Environmental Engineering Reference
In-Depth Information
Phytoremediation
Wetlands serve as distinct ecological areas with
high biodiversity and productivity and which
provide protection agencies against shoreline
erosion (Mitsch and Gosselink
1986
). Marshy
vegetation is easily damaged by fresh light oils
and oils tend to coat prop roots of mangroves
which are essential for their respiration. Man-
groves require decades to grow hence, once dam-
aged they cannot be easily replaced. In light of
the above discussion, it is of utmost importance
to save wetlands from detrimental effects of oil
spills (EPA
1999b
). Phytoremediation is one of
the bioremediation tools that can be utilized for
this purpose. It is defined as the employment of
plants and/or associated microbes to eliminate,
contain, or render harmless environmental pol-
lutants
in situ
. This strategy is environmentally
friendly, cost effective, and is proved to be effec-
tive for heavy metals, radionuclides, and organic
pollutants (Cunningham and Ow
1996
; Cunning-
ham et al.
1996
; Dzantor et al.
2000
; Njoku and
Oboh
2009
). However, in the context of oil spill
cleanup, phytoremediation is best suited for the
remediation of oil-contaminated marshes and
shorelines and not for open systems such as seas.
Plants through their roots oxygenate their rhizo-
sphere and exude organic compounds, which ul-
timately stimulate the activity, density, and diver-
sity of microbes in the rhizosphere. Plants have
also been documented to initiate fungal degra-
dation of PAHs through rhizosphere effect (Mc-
Genity et al.
2012
). Studies on salt marsh grass,
Sparta patens
indicate its potential to phytoreme-
diate oil spills.
S. Patens
could survive 320 mg
oil/g dry sediment and, at oil doses between 40
and 160 mg/g, the oil degradation was found to
be significantly higher than in control samples
(Lin and Mendelssohn
2008
). Rhizosphere-asso-
ciated bacteria of mangroves have been studied
and are found to promote plant growth as well as
oil degradation (do Carmo et al.
2011
).
Glycine
max
(soyabean) was reported to grow in oil-con-
taminated soil and also enhance the degradation
of crude oil. It was reported to reduce the oil tox-
icity as observed by the growth of weeds in soils
supplemented by
G.max
and their absence where
species utilize varied terminal electron acceptors
such nitrate, sulfate, or Fe (III) in place of oxygen
(Peixoto et al.
2011
). Phototroph-heterotroph in-
teractions are also significant in the context of
degradation of petroleum fractions. Many algae
produce hydrocarbons and nearly all produce
volatile hydrocarbons, isoprene which may be
essential in the sustenance of hydrocarbon de-
graders in absence of oil spill (Shaw et al.
2010
;
McGenity et al.
2012
), thus it is unsurprising that
Alcanivorax
spp. are often associated with micro-
and macroalgae (Green et al.
2004
; Radwan et al.
2010
). Moreover, PAHs tend to strongly adsorb
to the cell surface of marine microalgae encour-
aging the growth of associated microbes (Binark
et al.
2000
). Algae produce O
2
and encourage
the growth of hydrocarbon degraders, which in
turn produce CO
2
and reciprocally encourage
the growth of algae. Algal biosurfactants also
contribute to the emulsification of hydrocarbons
(Cohen
2002
).
Successful bioaugmentation requires the seed-
ing of microbes best suited to degrade the spilled
oil. Choice of the microbes is entirely based on
the type of oil, type of primary oil response un-
dertaken, and characteristics of the area under the
oil spill. Most of the organisms used for bioaug-
mentation are obtained from enriched cultures
from previously contaminated sites or similar
strains enriched in laboratories. Bioaugmenta-
tion has not been very effective in cleaning up
oil spills. Some of the reasons for this failure
are: poor survival or low activity of laboratory
strains because of sudden exposure to environ-
mental stress, absence of mutualistic interspe-
cies interaction that improves bioavailability and
biodegradation, biomass-limiting nutrients (N
and P), and predation by protozoa. Moreover, in
bioremediation strategies, the focus is on biodeg-
radation strains and use of a single species for
this purpose. Use of microbial consortium with
complementary catabolic pathways and the abil-
ity to adapt to local environment, disperse, and
increase the bioavailability of the pollutants has
been proved to be more successful in the biore-
mediation of simulated oil spills (Gallego et al.
2007
; Jacques et al.
2008
).
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