Clomazone 2-(2-chlorobenzyl)-4,4-dimethyl-1,2-oxazolidin-3-one is an isoxazolidinone herbicide commonly used for broadleaf weed control in agriculturally important crops such as soybeans (Glycine max (L.) Merr.), cotton (Gossypium hirsutum L.), corn (Zea mays L.), etc. (Liebl & Norman, Reference Liebl and Norman1991; Culpepper et al., Reference Culpepper, York, Marth and Corbin2001; TenBrook & Tjeerdema, Reference TenBrook and Tjeerdema2006; Đurović-Pejčev et al, Reference Đurović-Pejčev, Radmanović, Tomić, Kaluđerović, Bursić and Šantrić2020). The effectiveness of clomazone is based on its inhibition of carotenoid pigment synthesis, which protects chlorophyll and other plant constituents from photodegradation (Duke et al., Reference Duke, Kenyon and Paul1985). Clomazone's properties such as water solubility (1100 mg dm–3), moderate mobility (normalized organic carbon to water partition coefficient (K OC) = 150–562 dm3 kg–1), persistence in soil (half-life (DT50) = 30–135 days), photolytic half-life (>3 days) and microbial degradation half-life (>2–3 weeks) make it a possible environmental contaminant of agricultural relevance (Đurović-Pejčev et al., Reference Đurović-Pejčev, Radmanović, Tomić, Kaluđerović, Bursić and Šantrić2020).
Sorption of clomazone in soil depends mostly on the soil's organic matter content (Gunasekara et al., Reference Gunasekara, Dela Cruz, Curtis, Claassen and Tjeerdema2009) and clay minerals content (Li et al., Reference Li, Li, Yang, Guo and Liao2004). The surface of natural clay minerals is hydrophilic by nature, which makes these minerals weak adsorbents for organic pollutants, such as pesticides. On the other hand, the modification of natural clays with organic cations changes the nature of their surfaces from hydrophilic to organophilic. Such modified organoclays might be used as adsorbents for the removal of organic pollutants from soil and water (Hermosin & Cornejo Reference Hermosin and Cornejo1992; Nennemann et al., Reference Nennemann, Mishael, Nir, Rubin, Polubesova and Bergaya2001). Exchanged quaternary amine cations are relatively stable against desorption in aqueous salt solutions (Zhang et al., Reference Zhang, Sparks and Scrivner1993).
Montmorillonite is a clay mineral that is used widely in various industries due to its physical and chemical properties, such as its cation-exchange capacity (CEC), large specific surface area and pore volume, high swelling ability, etc. Some physicochemical properties of montmorillonite can be modified or adjusted depending on its usage. Intercalation of organic complexes into montmorillonite layers changes the surface properties from hydrophilic to hydrophobic (Sanchez-Camazano & Sanchez-Martin, Reference Sanchez-Camazano and Sanchez-Martin1996; Tomić et al., Reference Tomić, Ašanin, Antić-Mladenović, Poharc-Logar and Makreski2012). Pre-treatment of montmorillonite with Na+ cations is conducted to prepare the material for organic modification (Nasser et al., Reference Nasser, Gal, Gerstl, Mingelgrin and Yariv1997: He et al., Reference He, Frost, Bostrom, Yuan, Duong and Yang2006).
This paper aims to investigate the inorganic and organic modification of natural montmorillonite from the Bogovina mine (Serbia). Considering the agricultural importance and potential environmental impact of clomazone, the second aim of this paper is to investigate the adsorption of the herbicide clomazone on the surfaces and in the interlayer spaces of both natural and organic-modified montmorillonite from Bogovina. Montmorillonite was modified with hexadecyltrimethylammonium bromide (HDTMA) and phenyltrimethylammonium chloride (PTMA). This paper represents the first attempt at examining the adsorption of clomazone on an organically modified natural clay from Bogovina.
