Journal of Environmental Quality 31:1657-1664 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Organic Compounds in the Environment
Imazaquin Adsorbed on Pillared Clay and Crystal Violet–Montmorillonite Complexes for Reduced Leaching in Soil
Tamara Polubesova*,a,
Shlomo Nira,
Zev Gerstlb,
Mikhail Borisoverb and
Baruch Rubina
a Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot, 76100 Israel
b Institute of Soil, Water and Environmental Sciences, The Volcani Center, ARO, 50250 Bet Dagan, Israel
* Corresponding author (polubeso{at}agri.huji.ac.il)
Received for publication October 10, 2001.
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ABSTRACT
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Ground water pollution due to herbicide leaching has become a serious environmental problem. Imazaquin [2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)quinoline-3-carboxylic acid] is an herbicide used to control broadleaf weeds in legume crops. Imazaquin is negatively charged at the basic pH of calcareous soils and exhibits high leaching potential in soils. Our aim was to design formulations of imazaquin to reduce herbicide leaching. Imazaquin sorption on pillared clay (PC) and crystal violet (CV)–montmorillonite complexes was studied. The CV–montmorillonite complexes become positively charged with adsorption of CV above the cation exchange capacity (CEC) of montmorillonite, and thus can sorb imazaquin. The Langmuir equation provides a good fit to isotherms of imazaquin sorption on PC and CV–montmorillonite complexes, but for charged complexes an equation that combines electrostatics with specific binding was preferred. Maximal imazaquin sorption was 17.3 mmol kg-1 for PC and 22.2 mmol kg-1 for CV–montmorillonite complexes. The extents of imazaquin desorption into water were 21% for PC and 5% for CV–clay complexes. The presence of anions decreased imazaquin sorption on both sorbents in the sequence phosphate > acetate > sulfate. Reduction of imazaquin sorption by the anions and the extent of its desorption in electrolyte solutions were higher for PC than for CV–clay complexes. Leaching of imazaquin from CV–montmorillonite formulations through soil (Rhodoxeralf) columns was two times less than from PC formulations and four times less than that of technical imazaquin. The CV–montmorillonite complexes at a loading above the CEC appear to be suitable for preparation of organo–clay–imazaquin formulations that may reduce herbicide leaching significantly.
Abbreviations: CEC, cation exchange capacity • CV, crystal violet • PC, pillared clay
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INTRODUCTION
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GROUND WATER POLLUTION due to herbicide leaching has become a serious environmental problem. Imazaquin (Fig. 1)
is a pre- and postemergence herbicide used to control broadleaf weeds in legume crops (Ahrens, 1994). Imazaquin is amphoteric, having acidic and basic functional groups. The pKa of acidic carboxyl and basic quinoline moieties are 3.8 and 1.8, respectively (Loux and Reese, 1992). The molecular structure along with the physical and chemical properties of herbicides determine their potential for leaching through the soil profile (Wauchope et al., 1992; Carter, 2000). Imazaquin is negatively charged at the basic pH values commonly found in the calcareous soils of Mediterranean countries. Imazaquin exhibits low sorption and high leaching potential in soils (Goetz et al., 1986; Basham et al., 1987; Milanova and Grigorov, 1996). Imazaquin sorption is promoted by a decrease in pH and increase in organic matter and aluminum and iron oxide contents in soils (Loux et al., 1989; Stougaard et al., 1990; Che et al., 1992; Gennari et al., 1998; Regitano et al., 1997, 2000; Leone et al., 2001; Nègre et al., 2001).
