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ORNL quantifies important highly water-soluble mercury gas in air
OAK RIDGE, Tenn., Jan. 22, 1995 Mercury is a heavy liquid metal, but it can float through the air as a gas. Researchers at the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) have measured an important species of gaseous mercury in air that is highly soluble in water.
This finding may help explain the concentration of mercury in precipitation and, as a result, in fish in lakes far from industrial discharges of mercury. The discovery of a form of mercury in background air that is readily dissolved in water was made by Steve Lindberg, a geochemist in ORNL's Environmental Sciences Division, and Wilmer J. Stratton, professor of chemistry at Earlham College in Richmond, Ind., who conducted research in Lindberg's laboratory. Professor Stratton was visiting ORNL as the faculty director of the Oak Ridge Science Semester for students from the Great Lakes College Association.
"During dry weather," Lindberg notes, "this form of mercury would also be rapidly deposited to vegetation where it may be washed into soils and nearby streams."
They developed a novel sampling technique using a type of air scrubber, called a "high-flow refluxing mist chamber," to identify and measure reactive gaseous mercury, called Hg(II), in air. This type of mercury differs from elemental mercury, or Hg(0), which also exists as a vapor in air but is only sparingly soluble in water, in that the Hg(II) atom is missing two electrons. The actual compound in which Hg(II) resides is unknown, but Lindberg says it is most likely mercuric chloride.
Lindberg and Stratton's measurements indicate that approximately one to five percent of total gaseous mercury in air is the highly water-soluble species and the remainder is elemental mercury vapor. "Because this small fraction is highly soluble in water," Lindberg says, "it is important to explaining the observed concentration of mercury in rain and snow, as well as the high rates of mercury dry deposition measured in some areas. Rain and dry deposition are important mechanisms for depositing atmospheric mercury on the earth's surface, helping to account for the high levels of mercury in the tissue of fish in lakes remote from man-made mercury sources."
The results are of current interest because copies of a draft of an Environmental Protection Agency (EPA) report are now in the hands of members of the U.S. Congress. The EPA's Mercury Study Report, which is required by the Clean Air Act Amendments, noted the lack of data on airborne water-soluble mercury. EPA models indicate that even a slight difference in the amount of mercury dissolved in airborne vapor could have an enormous effect on the amount of mercury deposited on the earth's surface.
Sources of Hg(II) emitted directly to the air are the burning of municipal and medical waste in incinerators and coal combustion. Although some coal-fired power plants have scrubbers to remove pollutants from flue gases, Lindberg says their removal efficiency for mercury varies from about 30 to 70 percent.
Lindberg first learned about the mist chamber when he met Bob Talbot of the University of New Hampshire, one of its developers in 1985, during a global climate change field study in a Brazilian rain forest.
Then, in 1990, while participating on a panel to review Sweden's mercury program, Lindberg first heard speculation about whether water-soluble mercury vapor may exist in the atmosphere because it had been identified in laboratory studies. He saw an opportunity to determine whether this species exists in outside air by a novel application of the mist chamber.
In 1993 Lindberg contacted Talbot and persuaded him to send ORNL a mist chamber. Lindberg and Stratton then found they could trap water-soluble Hg(II) in an aerosol mist in the chamber. With support from the Electric Power Research Institute and DOE, they conducted a number of tests at Walker Branch Watershed near ORNL and near the Earlham College campus to verify that this species did not come from other sources such as oxidation of Hg(0) by ozone in the chamber.
"Swedish scientists had identified a water-soluble species of mercury in laboratory flue gases from coal combustion," Lindberg says. "But the director of the Swedish mercury program didn't think this species existed in outside air. It occurred to me that, if water-soluble mercury is in the air, it will dominate atmospheric deposition of mercury."
The finding is significant, Lindberg says, because the accuracy of predictions of computer models on atmospheric mercury transport and deposition depends largely on assumptions about the fraction of highly water-soluble mercury present.
"About 30 to 80 percent of mercury emitted to the air by combustion processes is in water-soluble form based on recent studies by Frontier Geosciences in Seattle, which is now collaborating with ORNL to test the mist chamber method," Lindberg says. "This Hg(II) is either deposited quickly to surfaces or is rapidly reduced to elemental mercury by sulfur dioxide dissolved in water. Elemental mercury is also dissolved in water, but Hg(II) is much more soluble in water and deposits much more rapidly."
Lindberg says the data suggest a link between atmospheric deposition and the accumulation of mercury in lakes far from industrial sources of mercury. The mercury is then transformed into methylmercury by bacteria. This compound, which is toxic to humans if consumed in even tiny amounts, is readily taken up by fish in these remote lakes.
Some geologists, however, argue that rock weathering, rather than atmospheric deposition, could be the chief source of mercury to these lakes. It is still not resolved whether mercury in waterways comes mainly from natural sources or from human activities such as waste incineration and coal combustion for electrical power production. The Electric Power Research Institute has just agreed to fund a new ORNL study of natural sources of mercury to the environment.
In the ORNL technique, a vacuum pump draws air through the mist chamber from an inlet at the bottom. A mist is sprayed into the chamber. As the air passes through to the top, the highly soluble mercury in the air is dissolved in the mist. In the laboratory, the mercury is then reduced to elemental mercury with tin chloride (which adds the two missing electrons), stripped from the water droplets by purging with nitrogen onto a gold trap (mercury is attracted to gold). After the gold surface is heated to remove mercury, the concentration of mercury is measured by atomic fluorescence spectroscopy.
The ORNL discovery of highly water-soluble mercury in the background atmosphere was made in 1993, reported in 1994 at a scientific meeting, and published in 1995.
Lindberg, who is co-chairman of the conference on "Mercury as a Global Pollutant" planned for 1996 in Hamburg, Germany, and the developer of a U.S. network for monitoring the movement of toxic mercury in the air, says: "Mercury is a very mobile metal because it exists so often in gaseous forms. It behaves less like a trace metal and more like some pesticides, PCBs, and other persistent organic pollutants. Because its various forms are volatile, it plays hopscotch-depositing on land and water from air, staying there awhile, and then reentering the air as a gas. In this way, mercury can be rapidly and widely distributed throughout the global ecosystem."
ORNL, one of the Department of Energy's multiprogram research laboratories, is managed by Lockheed Martin Energy Research Corp.