Introduction remediation technique. It uses plants that extract

Introduction

Beside the
natural activities, almost all human activities also have potential
contribution to Arsenic contamination in the environment  as side effects occurs in many parts of the
world and is a global problem. In many areas As level has crossed the safe
threshold level. Large-scale groundwater pollution by geogenic arsenic (As) in
West-Bengal and Bangladesh has recently promoted this element into an
environmental pollutant of prime concern. Epidemiological studies have documented
various adverse effects on the human population. Arsenic contaminated soils,
sediments, and sludge are the major sources of arsenic contamination of the
food chain, surface water, groundwater, and drinking water (WT Frankenberger & Arshad, 2002). Other potential
sources of arsenic contamination are the chemicals used extensively in
agriculture as pesticides, insecticides, defoliants, wood preservatives, and
soil sterilants (AZCUE & NRIAGU, 1994). Currently available
techniques for the remediation of As contaminated soil are very expensive and
time-consuming, often hazardous to workers, and capable of producing secondary
wastes (LOMBI, ZHAO, DUNHAM, &
MCGRATH, 2000).
Phytoextraction, the use of green plants to clean up contaminated soil, has
attracted attention as an environmentally- friendly, low-input remediation
technique. It uses plants that extract heavy metals from the soil and
accumulate it in the harvestable, above ground biomass.

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Several
comprehensive reviews on phytoremediation and phytoextraction have been
recently published. In the present review, phytoextraction of arsenic from
contaminated soils by the recently discovered arsenic hyperaccumulator ferns,
with emphasis on the recent studies developed in order to understand and
enhance the arsenic removal process.

 

ARSENIC

 

In the
environment As originates from both geochemical and anthropic sources. Concentration
of As normally varies from below 10 mg kg-1 in non-contaminated soils to as
high as 30,000 mg kg-1 in contaminated soils (ADRIANO, 1986). Through the
production or the use of arsenical pesticides (fungicides, herbicides, and
insecticides) human activities have caused an accumulation of arsenic in soils.
Because of manufacture of As-based compounds, smelting of arsenic-containing ores,
and combustion of fossil fuels, As contamination is affecting soils, water, and
atmosphere and has been identified as a major toxic contaminant in many
countries (AZCUE & NRIAGU, 1994). The form and
speciation of arsenic has great influence on bioavailability, toxicity, and
chemical behavior of arsenic compounds. Normally As can exit essentially in
four oxidation states: (-3), (0), (+3), and (+5). Arsenate (As(+5)) and
arsenite (As(+3)) are the main forms present in soils (HARPER &
HASWELL, 1988).

 

Remediation of arsenic contaminated soils

There are many remediation
techniques are available to address contamination problems but high capital expenditure, unsuitability for large
areas, and environmental disruption are some of the disadvantages of those
techniques, which differ from cost intensity and timeframe. No single
soil remediation technique is unique for all situations. Contaminated site
characteristics should be investigated carefully, contaminant problem,
treatment options, and treatment timeframe must be considered.

Some selected current remediation technologies for
arsenic-contaminated soil, adapted from USEPA (2002), follow:

a)
Excavation – Commonly used ex-situ remediation
method that involves the physical removal and disposal of contaminated soil in
designated landfill. Even though it produces rapid remediation results, excavation
is often expensive because of the operation, transport, and special landfill
requirements (USEPA, 2002).

b)
Capping – In-situ method.
A hard cover is placed on the surface of the contaminated soil. Capping is also
a rather simple method that reduces the contaminant exposure. However, it does
not remove contaminants from the soil (USEPA, 2002).

c)
Solidification and stabilization – In-situ method
where the contaminated soil is mixed with stabilizers reducing the mobility of
arsenic in soil. The drawbacks to these remediation techniques are that they
can be relatively costly (USEPA, 2002).

d) Vitrification – In-situ method where arsenic is
chemically bonded inside a glass matrix forming silicoarsenates.

e) Soil washing/Acid extraction – Ex-situ treatment based on the suspension
or dissolution of arsenic in a water-based wash solution to concentrate the
contaminant.

f) Soil
flushing: In-situ method that uses water, chemicals or
organics to mobilize arsenic and flush it from the soil.

g) Phytoremediation/phytoextraction: In-situ method
using plants to up-take arsenic from soil.

PHYTOREMEDIATION Phytoremediation includes any remediation process which utilizes plants
to either remove pollutants or render them harmless in soil and water systems,
it can be applied for both organic and inorganic pollutants present in soil,
water, and air (SALT, SMITH, & RASKIN, 1998).

The term phytoremediation includes several strategies:

Phytoextraction: Phytoextraction is a sub-process of phytoremediation in which plants
remove dangerous elements or compounds from soil or water, most usually heavy
metals, metals that have a high density and may be toxic to organisms even at
relatively low concentrations.

Phytostabilization: Phytostabilization involves the reduction of the mobility of heavy metals
in soil. Immobilization of metals can be accomplished by decreasing wind-blown
dust, minimizing soil erosion, and reducing contaminant solubility or
bioavailability to the food chain.

 

 

 

 

 

 

 

 

 

 

 

 

                  Fig: heavy metals uptake by
plant through phytoremediation

Phytoimmoblization: Phytoimmoblization is the use of plants
to reduce the bioavailability and mobility of pollutants by altering soil
factors that lower pollutant mobility by formation of precipitates and insoluble
compounds, as well as by sorption on roots.

Phytovolatilization:  A form of phytoremediation in which substances from the
soil are released into the air, sometimes after being broken down into volatile
components.

Rhizofiltration:
plants get rid of contaminants present in solution surrounding the root zone by
adsorption or precipitation onto their roots or absorption of contaminants into
their roots from the solution. This technique is used to clean contaminated water
such as groundwater or a waste stream.

 

Hypothesis:

 

Hyper-accumulators
plants can take up and concentrate in excess of 0.1% of a given element in
their tissue (Brooks, 1998). In higher plants
metal hyper-accumulation is a very rare phenomenon. Till now, only 400 plant
species have been identified as metal hyper-accumulators, representing

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