Materials and methods
Sample preparation
The bentonite was collected from the Bogovina locality. The clay fraction (<2 mm) was separated using the sedimentation method. The organic matter was removed from the sample using 30% hydrogen peroxide. The sample was then air dried (AD), ground in an agate mortar and sieved. This sample is denoted as R (raw sample).
The original montmorillonite was rendered homoionic by saturation with 1M NaCl. For this purpose, 10 g of the sample was saturated with 100 mL of 1M NaCl solution. The suspension was then placed on a rotary shaker for 24 h and centrifuged at 9000 rpm for 5 min. The excess Cl– ions were washed with deionized water. This procedure was repeated five times. The sample was then air dried, ground in an agate mortar and sieved. This sample is denoted as NaM (Na-montmorillonite).
In order to perform organic modification, CEC was determined using the Cu-triene (triethylenetetramine) complex (Meier & Kahr, Reference Meier and Kahr1999). This test was repeated twice, and the mean value was used for organic modification. Organic saturation was performed using two organic complexes: HDTMA and PTMA. Various amounts of HDTMA and PTMA were dissolved in 100 mL of distilled water. The concentrations of organic complexes in the prepared solutions were 25%, 50%, 75% and 100% of the CEC, respectively. A total of 7 g of the NaM sample was weighed into glass bottles and 100 mL of distilled water was added. The HDTMA and PTMA solutions were added dropwise to the NaM suspension under continuous stirring with a magnetic stirrer. The temperature was 50°C. After adding the organic complex, the suspension was agitated for 24 h, centrifuged at 9000 rpm for 5 min and washed with deionized water to remove excess Br– and Cl– ions. This procedure was repeated five times. The samples were then air dried, ground in an agate mortar and sieved. These samples are denoted as 1H, 2H, 3H and 4H (saturated with 25%, 50%, 75% and 100% of the CEC with HDTMA, respectively), as well as 1P, 2P, 3P and 4P (saturated with 25%, 50%, 75% and 100% of the CEC with PTMA, respectively).
Chemical analysis
The degree of saturation of raw montmorillonite with Na+ cations was determined by scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) analysis using a JEOL JSM-6610LV SEM device with a tungsten electron source. The samples were prepared in the form of tablets and covered with a 15 nm-thick layer of graphite using a LEICA EM SCD005 carbon coater. Characteristic X-ray spectra were collected with a counting time of 100 s per analytical point.
X-ray diffraction
X-ray diffraction (XRD) analyses were carried out using a Phillips PW 1710 X-ray diffractometer. X-ray powder diagrams were collected in the 2θ range from 2° to 30°, with a step of 2.45° min–1, using Ni-filtered Cu-Kα radiation (1.54184 Å), with 40 mA tube current and 35 kV voltage. Untreated montmorillonites were analysed in preparations that were air dried, ethylene glycol (EG) saturated and heated at 550°C. Modified samples were analysed as oriented samples on glass slides.
Scanning electron microscopy
A JEOL JSM-6610LV scanning electron microscope with a tungsten electron source was used for analysis. The electron acceleration voltage was 20 kV, the beam current strength was 0.1 nA and the working distance (focus) was 10 mm. The samples were covered with a 15 nm layer of gold to achieve electrical conductivity and were examined under high-vacuum conditions (150 μPa).
