Plant Power: Phytoremediation (Arsenic in soil, Part III)

Fern4 malibuwater.com
Chinese brake fern (Pteris vittata L.) Source: Brake fern remediation

Today I get to write about one of my absolute favorite gardening topics, and for once I’m not being ironic. Phytoremediation isn’t going to make it onto most people’s gardening hit lists, and it’s not a fad that’s going to take the nation’s gardens by storm the way a new rose or hellebore might. But for me it’s proof positive of the extraordinary power of plants; it’s hope in a polluted world; it’s a spot of green in the brownfields of industry; it’s good sense in the midst of madness.

Phytoremediation involves using plants (phyto) to remediate or restore contaminated soil or water. Some plants have the weird ability to tolerate very high levels of certain pollutants without being damaged. But wait, there’s more: these plants actually reduce pollution levels, or at the very least keep the contaminant from leaching into the nearest body of water. Different plants manage this by any of several methods: filtration, degradation, stabilization, transpiration, or my favorite, extraction. (Stick “phyto” in front of any of those words and you have the technical term.)

Phytoextraction uses plants known as hyperaccumulators that absorb the toxin and move (translocate) it into leaves and stems. These plant parts do need to be detoxified after harvesting, but as a USDA article about cadmium clean-up points out, “Phytoextraction costs about $250 to $1,000 per acre per year, while the alternative clean-up method—removal and replacement with clean soil—costs about $1 million per acre.”

Now that’s what I call real savings.

The fern that thrives on arsenic
Those of you who have been following the blog this week know where I’m going. (The heading just above kind of gives it away as well, I guess.) So, in answer to the burning question, Are there any plants that will remediate soils contaminated with arsenic? the answer is, Yes! In fact several.

The one used on industrial sites, the one you’ll see over and over (and over) if you Google “arsenic phytoextraction,” is Chinese brake fern (Pteris vittata L.), a.k.a. Chinese ladder brake fern, ladder brake fern, or just brake fern.

Brake ferns are perfect for phytoremediation: they grow quickly, they’re easy to care for, and unlike many ferns, they tolerate a wide range of light conditions, from full sun to partial shade. And they seem oblivious to arsenic: a plant doesn’t even qualify as a hyperaccumulator unless it absorbs at least 100 times as much of a toxin as can normal plants, and Chinese brake fern can absorb up to 200 times as much without any signs of phytotoxicity (plant ill-health.) Best of all, they’re a perennial, which means that the contaminated leaves can be harvested and removed each year for as long as is necessary to decontaminate a site. Disposal can be a serious issue though; large numbers of plant tops from seriously contaminated mining sites, for example, need to be treated as hazardous waste.

Native to Africa, Australia and Asia, brake ferns have become an invasive alien in Florida, which is where much of the work on them is being done. LenaMa, malibuwater.com This is not entirely a coincidence; a scientist at the University of Florida noticed the ferns growing happily on an arsenic-polluted site and decided that they would bear investigating. That scientist is Lena Quiying Ma, Professor of Biogeochemistry of Trace Metals (!) and a leader in the field; in 2001 she and several collegues published the groundbreaking paper “A Fern that Hyperaccumulates Arsenic” in Nature.*

Other plants that eat it up
Unfortunately, getting hold of brake ferns may not be that easy, unless you live in Florida. A company called Edenfern does sell them, but only in bulk, so it’s not cheap. (Details below.)**

There’s good news, though, for those not interested in investing a couple of hundred dollars in remediating a few square feet of possibly arsenic-contaminated soil: several other plants have turned up in the scientific literature as well. These include members of the Equisetales and Blechnales fern families; the former includes horsetails and the latter a wide range of ferns, amongst them spleenworts and maiden ferns. Neither actually qualifies as a hyperaccumulator, but “they still accumulated relatively high levels in their fronds, approaching 100 mg kg -1  when grown on a soil dosed with 100 mg kg -1  arsenic.” (see Meharg, 2002.)

Several studies have found that in addition to P. vittata, Cretan brake fern (P. cretica) serves as a hyperaccumulator; yet another adds P. longifolia  and P. umbrosa to the list. Sunflowers have also been used by at least two studies to extract and concentrate arsenic.

My personal favorite, however, is the study that used lupine. I’m sorry, I don’t know what kind; I’ve only looked at the abstract, which doesn’t include that detail. But the idea of using lupine to remove arsenic from soil—that I love. (Apparently lupine is a candidate because it can access forms of phosphorus unavailable to at least some other plants.)

Growing Tips
Research has shown that several things will increase the uptake of arsenic by brake fern: compost, phosphorus and mycorrhizal fungi, though the last may be important only at higher concentrations of arsenic (over 50 mg As kg–1). Phosphorus, being very similar to arsenic chemically, competes with it for binding sites in soil; when phosphorus is added, it takes some of those sites, freeing arsenic to be absorbed by plant roots. Mycorrhizal fungi, which essentially extend the root zone of plants, increase plant uptake of relatively immobile elements like phosphorus—and arsenic.

Now why compost facilitates arsenic uptake, rather than just providing lots of binding sites that would tend to fix arsenic in the soil, I don’t know.

My mission, and I do accept it, will be to find out if all these factors have similar effects on the other, perhaps more easily available plants that can be used to extract arsenic.

