Degradation of Fumigant Pesticides: 1,3-Dichloropropene, Methyl Isothiocyanate, Chloropicrin, and Methyl Bromide

Abstract

where C is chemical concentration (mg kg 1) at time t (h), C o is initial concentration (mg kg 1), and k 1 is the Soil fumigation using shank injection creates high fumigant concen-first-order rate constant (h 1) independent of C and C o. tration gradients in soil from the injection point to the soil surface. A temperature gradient also exists along the soil profile. We studied In deriving Eq. [2], it is assumed that only pesticide the degradation of methyl isothiocyanate (MITC) and 1,3-dichloro-concentration changes during the course of observation; propene (1,3-D) in an Arlington sandy loam (coarse-loamy, mixed, concentrations of other species, such as degrading mi-thermic Haplic Durixeralf) at four temperatures and four initial con-croorganisms, remain unchanged. A general equation centrations. We then tested the applicability of first-order, half-order, for nth-order kinetics (n 1) of this type is given by and second-order kinetics, and the Michaelis-Menten model for de-(Sparks, 1989): scribing fumigant degradation as affected by temperature and initial concentration. Overall, none of the models adequately described the 1 n 1 (1 C n1 1 C o n1) k n t [3] degradation of MITC and 1,3-D isomers over the range of the initial concentrations. First-order and half-order kinetics adequately described the degradation of MITC and 1,3-D isomers at each initial where k n is the rate constant for nth-order kinetics, concentration, with the correlation coefficients greater than 0.78 (r 2 independent of C and C o. For example, for second-order 0.78). However, the derived rate constant was dependent on the initial kinetics (n 2), plotting 1/C with respect to time (t) concentration. The first-order rate constants varied between 6 and should yield a straight line. 10 for MITC for the concentration range of 3 to 140 mg kg 1 , and between 1.5 and 4 for 1,3-D isomers for the concentration range of For most pesticides used in agricultural systems, the 0.6 to 60 mg kg 1 , depending on temperature. For the same initial effect of concentration on pesticide degradation may be concentration range, the variation in the half-order rate constants was reasonably approximated by pseudo first-order kinetics, between 1.4 and 1.7 for MITC and between 3.1 and 6.1 for 1,3-D as these pesticides are applied at rates from less than 1 isomers, depending on temperature. Second-order kinetics and the kg ha 1 to a few kg ha 1 (Grover et al., 1997; Wolt, Michaelis-Menten model did not satisfactorily describe the degrada-1997; Kearney and Wauchope, 1998). However, several tion at all initial concentrations. The degradation of MITC and 1,3-D studies have shown that even in a well-controlled envi-was primarily biodegradation, which was affected by temperature ronment, degradation of pesticides in soils did not al-between 20 and 40C, following the Arrhenius equation (r 2 0.74). ways follow first-order kinetics (Hamaker et al., 1968; Hance and McKone, 1971; Helweg, 1975; Reffstrup et al., 1998). A difficulty commonly encountered is that A ccurate assessments...

Properties of Soil Fumigants and Their Fate in the Environment

“…Bacteria have also been implicated by the oxidation of MeBr (Miller et al, 1997;Oremland et al, 1994;Ou et al, 1997;Rasche et al, 1990). This reaction is catalyzed by monooxygenase (Dungan and Yates, 2003): Rasche et al (1990) found that two soil ammoniaoxidizing nitrifiers, Nitrosomonas europaea and Nitrosolobus multiformis, consumed MeBr only in the presence of ammonium chloride. Inhibition of biodegradation by allylthiourea and acetylene, specific inhibitors of the ammonia monooxygenase, suggests that the enzyme catalyzed MeBr degradation.…”

“…Miller et al (1997), however, isolated a gram-negative aerobic bacterium that was able to utilize MeBr as a sole carbon and energy source. Detailed information about MeBr degradation can be found in Dungan and Yates (2003).…”

“…For example, although 1,3-D is effective against nematodes, it lacks herbicidal activity and is often formulated with chloropicrin, which is a better fungicide. MITC is effective against nematodes and a variety of weeds and fungal pathogens (Dungan and Yates, 2003). However, MITC fumigants should not be applied with 1,3-D or CP simultaneously because 1,3-D and CP transformation can be accelerated in the presence of MITC (Zheng et al, 2003).…”

“…Reviews of various soil fumigants, particularly MeBr and its alternatives, are available Dungan and Yates, 2003;Ruzo, 2006;Yates et al, 2003). This chapter provides basic and updated information on the chemical properties and environmental fate of currently used soil fumigants, environmental issues surrounding these fumigants, and strategies to minimize any negative impacts from soil fumigation.…”

Fumigant emission reductions with TIF warrant regulatory changes

“…Although TIF has the ability to retain 1,3-D in the soil for a few weeks, the degradation half-life of 1,3-D under field conditions is 5 to 7 days (Ajwa et al 2003(Ajwa et al , 2010, and a very small residual concentration, if any, is found in the soil when TIF is removed from the field after 10 days. Also, the final degradation products of 1,3-D are nontoxic (mainly carbon dioxide, 6 24 42 60 78 96 114 132 150 168 186 204 222 240 258 276 294 312 330 348 366 384 402 420 6 24 42 60 78 96 114 132 150 168 186 204 222 240 258 276 294 312 330 348 366 384 402 420 6 24 42 60 78 96 114 132 150 168 186 204 222 240 258 276 294 312 330 348 366 384 402 420 6 24 42 60 78 96 114 132 150 168 186 204 222 240 258 276 294 312 330 348 366 384 402 water and chlorine) and do not pose risk to humans and the environment (Dungan and Yates 2003). Emission data obtained from dynamic flux chambers agree with and strongly support the ambient monitoring data.…”