Environ. Sci. Technol. 2004, 38, 180-186
Adsorption of the Herbicide Simazine by Montmorillonite Modified with Natural Organic Cations M A R T A C R U Z - G U Z M AÄ N , R A F A E L C E L I S , M . C A R M E N H E R M O S IÄ N , A N D JUAN CORNEJO* Instituto de Recursos Naturales y Agrobiologı´a de Sevilla. CSIC, Avda Reina Mercedes 10, Apdo 1052, 41080 Sevilla, Spain
Three organic cations with a natural origin (L-carnitine, L-cystine dimethyl ester, and thiamine) were introduced at different loadings in the interlayer of a low-charge montmorillonite, and the performance of the modified clays as adsorbents of the herbicide simazine was investigated using batch adsorption-desorption experiments. The organic cations were selected on the basis of their natural origin and the presence of diverse functional groups in their structures, which was expected to influence simazine adsorption. Elemental analysis and spectroscopy results demonstrated the presence of the organic cations in the modified montmorillonites and their entrance in the clay mineral interlayers. Batch adsorption results showed that modification with thiamine (Kf ) 96-138), cystine dimethyl ester (Kf ) 400-753), and especially carnitine (Kf > 10 000) enhanced the adsorption of simazine by montmorillonite (Kf ) 28-47). It appeared that the specific interlayer microenvironment provided by the functional groups of each organic cation was an important factor controlling the adsorption efficiency of the modified clays. For carnitine and cystine dimethyl ester, the increase in simazine adsorption was considerably greater than that observed after montmorillonite modification with “classical” alkylammonium cations, such as phenyltrimethylammonium or hexadecyltrimethylammonium. This illustrated how modification of smectitic clay minerals with natural organic cations containing appropriate functional groups can be a useful strategy to improve the performance of organoclays for the removal of specific organic pollutants from the environment.
Introduction In the last 15 years, there has been much interest in the use of organoclays as adsorbents to prevent and remediate environmental contamination by pesticides and other organic pollutants (1-5). Because of the hydrophilic, negative character of their surfaces, clay minerals, particularly 2:1 phyllosilicates, have been shown to be very good adsorbents for cationic and highly polar pesticides, but their adsorption capacity for poorly soluble, nonionic organic compounds is usually low (3, 4, 6). Replacement of natural inorganic exchange cations with organic cations through ion exchange * Corresponding author phone: +34 954624711; fax: +34 954624002; e-mail:
[email protected]. 180
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reactions has been shown to yield organoclays with organophilic properties, and hence this simple modification has been proposed for the improvement of the adsorptive capacity of clays for nonionic organic compounds, including hydrophobic pesticides (2, 7, 8). The organic cations most commonly used for this purpose are quaternary ammonium ions of the general form [(CH3)3NR]+ or [CH3)2NR2]+, where R is an aromatic or aliphatic hydrocarbon (9). Incorporation of large alkylammonium cations, such as octadecylammonium, dioctadecyldimethylammonium, and hexadecyltrimethylammonium, in the interlayers of smectitic clays has resulted in organoclays with enhanced affinity for neutral (5, 10, 11) and even acidic pesticides (2, 3, 12-14). It appears that the interlayer phase formed from large organic cation alkyl groups functions as a partition medium for nonionic organic compounds and effectively removes such compounds from water (2, 4, 15). The adsorptive characteristics of