An ultrafine, ‘nanoscale’ powder made from iron is turning out to be a remarkably effective tool for cleaning up contaminated soil and groundwater- a trillion-dollar problem that encompasses more than 1000 still-untreated Superfund sites in the US, some 150,000 underground storage tank releases, and a staggering number of landfills, abandoned mines, and industrial sites.
The case for nanoscale iron was described in the September 3 issue of the Journal of Nanoparticle Research, where Lehigh University environmental engineer Wei-xian Zhang reviewed his eight years of work with the material.
Iron’s cleansing power stems from the simple fact that it rusts, or oxidises, explains Zhang. Ordinarily, of course, the only result is the familiar patina of brick-red iron oxide.
But when metallic iron oxidises in the presence of contaminants such as trichloroethene, carbon tetrachloride, dioxins, or PCBs, he says, these organic molecules get caught up in the reaction and broken down into simple carbon compounds that are far less toxic.
And that’s also true with dangerous heavy metals such as lead, nickel, mercury, or even uranium, says Zhang. The oxidising iron will reduce these metals to an insoluble form that tends to stay locked in the soil, rather than spreading through the food chain. And, iron itself has no known toxic effect. Indeed, says Zhang, 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, adds Zhang, these industrial reactors aren’t much help with the pollutants that have already seeped into the soil and water. That’s 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. Once there, the particles will flow along with the groundwater to decontaminate the soil.
In that sense, says Zhang, nanoscale iron is similar to in situ biological treatments that use specialised bacteria to metabolise the toxins. But unlike bacteria, he says, the iron particles aren’t affected by soil acidity, temperature, or nutrient levels. Moreover, because the nanoparticles are between 1 and 100 nanometres in diameter, which is about ten to a thousand times smaller than most bacteria, they can slip in between soil particles and not get trapped.
The cost of the nanoscale iron treatments is not nearly as big a barrier as it was in 1995, when Zhang and his colleagues first developed a means to make the particles. Then the nanoscale iron cost about $500 a kilogram; now, it’s between $40 to $50 per kilogram. (Decontaminating an area of about 100 square meters using a single ‘injection’ well requires 11.2 kilograms.)
Zhang is currently forming a company to mass-produce the particles.