Thermodynamics and Kinetics of Pretilachlor Adsorption on Organobentonites for Controlled Release.

Thermodynamics and kinetics of pretilachlor adsorption on organobentonites modified with hexadecyltrimethyl ammonium chloride were investigated to reveal the structural effects of organobentonites on the interaction with pretilachlor and the diffusion of the herbicide and were related to the controlled release from organobentonites. The adsorption of pretilachlor was entropically driven by hydrophobic interaction. The entropy change dropped with increasing surfactant loading from 0.4 to 1.50 times the cation exchange capacity (CEC) of the bentonite used, corresponding to a decrease in the degree of freedom of pretilachlor molecules due to the enhanced order of surfactant in the interlayer. The kinetics of pretilachlor adsorption was well fitted to the pseudo-second-order model and related to the structural features of organobentonites. The enhanced packing density of the surfactant in the interlayer generally resulted in a reduction of the rate constant of the pretilachlor adsorption onto organobentonites. However, the stepwise increase in the basal spacing due to the surfactant arrangement transition, from lateral-monolayer to lateral-bilayer at a loading level of more than 0.8 × CEC, benefited the diffusion of pretilachlor and diminished the influence of the increase in surfactant packing density. The release of pretilachlor from organobentonites was predominated by Fickian diffusion, which could be understood from the adsorption thermodynamics and kinetics. The time taken for the release of 50% of active ingredient was 16-23 times that for the control formulation and exhibited a linear increase with the relative value of the equilibrium constant to the rate constant of pretilachlor adsorption.

Introduction

Herbicides are the most commonly used agrochemicals for impeding the growth of weeds and improving the yield of crops. However, only part of herbicides reaches their target, whereas the rest remains attached to the soil colloids or undergoes degradation and transfer process such as migration to surface and groundwater, which has been reported to be one of the main sources of water pollution.[1−3] In the past decades, many efforts have been made to minimize the use and impact of herbicides on the environment by developing controlled release formulations (CRFs).[4−6]

The success of CRFs in agriculture practice is critically dependent on the development of the matrix of high loading efficacy and controllable release properties.[6] Clay minerals have been considered as good candidates for CRF because of their potential adsorbent properties in addition to their abundance and low cost. Bentonite is a natural material containing montmorillonite as a major constituent and has been considered as a potential platform for CRF of insecticides and herbicides due to the high specific surface area. Moreover, the surface properties and nanostructure in the interlayer of bentonite was readily regulated by organic modifications, which improved the adsorption capability toward several herbicides and reduce leaching in soil.[7−12]

The knowledge of the structural effects of the matrix on the release behavior of pesticides would be helpful in developing new formulations to fulfill the actual requirement of environmental control. The release of pesticides from the CRFs was usually understood from the adsorption capability and the permeability of the matrix. A higher adsorption capability and a lower permeability usually resulted in a slower release.[13,14] In the past decades, much attention has been paid to the development of organobentonites with high adsorption capacity and feasibility to regulate the adsorption capacity of organobentonites.[15−17] However, little is known about the effects of structural features, such as molecular structure, loading level, and orientation of modifying agents in the interlayer space, on the permeability of organobentonites. The release behavior from organobentonites was qualitatively interpreted from the adsorption capacity of the matrix.[18,19]

In a previous work, organobentonites modified with dodecyltrimethyl ammonium chloride (DTMA) and hexadecyltrimethyl ammonium chloride (HTMA) were used as carriers of pretilachlor [2-chloro-N-(2,6-diethylphenyl)-N-(2-propoxyethyl) acetamide], a selective pre-emergent herbicide widely applied to control annual grasses and broadleaf weeds in rice field, to obtain controlled release granules. The results indicated that the time taken for 50% of active ingredients to be released from organobentonites with surfactant loading level from 0.6 to 1.2 times the cation exchange capacity (CEC) increased with increasing adsorption coefficient by the Freundlich equation and is seemed to be related to the interlayer spacing and packing density of the surfactant in the interlayer. Unfortunately, the mechanism of structural effects of organobentonites on pretilachlor release remained unclear due to the lack of the physical significance of adsorption coefficient by the Freundlich equation and a suitable method to evaluate the permeability of the matrix.[20]

