Depleting methyl bromide residues in soil by reaction with bases.

Despite generally being considered the most effective soil fumigant, methyl bromide (MeBr) use is being phased out because its emissions from soil can lead to stratospheric ozone depletion. However, a large amount is still currently used due to Critical Use Exemptions. As strategies for reducing the postfumigation emissions of MeBr from soil, Ca(OH)(2), K(2)CO(3), and NH(3) were assessed as means of promoting MeBr degradation. Ammonia aqueous solution (NH(4)OH) was the most effective, because MeBr can be degraded by both hydrolysis and ammonolysis. At 20 °C, the half-lives (t(1/2)) of MeBr were 18.0, 2.5, and 1.3 h in 0.1, 1.0, and 2.0 M NH(4)OH, respectively. In 1.0 M NH(4)OH, increasing the solution temperature to 40 °C reduced the half-life of MeBr to 0.23 h. Ammonia amendment to moist soil also promoted MeBr transformation, and the MeBr degradation rate increased with increasing soil temperature. NH(4)OH (30%, 16 M) very effectively reacted with MeBr that was contained under plastic film. Under Hytibar (a virtually impermeable film, VIF), over 99.5% of the MeBr could be destroyed by 30% NH(4)OH in 8 h at 20 °C. On the basis of these results, good management practices (i.e., VIF plus NH(4)OH) could be developed for continued use of MeBr as a soil fumigant under Critical Use Exemptions, without significant emissions.