Dechlorination of chloropicrin and 1,3-dichloropropene by hydrogen sulfide species: Redox and nucleophilic substitution reactions

Article abstract of DOI:10.1021/jf0527100

The chlorinated fumigants chloropicrin (trichloronitromethane) and 1,3-dichloropropene (1,3-D) are extensively used in agricultural production for the control of soilborne pests. The reaction of these two fumigants with hydrogen sulfide species (H₂S and HS^-) was examined in well-defined anoxic aqueous solutions. Chloropicrin underwent an extremely rapid redox reaction in the hydrogen sulfide solution. Transformation products indicated reductive dechlorination of chloropicrin by hydrogen sulfide species to produce dichloro- and chloronitromethane. The transformation of chloropicrin in hydrogen sulfide solution significantly increased with increasing pH, indicating that H₂S is less reactive toward chloropicrin than HS ^- is. For both 1,3-D isomers, kinetics and transformation products analysis revealed that the reaction between 1,3-D and hydrogen sulfide species is an S_N2 nucleophilic substitution process, in which the chlorine at C3 of 1,3-D is substituted by the sulfur nucleophile to form corresponding mercaptans. The 50% disappearance time (DT₅₀) of 1,3-D decreased with increasing hydrogen sulfide species concentration at a constant pH. Transformation of 1,3-D was more rapid at high pH, suggesting that the reactivity of hydrogen sulfide species in the experimental system stems primarily from HS^-. Because of the relatively low smell threshold values and potential environmental persistence of organic sulfur products yielded by the reaction of 1,3-D and HS^-, the effects of reduced sulfide species should be considered in the development of alternative fumigation practices, especially in the integrated application of sulfur-containing fertilizers.

Full text of DOI:10.1021/jf0527100

2280 J. Agric. Food Chem. 2006, 54, 2280−2287
Dechlorination of Chloropicrin and 1,3-Dichloropropene by
Hydrogen Sulfide Species: Redox and Nucleophilic
Substitution Reactions
WEI ZHENG,*,†,‡ SCOTT R. YATES,‡ SHARON K. PAPIERNIK,§
†
MINGXIN GUO,†,‡ AND JIANYING GAN
Department of Environmental Sciences, University of California, Riverside, California 92521,
Soil Physics and Pesticides Research Unit, George E. Brown Jr. Salinity Laboratory, Agricultural
Research Service, U.S. Department of Agriculture, 450 West Big Springs Road,
Riverside, California 92507, and North Central Soil Conservation Research Laboratory, Agricultural
Research Service, U.S. Department of Agriculture, 803 Iowa Avenue, Morris, Minnesota 56267
The chlorinated fumigants chloropicrin (trichloronitromethane) and 1,3-dichloropropene (1,3-D) are
extensively used in agricultural production for the control of soilborne pests. The reaction of these
two fumigants with hydrogen sulfide species (H2S and HS-) was examined in well-defined anoxic
aqueous solutions. Chloropicrin underwent an extremely rapid redox reaction in the hydrogen sulfide
solution. Transformation products indicated reductive dechlorination of chloropicrin by hydrogen sulfide
species to produce dichloro- and chloronitromethane. The transformation of chloropicrin in hydrogen
sulfide solution significantly increased with increasing pH, indicating that H2S is less reactive toward
chloropicrin than HS- is. For both 1,3-D isomers, kinetics and transformation products analysis
revealed that the reaction between 1,3-D and hydrogen sulfide species is an SN2 nucleophilic
substitution process, in which the chlorine at C3 of 1,3-D is substituted by the sulfur nucleophile to
form corresponding mercaptans. The 50% disappearance time (DT50) of 1,3-D decreased with
increasing hydrogen sulfide species concentration at a constant pH. Transformation of 1,3-D was
more rapid at high pH, suggesting that the reactivity of hydrogen sulfide species in the experimental
system stems primarily from HS-. Because of the relatively low smell threshold values and potential
environmental persistence of organic sulfur products yielded by the reaction of 1,3-D and HS-, the
effects of reduced sulfide species should be considered in the development of alternative fumigation
practices, especially in the integrated application of sulfur-containing fertilizers.
