Adsorption from mixed systems

In systems containing two or more adsorbates, either competitive or synergistic effects may operate. The commonest synergistic effect is that of ternary adsorption (11.5.4). Competitive behaviour may involve competition for the same surface sites, indirect effects due to the change in the electrostatic properties of the oxide/water interface and in some cases, formation of non sorbing, metal-ligand complexes in solution. 

The actual extent of uptake of competing ions usually depends upon the proportions of the ions in the system (i.e. whether one is in excess) and on the order of addition to the system. In situ voltametry has recently been shown to be very useful for investigation of adsorption from mixtures of low levels (10 -10 M) of metal ions (Palmquist et ah, 1999). 

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Fig. XI-11. Relation of adsorption from binary liquid mixtures to the separate vapor pressure adsorption isotherms, system ethanol-benzene-charcoal (n) separate mixed-vapor isotherms (b) calculated and observed adsorption from liquid mixtures. (From Ref. 143.)...

Dilution. In many appHcations, dilution of the flocculant solution before it is mixed with the substrate stream can improve performance (12). The mechanism probably involves getting a more uniform distribution of the polymer molecules. Since the dosage needed to form floes is usually well below the adsorption maximum, a high local concentration is effectively removed from the system at that point, leaving no flocculant for the rest of the particles. A portion of the clarified overflow can be used for dilution so no extra water is added to the process. 

The performance of adsorption processes results in general from the combined effects of thermodynamic and rate factors. It is convenient to consider first thermodynamic factors. These determine the process performance in a limit where the system behaves ideally i.e. without mass transfer and kinetic limitations and with the fluid phase in perfect piston flow. Rate factors determine the efficiency of the real process in relation to the ideal process performance. Rate factors include heat-and mass-transfer limitations, reaction kinetic limitations, and hydro-dynamic dispersion resulting from the velocity distribution across the bed and from mixing and diffusion in the interparticle void space. 

Figure 16 Process flowsheet of a GAC system with regeneration. In this complete GAC adsorption and regeneration system, four GAC columns can be operated in parallel or in series. Spent carbon is transferred to a multiple-hearth furnace for thermal regeneration. Regenerated carbon is mixed with virgin makeup and pumped back to the GAC columns. The GAC columns are backwashed periodically. (From Ref. 27.)...

Scamehorn et. al. (20) also presented a simple, semi—empirical method based on ideal solution theory and the concept of reduced adsorption isotherms to predict the mixed adsorption isotherm and admicellar composition from the pure component isotherms. In this work, we present a more general theory, based only on ideal solution theory, and present detailed mixed system data for a binary mixed surfactant system (two members of a homologous series) and use it to test this model. The thermodynamics of admicelle formation is also compared to that of micelle formation for this same system. 

It would be of scientific interest to study the adsorption of mixed surfactant systems showing positive deviations from ideality, as has been discussed for mixed micelles and monolayers. 

The selectivity of adsorption (S = niyj/njyi) of water vapour (component 1, mole fraction yi) on aluminas over component j (mole fraction yj) of a gas mixture can be complex functions of adsorbate loadings (ni,nj), system temperature and pressure. There is a scarcity of published data on water adsorption from multicomponent gas mixtures on alumina. Typically, it is assumed that water is exclusively adsorbed on aluminas (S — oo,nj —> 0) from non- polar gases such as air or natural gas. The assumption may not be valid when the gas mixture contains polar components. The mixed gas Langmuir or Toth models may be used to describe multicomponent Type I equilibria on aluminas [6,7]. No isotherm model is available to describe adsorption of water from gas mixtures when there is partial condensation of water in the mesopores of the alumina. 

When producing hydrogen as the final product, impurities such as CO, sulfur compounds, and other trace contaminants must be removed, particularly for application in fuel cells. Currently, pressure swing adsorption (PSA) is commonly used for the separation and purification of hydrogen from mixed gas streams. PSA systems are based on selective adsorbent beds. The gas mixture is introduced to the bed at an elevated pressure and the solid adsorbent selectively adsorbs certain components of the gas mixture, allowing the unadsorbed components, in this case hydrogen, to pass through the bed as purified gas. 

The 77 values of (3-lg/PGA films (0.1 wt% PGA in the bulk phase) showed an antagonic behavior when compared to the tto( single (3-lg and PGA films, which should be attributed to their high degree of esterification (higher hydrophobicity) that allows them to rapidly adsorb at the interface. However, in the presence of KO at 0.5% (Figure 25.1a), the system showed a more cooperative adsorption. Similarly, KLVF at 0.5% increased tt of the mixed system. The increased cooperativity as PGA increased from 0.1 to 0.5% may be ascribed to an increase of segregation phenomena in the bulk solution. 

In the mixed systems, the behavior was similar to that observed for surface pressure. In the presence of surface-active PGA (Figure 25.3a and b) at low concentrations in the bulk phase (0.1 wt%), competition between the biopolymers at the interface results in a lower Ed than that expected from the observation of the single components. However, at higher concentrations of PS and long adsorption times, a cooperative adsorption can be deduced. This result could be explained by a concentration of (3-lg at the interface caused by the incompatibility with different biopolymers (that is more evident at higher concentrations). These phenomena would lead to an increase in the protein association in the film with the resultant increase in viscoelasticity. 

The theoretical models proposed in Chapters 2-4 for the description of equilibrium and dynamics of individual and mixed solutions are by part rather complicated. The application of these models to experimental data, with the final aim to reveal the molecular mechanism of the adsorption process, to determine the adsorption characteristics of the individual surfactant or non-additive contributions in case of mixtures, requires the development of a problem-oriented software. In Chapter 7 four programs are presented, which deal with the equilibrium adsorption from individual solutions, mixtures of non-ionic surfactants, mixtures of ionic surfactants and adsorption kinetics. Here the mathematics used in solving the problems is presented for particular models, along with the principles of the optimisation of model parameters, and input/output data conventions. For each program, examples are given based on experimental data for systems considered in the previous chapters. This Chapter ean be regarded as an introduction into the problem software which is supplied with the book an a CD. 

Fig. 6.3. Isotherms of adsorption of myo-inositol hexakisphosphate (IHP) on goethite, ferrihydrite, illite, kaolinite, calcite and ferrihydrite-kaolinite mixed systems at pH 4.5 and in 0.01 M KCI (adapted from Celi etaL, 1999,2000,2003).

According to Fig. 7.3.1 the isotherm slopes are approximately equal in formamide and methanol whereas the slope for the aqueous system is considerably larger. The mutual repulsion of adsorbed anions is therefore evidently stronger in methanol and formamide than in water. The interaction parameter is also found to depend strongly on the anion for a given solvent. For example in the formamide system the second virial coefficient (which is directly related to the interaction parameter) for adsorption of 1 ions is 310 A /ion compared with 2000 A /ion for Cl ions. Thus the simple adsorption model of point charges undergoing lateral coulombic repulsion represents a considerable oversimplification in non-aqueous solutions as in aqueous solutions. Studies of adsorption of halide anions from mixed electrolyte solutions in formamide and methanoF reveal complex behaviour which cannot be explained in terms of a simple model. 

Figure 15 Possible mechanism for adsorption (a) of bolaform cationic amphiphiles in mixed systems and (b) of single flexible bolaform cationic amphiphiles systems on glass. (Reprinted from J. Coll. Int. Sci. 337, Yun Yan, Ting Lu, Jianbin Huang Recent advances in the mixed systems of bolaamphiphiles and oppositely charged conventional snrfactants, 1, 2009 with permission from Elsevier.)...

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