Adsorption of inorganic anions

Adsorption of Inorganic Anions on Uncalcined and Calcined LDHs. 204... 

Standing of surface charge and surface potential, and their relationship to pH, ionic strength, and medium composition. Westall and Hohl (1980) provide an excellent review of alternative models for the electrical double layer, and James and Parks (1982) provide a detailed description zind explanation of the surface chemistry and electrostatics of hydrous metal oxides. Hayes et al. (1988) present an excellent discussion of the effect of the electrical double layer on the adsorption of inorganic anions. A similar approach is used by Zachara et al. (1990) to model the adsorption of aminonaphthalene and quinoline onto amorphous silica. 

Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosic fibres they may also be similarly used with vat or sulphur dyes in their leuco forms. In the case of anionic dyes on wool or nylon, however, their role is different as they are used to facilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreases dye uptake due to the competitive absorption of inorganic anions by the fibre and a decrease in ionic attraction between dye and fibre. In most discussions of the effect of electrolyte on dye sorption, attention is given only to the ionic aspects of interaction. In most cases, this does not create a problem and so most adsorption isotherms of water-soluble dyes are interpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however, some observed cases of apparently anomalous behaviour of dyes with respect to electrolytes that cannot be explained by ionic interactions alone. 

Shilov (23) attributed the adsorption of acids to genuine surface oxides of basic nature. According to Steenberg (109), adsorption of inorganic acids involved primary adsorption of protons, by physical forces, and secondary adsorption of anions. The adsorbed anions were... 

Chem. Min. 4 317-339 Hiller, D.A. (2000) Properties of urbic anthro-sols from an abandoned shunting yard in the Ruhr areas, Germany. Catena 39 245—266 Hiller, J.-E. (1966) Phasenumwandlungen im Rost. Werkst. und Korr. 11 943-951 Hingston, F.J. (1981) A review of anion adsorption. In Anderson, M.A. Rubin, A.J. (eds.) Adsorption of inorganics at solid-liquid interfaces. Ann Arbor Science, Ann Arbor Mich.,... 

Diagram (a) shows the results obtained with several inorganic salts, the adsorption of whose anions at the mercury surface increases in the order Cl < Br < CNS < I < S". The uppermost curve, for potassium hydroxide, is nearly symmetrical. The effects of the adsorption of the anions are a considerable depression of tension on the ascending part of the curve, a small or moderate depression at the maximum, and a shift of the applied voltage required to reach the maximum, to more negative values. The descending part of the curve is not appreciably altered a short distance past the maximum. 

Figure 34. Sequential adsorption scheme for producing multilayer films by alternate adsorption of inorganic two-dimensional anions and oligomeric or polymeric cations. Reprinted with permission from S.W. Keller, H.-N. Kim and T.E. Mallouk, J. Am. Chem. Soc. 1994, 116, 8817. Copyright 1994 American Chemical Society.

Hinoston, F. j. 1981. A review of anion adsorption. In Adsorption of inorganics at solid-liquid interfaces, ed M. A. Anderson and A. J. Rubin, pp. 51-90. Ann Arbor, MI Ann Arbor Science. 

An elegant example of SPE method based on ion-exchange retention was used for inline preconcentration of inorganic anions. A single capillary containing a preconcentration zone (adsorbed layer of cationic latex particles) and a separation zone (fused-silica modified by adsorption of a cationic polymer) was used. Analytes were retained in the preconcentration zone and eluted isota-chophoretically into the separation zone by means of an eluotropic gradient. This approach was used to determine nitrate in Antarctic ice cores at the 2.2-11.6 p,g L" level. 

Equilibrium uptakes of metallic cations, dyes, and even gold and anionic adsorbates are largely governed by electrostatic attraction or repulsion. It is suggested that further studies of the role of the carbon surface are needed to show whether this is indeed the case. If not, only then does the role of specific interactions (e.g. complex formation), or even nonspecific van der Waals interactions, become a reasonable alternative or complementary argument. Indeed, in the adsorption of inorganic solutes, the main fundamental challenge remains how to activate the entire carbon surface in order to achieve maximum removal efficiencies. The solution conditions, at which maximum uptake can be achieved, depend on the surface chemistry of the adsorbent. 

Similar reactions form the basis of the separation of cations. An example of the separation of inorganic anions at the ppm level is shown in Figure 3(b). Size exclusion chromatography (SEC). This is suitable for mixtures of solutes with relative molecular masses (J(MM) in the range lOMO Da. Stationary phases are either microparticulate cross-linked co-polymers of styrene and divinyl benzene with a narrow distribution of pore sizes, or controlled-porosity silica gels, usually end-capped with a short alkyl chain reagent to prevent adsorptive interactions with solutes. Exclusion is not a true sorption mechanism because solutes do not interact with the stationary phase (Topic D2). They can be divided into three groups ... 

Physical and ionic adsorption may be either monolayer or multilayer (12). Capillary stmctures in which the diameters of the capillaries are small, ie, one to two molecular diameters, exhibit a marked hysteresis effect on desorption. Sorbed surfactant solutes do not necessarily cover ah. of a sohd iaterface and their presence does not preclude adsorption of solvent molecules. The strength of surfactant sorption generally foUows the order cationic > anionic > nonionic. Surfaces to which this rule apphes include metals, glass, plastics, textiles (13), paper, and many minerals. The pH is an important modifying factor in the adsorption of all ionic surfactants but especially for amphoteric surfactants which are least soluble at their isoelectric point. The speed and degree of adsorption are increased by the presence of dissolved inorganic salts in surfactant solutions (14). 

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