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Arsenic
Threat and its Remediation Through Plants:
A Step For
Environmental Clean Up
By: Deepika
Sharma* and Sanjay Dwivedi**
Introduction
Geochemical
weathering and anthropo-genic activities contaminate the natural and ground
water with metals and metalloids, which poses a serious environmental
and health hazards for the 21st century.
Industrial
revolution has been a symbol of development throughout the world and has
led to the emergence of enormous number of facilities and commodities.
During
operation, Industries produce potentially toxic and hazardous wastes of
pesticides, dyes and pigments, organic chemicals, fertilizers, non ferrous
metals. This is creating a high risk in all compartment of the environment.
These waste and effluent of industries contain high amount of toxic metals and
metalloids.
Arsenic (As)
is considered one of the most important toxic elements. These days, it
has become a global concern owing to the ever-increasing contamination of
water, soil and crops in many region of the world. As is released into
environment by human activities as well as naturally from the earth crust,
through ground water irrigation. The international agency for research on
cancer (IARC) places inorganic As under highest health hazard category
i.e. a group carcinogen, and there is substantial evidence that it
increases risk of cancer of the bladder, lung, skin and prostate.
The permissible
limit of As- concentration in potable water is fixed at 50µg/l in India,
Bangladesh and many other countries, however WHO (1993) recommends
lowering this limit provisionally to 10µg/l, which has been endorsed by
Bureau of Indian standards(2003).
Groundwater
contamination by As has been reported from many countries, with the more
severe problems occurring in the south east Asia, mainly in Bangladesh, and
West Bengal in India. Arsenic can be harmful through inhalation, absorption
through skin, mucous membranes and ingestion. Accidental poisoning can occur
through breathing fumes, licking, paintbrushes to paint when using pigment
containing arsenic, or from wearing inadequate clothing when applying
arsenic based products.
Effects of
mild poisoning from inhalation include loss of appetite, nausea, and
diarrhoea. Effect of more severe, chronic or acute exposure includes
skin lesions, skin rash, and chronic headaches. Garlic odor on breath, a
metallic taste in the mouth, a bronzing pigment of the skin resembling
“raindrops on a dusty road” and possible damage to the liver. Arsenic and
As compounds are known cancer causing agents and have been implicated in lung
and skin cancer and associated with birth defects. It has different forms,
such as inorganic or organic form, inorganic arsenic being generally considered
more toxic.
Source of
Arsenic
A. Arsenic
in Groundwater
Arsenic is
found in groundwater which has flowed through arsenic-rich rocks. Recent work
has demonstrated that arsenic originates from ferrous oxides in the Holocene-
era aquifers tapped by the tube wells. Carbon deposits from ancient
mangrove swamps provide reducing conditions that cause the release of
arsenic. Contamination of groundwater with arsenic and its impact on humans
have been reported from 23 countries. The magnitude of this problem is
severe in Bangladesh and West Bengal in India. In recent years,
evidence of arsenic groundwater contamination has also emerged in other
Asian countries including Cambodia, The Lao People Democratic Republic,
Myanmar, Pakistan, Nepal, Vietnam, a province in Iran and Bihar state in the
middle Gangetic Plain in India.
B. Arsenic
in Surface water
Arsenic comes
in to surface water through industrial effluents, agricultural runoff, and it is
also used commercially in alloying agent and wood preservatives.
Arsenic
Transportation and Metabolism
Much of the
arsenic in the atmosphere comes from high-temperature processes such as
coal-fired power plants, burning vegetation and volcanic activity. The arsenic
is released into the atmosphere primarily as arsenic trioxide where it
adheres readily onto the surface of particles. These particles are
dispersed by the wind and eventually fall back to the earth due to their weight
or during rain. Microbes acting on arsenic in soils and sediments generate
arsine gas or other volatile arsenic compounds. Arsine reacts with oxygen in the
air and is converted back to non-volatile forms of arsenic, which settle back to
the ground. In well-oxygenated water and sediments, nearly all arsenic is
present in the stable form of arsenate. While in flooded conditions, arsenic
predominates and it is interchangeable, depending on the chemical and biological
conditions.
Arsenic is
absorbed from the lung, from the mucous membranes of the nose, and from the gut.
It passes through the body and is partially metabolized in the liver. It is
excreted in the urine, the sweat and in the keratin of skin and the nails. Its
disappearance rate from the blood is very rapid with a biological half-life
of one hour and from the body in to the urine with a biological half life
of four days.
Because of its
rapid elimination, arsenic dosages do not build up over time. The measurement of
arsenic in the urine will indicate the level of exposure over the past few days,
and in the hair, indicate past few months. It is known that children
eliminate arsenic from their system more rapidly than adults. A study of
about 400 children from the Anaconda smelter site in Montana found no
evidence of an increase in urinary arsenic level correlating with the
general contamination level for the area.
