Bacteria help precipitate arsenic from the surface water that springs out of mining sites
mining sites are important sources of pollution. Water released from mining sites is acidic and rich in metallic ions. The only forms of life that thrive in it are bacteria which grow by catalysing the oxidation reactions of the metals -- ferrous ion to ferric ion and sulphur into sulphate. A number of metals like arsenic, discarded by mines, are poisonous and pose a threat to the environment (Global Technoscan, October 1996).
In an abandoned mining site in Gard in the south of France, scientists from the 'Geofluids-Basins-Water' laboratory and the laboratory for bacteriology and microbiological control have discovered arsenic-rich (100-300 mg per litre) bacterial deposits in a stream of acidic water (ph 2.5-3.5).
Arsenic is very toxic in its soluble form. Ideally, it should be extracted from the water and stored in a relatively stable and a less toxic form. Bacteria of the Thiobacillus and Leptothrix types are responsible for precipitation of arsenic in the form of ferric arsenate. Consequently, the waters become less rich in arsenic and, therefore, less polluted.
Bacteria -- Thiobacillus ferrooxidans -- catalyse oxidation reactions of iron sulphide and are used industrially to retrieve metals. Arsenic contained in sulphides can be released from the mines in soluble (as3+) or insoluble (as5+) forms. That is, it will either be released as a poison in the medium or will get precipitated as a relatively stable form along with ferric iron.
At the mining site of Carnoules in Gard, wastes containing iron sulphide, lead and arsenic were found. Water from the Carnoules stream was acidic and rich in metals (iron, zinc and lead) and caused pollution over several kilometres. Scientists also noted high concentrations of arsenic (100 to 300 mg per litre, about 2,000 times greater than the limit approved for drinking water) in the upper portions of the stream. But downstream the concentration of arsenic was less, since it might have got deposited on the way. About nine to 20 per cent of arsenic formed yellow deposits of ferric arsenate. These sediments showed structures of bacterial construction -- stromatolites. Scientists discovered that these bacterial stromatolites could thrive in continental acidic waters. In a short period the bacterial colonies formed large bacterial carpets on sulphide sand deposits. Microscopic study revealed that the bacteria had a covering of arsenic and iron. Bacterial strains enabled precipitation of 80 per cent of arsenic, whereas only 25 per cent of arsenic precipitated without bacteria. The bacteria thus favour oxidation of arsenic and precipitation of arsenate along with lead.
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