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Reports

Silver Bullet or Tiny Terror?

Nanotechnology Promises to Clean Up Some of the Most Polluted Places in the United States. But Uncertainties about the Technology’s Effects Have Some People Worried about Creating and Unleashing a New Kind of Contaminant.

If all goes according to plan, in just a few years a host of green businesses will pack a section of a former naval complex known as “Parcel E,” part of the old, 500-acre Hunters Point Shipyard located in the industrial sector of southeast San Francisco. But before that can happen, the one-time naval shipyard needs to be cleared and cleansed. When the US Navy operated the site from 1941 to 1974, toxic compounds – such as tetrachloroethylene (TCE) – were used for the shipbuilding, repair, and maintenance operations. Since the shipyard’s closure, the Navy has been slowly cleaning up Hunters Point. Yet many pollutants remain intractable, which is one reason why the Navy plans to use a new technology known as “nanoremediation” to all but eliminate the toxic TCE plumes lurking deep below the dirt.

graphic of a spiny silver particle

Nanotechnology is the control of matter less than 100 nanometers in size; a nanometer is one-billionth of a meter, and nanoparticles are on an atomic or molecular scale. By way of comparison, a single human hair is about 80,000 nanometers wide. At such a small size, particles exhibit unique characteristics. Nano gold, for example, is purple. One of the defining traits of nanoparticles is a disproportionately large surface-to-volume ratio. Even though they are smaller, they are, in a sense, wider, which tends to make them more reactive. And it’s this enhanced reactivity that excites those who believe these tiny particles could spark the next big industrial revolution.

According to a 2008 survey by the Project on Emerging Nanotechnologies (a partnership between the Woodrow Wilson Center at Princeton and Pew Charitable Trusts), products containing nanoparticles are already flooding the market, with more than 800 products on shelves today. This includes food packaging, disinfectants, and clothing with nanosilver; cosmetics and sunscreen with nano zinc oxide; and nano cerium oxide used as a fuel catalyst. The industry’s boosters envision using nanotechnology to create carbon nanotubes that will conduct electricity 1,000 times more effectively than copper or using nanowires to perform delicate surgeries. The most sci-fi imaginings involve using nanotechnology to create computers or robots that could build and maintain themselves.

One application that’s already in use is nanoremediation. The idea is to use artificially small particles to clean up messes we’ve created with our other artificial technologies. So far, nanoremediation appears to show real promise: In many cases, it’s far more effective and cheaper than other clean-up methods.

Nanoremediation projects primarily use nanoscale elemental iron, known as zero valent iron or nZVI, to render contaminants benign by either reducing or absorbing them. As iron oxidizes in water and turns to rust, it releases electrons, which can quickly break down harmful contaminants, like dry cleaner fluids, into safer substances. Iron can also absorb certain contaminants, like arsenic, which easily bind to oxidized iron and are rendered benign.

The technology works by taking slurries of nZVI particles, mixing them with fluids like water, nitrogen gas, or vegetable oil, and then injecting the particles through small wells into a contaminated aquifer. In theory, because the particles are small, they can travel farther than larger-sized particles to reach contaminants buried deep below the surface. However, bare or uncoated nano iron doesn’t move very far by itself, so in the late 1990s Professor Wei-xian Zhang at Lehigh University in Pennsylvania began tweaking nano iron to make it more effective for groundwater remediation. “The initial objective was to create a mobile system where you could directly inject the iron into the groundwater or mix it with soil,” Zhang says.

In 2000, Trane Inc., an air conditioning manufacturer, used Zhang’s nano iron technology to remediate a toxic contaminant plume of TCE at one of its manufacturing plants. Water samples taken from monitoring wells 12 hours after the nanoparticles were injected showed that as much as 96 percent of the TCE was reduced to ethylene and ethane, both harmless chemicals. Since then, a number of nanoremediation projects have been conducted around the world. At an industrial facility in New York State, nanoremediation reduced source area concentrations of TCE from 1,900 to two micrograms per liter, which is below the federal safety limit. The Florida Department of Environmental Protection’s Dry Cleaner Remediation Program used nanoremediation to reduce total volatile organic compound concentrations by 90 percent. And at a commercial facility in the Czech Republic, nanoremediation reduced perchloroethylene (PCE) concentrations by 72 percent, TCE concentrations by 95 percent, and dichloroethene (DCE) concentrations by 85 percent.

