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

oot extract resulted in continuous linear disappearance of pyrene and
benzo[a]pyrene. They found that linoleic acid is a powerful stimulant of
pyrene and benzo[a]pyrene degradation by gram positive bacteria. They
hypothesized that (1) linoleic acid increased numbers of degrading
bacteria, or (2) linoleic acid acted as a surfactant to increase the
bioavailability of the PAH, or (3) linoleic acid formed a coating on soil
particles and increased the attachment of bacteria to hydrophobic sites,
which enhanced their proximity to PAH compounds.
Bioaugmentation
Bioaugmentation is the introduction (or inoculation) of a specific competent
microorganism or group of microorganisms to improve the metabolic capacity of
the indigenous population of microbes (Gentry et al., 2004). For example,
Mycobacterium sp. are known to have extremely lipophilic surfaces so they are
better equipped to directly take up organic hydrophobic contaminants such as
PAHs (Rehman et al., 1998; Bogan et al., 2003). Bioaugmentation can be
especially useful in sites with high PAH concentrations, in recently polluted soils
with limited adapted microbial populations, or in aged soils/sediments where the
PAH profile is dominated by HMW PAHs (Mueller et al., 1989).
Successful field-scale applications of bioaugmentation are limited (Alexander,
1999). Bioaugmentation is still experimental with most successful cases reported
in confined systems where conditions are controlled to favor the growth of added
microbes. There are several reports of the usefulness of bioaugmentation in
enhancing bioremediation of contaminated soils (Lendvay et al., 2003; Silva et
al., 2004), and others reporting that the procedure has failed to improve
biodegradation (Bouchez et al., 2000). Some lab-scale or demonstration-scale
experiments and summaries are presented below:
� Pritchard et al. (2002) found that introducing Sphingomonas paucinobilis
EPA 505 along with nutrients and biosurfactants in a lab microcosm
containing Superfund soils did not improve PAH degradation. They
suggested that the addition of microorganisms to improve biodegradation
in such highly contaminated soils might be limited by the microorganisms’
sensitivity to toxic components and competition with indigenous
community.
� Microbes introduced to a field setting in liquid cultures have poor survival
due to predation and competition (O’Reilly and Crawford, 1989). For
successful bioaugmentation, the added microorganism must fill a nonutilized
metabolic niche in the microbial community and be protected by
encapsulation or addition in a biofilm (Watanabe et al., 2002; El Fantroussi
and Agathos, 2005). Inert materials such as vermiculite can be used as
carriers to extend the survival of microbial cultures (Weir et al., 1995).
Other methods to assist microbial introduction include the addition of preadapted
strains or consortia of degraders, the addition of genetically
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