days). During the 76-day treatment, the different steps specific
to composting succeeded each other: mesophilic (30–40
C)
and thermophilic (40–53
C) phases, cooling, and maturation.
The decrease in PAH concentration that varied according to
STEP origin was in the range 15.8–57.9 %. Experiments of
this kind, conducted by other authors with other STEP
sludges, lead to similar results and sometimes better (70 %
decrease). The PAH elimination is explained by their biodegradation achieved by microorganisms, but a fraction was also
immobilized or sequestered within compost.
The treatment can also be implemented according to the
controlled spreading method (landfarming; cf. Sect. 16.2.2).
In that case, the polluted soil (particularly by gasoil, fuel oil,
oil), eventually enriched with nutrients and microorganisms,
is laid down in thin layers (10 to a few tens cm) over large
surfaces and regularly mechanically reversed.
This process enables the treatment of large soil volumes
(several thousands m
3 ) at relatively low coast. In 1987,
1.22 Â 10
6 tons hydrocarbon per year were treated by
landfarming (Amaral 1987).
16.9 Metals and Metalloids
16.9.1 Introduction
Metals constitute the majority of elements in the periodic
table of elements. With the exception of hydrogen, they
include all the elements on the left of the metalloids (to the
left of a line from boron to polonium). Thus, apart from
noble gases, halogens, nonmetal, and metalloid elements,
all the other elements are metals. Because of this diversity,
metals are classified into different groups such as
posttransition metals (e.g., aluminum and lead), transition
metals (the most numerous including iron, manganese, silver, cobalt), or alkaline earth metals (magnesium, calcium).
Metals are also characterized by their chemical properties,
including their ability to be oxidized (electron loss) in the
form of cationic ions. In addition to their classification is the
name of “heavy metals” which remain very empirical and
variable depending on the authors. There is often a confusion
between the terms “heavy metal”, “toxic metals,” or even
“metallic trace elements.” For example, arsenic, classified as
a metalloid, is often included among the heavy metals
because of its toxicity.
Metals and metalloids are present in soils, sediments, and
at lower concentrations in water. They naturally originate
from the weathering of rocks, but higher concentrations may
arose from anthropogenic origins such as mining, waste
storage and incineration, or industrial activities (smelting
plants). In the case of mining, microorganisms are used for
the recovery of metals of interest through a process called
bioleaching and bio-oxidation. These processes tend to
increase the levels of available metals in the ecosystem,
but can also be used for the removal of metals from soils
or sediments.
The majority of treatment techniques for polluted sites
focused on the removal of organic pollutants. Treatment of
wastes or sites containing metals is considered only recently
(Summers 1992). Unlike for organic compounds, it is not
possible to speak of metal degradation but only of modification of their chemical form and/or their precipitation. Toxicity, availability, and transport of metals in the environment
depend on their speciation, i.e., the chemical form in which
they are found. The microorganisms may be involved in
the speciation of metals and thus affect their fate in an
ecosystem. This change in chemical form may be related to
energetic metabolisms where metals are used as electron
donors or acceptors (cf. Chap. 14). The processes of metal
reduction most often lead to a decrease of their solubility and
toxicity and thus can be applied in the treatment of polluted
sites. The elimination of metals from ecosystems or wastes
can also make use of long-term trapping or immobilization
processes. In this case, both physicochemical and biological
processes are used: adsorption, immobilization with chelating agents, indirect precipitation, and accumulation. Finally,
some resistance mechanisms to metals can also participate to
their removal from a matrix.
16.9.2 Use of Microorganisms in Mining
Activities: Bioleaching and Bio-oxidation
Some microorganisms increase the solubility of metals
through oxidation processes and/or acidification. This is
Box 16.6 (continued)
the presence of plants reduced the risk of pollutant
dispersal. In a seven-year study using poplar, a reduction of PAH concentration was observed after a few
years but was limited to 3-ring PAHs and was limited
within the time, which was attributed to the low dissolution of PAHs from the solid phase (Widdowson et al.
2005). The challenge of such phytoremediation technique is high, since it has a low cost and could be well
adapted to the treatment of large contaminated areas,
which are not compatible with many other remediation
techniques. However, to control the feasibility and
potential of PAH rhizodegradation, it is still necessary
to further study and understand the processes involved
and the interactions of plant–microorganisms–pollutant. Many plants showed a potential for
phytoremediation, but the parameters such as nutrient
conditions, soil conditions, and PAH availability
should be taken into account.
