Ironing out a problem?

Water in acid lakes formed after the abandonment of mining activities can now be treated with a technique that manipulates the properties of iron-based heavy metal contaminants called ferrites

Published: Tuesday 15 October 1996

The ferrites in the acid lake< researchers from McGill University in Montreal, Canada, have found a new technique to treat water in acid lakes that form when mining operations are terminated. After the closure of mines, the water that fills up the pits becomes highly acidic. The traditional method of neutralising the water involves the addition of lime. But lime has not proved to be efficient because it creates a toxic sludge, packed with heavy metals. The Canadian scientists have now hit upon a novel method that would not only neutralise the water but also extract heavy metals such as small iron-based particles called ferrites ( New Scientist , Vol 151, No 2042).

Since ferrites are highly magnetic compounds, they can be easily separated from water and could find some commercial applications too. For instance, they could be used in the magnetic coating of recording tapes. Ferrites are more complex in nature than the other compounds found in the usual neutralised sludge and because their structure allows them to incorporate other metals such as zinc and aluminium, the water left behind is virtually free of metals.

The researchers, led by Zhenghe Xu, are not the first to hit upon the idea of using ferrites to clean up water in abandoned mines. There have been attempts to do so in the past but the earlier process of achieving the same was not very practical. It required the water to be heated to more than 60 c, for two days, while a precipitate of iron compounds formed. According to Xu, the new technique devised by his team does the job in less than half an hour and at ambient temperature.

The team has speeded up the reaction in the laboratory by manipulating the chemistry of iron in solution. The property of iron which helps it exist in two different electrical charges, also called the valence states, was utilised. The key to extracting ferrites from the water was controlling the ratio between the ferric (Fe 3+ ) and ferrous (Fe 2+ ) ions. If the ratio of ferrous to ferric ions is brought to two:one, with the subsequent addition of a neutralising agent like sodium hydroxide, a large proportion of ferrites begin solidifying immediately. The ferrites could then be separated by using a magnetic filter. The ratio between the ferrous and ferric ions in an actual acid lake could be manipulated by adding ferrous ions in the form of iron sulphate or by aerating the water to oxidise some of the ferrous ions to the ferric form.

However, Xu's process suffers from a major drawback. The use of lime or calcium hydroxide to neutralise the acid water is ruled out. This is because calcium molecules disrupt the reaction as they are too large to be incorporated smoothly into the structure of the ferrites. "I have managed to extract ferrites using sodium hydroxide, but this is more expensive than lime", says Xu. The scientists are therefore currently working to devise a way by which calcium can be prevented from inhibiting the reaction. In addition, they hope that promoting the commercial use of ferrites may also help solve the problem. If money were raised by doing so, enough resources could be generated to treat the acid in the lakes.

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