Geoscience Reference
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characteristics e.g., siderophores, IAA, and ACC deaminase, could potentially support plant
growth in As- contaminated areas and reduce stress symptoms/toxicity (Zaidi
et al
., 2006).
6.5
STRATEGIES FOR BIOREMEDIATION OF ARSENIC
A successful bioremediation program usually requires the application of strategies customized
for the specific environmental conditions of the contaminated sites. Further, bioremediation
utilizes the metabolic potential of microorganisms to clean up contaminated environments. Biore-
mediation is carried out in non-sterile open environments that contain a variety of organisms that
affect the process. Microbial and ecological information are useful for the development of strate-
gies to improve bioremediation and for evaluating its consequences. Strategies involving the
addition of seeded cultures (bioaugmentation) or the addition of nutrients (biostimulation), hold
the promise of fostering bioremediation/detoxification degradation rates (Atlas, 1995; Jimenez
et al
., 2006). The impending gap between laboratory trials and on-field studies are due to several
factors influencing the remediation process, like strain selection, indigenous microbial ecology,
environmental constraints and the procedures used for
in-situ
remediation. The fate and transport
of As in soil and groundwater also depends on the chemical form and speciation of As. Exam-
ples of bacteria and fungi, which have been investigated for environmental remediation of As
contamination, are shown in
Tables 6.2
and
6.3
.
6.5.1
Screening and selection of suitable microbes
Feasibility studies are the pre-requisite for any planned bioremediation intervention that includes
screening followed by tailoring of a microbial formulation for a particular site. The initial
screening/selection step is based on microbial metabolism, which is functionally active and
persistent under the desired environmental conditions. The best approach for selecting com-
petent microbes should be based on the prior knowledge of the microbial communities inhabiting
the target site (Thomson
et al
., 2005; Vander Gast
et al
., 2004). From the applied prospec-
tive, use of consortia of pure cultures for the bioremediation is more advantageous which could
have metabolic robustness in the field application (Ledin, 2000). Corsini
et al
. (2011) isolated
As-resistant bacteria (
Bacillus cereus, Pseudomonas azotoformans
and
Phodococcus erythro-
polis
) from citrate-amended soil, which were able to reduce 2 mmol As L
−
1
in liquid culture.
In a separate study, five bacterial isolates transformed arsenate to arsenite and volatile methyl
arsines. These strains belonged to the
Proteus, Escherichia, Flavobaterium, Corynebacterium
and
Pseudomonas
genera (Ordonez
et al
., 2005; Shariatpanahi
et al
., 1981). As given in Section 3.2.4.,
strain
R. palustris
was capable of forming of a number of methylated intermediates of As(III),
with trimethyl arsine as the end product (Qin
et al
., 2006). Many fungi have been found capa-
ble of As accumulation and/or volatilization such as
Scopulariopsis brevicaulis
(Gosio, 1892),
Phalolus schweinitzii
(Pearce, 1998),
Fusarium oxysporum
(Granchinho
et al
., 2002),
Sinorhi-
zobium melitoti
(300 mg L
−
1
As) (Carrasco
et al
., 2005). As uptake in
Aspergillus candidus
were
measured as 11.17, 4.09, and 8.00 mg g
−
1
on day 3, 6 and 9, respectively, when exposed to initial
50 mg L
−
1
arsenate (Vala, 2010). The mean percent removal as flux of biovolatilized As ranged
from 3.71-29.86% in
Trichoderma
sp.,
Necosmospora
sp.,
and Rhizopus
sp. can be effectively
used for the bioremediation of As contaminated agricultural soils (Srivastava
et al
., 2011). Su
et al
. (2010a) have found three fungal strains
Trichoderma asperellum
SM-12F1,
Penicillium
janthinellum
SM-12F4, and
Fusarium oxysporum
CZ-8F1 that are highly capable of As accumu-
lation and volatilization.
Trichoderma asperellum
SM-12F1,
Penicillium janthinellum
SM-12F4,
and
Fusarium oxysporum
CZ-8F1 were exposed to 50 mg L
−
1
of As(V), and the biotransforma-
tion of As and the concomitant variance of Eh and pH of media was studied after cultivation
for 2 or 3 days. The arsenate added to the media had been completely changed into arsenite,
whilst arsenate was predominate in fungal cells with concomitantly little arsenite during culti-
vation. After 15 days, the total As (t-As) content was the highest (as 41.5
µ
gmg
−
1
) in cells of
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