Environ. Sci. Technol. 2001, 35, 2226-2232
Influence of Clay Minerals on the Hydrolysis of Carbamate Pesticides
TABLE 1. Carbamate Pesticides Used in This Study
JIANG WEI,† GERHARD FURRER,* STEFAN KAUFMANN, AND RAINER SCHULIN Institute of Terrestrial Ecology, Swiss Federal Institute of Technology Zu ¨ rich (ETHZ), Grabenstrasse 3, CH-8952 Schlieren, Switzerland
Using batch experiments, we investigated the influence of clay minerals (montmorillonite, beidellite, illite, and vermiculite) on the hydrolysis of five carbamate pesticides: carbosulfan, carbofuran, aldicarb, pirimicarb, and chlorpropham. Compared to the other minerals, montmorillonite had the strongest influence on the hydrolysis of these carbamates. Montmorillonite enhanced the hydrolysis of carbosulfan and aldicarb. In contrast, the hydrolysis of chlorpropham was inhibited by montmorillonite, probably because of its strong adsorption on montmorillonite. The hydrolysis of pirimicarb was not affected by montmorillonite. The presence of organic substances, phosphate, and fluoride in suspensions decreased the catalytic activity of montmorillonite. Surface acidity of montmorillonite and/ or formation of surface chelates are probably the key factors of surface catalysis in the case of the hydrolysis of carbosulfan.
Introduction Carbamate pesticides have widespread application in agriculture as insecticides and herbicides. Among the pesticides studied in this work, aldicarb [2-methyl-2-(methylthio)propanal O-[(methylamino)carbonyl]oxime] is highly toxic for both mammals and insects (1). Carbosulfan [2,3-dihydro2,2-dimethyl-7-benzofuranyl [(dibutylamino)thio)]-N-methylcarbamate] and carbofuran [2,3-dihydro-2,2-dimethyl7-benzofuranol N-methylcarbamate] are systemic insecticides. They are used against soil-dwelling insects and foliar pests (2). Carbosulfan is a propesticide of carbofuran (3). Pirimicarb [2-(dimethylamino)-5,6-dimethyl-4-pyrimidinyl dimethylcarbamate] is an insecticide against organophosphorusresistant Myzus persicae (2). Chlorpropham [isopropyl 3-chlorophenylcarbamate] is a herbicide. The structures and water solubilities of these compounds are given in Table 1. While pirimicarb and carbosulfan are secondary carbamates, chlorpropham, aldicarb, and carbofuran are primary carbamates (see Table 1). Except for carbosulfan, all compounds are relatively soluble in water. Aldicarb has the highest water solubility (3.1 × 10-2 M), while carbosulfan has the lowest water solubility (7.9 × 10-8 M) (2). Soil and groundwater are under growing risk of pollution by the increasing use of carbamates. Therefore, it is important to understand the behavior including the transformation of carbamates in the environment. The transformation of carbamates can be biotic as well as abiotic, depending on the biotic activity under environmental conditions. A primary pathway for the transformation of carbamates in aquatic * Corresponding author e-mail:
[email protected]; phone: +41-1-6336009; fax: +41-1-6331123. † Present address: Paul Scherrer Institut, CH-5232, Villigen, PSI, Switzerland. 2226
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environments is hydrolysis. Hydrolysis reactions normally depend on several parameters. Besides homogeneous catalysis such as specific acid, base, or metal ion catalysis, heterogeneous catalysis is of particular importance for the hydrolysis of carbamates in soils and sediments. The latter can be due to catalytic effects of metal oxide and clay mineral surfaces. Clay minerals are abundant in most soils. In this paper, we focus on catalysis of hydrolysis of carbamate pesticides affected by clay minerals. Clay minerals have been used as catalysts in industry since a long time (4-6). They can catalyze several types of reactions: cracking, elimination, condensation, and addition in nonaqueous systems. Many of these processes are related to the acidic properties of mineral surfaces, especially in the case of ion-exchanged montmorillonites (4). Several papers