To get a further insight into the release properties of organobentonites, equilibrium adsorption of pretilachlor onto organobentonites with HTMA loading level from 0.4 to 1.5 × CEC was investigated. The adsorption thermodynamic parameters were used to reveal the interaction between the matrix and the herbicide. Kinetics of pretilachlor adsorption was also studied to illustrate the structural effects of organobentonites on the diffusion of pretilachlor molecules in the interlayer gallery. The release behavior of pretilachlor from organobentonites was understood from the thermodynamics and kinetics of pretilachlor adsorption. The results were helpful to evaluate the release properties of organobentonites and explore the organobentonite-based CRFs.

Results and Discussion

Preparation and Characterization of Organobentonites

Organobentonites modified with HTMA at a feeding level in the range from 0.4 to 1.5 × CEC were prepared. The proportion of HTMA adsorbed on the bentonite used was found to drop dramatically when the feeding level was more than 1.5 × CEC.[21] While those with a surfactant loading level less than 0.4 × CEC were subjected to suspension and blocked the filter membrane in the stirred-flow reactor system used in the adsorption kinetics and release of pretilachlor.

X-ray powder diffraction (XRD) is a useful method to characterize the arrangement model of the intercalated surfactants of organoclay, based on the basal spacing and the dimensions of the surfactant molecule. XRD patterns for the natural bentonite and organobentonites are presented in Figure , and the basal spacing d001 is listed in Table . The bentonite used was a typical Ca2+-bentonite, of which the d001-value was 1.48 nm.[22] A small quantity of HTMA of less than 0.8 × CEC slightly decreased the basal spacing of the bentonite. The interlayer separation, ca. 0.46 nm, agreed with the size of the head group of the quaternary ammonium, indicating that HTMA+ had intercalated into the interlayer of bentonite and replaced the exchangeable Ca2+ ion and water molecules adsorbed.[23] A stepwise increase in the basal spacing to about 1.8 nm for organobentonites with higher surfactant loading indicated that the intercalated HTMA+ underwent a transition from a lateral-monolayer (d001 = 1.45–1.47 nm) to a lateral-bilayer orientation (d001 = 1.75–1.85 nm) (Figure S1 in the Supporting Information).[23−26]

Figure 1

XRD patterns of HTMA-bent.

Table 1

Carbon Content foc, Basal Spacing d001, Stretching Vibration Wavenumber of Methyl Groups and Equilibrium Constant Ka for Pretilachlor Adsorption on HTMA-benta

sample	foc/%	d001/nm	νas (CH2)/(cm–1)	νs (CH2)/(cm–1)	Ka × 10–3/(mL·g–1)	R	Koc × 10–3/(mL·g–1)
DTMA			2919	2848
bentonite	1.23	1.48			0.15 (0.01)	0.992	12.5
BH0.4	7.41	1.42	2927	2854	0.68 (0.04)	0.992	9.2
BH0.6	10.34	1.42	2925	2854	1.43 (0.03)	0.999	13.9
BH0.8	13.24	1.74	2925	2852	2.25 (0.05)	0.999	17.0
BH1.0	15.72	1.79	2923	2852	2.56 (0.12)	0.995	16.3
BH1.25	18.23	1.81	2921	2850	2.91 (0.15)	0.993	16.0
BH1.5	20.56	1.82	2921	2850	3.30 (0.08)	0.998	16.0

Values in brackets represent the standard deviation.

