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TECHNICAL REPORT
Organic Compounds in the Environment

Sorption of Bentazone, Dichlorprop, MCPA, and Propiconazole in Reference Soils from Norway

^a The Norwegian Crop Research Institute, Plant Protection Centre, Høgskoleveien 7, N-1432 Ås, Norway
^b The Norwegian Crop Research Institute, Plant Protection Centre, Pesticide Laboratory, Osloveien 1, N-1430 Ås, Norway

* Corresponding author (christian.thorstensen{at}planteforsk.no)

Received for publication September 13, 2001.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Sorption–desorption kinetic and isotherm studies were performed by the batch equilibrium technique in three Norwegian soils. The soils were a fine sandy loam, a loam, and a soil of highly decomposed organic material. Two commercially formulations were used, Triagran-P and Tilt, containing either a mixture of bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], dichlorprop [(R)-2-(2,4-dichlorophenoxy)-propionic acid], and MCPA [(4-chloro-2-methylphenoxy)acetic acid], or propiconazole [(±)1-(2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-ylmethyl)-1H-1,2,4-triazole] alone. Sorption–desorption equilibrium occurred within 10 h for all pesticides. The Freundlich isotherms indicated nonlinear sorption of bentazone, dichlorprop, MCPA, and propiconazole. For all pesticides the highest Freundlich adsorption coefficient (K_F) values were in the soil with highest organic content and lowest pH. For the fine sandy loam and loam, which are representative Norwegian agricultural soils, the results indicate that bentazone, dichlorprop, and MCPA are mobile with K_F values ranging from 0.07 to 1.50 mg^1-1/n kg^-1 L^1/n. Propiconazole is much less mobile with K_F values ranging from 27.00 to 36.02 mg^1-1/n kg^-1 L^1/n in the agricultural soils.

Abbreviations: K_d, distribution coefficient • K_F, Freundlich adsorption coefficient

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
SORPTION of pesticides to soil is one of the main processes that influence their mobility and degradation in the soil environment. Sorption tends to decrease the degradation rate of organic chemicals by reducing their availability to microbial attack (Ogram et al., 1985; Smith et al., 1992). Quantitative determination of sorption is therefore essential for both the understanding of transport as well as sorption equilibrium in the soil–water system and also is an important parameter for predicting its fate using mathematical simulation models. Organic carbon is in general the most important soil property that affects pesticide sorption, but pH, cation exchange capacity (CEC), clay content, and type are also important (Kahn, 1978; Weber and Miller, 1989). In addition, pesticide properties must always be taken into account (Hamaker and Thompson, 1972; Kahn, 1980).

Bentazone , dichlorprop , and MCPA are the active ingredients in Triagran-P, which is used for weed control in cereals (Tomlin, 1997). They are all weak acids and ionize in aqueous solutions to form anionic species (A^-). Soil organic matter has been the major soil constituent reported to bind acidic pesticides, but metallic hydrous oxides are also likely to be involved, particularly in tropical soils (Hamaker and Thompson, 1972; Frear, 1976; Shea et al., 1983). As pH decreases, more molecular species are formed, and sorption is increased. This is because less-soluble molecular species more readily bind to the lipophilic fraction of the soil organic matter complex than the more soluble anionic species, which are repelled by negatively charged soil colloids (Frissel and Bolt, 1962; Weber, 1970, 1994).

Propiconazole is the active ingredient in the commercial product Tilt 62.5 used for control of fungal diseases in cereals. Propiconazole is a weak base with a pK_a value of 1.09 and solubility 110 mg L^-1 in water (Tomlin, 1997). Weakly basic pesticides are sorbed through ionic bonds and/or by physical adsorption, depending on the pH of the system. For both acidic and basic pesticides, sorption is inversely related to soil pH (Weber, 1988). Basic pesticides (B) associate with H⁺ ions in aqueous solutions and at acidic surfaces to form protonated species (HB⁺). Both soil organic matter and clay minerals have been reported to bind basic pesticides (Weber, 1994).

