Ion adsorption mechanisms

Two types of system are used for ion-pair liquid chromatography. When polar stationary phase materials, such as silica gel, are used an ion-pair partition mechanism is applied. When non-polar stationary phase materials, such as octadecyl-bonded silica gel and polystyrene gel, are employed a paired-ion adsorption mechanism is involved. The former is called normal-phase ion-pair partition liquid chromatography, and the latter is called reversed-phase ion-pair liquid chromatography. 

The competitive adsorption of metal ions is dependent on both the metal ions and the adsorbent, its magnitude being related to the adsorption mechanism. In the case of competition between metal ions for the same adsorption sites, it has been shown that the favored metal ion is that which presents the faster adsorption kinetics on the same activated carbon in a monocomponent solution. This is the case for the adsorption of Cu(II) and Pb(II) onto activated carbon cloths [17]. When metal ions present in solution do not interact with the same adsorption sites, the removal of both ions is not affected compared with monocomponent adsorption. For example, in a study performed with different activated carbons, nickel removal was not affected by the presence of cadmium, because the sites that interact with nickel do not strongly interact with cadmium [18]. Finally, due to the strong relationship that exists between the metal ion adsorption mechanism and pH (see Section 24.2.1.4), it as been demonstrated that competitive adsorption is also influenced by pH [19],... 

Stoop, W.A., Ion adsorption mechanisms in oxidic soils finpfications for point of zero charge determinations. Geoderma, 23, 303,1980. 

Actually, the surface area of graphite is much smaller than that of AC, but can store more anions than AC. In the high-voltage branch of charge curve, the storage mechanism of anions is different from the common ion adsorption mechanism in... 

Considering the promising properties, especially their cost, hazard, and the risks of these DESs series, their introduction as safer electrolytes could represent an important challenge for the realization of environmentally friendly EDLCs operating at high temperatures. The study of the original ions adsorption mechanism of desolvated lithium ions provides an interesting opportunity for fundamental smdies even beyond the applicative interest of DESs as electrolytes for supercapacitors. 

J. W. Novak, Jr., R. R. Burr, andR. Bednarik, "Mechanisms of Metal Ion Adsorption of Activated Alumina," Vol. 35, Proc. Int. Symp. on Metals Speciation, Separation, and Recorey, Chicago, lU., July 27—Aug. 1, 1986, Industrial Waste Elimination Research Center of the Illinois Institute of Technology, Chicago, lU. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

With regard to the anodic dissolution under film-free conditions in which the metal does not exhibit passivity, and neglecting the accompanying cathodic process, it is now generally accepted that the mechanism of active dissolution for many metals results from hydroxyl ion adsorption " , and the sequence of steps for iron are as follows ... 

The Heterogeneous Case. Hachiya et al. (1984) and Hayes and Leckie (1986) used the pressure-jump relaxation method to study the adsorption kinetics of metal ions to oxide minerals. Their results support in essence the same adsorption mechanism as that given for homogeneous complex formation. 

Mechanisms of Sorption Processes. Kinetic studies are valuable for hypothesizing mechanisms of reactions in homogeneous solution, but the interpretation of kinetic data for sorption processes is more difficult. Recently it has been shown that the mechanisms of very fast adsorption reactions may be interpreted from the results of chemical relaxation studies (25-27). Yasunaga and Ikeda (Chapter 12) summarize recent studies that have utilized relaxation techniques to examine the adsorption of cations and anions on hydrous oxide and aluminosilicate surfaces. Hayes and Leckie (Chapter 7) present new interpretations for the mechanism of lead ion adsorption by goethite. In both papers it is concluded that the kinetic and equilibrium adsorption data are consistent with the rate relationships derived from an interfacial model in which metal ions are located nearer to the surface than adsorbed counterions. 

Mechanism of Lead Ion Adsorption at the Goethite-Water Interface... 

If this mechanism is consistent with the experimental relaxation data, then a plot of xp versus the expression in the brackets of Equation 35 will give a straight line with a slope of kjnt and an intercept at the origin. As shown in Figure 11, the data fit this proposed mechanism quite well. Values for i i0, reactant and product concentrations, and K nt input into Equation 35 are from the equilibrium modeling results calculated at each pH value for which kinetic runs were made. Normally a variety of different mechanisms are tested against the experimental data. Several other more complex mechanisms were tested, including those postulated for metal ion adsorption onto y-A O (7) however, only the above mechanism was consistent with the experimental data. Hence it was concluded that the bimolecular adsorption/desorption reaction was the most plausible mechanism for Pb2+ ion adsorption onto a-FeOOH. 

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