Transformation of Chloropicrin and 1,3-Dichloropropene by Metam Sodium in a Combined Application of Fumigants

Article abstract of DOI:10.1021/jf0498564

Combined application of fumigants is a potential strategy to replace methyl bromide in the control of soil-borne pests. Unfortunately, abiotic and biotic interactions among fumigants restrict some combined application approaches. In this study, the kinetics and mechanisms of reaction between metam sodium (sodium methyldithiocarbamate) and the halogenated fumigants chloropicrin (trichloronitromethane) and 1,3-dichloropropene (1,3-D) were investigated in aqueous solution. For chloropicrin, an extremely rapid oxidation-reduction process occurred in the presence of metam sodium. The second-order rate constant for the reaction between chloropicrin and metam sodium was approximately 2 orders of magnitude greater than that for the reaction between 1,3-D isomers and metam sodium. Transformation of 1,3-D by metam sodium was associated with an aliphatic S_N2 nucleophilic substitution process. The nucleophilic reaction of ci-1,3-D with metam sodium was significantly faster than that of the trans isomer and was correlated with a lower reaction activation energy for the cis isomer in the transition state. Combining Telone C-35 (65% 1,3-D and 35% chloropicrin) and metam sodium in solution might yield some nucleophilic sulfur species, which played an important role in the dissipation of 1,3-D. The incompatibility of chloropicrin and 1,3-D with metam sodium was also examined in soil under different application scenarios. Simultaneous application of metam sodium with chloropicrin or 1,3-D accelerated the transformation of the two halogenated fumigants, reducing their availability in soil. A sequential strategy for multiple fumigants was developed, which could be applied without the loss of active ingredient that occurs due to the reaction between fumigants. The proposed methodology may enhance pest control while maintaining environmental protection.

Full text of DOI:10.1021/jf0498564

3002 J. Agric. Food Chem. 2004, 52, 3002−3009
Transformation of Chloropicrin and 1,3-Dichloropropene by
Metam Sodium in a Combined Application of Fumigants
WEI ZHENG,*,†,‡ SCOTT R. YATES,† MINGXIN GUO,†,‡
§
SHARON K. PAPIERNIK,† AND JUNG HO KIM
George E. Brown Jr. Salinity Laboratory, USDA-ARS, Riverside, California 92507,
Department of Environmental Sciences, University of California, Riverside, California 92521, and
Department of Environmental Science, Kyungsan University, Kyungsan, 712-240, Korea
Combined application of fumigants is a potential strategy to replace methyl bromide in the control of
soil-borne pests. Unfortunately, abiotic and biotic interactions among fumigants restrict some combined
application approaches. In this study, the kinetics and mechanisms of reaction between metam sodium
(sodium methyldithiocarbamate) and the halogenated fumigants chloropicrin (trichloronitromethane)
and 1,3-dichloropropene (1,3-D) were investigated in aqueous solution. For chloropicrin, an extremely
rapid oxidation-reduction process occurred in the presence of metam sodium. The second-order
rate constant for the reaction between chloropicrin and metam sodium was approximately 2 orders
of magnitude greater than that for the reaction between 1,3-D isomers and metam sodium.
Transformation of 1,3-D by metam sodium was associated with an aliphatic SN2 nucleophilic
substitution process. The nucleophilic reaction of cis-1,3-D with metam sodium was significantly faster
than that of the trans isomer and was correlated with a lower reaction activation energy for the cis
isomer in the transition state. Combining Telone C-35 (65% 1,3-D and 35% chloropicrin) and metam
sodium in solution might yield some nucleophilic sulfur species, which played an important role in
the dissipation of 1,3-D. The incompatibility of chloropicrin and 1,3-D with metam sodium was also
examined in soil under different application scenarios. Simultaneous application of metam sodium
with chloropicrin or 1,3-D accelerated the transformation of the two halogenated fumigants, reducing
their availability in soil. A sequential strategy for multiple fumigants was developed, which could be
applied without the loss of active ingredient that occurs due to the reaction between fumigants. The
proposed methodology may enhance pest control while maintaining environmental protection.
KEYWORDS: Fumigant; metam sodium; chloropicrin; 1,3-dichloropropene; transformation; simultaneous
and sequential application
INTRODUCTION
After the withdrawal of MeBr, a few registered soil fumigants
will remain: chloropicrin, 1,3-dichloropropene (1,3-D), metam
sodium and dazomet. None of these fumigants, however, can
be considered an equivalent replacement for MeBr in most field
applications because they lack the broad-spectrum activity of
MeBr. For example, chloropicrin has strong funigicidal activity
to control Verticillium wilt and beneficial nematicidal activity
(2, 3), but somewhat lacks activity against dormant weeds and
seeds in soil (4). 1,3-D is an effective nematicide, but is not as
effective as MeBr for weed and fungi control. To achieve the
same pest control efficacy and crop yield obtained with MeBr,
a stand-alone herbicide application would be required for ade-
quate weed control after soil fumigation. Previous field research
trials showed that Telone C-17 (83% 1,3-D + 17% chloropicrin)
at 35 gal/ac may be a viable replacement for MeBr when
combined with metolachlor or oryzalin at planting followed by
a midsummer application of oryzalin (5). Metam sodium may
be another option for weed control (6). According to a 1997 U.
S. EPA study, metam sodium and plastic mulch controlled most
Methyl bromide (MeBr) has been used extensively in
agriculture as an indispensable pre-plant fumigant for decades.
