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a Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521
b USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Road, Riverside, CA 92507
* Corresponding author (mingxin.guo{at}ucr.edu).
Received for publication January 31, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: 1,3-D, 1,3-dichloropropene • DOM, dissolved organic matter • IC, ion chromatography • OC, organic carbon • UV, ultraviolet
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Elimination of 1,3-D in the environment is mainly through biotic and abiotic decomposition processes (Batzer et al., 1996), and hydrolysis is the key mechanism for 1,3-D degradation in water and soil (Castro and Belser, 1966; Roberts and Stoydin, 1976; McCall, 1987). The hydrolysis product is 3-chloroallyl alcohol, which is further transformed to carboxylic acid intermediates (i.e., 3-chloroacrylic acid) and eventually to CO2. The hydrolysis process can be simply described as:
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Although hydrolysis is an important mechanism of 1,3-D degradation, the process is not well understood. Effects of environmental factors such as pH, photo irradiation, presence of suspended particles, soil moisture, particle size, mineralogy, and microorganisms on the hydrolysis reaction need to be investigated. In sterile aqueous solutions, the hydrolysis of 1,3-D is significant, and the reported half-life is approximately 11 d at 20°C (McCall, 1987). Predicted from its structure, 1,3-D hydrolysis should be a mixture of unimolecular (SN1) and bimolecular (SN2) nucleophilic substitution reactions, in which water molecule or hydroxide ion serves as the nucleophile (Schwarzenbach et al., 1993). Accordingly, the concentration of OH– is expected to be important in the reaction, especially at high pH values (i.e., pH 
10). van Dijk (1974) reported a higher hydrolysis rate of 1,3-D in pH 7.5 buffer solutions than in pH 5.5 buffer solution at both 15 and 29°C. Nevertheless, McCall (1987) observed an independence of the 1,3-D hydrolysis rate on the solution pH over a range of 5 to 9. To better understand the hydrolysis process, the effect of pH should be clarified.
Suspended particles and dissolved organic matter (DOM) are common components in natural water systems and may affect 1,3-D hydrolysis. Sunlight and microorganisms may also promote the reaction. So far, few studies have been conducted to investigate these aspects. Castro and Belser (1966) examined 1,3-D hydrolysis in phosphate buffer solutions and soil slurries (soil/water = 3:1 to 1:2), and found that hydrolysis rates (evaluated by the Cl– release) were significantly higher in the soil slurries than in water. It is unclear whether the rate enhancement in the presence of soil was due to soil microorganisms or to reaction with soil components.
In soil, 1,3-D is mainly degraded through biotic and abiotic hydrolysis (Castro and Belser, 1966; Roberts and Stoydin, 1976; Verhagen et al., 1995). The reported half-life of 1,3-D in soil ranges from 1.8 to 61 d at 25°C (van Dijk, 1980; Batzer et al., 1996). It is uncertain what factors control the hydrolysis rate. Considering that 1,3-D may react directly with OM (Gan et al., 1998) and be entrapped in soil matrix as persistent residues (Guo et al., 2003), its hydrolysis rate in soil is generally overestimated if evaluated by the disappearance. Since the hydrolysis process of organic halides is generally irreversible (Schwarzenbach et al., 1993), in sterile soils with low OM contents, the hydrolysis rate of 1,3-D may be determined by the Cl– release.
To control its residue in soil and persistence in water, hydrolysis of 1,3-D warrants systematic studies. The objectives of this study were to evaluate the effect of environmental factors such as pH, suspended particles, co-solutes, photo irradiation, soil particle size, moisture, organic matter, mineralogy, and microorganisms on 1,3-D hydrolysis and to determine half-lives of 1,3-D in water and soil.
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MATERIALS AND METHODS |
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Arlington sandy loam soil (coarse-loamy, mixed, thermic Haplic Durixeralfs) was collected from the Ap horizon (0–18 cm) of the University of California Agricultural Experiment Station in Riverside, CA. The soil was typical of that used in California fumigated agriculture, and the section from which the soil was collected was never treated with 1,3-D or other fumigants. The soil has an organic carbon (OC) content of 9.2 g kg–1, clay content of 74 g kg–1, and pH 7.20. It was air-dried, sieved to <2 mm, and stored at 20°C before use. A Florida muck soil (pH 7.16, OC 460 g kg–1) sampled from the Everglades Research and Education Center (Belle Glade, FL) was also used as an organic amendment.
