Physical and Chemical Properties of PAHs

PAHs are a group of compounds composed of two or more fused aromatic rings in linear, angular, and cluster arrangements as shown in Fig. 7.1. Based on structural diversity, PAHs can be broadly categorized into alternant and non-alternant classes. Alternant PAHs contain only fused benzenoid rings (e.g., anthracene, phenanthrene and pyrene), whereas non-alternant PAHs contain four or five carbon aromatic rings in addition to benzene ring (e.g., fluorene and fluoranthene) (Harvey 1991). PAHs can also be classified as low molecular weight (LMW) and high molecular weight (HMW) PAHs depending upon the number of aromatic rings. PAHs containing three or less than three aromatic rings are known as LMW PAHs (e.g., naphthalene and phenanthrene) and PAHs having greater than three aromatic rings are known as HMW PAHs (e.g., pyrene, fluoranthene, chrysene). Most of the PAHs have melting points above room temperature and boiling points above 100°C and are characterized by relatively low aqueous solubility.

Greater thermodynamic stability of PAHs arises from the delocalization of p-electron density. Thermodynamic stability and low aqueous solubility make them recalcitrant in the environment. The impact of PAH structure on its chemical behavior can generally be categorized as follows: PAHs with a linear structure are more unstable compared to their angular counterparts. Increased size and angularity of the PAH structure increase hydrophobicity and electrochemical stability which, in turn, affect their chemical and photochemical reactivity, as well as their ionization potential, vapor pressure, solubility and adsorption characteristics. The environmental fate of a PAH molecule depends on the number of aromatic rings and also on the pattern of ring linkage (Peters et al. 1999). The persistence of PAHs increases with an increase in the number of benzene rings and this is inversely correlated with the environmental biodegradation rates of PAHs.

Some PAHs contain a "bay-region" and a "K-region" (Mrozik et al. 2003). The bay- and K-region epoxides are reported to be highly reactive. Both these regions are present in phenanthrene as illustrated in Fig. 7.2. The bay-region of phenanthrene is between carbon atoms 4 and 5 whereas the K-region is between the carbon atoms 9, 10. The bay-region is a sterically hindered area. According to the Schmidt-Pullman electronic theory, K-region epoxides are likely to be more carcinogenic than the parent hydrocarbon.

LMW PAHs are characterized by higher volatility, higher solubility and greater ease of degradation than the higher molecular weight PAHs. HMW PAHs strongly sorb on to soil and sediments due to their high hydrophobicity and are therefore less available to indigenous microorganisms for degradation. The association of a contaminant with organic matter in soil (expressed as the organic matter

Naphthalene

Fluorene

Anthracene

Naphthalene

Fluorene

Anthracene

Pyrene

Fluoranthene

Chrysene

Pyrene

Fluoranthene

Chrysene

Benzo(a)pyrene Coronene

Fig. 7.1 Chemical structure of some polynuclear aromatic hydrocarbons

Fig. 7.2 Structure of phenanthrene showing bay-region and K-region

Bay-region

Bay-region

K-region normalized partition coefficient Koc) is a function of hydrophobicity of the compound which is expressed in terms of the octanol-water partitioning coefficient Kow. Some properties of selected PAHs are presented in Table 7.1.

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