organoclays formed using small quaternary ammonium cations, such as tetramethylammonium, are much different, because small organic cations exist as discrete species on the clay surface and do not form an organic partition phase (7, 9, 15-17). In these organoclays, the organic cations act as nonhydrated pillars that prop open the clay layers, exposing the abundant siloxane surface area (17). Organoclays based on large and small alkylammonium cations have been proposed not only as filters for water decontamination but also as carriers in pesticide formulations to retard pesticide leaching after soil application (14, 18-20). In a recent study, Nir et al. (21) pointed out the importance of the structural compatibility between the pesticide molecule and the alkylammonium cation preadsorbed on the clay mineral in determining the performance of organoclays as adsorbents of pesticides. Thus, alachlor and metolachlor, both containing a phenyl ring in their structure, displayed greater affinity for montmorillonite exchanged with organic cations with phenyl rings, such as benzyltrimethylammonium or phenyltrimethylammonium, than for hexadecyltrimethylammonium-exchanged montmorillonite. Because most pesticides contain polar functionalities, it can be hypothesized that organic cations with appropriate functional groups could make it possible to selectively modify the clay mineral surface to maximize its affinity for selected pesticides. In fact, polar interactions between the carboxylic group of acidic herbicides (2,4-D, dicamba, imazamox) and the protonated group of alkylammonium interlayer cations have been shown to contribute to pesticide adsorption by organoclays (3, 12, 13). Although a similar concept has been applied to develop organoclays with increased affinity and selectivity for heavy metal ions (22-24), there is very little information available about how organic cations containing different polar functionalities influence organic contaminant adsorption by organoclays. In this work, the ability of three organic cations containing polar functionalities, L-carnitine, L-cystine dimethyl ester, and thiamine, to improve the performance of montmorillonite as an adsorbent of the herbicide simazine has been evaluated. The organic cations were selected on the basis of (i) their natural origin, which should reduce concern about the incorporation of these materials into soil and aquatic environments, and (ii) the presence of diverse functional groups in their structures, with the aim to establish relationships between simazine adsorption and the structural characteristics of the organic cations. For comparison purposes, the adsorptive properties of two “classical” organoclays, hexadecyltrimethylammonium- and phenyltrimethylammon10.1021/es030057w CCC: $27.50
2004 American Chemical Society Published on Web 11/25/2003
TABLE 1. Amount of Organic Cation Added and Theoretical Organic Cation Saturation for Samples Prepared
sample SW (untreated) SW (blank) SW-CAR50 SW-CAR100 SW-CAR150 SW-CYSTI50
FIGURE 1. Molecular structures of the organic cations and the herbicide simazine. ium-montmorillonite, were also determined. Fourier transform infrared spectroscopic analysis of organoclay-simazine complexes was used to discuss possible interaction mechanisms.
SW-CYSTI100 SW-CYSTI150 SW-THIA50 SW-THIA100 SW-THIA150
organic cation none L-carnitine L-carnitine L-carnitine L-cystine dimethyl ester L-cystine dimethyl ester L-cystine dimethyl ester thiamine thiamine thiamine
amount organic cation added (mmol kg-1 clay)
OCtS (%)a
0 382 764 1146 191
0 50 100 150 50
382
100
573
150
191 382 573
50 100 150
Materials and Methods
a Organic cation saturation: theoretical percentage of the CEC compensated by the organic cations.