Batch adsorption experiment
To study the distribution of clomazone in the montmorillonite/water system, a batch equilibrium method was used (OECD, 1997). A total of 200 mg of the sample was mixed with 3 mL of the working solution of clomazone (0.5, 3, 6, 9, 12 and 15 μg mL–1) and then placed in 50 mL polypropylene centrifuge tubes with caps (Sarstedt, Germany). The working solutions of clomazone were prepared from a stock solution of clomazone with a purity of 99.5% (Dr Ehrenstorfer) and 0.01 M CaCl2. The suspensions were agitated on a rotary shaker for 24 h at 20 ± 1°C to achieve equilibrium and were centrifuged for 5 min at 3000 rpm using a UZ 4 centrifuge (Slovenia) to separate the aqueous phase. Its volume was recorded and it was extracted two times with 15 mL dichloromethane into a 50 mL separatory funnel (Sigma-Aldrich, Germany). The lower organophilic layer was collected and evaporated to dryness at 40°C using a rotary evaporator (Devarot, Slovenia). The residues were dissolved in 5 mL of acetone and were injected in 1 μL volumes and analysed with gas chromatography. Blank analyses (prepared using the same procedure without substrate) proved that no loss of acetochlor occurred during the experiment (i.e. no herbicide was observed on the tube wall). All measurements were carried out in triplicate.
The batch adsorption experimental data were collected using a Varian CP-3800 gas chromatograph, equipped with an electron capture detector (Agilent, 7890A). An HP-5 capillary column was used (30 m × 0.32 mm × 0.25 μm; Agilent). The injection temperature and detector temperature were 270°C and 280°C, respectively, and the column temperature was programmed as follows: initial temperature was set to 200°C (isothermally 1 min), then the temperature was increased to 260°C with a 15°C min–1 rate. Nitrogen with a flow rate of 2 mL min–1 was used as the carrier gas.
The sorption coefficients (K d, mL g–1) of the tested herbicide were calculated from equation (1) as the ratio of the average pesticide concentration adsorbed on the substrate (C s, μg g–1) and the average pesticide concentration that remained in the liquid solution under equilibrium (C e, μg mL–1):
$$K_{\rm d}= C_{\rm s}/C_{\rm e}$$Adsorption isotherms were obtained from both the linear form of the Freundlich equation (equation (2)) and the Langmuir model (equation (3)):
$$\log\lsqb {C_{\rm s}} \rsqb = \log K_{\rm f} + 1/n \log \lsqb {C_{\rm e}} \rsqb $$
$$C_{\rm s} = C_{{\rm s}\lpar 0\rpar }bC_{\rm e}/\lpar {1 + bC_{\rm e}} \rpar $$where the Freundlich coefficient K f (μg(n – 1)/n mL g–1) and n represent adsorption capacity and intensity, respectively, while the Langmuir parameter C s(0) (μg g–1) and b (mL μg–1) represent the maximum clomazone quantity adsorbed per unit mass of adsorbent to form a complete monolayer on the surface and the affinity of the binding sites, respectively. The separation factor derived from the Langmuir model (R L) was calculated as in equation (4):
$$R_{\rm L} = 1/\lpar {1 + bC_{{\rm s}\lpar 0\rpar }} \rpar $$Results and discussion
Characterization of montmorillonite
The mean CEC value of unmodified montmorillonite from Bogovina was 69 mmol 100 g–1. After saturation with the 1 M NaCl solution, the amount of Na on the surface of the mineral and in the interlayer space increased ~8.4-fold (Table 1). However, even after the Na treatment, montmorillonite was still mostly saturated with Ca2+ cations, suggesting that Ca is present mainly in Ca-bearing phases, except for montmorillonite.
Table 1. Chemical compositions (%) of R and NaM determined by SEM-EDS.
The XRD trace of the AD sample of NaM shows that the main d 001 peak of the AD sample occurs at 13.68 Å. After the EG saturation, this peak was shifted to 17.67 Å, and after heating (550°C), it was shifted to 9.91 Å (Fig. 1). This behaviour is typical of the smectite minerals montmorillonite and beidellite (Moore & Reynolds, Reference Moore and Reynolds1997). To this end, a Greene–Kelly test was performed (see Supplementary Material). From the peak positions after the Li treatment, it was concluded that montmorillonite was the smectite mineral present in the raw sample.
Fig. 1. XRD traces of NaM: AD (black), EG-saturated (blue) and heated (red).