have been published concerning the hydrolysis of organophosphorus pesticides in clay mineral suspensions (7-9). In most studies montmorillonite was used, whereas other clay minerals were more or less neglected. Mortland and Raman (7) observed a strong catalytic effect of Cu-montmorillonite on the hydrolysis of dursban, diazinon, ronnel, and zytron. Sanchez-Camazano and Sanchez-Martin (8) reported that phosmet hydrolyzed 500 times faster in Ca-montmorillonite suspensions than in aqueous solution at pH 6.0. The hydrolysis of quinaphos was found to be catalyzed in montmorillonite suspensions, and the hydrolysis mechanism was affected by exchangeable cations (9). However, the mechanisms of catalysis are not completely understood. It seems that different mechanisms are involved in the hydrolysis of different pesticides. The hydrolysis kinetics and mechanism of carbosulfan in aqueous solutions without minerals have been studied in our group (10). In this study, we attempt to compare the catalytic effects of the clay minerals montmorillonite, beidellite, illite, and vermiculite on the hydrolysis of the five carbamate pesticides listed above and to discuss possible mechanisms of surface catalysis. 10.1021/es000179d CCC: $20.00
2001 American Chemical Society Published on Web 04/26/2001
TABLE 2. Characteristics of Clay Minerals and Oxides Used in This Studya minerals
surface areab (m2/g)
pHzpc
montmorillonite beidellite illite vermiculite
23.1 41.5 21.9 8.6
2.5c nad na na
a The surface areas are N BET surface areas. 2 and Morgan (46). d na, not available.
b
This study. c Stumm
Materials and Methods Minerals and Chemicals. Montmorillonite (Swy-2), beidellite (SBCa-1), illite (Imt-2), and kaolinite were obtained from the Clay Mineral Society (USA). Vermiculite (2-4-mm fraction) was provided by Vermica AG (Switzerland). NaCl, acetonitrile, and HPLC-grade water were purchased from Merck. Humic acids and ethylenediamine were purchased from Aldrich Co. Certified carbosulfan (purity >97%), carbofuran (purity >98%), carbofuran phenol (purity >99.5%), aldicarb (purity >99%),), chlorpropham (purity >99%), and pirimicarb (purity >99%) were obtained from Dr. Ehrenstorfer GmbH (Germany). All other chemicals were analytical grade or better, obtained from either Fluka or Merck, and used without further purification. Specific surface areas of minerals were measured using the BET method (N2 adsorption) and are listed in Table 2. For montmorillonite, beidellite, illite, and vermiculite, the particle fraction Cu-vermiculite. This is the same sequence among the clay minerals as we observed for the influence of unexchanged montmorillonite, beidellite, and vermiculite on carbosulfan hydrolysis (Table 4). When the experimental data were fitted using the first-order model, we found that in most cases the first-order model is not sufficient to describe the reaction kinetics at pH 4.0 and pH 5.0. It seems that the reaction kinetics is composed of two processes, i.e., a fast reaction followed by a slow one. However, at pH 6.0, the experimental data fit the first-order model well except in the presence of montmorillonite. Since carbosulfan was not entirely dissolved in the experiments, dissolution of carbosulfan could be the slow process that limits the formation kinetics of carbosulfan after the initially dissolved carbosulfan has been used up. This is in agreement with our results from mineral-free solutions (10). The half-life of carbosulfan in montmorillonite suspension at pH 6.0 is about equal to that in aqueous solution at pH 4.0 (Table 4). Thus, the hydrolysis of carbosulfan is significantly accelerated if montmorillonite is present. In general, the catalytic effect of minerals was found to depend on pH. The catalytic effect was less significant at low pH. For instance, at pH 3.0, clay minerals had no visible catalytic effect. At this pH, acid catalysis in solution becomes dominant. The catalytic effect of minerals can be clearly observed at pH >4.0 (Table 