particularly the case of microorganisms involved in the
process of bacterial bioleaching.
Metals are found in nature in the form of ores and therefore trapped as oxides (bauxite), silicates (mica, feldspar),
carbonates (malachite, dolomite), or sulfides (galena, chalcopyrite, pyrite, cinnabar). Many metals of interest are
extracted in the latter form (Table 16.10), and the metal
production from sulfide ores can involve bacterial activity.
These processes use the capacity of microorganisms to oxidize sulfides and metals, especially iron. If the metabolisms
put at stake in both cases are the same, in terms of processes,
bioleaching and bio-oxidation have two distinct objectives:
(i) The bioleaching aims, from a sulfide ore, to leach the
metal of interest (copper or zinc) which is at the end of
the process present in the liquid phase, whereas the solid
residue is a waste.
(ii) The bio-oxidation aims to remove metal sulfides from
an ore rich in gold or silver, for example. In this case, the
metal of interest (gold or silver) is in the solid phase and
the leachate is a waste.
In the case of mining, copper, for example, is found in the
form (among others) of chalcocite (Cu 2 S). It must be
extracted from the ore and then reduced in the form of
zerovalent metal. Biological metabolisms that will allow
the separation of copper sulfide by oxidation of the latter
exist (Fig. 16.45, reactions 1 and 2). However, this process is
much slower than the chemical process (Fig. 16.45, reaction
3) involving an oxidation by ferric iron (Fe
3+ or FeIII). This
led to the production of copper in the soluble ionic form that
can be reduced to metal copper (Fig. 16.45, reaction 4).
Reactions 3 and 4 in Fig. 16.45 produce large amounts of
reduced iron (Fe
2+ ), a by-product of the oxidation–reduction
reactions that can be recycled in order to maintain the sulfide
oxidation process. This upgrading of ferrous iron (Fe
2+ ) is
realized via chemolithotrophic prokaryotic microorganisms
achieving the oxidation of iron (Fig. 16.45, reaction 5).
These microorganisms must be acidophiles since they must
grow under the highly acidic pH of the leachates that are rich
in sulfuric acid originating from the oxidation of sulfides
(Fig. 16.45, reaction 3). The acid leachate containing oxygen
and oxidized iron produced by these microorganisms will be
spread again on the ore to maintain the chemical oxidation of
sulfides and the release of copper. The best known microorganism, commonly cited for its involvement in this process,
is Acidithiobacillus ferrooxidans that can oxidize both iron
and sulfide. Other microorganisms are involved directly or
indirectly in bioleaching processes such as Leptospirillum
ferrooxidans or the thermophilic Acidimicrobium
ferrooxidans. The bioleaching process is accelerated by
microorganisms oxidizing elemental sulfur, a by-product of
the reaction between the sulfide ore (MS) and ferric iron.
MS þ 2Fe
3þ
! M
2þ
þ S
þ 2Fe
2þ
The oxidation of sulfur by specialized sulfur-oxidizing
microorganisms (Acidithiobacillus thiooxidans) prevents the
accumulation of sulfur that can decrease the oxidation of ore
and produces sulfuric acid, which contributes to improve the
bioleaching efficiency. These bioleaching processes can be
controlled during the leaching of ores (in this case they occur
in oxidation lagoons designed for this purpose), but they also
take place naturally when sulfide ores (e.g., mining wastes)
are exposed to air and rainfall. In all cases, the mining
production tends to produce acidic effluents, often
characterized by their color (Fig. 16.46) due to their richness
in metals, and among them iron. A large part of the metals
that are not of interest are not recovered and contaminate
mines catchments. One of the best known examples is the
Rio Tinto in Spain (Leblanc et al. 2000) which is the repository of metal contamination related to copper mining.
Acid mine drainage waters represent a risk for water
resources downstream of the mining areas. Their impact in
terms of metal contamination can be felt until their sea outlet
(Elbaz-Poulichet et al. 2001). Some of the treatments sometimes implemented aim to prevent natural bioleaching of
mining wastes in the presence of oxic water. Wastes (mine
tailings) can be covered in order to isolate them from rainfall
and water runoff. Another solution is to submerse them in a
water column under anoxic conditions. It is also possible to
treat these wastes through long-term and costly processes.
These treatments can include chemical neutralization/precipitation/flocculation (called active methods). Passive
methods, often preferred, are based on the following:
(i) Bacterial reduction and precipitation of metals in wetlands
(ii) Bio-oxidation and precipitation of the obtained leachate
(Roanne et al. 1996; cf. Sect. 16.9.5)
Despite their extensive use in mining, bioleaching
processes have been little used in bioremediation of soils
and sediments. Some successes were obtained for agricultural use, after decontamination, of wastewaters, and sludges
form wastewater treatment plants contaminated with metals
(Couillard and Zhu 1992). Some studies have also been
Table 16.10 Correspondence between some of the well-known
sulfide ores and their common name
Name
Formula
Argentite
Ag 2 S
Arsenopyrite
FeAsS
Calcocyte
Cu 2 S
Chalcopyrite
CuFeS
Covellite
CuS
Galena
PbS
Pyrite
FeS 2
Millerite
NiS
Orpiment
As 2 S 3
Sphalerite
ZnS
16 Applied Microbial Ecology and Bioremediation
741
process of bacterial bioleaching.