After the modification of montmorillonite with HDTMA+ cations, an increase in the interlayer space was observed, indicating that HDTMA+ cations entered the interlayer space (Fig. 2). This is important because the sorption capacity of organoclays increases with the surfactant loadings within the clay interlayer space (Wang et al., Reference Wang, Juang, Lee, Hsu, Lee and Chao2004). The loading of 25% of the CEC (1H) increased the basal space from 13.68 to 15.11 Å. Further increases in loading increased the basal space to 15.40, 18.07 and 21.87 Å in samples 2H, 3H and 4H, respectively. The perfectly straight chain of HDTMA+ looks like a ‘nail’, where the long alkyl chain is the ‘body’ and the chain end holding the three methyl groups is the ‘head’ (He et al., Reference He, Frost, Bostrom, Yuan, Duong and Yang2006). The length of the HDTMA+ cation is ~25.3 Å, consisting of the ‘head’ (4.3 Å) and the ‘body’ (21 Å). On the other hand, the height of the HDTMA+ cation varies depending on the cation arrangement. The height of the cation ‘head’ varies from 5.1 to 6.7 Å and that of the ‘body’ from 4.1 to 4.6 Å (He et al., Reference He, Frost, Bostrom, Yuan, Duong and Yang2006).
Fig. 2. XRD traces of HDTMA-modified montmorillonites.
If the thickness of a single montmorillonite tetrahedral–octahedral–tetrahedral (TOT) layer is 9.6 Å, the calculated interlayer spaces of samples 1H, 2H, 3H and 4H are 5.5, 5.8, 8.5 and 12.3 Å, respectively. This means that cations in the interlayer spaces in samples 1H and 2H take a lateral monolayer arrangement. In sample 3H, the HDTMA+ cations take a lateral bilayer arrangement, with apical methyl groups positioned in the cavity between the organic cations or in the hexagonal hole of the basal oxygen plane. Hence, the dimensions of the lateral bilayer depend on the height of the cation ‘body’ rather than on the cation ‘head’. In sample 4H, the HDTMA+ cations take the arrangement of a pseudotrilayer (Zhu et al., Reference Zhu, He, Guo, Yang and Xie2003; He et al., Reference He, Frost, Bostrom, Yuan, Duong and Yang2006).
On the other hand, in the case of the PTMA complex, an increment of the basal space from 13.68 to 14.60 Å in sample 1P (25% of the CEC saturated with PTMA complex) was observed (Fig. 3). Further increases in PTMA loading led to only small increments in the basal space to 14.93, 15.21 and 15.24 Å for 2P, 3P and 4P, respectively. A minimum interlayer space of 5.0 Å in sample 1P and a maximum space of 5.7 Å in sample 4P were calculated. The phenyl ring in a small concentration of PTMA complex takes an almost parallel position relative to the montmorillonite layer. These results are in agreement with previous works that indicate an interlayer distance of 5.1–5.7 Å for various loadings of PTMA montmorillonite (Sanchez-Camazano & Sanchez-Martin, Reference Sanchez-Camazano and Sanchez-Martin1996; El-Nahhal, Reference El-Nahhal2003).
Fig. 3. XRD traces of PTMA-modified montmorillonites.
Adsorption of clomazone caused a significant change in the d-spacing of the HDTMA-montmorillonite in sample 4H only, in which the 001 peak shifted from 21.87 to 24.34 Å (Fig. 4). This sharp increase is due to the arrangement of the HDTMA complex in the interlayer space, which formed a paraffin layer. In this arrangement, the HDTMA cations occupy a position at a sharp angle (30–60°) with respect to the surface of the montmorillonite layer.
Fig. 4. XRD traces of HDTMA-modified montmorillonites after clomazone sorption.