4). However, with a half-life of 1 h or so, the hydrolysis of carbosulfan in montmorillonite suspensions appeared to be almost independent of pH between pH 3.0 and pH 5.0. Effect of Exchangeable Cations. Na-, Al-, and Cu-montmorillonite were used to study the effect of exchangeable cations. The initial reaction rate in the presence of untreated montmorillonite suspension was much lower than that in Na-, Al-, and Cu-montmorillonite suspensions probably due to a smaller specific surface area of untreated montmorillonite (Table 5). However, the initial apparent half-lives measured in Na-, Al-, and Cu-montmorillonite suspensions at pH 6.0 were similar to each other (Table 5). This suggests that the choice of exchangeable cations have a small effect on the catalytic activity of montmorillonite. Effect of Phosphate and Fluoride. Phosphate and fluoride, which are present in soils and natural waters, can be adsorbed at mineral surfaces. Fluoride can replace surface hydroxyl groups. From fluoride adsorption isotherms, one can assess the number of fluoride-binding surface sites of minerals. Following the method of Sigg and Stumm (23), an excess of F- was added to Na-montmorillonite suspensions, and the pH was adjusted to 4.5. This yielded adsorption maximum of 2.0 × 10-4 mol of F-/g of montmorillonite. The hydrolysis experiments were carried out in Namontmorillonite suspensions (10 g/L) containing 0, 5, or 50 mM phosphate or fluoride at pH 7.0. The reaction rate decreased with increasing phosphate or fluoride concentrations (Table 6). Phosphate seems to inhibit the hydrolysis
a The hydrolysis experiments were performed in Na-montmorillonite suspensions containing 1% acetonitrile at pH 7.0 with total initial carbosulfan concentration of 1.3 × 10-4 M. b The half-lives were calculated assuming a pseudo-first-order reaction.
TABLE 7. Initial Apparent Half-Lives (h) of Carbosulfan in Montmorillonite Suspensions (10 g/L) with or without Humic Acids (HA) and Ethylenediamine (EDA)a media
half-lives
montmorillonite suspension montmorillonite suspension + 500 mg/L HA montmorillonite suspension + 100 mg/L EDA montmorillonite suspension + 500 mg/L EDA EDA soaked montmorillonite suspension
0.82 0.78 1.74 8.28 65.2b
a The hydrolysis experiments were performed in untreated montmorillonite suspensions (10 g/L) containing 1% acetonitrile with total initial carbosulfan concentration of 1.1 × 10-4 M at pH 5.0. b The halflife was calculated assuming a pseudo-first-order reaction.
more effectively than fluoride. In the presence of 50 mM phosphate, the hydrolysis was 10 times slower than that in absence of phosphate. As phosphate is anionic at pH 7.0, it will be repelled by the permanent negative charge of the plane sites and be adsorbed only as a specific chemical surface complex at the edge sites of montmorillonite. Inhibition of carbosulfan hydrolysis in the presence of phosphate suggests that edge sites are responsible for the catalytic activities of montmorillonite. Furthermore, the XRD results given in Table 9 suggest that carbosulfan did not penetrate into the interlayer space because the basal spacing d001 of montmorillonite increased only by 1 Å after montmorillonite interacted with carbosulfan. We conclude that interlayer sites of montmorillonite are not involved in the hydrolysis of carbosulfan. Effect of Organic Substances. We investigated the influence of two different types of organic compounds, HA and EDA, on the catalytic activity of montmorillonite. The synthetic chemical EDA strongly adsorbs at montmorillonite (24, 25). Table 7 shows half-lives of carbosulfan hydrolysis measured in montmorillonite suspensions in the presence of HA or EDA. HA showed no influence on the catalytic activity of montmorillonite even at a concentration of 500 mg/L, but EDA decreased the catalytic activity of montmorillonite substantially. Inhibitory effect of EDA on carbosulfan hydrolysis increased with its concentration. After soaking the clay mineral in EDA, the half-life of