740
J.-C. Bertrand et al.
to composting succeeded each other: mesophilic (30–40
C)
and thermophilic (40–53
C) phases, cooling, and maturation.
The decrease in PAH concentration that varied according to
STEP origin was in the range 15.8–57.9 %. Experiments of
this kind, conducted by other authors with other STEP
sludges, lead to similar results and sometimes better (70 %
decrease). The PAH elimination is explained by their biodegradation achieved by microorganisms, but a fraction was also
immobilized or sequestered within compost.
The treatment can also be implemented according to the
controlled spreading method (landfarming; cf. Sect. 16.2.2).
In that case, the polluted soil (particularly by gasoil, fuel oil,
oil), eventually enriched with nutrients and microorganisms,
is laid down in thin layers (10 to a few tens cm) over large
surfaces and regularly mechanically reversed.
This process enables the treatment of large soil volumes
(several thousands m
3 ) at relatively low coast. In 1987,
1.22 Â 10
6 tons hydrocarbon per year were treated by
landfarming (Amaral 1987).
16.9 Metals and Metalloids
16.9.1 Introduction
Metals constitute the majority of elements in the periodic
table of elements. With the exception of hydrogen, they
include all the elements on the left of the metalloids (to the
left of a line from boron to polonium). Thus, apart from
noble gases, halogens, nonmetal, and metalloid elements,
all the other elements are metals. Because of this diversity,
metals are classified into different groups such as
posttransition metals (e.g., aluminum and lead), transition
metals (the most numerous including iron, manganese, silver, cobalt), or alkaline earth metals (magnesium, calcium).
Metals are also characterized by their chemical properties,
including their ability to be oxidized (electron loss) in the
form of cationic ions. In addition to their classification is the
name of “heavy metals” which remain very empirical and
variable depending on the authors. There is often a confusion
between the terms “heavy metal”, “toxic metals,” or even
“metallic trace elements.” For example, arsenic, classified as
a metalloid, is often included among the heavy metals
because of its toxicity.
Metals and metalloids are present in soils, sediments, and
at lower concentrations in water. They naturally originate
from the weathering of rocks, but higher concentrations may
arose from anthropogenic origins such as mining, waste
storage and incineration, or industrial activities (smelting
plants). In the case of mining, microorganisms are used for
the recovery of metals of interest through a process called
bioleaching and bio-oxidation. These processes tend to
increase the levels of available metals in the ecosystem,
but can also be used for the removal of metals from soils
or sediments.
The majority of treatment techniques for polluted sites
focused on the removal of organic pollutants. Treatment of
wastes or sites containing metals is considered only recently
(Summers 1992). Unlike for organic compounds, it is not
possible to speak of metal degradation but only of modification of their chemical form and/or their precipitation. Toxicity, availability, and transport of metals in the environment
depend on their speciation, i.e., the chemical form in which
they are found. The microorganisms may be involved in
the speciation of metals and thus affect their fate in an
ecosystem. This change in chemical form may be related to
energetic metabolisms where metals are used as electron
donors or acceptors (cf. Chap. 14). The processes of metal
reduction most often lead to a decrease of their solubility and
toxicity and thus can be applied in the treatment of polluted
sites. The elimination of metals from ecosystems or wastes
can also make use of long-term trapping or immobilization
processes. In this case, both physicochemical and biological
processes are used: adsorption, immobilization with chelating agents, indirect precipitation, and accumulation. Finally,
some resistance mechanisms to metals can also participate to
their removal from a matrix.
16.9.2 Use of Microorganisms in Mining
Activities: Bioleaching and Bio-oxidation
Some microorganisms increase the solubility of metals
through oxidation processes and/or acidification. This is
Box 16.6 (continued)
the presence of plants reduced the risk of pollutant
dispersal. In a seven-year study using poplar, a reduction of PAH concentration was observed after a few
years but was limited to 3-ring PAHs and was limited
within the time, which was attributed to the low dissolution of PAHs from the solid phase (Widdowson et al.
2005). The challenge of such phytoremediation technique is high, since it has a low cost and could be well
adapted to the treatment of large contaminated areas,
which are not compatible with many other remediation
techniques. However, to control the feasibility and
potential of PAH rhizodegradation, it is still necessary
to further study and understand the processes involved
and the interactions of plant–microorganisms–pollutant. Many plants showed a potential for
phytoremediation, but the parameters such as nutrient
conditions, soil conditions, and PAH availability
should be taken into account.
740
J.-C. Bertrand et al.