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a Instituto de Recursos Naturales y Agrobiologia de Sevilla, CSIC, P.O. Box 1052, 41080 Sevilla, Spain
b USDA-ARS, Soil and Water Management research Unit, 1991 Upper Buford Cir. Rm 439, St. Paul, MN 55108
* Corresponding author (cornejo{at}irnase.csic.es)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CEC, cation-exchange capacity • HPLC, high prefomace liquid chromatography • OC, organic C • OCl, organoclay • TC, technical compound
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The use of controlled-release formulations of mobile herbicides has been suggested to restrict their movement in soil, thereby reducing their potential of surface and ground water contamination. Several synthetic (cationic surfactants and organic polymers) and natural (plant lignin and starch) materials have been proposed as supporting agents for pesticides in these formulations to reduce movement (Gish et al., 1994; Johnson and Pepperman, 1995a; Johnson and Pepperman, 1995b). These control-release formulations also decrease pesticides losses by offering protection from other processes such as volatilization (El-Nahhal et al., 1998; El-Nahhal et al., 1999). Clays have been used as an aid in powdered and granular formulations of pesticides for a long time. Recently, there has been an increased interest in the use of such natural soil components as possible herbicide carriers in controlled-release formulations (Margullies et al., 1993; Margullies et al., 1994; Gerstl et al., 1998; Gonzalez-Pradas et al., 1999; Cox et al., 2000). A recent comprehensive monograph on controlled-release formulation for pesticides has been edited by Scher (1999).
The sorptive capacity of clays for organic compounds can be enhanced by chemical modification, whereby the native inorganic-exchangeable cations are replaced with organic cations via ion-exchange reactions (Boyd et al., 1991; Xu et al., 1997). Organoclays have been shown to be good sorbents for removing polar pesticides from water (Hermosin and Cornejo; 1993; Zhao et al., 1996; Socias-Viciana et al., 1998; Celis et al., 1999; Aguer et al., 2000; Sheng and Boyd, 2000). Brixie and Boyd (1994) reported that organoclays have a strong immobilizing effect on nonionic organic chemicals with low water solubility such as benzene derivatives and phenols; other studies have shown OCls to be good sorbents for diverse nonpolar organic contaminants (Xu et al., 1997; Nir et al., 2000).
Polar chemicals that are soluble in water and weakly sorbed by soil particles can move rapidly with the infiltrating water and hence, are likely to be found in ground water (Goodrich et al., 1991). Acidic herbicides such as bentazone and dicamba have these characteristics, and have been used as models of very mobile and leachable herbicides (Romero et al., 1995; Ritter et al., 1996). Recent studies further suggest the use of OCls to protect soil and water from acidic herbicides such as bentazone (Carrizosa et al., 2000; Carrizosa et al., 2001), as well as potential carriers in slow-release formulations for polar (Hermosin et al., 2001) and hydrophobic (El-Nahhal et al., 1998; El-Nahhal et al., 1999) pesticides. In previous studies, we have shown OCls to be effective sorbents to remove 2,4-D (Hermosin and Cornejo, 1992), bentazone (Carrizosa et al., 2000), and dicamba (Carrizosa et al., 2001) from water. The sorption capacity of OCls is favored by high layer charge and saturation with bulky organic cations close to the cation-exchange capacity (CEC). Hydrophobic interactions with polar contributions are responsible for adsorption of these molecules, which need free polar space between alkylammonium groups in the OCl interlayer (Hermosin and Cornejo, 1993; Aguer et al., 2000; Carrizosa et al., 2000; Carrizosa et al., 2001).
The aim of this work was to assess the potential use of OCls selected on the basis of previous sorption-desorption studies (Carrizosa et al., 2000; Carrizosa et al., 2001) as matrices for controlled-release formulations of bentazone and dicamba to reduce herbicide concentrations in soil solution and leaching through soil columns. Organoclay characteristics and herbicide-OCl mode of preparation were related to the decrease in the leaching potential. A bioassay, to test the efficacy of dicamba as preemergence herbicide in these formulations, was also performed.
