Pesticide Decontamination and Detoxification - ACS Publications

Chapter 5

Downloaded by STANFORD UNIV GREEN LIBR on July 17, 2012 | http://pubs.acs.org Publication Date: October 10, 2003 | doi: 10.1021/bk-2004-0863.ch005

Detoxification of Some Halogenated Pesticides by Thiosulfate Salts J. Gan and S. Bondarenko Environmental Sciences Department, University of California, Riverside, C A 92521

Halogenated organic compounds are used for industrial and agricultural purposes as solvents, degreasing agents, and pesticides. One benign chemical method for remediation and contamination prevention of halogenated compounds in the environment is nucleophilic substitution by thiosulfate salts, in which the compound is dehalogenated and detoxified. This reaction has been shown to occur for methyl bromide (CH Br), 1,3-dichloropropene (C H Cl ), chloropicrin (CCl NO ), methyl iodide (CH I), and propargyl bromide (C H Br), all existing or potential fumigants, and for chloroacetanilide herbicides alachlor, propachlor, acetochlor, and metolachlor. Structural and kinetic analysis suggests that the reaction occurs by S 2-type nucleophilic substitution, in which thiosulfate (S O ) replaces the halogen in the molecule. This method has several potential advantages, i.e., rapid dehalogenation rate at ambient temperature, low toxicity of thiosulfate salts, removal of toxicity of parent compounds, and easy availability of common thiosulfate salts. This article is a review of the current state of knowledge about this reaction and its potential applications. 3

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© 2004 American Chemical Society In Pesticide Decontamination and Detoxification; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Halogenated organic compounds (HOC) have a multitude of uses in modem society. They are widely used in industrial applications as solvent degreasers and refrigerants, in the dry-cleaning industry, in the manufacture of plastics and dyestuffs, and in agricultural applications as active and inactive components of pesticides and fumigants (1-5). As a result of the large-scale industrial production and use of HOCs, these compounds have a potential for widespread contamination of natural resources. The Toxic Release Inventory reports releases of more than 1.92 χ 10 kg of chlorinated organic compounds in the U.S. for the years 1988 and 1994 to 1996 (4, 5). HOCs are chemicals with a wide range of physical and chemical properties. As a result, the various HOCs differ in their toxicity, persistence, or bioaccumulation potential (6). Many of these compounds are toxic or carcinogenic (3, 7, 8). Many HOCs resist either chemical or biological attack, which contributes to die environmental persistence of these materials. A lot of effort is being made to prevent pollution and to restore environmental systems that are already polluted from previous uses (9). One of the most attractive methods for remediation of HOCs in ground water and polluted soil is reductive dechlorination by metals such as iron, or zero valent bimetals such as mixtures of zinc-palladium, zinc-nickel, zinc-copper, ironpalladium, iron-nickel, and iron-copper (10). Haitko and Eykholt (//) reported dechlorination of chlorinated organic compounds by soluble iron citrate. Polychlorinated biphenyls, chlorinated aliphatic hydrocarbons, herbicides and pesticides were successfully dehalogenated by Na/NH or Ca/NH treatment in polluted clay, sandy or organic soils (12). The method has several advantages, such as rapid dehalogenation rates at ambient temperature, N H removal from soil, and easy recovery and recycling. Selective mechanochemical dehalogenation of chlorobenzenes was achieved using calcium hydride (13). Dechlorination approaches for contaminated soils also include the use of different soil microorganisms (14, 15), permanganate oxidation (16), new enhanced anaerobic methods (17), UV irradiation in the presence of liquid containing oxidation agents (18, 19), treatment with polyethylene glycol monomethyl ether potassium salt (20, 21), and reaction with sulfite (22). Many of these treatments, however, present some problems for decontaminating polluted natural media, and the disadvantages include limited breadth in reactivity, expensive reagents, toxic reaction products, and nonselective reactions. There is thus a need to develop new and selective approaches to decontaminate HOCs in the environment.