Metals are found in nature in the form of ores and therefore trapped as oxides (bauxite), silicates (mica, feldspar),
carbonates (malachite, dolomite), or sulfides (galena, chalcopyrite, pyrite, cinnabar). Many metals of interest are
extracted in the latter form (Table 16.10), and the metal
production from sulfide ores can involve bacterial activity.
These processes use the capacity of microorganisms to oxidize sulfides and metals, especially iron. If the metabolisms
put at stake in both cases are the same, in terms of processes,
bioleaching and bio-oxidation have two distinct objectives:
(i) The bioleaching aims, from a sulfide ore, to leach the
metal of interest (copper or zinc) which is at the end of
the process present in the liquid phase, whereas the solid
residue is a waste.
(ii) The bio-oxidation aims to remove metal sulfides from
an ore rich in gold or silver, for example. In this case, the
metal of interest (gold or silver) is in the solid phase and
the leachate is a waste.
In the case of mining, copper, for example, is found in the
form (among others) of chalcocite (Cu 2 S). It must be
extracted from the ore and then reduced in the form of
zerovalent metal. Biological metabolisms that will allow
the separation of copper sulfide by oxidation of the latter
exist (Fig. 16.45, reactions 1 and 2). However, this process is
much slower than the chemical process (Fig. 16.45, reaction
3) involving an oxidation by ferric iron (Fe
3+ or FeIII). This
led to the production of copper in the soluble ionic form that
can be reduced to metal copper (Fig. 16.45, reaction 4).
Reactions 3 and 4 in Fig. 16.45 produce large amounts of
reduced iron (Fe
2+ ), a by-product of the oxidation–reduction
reactions that can be recycled in order to maintain the sulfide
oxidation process. This upgrading of ferrous iron (Fe
2+ ) is
realized via chemolithotrophic prokaryotic microorganisms
achieving the oxidation of iron (Fig. 16.45, reaction 5).
These microorganisms must be acidophiles since they must
grow under the highly acidic pH of the leachates that are rich
in sulfuric acid originating from the oxidation of sulfides
(Fig. 16.45, reaction 3). The acid leachate containing oxygen
and oxidized iron produced by these microorganisms will be
spread again on the ore to maintain the chemical oxidation of
sulfides and the release of copper. The best known microorganism, commonly cited for its involvement in this process,
is Acidithiobacillus ferrooxidans that can oxidize both iron
and sulfide. Other microorganisms are involved directly or
indirectly in bioleaching processes such as Leptospirillum
ferrooxidans or the thermophilic Acidimicrobium
ferrooxidans. The bioleaching process is accelerated by
microorganisms oxidizing elemental sulfur, a by-product of
the reaction between the sulfide ore (MS) and ferric iron.
MS þ 2Fe
3þ
! M
2þ
þ S
þ 2Fe
2þ
The oxidation of sulfur by specialized sulfur-oxidizing
microorganisms (Acidithiobacillus thiooxidans) prevents the
accumulation of sulfur that can decrease the oxidation of ore
and produces sulfuric acid, which contributes to improve the
bioleaching efficiency. These bioleaching processes can be
controlled during the leaching of ores (in this case they occur
in oxidation lagoons designed for this purpose), but they also
take place naturally when sulfide ores (e.g., mining wastes)
are exposed to air and rainfall. In all cases, the mining
production tends to produce acidic effluents, often
characterized by their color (Fig. 16.46) due to their richness
in metals, and among them iron. A large part of the metals
that are not of interest are not recovered and contaminate
mines catchments. One of the best known examples is the
Rio Tinto in Spain (Leblanc et al. 2000) which is the repository of metal contamination related to copper mining.
Acid mine drainage waters represent a risk for water
resources downstream of the mining areas. Their impact in
terms of metal contamination can be felt until their sea outlet
(Elbaz-Poulichet et al. 2001). Some of the treatments sometimes implemented aim to prevent natural bioleaching of
mining wastes in the presence of oxic water. Wastes (mine
tailings) can be covered in order to isolate them from rainfall
and water runoff. Another solution is to submerse them in a
water column under anoxic conditions. It is also possible to
treat these wastes through long-term and costly processes.
These treatments can include chemical neutralization/precipitation/flocculation (called active methods). Passive
methods, often preferred, are based on the following:
(i) Bacterial reduction and precipitation of metals in wetlands
(ii) Bio-oxidation and precipitation of the obtained leachate
(Roanne et al. 1996; cf. Sect. 16.9.5)
Despite their extensive use in mining, bioleaching
processes have been little used in bioremediation of soils
and sediments. Some successes were obtained for agricultural use, after decontamination, of wastewaters, and sludges
form wastewater treatment plants contaminated with metals
(Couillard and Zhu 1992). Some studies have also been
Table 16.10 Correspondence between some of the well-known
sulfide ores and their common name
Name
Formula
Argentite
Ag 2 S
Arsenopyrite
FeAsS
Calcocyte
Cu 2 S
Chalcopyrite
CuFeS
Covellite
CuS
Galena
PbS
Pyrite
FeS 2
Millerite
NiS
Orpiment
As 2 S 3
Sphalerite
ZnS
16 Applied Microbial Ecology and Bioremediation
741