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MATERIAL AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Dicamba is a crystalline solid with a melting point of 114 to 116°C, vapor pressure 4.5 mPa, molecular weight 221, solubility 6.5 g L-1 water (25°C), Kow 0.29, and pKa 1.95 (Worthing and Hance, 1991). Pure analytical dicamba was purchased from Chem Service (West Chester, PA).
Organoclays
The reference clays SAz1 (Arizona Montmorillonite) and SWy2 (Wyoming montmorillonite) used in this study were supplied by the Clay Mineral Repository of the Clay Minerals Society (Columbia, MO) and were used as received in the preparation of the OCls. SAz1 and SWy2 were Ca- and Na-saturated smectites, respectively. Cation-exchange capacity was 120 cmol kg-1 for SAz1 and 76 cmol kg-1 for SWy2 (Van Olphen and Fripiat, 1979). The original clays were modified using the quaternary (hexadecyltrimethylammonium and dioctadecyldimethylammonium) and primary (octadecylammonium) alkylammonium cations listed in Table 1. The OCls were prepared by treating 100 g of the mineral clay with an ethanol/water (50:50) solution of the alkylammonium Cl/Br containing 50 or 100% of the sample CEC as described elsewhere (Carrizosa et al., 2000; Carrizosa et al., 2001). The organic C (OC) and N contents were determined on the OCls using an elemental C analyzer (LECO CHNS932, LECO Corp., St. Joseph, MI). Using the OC or N contents, the molecular weights of the alkylammonium cations, and the corresponding CEC values, the percentage of organic-cation saturation (%OctS) was calculated for each OCl. Selected properties of the OCls are summarized in Table 1.
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X-Ray Diffraction
The basal spacing of the organic clays or the distance between two consecutive silicate layers within the unit cell was measured by x-ray diffraction (Brindley and Brown, 1980). A Siemens D-500 equipment (Siemens, Germany) with Cu K
on a oriented film supported on a glass-slide and a goniometer rate of 1 or 2° min-1 was used for this purpose. The film was prepared by depositing aliquots of an approximately 2% methanol/water (50:50) suspension on the slide.
Herbicide-Organoclay Complex Synthesis
The OCls selected for this study were based on previous herbicide sorption-desorption results (Carrizosa et al., 2000, Carrizosa et al., 2001). Three OCls were selected for bentazone-OCl complexes: an OCl with high sorptive capacity (ADOD), a medium sorption capacity (WHDT) and a low sorptive capacity (AC18). For dicamba-OCl complexes, OCls with high (ADOD), medium (AHDT), and low (AC18) sorption capacity were used. To obtain a final active ingredient concentration of 4%, 24 mg of herbicide were mixed with 576 mg of OCl. Three types of OCl-herbicide complexes were prepared: (i) aged complexes (H-OClA), the OCl was mixed with a methanolic solution of the herbicide, shaken for 24 h and the methanol allowed to evaporate, (ii) nonaged complexes (H-OClNA), the same process was performed as in aged complexes, but without shaking to reduce the binding of the herbicide to the OCl; and (iii) dry mixing of the OCl and the herbicide (H-OClDM).
Herbicide Release from Organoclay Complexes
Closed System
The release of bentazone and dicamba from the complexes was monitored in water. Duplicate aliquots of the herbicide-OCl complexes (10 mg for bentazone and 55 mg for dicamba) were added to 300 mL of water, in 500-mL amber glass bottles. The suspensions were sampled periodically from 0 h to 4 d after shaking by hand. The samples were filtered, and the herbicide concentration in the supernatants determined by high performance liquid chromatography (HPLC).