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A HOC containing sp -hybridized carbon-halogenated bonds can undergo reaction by several routes: nucleophilic substitution, dehydrohalogenation (loss of HX; X = halogen), and reductive dehalogenation. Reductive dehalogenation

In Pesticide Decontamination and Detoxification; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

53 can react via either hydrogenolysis (replacement of halogen by hydrogen) or reductive elimination (loss of two vicinal or geminal halogens). It is known that mono- and dihalogenated organic compounds, which contain sp -hybridized carbon-halogenated bonds, are capable of bimolecular nucleophilic substitution (S 2) reactions (23, 24). For more highly substituted HOCs, dehydrohalogenation and reductive dehydrohalogenation predominate over Sn2 reactions. Taking into account these facts, our group has been exploring the use of thiosulfate salts to dehalogenate HOCs since 1997. So far, we have focused our research effort on halogenated pesticides. The reaction is S 2-type nucleophilic substitution, in which thiosulfate (S 0 ") replaces halogen in the HOC molecule (9, 25-29). This reaction has been shown to occur with methyl bromide (CH Br), 1,3-dichloropropene (C H C1 ), chloropicrin (CC1 N0 ), methyl iodide (CH I), and propargyl bromide (C H Br), all existing or alternative soil fumigants, and with chloroacetanilide herbicides including alachlor (2-chloro-2',6-diethyl-A/methoxy-methyl-acetanilide), propachlor (2-chloro-N-isopropylacetanilide), acetochlor (2-chloro-//-ethoxymethyl-6'-ethylacate-o-toluidide), and metolachlor [(2-chloro-6'-ethyl-AA.(2-methohy-l-methylethyl) acet-o-toluidide)]. These chemicals have extremely heavy use worldwide. For instance, the combined use of these chemicals in the United States alone exceeds 1.0 χ 10 kg each year. The general form of the reaction is as given below, 3

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RX + S 0 - — 2

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R S O f +X~, 2

where X can be CI, Br, or I. The use of thiosulfate salts has several advantages. First, thiosulfate is one of the most powerful nucliophiles (30). Secondly, thiosulfate salts are low in toxicity and relatively safe to use. For example, at 2.5 g/kg, the L D for rats of sodium thiosulfate is similar to that of sodium chloride (3.75g/kg). Lastly, thiosulfates are available at low cost, because, common thiosulfate salts, such as ammonium thiosulfate ((NH ) S 0 ), potassium thiosulfate (K S 0 ), and calcium thiosulfate (CaS 0 ), are commercial fertilizers or soil amendments. The other thiosulfate salt, sodium thiosulfate (Na S 0 ), is also easily available, as it is heavily used in the photographic industry. This review is a summary of our research findings over the last few years in characterization and application of this reaction. 5 0

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Transformation Kinetics in Solution Transformation of HOCs by thiosulfate was first studied in the aqueous phase. It was shown that as the molar ratio of HOCs to thiosulfate salt in water increased, the dissipation of the HOCs proportionally accelerated (25, 26). For example, in solutions containing ammonium thiosulfate at 2.0 mAf, the first-


54 order half-life {T ) of methyl iodide decreased from 7250 h (in water) to 8.5 h and that propargyl bromide decreased from 3100 h (in water) to 9.8 h. When the ammonium thiosulfate concentration was further increased to 8 mAf, the Ty of methyl iodide and propargyl bromide was only 1.4 and 1.6 h, respectively (Figure 1). The effect of different thiosulfate concentrations on the disappearance of a chloroacetanilide herbicide is illustrated with alachlor in Figure 2. The second-order reaction constant k was calculated for fumigants by fitting the measured data to the second-order kinetic model (Table 1). Good correlation was found for allfimigantsexcept for chloropicrin, with correlation coefficients r ^0.99 (Table 1). This suggests that the reaction between HOCs and thiosulfate salt in aqueous solution follows second-order kinetics typically exhibited by S 2 reactions. 1/2


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Figure 1. Dissipation offumigants (ImM) in thiosulfate solutions: (a) methyl iodide and (b) propargyl bromide (28)



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Time (hours)

