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Last Updated: Thursday, 26 July 2007, 20:41 GMT 21:41 UK
Foam targets heavy metal clean-up
Cambodian soldier standing next to toxic waste drums
Several industries can produce heavy metal pollution
A type of porous foam can be used to soak up heavy metals from water, a team has reported in the journal Science.

The rigid material could be used for environmental clean-ups and other jobs that require so-called "molecular sieves" that can trap heavy metals.

Previously, materials called oxides have been used as molecular sieves, but these prefer to form bonds with small metal ions such as magnesium and zinc.

But the new foam preferentially absorbs heavy metals like mercury.

Toxic heavy metals in air, soil and water are a growing problem, threatening the health of populations around the world.

A few cubic centimetres of this stuff has such a big surface area that if you were to unfold it, it would cover one football field
Mercouri Kanatzidis, Northwestern University
Sources of heavy metal pollution include the coal, natural gas, paper and the chlor-alkali industries - the latter manufactures chlorine and caustic soda.

Mercouri Kanatzidis, a chemist at Northwestern University in Illinois, US, and colleagues developed a type of "aerogel" - a low-density material derived from a gel in which the liquid component has been replaced by gas.

This highly absorbent aerogel is made by linking clusters of chemical groups called chalcogenides with charged metal atoms - called metal ions. The elements sulphur and selenium are examples of chalcogenides.

No way out

These porous "chalcogels" have remarkable properties. They are mostly made of air and have enormous surface areas.

"A few cubic centimetres of this stuff has such a big surface area that if you were to unfold it, it would cover one football field," Dr Kanatzidis told the BBC News website.

Aerogel, Science
The aerogels contain many small pores
"This surface area is so big that sooner or later, anything that goes into that material will hit the surface."

Importantly, the sulphur or selenium in the chalcogels preferentially bind to heavy metals.

"Heavy elements love to bind to sulphur atoms," said Dr Kanatzidis, "so if you are a heavy metal like cadmium, mercury, or lead, and you are travelling through the aerogel, you will see the sulphur atom, bind to it and never get out again. There you have the basis for purification."

Oxide aerogels are used as filters to capture nanoparticles and other contaminants from the air. But they cannot trap mercury, cadmium or lead; in place of sulphur or selenium, oxides contain the element oxygen which does not bind well to heavy metals.

The aerogels described in the Science paper could never be used for environmental clean-up tasks. They contain a platinum metal group which would make them far too expensive. But Dr Kanatzidis says his team has already made a gel with similar properties, in which the platinum is replaced by a cheaper alternative.

Pollution filters

"One way I can envisage chalcogenide aerogels being used is as a filter, in which you pass water in one side and out the other, capturing all the chemical species of interest," he told the BBC.

"If you have controlled waste that you could channel, you could purify it by passing it through filters and columns."

If contaminated water in a river could be passed through a similar system, it could - in principle - be purified in the same way.

The aerogels might also be able to absorb organic pollutants, such as dioxins and PCBs. The sulphur group in these gels can interact with parts of the molecules of these contaminants.

The researchers also expect to capitalise on other properties of chalcogenide aerogels. For example, they could be used as membranes to purify gases such as hydrogen that could be used in carbon-free fuel cells, cutting our reliance on fossil fuels.

"Firstly, you need to produce hydrogen in a cheap way - preferably from sunlight. Secondly, you need to have pure hydrogen to feed into a fuel cell and convert it into electricity," said Dr Kanatzidis.

"If the hydrogen is impure, the catalyst of the fuel cell could be poisoned and die out. We hope this material... might be able to help membranes that would purify hydrogen."

In addition, chalcogenide aerogels are made from some of the same materials as semiconductors and share semiconducting properties.

Stephanie Brock, a chemist at Wayne State University in Detroit, Michigan, commented: "Whether chalcogenide aerogels will find practical applications remains an unanswered question, although the few studies on these materials have yielded provocative results."

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