KEYWORDS: Fumigant; dechlorination; chloropicrin; 1,3-dichloropropene; hydrogen sulfide species
INTRODUCTION
likely to bioaccumulate, and more susceptible to further
biodegradation than the parent compounds (5-8). Generally,
halogenated organic compounds undergo three types of deha-
logenation via biotic or abiotic transformation: reductive
dehalogenation, nucleophilic substitution, and dehydrodehalo-
genation. Because of their high oxidation state, highly haloge-
nated compounds are subject to reductive dehalogenation in the
presence of appropriate reductants, as in hydrogenolysis (the
replacement of a halogen with a hydrogen), dihaloelimination
(elimination of vicinal halides to form a C-C double bond),
and radical coupling (9). For example, biotransformations of
tetrachloroethylene (5) and chloropicrin (10) in the environment
are recognized as sequential reductive dehalogenation processes.
In several cases, some halogenated intermediates resulting from
the reductive dehalogenation of polyhalogenated chemicals may
be more toxic and persistent than the parent compounds. For
example, tetrachloroethylene and trichloroethylene can be
sequentially dechlorinated to a more toxic vinyl chloride (11,
The chlorinated aliphatic fumigants chloropicrin and 1,3-
dichloropropene (1,3-D) have been widely applied in crop
production for several decades. Although these two fumigants
are not recognized as stratospheric ozone-depleting substances
such as methyl bromide, their rapid volatilization causes air
pollution (1-3) that is a public health concern. They may also
contaminate subsurface aquatic environments through leaching
(4), particularly in sensitive regions with shallow groundwater
tables.
Research has indicated that many dehalogenation reactions
produce transformation products that may be less toxic, less
* To whom correspondence should be addressed. Tel: 951-369-4878.
Fax: 951-342-4964. E-mail: wzheng@ussl.ars.usda.gov.
† University of California.
‡ George E. Brown Jr. Salinity Laboratory, Agricultural Research Service,
U.S. Department of Agriculture.
§ North Central Soil Conservation Research Laboratory, Agricultural
Research Service, U.S. Department of Agriculture.
10.1021/jf0527100 CCC: $33.50
© 2006 American Chemical Society
Published on Web 02/18/2006

Dechlorination of Chloropicrin and 1,3-Dichloropropene
J. Agric. Food Chem., Vol. 54, No. 6, 2006 2281
standards were purchased from Cansyn Chemical Corp. (New West-
minster, BC, Canada). Sodium sulfide (NaS‚9H2O, ACS reagent grade)
and flake sulfur (S, 99.99%) were purchased from Sigma-Aldrich Co.
(St. Louis, MO). All chemicals were used as received.
Experimental Systems. All aqueous solutions were prepared using
high-purity deionized water (E-pure, Barnstead, Dubuque, IA). All
glassware used with hydrogen sulfide solutions was washed with
alcoholic potassium hydroxide (KOH) to remove traces of sulfur
species, followed by soaking in 10% nitric acid (HNO3) and rinsing
with deionized water. The glassware was autoclaved prior to use to
inhibit microbial growth.
12). For mono- and dihalogenated compounds (e.g., primary
alkyl halides), the dehalogenation process is mainly associated
with the nucleophilic substitution reaction, especially in the
presence of strong nucleophiles. The fumigant methyl bromide
and chloroacetanilide herbicides, for instance, can be dehalo-
genated by the fertilizers ammonium thiosulfate and thiourea
via an SN2 nucleophilic reaction (6, 13, 14). The hydrolysis of
halogenated aliphatic compounds in aquatic ecosystems is also
often a nucleophilic dehalogenation process.
Some nucleophilic transformations may be exploited for the
remediation of halogenated organic contaminants. For example,
glutathione conjugation, an important natural detoxification
reaction involving the nucleophilic removal of a halogen through
an enzyme-mediated reaction, can potentially be used for in situ
bioremediation and phytoremediation of halogenated contami-
nants (8). Some glutathione nucleophilic metabolites, such as
ethane sulfonic acids of chloroacetanilide herbicides, have been
frequently detected in groundwater (15-17). To provide a more
complete risk assessment, it is of vital importance to determine
the environmental toxicity, distribution, and fate of pesticide
transformation products. Many nucleophilic transformation
products are relatively persistent and mobile, although their
toxicity may be decreased as compared to the parent compounds.
In hypoxic environments, many environmentally relevant
nucleophiles, such as reduced sulfur species, play a crucial role
in the abiotic dehalogenation process (18-23). Bisulfide (HS-),
a potential electron donor (reductant) and nucleophile that can
react with a wide array of halogenated organic compounds,
occurs ubiquitously in well water, salt marshes, anoxic bottom
layers of estuaries, and hypoxic soils at levels ranging from
0.2 µM to 5 mM. HS- is produced by anaerobic microbial
reduction of sulfate associated with the decomposition of organic
matter. Another important source of HS- in agricultural soils
results from the application of sulfur-containing agrochemicals.
