Arsenic- its toxicity and removal
Very often people in certain parts of Bangladesh and West Bengal who has been using the ground water for many years suffers from general weakness of the body, low body weight,lesions on their hands and feet. Such symptoms are usually associated with a toxic substance called arsenic. Though arsenic is usually present in water bodies all over the world, it is present in very large concentration in the ground water of Bangladesh and Bengal. According to WHO, the permissible limit of arsenic in the drinking water is 10microgram per liter of water.
Occurrence:
Arsenic is found naturally in small amount in the soil and water. The common sources of arsenic are rocks containing arsenic such as arsenic trioxide (As2O3), orpiment (As2S3), arsenopyrite (AsFeS) etc. In these rocks, arsenic is usually present in combination with sulphur and iron. When ground water passes through these rocks, arsenic is transported to different regions. Some amount is also transported during the weathering of these rocks. Various human activities have also increased the amount of arsenic content in the environment. For instance, a significant amount is contributed from the mining of precious metals like copper, silver etc as the ores of these metals also contain arsenic. During the extraction of these metals, arsenic is left behind as waste which then enters the water bodies or penetrates into the soil during surface water run-off.
Uses:
Arsenic is a very toxic element and so its compounds are useful for making fungicides, insecticides, wood preservatives etc. Besides these, it has also various other applications such as in paint pigments, light emitting diodes, semiconductors, treatment of cancer etc. It is also mixed with other metals to form alloys.
Effects of arsenic on health:
Arsenic is one of the most toxic elements known. In nature it can occur either as organic arsenic or inorganic arsenic. When arsenic compounds are formed in combination with carbon and hydrogen only it is called organic arsenic. They are comparatively less toxic. When arsenic forms compound in combination with elements such as oxygen, chlorine, and sulfur, it is called inorganic arsenic.
Generally human beings may get exposed to arsenic by working or living in the arsenic containing environment such as pesticides factories, copper or lead smelting or using ground water contaminated with it. Surface water can also be sometimes contaminated by the surface run-off from the pesticides applied fields etc. A small amount may also be contributed by eating sea food. Sea organisms like shrimp, fish, shellfish etc absorb arsenic from the water and convert them into organic compounds such as arsenobetaine and arsenocholine. Though these compounds are not toxic to living beings and can be easily eliminated from the body along with the urine, it is believed that prolong exposure to it can cause stomach irritation and also damage nerves.
But the inorganic arsenic compounds are very toxic. Unlike organic arsenic, inorganic arsenic compounds tend to settle in the body organs so that with times, its concentration goes on increasing, causing various ailments. Some of the health problems associated with arsenic poisoning are sore throats, hallucinations, cancer of skin, lungs, kidneys etc. It also causes miscarriages, hypertension, decrease the RBC and WBC formation, etc. However immediate death does not occur unless taken at a very high dose.
The fact that it is colorless and has neither smell nor taste from which its presence can be known makes it difficult to detect its presence by simple physical observation. So, the cases of arsenic poisoning usually remain undetected for long. Its presence can be confirmed only from the chemical test.
Mechanism of toxicity:
Arsenic causes malfunctions in the body mainly by two different mechanisms:
As it has an affinity for binding with sulphur it binds with the sulphur atom present in different enzymes. This leads to the malfunctioning of that enzyme and subsequently the function of the cell related with those enzymes is disrupted.
Another mechanism of cell disruption is by replacing the phosphate. Arsenate resembles phosphate, which is needed by cells for energy production and signaling. By displacing phosphate in enzymes, arsenic reduces the energy production and interferes with cell functioning.
Remediation methods:
Various methods are employed for the effective removal of the arsenic. In the water, arsenic can exist either as arsenite ion as arsenate ion. Arsenate compounds are more easily removed than arsenite compounds. So, prior to removal, arsenite compound are converted to arsenate using oxidants such as ozone, chlorine, hydrogen peroxide etc. This is followed by other treatment such as Coagulation, filtration, adsorption etc.
Coagulation: Groundwater usually contains dissolved metal salts such as iron and aluminum. So, appropriate chemicals are used to coagulate the dissolved metal. Arsenic present in the water then attaches itself to these coagulants and forms a complex. The complex is then allowed to settle down by storing for about twenty four hours. The clear water is then filtered and used.
Membrane filtration: In this method, porous membranes are used to filter out the contaminated water under high pressure. Though it is quite costly, it has the benefits of removing other contaminants as well.
Adsorption: in this method, adsorbents such as activated alumina, activated charcoal, iron oxide coated sand etc are used to adsorb the arsenic from the water. When water passes through the adsorbents, arsenic is held back at the surface of the adsorbent either by physical or chemical processes. The adsorbents should have a large surface area for effective removal.
Ion exchange: It is an advanced method of water treatment in which the contaminated water is passed through a column containing a high amount of chloride ion. As the water passes through this column, the chloride ion is exchanged for arsenic ion so that the water leaving the column has higher amount of chloride while the amount of arsenic is considerably reduced.
Phytoremediation: It is a method which is slowly becoming popular. In this method, plants which can adsorb arsenic from the soil and water bodies without being affected are planted in the affected areas and after maturation are harvested and disposed off somewhere. Some of the plants that can be used for the removal of arsenic from the soil are bracken ferns, mustard plants, duckweed, watercress etc. This method is quite cheap compared to other methods.
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