Unlike existing remediation methods, such as pump-and-treat – which involves the arduous and costly task of digging up contaminants, treating them aboveground, and then placing the treated material back below the earth – or the use of permeable reactive barriers (PRBs, essentially mesh filters) to filter out contaminants, nanoremediation is heralded as a quick and cost-effective process. In 2004, environmental consulting firm PARS Environmental Inc. conducted a cost comparison of methods for cleaning up a contaminated manufacturing site in New Jersey and found that using pump-and-treat would cost approximately $4.1 million and PRBs approximately $2.2 million, while nZVI would cost about $450,000, representing a considerable cost savings.

Given that the United States is littered with thousands of Superfund sites (including 1,300 filled with contaminated water on the National Priorities List) and 450,000 brownfield sites, the lure of a fast, cost-effective remediation strategy is almost impossible to resist. In 2007, the Superfund program spent $380 million on cleanup, but that’s only a fraction of the estimated $250 billion and 30 to 35 years that the US EPA estimates it will take to fully decontaminate all of the nation’s hazardous waste sites. Using nanoremediation could result in a potential savings of $87 billion to $98 billion over the next 30 years.

There’s just one large splinter in this silver bullet technology. No one – including the EPA, which doesn’t regulate nanomaterials – knows what exactly the risks may be of unleashing these particles into the environment. Skeptics caution that nanoparticles require far more testing, and they worry that the tiny particles could end up forming new and unstable chemicals, infesting drinking water sources, and harming the health of humans and other animals.

“Generally we know that nanoparticles are more chemically reactive than their larger counterparts, so not only is their reactivity different, but also they exhibit totally different properties,” says Ian Illuminato, a health and environment campaigner at Friends of the Earth, which has called for a moratorium on the manufacture and use of all nanoparticles until further studies are conducted.

Although the EPA is allowed to regulate new commercial chemicals before they enter the market, under the Toxic Substances Control Act, most nanomaterials don’t qualify as new materials, but rather smaller versions of their larger chemical equivalents. For example, nano iron is considered to be the same chemical substance as regular iron so it’s not subject to additional testing requirements. And that has Illuminato and many others worried.

“A lot of our regulators would like us to think that these materials at the nanoscale are the same as their larger equivalents, but the science isn’t showing that. So really what we’re dealing with is a whole new line of chemicals.”

A Wonder Material?

To its critics, nanotechnologies are eerily reminiscent of the “wonder” materials of our past such, as DDT and asbestos – innovations that were heralded as world-changing solutions to long-standing problems, and which ended up becoming problems themselves, leaving behind toxic legacies that continue to haunt us today. Part of the fear comes from a lack of knowledge about nanotechnology. The potential risks of zero valent iron used in nanoremediation are poorly understood, a disconcerting fact acknowledged by regulators, researchers, and even some industry leaders. While some nanoparticles, such as carbon nanotubes, have been studied and already shown to carry potential health risks, research into nano iron’s health and environmental effects has barely scratched the surface.

“In order to prevent any potential adverse environmental impacts, proper evaluation, including full-scale ecosystem-wide studies, of these nanoparticles needs to be addressed before this technique is used on a mass scale,” wrote the authors of “Nanotechnology and in Situ Remediation: A Review of the Benefits and Potential Risks,” a recent report published in Environmental Health Perspectives.

One of the authors, Martha Otto, an EPA environmental engineer who has long followed the development of environmental cleanup applications of nanotechnology, believes that more research is needed to determine where these particles end up and what they become when used in environmental remediation. However, she says, “We don’t have any clear evidence that nanoremediation technology poses an ecotoxicity hazard, so we don’t have reason right now to believe that there’s a concern.”

Other organizations, including the United Kingdom’s Royal Society and Royal Academy of Engineering, have taken a harder line on the use of nanomaterials. In their 2004 joint report, “Nanoscience and Nanotechnologies: Opportunities and Uncertainties,” they recommend that, because there is “virtually no information available about the effect of nanoparticles on species other than humans or about how they behave in the air, water or soil, or about their ability to accumulate in food chains,” the release of these particles into the environment should be avoided as much as possible until more is known. Concerning nanoremediation, the authors recommend that, “the use of free nanoparticles in environmental applications such as remediation of groundwater be prohibited.”

Even some corporations have hesitated to unleash nanoremediation onto the environment because they worry that the uncertainties of the technology could lead to litigation. In 2007, DuPont ruled out using nZVI for remediation at any of its sites until questions concerning the fate of these materials once they’re injected into the environment are answered.

