Oxidation PAHs

These results clearly illustrate the care that must be exercised in interpreting the occurrence of PAH oxidation products as evidence of biodegradation. 

Wischmann H, H Steinhart (1997) The formation of PAH oxidation products in soils and soil/compost mixtures. Chemosphere 35 1681-1698. 

Hemoglobin is another heme-containing protein, which has been shown to be active towards PAH, oxidation in presence of peroxide [420], This protein was also modified via PEG and methyl esterification to obtain a more hydrophobic protein with altered activity and substrate specificity. The modified protein had four times the catalytic efficiency than that of the unmodified protein for pyrene oxidation. Several PAHs were also oxidized including acenaphthene, anthracene, azulene, benzo(a)pyrene, fluoranthene, fluorene, and phenanthrene however, no reaction was observed with chrysene and biphenyl. Modification of hemoglobin with p-nitrophenol and p-aminophenol has also been reported [425], The modification was reported to enhance the substrate affinity up to 30 times. Additionally, the solvent concentration at which the enzyme showed maximum activity was also higher. Both the effects were attributed to the increase in hydrophobicity of the active site. 

Oxidation is intimately linked to the activation of polycyclic aromatic hydrocarbons (PAH) to carcinogens (1-3). Oxidation of PAH in animals and man is enzyme-catalyzed and is a response to the introduction of foreign compounds into the cellular environment. The most intensively studied enzyme of PAH oxidation is cytochrome P-450, which is a mixed-function oxidase that receives its electrons from NADPH via a one or two component electron transport chain (10. Some forms of this enzyme play a major role in systemic metabolism of PAH (4 ). However, there are numerous examples of carcinogens that require metabolic activation, including PAH, that induce cancer in tissues with low mixed-function oxidase activity ( 5). In order to comprehensively evaluate the metabolic activation of PAH, one must consider all cellular pathways for their oxidative activation. 

Significance of Fatty Acid Hydroperoxide-Dependent PAH Oxidation... 

Laboratory experiments have shown that PAHs are photoreactive under atmospheric conditions (Zafiriou, 1977 Behymer and Hites, 1988 Schwarzenbach et al., 1993 Reyes et al., 2000) and in the photic zone of the water column (Zepp and Schlotzhauer, 1979 Payne and Phillips, 1985 Paalme et al., 1990). The existence of PAH oxidation products in atmospheric particulate matter indicates that PAHs react with oxygen or ozone in the atmosphere (Schwarzenbach et al., 1993 Howsam and Jones, 1998), but the reaction with... 

Fig. 2. Overall schematic of solid fuel combustion (1). Reaction sequence is A, heating and drying B, solid particle pyrolysis C, oxidation and D, post-combustion. In the oxidation sequence, left and center comprise the gas-phase region, tight is the gas—solids region. Noncondensible volatiles include CO, CO2, CH4, NH, H2O condensible volatiles are C-6—C-20 compounds oxidation products are CO2, H2O, O2, N2, NO, gaseous organic compounds are CO, hydrocarbons, and polyaromatic hydrocarbons (PAHs) and particulates are inerts, condensation products, and solid carbon products.

Aerobic, Anaerobic, and Combined Systems. The vast majority of in situ bioremediations ate conducted under aerobic conditions because most organics can be degraded aerobically and more rapidly than under anaerobic conditions. Some synthetic chemicals are highly resistant to aerobic biodegradation, such as highly oxidized, chlorinated hydrocarbons and polynuclear aromatic hydrocarbons (PAHs). Examples of such compounds are tetrachloroethylene, TCE, benzo(a)pyrene [50-32-8] PCBs, and pesticides. 

Benzene monoxide-oxepin and its sulfur analog are treated elsewhere (Chapter 5.1.7) (67AG(E)385). However, we point out here that electron-withdrawing substituents often favor the benzene oxide tautomer. The first study on oxides of the environmentally hazardous polychloro- and polybromo-biphenyls shows that they exist mainly in the benzene oxide form (81JOC3721). Oxides of polynuclear aromatic hydrocarbons (PAH) also exist mainly in the fused-ring oxirane form. 

Several industrial facilities near a residential area emit tlie inhalable pollutants ethylene oxide, polychlorobiphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). Tlie aimual average concentration of ethylene oxide, PCBs, and PAHs are 10 pg/in, 2 pg/m, and 5 pg/m, respectively. 

The branched oligo(arylene)s 37 and 40 can undeigo a further oxidative cyclization with copper(ll) chloride or triflate/aluminum trichloride leading to the formation of large, hitherto unknown polycyclic aromatic hydrocarbons PAHs 41 and 42. 

Major unknowns in the mechanism by which a hydrocarbon fuel bums concern the pyrosynthesis reactions that lead to the formation of polycyclic aromatic hydrocarbons (PAHs) and soot and the oxidation chemistry of atoms other than carbon and hydrogen (heteroatoms) in the fuel, particularly nitrogen, sulfur, and halogens. 

In terrestrial animals, the excreted products of PAHs are mainly conjugates formed from oxidative metabolites. These include glutathione conjugates of epoxides, and sulfate and glucuronide conjugates of phenols and diols. 

PAHs can be bioconcentrated or bioaccumulated by certain aquatic invertebrates low in the food chain that lack the capacity for effective biotransformation (Walker and Livingstone 1992). Mollusks and Daphnia spp. are examples of organisms that readily bioconcentrate PAH. On the other hand, fish and other aquatic vertebrates readily biotransform PAH so, biomagnification does not extend up the food chain as it does in the case of persistent polychlorinated compounds. As noted earlier, P450-based monooxygenases are not well represented in mollusks and many other aquatic invertebrates (see Chapter 4, Section 4.2) so, this observation is not surprising. Oxidation catalyzed by P450 is the principal (perhaps the only) effective mechanism of primary metabolism of PAH. 

It appears that organisms at the top of aquatic food chains are not exposed to substantial levels of PAH in food because of the detoxifying capacity of organisms beneath them in the food chain. On the other hand, fish, birds, and aquatic mammals feeding on mollusks and other invertebrates are in a different position. Their food may contain substantial levels of PAH. Although they can achieve rapid metabolism of dietary PAH, it should be remembered that oxidative metabolism causes... 

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