Fourier transform infrared spectroscopy (FTIR) has been widely used to probe the conformation of the adsorbed surfactant on clay minerals. The frequency of the CH2 absorption is sensitive to the ordering and packing density of the surfactants, the environment, and interactions between alkyl chains. The FTIR spectra of organobentonites are presented in Figure . The CH2 asymmetric stretching (νas(CH2)) and symmetric stretching vibrations (νs(CH2)) of the adsorbed surfactant were found to be around 2927 and 2854 cm–1 and slightly shifted to 2921 and 2850 cm–1 due to the increasing surfactant loading level up to 1.5 × CEC, close to those of pure HTMA (around 2919 and 2848 cm–1, respectively), indicating that the adsorbed surfactant developed from a less ordered conformation of low packing density to an ordered one of high packing density.[25−28] An increase in hydrophobicity of organobentonite was also observed, as indicated by the decrease in the relative absorption of the adsorbed hygroscopic water around 3450 cm–1 to the structural −OH of organobentonites at 3630 cm–1.[10]

Figure 2

FTIR spectra of HTMA-bent.

Adsorption Isotherms

The isotherms of pretilachlor adsorption are shown in Figure . The linear increase of pretilachlor adsorption on organobentonites with the equilibrium concentration in the solution indicated that the adsorption was predominated by the partition of pretilachlor between the aqueous phase and the organic phase created by long hydrocarbon chain surfactant in the interlayer.[29,30] The apparent adsorption equilibrium constant (Ka) was thus derived from the slope of isotherms and found to increase from 0.68 × 103 mL·g–1 for BH0.4 to 3.30 × 103 mL·g–1 for BH1.5, much more than that for bentonite (Table ).

Figure 3

Isotherms for pretilachlor adsorption on HTMA-bent.

The adsorption capability was usually related to the organic carbon content of the clay. To evaluate the impact of organic carbon content on the adsorption capability of organobentonites toward pretilachlor, the organic carbon contents of organobentonites (foc) were determined by elemental analysis (Table ). The adsorption equilibrium constants of pretilachlor normalized to the carbon content of the organoclay (Koc) were derived by the following equationKoc values of HTMA-bent first increased and then slightly decreased with increasing surfactant loading level, reaching a maximum of around 0.8 × CEC (Table and Figure S2 in the Supporting Information). Similar results were obtained for the adsorption of other organic compounds on organobentonites modified with quaternary ammonium surfactants. It has been suggested that the increase of the surfactant benefits the formation of the organic phase in the interlayer gallery. However, quaternary ammonium surfactant in molecule form appeared in the interlayer via a hydrophobic interaction when the loading exceeded 0.8 × CEC. In the confined clay interlayer, the available space to accommodate the organic solutes reduced when the surfactant loading increased to a higher level. Moreover, the ammonium heads and the counterions of the surfactant interfered with the hydrophobic interaction between the surfactant molecules, resulting in a reduction of the adsorption capability of organobentonite toward hydrocarbon organic compounds.[30−32]

Adsorption Thermodynamics

To gain further insight into the structural effects of organobentonites on pretilachlor adsorption, equilibrium adsorption was conducted for different temperatures at 293, 303, and 313 K. The equilibrium constant (Ka) was obtained from the equilibrium concentration of pretilachlor in the solutions (ce) and the adsorbed amount onto organobentonites (qe). The thermodynamic parameters of pretilachlor adsorption were determined from Van’t Hoff equation by plotting ln Ka vs 1/T (Figure )where T and R are the thermodynamic temperature and gas constant, respectively; ΔG, ΔH, and ΔS are the changes in Gibbs free energy, enthalpy, and entropy, respectively. The results are listed in Table .

Figure 4

Van’t Hoff plots of pretilachlor adsorption on HTMA-bent.

Table 2

Thermodynamic Parameters for Pretilachlor Adsorption on HTMA-Benta

			ΔG/(kJ·mol–1)
sample	ΔH/(kJ·mol–1)	ΔS/(J·mol–1·K–1)	293 K	303 K	313 K
BH0.4	27.7 (2.1)	148.5 (7.0)	–15.8 (0.3)	–17.4 (0.5)	–18.8 (0.2)
BH0.6	19.6 (1.2)	127.1 (3.8)	–17.7 (0.2)	–18.9 (0.2)	–20.2 (0.3)
BH0.8	10.6 (0.8)	100.1 (2.6)	–18.7 (0.1)	–19.8 (0.3)	–20.7 (0.3)
BH1.0	7.7 (1.2)	93.2 (3.8)	–19.6 (0.3)	–20.6 (0.1)	–21.5 (0.1)
BH1.25	8.2 (2.2)	96.0 (7.4)	–19.9 (0.3)	–21.0 (0.2)	–21.8 (0.2)
BH1.5	6.6 (0.3)	91.0 (1.1)	–20.1 (0.4)	–21.0 (0.3)	–21.9 (0.3)

Values in brackets represent the standard deviation.