However, MeBr has been scheduled for phase-out in the United
States and other industrialized countries because of its potential
to deplete stratospheric ozone (1). The enormous economic
impacts of the methyl bromide ban have triggered a feverish
search for alternatives to MeBr. Although numerous nonchemi-
cal approaches have been proposed and developed as replace-
ments for MeBr, unfortunately, most of these alternatives are
less efficient or economical to meet current needs in soil-borne
pest control. Therefore, chemical fumigation will likely continue
to serve as a primary pest control practice in many economically
important crops in the foreseeable future.
* To whom correspondence should be addressed. Tel.: (909)-369-4878.
Fax: (909)-342-4964. E-mail: Wzheng@ussl.ars.usda.gov.
† George E. Brown Jr. Salinity Laboratory.
‡ University of California.
§ Kyungsan University.
10.1021/jf0498564 CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/10/2004

Chloropicrin and 1,3-Dichloropropene
J. Agric. Food Chem., Vol. 52, No. 10, 2004 3003
weeds tested in the southeastern United States. Koster and van
der Meer (7) found that soil treatment with metam sodium led
to 90% yellow nutsedge control in The Netherlands. Therefore,
metam sodium applied in combination with chloropicrin or
1,3-D has been attempted in some field trials to evaluate its
potential to serve as a MeBr replacement (4, 8).
instruction and regulation of multi-fumigant application in the
field but is also necessary to accurately assess and predict the
environmental fate of fumigants, to avoid or mitigate the risk
to human and environmental health associated with fumigant
application.
MATERIALS AND METHODS
Metam sodium is a precursor of methyl isothiocyanate
(MITC), which has pesticidal activity against nematodes, weeds,
and fungi. Since 1995, the use of metam sodium has exceeded
MeBr and it consistently ranks 3rd among all pesticides used
in the U. S. (9). Metam sodium is relatively stable in water
(e.g., Vapam HL, 42% metam sodium aqueous solution). Once
in contact with warm soil or sediment, however, metam sodium
decomposes rapidly to MITC. The conversion process is usually
completed within a few hours to a day (10-12). Due to its high
solubility, metam sodium can be applied with irrigation water
through drip irrigation systems. Application of metam sodium
via subsurface drip irrigation is attracting substantial attention,
because this strategy may reduce application costs and mitigate
the environmental and human health risks associated with
fumigant emissions. However, problems achieving a uniform
distribution of metam sodium or MITC in soil have resulted in
inconsistent pest control and a lower crop yield than with MeBr
treatment.
Chemicals. Chloropicrin (99%) and metam sodium (dihydrate, 99%)
were purchased from Chem Service (West Chester, PA). 1,3-Dichlo-
ropropene (Telone II, 50.5% cis and 46.9% trans isomer) was donated
by Dow AgroSciences LLC (Indianapolis, IN). Methyl isothiocyanate
(MITC) was obtained from Sigma Chemical Co. (St. Louis, MO). All
chemicals were used as received.
Soil. The soil used in the incubation study was an Arlington sandy
loam (coarse-loamy, mixed, thermic Haplic Durixeralf) that was
collected from the University of California, Riverside Agricultural
Experiment Station. Fresh soils were sampled from the top 15-cm (A
horizon) of a field that has no history of fumigant application. Moist
soils were passed through a 2.0-mm sieve without complete air-drying
and stored at low temperature before use. Soil organic matter content
was 0.92% and pH was 7.2.
Aqueous System Experiments. Transformation experiments be-
tween metam sodium and chloropicrin or 1,3-D were conducted to
determine the reaction kinetics in pH 6.9 phosphate buffer solution at
21 ( 0.5 °C. Briefly, 1.0 mM solutions of chloropicrin, 1,3-D, or Telone
C-35 (65% 1,3-D and 35% chloropicrin standard incorporated in lab)
were prepared in aqueous solution, and aliquots were placed in 55-mL
serum bottles. To initiate the reaction, all bottles were spiked with
metam sodium stock solution using a gastight syringe. The initial molar
ratio of metam sodium to chloropicrin or 1,3-D was 1:1 in the mixed
aqueous solution. At regular intervals, aliquots (0.5 mL) were withdrawn
from triplicate bottles and transferred into sealed glass vials containing
ethyl acetate (3.0 mL) and anhydrous sodium sulfate (2.5 g). The vials
were vigorously shaken for 10 min, and an aliquot of the ethyl acetate
extract was immediately transferred to a GC vial for fumigant analysis.
Preliminary experiments revealed that ethyl acetate efficiently extracted
chloropicrin, 1,3-D and MITC from aqueous solution, while metam
sodium remained in the aqueous phase. The reactions were quenched
when solvent was added. Control experiments were concurrently
performed in phosphate buffer solutions containing only chloropicrin,
1,3-D, or metam sodium to determine the hydrolysis of pesticides during
the experimental period.
To identify transformation products, 10 mM metam sodium and 10
mM chloropicrin or 1,3-D were mixed in the aqueous solution and
then incubated at room temperature. Aliquots of the solution were
periodically extracted by ethyl acetate and analyzed by GC/MS.