Our aim was to design formulations of imazaquin to reduce herbicide leaching and migration to nontarget areas. Pillared clays (PC) were found to be efficient sorbents for herbicides and environmental pollutants, due to the geometrical features of the porous spaces and to the specific interactions of the adsorbates with the pillars and the clay layers (Lahav et al., 1978; Van Damme and Fripiat, 1985; Bergaya, 1995; Lagaly, 1995; Konstantinou et al., 2000; Matthes and Kahr, 2000). Pillared clays are effective sorbents for anions at the positive sites of clays that were characterized as Bronsted and Lewis acid sites (Occelli and Tindwa, 1983; Zubkov et al., 1994; Polubesova et al., 2000; Sun Kou et al., 2000). The PC–herbicide formulations exhibited slow release for the nonionic herbicides alachlor and metolachlor (Gerstl et al., 1998; Nennemann et al., 2001). Crystal violet (CV)–montmorillonite complexes become positively charged with adsorption of CV above the cation exchange capacity (CEC) of montmorillonite (Rytwo et al., 1995) and thus might adsorb organic anions. We present results that show the suitability of both PC and CV–montmorillonite complexes for preparation of formulations that can reduce herbicide leaching.
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MATERIALS AND METHODS
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The pillared acid-activated clay was prepared in large quantities at the National Technical University of Athens and Straton Hi-Tec, as described in Jones et al. (1997). The acid-activated precursor montmorillonite is sold under the name Fulcat F22B (Laporte). The Al-pillaring of this matrix was performed with aluminum chlorohydrate (ACH) solutions. Typically, the intercalation of the polycations was performed in solutions of 6 mmol of aluminum per gram of clay. Exchange was performed at room temperature after aging of the aluminum solution at 80°C. The amount of intercalated aluminum is always lower in Fulcat F22B than in the parent clay prior to acid activation, reflecting the decreased cation exchange capacity of the clay after acid treatment. After calcination between 400 and 600°C, a 001 reflection is observed at 1.72 nm. The CEC of the clay is 0.53 molc kg-1, and the specific surface area is 256 m2 g-1 (Jones et al., 1997). The pH of a 0.5% suspension in distilled water was 4.63.
Wyoming Na–montmorillonite complex SWy-1 was obtained from the Source Clays Repository (Clay Minerals Society, Columbia, MO) and had a CEC of 0.80 molc kg-1.
Imazaquin, 97% purity, was obtained from AGAN Chemical Manufacturers (Ashdod, Israel). Acetochlor (2-chloro-2'-methyl-6'-ethyl-N-ethoxymethyl-acetanilide), 99% purity, was obtained from Chem Service (West Chester, PA). Crystal violet (CV) as the chloride salt was purchased from Fluka Chemie AG (Buchs, Switzerland). We purchased analytical-grade NaCl from Frutaron (Haifa, Israel); Na2SO4 from J.T. Baker (Phillipsburg, NJ); and CH3COONa and Na3PO4 from Merck (Darmstadt, Germany). The high performance liquid chromatography (HPLC)-grade methanol was obtained from BDH Chemicals (Poole, UK) and trifluoroacetic acid from Merck.
Sorbent Preparation
Imazaquin sorption on the sorbents was measured in the presence and absence of sulfate, acetate, or phosphate. Prior to imazaquin sorption measurements in the presence of other anions, PC was washed with distilled water (clay to water ratio was 1:30) to remove sulfate from the clay surface (unwashed clay contained 0.41 molc kg-1 of sulfate). Suspensions were centrifuged for 10 min at 15000 x g and the supernatant was discarded. After 10 washings the clay was air-dried. The amount of sulfate in the washed clay was reduced to 0.008 molc kg-1 and the pH of a 0.5% suspension of washed clay was 5.67 ± 0.05. X-ray diffraction (XRD) measurements of the clay-oriented samples showed that the c-spacing did not change after washing. Imazaquin sorption as described below in the presence of sulfate, acetate, or phosphate was measured on washed PC; sorption of the herbicide in the absence of these anions was determined on washed and unwashed PC.
The CV–montmorillonite complexes, containing 40 and 70% of CV above the CEC, were prepared according to Rytwo et al. (1995). In brief, CV solutions were added dropwise to clay suspensions under continuous agitation. The clay concentration was 1.67 g L-1. The suspensions were kept for 7 d under stirring at 25 ± 1°C. Supensions were centrifuged for 30 min at 15000 x g, supernatants were removed, and precipitates were freeze-dried and stored in plastic bottles at room temperature.