Figure 2. Dissipation of alachlor (0.2 mM) in aqueous solutions with different initial concentrations of ammonium thiosulfate (29) 1

Table 1. Reaction rate constant k (M'Y ) and regression coefficient r of fumigants and ammonium thiosulfate in aqueous phase at 20°C (28) Fumigant Methyl bromide Methyl iodide Propargyl bromide Chloropicrin 1,3-dichloropropene

k

R 1.00 1.00 0.99 0.70 0.99

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2.11xl(r 1.87X10' 1.59xl0 3.5X101.95x1ο

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Reaction Pathways Analysis of reaction mixtures by ion chromatography (IC) showed that as each HOC reacted with thiosulfate, X~ (X~= CP, Br~, or I ) was liberated into the solution. The rate of X release always equaled the rate of HOC consumption, as shown in Figure 3 for propargyl bromide and Figure 4 for alachlor. This analysis provided evidence that X"" was displaced from the HOC molecule during the reaction, indicating that the reaction was a stoichiometric substitution. However, in the transformation of chloropicrin, consumption of S 0 ~~ was four times greater than that of chloropicrin, while accumulation of CT was about two times faster than the dissipation of chloropicrin (28). It is likely that more than one of three CI atoms were replaced by S 0 ~ in the transformation or that other reactions occurred that consumed additional S 0 ~. -

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Figure 3. Consumption ofpropargyl bromide (ImM) and thiosulfate (2mM) and accumulation of bromide in aqueous phase during propargyl bromide transformation by thiosulfate salt (28)

Reaction Time (hr)

Figure 4. Consumption of alachlor (ImM) and thiosulfate (2mM) and accumulation of chloride in aqueous phase during alachlor transformation by thiosulfate salt (29)


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-CCH -S 0 2

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Figure 5. The initial step of nucleophilic dehalogenation reaction between alachlor and thiosulfate salts (A) andfumigants and thiosulfate salts (B)

Since the reaction between HOCs and thiosulfate followed second-order kinetics, and that X~ and dehalogenated HOC-thiosulfate derivative was formed as one of the initial products, it may be concluded that the reaction followed a pathway as depicted for alachlor in Figure 5A or for fumigants in general in Figure 5B.

Relative Reactivity The relative reactivity was compared under the same conditions for fumigants, where the initial concentration of fumigants was 1 mM and the initial concentration of ammonium thiosulfate was variedfrom0 to 10 mM. The rate of fumigant dissipation followed an order of methyl bromide « methyl iodide > propargyl bromide > 1.3-D « chloropicrin. The relative reactivity among the fumigants is shown also in the difference in the second-order rate constants (Table 1). The relative reactivity of these compounds with thiosulfate may be explained by the steric hindrance of substitution groups on the primary carbon and the tendency for the leaving group to leave dining nucleophilic substitution. As the nuchleophile would attack a HOC molecule from the direction opposite to the leaving group, a bulky substitution on the primary carbon would prevent easy approaching of the incoming nucleophile, rendering the reaction slower. Thus, methyl bromide and methyl iodide are more susceptible to nucleophilic attack. In addition, Br and I are better leaving groups than CI. The reactivity of the herbicides decreased in an order of propachlor > alachlor > acetochlor > metolachlor (Figure 6). This dependence suggests again that the substitutions at the nucleophilic center, i.e., the chlorinated carbon, influenced the reactivity. The bulky substitutions at this position may have resulted in greater steric hindrance for metolachlor, rendering its primary carbon less accessible by s o -. 2

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0.20 î

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Figure 6. Disappearance of chloroacetanilide herbicides in 10 mM thiosulfate solutions at 20° C (29)

