Environ. Sci. Technol. 1999, 33, 4221-4225
Adsorption-Desorption of Simazine on Montmorillonite Coated by Hydroxy Aluminum Species F. SANNINO, M. T. FILAZZOLA, A. VIOLANTE, AND L. GIANFREDA* Dipartimento di Scienze Chimico-Agrarie, Via Universita` 100, 80055 Portici, Napoli, Italy
Adsorption-desorption of simazine on model systems which closely simulate those encountered in soils [i.e., pure montmorillonite and montmorillonite covered by different amounts of OH-Al species (chlorite-like complexes)] was investigated. The adsorption data were analyzed according to Langmuir and Freundlich equations. Results obtained indicate that the adsorption of simazine was strongly influenced by the charge characteristics of clay minerals and the pH of buffer (3.7 and 5.6). At pH 3.7, simazine was adsorbed in large amounts on montmorillonite, whereas adsorption significantly decreased with increasing amounts of Al(OH)x species on montmorillonite surfaces. When experiments were carried out at pH 5.6, the adsorption of the herbicide on montmorillonite drastically decreased, whereas no severe differences were observed with AM18 (a montmorillonite loading 18 mequiv g-1 M). Desorption tests showed that negligible and very little amounts of simazine were desorbed from montmorillonite and AM18, respectively. Strong electrostatic interactions were involved in the adsorption process. Simazine molecules, arriving at support interfaces mostly as molecular species, dissociated as cations by the microenvironmental pH (“surface acidity”) and were adsorbed by cation-exchange mechanism.
Introduction The adsorption of pesticides on soil constituents (phyllosilicates, humic acids, and so on) has been extensively studied (1, 3). Although organic matter fraction might be a significant factor in the adsorption of pesticides (in particular triazine herbicides), clay minerals may also have a greater potential for adsorption of pesticides due to their large surface area and their predominance in agricultural soils. In acid soils, such as Ultisols, Alfisols, and Podsols, hydrolytic products of aluminum or iron usually coat the surfaces of phyllosilicates. Hydroxy interstratified minerals show modified physicochemical properties (4) and differently interact with nutrients and agrochemicals. Limited information is available on the interaction of pesticides with Al or Fe oxides or Al(OH)x-clay complexes. In a recent paper, Sannino et al. (5) have demonstrated that a montmorillonite and chlorite-like complexes [Al(OH)xmontmorillonite], obtained by coating montmorillonite surfaces with different amounts of Al(OH)x species, showed different capacities with regard to adsorb 2,4-dichlorophenoxyacetic (2,4-D), a weakly acid herbicide. * Corresponding author phone: (39-81)7885211/7885225; fax: (3981)7755130; e-mail:
[email protected]. Dipartimento di Scienze Chimico-Agrarie, Universita` di Napoli “Federico II”, Via Universita` 100, 80055 Portici, Napoli, Italy. 10.1021/es9813313 CCC: $18.00 Published on Web 10/16/1999
1999 American Chemical Society
The purpose of the present paper was to study the adsorption/desorption processes of a weakly basic herbicide such as simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] on the same supports. Simazine, a triazine herbicide, is a weakly basic organic molecule, easily adsorbed by natural inorganic clays (6). This herbicide presents a soil and foliar action on young weeds and is generally used to control broadleaved and grass weeds and in deep-rooted crops such as asparagus, berry crops, broad beans, and coffee (7).