Sorption Experiments
The sorption experiments were performed in 50-mL centrifuge tubes by mixing 0.1 g of PC or CV complex with 20-mL solutions containing given amounts of imazaquin stock solutions from 11.45 x 10-3 to 121.5 x 10-3 mmol L-1. To study the effect of anions on imazaquin sorption, one of the following electrolytes was added at concentrations from 23.45 x 10-3 mmolc to 93.8 mmolc: Na2SO4, CH3COONa, and Na3PO4. Sorption of the neutral herbicide acetochlor on pillared clay in the presence of phosphate was also studied. Both acetochlor and phosphate were added in the same amounts as in the experiments with imazaquin, and the same experimental procedure was performed. In all experiments, the tubes were kept at 25 ± 1°C under continuous agitation for 24 h, which was sufficient for reaching sorption equilibrium of imazaquin, since the sorbed amounts were the same after 24, 48, and 72 h (data not shown). The tubes were then centrifuged for 20 min at 15 000 x g and imazaquin was measured in the supernatants as described below.
Desorption Measurements
Imazaquin desorption was determined in water and in electrolyte solutions. Desorption kinetics for another imidazolinone herbicide (imazethapyr) from soil was found to be much slower than adsorption (O'Dell et al., 1992; Johnson et al., 2000). It was supposed that there is some amount of herbicide that is extremely tightly or nearly irreversibly bound (Johnson et al., 2000). Three-step imazaquin desorption by water was studied. We assumed that loosely bound imazaquin will be desorbed within 2 h (first step), while strongly bound herbicide will be released in the next 24 h (second step) and after 7 d (third step). Twenty milliliters of distilled water were added to the samples with pre-adsorbed herbicide. The PC or CV–clay–imazaquin complex concentration was 5 g L-1. The tubes were kept at 25 ± 1°C under continuous agitation for 2 h, then centrifuged for 20 min at 15 000 x g, and imazaquin was measured in the supernatants as described below. The clay complexes were resuspended with 20 mL of distilled water, kept at 25 ± 1°C under continuous agitation for 24 h, and centrifuged, and imazaquin was measured in supernatants. Then, the clay complexes were again resuspended with 20 mL of distilled water and kept under agitation for 7 d, and imazaquin was measured in supernatants.
Imazaquin desorption by electrolyte solutions was measured after 24 h of agitation of PC and CV70%–clay–imazaquin complexes in 10 and 100 mmolc solutions of Na2SO4, CH3COONa, and Na3PO4. Sorption–desorption experiments were performed in triplicate, and pH was monitored in supernatant solutions.
For analysis, supernatants were passed through Teflon filters (ISI, Israel) of 0.2 µm pore diameter. Imazaquin was analyzed by high performance liquid chromatography (Merck Hitachi 6200) equipped with a diode array detector set at 242 nm. The column was a LiChrospher 100 RP-18 (5 µm; Merck) and the mobile phase was a mixture of 60% methanol and 40% of water with 0.65 mM trifluoroacetic acid at a flow rate of 1.0 mL min-1. Acetochlor was analyzed by high performance liquid chromatography with the same column; the mobile phase was 90% acetonitrile and 10% water at a flow rate of 0.5 mL min-1; UV detection at 216 nm was used.
Binding coefficients of imazaquin sorption were calculated with the Langmuir equation (Nir et al., 1994) and a model for ion adsorption, which is described later.
Preparation of Formulations
Imazaquin–PC and imazaquin–CV–montmorillonite formulations were prepared by adding aqueous solutions of imazaquin to the PC or CV–montmorillonite complexes. The concentrations of PC and CV–clay suspensions were 5 g L-1. Suspensions were kept at 25 ± 1°C under continuous agitation for 24 h and then centrifuged for 20 min at 15 000 x g. After centrifugation, the PC and CV–clay–imazaquin formulations were freeze-dried and stored in a plastic bottle at room temperature for the leaching experiments. Imazaquin load was 16.44 mmol kg-1 on PC, 22.1 mmol kg-1 on the CV70%–clay complex, and 22.7 mmol kg-1 on the CV40%–clay complex.
