Bacteria eat sulphate

The arsenic killers: sulphate-

during a recent study, researchers from the us -based University of Illinois have found that adding a commonly found mineral in water contaminated by arsenic is a quick and cost-effective way to remove the toxic chemical. Water tainted by arsenic threatens the health of millions of people, mainly in Bangladesh and India. As per some researchers, the problem exists because bacteria trigger the leaching of arsenic from underground rocks (see 'More arsenic', Down To Earth, September 15, 2004).

The Illinois University researchers analysed 21 wells in central Illinois; they found the levels of the chemical vastly varied from well to well. Where arsenic concentration was low, levels of the mineral sulphate were high, and vice versa. A further probe revealed the reason behind the phenomenon -- when sulphate levels are high, bacteria consuming the mineral produce sulphide as a by-product. Sulphide converts most of the arsenic present in the water into a solid form.

According to the researchers, their finding indicates that adding sulphate to contaminated wells could be one of the best ways to combat the arsenic menace. Sulphate salts, such as gypsum, readily dissolve in water; they are widely available and cost peanuts -- less than us $1 per 10 kilogrammes. Thus, they can be used by the worst-affected countries, which mostly face financial constraints.

Furthermore, adding sulphate to drinking water will not bring any additional health risks. As per the World Health Organization guidelines for drinking water, a level of 400 milligrammes (mg) of sulphate per litre (l) of water is safe; the concentration of sulphate in the Illinois wells was less than 50 mg/l. Well water in Bangladesh and other nations has the same sulphate levels.

There is yet another advantage of the method -- since the arsenic gets immobilised within the aquifer, there would be no arsenic-bearing hazardous waste to be disposed of. The method could also make identifying contaminated wells easier. "Unlike detecting the presence of arsenic, testing for sulphate is simple," the researchers write in the November 2004 edition of the journal Geology.