Benzimidazole adsorption on the external and interlayer surfaces of raw and treated montmorillonite

Applied Clay Science 53 (2011) 366–373

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y

Research Paper

Benzimidazole adsorption on the external and interlayer surfaces of raw and treated montmorillonite Rosa M. Torres Sánchez a,⁎, Michel J. Genet b, Eric M. Gaigneaux c, Maria dos Santos Afonso d, Simón Yunes e a

CETMIC (Centro de Tecnología en Minerales y Cerámica), Camino Centenario y 506 CC (49) (B1897ZCA) M. B. Gonnet, Argentina Unité de chimie des Interfaces Université Catholique de Louvain, Croix du Sud 2/18,. B-1348 Louvain-la-Neuve, Belgium Unité de catalyse et de chimie des matériaux divisés, Université Catholique de Louvain, Croix du Sud 2/17., B-1348 Louvain-la-Neuve, Belgium d Dep. de Química Inorgánica, Analítica y Química Física e INQUIMAE, FCEN, UBA, Ciudad Universitaria, Pabellón II, C1428EHA, Buenos Aires, Argentina e YUMA Consulting, Calle 5 Terrazas del Ávila, Caracas-107, Venezuela b c

a r t i c l e

i n f o

Article history: Received 22 September 2009 Received in revised form 23 June 2010 Accepted 25 June 2010 Available online 3 July 2010 Keywords: Montmorillonite Benzimidazole Sorption mechanism Thermal and mechanical treatments

a b s t r a c t The purpose of this study is to evaluate the efficiency of benzimidazole removal using raw, thermally and mechanically treated montmorillonite surfaces. The comparison of benzimidazole adsorption curves on these montmorillonite surfaces was made. A two step sorption mechanism that describes the data was corroborated by X-ray diffraction (XRD) results. The removal efficiency for the different sorbents at pH 6 follows the sequence raw montmorillonite N thermally treated montmorillonite N mechanically treated montmorillonite. The external and the interlayer surfaces were proposed as adsorption sites. On the latter, benzimidazole adsorbs in a planar monolayer arrangement with a consequent reduced water uptake. Two different foms of benzimidazole nitrogen surface coordination were indicated by X-ray photoelectron spectroscopy (XPS) results and isoelectric point (IEP) values. Protonated and deprotonated nitrogen were equally coordinated to the surface for raw and thermally treated samples at low temperature (350 °C). Stronger coordination of deprotonated nitrogen was found for samples treated at higher temperature (550 °C), while only a cationic exchange and no coordination to the surface was observed for the mechanically treated sample. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Raw clays (Rodríguez-Cruz et al., 2007) and clays modified by (i) ion exchange with organic cations (El-Nahhal et al., 1998, 1999a,b; Undabeytia, et al., 2000; Nennemann et al., 2001; Carrizosa et al., 2000. Hermosín et al., 2001; Azejjel et al., 2009), (ii) thermal treatment (Bojemueller et al., 2001) or (iii) mechanical processing (Maqueda et al., 2007) have been proposed as promising materials to decrease the environmental impact of pesticides. Such clays would indeed offer the possibility to limit the amount of pesticide available (slow release formulations) for undesirable processes such as runoff, leaching and consequent ground water contamination. Understanding the pesticides interaction with soil components will allow diminishing the dose of active ingredient applied to assure good yields of cereals and a reduction of the risks of soil, water and atmospheric contamination. The economic saving and environmental protection resulting from the understanding of these interactions are not negligible. ⁎ Corresponding author. Tel./fax: +54 0221 4710075. E-mail addresses: [email protected], [email protected] (R.M. Torres Sánchez). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.06.026

Thermally treated montmorillonites can provide a simple possibility to change the adsorption properties of clay minerals. New adsorption sites can originate from the movement of octahedral cations produced from dehydration and dehydroxylation processes during calcination of clay minerals (Odom, 1984; Heller-Kallai and Rozenson, 1980; Rozenson and Heller-Kallai, 1978; Drits et al., 1995; Emmerich et al., 1999; Emmerich, 2000). Enhanced hydrophobic herbicide metolachlor adsorption was linked to the increase of the montmorillonites specific mesopore surface area produced by calcination, which was promoted by the exposure of the aluminum ions at the surface edges (Bojemueller et al., 2001). Different milling processes were also used to obtain clay structure transformations: thus processing in an oscillating mill changes the clay crystal structure (Torres Sánchez et al., 1999). High energy ball milling produces sub-micrometer rounded particles, as well as nanometric particles of montmorillonite (Dellisanti and Valdre, 2005). Ionized argon interaction generates mechanical deformations with modification of surface characteristics and particle size distribution, but without particle agglomeration or compaction (Dellisanti and Valdre, 2005). High energy ultrasound generates exfoliation in the c-axis with some sheet rupture in other directions and minimal crystallinity transformation (Pérez-Maqueda et al., 2001) which has