Elemental sulfur is an essential nutrient in soils and is also used
as a pesticide. Sulfur is currently the most heavily used pesticide
in California at 24 million kg of active ingredient in 2003. Under
hypoxic conditions, elemental sulfur can undergo reduction by
soil microorganisms to generate HS- (23). HS- and other
reduced sulfur species often occur in fumigated soil, especially
in soil treated with metam sodium (currently the most heavily
used soil fumigant in the United States) because metam sodium
rapidly decomposes to methyl isothiocyanate (MITC) and HS-
in soil (24). HS- was also detected as one of the main
degradation products of MITC in soil and aquatic media (25).
Hence, the study of the effect of HS- on the fate of halogenated
fumigants may be of considerable environmental significance,
particularly in soils treated previously with metam sodium or
sulfur-containing fertilizers.
Buffer solutions of pH 6-8 were prepared by mixing phosphate
buffer (0.1 M KH2PO4) and different amounts of sodium hydroxide
(NaOH) according to ref 26. A buffer solution of pH 9 (0.1 M, Fisher
Scientific) consisted of boric acid (H3BO3) and NaOH. Sufficient NaCl
was added to buffer solutions to establish an ionic strength of 0.15
equiv/L. All buffer solutions were deoxygenated by purging with argon
for 1 h. Stock sulfide solutions (∼0.1 M) were prepared by rinsing
NaS‚9H2O crystals with deoxygenated water to remove the surface-
oxidized products, wiping them dry with a cellulose tissue, and then
dissolving in deoxygenated water. Serum bottles (55 mL) sealed with
Teflon-faced butyl rubber septa served as reactors. The sealed reactor
may avoid losses of fumigant by volatilization and prevent the oxidation
of hydrogen sulfide species by adventitious O2. Fumigant solutions of
chloropicrin (0.5 mM) and 1,3-D (1.0 mM) were prepared by spiking
the liquid fumigant standards to deoxygenated buffer solutions (50 mL)
in reactors. Kinetics experiments were initiated by spiking stock sulfide
solutions to fumigant solutions. Experimental processes were conducted
within an anaerobic glovebag enriched with 5% H2/95% N2 to maintain
anoxic conditions. All reactors were vigorously shaken and then
incubated in the dark at 21 ( 0.5 °C for chloropicrin and 25 ( 0.5 °C
for 1,3-D. At regular time intervals, a 0.5 mL aliquot of reaction solution
was withdrawn from each reactor using a gastight syringe and
transferred into a sealed glass vial containing ethyl acetate (3.0 mL)
and anhydrous sodium sulfate (2.5 g). Simultaneously, 0.5 mL of N2
was injected into the reactor to avoid the introduction of headspace.
The sealed vials were shaken for 5 min, and then, an aliquot of the
ethyl acetate extract was transferred to a gas chromatography (GC)
vial for fumigant concentration determination by GC/electron capture
detection (ECD). Preliminary experiments revealed that the reaction
was quenched immediately when ethyl acetate was used as an extraction
solvent because chloropicrin or 1,3-D was efficiently extracted into
the organic phase, while hydrogen sulfide species remained in the
aqueous phase. Control experiments were concurrently performed in
deoxygenated buffer solutions containing only chloropicrin or 1,3-D
to measure their hydrolysis. The recovery of two fumigants ranged from
95 to 105% using the above-described procedures.
To identify primary reaction products and to propose the transforma-
tion pathway, 10 mM chloropicrin or 1,3-D and 10 mM hydrogen
sulfide species were mixed in buffer solution (pH 9). Aliquots of the
reaction solution were periodically extracted according to the above
extraction procedure and analyzed by GC/mass spectrometry (MS).
Fresh stock sulfide solutions were prepared daily. The total
concentration of hydrogen sulfide species [H2S]T was determined by
iodometric titration, representing the sum of all hydrogen sulfide species
([H2S]T ) [H2S] + [HS-] + [S2-]). The exact pH values in the reaction
sulfide solutions were measured using an Accumet pH meter (Fisher
Scientific).
GC/ECD and GC/MS Analysis. Ethyl acetate extracts were
analyzed for chloropicrin and 1,3-D using a Hewlett-Packard HP 6890
GC equipped with an on-column injector, a micro-ECD, 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 detector
temperature. 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, chloroni-
tromethane, and dichloronitromethane were 10.9, 12.2, 13.6, 10.1, and
11.3 min. Data were subjected to analysis of variance, and means were
compared by least significant difference.