Open System: Column Leaching
The leaching of bentazone and dicamba through hand-packed soil columns was investigated for the herbicide-OCl complexes and free technical compound. The soil used was a sandy clay soil, pH 7.9, 1.0% organic matter, 12% illite, 4% montmorillonite, 4% kaolinite, 1.4% Fe2O3, and CEC 9 cmol kg-1. The columns were constructed using six methylacrylate rings (5 cm diam. by 5 cm long) and a plastic funnel at the bottom, sealed together with silicone. The top ring was filled with sea sand and the bottom ring with sea sand plus glass wool, to minimize losses of soil and contamination of leachates with soil particles. The other four rings were packed with soil at a bulk density of 1.25 g cm-3. Before herbicide application, soil columns were saturated with 0.01 M CaCl2 and then allowed to drain for 24 h. Technical bentazone and dicamba were added to duplicate soil columns as methanolic solutions (1 and 2 mM, respectively) in amounts equivalent to the maximum application rates, 2.2 kg ha-1 (0.4 mg, a.i) and 11.2 kg ha-1 (2.0 mg, a.i) for bentazone and dicamba respectively (Worthing and Hance, 1991). Organoclay-formulated bentazone (10 mg) and dicamba (55 mg) complexes were applied at the same rates to the top of duplicate columns. The columns were leached with 25 mL of 0.01 M CaCl2 added each of the first 15 d and then 50 mL daily until no herbicide was detected in the leachates. This was achieved within a 30-d period. All column experiments were performed at room temperature (20 ± 1°C). The amount of water was added at once on the top ring of the column. The leachates were collected daily and the concentration of herbicide determined by HPLC until the herbicide concentration measured was <0.1 µM. After pesticide elution, the soil columns rings were separated and soil (2 g) extracted with methanol/0.01 M CaCl2, (80:20 v/v for bentazone and 50:50 v/v for dicamba), filtered, and analyzed for herbicide residues.
Analytical Methods
The aqueous solutions of the pesticides from these experiments were analyzed by HPLC using Nova-Pack C18 column (Waters, Milford, MA), 150 by 3.9 mm; flow rate 1 mL min-1; injection volume 25 µL under the following conditions: (i) bentazone: mobile phase 20:80 methanol/sodium acetate; wavelength 332 nm; (ii) dicamba: mobile phase 45:55 methanol/H3PO4 (pH = 2); wavelength 220 nm.
Dicamba Bioassay Experiment
Bioassays were conducted using with the soil used in the column-leaching experiments to investigate the herbicidal activity of dicamba–OCl complexes. Bioassays were performed in duplicate with air-dried soil (250 g) in plastic pots (7 by 8.5 cm diameter), saturated with water, and allowed to drain for 24 h. Treatments consisted of four different formulations of dicamba: technical compound (TC), dicamba–AHDT aged complexes (D-AHDTA), dicamba–AC18 aged complexes (D-AC18A), and the physical solid mixture of AHDT and dicamba (D-AHDTDM). The TC (5.4 mL methanolic solution 2 mM) or OCl (60 mg of complexes 4% a.i.) formulations of dicamba were applied as preemergence at the average of 4.2 kg ha-1. Watercress (Lepidum sativum) was selected as a test plant for measuring the herbicidal activity of dicamba because of its sensitivity to these herbicide applications. Soil moisture was maintained during the experiment by spraying 50 mL of water on the top of the soil twice a week. For the bioassay, 20 seeds were sown and herbicide was applied onto the soil surface. After 2 wk, plants were cut off at soil level and the dry matter weights of shoots were determined.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Herbicide Release Kinetics
The amount of herbicide released from the different herbicide complexes as H-OClA and H-OClNA forms are shown in Fig. 2
for bentazone and Fig. 3
for dicamba. The differences in the percentage of bentazone released from the B-ADOD, B-WHDT, and B-AC18 complexes were significant. The percentage released decreased in order B-AC18 > B-WHDT > B-ADOD, which can be attributed to an increase in the sorption capacity. Both B-OClA and B-OClNA preparations of the weakly sorbing OCl, AC18, gradually release bentazone from 35 and 40% respectively, after 10 min., to 80 and 100%, after 10 h. Concentrations remain constant through the rest of the experiment (96 h). Initial percentages released from the aged and nonaged formulations of the OCl with intermediate sorption capacity (WHDT) were smaller (11 and 16%, respectively). Percentages released were 60% for the aged and 80% for the nonaged complex after 24 h and increased somewhat by the end of the experiment. The release profiles of the highly sorptive ADOD showed an initial amount of bentazone in solution of only 3 and 9% for the B-OClA and B-OClNA, respectively. The bentazone released reached the maximum of 20% amount rapidly (after 10 h) for B-OClA and 35% for B-OClNA. These results suggest that sorption capacity of OCl for bentazone is the main factor influencing the release of the herbicide-OCl complexes. However, the aging process greatly influenced the release behavior of bentazone-OCl associations as greater amount were released from B-OClNA than B-OClA, probably because of stronger H-OCl interactions or bonds for aged complexes.
