Using Plants to Clean Up Soil

Using Plants to Clean Up Soil

By Sharon Durham

Raising soil acidity to a pH level of 5.8 to 6 to help alpine pennycress absorb heavy metals from soil doesn't harm beneficial soil microbes, according to a recent study by Agricultural Research Service (ARS) scientists and cooperators.

The researchers have been conducting ongoing studies on using alpine pennycress (Thlaspi caerulescens) to remove cadmium and other heavy metals as part of a soil remediation process known as phytoextraction. Previously, they found that lowering the pH helped the plant remove toxic metals, but they were concerned that increasing soil acidity too much could harm beneficial soil microbes.

ARS agronomist Rufus Chaney, with the Environmental Management and Byproduct Utilization Laboratory, Beltsville, Md., has been a leader in using metal-accumulating plants to clean contaminated soil. He and others have shown that T. caerulescens can concentrate up to about 8,000 parts per million of toxic cadmium in its leaves.

Harvesting the aboveground vegetation annually makes it possible to reduce the concentration of cadmium in soil to safe levels in three to 10 years. 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.

The University of Maryland filed a patent on the use of T. caerulescens for the phytoextraction of cadmium in 2000, with Chaney as a cooperator. A patent for the process was granted in 2006 in the United States and Australia. No other similar technologies currently exist for remediation of cadmium-contaminated soils using plants.

To measure how pH affects soil microbes, Chaney and University of Maryland colleagues Shengchun Wang and Scott Angle adjusted two smelter-contaminated, high-metals soils to a range of pH levels, grew T. caerulescens in them for six months, and then analyzed soil microbe populations and activity. Then they adjusted the soils back to normal pH levels and incubated them for six months, to see if previously observed reductions in microbes persisted under normal soil management.

The scientists found that if the soil pH was adjusted no lower than that needed to maximize annual cadmium removal—a pH of about 5.8 to 6—there was no lasting adverse effect on soil microbes. And in both test soils, T. caerulescens tended to protect the soil microbes, compared to unplanted soils at the same pH levels.

Acidifying Soil Helps Plant Remove Cadmium, Zinc Metals

By Sharon Durham

Acidifying cadmium-contaminated soil can help a plant called alpine pennycress to remove even more cadmium and zinc from contaminated soil, Agricultural Research Service (ARS) and cooperating scientists report.

ARS agronomist Rufus Chaney and University of Maryland colleagues Shengchun Wang and Scott Angle have found lowering the soil pH—increasing its acidity—can maximize the ability of alpine pennycress (Thlaspi caerulescens) to remove cadmium and zinc metals from soil.

The scientists used a particular strain of alpine pennycress from southern France in their research. In the study, they increased the acidity of two soils collected at different fields near a zinc smelter at Palmerton, Pa. The pH of the soils was lowered from a neutral level of 7 to an acidic level of about 4.7 by using sulfur.

Alpine pennycress was grown on these soils for six months and then analyzed. As the pH was lowered, concentration of cadmium in the plant shoots rose. But if the pH was lowered below 6, the soils were so acidic that the alpine pennycress yields were reduced.

Alpine pennycress can concentrate cadmium in its leaves up to about 8,000 parts per million. Harvesting the above-ground vegetation annually makes it possible to gradually reduce the soil concentration of cadmium to safe levels. The cost of this remediation method, called phytoextraction, costs about $250 to $1,000 per acre per year, according to Chaney. He's based at the ARS Animal Manure and By-Products Laboratory in Beltsville, Md.

The alternative clean-up method—removal and replacement with clean soil—costs about $1 million per acre. Most highly contaminated soils can be deemed safe after three to 10 years of phytoextraction, an effective clean-up at far lower cost. The technology will be especially useful in cleaning up rice paddy soils in Asia, where mine waste contamination is causing human health effects from cadmium accumulated in rice grain.

In 2000, a patent was filed by the University of Maryland on the use of alpine pennycress for the phytoextraction of cadmium from soil, and a patent has been granted in Australia. No other similar technologies currently exist for remediation of cadmium contaminated soils using plants.