Soils all over the world are exposed to chemicals, whether these are pesticides intentionally spread out on fields and forests, evaporated industrial chemicals, combustion remnants from traffic and industry, or evaporated pesticides distributed by the atmosphere. It is a major issue to be able to assess how the chemicals will be distributed and degraded in the soil environment. It is also important to determine the risk of pollution of the closely connected aquatic environment. An important factor is the ability of soils to retain the chemicals so that they do not run off or leach, which may lead to the pollution of surface and ground water. To obtain reliable fate information for chemicals in the Nordic countries, selection of representative soils is important. Nordic soils differ from soils of other regions due to the colder climate, which retards decomposition. It is assumed that the effect of the colder climate on the decomposition of the soil, in turn, affects both sorption and degradation. From Norway, three different soils have been selected. Two of the soils, fine sandy loam (alluvial deposit) from Hole and loam (marine deposit) from Kroer, were selected based on information from the Norwegian Institute of Land Inventory's Digital Soil Database, which currently covers about 2500 km² of agricultural land. These are representative agricultural soils in Norway. A forest soil of highly decomposed organic matter from Froland was selected as a third soil due to its high organic content. The selected soils serve as reference soils in ecotoxicological tests of pesticides in the Norwegian environment.

The objective of this work was to investigate sorption of a formulation containing a mixture of bentazone, dichlorprop, and MCPA and a formulation containing propiconazole alone in three reference soils from Norway.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Soil
Samples from three different locations in Norway were used in this study. Soils from Kroer (loam) and Hole (fine sandy loam) represent the most frequently found soil types of the cultivated areas in Norway. The Froland soil represents a forest soil with highly decomposed organic material. Soils were air-dried, passed through a 2-mm sieve, and stored in the dark. The soils differ with respect to organic matter content, pH levels, and sand, silt, and clay content (Table 1). Gamma irradiation from a ⁶⁰Co source with an exposure dose of 32 kGy (three 8-h exposures) was used for sterilizing a subset of the Froland gleyic podozol soil.

Experimental
According to the Organisation for Economic Co-Operation and Development (OECD) Guideline 106 (Organisation for Economic Co-Operation and Development, 1997), the batch study was carried out in four tiers. A screening test was first conducted to determine the proper (best) soil to water ratio. Second, a kinetic test determined when adsorption equilibrium was reached. Step 3 used the most convenient soil to water ratio and equilibration time to conduct sorption isotherms. A desorption kinetic study was performed following adsorption equilibrium as Step 4.

In all steps, the soils were pre-equilibrated (to achieve equilibrium between CaCl₂ and soil before the addition of pesticide solution) for 12 h with a 0.01 M CaCl₂ solution to give a better phase separation and to keep ionic strength similar to that of a natural soil solution before the addition of pesticides. All experiments were conducted in Teflon centrifuge tubes at 20°C using a horizontal shaker. Centrifugation was performed at 16270 x g for 10 min. Supernatants were transferred to glass tubes and stored in the refrigerator at 4°C prior to analyses.

Analysis
The analyses were performed with high performance liquid chromatography with ultraviolet detection (HPLC–UV) composed of a Spectra-Physics (San Jose, CA) Model SP8000 pump, a SP4270 integrator, a Gilson (Middleton, WI) autosampler, and a Milton Roy (Staffordshire, UK) UV detector. For determination of phenoxyacids and bentazone the detector was set at 200 nm, and a C₈ analytical column was used (stainless steel, 25 cm x 4.6 mm i.d., packed with Spherisorb 5-µm particles from Phase Separations Ltd. [Deeside, UK]). The mobile phase was 20 mM phosphate buffer (pH 3) and acetonitrile (65:35 v/v) at a flow rate of 1 mL min^-1. The injection volume was 50 µL. Determination of propiconazole was performed with the UV detector set at 208 nm and a C₁₈ analytical column (stainless steel, 25 cm x 4.6 mm i.d., packed with Spherisorb 5-µm particles from Phase Separations Ltd.). The mobile phase was 1 mM ammoniumacetat and acetonitrile (1:1 v/v) at a flow rate of 1 mL min^-1 and the injection volume was 50 µL. Quantification was performed by external calibration by measuring the peak areas. The precision given as the relative standard deviation (r.s.d.) of the injection of standards was <1%. The precision of the control and non-control samples was <10%.

Soil to Solution Ratio
Different soil to solution ratios in the range of 1:100 to 1:1 were tested. It was desirable to obtain more than 20% sorption because of the analytical method and preferably >50% sorption (Boesten, 1990). After pre-equilibration, a volume of stock solution, not exceeding more than 10% of final volume, was added. The final concentrations of the pesticide mixture were: 7.1 mg L^-1 of bentazone, 5.1 mg L^-1 dichlorprop, and 6.2 mg L^-1 MCPA. The concentration of propiconazole was 5.1 mg L^-1.

Duplicate samples were shaken for 24 h on a horizontal shaker and then centrifuged at 16270 x g for 10 min. The supernatants were transferred to glass tubes and stored in the refrigerator at 4°C prior to analyses. Sorption was calculated as the difference between initial and final concentration.