Enhanced Transformation in Soil The reaction was further tested in soils when a known rate of ATS was added to soil with selected fumigants. Greatly enhanced transformation of fumigants was consistently observed. For instance, first-order T for methyl bromide transformation was 1300 h in a unamended sandy loam soil, but was reduced to 21 h when ATS was added to the soil at 1.0 mmol kg" (Table 2). Under the same conditions, the persistence of the other fumigants decreased by factors of 5-65. The reduction was greater for methyl bromide, methyl iodide, and propargyl bromide as compared to 1,3-D or chloropicrin (Table 2). Comparing fumigant dissipation in sterile and nonsterile soils showed that the enhanced degradation in thiosulfate-amended soils was a chemically derived transformation (25, 26). Comparing thiosulfate-induced fumigant transformation in different types of soil showed that the enhancement was similar in different soils. Thiosulfate-induced fumigant transformations were consistently accelerated as the temperature increased. This temperature dependence implies that the reaction would be more effective when the soil is warm, a condition that may be created under plastic covers that are commonly used in soil fumigation (25, 26). Thiosulfate-induced fumigant transformation in soil was relatively insensitive to moisture variations within the normal soil moisture range (27). These findings imply that thiosulfate salts may be introduced into soil to accelerate fumigant transformation, and the application should be effective under common field conditions. m

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59 Table 2. Half-lives (h) of fumigants (0.5 mmol kg "*) in Arlington Sandy Loam with and without addition of ammonium thiosulfate (ATS) at 1.0 mmol kg (28) 1

Fumigant

T,

R

Tl/2

r

20.5 23.5 12.9 30.1 29.7

0.99 0.98 0.97 0.97 0.98

1300 495 290 139 162

0.53 0.91 0.93 0.95 0.99


a

Methyl bromide Methyl iodide Propargyl bromide Chloropicrin 1,3-dichloropropane

Blank control

ATS-amended

The reaction was also tested in soils and sand for chloroacetanilide herbicides. Enhanced herbicide dissipation occurred in sand or sandy soils, but became insignificant in clayey soils (29). The reduced enhancement in clayey soils may be attributed to the strong adsorption of herbicides to soil, which may have limited the interaction between the herbicide (adsorbed) and thiosulfate anion (in solution). In sand or sandy soils, the transformation of propachlor was the most rapid, which was followed by acetochlor and alachlor. Metolachlor transformation was only marginally enhanced.

Detoxification Bacteria-based bioassays were used to evaluate changes in toxicity of the HOCs caused by the reaction. The acute toxicity test was carried out using a illumescent bacterium Vibriofisheri.The measurement is based on the principle that the emitted illuminescence from the bacteria is suppressed under toxicity stress, and E C can be obtained from the relationship of illuminescene and the concentration of test compound. Bacterial E C increased significantly after the fumigants were reacted with sodium or ammonium thiosulfate, suggesting that the acute toxicity greatly decreased after the reaction (Table 3). For instance, bacterial toxicity decreased by nearly 200 times for chloropicrin, >300 times for methyl iodide, and >700 times for 1,3-dichloropropene (Table 4). The bacterial E C increased from 300-550 mM for herbicides to 1,360-4,400 mM after transformation by thiosulfate salts. When the bacterial E C is >1,500 mM, the test compound may be considered nontoxic or only slightly toxic. Therefore, it may be concluded that transformation of herbicides by thiosulfate is also a detoxification process. The biological activity of many HOCs correlates with their alkylating ability. The activity arisesfromdirect alkylation of critical biological molecules by fumigants. It is likely that transformation by thiosulfate removes the nucleophilic center (i.e., CI, Br, or I) from the HOC, thus preventing the HOC 5 0

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5 0


60 from reacting further with biological systems. These preliminary assays suggest that thiosulfate-induced dehalogenation is a detoxification reaction, and the use of this reaction would be environmentally compatible. This, when combined with the easy availability of common thiosulfate salts, implies that the use of this reaction for environmental decontamination holds great promises.

