Carbon nanotubes (CNTs) add strength and design flexibility to many materials, from reinforcing adhesives and materials used in electronics to enhancing the design of carbon fiber composites for use in cars and aerospace applications. But there's been increasing evidence that they, along with other nanoscale materials, may pose risks to human health and the environment. In ongoing research efforts to determine just how toxic they actually are, researchers at Texas Tech University have come up with a new method for detecting CNTs in soils.
The university's environmental biologists built an apparatus that can quantify how much CNT is present in a given soil sample. That's not an easy task since they're so small: mean outer diameters of 13nm to 16nm are common in multi-walled tubes. The apparatus was developed as part of the researchers' ongoing work in locating CNTs in biological environments and examining how they to accumulate in soil, plants such as food crops, or other organisms.
Researchers at Texas Tech University have come up with a new method for detecting CNTs in soils, which will help determine their toxicity. CNTs are so small that mean outer diameters of 13nm to 16nm are common in multi-walled tubes, shown here as grains partially smeared on paper (scale in centimeters). (Source: Shaddack/Wikimedia Commons)
The testing apparatus works by exposing soil samples to microwaves, which can reveal even mere trace quantities of CNTs. That's because, in the presence of microwaves, CNTs produce very high quantities of heat, much higher than most materials. The effect is so extreme that, if you put CNTs in your kitchen microwave oven, the carbon will spontaneously ignite. The researchers used this fact to heat samples to different temperatures to determine different concentration levels. The method has also been used to determine the amount of CNT loading in plant samples and earthworms.
The news about CNTs' toxicity is more than a little unwelcome, since graphene, the tubes' constituent substance, is the hardest known substance. When layers of it are rolled up into tubes, the resulting CNTs constitute a fiber that is 100 times stronger than steel and weighs one-sixth as much. That combination, along with qualities such as mechanical strength and electrical and thermal conductivity, makes them highly unusual and ideal building blocks for industrial uses.
Earlier this year, a joint study by the University of Missouri and the US Geological Survey showed CNTs to be toxic to various species of invertebrate aquatic organisms that live in sediment, including mussels, worms, and crustaceans. These researchers note that CNTs, which may contain metals as well as carbon, tend to accumulate in sediment when released into water. Both the metals -- including nickel, chromium, and other metals used in manufacturing that may remain as impurities -- and the carbon in CNTs can reduce growth rates or even kill certain types of marine life. The degree of toxicity varied depending on the type and source of CNT, the species of test organism, whether the materials had been cleaned using acid, and what method was used for dispersing the materials.
Researchers of the joint study say one of the biggest potential contamination risks occurs during the manufacture of carbon composites, but careful waste management and handling procedures can reduce that risk. More information is also needed on what happens when the composites begin to break down.
Damascus steel is certainly some impressive stuff, but my understanding is that it is created through a great deal of work, bonding and folding and rebonding to get steel with alternating layers of hard and tough steel. Definitely not a naturally occurring material. Free lead, nickle, cadmium, and copper all occur in their free state and can be found naturally. Likewise asbestos, although it is usually in with mica, I think.
Of course heating to ignition with microwaves would be an effective means of disposal for CNT waste, but first it would need to be located. That is probably going to be the challenge if quantities of CNT material are spilled.
If microaves can heat up the CNT then they can destroy them. Collect the waste and bath it with microwaves, waste disposal problem solved. Light them up! A plasma furnace would also work and other metals can be recovered. iRobot can probably build a Roomba robot that will do manufacturing cleanup and CNT destruction at the same time.
It seems that the alleged toxicity of these nanotubes must be a mechanical thing, although none of the reports bothers to say anything about the mechanism of toxicity. The good news is that these nanotubes don't occur naturally, at least I have not heard of them being natural. So the proliferation should be quite a bit less. Also, they are kind of expensive, I think, so perhaps users may be motivated to avoid spilling them.
It would be good if those who go around bleating out noninformational phrases could somehow be motivated to provide more actual information and less intention toward causing hysteria.
Of course, if the nanotubes are locked into a composite material they may be a lot less free to cause any type of problems, although it would seem that machining the composite could be a bit hazardous.
Interesting comments. This is the first I've heard about toxicity concerns in CNT's. Guess it's time to dump my stock in graphene futures! When I hear about toxicity concerns, the scientist voice inside my head always asks, "But what is the mechanism at work?". As you point out, it may be an issue of mechanics rather than chemistry. Once this is understood, then a potential solution is usually at hand. In the meantime I'll start using a HEPA filter when I sharpen my #2 pencil.
Asbestos is absolutely safe (inorganic and non-reactive). It's the genetic material that rides along with it when asbestos (because it's ends are needle sharp) puncture a cell's wall that kills. Which is why asbestos was used everywhere without any concern for human health (in schools, ships, tile floors). Human history is full of examples of "safe" technology that that was over-exploited before realizing the dangers.
Carbon nanotubes probably have similar mechanical effects. I was told by an ME friend that carbon fiber (yes, not nanotubes, but still relevant) is much more dangerous than fiberglass. Where you get fiberglass on or in your skin, it works its way out. Carbon fiber works their way in ...
I read in Science News a few years ago, nanotubes in the environment do NOT disperse like other polutants (perhaps this is a good thing?)
In any case, if they are so reactive to microwave's, perhaps this is the solution. Just blast the contaminated soil with a high enough dose to destroy the bonds and turn them back into simple carbon.
It's good to have that voice of reason to set some perspective. Still, it's better to find it now and accomodate its problems than much, much later when it's everywhere. Asbestos comes to mind in that regard.
Its carbon, the thing that loves to react with oxygen... Light it up :) Let's find out how fast this material will decay or adhere to larger particles. How much of it is released in the industry, as well as from end use products and compare it with the amount of material required for it to be a threat prior to hitting the panic button.
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