Forum—Green Remediation: Letting the Earth Heal Itself

Agronomist Rufus Chaney and geneticist Yin Li have the largest collection of Alpine pennycress in the world—24 types, from more than 10 countries.

They're banking on this herb to launch a new industry within a decade. The new industry, discussed on pp. 4-7, is phytoremediation—popularly called green remediation. It is based on the concept of using rare plants to clean contaminated soils at toxic waste sites. Plants like pennycress evolved on metal-laden soils, selectively absorbing and transporting the metals to their stems and leaves to protect against diseases and chewing insects.

The two scientists' confidence in the industry's future comes partly from their discovery that one of the pennycresses in their collection literally towers above other Alpine pennycress—growing up to 1-½ feet tall in the same time it takes others to grow only half a foot.

The first generation of breeding crosses of this plant is now growing in a greenhouse near Chaney and Li's lab, which is part of the ARS Environmental Chemistry Laboratory in Beltsville, Maryland. The plant is also growing at a zinc smelter-contaminated site in Palmerton, Pennsylvania.

With its higher yield and harvestability, the superior plant can remove three to four times the amount of zinc and cadmium taken in by normal pennycress types. Chaney and Li expect to develop a commercial variety in 3 to 5 years, through conventional breeding.

To do inheritance studies—a prerequisite for both conventional breeding and genetic engineering—they are crossing the giant pennycress with pennycress types that don't hyperaccumulate heavy metals. Scientists define hyperaccumulation as a plant's capacity to take up and store more than 2.5 percent of its dry weight in heavy metals without a reduction in yield.

The inheritance studies will tell the scientists how many or how few genes control the hyperaccumulation trait. Chaney and Li need this information in order to choose the appropriate techniques for both conventional breeding and genetic engineering of a new hyperaccumulating plant. They plan to do both simultaneously, adding at least another 5 years to their timetable to allow for genetic engineering.

But rewards from genetic engineering might be worth the wait. Transferring the hyperaccumulator gene or genes from pennycress into crops such as hay-type canola or Indian mustard could lead to hyperaccumulator plants with even better agronomic properties and higher yield.

With either route, the future commercial hyperaccumulator plant would have to combine the metal accumulation ability of pennycress with the high yield, fast growth, deep rooting, and normal growing qualities and pest resistance of standard hay crops. They would also have to not shed their leaves so that the leaves could be easily harvested with the plant.

In 1980, Chaney was the first scientist to publish a paper describing the concept of putting plants to work cleaning vast acreages of contaminated soils. But until a few years ago, interest in the subject was merely academic—a matter of curiosity as to how plants could store heavy metals or other toxins and still survive.

Now Chaney proposes growing the crops as hay, cutting and baling the plants when they've done their work, and burning them to generate electricity to partially offset production costs. But the real payoff would be in the ashes: Ashes of Alpine pennycress grown at the Palmerton site contain 30 to 40 percent zinc, the same as high-grade ore. Chaney believes they can be marketed as "Bio-Ore," possibly turning an overall profit.

Engineers once believed they would have by now developed cost-effective technology for large-scale soil cleanups—but they haven't. The engineers and others have seen that removing and replacing contaminated soils or washing them in acid are only practical for a few 10's or 100's of square yards. Society can't afford cleanup bills of $1 million—or more—per acre on sites measured by the square mile. Nor can it manufacture new topsoil to replace the old.

But major companies are seeing profits in green remediation. Chaney and Li have been approached by a few of them and are working out a cooperative research and development agreement that would bring in over $1 million in funding. Within 5 to 10 years, they expect to move the idea from basic research and develop it into an industrial technology for several metals.

The beauty of this type of work is that the R&D stage could help pay for itself by bio-mining the field experiments' ashes—making more than hay while the sun shines.

Jan van Schilfgaarde

Associate Deputy Administrator
Natural Resources and Systems