Leaching Experiments
Leaching experiments were performed in a Rehovot sandy soil (Rhodoxeralf; pH = 7.5, organic matter content = 0.2%, sand = 95.5%, silt = 3.3%, clay = 1.2%). Plexiglass columns (25 cm in length with a 5-cm i.d.) with 25-cm-long glass fiber wicks attached to the bottom were packed with air-dry soil. The bulk density of the column was 1.36 g cm-3. We then applied CaCl2 (0.01 M) at a rate of 47 mL h-1 for 29 h with a high performance liquid chromatography pump to establish steady state conditions. The flow was temporarily interrupted and solutions of either technical imazaquin or suspensions of the PC and CV70%–montmorillonite–imazaquin formulations were applied to the surface of columns, after which the application of the 0.01 M CaCl2 was resumed. The amount of imazaquin applied was 800 µg column-1 (800 µg 20 cm-2). Effluents were collected with an automated fraction collector and analyzed by high performance liquid chromatography. The water content of the soil was determined at the end of experiment. Pore volumes were calculated by dividing corresponding cumulative volumes of the collected effluent by the water content of the soil. One pore volume was 132 ± 8.2 mL. The amount of applied CaCl2 was about 1350 mL (10 pore volumes), which corresponded to 680 mm of rain. Imazaquin concentration approached very low values already at 3 pore volumes, that is, about 9 h. Two to four replicates were performed.
Modeling of Anion Adsorption to Positively Charged Surfaces
We used the equations and programs that were developed for cation adsorption (Nir, 1986), except for inverting the role of cations and anions. Briefly, the program considers anion binding to the positively charged surface sites of the organo–clay complex (e.g., CV–montmorillonite) and electrostatic Gouy–Chapman equations and solves iteratively for the solution concentrations of all anions in a closed system. Anion adsorption is obtained by considering binding and residence in the double-layer region. The enhancement in the concentrations of anions at the surface is given by:
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in which C-i
is the concentration of the given anion at the surface, C-i is its concentration far away from the surface, e is the absolute magnitude of the charge of an electron, k is Boltzmann's constant, T is absolute temperature, and 
0 is the surface potential. For a monovalent anion this factor amounts (at room temperature) to exp(0/25.2), in which 0 is given in mV.
The program takes into account explicitly that C-i is less than the anion concentration at the surface according to Eq. [1], and due to the complexation reaction with the positively charged sites (P+) of the surface, which is given by:
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with a binding coefficient Ki given by:
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RESULTS AND DISCUSSION
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Pillared Clay
The sorption isotherm of imazaquin on PC is presented in Fig. 2 . The imazaquin sorbed reached 72 to 93% of the amount added. The isotherm is nonlinear and Langmuir-type. The distribution coefficient Kd (ratio of the sorbed amount of imazaquin to the herbicide concentration in the equilibrium solution) decreased fivefold as sorption increased (Fig. 2). The binding coefficient (k) (the constant that characterizes the affinity of the sorbate to the sorbent) calculated with the Langmuir equation was 1500 M-1, which is significantly less than the binding coefficient previously found (12 000 M-1) for sorption of another anionic herbicide, sulfometuron, on pillared clay (Polubesova et al., 2000). The nonlinearity of the isotherm demonstrates the restricted number of sorption sites on the PC surface. Helmy et al. (1983) found nonlinear isotherms of quinoline sorption on Na–montmorillonite and silica–alumina. They explained the pattern in these isotherms by the reorientation of adsorbed molecules, which enables more dense packing. According to their X-ray diffraction data, only one layer of molecules is formed on the surface and thus the isotherms reached a plateau. It is possible that steric hindrance of the quinoline constituent of imazaquin (Fig. 1) can restrict the imazaquin adsorption on PC.
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Fig. 2. Imazaquin sorption on pillared clay. Broken and solid lines denote sorbed amount and Kd, respectively.
