Chad Jafvert and then-PhD student Wen-Che Hou in the field with some carbon nanomaterials, testing the effects of exposure.
A new study has found sunlight may help break down carbon nanotubes — but also suggests the products of the photochemical reaction could be toxic to aquatic organisms.
“This study is one of the first to address whether [carbon nanotubes] undergo reactions in the environment,” said environmental engineer Chad Jafvert of Purdue University, one of the researchers. He said exposure to sunlight seems to produce highly reactive oxygen molecules that could degrade the carbon nanotubes; but those same molecules also are known to damage cells and DNA.
The results mirror the complexity of the effort to understand how manufactured nanoparticles — materials measured in billionths of a meter — might affect the environment and human health.
The growing use of nanomaterials in hundreds of industrial and consumer products, and their huge potential in medicine, electronics, energy and other fields, have lent urgency to that effort. Scientists are concerned about what happens to these substances when workers and consumers are exposed to them, and when they enter the environment.
Because of their tiny size, the materials can take on extraordinary properties. Carbon nanotubes, for instance, can be lighter and stronger than steel and are already used to make bicycles, tennis racquets, car bumpers and other products.
But nanoparticles also can react differently with living tissue than their larger-sale counterparts. They are small enough to infiltrate human cells and potentially cause health problems. Some carbon nanotubes have been found to behave similarly to deadly asbestos fibers when inhaled.
In this study, published in August in the journal Environmental & Science Technology, Jafvert and his graduate student dispersed chemically altered single-walled carbon nanotubes in water. They exposed the solutions to sunlight and found they produced several types of reactive oxygen species — that is, molecules containing oxygen that because of their electrical properties react with other molecules.
These types of molecules occur naturally in the environment and are produced by normal photolytic processes; they play both useful and destructive roles. If unchecked they can cause cell damage. The study also raises questions about how the chemical properties of carbon nanotubes might change when exposed to environmental influences.
When the researchers exposed an acidic solution of the nanotubes to light, the nanotubes separated out of solution and clumped together. This suggests that these materials may be chemically altered by this process, which could also affect the materials’ toxicity.
“If you’re going to assess this material, you’ve got to ask the question whether this interaction with sunlight is going to increase the environmental harm,” said Andrew D. Maynard, director of the risk science center at the University of Michigan. “Maybe exposure to sunlight starts to degrade the nanotubes; maybe these nanotubes aren’t going to be as long-lived in the environment as we thought, and that’s a good thing.”
Maynard said the results point to a possible positive use of carbon nanotubes: If they can be induced to produce reactive oxygen species, they could be used to kill off microbes in water supplies.
On the other hand, “if these carbon nanotubes get out in an uncontrolled manner, I would be worried about what it does.”
A 2007 review of carbon nanotube studies published in Environmental Health Perspectives concluded: “Once taken up by humans or other species, [carbon nanotubes] may cause oxidative stress, inflammation, cell damage, adverse effects on cell performance, and, in a long-term perspective, pathological effects like granulomas, fibrosis, and wall thickening. These effects have been observed time and dose dependently in the majority of toxicology studies.”
That review noted there are many types of carbon nanotubes, and each needs to be assessed for its potential toxicity. There are single- and multi-walled carbon nanotubes, different shapes, and different chemicals attached to them to give them particular properties.
Jafvert, of Purdue, said it’s important to follow up his study with research to see whether the nanotubes actually do degrade, and how oxidation might affect their chemical properties.
“Information is needed on [carbon nanotubes’] persistence in the environment,” he said. “For example, if they contact soil, how long do we expect them to last in the soil, or will they quickly be degraded? If they are degraded, how do their chemical properties change as reactions on their surfaces occur? Are these altered CNTs more toxic than the original materials? Do they completely break down in the environment, or do products persist in the environment for long times?”
Jafvert said the nature of these tiny particles makes it very hard to separate them out from the environment and measure their effects. He and his colleagues are continuing to study the photochemical reactivity of other forms of carbon nanotubes, he said.
“The most critical thing [in the study] is that it’s proof of the principle that something might happen if [carbon nanotubes] get into the wrong place at the wrong time,” said Maynard, the Michigan researcher.
For more on nanotechnology and its impact on health and environment, go here.