David Tsao specializes in phytoremediation, the use of plants to clean up pollution. Working from the business center of oil giant BP, in west-suburban Warrenville, he's in charge of developing, designing, implementing, and monitoring more than 150 phytoremediation sites on four continents.
Harold Henderson: When we say "clean up," that translates into a lot of different activities by plants, right?
David Taso: Yes. For organic chemicals like oil products, plants can break them down in the root zone (rhizodegradation) or in the plant (phytodegradation), or release them into the air (phytovolatilization). For inorganics like metals, plants can store them in the roots or soil (phytosequestration) or accumulate them aboveground (phytoextraction). The whole field of phytotechnology is really pieces of other areas of expertise.
HH: Meaning you have lots of journals to keep up with?
DT: Yes, from botany to forestry to agriculture and even agricultural engineering, since we often use farm equipment to begin dealing with a contaminated site.
HH: So many pollutants are new to nature or occur in unnaturally large concentrations. Are you surprised that plants can help clean them up?
DT: Most contaminated sites are already covered in vegetation of some sort, so it's not too surprising--nature is cleaning up already. The environment is cleaning the environment. We're just supplementing nature.
HH: I was surprised to read how much microbes and fungi are involved in these processes. It seems like they don't get the credit they deserve.
DT: I agree. A plant root might extend a couple of meters. Fungi extend tens of meters and form a network among plants. In some cases we don't know a lot about the exact processes. For instance, certain fungi that live in the root zone, or rhizosphere, enhance plant health by restricting the uptake of cadmium, nickel, lead, and other nonessential elements. But we don't know exactly how they do it.
HH: Is that a problem?
DT: Not necessarily. Phytoremediation is sometimes like a black box: we can measure what's there before and we can measure what's there afterwards, but we don't always know what happens in between. In a way, I don't care how it works as long as it does work. But some regulators want to know how it works to make sure it won't make things worse.
HH: Plus, the more you know the more design options you have.
DT: Of course. For example, we know that mulberry trees exude a specific chemical that helps soil microbes degrade PCBs.
HH: But do mulberries have a deep enough root system to bring oxygen down into the soil? You wrote that oxygen-using microbes are much faster at breaking down chemicals than anaerobic microbes.
DT: The mulberry does have a good root system. But I would like all the species we use to be deeper rooted, more extensive, longer lived, etc. These are limitations to even the best plants.
HH: Is it possible to change your tools with genetic engineering?
DT: That's a very active area of research. But as far as I know there's been only one application in the real world, due to a lack of public acceptance. Genetically engineered poplar trees have been developed that can remove methylmercury--the most toxic kind of mercury--from the soil. They were also engineered to be able to convert it to much less toxic forms, which they then volatilize into the atmosphere. Although it's just one small site, I'm not sure how acceptable it will be in the long run, since the mercury might then redeposit back onto soil and become methylated again. Genetic engineers are now working on preventing that last step, so that the poplar trees would sequester the mercury in their own tissues, making it available for use--phytomining!--or safe disposal.
HH: You mentioned farm equipment before. I was struck by one experiment you did that showed plant roots open up the soil better than plowing it.
DT: That's a project I have in Texas. We compared "land farming," where we plow the ground and add water and fertilizer for the microbes, with a designed mix of plants and with natural revegetation. We got a significantly better reduction in contamination from each of the two planted plots than from the tilled plot.
HH: Are there any downsides to phytoremediation?
DT: It's generally approved of, since even when it doesn't help it usually won't hurt. One major regulatory concern is whether we might impact the food web. Some newly published research, for instance, shows that pumpkins are very good accumulators of DDT--but that poses a problem if small creatures eat the pumpkins and are themselves eaten by larger animals, in which case the DDT returns to the environment. And there are limits to the technology: roots only go so deep, their growth and activity depend on daily and seasonal cycles, and they can take a long time both to grow and do the job.
HH: Is there any contaminant that defeats phytoremediation?
DT: I don't know if I would say "defeats," but a couple haven't worked well. One is lead. Plants don't want to take it up at all--for phytoremediation to work, the plants have to concentrate the contaminants more than they're concentrated in the soil. In early experiments they took lead up only if the soil was acidified and had chemicals added to make it more soluble, but these treatments enabled the lead to migrate through the soil and escape from the site. Some five- and six-ringed polynuclear aromatic hydrocarbons (PAHs) are very recalcitrant as well. Genetic engineering may help solve these problems. But for most contaminants, Mother Nature has some pretty amazing tools out there. It's just a question of finding the right ones.