The primary objective of this study was to investigate the
potential impact of hydrogen sulfide species (H2S and HS-) on
the abiotic transformation of the halogenated fumigants chlo-
ropicrin and 1,3-D by determining the mechanism and kinetics
of reaction in aqueous solution. These studies not only provide
important insights into the transformation and fate of the
fumigants but also provide useful information for further
evaluation of the potential environmental effects of degradation
products.
MATERIALS AND METHODS
Reagents. Standards of chloropicrin (99%) were obtained from Chem
Service (West Chester, PA), and 1,3-D (Telone II, 50.5% cis and 46.9%
trans isomer) was donated by Dow AgroSciences LLC (Indianapolis,
IN). Chloronitromethane (95%) and dichloronitromethane (>95%)

2282 J. Agric. Food Chem., Vol. 54, No. 6, 2006
Zheng et al.
Figure 2. Example time courses for dissipation of chloropicrin in different
Figure 1. Dissipation of chloropicrin (0.5 mM) in hydrogen sulfide solutions
initial (H2S)T concentration solutions at constant temperature (21
± 0.5
(0.5 mM) at different pH values at 21
standard deviation of triplicate samples.
± 0.5 °C. Error bars represent
°
C) and approximately constant pH (6.9 0.1). C0 is the initial
±
concentration of chloropicrin (0.5 mM) in the solutions.
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.
pH (Figure 1) clearly indicates that the reactivity of chloropicrin
with H2S is much lower than that with HS-.
Figure 2 depicts chloropicrin dissipation at different (H2S)T
concentrations at a pH of 6.9 ( 0.1. The rate of chloropicrin
dissipation in buffer solutions at constant pH increased with
increasing total hydrogen sulfide concentration. These results
show that, at a given pH, the rate of reaction between
chloropicrin and hydrogen sulfide species is clearly dependent
on (H2S)T concentration.
Transformation Mechanism of Chloropicrin and Hydro-
gen Sulfide Species. A precipitate was immediately formed
when chloropicrin was mixed with hydrogen sulfide in the buffer
solutions (pH 6-8). The elemental composition of the solid
reaction product was analyzed using an elemental analyzer (CE-
ELANTECH Flash 1112, Thermo) after separating the precipi-
tate from the reaction solution. Microanalysis results showed
that the contents of both C and N were <0.3% and that the
major constituent of the solid product was S (>85.0%), which
is close to the microanalysis of the standard sulfur.
RESULTS AND DISSCUSSION
Chloropicrin Transformation by Hydrogen Sulfide Spe-
cies. The reaction of chloropicrin with hydrogen sulfide in buffer
solution was assessed at different pH values. Figure 1 illustrates
the disappearance of chloropicrin in the presence of 0.5 mM
hydrogen sulfide species at pH 5.8-8.9. Chloropicrin dissipated
rapidly (within <1 h) when in contact with hydrogen sulfide
species at all pH values and within a couple of minutes at high
pH (Figure 1). No discernible fumigant degradation occurred
in the control buffer solutions containing only chloropicrin at
these short time scales. This observation is consistent with
previous reports that chloropicrin undergoes extremely slow
hydrolysis in the absence of light and microorganisms (24, 27).
The dissipation of chloropicrin in hydrogen sulfide solutions
follows an exponential decay (Figure 1). The dissipation half-
life of the fumigant was approximated based on the description
of chloropicrin reduction by iron-bearing clay minerals (28, 29).
A similar calculation was utilized to describe the reductive
dehalogenation of polyhalogenated hydrocarbons (30, 31). The
dissipation half-lives (t1/2) of chloropicrin in solutions containing
0.5 mM hydrogen sulfide species decreased with increasing pH.
For example, the half-life of chloropicrin decreased by more
than a factor of 20 when hydrogen sulfide solution pH increased
from 5.8 to 8.9 (t1/2 decreased from approximately 10 to 0.45
min). In these experimental systems, the total hydrogen sulfide
(H2S)T concentration represents the sum of three hydrogen
sulfide species (H2S, HS-, and S2-). The proportion of these
individual species in hydrogen sulfide solution is pH-dependent
(18, 32). At pH < 6, H2S is the predominating sulfide species.
As the pH increases, the proportion of H2S that dissociates to
HS- gradually increases, and HS- accounts for 99% of the total
at pH 9 (33). At the pH range used in these experiments (5.8-
8.9), the concentration of S2- is very small (pK2 ) 14.6) (32).
