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J. Agric. Food Chem. 2004, 52, 3888−3893
Use of Activated Bentonites in Controlled-Release Formulations of Atrazine MANUEL FERNAÄ NDEZ-PEÄ REZ,* FRANCISCO FLORES-CEÄ SPEDES, EMILIO GONZAÄ LEZ-PRADAS, MATILDE VILLAFRANCA-SAÄ NCHEZ, SUSANA PEÄ REZ-GARCIÄA, AND FRANCISCO JAVIER GARRIDO-HERRERA Departamento de Quı´mica Inorga´nica, Universidad de Almerı´a, Almerı´a 04120, Spain
The herbicide atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) was incorporated in alginate-based granules to obtain controlled-release (CR) properties. The basic formulation [sodium alginate (1.40%)-atrazine (0.60%)-water] was modified by the addition of sorbents. The effect on atrazine release rate caused by the incorporation of acid-treated bentonite (0.5 and 2.5 M H2SO4) in alginate formulation was studied by immersion of the granules in water under static conditions. The water uptake, sorption capacity of the sorbent, permeability, and time taken for 50% of the active ingredient to be released into water, t50, were calculated for the comparison of the preparations. t50 values were longer for those formulations containing acid-treated bentonite (36.78 and 29.01 days for 0.5 and 2.5 M H2SO4 treatments, respectively) than for the preparation without bentonite (9.69 days). On the basis of a parameter of an empirical equation used to fit the herbicide release data, it appears that the release of atrazine from the various formulations into water is controlled by diffusion mechanism. The sorption capacity of the sorbents and the permeability of the formulations (ranging from 4.99 to 20.83 mg day-1 mm-1) were the most important factors affecting herbicide release. KEYWORDS: Atrazine; controlled release; activated bentonite; sorption
INTRODUCTION
Public awareness of the health hazards of pesticides has increased as a consequence of the pollution of the water environment by these compounds. This fact has stimulated interest in developing new formulations to reduce the impact of pesticides in the environment. Controlled-release (CR) pesticide formulations have several advantages over standard formulations, including increased safety for the user and nontarget organisms, a reduction in the amount of pesticide applied, and reduced leaching potentials (1-4). Many different types of CR systems have been developed for use in agriculture (5-7) including those that use a natural polymer such as alginate as a matrix for controlled release of pesticides (8-11). Besides the benefits of delayed release profiles of the pesticide, alginate formulations do not disintegrate in soil as readily as other formulations of a related nature (12). Atrazine is a generally applied herbicide (13) and has been identified as a potential leacher by using the groundwater ubiquity scores (GUS) modeling technique (14). Furthermore, atrazine has been widely detected in groundwater (15, 16). CR formulations of atrazine would potentially extend its commercial lifetime, reduce risks of leaching or runoff on application, and be safer for the user and nontarget organisms (17). Several * Address correspondence to this author at the Departamento de Quı´mica Inorga´nica, Universidad de Almerı´a, La Can˜ada de San Urbano s/n, 04120 Almerı´a, Spain (telephone 34 950 015961; fax 34 950 015008; e-mail
[email protected]).
