Oxidations mineral formation

Uranium, too, is widely distributed and, since it probably crystallized late in the formation of igneous rocks, tends to be scattered in the faults of older rocks. Some concentration by leaching and subsequent re-precipitation has produced a large number of oxide minerals of which the most important are pitchblende or uraninite, U3O8, and camotite, K2(U02)2(V04)2.3H20. However, even these are usually dispersed so that typical ores contain only about 0.1% U, and many of the more readily exploited deposits are nearing exhaustion. The principal sources are Canada, Africa and countries of the former USSR. 

Another specific and important aspect to consider is the possibility that an environmentally heterogeneous photocatalyst can lead to the undesirable formation of reaction intermediates which are more toxic than the starting reagents. For instance, the Ti02-based photodegradation of ethanol, a relatively innocuous air pollutant, occurs through its transformation into the more toxic acetaldehyde. Condensation reactions can also lead to the formation of traces of methyl formate, ethyl formate, or methyl acetate. Catalyst design is therefore important to increase the overall oxidation rate to ensure complete mineralization (formation of C02 and H20). 

These minerals can occur separately (e.g. cemssite) or as mixtures of two or more oxide minerals. Depending on the formation of the ore body with oxide minerals, the ore may... 

The Heterogeneous Case. Hachiya et al. (1984) and Hayes and Leckie (1986) used the pressure-jump relaxation method to study the adsorption kinetics of metal ions to oxide minerals. Their results support in essence the same adsorption mechanism as that given for homogeneous complex formation. 

From the point of view of electrochemistry of flotation, a depressant is, however, defined as a reagent by the addition of which the oxidation of the mineral surface occurs at lower potential than collector oxidation or formation of metal collector salt which may be also decomposed imder the conditions given in the discussions which follow. Under these conditions, the mixed potential model becomes one of mineral oxidation and oxygen reduction, the oxidation of the thio collector or the formation of the metal collector is suppressed, and the mineral will remain... 

Additional information is required to determine what is causing this net removal. In the case of iron, research has demonstrated that its solubility decreases with increasing salinity leading to the formation of two types of solids (1) iron oxide minerals, and (2) organic floes. Some iron is also removed by uptake as a micronutrient by plankton. The floes form from the co-precipitation of iron with the high-molecular-weight dissolved organic compounds naturally present in river water. 

During soil generation, the breakdown of rock to soil mimics the general supergene process described above with the oxidation of rock, formation of clays and the generation of oxide minerals. 

Korzhinskii, D.S. 1963. Correlation between activity of oxygen, oxidity and reduction potential in endogenic mineral formation. Izvestiya of Academy of Sciences of USSR. Geological seria. No. 3. 54-61. (In Russian)... 

By convention, rocks are divided into three groups magmatic (volcanic or extrusive and plutonic or intrusive), metamorphic and sedimentary rocks. Iron ores being the source of iron as a metal, are also rocks and are common in all three groups. Most rocks contain iron oxide minerals of varying nature and abundance. This chapter collects information about their occurrence (Tab. 15.1), properties and formation. 

Feltz, A. Martin, A. (1987) Solid-state reactivity and mechanisms in oxide systems. 11 Inhibition of zinc ferrite formation in zinc oxide - a-iron(lll) oxide mixtures with a large excess of a-iron(lll) oxide. In Schwab, G.M. (ed.) Reactivity of solids. Elsevier, 2 307—313 Fendorf, S. Fendorf, M. (1996) Sorption mechanisms of lanthanum on oxide minerals. Clays Clay Miner. 44 220-227 Fendorf, S.E. Sparks, D.L. (1996) X-ray absorption fine structure spectroscopy. In Methods of Soil Analysis. Part 3 Chemical Methods. Soil Sd. Soc. Am., 377-416 Fendorf, S.E. Eick, M.J. Grossl, P. Sparks, D.L. (1997) Arsenate and chromate retention mechanisms on goethite. 1. Surface structure. Environ. Sci. Techn. 31 315-320 Fendorf, S.E. Li,V. Gunter, M.E. (1996) Micromorphologies and stabilities of chromiu-m(III) surface precipitates elucidated by scanning force microscopy. Soil Sci. Soc. Am. J. 60 99-106... 