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These results indicate that part of the bentazone and dicamba associated with the organic clay may be unavailable, particularly in the case of ADOD complexes, suggesting that this OCl may be not appropriate for slow release formulations.
Soil Column Leaching
To test the mobility of the herbicides in soil columns, WHDT and AC18 aged and nonaged complexes were selected for the herbicide bentazone, and AHDT and AC18 aged complexes were used for dicamba. For both herbicides, a dry mix of an OCl and the herbicide was applied to soil columns (AC18 for bentazone and AHDT for dicamba).
Herbicide–OCl complexes and technical herbicide applied to soil columns resulted in the breakthrough curves presented in Fig. 4 . Breakthrough and peak timing and peak concentrations as relevant features of the Fig. 4 are summarized in Table 2. Breakthrough of bentazone occurred in the following order: TC < B-AC18DM < B-AC18NA < BAC18A < B-WHDTNA < B-WHDTA (Fig. 4A). Peak (maximum) concentrations were reached at greater water volumes for the two B-WHDT complexes (Table 2). The broadness and flatness of these two profiles reflect the high sorption capacity and the strength of interaction between this sorbent and bentazone. Free bentazone gave a maximum concentration of 20 µM, which was reduced to 14 µM for the dry mix B-AC18, 13 µM for B-AC18NA, 9 µM for B-AC18A, 7 µM for B-WHDTNA, and 6 µM for B-WHDTA complexes (Table 2). The strongest decrease in peak concentrations was found for OCl with the largest sorption capacity, WHDT. In agreement with bentazone released in water (Fig. 2), there was an inverse relationship between peak maximum concentrations and sorption capacity of the OCl for bentazone. Peak concentrations increased in the order dry mix < nonaged complexes < aged complexes, which further demonstrates that the effect on pesticide leaching is inversely related to the strength of the interaction between the chemical and the OCl.
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The profiles in Fig. 4 clearly show the ability of OCls to reduce the peak herbicide concentration associated with the initial infiltration of water and subsequent leaching, and hence herbicide mobility.
Figure 5 show cumulative bentazone and dicamba recovered in leachate from the soil columns. Herbicide losses were smaller for OCl complexes than for the technical compounds in most cases. Again differences of bentazone released were observed between losses for aged and nonaged complexes and the dry mix (90% B-AC18DM, 85% B-AC18NA, 55% B-AC18A, 65% B-WHDTNA, and 55% B-WHDTA). No clear differences were found between B-AC18DM and B-AC18NA, showing the similarity of both preparations. Percentages of dicamba leached differed between D-AHDTA (50%) and D-AC18A (75%) as a consequence of the difference in the sorption capacity of both OCls, and in the case of the dry mixing complex, 100% of the herbicide was released. For the dry-mixed complexes, most of the herbicide is recovered but the delay in the recovery may contribute to a slower contamination rate of ground water, which it could be advantageous for natural attenuation process. No residual herbicide (bentazone and dicamba) was detected in the soil column after extraction. Hence the differences between pesticide recovery from the technical compound and pesticide-OCl complexes should correspond with the herbicide tightly bound to OCl and hence unavailable.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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Received for publication June 22, 2001.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIAL AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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