Posted by: unlikelygrad | January 13, 2011

cool environmental stuff: phytoremediation

For the last year I’ve intended to write a bit about science, to try to explain the sorts of things that get me all excited. It took a while, but here’s my first installment.

When I was first thinking about the sort of research I wanted to do, my mind jumped almost instantly to remediation of contaminated soil. I love to garden, though, sadly, I haven’t been able to have a “real” garden in grad school. I think this is genetic: my grandmother was an avid gardener, her eldest son (my father) was too, and now I am as well. During my early adult years when I didn’t even have a balcony for potted plants, I spent time pulling my hair out, fantasizing about (I kid you not) plunging my hands into freshly turned soil.

Before I started grad school I spent hours pondering the fate of metals in soil. Organic contaminants might take thousands of years to break down, but they would eventually decompose. Metals, on the other hand, would never disappear.

Thus, the remediation of sites contaminated with toxic metals generally consists of removing all of the contaminated soil and disposing of it in a hermetically-sealed landfill. As you can imagine, this is expensive and very disruptive to the area. But what is the alternative?

The articles I read for fun when I was working in SL’s lab helped me realize that it was possible to change metals into other forms which might be less bioavailable. In other words, the toxins would still be there, but would be less likely to poison plants, animals, or people. I thought this was a good start, but I knew enough about chemistry to realize that most reactions are reversible; that a systemic change might well return the metal to its more bioavailable form. I still wanted to find a way to get the metal out once and for all!

Over the past few months I’ve been reading about phytoremediation, and I am so, so impressed. In a nutshell: some plants concentrate certain kinds of metals. Even better, some plants concentrate a particular metal in their shoots (the parts above the ground) rather than in their roots.

Thus, you can take a soil contaminated with lead, plant it with ragweed (which is a lead-accumulating plant), let the ragweed do its thing, and then harvest the plants…along with a fair amount of the lead that used to be in the soil. At this point, you can use standard chemistry to extract the lead from the ragweed, so it can be used in industrial processes. Nifty, huh?

The one downside to phytoremediation is that it is slow. In the past, people tried to accelerate the process by injecting the soil with chelators like EDTA to increase metal solubility. But this wasn’t as effective as they would have liked. It sped up the process by a factor of 2, but it still required 5+ years to get the soil levels of toxic metals down to an acceptable concentration.

The latest twist: microbially-enhanced phytoremediation. Instead of injecting chemicals, the soil is seeded with microbes that enhance metal mobility. (I read a suggestion that the microbes could be fixed into a matrix along with a food source and essential trace metals so that they could do their work more efficiently. Awesome!) This is much faster; we are now talking just a couple of years to remove 90% of toxic metals from a site.

Isn’t this cool? This is exactly why I wanted to go into an interdisciplinary program. I need the tools to be able to come up with stuff like this.


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