Soil System Experiments. A series of experiments was conducted
to elucidate the incompatibility between metam sodium and chloropicrin
or 1,3-D when applied simultaneously to soil. In all cases, fumigant
transformation was determined in fresh Arlington sandy loam at 21 (
0.5 °C. Soil (10-g dry weight, initial moisture 4.6%) was weighed into
20-mL headspace vials. Soil vials were treated with chloropicrin, 1,3-D
or Telone C-35 at 0.5 mmol/kg. The treated vials were sealed
immediately with Teflon-faced butyl rubber septa and aluminum seals
and then shaken to achieve uniform fumigant distribution in soil. Metam
sodium solution (0.5 mL, 10 mM) was injected through the septum
into the soil sample. Soil vials treated with only chloropicrin, 1,3-D or
Telone C-35 were prepared and used as controls. All treated soils were
shaken for 2.5 h at 21 ( 0.5 °C, and then chilled at -21 °C for 3h. To
analyze the fumigant remaining in soil, samples were decapped when
the soil was still frozen, anhydrous sodium sulfate (10 g) and ethyl
acetate (10 mL) were added, and vials were resealed immediately. The
samples were shaken for 1 h and vortexed for 2 min at room
temperature. A portion of the ethyl acetate was transferred to a GC
vial and analyzed using GC/ECD/NPD. The recovery of fumigants by
this procedure ranged from 95 to 105% according to preliminary
experiments.
The realization that combinations of fumigants may broaden
the spectrum of pest control has provided much of the impetus
in the use of multi-fumigant formulations, such as commercial
mixtures of 1,3-D with 17 and 35% chloropicrin (Telone C-17
and Telone C-35, respectively). However, the environmental
impacts of a multi-fumigant formulation need to be determined
and assessed prior to widespread adoption and use. Previous
studies have shown that the application of multiple fumigants
may have different effects on the soil environment compared
to the individual fumigants, particularly on soil microorganisms
(13, 14). Furthermore, the different impacts of the fumigant
mixtures on soil microbial communities may alter pesticide
biotic transformation rates and affect their ultimate fate in the
environment (15). Stiles et al. (16) reported that metam sodium
would reduce soil microbial activity and thus protract the
persistence of the herbicides EPTC and pebulate when applied
in conjunction with metam sodium. Additionally, simultaneous
application of multiple fumigants may lead to competitive
degradation on the soil surface, which may alter the abiotic
transformation rate in soil (15). Most noticeably, recent studies
have shown that soil fumigation with metam sodium plus a
mixture of 1,3-D and chloropicrin (Telone C35) did not increase
strawberry yields compared to fumigation with metam sodium
alone or Telone C35 alone, probably because of the reaction
between the fumigants in aqueous solution (4). Therefore,
extensive knowledge of the influence of fumigant mixture use
on pest control efficiency and their transformation and persis-
tence in the environment is necessary for successful application
and operation of multiple fumigants in agriculture.
The objectives of this research were to (1) characterize the
transformation of chloropicrin and 1,3-D by metam sodium in
an aquatic environment and soil, (2) elucidate the incompatibility
of simultaneous application for halogenated fumigants with
metam sodium, and (3) propose a sequential strategy for multi-
fumigant application. To comprehensively understand the reac-
tion between metam sodium with chloropicrin and 1,3-D, the
transformation processes were investigated systematically in the
aqueous solution and soil, transformation products were identi-
fied and reaction mechanisms were proposed. The information
obtained from these experiments is not only required in the
A sequential treatment was also conducted to determine the fumigant
transformation and demonstrate the application feasibility of metam
sodium in conjunction with chloropicrin and 1,3-D in soil. Soil (10 g)

3004 J. Agric. Food Chem., Vol. 52, No. 10, 2004
Zheng et al.
Table 1. Second-Order Transformation Rate Constant (µ) and 50%
Dissipation Time (DT50) for Chloropicrin and 1,3-D (1.0 mM) in
Aqueous Solution Containing an Equal Initial Concentration Metam
Sodium (1.0 mM) at 21 °C
fumigant
µ (mM-1 min-1
)
DT50 (min)
r2
chloropicrin
cis-1,3-D
trans-1,3-D
1.73 (± 0.05)
0.58
153.8
532.0
0.996
0.996
0.984
1.30 (± 0.03) × 10-2
3.90 (± 0.14) × 10-3
coefficient (µ) and the 50% disappearance time (DT50) for
chloropicrin were obtained according to the equations described
by Zheng et al. (19, 20) (Table 1). The results indicated that
the rapid reaction between chloropicrin and metam sodium
resulted in a significant reduction of the aqueous persistence of
both chemicals.
As the reaction of chloropicrin with metam sodium proceeded,
ethyl acetate extracts of the reaction mixture were analyzed by
GC-MS. Three major metabolites were identified in the reaction
process of chloropicrin and metam sodium; their mass spectra
are exhibited in Figure 2. The EI spectrum indicates a significant
molecular ion M+• at m/z 73 (Figure 2A) and is consistent with
an authentic standard of MITC. The identification of this product
was further confirmed by its retention time on GC compared
to that of authentic MITC. The MITC peak was invariably
detected throughout the reaction process, and yields of MITC
ranged between 97 and 103% in the final reaction solution. The
dissipation of chloropicrin was accompanied by an increase in
MITC (Figure 1), and the rate of MITC formation always
equaled the rate of chloropicrin consumption.
Figure 1. Transformation reaction of chloropicrin (1.0 mM) with metam
sodium (1.0 mM) in pH 6.9 buffer solution at 21 °C. Error bars represent
standard deviation of triplicate samples.
was treated with metam sodium at 0.5 mmol/kg and then incubated at
21 °C in the dark. After incubation for 2.5 h and 1 d, chloropicrin,
1,3-D or Telone C-35 solution (0.5 mL) was injected into sample vials
using a gastight syringe, resulting in a concentration of 0.5 mmol/kg.
The procedures described above were used for sampling, extraction,
and analysis of residual fumigants in soils.
