the problem of arsenic poisoning in ground water

8/8/2019 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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the problem of arsenic poisoning in ground water

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