Experimental Section Clay Minerals. A Na-saturated montmorillonite (M) from Crook (U.S.A.) and three Al(OH)x-montmorillonite (chloritelike) complexes, obtained by covering montmorillonite surfaces with increasing quantities of OH-Al species, 3 (AM3), 9 (AM9), and 18 (AM18) mequiv Al g-1 clay, were prepared according to the procedure described by Sannino et al. (5). The chemico-physical properties of samples were determined as described elsewhere (5). The values of both the cation exchange capacity (CEC) and surface area drastically decreased (CEC from 59.08 to 16 mequiv 100 g-1 and surface area from 812 to 180 m2 g-1) by increasing Al(OH)x sorbed on montmorillonite surfaces from 0 (M) to 18 mequiv of Al g-1 clay (AM18), respectively. Due to the interlayering of Al(OH)x species in the complexes, the d spacings increased from 1.170 (M) to 1.280 (AM3), 1.470 (AM9), and 1.508 nm in the case of AM18. Chemicals. 2-Chloro-4,6-bis (ethylamino)-s-triazine (simazine; 99.6% purity) was purchased from Dr. Ehrenstorfer, GmbH, Germany. HPLC solvents were from Lab Scan, Ireland. All other chemicals (analytical grade) were from Serva GmbH (Germany). Experimental Conditions. Kinetic experiments were carried out with montmorillonite in 0.01 M acetate buffer at pH 3.7 and using an initial concentration of 4 µg mL -1 simazine. Samples were kept at 25 °C for different incubation times, and the equilibrium concentration was measured as described below. Adsorption of simazine on clays was performed by incubation of clay (50 mg) and pesticide (0.5-7 µg mL-1 simazine) in 2.5 mL (solid/liquid ratio of 20) of 0.01 M acetate buffer (pH 3.7 and 5.6) for 7 h at 25 °C. The amounts of simazine used per gram of clay were not so far from usual field application rates (6-8 Kg/ha) (8). The following experimental procedure was used. A clay-buffer suspension (25 mg mL-1) was prepared and sonicated for 10 min prior to simazine addition. Different volumes (6.25-87 µL) of a stock simazine solution in methanol (200 µg mL-1) were added to 2 mL of the clay-buffer suspension, and buffer was added to a 2.5 mL final volume. After centrifugation at 10 000g for 15 min, the residual herbicide in the supernatants was measured by HPLC analysis. The pH refers to clay-buffer-simazine suspensions, and no changes were measured in the supernatants after centrifugation. The HPLC analyses were carried out with a Varian apparatus, equipped with a Varian Mat pump and vari-Chrom variable wavelength absorbance detector set at 220 nm. A Speri-5-RP18 22 cm × 4.6 mm C18-80 column (BrownLee) of 5 µm particle size was utilized; isocratic elution was performed at a flow rate of 1 mL min-1 with acetonitrile + water (65 + 35 by volume) as mobile phase. The amount of simazine sorbed on clays was calculated as the difference between the initial quantity of herbicide added and that present in the equilibrium solution. Adsorption isotherms were calculated plotting the amount of VOL. 33, NO. 23, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Kinetics of adsorption of simazine on M at 25 °C and pH 3.7 in 0.01 M acetate buffer (a) and amount of simazine adsorbed on M plotted against square root of time (b). simazine sorbed on matrices versus equilibrium herbicide concentration. The adsorption data over the range of concentrations studied were plotted according to the Langmuir and Freundlich adsorption equations (9). The values of distribution coefficients Kd, defined as the ratio of the concentration of simazine sorbed per unit weight of the clay to its equilibrium concentration (mL g-1), were determined at saturation levels (at the maximum adsorption value). Desorption experiments were performed with M and AM18, initially treated with 2-4 and 1-3.5 µg mL-1 of herbicide, respectively. After incubation for 7 h at 25 °C, 2.5 mL of equilibrium solution was removed from supernatants, and equal volumes of acetate buffer 0.01 M pH 3.7 were added to the samples. The suspensions were shaken at 25 °C, centrifuged for different contact times (1, 3, 5, 7, and 24 h), and centrifuged at 10 000g for 15 min. The procedure was repeated twice. The residual concentration of herbicide still adsorbed on clays was calculated as the difference between the initial adsorbed and the desorbed amount evaluated in both the washing solutions. Oriented aggregate specimens of the K-saturated samples for X-ray diffraction (XRD) patterns were obtained by drying aliquots of samples on glass slides. XRD patterns were obtained with a Rigaku diffractometer with Co-Ka radiation generated at 40 kV and 30 mA. Control tests at both pH 3.7 and 5.6 were carried out to evaluate, if any, the hydrolysis of simazine in the presence of acetic acid. All experimental data were conducted in triplicate, and the relative standard deviation (SD) was lower than 4%.
