Polycyclic Aromatic Hydrocarbons PAHs

The PAHs are substances mainly formed from incomplete combustion processes (e.g. pyrolysis) of petroleum and coal in connection with energy production, industrial activities, transport, etc. but also from graphite electrodes in industrial processes, from wood preservatives made up from coal tar and from products such as car tyres. The occurrence of PAHs is widespread, although often more or less concentrated to hot spots associated with anthropogenic activities. Of course, PAHs being products from combustion processes, are also formed in natural combustion processes such as forest fires, volcanic activities, etc. and they are 

The PAHs are composed of carbon and hydrogen, and different substituted substances can also be assigned to this group of environmental toxicants. The aromatic structure and the absence of polar groups constitute molecules of lipophilic character that are prone to biotransformation or degradation. 

The PAHs, like many other groups of envirotoxicants, constitute an immense number of different substances, each one expressing its own toxicity (Bispo et al., 1999). However, certain PAHs express planar structures and affinity for the Ah-receptor and thus interact with certain PCDDs and planar PCBs, adding toxicity equivalents (TEQs) to the overall impact on the receptor. Also, for different PAHs, additive effects towards the Ah-receptor have been documented, as have competition and antagonistic effects from naphthalene in combination with benzo[a]pyr-ene (White, 2002). Exposure to PAHs gives rise to a panorama of different toxic 

Other effects of environmental concern from different PAHs are immunotoxi-city, resulting in increased sensitivity towards bacteria and virus. These effects seem to be mediated through the Ah-receptor and expressed at the DNA level (Grundy et al., 1996 Burchiel and Luster, 2001). 

Contamination of food with polycyclic compounds can be caused by fall-out from the 

This class of compounds is ubiquitous in the atmosphere with more than 100 PAH compounds having been identified in urban air. PAHs observed in the atmosphere range from bicyclic species such as naphthalene, present mainly in the gas phase, to PAHs containing seven or more fused rings, such as coronene, which are present exclusively in the aerosol phase. Intermediate PAHs such as pyrene and anthracene are distributed in both the gas and aerosol phases. The major compounds that have been identified in the atmosphere with their abbreviations and some physical properties are shown in Table 14.15 (Baek et al. 1991). The ambient concentrations of PAHs vary from a few ngm with values as high as 100 ngm-3 reported close to their sources (traffic, combustion sources). Concentrations are usually higher during the winter compared to the summer months (Table 14.16). 

TABLE 14.16 Concentrations (ngm 3) of Selected PAHs in the Ambient Air in Los Angeles (United States) and Munich (Germany) 

McMurry and Grosjean (1985) estimated saturation vapor concentrations for condensable products from the oxidation of 1-heptene (0.14-1.28 ppb), o-cresol (0.06 to 1.6 ppb), and nitrocresol (1.7 to 2.2 ppb). Odum et al. (1996) fitted their smog chamber data to (13.39) and derived effective vapor pressures for the aerosol products of m-xylene, 1,2,4-trimethylbenzene, and a-pinene. Available relevant vapor pressures are shown in Table 13.13. 

386 14 Reducing Carcinogenicity and Mutagenicity Through Molecular Design 

---

Unbumed Hydrocarbons Various unburned hydrocarbon species may be emitted from hydrocarbon flames. In general, there are two classes of unburned hydrocarbons (1) small molecules that are the intermediate products of combustion (for example, formaldehyde) and (2) larger molecules that are formed by pyro-synthesis in hot, fuel-rich zones within flames, e.g., benzene, toluene, xylene, and various polycyclic aromatic hydrocarbons (PAHs). Many of these species are listed as Hazardous Air Pollutants (HAPs) in Title III of the Clean Air Act Amendment of 1990 and are therefore of particular concern. In a well-adjusted combustion system, emission or HAPs is extremely low (typically, parts per trillion to parts per billion). However, emission of certain HAPs may be of concern in poorly designed or maladjusted systems. 

Polycyclic aromatic hydrocarbons (PAH) Apply sample solution and dry. Place TLX2 plate for 20 min in a twin-trough chamber containing phosphorus pentoxide to which 2 to 3 ml cone, nitric acid have been added. PAH nitrated by nitrous fumes. [20]... 

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. 

An alternative way of eliminating water in the RPLC eluent is to introduce an SPE trapping column after the LC column (88, 99). After a post-column addition of water (to prevent breakthrough of the less retained compounds), the fraction that elutes from the RPLC column is trapped on to a short-column which is usually packed with polymeric sorbent. This system can use mobile phases containing salts, buffers or ion-pair reagents which can not be introduced directly into the GC unit. This system has been successfully applied, for example, to the analysis of polycyclic aromatic hydrocarbons (PAHs) in water samples (99). 

