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Water/shale mix neutralises mine arsenic

GEOSCIENTISTS HAVE helped the mining district of Zimapan, 200km north of Mexico City, find a cheap way of cleaning its drinking water, which is contaminated with arsenic.

All that is needed is a bucket. Researchers believe that one or two kilograms of crushed local rock, added to 20 litre of contaminated water and stirred frequently for 24 hours, will reduce arsenic concentrations to below acceptable levels.

Supplies are polluted by natural sources of lead, silver and zinc as well as water leached through mine tailings. Some leachates are highly acidic, with arsenic concentrations of 16g/litre.

Researchers from the Androscoggin Valley Environmental Center, Lewiston, USA and the Instituto di Geofisica, Nacional Autonoma de Mexico found that arsenic levels in water were reduced to below detectable levels when it reacted with rocks from the local Soyatal Formation. The work was published in the Geological Society's Geochemistry, Exploration, Environment, Analysis journal earlier this year.

This formation is a calcareous shale containing up to 15% clay minerals (kaolinite and illite).

Field testing is now planned to quantify the effect before residents try it for themselves.

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