The negative value of ΔG at all temperatures indicated that pretilachlor adsorption on HTMA-bent was spontaneous and that the solute was favored to stay in the stationary phase rather than in the mobile phase. Generally, the physisorption was characterized by a standard Gibbs free energy change (ΔG°) of the absolute magnitude between 0 and −20 kJ·mol–1, while for chemisorption ΔG° is in the range from −80 to −400 kJ·mol–1.[33] In the temperature range studied, ΔG for pretilachlor adsorption on HTMA-bent was found to be between −15.8 and −21.9 kJ·mol–1, indicating that the adsorption was physical in nature. The noncovalent binding usually originated from hydrophobic interaction, electrostatic interaction, hydrogen bond, or van der Waals interaction, which was reflected by the thermodynamic parameters. Hydrophobic interactions between two apolar molecules have been known as entropy-driven processes with a favorable entropy and unfavorable enthalpy (ΔH > 0, ΔS > 0, ΔH < TΔS), opposite to the enthalpy-driven hydrogen bond and van der Waals interaction (ΔH < 0, ΔS < 0, |ΔH| > |TΔS|). The interaction dominated by electrostatic attraction is characterized by a positive entropy and a minor unfavorable enthalpy (ΔH ∼ 0 or ΔH > 0, ΔS > 0).[34] The positive values of enthalpy and entropy change indicated that pretilachlor adsorption was driven by entropy through hydrophobic interaction. Despite the decrease in entropy change, the reduction of the unfavorable enthalpy change led to an enhancement of the adsorption capability of HTMA-bent with increasing surfactant loading, as indicated by an increase in |ΔG| (Table ).

The adsorption of pretilachlor on organobentonites via partition can be considered as a transfer from the aqueous phase to the organic phase in the interlayer gallery. The decrease in the degree of freedom of the adsorbed species was usually accompanied by a negative entropy change.[35] However, the strong hydrophobicity of pretilachlor (octanol–water partition coefficient log Kow = 4.08)[36] made it a structure maker to induce an enhancement in the hydrogen-bonding network in the hydration cosphere, as compared to the bulk.[37−39] The transfer of pretilachlor to the organic phase in the interlayer gallery of organobentonite would release the solvating water molecules to the bulk, bringing about an increase in the entropy of the whole system, which can be attributed to the positive entropy change for pretilachlor adsorption (Table ). Since the intercalation of quaternary ammonium surfactants was dominated by ion exchange at the loading level less than 1.0 × CEC,[30] an increase in surfactant loading usually resulted in an enhanced hydrophobicity of organobentonite, as illustrated by the decreased relative IR absorption intensity of the adsorbed water to that of the structural −OH of organobentonites (Figure ). However, the value of ΔS dropped dramatically with increasing surfactant loading up to 0.8 × CEC (Table ), indicating that the hydrophobicity of organobentonite had little influence on the dehydration of pretilachlor molecules. The reduction of entropy change may be related to the decrease in the degree of freedom of pretilachlor molecules due to the enhancement in structure order and packing density of surfactant in the interlayer gallery. Following the suggestion by Xu et al.,[30] quaternary ammonium surfactant in molecular form intercalated in the clay via hydrophobic interaction when the loading exceeded 0.8 × CEC. The ammonium heads and the counterions would interfere with the hydrophobic interaction of the surfactant and destroyed the close packing in clay galleries. As a result, the entropy decrease due to the solidification of pretilachlor in the organic phase of HTMA-bent became less evident and quickly leveled off when the surfactant loading level was more than 0.8 × CEC (Table and Figure S3 in the Supporting Information).