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. 

Hydrocarbons. In other publications the historical trend of organic pollutant concentrations, namely polychlorinated biphenys (PCBs), chlorinated pesticides DDT and metabolites DDE, DDD, and polycyclic aromatic hydrocarbons (PAHs), have been reconstructed. For this purpose the sediments of the core sampled in the Lagoon area close to the industrial district were employed (16,17). 

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. 

FIGURE 7.9 Mechanism of formation of polycyclic aromatic hydrocarbons (PAHs) during combustion. 

Benzo[a]pyrene, a molecule with five, fused, hexagonal rings, is among the most carcinogenic of the polycyclic aromatic hydrocarbons (PAHs). Such biological activity may be related to the electronic structure of benzo[a]pyrene and its metabolites. Ionization energies of these molecules therefore have been investigated with photoelectron spectroscopy [28]. 

Table VII. Standard Reference Materials (SRM s) for the Determination of Polycyclic Aromatic Hydrocarbons (PAH s) ...

The synthesis of toxic organic compounds by humans, and their release into the natural environment began to assume significant proportions during the 20th century, especially after the Second World War. Prior to 1900, the chemical industry was relatively small, and the largest chemical impact of humans on the environment was probably dne to the release of hydrocarbons, especially polycyclic aromatic hydrocarbons (PAHs), with the combnstion of coal and other fuels. 

The next eight chapters will be devoted to the ecotoxicology of groups of compounds that have caused concern on account of their real or perceived environmental effects and have been studied both in the laboratory and in the field. These are predominantly compounds produced by humans. However, a few of them, for example, methyl mercury, methyl arsenic, and polycyclic aromatic hydrocarbons (PAHs), are also naturally occurring. In this latter case, there can be difficulty in distinguishing between human and natural sources of harmful chemicals. 

The largest releases of polycyclic aromatic hydrocarbons (PAHs) are due to the incomplete combustion of organic compounds during the course of industrial processes and other human activities. Important sources include the combustion of coal, crude oil, and natural gas for both industrial and domestic purposes, the use of such materials in industrial processes (e.g., the smelting of iron ore), the operation of the internal combustion engine, and the combustion of refuse (see Environmental Health Criteria 202, 1998). The release of crude oil into the sea by the offshore oil industry and the wreckage of oil tankers are important sources of PAH in certain areas. Forest hres, which may or may not be the consequence of human activity, are a signihcant... 

Geiselbrecht AD, BP Hedland, MA Tichi, IT Staley (1998) Isolation of marine polycyclic aromatic hydrocarbon (PAH)-degrading Cycloclasticus strains from the Gulf of Mexico and comparison of their PAH degradation ability with that of Puget Sound Cycloclasticus strains. Appl Environ Microbiol 64 4703-4710. 

Although these issues have already been briefly noted, they deserve a few additional comments. For freely water-soluble substrates that have low volatility, there are few difficulties in carrying out the appropriate experiments described above. There is, however, increasing interest in xenobiotics such as polycyclic aromatic hydrocarbons (PAHs) and highly chlorinated compounds including, for example, PCBs, which have only low water solubility. In addition, attention has been focused on volatile chlorinated aliphatic compounds such as the chloroethenes, dichloromethane, and carbon tetrachloride. All of these substrates present experimental difficulties of greater or lesser severity. 

Bastiaens L, D Springael, P Wattiau, H Harms, R deWachter, H Verachtert, L Diels (2000) Isolation of adherent polycyclic aromatic hydrocarbons (PAH)-degrading bacteria using PAH-sorbing carriers. Appl Environ Microbiol 66 1834-1843. 

Kastner M, M Breuer-Jammali, B Mahro (1998) Impact of inocnlnm protocols, salinity, and pH on the degradation of polycyclic aromatic hydrocarbons (PAHs) and survival of PAH-degrading bacteria introduced into soil. Appl Environ Microbiol 64 359-362. 

Potin O, C Rafin, E Veignie (2004) Bioremediation of an aged polycyclic aromatic hydrocarbons (PAHs)-contaminated soil by filamentous fungi isolated from the soil. Int Biodet Biodeg 54 45-52. 

Spectra at p (=20) wavelengths. Because of the Lambert-Beer law, all measured spectra are linear combinations of the two pure spectra. Together they form a 15x20 data matrix. For example the UV-visible spectra of mixtures of two polycyclic aromatic hydrocarbons (PAH) given in Fig. 34.2 are linear combinations of the pure spectra shown in Fig. 34.3. These mixture spectra define a data matrix X, which can be written as the product of a 15x2 concentration matrix C with the 2x20 matrix of the pure spectra ... 

---