Critics charge that despite the push for more information, not enough research is being conducted. According to a 2008 Government Accountability Office report on federal nanotechnology expenditures, federal agencies in 2006 devoted only $37.7 million – a mere three percent of the $1.3 billion total nanotechnology research funding – to studies focused specifically on the environmental, health, and safety risks of nanotechnology.

“Our main issue with the US government is that we’re investing a billion and a half of our taxpayer dollars in research and development of nanotech per year, and out of that very little is spent on environmental, health, and safety studies, so that’s really pennies in terms of the science that that’s producing,” says Illuminato of Friends of the Earth.

Although studies on the effects of nano iron are limited, the research conducted so far has raised some eyebrows. A study published in Environmental Science and Technology in 2009 found that nano iron particles could be toxic to human lung cells due to nano iron’s high reactivity, which can release free radicals that damage cell DNA through a process called oxidative stress. Another study in March 2009 found that bare nano iron particles can accumulate on fish gills. In addition, bacteria and living cells seem to be able to absorb nanoparticles such as nZVI, which implies that these substances could bioaccumulate in the food web, slowly making their way up the food chain and eventually reaching humans. In 2006, researcher M.N. Moore cautioned that nanoparticle surfaces might serve as vehicles for transporting other toxic pollutants to critical areas in cells and organisms, though nano iron specifically has not yet been found to exhibit this trait.

Other nanomaterials have a similarly troubling rap sheet. For example, a 2009 study found that nanosilver can kill and mutate fish embryos, and a major study published in Nature Nanotechnology in 2008 discovered that long, thin, multi-walled carbon nanotubes can behave like asbestos, causing cancer in mice. However, many researchers point out that no two nanomaterials are alike and therefore should not be lumped together.

To help shed light on some of these murky areas, the EPA is funding two Centers for Environmental Implications of Nanotechnology, headquartered at Duke University and at University of California, Los Angeles. Gregory Lowry, deputy director of the Duke Center and a leading researcher in nZVI technology, is examining the fate of select nanoparticles (though not nano iron) in the center’s soil mesocosms, which are highly controlled wetland ecosystems used for studying the interactions between nanoparticles and plants, fish, bacteria, and other elements.

Though previous experiments indicated that the introduction of nZVI might negatively affect microbes, Lowry’s lab research has found that’s not the case, and in fact the microbial community is able to rebound after nano iron is introduced.

“You see a slight shift in native microbial populations, but certainly not a massive kill off of bacteria, even at several grams per liter of nZVI,” Lowry says. “Moreover, when the iron fully oxidizes, the bacteria tend to shift back to their original populations.”

Lowry has no plans to test nZVI in the mesocosms because he believes nano iron’s relatively short reactive lifetime and limited mobility suggest that the nano iron’s toxic component, iron zero, will break down long before it could pose a health concern. Of course, that’s assuming the nano iron is delivered correctly.

“With iron zero in the particles, there’s clearly some biocidal activity, and you can find reports in the literature that show that it is toxic,” explained Lowry. “However, many ecotoxicity studies on aquatic and terrestrial organisms use concentrations of nanoparticles much higher than one would expect in the environment, so the relevance of these studies needs to be examined.”

nano silver graphicnanosilver graphicphoto of zero-valent iron

Nano Material Benefits Risks
Carbon Nanotubes
  • Carbon sequestration
  • Hydrogen fuel cells
  • Remove arsenic
  • Free-radical production
  • Aerosol formation
  • Soil contaminant
Nanosilver
  • Zero-emission disinfectant
  • Treatments for immunological diseases
  • Immune system effects
  • Potential to cause wasting
Zero-Valent Iron
  • Soil and water remediation
  • Toxins replaced with other toxins
  • Little known environmental effects
Nano Titanium Oxide (TiO2)
  • Pollutants detected and neutralized
  • Negative respiratory effects
Quantum Dots and Nanocrystals
  • Efficient, low-cost solar cells
  • Contamination detection
  • Self-cleaning products
  • Bactericidal
Nano 2.0

Although nZVI is already being deployed in the field, practitioners don’t entirely understand the iron particles themselves, including how to maximize their potential and how optimizing certain characteristics affects the particles’ toxicity.

“There were a lot of initial reports based on simple lab experiments that showed very rapid destruction of contaminants of concern,” says Jeffrey Marqusee, the executive director of a Department of Defense research program into uses of nZVI. “But I think it quickly became clear that it’s a little more complicated than nZVI just being some magic answer for cleaning up these sites cost-effectively.”