Adsorption Kinetics

Adsorption kinetics of pretilachlor on HTMA-bent is presented in Figure . The profiles were fitted to two commonly used kinetics models, pseudo-first-order (eq ) and pseudo-second-order (eq )[40]where q (mg·g–1) is the amount of pretilachlor adsorbed at time t (min); q1 (mg·g–1) and q2 (mg·g–1) are the maximum adsorption capacities for the pseudo-first-order adsorption and pseudo-second-order adsorption, respectively; and k1 (min–1) and k2 (g·mg–1·min–1) are rate constants for the pseudo-first-order adsorption and pseudo-second-order adsorption, respectively. The values of the maximum adsorption capacity and rate constant were obtained by a nonlinear fit using the least-squares method and are summarized in Table . Based on the R2 values, both pseudo-first-order and pseudo-second-order models could be well used to describe the kinetics of pretilachlor adsorption onto organobentonites. The maximum adsorption capacity obtained by both models increased, while the rate constant decreased and leveled off with increasing surfactant loading. However, a further comparison showed that the latter generally provided better results than the former. The adsorption capacity q2 agreed well with those from adsorption isotherms (Table S1 in the Supporting Information). Thus, the structural effects of organobentonites on the kinetics of adsorption and the release of pretilachlor were discussed based on the results from the pseudo-second-order model in the following text.

Figure 5

Kinetics profiles of pretilachlor adsorption on HTMA-bent.

Table 3

Kinetic Parameters for Pretilachlor Adsorption onto HTMA-Benta

	pseudo-first-order model			pseudo-second-order model
sample	q1/(mg·g–1)	k1 × 103/min–1	R2	q2/(mg·g–1)	k2 × 105/(g·mg–1·min–1)	R2
BH0.4	25.8 (0.4)	13.5 (0.7)	0.996	31.5 (0.3)	45.8 (2.1)	0.999
BH0.6	43.0 (0.7)	11.6 (0.5)	0.997	53.6 (0.6)	22.2 (1.0)	0.999
BH0.8	57.0 (0.5)	11.1 (0.3)	0.999	72.5 (1.9)	14.5 (1.4)	0.995
BH1.0	67.7 (0.9)	10.6 (0.4)	0.998	85.8 (1.6)	12.1 (0.8)	0.997
BH1.25	72.3 (0.5)	10.5 (0.2)	0.999	93.2 (2.0)	10.4 (0.8)	0.997
BH1.5	78.1 (0.9)	10.0 (0.3)	0.998	100.3 (1.9)	9.4 (0.7)	0.997

Values in brackets represent the standard deviation.

Several rate-limiting steps and mechanisms were involved in the kinetics of adsorption onto organobentonites, including (1) diffusion of adsorptive from the bulk solution to the surface of clay particles, (2) migration into the interlayer gallery, and (3) binding to the active side.[41,42] It was apparent that an increase of basal spacing would result in an increase of interface between the bulk solution and the organic phase created by the surfactant and thus benefit the diffusion of the active ingredient into the interlayer gallery of organobentonite. However, the increase in surfactant packing density would slow down the migration of adsorptive. As illustrated by XRD, the intercalated HTMA adopted a lateral-monolayer arrangement mode at a loading level less than 0.8 × CEC and the increase in surfactant loading had little influence on the basal spacing of HTMA-bent (Table ). Thus, the enhanced packing density of the surfactant retarded the migration of pretilachlor molecules in the interlayer, resulting in a dramatic drop of the rate constant of adsorption (Table ). A similar trend was also observed for the adsorption onto organobentonites with surfactant loading more than 0.8 × CEC. However, the decrease in the rate constant was much less evident than that for organobentonites with a lower surfactant loading (Table and Figure S4 in the Supporting Information), as the stepwise increase in the basal spacing (Table ) benefited the diffusion of pretilachlor into the interlayer gallery and diminished the influence of surfactant loading on the increase in surfactant packing density. The results were quite consistent with the entropy change of pretilachlor adsorption (Table and Figure S3 in the Supporting Information).