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been demonstrated to be important in order to obtain a controlled release of herbicides (Maqueda et al., 2009). Thermal and mechanical treatments on raw kaolinite are claimed to modify the aluminium coordination that changes from octahedral to tetrahedral (Torres Sánchez et al. 1999), accounting for the isoelectric point (IEP) increase from pH around 3 to 7. The same shift of IEP values was found in montmorillonites with analogous treatments (Torres Sánchez, 1997) and also a corresponding Al(VI) turned into Al(IV) in octahedral sheets was found for thermal modified montmorillonites (Wu et al., 2005). The similar structure and the IEP shift of kaolinite and montmorillonite, suggests changes in the aluminium coordination of montmorillonites comparable to those observed in kaolinites after these treatments. The changes in IEP should modify the electrostatic attraction of organic molecules and consequently the adsorption capacity of the clay. In 1975, Aharonson and Kafkafi, in order to explain the strong adsorption of benzimidazole derivative fungicides on the soil, suggested cationic adsorption caused by the protonation of these molecules. Lombardi et al. (2006) demonstrated the co-adsorption of benzimidazole or thiabendazole and protons in aqueous acidic solutions. The electrophoretic mobility indicated the adsorption of both molecules on montmorillonite. In addition, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) identified the inclusion in the interlayer of both fungicides in a planar monolayer arrangement. This indicated that the interlayer sodium ions were replaced by the organic molecule due to the presence of positive charge on the protonated nitrogen of both fungicides. The intercalation of benzimidazole into the interlayer space of Ni-montmorillonite was also confirmed recently by Jóna et al. (2008). Further to its use as a precursor of fungicide, benzimidazole acts as a ligand in a variety of biological molecules (Tong et al., 2009) and is used to synthesize molecules of pharmaceutical or biological interest (Li et al., 2008). Its derivative 2-mercaptobenzimidazole adsorbed on Hectorite was used to remove Cr(VI) from aqueous solution (Guerra et al., 2009). The silver chelate of 2-(4-thiazolyl) benzimidazole was exchanged in clays to prepare antimicrobial and antifungal agents (Ohashi et al., 1998). Recently, complexes of benzimidazole with some transition metal were shown to exhibit higher fungicidal activity than the standard fungicide, benzimidazole dithiocarbamate (Mohamed et al., 2009). The purpose of this study is to evaluate the efficiency of montmorillonite (raw, thermally and mechanically treated) for benzimidazole removal. This study would provide evidence of the general adsorption characteristics of the benzimidazole fungicide family (benomyl, carbendazim, chlorfenazole, cypendazole, debacarb, fuberidazole, mecarbinzid, rabenzazole, thiabendazole, etc.). To attain this objective an Argentine montmorillonite was heated at different temperatures during two fixed times or milled for 300 s. Thermodynamic parameters were calculated and a hypothesis concerning the sorption mechanism was proposed. 2. Materials and methods 2.1. Materials and thermal and mechanical treatments A bentonite sample from Argentine North Patagonia (Rio Negro) was used as raw material. The montmorillonite fraction b2 μm was isolated from the raw bentonite by centrifugation at 14,000 rpm and denoted as Mo. CEC was 105 meq/100 g (Lombardi et al., 2003). The Mo mineralogy was evaluated by X-ray diffraction and chemical analysis in previous works (Viseras Iborra et al., 2006; Lombardi et al., 2002) and consists of Na-rich montmorillonite with minor phases as quartz and feldspars. Some fractions of Mo were mechanically deformed in an oscillating mill (Herzog HSM 100) for 300 s (denoted Mo300 s). Other fractions of Mo were heated at 350 °C and 550 °C during 3 h and 12 h

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periods at each temperature. The products obtained were named with a subscript indicating the treatment (temperature and time, Mo350 3, Mo350 12, Mo550 3 and Mo550 12). Chemical analyses of all samples without previous washing were made by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). The structural formula determination requires the knowledge of percent metal oxide composition. The Al+ 3 released by both kinds of treatment (thermal and mechanical) were determined by Atomic Absorption Spectroscopy (AAS) in the aqueous phase of the corresponding suspensions (0.5 g clay/L). The 2-(thiazol-4-yl) benzimidazole supplied by Chem Service, West Chester, PA (99% purity) was used as received. It is a crystalline substance with a molecular weight of 118.14, melting point of 170.5 °C, and the solubility in water at 30 °C is 2.01 × 103 mg/L (Weber et al., 1969). 2.2. Methods 2.2.1. Materials characterization The pH of the samples was determined on 200 mg/L suspensions. Isoelectric point (IEP) determinations were performed by means of diffusion potential measurement as described elsewhere (Torres Sánchez, 1996) in KCl as support electrolyte. The swelling capacity was determined on 2 g of bentonite following the procedure indicated in Besq et al. (2003). The swelling index value (SI) corresponds to the volume, expressed in cm3, of the swollen bentonite measured after 24 h. The specific surface area was determined by water adsorption at room temperature (Sw) (Ormerod and Newman, 1983; Torres Sánchez and Falasca, 1997) and nitrogen adsorption at 77 K (SN2) using a Micromeritics model TriStar 3000. Internal or interlayer surface was determined as the difference between total and external surfaces, Sw and SN2 respectively (Michot and Villieras, 2006). To determine the BET surface areas (SN2), all samples were dried at 100 °C for 6 h at high vacuum before nitrogen sorption. BJH (Barrett– Joyner–Halenda) pore size and micropore distributions (Dubinin– Astakhov) were computed using the Micromeritics software package associated with the instrument. In the case of the BJH pore distributions, the data derived from the desorption branch of each isotherm was used (Barrett et al., 1951). Standard errors of surface values determination were 10% for Sw and 1% for SN2. Errors of interlayer surface values were similar to those of Sw. 2.2.2. Benzimidazole adsorption experiments The benzimidazole adsorption (range between 0.3 to 1.5 mM) was performed in 0.005% w/w sample suspension at 20 °C, with continuous shaking, without pH control and without ionic strength adjustment. After 24 h contact, the supernatants were isolated from solution by centrifuging at 3500 rpm and further filtering with Microclar nitrocellulose membranes of 0.2 m pore size. The solutions were analyzed without any treatment by UV spectrophotometry (maximum absorption λ = 267 nm) using a Hewlett-Packard 8453 UV–visible spectrophotometer. The dynamic linear range was within 0.1–0.7 mM (R2 = 0.99) of benzimidazole concentrations. The amount of benzimidazole adsorbed was determined as the difference between initial concentration and that of the supernatant in equilibrium. All the isotherms were determined in triplicate. In this study, the Langmuir isotherm model was used to fit the data. Solid samples with indicated benzimidazole adsorption were washed three times with distilled water, air dried and stored in desiccators over silica gel, at room temperature, for use in characterization experiments (XRD and XPS). 2.2.3. X-ray diffraction Crystallographic data of samples without and with benzimidazole adsorbed were obtained by X-ray diffraction (XRD) on semioriented

