Halloysite tubes as nanocontainers for herbicide and its controlled

the controlled release of herbicide also reduced its leaching through soil layer, ... Keywords: Controlled release; Herbicide; Biodegradable polymer; ...
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Halloysite Tubes as Nanocontainers for Herbicide and Its Controlled Release in Biodegradable Poly(vinyl alcohol)/Starch Film Bangchao Zhong,† Song Wang,† Huanhuan Dong,† Yuanfang Luo,† Zhixin Jia,*,† Xiangyang Zhou,*,‡ Mingzhou Chen,§ Dong Xie,§ and Demin Jia†

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Key Lab of Guangdong for High Property and Functional Polymer Materials, South China University of Technology, Guangzhou 510640, China ‡ Department of Chemistry and Chemical Engineering, Zhongkai University of Agriculture and Engineering, Guangzhou 510225, China § Guangzhou Sugarcane Industry Research Institute, Guangdong Academy of Sciences, Guangzhou 510316, China S Supporting Information *

ABSTRACT: Commercial herbicide atrazine (AT) was first loaded into the lumen of halloysite nanotubes (HNTs) in the amount of 9 wt %, and then the AT-loaded HNTs (HNTs-AT) were further incorporated into poly(vinyl alcohol)/starch composites (PVA/ST, with the weight ratio of 80/20) to construct a dual drug delivery system. AT loaded in nanotubes displayed much slower release from PVA/ST film in water than free AT; for example, the total release amount of AT from PVA/ ST film with loaded AT was only 61% after 96 h, while this value reached 97% in PVA/ST film with free AT. The release behavior of AT from PVA/ST film with HNTs-AT was first dominated by the mechanism of matrix erosion and then by the mechanism of Fickian diffusion. In addition, combining HNTs and PVA/ST blends together in the controlled release of herbicide also reduced its leaching through the soil layer, which would be useful for diminishing the environmental pollution caused by pesticide. KEYWORDS: controlled release, herbicide, biodegradable polymer, halloysite nanotubes, leaching

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INTRODUCTION Pesticides are effective in controlling weeds and pests, which significantly improves the crop yields.1−3 However, the extensive use of pesticides in agronomic practices is becoming a serious environmental concern because of the potential runoff and leaching of these toxic compounds through the soil, leading to contamination in water and food.4,5 Therefore, the improvement of pesticide safety has always been a highly useful and challenging topic in agriculture.6,7 Many efforts have been devoted to minimizing the risk of pesticides.5,8 Typically, Dumée et al. synthesized a novel hybrid thin film composite catalytic membrane.9 In their work, catalytic silver−metal nanomaterials were uniformly templated and encapsulated across metal organic frameworks (MOF) nanoparticles and incorporated across the top surface of poly(amide) thin films during interfacial polymerization for the first time. Results showed that the kinetics of Ag@MOF nanocrystal-catalyzed degradation of 2,4-dichlorophenoxyacetic acid was extremely fast and complete degradation was achieved within a very short time. The utilization of sorbents to carry pesticides is an effective approach to avoid the runoff and leaching of pesticides. Many kinds of sorbent materials were synthesized to construct controlled release systems for pesticides or other active materials. Sonawane, Bhanvase, and their co-workers have done a remarkable job in this field. They synthesized a series of novel nanocontainers, such as calcium zinc phosphate pigment,10 ZnO,11 iron oxide-blended sodium zinc molybdate,12 silica,13 cerium zinc molybdate,14 and zinc phosphate.15 Those nanocontainers were mainly prepared by the method of © 2017 American Chemical Society

layer-by-layer assembly or with the aid of ultrasound. Recently, natural clays have received increasing attention for their biocompatibility, thermal stability, abundance, low-cost, and other environmentally friendly characteristics.16−19 As a kind of naturally occurring aluminosilicate mineral, halloysite nanotubes (HNTs) were used as sorbent materials for the controlled release of active molecules20−25 because of their biocompatibility and tubular structure with appropriate size.26 Nonetheless, the ability to retard the release of pesticides with single nanoclay delivery system was still not very satisfactory.27 Agricultural mulching film has been widely used to promote the growth of crop. Polyethylene (PE) is the most common material used for agricultural mulch. However, PE does not decompose in soil, leading to deleterious effects on the environment. Hence, developing degradable plastic mulch is a pressing environmental demand for the increasing nondegradable waste and the corresponding disposal problem.28 Starch (ST) is a potential candidate of biodegradable mulch for being natural, renewable, and inexpensive, but the weak mechanical strength limits its application. While biodegradable poly(vinyl alcohol) (PVA) possesses outstanding mechanical properties (strength and flexibility, etc.), the high cost restricts its commercialization as agricultural mulch. Received: Revised: Accepted: Published: 10445