Release Studies

The release of pretilachlor from organobentonites was investigated in the stirred-flow reactor system, which was also used in adsorption kinetics. The technical pretilachlor was mixed with fine sands and used as control formulation, and the dissolution profile was obtained under the same conditions. The cumulative release of pretilachlor from organobentonites and the control formulation are summarized in Figure .

Figure 6

Cumulative release of pretilachlor from HTMA-bent in water. Inset: dissolution profile of pretilachlor in water.

The release profiles were fitted to the Rigter–Peppas equation[43,44]where M/M0 is the cumulative release of the active ingredient at time t, k is a characteristic constant of the system, and n is the diffusional exponent to describe the kinetics and the release mechanism. The release from slab, cylinder, and sphere systems via the Fickian diffusion mechanism was characterized by n values of 0.5, 0.45, and 0.43, respectively. For the release via “case II transport” mechanism, a non-Fickian release, the values of n were 1.0, 0.89, and 0.85, respectively.[43,44] The values of k and n were obtained by a nonlinear fit, from which the time taken for the release of 50% of the active ingredient, T50, was calculated (Table ). The T50 value for the control formulation was estimated by an interpolation method, as the Rigter–Peppas equation did not fit well to the fast dissolution of pretilachlor in water and in the stirred flowing condition. The release of the herbicide was greatly retarded by the organobentonites, of which the T50 value was 16–23 times that of the control formulation and increased linearly with increasing surfactant loading level (Figure S5 in the Supporting Information).

Table 4

Parameters Obtained from the Rigter–Peppas Equation for Pretilachlor Release from HTMA-Benta

sample	pretilachlor/(mg·g–1)	k	n	R2	T50/h
control					0.7
BH0.4	12.6	15.0 (0.2)	0.50 (0.01)	0.999	11.2
BH0.6	12.1	13.5 (0.7)	0.53 (0.03)	0.991	11.8
BH0.8	10.6	11.0 (0.6)	0.59 (0.03)	0.991	13.2
BH1.0	10.9	11.3 (0.4)	0.57 (0.02)	0.996	13.5
BH1.25	11.0	11.1 (0.4)	0.56 (0.02)	0.995	15.1
BH1.5	11.7	10 (0.2)	0.57 (0.01)	0.999	16.1

Values in brackets represent the standard deviation.

The release of pesticides from CRF was usually understood from the adsorption capability and the permeability of the matrix. A higher adsorption capability would result in a slower release, a consequence of the lower equilibrium concentration in the medium.[13,14] The T50 values for the pretilachlor release from organobentonites totally increased with increasing adsorption equilibrium constant Ka (Tables and 4). However, the upward curvature of the T50 vs Ka plot suggested a possible alteration of matrix permeability induced by the structural change in the interlayer (Figure S6 in the Supporting Information).

The value of diffusional exponent n from 0.50 to 0.59 (Table ) indicated that the pretilachlor release from organobentonites was predominated by a Fickian diffusion mechanism.[43,44] In an aqueous environment, the active ingredient migrated in the interlayer space and diffused into the release medium due to the concentration gradient and was sustained by the lamellar of bentonite and surfactant molecules intercalated. Thermodynamic studies have indicated that the adsorption of pretilachlor on HTMA-bent is physical and thus a reversible process of the release. The rate constant of adsorption ka and that of desorption kd could be related to the equilibrium constant KaTherefore, a decrease in the rate constant or an increase in the equilibrium constant of adsorption would result in a slower desorption or release of pretilachlor, which was confirmed by the linear increase of the T50 value for pretilachlor release with the relative value of the equilibrium constant Ka to the rate constant k2 for pretilachlor adsorption on HTMA-bent (Figure ). The results from this work indicated that the structure of organobentonites had a similar influence on the kinetics of release and adsorption of pretilachlor and that the adsorption rate constant could be used to describe the permeability of organobentonites. A combinational study of thermodynamics and kinetics of adsorption would be helpful to evaluate the release properties of organobentonites and explore the organobentonite-based CRFs.

