Surface exchange adsorption

Adsorption beds of activated carbon for the purification of citric acid, and adsorption of organic chemicals by charcoal or porous polymers, are good examples of ion-exchange adsorption systems. Synthetic resins such as styrene, divinylbenzene, acrylamide polymers activated carbon are porous media with total surface area of 450-1800 m2-g h There are a few well-known adsorption systems such as isothermal adsorption systems. The best known adsorption model is Langmuir isotherm adsorption. 

In this picture, the kinetic barriers hindering the exchange between the two adlayers are related to the presence of metastable, but rather strongly bound, adsorbed species (Hupd and OHad), which cannot be removed easily, and which block the surface for adsorption of the respective other species. The nonequilibrium situation is also reflected in the shape of the corresponding peaks A and A, where the anodic one (A) is less sharp and extends over a larger potential range. 

The reaction is carried out in close-loop reactor connected to a mass spectrometer for 1S02, 180160 and 1602 analyses as a function of time [38], The gases should be in equilibrium with the metallic surface (fast adsorption/desorption steps 1 and f ) If the bulk diffusion is slow (step 6) and the direct exchange (step 5) does occur at a negligible rate, coefficients of surface diffusion Ds can be calculated from the simple relationship between the number of exchanged atoms Ne and given by the model of circular sources developed by Kramer and Andre [41] ... 

The solid-water interface, mostly established by the particles in natural waters and soils, plays a commanding role in regulating the concentrations of most dissolved reactive trace elements in soil and natural water systems and in the coupling of various hydrogeochemical cycles (Fig. 1.1). Usually the concentrations of most trace elements (M or mol kg-1) are much larger in solid or surface phases than in the water phase. Thus, the capacity of particles to bind trace elements (ion exchange, adsorption) must be considered in addition to the effect of solute complex formers in influencing the speciation of the trace metals. 

The influence of the organocation structure on the exchange adsorption becomes evident from the data in table V. 4,4 Bipyridinium cations adsorb two times more energetically (AH s 2j2 kJ Eq ) than do 2,2 bipyridinium cations (AH° = 11 f5 Eq ). The former adapt a planar orientation (dnm = f 26 nm) in contrast to the inclined position of the latter ( qq = 1.4 nm), despite the fact that sufficient surface is available for adsorption in a flat configuration. Smaller enthalpy terms are consistent with smaller electrostatic interaction energies. The reason for the tilting is unknown however. 

For this estimate, values for the surface diffusion coefficient (D) and the surface exchange coefficient (i) in eq 2 were obtained by linearizing Mitterdorfer s rate expressions for surface transport and adsorption/desorption (ref 84) and re-expressing in terms of the driving forces in eq 2. 

The release of cations is interpreted to have resulted chiefly from two processes an initial release caused by rapid exchange of surface cations for hydrogen followed by a slow release due to structural attack and disintegration of the aluminosilicate lattice. Other processes which could complicate the form of the dissolution curves are adsorption of cations released by structural breakdown, ion exchange on interlayer sites of cations released by structural breakdown and surface exchange (shale only), precipitation of amorphous or crystalline material, and dissolution rate differences among the various crystalline phases. 

The three types of adsorption are (1) physical, (2) chemical, and (3) exchange adsorption. Especially important to the success of in situ treatment by Fe° are the soil characteristics, which affect soil sorptive behavior such as mineralogy, permeability, porosity texture, surface qualities, and pH. Physical adsorption is due to van der Waal s forces between molecules where the adsorbed molecule is not fixed on the solid surface but is free to move over the surface and may condense and form several superimposed layers. An important characteristic of physical adsorption is its reversibility. On the other hand, chemical adsorption is a result of much stronger forces with a layer forming, usually of one molecule thickness, where the molecules do not move. It is normally not reversible and must be removed by heat. The exchange adsorption and ion exchange process involves adsorption by electrical attraction between the adsorbate and the surface (Rulkens, 1998). 

The strong interaction of dextran sulfates with cationic functions in porous support materials is exploited to create new highly charged surfaces for adsorption of proteins. It was revealed that new and strong ionic exchange resins are accessible by simple and rapid deposition of dextran sulfates on commercial DEAE- or MANAE-agarose. The material is characterised by an increased charge density on the porous surface of the support, which can perfectly bind protein material, as demonstrated in Fig. 15 [153]. 

The type of the oxidation product on galena is independent of the chemical environment during preparation. Rao152) measured the adsorption heat of K amyl xanthate (KAX) on unactivated and Cu2+-activated pyrrhotite (FeS) and compared his results with heats of the reaction between KAX and Fe2+ or Cu2+ salts. With the unactivated mineral, the interaction involves a chemical reaction of xanthate with Fe2+ salts present at the interface (i.e. not bound to the crystal surface). The adsorption enthalpy is identical with the formation of Fe2+ amyl xanthate FeS04 + 2 KAX —> FeX2 + K2S04, and -AH = 97.45 kJ/mol Fe2+). As revealed from the enthalpy values and the analysis of anions released into the solution, the interaction of xanthate with Cu2+-activated pyrrhotite consists of xanthate adsorption by exchange for sulfate ions (formed by an oxidation of sulfides) at isolated patches (active spots), and by further multilayer formation of xanthate. The adsorption heat of KAX on pyrrhotite at the initial pH 4.5 was - AH (FeS unactivated) = 93.55 kJ/mol Fe2+ and - AH (FeS activated) = 70.03 kJ/mol Cu2+. 

Atoms adsorbed on a metal surface exchange electrons with it and, as a result, may be desorbed as either atoms or ions. Only those ions and atoms with enough energy to break the adsorption bond will leave the surface. The strength of this bond is measured by the desorption energy, Ea and ., for atoms and singly... 

The surface after adsorption will be chained with a potential, as in Figure 9.14, so that primary adsorption can be treated in terms of a capacitor model called the Stem model [43]. The other type of adsorption that can occur involves an exchange of ions in the diffuse layer with those of the surface. In the case of ion exchange, the primary ions are chemically bound to the structure of the solid and exchanged between ions in the diffuse double layer. 

Clearly, this is the direction in which further fundamental studies should be oriented. For example, it will be interesting to find out whether much higher surface coverages can be accomplished on a carbon whose maximum number of (cation-exchangeable) adsorption sites, e.g., 3 mmol COO /g C, is not only created but made electrostatically accessible by adjusting the solution chemistry. Under these conditions, for example, the theoretical uptake of a divalent cation is 1.5 mmol/g, which translates into 450 mVg, which in turn is a large fraction of the total surface area. This is obtained by assuming a radius of 0.4 nm for a hydrated divalent cation, which is usual for heavy metals [309], Indeed, in the study of Cr(IlI) adsorption by activated carbon MO (see Table 3), the surface covered by a monolayer of the adsorbed eations was 196 mVg on a sample whose Nt and CO2 surface areas were 164 and 537 mVg, respectively. 

This value indicates that AAH has not only many active sites for ion-exchange process, but also a very well-developed specific surface area. Exchange adsorption is effective at very low equilibrium concentrations. 

I. A. H. Schneider, J. Rubio and R. W. Smith, Biosorption of Metals onto Plant Biomass Exchange Adsorption or Surface Precipitation Int. J. Miner. Process. 62, 111-120 (2001). 

Like layer silicates, the porous palygorskite can also be organophilized. X-ray studies, however, do not reveal any structural changes in the organocomplexes, since cationic surfactants are adsorbed only on the external surfaces. The amount of surfactant bound by ion-exchange adsorption and the extent of organophilicity can be quantified by the liquid sorption studies and microcalorimetry [19-21]. 

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