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Dealing with Nanopollution
October 1, 2010   
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Researchers at the Polish Academy of Sciences’ Institute of Physical Chemistry in Warsaw have developed an innovative method to separate nanopollutants from sewage and recover some of the nanoobjects. The method can also be used to produce nanocomposites, the researchers say.

The rapid development of nanotechnology—which deals with materials sized between 1 and 100 nanometers—and the production of nano-based devices create a type of microscopic pollution that is extremely difficult to detect or contain. Researchers are afraid of the effect that nanopollution might have on humans, animals and other living organisms.

Nanopollutants are nanoparticles that are so small that they easily penetrate cells. They can enter people’s lungs or be absorbed by the skin. Nanoparticles are used in some of the products available in stores today, such as cosmetics. The highest risk is to the workers in nano-technology research and manufacturing processes.

An invisible enemy

Some nanometric structures such as carbon nanotubes are just above 10 micrometers in length and several nanometers in diameter, meaning they are like tiny needles that could be extremely hard to remove from the body.

Nanostructures that have a round shape and are made of hazardous materials, such as cadmium selenide particles, are about as dangerous. Traditional mechanical and chemical treatment of sewage fails to eliminate nanopollutants, while laboratory methods only work for small volumes of liquids.

The innovative sewage treatment process developed at the Institute of Physical Chemistry makes it possible to separate nanometric-size pollutants on an industrial scale. According to the researchers, the method, protected by four patents, is simple, cheap and effective.

Targeting tiny particles

The process, which took five years to develop, separates nanometric particles from solutions using two substances. One of them is a surface active agent, or surfactant for short, which can be ordinary soap, for example. The other is a polymer, such as the inexpensive and environment-friendly polyethylene glycol (PEG).

“If we match the concentrations properly, all the tiny pollutants will gather in the upper, floating layer,” says Marcin Fiałkowski, D.Sc., from the Institute of Physical Chemistry. The upper layer of the surfactant can be easily collected using mechanical methods and then disposed of safely or processed to recover the substances it contains.

The physical mechanism which separates the substance in the solution takes advantage of the different sizes and shapes of the surfactant and polymer particles. When introduced to solutions, surfactants, which are the dominant components of all cleaning agents, form aggregates called micelles. They can take various shapes, but they usually resemble spheres. Polymers, on the other hand, have the form of a ball of wool, which is basically spherical as well, Fiałkowski says. When the two “spheres” of a surfactant get close enough to each other, the tinier polymer “sphere” will not be able to squeeze between them and will remain at a certain distance, which is called a radius of gyration. However, when surfactant micelles approach each other and the distance between them is smaller than twice the radius of gyration, an empty space is created between them, causing a difference in the concentration of the polymer and, consequently, osmotic pressure. Water flows out from among the micelles; they get closer and within an hour or so, the phases in the solution become separated, Fiałkowski says. The process occurs even when an electrically charged polymer, or polyelectrolyte, is added into a solution containing an ionic surfactant of the same sign.

The research has shown that where there are any particles in the original solution, they gather in the layer which is rich in the surfactant.

Soap and polymer

Ewelina Kalwarczyk, a Ph.D. student at the Institute of Physical Chemistry’s Department of Soft Condensed Matter and Fluids, says, “In one of the experiments, we studied a solution of gold nanoparticles which were the size of five nanometers. After several percent of soap and around 10 percent of polymer were added, a viscous and elastic layer with the surfactant formed on the surface. It contained particles of gold, which, being heavier than water, would have fallen to the bottom under different conditions.”

The method is also effective in the case of detergents which thicken and are raised to the surface, according to Kalwarczyk.

Once the process is completed, the polymer stays in the water from which it can be almost fully recovered. The only substance that is completely used up is the surfactant, that is the soap which captures the nanopollutants.

Possible uses

An 11-month-long battery of tests has not revealed any changes in the physical and chemical stability of the collected surfactant, which means the particles contained in it are effectively isolated from the environment, Kalwarczyk says.

The layer which forms on the surface of the solutions has an organized hexagonal structure. Since the type of organization depends solely on the concentration level, structures of this type are called lyotropic liquid crystals. The newly developed method can be easily used to introduce carefully selected nanoparticles into the framework that forms the structures, according to Kalwarczyk. The framework can then be solidified and the organic part can be removed.

The method developed at the Institute of Physical Chemistry can thus be used not only to treat sewage, but also to produce composite materials containing admixtures of gold, platinum, silver, semiconductors, and carbon nanotubes. Materials of this kind can find application in the production of solar cells and various types of catalytic converters, including those used in cars.
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