Local rock formations help Mexicans to remove arsenic from water
the inhabitants of Zimapan, a district in Mexico, consume water contaminated with arsenic (as), a poisonous chemical. But, a remedy may be locally available to remove the contamination.
About half of the water supply samples tested by Mexico's National Water Commission in the region contained arsenic concentrations above current World Health Organisation limit of 0.01 milligramme per liter of water. Zimapan lies 200 kilometres north of Mexico City and is a mining district famous for its lead, silver and zinc ores. But, these naturally found chemicals contaminate the water supplies and also pollute the groundwater due to leaching from the mines.
The leachates have been found to have a s concentration of almost 16 gramme per litre and high acidity with ph as low as two. The lower the p h level greater is the acidic nature of the chemical tested.
In a region where 72 per cent of the population has been recorded to earn less than us $3.00 per day in 1994, commercial 'point-of-use' purification systems lie well beyond the reach of the people. They are exhorbitantly costly solutions for the impoverished region.
Now geoscientists have discovered that the answer could lie all around them . A stufy conducted by a group of geoscientists shows that a s levels are markedly reduced when the contaminated water is mixed with rocks of the local 'Soyatal formation'. The Soyatal formation, which crops out at surface throughout the area of contamination, is made up of calcareous shale (a loose rock containing calcium carbonate) 15 per cent of which is made up of clay minerals such as kaolinite and illite. Both of these are known to adsorb as. The finding is supported by the fact that water sources emerging through the Soyatal Formation are uniformly low in as.
Commercial purification mechanisms, which use ion exchange process, resins, green sand filtration and reverse osmosis processes as mechanism are too expensive. In comparison the use of rocks from the soyatal formation would be well within the pockets of local residents, requiring no more sophisticated equipment than a bucket (Geochemistry: Exploration, Environment, Analysis , Vol 1, No 1). The researchers believe that one or two kilogrammes of crushed rock, added to about 20 litres of contaminated water, stirred frequently over 24 hours, will effectively remove a s to below acceptable levels.
However, they warn that further research will be needed before residents are asked to try it for themselves at home.
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