Reviving Propargyl Bromide, A Chemical From the Past

As the January 1, 2001, deadline for the ban on methyl bromide looms nearer, scientists from around the world intensify their search for viable alternatives.

Scott R. Yates is reviving interest in propargyl bromide, a chemical used with chloropicrin and methyl bromide in Trizone, a fumigant developed in the l960s. Yates is a soil scientist with the Agricultural Research Service's U.S. Salinity Laboratory in Riverside, California. He reported this research at the Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions held in San Diego, November 3–5, 1997.

"The increasing popularity of methyl bromide back in the '60s is one reason that propargyl bromide was never commercialized," Yates reported. "Since the compound is not currently registered by the Environmental Protection Agency as a pesticide, its environmental behavior is essentially unknown."

In tests, Yates and colleagues at Riverside found that under typical agricultural conditions, this chemical appears to pose no serious environmental risk. "It degrades quickly in the soil. This would help limit the amount of the chemical that reaches groundwater or escapes into the atmosphere," he said.

Since virtually no information exists on propargyl bromide, Yates concentrated research efforts on basic parameters such as solubility, saturated vapor density, and the ease with which the chemical moves through the soil and escapes into the atmosphere. This knowledge is critical before a decision can be made on whether the chemical should be used as a soil fumigant, Yates said. The data are also needed to design methods that minimize application rates and optimize control of target organisms.

Yates compared the solubility of propargyl bromide (3BP) with that of methyl bromide and 1,3–dichloropropene (1,3–D), a potential methyl bromide alternative. Solubility determines how quickly a chemical is transported with water moving through porous substances.

"At low temperatures, the solubility of propargyl bromide increases rapidly as temperatures rise, and then levels off. We found that this chemical dissolves into water at a rate comparable to methyl bromide," Yates said. "And under high leaching conditions, more propargyl will move with water than 1,3–D, since 1,3–D is less soluble."

According to Yates, since it is important that a chemical move rapidly through the soil, all agricultural fumigants generally have high saturated vapor density. The saturated vapor density gives an indication of the tendency of a chemical to prefer the vapor phase.

In soil tests, 3BP degraded at a rate similar to 1,3–D, but much faster than methyl bromide. In theory, Yates said, that part of a chemical applied to the soil which doesn't degrade will eventually enter groundwater or the atmosphere. Since 3BP degrades rapidly in soil, it will have lower volatilization and is less likely to leach into groundwater than methyl bromide. "Because of its easy movement through and low retention rate in the soil, 3BP should provide a fairly uniform fumigation," said Yates. "Unless the soil is covered with a barrier like an impermeable plastic film, most agricultural fumigants will have a high rate of emission into the air. In fact, very high emission rates are reported for both methyl bromide and 1,3–D in field and lab tests.

"On one hand, high emission rates would reduce phytotoxicity and possible groundwater contamination. But on the other hand, air emissions could produce undesirable health effects on people living or working near treated fields. Since propargyl bromide is highly degradable, little would reach the atmosphere or groundwater, compared to methyl bromide," he reported.

Propargyl bromide has characteristics which suggest that it would be environmentally safe and, therefore, warrants consideration as a methyl bromide alternative, Yates said. "However, further research is needed to determine its effectiveness in controlling pests and to further study its fate and transport behavior under typical farming conditions."