Hydrolysis in Water
To test 1,3-D hydrolysis in water, 8.00 mL of deionized water was pipetted into 8.5-mL headspace vials (clear borosilicate glass). The vials were capped with aluminum seals and Teflon-faced butyl rubber septa, autoclaved at 121°C for 60 min, and spiked with 40 µL of ethyl acetate solution containing 105 mM cis-, trans-, or isomeric mixed 1,3-D, using a 100-µL gas-tight syringe. The final concentration of 1,3-D in solution was 58.6 mg L–1. For better sealing effects, the vial heads were further dipped into melted paraffin wax to cover with a thin layer of the material. The vials were wrapped with Al foil to exclude light, and incubated at 20 ± 1°C with constant shaking. At predetermined times, triplicate vials were removed, 0.5 mL of the solution was taken from each vial with a 0.5-mL gas-tight syringe, and extracted with 5 mL ethyl acetate and 3.0 g anhydrous Na2SO4. The extracts were analyzed by GC for cis- and trans-1,3-D contents.
To investigate the effect of photo irradiation on 1,3-D hydrolysis in water, vials spiked with cis-, trans-, or isomeric mixed 1,3-D, with and without Al foil wraps, were placed under fluorescent light of 3 W m–2 in the laboratory at 23°C and under direct sunlight (550–720 W m–2 at 1200 h) outdoors without temperature control (9–32°C). At scheduled times, triplicate vials were taken out, and solutions were extracted and analyzed for remaining 1,3-D.
To determine the effect of pH, 0.05 M H2SO4, pH 4.00 buffer (0.1 M formic acid–sodium formate), pH 7.00 buffer (0.1 M NaH2PO4–Na2HPO4), pH 10.00 buffer (0.1 M NaHCO3–Na2CO3), and 0.1 M NaOH solutions were used instead of deionized water to carry out the experiment following the procedures described above.
To examine effects of clay and organic particle suspensions, 40 mg of Na+-montmorillonite (treated to <2 µM, Clay Minerals Repository, Univ. of Missouri, Columbia, MO) or oven-dried Florida muck (sieved to <75 µM) was added into 8.00 mL deionized water, autoclaved, and incubated with 1,3-D as described above.
To investigate inorganic salt and DOM effects, Arlington soil–water extracts (pH 7.20, EC 4.13 dS m–1, Cl– 1.24 mM, SO42– 1.13 mM, H2PO4––HPO42– 24.11 mM, DOC 47.8 mg L–1) and Florida muck–water extracts (pH 7.16, EC 1.09 dS m–1, Cl– 0.52 mM, NO3– 0.02 mM, SO42– 1.82 mM, H2PO4––HPO42– 4.63 mM, DOC 848.2 mg L–1) were used in place of deionized water in the hydrolysis experiment. Nonsterilized water extracts of Arlington soil and Florida muck were also employed to investigate the microbial effect.
Hydrolysis in Soil
Hydrolysis of 1,3-D in soil was examined on the basis of Cl– release from fumigated sterile Arlington sandy loam. Briefly, air-dried soil was adjusted to 10% gravitational moisture content, and aliquots of 11-g moist soil were weighed into 25-mL serum bottles, capped with aluminum seals and Teflon-faced butyl rubber septa, and autoclaved at 121°C for 60 min. Cis-, trans-, or isomeric mixed 1,3-D (82 µL) was injected into the sterilized soil with a 100-µL gas tight syringe. The 1,3-D application rate was 10 g kg–1 soil. The bottles were further sealed with a thin layer of paraffin wax on heads, and set in the dark at 20°C. At scheduled times, triplicates were taken out and soils were spread on Al foil in a fume hood and evaporated for 24 h to dissipate remaining 1,3-D. The soils were then put back into their original bottles, and 10.00 mL of 0.01 M NaNO3 water solution was added to extract Cl– for 1 h under shaking. Following extraction, the slurries were centrifuged at 10900 x g for 15 min, and supernatants were analyzed for Cl– concentrations by ion chromatography (IC). Soils without 1,3-D spiking were treated as controls and were handled using exactly the same procedures. Amounts of Cl– resulting from the chemical application were used to index 1,3-D hydrolysis.
To investigate the effect of soil moisture on 1,3-D hydrolysis, air-dried Arlington soil was adjusted to gravimetric moisture contents of 5, 10, and 15% with deionized water, and incubated with 1,3-D as described above.
Air-dried Arlington soil was further ground to completely pass through a 0.25- or 0.075-mm sieve, and the three particle-sized soils (<2, <0.25, and <0.075 mm) were adjusted to 10% moisture content and treated as described above to examine the effect of soil particle size on 1,3-D hydrolysis. The chemical composition of the three particle-sized soils was the same.
