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. 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.)
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.
** 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.