Organic Cations and Herbicide. The three natural organic cations, L-carnitine, L-cystine dimethyl ester, and thiamine (purity > 98%), were purchased as high-purity chloride salts from Sigma (Germany). High purity simazine (purity ) 99%) was supplied by Riedel-de Hae¨n (Germany). Simazine [2-chloro-4,6-bis(ethylamino)-1,3,5-triazine] is a nonselective herbicide of the s-triazine group with a low water solubility (5 mg L-1 at 20 °C) and weakly basic character (pKa ) 1.7) (25). The structural formulas of simazine and the organic cations used are shown in Figure 1. Synthesis of Organoclays. SWy-2 Wyoming montmorillonite (SW) from The Clay Minerals Society (Columbia, MO) was exchanged with the three natural organic cations through ion exchange reactions. For the synthesis, the amount of L-carnitine, L-cystine dimethyl ester, or thiamine (chloride salts) corresponding to 50%, 100%, or 150% of the cation exchange capacity of SW (CECSW ) 764 mmol kg-1) was dissolved in 50 mL of 1 mM HNO3 and added to 1 g of SW. Acidic conditions were used to ensure the protonation of the biomolecules, thus favoring the exchange reaction. The suspensions were shaken for 24 h, centrifuged, washed three times with 100 mL of distilled water, and then freeze-dried. A blank clay sample [SW(Blank)] was also prepared by shaking 1 g of clay in 50 mL of 1 mM HNO3 for 24 h, washing three times with 100 mL of distilled water, and then freeze-drying. This sample, free of organic cations, served as a control and helped evaluate the effects of the acid treatment on the clay mineral during the synthesis procedure. The nomenclature of the different samples prepared in this work is summarized in Table 1. The two alkylammonium-exchanged clays used as reference materials in the adsorption-desorption experiments were hexadecyltrimethylammonium-SW and phenyltrimethylammonium-SW, containing an amount of alkylammonium cation equal to 50% and 100% of the CEC of SW. The preparation and characteristics of these samples have been reported elsewhere (20). Characterization of Organoclays. Elemental analyses (C, N, S) of the unexchanged and exchanged montmorillonite samples were performed using a Perkin-Elmer, model 1106, elemental analyzer (Perkin-Elmer Corp., Norwalk, CT). In addition, all samples were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction, and specific surface area measurements. FTIR spectra were obtained on KBr disks in a Nicolet 5 PC spectrometer (Nicolet Instrument Corp., WI). Basal spacing values (d001) were obtained by X-ray diffraction on oriented specimens using a Siemens D-5000 diffractometer (Siemens, Stuttgart) with Cu KR radiation. Specific surface areas (SSA) were obtained by N2 adsorption at 77 K using a Carlo Erba Sorptomatic 1900 (Fisons
Instruments, Milan). The samples were outgassed at 80 °C and equilibrated under vacuum for 4 h before measuring the N2 adsorption isotherm. Simazine Adsorption-Desorption Experiments. Simazine adsorption-desorption isotherms were obtained using the batch equilibration procedure. Duplicate 20-mg adsorbent samples were equilibrated for 24 h at 20 ( 2 °C with 8 mL of aqueous solutions of simazine with pesticide concentrations ranging from 1 to 20 µmol L-1. After equilibration, the suspensions were centrifuged, and 4 mL of the supernatant solution was removed for analysis. The concentration of simazine in the supernatant was determined by HPLC using a Waters 600E chromatograph coupled to a Waters 996 diode-array detector. The following conditions were used: Novapack C18 column (150 mm length × 3.9 mm i.d.), acetonitrile-water (30:70) eluent mixture at a flow rate of 1 mL/min, 25 µL injection volume, and UV detector at 225 nm. External calibration curves with standard solutions between 1 and 20 µM were used in the calculations. The amount of pesticide adsorbed was calculated by difference between the initial and final solution concentrations. Pesticide solutions without adsorbent were also shaken for 24 h and served as controls. Desorption was measured immediately after adsorption from the 20 µM initial concentration point of the adsorption isotherms. The 4 mL of supernatant removed for the adsorption analysis was replaced with 4 mL of distilled water. After shaking at 20 ( 2 °C for 24 h, the suspensions were