Remediation of PAH-Contaminated Soils and Sediments: A ...

soil is the primary environmental repository (Dabestani and Ivanov, 1999; Juhasz
and Naidu, 2000).
Molecular weight, structure, water solubility, and vapor pressure of each
PAH compound affects the potential for PAH transfer (i.e. volatilization,
absorption, leaching and erosion) degradation (biological and chemical), and
sequestration (adsorption and diffusion) (Jones et al., 1996; Pierzynski et al.,
2000; Reid et al., 2000a). Understanding the fate of each PAH compound in the
natural environment based on its specific characteristics is important in
determining appropriate remediation techniques.
Molecular weight
Polycyclic aromatic hydrocarbons are classified as low molecular weight (LMW) if
they have two or three fused rings or high molecular weight (HMW) if they have
four or more fused rings. LMW PAHs are degraded and volatilized more rapidly
than HMW PAHs (Harvey, 1997). As molecular weight increases,
hydrophobicity/lipophilicity increases, water solubility decreases, vapor pressure
decreases, and the compound will have a more recalcitrant (difficult to degrade)
structure. High molecular weight PAHs persist in the environment because of
low volatility, resistance to leaching, and recalcitrant nature (Jones et al., 1996;
Wild and Jones, 1995). Molecular weights of select PAHs are provided in Table
1.
Structure
Polycyclic aromatic hydrocarbons are also classified into two groups based on
ring structure: alternant and nonalternant. Alternant PAHs such as anthracene,
phenanthrene, and chrysene are derived from benzene by fusion of additional
six-membered benzoid rings, and contain fewer than eight benzoid rings (Harvey,
1998). Nonalternant PAHs may contain rings with fewer than six carbon atoms in
addition to six membered rings. This group is extremely broad in structure and
greatly increases PAH diversity (Harvey, 1998). Examples of four-, five-, and sixmembered
rings are fluorine and fluoranthene (Harvey, 1997; Dabestani and
Ivanov, 1999).
Notice the difference in the two structures of alternate and nonalternate PAHs in
the below diagrams. Chrysene consists of four fused, six carbon benzene rings.
Fluoranthene, on the other hand, contains naphthalene and a benzene unit
connected by a five-membered ring (in the center of the structure) and is
indicative of lower temperature and less efficient combustion. Information on
number of rings for select PAH compounds are provided in Table 1.
Alternate (Chrysene) Nonalternate (Fluoranthene)
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