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The moderate increase in pH value from 5.7 ± 0.1 to 6.2 ± 0.1 with imazaquin sorption was not significant enough to assume inner-sphere complexation (exchange of imazaquin anion with surface hydroxyls) as a dominant mechanism of imazaquin sorption. We suggest the following mechanism for imazaquin sorption on PC: (i) electrostatic interaction of the anion with positively charged sites of pillars that are attributed to Lewis acidities (Schutz et al., 1987; Zubkov et al., 1994); (ii) hydrogen bonding of Bronsted acid sites with the carboxylate anion, oxygen, and nitrogen atoms of imazaquin. Both mechanisms provide less tight binding than inner-sphere complexation and may facilitate imazaquin desorption from the PC formulations.
The effect of sulfate, acetate, and phosphate on imazaquin sorption on PC is presented in Table 1. The experiment was performed with prewashed PC. Washing alone decreased imazaquin sorption on PC to 53 to 73% of the amount added as compared with unwashed clay due to a decrease of the amount of Al on the surface, which is responsible for the positively charged surface sites (Polubesova et al., 2000). Sulfate and acetate did not significantly affect the imazaquin sorption when added at 1:1 molar ratios, whereas the same amounts of phosphate decreased the imazaquin sorption by 32.6%. The same trend was observed whether 4.69 or 18.76 mmol kg-1 of imazaquin was added. The maximal reduction of imazaquin sorption by sulfate was 22.1 and 30.1% for 4.69 and 18.76 mmol kg-1 of imazaquin added at an imazaquin to sulfate ratio of 1:1000. Acetate more effectively reduced the imazaquin sorption on pillared clay than did sulfate. The maximal reduction of imazaquin sorption was 70.9 and 88.7% for 4.69 and 18.76 mmol kg-1 of herbicide added at an imazaquin to acetate ratio of 1:1000. Simultaneous addition of imazaquin and phosphate at imazaquin to phosphate ratios of 1/100 and 1/1000 resulted in negative imazaquin adsorption (Table 1), the magnitude of which increased with the increase in phosphate to imazaquin ratio. It appears that sulfate, acetate (in high concentrations), and phosphate compete with imazaquin for the surface sites. Phosphate competes with imazaquin more effectively than the other anions. Inner-sphere complexation was suggested as a mechanism for phosphate adsorption on oxides, and in particularly on 
-alumina (Sposito, 1984; He et al., 1997). Negative adsorption of the imazaquin anion can be explained by the reduction of the positive potential of the PC surface and by the increase of the negative charge of the surface due to phosphate adsorption on PC. A significant decrease in the imazaquin sorption to soils containing Al and Fe oxides in the presence of H2PO-4 was found by Regitano et al. (1997). In all cases, an increase in ionic strength enhanced the reduction of imazaquin sorption. Regitano et al. (1997) observed an increase in imazaquin sorption to soils with an increase in the ionic strength of background KCl and CaCl2 solutions from 0.03 to 0.3 M. This phenomenon could be explained by the release of anions (i.e., sulfates from the soil) by chloride at high ionic strength. Imazaquin was sorbed instead of sulfates; chloride anions probably do not compete with imazaquin as they do not compete with sulfometuron (Polubesova et al., 2000). The surface of washed PC was free from sulfates and increasing sulfate concentrations decreased the imazaquin sorption.
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Table 1. Effect of sulfate, acetate, and phosphate on the imazaquin sorption on pillared clay (4.69 mmol kg-1 imazaquin added).
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The changes in pH of the initial solutions (before they were combined with PC) with the increase of electrolyte concentrations were similar to the changes in pH of the supernatants (Table 1). The small increase of pH in supernatants containing sulfate as compared with initial solutions could be explained by the removal of the imazaquin acid from solution by its sorption. The increase of sulfate concentration in supernatant resulted in the decrease of imazaquin sorption and the decrease of supernatant pH. The formation of the sodium aluminum sulfate salts on the PC surface resulted in increased concentration of Al species in the supernatant. Hydrolysis of these species added to a decrease in the pH of supernatant solutions (Polubesova et al., 2000). Adsorption of the acetate on PC resulted in the partial removal of the anion from supernatant solution and thus less amount of acetate was available to hydrolysis. Hydrolysis of acetate added to the increase of solution pH. Decrease of acetate anions in supernatants resulted in lower pH values of supernatant solutions as compared with initial solutions. The same phenomenon was observed for the solutions containing sodium phosphate. The lower pH values of the supernatant solutions than those of initial solutions can be explained by the removal of phosphate from the solution due to its adsorption on PC. Thus, changes in pH did not mask the effect of anion competition with imazaquin, which is the dominant mechanism for the reduction of imazaquin sorption in the presence of anions.
