Attack of the Mustard Plants!
Ravenous plants are the stuff of science fiction, but today's scientists are using feisty flora to munch on a variety of manmade toxins.
“Here Come the Hyper-Accumulators!” by Niall Kirkwood, in Harvard Design Magazine (Fall 2002–Winter 2003), 48 Quincy Street, Cambridge, Mass. 02138.
Aficionados of 1950s horror flicks who think they know everything there is to know about voracious plants might be surprised to learn that scientists are now enlisting certain strains of feisty flora in the fight against artificial toxins. This budding field is known as phytoremediation.
Kirkwood, director of the Harvard Design School’s Center for Technology and Environment, says there are three main branches of natural environment-scrubbers. First are the plants known collectively as phyto-accumulators, such as the Indian mustard plant, whose leaves and shoots can absorb toxic substances from soil; the leaves can then be harvested and disposed of several times during the growing season. This process has been used to extract lead from the grounds of a former battery factory, and was also used after the 1986 Chernobyl nuclear accident to remove radioactive cesium and strontium from the soil.
A second, much slower process, called phytodegradation, utilizes the enzymes secreted by certain toxin-resistant plants to break down harmful chemicals in the soil around their roots.
The final group of cleaners, represented by willow and poplar trees, uses hydraulic control to pump contaminated water up from their deep root systems to transpire it through their leaves.
Why turn to plants when there is a billion-dollar cleanup industry already in place? Because plants can be just as effective in dealing with some toxins, and at a fraction of the cost. Kirkwood cites a 1998 Environmental Protection Agency study demonstrating that mustard plants could reduce lead levels from 1,200 parts per million to below 400 parts per million (an acceptable level) at a projected cost of $60,000 to $100,000 per acre. Cleaning an acre this way requires the disposal of just 500 tons of mustard plants. The conventional approach would require hauling away 20,000 tons of contaminated soil, at a cost of $600,000. Small wonder that the domestic market for phytoremediation is expected to grow from well under $100 million in 2000 to between $235 million and $400 million by 2005.
The downside to phytoremediation is that it takes time for the plants to do their work. Such techniques, says Kirkwood, “will make sense only if there are appropriate growing conditions, contaminant densities, and aeration of the soil.” But phytoremediation can also allow contaminated sites to be partially inhabited even while the cleanup is going on.