Table 3. Changes in bacterial E C values of fumigants before and after reaction with ammonium thiosulfate in aquoues phase (28) Downloaded by STANFORD UNIV GREEN LIBR on July 17, 2012 | http://pubs.acs.org Publication Date: October 10, 2003 | doi: 10.1021/bk-2004-0863.ch005

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Fumigant Methyl bromide Methyl iodide Propargyl bromide 1,3-dichloropropene Chloropicrin Ammonium thiosulfate

EC50 (mM) Fumigant Reacted solution solution 1.22 2.95 0.38 0.13 9.4x1ο

4.38 >1000 2.58 >100 0.18

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Difference (times) 3.6 >300 6.7 >700 191

248

Table 4. Changes in bacterial E C values of herbicides before and after reaction with thiosulfate salt in aquoues phase 50

Herbicide Alachlor Propachlor Acetochlor Metholachlor Phenol

EC50 (mM) Herbicide Reacted solution solution 494 4376 358 1361 297 1839 544 1395 5177

Difference (times) 8.9 3.8 6.2 2.6

Application Examples Fumigant Emissions Reduction Methyl bromide is known for its very high volatility. Methyl bromide emission during soil fumigation has been suggested to contribute to stratospheric


61 ozone depletion. For this reason, methyl bromide is undergoing phase out in the United States and some other countries. In our earlier studies, we explored the use of surface amendment of thiosulfate fertilizers to reduce methyl bromide emission by neutralizing it before it is emitted into the atmosphere. It was observed that rapid CH Br volatilization occurred shortly after it was injected into the soil columns (25). However, in thiosulfate-amended soil, the magnitude of volatilizationfluxeswas greatly reduced as compared to the control column. For example, the maximum flux detected from the control column without thiosulfate amendment was 1,400 μg h" , while that from the thiosulfateamendment column was only about 300 μg h" . The overall emission loss of methyl bromide was 61%fromthe control treatment, but decreased to only about 8% in the ammonium thiosulfate amended columns (25). In preliminary field tests, no adverse effect on pesticide control efficacy was observed when ammonium thiosulfate was applied to the soil surface. It is likely that methyl bromide transformation was limited only to the surface soil layer, and the depletion of methyl bromide did not compromise the control of pesticides dwelling in the root zone below the soil surface. Surface amendment of thiosulfate products was also tested for reducing 1,3D emission after soil treatment using packed soil columns or field plots (27). In packed soil columns, it was found that the reduction in 1,3-D emission was proportional to the amount of ammonium thiosulfate used when the amount of water (as carrier for ammonium thiosulfate) was fixed, and to the amount of water sprayed when the amount of ammonium thiosulfate was fixed (Table 5). When ammonium thiosulfate was used at 64 g m' , the emission of 1,3-D decreasedfrom33% to 12% when the amount of water was increased from 1 mm to 9 mm (Table 5). When the water application rate was kept at 9 mm, 1,3-D emission loss decreased from 15% to only 3% when the rate of ammonium thiosulfate was increased from 64 g m" to 193 g m" . Under similar conditions, the total emission loss of 1,3-D from untreated columns was 43-47%. In field plots, the emission loss of 1,3-D was observed to decrease from 25% in unamended plots to about 5% in ammonium thiosulfate-amended plots. In addition, application of either ammonium or potassium thiosulfate to field plots did not significantly inhibit the effectiveness for nematode control. 3

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Soil Remediation In subsequent studies, we also tested the use of sodium or ammonium thiosulfate to remove HOCs from soil or sand, with the objective for soil or aquifer remediation. Small soil columns were packed with sandy soil or sand, and contaminated with herbicides at a known concentration. In one group of columns, ammonium thiosulfate was introduced into the column, and in another group of column, only water was injected into the column. The columns were then allowed to sit at ambient temperature for about a week. Water (0.01 M


62 Table 5. Total emissions (% of applied) of 1,3-D from soil columns following surface application of ammonium thiosulfate (ATS) in different amounts of water and at different rates


Treatment Water application rate: 1,3-D only 1,3-D + ATS in 1mm water* 1,3-D + ATS in 3mm water 1,3-D + ATS in 9mm water ATS application rate: 1,3-D only 1,3-D+lmLThio-Sul 1,3-D+ 2mLThio-Sul 1,3-D+ 3mLThio-Sul §

T

Pesticide Decontamination and Detoxification - ACS Publications

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