To investigate the effect of soil OM, Arlington soil was amended to OC 30.7 g kg–1 with oven-dried Florida muck, adjusted to 10% moisture content, sterilized, and incubated with 1,3-D. The hydrolysis experiment was also conducted with nonsterilized Arlington soils to examine the microbial effect. The effect of soil mineralogy was tested by conducting the hydrolysis experiment with Na+-montmorillonite (Univ. of Missouri Source Clay Minerals Repository, Columbia, MO), Na+–kaolinite (Ward's Natural Science Establishment, Macon, GA), hematite (Fisher Scientific, Fair Lawn, NJ), and quartz sand (<0.075 mm) at 20% water content.
Chemical Analysis
Cis- and trans-1,3-D in ethyl acetate extracts were analyzed with a HP5890 GC system (Hewlett-Packard, Avondale, PA) with an electron capture detector and an DB-VRX fused silica capillary column (30 m long by 0.25 mm i.d. by 1.4 µm film thickness). The carrier gas (He) flow rate, inlet temperature, oven temperature, and detector temperature were set as 1.2 mL min–1, 230°C, 120°C, and 280°C, respectively. The analysis time for each sample was 15 min, and the injection volume was 2.0 µL without split. The method detection level for cis- or trans-1,3-D isomer in water was 40 µg L–1.
Chloride ion in soil water extracts were analyzed by an IC DX-100 system (Dionex Corp., Sunnyvale, CA) consisting of an IonPac AS14 ion exchange column, two AG14 guard columns, a conductivity detector, and an AS40 automated sampler. A mobile phase comprising 7.5 mM Na2CO3 and 2.5 mM NaHCO3 was employed, the flow rate was 1.2 mL min–1 and the run time was 13 min per sample. The method detection level for Cl– in soil was 3 µmol kg–1.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The Effect of Suspended Particles
Hydrolysis of 1,3-D in the montmorillonite suspension followed a similar rate as in deionized water (P > 0.53, Table 1), demonstrating no significant effect of added clay particles. Evidently, clay mineral surface-catalyzed 1,3-D hydrolysis, if any, was insignificant. The hydrolysis experiment was further conducted with 1:1 Arlington soil water slurries, and a similar hydrolysis rate as in water was observed.
In water with Florida muck addition at 5 g L–1, concentrations of residual 1,3-D were slightly yet consistently lower than in deionized water in a range of 0.4 to 2.5 mg L–1 through the experiment period. However, the degradation rate constant was not significantly different from that in deionized water (Table 1), suggesting little catalytic effect of organic particles on 1,3-D hydrolysis. The low concentrations may be a result of rapid reactions of 1,3-D with the added organic matter or sorption on the solids.
The Effect of Microorganisms
Hydrolysis of 1,3-D occurred at similar rates in sterilized and nonsterilized Arlington soil water extracts or Florida muck water extracts (P > 0.50, Table 1), demonstrating that microorganisms in the extracts did not promote 1,3-D degradation. It was suspected that microbial activities might be inhibited due to the high initial concentration of applied 1,3-D (59 mg L–1) or lack of O2 in the vials, so the hydrolysis experiment was reconducted with 10 mg L–1 1,3-D in 5-mL extracts (headspace 3.5 mL), and similar results were obtained. Microbial species capable of rapidly decomposing 1,3-D have been reported in soils regularly treated with the fumigant (Verhagen et al., 1995). Such microbes may not abundantly exist in the soil and muck used in these experiments, which were never treated with 1,3-D. Deionized water may not be able to extract microbes that can use 1,3-D as substrate from the Arlington soil or Florida muck, or the extracted microorganisms may not have adapted to the presence of 1,3-D in the time required for these experiments. In waste water contaminated by 1,3-D, bacteria that use the chemical as carbon and energy sources may abound after a certain time period of adaptation. Katsivela et al. (1995) isolated five species of 1,3-D-degrading bacteria on biofilms after long adaptation phases of the community in mineral salt water containing 1 mM 1,3-D. However, these bacteria were fairly susceptible to environmental changes, and could not be enriched using standard batch-enrichment techniques. In our experiments, hydrolysis of 1,3-D was mainly an abiotic process.
