Since their invention a decade ago at Lehigh University, iron nanoparticles 1,000 times thinner than a human hair have demonstrated an unprecedented ability to clean contaminated groundwater. The palladium-coated particles have remediated more than 50 toxic waste sites in the US and other countries in 10 times faster than traditional pump-and-treat methods.
Researchers at Lehigh University have been utilising scanning transmission electron microscopy (STEM) and X-ray energy dispersive spectroscopy (XEDS) in order to extend and improve the applications of the powerful nanoparticles. They have captured, for the first time, the evolution in the nanostructure of the bimetallic particles as they remove contaminants in water. As they react with pollutants such as trichloroethene (TCE), a toxic industrial solvent, the nanoparticles display huge structural changes. The particle core hollows out, the iron diffuses outward and the palladium, a catalyst that makes up 1% of particle mass, migrates from the outer surface to the interior surface of the iron.
Writing earlier this month in Environmental Science and Technology (ES&T), the Lehigh researchers reported that the ability of the nanoparticles to remove toxins decreases as the particles ‘age' and undergo structural change with exposure to water. Their results, they wrote, suggest that the age and storage environment of the nanoparticles play a critical role in influencing their effectiveness as remediation agents. The nanoparticles, which were invented by co-author Zhang, average 50 nanometres in diameter (1 nm equals a billionth of a metre). Islands of palladium on the outer surface of the iron measure 2-5nm in diameter. The particles have removed pesticides, vinyl chloride, TCE and other contaminants in 10 states and in Europe and Asia. Treated sites include landfills, an electronics manufacturing plant, chemical plants and military facilities.
When injected into groundwater, the nanoparticles flow with the water and react with and detoxify contaminants. Their small size and greater proportional surface area give them more reactivity with toxins than larger quantities of the same catalyst. According to Harch Gill (president of Lehigh Nanotech LLC, a Bethlehem company which owns the commercial rights to the particles), this superior reactivity enables the particles to remediate a toxic site in less than a year; treating such a site with traditional pump-and-treat methods would take 10-20 years. According to the Association of University Technology Managers, which named Lehigh Nanotech one of the top 25 technology-collaboration stories in 2008, 'it takes only six ounces of the tiny nanomaterials, versus a ton of larger compounds, to make sweeping changes in cleaning up contaminated environments.' As a result, the nanoparticles are now one of the world's most widely used nanomaterials.
Lead author Yan, who has studied the nanoparticles since 2007, says the experimental results will help researchers develop better methods of handling and storing the particles, and of collecting and reusing the palladium after it has neutralized contaminants. Palladium, used in catalytic converters, electronic devices and fuel cells, is a rare and often expensive metal.
The results can also be used to improve the ability of iron-based nanoparticles to capture and remove heavy-metal toxins from contaminated sites. 'This paper is just a starting point', she says. 'Using the same suite of tools, we can study metal species and nZVI to learn how heavy metals are captured by nZVI, where they interact and where the final destination of the heavy metals is inside the nZVI.'