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(powder samples), and also on oriented samples. These were prepared by spreading the sample suspension on glass slides, followed by drying at room temperature over night and with atmospheric humidity control rh = 0.47. Analyses were performed using a Philips PW 1710 diffractometer using CuKα radiation. The measurement conditions were power supply at 40 kV and 30 mA; 1° divergence and detector slits. 0.02° (2θ) step size, counting time of 10 s/step and patterns collected from 3° to 14° (2θ). Some d(001) peaks were deconvoluted using the origin Pro 7.0 software (fitting wizard method). The fit quality was controlled by the R2 value (N0.98). 2.2.4. XPS XPS analyses were performed on a Kratos Axis Ultra spectrometer (Kratos Analytical, Manchester, UK) equipped with a monochromatized aluminium X-ray source (powered at 10 mA and 15 kV) and an eight channeltrons detector. The sample powders were pressed into small stainless steel troughs of inner diameter 4 mm and 0.5 mm depth, mounted on a multi specimen holder. The pressure in the analysis chamber was about 10−6 Pa. The angle between the normal to the sample surface and the direction of photoelectrons collection was about 0°. Analyses were performed in the hybrid lens mode with the slot aperture and the iris drive position set at 0.5, the resulting analyzed area was 700 μm × 300 μm. The pass energy was set at 160 eV for the wide scan and 40 eV for narrow scans. In the latter conditions, the full width at half maximum (FWHM) of the Ag 3d5/2 peak of a standard silver sample was about 0.9 eV. Charge stabilization was achieved by using the Kratos Axis device. The following sequence of spectra was recorded: survey spectrum, C1s, O1s, Al 2p, Si 2p, Mg 2s, Fe 2p, N 1s, Na 1s and C1s again to check for charge stability as a function of time and the absence of degradation of the sample during the analyses. The C–(C, H) component of the C1s peak of carbon has been fixed to 284.8 eV to set the binding energy scale. Spectra were decomposed with the CasaXPS program (Casa Software Ltd., UK) with a Gaussian/Lorentzian (70/30) product function and after subtraction of a linear baseline. Molar fractions were calculated using peak areas normalized on the basis of acquisition parameters after a linear background subtraction, experimental sensitivity factors and transmission factors provided by the manufacturer. Elemental mole fractions are provided, excluding hydrogen which is not detected by XPS. 3. Results and discussion 3.1. Sample characteristics The structural formulae of the samples were determined from the chemical analysis, presented in Table 1, following the method of Siguin et al. (1994), and summarized in Table 2. The aluminium losses in the washing suspensions, for the Mo and the thermally treated (Mo350 3, and Mo550 3) samples were low (around 0.1 mg/g clay) resulting in a similar structural formula for both samples. On the contrary aluminium loss in the washing

Table 2 Structural formula. Sample

Structural formula

Mo Mo Mo

[(Si3.88 Al0.12)(Al1.44Fe3+0.28 Mg0.28)O10(OH)2] M+0.40 [(Si3.92 Al0.08)(Al1.44 Fe3+0.23 Mg0.33) O10(OH)2] M+0.42 [(Si3.91 Al0.09)(Al1.44 Fe3+0.22 Mg0.34) O10(OH)2] M+0.43

350 3 550 3

suspension of the Mo300 s sample is high (16.9 mg/g clay), making it difficult to determine accurately its structural formula. Table 3 shows the suspension pH, IEP and swelling index values of all samples. The suspension pH values decrease when the sample is subjected to a thermal treatment, the decrease being more pronounced when the time of the treatment increases. Additionally the temperature of the treatment seems to affect the suspension pH: the higher the temperature, the lower the pH at the end of a long treatment. The suspension pH also decreases for mechanical treated sample. These pH variations were previously assigned to aluminium release into the solution, and/or the enrichment of aluminium ions or hydroxoaluminium species at the edges and/or to the face (+) edge (−) contacts (Bojemueller et al., 2001) and/or the migration of Al cations from the original trans-octahedral sites to formerly unoccupied five-fold prisms (Drits et al., 1995). Thermally and mechanically treated products exhibit an increase at the IEP pH value in agreement with that found for homoionic smectites with similar treatments (Torres Sánchez, 1997). The swelling index value also decreases upon treatment, suggesting that the rehydration of the interlayer cations decreases with the increase of the treatment temperature and time and that the mechanical treatment induces some breakdown of the layered structure, in agreement with that found by Hrachová et al. (2007). 3.2. Specific surface area and porosity The specific surface area (SN2) values of Mo and thermally treated samples decrease with temperature increase and 3 h treatment (Table 4) following a similar behaviour to that found by Bojemueller et al. (2001) for a Wyoming bentonite and their thermally treated products. The thermally treated samples showed a decrease of micropore volume (Table 4) and a coherent increase of mesoporosity with increase of treatment time and temperature. The Mo300 s sample shows a much higher specific surface area (SN2) compared to the other samples. The increase of SN2 value found for the Mo300 s sample was also consistent with the micropore volume increase and lower average pore diameter than those found for thermal or raw samples. Compared to Mo, the total specific surface area (Sw) value is decreased for the thermally treated samples, which demonstrates that the interlayer collapses with the thermal treatment. For the Mo300 s sample the decrease of Sw indicated not only the interlayer collapse, but also some structure deterioration by distortion of the layers, which in turn facilitates dehydroxylation of montmorillonite (Hrachová et al., 2007). These results are in agreement with those found for mechanically treated kaolinite (Torres Sánchez et al., 1988), and were also confirmed by XRD measurements as discussed in the next paragraph.