The dissipation kinetics of chloropicrin in solutions of different
The reaction solution was periodically extracted and analyzed
by GC/MS to monitor the progress of the reaction. Two reaction
products were detected and identified as dichloronitromethane
(Cl2CHNO2; Figure 3a) and chloronitromethane (ClCH2NO2;
Figure 3b) according to the analysis of their mass spectra (27).
These two reaction products were further verified using authentic
standards. The products dichloronitromethane and chloroni-
tromethane were also detected as the reduction products of
chloropicrin in aqueous solution containing metam sodium (24).
On the basis of these identified reaction products, chloropicrin
is believed to react via a redox reaction mechanism in hydrogen
sulfide solution. The major transformation pathways of chlo-
ropicrin in the presence of hydrogen sulfide species are proposed
in Scheme 1. Chloropicrin, as an electron acceptor, initially
obtains an electron (9), resulting in the formation of a carbon-
centered radical (Cl2C‚NO2), which would further transform to
Cl2CHNO2 (Scheme 1a). Simultaneously, the reductive dechlo-
rination of chloropicrin may be initiated by a two-electron
transfer, which liberates two chlorine ions and produces a
carbene(oid) intermediate (ClC:NO2) (34). Subsequent reaction
of this radical intermediate would lead to the formation of
ClCH2NO2 (Scheme 1b). The proposed mechanism is similar
to the reduction of chloropicrin by ferruginous smectite (28,
29) and zerovalent iron (34) to simultaneously produce Cl2-
CHNO2 and ClCH2NO2 via parallel reaction pathways. In this

Dechlorination of Chloropicrin and 1,3-Dichloropropene
J. Agric. Food Chem., Vol. 54, No. 6, 2006 2283
Figure 4. Dissipation of chloropicrin (0.5 mM) and formation of dichlo-
ronitromethane (Cl2CHNO2) and chloronitromethane (ClCH2NO2) in
hydrogen sulfide solutions (2.0 mM) at 21
± 0.5 °C (pH 7.0 ± 0.1).
then decreased gradually, with a concurrent gradual increase in
ClCH2NO2 concentration (Figure 4), inferring that Cl2CHNO2
was further reduced by hydrogen sulfide species via a redox
reaction to yield ClCH2NO2 (Scheme 1c). The latter would
further transform to the nonhalogenated product nitromethane
(CH3NO2) in hydrogen sulfide solution (Scheme 1d), which
could also be reduced to methylamine (CH3NH2) (34). Because
the oxidation potential of chloronitromethane compounds
decreases with an increasing number of hydrogen atoms in the
molecule, a reactivity trend of Cl3CNO2 > Cl2CHNO2 > ClCH2-
NO2 is expected for hydrogen sulfide species in the reaction
system. This is similar to the degradation of chloropicrin in the
presence of zerovalent iron (34).
The experimental results presented in Figure 4 clearly
indicate a lack of mass balance among the chloronitromethane
compounds after the initial reaction time, which suggests that
chloropicrin and dichloronitromethane might also directly reduce
to nitromethane via the involvement of a series of carbene(oid)
intermediates (such as ClC:NO2 and HC:NO2). The occurrence
of these intermediates species in the process of chloropicrin
reduction had been verified by using a carbene(oid) trapping
agent as performed by Pearson et al. (34).
Figure 3. Mass spectra (EI) of transformation products obtained from
the reaction of chloropicrin with hydrogen sulfide species: (a) dichloroni-
tromethane (Cl2CHNO2) and (b) chloronitromethane (ClCH2NO2).
Scheme 1
Previous research has shown that elemental sulfur can readily
combine with HS- to form polysulfides (19, 20). Polysulfides
are considered to be substantially more reactive than other
reduced sulfide species such as HS-. Additional experiments,
in which reaction solutions of chloropicrin and hydrogen sulfide
were immediately mixed with excess S8, did not produce an
increase in the rate of chloropicrin dissipation (data not shown).
These results suggest that the effect of polysulfide on chloropi-
crin reduction was negligible in the anaerobic experimental
system because of rapid reaction of chloropicrin with hydrogen
sulfide species (occurring within minutes) and the relatively slow
formation of polysulfide from S8 and HS- (31).
redox reaction system, hydrogen sulfide species (HS- and H2S)
serve as electron donors and are oxidized to elemental sulfur
(S) (Scheme 1), in agreement with the microanalysis of the solid
precipitate.
