Natural oxide, dissolution

Therefore, other factors that have not yet been studied and are not easily quantifiable, such as the absorption properties of the C.-T. adduct at the surface of the metal powder and the solubility of the formed species should be important in determining the oxidation properties of C.-T. adducts towards metal powders. Furthermore, some extrinsic factors inherent to the experimental conditions, such as reaction temperature, reagent concentration, and nature of the solvent have been reported to affect the overall yield or the course of the reaction, and led to separation of different products in some cases.55 59 In any case, it appears that the simultaneous presence of the donor molecule and the di-/inter-halogen lowers the oxidation potentials of the metals, allowing their oxidation, dissolution, and complexation. 

Natural Systems Dissolution of Oxides, Silicates and other minerals... 

Hoffman et al. (18) conducted a parametric study to determine the effect of bacterial strain, N/P molar ratio, the partial pressure of CO2, the coal source and the total reactive surface area on the rate and extent of oxidative dissolution of iron pyrite at a fixed oxygen pressure. The bacterial desulfurization of high pyritic sulfur coal could be achieved in 8 to 12 days for pulp densities upto 20% and particle size of less than 7 um. The most effective strains of T. ferrooxidans were isolated from the natural systems, and the most effective nutrient medium contained low phosphate levels, with an optimal N/P molar ratio of 90 1. 

Assumptions underlying the adsorption models are not often discussed in the literature, since the exact nature of the relevant surface complexes or phases is difficult to identify. In particular, lateral interactions between adsorbed ions, site heterogeneity as well as phenomena involving the oxide dissolution or rchy-dration are not taken into account systematically. The latter phenomena are discussed in section D.d. Lateral interactions between adsorbed ions (ion coadsorption) have been reported [27, 28] and make questionable the use of mass action equations at interfaces. The effect of surface structure, site heterogeneity and surface composition, in particular on the ZPC value, were also pointed out [29, 30]. 

Acid-base accounting has been criticized because it does not consider differences between the rate of pyrite oxidation and the rate of carbonate dissolution. Furthermore, the technique considers that all pyrite present in the sample is oxidizable under natural oxidizing conditions. 

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

Ligands and metal complexes present in aqueous systems in contact with natural oxides can affect their dissolution either by promoting or inhibiting it. For example, some metal—EDTA complexes react with Fe2C>3 and dissolve it, producing [Fe(III)EDTA]. Other minerals like Co(III)OOH and Mn(III)OOH reductively dissolve by oxidizing ligands and metal complexes. Dissolution rates can... 

Partitioning and mobility of metal ions, metal complexes, and ligands in soils or sediments are affected by their adsorption onto a variety of substrates. As mentioned earlier (see Section 6.3.1), natural oxides offer suitable adsorption sites for some of these species and may even undergo dissolution as a result. Here, an understanding of the bonding phenomena is crucial. For example, the adsorption of [Co(III)EDTA] (here written as [ML]-) on hydrated aluminum oxide surfaces (written as =A10H) can be represented as ... 

Oxidizers For metal CMP, most of the chemical reactions are electrochemical in nature. Oxidizers react with metal surfaces to raise the oxidation state of the metal via a reduction-oxidation reaction, resulting in either dissolution of the metal or the formation of a surface film on the metal. For both tungsten and copper, polish rate has been shown to be proportional to the rate of these reduction-oxidation reactions (see Chapters 6 and 7). 

Various chemical extraction techniques have been introduced in order to selectively remove metals from the different adsorption or complexation sites of natural sediments (e.g., Tessier et al, 1979 Erel et al, 1990 Leleyter et al., 1999). It is, for example, shown by Leleyter et al. (1999) that between 20% and 60% of REE in various suspended river sediments are removed by successive extractions by water, by Mg(N03)2 (exchangeable fraction), sodium actetate (acid-soluble fraction), NH2OH - - HCl (manganese oxide dissolution) ammonium oxalate (iron oxide dissolution) and a mixture of H2O2 + HNO3 (oxidizable fraction). The complexity of... 

Weathering — A series of processes whereby the physical and chemical properties of oil change after a spill. These processes begin when the spill occurs and continue indefinitely while the oil remains in the environment. Major processes that contribute to weathering include evaporation, emulsification, natural dispersion, dissolution, photo-oxidation, sedimentation, adhesion to materials, interaction with mineral fines, microbial biodegradation, and the formation of tar balls or tar mats. (See also Biodegradation, Dispersion, Dissolution, Emulsification, Photooxidation, Sedimentation, Tar balls or mats.)... 

A role of microbial processes in release of arsenic into groundwater concomitant with the reductive dissolution of Fe(ni) oxyhydroxides has been suggested based on the observed correlation between dissolved arsenic and bicarbonate concentrations (94,95). Increased bicarbonate concentrations are attributed to the oxidation of organic matter with Fe(III) oxyhydroxides as the terminal electron acceptor. Like oxidative dissolution, reductive dissolution may be kinetically limited. Rates of microbial reduction may be limited by the supply (and nature) of organic carbon. 

Chrome baths always contain a source of hexavalent chromium ion (e.g., chromate, dichromate, or chromic acid) and an acid to produce a low pH which usually is in the range of 0-3. A source of fluoride ions is also usually present. These fluoride ions will attack the original (natural) aluminum oxide film, exposing the base metal substrate to the bath solution. Fluoride also prevents the aluminum ions (which are released by the dissolution of the oxide layer) from precipitating by forming complex ions. The fluoride concenfration is critical. If the concentration is too low, a conversion layer will not form because of the failure of the fluoride to attack the natural oxide layer, while too high a concentfa-tion results in poor adherence of the coating due to reaction of the fluoride with the aluminum metal substrate. 

However, this nature oxide film does not offer sufficient protection against aggressive anions and dissolution of aluminium substrate occurs when exposed to corrosive solution. 

The dissolution potential of aluminium is measured on a surface which is always covered by a natural oxide layer. This oxide layer consists of three very different elements anodic pores (about 0.5% of the total surface), the cathodic barrier layer, and thicker areas which are neutral [9]. All parameters that modify the properties of the natural oxide layer will also modify the potential of aluminium. 

However, the pH value is not the sole parameter to be considered when predicting the stability of the natural oxide film in aqueous media, and therefore, of aluminium itself at acidic or alkaline pH, the dissolution rate of aluminium also depends on the nature of the acid or of the base dissolved in water, as shown in Figure B.1.19. 

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