carbosulfan was about 80 times longer than in the montmorillonite suspension without organic substances and corresponded to that in mineral-free solution (Table 4). If surface sites are fully occupied by EDA molecules, carbosulfan hydrolysis occurs only in solution; therefore, half-life becomes about the same as in aqueous solution without minerals. Hydrolysis of Carbofuran. The kinetics of carbofuran hydrolysis was investigated at pH 7, pH 8, and pH 10 in aqueous solutions and in suspensions with montmorillonite, beidellite, illite, and vermiculite. Half-lives for all experiments are exhibited in Table 8, indicating that the hydrolysis kinetics in the homogeneous solution and in all kind of suspensions clearly depends on the pH value. The slope of the log-log VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 8. Half-Lives (h) of Carbofuran in Aqueous Solutions and Suspensions with 23.9 g/L Montmorillonite, 4.9 g/L Beidellite, 9.2 g/L Illite, or 8.5 g/L Vermiculite with an Initial Carbofuran Concentration of 1.0 × 10-4 M at Different pH Valuesa media
pH 7.0
pH 8.0
pH 10.0
aqueous solutions montmorillonite suspensions beidellite suspensions illite suspensions vermiculite suspensions
737 630 937 889 753
93.7 133 76.2 113 80.6
1.17 0.87 0.98 0.78 0.91
a
TABLE 9. Basal Spacing d001 of Montmorillonite Treated with Different Solutionsa solutions
d001 Å
0.1 M NaCl + 0.005 M MES 0.1 M NaCl + 0.005 M MES + 10-4 M carbosulfan 0.1 M NaCl + 0.005 M MES + 10-4 M carbofuran 0.1 M NaCl + 0.005 M MES + 10-4 M chlorpropham 0.1 M NaCl + 0.005 M MES + 10-4 M pirimicarb
11.3 12.3 11.9 14.9 15.0
a Experimental procedure: montmorillonite was added to different solutions. After 1 h, montmorillonite was air-dried and used for X-ray diffraction analysis.
Half-lives were calculated assuming a pseudo-first-order reaction.
FIGURE 2. Hydrolysis of carbofuran in aqueous suspensions with 23.9 g/L montmorillonite at pH 10. Carbofuran phenol appears as an intermediate product in solution. plot (not shown) of half-lives versus the concentration of hydroxide ions is almost exactly 1.0 and indicates that the decomposition rate of carbofuran is first order with respect to the concentration of hydroxide ions. None of the clay minerals seemed to increase or decrease the kinetics of carbofuran hydrolysis, i.e., the influence of the clay minerals remained within the experimental error. Thus, the rate of carbofuran hydrolysis is independent of any of the investigated minerals, and the kinetic rate equation can be written as
rate ) k[OH][carbofuran]
(2)
with the rate constant k ) 5.0 × 10-3 M-1 h-1 (for 24 °C and I ) 0.1). In almost all experiments, which are represented by Table 8, monitoring of the dissolved concentrations of both the educt carbofuran and the product carbofuran phenol revealed preservation of mass balance. Only at pH 10 and in the presence of beidellite or montmorillonite, the sum of the dissolved concentrations of the two compounds decreased in the course of the first 8 h. In the suspension with beidellite, about 12% of the produced carbofuran phenol disappeared again (data not shown). In the presence of montmorillonite, the concentration of carbofuran phenol first increased slightly and then decreased again and depleted completely in the time span of 8 h (Figure 2). The curves in Figure 2 represent the results of the least-squares fits based on the following model assumptions: (i) decomposition of carbofuran is a pseudo-first-order reaction ([cf] model), (ii) the concentration of the produced carbofuran phenol is depleted in a consecutive first-order reaction ([cfp] model without adsorption), and (iii) a constant fraction of about 70% of the product carbofuran phenol is adsorbed instantly onto the montmorillonite surface ([cfp] model). 2230