Table 1 Chemical analysis and Al+ 3 released. Sample

SiO2 %

Al2O3 %

Fe2O3 %

CaO %

MgO %

Na2O %

K2O %

TiO2 %

Al+ 3 (mg/g clay)

Mo Mo 350 3 Mo 550 3 Mo300 s

58.2 57.3 59.6 54.1

19.9 18.8 19.8 17.8

5.62 4.47 4.48 4.12

0.46 0.46 0.39 0.34

2.77 3.28 3.45 3.02

2.31 1.64 1.77 1.71

0.07 0.17 0.16 0.19

0.55 0.15 0.16 0.15

– 0.10 0.16 16.9

Table 3 Suspension pH, IEP and swelling index (SI).

Suspension pH IEP (pH) SI (cm3/g)

Mo

Mo350

8.0 3.9 37.3

7.7 4.3 31.6

3

Mo350 7.6 4.5 25.0

12

Mo550 7.9 8.3 6.89

3

Mo550 7.3 8.4 3.65

12

Mo300 s 6.7 8.0 10.0

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Table 4 Specific surface areas: SN2, total (Sw) and internal (Sw–SN2), micro pore area, average pore diameter and micropore volumes. Sample

SN2 (m2/g)

Sw (m2/g)

Internal Surf. (m2/g)

Micropore Area (m2/g)

Micro pore Vol (cm3/g)

Average Pore Diam Desor BET (nm)

Average Pore Diam Desor. BJH (nm)

Mo Mo350 3 Mo350 12 Mo550 3 Mo550 12 Mo300 s

65.1 ± 0.4 61.1 ± 0.3 61.2 ± 0.3 54.2 ± 0.5 49.5 ± 0.2 146 ± 0.7

792 ± 8.0 415 ± 4.2 319 ± 3.2 130 ± 1.3 113 ± 1.1 222 ± 2.2

727 354 258 76 64 76

40.7 34.4 34.5 27.6 23.0 92.1

0.0196 0.0158 0.0160 0.0133 0.0113 0.0423

6.3 6.8 6.7 8.2 8.7 4.3

4.7 5.8 5.7 7.7 8.1 6.2

3.3. X-ray powder diffraction of montmorillonite samples without benzimidazole The X-ray diffraction pattern (Fig. 1) of the Mo sample shows a (001) peak at 1.35 nm (6.5° 2θ) suggesting the predominance of more than one-layer hydrate, taking into account that a (001) diffraction of montmorillonite at 1.26 nm indicates a water monolayer in the interlayer space (Borchardt, 1977; Lagaly, 1993; Magaña et al., 2008). The XRD pattern of Mo sample dried overnight at 80 °C (not shown), showed a (001) peak at 0.96 nm. For the thermally treated samples the (001) peak shift from 1.35 to 0.96 nm (6.5 to 9.2θ) indicates a loss of water and consequently a decrease of the interlayer space close to that of fully dehydrated montmorillonite at 0.96 nm (Ormerod and Newman, 1983) or pyrophyllite at 0.92 nm (Erdemoglu et al., 2004). This shift indicates a progressive decreasing of structural order due to the continuous removal of interlayer water molecules followed by destabilization of structural OH as a non-linear function of the treatment time (Bojemueller et al., 2001). For the Mo300 s sample, the semioriented pattern differs from those of thermally treated samples (insert of Fig. 1) (Magaña et al., 2008). Generally, the mill treatment by conventional ball or planetary mills yields almost totally amorphous material after 20 h grinding (Volzone et al., 1987). The remarkable broadening from 0.96 to 1.23 nm for the Mo300 s sample and the almost disappearance of the (001) diffraction peak indicate a change of the c-axis. Also the low amorphisation shown previously by Magaña et al. (2008) for this sample indicates that the milling process seems to break montmorillonite crystallites along the ab plane, as was suggested by Christidis et al. (2005).

following a cation exchange mechanism. This mechanism was established by both the nitrogen protonation of the fungicide determined by XPS analysis and the exchange of the interlayer sodium ions. Adsorption of benzimidazole normalized by montmorillonite weight (Fig. 2A) decreases with increasing temperature and time of adsorbent pretreatment. Similar adsorption behaviour is observed in the Mo300 s sample with respect to Mo sample. The removal efficiency for the different sorbents at pH 6 follow the sequence raw montmorillonite N thermally treated montmorillonite N mechanically treated montomorillonite. The adsorption shapes on all samples seem to indicate a two steps behaviour. The first step is observed from 0 to around 20 mmol/g surface coverage values and the second step from 20 mmol/g to higher surface coverage values. This behaviour suggests that a second monolayer adsorption starts after the first monolayer is completed. Then, for a better comparison, in Fig. 2B, the surface coverage values (Γ) were normalized to the specific surface area. The external specific surface (SN2) or total surface (Sw) values were used for low or high benzimidazole concentrations, respectively. This normalization shows the relative contribution of each kind of surface site to the total benzimidazole adsorption pointing out that the interlayer adsorption

3.4. Adsorption isotherms Lombardi et al. (2006) found that a maximum amount of 0.10 g/g of benzimidazole was adsorbed on a similar raw montmorillonite at pH 5

Fig. 1. X-ray diffraction patterns in semioriented condition without benzimidazole.

