Paul et.al.
et al. , 2007). RO membrane contains extremely small
resistant strains also utilizes As in their metabolism,
pores (< 0.001 μm) (Schneiter 1983) and a very high
either as a means of generating energy through
(often close to 100%) rejection of low-molecular
chemoautotrophic arsenite oxidation (Santini et
mass compounds and ions can be achieved. With
al. , 2000) or using arsenate as a terminal electron
the invent of cellulose acetate RO membrane in the
acceptor in an aerobic respiration (Ahmann et al. ,
1980’s, arsenate removal efficiency of above 90%
1994; Stolz et al. , 1999). Moreover microbes are also
have been achieved with the RO system operated at
capable of transforming inorganic As compounds by
high-pressure around 400 psi (Fox 1989). However,
biomethylationintomonomethylarsonicacid(MMA),
arsenite removal efficiency is less than 70%.
dimethylarsenic acid (DMA), trimethylarsine (TMA)
Despite the development of numerous conventional
and trimethylarsine-oxide (TMAO) (Ridley et al. ,
methods, the application of these techniques is
1977; Woolson 1977; Cullen and Reimer 1989; Gadd
not very economical and requires very high tech
1993). Methylation of inorganic arsenic has actually
operation and maintenance. Many of these processes
been described as the most important detoxification
remove As partially and the byproducts resulted
process in the human body since it reduces the
from these physiochemical processes can be harmful
affinity of the compound for tissue (Vahter and
and difficult to remove from soil and water.
Marafante 1988). The three main approaches used in
bioremediation of As are as follows.
Bioremediation of As
Bioaccumulation
Bioremediation is defined as degradation of
environmental pollutant to less toxic form using
Microorganismsexhibitastrongabilitytoaccumulate
living organisms like bacteria, fungi or plant.
(bioaccumulation)Asfromasubstratecontainingvery
low concentrations of this element. Bioaccumulation
Microbial Bioremediation of As
is activated by two processes, namely biosorption
of As by microbial biomass and its byproducts and
Currently bioremediation of heavy metals using
physiological uptake of As by microorganisms
microorganisms has received immense interest for
through metabolically active and passive processes.
its potential, cost effectiveness, and environmental
Factors such as soil pH, moisture and aeration,
friendly way for heavy-metal removal (Valls
temperature, concentration and speciation of As,
and Lorenzo 2002, Rakshit and Ghosh 2009).
soil amendments, and rhizosphere are known to
Microorganisms control the environmental fate of
influence the process of bioaccumulation of As in
As through various mechanisms resulting changes
microbial cells (Mahimairaja et al. , 2005). Scientists
in solubility and/or toxicity of different As species
have isolated a non-genetically engineered potent
(Ghodsi 2011, Rakshit et al. , 2009). Researchers
As accumulating bacteria Marinomonas communis
have successfully isolated and characterized As
from marine and non marine environment in Japan
resistant bacteria from different environmental
with high removal efficiency (Takeuchi et al. , 2007).
samples and concluded that they have evolved
Furthermore, Ghodsi et al. , (2011) concluded that
several mechanisms to tolerate the uptake of heavy
the cellular accumulation of As could be a result
metal such as As. Microorganisms have evolved
of higher uptake and lower efflux by a regulatory
several strategies for dealing with the toxicity of As
protein of the ars operon that has a specific binding
which could be utilized as attractive approaches for
site available for arsenite (Ghodsi et al. , 2011). In
removal of As from contaminated sites. It mainly
addition to bioaccumulation of As by bacterial and
involves efflux of heavy metal ions outside the cell,
fungal species, some algal species (Fucus gardneri
accumulation and complexation of the metal ions
and Chlorella vulgaris) are also known to accumulate
inside the cell and oxidation-reduction of the heavy
(Granchinho et al. , 2001; Maeda et al. , 1985).
metal ions to a less toxic state (Nies 1999). Arsenic-
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