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Synthetic clay eliminates waste in the processing of carbon nanotubes

Purified multiple-walled nanotubes are detected by a transmission electron microscope at Unicamp

LQES / UNICAMP Purified multiple-walled nanotubes are detected by a transmission electron microscope at UnicampLQES / UNICAMP

A simple and innovative method for treating laboratory waste after the purification of carbon nanotubes – comprised of layers of carbon atoms coiled in a tube-shape – was developed by the research group led by professor Oswaldo Alves, from the Solid State Chemistry Laboratory of the State University of Campinas (LQES-Unicamp). Using nanoparticles of synthetic hydrotalcite (a type of clay), the researchers managed to remove about 99% of the impurities from the effluent generated by the purification process.

Hydrotalcite is a highly adsorbent type of clay made up of positively charged layers – ions with a positive electrical charge – of mixed hydroxide of metals, generally aluminum and magnesium, interspersed with layers of anions (negatively charged ions), such as carbonate. During the process of adsorption, the molecules or ions are detained on the surface of the hydrotalcite by chemical or physical reactions.

This unprecedented method for cleaning effluents generated by nanotube purification systems led to the filing of a Brazilian patent application and its international extension by Unicamp’s Innovation Agency, Inova. The international patent was requested to safeguard the results of the paper presented at the NanoSafe 2010 Congress in Grenoble, France, attended by researchers who work with nanotechnology risks and by companies that produce nanomaterials.

The need to purify nanotubes in laboratories first arose in 2003, when the researchers were planning to study the interaction of the nanostructures with living organisms. One of the studies conducted along this line of research, which is essential for developing the purification process, was carried out by the biologist Diego Stéfani Martinez while doing his PhD under Alves’ guidance. The researchers Antonio Gomes de Souza Filho and Natália Parizotto also played an active part in the research, which covered everything from the purification and characterization of the nanostructures to the interaction between them and the different organization levels of the bio-systems. The objective was to analyze the impact of nanotubes on, for instance, the aquatic ecosystem.

The micro crustacean bio-indicator Daphnia similis, more commonly known as the water-flea, was used for the study. Different concentrations of nanotubes placed in mineral water for up to 48 hours were evaluated to find out if this interfered in the water-flea’s mobility, this being regarded as an adverse effect. The result pointed to an absence of acute toxicity for the micro-crustacean up to a concentration of 30 milligrams per liter.

To conduct this and other similar studies, it was necessary to have high quality nanotubes, free of residues of amorphous carbon or of the metallic catalysts used in the synthesizing process. “Purification is an essential stage for us to be able to create new chemical uses for the nanotubes and also so that we can use the nanostructures in studies about interaction with biological systems,” comments Alves. At that time, the nanotubes found in the market were highly heterogeneous. One could find structures with different shapes, diameters and impurity content levels, all within a single sample.

It was necessary to set a standard. It took four years to arrive at a consistent purification protocol, but a new waste product appeared and needed to be treated. “We managed to eliminate the impurities that were due to the synthesis, but not those caused by oxidation, which are known as oxidation debris,” states Alves. This occurs because, to enable nanotubes to be scattered in water and be compatible with different materials, they have to be treated with highly oxidizing mixtures that contain sulfuric acid and nitric acid.

To eliminate these impurities, it was necessary to add to the process a solution of diluted caustic soda (sodium hydroxide), used by industry in the production of paper, fabrics, detergents, foodstuffs and bio-fuels. However, without some kind of treatment, it was impossible to get rid of the resulting effluent, a dark liquid composed of a complex mixture of polyaromatic substances and organic material. If not properly treated, this can contaminate water tables and rivers with chemical substances that are hard to remove at water treatment stations prior to human consumption.

“So we decided to turn to the purification of the purification,” explains Alves. That was when the researchers decided to test hydrotalcite. Since the 1990s, it has been studied in the laboratory for its physical and chemical properties. So much so that it had been used in a textile industry effluent treatment process, developed in partnership with the company Contech (see issue 155 of Pesquisa FAPESP ).

During the trials, the clay eliminated the waste products created during the nanotube purification process, giving rise to a dark solid that can be separated by decanting. In addition to the advantage of eliminating the oxidation impurities, the sodium hydroxide solution that is left over can be put back into the process and re-used with the same level of effectiveness. Once it undergoes thermal treatment, the dark powder obtained eliminates the organic matters and becomes white once again, so that it too can be used in the removal process again, with no loss of its efficiency.

Scientific articles
STÉFANI, D. et al. Structural and proactive safety aspects of oxidation debris from multiwalled carbon nanotubes. Journal of Hazardous Materials. v. 189, p. 391-96. 2011.
ALVES, O. L. et al. Hydrotalcites: a highly efficient ecomaterial for effluent treatment originated from carbon nanotubes chemical processing. Journal of Physics: Conference Series. 304 012024. 2011.

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