The Problem of Arsenic Poisoning in Ground Water
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Transcript of The Problem of Arsenic Poisoning in Ground Water
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The Problem of Arsenic
poisoning in Ground Water
Siddhartha Sinha
Roll No- 075515
Department of Chemical Engg.
HIT- Kolkata
_________________________________________________
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Contents
Introduction
How it began?
Sources
Health effects
Mitigation techniques and strategies
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Magnitude of the problem- The present
situation we are inApproximately 137 million people in more than 70 countries around
the globe are possibly affected by Arsenic poisoning
Nine out of 19 districts in West Bengal are having arsenic above the safe
concentration level.- World Health Organization
59 out of 64 districts of Bangladesh are exposed to the risk. That is equivalent
to 60% of the people in the country.
WHO has declared it the biggest mass poisoning in human history.
Every year thousands of cases are reported of cancer caused due to Arsenic
poisoning in South East Asia.
No comprehensive step has so far been taken.
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Map of Bangladesh showing probability of Arsenic exceeding the
recommended safe concentration level of0.05 microgram/lit or 5 ppb.
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How did this problem begin?
The period was 1980s. There was a remarkable change in the minor waterpolicies. This was mainly because of two reasons -
Green Revolution had began and there was a rapid and unpredictable growth in
agriculture. Rice which is the summer crop is a water intensive crop. So lots of
tube wells were installed. The immediate effect was lowering of ground water
level.
Secondly , lots of cases of water borne diseases such as dysentery ,cholera
,diarrhea were being reported. This happened as villagers had no domestic water
supply and had to share surface water resources with cattle etc. So installing
tube-wells was necessary.
So what happens if the ground water table gets
depleted?
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The root sources of Arsenic can be traced to the places where the river systems
namely Ganga and Brahmaputra originate.
One important property of Arsenic is that it can move in adsorbed phase with
Iron.
Iron on the other hand is plentiful in the Himalayas.
The sediments containing this Iron and Arsenic with it has been coming in with
the rivers for thousands of years and getting deposited.
The sulphate available in the Ganga basin area reduced to hydrogen sulphide in
presence ofsulphur reducing bacteriaThe iron and hydrogen sulphide react to form iron pyrites and since they contain
arsenic they are also known as arsenopyrites.
These are water insoluble. As long as they are under water they are in reducing
environment.
As the water table goes down these minerals get exposed to air i.e. an oxidizing
environment. Thus they get oxidized into hydrated iron arsenate which is watersoluble.
Then during the next rainfall they dissolve in water and percolate down into the
subsoil. The pressure applied during operation of a tube well breaks it up into
smaller particles and this way arsenic enters ground water.
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To summarize the entire matter
SULPHATES HYDROGEN SULPHIDE
IRON + HYDROGEN SULPHIDE IRON
PYRITES (or ARSENOPYRITES)
ARSENOPYRITES + OXYGEN IRONARSENATE HYDRATE
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Effects of Arsenic on human health -
Arsenic is known as the king of all poisons. One interesting fact about arsenicis that it is not harmful if ingested in the elemental state. Most of the arsenic
ingested is excreted through urine stools,skin,hair,nail etc.
The lethal dose on humans is 125 micrograms.
Usually it takes 8 to 15 years for the first symptoms to appear.
Recent studies show that it also causes cardiac diseases, cerebrovascular
diseases, pulmonary diseases, diabetes mellitus etc.Apart from these it is common to hear of melanosis, keratosis, oedema,
gangrene too.
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Techniques of mitigation.
Coagulation- A small amount of potassium permanganate and alumin powdered form is mixed in some quantity of water and kept
untouched for some hours. After some hours precipitate is seendepositing in the bottom of the container. The clear water is collected
carefully from the bottom and is safe to drink. The demerits of the
process is that the illiterate villagers may not take proper care in the
measurement of the ingredients taken. Secondly, the sediment should
be disposed off in a proper manner such as buried underground where
nothing else is grown. Ferric sulphate is also often used.
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Adsorption on activated alumina- This method is usefulwhen the water being treated has high amount of dissolved solids in
it. However if amongst these solids selenium, chloride, sulphate or
fluoride is present they may compete for adsorption sites. Activated
alumina is highly selective towards Arsenic and this poses
regeneration problems. On every use the adsorptive capacity of
activated alumina gets reduced.
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Ion exchange - This is highly effective in removing arsenic. Howeverother solids may compete with arsenic and this may affect the run
length. Suspended solids and cause clogging in the ion exchange bed.
Sometimes pretreatment of feed water may also be required. The bye-
product stream is highly concentrated which causes disposal problems.
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Lime softening This method works well above the pH level of 10.5However it is difficult to reduce the concentration to safe levels by using
this technique alone. Some secondary treatment may be required.
Here the main disadvantage is regarding where to dispose the sludge.
Besides we need skilled operators to handle the equipment.
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Some other practical measures
Restoring the normal flow of the rivers which got
diverted due to Farakka and Teesta barrage.
Popularizing rain water harvesting in rural areas and also
other sources of drinking water. Research on exact cause of arsenic found in water.
Tube wells giving arsenic contaminated water should be
marked for identification or disconnected.
Changing the cropping pattern in the area to reduce
usage of ground water. Setting up of Arsenic Removal Plant and proper
maintenance of it.
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