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The modeling procedure made clear that the depletion of carbofuran phenol in the montmorillonite suspensions is caused by two processes: a fast adsorption onto the mineral surface, which can be described in terms of equilibria, and a slow process, which is best represented by a consecutive first-order reaction. The strong adsorption of carbofuran phenol onto the montmorillonite surface at pH 10 probably occurs in the form of a surface chelate complex and is based on the partial deprotonation of the phenolic functional group at this pH value. The product of the decomposition of carbofuran phenol has not been identified in this work. However, it is plausible that carbofuran phenol that is adsorbed to the montmorillonite surface is further degraded to a substituted catechol with a tert-butyl alcohol group at the number three carbon. After being dehydrated, the tertbutyl alcohol is converted into an alkene group. Both compounds were identified by Bachmann and Peterson (26) as products from photodecomposition of carbofuran in aqueous solution. Effects of Clays on Hydrolysis of Other Carbamates: Aldicarb, Chlorpropham, and Pirimicarb. Since aldicarb has a high water solubility, it was totally dissolved in aqueous solutions or suspensions at the concentration used. Huang (27) reported half-lives of aldicarb at room temperature to be about 80 d and 1 yr at pH 3.7 and pH 7.0, respectively. This pH dependence suggests that hydrolysis of aldicarb is an acid-catalyzed reaction. We performed a hydrolysis experiment in aqueous montmorillonite suspensions (10 g/L) at pH 3.7 and found a half-life of 16 d for aldicarb. Although the reaction was catalyzed by montmorillonite, we found that the adsorption of aldicarb on montmorillonite was negligible (less than 5%), indicating that strong adsorption is not necessarily required for surface catalysis. Nevertheless, the decrease in half-life by a factor 5 suggests a strong influence of montmorillonite. The hydrolysis of herbicide chlorpropham in solution appeared to be a hydrolysis reaction because we could identify 3-chloroaniline, the expected product of the hydrolysis of clorpropham. The hydrolysis experiments with chlorpropham in aqueous solution at pH values of 3.0, 5.0, 6.5, and 10.0 revealed that the hydrolysis is more or less pH independent with a half-life of about 1 d. The hydrolysis experiments of chlorpropham in montmorillonite suspensions (10 g/L) at pH 5.0 and pH 6.5 with an initial chlorpropham concentration of 1.0 × 10-4 M showed that chlorpropham concentration in suspension decreased to almost zero within about 30 min. However, we found no hydrolysis product. After centrifugation, the added chlorpropham was completely recovered from montmorillonite by extraction with hexane. After the adsorption of chlorpropham, montmorillonite was analyzed by X-ray diffraction (XRD). The basal spacing d001 increased from 11.3 to 14.9 Å (Table 9), indicating that chlorpropham was sorbed in interstitial space of montmorillonite. Even after 30 d no 3-chloroaniline could be detected in montmorillonite suspensions with chlorpropham, indicating that no hydrolysis of chlorpropham occurred in the presence of montmoril-
FIGURE 3. Two hypothetical mechanisms of surface catalysis for the hydrolysis of carbosulfan in mineral suspensions. lonite. The sorption of chlorpropham in the interstitial space of montmorillonite probably prevented the attack by water molecules. Macalady and Wolfe (28) reported that the hydrolysis of the insecticide chlorpyrifos was significantly retarded by the adsorption on sediment material. The hydrolysis of primary carbamates is much faster than that of secondary carbamates (14). As expected, we did not observe the hydrolysis of pirimicarb in solutions at pH 3.0, 5.0, 6.5 up to 50 days, neither in montmorillonite suspension (10 g/l) at pH 6.0. Like chlorpropham, pirimicarb was completely adsorbed by montmorillonite at concentration of 1.0 × 10-4 M. The basal spacing d001 of montmorillonite increased from 11.3 to 15.0 Å due to the adsorption of pirimicarb (Table 9), indicating that pirimicarb was also sorbed in interstitial surface of montmorillonite.
