Aluminum oxides and hydroxides

The coating composition is a combination of hydrated chromium and aluminum oxides and hydroxides, eg, Cr202 XH2O, x — 1, 2. 

The application of infrared photoacoustic spectroscopy to characterize silica and alumina samples is reported. High quality infrared photoacoustic spectra illuminate structural changes between different forms of silica and alumina, as well as permit adsorbate structure to be probed. Adsorption studies on aerosil suggest adsorbed species shield the electric fields due to particle-particle interactions and induce changes in the vibrational spectra of the adsorbates as well as in the bulk phonon band. It is shown that different forms of aluminum oxides and hydroxides could be distinguished by the infrared spectra. 

The infrared spectra for various aluminum oxides and hydroxides are shown in Figure 3. Figure 3a is a-alumina (Harshaw A13980), ground to a fine powder with a surface area of 4 m /g. The absorption between 550 and 900 cm is due to two overlapping lattice modes, and the low frequency band at 400 cm is due to another set of lattice vibrations. These results are similar to those obtained by reflection measurements, except that the powder does not show as... 

The results obtained for the various aluminum oxides and hydroxides indicate that infrared photoacoustic spectroscopy may be useful in characterizing structural transformations in these species. Very clear differences between a-alumina and y-alumina were noted in the region of the lattice vibrations. The monohydrate, boehmite, showed a very distinct Al-OH stretching feature at 1070... 

The results presented here for silicas and aluminas illustrate that there is a wealth of structural information in the infrared spectra that has not previously been recognized. In particular, it was found that adsorbed water affects the lattice vibrations of silica, and that particle-particle Interactions affect the vibrations of surface species. In the case of alumina, it was found that aluminum oxides and hydroxides could be distinguished by their infrared spectra. The absence of spectral windows for photoacoustic spectroscopy allowed more complete band identification of adsorbed surface species, making distinctions between different structures easier. The ability to perform structural analyses by infrared spectroscopy clearly indicates the utility of photoacoustic spectroscopy. 

General reference J.A. Davis and J.D. Hem, The Surface Chemistry of Aluminum Oxides and Hydroxides, in G. Sposito, The Environmental Chemistry of Aluminum, CRC Press, Boca Raton (1989). 

Activated carbon, silica gel, and alumina are the most popular adsorbent materials in industry due to the fact that they provide large surface areas per unit weight. Activated carbon is produced from coconut shell, wood, and bone, whereas silica gel is made of hydrated silicon dioxide. Alumina can be either mined or produced by precipitated aluminum oxide and hydroxide. In the following sections, the most important adsorbents are presented in detail. 

Figure 8.7 The aluminum oxides and hydroxides and their temperatures of interconversion. Hydroxide materials are named along with their chemical formulae oxides all have the composition AI2O3 and so are indicated by the Greek letter associated with the phase. The a-phase is indicated in large font and bold as it is the thermodynamically most stable oxide. Adapted from reference [61].

In general, minerals in sedimentary and meta-morphic rocks contain ferrous iron (Velde, 1985) which is destined to become iron oxide under conditions of weathering. Oxidation under surface conditions has a tendency to produce iron in the ferric state. Most often the process takes iron out of the silicates and puts it into an oxide phase. In the uppermost layers of mature soils, iron oxide and various silicates, usually non-iron-bearing, are produced. In silicates containing iron, the majority is in the ferric state. The extent of the transformation of iron oxidation state is a rough measure of the maturity of the soil. In the extremely weathered soils one finds only ferric iron and aluminum oxides and hydroxides. These soils are typically red. 

Goldberg, S., Davis, J. A., and Hem, J. D. (1996a). Tlie surface chemistry of aluminum oxides and hydroxides. In The Environmental Chemistry of Aluminum, 2nd ed.. ed. Sposito, G., Lewis Publishers, Boca Raton, FL, 271-331. 

ELECTRON SPIN RESONANCE SPECTROSCOPY Electron spin resonance (ESR) is a technique that can also be used on aqueous samples and has been used to study the adsorption of copper, manganese, and chromium on aluminum oxides and hydroxides. Copper(II) was found to adsorb specifically on amorphous alumina and microcrystalline gibbsite forming at least one Cu-O-Al bond (McBride, 1982 McBride et al., 1984). Manganese(II) adsorbed on amorphous aluminum hydroxide was present as a hydrated outer-sphere surface complex (Micera et al., 1986). Electron spin resonance combined with electron spin-echo experiments revealed that chromium(III) was adsorbed as an outer-sphere surface complex on hydrous alumina that gradually converted to an inner-sphere surface complex over 14 days of reaction time (Karthein et al., 1991). 

The partial orders with respect to [OH ] observed for most silicate mineral dissolution reactions can be explained by the surface complexation model (Blum and Lasaga, 1988 Brady and Walther, 1989). Brady -and Walther (1989) showed that slope plots of log R vs. pH for quartz and other silicates at 25 °C is not inconsistent with a value of 0.3. Plots of the log of absorbed OH vs. pH also have slopes of about 0.3, suggesting a first-order dependence on negative charge sites created by OH adsorption. Because of the similarity of quartz with other silicates and difference with the dependence of aluminum oxides and hydroxide dissolution on solution [OH ], Brady and Walther (1989) concluded that at pH >8 the precursor site for development of the activated complex in the dissolution of silicates is Si. This conclusion is supported by the evidence that the rates (mol cm s ) at pH 8 are inversely correlated with the site potential for Si (Smyth, 1989). Thus it seems that at basic pH values, silicate dissolution is dependent on the rate of detachment of H3SiO4 from negative charge sites. 