Sorption Kinetic
The optimum soil to solution ratio from Step 1 was chosen for the sorption kinetics experiment. The soil–solution suspensions were shaken for different time intervals in triplicate. Blanks with the same amount of soil and volume of 0.01 M CaCl₂ were subjected to the same test procedure for all three soils. Control samples with the test pesticides in 0.01 M CaCl₂ were subjected to the same steps as the test system, in order to check pesticide stability in 0.01 M CaCl₂ solution and possible adsorption to the test vessels.

Desorption Kinetic
Desorption experiments were conducted immediately after the 24-h sorption experiments. Supernatant was removed and replaced by the same amount of fresh CaCl₂. Shaking, subsequent separation of soil and aqueous phase, and storing of solutions were conducted as described above. One sample from each of three separate centrifuge tubes was taken each time.

Freundlich Isotherm Study
After equilibration, different initial concentrations (Table 2) of pesticide solutions were shaken with soil for 24 h. The remaining concentration in the pesticide solution was measured. At each concentration level, one sample from three separate centrifuge tubes was taken. Sorption data were calculated using the Freundlich equation:

[1]

[2]

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Soil to Solution Ratio
Soil to solution ratios in the range of 1:1 to 1:100 were tested. For the mixture of bentazone, dichlorprop, and MCPA, a soil to solution ratio of 1:1 was selected for the Hole fine sandy loam and the Kroer loam. In general, at high soil to solution ratios, the number of sorption sites relative to the amount of pesticide increases, which results in higher sorption (Celis et al., 1999). Due to a strong sorption of the acidic pesticide mixture to the Froland gleyic podozol, a soil to solution ratio of 1:50 was selected. A soil to solution ratio of 1:25 was selected for propiconazole in the Hole fine sandy loam and the Kroer loam. For propiconazole in the Froland gleyic podozol, it was necessary to select a soil to solution ratio of 1:100 due to strong sorption.

Sorption and Desorption
Blanks did not reveal any interfering peaks or changes due to adsorption or degradation for all pesticides. Sorption and desorption of Triagran-P were conducted with sterilized and nonsterilized Froland gleyic podozol soil. There was no significant difference (p = 0.05) between the distribution coefficient (K_d) values of sterilized and nonsterilized Froland gleyic podozol after 48 h of shaking. This indicated that no microbial degradation was occurring over the course of the equilibration. Propiconazole has a long half-life (t_1/2 > 200 d) (Bromilow et al., 1999; Thorstensen and Lode, 2000) and therefore, no degradation experiments with this pesticide in sterilized Froland gleyic podozol were performed.

The sorption experiments (Fig. 1) showed an immediate rapid sorption by which about 8 to 89% of the added pesticide was sorbed within 5 h. The rapid sorption was followed by a slow sorption of all pesticides in the Froland gleyic podozol. This could also be observed for propiconazole in the fine sandy loam and loam. Presumably there is an initial quick sorption, a surface phenomenon, followed by slow migration and diffusion of the pesticides into the organic matter matrix and mineral structure (von Oepen et al., 1991).

View larger version (31K): [in this window] [in a new window]   Fig. 1. Percent sorption of (a) bentazone, (b) dichlorprop, (c) MCPA, and (d) propiconazole on three different soils versus time. The initial concentrations were 7.1, 5.1, 6.2, and 5.1 mg L-1 of bentazone, dichlorprop, MCPA, and propiconazole, respectively.

In the Froland gleyic podozol (pH = 2.9) the acidic pesticides will be undissociated, while in soil from fine sandy loam (pH = 6.3) and loam (pH = 5.5) they will be anionic. A pH near neutral can decrease sorption due to electrostatic repulsion between negatively charged soil particles and anionic herbicides and result in a lower sorption (Burnside and Lavy, 1966; Helweg, 1987; Eklo and Lode, 1991; Helweg and Fomsgaard, 1995). The difference in sorption between loam and fine sandy loam was small, which can be explained by little variation in soil properties.

The sorption of propiconazole was highest in the Froland gleyic podozol, which had an organic content of 37% and a pH 2.9. A higher organic matter and clay content and a lower pH can be the reason for a higher sorption in loam compared with fine sandy loam. For weakly basic molecules such as propiconazole, sorption maxima are often found at pH values near their pK_a. Lowering the pH increases the fraction of protonated molecules having interaction with the soil matrix based not only on van der Waals forces but also on cation exchange mechanisms (Weber, 1982).

