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Green plants emit mercury, ORNL discovers
OAK RIDGE, Tenn.,
Jan. 17, 1996
In the 1970s, researchers at the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) discovered that green plants can take up mercury from the soil and from the air. Now, ORNL researchers have scientific proof that plants can also emit mercury to the air.
In a study conducted for the Electric Power Research Institute, the research arm of the electric utility industry, Paul Hanson of ORNL's Environmental Sciences Division (ESD) discovered that green plants can give off mercury. The goal of the study, designed by Steve Lindberg of ESD and Hanson, was to determine if the landscape is primarily a source of or a "sink" for mercury-that is, whether it mainly emits mercury to the air or stores mercury deposited on it from air.
The project used two independent methods to measure the mercury fluxes, one in the laboratory and one in the field. To conduct the study, Lindberg and Hanson used a technique pioneered by Lindberg and Ki Kim, a postdoctoral scientist in his laboratory. This high-precision sampling technique measures the exchange of mercury between air and land.
"In the field, we measure mercury concentrations at various heights above the ground to get a concentration profile," Lindberg said. "If the mercury concentrations are higher close to the surface than farther above it, then the surface is a mercury source. If the reverse is true, more mercury is being deposited than emitted. These fluxes are mapped over various areas, and the data are used to answer such questions as whether the surface is primarily a source of or sink for mercury."
In his laboratory experiments, Hanson studied maple, oak, and spruce saplings in a chamber into which low-mercury air was introduced. Hanson developed the mercury chamber method using equipment developed for ground-level ozone studies in the 1980s at ESD. The soil the saplings were planted in was isolated from the chamber. Hanson sampled the air for mercury vapor. "To my surprise," he said, "mercury was coming from the plants!" .
Further experiments showed that the plants take mercury from the air when the air's mercury level is above about 20 nanograms per cubic meter. When the mercury level in air is only 2 nanograms per cubic meter, the plants emit mercury. These levels of mercury are common near pollution sources and at background sites, respectively.
"Our theory," Hanson said, "is that elemental mercury in soil gas is pulled into the plant when the plant's mercury level is low. The plant tries to achieve equilibrium with respect to mercury levels in the air. When the plant's mercury level rises and the air mercury level decreases, at some point the plant releases some of its mercury by transpiration-the process of giving off vapor containing waste products through the stomata of plant tissues."
While Hanson was performing his studies, Lindberg, Kim, and Jim Owens of ESD climbed the 44-meter meteorological tower at Walker Branch Watershed near ORNL to measure gradients in mercury concentrations over the forest.
"The tower data also indicated significant emission of mercury from the oak, hickory, and maple trees below," Lindberg said. "However, we were concerned that our data were collected at a site that was near the Oak Ridge Y-12 Plant, a former source of mercury to the atmosphere, possibly biasing our results."
So Lindberg studied trees at a Christmas tree farm in Wartburg, Tenn., too far away from Oak Ridge to have been exposed to mercury emissions. He measured mercury concentrations in air, soils, water, and vegetation at the Wartburg farm.
"We found that mercury deposits from the air to the trees when they are wet," he says. "But we also observed that the trees are a strong source of mercury to the air when they are dry, supporting the data from the ORNL studies."
In ORNL studies in the 1970s of mercury-rich soils near a large mercury mine in Spain, Lindberg, Danny Jackson, and John Huckabee found that these soils emit mercury vapors at a rate that depends on temperature and vegetation cover. "We found that crops grown on these soils accumulate mercury in two ways," Lindberg says. "The roots take up mercury from the soil, and the leaves absorb mercury vapor from the air. These pathways may provide important exposure mechanisms if humans consume either leafy or root-type vegetables grown on these soils."
ORNL's pioneering method to measure fluxes of mercury over the landscape has also resulted in another discovery. Anthony Carpi, a graduate student from Cornell University working in Lindberg's laboratory, found large gaseous fluxes of both inorganic and organic mercury from sewage sludge applied to forest and farm soils.
"Sewage sludge used to fertilize soil is a previously unmeasured source of elemental mercury to the air," Lindberg says. "Carpi found that more mercury comes off agricultural soil than forest soil to which the same amount of sewage sludge is applied. The emission rates measured over sludge-amended soils exceeded those measured over forest soils at Walker Branch Watershed by a factor of 100 or more. He also discovered and measured emissions of methylmercury from sewage sludge."
This is significant because sewage sludge applied to soil is the only known terrestrial source of methylmercury to the atmosphere. Methylmercury is a highly toxic compound which accumulates in the environment and which can be a health hazard for humans when consumed with food.
In the ORNL studies, mercury vapor is collected in quartz glass tubes that contain acid-washed sand coated with metallic gold. When air is drawn through tubes at a prescribed flow rate, mercury vapor adheres to the gold. By heating the gold, researchers can measure the amount of mercury vapor released at a precision level of a few trillionths of a gram.
ORNL, one of the Department of Energy's multiprogram research laboratories, is managed by Lockheed Martin Energy Research Corp.