Journal of Environmental Quality 31:1630-1635 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Organic Compounds in the Environment
Sorption and Degradation Characteristics of Phosmet in Two Contrasting Australian Soils
H. C. Suter*,
R. E. White,
L. K. Heng and
L. A. Douglas
School of Resource Management, The University of Melbourne, Victoria 3010, Australia
* Corresponding author (hsuter{at}mira.net)
Received for publication October 27, 2000.
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ABSTRACT
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The organophosphate insecticide phosmet [phosphorodithioic acid, s-((1,3-dihydro-1,3-dioxo-2H-isoindol-2yl)methyl), o,o-dimethyl ester] is used to control red-legged earth mites (Halotydeus destructor), lucerne flea (Sminthurus viridis), and Oriental fruit moth (Cydia molesta) in horticulture and vegetable growing. This study was undertaken with two soils of contrasting properties to determine the extent to which sorption and degradation of the insecticide might influence its potential to leach from soil into receiving waters. Two soils were used: a highly organic, oxidic clay soil (Ferrosol) and a sandy soil low in organic matter (Podosol), sampled to 0.3 m depth. The extent of sorption and decomposition rate of a phosmet commercial formulation were measured in laboratory experiments. Sorption followed a Freundlich isotherm at all depths. The Freundlich coefficient K was significantly correlated (p = 0.005) with organic C content in the Podosol, and significantly correlated (p = 0.005) with organic C and clay content in the Ferrosol. K was highest (48.8 L kg-1) in the 0- to 0.05-m depth of the Ferrosol, but lowest (1.0 L kg-1) at this depth in the Podosol. Degradation followed first-order kinetics, with the phosmet half-life ranging from 14 h (0–0.05 m depth) to 187 h (0.2–0.3 m depth) in the Ferrosol. The half-life was much longer in the sandy Podosol, ranging from 462 to 866 h, and did not change significantly with depth. Soil organic C and to a lesser degree clay content influenced phosmet sorption and degradation, but the interaction was complex and possibly affected by co-solvents present in the commercial formulation.
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INTRODUCTION
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PHOSMET is an organophosphate insecticide used in Victoria (Australia) to control red-legged earth mites, lucerne flea, and Oriental fruit moth. Phosmet use in Australia and New Zealand has declined and now it is considered a minor pesticide. It often has a limited activity time on pests (Greaves et al., 1992; James and O'Malley, 1992), suggesting that it is rapidly degraded. Menn et al. (1965) found that phosmet degradation was pH dependent in soil and solution, and that compared with other organophosphate pesticides, phosmet was relatively unstable to water hydrolysis. Phosmet is considered to be moderately mobile (USEPA, 1988). Interactions of phosmet with clays have been studied previously (Sánchez Camazano and Sánchez Martín, 1980, 1983).
Sorption, which removes a compound from the mobile phase, is one of the major processes influencing pesticide mobility in soil (Rao and Davidson, 1979; Khan and Khan, 1986; Singh et al., 1988; Kookana and Aylmore, 1994). Sorption has often been described by the Freundlich equation:
 | [1] |
where S is the concentration of sorbed pesticide at equilibrium (mg kg-1 soil), C is the solution concentration (mg L-1), K is the sorption coefficient, and n is a coefficient (usually in the range 0 < n 
1) (Singh et al., 1989; Boesten and van der Linden 1991; Sánchez Martín and Sánchez Camazano, 1991).
Degradation of a pesticide will influence its persistence in soil, and hence the potential for the pesticide to be leached from the soil. Sorption can influence degradation by protecting the compound (Menn et al., 1965; Yaron, 1989) or catalyzing degradation (Sánchez Camazano and Sánchez Martín, 1980, 1983). Degradation also may be influenced by the microbial population, and consequently by the soil organic matter content. Pesticide degradation can often be described by first-order kinetics (Yaron, 1989; Locke et al., 1994). Commercial pesticide formulations applied in the field contain the pesticide mixed with other solvents. These other solvents may play an important role in both pesticide sorption and degradation reactions in the soil.
