Role of atmospheric oxygen

Acid drainage results from the reaction of sulfide minerals with oxygen in the presence of water. As we show in this section, water in the absence of a supply of oxygen gas becomes saturated with respect to a sulfide mineral after only a small amount of the mineral has dissolved. The dissolution reaction in this case (when oxygen gas is not available) causes little change in the water s pH or composition. In a separate effect, it is likely that atmospheric oxygen further promotes acid drainage because of its role in the metabolism of bacteria that catalyze both the dissolution of sulfide minerals and the oxidation of dissolved iron (Nordstrom, 1982). 

The resulting fluid contains about 8 mg kg-1 (0.25 mmol) of dissolved oxygen. 

The fluid s pH in the model changes slightly (Fig. 31.1), decreasing from 6.8 to about 6.6. 

To see how contact with atmospheric oxygen might affect the reaction, we repeat the calculation, assuming this time that oxygen fugacity is fixed at its atmospheric level 

In this case, the reaction proceeds without exhausting the oxygen supply, which in the calculation is limitless, driving pH to a value of about 1.7 (Fig. 31.1). We could, in fact, continue to dissolve pyrite into the fluid indefinitely, thereby reaching even lower pH values. 

1 Calculated variation in pH during reaction of pyrite with a hypothetical groundwater at 25°C, assuming that the fluid is isolated from (fine line) and in contact with (bold line) atmospheric oxygen. 

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The role of atmospheric oxygen in the crystallization of [Ca6(/x4-0)2(/t3-OEt)4(OEt)4] 14EtOH from a refluxing solution of Ca(OEt)2 in ethanol was emphasized by Turova et al. (483) as this oxo-ethoxide species does not crystallize out under complete isolation from oxygen (deaerated alcohol under an helium atmosphere). It was suggested that the oxo groups originate from the oxidation of Ca(OEt)2 with the formation of peroxide derivatives and their subsequent decomposition. 

E.M. Moujahid, M. Dubois, J.-P. Besse, and F. Leroux, Role of atmospheric oxygen for the polymerization of interleaved aniline sulfonic acid in LDH, Chem. Mater., 14, 3799-3807 (2002). 

It is noteworthy that metallic copper or cuprous bromide used under nitrogen atmosphere shows only a very short induction time. This last result points out the inhibitor role of the oxygen of the air atmosphere and most likely the important role taken either by reduced species or by radical intermediates in the catalytic cycle. 

Like CO oxidation on Ru, the understanding for ethylene epoxidation on Ag has continued to evolve. Many questions remain open, including the reaction mechanism on the Ag structures, and the role of intercalated oxygen atoms. Another dimension that is little explored so far is the surface states in a combined oxygen-ethylene atmosphere. Greeley et al. have reported recently that an ethylenedioxy intermediate may be present at appreciable coverage under industrial reaction conditions, the effect of which on the structure of the surface is unknown. More importantly, the implication of a dynamic co-existence of various surface oxides under reaction conditions for the reaction mechanism needs to be explored and understood at greater depth. 

Oxygen can also cycle between the biosphere and the lithosphere — when marine organisms with calcium carbonate (CaC03) shells die, the shells are buried on the shallow waters of the sea floor, becoming the limestone of the lithosphere. A small amount of atmospheric oxygen is transformed to ozone, O3, and the ozone layer in the stratosphere plays an important role in shielding our planet from harmful ultraviolet radiation. 

Another example of the cyclodextrin cavity s role in complexation photochemistry is given by Weber et al. [96] who reported the photosensitized enantio-differentiating oxygenation of racemic a-pinene (31), using a porphyrin tethered to a p-cyclodextrin derivative (32) (Fig. 6). Upon irradiation with visible light (A, > 350 nm) in the presence of atmospheric oxygen for 8 h, several oxidation products were obtained. The ratios of these products were found to differ depending on the solvent employed, but it was noted that the (S) enantiomers were always formed in excess. For the reaction in the presence of 5 equivalents of 2-methylpyridine, excellent ee s of up to 67% for the (S)-1,2-epoxide (33) and 57%... 

During the last three years the role of singlet oxygen has received increasing attention as an environmental oxidant [Refs. 4, 340, 356, 478, 479, 558, 659]. Atmospheric contaminants absorb solar radiation and... 

Dohmen R, Chakraborty S, Palme H, Rammensee W (in press) The role of element solnbihty on the kinetics of element partitioning in situ observations and a thermodynamic model. J Geophys Res Dole M, Lane GA, Rudd DP, Zaukelies DA (1954) Isotopic composition of atmospheric oxygen and nitrogen. Geochim Cosmochim Acta 6 65... 

In 1766 Cavendish separated hydrogen from other gases and showed that it burned to water. In connection with Lavoisier s discovery of the role of airy oxygen (1777) it became clear that water is a chemical compound. Only in 1900 was Armand Gautier the first to proclaim the presence of hydrogen in atmospheric air. This was verified in 1902 by Rayleigh s spectroscopic studies in air. 

Schiff, H. I. (1970) Role of singlet oxygen in the upper atmosphere. Annals of the New York... 

Saito K, Okuda K, Ikenaga N, Miyake T, Suzuki T (2010) Role of lattice oxygen of metal oxides in the dehydrogenation of ethylbenzene under a carbon dioxide atmosphere. J Phys Chem A 114 3845-3854... 

The principal components of atmospheric chemical processes are hydrocarbons, oxides of nitrogen, oxides of sulfur, oxygenated hydrocarbons, ozone, and free radical intermediates. Solar radiation plays a crucial role in the generation of free radicals, whereas water vapor and temperature can influence particular chemical pathways. Table 12-4 lists a few of the components of each of these classes. Although more extensive tabulations may be found in "Atmospheric Chemical Compounds" (8), those listed in... 

The polymer plays several roles in this composition. First, it reduces shrinkage. Second, it increases the viscosity of the adhesive to the point where it can be easily applied. It also speeds the rate of cure. As will be discussed in the section on initiators, the boron compound reacts with atmospheric oxygen to form free radicals. 

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