Sorption of the nonionic herbicide acetochlor also decreased in the presence of phosphate (Table 2), but reduction of acetochlor sorption was much less pronounced than that of imazaquin, and no negative adsorption of acetochlor was found. Acetochlor can be sorbed on PC by the interaction of its oxygen atom with Bronsted sites of PC, which provide hydrogen bonding, and with Lewis sites of surface hydroxy-Al (Yariv, 1996; Nennemann et al., 2001) similar to the nonpolar metolachlor (Nennemann et al., 2001). The inner-sphere complexation of phosphate via exchange of surface hydroxyls decreases the possibility of hydrogen bonding, thus reducing acetochlor sorption, but to a smaller degree than in the case of the anion imazaquin. Sorption of acetochlor on PC without phosphate was found to be 67.1 to 68.3% of the added amount (Table 2), which is higher than previously reported for metolachlor (Nennemann et al., 2001). The pH values of supernatants in the experiment with acetochlor were similar to those in the experiment with imazaquin. As in the former case the lower pH values of the supernatant solutions than those of initial solutions can be explained by the removal of phosphate from the solution due to its adsorption on PC.
The results of imazaquin desorption in water from PC are presented in Table 3. After 2 and 24 h the percent desorbed increased from 10% for 4.32 mmol kg-1 to 21% for 17.3 mmol kg-1 imazaquin sorbed. After 7 d desorption increased to 15.3% for 4.32 mmol kg-1 and to 24.6% for 17.3 mmol kg-1. These results demonstrate that some amount of imazaquin is more strongly bound to PC and desorbed more slowly as was found for imazethapyr (O'Dell et al., 1992; Johnson et al., 2000). As a whole, imazaquin is less tightly bound to PC than the previously studied anionic herbicide sulfometuron, which under similar conditions desorbed only 3% (Polubesova et al., 2000).
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Table 3. Imazaquin desorption from crystal violet (CV)–mont-morillonite complexes and pillared clay (the suspension concentrations are 5 g L-1).
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It is important to study imazaquin desorption from formulations by electrolyte solutions since the soil solution contains electrolytes that may increase imazaquin desorption from formulations as compared with distilled water. Imazaquin desorption from PC by electrolyte solutions is presented in Table 4. Anions enhanced imazaquin desorption as compared with water. The most effective anion was phosphate. The explanation is that all three anions have strong affinity to the PC surface. The pH values of the supernatants in the desorption experiments were lower than those of desorbing electrolyte solutions due to the presence of desorbed imazaquin (Table 4).