centrifuged, and the pesticide concentration was determined in the supernatant. This desorption procedure was repeated three times. All adsorption and desorption studies were conducted in duplicate. Simazine adsorption-desorption isotherms were fit to the Freundlich equation: Cs ) KfCeNf, where Cs (µmol kg-1) is the amount of pesticide adsorbed at the equilibrium concentration Ce (µmol L-1), and Kf and Nf are the empirical Freundlich constants, which can be calculated from the linear plot of log Cs vs log Ce. Hysteresis coefficients, H, were calculated according to H ) Nf-des/Nf, where Nf and Nf-des are the Freundlich Nf constants obtained from the adsorption and desorption isotherms, respectively (26, 27). Successive Saturation for Spectroscopic Analysis. Twenty milligrams of SW-CAR150 sample was saturated with simazine by seven successive treatments with a 60 µM herbicide solution prepared in 20% methanol. Control and simazinesaturated samples were washed twice with distilled water, air-dried, and analyzed by FTIR spectroscopy. VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Elemental Analysis Results of Unexchanged and Biomolecule-Exchanged Montmorillonite Samples sample
C (%)
N (%)
S (%)
organic cation content (mmol kg-1)
OCtS (%)a
SW (untreated) SW (blank) SW-CAR50 SW-CAR100 SW-CAR150 SW-CYSTI50 SW-CYSTI100 SW-CYSTI150 SW-THIA50 SW-THIA100 SW-THIA150
0.31 ( 0.01b 0.30 ( 0.01 0.38 ( 0.04 3.09 ( 0.02 5.21 ( 0.03 1.84 ( 0.02 3.74 ( 0.04 4.55 ( 0.01 2.93 ( 0.12 5.69 ( 0.08 5.67 ( 0.11
ndc nd nd 0.43 ( 0.01 0.84 ( 0.07 0.47 ( 0.03 1.00 ( 0.04 1.23 ( 0.01 1.10 ( 0.06 2.07 ( 0.03 2.06 ( 0.04
nd nd nd nd nd 1.00 ( 0.01 2.26 ( 0.02 2.78 ( 0.03 0.54 ( 0.06 1.19 ( 0.05 1.08 ( 0.02
0 0 8 330 554 159 357 441 183 374 372
0 0 0.96 (Table 4). The improvement of the adsorptive properties of montmorillonite after treatment with carnitine, cystine, and thiamine is evidenced by the higher Freundlich Kf values of the exchanged montmorillonites compared to the unexchanged clay mineral (Table 4). The blank (acid) treatment had little effect on the adsorption behavior of SW. Because of the very high adsorption of simazine on SW-CAR samples, simazine adsorption isotherms on these samples showed poor fitting to the Freundlich equation. The improvement of the adsorption capacity of montmorillonite for simazine after treatment with carnitine, cystine, and thiamine cations can be attributed to an increase in the hydrophobicity of the clay mineral, which enhanced the affinity of its surface for the pesticide molecules (2, 5, 7, 13). Introduction of small organic cations, such as those used in this study, in the interlayers of low-charge smectites, such as SW, has been shown to result in considerable interlayer space not covered by the organic cation and hence available for pesticide adsorption (20). The unoccupied interlayer space has been suggested to have hydrophobic properties bestowed on it by the organic cations and by the areas of exposed silicate oxygens (1, 15, 34). These characteristics may have allowed simazine molecules to effectively compete with water molecules for adsorption sites on SW-CAR, SW-CYSTI, and SW-THIA. The different adsorption capacities provided by the different organic cations are likely due to a combination of functionality and steric effects. Carboxylic, ester, and hydroxyl functionalities have been shown to form, respectively, strong, moderately strong, and weak hydrogen-bond complexes with s-triazine herbicides (35). In their work on the interaction of atrazine with functional groups commonly found in soil organic matter, Welhouse and Bleam (35) reported formation constants for hydrogen bond complexes between atrazine and carboxylic, ester, and hydroxyl functionalities of 210, 4.8, and 3.3 L mol-1, respectively. Thus, the presence of these functionalities in carnitine, cystine dimethyl ester, and thiamine (Figure 1) should have strongly influenced the affinity of the modified clays for the herbicide simazine in 184
