The US Environmental Protection Agency (EPA) has published an upbeat fact sheet on the potential of nanotechnology for remediating contaminated land.1 Trials have been carried out at 26 sites in the US and Canada, including the space launch complex at Cape Canaveral, Florida. But the method has yet to emerge from the lab in the UK.
Most research and applications focus on nano-scale zero-valent iron (nZVI) and related products, used at 25 of the 26 trials. The fact sheet says "nZVI may prove more effective and less costly than macroscale ZVI under similar environmental conditions".
The aim is for particles of nZVI to react with soil contaminants, producing far less toxic compounds. They can be modified in various ways to meet specific needs. Small amounts of another metal, such as platinum or palladium, can be added to help them destroy contaminants more effectively. Or the nZVI particles can be suspended in a vegetable oil emulsion to stops the iron from rusting in air or water.
There are ongoing laboratory studies into more advanced products, including carbon nanotubes and porphyrins, which are cyclic organic compounds that bind to metals. They are related to natural structures found in chlorophyll and haemoglobin.
The contaminants which particles can deal with include dense non-aqueous phase liquids such as many chlorinated solvents, polychlorinated biphenyls, halogenated aromatics and metals such as arsenic or chromium. Most trials so far have involved chlorinated solvents.
Seven full-scale field trials have been carried out. Five have met their clean-up goals and the remaining two have shown decreasing trends in contaminant levels. Each project can involve hundreds of kilograms of nanoscale material.
One of these trials was in Hamilton Township, New Jersey. A former waste site contaminated with various chlorinated solvents was treated with more than two tonnes of a nZVI-water slurry injected into the ground. Contractor PARS Environmental reported solvent concentrations were reduced by up to 90%.
In the UK nanoremediation activities are more circumspect. In their 2004 report, the Royal Society and Royal Academy of Engineering raised concerns about the release of ‘free’ nanoparticles into the environment, which they said should be prohibited. In its response to the report, the government said it would work with industry to prevent nanoremediation projects "until there is sufficient evidence that the benefits outweigh any adverse effects" (ENDS Report 362, p 54 ).
Although no legislative prohibition is in place, a spokesman for the UK’s Environment Agency said it was not aware of any schemes using nanomaterials in remediation work. "We’ve had enquiries about using it in pilot projects for groundwater remediation, but no one has come back to say they want to proceed," he said. "They would need to approach us for licensing, and we would have to be satisfied that benefits outweighed risks."
Environmental risk assessments can be applied to nanomaterials as for other potentially hazardous substances, according to Professor Steven Banwart at the University of Sheffield and a member of the UK Nanotechnologies Environmental Risk Assessment Task Force. "But there may be knowledge gaps we need to fill in," he says, and "because this is such a new area, you would probably have to do a whole range of site-specific trials."
Professor Banwart is using bench-top reactors to try to understand nanoparticle interactions with bacteria. "I don’t think anyone [in North America] is seeing any really bad side effects," he said. But going to the nano-scale means that particles "could potentially enter cells, and the biological impacts are not well known. Are they potentially toxic for instance?" he asks.
His research is also using porous sand bed reactors to look at how nanoparticles move in soils compared with their larger counterparts. More information is needed on nanoparticle persistence, whether they stick to soils or are mobile, potentially even transporting contaminants further from the site.
Professor Banwart’s project is one of several funded under the Environmental Nanosciences Initiative, a joint programme backed by the Natural Environment Research Council, the Environment Department (DEFRA) and the Environment Agency.
Professor Banwart said nanoremediation methods are more advanced in North America than the UK because there is a larger body of prior research and more commercial experience.
"Regulatory barriers here in the UK aren’t inherently a problem," he says, but cautions that "as with any new technology it will not be a panacea, and it may only be effective in certain circumstances".
Almost any subsurface remediation technology is subject to hydrogeological constraints, Professor Banwart said. "It’s hard to get things where you want them to go underground due to site geology, water flows and other factors. The whole process is not completely predictable."