Schwertmann, U. Friedl, J. Stanjek, H. (1999) From Fe(III) ions to ferrihydrite and then to hematite. J. Coll. Interface Sci. 209 215-223 Schwertmann, U. Friedl, J. Stanjek, H. Schulze, D.G. (2000) The effect of A1 on Fe oxides. XIX. Formation of Al-substituted hematite from ferrihydrite at 25°C and pH 4 to 7. Clays Clay Miner. 48 159-172 Schwertmann, U. Friedl, J. Stanjek, H. Schulze, D.G. (2000a) The effect of clay minerals on the formation of goethite and hematite from ferrihydrite after 16 years ageing at 25 °C and pH 4-7. Clay Min. 35 613-623... 

Many mineral species are known to be selectively crystallized by the presence of bacteria. Carbonate minerals, such as calcite, aragonite, hydroxycalcite, and siderite oxide minerals, such as magnetite and todorokite oxalate minerals, such as whewellite and weddellite sulfide minerals, such as pyrite, sphalerite, wurtzite, greigite, and mackinawite and other minerals, such as jarosite, iron-jarosite, and g3q>sum, are known to precipitate in the presence of bacteria. Therefore, investigations have been developed to analyze the formation of banded iron ore by the action of bacteria, and to analyze the ancient environmental conditions of the Earth through the study of fossilized bacteria. 

A number of attempts have been made to understand the mechanism of the adsorption of chelates on oxide minerals. For instance, IR spectroscopic studies10 have indicated the presence of a basic monosalicylaldoximate copper complex as well as the bis-salicylaldoximate complex on the surface of malachite (basic copper carbonate) treated with salicylaldoxime. However, other workers4 have shown that the copper chelate is partitioned between the surface and dispersed within the solution, and that a dissolution-precipitation process is responsible for the formation of the chelate. Research into the chemistry of the interaction of chelating collectors with mineral surfaces is still in its infancy, and it can be expected that future developments will depend on a better understanding of the surface coordination chemistry involved. 

Some of the extensive literature surrounding a relatively simple reaction, exchange on silica will be summarized in this section and contrasted with that for magnesium oxide in the next section. This reaction proceeds via O -centers on both catalysts, but the sites, modes of production and the reaction mechanisms themselves are very different. This reaction is very useful for illustrating the possible effects, on mineral catalysts, of processes common in natural systems, i.e., artifacts of mineral formation and weathering and electronic excitation. 

Therefore, as an oxide mineral weathers, reactions of C02 and water at the surface lead to the formation of carbonates and bicarbonates. The presence of OH can eventually cause... 

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. 

Mandernack, K.W., Post, J., and Tebo, B.M., Manganese mineral formation by bacterial spores of the marine Bacillus, strain SG-1 Evidence for the direct oxidation of Mn(II) to Mn(IV), Geochim. Cosmochim. Acta, 59, 4393, 1995. 

Tebo, B.M. et al., Bacterially mediated mineral formation Insights into manganese(II) oxidation from molecular genetic and biochemical studies, in Geomicrobiology Interactions Between Microbes and Minerals, Banfield, J.F. and Nealson, K.H., Eds., Mineralogical Society of America, Washington, D.C., 1997, p. 225. 

Metals such as iron and copper are generally most soluble in acidic water (i.e. pH < 7), and solubility increases as the pH drops. Other metals, such as aluminium and zinc, are more soluble in alkaline water, especially when the pH is above 10. In mildly acidic water (i.e. pH 45-6.5), metals such as iron and copper have a low solubility under extreme anaerobic and aerobic conditions. This is due to the formation of sulfide minerals that have a low solubility under highly anaerobic conditions, and the formation of low-solubility hydroxide and oxide minerals under highly aerobic conditions. 

Aerobic oxidation of dissolved inorganic substances resulting in end-product immobilization and mineral formation Aerobic bacterial oxidation of dissolved Fe to a Fe(III) oxide or oxyhydroxide and of Mn " " to Mn(IV) oxide are examples of end-product immobilization by mineral formation (Ehrlich, 1999). 

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