September 20, 2003
Iron Nanoparticles More Effective Way To Clean Up Polluted Sites

Lehigh University environmental engineer Wei-xian Zhang has developed techniques to use iron nanoparticles to destroy dangerous organic compounds in soil and to neutralize toxic heavy metals in soil.

Iron's cleansing power stems from the simple fact that it rusts, or oxidizes, explains Zhang. Ordinarily, of course, the only result is the familiar patina of brick-red iron oxide. But when metallic iron oxidizes in the presence of contaminants such as trichloroethene, carbon tetrachloride, dioxins, or PCBs, he says, these organic molecules get caught up in the reactions and broken down into simple carbon compounds that are far less toxic.

Likewise with dangerous heavy metals such as lead, nickel, mercury, or even uranium, says Zhang: 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. And, iron itself has no known toxic effect--just as well, considering the element is abundant in rocks, soil, water, and just about everything else on the planet. 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, says 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 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, says Zhang, nanoscale iron is similar to in situ biological treatments that use specialized bacteria to metabolize 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 nanometers in diameter, which is about ten to a thousand 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 drastically 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 now meet federal groundwater quality standards. The tests also show that the nanoscale iron will remain active in the soil for 6 to 8 weeks, says Zhang, or until what's left of it dissolves in the groundwater. And after that, of course, it will be essentially undetectable against the much higher background of naturally occurring iron.

Finally, says Zhang, the cost of the nanoscale iron treatments is not nearly as big a barrier as it was in 1995, when he and his colleagues first developed a chemical route for making the particles. Then the nanoscale iron cost about $500 a kilogram; now, it's more like $40 to $50 per kilogram. (Decontaminating an area of about 100 square meters using a single injection well requires 11.2 kilograms.)

United States federal "Superfund" clean-up costs for polluted sites run over $1 billion per year and additional money is spent by state governments and private interests. Other countries face similar problems. Superfund costs are expected to continue for years to come. This technique holds the promise of much lower cost and even more effective clean-up of polluted sites.

What would be more exciting and potentially much more beneficial for human health is a way to clean up organic pollutants that concentrate in fish. In particular, I'd love to see a nanotech solution to the problem of PCB build-up in farmed salmon.

Seven of ten farmed salmon purchased at grocery stores in Washington DC, San Francisco, and Portland, Oregon were contaminated with polychlorinated biphenyls (PCBs) at levels that raise health concerns, according to independent laboratory tests commissioned by Environmental Working Group.

These first-ever tests of farmed salmon from U.S. grocery stores show that farmed salmon are likely the most PCB-contaminated protein source in the U.S. food supply. On average farmed salmon have 16 times the dioxin-like PCBs found in wild salmon, 4 times the levels in beef, and 3.4 times the dioxin-like PCBs found in other seafood. The levels found in these tests track previous studies of farmed salmon contamination by scientists from Canada, Ireland, and the U.K. In total, these studies support the conclusion that American consumers nationwide are exposed to elevated PCB levels by eating farmed salmon.

The problem is coming from their food. I'm guessing that iron nanoparticles would be both too expensive and too generally destructive if applied to the feedstock used for farmed salmon. Though the PCB concentration problem may even be a problem for some wild Sockeye salmon.

The farmed fish industry needs to grow because ocean fish are being depleted even as the demand for fish looks set to grow enormously as the health benefits of omega-3 fatty acids become more generally known. As fish go salmon is otherwise an attractive choice because salmon are an excellent omega-3 fatty acid source and salmon do not appear to concentrate mercury. So a cheap way to eliminate PCBs from farmed salmon feedstock would be great.

Nature has a good article on the problem of the bioaccumulation of PCBs due to salmon returning to lakes to spawn.

The problem is bioaccumulation - the build-up of contaminants in creatures at the top of the food chain. The North Pacific contains about 1 nanogram of PCBs per litre. By the time the average salmon has finished bulking up for its journey, its fat contains about 160 micrograms, Blais and co-workers report.

Incredibly low concentrations of a pollutant in the environment can be concentrated enormously by the food chain.

Share |      Randall Parker, 2003 September 20 02:34 PM  Engineering Environmental

Patrick said at September 23, 2003 12:07 AM:

Pity Iron nanoparticles are highly explosive and ignite on contact with air.

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