The concentrations of Cl2CHNO2 and ClCH2NO2 monitored
by GC are shown in Figure 4 along with the loss of chloropicrin
(0.5 mM) in solutions containing excess hydrogen sulfide (2.0
mM). Results indicate that Cl2CHNO2 and ClCH2NO2 were
immediately yielded when chloropicrin was mixed with hydro-
gen sulfide species. The simultaneous formation of Cl2CHNO2
and ClCH2NO2 supports the proposed reaction mechanism in
which the reduction of chloropicrin involves the formation of
radicals prior to homolysis. The formation of Cl2CHNO2
achieved a maximum when chloropicrin was completely reduced
by hydrogen sulfide species. The concentration of Cl2CHNO2
1,3-D Transformation by Hydrogen Sulfide Species. The
reaction of 1,3-D with hydrogen sulfide was investigated at
different pH values and different (H2S)T concentrations. An
example for 1,3-D dissipation of 1,3-D in hydrogen sulfide
solution at pH 7.9 ( 0.1 is shown in Figure 5. As compared to
the rate of 1,3-D hydrolysis [0 mM (H2S)T], the dissipation of
both 1,3-D isomers was significantly accelerated in all anoxic
aqueous solutions containing hydrogen sulfide species. At a
given pH, the rate of 1,3-D dissipation increased with increasing
initial total concentration of hydrogen sulfide (Figure 5),

2284 J. Agric. Food Chem., Vol. 54, No. 6, 2006
Zheng et al.
Upon rearrangement and integration, a solution of eq 2 is
obtained
(X0 - C0) exp[-k2′(X0 - C0)t]
C ) C0
(3)
X0 - C0 exp[-k2′(X0 - C0)t]
where k2′ is the apparent second-order rate constant and C0 and
X0 are the initial concentrations of 1,3-D and hydrogen sulfide
species, respectively. Note that eq 3 is valid only if C0 * X0.
The second-order rate constant (k2′) may be obtained from
nonlinear least-squares fit of the experimental data to eq 3
(Table 1). If one inserts C0/2 for C in eq 3, one obtains
ln(2 - C0/X0)
DT50
)
(4)
k2′X0(1 - C0/X0)
where DT50 represents the 50% disappearance time of pesticide
under a given set of reaction conditions (6, 35).
To distinctly compare the rate of 1,3-D dissipation in buffer
solutions in the presence and absence of hydrogen sulfide
species, 50% disappearance times of both 1,3-D isomers at
different pH values are summarized in Table 1. The half-life
of 1,3-D hydrolysis was calculated according to pseudo-first-
order kinetics, whereas the DT50 for 1,3-D in hydrogen sulfide
solution was obtained from eq 4. Table 1 shows that the DT50
of 1,3-D decreased with increasing initial hydrogen sulfide
concentration at a constant pH. The reaction rate constants of
both 1,3-D isomers with hydrogen sulfide species increased with
increasing pH values (Table 1). As observed for chloropicrin,
these results indicate that HS- is more reactive toward 1,3-D
than H2S in the pH range investigated (5.9-8.9). Note that the
reported rate of hydrolysis of both 1,3-D isomers in neutral
buffer solution is much faster than that in previous reports (36,
37) because the phosphate buffer (in solutions of pH 6-8) can
accelerate the hydrolysis of 1,3-D. A similar phosphate-catalyzed
hydrolysis of 1,2-dichloroethane and 1,2-dibromoethane has
been reported (38). Because no phosphate anion was present in
the pH 8.9 buffer solution, the hydrolysis rates of 1,3-D isomers
were lower than those at pH 6.9 or 7.9 (Table 1).
Figure 5. Dissipation of 1,3-D (1.0 mM) in different (H2S)T concentration
solutions at 25
± 0.5 °C and pH 7.9 ± 0.1. Error bars represent standard
deviations of triplicate samples. Lines indicate fit to a second-order kinetic
model.
Transformation Mechanism of 1,3-D and HS-. To eluci-
date the pathway of 1,3-D reaction with hydrogen sulfide,
aliquots of the reaction mixture at pH 8.9 ( 0.1 were
periodically extracted and analyzed by GC/MS. Six reaction
products were characterized based on their mass spectra and
retention times. The reaction of 1,3-D and HS- is proposed as
a bimolecular nucleophilic substitution (SN2) mechanism. HS-
as a “soft” nucleophile attacks 1,3-D to form a RSH mercaptan,
liberating a chlorine at C3 in the initial process (Scheme 2).