September 10, 2017 November 9, 2017 November 13, 2017 November 13, 2017 DOI: 10.1021/acs.jafc.7b04220 J. Agric. Food Chem. 2017, 65, 10445−10451

Article

Journal of Agricultural and Food Chemistry

AT) were added into the PVA/ST solution. Sequentially, stirring was employed until a homogeneous gel-like solution was formed. The gellike solution was poured on a warmed glass plate (90 °C) and dried for 24 h at 60 °C. Characterization. Fourier transform infrared (FTIR) spectra were recorded by a Bruker Vector 33 spectrometer from 400 to 4000 cm−1. Transmission electron microscopy (TEM) images were taken with a JEOL2100. Scanning electron microscopy (SEM) was employed to examine the surface morphologies of the biodegradable polymer films using a Zeiss Merlin scanning electron microscope. X-ray diffraction (XRD) measurements were conducted on an Bruker D8 Advance X’Pert-Pro X-ray diffractometer. Thermogravimetric analysis (TGA) was conducted with a TA TGAQ500 at the heating rate of 20 °C/min from 30 to 700 °C under dry N2 atmosphere. Ultraviolet−visible (UV−vis) spectra were monitored on an Agilent Cary 60 spectrophotometer and used to analyze the amount of the released drug from the biodegradable films. The tensile strength and elongation at break of polymer films were tested with a Zwich/Roell instrument following ASTM D 882012. To investigate the swelling degree and solubility of the biodegradable films, dried films were immersed in distilled water at room temperature. After equilibrium was reached, free water on the surface of the films was removed, and the weight of the samples was measured. The swelling degree was calculated by the following equation:31

In this work, a dual drug delivery system was constructed by first loading commercial herbicide atrazine (AT) into the lumen of HNTs and then adding AT-loaded HNTs into PVA/ST blends. The release kinetics of herbicide from this double delivery system was systematically investigated.

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MATERIALS AND METHODS

Materials. PVA was supplied by Pritt Plastics Chemical Co, Japan. Corn ST was produced by Hebei Ruide Starch Co., China. HNTs were mined in Hubei, China, and purified according to a previous method.29 The typical dimensions were 10−50 nm in outer diameter, 5−20 nm in inner diameter, and 0.5−2 μm in length. AT was purchased from Shanghai Macklin Biochemical Co., Ltd., and was used without further purification. Methanol, glycerol, and glyceryl monostearate from Sinopharm Chemical Reagent Co., Ltd., China, were of analytically pure grade and were used as received. Preparation of Herbicide-Loaded HNTs. The loading process of herbicide into the lumen of HNTs is shown in Figure 1. Specifically, 3

swelling degree (%) =

Figure 1. Schematic diagram of loading halloysite nanotubes with herbicide.

Wt − W0 × 100 W0

(1)

where Wt is the equilibrium weight of films after immersion and W0 is the initial weight of the dried film. The swollen films were dried again at 60 °C, and the solubility of films in water was calculated according to the following equation:

g of HNTs was dispersed into 300 mL of AT methanol solution (0.015 g/mL) with the aid of a magnetic stirrer. The suspension was first placed under vacuum condition (∼100−200 Torr). The fizzing of the suspension indicated that air was being removed from the lumen of HNTs and replaced with herbicide solution. The suspension was kept under vacuum for 30 min and then was cycled back to atmospheric pressure for another 30 min. This process was repeated five times to increase the loading efficiency of herbicide. This cycle of vacuumatmospheric pressure was conducted at room temperature. Subsequently, the suspension was centrifuged and washed 10 times with water to remove the surface adsorbed herbicide. The product was finally dried to constant weight with a vacuum oven at 40 °C to obtain AT-loaded HNTs (HNTs-AT). The amount of loaded AT in HNTs lumen was determined by the residue from thermogravimetric analysis at 700 °C. Preparation of the Biodegradable Polymer Films with Herbicide. Biodegradable polymer films were fabricated via the casting method.30 The detailed procedure was as follows: 8 g of PVA and 2 g of ST were dissolved in distilled water under stirring at 90 °C. Next, glycerol (20% of total weight of PVA and ST), glyceryl monostearate (1 wt %), and herbicide (0.2 g of AT or 2 g of HNTs-

solubility (%) =

W0 − Wd × 100 Wd

(2)

where Wd is the weight of the dried swollen film. The release measurements of pesticide were carried out in a flask at room temperature. The drug-loaded biodegradable film was immersed in 200 mL of distilled water. Next, 5 mL of supernatant was removed from the solution at selected time intervals and replaced by 5 mL of distilled water. The amount of AT in the supernatant was determined by the UV−vis spectrum at 223 nm. The release data were analyzed by applying the Higuchi equation32 (eq 3), Hixson−Crowell equation33 (eq 4), Hopfenberg equation34 (eq 5), and Ritger−Peppas equation35 (eq 6) as follows: M t /M∞ = kt 1/2

(3)