Discussion Mechanism of Surface Catalysis of Carbosulfan Hydrolysis. Our results clearly show that the rates of carbosulfan hydrolysis are significantly enhanced in the presence of clay minerals. The catalytic effect was most distinct in montmorillonite suspensions with only little acetonitrile in the aqueous solution (1% (v/v)) (Tables 4 and 5). Furthermore, the enhancement of the rate constant by montmorillonite is much more pronounced at pH 6 than at pH 4. At pH 6.0 it was about equal to that in particle-free aqueous solutions at pH 4.0 (Table 4), i.e., in the presence of 10 g/L montmorillonite the rate constant increased by 2 orders of magnitude. The catalytic effect of montmorillonite was more manifest at higher pH values, where the rate constants of carbosulfan in particle-free solutions were slower. In suspensions where the solution contained a large amount of acetonitrile (40% (v/v)), the influence of montmorillonite was only minor (Table 3), i.e., the enhancement of rate constant by the same particle concentration was only about 2 times and almost independent of pH. Here, we discuss two alternative mechanisms that may be responsible for the surface catalysis of carbosulfan hydrolysis (Figure 3). One involves the formation of a surface complex (surface-chelate mechanism). On the basis of the molecular structure of carbosulfan, it is possible that a sixmembered chelate complex is formed between carbosulfan
and a surface metal center. The complexation of carbosulfan by an aluminum metal center at the montmorillonite surface may polarize the hydrolyzable bond to such a degree that hydrolysis occurs without the need of a proton (Figure 3a). A similar mechanism for TiO2 surface-catalyzed reaction has been proposed (29). The other mechanism that might lead to enhanced carbosulfan hydrolysis does not involve a direct interaction with the mineral surface. It is simply the same process that also occurs in mineral-free aqueous solutions (10). The role of the mineral surface is limited to a strong increase of the proton activity at the mineral-water interface, which leads to an enhanced rate of proton-promoted hydrolysis of carbosulfan (surface-acidity mechanism; Figure 3b). Several studies have demonstrated that acid-catalyzed reactions are promoted by adsorption to clay surfaces (30-33), although these studies were not performed in clay suspensions. Surface-Acidity Mechanism. Evidences for the surfaceacidity mechanism arise from studies that suggested surface pH of clay minerals to be 2 to 3 units lower than the pH in the bulk solution (30, 34-37). The Brønsted acidity of clay minerals originates primarily from the dissociation of water coordinated to exchangeable cations (38-41). Although methods to determine the surface acidity of clay minerals in suspensions have been proposed (42-44), it is difficult to determine surface acidity in aqueous suspensions quantitatively. We tested the acidity of the investigated minerals qualitatively by adding methyl red (pKa ) 4.8) to the suspensions in aqueous solutions with 0.1 M NaCl. In suspensions with a bulk pH value of about 7, the presence of montmorillonite exhibited a bright red color (acid color of methyl red), beidellite exhibited a light red color, and illite and vermiculite exhibited a yellow color (alkaline color). This indicated that montmorillonite and beidellite exhibit stronger surface acidity than illite and vermiculite and is in agreement with Bailey et al. (34), who found that the surface acidity of illite in aqueous suspensions was lower than that of montmorillonite. Surface acidity is affected by the position of the negative charge. Isomorphic substitution in octahedral sheets seems to cause stronger acidity than in the tetrahedral sheets (36). In clay minerals with tetrahedral substitution, the negative charge is closer to the interlayer space, reducing the polarizing power of exchangeable cations more than in case of octahedral substitution (45). Octahedral substitution dominates in montmorillonite, while tetrahedral substitution is characteristic for beidellite, illite and vermiculite. This might cause stronger surface acidity and could explain the particularly pronounced catalytic effect in the case of montmorillonite. We found that the rate of carbosulfan hydrolysis in montmorillonite suspensions at pH 6.0 was approximately equal to that in aqueous solution at pH 4.0 (Table 4). In the case that surface catalysis occurs entirely via the surfaceacidity mechanism, it means that all carbosulfan must be localized near the mineral surface, where the increased proton activity occurs. However, a substantial amount of carbosulfan might be adsorbed also at the edge sites or somewhat further away from the