What is Arsenic?
Arsenic (As) is a Group V element with atomic number 33. It is a metalloid with properties in between metals and nonmetals. Arsenic is a silver-grey brittle crystalline solid with atomic weight 74.9 amu (atomic mass unit); specific gravity 5.73 g/cm3, melting point 817 ºC (at 28 atm), boiling point 613 ºC and vapor pressure of 1 mm Hg at 372 ºC.
Arsenic is a deadly poisonous water contaminant. It cannot be detected easily by colour, odour or sedimentation. It is present in groundwater in some areas around the world including the valleys of the rivers originating from the Himalayas and the Barail ranges of mountains. Arsenic is known to cause several health problems including cancer in human if ingested through drinking water for a long time. This is because arsenic affects enzymes, which control most of the biological process in human body, leading to their malfunctions.
Arsenic in the environment:
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Arsenic occurs in both inorganic and organic forms in the environment in oxidation states of -3, 0, +3 and +5. Trivalent arsenic can exist as arsenous oxide (As2O3), arsenious acid (HAsO2), arsenite (H2AsO3–, HAsO32–, AsO33–) ions, arsenic trichloride (AsCl3), arsenic sulfide (AsS3) and arsine gas (AsH3). Pentavalent arsenic commonly exist as arsenic pentoxide (As2O5), orthoarsenic acid (H3AsO4), metaarsenic acid (HAsO3) and arsenate (H2AsO4–, HAsO42–, AsO43–) ions. The presence of different forms of organic arsenic, such as monomethylarsonic acid [CH3AsO(OH)2], dimethylarsinic acid [(CH3)2AsO(OH)], trimethylarsine oxide [(CH3)3AsO], methylarsine (CH3AsH2), dimethylarsine [(CH3)2AsH] and trimethylarsine [(CH3)3As], has also been observed in contaminated soil and water.
Arsenic in groundwater:
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In natural waters, arsenic is mostly found in inorganic form as oxyanions of trivalent arsenite [As(III)] or pentavalent arsenate [As(V)]. Both As(III) and As(V) compounds are highly soluble in water and may change valency states depending on the pH and redox conditions.In surface waters under oxidizing conditions, the predominant species is pentavalent arsenic, which is mainly present in the protonated oxyanionic forms (H2AsO4-, HAsO42-). However, under mildly reducing conditions and lower redox potential such as in anoxic groundwater, As(III) is the thermodynamically stable form, which at the pH values of most natural waters is present as nonionic arsenious acid (H3AsO3). As0 and As3– are rare in aquatic environments. Organic arsenic forms may be produced by biological activity, mostly in surface waters and aquatic plants and animals.
Toxicity of Arsenic
The toxicity of the different arsenic species varies in the order: arsenite [As(III)] > arsenate [As(V)] >organic arsenic. Toxicity of arsenic in trivalent state [As(III)] is almost a thousand times higher than that of its pentavalent [As(V)] species.
The World Health Organization (WHO) recommends a maximum arsenic level of 10 microgram per liter (10 ppb) in drinking water. However, the WHO also says that presence of arsenic even below 10 ppb, even as low as 0.17 ppb, also can cause cancer.
Surface water and rain water does not contain arsenic. Therefore, sourcing alternate arsenic-free water from rivers, streams, lakes and rain is the first option to be explored for mitigation of the problem of arsenic contamination of groundwater.
The maximum 10 ppb level is recommended by the WHO because it is difficult to detect arsenic below that and it is also difficult to remove arsenic to below that level. In fact, it is better to remove arsenic from drinking water as much as possible or to below the detection limit. Now, there are ways to remove arsenic to below the detection limit or to nearly 0 ppb. One of them is Arsiron Nilogon, the method presented here.
Arsenic Problem in Assam
In Assam, groundwater is contaminated with excess arsenic in most of the districts in the Brahmaputra and Barak valley, the worst affected districts being Jorhat, Golaghat, Majuli, Lakhimpur, Dhemaji, Nalbari, Baksa, Borpeta, Nagaon, Hailakandi, Kachar and Karimganj. In fact, the entire Assam is affected by groundwater arsenic except probably the three hills districts and Hojai and Tinsukia districts. However no one can rule out presence of arsenic in groundwater of Tinsukia district and the south-western valleys of Dima Hasao district without a detail study.
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Arsenic has been detected up to as high as 1000 ppb in groundwater of Assam. There are some districts which are reported to have over 600 ppb of groundwater arsenic. More importantly, there are vast areas with groundwater having high level of arsenic contamination in several districts affecting tens of millions of people in Assam.
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Lack of (i) detail information about arsenic contamination of groundwater of Assam, (ii) detail study of impact of drinking water arsenic on public health in Assam and (iii) lack of public awareness of arsenic contamination of groundwater and its health-effects are the banes cursed upon the arsenic-affected people of Assam.
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While the Governments are responsible and are often accused for failure to solve the problem, there are also solutions to the problem that can be adopted by the arsenic affected people themselves for saving their own families from the menace of arsenic.