Nanotubes persist in natural environments

[Guest guest]

Natural organic matter stabilizes carbon nanotubes in aqueous

environments, an interaction that increases the potential for their

transport downstream.

The explosion in the creation of better and more complicated

nanomaterials is moving far ahead of research on such materials'

environmental safety, leaving many to wonder about their potential

effects on human health and the environment. New research published

today on ES & T's Research ASAP website (DOI: 10.1021/es061817g)

underscores that these anthropogenic nanomaterials readily interact

with natural materials.

 

 

Courtesy of Jae-Hong Kim et al.

Multiwalled carbon nanotubes suspended in Suwannee River water

remained stable after 4 days, in the bottle on the left. The

relatively clear bottle on the right contains no nanotubes.The data

are the first to show how multiwalled carbon nanotubes (MWNTs) might

behave in natural aquatic environments. Researchers say that because

organic material stabilizes the nanotubes, their potential for

dispersal increases dramatically, but the toxicity of the new

materials in natural environments remains relatively unknown.

 

Industrial manufacturers have created many types of nanomaterials for

use in a broad spectrum of applications. In addition to " buckyballs " —

the C60 molecules that are also called fullerenes—nanotubes can be

tiny straw-shaped structures made with single or multiple layers of

carbon atoms. Both the particles and the tubes tend to clump

together, but surfactants and added polymers can keep them from

cohering in laboratory settings. Even so, previous experiments

demonstrate that C60 aggregates in natural settings.

 

A team led by Jae-Hong Kim of the Georgia Institute of Technology

examined how MWNTs interact with Suwannee River natural organic

matter suspended in a 1% solution of sodium dodecyl sulfate (a

surfactant known to keep nanotubes and nanoparticles discrete).

Electron microscopy, measurements of opacity and turbidity, and other

analyses were used to determine the behavior of the MWNTs in various

mixtures left to settle over hours and days.

 

In clean water, the nanotubes settled to the bottom within an hour.

In contrast, the surfactant-only solution kept the tubes suspended

for a day or more. But the water containing organic matter remained

cloudy for more than 4 days, and a month later the mixture retained a

grayish background color from suspended nanotubes.

 

Natural organic matter " suspends [MWNTs] in water and becomes a

vehicle for transport, " says a coauthor of the paper, Joseph Hughes

of the Georgia Institute of Technology. The team's finding " clearly

demonstrates the fact that as we produce these anthropogenic

nanomaterials, they're going to interact with natural materials, " he

says. Using electron microscopy, the researchers showed that natural

organic matter not only acts to stabilize individual nanotubes in

solution but also sometimes even creates bridges between two tubes.

 

Corresponding author Kim calls the results intriguing. " The fact that

natural organic matter stabilized the model carbon nanotube (MWNT) in

the aqueous phase more efficiently than surfactant was pretty

surprising to us, " Kim said, " since similar behavior was not observed

with another widely investigated carbon nanomaterial—C60. "

 

" It's interesting that the nanotubes remain as discrete units, " says

Ron Turco of Purdue University. " I would have said they would cluster

up. " This experiment is the first basic research to address carbon

nanotubes in the environment, and even then only the most basic form

of such nanomaterials, he continues. " It is going to have to be

repeated, " he adds, in flowing systems with sediments. Researchers

will also need to track whether the nanotubes degrade, he says.

 

A question for future research, Turco notes, is whether single tubes

are more toxic than an aggregate of MWNTs. The natural organic

matter " may allow them to transfer farther, and [eventually] interact

with cells, bacteria, or fungi, or higher up the food chain,

differently than they would as a cluster. " Research so far shows that

single-walled carbon nanotubes can behave much like silica fibers in

pulmonary tissues, causing inflammation associated with lacerations;

this effect makes them an occupational health hazard. Cell cultures

and other experiments indicate varying levels of toxic effects.

 

Turco says that the field is " wide open " and constantly changing, as

industrial manufacturers introduce ever more " flavors " of nanotubes

and particles with various components attached to their outer

coatings. " Everyone's looking for a material with a substitution that

will do something new and unique to replace something else. It's

great chemistry, " he says. " But in terms of environmental effects,

we're not able to keep up right now. We're still trying to figure out

plain carbon nanotubes. " —NAOMI LUBICK