Microbial in-situ mitigation of arsenic contamination in plants and soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 117

also evolved biochemical mechanisms to exploit As oxyanions, either as an electron acceptor
[As(V)] for anaerobic respiration, or as an electron donor [As(III)] to support chemoautotrophic
fixation of carbon dioxide into cell carbon (Santini et al ., 2000; Silver and Phung, 2005; Stolz
and Oremland, 1999; Wang and Zhao, 2009; Zobrist et al ., 2000). A number of methodologies
have been investigated for bioremediating As-contaminated soils.
6.3.1 Biostimulation
Biostimulation include the introduction of adequate amounts of water, nutrients and oxygen into
the soil to enhance the activity of bioremediating indigenous microbial agents (Couto et al ., 2010)
and/or to promote co-metabolism (Lorenzo, 2008). The concept of biostimulation is to boost the
intrinsic production of enzymes participating in bioremediation process of a polluted matrix and
has been used for a wide variety of xenobiotics (Abdulsalam et al ., 2011; Kadian et al ., 2008).
Even though the diversity of natural microbial populations apparently displays the potential
for contaminant remediation at polluted sites, the factors such as lack of electron acceptors or
donors, low nitrogen or phosphorus availability, or a lack of stimulation of the metabolic pathways
are responsible for inhibition or delay of the bioremediation process. In these cases, accumula-
tion of exogenous nutrients can enhance the remediation of the toxic materials (Cosgrove et al .,
2010). Elemental sulfur can be added as an energy substrate and an acid source in aerobic
conditions to stimulate As leaching from soil and sediments by microbes. Seidel et al . (2002)
studied the bioleaching of As from a highly polluted lake sediment (753 mg As kg − 1 ) by indige-
nous Thiobacillus sp. under aerobic and anaerobic conditions. Under aerobic conditions without
adding elemental sulfur, the As solubility ranged between 0.6-3.5 mg kg − 1 . However, stimu-
lating the aerobic bioleaching with elemental sulfur, the total soluble As was increased up to
80% (660 mg kg − 1 ) in the form of As(III) and As(V) species. In the presence of sulfur, soluble
As gets immobilized. Kohler et al . (2001) reported that release of As ions and soluble organo
As compounds from contaminated soil by autochthonic soil bacteria and a mixture of isolated
pure cultures were increased after addition of nitrogen, phosphorus, sodium acetate and ethanol.
Chatain et al . (2005) found an increase of As bioleaching by 28-folds by indigenous anaerobic
bacteria by addition of carbon sources and this may help in mitigating As contamination in plants.
Biostimulation technique has already been extended to the removal of wide array of environ-
mental contaminants. The processes of biostimulation can be successfully applied for remediation
of As-contaminated soil and water.
6.3.2 Bioaugmentation
Addition of previously grown microbial cultures to enhance the remediation of contaminants
is called bioaugmentation. Bioaugmentation, an economical and eco-friendly approach, has
emerged as the most advantageous soil and water clean-up technique for contaminated sites
containing heavy metals and/or organic pollutants (Tyagi et al ., 2011). It is done in conjunction
with the development and monitoring of an ideal growth environment, in which selected microbes
can live and work. Bioaugmentation with microbes plays an important role in biogeochemical
cycling of As and in As detoxification by various processes, such as oxidation-reduction, efflux,
biosorption, bioaccumulation and biomethylation.
6.3.2.1 Microbially mediated As(V) reduction and As(III) oxidation
The understanding of biogeochemical processes is crucial for predicting and protecting envi-
ronmental health and can provide new opportunities for remediation strategies. Energy can be
released and stored by means of redox reactions via the oxidation of labile organic carbon or
inorganic compounds (electron donors) by microorganisms coupled with the reduction of elec-
tron acceptors including humic substances, iron-bearing minerals, transition metals, metalloids,
and actinides. Environmental redox processes play key roles in the formation and dissolution