GC and GC/MS Analysis. Ethyl acetate extracts were analyzed
for chloropicrin, 1,3-D and MITC using a Hewlett-Packard HP 6890
GC equipped with an on-column injector, a micro-electron capture
detector (ECD) and a flameless nitrogen-phosphorus detector (NPD),
and a 30 m DB-VRX, 0.25-mm i.d. × 1.4-µm film thickness fused-
silica capillary column (J&W, Folsom, CA). The GC conditions were
1.4 mL min-1 carrier gas flow rate (He), 240 °C inlet temperature, and
290 °C for both detectors. The initial oven temperature was 45 °C for
1 min and the temperature was increased to 80 °C at 2.5 °C/min, then
increased to 120 °C at 30 °C/min and held for 2 min. Under these
conditions, the retention times for cis-1,3-D, trans-1,3-D, chloropicrin,
monochloronitromethane, and dichloronitromethane were 10.97, 12.22,
13.56, 10.09, and 11.26 min on the ECD detector, respectively. MITC
was analyzed simultaneously by the NPD detector, and its retention
time was 11.10 min. Data were subjected to analysis of variance, and
means were compared by least significant difference.
Chloropicrin and 1,3-D transformation products were analyzed using
an HP 5890 GC in tandem with an HP 5971 quadrupole mass selective
detector equipped with an on-column injector. Chloropicrin transforma-
tion products were separated using a 30 m DB-VRX, 0.25-mm i.d. ×
1.4-µm film thickness fused-silica capillary column; 1,3-D transforma-
tion products were separated using a 30-m HP-5MS (Wilmington, DE),
0.25-mm i.d. × 0.25-µm film thickness fused-silica capillary column.
The EI mass spectra were generated using an electron energy of 70 eV
and were monitored for ions m/z 10-150 for chloropicrin and m/z 50-
300 for 1,3-D transformation products.
Although not confirmed by an authentic standard, the other
two major metabolites were identified as dichloronitromethane
(Cl2CHNO2, Figure 2B) and monochloronitromethane (ClCH2-
NO2, Figure 2C) according to their mass spectra, which were
consistent with those provided by Zheng et al. (15) and Castro
et al. (21). The EI spectra of these two products do not
demonstrate the formation of molecular ion M+‚, but yield an
intense fragment (parent - NO2). This type of fragmentation
is a typical characteristic of EI spectra of the family of
chloronitromethanes, in which the parent ion is rarely observed
(21).
Previous studies demonstrated that metam sodium could
decompose to its breakdown product (MITC) via an oxidation
process (12, 22) and undergo an oxidation reaction in the
presence of an oxidant (e.g., I2, FeCl3, or Br2) (23). The
oxidation potential of chloropicrin makes it susceptible to
degradation via successive reductive dechlorination from chlo-
ropicrin to nitromethane under the reduction system. In the
experiment, a rapid oxidation-reduction (redox) reaction oc-
curred immediately once chloropicrin and metam sodium were
combined in aqueous solution. Proposed reaction pathways of
chloropicrin and metam sodium are depicted in Scheme 1:
chloropicrin is rapidly decomposed to dichloronitromethane
(Scheme 1a) and monochloronitromethane (Scheme 1b) within
a few minutes via a reduction process (Cl3CNO2 + H+ + e-
f Cl2CHNO2 and Cl2CHNO2 + H+ + e- f ClCH2NO2).
Because the oxidation potential of chloronitromethane com-
pounds decreases with an increasing number of hydrogen atoms
in the molecule, a reactivity trend of Cl3CNO2 > Cl2CHNO2
. ClCH2NO2 is deduced for metam sodium. In the redox
reaction system, metam sodium can completely convert to its
expected product (MITC) via an oxidation process (CH3NHCSS-
f CH3NCS + H+ + S +2e-). Overall, the pest control efficacy
and persistence of chloropicrin in the environment may be
RESULTS AND DISCUSSION
Reaction of Chloropicrin with Metam Sodium in Aqueous
Solution. Initial experiments focused on the kinetics of chlo-
ropicrin reaction with metam sodium. In the control solutions
containing only chloropicrin or metam sodium, no discernible
pesticide degradation occurred during the experimental period.
This observation is consistent with other reports that chloropicrin
does undergo extremely slow hydrolysis without light and
microorganisms (17, 18). In buffer solution containing metam
sodium, a substantially rapid disappearance of chloropicrin was
observed (Figure 1), so that chloropicrin was almost completely
dissipated within 10 min in the presence of metam sodium. In
aqueous solution containing equimolar concentrations of chlo-
ropicrin and metam sodium, the second-order reaction rate

Chloropicrin and 1,3-Dichloropropene
J. Agric. Food Chem., Vol. 52, No. 10, 2004 3005
Figure 3. Transformation reaction of 1,3-D (1.0 mM, 50.5% cis- and 46.9%
trans-1,3-D) with metam sodium (1.0 mM) in pH 6.9 buffer solution at 21
°C. Error bars represent standard deviation of triplicate samples.
the environmental behavior and health effects of chloropicrin
when it is applied in conjunction with metam sodium.
Reaction of 1,3-D and Metam Sodium in Aqueous Solu-
tion. An example timecourse for 1,3-D reaction with metam
sodium in aqueous solution is depicted in Figure 3. The
transformation of 1,3-D by metam sodium was slow compared
to that of chloropicrin, but dissipation of 1,3-D in metam sodium
solution was more rapid than the hydrolysis rate of 1,3-D
isomers (hydrolysis half-life >11 d) (24, 25). At an initial
concentration of 1.0 mM for both 1,3-D and metam sodium
solution, the times of 50% cis- and trans-1,3-D dissipation were
approximately 2.6 and 8.9 h, respectively (Table 1).