Results Adsorption Experiments. Kinetic studies were conducted to establish the time needed to reach adsorption equilibrium. The herbicide was adsorbed rather rapidly reaching the adsorption equilibrium within 2-3 h (Figure 1a), and no appreciable changes in the sorbed amount were observed after more than 24 h. When the amounts of sorbed simazine were plotted against the square root of time (Figure 1b), a 4222
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linear segment in the curve was observed after 2 h incubation. An incubation period of 7 h was then adopted to ensure that equilibrium was reached. The adsorption isotherms for simazine at 25 °C and pH 3.7 in 0.01 M acetate buffer with M, AM3, AM9, and AM18 complexes showed typical Langmuir characteristics (Figure 2). The best-fitting curves through the experimental data were calculated by a computed nonlinear regression analysis, according to the Langmuir equation by the least-squares method. Simazine isotherms showed different shapes and were distinguished by the initial slope. The adsorption of simazine on M and AM3 increased more slowly with increasing herbicide equilibrium concentration, thus approximating a L-form (9). On the contrary, adsorption of simazine on AM9, and mainly on AM18, followed a high-affinity behavior (Htype isotherm), because it rapidly rose at low pesticide equilibrium concentration and suddenly reached a constant asynthotic value (Figure 2c,d). The data were also fitted by Freundlich equation, as demonstrated by the high values of correlation coefficients r 2 (Table 1). The values of Langmuir and Freundlich constants indicate that M showed the highest sorptive capacity, i.e., xm and K values which decreased by increasing the amounts of OHAl species coating the surfaces of montmorillonite. On the contrary, the trend of k and n values seems to indicate that simazine was adsorbed with both higher binding energy and sorptive intensity when greater amounts of Al species were present in the mineral. The amount of herbicide adsorbed on clays at pH 3.7 followed the order M ∼ AM3 > AM9 . AM18. For example, at 0.6 µg mL-1 equilibrium concentration of herbicide, the amount of sorbed simazine was almost equal for M (260.5 µg g-1clay) and AM3 (258.7 µg g-1clay), while it significantly decreased by 4-fold for AM18 (63.5 µg g-1 clay). This behavior was widely confirmed by the distribution coefficients Kd which decreased with increasing quantities of Al(OH)x species held on montmorillonite surfaces (Table 1). When experiments were carried out in the same buffer but at pH 5.6, the adsorption of simazine on montmorillonite drastically decreased, in contrast to the same differences observed with AM18 (Figure 3a,b). In particular, the maximum amount (xm) of adsorbed simazine on montmorillonite decreased 9-fold from 373.7 at pH 3.7 to 41.0 at pH 5.6 (Figure 3a). A concomitant reduction (5-fold) of Langmuir-k value was measured. The Freundlich parameters K and n accordingly decreased, thus confirming the Langmuir parameter trend (Table 1). The amount of simazine adsorbed on AM18 at pH 5.6 was only 1.2-fold lower than that adsorbed at pH 3.7. Accordingly, the distribution coefficient Kd for M decreased significantly, whereas that for AM18 remained practically unchanged. Desorption results obtained with M and AM18 showed that very little amounts of simazine were desorbed from the two samples. Negligible quantities (not detectable) of the herbicide were removed by simazine-montmorillonite complexes containing 116.05 and 210.25 µg of herbicide adsorbed per g-1 M after the first washing (time ) 0 h) and varying the exposure time (1, 3, 5, 7, and 24 h) between the herbicide-M complex and washing solution. A similar behavior was observed with two simazine-AM18 complexes containing respectively 28.6 and 64.0 µg of adsorbed herbicide g-1 AM18. Only after 24 h contact, about 2.0 and 7.0 µg g-1 clay corresponding to 7.0 and 11.0% of simazine initially adsorbed were removed from the two complexes. This behavior cannot be ascribed to a possible acidcatalyzed hydrolysis of simazine adsorbed by two minerals. As reported by Grayson (10), acetic acid may behave as a catalyst in the hydrolysis of chlorotriazines. Control tests performed under experimental conditions similar to those
FIGURE 2. Adsorption isotherms of simazine at 25 °C and pH 3.7 in 0.01 M acetate buffer on M (a), AM3 (b), AM9 (c), and AM18 (d).
TABLE 1. Langmuir and Freundlich Parameters and Kd Values for the Adsorption of Simazine by Clay Minerals at 25 °C Langmuir parameters complex
xm
k
M AM3 AM9 AM18
373.75 352.23 260.36 80.59
3.66 3.37 4.05 4.11
M AM18 a
Kd (mL g-1)
Freundlich parameters
r 2a
K
n
r 2a
SD
pH 3.7 0.98 427.46 1.42 0.94 458.0 0.50 0.99 311.31 1.94 0.99 383.3 0.10 0.97 260.55 4.24 0.96 269.5 0.50 0.97 58.65 4.39 0.83 19.3 0.40
41.07 0.78 0.98 66.20 5.12 0.99
pH 5.6 16.43 1.25 0.99 52.05 5.20 0.98
16.0 0.22 21.1 0.06
Correlation coefficients.
adopted in desorption experiments ruled out any hydrolysis of simazine molecules.
