|Allen, Leon - Hartwell|
Submitted to: Chemosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2005
Publication Date: 2/1/2006
Citation: Thomas, J.E., Ou, L., Allen Jr, L.H., Vu, J.C., Dickson, D.W. 2006. Henry’s law constants and mass transfer coefficients for methyl bromide and 1,3-dichloropropene applied to Florida sandy field soil. Chemosphere 62:980-988. Interpretive Summary: Methyl bromide is scheduled to be phased out as a preplant soil fumigant in the USA by 2005 except for critical uses. However, in general, the problem is that alternative fumigants such as 1,3-dichloropropene (1,3-D) have been less effective than methyl bromide. Using field data, ARS and University of Florida scientists calculated Henry’s Law Constants for methyl bromide, and for cis and trans isomers of 1,3-D. They also measured mass transfer coefficients of each chemical through high density polyethylene (PE) film. At 16.4 Celsius, calculated Henry’s Law Constants were 0.21 for methyl bromide, 0.041 for cis-1,3-D, and 0.027 for trans-1,3-D. Mass transfer coefficients through PE were 1.08, 3.25, and 4.13 cm/h for methyl bromide, cis-1,3-D, and trans-1,2-D, respectively. In summary, the much greater value of Henry’s Law Constant for methyl bromide indicates more chemical in soil air pores and less dissolved in soil water, resulting in faster, more effective soil fumigation, which explains in part why methyl bromide is the most effective fumigant. Furthermore, all values of mass transfer coefficients were large and indicated that PE film is inadequate for retaining fumigants in field soil.
Technical Abstract: Methyl bromide (MBr) is scheduled to be phased out as a preplant soil fumigant in the USA by 2005 except for critical uses. In this study, physical constants related to distribution and retention of MBr as well as (Z)- and (E)-1,3-dichloropropene (1,3-D) isomers in soil were compared. The Henry’s Law Constants of these chemicals and their mass transfer coefficients for movement through a plastic mulch of high density polyethylene (PE) were evaluated using field data. At a soil temperature of 16.4 C, calculated Henry’s Law Constant gave a ranking of MBr = [0.21] >> (Z)-1,3-D = [0.041] > (E)-1,3-D = [0.027]. Since rapid subsurface distribution of a fumigant is dependent on the amount in the gas phase, the greater value for Henry’s Law Constant implies faster distribution throughout the soil. After distribution in the soil, retention of the fumigant becomes imperative. Calculation of the fumigant’s mass transfer coefficients through PE from field data gave a ranking of: MBr = [1.08 cm/h] < (E)-1,3-D = [3.25 cm/h] < (Z)-1,2-D = [4.13 cm/h]. Mass transfer coefficients of this magnitude indicated that PE film was inadequate for retaining these fumigants in field soils.