TRANSPORT AND TRANSFORMATION OF METHYL BROMIDE IN SOILS

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Abstract
Although methyl bromide (CH3Br) is used extensively as a soil fumigant, quantitative descriptions of its behavior after fumigation are limited. Methyl bromide undergoes a variety of processes, such as sorption and dissolution, and irreversible sink processes, such as hydrolysis. To evaluate these simultaneously occurring processes, a miscible displacement technique was used in which a pulse of pure CH3Br gas was passed through 10-centimeter-long soil columns at flow velocities ranging from 6 to 60 centimeters per hour (cm h-1). The concentration of the effluent gas was measured using gas chromatography. Experiments were conducted at one flow rate on dry sand, moist sand, a loam, and a peaty muck soil to determine the effect of water, clay minerals, and organic matter on the reversible sink processes. Additional experiments were conducted on Yolo loam at four different flow velocities to determine the effect of flow rate on the reversible sink terms. Mathematical models of transport and transformation of a reactive gas were evaluated by fitting calculated to measured effluent curves. A first-order kinetic model for the reversible sink term more adequately described effluent curves than did a linear, equilibrium model, although both models were apparently inadequate in completely describing the adsorption-desorption process. Irreversible sink processes had negligible effect on the quantitative description of the flow of gas in the columns. Although the rate of transformation of CH3Br gas to Br− had negligible effect on the shape and position of effluent curves, rates of Br− production were significant and influenced by soil type, as determined from incubation experiments. Rates of Br− production were greatest with muck, intermediate with loam, and least with sand. Although methyl bromide (CH3Br) is used extensively as a soil fumigant, quantitative descriptions of its behavior after fumigation are limited. Methyl bromide undergoes a variety of processes, such as sorption and dissolution, and irreversible sink processes, such as hydrolysis. To evaluate these simultaneously occurring processes, a miscible displacement technique was used in which a pulse of pure CH3Br gas was passed through 10-centimeter-long soil columns at flow velocities ranging from 6 to 60 centimeters per hour (cm h-1). The concentration of the effluent gas was measured using gas chromatography. Experiments were conducted at one flow rate on dry sand, moist sand, a loam, and a peaty muck soil to determine the effect of water, clay minerals, and organic matter on the reversible sink processes. Additional experiments were conducted on Yolo loam at four different flow velocities to determine the effect of flow rate on the reversible sink terms. Mathematical models of transport and transformation of a reactive gas were evaluated by fitting calculated to measured effluent curves. A first-order kinetic model for the reversible sink term more adequately described effluent curves than did a linear, equilibrium model, although both models were apparently inadequate in completely describing the adsorption-desorption process. Irreversible sink processes had negligible effect on the quantitative description of the flow of gas in the columns. Although the rate of transformation of CH3Br gas to Br− had negligible effect on the shape and position of effluent curves, rates of Br− production were significant and influenced by soil type, as determined from incubation experiments. Rates of Br− production were greatest with muck, intermediate with loam, and least with sand. © Williams & Wilkins 1980. All Rights Reserved.

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