Hydrous features

SnO exists in several modifications. The commonest is the blue-black tetragonal modification formed by the alkaline hydrolysis of Sn salts to the hydrous oxide and subsequent dehydration in the absence of air. The structure features square... 

Some emphasis is given in the first two chapters to show that complex formation equilibria permit to predict quantitatively the extent of adsorption of H+, OH , of metal ions and ligands as a function of pH, solution variables and of surface characteristics. Although the surface chemistry of hydrous oxides is somewhat similar to that of reversible electrodes the charge development and sorption mechanism for oxides and other mineral surfaces are different. Charge development on hydrous oxides often results from coordinative interactions at the oxide surface. The surface coordinative model describes quantitatively how surface charge develops, and permits to incorporate the central features of the Electric Double Layer theory, above all the Gouy-Chapman diffuse double layer model. 

The low-albedo C-, B-, G-, and F-class asteroids dominate the middle part of the main belt. The sharp 3 pm feature in the spectra of these objects indicates that hydrous... 

The most general feature of the adsorption behavior of metal ions at solid-aqueous solution interfaces is the abrupt rise in adsorption over a narrow pH range. This has been illustrated, for example, for manganese adsorption on glass (2), cobalt on hydrous ferric oxide (8), manganese on hydrous manganese oxide (12), protactinium on glass (14), and... 

The results of all of these measurements can be approximated by an empirical expression that explains most of the features of our experiments on oxides. In this paper, we will not attempt to give theoretical justification, but we and others (2, 4 ) have shown that most of the five characteristics given follow directly from equilibrium ion exchange considerations if certain assumptions are made relative to the origin of the lattice charge on the oxide. Other approaches have been used to explain these observations on hydrous oxides. Among these are approaches which associate enhanced sorption with hydrolysis of the nuclide (5-7) and with formation of surface complexes to specific sites (8-10). Some of these approaches are quite elaborate making extensive use of computer calculations and include double layer theory. The approach that we have used (2) is relatively simple, and explains many of the characteristics of sorption on hydrous oxide with equilibrium theory. 

The crystal structure of the natural mineral tobermorite was partly worked out by Megaw and Kelsey (22). Taylor and coworkers (15, 17, 26, 27) have carried out detailed investigations of tobermorite prepared synthetically, and correlated their structural features with those of the natural mineral structure. However, little was known about the nature of the surface, except what may be deduced from the surface energy results, the surface energy being approximately the geometric mean of the surface energies of calcium hydroxide and hydrous amorphous silica. 

Halse and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8°C or 23 C, the main feature was fibrous material that was considered to be hydrous alumina, but which could also have been partly dehydrated CAH,q. The hydrating grains of cement were surrounded by shells of hydration products, from w hich they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40"C showed spheroidal particles of CjAH and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed. 

A typical feature of carbonate rocks is the siderite-plus-quartz association, and usually also the presence of iron oxides, both magnetite and hematite it is assumed that there is no Fe in hydrous silicates. 

The characteristic feature of CM chondrules is the coexistence of roughly equal amounts of high-temperature anhydrous silicates (olivine and pyroxene) and low-temperature hydrous clay minerals. It is generally believed that the clay minerals were formed by aqueous alteration of the high-temperature phases, either in space or in the parent body. Figure 7 shows that the phyllosilicate matrix is systematically enriched in the heavy isotopes of oxygen, relative to the whole rock. The tie-lines between whole-rock compositions and matrix compositions have slopes of —0.7, implying that the water reservoir had a composition with more positive than the silicate reservoir. Clayton and Mayeda (1999) showed that the observed patterns can be accounted for with a simple closed-system hydration reaction at temperatures near 0°C,... 

Ferritins present several of the features of iron biochemistry mentioned above. They sequester iron in a safe form as a hydrous ferric oxide-phosphate mineral inside a protein coat. The protein, a hollow shell, has a capacity for up to about 4000 Fe atoms by virtue of its ability to pack this iron in its interior in a mineral form (11). The molecular design renders an otherwise insoluble mineral soluble and, by enabling iron to traverse the shell, allows a relatively high surface area of mineral to equilibrate with cytosolic iron. In eukaryotes, iron acquisition and release by ferritin may be to some extent compartmentalized. In higher organisms, iron may be deposited... 

In simple adsorption from aqueous solution, Hg has features in contrast and in common with the base metals. The hydroxy-cation is the active species in the model for heavy-metal adsorption and this also appears to be true for Hg. However, in contrast with Cu, Pb and Zn, the adsorption is less efficient and is strongly inhibited by the formation of halide complexes, as has been shown by Forbes et al. (1974) (Fig. 12-1). These authors also demonstrate that the adsorption of Hg to goethite is effective at pH as low as 4, allowing it to be trapped subsequent to sulphide oxidation. Whilst many minerals in weathered rocks and soils may each adsorb Hg, the relative efficiency of the hydrous iron oxides (Andersson, 1979) implies that these phases will be the dominant host in most exploration samples. However, the soil organic matter is also of importance and, although the association with Hg has been described as adsorption, it seems more... 

Alumina is a good example with which to demonstrate all of these features. Figure 6.9 shows the progression through all steps from precipitation to calcining. After drying, a hydrous oxide known as boehmite is produced with a structure AlaO rt H O. With pure boehmitc it 1, but values up to 1.8 are found. The structure of bochmite is distinctive and the surface hydrated as shown in Fig. 4.17. Upon calcination above 300 C, a series of... 

The essential features of the IHOAM theory are the suggestions that (1) premonolayer incipient hydrous oxide formation commences at noble metal surfaces at approximately the same potential as that at which thick hydrous oxide films are reduced and (2) the resulting oxyspecies function as mediators or inhibitors in many electro-catalytic processes. 

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