Saturation with clomazone led to small changes in the basal spaces of PTMA-montmorillonites (Fig. 5). Clomazone is probably adsorbed on the mineral surface and in the interlayer space between PTMA+ cations, and as the clomazone molecule is not significantly larger than the PTMA+ cation, there is no significant change in XRD traces. The same mechanism has been reported for other herbicides (Sanchez-Camazano & Sanchez-Martin, Reference Sanchez-Camazano and Sanchez-Martin1996). On the other hand, HDTMA adsorbed on montmorillonite surfaces acts as a partitioning medium for removing non-ionic organic herbicides from water (Zhu et al., Reference Zhu, Chen, Zhou, Xi, Zhu and He2016).
Fig. 5. XRD traces of PTMA-modified montmorillonites after clomazone sorption.
SEM analysis
The changes in the montmorillonite morphology that occurred before and after the modification with organic complexes may be observed in the SEM images (Fig. 6). Inorganic montmorillonite (Fig. 6a) shows a smooth surface with flat crystals or slightly curved grains with grain sizes between 1 and 3 μm. The modification with a small amount of organic cations did not cause a significant change (Fig. 6b). However, when the amounts of cationic surfactant increased, the clay layer appeared swollen and fluffy, with curved edges to the grains (Fig. 6c,d). The grains became more curved as the concentration of organic complex in the interlayer space increased. This might be due to the increase in the basal spacing of the clay, which is in agreement with previous work (Dutta & Singh, Reference Dutta and Singh2015).
Fig. 6. SEM images of inorganic and organically modified montmorillonites: (a) NaM, (b) 1H, (c) 4H and (d) 4P.
The layer charge density and charge distribution on the montmorillonite surface affects the distribution of adsorbed organic complexes. Previous studies on smectites with low layer charge and CECs <100 cmol(+) kg–1, such as the SWy-1 montmorillonite, showed that the smectite particle shape changes progressively with increasing saturation of the HDTMA+ cations and that curved particle edges occur. On the other hand, the high-layer-charge SAz-1 montmorillonite formed aggregates with increasing saturation of HDTMA+ cations (Lee et al., Reference Lee, Cho, Hahn, Lee, Lee and Kim2005).
Adsorption of clomazone
The isotherm constants and the maximum adsorption capacities of the sorbents tested were calculated using Freundlich and Langmuir equilibrium models (equations (2) and (3)). The adsorption isotherms of clomazone for R, NaM, 1H–4H and 1P–4P using the Freundlich and Langmuir equilibrium models are shown in Figs 7 and 8, respectively, while the isotherm parameters obtained are listed in Table 2. Both models fitted well the experimental data, with the Langmuir model being slightly better (greater coefficients of determination R 2). In addition, little adsorption of clomazone was observed on R and NaM. Clearly, saturation with Na+ cations did not affect significantly the clomazone sorption. The Freundlich parameter (K f) and the Langmuir parameter (C s(0)) demonstrated the significantly greater sorption in the organically modified montmorillonites. If we compare the K f values for clomazone sorption on HDTMA-montmorillonites and PTMA-montmorillonites with those for clomazone sorption in natural soils, organically modified montmorillonites displayed greater sorption capacity (Đurović-Pejčev et al., Reference Đurović-Pejčev, Radmanović, Tomić, Kaluđerović, Bursić and Šantrić2020). Moreover, greater sorption occurred in HDTMA-montmorillonites than in PTMA-montmorillonites. These results are at odds with previous studies of NaM modified with PTMA and HDTMA complexes, in which the replacement of inorganic cations in the interlayer space of montmorillonite with aromatic cations (PTMA+ or benzyltrimethylammonium) increased the sorption of acetochlor compared to montmorillonite modified with aliphatic cations (HDTMA+) (El-Nahhal et al., Reference El-Nahhal, Nir, Serban, Rabinovitz and Rubin2000; El-Nahhal & Safi, Reference El-Nahhal and Safi2004). However, it should be noted that these results were obtained from pure NaM from The Clay Minerals Society repository (SWy-1). On the other hand, in this paper, natural montmorillonite was separated and treated with NaCl in order to prepare the sample for organic modification. Chemical analysis showed that even after the NaCl treatment, significant amounts of Ca2+ cations remained in the interlayer space. Ca-montmorillonite modified with HDTMA+ cations showed greater sorption of metolachlor compared to PTMA+-modified Ca-montmorillonite (Cruz-Guzman et al., Reference Cruz-Guzman, Celis, Hermosin, Koskinen and Cornejo2005). The ion-exchange reaction between PTMA+ cations and Na+ cations occurs more easily due to the greater exchangeability of monovalent cations (Jaynes & Boyd, Reference Jaynes and Boyd1990).