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our experiments. In addition, steric factors have been shown to play an important role in the adsorptive properties of organoclays (10, 20). As indicated by the SSA values reported in Table 3, the interaction of the divalent cations (thiamine and cystine) with the clay mineral surface through two points of contact results in greater covering of the SW surface compared to the singly charged carnitine cation. Consequently, the presence of most interlayer surface unoccupied in SW-CAR should have provided additional space available to host herbicide molecules, exacerbating the adsorption of simazine by this sample. The low and very similar adsorption of simazine by SW-THIA50 and SW-THIA100 samples suggests adsorption occurred only on highly accessible, external adsorption sites, equally present in both samples, whereas the moderate adsorption of simazine by SW-CYSTI, increasing with the amount of organic cation in the sample, is indicative of some interlayer adsorption. As mentioned above, the presence of the carboxylic functionality is likely to explain much of the extremely high adsorption capacity of SW-CAR samples. Carboxylic groups have been demonstrated to be one of the most reactive functionalities of organic matter for s-triazine herbicides (35, 36), and the importance of the surface acidity of montmorillonite in the retention of weak bases, such as simazine, has also been stressed (37-39). In particular, acidic functional groups associated with montmorillonite have been proposed to play a major role in the retention of s-triazines by soil components (37, 39). Accordingly, interaction of the weakly basic simazine molecules with the -COOH functionality of SW-associated carnitine may have played a major role in the retention of simazine by SW-CAR samples, explaining the very high adsorption measured on these samples. The improvement of the adsorptive properties of montmorillonite provided by carnitine, cystine, and thiamine cations can also be compared in Figure 3 with that provided by two “classical” alkylammonium cations, hexadecyltrimethylammonium (HDTMA) and phenyltrimethylammonium (PTMA). The characteristics of SW-HDTMA and SW-PTMA organoclays have extensively been reported in the literature (9, 15, 16, 20). The low adsorption of simazine measured on SW-HDTMA (Figure 3, Table 4) can be related to the horizontal arrangement of the large HDTMA cation in the interlayer of low-charge smectites, such as SW, which results in a bilayer structure with low interlayer thickness and little space available for the retention of organic compounds (4, 5, 9). Accordingly, SSAs of our SW-HDTMA samples were found to be low (12-13 m2 g-1). The small adsorption of simazine measured on SW-HDTMA100 (Kf ) 100) is likely to be due to some disruption of the bilayer structure at high loading of HDTMA, caused by surface charge heterogeneity in the clay (4), which may have created some space available for simazine adsorption. The small PTMA cation led to some increase in the adsorption capacity of SW montmorillonite, especially when incorporated at 50% of the CEC of SW (Table 4, Figure 3). At this organic cation loading, the low surface charge density of SW combined with the small size of PTMA cation result in considerable interlayer space not covered by the organic cation and hence available for herbicide adsorption (20). Nevertheless, the increase in adsorption observed upon treatment with PTMA was small compared to that observed for carnitine or cystine cations, which further illustrates the importance of the functionality effect. In addition to higher adsorption capacity for simazine, SW-CAR and SW-CYSTI also displayed irreversibility of the adsorption-desorption process greater than that of samples with lower affinity for the herbicide, such as SW-THIA or SW-PTMA (Figure 4). The low hysteresis coefficients (H ) Nf-des/Nf) calculated for simazine adsorption-desorption by SW-CAR and SW-CYSTI revealed the resistance of simazine to desorb from these samples (Figure 4). It appears, therefore,
TABLE 4. Freundlich Parameters for Simazine Sorption-Desorption by Unexchanged and Organic Cation-Exchanged Montmorillonite Samples
a
sorbents
Kf
SW(untreated) SW(blank) SW-CAR100 SW-CAR150 SW-CYSTI50 SW-CYSTI 100 SW-THIA50 SW-THIA100 SW-HDTMA50 SW-HDTMA100 SW-PTMA50 SW-PTMA100
(24-33)a
28 47 (40-60) 50000 (10000-280000) 120000 (58000-250000) 400 (331-485) 753 (733-775) 138 (113-168) 96 (86-107) 0b 100 (97-103) 243 (241-244) 96 (95-98)
Values in parentheses are standard error ranges about the mean.
