adsorb specifically on hematite at low solute concentrations at high enough concentrations hydrophobic ef... 

For lipase, initial activity corresponds to the amount of protein that was adsorbed. Specific activity is constant at 1 mmoFs gE for this carrier-enzyme system, which compares to 27% of the free enzyme activity. The trypsin system shows a lower specific activity that is only 10% of the free enzyme. The reason for the lower recovered activity of this system is not known. To rule out possible internal diffusion limitations, the Wheeler-Weisz modulus was estimated, assuming a carrier layer thickness of 0.1 mm for all carriers. Using the data of the experiments performed at 150 rpm, one finds ... 

Column adsorbants, specifications and eluent solutions are discussed in determining generator performance for yield, breakthrough, and stability for long term use. Some examples of clinical applications in positron emission tomography are presented. 

Anions bind also to other metals, like gold, platinum, or silver [74,81], Why do anions adsorb specifically to metals, while cations do not The explanation is a strong hydration of cations. A cation would have to give up its hydration shell for an adsorption. This is energetically disadvantageous. Anions are barely hydrated and can therefore bind more easily to metals [82], Another possible explanation is the stronger van der Waals force between anions and metals. The binding of ions to metallic surfaces is not yet understood and even the idea that cations are not directly bound to the metal, was questioned based on molecular-dynamics simulations [83],... 

Apart from this hydroxyl-specific interaction, also non-hydroxyl specific adsorption was demonstrated to occur at high concentration and long reaction times. This includes physical interactions with the surface or other adsorbed molecules. Only the molecules, adsorbed specifically in the first monolayer were shown to remain chemisorbed on the surface after curing. Other molecules desorb upon curing. 

Redox potential strongly affects the rate and products resulting from explosives transformation [193], This effect was found for RDX in the study of adsorption and transformation of RDX in low-carbon aquifer soils [194], In this context, it is interesting to note that nonaromatic polynitro compounds such as RDX do not show any tendency to adsorb specifically on clay mineral. 

The only practical method for the measurement of specific hydrocarbons is by gas chromatography (GC), which can be automated, but is expensive and often used as a laboratory analyzer. Certain specially prepared columns can be used to adsorb specific hydrocarbon classes. For atmospheric hydrocarbons, a common method is to pass a sample of air through a small freeze-out trap, sweep out the air with helium, and then warm the trap and introduce the condensables into the GC column in one concentrated slug. 

In a broad sense, an acid site can be defined as a site on which a base is chemically adsorbed. Conversely, a basic site is a site on which an acid is chemically adsorbed. Specifically, a Bronsted acid site has a propensity to give a proton, and a Bronsted base has the tendency to receive a proton. Additionally, a Lewis acid site is capable of taking an electron pair and a Lewis basic site is capable of providing an electron pair. These processes can be studied by following the color modifications of indicators, and by using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and calorimetry of adsorption of the probe molecules (see Chapter 4). 

In the four layer model of the edl, proposed by Charmas et al. (Fig. 4), the separate planes are assumed for the adsorbing specifically ions (cation and anion) of the background electrolyte [63]. According to this, for the compact layer the separate capacity is assumed, concerned with the adsorbing... 

Adsorptive Properties. Under certain conditions of pH and low ionic strength, certain proteins will be adsorbed by various substances. Calcium phosphate gel and alumina Cy gel, for example, are frequently used to adsorb specific proteins from heterogeneous mixtures. The adsorbed proteins can frequently be released from the insoluble support either by altering the pH or increasing the ionic strength. Thus, purification can be obtained by removing either the extraneous unwanted proteins or the desired protein with the gel. 

All of these changes in ciystal habit caused by kinetic factors are drastically effected by the presence of impurities that adsorb specifically to one or another face of a growing ciystal. The first example of crystal habit modification was described in 1783 by Rome de Lisle [77], in which urine was added to a saturated solution d NaCl changing the crystal habit from cubes to octahedra. A similar discovery was made by Leblanc [78] in 1788 when alum cubes were changed to octahedra by the addition of urine. Buckley [65] studied the effect of organic impurities on the growth of inoiganic crystals from aqueous solution, and in Mullin s book [66] he discusses the industrial importance of this practice. 

Ions like Br and I, which are not solvated in aqueous solution, have been shown to be adsorbed specifically. One should not expect to... 

The adsorption of alkali Ions (and of earth alkali ions, not shown) differs from that of the anions SO and HPO In that the latter adsorb specifically on uncharged silver Iodide, with the concomitant change In p.z.c. (sec. 3.8. fig. 3.23 -25), whereas the former do not shift the p.z.c. For alkali ions, specificity starts only when there Is already 1 on the surface. This Is an example of specific adsorption of the second kind, as defined in sec. 3.6d. Apparently, the alkali Ions only adsorb on 1 sites, so that there will be some analogy with water structure-originating alkali lon-iodlde Ion interaction In solution. We will come back to this in sec. 3.10g. 

To the far negative side, no difference between the various cations is seen on this scale, but to the left, where the anions adsorb specifically, the curves... 

---