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Helping The Carbon Nanotube Industry Avoid Mega-Mistakes Of The Past

Toxic by-products of carbon nanotube manufacturing. These spaghetti-like structures are carbon nanotubes viewed under an electron microscope. The manufacture of these nanotubes can produce toxic byproducts, researchers report. Photo by Anastasios John Hart
by Staff Writers
Washington, DC (SPX) Sep 21, 2007
A new analysis of by-products discharged to the environment during production of carbon nanotubes (CNTs) - expected to become the basis of multibillion-dollar industries in the 21st Century - has identified cancer-causing compounds, air pollutants, and other substances of concern, researchers reported here today at the 234th national meeting of the American Chemical Society.

Study co-author Desiree L. Plata and colleagues described their work as "totally new," noting that past analyses of the environmental impact of the emerging nanomaterials industry have been based on the toxicity of ingredients used in the recipes, rather than the actual pollutants formed during CNT manufacture.

While expressing concern about the possible health and environmental effects of nanotechnology by-products, Plata said the new data may be crucial as the nanotechnology industry seeks to avoid the kind of unanticipated health and environmental problems that have accompanied emergence of other new technology.

Researchers said, for instance, that they foresee developing, in collaboration with the CNT industry, "green chemical" reactions and filtration systems to substitute for those with potentially hazardous by-products and other ways of manufacturing carbon nanotubes that minimize potentially adverse impacts.

"Without this work, the environmental and health impacts of the carbon nanotube industry could be severe and costly to repair," said Plata, a doctoral student in chemical oceanography at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution. "We would like to help it develop in an environmentally sustainable fashion."

Recent experiences with other industrial pollutants underscore the need to try to improve nanotube manufacturing methods before serious problems arise, said Plata. These pollutants include Freon refrigerants, the gasoline additive methyl t-butyl ether (MTBE), flame retardants like polybrominated diphenyl ethers (PBDEs), and the surfactant perfluoroctane sulfanate (PFOS), she noted. Her collaborators include graduate advisor Christopher M. Reddy, Ph.D., of the Woods Hole Oceanographic Institution in Woods Hole, Mass. and Philip M. Gschwend, Ph.D., of MIT.

Carbon nanotubes, submicroscopic cylinders of carbon that are thousands of times smaller in diameter than the width a single human hair, possess characteristics not found in their larger, bulk counterparts, including enhanced strength and high electrical conductivity.

Studies by other scientists have shown that carbon nanotubes, which come in many sizes and shapes, can damage the lungs of mice, but their exact risk to human health remains unknown. Even less is known about the potential effects of the by-products of nanotube production, the researchers said.

To evaluate the emission products formed during nanotube production, Plata and her associates utilized a small-scale device to simulate "chemical vapor deposition," one of the main methods for making CNTs. Using a carbon vapor source, the researchers produced CNTs and analyzed chemical by-products from the reaction.

They found at least 15 aromatic hydrocarbons, including four different kinds of toxic polycyclic aromatic hydrocarbons (PAHs) similar to those found in cigarette smoke and automobile tailpipe emissions. The most harmful PAH identified was benzo[a]pyrene, a known human carcinogen, the researchers said. They also saw release of other hydrocarbons that can contribute to smog formation and can trigger the formation of ozone in the lower atmosphere, which can in turn cause respiratory problems in humans, Plata said.

"If nanotubes are produced in the tons, there will also be tons of PAHs produced," Plata notes. She said that the key solution to the problem may also be to employ special filters or 'scrubbers' in the production process to reduce the formation of harmful by-products.

Another possible solution is to develop new nanotube manufacturing processes that produce fewer toxins, said Plata, who notes that the research team is currently working with four of the major nanotube producers in the United States to help develop strategies to make production more environmentally friendly.

CNTs are already produced on a small industrial scale, and the researchers plan to measure actual emissions at several industrial sites in the near future to get a clearer picture of real-world pollutant emissions.

earlier related report
Coproducts of carbon nanotube synthesis: Emerging contaminants associated with the nanomaterial revolution
The carbon nanotube (CNT) industry is rapidly expanding, yet by-products of CNT synthesis are almost completely uncharacterized. The low purity of the bulk material suggests that by-products are formed in quantities comparable to CNTs themselves, and the limited efficiency of production suggests that large amounts of carbonaceous by-product or un-reacted feedstock may be released to the atmosphere, posing a threat to air quality and contributing to climate change.

To avoid environmental damages from by-products and to optimize the resource use in CNT production, we characterized the elemental composition of commercial CNTs, including catalyst impurities and associated extractable aromatic hydrocarbons. In addition, we analyzed the effluent of a representative CNT synthesis. Using a mass-balance approach, we account for formation of organic co-contaminants and estimate the contribution of CNT manufacture as a new source of these chemicals to the environment. We discuss implications for biogeochemical cycling of select trace chemicals (e.g., PAHs).

Researcher Provided Non-Technical Summary

Briefly explain in lay language what you have done, why it is significant and what are its implications (particularly to the general public)

Carbon nanotubes (CNTs) are new materials that are expected to have broad and bountiful applications in many fields, ranging from biomedicine to electronics. Over the next decade, their global production will increase dramatically. In the past, when such production has ramped up, both new materials (e.g., MTBE in gasoline) and associated by-products (e.g., dioxins formed during PVC plastic manufacture and paper production) have become undesirable threats to ecological and human health. In order to minimize the likelihood of a similar environmental catastrophe occurring due to carbon nanotube synthesis and use, we have begun to examine CNT syntheses methods and identified the by-products formed during CNT production.

Some of the materials formed during the synthesis include cancer-causing toxins and smog-forming compounds that threaten local air quality and public health, as well as green house gases that contribute to global warming. We are working with research colleagues to design new CNT synthesis methods and industrial collaborators already producing CNTs to try to limit the formation and release of these materials to the environment before the industry applications grow to large scale. Without this work, the environmental and health impacts of the carbon nanotube industry could be severe and costly to repair.

How new is this work and how does it differ from that of others who may be doing similar research?

This is the first time that anyone has identified these types of materials in a discharge stream from carbon nanotube production. Also, this is the first time that anyone has considered the environmental implications of the co-products formed during carbon nanotube manufacture.

Other environmental analyses have used the toxicity of the ingredients needed for the production of carbon nanotubes to estimate (roughly) the relative danger of the process, and our study is completing the story by determining what happens after those starting materials react together. This work is very new, and it will continue as the industry develops.

Special Instructions/feedback: The truly unique aspect of this work is that it represents a paradigm shift from the traditional development of industrially important materials. In the past, we have produced materials until an environmental or health problem is observed, and then we ban or limit the chemical (e.g., tetraethyl lead, DDT, freons, MTBE...).

Now, we are encouraging environmental and industry scientists to work together during the birth and growth of new materials, carefully selecting a synthesis that has minimal environmental and public health effects. By working with carbon nanotube scientists before problems arise, we will co-optimize the production of nanotubes for environmental friendliness and industrial performance. We hope that this work will set a new precedent for the development of all novel chemicals.

Editor's Note: This report was present in later August.

Desiree L. Plata, is a doctoral student in chemical oceanography at the Massachusetts Institute of Technology in Cambridge, Mass., and the Woods Hole Oceanographic Institution in Woods Hole, Mass.

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