Figure 7

Relationship of T50 values for pretilachlor release from HTMA-bent with the relative value of adsorption equilibrium constant Ka to adsorption rate constant k2 from the pseudo-second-order model.

Conclusions

Organobentonites modified with HTMA were excellent adsorbents for pretilachlor and potential platforms for CRF. The adsorption of pretilachlor on HTMA-bent was dominated by the hydrophobic interaction accompanying a positive enthalpy and entropy change. With increase in the loading level of HTMA from 0.4 to 1.5 × CEC, the intercalated surfactant developed from a less ordered conformation of low packing density to an ordered one of high packing density, leading to a decrease in the degree of freedom of the adsorbed pretilachlor molecules and thus a decrease in the entropy change. However, an enhancement of the adsorption capability of organobentonite toward pretilachlor was achieved due to the reduction of the unfavorable enthalpy change.

The kinetics of pretilachlor adsorption was related to the basal spacing of organobentonites and the surfactant packing density in the interlayer gallery. The intercalated HTMA adopted a lateral-monolayer arrangement at the loading level less than 0.8 × CEC. The increase in surfactant loading had little influence on the basal spacing of HTMA-bent. However, the enhanced packing density of the surfactant weakened the diffusion of pretilachlor molecules in the interlayer, resulting in a dramatic drop in the adsorption rate constant. The surfactant arrangement underwent a transition from lateral-monolayer to lateral-bilayer at the loading level of more than 0.8 × CEC. A decrease in the rate constant of pretilachlor adsorption on organobentonite was also observed, although much less evident than that for organobentonites with lower surfactant loading, as the stepwise increase in the basal spacing benefited the diffusion of pretilachlor into the interlayer gallery of organobentonite and reduced the influence on the increase in surfactant packing density.

The release of pretilachlor was greatly retarded by incorporating it into HTMA-bent. The release was dominated by Fickian diffusion, which could be understood from the thermodynamics and kinetics of pretilachlor adsorption on the matrix. The time taken for the release of 50% of active ingredient from organobentonites was 16–23 times that for the control formulation and increased linearly with the relative value of the equilibrium constant to the rate constant of pretilachlor adsorption.

Materials and Methods

Materials

Bentonite was obtained from Guangxi Ningming Minerals Co. (China). The cation exchange capacity (CEC) of the bentonite was about 0.82 mmol·g–1. Analytical grade hexadecyltrimethyl ammonium chloride (HTMA) and methanol were obtained from Sinopharm Chemical Reagent Co. Ltd. (China). Technical grade pretilachlor (95%) was kindly supplied by Guangxi Tianyuan Biochemical Co. (China).

Methods

Preparation of Organobentonites

Organobentonites were prepared following the process described in previous work.[21] Briefly, a stock solution containing HTMA of the amount equivalent to 0.4–1.5 × CEC of the clay was added to the dispersion containing ca. 2% bentonite under agitation and kept at 60 °C for 24 h. The resulting organobentonites were then separated by vacuum filtration, dried to constant weight at 60 °C, ground in an agate mortar and sifted through a 150 μm mesh sieve. The organobentonite is denoted as HTMA-bent and labeled as BH, where x is the quantity of HTMA used and presented as the fractions of the clay’s CEC. The carbon content of organobentonites was determined on a PerkinElmer PE2400II elemental analyzer.

XRD

XRD patterns of organobentonites were recorded on a Rigaku D/Max-2500V diffractometer. The Ni-filtered Cu Kα radiation (λ = 1.5405 Å) was used as the X-ray source. All scans were performed in the range of 2θ = 3–10° at a speed of 4°·min–1, with a step size of 0.02°. The basal spacing of organobentonites was calculated from the 2θ values of the (001) plane, using the Bragg equation.