This paper describes research undertaken to study the potential for phosmet movement in soils, using simple parameters that describe phosmet sorption and degradation. The aim of the work presented here was to:
- Measure phosmet sorption and degradation in two contrasting Australian soils.
- Determine whether sorption was described well by the Freundlich equation.
- Investigate the relationship between soil properties and phosmet sorption and degradation.
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MATERIALS AND METHODS
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Phosmet
The chemical structure of phosmet is shown in Fig. 1
. Analytical-grade phosmet is a white crystalline compound. The commercial formulation of phosmet used was Imidan EC 150 (Crop Care Australasia, Pinkenaba, QLD, Australia), a liquid formulation containing 1.5 x 105 mg L-1 of active phosmet in a solvent carrier (6.85 x 105 mg L-1 of toluene) and a surfactant. Phosmet solubility in water is 25 mg L-1, and toluene solubility in water is 470 mg L-1. The octanol partition coefficients (Kow) at 25°C are similar for phosmet (676) and toluene (603) (Chiou et al., 1977), indicating that they will act similarly in an organic solvent–water mixture. This is an important factor to consider when looking at sorption of phosmet applied to soil as a commercial formulation.
Soils
Two soils from Victoria, Australia were studied. One was a Ferrosol (Isbell, 1996), collected from the Ellinbank Research Station in Gippsland, in a dairying region with high-quality pasture production. The other was a Podosol (Isbell, 1996) collected from Cranbourne, southeast of Melbourne, in a region used for intensive horticulture and vegetable growing. The soils were chosen for their contrasting properties, as shown in Table 1. The Ferrosol had a higher clay and organic matter content than the Podosol. Clay content increased with depth in the Ferrosol and decreased with depth in the Podosol, while the reverse trend existed for organic matter. The increase in organic C with depth in the Podosol was due to the decomposition in the subsoil of roots of dense native vegetation that covered the region prior to recent site disturbance. Surface disturbance led to a reduction in the organic C content in the topsoil. The Podosol developed hydrophobic characteristics when dried and so it was stored at its field moisture content at 4°C. The Ferrosol was air-dried and stored at room temperature.
Analytical Method for Phosmet
Phosmet samples were analyzed by high performance liquid chromatography (HPLC) with a reverse phase C18 column and an ultraviolet (UV) detector set at 254 nm. The gradient mobile phase was 400 to 850 g kg-1 CH3CN in H2O at a rate of 0.0016 L min-1, for a run time of 45 min. Standards were made in 650 g kg-1 CH3CN in H2O with both the analytical phosmet and the commercial formulation. Preparation of aqueous samples involved liquid–liquid extraction with 300 g kg-1 ethyl acetate in hexane in a 1:1 ratio with the sample. Samples were then evaporated to dryness by rotary evaporation (50°C), reconstituted in 650 g kg-1 CH3CN in H2O, and filtered before analysis.
Phosmet was extracted from soil samples with the method of Menn et al. (1965). Soil samples (0.01 kg) were shaken with acetone (0.02 L) three times, and the supernatants from each extraction were pooled. Samples were then evaporated to dryness by rotary evaporation (65°C) and reconstituted in 0.01 L hexane. Sample preparation was performed with 50 g kg-1 deactivated Florisil columns prewashed with 0.002 L hexane. Following sample loading onto the columns, the columns were washed with 0.003 L hexane, 0.003 L of 50 g kg-1 ethyl acetate in hexane, and 0.005 L of 300 g kg-1 ethyl acetate in hexane. Phosmet was eluted in the 300 g kg-1 ethyl acetate in hexane wash only, which was collected and evaporated to dryness by rotary evaporation (50°C). The phosmet was reconstituted in 650 g kg-1 CH3CN in H2O and filtered before analysis. A 90% recovery of phosmet from the sample preparation phase was achieved. Phosmet recovery from soil, including the sample preparation phase, was 80% for the Podosol and 70% for the Ferrosol.
