Taming Methyl Bromide

Methyl bromide is a popular agricultural fumigant that can kill fungi, insects, weeds, nematodes, and bacteria. Unfortunately, it also depletes stratospheric ozone. Now researchers have devised a plastic film that reacts with or captures most methyl bromide before it enters the atmosphere (Environ. Sci. Technol. DOI: 10.1021/es103713k).

When planting crops such as strawberries, farmers fumigate their fields by injecting methyl bromide into soil. The United Nations' Montreal Protocol placed restrictions on methyl bromide, and the U.S. planned to phase it out by 2005. But because no substitute has such wide efficacy, exemptions allow methyl bromide use on certain crops.

Attempts to limit methyl bromide's escape into the environment have included applying plastic tarps to physically hold the fumigant in the soil, or applying a reactive compound to the soil to degrade the pesticide. But the two approaches had not been combined.

So Richeng Xuan, Scott Yates, and colleagues at the Department of Agriculture placed soil spiked with methyl bromide in a stainless steel container and covered it with a three-layer film. The film's outer layers were plastic. Its middle layer was tissue paper infused with an aqueous solution of ammonium thiosulfate. The chemical reacts quickly with methyl bromide to produce methyl thiosulfate and bromine, which Xuan says are easy to trap and remove.

The scientists found that after 72 hours at 40 °C, nearly all the methyl bromide had moved into the tissue paper and reacted with the ammonium thiosulfate. Only 0.15% of the methyl bromide seeped through all three layers. By contrast, 50% of the fumigant typically escapes to the atmosphere after application on agricultural fields.