Because 1,3-D is a mixture of cis and trans isomers, two
corresponding mercaptans (RSH) were detected with the same
mass spectrum (Figure 6a) but at different retention times on
the total ion chromatograms. These two mercaptan isomers may
convert to related mercaptides (RS-) in aqueous solution, which
should further react with any remaining 1,3-D to yield thioethers
ClCHdCHCH2-S-CH2CHdCHCl (RSR). Consistent with the
stereochemistry of the reaction between the mercaptides and
the 1,3-D isomers, three thioether isomer products were observed
that had different retention times but the same mass spectrum
(Figure 6b). Additionally, small amounts of dialkyl disulfide
(RSSR) were also detected (Figure 6c), which stemmed from
the oxidation combination of partial mercaptans (RSH). How-
ever, the dialkyl disulfide (RSSR) gradually degraded with
increasing incubation time. The final reaction products observed
in the reaction mixture of 1,3-D and HS- were thioether isomers
(RSR).
suggesting that the reaction rate of 1,3-D with hydrogen sulfide
species is clearly dependent on the their initial concentrations.
The experimental results indicate that the reaction mechanism
between 1,3-D and hydrogen sulfide species is consistent with
an SN2 nucleophilic substitution reaction, which follows second-
order kinetics. In these experimental systems, the disappearance
of 1,3-D is mainly attributed to the reaction with hydrogen
sulfide species and hydrolysis. Therefore, the overall 1,3-D
dissipation rate in hydrogen sulfide solution would be the sum
of two concurrent reactions, which may be expressed as
-d[1,3-D]
) (ka[H+] + kN + kb[OH-])C + k2CX )
dt
khC + k2CX (1)
where ka, kN, and kb represent the rate constants for acid-
catalyzed, neutral, and base-catalyzed hydrolysis (35), kh is the
overall hydrolysis rate constant, k2 is the second-order rate
constant, and C and X are the concentrations of 1,3-D and
hydrogen sulfide species, respectively. Because the reaction
between 1,3-D and hydrogen sulfide species is much faster than
1,3-D hydrolysis (Figure 4), the equation may be simplified to
a second-order reaction.
-d[1,3-D]
) k2′CX ) k2′C(C - C0 + X0)
(2)
dt

Dechlorination of Chloropicrin and 1,3-Dichloropropene
J. Agric. Food Chem., Vol. 54, No. 6, 2006 2285
Table 1. Pseudo-First-Order Hydrolysis Rate Constants and Half-Lives (t1 2) of 1,3-D and Second-Order Reaction Rate Constants and 50%
/
Disappearance Times (DT50) of 1,3-D (1.0 mM) with Hydrogen Sulfide Species at 25
± 0.5 °C
cis-1,3-D
trans-1,3-D
initial [H2S]T
(mM) a
kh
(h
t1
(h)
k2
′
DT50
(h)
kh
(h
t1
/2
(h)
k2
′
DT50
(h)
/
2
-
1
-
-1
-
1
-
-1
ppH
)
(mM 1 h
)
)
(mM 1 h
)
-
-
-
-
3
3
3
3
-3
-3
-3
-3
5.9
±
±
±
±
0.1b
0
5.26
7.36
7.68
7.10
×
×
×
×
10
131.7
5.43
7.54
7.61
7.26
×
10
127.7
-
-
-
-
-
2
2
2
2
2
-
-
-
-
-
2
2
2
2
2
1.01 (1:1)
2.03 (1:2)
3.04 (1:3)
4.05 (1:4)
average
0
0.97 (1:1)
1.94 (1:2)
2.91 (1:3)
3.88 (1:4)
average
0
0.51 (2:1)
1.02 (1:1)
2.03 (1:2)
3.05 (1:3)
average
0
0.51 (2:1)
1.02 (1:1)
1.53 (2:3)
2.04 (1:2)
average
3.22
3.06
3.68
4.51
×
×
×
×
×
10
10
10
10
10
24.96
12.06
6.50
1.16
0.88
0.97
1.06
×
×
×
×
×
10
10
10
10
10
68.55
41.49
24.70
16.74
3.93
3.62 (
1.30 (
3.50 (
4.49 (
±
±
±
±
0.65)
1.02 (
2.92 (
6.83 (
7.96 (
±
±
±
±
0.12)
6.9
7.9
8.9
0.1b
0.1b
0.1
10
10
10
94.22
90.24
97.60
×
×
×
10
10
10
91.92
91.10
95.44
-1
-1
-1
-1
-1
-2
-2
-2
-2
-2
1.39
1.23
1.17
1.41
0.11)
×
×
×
×
×
10
10
10
10
10
5.92
3.13
2.16
1.31
3.34
2.75
2.73
2.86
0.28)
×
×
×
×
×
10
10
10
10
10
25.01
13.98
9.14
6.48
-1
-1
-1
-1
-1
-2
-2
-2
-2
-2
3.73
3.06
2.92
4.30
0.63)
×
×
×
×
×
10
10
10
10
10
5.33
2.58
1.26
0.56
6.71
5.97
6.24
8.41
1.09)
×
×
×
×
×
10
10
10
10
10
28.57
13.25
5.87
2.83
-1
-1
-1
-1
-1
-2
-2
-2
-2
-2
6.01
4.05
4.04
3.85
1.01)
×
×
×
×
×
10
10
10
10
10
3.28
1.97
1.24
0.96
7.32
8.33
8.17
8.02
0.44)
×
×
×
×
×
10
10
10
10
10
26.01
9.44
6.08
4.52
a Values in parentheses are approximate initial molar ratios of 1,3-D to [H2S]T in solutions. b Phosphate buffer may have catalyzed 1,3-D hydrolysis.