Crystal Violet–Montmorillonite Complexes
The amounts of imazaquin sorbed on CV–clay complexes were higher than on PC and reached 93 to 95% of the amount added for CV40% and CV70% complexes (where 40% and 70% indicate that sorption of CV exceeds the CEC by 40 and 70%, respectively). Pure montmorillonite sorbed only 2% of the amount added. The Langmuir equation provides a good fit to the isotherms of imazaquin sorption on CV–montmorillonite complexes with R2 close to unity. Binding coefficients calculated with the Langmuir equation were higher than for imazaquin sorption on PC, with k = 3800 and 4000 M-1 for CV40%– and CV70%–montmorillonite complexes, respectively. The binding coefficients calculated with the model for ion adsorption (Nir, 1986) were 40 M-1 for CV40%– and 20 M-1 for CV70%–montmorillonite complexes with R2 = 1. It should be noted that the model considers explicitly larger anion concentrations near the surface for CV70%– than for CV40%–montmorillonite complexes. According to Eq. [1] the factor of enhancement of the concentrations of anions at the positively charged surface of a CV–montmorillonite complex is exp (o/25.2), in which o is the surface potential in mV. For instance, for 24 µmol of imazaquin added to CV40%–clay complex the calculations gave o = 114 mV, whereas for the CV70%–clay complexes, o = 143 mV. The enhancement of imazaquin concentration at the surface yields a larger binding coefficient for the CV40%– than for CV70%–montmorillonite complex and two orders of magnitude below the value deduced from the Langmuir equation, which does not account for this effect. At higher CV loadings, a significant number of the CV molecules interact among themselves and are less available for interactions with imazaquin, and consequently a larger binding coefficient is observed for the CV40% complex. Similar results were obtained for the interaction of benzyltrimethylammonium– and benzyltriphenylammonium–montmorillonite complexes with alachlor, metolachlor, and norflurazon (Nir et al., 2000) and for the interaction of rhodamine B–montmorillonite complex with atrazine (Borisover et al., 2001). The proposed mode for imazaquin sorption on CV–clay complexes is a combination of electrostatic adsorption and hydrophobic interactions. The aromatic nature of the CV molecules probably contributes to the hydrophobic interactions with imazaquin. The calculations of the binding coefficients with the ion adsorption model were preferred because they reflect both mechanisms of imazaquin sorption.
The effects of sulfate, acetate, and phosphate on imazaquin sorption on CV70%–clay complex are presented in Table 5 for 4.69 mmol kg-1 of imazaquin added. The same trend was found for 18.76 mmol kg-1 of imazaquin added (data not shown). Imazaquin sorption on the CV–clay complex, as observed on PC, was not significantly affected by small amounts of sulfate or acetate. Small amounts of phosphate added decreased imazquin sorption on the CV–clay complex much less than on PC (Tables 1 and 5). In all cases, an increase in ionic strength of electrolyte solutions reduced imazaquin sorption. When these anions were added in amounts that exceeded those of imazaquin by 100 and 1000 times, they competed with imazaquin for the surface sites of the CV–clay complex, and significantly reduced herbicide sorption according to the order phosphate > acetate > sulfate. Reduction of herbicide sorption on the CV–clay complex was lower than that on PC; no negative imazaquin adsorption was observed with phosphate on the CV–clay complex (Tables 1 and 5). The maximal reduction of imazaquin sorption with sulfate was 15.7% for 4.69 mmol kg-1 of imazaquin added, whereas with phosphate the reduction was 78.3% at an imazaquin to anion ratio of 1:1000.
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Table 5. Effect of sulfate, acetate, and phosphate on the imazaquin sorption on crystal violet (CV70%)–montmorillonite complexes (4.69 mmol kg-1 imazaquin added).
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Similar to the results obtained for PC the changes in pH of the initial solutions with the increase of electrolyte concentrations were akin to the changes in pH of the supernatants of CV–clay complexes. The pH values of supernatants in the presence of sulfate were higher than in the initial solutions due to the removal of imazaquin from solution by sorption. The pH values of the supernatants containing acetate and phosphate were lower than that of initial solutions due to the partial adsorption of acetate and phosphate on CV–clay complexes. The pH values of the supernatants are higher for all anions than that for the similar supernatants for PC. Hence, lower amounts of anions were adsorbed on the CV complex than on PC, which is consistent with lower reduction of imazaquin sorption on CV–montmorillonite complex than on PC in the presence of anions.
Imazaquin desorption from CV–clay complexes in water was less than from PC (Table 3) due to the higher affinity of imazaquin to the organo–clay surface containing aromatic molecules of CV. The desorbed amount, in the first and second step (loosely and strongly sorbed imazaquin), was 3.8 to 5.5% of sorbed imazaquin for both complexes. Crystal violet release from the complexes was 3.5 and 2.4% for CV70% and CV40% clays. After 7 d desorption increased to 7.1 to 7.8% for CV70%–clay complex. A certain fraction of imazaquin was more strongly bound to CV–clay complex and desorbed slower from organo–clay complex, similar to the desorption from PC.