Fig. 2. Adsorption isotherms of benzimidazole at pH 6 and 1 %w/w suspensions. (▲) M; ( )Mo350 3; ( ) Mo350 12; ( ) Mo550 3; ( ) Mo550 12 and ( ) Mo300 s. The symbols represent averages from two replicates and the maximum scatter was less than ±5% of the average values. (A) Γ in weight basis and (B) Γ normalized with respect to the specific surface area of each sample (Sw). Solid lines were calculated using the Langmuir and SN2 and Sw data from Table 4.

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process is energetically unfavorable as compared to the adsorption process on the external surface. Now, the adsorption sequence is Mo N Mo550 3 ~ Mo350 3 ~ Mo550 12 N Mo350 12 N Mo300 s. This sequence shows that the thermal treatment decreases the adsorption as the heating time increases. This thermal effect is more evident at 350 °C than at 550 °C while the changes after 3 h heating are little noticeable. The mechanically treated sample shows always the lower adsorption efficiency. This adsorption behaviour would be directly related to the surface changes due to thermal or mechanically treatment. In this sense, the samples heated at 350 °C show a similar external surface than that of Mo but the internal surface decreases with heating time increase leading to a decrease in adsorption in the interlayer that is represented by the isotherm drop. For the samples heated at 350 °C the external surface area decreases about 6%. In contrast the internal surface area decreased 50% for the sample heated for 3 h and 65% for the sample heated 12 h (see Table 4). For the samples heated at 550 °C the internal and external surface areas are similar, leading to a slight difference in the adsorption isotherms but the external surface area decreases from 70 to 80% and the interlayer surface area decreases around 99.9% reaching the maximum collapsing value. The most important differences in these last samples consist of the external surface change which leads to small changes in the adsorption process due to a much smaller number of available sites. In this way the changes are scarcely noticeable. The adsorption isotherms were fitted to the Langmuir isotherm equation using Excel solver package. The experimental and mathematically calculated isotherms (Fig. 2) present a regression coefficient N0.99 and a significant p-level of 0.001 for the first branch of the adsorption isotherms. The adsorption constants (K1 and K2) and the maximum surface coverage (Γ1∞ and Γ2∞) are shown in Table 5 where subscripts 1 and 2 correspond to the thermodynamic parameters obtained by normalizing by external surface and external + interlayer space surfaces, respectively. The adsorption constants, K1 and K2, have values varying between 1.1 and 1.2 for all the samples showing that the adsorption sites have the same identity and the adsorption differences are due to the differences on the available number sites for the adsorption process. This difference is mainly due to Γ2∞, which shows the adsorption extent on the interlayer. Thus, the determined values show the thermal interlayer collapses while, for the Mo300 s sample, surface modification is also evidenced since the value of K1 shows different affinity to the surface indicating that the adsorption sites have been structurally modified. On the other hand, the K1 value for the Mo300 s sample also shows that the number of sites susceptible to be coordinated is 40% lower than that of the Mo sample. Accordingly while the external surface area increases, the affinity diminishes and only 40% of the sites are coordinated leading to a significant decrease in the adsorption capacity of this sorbent. With regards to the processes in the interlayer of Mo300 s sample, while the affinity value is the same as for other samples, the Γ2∞ value is very small compared with that of the Mo550 12 sample having similar value of interlayer area. This behaviour indicates an much higher affinity for the obtained surface coverage of the Mo300 s sample. Then, the mechanical treatment is somehow modifying the interlayer access channels for the molecule under study.

The differences among Mo and their thermally treated products cannot be assigned to different cationic exchange capacities, because heating Na+-montmorillonite up to 400 °C did not affect the CEC (Emmerich et al., 1999). The reduced adsorption may be explained by the decrease of the number of exchange sites and by Al3+ release from the structure during treatments which also generates the higher IEP of thermal and mechanical products, (Table 3). The Al3+ released from the structure in Mo300 s sample would occupy the exchange sites leading to a decrease of the available adsorption sites number. Additionally a lower CEC value (Christidis et al., 2005) than those indicated for raw and thermally treated samples would limit the adsorption of the organic cations. The two surface sites, indicated by the successive adsorption steps, were also confirmed by the CEC of Mo (105 meq/100 g, Lombardi et al., 2003) which allows a maximum adsorption of 1 mmol/g in the interlayer sites by exchange reaction mechanisms. The remnant amount of benzimidazole/g clay (up to 50 mmol/g for Mo sample) must be adsorbed on a second surface site (Damonte et al., 2007). This behaviour is also evidenced by the thermally treated samples due to the low CEC value changes with thermal treatment observed for montmorillonites as indicated above. 3.5. XRD of montmorillonite samples adsorbed with benzimidazole The changes in the (001) peak of montmorillonite samples with adsorbed benzimidazole were used to validate the hypothesis of two different surface sites on the montmorillonite surface. To attain this objective , firstly the XRD patterns of powder samples without benzimidazole (Fig. 1) were analyzed and compared with those obtained under similar conditions with a low benzimidazole adsorption (around 16 mmol/g clay) (Fig. 3). The presence of benzimidazole in the interlayer surface was determined by the shift of the d(001) diffraction peak. Secondly, the XRD patterns on oriented condition at constant humidity (rh = 0.47) that allow a higher accuracy of the peak width estimation by the intensity enhance of the d(001) diffraction were used for further analysis. The XRD patterns of oriented samples without benzimidazole were shown in Fig. 4A and those of samples with two different benzimidazole amount adsorbed (around 16 and 30 mmol/g clay) were shown in Fig. 4B and C, respectively. The comparison of the d (001) diffraction originated by the different benzimidazole amount adsorbed, would suggest the saturation of the interlayer surface site by benzimidazole molecules. The decomposition of the d(001) peak would demonstrate the inability of water to penetrate in the interlayer due to the presence of a high amount of benzimidazole.

