Hydroxyl radical aromatic hydrocarbons

The transformation of arenes in the troposphere has been discussed in detail (Arey 1998). Their destruction can be mediated by reaction with hydroxyl radicals, and from naphthalene a wide range of compounds is produced, including 1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and with less certainty 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both 1- and 2-nitronaphthalene were formed through the intervention of NO2 (Bunce et al. 1997). Attention has also been directed to the composition of secondary organic aerosols from the photooxidation of monocyclic aromatic hydrocarbons in the presence of NO (Eorstner et al. 1997) the main products from a range of alkylated aromatics were 2,5-furandione and the 3-methyl and 3-ethyl congeners. 

Lloyd, A.C., Darnall, K.R., Winer, A.M., Pitts, Jr., J.N. (1976) Relative rate constants for reaction of the hydroxyl radical with a series of alkanes, alkenes, and aromatic hydrocarbons. J. Phys. Chem. 80, 189-794. 

Ohta, T., Ohyama, T. (1985) A set of rate constants for the reactions of hydroxyl radicals with aromatic hydrocarbons. Bull. Chem. Soc. Jpn. 58, 3029-3030. 

Perry, R.A., Atkinson, R., Pitts, J.N. (1977) Kinetics and mechanisms of the gas phase reaction of the hydroxyl radicals with aromatic hydrocarbons over temperature range 296 473 K. J. Phys. Chem. 81, 296-304. 

In addition, phenols are formed by the reaction of hydroxyl radical addition to the aromatic ring of oxidized alkylaromatic hydrocarbon [56]. 

Finally, it makes possible the oxidation of hydrocarbon to a significant depth, and when the RH molecule contains several methyl groups, the catalyst allows all these groups to be transformed into carboxyls. This last specific feature is insufficiently studied so far. Perhaps, it is associated with the following specific features of oxidation of alkylaromatic hydrocarbons. The thermal decomposition of formed hydroperoxide affords hydroxyl radicals, which give phenols after their addition at the aromatic ring... 

FIGURE 3 12 Possible initial steps for ozone, atomic oxygen, nitrogen dioxide, and hydroxyl-radical reaction with aromatic hydrocarbons. 

From all available evidence, the hydroxyl radical plays a major role in the photooxidation and aerosol formation processes for aromatic hydrocarbons. However, much research remains to be done to improve our knowledge in this field. 

Solaqua is a patented, ex situ process for the removal of organic contaminants from wastewater or groundwater. The technology uses ferric oxalate and hydrogen peroxide in the presence of light to produce hydroxyl radicals, which destroy organic contaminants such as aromatic hydrocarbons, phenols, alkanes, aUcynes, ethers, and ketones. Solaqua is not yet commercially available. 

The most widespread environmental carcinogens are the polycyclic aromatic hydrocarbons (PAHs), which are found, among other places, in automobile exhaust, cigarette smoke, and broiled meats. PAHs undergo two main pathways of bioactivation one-electron oxidation and oxygenation. The former yields cation radicals the latter produces hydroxyl derivatives. 

Conversely, nucleophilic molecules (Nu) [Lewis bases e.g., catechols, hy-droquinones, phenols, alcohols, and thiols (and their anions) aromatic hydrocarbons and amines (benzene, toluene, pyridine, bipyridine)] can be oxidized by (1) direct electron-transfer oxidation [Eq. (12.3)] or (2) by coupling with the oxidation product of H20 (or HO-), hydroxyl radical (HO-) [Eq. (12-4)] ... 

On the other hand, the indirect type of ozonation is due to the reactions of free radical species, especially the hydroxyl radical, with the organic matter present in water. These free radicals come from reaction mechanisms of ozone decomposition in water that can be initiated by the hydroxyl ion or, to be more precise, by the hydroperoxide ion as shown in reactions (4) and (5). Ozone reacts very selectively through direct reactions with compounds with specific functional groups in their molecules. Examples are unsaturated and aromatic hydrocarbons with substituents such as hydroxyl, methyl, amine groups, etc. [45,46],... 