To investigate the reaction mechanism, transformation prod-
ucts were analyzed by periodically extracting the reaction
solution. Four transformation products were characterized by
GC/MS analysis. The mass spectrum and retention time of one
transformation product was consistent with authentic MITC,
which indicated that the major breakdown product of metam
sodium was MITC. Two products have different retention times
on the total ion chromatograms (TICs), but present consistent
EI mass spectra (Figure 4A). These two transformation products
gradually disappeared with the proceeding reaction between
1,3-D and metam sodium, and simultaneously, an additional
product (Figure 4B) was formed.
On the basis of the identification of reaction intermediate
products, a transformation mechanism is proposed for 1,3-D
by metam sodium, which is different from chloropicrin. Similar
to thiolate, a metam sodium anion (CH3HNCSS-) contains a
strong nucleophilic center and is susceptible to SN2 nucleophilic
substitution reaction with halogenated hydrocarbons. It is
proposed that metam sodium reacts with 1,3-D via the displace-
ment of the chlorine at C3 to yield a dithiocarbamate substituted
1,3-D product (Scheme 2). Because 1,3-D is a mixture of cis
and trans isomers, two corresponding transformation products
(product 1 and 2 in Scheme 2) were observed to have different
retention times but the same EI mass spectra (Figure 4A). The
two sulfide products decompose to MITC and thiol, which may
further convert to a symmetrical disulfide compound (product
3 in Scheme 2). The disulfide product peak increased in area
with reaction time, and the disulfide compound and MITC were
detected as major transformation products in the mixed solution
of metam sodium and 1,3-D.
Figure 2. Mass spectra of products obtained from the reaction of
chloropicrin with metam sodium.
Scheme 1
significantly altered because of the rapid transformation in the
mixed fumigation system. The two principal reaction products
(Cl2CHNO2 and ClCH2NO2) must be considered in evaluating
It was apparent that the transformation of cis-1,3-D by metam
sodium was significantly faster than that of trans-1,3-D (Figure

3006 J. Agric. Food Chem., Vol. 52, No. 10, 2004
Zheng et al.
Figure 5. Transformation reaction of Telone C-35 (1.0 mM, 65% 1,3-D
and 35% chloropicrin) with metam sodium (1.0 mM) in pH 6.9 buffer
solution at 21 °C. Error bars represent standard deviation of triplicate
samples.
nucleophilic reactions, for example, the reaction between 1,3-D
and ATS (25, 26) or Na-DEDTC (15). Also, GC/MS analysis
showed that cis-1-chloro-3-dithiocarbamate-propene disappeared
faster than the trans transformation product (product 2 in
Scheme 2). The rapid dissipation of the cis intermediate product
also facilitated the acceleration of the reaction of cis-1,3-D with
metam sodium.
Reaction of Telone C-35 with Metam Sodium in Aqueous
Solution. Telone C-35 solution in the experiment consists of
1,3-D (65%) and chloropicrin (35%). At an initial Telone C-35
concentration of 1.0 mM (sum of cis-1,3-D, trans-1,3-D, and
chloropicrin), 1,3-D and chloropicrin exhibited different reactiv-
ity toward metam sodium (1.0 mM) in aqueous solution (Figure
5). The reactivity of the fumigants decreased in the following
order: chloropicrin . cis-1,3-D > trans-1,3-D. This reactivity
is attributed to the different reaction mechanism between metam
sodium and chloropicrin versus 1,3-D. The rapid oxidation-
reduction reaction between chloropicrin and metam sodium
preferentially occurred in the reaction system containing chlo-
ropicrin, 1,3-D, and metam sodium, so that chloropicrin was
almost completely dissipated within a few minutes (Figure 5).
The nucleophilic transformation of 1,3-D by metam sodium was
much slower than the reduction of chloropicrin. These observa-
tions are consistent with the reaction rate constants shown in
Table 1, in which the second-order transformation constant of
chloropicrin is approximately 2 orders of magnitude higher than
that of both 1,3-D isomers.
Figure 4. Mass spectra of products obtained from the reaction of 1,3-D
with metam sodium.
3 and Table 1). According to the stereochemistry, the CdC
bond and the four connected single bonds of 1,3-D are all in
the same plane, suggesting that the spatial hindrance for
approach of the nucleophile should be similar for both iso-
mers (26). When the monomethyl dithiocarbamate anion
(CH3HNCSS-) attacks 1,3-D (Scheme 2), a crowded transition
state will be formed. In the transition state, the steric hindrance
of cis form is greater than that of trans form. The greater static
hindrance may facilitate further transformation of the cis
transition state, yielding cis-1-chloro-3-dithiocarbamate-propene
(product 1 in Scheme 2). The transition state of trans-1,3-D
has higher stability in comparison with cis isomer, and the
reaction with metam sodium shows a higher activation energy.
This reaction mechanism is consistent with other similar
On the basis of the reaction process of chloropicrin and metam
sodium (Scheme 1), one mole of chloropicrin may consume,
theoretically, three moles of metam sodium. A stoichiometric
Scheme 2

Chloropicrin and 1,3-Dichloropropene
J. Agric. Food Chem., Vol. 52, No. 10, 2004 3007
analysis in this system indicated that a major proportion of the
metam sodium reacted with chloropicrin in this experiment.