Table 5 Adsorption constants (K) and the maximum surface coverage (Γ∞). Sample

K1 (L/mmol)

Γ1∞ (mmol/m2)

K2 (L/mmol)

Γ2∞ (mmol/m2)

Mo Mo350 3 Mo350 12 Mo550 3 Mo550 12 Mo300 s

1.20 1.25 1.10 1.20 1.10 0.80

0.51 0.50 0.50 0.50 0.50 0.35

1.10 1.10 1.20 1.10 1.20 1.20

0.45 0.20 0.15 0.22 0.22 5 × 10−3

Fig. 3. X-ray diffraction patterns in semioriented condition with around 16 mmol benzimidazole/g clay.

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interlayer, the difference between the measured basal spacing with benzimidazole and that of pyrophyllite (1.32 nm–0.92 nm= 0.40 nm) might be taken into account to determine whether the benzimidazole could really penetrate the interlayer space or not. This hypothesis seems realistic considering the benzimidazole molecular size of 0.45 nm estimated using the Molecular Modeling and Geometry Optimization Tools for HyperChem (Hyperchem software v 7.0), and its arrangement in a planar monolayer in the interlayer of the Mo sample. The same interlayer space shift was found for a montmorillonite of same characteristics from the same region and was also assigned to a close approach of the benzimidazole to the surface oxygen atoms (Lombardi et al., 2006). The examination of the Mo sample d(001) peak (insert of Fig. 3) revealed a shoulder at 6.20°(2θ) (1.42 nm). The decomposition of the Mo d(001) peak into with two Gaussian components suggested the existence of at least two populations of layer spacings at 1.42 and 1.30 nm, respectively (6.20 and 6.78°(2θ)) attributed to the presence of benzimidazole and also some water molecules in the interlayer. For thermally and mechanically treated samples (Fig. 3), the entrance of the benzimidazole molecules in the interlayer space produces a d(001) diffraction peak shift from 0.96 nm (value close to that of pyrophyllite) to around 1.28 nm. Fig. 3 also indicates a net decrease of the (001) diffraction peak intensity of the Mo550 and Mo300 s samples with respect to the Mo and Mo350 samples, which suggests different benzimidazole entrance capacities. The second series of XRD spectra recorded on samples oriented and equilibrated at constant humidity (rh = 0.47), without and with two different benzimidazole amounts, shows the influence of adsorbed water on the d(001) peak. XRD of samples without benzimidazole obtained in oriented form (Fig. 4A), showed the classical intensity enhancement of the d(001) diffraction and a shift of this diffraction towards lower values of 2θ angle (1.45 nm), with respect to same semioriented sample patterns (Fig. 1). The latter behaviour indicates the reversibility of water entrance in the interlayer space (Emmerich et al., 1999). In particular for the Mo550 12 sample the d(001) peak of the oriented form (inset of Fig. 4A) showed a width increase and for the Mo300 s sample a loss of the first shoulder was observed with respect to the XRD pattern of semioriented form analysis. XRD patterns of oriented samples with 16 mmol of benzimidazole/g clay (Fig. 4B), show a d(001) peak shift towards higher 2θ angle (1.29 nm) when benzimidazole penetrates into the interlayer. The same shoulder at 6.20°(2θ) (1.42 nm) appeared on the left hand side of the peak which was assigned to one layer of water molecules, as already observed for semioriented samples (Fig. 3). On samples with a higher amount of benzimidazole (30 mmol/g clay) (Fig. 4C) the peaks are more symmetrical, with the disappearance of the shoulder at 6.20°(2θ) indicating that fewer water molecules can access the interlayer space. The d(001) diffraction intensity increases with the increase of the concentration of benzimidazole. This observation is in agreement with that found by Suquet et al. (1975) and Brindley (1980) and was indicative of less randomly arranged layers (or progressive ordering of the layers). 3.6. XPS analysis Fig. 4. X-ray diffraction patterns in oriented condition and constant humidity (rh = 0.47). (A) Without benzimidazole, (B) with around 16 mmol benzimidazole/g clay and (C) with around 30 mmol benzimidazole/g clay.

