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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 Soil
 
Hydrolysis in water is not the only pathway, and in many cases, not even an important pathway, that causes MeBr degradation in soil. This is evidenced by shorter t1/2 values obtained in soil degradation studies, particularly with organic matter-rich soils, as opposed to the t1/2 in water. Two other pathways, i.e., reaction with soil organic matter and microbial degradation, have been identified as contributing to MeBr degradation in soil.
 
Soil organic matter contains nucleophilic sites such as the -NH2 , -NH, -OH, and -SH functional groups. Methyl bromide may react with these groups via nucleophilic substitution, as in the hydrolysis reactions:
 
CH3Br  +  OM-NH2  →  OM-NH-CH3  +  Br¯  +  H+
 
CH3Br  +  OM-SH  →  OM-S-CH3  +  Br¯  +  H+
 
As a result of these reactions, soil organic matter is methylated, and inorganic bromide ion is released. The reaction of MeBr with soil organic matter is confirmed by the general observation of the close dependence of MeBr degradation on soil organic matter content (OM): in organic-matter-rich soils, degradation is consistently more rapid than in organic matter-poor soils (Brown and Jenkinson, 1971; Brown and Rolston, 1980; Arvieu, 1983; Arvieu and Cuany, 1985; Gan et al., 1994). For a soil containing 2.81% organic matter, 63 ppm of Br¯ was generated after exposure to 500 ppm MeBr in closed flasks for 24 h, while only 25 ppm of Br¯ was produced in a soil with 0.93% organic matter (Brown and Jenkinson, 1971). Arvieu (1983) studied the rate of MeBr degradation rate in 8 soils, and that t1/2 decreased with increasing organic matter content. In a soil with 0.23% organic matter, the t1/2 was 49 d; but in a soil with 5.11% organic matter, the t1/2 was shortened to only 3.6 d. After measuring MeBr degradation rates in selected soils, Gan et al. (1994) found that the MeBr degradation rate (based on Br¯ production) and soil organic matter content were highly correlated, and the measured correlation coefficient (r2) ranged from 0.95 to 1.00. Papiernik et al. (2000) also reported an increase in MeBr degradation rate with increasing soil organic matter content. Spiking soil samples with 14C-labeled MeBr resulted in the formation of non-extractable (bound) residues of MeBr, which increased as extractable MeBr decreased. These bound residues represented transformed (not sorbed) MeBr, as evidenced by the release of equimolar amounts of Br¯ for each mole of MeBr lost (Papiernik et al., 2000).
 
The reliance of MeBr degradation on soil organic matter content also has implications for MeBr degradation in subsurface layers. Since soil organic matter content normally decreases with increasing depths, MeBr degradation may be much slower in the deep layers, and the overall persistence could be much longer than what the degradation data generated from surface soils would suggest. This was verified in laboratory incubation studies using soils collected from 0 to 300 cm (Gan et al., 1994). In a Greenfield sandy loam, the t1/2 for MeBr degradation was about 8 d for the 0-30 cm layer, but increased gradually with depth to a t1/2 of 21 d for the 270-300 cm layer. This decrease closely corresponded to the decrease in soil organic matter content (Gan et al., 1994).
 
Biodegradation of MeBr has been documented for isolated bacteria, including the nitrifying bacteria Nitrosomonas europaea, Nitrosolobus multiformis, and Nitrosococcus oceanus (Rasche et al., 1990; Hyman and Wood, 1984), and Methylomonas methanica and Methylococcus capsulatus (Colby et al., 1975, 1977; Meyers, 1980; Oremland et al., 1994b). The nitrifier-catalyzing degradation suggests the involvement of ammonia monoxygenase, while the consumption of MeBr by the methane-oxidizing bacteria indicates that methane monooxygenases are responsible. Increasing the activity of nitrifying bacteria may increase the rate of biodegradation of MeBr in soil (Ou et al, 1997). Other bacteria capable of degrading MeBr have been isolated from soil. Miller et al. (1997) isolated a facultative methylotroph which could use MeBr as a source of carbon and energy. Oremland et al. (1994b) demonstrated that at high concentration, biodegradation of MeBr in methanotrophic soils was inhibited due to the toxicity of MeBr itself, but became significant at concentrations lower than 1000 ppm. Shorter et al. (1995) suggested that microbial degradation of MeBr at low concentrations (ppb) in surface soils may be important in removing MeBr from the atmosphere, thus reducing its lifetime in the atmosphere and lowering its ozone-depletion potential. They observed that MeBr removal from the headspace in closed systems containing soil was more rapid in live soils than in autoclaved soils, and the degradation rate decreased with the depth from which the soil was sampled, which corresponded to the methane oxidation activity of the soil. Hines et al. (1998) reported that at low atmospheric mixing ratios (5 pptv to 1 ppmv), rapid degradation of MeBr was effected by aerobic soil bacteria, resulting in half-lives on the order of minutes for a variety of soils. Since the initial concentrations around the injection point are normally in the order of 104 ppmv (Kolbezen et al., 1974) and the MeBr-degrading bacteria are low in population in normal agricultural soils, bacteria-mediated degradation may be insignificant under typical circumstances. However, studies have indicated that MeBr oxidation can occur in field-fumigated soil. High rates of 14C-MeBr oxidation to 14CO2 were observed in the first few days following soil fumigation where the MeBr concentration was >9.5 µg/g soil (Miller et al., 1997). This oxidation was inhibited by the addition of chloropicrin at concentrations >1.6 µg/g soil.
 
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Last Modified: 10/20/2005
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