Discussion The shape of the curve shown in Figure 1a reveals the presence of two kinds of retention, a fast one followed by slow one. This could indicate the presence of a diffusioncontrolled process. The linearity observed after ∼3 h incubation in Figure 1b confirmed this assumption. A similar behavior was observed by Kalouskova (11) and Sannino et al. (5) in their studies on the adsorption of simazine on two humic acids and of 2,4-D on AM18, respectively. The charge characteristics of clay minerals and the pH of the buffer (3.7 or 5.6) strongly influenced the adsorption process. According to results reported by several authors (12, 13) simazine was adsorbed in large amounts on montmorillonite, whereas adsorption significantly decreased with increasing amounts of Al(OH)x species on montmorillonite surfaces. The adsorption of large quantities of herbicide on montmorillonite observed at pH 3.7 is widely supported by the results reported previously (14). Simazine is a basic compound belonging to an important family of herbicides, the s-triazines. These herbicides, heterocyclic organic bases, are strongly adsorbed by smectites.
FIGURE 3. Adsorption isotherms of simazine at 25 °C and pH 5.6 in 0.01 M acetate buffer on M (a) and on AM18 (b). Lines refer to the adsorption isotherms obtained on M and AM18 at pH 3.7 and illustrated in Figure 2 (parts a and d, respectively). Plant studies showed that these compounds exhibited less phytotoxicity in soils of low pH compared to soils at higher pH levels. This effect was attributed to the higher adsorption by soil colloids at the lower pH (15). The adsorption of s-triazines on clays increases as the suspension pH is lowered, reaching a maximum when the pH approaches the pKa of the compound and then decreases. Ainsworth et al. (16) suggested that when the pH of the smectite suspension is greater than the pKa of the pesticide, protonated species are adsorbed preferentially over neutral species. This prefVOL. 33, NO. 23, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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erential adsorption of protonated species is ascribed to “surface acidity” (17) that promotes protonation of weak bases as they are adsorbed (18, 19). The surface acidity of smectites, which can be up to four pH units lower than the pH of the bulk solution (20, 21) is due probably to Bronsted acidity that arises from the enhanced ionization of water molecules in solvation spheres of adsorbed inorganic cations. Therefore at pH 3.7, adsorption mechanism of simazine by montmorillonite as protonated species could be preceded by initial adsorption as molecular species. The contribution to adsorption deriving from simazine molecules which preexist as protonated species in the bulk solution is practically zero. On the basis of a pKa) 1.65 (22) at pH 3.7 and in the range of herbicide concentrations used in adsorption isotherm experiments, the concentration of protonated simazine is lower by 2 orders of magnitude than that of neutral molecules. In conclusion, simazine molecules arrive at M interface mostly as molecular species, dissociate as cations by the microenvironmental pH, and are adsorbed by cation-exchange mechanism. This point of view is in agreement with the results reported by Celis et al. (23) who found that simazine was highly adsorbed (∼100%) on an Fe3+-saturated montmorillonite by a similar cation-exchange mechanism. They also demonstrated that the protonation of atrazine and simazine at clay interfaces would involve a movement from hydrophobic to hydrophilic sites on the clay surface, so new hydrophobic sites would become available for molecular species in solution (24). Russell et al. (25), showed that uncharged atrazine molecules can be protonated on the heterocyclic ring nitrogen and then adsorbed on the negatively charged surfaces of soil particles. The shape of adsorption isotherms (Figure 2) and the values summarized in Table 1 indicate that the covering of M surface with OH-Al species determined a negative effect on the quantities of herbicide adsorbed. In particular, decreasing amounts of the herbicide were held on Al-coated clays, i.e., xm values decreased by increasing OH-Al species from 3 to 18 mequiv of Al g-1 clay. Apparently, a higher affinity between sorbent and sorbate occurred. To explain the different behavior of Al(OH)x-montmorillonite complexes to adsorb simazine, it might be considered that several factors contribute to the adsorption process. Al(OH)x complexes are characterized by an elevated surface acidity, which is probably higher than that of montmorillonite. The simple and double aluminum-coordinated hydroxyl groups, present on their surfaces, may behave as weak and strong acids, respectively, because of the weak and strong polarization induced by Al cation on the oxygen atoms (26). As a consequence, at clay interface higher concentrations of protons may occur deriving not only from the dissociation of water molecules surrounding hydroxy Al species but also from the proton donor functional groups. Thus, the protonation of simazine species arriving at the surface of AM complexes is enhanced with respect to “pure” montmorillonite. This could explain a higher affinity between simazine [present as double protonated species? (27)] and AM18, as indicated by high values of both K and n parameters (Table 1). On the other hand, OH-Al species determine the presence of numerous positive sites on M surfaces, not available to interact with other positively charged molecules, as well as a drastic reduction of both surface area and possibility of interlayering. All these factors will result in adsorption of simazine on AM18 to a smaller extent than on M in terms of absolute quantities (80.59 µmol g-1 against 373.75 µmol g-1 at saturation levels). However, if we refer to the quantity of simazine adsorbed per m2 of clay, we observe that the amount of herbicide held on AM18 (0.45) at pH 3.7 was quite similar 4224