In contrast to the results of Bossmann et al., Lindsey and Tarr [31,32] observed equivalent rate constants for polycyclic aromatic hydrocarbon (PAH) degradation with Fenton systems as had been previously observed for PAH reaction with hydroxyl radicals as generated by pulse radiolysis techniques [33], Such kinetic agreement suggests, but does not confirm, equivalent mechanisms. PAHs, unlike 2,4-dimethylaniline, are not expected to directly coordinate iron in aqueous solutions. 

Lindsey ME, Tarr MA. Inhibited hydroxyl radical degradation of aromatic hydrocarbons in the presence of dissolved fulvic acid. Water Res 2000 34 2385-2389. 

Keywords Aqueous photochemistry Hydroxyl radicals Ice photochemistry Photochemistry Polycyclic aromatic hydrocarbons Polychlorobiphenyls Surfactants Titanium dioxide... 

Krauss and Wilcke examined the TiC -photocatalyzed oxidation of 12 PCB congeners [and 20 polycyclic aromatic hydrocarbons (PAHs)] on various soil samples (four mineral topsoil horizons, six organic horizons, and four particle-site fractions in three different soils) [107]. When the Ti02/soil mixture was irradiated in the absence of H2O, no photooxidation of the chlorobiphenyls occurred. When slurried with water, however, chlorobiphenyl concentrations decreased by 40-50% after 48 hours of irradiation, while the PAH concentrations were unchanged. By way of contrast, PAHs and PCBs doped onto quartz sand diminished by 95-100% after 8 hours of photolysis. The pollutants are clearly more accessible to hydroxyl radicals on sand than on soil. It is also clear that the photooxidation occurred in the soil and not in solution. Thus, OH is generated on one surface (Ti02), diffuses in the water to the other surface (soil), where the oxidation occurs. 

Aromatic hydrocarbons can be oxidized to the corresponding phenols by transition metal peroxo complexes and, in particular, vanadium(V) peroxo complexes, which act either as electrophilic oxygen transfer reagents or as radical oxidants -, depending on the nature of the ligands coordinated to the metal and on the experimental conditions. Vanadium picolinato peroxo complex (V0(02)PIC(H20)2) (39) (PIC = picoUnic acid anion) has been reported to be particularly effective in the hydroxylation of benzene and substituted benzenes (equation 50) . Accordingly, 39 smoothly oxidizes substituted benzenes 38 to the corresponding monophenols 40 in acetonitrile at room temperature. 

Synthetic Organic Micropollutants. Micropollutants that frequently presented in water include chlorobenzenes, PAHs, polychlorinated biphenyls (PCBs), and pesticides. Polycyclic aromatic hydrocarbons can be degraded effectively by ozone. But oxidation of chlorobenzenes and PCBs is slow by molecular ozone. However, they are more reactive with hydroxyl radicals. Therefore, it is recommended that either ozonation be conducted at high pH or advanced oxidation processes (AOP) be used. As for the pesticides, their removal by ozonation may be related to their water solubility. However, faster degradations are usually obtained with AOP. ... 

The kinetics of the reactions of many xenobiotics with hydroxyl and nitrate radicals have been examined under simulated atmospheric conditions and include (1) aliphatic and aromatic hydrocarbons (Tuazon et al. 1986) and substituted monocyclic aromatic compounds (Atkinson et al. 1987c) (2) terpenes (Atkinson et al. 1985a) (3) amines (Atkinson et al. 1987a) (4) heterocyclic compounds (Atkinson et al. 1985b) and (5) chlorinated aromatic hydrocarbons (Kwok et al. 1995). For PCBs (Anderson and Hites 1996), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 days for 3-chlorobiphenyl to 34 days for 2,2, 3,5, 6-pentachlorbiphenyl. It was estimated that loss by hydroxylation in the atmosphere was a primary process for removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a... 

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