Sorption Isotherm Method
Preliminary studies of the sorption kinetics (with 0.7 mg L-1 phosmet in a 1:2.5 soil to solution ratio) showed rapid initial phosmet sorption within 10 min, and sorption reaching equilibrium within 30 min on the Ferrosol and 60 min on a Kurosol (11% silt, 79% sand, 10% clay, and 6 g kg-1 organic C), which had similar properties to the Podosol used subsequently. A 60-min equilibrium time was considered adequate for sorption to attain equilibrium.
Phosmet sorption isotherms were obtained with the batch equilibrium technique. The commercial phosmet formulation was diluted to produce a concentration range of 0 to 7 mg L-1, which covers the recommended phosmet application rate (0.1 kg ha-1). Isotherms were obtained for soil depths of 0 to 0.05, 0.05 to 0.1, 0.1 to 0.2 and 0.2 to 0.3 m. Phosmet solutions of different concentration were added to 0.005 and 0.010 kg of air-dried soil in a ratio of 2.5:1 (solution to soil) and shaken for 1 h. The phosmet solutions were made up in 0.005 mol L-1 KCl for the Podosol, and 0.005 mol L-1 CaCl2 for the Ferrosol. The presence of Ca2+ or K+ as the dominant cation may alter the phosmet sorption characteristics (Sánchez Camazano and Sánchez Martín, 1980).
Desorption isotherms were determined with the sequential method of Singh et al. (1989), with three sequential desorptions (1 h shaking, 1:5 soil to solution ratio) on each sample performed following the sorption measurements. In all measurements, triplicate samples were analyzed and standard errors calculated for all means. Isotherms were fitted to the data by least squares regression where appropriate. Values for K and n were obtained by simultaneous optimization of the log–log form of the Freundlich equation, except in the case where a linear isotherm was appropriate (n = 1).
Degradation Measurement
Incubation experiments were performed at 25°C in the dark with the soil at field capacity (-10 kPa water potential). Phosmet in 0.005 mol L-1 KCl was applied to the Ferrosol at a rate of 0.025 mg per 0.5 kg soil (equivalent to the recommended application rate of 0.1 kg ha-1). Phosmet was applied in 0.05 mol L-1 KBr solution to the Podosol at a rate of 2.5 mg per 0.5 kg soil (equivalent to 10 kg ha-1). The bromide was intended for use as a conservative nonreactive tracer for water movement through the soil.
Samples (0.01 kg) were taken over time and the phosmet concentration was measured after extraction at each time. Sampling times for the Podosol were 15 and 30 min, 1, 4, 10, and 24 h, and 3 and 7 d, and then weekly until no phosmet could be detected in the extracts. The initial sampling frequency was less intense for the Ferrosol.
The degradation data were fitted to the first-order reaction equation:
 | [2] |
where C is the pesticide concentration in the soil at time t, Co is the pesticide concentration in the soil at the application time, and µ is the first-order rate coefficient. The data were corrected to account for losses associated with the extraction and sample preparation technique by setting time zero at the first extraction (15 min), and C was measured at that time as Co. The half-life t1/2 was calculated from µ with the equation:
 | [3] |
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RESULTS
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Sorption Isotherms
The sorption isotherms, fitted by the Freundlich equation, are shown in Fig. 2 and 3
. For the 0- to 0.05-m depth only in the Ferrosol, the linear form of the Freundlich equation:
 | [4] |
gave the best fit. Standard error bars are shown on all figures. The derived K and n values are given in Table 2. The slope of the sorption isotherm decreased with depth in the Ferrosol (Fig. 2, Table 2), but increased with depth in the Podosol (Fig. 3, Table 2). Sorption was well described by the Freundlich equation in both soils.
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Table 2. Freundlich K and n values for phosmet sorption and desorption on the Ferrosol and Podosol, and the coefficients of determination, R2.
