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United States Department of Agriculture

Agricultural Research Service

Methyl Bromide
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1 - Background
2 - Chemical and Physical Properties
3 - Reactions with Stratospheric Ozone
4 - Solubility
5 - Henry's Law Constant
6 - Vapor Pressure
7 - Adsorption
8 - Diffusion Coefficient
9 - Air Sampling
10 - Field Experiments
11 - Transformation of MeBr in Water
12 - Transformation of MeBr in Soil
13 - Transport Model
14 - Simulating MeBr Volatilization
15 - Fumigation
16 - Post-Fumigation
17 - Further Reading
Transformation of MeBr in Water
Transformation or degradation of MeBr is an irreversible process that depletes MeBr from the soil-water-air system before it reaches the soil surface and volatilizes into the air. Extremely rapid transformation may deplete MeBr concentrations so quickly that efficacy is compromised. The actual transformation of MeBr in an agricultural soil is the sum of its hydrolysis in water, reactions with soil constituents, and decomposition by soil microorganisms.
Degradation of MeBr in water is important since it contributes to MeBr degradation in moist soil as well as to its fate in the overall environment. Based on its chemical structure, MeBr is an electrophile, and -Br is reactive as a leaving group and may participate in various nucleophilic substitution reactions (SN1 and SN2 types) in the environment. Water is a weak nucleophile, and therefore hydrolysis of MeBr in water is anticipated:
(Note: if you see a "ÿ" is should be an arrow "==>")
CH3Br  +  H2O  →  CH3OH  +  Br¯  +  H+ Reaction I
CH3Br  +  OH¯  →  CH3OH  +  Br¯ Reaction II
The reaction rate constants for reactions I and II are approximately 5 × 10-9 and 10-4 M-1s-1 respectively (Schwarzenbach et al., 1993). In pure water where the OH¯ concentration is extremely low, reaction I dominates, and the calculated pseudo first-order half-dissipation time (t1/2) of MeBr should be around 30 days. Mabey and Mill (1978) and Papiernik et al. (2000) report a t1/2 of 20 days, Arvieu (1983) reported a t1/2 of 46 d for MeBr in water at 20EC, and Gentile et al. (1989) reported t1/2 of 36-50 days in well waters at 18 EC. The relatively slow hydrolysis of MeBr in water was also noted by Herzel and Schmidt (1984). In an attempt to correlate MeBr hydrolysis and pH, Gentile et al. (1992) measured MeBr degradation in buffer solutions with pH 3.0 to 8.0, and found MeBr hydrolysis rates generally increased with increasing pH. However, in their experiments, they used buffer solutions comprised of phosphate and citrate, and apparently nucleophiles other than OH¯ caused the enhanced hydrolysis in solutions with elevated pH. From the rate constant of reaction II, MeBr hydrolysis rate should not increase significantly when pH is changed from 7 to 10.
In waters which are rich in nucleophiles, such as the supernatant of a salt marsh containing sulfide, MeBr degradation may be accelerated (Oremland et al., 1994a). The reaction produces methanethiol:
CH3Br  +  HS¯  →  CH3SH  +  Br¯
and further reaction with MeBr produces dimethylsulfide
CH3SH  +  CH3Br  →  (CH3)2S  +  Br¯  +  H+
MeBr was observed to degrade rapidly in anaerobic salt marsh slurries containing sulfide, with a reported transformation half life of /1 d. Production of methanethiol in slurries doped with sulfide exhibited very rapid reaction, with a MeBr half life of ~1 h (Oremland et al., 1994a). Accelerated transformation by MeBr in aqueous solution containing other nucleophiles (for example, aniline) has also been reported. Reaction with aniline in aqueous solution with a molar ratio of aniline:MeBr of 10:1 formed N-methylaniline and N,N-dimethylaniline with a MeBr transformation half life of 2.9 d (Gan and Yates, 1996).
The mechanism of photo-induced hydrolysis of MeBr in water was first reported by Castro and Belser (1981). When a pen-ray UV lamp emitting UV at 254 nm was used to irradiate MeBr-water solution in a 4-L closed flask, MeBr was gradually converted to methanol and Br¯. The following mechanism was proposed by these authors:
CH3Br  +  hv  →  (CH3Br)*  +  H2O  →  CH3OH  +  H+  +  Br¯
Photohydrolysis caused faster dissipation of MeBr in sunlight under sealed conditions (Castro and Belser, 1981), and under UV irradiation (Gentile et al., 1989). The significance of reactions with nucleophiles and UV in MeBr transformation in soil water, however, has not been investigated.
Hydrolysis of MeBr in aqueous solutions may bear limited significance in determining its fate as a water contaminant. The loss of MeBr in stirred and ventilated waters, or water that had a high surface-to-volume ratio, was found to be very rapid due to volatilization (Gentile et al., 1992). In a study to follow MeBr kinetics in surface water, Wegman et al. (1981) found that the average half-life for MeBr in surface water at a water temperature of 11 EC was only 6.6 h. Over its many years of use, contamination of water sources with the parent MeBr has never been a topic of concern.
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Last Modified: 10/20/2005
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