Other bacteria that oxidize As-reduced compounds (Bacillus arsenoxydans,
Alcaligenes faecalis) cannot increase chemolithoautotroph.
Therefore, the discovery of this strain of P. arsenitoxidans represents a breakthrough in understanding the interactions between MOs and As compounds, so wellknown for their strong antibiotic action (Sun et al. 2009).
Cr is present in both living organisms and in rocks, waters, and soils. There is
only nature in the nature of compounds, not in elementary form. The most common
forms of Cr in nature are the bivalent (Cr
2+ ), trivalent (Cr
3+ ), and hexavalent (Cr
6+ )
compounds. For the manufacture of steel, it is used Cr
0 . From the activities of the
metallurgical industry results Cr
2+ and Cr
6+ , while Cr
2+ occurs naturally in the
environment. From the point of view of the negative impact of Cr compounds on
the health of the environment, the greatest interest is represented by the compounds
with Cr
6+ , most commonly encountered in contaminated sites. Cr
6+ can be reduced
to Cr
3+ , by OM and the ions S
2À and Fe
2+ , under anaerobic conditions, frequently
encountered in groundwater, respectively, in flooded soils. In the presence of
chromates (CrO 4
2À ) and dichromates (Cr 2 O 7
2À ), metallic cations, such as Pb
cations, precipitate. In the presence of chromates and dichromates, Fe and Al oxides
are adsorbed by the soil particles. Toxicity and mobility of Cr depend on soil
characteristics and the amount of OM incorporated by it. Cr hexavalent is more
toxic and more mobile than all other forms.
Trivalent Cr is also mobile, but its mobility decreases with adsorption by clay
minerals and a decrease in pH below 5. The increase in pH stimulates the leaching,
solubilization of hexavalent Cr compounds. Normally, when Cr is discharged into
natural waters, it accumulates in sediments, which can be subjected to bioremediation procedures.
The effects of minerals present in the underground soil on the efficiency of Pb
removal from groundwater using biofilms composed of reducing sulfate MOs were
evaluated and examined the stability of metal deposits after biofilms were temporarily exposed to air. To quantify the effects, Pb was immobilized in Desulfovibrio
desulfuricans biofilms, grown under anaerobic conditions in two bioreactors filled
with one hematite (redox-active) and the other with quartz (redox-inert). Biofilms
grown on hematite were denser, thicker, and more porous than those grown on
quartz. The average H 2 S concentrations were higher in quartz biofilm than in
hematite. Pb was more efficiently immobilized in quartz biofilm than in hematite
(Shahid et al. 2017).
During the reported experiment, H 2 S was produced under the action of
desulfurizing bacteria, which reacted with Pb present to form precipitating PbS. It
has been shown that Pb precipitates more in the presence of biofilm located near
redox-inerts (quartz).
Pb deposits were partially reoxidated, especially in biofilms grown on hematite.
In both bioreactors, biofilms responded to the presence of O 2 by lowering their
density and by increasing the production rate of H 2 S. Although the reduction of Fe
3+
to Fe
2+ was not quantified, it was found that Fe was continuously released from
hematite throughout the experiment.
100
M. Butu et al.
Alcaligenes faecalis) cannot increase chemolithoautotroph.
Therefore, the discovery of this strain of P. arsenitoxidans represents a breakthrough in understanding the interactions between MOs and As compounds, so wellknown for their strong antibiotic action (Sun et al. 2009).
Cr is present in both living organisms and in rocks, waters, and soils. There is
only nature in the nature of compounds, not in elementary form. The most common
forms of Cr in nature are the bivalent (Cr
2+ ), trivalent (Cr
3+ ), and hexavalent (Cr
6+ )
compounds. For the manufacture of steel, it is used Cr
0 . From the activities of the
metallurgical industry results Cr
2+ and Cr
6+ , while Cr
2+ occurs naturally in the
environment. From the point of view of the negative impact of Cr compounds on
the health of the environment, the greatest interest is represented by the compounds
with Cr
6+ , most commonly encountered in contaminated sites. Cr
6+ can be reduced
to Cr
3+ , by OM and the ions S
2À and Fe
2+ , under anaerobic conditions, frequently
encountered in groundwater, respectively, in flooded soils. In the presence of
chromates (CrO 4
2À ) and dichromates (Cr 2 O 7
2À ), metallic cations, such as Pb
cations, precipitate. In the presence of chromates and dichromates, Fe and Al oxides
are adsorbed by the soil particles. Toxicity and mobility of Cr depend on soil
characteristics and the amount of OM incorporated by it. Cr hexavalent is more
toxic and more mobile than all other forms.
Trivalent Cr is also mobile, but its mobility decreases with adsorption by clay
minerals and a decrease in pH below 5. The increase in pH stimulates the leaching,
solubilization of hexavalent Cr compounds. Normally, when Cr is discharged into
natural waters, it accumulates in sediments, which can be subjected to bioremediation procedures.
The effects of minerals present in the underground soil on the efficiency of Pb
removal from groundwater using biofilms composed of reducing sulfate MOs were
evaluated and examined the stability of metal deposits after biofilms were temporarily exposed to air. To quantify the effects, Pb was immobilized in Desulfovibrio
desulfuricans biofilms, grown under anaerobic conditions in two bioreactors filled
with one hematite (redox-active) and the other with quartz (redox-inert). Biofilms
grown on hematite were denser, thicker, and more porous than those grown on
quartz. The average H 2 S concentrations were higher in quartz biofilm than in
hematite. Pb was more efficiently immobilized in quartz biofilm than in hematite
(Shahid et al. 2017).
During the reported experiment, H 2 S was produced under the action of
desulfurizing bacteria, which reacted with Pb present to form precipitating PbS. It
has been shown that Pb precipitates more in the presence of biofilm located near
redox-inerts (quartz).
Pb deposits were partially reoxidated, especially in biofilms grown on hematite.
In both bioreactors, biofilms responded to the presence of O 2 by lowering their
density and by increasing the production rate of H 2 S. Although the reduction of Fe
3+
to Fe
2+ was not quantified, it was found that Fe was continuously released from
hematite throughout the experiment.
100
M. Butu et al.