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Vol. 15 No. 4 - October 2009

Arsenic Threat and its Remediation Through Plants:
A Step For Environmental Clean Up

By: Deepika Sharma* and Sanjay Dwivedi**


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: deepikadesire@yahoo.in

**National Botanical Research Institute, Lucknow, India. E-mail:  drs_dwivedi@yahoo.co.in

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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