Fig. 7. Freundlich adsorption isotherms for clomazone on: raw sample (R), Na-montmorillonite (NaM), HDTMA-montmorillonite saturated with HDTMA equal to 25% of the CEC (1H), HDTMA-montmorillonite saturated with HDTMA equal to 50% of the CEC (2H), HDTMA-montmorillonite saturated with HDTMA equal to 75% of the CEC (3H), HDTMA-montmorillonite saturated with HDTMA equal to the CEC (4H), PTMA-montmorillonite saturated with PTMA equal to 25% of the CEC (1P), PTMA-montmorillonite saturated with PTMA equal to 50% of the CEC (2P), PTMA-montmorillonite saturated with PTMA equal to 75% of the CEC (3P) and PTMA-montmorillonite saturated with PTMA equal to the CEC (4P).
Fig. 8. Langmuir adsorption isotherms for clomazone on: raw sample (R), Na-montmorillonite (NaM), H1, H2, H3 H4, P1, P2, P3 and P4 montmorillonites. The symbols H1, H2, H3 H4, P1, P2, P3 and P4 are similar to those shown in Fig. 7.
Table 2. Freundlich and Langmuir isotherm parameters for the removal of clomazone by R, NaM, 1H–4H and 1P–4P samples.
a Freundlich constant that represents adsorption capacity.
b Freundlich constant that represent intensity.
c Correlation coefficient.
d Maximum amount of clomazone per unit mass of adsorbent to form a complete monolayer on the surface.
e Langmuir constant related to the affinity of the binding sites.
f Separation factor derived from the Langmuir isotherm.
A long-chain organic cation such as HDTMA+ acts as a partitioning agent for non-ionic hydrophobic components such as clomazone. It can also act as an efficient environment for the sorption of aromatic compounds (Lee et al., Reference Lee, Kim, Chung and Jeong2004). The partitioning is an extremely important mechanism as it determines the fate and the bioavailability of chemical compounds. The sorbate is distributed depending on its affinity for the sorbent phases. Therefore, the distribution of chemical contaminants in the environment depends directly on their physicochemical properties and on environmental factors (Tourinho et al., Reference Tourinho, Kočí, Loureiro and & van Gestel2019).
The samples modified with the HDTMA and PTMA complexes showed a decrease in sorption with a decrease in the organic cation content. The observed trend in clomazone sorption obtained by the Freundlich (K f) and Langmuir (C s(0)) models was: 4H > 3H > 4P > 3P > 2H > 1H > 2P > 1P > NaM > R. In agreement with the present study, previous work showed that there is an increase in the non-ionic pesticide sorption in soils with greater contents of organic matter (Kirskey et al., Reference Kirksey, Hayes, Krueger, Mullins and Mueller1996; Liu et al., Reference Liu, Gan, Papiernik and Yates2000; Gunasekara et al., Reference Gunasekara, Dela Cruz, Curtis, Claassen and Tjeerdema2009; Pena et al., Reference Pena, Lopez-Pineiro, Albarran, Sanchez-Lerena, Cox and Rato-Nunes2015). Thus, the K d of the adsorption of the non-ionic herbicide alachlor on natural clays increased from 1.77 to 2.78 mL g–1 when initial concentration of alachlor decreased from 120 to 12 μmol L–1 (Liu et al., Reference Liu, Gan, Papiernik and Yates2000).