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In both soils, sorption appeared to increase with increasing organic C content and decrease with increasing clay content. The importance in phosmet sorption of clay and organic C, and a combination of these two properties, was determined by simple and multiple linear regression for each soil type. In the Ferrosol, the relationship between sorption and clay and between sorption and clay and organic C combined had equal significance (p = 0.005), while the relationship between sorption and organic C alone was weaker (p = 0.025). This suggests that clay content had the major influence on sorption in this soil, and organic C content contributed less. In the Podosol, the relationship between sorption and organic C was equally significant (p = 0.005) when considered alone or in combination with clay content, and the relationship between sorption and clay content was not significant. This suggests that in this soil, organic C had the major influence on sorption and clay content contributed little.
For the Podosol, the sorption–soil property relationship was simple, because there was little change in clay content with depth (Table 1), and sorption increased as a result of an increase in organic C. In the Ferrosol, the relationship was more complex. Sorption was directly related to organic C content and yet was also dependent on the clay content. The complexity arises because clay content increased with depth in the Ferrosol, at the same time that organic C content decreased. The relationship between clay and organic C on this soil was inverse and significant (p = 0.025).
The Freundlich equation was fitted to desorption isotherms for the Podosol only (Fig. 3), and K and n values were obtained (Table 2). There was little evidence for hysteresis in the Podosol because the K values were similar for both adsorption and desorption (Table 2).
Degradation
Phosmet degradation followed first-order kinetics in both soils (Fig. 4 and 5)
. The decay coefficient µ, and the phosmet half-life t1/2, as determined from the fitted relationship with Eq. [2] and [3], are shown in Table 3. The degradation data for the 0.2- to 0.3-m depth in the Podosol were highly variable and therefore were not included.
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Table 3. Decay coefficient (µ) and half-life (t1/2) values for phosmet in the Ferrosol (application equivalent to 0.1 kg ha-1) and the Podosol (application equivalent to 10 kg ha-1).
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Simple linear regressions were performed to assess the importance of clay, organic C, and sorption on degradation. Insufficient data existed to perform multiple linear regressions. The results showed that, in the Ferrosol, the relationship between degradation and the sorption coefficient K gave high R2 values (0.992) and was significant (p = 0.005). In the Podosol, the highest R2 value (0.981) was obtained for the relationship between degradation and clay content, but this was not significant (p = 0.1), mainly because the range in clay content in this soil was very small.
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DISCUSSION
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Sorption Isotherms
Nature of the Sorption Process
Sánchez Camazano and Sánchez Martín (1983) postulated that phosmet sorption on montmorillonite occurred simultaneously through the sulfur atom of the P=S group and the oxygen of one of the C=O groups interacting with an exchangeable cation on the clay (see Fig. 1). Because of the differences in the electronegativity of the two atoms in each group, the S and O atoms develop a slight negative charge, and the molecule is therefore attracted to positive charges present on the clay or organic matter complexes.
The measured sorption may cover a number of processes that are occurring in the soil because the phosmet was applied in a commercial formulation containing toluene as a co-solvent. The interaction between phosmet, toluene, water, and the soil sorption sites is probably complex. At sites on organic matter, toluene and phosmet may behave similarly because of their similar Kow values. Because of toluene's higher solubility in water and hence its higher effective concentration, where toluene and phosmet are competing for the same sorption sites, phosmet sorption may be inhibited. However, because of phosmet's greater affinity for toluene than water, toluene sorption at a site may also enhance phosmet sorption at that site. Thus, the measured sorption may be the result of simple sorption, competitive or cooperative sorption involving toluene, or even precipitation, if the phosmet solubility was exceeded in the equilibrating solution. At the low concentrations applied, there was no evidence that phosmet precipitation occurred; nor that sorption was influenced by the presence of co-solvents, because the isotherms were of the typical L type (Giles et al., 1960), and were well fitted by the Freundlich isotherm. This suggests that there were sufficient sorption sites on both soils for the sorption of all components within the commercial formulation separately.