The first XRD analysis, on powder samples, indicates that low benzimidazole adsorption in all samples (Fig. 3) induced changes in the (001) peak, indicating a rearrangement in the interlayer space due to the presence of the fungicide. The (001) peak of the Mo sample in Fig. 3 is narrow, its intensity increases and its position was shifted slightly (from 1.35 nm, see Fig. 1, to 1.32 nm) with respect to Mo without benzimidazole. Considering that samples can have different amounts of water molecules in the

Lombardi et al. (2006) using a montmorillonite sample of similar characteristics, found that sodium was detected in pure benzimidazole. After fungicide adsorption, sodium was only detected on samples treated at pH 7.0 but not at pH 5.3. The surface elemental composition, determined by XPS for all samples after benzimidazole adsorption is presented in Table 6. The sodium concentration increases with temperature and time thermal of treatment, while the N concentration, characteristic of the adsorbed amount of benzimidazole, decreases in agreement with data published on thiabendazole and benzimidazole on raw sample at pH 7 (Lombardi et al., 2006). The N/C ratio calculated from the wide scan recorded on a

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Table 6 Surface elemental composition determined by XPS (molar fractions in %, and Ntotal/Ctotal molar ratio).

Ctotal Ntotal O Mg Si Al Fe Na N total/C

total

Mo

Mo350

13.6 0.7 57.0 1.3 18.4 7.5 1.1 0.3 0.05

11.1 1.1 57.7 1.3 19.5 7.9 1.1 0.3 0.10

3

Mo350 10.7 0.8 57.8 1.5 19.9 7.9 0.7 0.6 0.08

12

Mo550 7.6 0.9 59.9 1.8 20.2 8.1 1.0 0.5 0.12

3

Mo550 6.9 0.8 58.9 1.6 20.9 8.6 1.7 0.7 0.12

12

Table 7 N1s components binding energies (eV) and Nprot/Nnon prot ratio. Sample

Binding energy N1s Nprot+

Binding energy N1s Nnon prot

Nprot/Nnon prot (401/399)eV

Mo Mo350 3 Mo350 12 Mo550 3 Mo550 12 Mo300 s

401.9 401.9 402.0 401.6 400.6 400.9

400.2 400.1 400.3 399.8 398.6 399.2

1.72 1.99 1.76 2.99 3.08 0.82

Mo300 s 9.3 1.0 57.7 1.7 20.0 8.5 1.3 0.5 0.11

pure benzimidazole was 0.22, not far from 0.28 expected from the stoichiometry (Lombardi et al., 2006). The N/C ratios on all adsorbed samples (Table 6) were significantly lower than 0.28 expected on pure benzimidazole, as already found in a previous study (Lombardi et al., 2006). But an increase by a factor 2 was found between raw (Mo) and treated samples. This systematic lower N/C ratio could be explained by the presence of an adventitious carbon layer on the samples surface or by the decomposition of the benzimidazole molecule which may proceed differently for free and adsorbed fungicide. The N1s peaks of samples Mo, Mo350 3, Mo350 12, Mo550 3, Mo550 12 and Mo300 s were decomposed into two components (Fig. 5, Table 7) centered at about 399 and 401 eV which were attributed to non protonated and protonated nitrogen (Canesson et al., 1978), respectively. The theoretical protonated/non protonated nitrogen ratio (Nprot/ Nnon prot) expected for benzimidazole is 1, while those obtained in Lombardi et al. (2006) for benzimidazole adsorbed on a similar montmorillonite were 2.1 and 1.89 at pH 5.3 and 7.0, respectively. Similar Nprot/Nnon prot ratios were obtained for raw and thermally treated samples at 350 °C (Fig. 5). However, the ratio increased from around 2 to around 3 for samples treated at 550 °C and a significant decrease was observed (from around 2 to 0.82) for the Mo300 s sample. These results are indicative of the different N coordinative demeanor to the surface. For the Mo and Mo350C samples, Nprot and Nnon prot are similarly coordinated such as those previously reported in agreement with data found for other raw montmorillonites

(Lombardi et al., 2006). For samples treated at 550 °C an increase of the Nprot/Nnon prot ratio indicates a higher N coordination to the surface. For the Mo300 s sample the Nprot/Nnon prot ratio was close to 1 indicating that the benzimidazole is not coordinated to the surface by the N and that only a cationic exchange is involved in the process. The IEP data (Table 3) indicate similar behaviour of electrostatic attraction of the sample surface to NH and N. The IEP of both Mo and Mo350 °C samples is close to 4, indicating a similar electrostatic attraction by the surface of these samples to nitrogen. Thermally treated Mo550 °C samples, with an IEP around 8.4, will develop a higher surface electrostatic attraction to nitrogen whilst Mo300 s with a similar IEP value to Mo550 °C, the structural damage seems to modify the nitrogen adsorption process.

4. Conclusions Thermal and mechanical treatments of montmorillonite produced a decrease of specific surface area. However a micropore volume decrease and increase was found respectively for thermally and mechanically treated samples. A two step sorption mechanism hypothesis is proposed to explain the adsorption isotherms and the XRD results. This successive adsorption corresponds to the adsorption on surface sites that are structurally different, namely the external and the interlayer surfaces. In the latter a planar monolayer arrangement of benzimidazole can be attained with a consequent decrease of water molecules access. The XPS results, in agreement with IEP values, indicate a different N coordination of the benzimidazole molecule to the surface. For raw

Fig. 5. N 1s XPS peak recorded with a pass energy of 40 eV on benzimidazole adsorbed (around 30 mmol benzimidazole/g clay) on the different samples.