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to that adsorbed on M (0.46). This indicates that the number of sites available on AM18 surfaces for simazine adsorption is significantly lower than those on montmorillonite. X-ray diffraction analyses of simazine-clay complexes seem to support these findings. The d-basal spacing of montmorillonite increased from 1.170 to 1.270 nm after simazine adsorption, thus indicating a measurable intercalation of simazine molecules in the interlamellar spaces of montmorillonite. By contrast, a negligible variation was evaluated for simazine-AM18 complex. As expected by the numerous findings reported in the literature, increasing the pH of the solution from 3.7 to 5.6 lowered the adsorption of simazine to montmorillonite (comparison between Figure 2a and Figure 3b). A corresponding decrease, however, was not observed with AM18 (Figure 3b and Table 1). Although the concentration of acetate ions was higher at pH 5.6 than 3.7, no significant competition between acetate ions and simazine should be considered on both matrixes. Adsorption of organic acids including acetic acid is difficult to occur on negatively charged matrices such as montmorillonite. Furthermore, as largely reported in the literature (28), the adsorption of organic acids on Al(OH)x complexes decreases by increasing pH, and it is usually at the maximum value at a pH equal to pKa (in this case pKa ) 4.6). This completely different behavior could be explained by drastic changes in surface acidity occurring on M but not on AM18 surfaces by rising the bulk pH by about two units. Just as a pure speculation: assume we have a local pH at montmorillonite surfaces about two units lower than that in the bulk solution and this pH-difference independent of the absolute pH value. An increase of surface pH from a value of approximately 1.7-3.6 should be expected when the pH of bulk solution is raised from 3.7 to 5.6. At a local pH of 1.7, simazine is dissociated in protonated species by 47% (pKa ) 1.65). On the contrary, at a pH of 3.60 the amount of ionized simazine molecules drastically decreases, becoming about 100-fold lower than the initial herbicide concentration. A similar consistent variation of pH, and consequently of concentration of ionized simazine, would not occur at AM18 surfaces. As previously indicated, the dissociation of proton donor functional groups of AM18 contributes to a considerable extent to the formation of protons, which in turn give rise to protonated simazine species. If the dissociation of these groups is quite constant by changing pH, the concentration of simazine cations forming at AM18 interface at pH bulk of pH 5.6 would not differ very much from that at pH bulk of 3.7. A validation of this adsorption mechanism could stem from measurement of pH and concentration at the interface of ionized simazine. However, this kind of experiment requires very complex and not easily available instruments. Furthermore, with increasing pH from 3.7 to 5.6 the surface charge of AM18 becomes less positive, i.e., more negative, so that on AM18 surfaces a higher number of negative sites will be available to interact with positively charged species of simazine. No similar changes occur in the surface charge of M, which remains negative throughout the pH range (29). The absence of simazine desorption from montmorillonite surfaces supports the hypothesis that strong coloumbic forces are involved in the interaction between the herbicide and clay. The results obtained with AM18 (10% desorption after 24-h washing) could indicate that very few molecules of simazine adsorbed to AM18 surfaces through weak interactions (van der Waals). In conclusion, the main factors controlling the adsorption of simazine on montmorillonite, pure and coated by hydroxy aluminum species, were electrostatic interactions occurring between oppositely charged herbicide and surface sites of supports.
Both clays display a surface acidity that may promote the protonation of weak bases, such as simazine, as they are adsorbed. At pH 3.7, the surface acidity of montmorillonite as well as its higher available surface area accounted for the greater amount of herbicide adsorbed as respect to that held on AM18. Increasing the pH to 5.6 determined a significant decrease of the acidity at montmorillonite surfaces, which reflected in a drastic reduction of herbicide adsorption. By contrast, the surface acidity of Al(OH)x-complexes did not change at higher pH, and only a low decrease (1, 2-fold) in the adsorption of simazine was observed on AM18. These results seem to imply that in acid soils, reach in aluminum and/or iron coated minerals, adsorption of simazine may be retarded, with a consequent low accumulation of the pesticide in soil.
Acknowledgments Part of this work was carried out with the support of the Environment Research “Programme 1990-1994 of the European Community, Contract EV5V-0470”, D.I.S.C.A. publication no. 175.
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Received for review December 22, 1998. Revised manuscript received July 2, 1999. Accepted September 2, 1999. ES9813313
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