modifying agents have been used in CR systems to obtain different release profiles (9, 18) including those that used natural clays (11, 19, 20). Previous studies have shown that acid treatment given to the clays produced an increase in the specific surface area of the samples (21-23) and also improved their adsorptive characteristics (24, 25), so the present study evaluated the potential use of two acid-treated bentonite samples as modifying agents in alginate-based CR formulations of atrazine. The acid-treated bentonite samples were obtained from native bentonite clay from the region of Almerı´a and were previously studied and characterized by the present authors (21). The objective of the present work was to evaluate the effect of incorporation of acid-treated bentonite into alginate-based formulations on the rate of atrazine release. We intend also to obtain a deeper understanding of the release mechanism of the herbicide from the investigated formulations. Therefore, CR formulations were prepared by incorporating the herbicideclay mixtures into alginate granules by using calcium chloride as gellant. The release rate of active ingredient from alginatebased CR granules was studied using a static immersion water test. The factors influencing the controlled release of atrazine were examined by studying the interactions between the active ingredients and the sorbents, in particular, the sorption processes of atrazine with acid-treated bentonite samples. MATERIALS AND METHODS Materials. A natural bentonite (98% montmorillonite, containing sodium as exchange ion), previously described by Gonza´lez-Pradas et
10.1021/jf030833j CCC: $27.50 © 2004 American Chemical Society Published on Web 05/22/2004
Activated Bentonites in Controlled-Release Atrazine
J. Agric. Food Chem., Vol. 52, No. 12, 2004
Table 1. Characteristics of the Sorbents
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Table 3. Characteristics of Controlled-Release Granules Containing
Atrazine sample
pH
surface area (m2 g-1)
B-0.5 B-2.5
2.78 3.31
183.0 484.8
cation exchange capacity (mequiv/100 g)
exchange acidity (mequiv/100 g)
76.9 54.4
67.4 50.4
AAL AALB-0.5 AALB-2.5
Controlled-Release Formulations Containing Atrazine
AAL AALB-0.5 AALB-2.5 a
sodium alginate (%)
0.60 1.20 1.20
1.40 1.40 1.40
B (%)
B-0.5 (%)
B-2.5 (%)
water (%)
pH
5.00
98.00 92.40 92.40
7.51 (0.13)a 3.98 (0.03) 4.13 (0.05)
5.00
Ca2+ (%)
av wt (mg/granule)
encap effica (%)
19.34 (0.22)b 12.33 (0.12) 13.01 (0.18)
10.85 (1.25) 10.54 (0.84)
0.93 (0.05) 2.62 (0.08) 2.92 (0.12)
90.91 96.32 97.30
a Encapsulation efficiency) (amount of pesticide in dry product/amount of pesticide in formulation processed) × 100. b Values in parentheses represent the standard deviation.
Table 2. Percentage (by Weight) of Component and pH of atrazine (%)
atrazine (%)
Values in parentheses represent the standard deviation.
al. (26), was used for the preparation of acid-treated bentonite samples. Acid activation of the bentonite was carried out in a boiling solution of H2SO4 (concentrations ) 0.5 and 2.5 mol L-1) for 1 h. In all cases the resulting samples were heated at 105 °C to constant weight. Chemical composition and textural properties of the samples were also previously studied by the present authors (25). The two products so obtained are labeled in the text as B-0.5 and B-2.5. Bentonite samples were analyzed by standard methods. Sample pH was determined in a 1:2.5 clay/water suspension using a glass electrode (27). Specific surface area was determined from N2 adsorption isotherms using a surface analyzer from Micromeritics, model II-2370; cation exchange capacity (CEC) and exchange acidity were calculated by using the barium acetate method (28). Properties of the acid-treated bentonite samples are shown in Table 1. Technical grade atrazine (98.0%) was kindly supplied by Ciba-Geigy, S.A. Solvents used in the mobile phase for high-perfomance liquid chromatography (HPLC) determinations were HPLC grade acetonitrile (Merk) and water of demineralized Milli-Q quality (Millipore Corp.). Chemical products used in the preparation and evaluation of CR formulations were sodium alginate (medium viscosity ) 3.5 kg m-1 s-1 for 2% solution) and tripolyphosphate (90-95%) obtained from Sigma Chemical Co. (St. Louis, MO) and calcium chloride (95%) from Panreac (Barcelona, Spain). Preparation of Controlled-Release Formulations. The CR formulations prepared were based on the gelling properties of the alginate in the presence of divalent cations. Formulations containing different percentages of technical grade atrazine (A), sodium alginate (AL), and acid-treated bentonite samples (B-0.5 and B-2.5) were made up in water (as shown in Table 2) and vigorously stirred for 1 h. The alginate mixtures (100 g) were dropwise added to a 300 mL gellant bath of 0.25 M CaCl2 using the apparatus described by Connick (29). The resulting beads were allowed to gel in the 0.25 M CaCl2 solution for a total of 20 min, then they were filtered and allowed to dry first at room temperature and then in an oven (40 °C) to constant weight. The products so obtained will be labeled in the text as AAL, AALB-0.5, and AALB-2.5. Analysis of Herbicide and Calcium in Granules. The actual concentration of atrazine in the dry products was determined by dissolving 20 granules in a 0.03 M tripolyphosphate solution (20 mL) following an extraction into a water/methanol (80:20) mixture (300 mL) using an ultrasound bath for 15 min. The resulting extract was filtered using nylon filters (0.45 µm) and the herbicide concentration determined by high-performance liquid chromatography (HPLC). The HPLC operating conditions were as follows: separation by isocratic elution was performed on a 150 × 3.9 mm Nova-Pack LC-18 bondedphase column (Waters, Millipore Corp.): sample volume, 20 µL; flow rate, 1.0 mL min-1; mobile phase, acetonitrile/water mix (60:40). Atrazine was analyzed at 222 nm, its wavelength of maximum absorption. External standard calibration was used, and three replicates were carried out for each formulation.
