Species hydrous oxide

SnO and hydrous tin(II) oxide are amphoteric, dissolving readily in aqueous acids to give Sn" or its complexes, and in alkalis to give the pyramidal Sn(OH)3 at intermediate values of pH, condensed basic oxide-hydroxide species form, e.g. [(OH)2SnOSn(OH)2] and [Sn3(OH)4] +, etc. Analytically, the hydrous oxide frequently has a composition close to 3Sn0.H20 and an X-ray study shows it to... 

When produced by such dry methods it is frequently unreactive but, if precipitated as the hydrous oxide (or hydroxide ) from aqueous chromium(III) solutions it is amphoteric. It dissolves readily in aqueous acids to give an extensive cationic chemistry based on the [Cr(H20)6] ion, and in alkalis to produce complicated, extensively hydrolysed chromate(III) species ( chromites ). 

Arsenate is readily adsorbed to Fe, Mn and Al hydrous oxides similarly to phosphorus. Arsenate adsorption is primarily chemisorption onto positively charged oxides. Sorption decreases with increasing pH. Phosphate competes with arsenate sorption, while Cl, N03 and S04 do not significantly suppress arsenate sorption. Hydroxide is the most effective extractant for desorption of As species (arsenate) from oxide (goethite and amorphous Fe oxide) surfaces, while 0.5 M P04 is an extractant for arsenite desorption at low pH (Jackson and Miller, 2000). 

Employing potential-pH and cyclic voltammetric characterization techniques, Burke and coworkers [48, 73] have advanced the notion that incipient surface hydrous oxide species mediate the electrooxidation reactions of reducing agents of interest to electroless deposition. Thus, in the case of electroless Ni-B, an interfacial, basic, cationic Ni(I) hydrous oxide species would mediate the oxidation of DMAB, as shown in part here ... 

When water is added to a metal oxide, it may react to produce the hydroxide, but the reaction may not be complete. Thus, if the metal has a +3 charge, the product may consist of a mixture of M203, M(OH)3, M(OII)/, and M203 -xH20. The first of these is an oxide, the second and third are hydroxides, and the last is a hydrated oxide (also known as a hydrous oxide). In many cases there is a complex equilibrium involving all of these species, so the exact nature of the products when a metal oxide reacts with water may be variable in composition. 

Hydrolysis and Adsorption. Some years ago, a theory was advanced, that hydrolyzed metal species, rather than free metal ions, are adsorbed to hydrous oxides. The pH-dependence of adsorption (the pH edge for adsorption is often close to the pH for hydrolysis) was involved to account for this hypothesis. As Figs. 2.7b and c illustrate, there is a correlation between adsorption and hydrolysis but this correlation is caused by the tendency of metal ions to interact chemically with the oxygen donor atoms with OH, and with S-OH. The kinetic work of Hachiya et al. (1984) and spectroscopic information are in accord with the reaction of (free) metal ions with the surface. 

Does Surface Precipitation occur at Concentrations lower than those calculated from the Solubility Product As the theory of solid solutions (see Appendix 6.2) explains, the solubility of a constituent is greatly reduced when it becomes a minor constituent of a solid solution phase (curve b in Fig. 6.10).Thus, a solid species, e.g., M(OH)2 can precipitate at lower pH values in the presence of a hydrous oxide (as a solid solvent), than in its absence. 

Similarly, the rate of oxygenation of Fe(II) bound to the surface hydroxyl groups of a hydrous oxide can be expressed in terms of the surface species. Thus,... 

In the formation of metal (hydrous) oxides, hydrolyzed metal ions are the primary constituent species. In the "forced hydrolysis" method, the latter are generated by deproionation of the coordinated water of the hydrated cation at elevated temperatures, according to ... 

The isoelectric point may be conveniently defined as the ZPC expected for a pure, single component solid oxide, hydrous oxide, or hydroxide with a nondefective structure in an electrolyte totally devoid of specifically adsorbed polar or ionic species. An IEP(s) can be calculated from the charge and size of the cation using Equation 13 and the constants in Tables I and II. The maximum accuracy to be expected may be judged from the graphical correlation given in Figure 3. 

In the range of pH exhibited by most natural water and in the concentration range greater than millimolar few metal ions exist as simple hydrated cations, and relatively few oxyanions exist as simple monomeric species. The hydrated cations are good buffers toward bases, the metallate ions toward acids. As pH is raised in solutions of many hydrated cations, isopolycations are produced, and ultimately, hydrous metal oxides precipitate. As pH is lowered in solutions of many metallate ions, isopolyanions are produced, and ultimately, hydrous oxides precipitate. Salts of the intermediate isopolyions precipitate in some cases. Where the results are unambiguous, the nature of the intermediate species can be described. Kinetics are thought to have been neglected in studies of such solutions up to the present time. 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

In addition to the soluble chemical species and possible solid phase species described in the previous sections no discussion on speciation can be complete without the consideration of surface species. These include the inorganic and organic ions adsorbed on the surface of particles. Natural systems such as soils, sediments and waters abound with colloids such as the hydrous oxides of iron, aluminium, manganese and silicon which have the potential to form surface complexes with the various cationic and anionic dissolved species (Evans, 1989). 

The successive chemical extractions are designed to indicate the type of component phase to which the trace component is bound and from this the nature of the bound species may theoretically be inferred. But as Cremers and Henrion (1985) have pointed out, the whole basis of this type of approach is controversial and the literature contains many examples of conflicting conclusions regarding the relative contribution of the various phases. For example, in some cases plutonium and americium are reported to be solely associated with hydrous oxides (Edgington et al., 1976 Aston and Stanners, 1981) whilst, in others, these radionuclides are claimed to be fairly uniformly distributed throughout the various phase components of what appear to be similar sediments (Hetherington, 1978). 

The analysis of samples extracted with various solvents will provide information on the most easily removed metal species, the less available, and the most refractory metal content, which is dissolved only by the strongest acid extractants. There are at least a dozen different published speciation schemes for metals in soils and sediments. Many are based on the pioneering work by Tessier et al. [125]. Most include releasing metals from carbonates and hydrous oxides with acids, and an oxidation step to destroy organic... 

Numerous other species such as oxides and hydrous oxides are not shown. A really complete diagram for iron would need to have at least two additional dimensions showing the partial pressures of 02 and C02. 

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