Oct. 30, 2007

Plants, from pennycress to willow, have potential to clean up polluted soils, researchers are finding

The ground beneath our towns and cities harbors a legacy of contaminated soils that threatens to endure for decades, if not centuries. In many places, the soil has high concentrations of organic toxins and heavy metals from smelting, manufacturing and other industrial processes as well as the burning of fossil fuels. In several states groundwater fills thousands of abandoned mines, creating toxic soups that endanger whole watersheds. And much farmland is contaminated from the application of phosphate fertilizers and sewage sludge.

Though science is far from able to undo so much damage, one promising route is phytoremediation, using plants, which are inexpensive and renewable, to clean up polluted soils.

"There are some really cool plants out there that will not only tolerate very toxic soils but will accumulate some of those metals to very high levels in the shoot," says Leon Kochian, director of the U.S. Plant, Soil and Nutrition Laboratory located on Cornell's campus and a Cornell professor of plant biology and crop and soil sciences. Thlaspi caerulescens, a member of the cabbage family known to some as Alpine pennycress, for example, "accumulates metals to astounding levels," says Kochian.

Thlaspi can pack its leaves with 30,000 to 40,000 parts per million of zinc and 10,000 to 20,000 parts per million of cadmium, says Kochian, who has studied the plant for many years. Although this weed is too small to have a meaningful impact on a heavily polluted site, Kochian is working to figure out the molecular mechanisms that allow the plant to tolerate having up to four percent of its leaf mass taken up with very toxic metals.

"We have found a couple of membrane transporter genes that look really interesting," says Kochian. "The idea is to identify a suite of hyperaccumulation genes and transfer them into a plant with bigger biomass."

Willows, for example, have enough biomass to be useful for phytoremediation. While they do not accumulate metals to anywhere near the levels of Thlaspi, they can accumulate cadmium and zinc from both soil and groundwater. Murray McBride, a professor of soil chemistry, who has tested an array of Salix species for metal tolerance by growing them in hydroponic solutions laced with cadmium and zinc, has identified two that are looking like excellent candidates for field remediation.

"We're getting at least 1,000 parts per million zinc in the leaf tissue, and they have no problem with cadmium levels at 100 parts per million, which for cadmium is very high."

Furthermore, McBride notes, willows "grow like a weed, and they generate a lot of biomass that can be used for biofuel."

Another major pollutant, lead, is nearly ubiquitous in inner-city soils. To add to the problem, "it's not very soluble, so it's very hard to move it from soil," says Tim Vadas, M.S. '06, Ph.D. '07, who worked as a doctoral student with Beth Ahner, associate professor of biological and environmental engineering. Lead has been solubilized, says Vadas, using synthetic chelators such as EDTA. "But the compound is pretty stable and not as biodegradable as biological compounds, so it has a greater potential to leach away into the groundwater if it isn't taken up by a plant."

Working primarily with hydroponically grown Brassica napus, the plant that yields canola oil, Ahner and Vadas are focusing on cysteine and glutathione, two important biochemicals in plants that not only are able to bind with lead and cadmium so that the plants will take them up into their roots but are biodegradable. In collaboration with Professor Roger Spanswick, a department colleague with expertise in membrane transport, they are homing in on determining under what conditions plants will take up more lead.

Their progress will ultimately depend in part on the progress of plant genomics research, to find which genes might be useful in phytoremediation strategies.

Filtering toxins with plant material

Gary Harman, a professor of plant pathology at Cornell's New York State Agricultural Experiment Station in Geneva, studies phytoremediation using the common soil fungus Trichoderma. He has created an asexual hybrid of Trichoderma with greatly enhanced antifungal properties and started a company in Geneva in the 1980s to market it. In addition to controlling plant diseases and increasing nitrogen utilization, he says, his hybrid significantly increases plant root growth, which is very important in phytoremediation.

He also has been searching for plant-based solutions for cleaning up metals and chemicals too toxic for live plants to tolerate.

"What you want is something that still has its cellular structure but has the cellulose degraded away and still has lots and lots of lignin left," he explains of his search for a material to mop up spills and filter polluted groundwater.

"Plants and other biomass have been known for years to sequester heavy metals in soils," he says. "Lignocellulose, the active component, will absorb both oils and heavy metals from polluted water. The best materials turn out to be those that have a low level of cellulose; manure is well suited to this since cows digest cellulose and leave the lignin."

Harman developed a way to process manure that is very lightweight and 10 times as absorbent as cat litter for uptake of oil. Aged tree bark, he says, is also a good material to remove heavy metals from polluted water because it has few contaminants that will leach into water.

"The best bark is the stuff you get for mulching around your vegetables," he says, "because landscaping companies put it in enormous piles, and the anaerobic process gets rid of the cellulose. This material is particularly good at removing heavy metals from polluted water."

Harman's group and the company he co-founded expect to begin commercial application of his bark product, cleaning up a former industrial site in Rochester that is polluted with chromate, a form of chromium that is highly toxic to humans. He has also been successfully pilot testing a modification of the process that is designed to remove hydrogen sulfide gas from landfills.

The full version of this article was originally published in CALS News.