Scientists and environmental regulators are hoping that some extremely tiny materials might make a huge difference in hazardous waste cleanups.
Researchers are exploring whether nanoscale materials -- so named because they are as small as 1/100,000 the width of a human hair -- can be cleanup assets. They have two reasons for optimism: Nanomaterials' size lets them penetrate otherwise impossible-to-reach groundwater or soil, and their engineered coatings allow them to stay suspended in groundwater, a major asset in cleanups.
If they work, nanomaterials could slash cleanup prices by avoiding the extraordinary costs and risks of hauling materials away for burning or burial.
"There is significant potential to target a number of very complicated sites that to date we have been unable to remediate adequately," said Denis O'Carroll, an assistant professor of civil and environmental engineering at the University of Western Ontario.
The Woodrow Wilson Center's Project on Emerging Nanotechnologies (PEN) has identified 45 sites in several countries that are currently using nanomaterials for waste cleanups. But there is little data on cost and performance of nano-remediation, since most projects are in their infancy. Moreover, companies involved in the work are tight-lipped, concerned about releasing cost and performance information they consider proprietary.
"It's difficult to quantify operational costs," said Jeffrey Marqusee, executive director of the Defense Department's Strategic Environmental Research and Development Program and the Environmental Security Technology Certification Program. Marqusee's programs are tasked with researching and developing environmental cleanups.
Most DOD nano-remediation research projects are focused on cleaning up groundwater contaminated by chlorinated solvents such as trichloroethylene, Marqusee said. His teams have used nano-scale emulsified zero-valent iron at the Marine Corps base in Parris Island, S.C., among others.
Iron is a key element in cleanups, both traditional and nano, because it strips down toxic compounds and leaves behind individual, nontoxic elements. It has been used for some time at cleanups to form permeable barriers that keep contaminants from being carried off the site with groundwater. In nano-cleanups, nano-scale zero-valent iron is injected in toxic "hot spots," the source of contamination.
So far, results are promising, Marqusee said. In most demonstrations, he said, most of the contaminant has been destroyed, a finding that has been replicated by researchers elsewhere.
But there are complications. For one, iron also reacts with nontargeted materials, making it degrade too quickly -- before remediation is complete, O'Carroll said. Another problem: Iron particles clump after they are released, making it difficult for them to travel beyond where they are injected.
"Mobility is a really big deal," O'Carroll said. "With any sub-surface remediation technology, the big deal is getting it to the contaminant. You can't see it, so knowing where it is is a challenge."
So scientists are trying coatings that might enable nano-particles to travel. Researchers are also trying to learn how to make the nano-iron react only with target contaminants and not the surrounding environment. They are also trying to get it to self-destruct after it has done its job.
Such "smarter" nanomaterials will ultimately make the technology less risky, according to a paper published recently by U.S. EPA and the Wilson Center's PEN in the journal Environmental Health Perspectives.
But nanotechnology won't be a silver bullet for cleanups because most contaminated sites are fouled by more than a single contaminant, O'Carroll said. Scientists will probably always need to have a phased approach, in which one technology is used to get rid of certain pollutants, followed by a different approach and then another as the site requires, he said.
And there is still a long way before nanotechnology becomes a mainstay at hazardous waste cleanups -- if it ever does. For one thing, regulatory uncertainties abound.
"It can save years of time and billions of dollars in estimates for cleanup for some Superfund sites," said Jane Warren, a lawyer at McCarter & English. "The problem is, as with any new technology, it hasn't been studied enough. There aren't even reliable ways to measure whether nanomaterials are achieving the goals they claim to be able to achieve."
And companies that do use nanomaterials in cleanups have been reluctant to discuss their efforts openly.
For example, a source who worked with one company that used nanomaterials in a cleanup agreed in an interview to discuss the work on the condition that neither the company nor the project be identified by name.
That company used nano-scale, zero-valent iron in two projects to clean up the solvent trichloroethane, or TCA, at a site that stayed frozen most of the year, the source said. The projects saw mixed success, depending on how the iron was delivered.
Delivering iron with water destroyed 99 percent of the contaminant after six weeks, and the site remained clean a year later, the source said. But EPA decided not to approve the use of nanomaterials outside of pilots on the site because TCA levels were so high and because of concerns about how nanomaterials might interact with the environment, the source said.
"One of the main concerns is whether the particles are going to be releasing any other contaminants and making them mobile," said Todd Kuiken, a research associate at PEN, who was speaking generally about nano-scale iron.
"The iron is not just interacting with the chemical it wants to go after, it's reacting with anything in the area. It's a similar story with all nanoparticles -- no one's sure if they're going to be harmful or not, so we're advocating that people look into it."
Martha Otto, an EPA environmental engineer, said the agency evaluates cleanup technologies on a site-specific basis. Each site overseen by EPA has a remedial project manager who evaluates the particular site and weighs all possible options to determine the best available technology to use.
In that process, it is important to remember that different nanomaterials have varying degrees of potential risks and behaviors, said Barbara Karn, an EPA scientist.
"Not all nanomaterials are the same," Karn said. "You almost have to go, at this point, on a nanomaterial-by-nanomaterial basis. You really have to look at what kinds of materials we're using for the cleanup, then look at the end. I'd much rather have some excess active iron in the soil than a chlorinated hydrocarbon [the contaminant]."
And Otto pointed to the long track record iron has at successfully cleaning up sites. "The nano-scale zero-valent iron is just a small version of a material that's been used for years in site remediation," she said.
'Better of two evils'?
Eventually, Warren said research will likely show that nanomaterials are less harmful than the contaminants they are cleaning up.
"I suspect and hope that nanotech proves to be the better of two evils," Warren said.
Long-term concerns might become an issue once the technology develops and particles can remain suspended underground for a year or more, said O'Carroll, the Western Ontario engineering professor.
For now, the technology is not ready.
"So much fundamental work needs to be done," EPA's Karn said. "We are continuing our research in risk assessment, toxicity, exposure routes, fate and transport."
EPA provides grants to spur that kind of research. Since the agency first began to fund grants for nanotechnology in 2000, it has spent more than $37 million, Karn said. Of that, $10.5 million has focused on fate and transport, and $3.3 million on remediation. The remaining grants have looked at toxicity, exposure, with much smaller amounts in sensors, treatment and green manufacturing.
Because of these concerns and gaps in knowledge, there are some who think the technology's development should be dramatically slowed down. In the go-slow crowd is George Kimbrell, a staff attorney at the International Center For Technology Assessment, a group urging a moratorium on nanotechnology until its risks are better understood.
"I don't think we know enough yet to do so safely," Kimbrell said in an e-mail, "whether the release is unintentional or intentional."
Click here to read the Environmental Health Perspectives paper.