Many dissolution reactions, however, are not dependent on adsorbed H or OH , but are first-order with respect to solution concentrations of these ions. Examples of reactions that are first-order with respect to [H ] include feldspar dissolution at pH <2.9 nepheline dissolution in the pH range of 3 to 6 and the dissolution of naturally weathered olivine (Grandstaff, 1986). The results of Grandstaff (1986) for olivine contradict the previously mentioned conclusions of Blum and Lasaga (1989) concerning a first-order dependence on H adsorption sites for freshly crushed olivine. The dissolution of aluminum oxides and hydroxides is first-order with respect to [OH ] at pH >8. 

Nevskaya, E.Yu., et al.. Calculations of acid-base equilibrium constants from the pH dependence of the electrokinetic potential and potentiometric titration data on aluminum oxides and hydroxides, Russ.. 1. Phys. Chem., Ti, 1421, 1999. 

Silicon and aluminum oxides and hydroxides are the most abundant compounds on our earth. They exist in rock, soil, the dust we breathe, and as components or trace components in just about all we touch or consume. Over decades, they have made their appearance in a number of segments of medicine such as fibrotic diseases, tumor induction, Alzheimer s disease, and aluminum related bone diseases. Of these Alzheimer s disease is perhaps one of the most serious diseases in our society. Here a still mysterious role of "aluminum" is observed and acknowledged by many researchers. 

A central issue in the attempt to establish a reliable database is the requirement of critically evaluated thermodynamic data for several key species. One such pivotal element is aluminum, which has an extensive literature of solubility and thermochemical data from which to choose, for each of the aqueous species or complexes. The aluminum species are fundamental to the calculation of solubility and reaction state with respect to many silicates and aluminum oxides and hydroxides and are principal components in numerous surface chemical reactions in the environment. Two key chapters in this volume address this fundamental problem Apps and Neil give a critical evaluation of the data for the aluminum system and Hem and Roberson present the kinetic mechanisms for hydrolysis of aluminum species. 

Coatings can form naturally by reaction with the surrounding atmosphere aluminum is quickly covered by an aluminum oxide and hydroxide layer in the presence of oxygen and water. This inert and protective layer can be formed artificially, as is done for window and door frames. A porous layer of alumina can be formed by reaction with oxygen. Pigments can be included in the pores for decorative purposes. After inclusion of the pigments, a suitable treatment of the layer transforms its porous structure to a continuous nonporous one (sealing). These coated frames can withstand relatively harsh conditions for years. 

Alumina is a commonly used term for aluminum oxides and hydroxides, which exist as at least five thermodynamically stable phases and many more meta-stable transition forms. Other chapters include discussions of the physical forms of these materials and their methods of production. With the exception of some alpha aluminas, all forms possess surface hydroxyls which can have a certain degree of activity in adsorption. The treatise by Wefers and Bell is considered to be a good review of aluminum oxide phase chemistry [10]. 

The interest in the uptake of phosphate by metal oxides such as iron, alumina, and titania arises from current problems found in the study of soils, corrosion, and biomimetic materials. Phosphate reactions with iron and aluminum oxides and hydroxides have been extensively studied by soil chemists because these soil conq)onents are the most abundant of the naturally occurring metal oxides (7, 2) and are the inorganic soil constituents primarily responsible for phosphate reactions in... 

Goodboy and Fleming (1984) point out that the term alumina is a misused expression for aluminum oxides and hydroxides that exist in at least flve thermodynamically stable phases and many more metastable transition forms. The commercial activated alumina desiccants arc generally manufactured by a process involving the following. steps (Woo.sley, 1990) ... 

One concern is that Cr(III) and Al(III) compounds are both capable of forming octahedral complexes, and the introduction of these ions into an aqueous electrolyte will interfere with conversion of the hydrous alumina into the aluminum hydroxide film by bonding to the active film sites. Therefore, similar to their application in hexavalent chromate conversion coatings, fluoride ions are used to remove aluminum oxide and hydroxide films on the substrate surface before forming trivalent chromium conversion coatings. ... 

Aluminum Oxide and Hydroxide Aluminum oxide is often referred to as /m-mina when in crystalline form, it is called corundum. 

Ernst et al. [1959] have proposed a shorthand system for naming clays. This is really concerned with the whole clay rather than with individual clay minerals and makes use of the following abbreviations K = kaolin, M = montmorin (or montmorillonoid, or smectite), I = micas, A = aluminum oxides and hydroxides, Q = Si02, F = felspar, S = salts, C = carbonates, X = other silicates, Y = accessories, Z = aggregates, fossils, etc., O = organic. 

Aluminum Oxides and Hydroxides Caillere and Pobeguin [1966], Duffin and Goodyear [1960], Frederickson [1954], Fripiat et al [1967], Kolesova and Ryskin [1959, 1962], Lyon [1962b], Marel [1966], Moenke [1962a], Takamura and Koezuka [1965], Tarte [1963a, 1967]. 

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