Source Characterization of Polycyclic Aromatic Hydrocarbons by Using Their Molecular Indices: An Overview of Possibilities - Reviews of Environmental Contamination and Toxicology - page 59

Oil degradation processes, including evaporation, dissolution, and microbial
oxidation, are controlled by factors such as oil type, environmental conditions and
microbiological activity (Short et al. 2007 ; Sicre et al. 1987 ; Wang et al. 1999a ). The
petrogenic PAHs degrade at much faster rates than pyrogenic ones, because the
former are more bioavailable and associate less with carbon particles after their
release, e.g., oil spills, discharges (Gogou et al. 2000 ; Zakaria et al. 2002 ). Figure 3
shows the PAH fingerprint of the spilled Exxon Valdez crude (EVC) at different
states of degradation (Burns et al. 1997 ). The weathering largely resulted from
evaporation and dissolution, which first impacted the more volatile and soluble
LMW compounds (Hostettler et al. 2007 ; Table 2 ; Wang et al. 1999a ).
Evaporation of the PAHs after their incorporation into sediments is not a signifi-
cant process (Stout et al. 2001b ), but the evaporation of PAHs before they associate
with sediments or particles (e.g., oil spills) is the most important short-term (hours-
days) weathering factor (Wang and Fingas 2003 ). The degree of loss from evapora-
tion depends on the kind of petroleum product involved. Light products containing
light PAHs evaporate readily, whereas heavier ones lose as little as 5-10% of their
total volume (Philp 2007 ; Wang and Fingas 2003 ). Thereafter a PAH0 < PAH1
< PAH2 < PAH3 profile emerges for each alkylated PAH family (Iqbal et al. 2008 ;
Stout 2007 ), starting from the LMW PAHs (Fig. 3c-g ).
Dissolution depends mainly on the structure of the PAHs and decreases as the
ring number and alkylation level of the PAHs increases, although exceptions exist
(e.g., chrysene is less soluble than its methyl and dimethyl homologues) (Stout et al.
2001b ). Furthermore, the linear PAHs are less soluble than their angular equivalents
(anthracene is less soluble than phenanthrene) (Stout et al. 2001b ; Wang and Fingas
2003 ). As dissolution proceeds, biodegradation may begin to affect the distribution
of individual compounds. The biodegradation rate depends on the nature of the spill
and environment (O 2 , pH, microbial populations, etc.) into which the oil is spilled
(Philp 2007 ; Stout et al. 2002 ); biodegradation is slower and less predictable than
abiotic degradation. Biodegradation can even alter the distribution of PAHs within
a homologue category, because individual isomers have different susceptibilities to
microbial activity (Wang et al. 1998 , 1999a , 2004 ). Utilizing a biodegradation index
for oils has been proposed to assist in evaluating their potential for biodegradation
in the lab (Christensen et al. 2004 ; Wang et al. 1998 , 2004 ). It is generally accepted
that PAH biodegradation decreases concomitantly with increasing PAH ring num-
bers and alkylation (Stout et al. 2001b ).
Further degradation (Fig. 3c-g ) leads to the enhancement of chrysenes relative to
other PAH series, and to a significant decrease in the relative ratios of the sum of
naphthalenes, phenanthrenes, dibenzothiophenes, and fluorenes, to chrysenes (most
stable) (Wang et al. 1998 ). In Fig. 3 , there are two distinct and apparent features of
PAH degradation. The first is the more rapid depletion of the less alkylated homo-
logues, together with faster degradation of the parent PAH. The second is the
enhancement of the more alkylated homologues at certain degrees of degradation,
not only relative to the less alkylated homologues, but also in absolute terms.