Oxidation, abiotic

These have already been noted in the context of hydroxyl radical-initiated oxidations, and reference should be made to an extensive review by Worobey (1989) that covers a wider range of abiotic oxidations. Some have attracted interest in the context of the destruction of xenobiotics, and reference has already been made to photochemically induced oxidations. 

Processes which generate heat in organic materials are reviewed. At ordinary temperatures, respiration of living cells and particularly the metabolism of microorganisms may cause self-heating, while at elevated temperatures pyrolysis, abiotic oxidation, and adsorption of various gases by charred materials drive temperatures up whenever the released heat is unable to dissipate out of the material. The crucial rate of pyrolytic heat release depends on exothermicity and rates of the pyrolysis process. 

Figure 4.15 indicates the range of rates of O2 consumption in different soils. Oxygen is consumed in oxidation of inorganic reductants, such as Fe(II), as well as in oxidation of organic matter by microbes. Bouldin (1968) and Howeler and Bouldin (1971) compared measured rates of O2 movement into anaerobic soil cores with the predictions of various models, and obtained the best fits with a model allowing for both microbial respiration and abiotic oxidation of mobile and immobile reductants abiotic oxidation accounted for about half the O2 consumed. The kinetics of the abiotic reactions are complicated. They often depend on the adsorption of the reductant on solid surfaces as, for example, in... 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

In these reactions oxygen serves as the electron acceptor. Micro-organisms may or may not be involved in the Fe" to Fe " oxidation. The higher the pH of the solution, the more rapidly will the dissolved Fe " ions be oxidized and the more likely is it that abiotic oxidation will prevail. On the other hand, under very acid conditions (pH <3), oxidation and the formation of Fe " minerals must be mediated by micro-organisms (see Chap. 17). 

Abiotic oxidation of sulfide by oxygen cannot supply sulfate at rates comparable to rates of sulfate reduction. Unless high concentrations of sulfide develop and the zone of oxidation is much greater than 1 cm, rates of chemical oxidation of sulfide by oxygen will be much less than 1 mmol/m2 per day (calculated from rates laws found in refs. 115-118). Such conditions can exist in stratified water columns in the Black Sea water column chemical oxidation rates may be as high as 10 mmol/m2 per day (84). However, in lakes in which sulfide is undetectable in the water column and oxygen disappears within millimeters of the sediment-water interface (e.g., 113), chemical oxidation of sulfide by oxygen is unlikely to be important. 

Abiotic formation, hydrogen peroxide, 411 Abiotic oxidation... 

Amirbahman, A., Kent, D.B., Curtis, G.P. and Davis, J.A. (2006) Kinetics of sorption and abiotic oxidation of arsenic(III) by aquifer materials. Geochimica et Cosmochimica Acta, 70(3), 533-47. 

The oxidation rate of As(III) in the presence of manganese and water may be substantially enhanced by manganese-oxidizing bacteria, such as Leptothrix ochracea (Katsoyiannis, Zouboulis and Jekel, 2004). Katsoyiannis, Zouboulis and Jekel (2004) found that the bacteria are important in oxidizing Mn(II) to Mn(IV), Fe(II) to Fe(III), and As(III) to As(V). The oxidation of Mn(II) leads to the precipitation of Mn(IV) (oxy)(hydr)oxides, which then abiotically oxidize additional As(III) and significantly sorb the As(V) that results from both abiotic and biotic oxidation. 

Aside from oxygen and the activated oxygen species,there are several other oxidants that cause abiotic oxidation reactions involving environmental contaminants. In engineered systems, these include chlorine (49), chlorine dioxide (50-52), permanganate (53, 54) and ferrate (55, 56). At highly contaminated sites, anthropogenic oxidants such as chromate, arsenate, and selenate may react with co-contaminants such as phenols (57, 58). 

In the dissolved phase, few alternative abiotic oxidants are available in the natural environment. Nitrate, sulfate, and other terminal electron acceptors used by anaerobic microorganisms are thermodynamically capable of oxidizing some organic contaminants, but it appears that these reactions almost always require microbial mediation. 

Since the environmental degradation of polyethylene starts with abiotic oxidation, the determination of abiotic oxidation products is an important step towards establishing the environmental degradation mechanisms and environmental impact of the material. In a secondary process, microorganisms may utilise these abiotic degradation products and the low molecular weight... 

The implications of these results are extremely important, as they show that abiotic oxidation by Mn(III/IV) oxides can be a degradation mechanism for substituted phenols, which are so deleterious to environmental quality. 

Oxidation/Reduction Reactions. Reactions of chemicals via abiotic oxidation or reduction involve a transfer of electrons and result in a change in oxidation of the state of the product compared to its parent compound. As a general rule, reduction reactions are prevalent in soil sediments, while oxidation reactions are more important in surface waters and in the atmosphere.28... 

Abiotic oxidation of Fe2+ with 02 is extremely pH-sensitive. The reaction is rapid above pH 5, and becomes extremely slow in very acidic solution. In the presence of T. ferrooxidans, however, Fe2+ oxidation is very rapid in acidic conditions. The conditions under which T. ferrooxidans activity is optimized is shown in Figures 6.3-6.5. 

Millero and Izaguirre (1989) examined the effect various anions have on the abiotic oxidation rate of Fe2+ at constant ionic strength (/- 1,0) and found that this effect was on the order of HCOj > Br > N03- > C104- > Cl" > SO2" > B(OH)4 (see also Fig 7.18). Strong decrease in the rate of Fe2+ oxidation due to the addition of SO2 and... 

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