Effects of
Arsenic on Aquatic System
Arsenic is an
essential compound for many animal species, because it plays a role in protein
synthesis. It is unclear whether arsenic is a dietary mineral for humans.
Arsenic toxicity is another important characteristic. The boundary concentration
of arsenic is 2.46 ppm for fresh water algae. This compound also blocks
enzymatic processes, increasing the toxicity. Large amount of arsenic end up
the environment and living organisms. It is mainly emitted by the copper
producing industries, but also during lead and zinc production and in
agriculture. It cannot be destroyed once, entered the environment, so that
the amounts that we add can spread and cause health effects to humans and
animals on many locations on earth.
Plants absorb
arsenic fairly easily, so that high ranking concentration may be present in
food. The high concentrations of dangerous inorganic arsenic that are currently
present in surface water enhance the chances of alteration genetic materials of
fish. This is mainly caused by accumulation of arsenic in the bodies of
plant-eating freshwater organisms. Birds eat the fish that already contain
eminent amounts of arsenic and will die as a result of arsenic poisoning as
the fish is decomposed in their bodies.
Effect of
Arsenic on Human Health
Arsenic is a
well known carcinogen. It is also causes melanosis, leucomelanosis,
hyperkeratosis, hepatomangoly, neuropathy, odema. Environmental exposure to
arsenic has also been well linked to the development of a variety of cancers
like skin, lung, bladder and urinary tract cancers.
The study was
reported from Taiwan in the 1960s and concerned an area with a previously
unknown disease-Blackfoot Disease. It was the investigation of Blackfoot
disease and its associated skin cancer that led to the identification of
arsenic as the probable or possible cause of both diseases.
Phytoremediation: From Green to Clean
Various
efforts have been made to enhance the remediation capacity of plants
through genetic engineering, for example, through over expression of
arsenate reductases and phytochelatins synthases. Although, As tolerance in
some transgenic was enhanced, but accumulation factors particularly in
shoot tissue have remained low.
Even though As
and most other heavy metals are toxic to plants, a range of plants have
been described as so called metallophytes or hyperaccumulators.
Hyperaccumulator ferns, which accumulate very high concentration of
arsenic specifically in above ground tissues.
Fresh water
macrophytes such as, Ceratophyllum demersum, Alisma plantago,
Collitriche stagnalis, Eigeria densa, Elodea canadensis, Juncus spp.,
Potamogeton orchjreatus, Oscillataria, Chara etc. are potent
hyperaccumulator of arsenic.
Various
methods of phytoremediation are as follows:-
1.
Phytoextraction (phytoaccumulation):
Is the name given to the process where plant roots uptake metal contaminants
from the soil and translocate them to their above ground tissues.
2.
Rhyzofiltration:
It is similar to the phytoextraction but is concerned with the remediation of
the contaminated ground-water rather than the remediation of polluted
soils. The contaminants are either adsorbed on to the root surface or are
absorbed by the plants roots.
3.
Phytostabilization:
Is the use of certain plants to immobilize soil and water contaminants are
precipitated in the rhyzosphere.
4.
Phytodegradation (phytotransformation):
Is the degradation or breakdown of organic contaminant by internal and external
metabolic processes driven by the plant.
5.
Rhyzodegradation:
Also called enhanced rhyzosphere biodegradation, Phytostimulation and plant
assisted bioremediation is the breakdown of organic contaminants in the soil by
soil dwelling microbes which is enhanced by the rhyzospher's presence.
6.
Phytovolatilization:
Is the process where plants uptake contaminants which are water soluble
and release them into the atmosphere as they transpire the water.
Extensive
efforts have been made to reduce the negative effects of arsenic
contamination on the environment and human health. Among these, phyto-remediation
has been proved as a promising new technology for environmental clean-up. The
term phytoremediation consists of a Greek prefix phyto (plant) and the Latin
root remedium (remove an evil). Thus, phytoremdiation is a technology that
removes contaminants or pollutants by growing particularly selected
plants. Pteris vittata, the first identified arsenic hyperaccumulator,
has received extensive attention since its discovery in 2001. P. vittata
belongs to the Pteris genus and family Pteridaceae. It meets all the
required criteria to qualify for being a natural phyto-extractor. The amount
of arsenic accumulated in fronds can be up to 93% of the total
arsenic content in the plants and 25 times more than that in the
roots.
*International Society of Environmental Botanists, NBRI, Lucknow, India. E-mail:
[email protected]
**National
Botanical Research Institute, Lucknow, India. E-mail: [email protected]
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