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and thermally (350 °C) treated samples, similarly coordinated Nprot and Nnon prot are proposed. A stronger coordination of N to the surface of thermal (550 °C) treated samples was found. Only cationic exchange, without N coordination to the surface, was indicated for the mechanical treated sample. Modified montmorillonites by thermal and mechanical treatments showed a decrease in the benzimidazole adsorption capacity when comparing with the adsorption capacity of raw montmotillonite. Consequently, these modificated montmorillonites do not show an improvement as fungicide sequestrant with respect to the raw montmorillonite and the potential use of the treated clays as sorbents of the studied chemicals are relative. Acknowledgments The authors acknowledged the Secretaria de Ciencia y Técnica (SECyT-Argentina) and the National Foundation for Scientific Research (FNRS, Belgium) (BE/PA05-EXIII/001) and ANPCyT-FONCyT PICT 2332678 and 1360 for the financial support. The authors also want to thank R. Maley for the English improvement. References Aharonson, N., Kafkafi, U., 1975. Adsorption of benzimidazole fungicides on montmorillonite and kaolinite clay surfaces. J. Agric. Food Chem. 23 (3), 434–437. Azejjel, H., del Hoyo, C., Draoui, K., Rodríguez-Cruz, M.S., Sánchez-Martín, M.J., 2009. Natural and modified clays from Morocco as sorbents of ionizable herbicides in aqueous medium. Desalination 249, 1151–1158. Barrett, E., Joyner, L., Halenda, P., 1951. The determination of pore volume and area distributions in pure substances. J. Amer. Chem. Soc. 73, 373–380. Besq, A., Malfoy, C., Pantet, A., Monneta, P., Righi, D., 2003. Physicochemical characterization and flow properties of some bentonite muds. Appl. Clay Sci. 23, 275–286. Bojemueller, E., Nennemann, A., Lagaly, G., 2001. Enhanced pesticide adsorption by thermally modified bentonites. Appl. Clay Sci. 18, 277–284. Borchardt, G., 1977. In: Dixon, J., Weed, S., Kittrick, J., Milford, M., White, J. (Eds.), Minerals in Soil Environments. S.S.S. Amer, Madison, WI, p. 293. Brindley, G. W., 1980 in: Brindley G. and Brown G. (Eds.), Crystal structures of clay minerals and their X-ray identification. Chap. Order disorder in clay mineral structure (.) Miner. Soc. of London. 125–129. Canesson, P., Cruz, M.I., van Damme, H., 1978. XPS study of the interaction of some porphyrins and metalloporphyrins with mont. In: Mortland, M.M., Farmer, V.C. (Eds.), Proc. VI Internat. Clay Conference. : Developments in Sedimentology, 27. Elsevier, Amsterdam, pp. 217–222. Carrizosa, M.J., Calderón, M.J., Hermosín, M.C., Cornejo, J., 2000. Organosmectites as sorbent and carrier of the herbicide bentazone. Sci. Total Environ. 247, 285–293. Christidis, G.E., Dellisanti, F., Valdre, G., Makri, P., 2005. Structural modifications of smectites mechanically deformed under controlled conditions. Clay Miner. 40 (4), 511–522. Damonte, M., Torres Sánchez, R.M., dos Santos Afonso, M., 2007. Some aspects of the glyphosate adsorption on montmorillonite and its calcined form. Appl. Clay Sci. 36, 86–94. Dellisanti, F., Valdre, G., 2005. Study of structural properties of ion treated and mechanically deformed commercial bentonite. Appl. Clay Sci. 28, 233–244. Drits, V., Besson, G., Muller, F., 1995. An improved model for structural transformations of heat treated aluminous dioctaedral 2:1 layer silicates. Clays Clay Miner. 43, 718–731. El-Nahhal, Y., Nir, S., Polubesova, T., Margulies, L., Rubin, B., 1998. Leaching, phytotoxicity, and weed control of new formulations of alachlor. J. Agric. Food Chem. 46, 3305–3313. El-Nahhal, Y., Nir, S., Margulies, L., Rubin, B., 1999a. Reduction of photodegradation and volatilization of herbicides inorgano-clay formulations. Appl. Clay Sci. 14, 105–119. El-Nahhal, Y., Nir, S., Polubesova, T., Margulies, L., Rubin, B., 1999b. Movement of metolachlor in soil: effect of new organo-clay formulations. Pestic. Sci. 55, 857–864. Emmerich, K., 2000. Spontaneous rehydroxylation of dehydroxylates cis-vacant montmorillonite. Clays Clay Miner. 48, 405–408. Emmerich, K., Madsen, F., Kahr, G., 1999. Dehydroxylation behaviour of heat treated and steam treated homoionic cis-vacant montmorillonites. Clays Clay Miner. 47, 591–604. Erdemoglu, M., Erdemoglu, S., Sayilkan, F., Akarsu, M., Sener, S., Sayilkan, H., 2004. Organo-functional modified pyrophyllite: preparation, characterization and Pb(II) ion adsorption property. Appl. Clay Sci. 27, 41–52. Guerra, D.L., Viana, R.R., Airoldi, C., 2009. Application of natural and modified hectorite clays as adsorbents to removal of Cr(VI) from aqueous solution—thermodynamic and equilibrium study. J. Hazard. Mater. 172, 507–514. Heller-Kallai, L., Rozenson, I., 1980. Dehydroxylation of dioctahedral phyllosilicates. Clays Clay Miner. 28 (5), 355–368. Hermosín, M.C., Calderón, M.J., Aguer, J.P., Cornejo, J., 2001. Organoclays for controlled release of the herbicide fenuron. Pest Manage. Sci. 57, 803–809.

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