Clay and Organic Carbon Influence on Sorption
Organic pesticide sorption is generally correlated with the soil organic C content (Wood et al. 1987; Grundl and Small, 1993). Sánchez Martín and Sánchez Camazano (1991) observed that thiophosphate (such as phosmet) sorption was related to the soil organic matter content. In this work, phosmet sorption was found to relate directly to organic C content, which decreased with depth in the Ferrosol and increased with depth in the Podosol (Tables 1 and 2). The relationship between sorption and organic C was stronger than between sorption and clay in the Podosol, and vice versa in the Ferrosol. The relationship between sorption and both soil parameters was not improved relative to clay alone (in the Ferrosol), or relative to organic C alone (in the Podosol).
The overall lower phosmet sorption by the Podosol relative to the Ferrosol (Table 2, Fig. 2 and 3) was due to the lower organic C content of the Podosol (Table 1). The fact that phosmet was applied in 0.005 mol L-1 CaCl2 to the Ferrosol, but that 0.005 mol L-1 KCl was used for the Podosol experiments, may have led to increased sorption on the Ferrosol (Sánchez Camazano and Sánchez Martín, 1980). But the differences in measured sorption were larger than would be expected from changing the cation in solution. At the 0.2- to 0.3-m depth, the two soils had similar K values, despite a big difference in clay content (Table 1).
Degradation
The decay coefficient was much smaller in the Podosol than the Ferrosol at all depths. The decay coefficient was greatest in the upper 0.05-m depth of both soils and decreased with depth, although the trend was not significant in the Podosol (Table 3). The decrease in decay coefficient with depth in the Ferrosol corresponded with a decrease in organic C content, and an increase in clay content (see Table 1). This suggests that a high organic C content accelerated phosmet decay, but sorption onto clay protected phosmet from decay. In the Podosol, however, there was little change in clay content with depth, and the change in organic C content of 2.1 to 9.1 g kg-1 from 0 to 0.2 m depth appeared insufficient to change the decay coefficient significantly. Overall, the smaller decay coefficient (and longer half-life) in the Podosol must reflect the lower organic matter content, and lower biological activity, in this soil.
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CONCLUSIONS
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The sorption coefficient K for phosmet was greater in the Ferrosol than the Podosol. There was a direct relationship between sorption and organic C content, and an inverse relationship between sorption and clay content, in both soils. The factor most important for sorption was clay content in the Ferrosol and organic C content in the Podosol. Sorption was described well by the Freundlich isotherm.
Phosmet degradation was described by first-order kinetics, although this was not well defined in the 0.2- to 0.3-m depth for both soils. Degradation was more rapid in the Ferrosol at all depths than in the Podosol, which appeared to be due to the higher organic C content down to 0.3 m in the Ferrosol. Sorption onto clay may have protected phosmet to some extent from degradation in the Ferrosol, but there was no evidence for this effect in the Podosol.
The results of the phosmet sorption and degradation experiments, conducted on two contrasting soils, showed that the relative importance of key soil properties—organic C and clay content—on these processes was a function of the soil type. This means that it is very difficult to obtain a single parameter to describe phosmet sorption and degradation across a range of soil types. In addition, complex interactions may have occurred between the various components of the phosmet commercial formulation that could influence sorption, and hence degradation. Therefore, studying the reactions of a pure ingredient may not provide satisfactory predictions of the reaction of the pesticide under field conditions.
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ACKNOWLEDGMENTS
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We would like to acknowledge the help provided by Dr. P.M. Chalk, Dr. R. Edis, Dr. A.J. Weatherley, and Dr. D. Chen. Acknowledgment is also made for the financial support provided by the Rowden White Prize, the E.A. Crespin Scholarship, the R.W. Nicholas Scholarship, and the A.M. White Scholarship.
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REFERENCES
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ABSTRACT
INTRODUCTION
), 0.05 to 0.1 m (
), 0.1 to 0.2 m (
), and 0.2 to 0.3 m (•), with fitted Freundlich isotherms (0.05–0.1, 0.1–0.2, 0.2–0.3 m) and a fitted linear isotherm (0–0.05 m) (——).
); 0.05 to 0.1 m (ads: 
); 0.1 to 0.2 m (ads: 
); and 0.2 to 0.3 m (ads: •, des: 
); with fitted Freundlich isotherms for sorption (——) and desorption (——).