Calcium content was also determined in the extract by atomic absorption spectrometry using a 1100 B Perkin-Elmer spectrometer. Water Uptake Tests. The water uptake was measured for granules using the method of Franson and Peppas (30). Granules were immersed in water using stoppered conical flasks and shaken in a thermostated bath at 25 ( 0.1 °C. Then, they were removed periodically (for intervals see Results and Discussion). The excess water was blotted from the surface of the granules using filter paper and, after that, granules were weighed. Three replicates (10 granules in each replicate) were carried out. The granules were then allowed to dry first at room temperature and then in an oven (40 °C) to constant weight, so the water uptake (grams per gram of dry granule) could be calculated. Sorption Studies. The sorption experiments were carried out as follows: Aqueous calcium chloride solutions (0.02 M) containing initial atrazine concentrations (C0) ranging between 2 and 24 mg L-1 were prepared. Amounts of 0.5 g of each bentonite sample and atrazine solution were placed in stoppered conical flasks and shaken in a thermostated bath at 25 ( 0.1 °C. Preliminary experiments were conducted for various time intervals to determine when sorption equilibrium was reached. The time required for equilibrium to be reached between atrazine sorbed and atrazine in solution was 24 h. After shaking, the solutions were centrifuged and the concentration of atrazine in the supernatant was determined by HPLC using the method described above. The atrazine sorbed was calculated from the difference between initial and final solution concentrations. Blanks containing no atrazine and three replicates of each sorption point were used for each series of experiments. Water Release Kinetics. An accurately weighed quantity of dry CR granules containing 20.0 mg of atrazine in each sample (three replicates) was immersed in water (50 mL), whereas 5.3 mg of technical grade atrazine (three replicates) was added to 500 mL of distilled water. The systems were shaken in a thermostated bath at 25 ( 0.1 °C. At different time intervals, aliquots of 1 mL were removed for determination of atrazine by HPLC as described above. For CR granules, all water was replaced with fresh to minimize saturation effects, whereas for the technical grade herbicide, unused sample portions were returned to the flasks. The experiment was concluded once 90% of the active ingredient initially present in the granules was released. To establish the mass balance, the concentration of atrazine remaining in the granules was calculated. For that, granules were removed from each flask and the herbicide concentration was determined as above-described. The amounts of atrazine released plus atrazine remaining in the granules at the end of the experiment were in agreement with the total atrazine initially present in the granules. Total averaged recovery of atrazine from all the samples was 99.9 ( 0.2%. RESULTS AND DISCUSSION
Controlled-Release Formulations. Characteristics of alginate-based CR granules containing atrazine are presented in Table 3. The granules were generally spherical in shape, and the technical grade atrazine was readily incorporated in the alginate matrix, obtaining granules of size between 0.3 and 0.91 mm. The herbicide content of the dried granules is adequate for a practical agricultural application where good coverage is needed. The addition of acid-treated bentonite samples to the
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Ferna´ndez-Pe´rez et al.