Desorbed amounts of imazaquin from the CV–clay complex by electrolyte solutions are presented in Table 6. Phosphate was the most effective anion for imazaquin desorption. In all cases, herbicide desorption from CV–clay complexes was less than from PC. The explanation is that affinity of imazaquin to the aromatic part of CV molecules is higher than to PC surface, hence imazaquin is more tightly bound to CV–clay complex than to PC. Acetate caused desorption of a smaller amount of imazaquin from CV–clay complex than sulfate, though the reverse was found for PC. We found significant adsorption of acetate on PC (Polubesova et al., 2000). It is probable that acetate has lower affinity to CV than to PC and is less effective to displace strongly bound imazaquin to CV–clay complex than imazaquin bound to PC.
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Table 6. Imazaquin desorption from the crystal violet (CV70%)–montmorillonite complex in the presence of electrolytes.
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The higher pH values of the supernatants in the desorption experiments for CV clay than those for PC (Tables 4 and 6) are consistent with the lower amount of imazaquin desorbed from CV–montmorillonite complex.
Leaching of imazaquin from CV formulations is less than that from PC (Fig. 3)
and significantly lower than that of technical imazaquin. Only 25% of imazaquin was released from CV70%–montmorillonite formulations after 1.56 pore volumes versus 57% of imazaquin from PC formulations and 93% of technical imazaquin. At 0.86 pore volumes (equivalent to 59 mm rain), leaching of imazaquin from CV70%–clay complexes was 2%; at 1.03 pore volumes (equivalent to 70 mm rain), leaching of imazaquin from formulations was 11 versus 60% of technical imazaquin. Thus, the consistent results of leaching and batch experiments demonstrate stronger sorption and slower release of imazaquin from CV–montmorillonite formulations than from PC formulations.
Experiments performed with PC and CV70%–clay–imazaquin complexes showed that PC as well as CV70%–montmorillonite–imazaquin formulations were still as bioactive as the commercial formulation Scepter (American Cyanamid, Princeton, NJ) and supplied enough imazaquin to inhibit the root growth of sorghum [Sorghum bicolor (L.) Moench cv. RS610]. These data demonstrate the suitability of CV–montmorillonite complexes for preparation of herbicide formulations with reduced herbicide leaching.
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CONCLUSIONS
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- The isotherm of imazaquin sorption on PC is nonlinear, indicating a restricted number of sites for imazaquin sorption. The Langmuir equation provides a good fit to the isotherm.
- The Langmuir equation provides good fit to the isotherm of imazaquin sorption on CV–montmorillonite complexes. However, for charged complexes an equation that combines electrostatics with specific binding is preferred. The proposed mode for imazaquin interaction with CV–clay complexes is a combination of electrostatic adsorption and hydrophobic interactions.
- The rate of imazaquin desorption in water was much less from the CV complex than from PC.
- Sulfate, acetate, and phosphate decreased imazaquin sorption on both sorbents according to the sequence: phosphate > acetate > sulfate. The reduction was higher for PC than for the CV–clay complex. Imazaquin desorption in the presence of large concentrations of these anions is more extensive from PC than from CV–clay complex.
- Leaching of imazaquin from CV–montmorillonite formulations through soil columns was significantly less than leaching of the herbicide from PC formulations and that of technical imazaquin.
- The CV–montmorillonite complexes are more effective sorbents for imazaquin than PC. The CV70%–imazaquin formulations were as active as the commercial formulation Scepter and supplied enough imazaquin to inhibit the root growth of sorghum. The CV–montmorillonite complexes appear to be suitable for preparation of organo–clay–imazaquin formulations, which may reduce imazaquin leaching significantly.
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ACKNOWLEDGMENTS
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This research was supported by Grant 1317 from Israeli Ministry of Science, Culture and Sport and by Grant G-641.106.8/1999 from G.I.F., the German–Israeli Foundation for Research and Development.
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