Metal oxides adsorption

Wu M-C, Estrada C A, Corneille J S and Goodman D W 1996 Model surface studies of metal oxides adsorption of water and methanol on ultrathin MgO films on Mo(IOO) J. Chem. Phys. 96 3892... 

Clifford, D.A. and Ghurye, G.L. (2002) Metal-oxide adsorption, ion exchange, and coagulation-microfiltration for arsenic removal from water, in Environmental Chemistry of Arsenic (ed. W.T. Frankenberger Jr.), Marcel Dekker, New York, pp. 217-45. 

Metal-Oxide Adsorption, Ion Exchange, and Coagulation-Microfiltration for Arsenic Removal from Water... 

Arsenic in drinking water supplies can be removed by a variety of treatment processes including those cited in Table 1, which also lists the typical applications of each process. All these processes do a much better job of removing As(V) compared with As(III). Thus, before using these processes, it will often be necessary to oxidize As(in) to As(V) using chlorine or an alternative oxidant. This chapter focuses arsenic treatment by metal-oxide adsorption (MOA), ion exchange (IX), and iron (III) coagulation-microfiltration (C-MF), because these processes have proven to be the most efficient and cost effective in bench- and pilot-scale studies, especially for point-of-use (POU), point-of-entry (POE), wellhead, and small community treatment systems. 

Metal-oxide adsorption using packed beds of activated alumina, modified activated alumina, granular ferric hydroxide, iron-oxide coated sands, and other specialty adsorbents... 

The earliest derivations of solute-metal oxide adsorption equations using die TMAB adsorption model appeared in the literature in die mid-1990s [2—4], The details of the derivation of the TMAB adsorption equations have been explored in detail in previous sections of this chapter so no further review of the model is necessary. However, it is useful to compare the subtle but important differences between die TMAB adsorption models and all the other adsorption models discussed in diis section as shown in Table 20. 

TMAB adsorption equations are shown to give excellent agreement with the pH and solute concentration dependence of solute adsorption for different solute-metal oxide adsorption data. The effects of different solute-surface site complex stoichiometries are examined for TMAB adsorption equations derived to include many different stoichiometries. In most cases these additional stoichiometries have no signifieant effect on total solute adsorption, but no general statement is possible without a more in-depth study. Of partieular importance is the potential interference of adsorption of a particular ion by the eounter ion in the salt. No experimental data were found to eon-firm this effect, but TMAB adsorption equations derived for adsorption of a single ion from a solution of a salt of that ion predieted signifieant adsorption interferenee between the counter ion and ion when both ion and eounter ion have mass aetion adsorption constants of similar magnitude. 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

Surface heterogeneity is difficult to remove from crystalline inorganic substances, such as metal oxides, without causing large loss of surface areas by sintering. Thus in Fig. 2.14 in which the adsorbent was rutile (TiO ) all three adsorbates show a continuous diminution in the heat of adsorption as the surface coverage increases, but with an accelerated rate of fall as monolayer completion is approached. 

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

A wide range and a number of purification steps are required to make available hydrogen/synthesis gas having the desired purity that depends on use. Technology is available in many forms and combinations for specific hydrogen purification requirements. Methods include physical and chemical treatments (solvent scmbbing) low temperature (cryogenic) systems adsorption on soHds, such as active carbon, metal oxides, and molecular sieves, and various membrane systems. Composition of the raw gas and the amount of impurities that can be tolerated in the product determine the selection of the most suitable process. 

The H2S comes out with the reactor products, goes through the product-recovery system of the FCCU, and eventually goes to a Claus plant for sulfur recovery. The metal oxide adsorbent recirculates with the spent cracking catalyst back to the regenerator for the next SO adsorption cycle. 

A sohd solution of a pure magnesium aluminate spiael (MgAl20 with MgO is an effective metal oxide adsorbent (43). Such a soHd solution (Mg2A10 ) does not destroy the spiael framework the MgO adsorbs SO and the adsorption activity of the dispersed MgO ia the spiael is much greater than that of pure MgO itself (43). 

COLLECTION AND IDENTIFICATION OF BIOACTIVE ORGANIC COMPOUNDS OCCURRING IN RIVERS AND LAKES. ADSORPTION SELECTIVITY OF MONOSACCHARIDES ONTO HYDROUS METAL OXIDES... 

As a possible method of concentrating trace amounts of bioactive organic compounds occurring in the hydrosphere, adsorption properties of various compounds have been explored by employing hydrous metal oxides as the adsorbents. To date, a family of organophosphoms compounds and carbonic acids were adsorbed onto hydrous iron oxide, along with the adsoi ption of monosaccharides onto hydrous zirconium oxide. 

The lack of a well-defined specular direction for polycrystalline metal samples decreases the signal levels by 10 —10, and restricts the symmetry information on adsorbates, but many studies using these substrates have proven useful for identifying adsorbates. Charging, beam broadening, and the high probability for excitation of phonon modes of the substrate relative to modes of the adsorbate make it more difficult to carry out adsorption studies on nonmetallic materials. But, this has been done previously for a number of metal oxides and compounds, and also semicon-... 

There are various theories on how passive films are formed however, there are two commonly accepted theories. One theory is called the oxide film theory and states that the passive film is a diffusion-barrier layer of reaction products (i.e., metal oxides or other compounds). The barriers separate the metal from the hostile environment and thereby slow the rate of reaction. Another theory is the adsorption theory of passivity. This states that the film is simply adsorbed gas that forms a barrier to diffusion of metal ions from the substrata. 

Surface adsorption reaction, which occurs on the surface of the metal and metal oxides ... 

The importance of one other type of reaction that metal ions undergo has been recognized and studied extensively in the past 40 years. This reaction is adsorption, in which metal ions bind to the surface of particulate matter and are thereby transported as part of a solid phase even though they do not form an identifiable precipitate. Conceptually, these reactions can be thought of as hybrids between complexation and precipitation reactions. Most studies of these reactions have used metal oxides or hydroxides as the solid (adsorbent) phase, and the... 

The effects on metal ion adsorption of ligands that can themselves adsorb strongly can be quite different from that described above. Many multi-atomic ligands can bond to oxide surfaces through atoms different from those they use to... 

It is unlikely in real tribological events that adsorbed mono-layers work solely to provide lubrication. Instead, adsorption and chemical reactions may occur simultaneously in most cases of boundary lubrication. For example, fatty acid is usually regarded as a friction modiher due to good adsorp-tivity, meanwhile its molecules can react with metal or a metal oxide surface to form metallic soap which provides protection to the surface at the temperature that is higher than its own melting point. 

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