Aluminum oxide, dissolution mechanism

There are various concepts about the aluminum silicates dissolution mechanism. Relatively recently a low rate of their dissolution was explained by inner diffuse regime. Currently more substantiated appears hydrolysis with the formation of activated complexes. According to this theory, the dissolution begins with the exchange of alkaline, alkaline-earth and other metals on the mineral surface of H+ ions from the solution (see Figure 2.26). At that, metals in any conditions are removed in certain sequence. In case of the presence of iron and other metals with variable oxidation degree the process may be accompanied with redox reaction. Hydrolysis is a critical reaction in the dissolution of aluminum silicates. It results in the formation on the surface of a very thin layer of activated complexes in Na, K, Ca, Mg, Al and enriched with H+, H O or H O. The composition and thickness of this weakened layer depend on the solution pH. These activated complexes at disruption of weakened bonds with mineral are torn away and pass into solution. For some minerals (quartz, olivine, etc.) the disruption of one inner bond is sufficient, for some others, two and more. The very formation of activated complexes is reversible but their destruction and removal from the mineral are irreversible. 

There is no stable entity Al2+(aq) to compare with Fe2+(aq) consequently, the mechanism that causes rust to be nonprotective because of migration of Fe2+(aq) through the water before precipitation as FeO(OH) does not apply to aluminum, on which Al(OH)3 or AIO(OH) forms, at once, on the anodic site. Conversely, removal of the protective aluminum oxide film cannot occur by the reductive dissolution mechanism described for iron. 

Studies of the dissolution rates of alum inum oxide and beryllium oxide (Furrer and Stumm, 1986) showed that the rates of these reactions are facilitated by increased concentrations of protons and various aliphatic and aromatic ligands at the oxide surface. The key link for determining the mechanism of reaction catalysis is establishing the relation between the concentration of these species in solution and the concentration at the solid surface. This relation is determined by potentiometric titration of the solution-solid mixture, just as described earlier for the surface catalysis of Mn oxidation. The relations between the dissolution rate of aluminum oxide and the hydrogen concentration both in solution and on the solid surface are presented along with the proposed mechanism in Fig. 9.9. The solution dependence of the dissolution rate on pH (Fig. 9.9A) has a fractional order of 0.4, which is similar to that of other oxide dissolution reactions (Table 9.4). But when the same relation is plotted as... 

The proposed mechanism for the ligand-promoted dissolution of aluminum oxide involves three general steps (1) ligand adsorption and surface complex formation, (2) slow detachment of a surface metal center (as a complex with the ligand), and (3) regeneration of the surface (shown schematically in Figure 1). 

Minerals with Kinetic Dissolution Condition Minerals of this group are considered in everyday life insoluble. Ihey include mostly metal oxides, hydroxides, sulphides and aluminum sihcates. The mechanism of their dissolution is dominated by hydrolysis whose nature depends on the structure and composition of minerals. Their dissolution under any conditions has kinetic condition, i.e., it is controlled by extremely slow chemical reactions of surface complexation. The rate of their dissolution is noticeably lower than 10 ° mole m s and the solubility does not exceed 10" mole l Besides, both their dissolution rate and solubility depend on pH values. These minerals are most common in the Earth crust and often play a leading role in the formation of imderground water composition. It is convenient to subdivide minerals with kinetic dissolution regime into three groups 1- silica, 2 - oxides, hydroxides and sulphides of metals, 3-aluminum silicates. 

Similar to the formation of porous aluminum oxide a passivation - dissolution mechanism can be used to form nanopo-rous structures on InP. If (OOl)n-InP is polarized anodically under illumination in HCl solutions, nanoscaled pores are formed [117]. For potentials up to 1.2 V vs. SGE the main reaction is uniform anodic dissolution. Above this potential porous InP with a surface oxide is formed. The overpotential and anodizing time influence pore diameter (110-250 nm), wall thickness (16-50 nm) and pore length... 

Ni Sorption on Clay Minerals A Case Study. Initial research with Co/clay mineral systems demonstrated the formation of nucleation products using XAFS spectroscopy, but the stmcture was not strictly identified and was referred to as a Co hydroxide-like stmcture (11,12). Thus, the exact mechanism for surface precipitate formation remained unknown. Recent research in our laboratory and elsewhere suggests that during sorption of Ni and Co metal ions, dissolution of the clay mineral or aluminum oxide surface can lead to precipitation of mixed Ni/Al and Co/Al hydroxide phases at the mineral/water interface (14,16,17,67,71). This process could act as a significant sink for metals in soils. The following discussion focuses on some of the recent research of our group on the formation kinetics of mixed cation hydroxide phases, using a combination of macroscopic and molecular approaches (14-17). 

The mechanism of corrosion inhihition, with these heavy metal chromates, hinges on the fact that they can passivate aluminum. When such a corrosion-inhibited bonded joint is attacked, a mixture of hydrated aluminum oxide and chromic oxide (Cr203) is formed (cf. the Alocrom process). This not only seals the oxide film, repairing the damage caused by the ingress of the electrolyte, but the presence of the stable chromic oxide also reduces the rate of dissolution of the aluminum oxide. The longevity of such a protection is due to the low solubility ( 1.2 g/L at 15 °C) of the chromate in water, which means that the chromate remains active for a considerable period of time. 

Incorporation of promoters can occur via two distinct mechanisms. A local pH drop at the leach front caused by the aluminum dissolution can cause a solvated promoter to deposit via a shift in the solubility equilibrium. Zincate shows this behavior, depositing as zinc oxide as the pH drops at the leach... 

The results obtained here also indicate that the acidity of the oxidized surface of a metal depends on the pretreatment of the surface. For a take-off angle of 75°, about 42% of the amino groups were protonated when y-APS films were applied to mechanically polished 1100 aluminum. That increased to about 57% when the silane films were rinsed, probably because of the dissolution of a small amount of silane from the free surface of the film on which the amino groups were mostly not protonated. However, for a take-off angle of 75°, about 48% of the amino groups were protonated when y-APS was applied to FPL-etched aluminum substrates. That increased to about 65% when the silane films were rinsed. These results imply that the FPL-etched surface is more acidic than the polished surface. 

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