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.
Good article explaining the detection method for CNT's in soil. Are there toxicity concerns for CNT's in product? Also, are there concerns with the processing method used to add the CNT's to the base material?
Ann, this is indeed a concern. Like many of the clever solutions to engineering problems, we have to think of the effect on living organisms, not just humans. Semiconductor manufacturing also uses many toxic chemicals, for example, and these have to be controlled. This is true at the point of manufacture and at the point of disposal. I recall that even the ink used in thermal printers, such as those that are used to print receipts at stores, can be toxic. We need to be careful in handling exotic, engineered materials.
One question I do have is about the detection method. Since microwaves are used, I assume that the tests done on earhtworms are destructive. Soil, even after being exposed to microwaves, is still just soil. An earthworm on the other hand...
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