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Solving a Problem with a Problem

Used tires can reduce mercury emissions.

Excessive levels of mercury in some water bodies have resulted at various times in the closure of both sport and commercial fishing seasons. Sometimes the mercury had a natural origin - but often it came from an industrial source, often a pulp and paper mill or a power plant.

Disposing of old tires is another environmental challenge - and a big one. Each year, more than 200 million tires are disposed of in the United States alone. On several occasions, large, difficult to extinguish fires have burned in piles of old tires.

A recent study conducted by the University of Illinois and the Illinois State Geological Survey has found that some of those used tires could be put to use cleaning up mercury emissions from coal fired power plants. The tires, when combined with pistachio shells, were found to work as well as or better than current commercial products and might be cheaper to produce.

"Mercury is a toxic pollutant that can enter rivers, lakes and the human food chain," said researcher Massoud Rostam-Abadi. "Coal-fired power plants are one of the largest sources of mercury emissions. We looked for materials that could effectively remove mercury from the combustion flue gases."

In the study, adsorbents were prepared from a variety of materials. The adsorbents were then evaluated for their effectiveness at removing two forms of mercury emissions - elemental mercury and mercuric chloride - from several different simulated combustion gas streams.

"We found that mercury removal was affected by both the properties of the adsorbent and the flue gas compositions," Rostam-Abadi said. "In one flue gas, the adsorbents were equally effective in removing both forms of mercury. In another flue gas, the tire and pistachio carbons had nearly five times larger capacity for the adsorption of mercuric chloride" than other substances tested. Because utilities produce different amounts of the two mercury emissions, some tailoring of the tire and pistachio substance will be required to achieve effective mercury removal for individual power plants.

"We are currently examining the mixing of adsorbents to more effectively remove both forms of mercury from individual power plants," Rostam-Abadi said. " Also, depending on plant location, certain adsorbents may be more economical than others."

In their work, the researchers also showed that activated carbons containing sulfur additives were significantly more effective at removing mercury emissions from the flue gases - but adding the sulfur usually requires additional processing steps and production costs. Tire rubber already contains sulfur - which makes the rubber more durable - so activated carbon from tires might prove more cost-effective than existing products.