Desorption equilibrium was achieved almost immediately (Fig. 2) . When sorption was high, desorption was low, as expected. Sorption and desorption were inversely correlated for dichlorprop, MCPA, and propiconazole. Ratios of K_d values for sorption after 24 h compared with the corresponding K_d values for desorption after 24 h were always different from 1, indicating sorption nonideality (Beck et al., 1993; Beck et al., 1996a). Intraparticle diffusion and intrasorbent diffusion are frequently cited as causes for not reaching equilibrium (Beck et al., 1993). However, hysteresis can be caused by methodological artifacts, biological and chemical degradation, failure to establish equilibrium, and competitive sorption.

View larger version (31K): [in this window] [in a new window]   Fig. 2. Percent desorption of (a) bentazone, (b) diclorprop, (c) MCPA, and (d) propiconazole on three different soils versus time.

Sorption Isotherms
Data were fitted to Freundlich isotherms (Fig. 3 and Table 3) according to Equation 1 through nonlinear regression analysis.

View larger version (33K): [in this window] [in a new window]   Fig. 3. Freundlich isotherms fitted to sorption data of (a) bentazone, (b) dichlorprop, (c) MCPA, and (d) propiconazole on three different soils using nonlinear regression analysis.

The K_F values of bentazone were 0.09 and 0.07 mg^1-1/n kg^-1 L^1/n in fine sandy loam and loam, respectively. For dichlorprop, the calculated K_F in fine sandy loam soil was 0.67 and 0.86 mg ^1-1/n kg^-1 L^1/n in loam. The K_F of MCPA was in the range 1.41 to 1.50 mg ^1-1/n kg^-1 L^1/n in the agricultural soils. The K_F values of propiconazole were 27.00 and 36.02 mg^1-1/n kg^-1 L^1/n in fine sandy loam and loam, respectively. It may be difficult to compare results if K_F values are derived from different units of C_s and C_w or if C_w 1 or if 1/n values differ (Chen et al., 1999). However, the results are roughly within the range of previously reported values (Grey et al., 1996; Gaston et al., 1996; Matallo et al., 1998; Riise and Salbu, 1992; Ghorayshi and Bergström, 1991; Helweg, 1987; Susarla et al., 1992; Riise et al., 1994, Shang and Arshad, 1997; Socías-Viciana et al., 1999; Liu and Weber, 1986). Higher K_F values for all pesticides were found in soil from Froland with a high organic content and a low pH, which is consistent with previously mentioned literature that also reports higher increasing K_F in soil with increasing organic content.

For the same soil, the K_F value increased among the pesticides in the order: bentazone < dichlorprop < MCPA < propiconazole. The magnitude of sorption of acidic pesticides is much lower than that of cationic or basic pesticides (Weber, 1972). For each pesticide, K_F increased in the following order: fine sandy loam < loam < highly decomposed organic material. In all soils, the K_F values were significantly different (P = 0.01) with the exception of bentazone in fine sandy loam and loam. The K_F values for bentazone, dichlorprop, and MCPA were low in loam and fine sandy loam, which implies that they are mobile. Abernathy and Wax (1973) reported that bentazone was very mobile in 12 soils from Illinois. The K_F values for the weakly basic propiconazole were a factor of 100 higher in fine sandy loam and loam than for the acidic pesticides. This implies that propiconazole is strongly sorbed and hence only slightly mobile. High K_d values for propiconazole have also been reported by Liu and Weber (1986), who also concluded that propiconazole is not mobile. This does not necessarily imply that propiconazole would be immobile. Erosion of propiconazole bounded to small particles or transport through macropores may lead to accidental leaching but the concentrations transported would be very low (Eklo and Lode, 1994). Dichlorprop and MCPA may be considered as mobile based on this batch study. However, other factors like half-lives should be considered in the evaluation of the potential risk of contamination of the aquatic environment. In the highly decomposed soil, K_F values for all pesticides were much higher than for the two other soils and hence none of the pesticides would be mobile.

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Adsorption and desorption kinetics of all investigated pesticides showed an initial steep slope reaching a plateau. The low K_F value of bentazone, dichlorprop, and MCPA indicates that these pesticides are potentially mobile, while propiconazole is not mobile in the soils evaluated. All four pesticides were strongly bound to a soil consisting of highly decomposed material and are not mobile, which is probably a result of the low pH or the high organic content. The significance of 1/n > 1 is that the percent change in sorption (slope) with increasing C_W (equilibrium concentration) is greater in the higher concentration range than at the lower concentration range. Results of the tested pesticides in Norwegian reference soils were within the range of results from other countries, and this indicates that the effect of a colder climate on the soil formation did not affect sorption.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 

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