Scheme 2
It was apparent that the transformation of cis-1,3-D by
formulations of fumigants through drip irrigation systems is
being proposed to replace the conventional shank method (40).
This approach may alleviate some environmental risks of
fumigant applications by reducing atmospheric emissions and
may improve pest control efficacy by extending the fumigant
residence time in soil. However, drip fumigation may impose
an increased risk for groundwater contamination, as leaching
of water may facilitate the downward transport of fumigants to
groundwater, particularly in regions with shallow aquifers and
sandy soils. Fumigant application with large volumes of water
by drip application increases the potential of fumigant occur-
rence in hypoxic environments. In anoxic aquatic and soil
systems, hydrogen sulfide species such as HS- are ubiquitous
and are commonly present at sufficiently high concentrations
(32) to undergo the abiotic reactions indicated by our results.
hydrogen sulfide species was significantly faster than that of
trans-1,3-D (Figure 5 and Table 1). In 1,3-D, the CdC bond
and four connected single bonds are all in the same plane,
suggesting that the spatial hindrance for approach of hydrogen
sulfide species such as HS- should be similar for both isomers.
When HS- attacks 1,3-D isomers, a crowded transition state
will be formed. In the transition state, the potential rotation
around the C-C bond of cis-1,3-D may be hindered by the
chlorine attached at the CdC bond. The greater steric hindrance
facilitates breakage of the C-Cl bond in cis-1,3-D, yielding a
corresponding RSH mercaptan. This reaction mechanism is
consistent with those proposed for nucleophilic reaction pro-
cesses between 1,3-D and metam sodium (24) or thiourea (39).
Environmental Implications. Our study of the rapid reaction
of chloropicrin and 1,3-D with hydrogen sulfide species indicates
that the abiotic transformation of halogenated fumigants by HS-
may be of considerable environmental significance in fumigated
soil. To reduce fumigant volatilization, application of soluble
As illustrated in the major transformation pathways of
chloropicrin and 1,3-D with HS- (Schemes 1 and 2), the
reaction mechanisms and products are dependent on the
individual fumigant properties. The concentration of chloropicrin

2286 J. Agric. Food Chem., Vol. 54, No. 6, 2006
Zheng et al.
containing chlorine atoms, may be hazardous to public health
because of their potential toxicity and also be more persistent
than the parent compounds (9, 21, 41). In addition, these low
molecular weight organic sulfur compounds have relatively low
smell threshold values and have great potential to enter the air
through gradual volatilization from soil and aquatic systems.
This may limit the potential for many other inorganic sulfur
nucleophiles (such as thiosulfate, polysulfide, and sulfite) to be
used as chemical remediation reagents to control fumigant
emissions. For example, the fertilizer ammonium thiosulfate may
react rapidly with the volatile halogenated fumigants 1,3-D (36)
and methyl bromide (14). However, these reaction products may
further degrade to volatile organic sulfur contaminants in soils.
The results of these studies indicate that the environmental
benefits of integrated fumigation practices in which inorganic
sulfur chemicals serve as amendment reagents may be limited
by the volatilization and toxicity of the reaction products
liberated.
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Received for review November 1, 2005. Revised manuscript received
January 14, 2006. Accepted January 16, 2006.
JF0527100