[permaculture] Phytoremediation of heavy metal contaminants in soil.

Lawrence London lfljvenaura at gmail.com
Wed Jul 8 12:37:18 EDT 2015


Phytoremediation of heavy metal contaminants in soil.
<>
There's Lead in Your Farm, But Here's How to Get It Out
http://io9.com/theres-lead-in-your-farm-but-heres-how-to-get-it-out-1715106124

Urban farmers who have their soil tested for heavy metals and other
contaminants can get a nasty shock when they realize what would be coursing
through the food they grow on their land. Establish an innocent little
vegetable patch and you’ll be serving your family a salad full of fresh
lead.

Happily, contaminated soil doesn’t mean farming is out of the question. A
relatively small investment in compost and new topsoil can mean a
relatively large drop in contaminants. Some urban farmers put in raised
beds that keep the plants they intend to eat out of contact with the soil.
And then there’s another solution: phytoremediation.

Phytoremediation means using plants to remove contaminants from the soil.
The growing plants take up contaminants in the soil and produce and scatter
new seeds, at which point the grown plants are harvested and disposed of,
leaving the soil that much cleaner. Plus, just because plants take up
materials such as lead, doesn’t mean that the lead ends up spread out
equally in every part of the plant. As one report states
<http://nature.berkeley.edu/classes/es196/projects/2008final/Green_2008.pdf>,
“Fruits can generally be grown in soil with high lead levels without
accumulating dangerous amounts of lead in edible tissues.” The lead
bypasses the fruit and accumulates in the leaves, which drop off every
year. Small community gardens, or household farms, get the contaminated
leaves shipped out and keep the fruit to eat.

If you get phytoremediation right, you can farm contaminated land. To
remove contaminants, though, you first have to pick the right plants. If
your garden is saturated with arsenic, there are famous hyperaccumulator
ferns
<http://www.researchgate.net/publication/7906480_Phytoremediation_novel_approaches_to_cleaning_up_polluted_soils>
which evolved in arsenic-rich soils and which take up a thousand times the
arsenic of other plants. St. John’s Wort accumulates
<http://wwwlib.teiep.gr/images/stories/acta/Acta%20576/576_39.pdf> cadmium
and other heavy metals
<http://scialert.net/fulltext/?doi=jps.2006.182.186&org=10> (something to
watch out for if you’re taking supplements). Hydrangeas take up aluminum.
Water hyacinths take up mercury
<http://www.ncbi.nlm.nih.gov/pubmed/22567701>.

But the work-horses of phytoremediation are Indian mustard and Chinese
cabbage. Both in the *Brassica *genus, they are related to, look like, and
are often served like bok choy. You’ve almost certainly eaten them at some
point. You may have been eating more than you bargained for, which isn’t
necessarily a bad thing. They tolerate, and thus accumulate, nearly
anything, including nutritional metals like zinc
<http://www.sc.chula.ac.th/nrc-ehwm-fs/research/journal/5.pdf>, so they’re
sometimes used as natural supplements.

They also hyperaccumulate cadmium, chromium, copper, selenium, manganese,
and lead. And with just a little juice, they’ll start accumulating uranium.
Scientists found that, although neither the mustard nor the cabbage were
spectacular at accumulating uranium under normal circumstances, adding
citric acid to topsoil <http://pubs.acs.org/doi/abs/10.1021/es971027u>
impressively increased their uptake, and speculated that this might be true
for all plants. So dousing your uranium-contaminated farm in acid might
actually improve things . . . eventually.

The “eventually” is what is holding phytoremediation back, as a widespread
commercial technique. It does work, provided that farmers and gardeners
take contaminated plants away to be properly stored every year. It doesn’t
work fast enough to be a good financial option for small commercial farms.
Farmers tend to have to turn to chemical solutions, or abandon the idea of
farming. Urban gardeners, however, can do a lot of good, over the course of
their lives, steadily decontaminating the soil underneath them.


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