Box 16.6: Phytoremediation of Polycyclic Aromatic
Hydrocarbons (PAHs): From Laboratory to Field Trials
Corinne Leyval
PAH Origins and Properties
PAHs are major pollutants of the environment. They
are formed by incomplete combustion of organic matter and can be found, for example, during forest fires.
However, the high PAH concentrations in soils are
mainly due to industrial activities (coking plant, gas
plant, and wood treatment). PAHs are a group of
compounds with 2–6 aromatic rings. PAH properties
depend on their molecular weight, but all of them are
characterized by a low solubility and high
hydrophobicity; they are highly toxic, especially the
ones with high molecular weight, that are mutagenic
and carcinogenic. In contaminated soils, PAHs can
reach high concentrations up to one to tens of gram
per kilo (Leyval 2005).
PAH Biodegradation
Contrary to metallic pollutants that cannot be
eliminated and can only be transferred from a compartment to another, PAHs can be transformed and
biodegraded. The first bacteria able to degrade PAHs
were described fifty years ago. However, if the ability
to degrade low molecular weight PAHs, from 2 to 4
cycles, has been described for a large number of bacterial species, the biodegradation of 5 to 6 cycles
PAHs is far less documented and seems to imply
cometabolism processes. The metabolic pathways
and the genes involved in the biodegradation are well
described in the literature. The quantification of genes
and their expression in situ is currently developing
using quantitative PCR or microarrays to estimate the
potential of soils to biodegrade these pollutants. Some
remediation techniques for PAH-contaminated soils
use microbial degradation activity, either stimulated
by the addition of nutrients or oxygen (biostimulation)
or improved by addition of microorganisms
(bioaugmentation). Such techniques, for example,
biopiles, landfarming, and bioreactors, are efficient
and can reduce the concentration of PAHs from
thousands to hundreds of milligrams per kg within a
few months. However, the low molecular weight
PAHs are mostly degraded.
C. Leyval (*)
Laboratoire Interdisciplinaire des Environnements
Continentaux (LIEC, UMR 7360), Universite ´ de Lorraine,
CNRS, Vandoeuvre-les-Nancy, France
(continued)
Box 16.6 (continued)
Biodegradation of PAHs in Plant Rhizosphere
The rhizosphere is the zone of soil influenced by the
presence of plants, especially root exudates that promote microbial activity. It is a zone of intense microbial activity, where microorganism density and
activity is higher than in bare soil and microbial diversity is also different. It has been shown that the presence of plants increase PAH degradation in plant
rhizosphere. Such activity has been mainly attributed
to the stimulation of microbial degradation activity,
since the plants take up little or no PAHs (Binet et al.
2000). This is called rhizodegradation.
However, the results published in the literature
show variable results depending on the experiments:
in some of them, the biodegradation is significantly
increased in presence of plants, and in others there is
no or little effect of plants. Such differences in the
results can be due to the use of different plants, different PAHs, different experimental conditions in terms
of duration, and nutrient conditions. Many studies
were performed with soil and simplified substrates
such as sand artificially spiked with one or many
PAHs. In order to better understand the dissipation of
PAHs in the rhizosphere, experiments using columns
or compartment devices were performed. They clearly
showed a gradient in PAH concentration that
decreases when the distance to roots increases. Such
decrease in PAH concentration was associated with
changes in microbial community structure. The biodegradation also differed with the type of PAH, for
example, phenanthrene, a 3-ring PAH, is more rapidly
degraded than 5- or 6-ring PAHs, such as benzo(a)
pyrene. The artificial spiking of soils or substrates with
PAHs allows to have non-contaminated controls; however, the bioavailability of pollutants is much higher
than in historically contaminated industrial soils,
where the contamination is often complex. The socalled aging phenomena have a strong influence on
PAH degradation in soil and in rhizosphere.
PAH-degradation phenomena in the rhizosphere
are complex, since microorganism and plant are
involved. In laboratory conditions, it was shown that
PAH biodegradation was increased in presence of
plants inoculated with PAH-degrading bacteria such
as Pseudomonas or Mycobacterium species (Ho et al.
2007). The selection of plants for phytoremediation
can therefore be focused on their ability to select or
promote the bacterial community harboring PAHdegradation genes. The release of root exudates
could favor cometabolism processes and promote the
(continued)
738
J.-C. Bertrand et al.