Therefore, the release of MITC rapidly reached a maximum
(1.0 mM) with the complete dissipation of chloropicrin, indicat-
ing metam sodium was completely consumed within a short
reaction period (Figure 5). Note that even though the metam
sodium was exhausted, dissipation of 1,3-D continued (Figure
5). Scheme 1 showed that sulfur (S) was liberated as an
inorganic product of metam sodium transformation by chlo-
ropicrin. In the redox reaction system, partial sulfurs can be
further oxidized to yield a series of aqueous sulfur species, for
example, sulfite (SO32-), bisulfite (HSO32-), thiosulfate (S2O32-),
tetrathionate (S4O62-), and sulfate (SO42-). Evidence suggests
2-
several of these oxidized sulfur species (such as SO32-, HSO3
,
and S2O32-) are powerful nucleophiles, and nucleophilic
substitution reactions with halogenated aliphatic compounds may
readily occur (25, 27, 28). The dissipation of 1,3-D observed
in the absence of metam sodium in these experiments may have
been due to SN2 nucleophilic reaction between 1,3-D and certain
nucleophilic sulfur species. The dissipation of cis-1,3-D was
faster than that of the trans isomer, further suggesting the
occurrence of a nucleophilic reaction in this system.
Simultaneous and sequential application of metam sodium
with chloropicrin and 1,3-D in Soil. Metam sodium is highly
unstable in the soil environment, undergoing rapid transforma-
tion to MITC. The decomposition of metam sodium and reaction
with chloropicrin or 1,3-D will occur simultaneously when fumi-
gant mixtures are applied to soil. The influence of combined
application on the concentration of chloropicrin and 1,3-D in
soil is depicted in Figure 6. Compared to control samples (no
metam sodium, Figure 6A-a), approximately 40 to 50% of the
applied chloropicrin was dissipated when applied simultaneously
with metam sodium (Figure 6A-b), suggesting that a reaction
occurred between metam sodium and chloropicrin in addition
to the decomposition of metam sodium in soil. A sequential
application of metam sodium and chloropicrin, however, may
be effective in reducing the loss of chloropicrin to preserve pest
control efficacy. The concentration of chloropicrin was sub-
stantially higher where metam sodium was applied to soil 2.5
h before addition of chloropicrin (Figure 6A-c). When metam
sodium was added 24 h before chloropicrin, chloropicrin
concentration (Figure 6A-d) was not significantly different from
that in the control soil without metam sodium. These results
suggest that metam sodium was completely dissipated within
24 h in soil, so that the reaction with chloropicrin did not occur.
The yield of MITC was higher in the simultaneous application
than in the sequential application (Figure 6A-b). Generally, the
conversion of metam sodium to MITC ranges from ap-
proximately 70 to 90% in different soils (22, 29). The reaction
of metam sodium and chloropicrin could increase the conversion
of metam sodium to MITC because its direct reaction product
was MITC.
Figure 6. Influence of fumigant combined application on chloropicrin
dissipation and MITC yield (A) and 1,3-D dissipation (B) in the Arlington
sandy loam: (a) control (only chloropicrin or 1,3-D), (b) simultaneous
application by metam sodium (0.5 mmol/kg), (c) sequential application by
metam sodium 2.5 h early, (d) sequential application by metam sodium
(0.5 mmol/kg) 24 h early.
reported the reactivity of HS- associated with nucleophilic
substitution with halogenated aliphatic compounds (28, 30-
32), and even with nucleophilic aromatic substitution with
chlorozaines (33). The reduction of 1,3-D may be a result of
HS- nucleophilic reaction with 1,3-D. However, bisulfate is not
stable in soil and gradually oxidized to SO42-, so that when the
time delay between metam sodium and 1,3-D application was
increased to 24 h, no effect of metam sodium application on
1,3-D concentrations was observed (Figure 6B-d).
The influence of simultaneous and sequential application of
Telone C-35 and metam sodium to soil was also investigated.
The impact of metam sodium on the concentration of chloropi-
crin and 1,3-D in Telone C-35 was similar to that observed for
chloropicrin and 1,3-D in isolation (Figures 6 and 7). Concen-
trations of chloropicrin and 1,3-D were decreased when metam
sodium and Telone C-35 were applied simultaneously (Figure
7-b) as well as sequentially with a 2.5-h delay (Figure 7-c).
The influence on chloropicrin concentration was more significant
because of its high reactivity with metam sodium. The reaction
between metam sodium and chloropicrin or 1,3-D was negligible
if metam sodium was applied to soil 24 h before addition of
Telone C-35 (Figure 7-d). It implies that sequential application
A similar influence of simultaneous application on the
concentration of 1,3-D was observed (Figure 6B), although the
decrease in 1,3-D concentration was not as significant as that
of chloropicrin. These results suggest that the reaction mech-
anisms in soil were consistent with those in aqueous solution
described above. The influence of metam sodium in sequential
treatment with 1,3-D was different from that observed for
chloropicrin. The maximum reduction of 1,3-D concentration
in soil occurred when metam sodium was applied 2.5 h before
the addition of 1,3-D (Figure 6B-c). The primary breakdown
products of metam sodium in soil were HS- and MITC
(CH3NHCSS- f CH3NCS + HS-). A wealth of literature has

3008 J. Agric. Food Chem., Vol. 52, No. 10, 2004
Zheng et al.
addition of chloropicrin or 1,3-D. Furthermore, the influence
of MITC as a major breakdown product of metam sodium on
the abiotic and biotic transformation of chloropicrin and 1,3-D
should be evaluated.
ABBREVIATIONS USED
1,3-D, 1,3-dichloropropene; MITC, methyl isothiocyanate;
ATS, ammonium thiosulfate; Na-DEDTC, sodium diethyldithio-
carbamate.