Figure 2. Sorption isotherms for atrazine on the B, B-0.5 and B-2.5
samples (error bars represent standard deviation of three replicates). Figure 1. Water uptake of atrazine granules over time (error bars represent
Table 4. Freundlich Coefficients, Kf and nf, for the Sorption of Atrazine
on the Bentonite Samples
the standard deviation of three replicates).
alginate formulation led to larger and heavier granules that were more spherical and dried with less aggregation. In all cases, the encapsulation efficiency was >90.91%, the higher values corresponding to the formulations prepared with acid-treated bentonite samples. Water Uptake. The water uptake of the granules versus time is shown in Figure 1. Water uptake curves were characterized by a fast initial uptake of water by the granules, after which an apparent equilibrium or slow water uptake was observed. Water uptakes by granules containing acid-treated bentonite samples (AALB-0.5 and AALB-2.5) were less than that obtained for the alginate formulation prepared without bentonite (AAL). Lack of structural expansion of the bentonite, together with the possible formation of bonds with the other components of the matrix on drying, may be responsible for the lower water uptake. This behavior was also observed and reported for other clays where swelling of extruded starch formulations of atrazine was reduced by the addition of increasing amounts of attapulgite, and no swelling was observed when the clay and herbicide were the only components extruded (19). Granules containing B-2.5 as modifying agent took up more water than the formulation containing B-0.5. This fact may be explained, according to a previous paper (21), by the gradual destruction of the bentonite periodic structure as the concentration of sulfuric acid used for the bentonite acid treatment increases (from 0.5 to 2.5 M), which occurs together with the formation of free silica, generating a higher percentage of microporosity and producing an increase in the specific surface area of the samples. The presence of a higher amount of SiO2sthat could be solvated by watersin B-2.5 could increase the amount of water uptake by AALB-2.5 formulations. The extent of swelling of the bentonite determines the volume occupied in the matrix and the area that can interact with the diffusing molecules of herbicide. Thus, water uptake and intensity of interactions of the herbicide with either B-0.5 or B-2.5 could affect the diffusion through the granules and, hence, the release of the active ingredient. Sorption Studies. The sorption isotherms of atrazine on the natural untreated bentonite (B) and two acid-treated bentonite samples studied are shown in Figure 2. According to the initial portion of the curves, these isotherms may be classified as L type of the Giles classification (31), which suggests both that the samples have an average affinity for the herbicide atrazine and that there is no strong competition from the solvent for sorption sites.
sample
Kf (mg kg-1)
nf
ra
B B-0.5 B-2.5
0.28 ± 0.11b 80.86 ± 6.39 46.34 ± 4.53
1.15 ± 0.14b 0.62 ± 0.03 0.54 ± 0.04
0.998* 0.996* 0.994*
a
An asterisk indicates significance at the 0.001 probability level. b These values represent the 95% confidence limits.
The sorption isotherms in Figure 2 were compared using the Kf parameter of the Freundlich equation (32). The linear form of this equation is
log X ) log Kf + nf log C
(1)
where X is the amount of atrazine sorbed (mg kg-1 of sorbent), C is the equilibrium solution concentration (mg L-1), and Kf and nf are constants that characterize the sorption capacity of the herbicide. The constant Kf is the amount of herbicide sorbed for an equilibrium concentration of 1 mg L-1, and nf is a measure of the intensity of adsorption and reflects the degree to which adsorption is a function of the concentration (33). The Kf and nf values were calculated from the least-squares method applied to the linear form of the Freundlich equation, and their values are summarized in Table 4. The correlation coefficients (r) were in all cases >0.99, the correlation being significant at p ) 0.001. As can be seen from Table 4, Kf values are 80.9 mg kg-1 for B-0.5 and 46.3 mg kg-1 for B-2.5, these values being much higher than that obtained for adsorption of atrazine on natural untreated bentonite (34). Increased adsorption of atrazine on bentonite samples is probably due to the decrease of the pH generated (35). The decrease of the pH leads to a greater protonation of atrazine, and then a greater sorption takes place by a cation exchange mechanism. This mechanism could operate complementarily to the sorption of nonprotonated species throughout relatively weak interactions such as van der Waals forces and hydrogen bonds (36). The lower pH generated and the higher cation exchange capacity of the B-0.5 sample compared with those observed for B-2.5 could lead to a higher sorption capacity of B-0.5 sample for atrazine. Release Studies. The cumulative release of atrazine from alginate-based CR granules and the solubility profile for technical grade atrazine are shown in Figure 3. As can be seen, 89.78% of technical grade product is dissolved in
