Nanoscale Iron Could Help Clean the Environment

The ultrafine particles will flow underground and destroy toxic compounds in place

Years of pioneering research by environmental engineer Wei-xian Zhang of Lehigh University have demonstrated the potential of nanoscale powder made from iron to clean up soil and groundwater contaminated by industrial pollutants. Iron, one of the most abundant metals on Earth, could thus prove to be an invaluable part of the solution to the trillion dollar problem posed by more than 1000 untreated Superfund sites in the United States and contamination associated with other industrial sites, underground storage tank leakage, landfills, and abandoned mines.

Iron's capability for remediating toxic substances stems from the simple fact that it oxidizes readily. Ordinarily the only result is the familiar patina of brick-red iron oxide. But when metallic iron oxidizes in the presence of contaminants such as trichloroethylene, carbon tetrachloride, dioxins, or PCBs, these organic molecules are caught up in the reactions and broken down into simple carbon compounds that are far less toxic.

Site remediation with iron nanoparticles. Credit: Lehigh University

Likewise with dangerous heavy metals such as lead, nickel, mercury, or even uranium: the oxidizing iron will reduce these metals to an insoluble form that tends to stay locked in the soil, rather than spreading through the food chain. Iron itself has no known toxic effect. This is important considering the element is abundant in rocks, soil, water, and just about everything else on earth. Indeed, for all those reasons, many companies now use a relatively coarse form of metallic iron powder to purify their industrial wastes before releasing them into the environment.

Unfortunately, these coarse powders are not much help with the pollutants that have already seeped into the soil and water. That is the beauty of the nanoscale iron particles. Not only are they some 10 to 1000 times more reactive than conventional iron powders, because their smaller size collectively gives them a much larger surface area, but they can be suspended in a slurry and pumped straight into the heart of a contaminated site like an industrial-scale hypodermic injection. Once there, the particles will flow along with the groundwater to work their decontamination magic in place – a vastly cheaper proposition than digging out the soil and treating it shovelful by shovelful, which is how the worst of the Superfund sites are typically handled today.

In that sense, nanoscale iron is similar to in situ biological treatments that use specialized bacteria to metabolize the toxins. But unlike bacteria, the iron particles are not affected by soil acidity, temperature, or nutrient levels. Moreover, because the nanoparticles are between 1 and 100 nanometers in diameter, which is about 10 to 1000 times smaller than most bacteria, the tiny iron crystals can actually slip in between soil particles and avoid getting trapped.

Laboratory and field tests have confirmed that treatment with nanoscale iron particles can dramatically lower contaminant levels around the injection well within a day or two, and will all but eliminate them within a few weeks – reducing them so far that the formerly polluted site will meet federal groundwater quality standards. The tests also show that the nanoscale iron will remain active in the soil for 6 to 8 weeks, or until it completely dissolves in the groundwater. After that, the special particles will be essentially undetectable against the much higher background of naturally occurring iron.

The cost of the nanoscale iron treatments is not nearly as big a barrier as it was when Professor Zhang first began his research. In 1995, Zhang and his colleagues first developed a chemical route for making the particles. Then the nanoscale iron cost about $500 a kilogram; now, with improved manufacturing techniques, the cost is about $40 to $50 per kilogram. Decontaminating an area of about 100 square meters using a single injection well requires about 11 kilograms of the nanoscale particles.

Professor Zhang is forming a company to mass-produce the nanoscale iron particles. In the meantime, he and his colleagues are consulting with multiple clients.

Principal Investigator: Wei-xian Zhang, Lehigh University, (610)-

758-5318, wez3@lehigh.edu