Submitted to: Environmental Science and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/19/2010
Publication Date: 12/1/2010
Citation: Xuan, R., Ashworth, D.J., Luo, L., Wang, H., Yates, S.R. 2010. Depleting methyl bromide residues in soil by reaction with bases. Environmental Science and Technology. 44(23):9080-9085. Interpretive Summary: Methyl bromide (MeBr) has been used for several decades to control plant pests prior to production of many food crops. MeBr has also been identified as an ozone depleting compound and was scheduled for phase out during 2005 in developed countries, and 2015 elsewhere. However, MeBr use in the United States continues due to the submission and approval of Critical Use Exemptions (CUE). This will likely also occur in less developed nations as the 2015 phase out date approaches. To obtain a CUE, an emission mitigation methodology must be used; therefore, a great need exists for low-cost and effective methods to reduce MeBr emissions, which in turn, will protect stratospheric ozone. Combining gas-tight films and soil amendments provides an effective method to reduce fumigant emissions. The objective of this study was to find a feasible way of quickly decomposing MeBr when it reaches the soil surface. The work aims to determine (a) the effect of pH on MeBr hydrolysis; (b) the effects of different bases on MeBr decomposition; (c) select the most feasible base for degradation of MeBr; (d) the effect of temperature on MeBr degradation; and (e) explore possible ways to deplete MeBr quickly while maintaining its efficacy. This approach to reduce MeBr emissions will help growers, fumigators and regulators to protect stratospheric ozone from depletion during the phase-out period.
Technical Abstract: 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, K2CO3, and NH3 were assessed as means of promoting MeBr degradation. Ammonia aqueous solution (NH4OH) was the most effective, because MeBr can be degraded by both hydrolysis and ammonolysis. At 20 °C, the half-lives (t1/2) of MeBr were 18.0, 2.5, and 1.3 h in 0.1, 1.0, and 2.0 M NH4OH, respectively. In 1.0 M NH4OH, 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. NH4OH (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% NH4OH in 8 h at 20 °C. On the basis of these results, good management practices (i.e., VIF plus NH4OH) could be developed for continued use of MeBr as a soil fumigant under Critical Use Exemptions, without significant emissions.