R2
pH
0.79 (0.71-0.87) 0.89 (0.78-1.00) 1.05 (0.52-1.58) 1.76 (1.44-2.08) 0.85 (0.73-0.69) 0.67 (0.65-0.69) 0.75 (0.74-0.76) 0.86 (0.80-0.92)
0.980 0.970 0.666 0.940 0.962 0.999 0.963 0.991
0.22 (-0.29-0.74) 0.67 (0.61-0.74) 0.87 (0.75-0.98)
0.158 0.981 0.967
7.8 7.4 5.1 4.8 6.5 6.8 7.3 6.8 7.9 8.1 7.7 8.3
Nf
b
Negligible adsorption.
FIGURE 4. Simazine adsorption-desorption isotherms and hysteresis coefficients, H ) Nf-des/Nf, for exchanged montmorillonite samples. that the samples with higher affinity for simazine also retained the herbicide more strongly. It is likely that contribution of polar interactions to the retention mechanism, such as cooperative hydrogen bonding involving carboxylic and ester groups (5, 13, 35), resulted in reduced desorption of simazine from SW-CAR and SW-CYSTI samples. On the other hand, simazine adsorption on lowly adsorptive organoclays may have been limited to highly accessible, external adsorption sites, facilitating desorption. FTIR Spectroscopy Study of Simazine-Organoclay Complexes. To corroborate possible interaction mechanisms, the FTIR spectrum of SW-CAR150 repeatedly treated with a simazine solution (total simazine sorbed in the complex ) 119 µmol g-1) was recorded and compared with those of simazine and the untreated SW-CAR150 sample (Figure 5). A similar study was conducted with SW-CYSTI and SW-THIA
samples, but the amounts of simazine adsorbed by these organoclays were too low to enable the FTIR analysis of the complexes. As mentioned above, the band at 1728 cm-1 in the FTIR spectrum of SW-CAR150 is assigned to the CdO stretching vibration of protonated carboxylic groups of carnitine. Upon simazine treatment, a considerable decrease in the intensity of this band was observed, accompanied by the appearance of a band at about 1674 cm-1, which is typically assigned to the antisymmetric stretching vibration of ionized COO- groups (30, 40). This result strongly indicates that simazine adsorption on SW-CAR150 is accompanied by an increase in the ionization of the carnitine COOH groups, probably as a result of proton transfer or formation of hydrogen bonds between the carboxylic group of carnitine and the basic N atoms of simazine (35, 40, 41). A similar mechanism has been proposed for the adsorption of triazine VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Fourier transform infrared spectra of simazine, SWCAR150, and SW-CAR150-simazine complex. herbicides by soil organic matter, where involvement of carboxylic groups has been demonstrated through spectroscopic studies and by determining changes in adsorption after methylation of acidic groups (35, 40, 42). The presence of a carboxylic group in the carnitine molecule is confirmed therefore as a major cause for the high retention capacity of SW-CAR samples for the herbicide simazine and for the observed irreversibility. The polar interaction between carboxylic groups and protonable nitrogen atoms has been described in previous work, where pesticide molecules containing carboxylic groups displayed enhanced affinity for alkylammonium cations containing monosubstituted ammonium groups (3, 12, 13). In summary, our results illustrate how the chemical nature of the interlayer organic cation can greatly influence the adsorptive characteristics of exchanged clays, probably through a combination of functionality and steric effects. This suggests the possibility to selectively modify the clay mineral surfaces with organic cations containing appropriate functional groups to create an interlayer microenvironment designed to improve the affinity of the clay mineral for a given organic compound. The suitability of natural organic cations for this purpose appears particularly interesting to minimize the environmental impact of the adsorbent when incorporated into natural ecosystems for practical applications.
Acknowledgments This work has been partially supported by the MCYT Project REN2001-1700-CO2-01/TECNO, the FP5 EU Project EVK1CT-2001-00105, and by Junta de Andalucı´a through Research Group RNM124. M.C.-G. gratefully acknowledges the Spanish Ministry of Education and Culture for her F.P.U. fellowship.
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Received for review May 5, 2003. Revised manuscript received October 3, 2003. Accepted October 14, 2003. ES030057W