Hydrolysis of 1,3-D in Soil
Hydrolysis of 1,3-D in soil was determined from Cl– release. When 1,3-D was spiked at 10 g kg–1 (90 mmol kg–1) into sterile Arlington soil (10% moisture content, 20°C), 8.5 mmol kg–1 of Cl– was generated in 30 d, which accounted for 9.5% of the potential Cl– production assuming a complete initial hydrolysis (removal of one Cl atom from each 1,3-D molecule). It is postulated that at such a high spiking rate, most of the 1,3-D remained in the headspace, and only the portion that diffused into the water layer on soil particle surfaces underwent hydrolysis. The static sealed vials used in these experiments provided poor gas-exchange conditions, and the diffusion of spiked 1,3-D into soil water was highly dependent on the initial concentration. Consequently, the absolute amount of 1,3-D that experienced hydrolysis (based on Cl– production) increased with the initial concentration, yet the relative percentage decreased. When we reduced the application rate to 0.61 g kg–1, approximately 62% of the applied 1,3-D hydrolyzed in 30 d, and the resulted Cl– production in soil was 3.4 mmol kg–1, equivalent to 40% of that at an application rate of 10 g kg–1. Although the cumulative Cl– release followed first-order kinetics (Fig. 3), it is inappropriate herein to estimate the hydrolysis rate constant or half-life of 1,3-D in soil because of the initial-concentration dependence. Ma et al. (2001) also found that the 1,3-D disappearance rate was highly dependent on the initial concentration, with first-order rate constants varying 1.5- to 4-fold for the concentration range of 0.6 to 60 mg kg–1. It is expected that a large proportion of spiked 1,3-D will hydrolyze in soil (10% moisture) in 30 d at a typical field application rate of 0.16 g kg–1.
The Effect of Moisture
Moisture plays an important role in 1,3-D hydrolysis. Tests with Arlington sandy loam at 5, 10, and 15% moisture show that the Cl– release from 1,3-D hydrolysis was remarkably greater under higher soil moisture conditions (P < 0.01, Fig. 3). The effect of soil moisture content may be ascribed to two aspects. On the one hand, dissolution of 1,3-D in water was limited at low moisture contents, and the hydrolysis reaction was hindered. On the other hand, sorption of 1,3-D onto soil matrix was inhibited at high moisture contents, which in turn, promoted the hydrolysis reaction. In soils from four different locations, van Dijk (1980) observed significantly higher dissipation rates of 1,3-D at the moisture content of field holding capacity than at plant withering points. Gan et al. (1999) reported that degradation of 1,3-D in a Carsitas loamy sand (mixed, hyperthermic Typic Torrpsamments) increased linearly with soil moisture content over a range of 2 to 16%. The more rapid hydrolysis of 1,3-D at higher moisture contents suggests that water sealing after 1,3-D application is an effective method via physical blocking and chemical degradation to decrease fumigant atmospheric emission from the field.
The Effect of Soil Particle Size
Arlington soils ground to <2, <0.25, and <0.075 mm were incubated individually with 1,3-D. At 10% moisture content, Cl– releases from the three soils were similar (P > 0.1), suggesting that particle size had little effect on 1,3-D hydrolysis. It is inferred that clay minerals do not contain catalytic sites for 1,3-D hydrolysis reactions, and soil particle size is unimportant in the process. In closed systems, van Dijk (1980) observed that 1,3-D disappeared much more rapidly in clay soils than in sandy soils, but this effect may have been due to the higher pH of the former (pH 7.3) than the latter (pH 4.6).
The Effect of Soil Mineralogy
When 1,3-D was incubated at 10 g kg–1 with montorillonite, kaolinite, hematite clay, and fine quartz sand (20% moisture), the rate of Cl– release was similar (P > 0.1, Fig. 4)
, suggesting that soil mineralogical effects on 1,3-D hydrolysis may be insignificant. The hydrolysis reaction may occur on the surface of soil particles, but these clay minerals apparently do not function as catalysts, and the specific surface area is not important. This contention is strengthened by the fact that in water–clay suspensions and 1:1 soil slurries, 1,3-D dissipated at similar rates as in deionized water. Combined with the insignificant effect of soil particle size, it may be deduced that soil texture does not affect 1,3-D hydrolysis.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In soil, hydrolysis of 1,3-D is comparatively slow, and is concentration dependent. At an application rate of <0.60 g kg–1, >60% of the applied 1,3-D was hydrolyzed within 30 d in 10% moisturized Arlington soil at 20°C, judged by the Cl– release. Soil particle size and mineralogy had little effect on 1,3-D hydrolysis, while moisture content influenced the process significantly. 1,3-D hydrolysed more rapidly as soil moisture increased in a range of 5 to 15%. Organic matter promoted 1,3-D degradation via direct substitution reactions, but did not affect the hydrolysis reaction. Trans- and cis-1,3-D hydrolyzed in soil at equivalent rates, and the former showed preference over the latter to react directly with particular organic molecules in soil. Microbial accelerated hydrolysis was initially insignificant, and became important as soil microorganisms adapted to the fumigant.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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