Hydrocarbons (PAHs): From Laboratory to Field Trials
Corinne Leyval
PAH Origins and Properties
PAHs are major pollutants of the environment. They
are formed by incomplete combustion of organic matter and can be found, for example, during forest fires.
However, the high PAH concentrations in soils are
mainly due to industrial activities (coking plant, gas
plant, and wood treatment). PAHs are a group of
compounds with 2–6 aromatic rings. PAH properties
depend on their molecular weight, but all of them are
characterized by a low solubility and high
hydrophobicity; they are highly toxic, especially the
ones with high molecular weight, that are mutagenic
and carcinogenic. In contaminated soils, PAHs can
reach high concentrations up to one to tens of gram
per kilo (Leyval 2005).
PAH Biodegradation
Contrary to metallic pollutants that cannot be
eliminated and can only be transferred from a compartment to another, PAHs can be transformed and
biodegraded. The first bacteria able to degrade PAHs
were described fifty years ago. However, if the ability
to degrade low molecular weight PAHs, from 2 to 4
cycles, has been described for a large number of bacterial species, the biodegradation of 5 to 6 cycles
PAHs is far less documented and seems to imply
cometabolism processes. The metabolic pathways
and the genes involved in the biodegradation are well
described in the literature. The quantification of genes
and their expression in situ is currently developing
using quantitative PCR or microarrays to estimate the
potential of soils to biodegrade these pollutants. Some
remediation techniques for PAH-contaminated soils
use microbial degradation activity, either stimulated
by the addition of nutrients or oxygen (biostimulation)
or improved by addition of microorganisms
(bioaugmentation). Such techniques, for example,
biopiles, landfarming, and bioreactors, are efficient
and can reduce the concentration of PAHs from
thousands to hundreds of milligrams per kg within a
few months. However, the low molecular weight
PAHs are mostly degraded.
C. Leyval (*)
Laboratoire Interdisciplinaire des Environnements
Continentaux (LIEC, UMR 7360), Universite ´ de Lorraine,
CNRS, Vandoeuvre-les-Nancy, France
(continued)
Box 16.6 (continued)
Biodegradation of PAHs in Plant Rhizosphere
The rhizosphere is the zone of soil influenced by the
presence of plants, especially root exudates that promote microbial activity. It is a zone of intense microbial activity, where microorganism density and
activity is higher than in bare soil and microbial diversity is also different. It has been shown that the presence of plants increase PAH degradation in plant
rhizosphere. Such activity has been mainly attributed
to the stimulation of microbial degradation activity,
since the plants take up little or no PAHs (Binet et al.
2000). This is called rhizodegradation.
However, the results published in the literature
show variable results depending on the experiments:
in some of them, the biodegradation is significantly
increased in presence of plants, and in others there is
no or little effect of plants. Such differences in the
results can be due to the use of different plants, different PAHs, different experimental conditions in terms
of duration, and nutrient conditions. Many studies
were performed with soil and simplified substrates
such as sand artificially spiked with one or many
PAHs. In order to better understand the dissipation of
PAHs in the rhizosphere, experiments using columns
or compartment devices were performed. They clearly
showed a gradient in PAH concentration that
decreases when the distance to roots increases. Such
decrease in PAH concentration was associated with
changes in microbial community structure. The biodegradation also differed with the type of PAH, for
example, phenanthrene, a 3-ring PAH, is more rapidly
degraded than 5- or 6-ring PAHs, such as benzo(a)
pyrene. The artificial spiking of soils or substrates with
PAHs allows to have non-contaminated controls; however, the bioavailability of pollutants is much higher
than in historically contaminated industrial soils,
where the contamination is often complex. The socalled aging phenomena have a strong influence on
PAH degradation in soil and in rhizosphere.
PAH-degradation phenomena in the rhizosphere
are complex, since microorganism and plant are
involved. In laboratory conditions, it was shown that
PAH biodegradation was increased in presence of
plants inoculated with PAH-degrading bacteria such
as Pseudomonas or Mycobacterium species (Ho et al.
2007). The selection of plants for phytoremediation
can therefore be focused on their ability to select or
promote the bacterial community harboring PAHdegradation genes. The release of root exudates
could favor cometabolism processes and promote the
(continued)
738
J.-C. Bertrand et al.