So far, so encouraging - or perhaps not. According to a study presented at the national meting of the American Chemical Society (ACS) in Boston earlier this week, the by-products discharged to the environment during carbon-nanotube production include a cocktail of carcinogens, smog-forming compounds and greenhouse gases.
In a volume manufacturing scenario, those by-products will have environmental impacts that "could be severe and costly to repair", said study co-author Desirée Plata, who carried out the research along with colleagues at Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution. Previous investigations in this field focused on the toxicity of the reactants used in nanotube production. The MIT-Woods Hole work is different, highlighting the dangers associated with by-products and unreacted feedstock.
Specifically, Plata and co-workers examined the emission products of a representative carbon-nanotube synthesis. They used a small-scale device to simulate chemical-vapour deposition, one of the main methods for making carbon nanotubes. Analysis of the by-products showed the presence of at least 15 aromatic hydrocarbons, including four types of toxic polycyclic aromatic hydrocarbons (PAHs), similar to those found in cigarette smoke and car-exhaust emissions.
The most harmful PAH identified was benzo[a]pyrene, a known human carcinogen. Other by-products included hydrocarbons that contribute to smog and ozone formation in the lower atmosphere. "If nanotubes are produced in the tons, there will also be tons of PAHs produced," Plata explained.
The MIT-Woods Hole scientists are currently working with four US nanotube producers to define strategies for environmentally sustainable nanotube production. Technology fixes are likely to include "green" chemical reactions and the integration of special filters or "scrubbers" in the production process. Another option, says Plata, is to pursue alternative manufacturing processes that produce fewer toxins.
Carbon nanotubes are already produced on a small industrial scale. In the next stage of their research, the scientists plan to measure actual emissions at several production facilities to get a picture of real-world emissions. "Without this work," added Plata, "the environmental and health impacts of the carbon-nanotube industry could be severe and costly to repair. We would like to help it [the nanotube industry] develop in an environmentally sustainable fashion."