The fact that the Freundlich constant 1/n was <1 suggests that clomazone sorption decreases with increases in the initial concentration. This is confirmed when comparing the K d (mL g–1) values (Table 3). The K d values in all of the examined samples decrease with increasing clomazone concentration. This is probably due to the adsorption of pesticide molecules at the high-energy sorption sites of these substrates at lower pesticide concentrations (specific adsorption), in contrast to greater concentrations, where adsorption occurs at low-energy sorption sites (non-specific adsorption).
Table 3. Distribution coefficients K d (mL g–1) for six concentrations of clomazone applied on tested samples R, NaM, 1H–4H and 1P–4P.
As the number of clomazone molecules increases, the number of available active sorption sites decreases, which is manifested by reduced sorption. Similar decreases in the sorption constant of clomazone with the increasing concentration of this compound have been determined previously in soils with various physicochemical characteristics (Mervosh et al., Reference Mervosh, Sims, Stoller and Ellsworth1995; Gunasekara et al., Reference Gunasekara, Dela Cruz, Curtis, Claassen and Tjeerdema2009).
The increased sorption of clomazone by HDTMA-montmorillonite may be explained on the basis of the hydrophobicity and partition coefficient of clomazone. The partition coefficient of clomazone is logP ow = 2.55 at room temperature, with high affinity towards hydrophobic groups in HDTMA-montmorillonites. Similar results were obtained for acetochlor (Li et al., Reference Li, Li and Dong2007; Tomić et al., Reference Tomić, Ašanin, Đurović-Pejčev, Đorđević and Makreski2015).
The separation factors R L (Table 2) obtained by the Langmuir model were <1 for all of the materials studied, indicating that clomazone adsorption is favourable in all of these cases. Generally, the R L value indicates that adsorption could be either unfavourable (R L > 1), linear (R L = 1), favourable (0 < R L < 1) or irreversible (R L = 0). Comparing the obtained values for all of the samples studied, the following trend for R L was observed: 4H < 3H < 4P < 3P < 2H < 1H < 2P < 1P < NaM < R. This indicates that the most favourable clomazone adsorption was on sample 4H (the smallest R L value) and the least favourable clomazone adsorption was on sample R (the highest RL value).
Conclusion
The sorption of non-ionic herbicides such as clomazone depends heavily on the nature of the surface of montmorillonite. Changing the nature of the surface of montmorillonite from hydrophilic to hydrophobic is crucial for clomazone sorption. Organic modification increases the interlayer space. Depending on the degree of saturation with organic complexes, long HDTMA+ cations may take various arrangements in the interlayer space. The HDTMA adsorbed on montmorillonite surfaces acts as a partitioning medium for removing non-ionic organic herbicides from water. On the other hand, PTMA+ cations act like pillars, which means that, regardless of their concentration, no significant change in the interlayer space occurs.
The modification of montmorillonite with HDTMA and PTMA complexes led to improved sorption of clomazone. Greater HDTMA+ cation contents in organically modified montmorillonites were more effective for the sorption of clomazone. The sorption of clomazone was greatest in the montmorillonite saturated with HDTMA+ cations with a concentration equal to the CEC (sample 4H). The increased sorption of clomazone by HDTMA-montmorillonite may be explained by the hydrophobicity and partition coefficient of these pesticides.
The change in the nature of the surface of montmorillonite from hydrophilic to hydrophobic led to improved sorption of clomazone. It is concluded that organically modified montmorillonite from Bogovina might be an effective adsorbent for clomazone.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1180/clm.2021.3
Acknowledgements
The authors are grateful to Kristina Radović for proofreading and editing the manuscript.
Financial support
This research was funded by the Ministry of Education, Science and Technological Development of the Republic of Serbia (grant no. 451-03-9/2021-14/200116 and grant no. 451-03-9/2021-14/200214).