One snag is getting the nano iron to the contamination zone before it clumps together to form aggregates too large to move underground. Another issue is making sure the nano iron doesn’t react with water and/or oxygen in the ground before reaching the contaminant. If that happens, the nano iron may not actually reduce a chemical to a benign substance. In addition, if the nano iron is unable to completely reduce a chemical such as TCE, what might be left behind is a chemical like vinyl chloride, a carcinogenic compound that’s toxic even at low doses.

“In principle iron can degrade these contaminants all the way down into carbon dioxide and water, but that doesn’t always happen,” says Andrew Maynard, a chief science advisor for the Woodrow Wilson International Center for Scholars’ Project on Emerging Nanotechnologies. “The question is whether we’re actually producing less harmful chemicals or just different chemicals.”

Factors such as insufficient contact times between nZVI and the contaminants, low pH levels, or simply not using enough nZVI can all lower nano iron’s effectiveness. To address these issues, researchers are enhancing nano iron’s mobility and reactivity by coating the iron in polymers.

“We’ll be injecting nano iron into the ground that will be encapsulated into a polymer type matrix to make it more stable in liquids,” says Denis O’Carroll, an environmental engineering professor at the University of Western Ontario. “If you don’t do that, the nano iron would rapidly settle to the bottom, so it won’t flow through the ground.”

Different types of nano iron particles also exist. Emulsified zero valent iron, or EZVI, is an iron particle encapsulated in a biodegradable oil membrane that can soak up insoluble contaminants and break them down into harmless compounds. Since the emulsifier is typically made of organic compounds such as vegetable oil, EZVI also provides food for naturally occurring microbes, which can then further eat away at contaminants. According to the EPA, EZVI was used to clean up TCE-contaminated soil and groundwater at an industrial site on Patrick Air Force Base in Florida where TCE concentrations were reduced from 150,000 micrograms per liter to approximately 3,580 micrograms per liter.

Bimetallic nanoscale particles (BNP), which are elemental iron particles coated with a catalyst such as platinum to increase reactivity, are also being explored. Todd Rees, a principal at Golder Associates, one of the leading consultancy firms in using nZVI technology, says that BNPs’ enhanced reactivity increases their ability to break down even PCBs, which pollute our nation’s waterways and our bodies despite being banned more than 30 years ago.

“We’re trying to take these iron particles that we already know what they can and can’t do and modify them to work on a whole new set of chemical compounds,” said Rees. “That’s a big quantum leap in the technology.”

But with new coatings come new unknowns. For example, increased mobility of nZVI may increase the possibility that nano iron could slip past the contaminated area and seep into drinking water aquifers.

“It’s quite tricky because obviously you want to make the particles mobile enough so that if you inject them in the ground they can transport far enough to the contamination that they’ll be able to do some good,” says Paul Tratnyek, a professor in the Division of Environmental and Biomolecular Systems at Oregon Health & Science University. “On the other hand, if they’re too mobile then that’s a problem because you don’t really want them coming out in the stream. So there’s this trade-off. It’s still a little bit of an evolving story.”

Tratnyek says that the real question lies in whether the technology actually works, not just in computer models, but also on the ground.

“The real debate is, does this really work? And is it just a waste of money? That’s actually a much more open question at this point,” Tratnyek says. “The lab results are relatively easy to trace and interpret, but when people go out and try to do this in the real world, then it’s often quite difficult how to figure out exactly what happened, or for that matter exactly what was done.

“The broader picture is that there’s a lot of work being done by academic types in the lab and then there’s a fairly surprisingly large communion of people that claim to have jetted a bunch of this stuff into the ground in real full-scale field sites, and there’s kind of a gap in between in terms of reconciling the lab-based results versus the apparent performance of these things. So we won’t really know for a few years until the dust settles whether this technology really works.”

In the meantime, what is clear is that the number of nZVI applications is steadily multiplying. So far, the EPA lists 44 sites in seven countries and 12 US states where nanoremediation has been released into the environment to clean up oil fields, manufacturing sites, military installations, private properties, and residences.

“It’s a very promising and economically viable technology, but this whole field is plagued by unknowns,” said Maynard of the Woodrow Wilson Center. “It’s just a case of treading carefully and treading cautiously.”

Jessica A. Knoblauch is a freelance environmental journalist living in the San Francisco Bay Area. Her work has appeared on Grist.org and in Environmental Health News and E: The Environmental Magazine.

   

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