Attack of the Mustard Plants!

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.

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“Here Come the Hyper-Accumulators!” by
Niall Kirkwood, in
Harvard Design Magazine class="text14"> (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
class="text57">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 class="text73">, 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 class="text69">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
class="text4">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.