LITERATURE CITED
(1) U. S. Environmental Protection Agency. Protection of strato-
spheric ozone: Incorporation of Clean Air Act Amendments for
Reductions in Class I, Group VI controlled substances. Fed.
Regist. 2000, 65 (229), 70795-70804.
(2) Wilhelm, S. Chemical treatments and inoculum potential of soil.
Annu. ReV. Phytopathol. 1966, 4, 53-78.
(3) Wilhelm, S.; Paulus, A. O. How soil fumigation benefits the
California strawberry industry. Plant Dis. 1980, 64, 264-270.
(4) Duniway, J. M. Status of chemical alternatives to methyl bromide
for pre-plant fumigation of soil. Phytopathology 2002, 92,
1337-1343.
Figure 7. Influence of fumigant combined application on Telone C-35
(0.5 mmol/kg) in the Arlington sandy loam: (a) control, (b) simultaneous
application by metam sodium (0.5 mmol/kg), (c) sequential application by
metam sodium 2.5 h early, (d) sequential application by metam sodium
(0.5 mmol/kg) 24 h early.
of chloropicrin and 1,3-D with metam sodium is likely a more
effective strategy for multi-fumigant applications in the field.
Environmental Significance. Our results suggest that metam
sodium is sufficiently reactive to alter the environmental fate
of chloropicrin and 1,3-D, if they are simultaneously applied
to soil or water. Metam sodium can accelerate the degradation
of chloropicrin in the soil and aquatic environment by an
oxidation-reduction reaction. The rapid transformation will
have a profound effect on the suitability of multi-fumigant
formulations in agronomic systems and could be a severe
impediment to development of an adequate replacement for
MeBr. However, the problem can be solved by a sequential
application strategy. Sequential application of metam sodium
followed by chloropicrin with a delay of 1 day may provide
time for the conversion of metam sodium to MITC, which
reduces the unnecessary loss of chloropicrin. On the other hand,
chloropicrin may be first applied, and then metam sodium is
followed after reaching the pest control target of chloropicrin.
The transformation pathway of chloropicrin in mixture with
metam sodium is similar to the degradation of chloropicrin itself
in the environment via reductive dehalogenation (18, 21). The
sequential application practice could not only improve the
convention efficacy of metam sodium to MITC but also
eliminate the chloropicrin residue in soil and mitigate the
potential phytotoxicity and negative environmental risk of
fumigant. Therefore, further research is needed to comprehen-
sively assess the impact of the sequential strategy on environ-
ment pollution prevention and pest control efficacy.
(5) Gilreath, J. P.; McSorley, R.; McGovern, R. J. Soil Fumigant
and herbicide combinations for soilborne pest control in cala-
dium. Methyl bromide alternatiVes newsletter. USDA, Agricul-
tural Research Service 6 (4), 2000. http://www.ars.usda.gov/is/
np/mba/oct00/index.htm.
(6) Gilreath, J. P. MeBr alternatives and their current limitations in
Florida. Methyl bromide alternatiVes newsletter. USDA, Agri-
cultural Research Service 8 (1), 2002. http://www.ars.usda.gov/
is/np/mba/jul02/index.htm.
(7) Koster, A. Th. J.; van der Meer, L. J. Effects of sheeting with
black plastic or flooding of soils on the control of Cyperus
esculentus. Acta Hortic. 1990, 266, 569-573.
(8) Dickson, D. W.; Locascio, S. J.; Mitchell, D. J. Evaluating methyl
bromide alternative fumigants on tomato under polyethylene
mulch in Florida. Methyl bromide alternatiVes newsletter. USDA,
Agricultural Research Service 5 (1), 2000. http://www.ars.us-
da.gov/is/np/mba/jan99/index.htm.
(9) Donaldson, D.; Kiely, T.; Grube, A. Pesticide industry sales and
usage 1998 and 1999 market estimates. Office of Prevention,
Pesticides, and Toxic Substances, EPA: Washington, DC. 2002.
(10) van den Berg, F.; Smelt, J. H.; Boesten, J. J. T. I.; Teunissen,
W. Volatilization of methyl isothiocyanate from soil after
application of metam-sodium with two techniques. J. EnViron.
Qual. 1999, 28, 918-928.
(11) Gerstl, Z.; Mingelgrin, U.; Yaron, B. Behavior of Vapam and
methylisothiocyanate in soils. Soil Sci. Soc. Am. J. 1977, 41,
545-548.
(12) Turner, N. J.; Corden. M. E. Decomposition of sodium N-
methyldithiocarbamate in soil. Phytopathology 1963, 53,
1388-1394.
Reaction of metam sodium and 1,3-D followed an entirely
different transformation pathway compared to the transformation
of 1,3-D itself in the environment. Some sulfur-substituted
transformation products are formed, which may necessitate
future eco-toxicological assessments if metam sodium and 1,3-D
are combined in field application. In addition, the high nucleo-
philicity of metam sodium indicates that it may also react with
halogenated aliphatic compounds or pesticides, such as chlo-
roacetanilide and triazine herbicides, via nucleophilic substitu-
tion processes.
Overall, the results of our experiments demonstrated the
simultaneous application of metam sodium with chloropicrin
and 1,3-D failed in maintaining fumigant efficacy. No significant
influence on fumigant transformation, however, is expected if
metam sodium is sufficiently converted to MITC prior to the
(13) Ibekwe, A. M.; Papiernik, S. K.; Gan, J.; Yates, S. R.; Yang,
C.-H.; Crowley, D. E. Impact of fumigants on soil microbial
communities. Appl. EnViron. Microbiol. 2001, 67, 3245-3257.