surface, where the proton activity is more similar to that in the bulk phase. In this case, the surface proton activity must be correspondingly higher. Surface-Chelate Mechanism. Evidences for the surfacechelate mechanism seems to arise from the hydrolysis experiments in montmorillonite suspensions in the presence of phosphate and fluoride (Table 6). Both anions exhibit a strong chemical affinity for oxide and clay mineral surfaces. On montmorillonite, these ligands probably adsorb at the edge sites, where surface metal centers are accessible similar to surfaces of oxide minerals. This might reduce the surface sites available for chemically specific interaction with carVOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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bosulfan and lead to an inactivation of the mineral surface. Inhibition of surface catalysis was also observed after the pretreatment of montmorillonite with EDA or in the presence of EDA, a ligand that can form a five-membered chelate ring with surface metal centers. The experiments with all three ligands seem to support more the hypothesis with the surfacechelate mechanism. With some FTIR investigations we tried to find further evidence for or against the surface-chelate mechanism. However, the spectra of the KBr pellets showed that the absorption peak assigned to the stretching vibration of the carbonyl group of carbosulfan shifted only 2 cm-1, which could be caused by a weak coordination between the mineral surface and carbosulfan. The formation of a surface complex with carbosulfan is expected to cause a significantly stronger shift. Nevertheless, we do not exclude the possibility that a small amount of carbosulfan could form surface-chelate complexes, which is not detectable with the FTIR method. The effect of increased amounts of acetonitrile in solution (compare Tables 3 and 4) does not support one or the other hypothesis. Acetonitrile simply improves the solubility of carbosulfan, i.e., it causes desorption from the mineral surface, which reduces the catalytic surface effect. In summary, it appears that we are not able at this point to decide whether the surface catalytic effect of montmorillonite on carbosulfan hydrolysis is caused by a surface-chelate mechanism or by a surface-acidity mechanism. In principle, it is also possible that surface catalysis occurs via a combination of the two mechanisms (surface-chelate and surface-acidity). Anyway, further experiments are needed to clarify the mechanisms responsible for the observed surface catalysis. It is possible that besides the two mechanisms discussed here other mechanisms may play a role. The mechanisms of surface catalysis can be manifold and depend on the chemical properties of the pesticides as well as on the physicochemical properties of the involved mineral surfaces. Environmental Significance. With respect to environmental conditions, the half-life of carbosulfan can be estimated to be about 1 yr in aqueous solutions at pH 7.0, while the half-life in montmorillonite suspensions (10 g/L) was only about 5 d. In soils, when the solid/solution ratio is higher than in our suspension experiments, an even stronger catalytic surface effect can be expected. This shows that mineral surfaces can contribute substantially to the hydrolysis kinetics of pesticides. However, reactions with mineral surfaces should be individually assessed for each type of soil (mineral composition) and pesticide.
Acknowledgments The authors thank Prof. Alan Stone (Johns Hopkins University), Dr. Felix Funk (Institute of Terrestrial Ecology, ETHZ), and Dr. Thomas Lippert (Paul Scherrer Institut) for helpful discussion and Dr. Peter Weidler (Institute of Terrestrial Ecology, ETHZ) for the XRD analysis. We also thank three reviewers for comments that helped to greatly improve the paper. The work was supported by ETHZ Research Grant 41-2713.5.
Literature Cited (1) Hassall, K. A. The Chemistry of Pesticides, Their Metabolism, Mode of Action and Uses in Crop Protection; Verlag Chemie: Weinheim, 1982. (2) Worthing, C. E.; Hance, R. J. The Pesticide Manual, British Crop Protection Council: Surrey, 1991. (3) Coats, J. R. In Pesticide Transformation Products, Fate and Significance in the Environment; Somasundaram, L., Coats, J. R., Eds.; American Chemical Society: Washington, DC, 1991. (4) Adams, J. M.; Clapp, T. V.; Clement, D. E. Clay Miner. 1983, 18, 411-421. (5) Rupert, J. P.; Granquist, W. T.; Pinnavaia, T. J. In Chemistry of Clay and Clay Minerals; Newman, A. C. D., Ed.; Longman: Harlow, 1987. 2232
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Received for review August 8, 2000. Revised manuscript received January 22, 2001. Accepted January 30, 2001. ES000179D