FTIR

FTIR spectra were collected from 400 to 4000 cm–1 with a resolution of 4 cm–1 on a Shimadzu FTIR-8400S FTIR spectrometer in the transmission mode. Organobentonites were ground with KBr in the weight ratio of about 2:100 and compressed into pellets before the performance.

HPLC

The concentration of pretilachlor in the solutions was determined by a Shimadzu SPD-10A high-performance liquid chromatograph equipped with a UV–vis detector set at 225 nm and an ODS column (250 mm × 4.6 mm, 5 μm). The mobile phase was a methanol–water mixture (volume ratio of 90:10) at a flow rate of 1.0 mL·min–1. Solutions containing pretilachlor were passed through nylon filters of 0.22 μm pore diameter before the performance.

Adsorption Isotherms

Fifty milligrams of organobentonite was mixed with 50 mL of the aqueous solution of pretilachlor of concentration ranging from 5 to 40 μg·mL–1. The dispersions were kept on a multichannel magnetic stirrer equipped with a water bath thermostated to 25 °C for 4 h to reach an equilibrium. The supernatants were analyzed by HPLC and the adsorbed amount of pretilachlor was calculated from the difference between initial and final concentrations. The performance was carried out duplicate, and the average results were used to obtain the adsorption isotherms of pretilachlor on organobentonites.

Adsorption Thermodynamics

Similar to the performance of adsorption isotherms, 50 mL of a solution containing pretilachlor in ca. 40 μg·mL–1 concentration was mixed with 50 mg of organobentonite under agitation and kept for 4 h to reach an equilibrium. The equilibrium concentration of pretilachlor in the solution was determined by HPLC, and the adsorption equilibrium constant was evaluated. The performance was carried out at 20, 30, and 40 °C and for least 5 times under the same conditions. The average results were used to evaluate the thermodynamic parameters of pretilachlor adsorption onto organobentonites.

Adsorption Kinetics

Adsorption kinetics of pretilachlor was carried out on a stirred-flow reactor system made in our laboratory (Figure S7 in the Supporting Information). The ca. 1.0 cm3 propylene stirred-flow reactor had a side inlet at the bottom and was connected to an LC-10AD pump (Shimadzu, Japan) by 0.5 mm i.d. poly(tetrafluoroethylene) tube. The outlet in the lid was covered with a nylon filter membrane of pore size 0.45 μm to retain the samples in the reactor. The reactor carrying 25 mg of organobentonite and a magnetic stirring bar was immersed in a water bather thermostated to 25 °C. The stock solution containing pretilachlor in the concentration of ca. 40 μg·mL–1 was circulated at a flow rate of 1 mL·min–1. The output solution fractions were collected in predefined time duration, and the mass was determined on a Mettler AE200 analytical balance. The herbicide was analyzed by HPLC, and the adsorbed amounts were obtained from the concentration difference between the input and the output solutions.

Preparation of Pretilachlor-Loaded Organobentonites

A stock solution of 1 g of pretilachlor (a.i.) dissolved in 5 mL of methanol was dropped into aqueous dispersions containing 5 g of organobentonites under rigorous agitation. The dispersions were kept at room temperature for 12 h and separated by vacuum filtration. The resulting pretilachlor-loaded organobentonites were dried to a constant weight at 60 °C, ground in an agate mortar, and sifted through a 150 μm mesh sieve. Fifty milligrams of pretilachlor-loaded organobentonites were extracted by 50 mL of methanol 2 times. The extract was separated by centrifugation, and the supernatant was analyzed by HPLC to evaluate the amount of pretilachlor loaded. Organobentonites incorporating pretilachlor were labeled as BHP.

Pretilachlor Release in Water

The release of pretilachlor from organobentonites in water was conducted in the stirred-flow reactor system using a method similar to that used for adsorption kinetics. Twenty-five milligrams of pretilachlor-loaded organobentonite was placed in the reaction chamber kept at 25 °C and water was circulated at a flow rate of 1 mL·min–1. The output solution fractions were collected in predefined time duration. The herbicide was analyzed by HPLC and the amount released was calculated to obtain a cumulative release profile.