(14) Dungan, R. S.; Ibekwe, A. M.; Yates, S. R. Effect of propargyl
bromide and 1,3-dichloropropene on microbial communities in
an organically amended soil. FEMS Microbiol. Ecol. 2003, 43,
75-87.
(15) Zheng, W.; Papiernik, S. K.; Guo, M.; Yates, S. R. Competitive
degradation between the fumigants chloropicrin and 1,3-dichlo-
ropropene in unamended and amended soil. J. EnViron. Qual.
2003, 32, 1735-1742.
(16) Stiles, C. L.; Sams, C. E.; Robinson, D. K.; Coffey, D. L.;
Mueller, T. C. Influence of metam sodium on the dissipation
and residual biological activity of the herbicides EPTC and
pebulate in surface soil under black plastic mulch. J. Agric. Food
Chem. 2000, 48, 4681-4686.

Chloropicrin and 1,3-Dichloropropene
J. Agric. Food Chem., Vol. 52, No. 10, 2004 3009
(17) Castro, C. E.; Belser, N. O. Photohydrolysis of methyl bromide
and chloropicrin. J. Agric Food Chem. 1981, 29, 1005-1008.
(18) Wilhelm, S. N.; Shepler, K.; Lawrence, L. J.; Lee, H. Environ-
mental Fate of Chloropicrin. In Fumigant: EnVironmental Fate,
Exposure, and Analysis. ACS Symposium Series 652; Seiber, J.
N., Knuteson, J. E., Woodrow, J. E., Wolfe, N. L., Yates, M.
V., Yates, S. R., Ed.; American Chemical Society: Washington,
DC, 1996; pp 79-93.
(19) Zheng, W.; Papiernik, S. K.; Guo, M.; Yates, S. R. Accelerated
degradation of methyl iodide by agrochemicals. J. Agric. Food
Chem. 2003, 51, 673-679.
(20) Zheng, W.; Papiernik, S. K.; Guo, M.; Yates, S. R. Remediation
of methyl iodide in aqueous solution and soils amended with
thiourea. EnViron. Sci. Technol. 2004, 38, 1188-1194.
(21) Castro, C. E.; Wade, R. S.; Belser, N. O. Biodehalogenation.
The metabolism of chloropicrin by Pseudomonas sp. J. Agric.
Food Chem. 1983, 31, 1184-1187.
(22) Draper, W. M.; Wakeham, D. E. Rate constants for metam-
sodium cleavage and photodecomposition in water. J. Agric.
Food Chem. 1993, 41, 1129-1133.
(23) Joris, S. J.; Aspila, K. I.; Chakrabarti, C. L. Decomposition of
monoalkyl dithiocarbamates. Anal. Chem. 1970, 42, 647-651.
(24) McCall, P. J. Hydrolysis of 1,3-dichloropropene in dilute aqueous
solution. Pestic. Sci. 1987, 19, 235-242.
(27) Croue, J.-P.; Reckhow, D. A. Destruction of chlorination
byproducts with sulfite. EnViron. Sci. Technol. 1989, 23, 1412-
1419.
(28) Roberts, A. L.; Sanborn, P. N.; Gschwend, P. M. Nucleophilic
substitution reactions of dihalomethanes with hydrogen sulfide
species. EnViron. Sci. Technol. 1992, 26, 2263-2274.
(29) Smelt, J. H.; Crum, S. J. H.; Teunissen, W. Accelerated
transformation of the fumigant methyl isothiocyanate in soil after
repeated application of metam-sodium. J. EnViron. Sci. Health.
1989, B24, 437-455.
(30) Loch, A. R.; Lippa, K. A.; Carlson, D. L.; Chin, Y. P.; Traina,
S. J.; Roberts, A. L. Nucleophilic aliphatic substitution reactions
of propachlor, alachlor, and metolachlor with bisulfide (HS-)
and polysulfides (Sn2-). EnViron. Sci. Technol. 2002, 36, 4065-
4073.
(31) Larson, R. A.; Weber, E. J. In Reaction Mechanisms in
EnVironmental Organic Chemistry; Larson, R. A., et al., Eds.;
CRC Press: Boca Raton, FL, 1994. pp136-143.
(32) Barbash, J. E.; Reinhard, M. Abiotic dehalogenation of 1,2-
dichloroethane and 1,2-dibromoethane in aqueous solution
containing hydrogen sulfide. EnViron. Sci. Technol. 1989, 23,
1349-1358.
(33) Lippa, K. A.; Roberts, A. L. Nucleophilic aromatic substitution
reactions of chloroazines with bisulfide (HS-) and polysulfides
(Sn2-). EnViron. Sci. Technol. 2002, 36, 2008-2018.
(25) Gan, J.; Yates, S. R.; Knuteson, J. A.; Becker, J. O. Transforma-
tion of 1,3-dichloropropene in soil by thiosulfate fertilizers. J.
EnViron. Qual. 2000, 29, 1476-1481.
(26) Wang, Q.; Gan, J.; Papiernik, S. R.; Yates, S. R. Isomeric effects
on thiosulfate transformation and detoxification of 1,3-dichlo-
ropropene. EnViron. Toxicol. Chem. 2001, 20, 960-964.
Received for review January 28, 2004. Revised manuscript received
March 7, 2004. Accepted March 8, 2004. This study was funded by
USDA-CRSEES Methyl Bromide Transitions Program grant No.
2003-51102-02049.
JF0498564