__________________________________________

* The only way I’ve found to see or download the whole article is to go to the 2001 section of the Publications page of Ma’s resumé, and then click on the icon beside the article title.

** A company called Edenspace sells several varieties of brake fern commercially under the name “edenfern,” including one, the Arctic, which is bred to withstand cooler temperatures. Unfortunately, it’s not cheap, because it’s only available in bulk: cell packs (72 plugs for $212.40 plus shipping) or boxes (30 4” pots for $178.50 plus shipping), and the minimum shipping charge is $50. It’s not an arrangement designed for the small-time gardener.

17 Responses to Plant Power: Phytoremediation (Arsenic in soil, Part III)

  1. Interesting info….
    At one corner in our plot, there is a fern. And we know that fern survives in the most adverse soil.. I will do a posting on that fern sometime later.
    ~Bangchik and Kakdah,
    Putrajaya,
    Malaysia

  2. I am AMAZED ! .. I have not read about this and wow ! How “green” can you get with plants that can detox soil like this ! .. These plants are such a fantastic “band-aid” for the earth .. provides healing from pollutants.
    I hope this becomes a lot more popular and widespread !

  3. That is very interesting. Right up there with the house plants for sick buildings.
    thanks!

  4. Very cool. Plants are so amazing in so many ways, aren’t they!

  5. We have traces of arsenic in our soil, volcanic, bu tnot harmful so they say. It good to know that ferns our our allies. Thanks for the great reporting.

  6. wow Kate, this is certainly an uplifting post. Of course we plantophiles knew this all along, that green just makes the world a cleaner better place, but you show us how science has also demonstrated it. Cheers, catmint

  7. Hi Kate, enjoyed this a lot. btw, when I hear bozeman, I think of the ceramics/art dept at the Univ, or is it the Archie Bray Foundation. (I get bozeman & helena mixed up). Years ago when I was working in clay full time, friends of mine were moving to Montana to study. Any clay in your background? Alice

  8. Thanks for this article. I have been mentioning phytoremediation on my blog too, several months ago, but this article is really excellent.

  9. Bangchik and Kakdah–Do tell us about your fern!
    Joy–You react the same way I do. I was blown away. It still sounds like science fiction to me.
    TM–If I’d heard of plants for sick buildings, I’d forgotten, so thanks for mentioning it. It does make sense, doesn’t it?
    VW–They are indeed. And then on top of all the great stuff they do, they’re so pretty!
    Daniel–Yup, volcanoes will do that. But as you clearly realize, “some” isn’t necessarily a dangerous amount.
    catmint–Exactly. Just being a plantophile is enough to keep me gardening, but when I get into the business of writing articles for others, I feel I need to offer more than my gut instinct. And it keeps amazing me, what science is discovering in these areas.
    Alice–My my, it is a small world. The Archie Bray Foundation is in Helena, along with the Holter–do you know it? Bozeman does have a good art and ceramics dep’t at the U., and one of the profs there used to direct the Archie Bray. My bet is that if people were moving here to study, they were headed for Helena.
    Not much clay for me; I’m the music member of the home team, while my husband upholds the visual arts (b&w photography)–so we get over to Helena several times a year for shows. Did you ever get up to Montana?
    Anne–Good to know that someone else has been posting about this! I’ll have to look up what you wrote.
    Thanks for coming by and commenting, everyone!
    –Kate

  10. Fascinating post, Kate, as ever. But what I REALLY want to know is what your choir was singing at the concert…

  11. Well Holy Moly, that was brilliant Kate. I feel dead brainy now and shall have to drop that info into as many conversations as I can. Plants are fastantic, they really are like mothers: looking after us and clearing up our messes. Thank goodness for them, I didn’t think I could love ferns more….

  12. I remember reading something about alfalfa being good for arsenic (or was it Pb?) – and one of the reasons it was a good choice (besides the observation that it hyperaccumulates the metal) is that you could take several cuttings of the alfalfa ‘hay’ — so like the brake fern, it would grow back, facilitating removal of the metal from soil. The problem with all of these phyto approaches is you still have the metal – it’s easier to ‘dispose’ of plant material than digging up a bunch of soil – but it’s still what’s keeping the method from taking over.

  13. Really interesting. It seems like I don’t hear too much about the benefits of ferns, but they do seem to have their uses.

  14. Kate, that was very interesting. It’s nice to know that nature keeps renewing itself in spite of us. I’m glad scientists are doing research on this, and that the plant is being used.~~Dee

  15. Victoria–Well, I was hoping to spare everyone, but since you ask–an all Bach concert featuring parts of the B minor Mass including the incomparable Sanctus Wonderful.
    Carrie Oh, I love that–all over the place, people spouting words like “phytoremediation” at every available coctail party. The jet set won’t know what hit them.
    Pam— It’s so much easier to deal with the contaminated foliage than the contaminated soil that I don’t see why that would slow down adoption of this method.
    Ryan— Indeed. Every plant plays its part.
    Dee— Me too. As I said, this made me feel as hopeful about the world as anything has recently.
    –Kate

  16. But HOW LONG do these processes take? Every organic gardening article espousing phytoremediation leaves this out. One person even told me 24 hours – well let’s be honest about this, it takes more than 24 hours for the sunflower seed to germinate.

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