W01/3 − Wt 1/3 = kt

(4)

Figure 2. (a) FTIR spectra of HNTs and HNTs-AT; and (b) XRD spectra of pure AT, HNTs, and HNTs-AT. 10446

DOI: 10.1021/acs.jafc.7b04220 J. Agric. Food Chem. 2017, 65, 10445−10451

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Journal of Agricultural and Food Chemistry 1 − (1 − M t /M∞)1/ p = kt

(5)

ln(M t /M∞) = n ln t + C

(6)

attributed to the stretching vibration of Si−O−Si. In the spectrum of HNTs-AT, apart from the characteristic peaks of HNTs, the vibrations of organic groups from the structure of AT molecule are noticeable, such as the vibration of triazine at 1554 cm−1, and the vibrations of CH3 and CH2 at 2974 and 2940 cm−1, respectively. To further confirm the loading of AT into HNTs lumen, XRD was employed to provide more information. As shown in Figure 2b, characteristic diffraction peaks of pristine HNTs corresponding to 12.2°, 19.9°, and 24.9° appear, which confirms that the used HNTs are dehydrated halloysite with an interlayer space and nanotubular structure.36,37 As for pure AT, the diffraction peaks at 12.7°, 18.1°, 19.4°, 20.2°, and 22.2° are very intensive, reflecting the significantly high crystallinity of AT molecules.38 After AT was loaded into HNTs lumen, the main peak positions for pristine HNTs remain unchanged in the spectrum of HNTs-AT, proving that the crystalline structure of pristine HNTs is not affected in the presence of guest molecules. More importantly, new diffraction peaks at 12.7°, 18.1°, 19.4°, and 22.2° attributed to the crystalline structure of pure AT appear as well. However, the intensities of those new peaks in the spectrum of HNTs-AT are much weaker than those in the spectrum of pure AT, which should be ascribed to the low loading amount of AT. Considering the fact that scarcely any surface adsorbed AT molecules were left after sufficient washing, it can be concluded that herbicide molecules were successfully loaded into the lumen of HNTs on the basis of the results of FTIR and XRD. The loaded AT in the lumen of HNTs was observed by TEM. From Figure 3a, pristine HNTs show a typical nanotubular structure. However, in the case of HNTs-AT before the deloading of AT, the lumen of HNTs is blocked by AT (as denoted by the arrow in Figure 3b) and is interrupted by voids. After the deloading of AT, the morphology of HNTsAT shown in Figure 3c is similar to that of the pristine HNTs, and the density in the nanotubes is decreased as compared to HNTs-AT before deloading, indicating the successful release of AT from nanotubes. The TGA curves of HNTs and HNTs-AT are presented in Figure 4. HNTs-AT shows worse thermal stability than HNTs, because AT begins to decompose at ca. 150 °C. According to the weight loss of HNTs and HNTs-AT in the range of 150− 700 °C, the amount of AT loaded in the lumen of HNTs is ca. 9 wt %. Effect of ST Content on the Biodegradable and Mechanical Performances of PVA/ST Film. Figure 5a shows the weight loss of PVA/ST films with different ST contents during soil burial test. All polymer films degrade in soil, and the degradation elevates strongly with increasing ST, because many bacteria are capable of efficient degradation of ST.39 For example, when the ST content is 40 wt %, the weight loss of PVA/ST film exceeds 60%, verifying that the biodegradation of PVA/ST film is improved by the incorporation of ST. However, as can be found from Figure 5b, the mechanical performances including tensile strength and elongation at break of PVA/ST films sharply decline when the ST content is more than 20 wt %. To balance the biodegradable and mechanical performances of PVA/ST film, the ST content was fixed at 20 wt % to prepare herbicide-loaded biodegradable film. Morphologies of Biodegradable Films. Figure 6 shows the SEM images of different biodegradable films. As can be seen from Figure 6a, PVA/ST/AT film shows a rough surface with obvious rod-like protrusions. From the high-resolution image in

where Mt and M∞ are the amounts of drug released into water at time t and time ∞, respectively. W0 and Wt are the initial drug amount and remaining drug amount at time t, respectively. k is a constant related to the characteristics of the sorbent materials and the drug, and n is a parameter that indicates the transport mechanism. p is an exponent that varies with geometry. HNTs have cylindrical-shaped tubular morphology, hence p = 2. Soil burial degradation tests of PVA/ST films with different contents of ST were carried out at room temperature, under moisturecontrolled conditions. At indicated intervals, samples were recovered and fully washed with distilled water, then dried to constant weight under vacuum at room temperature. The degree of degradation was estimated from the weight loss normalized with respect to the initial weight of polymer film. The leaching potential of AT from the biodegradable films was tested with an experiment through a thin soil layer carried out in a Buchner funnel.31 Briefly, a thin layer of red soil was deposited on filter paper in a funnel. The biodegradable film then was cut into a disc with a diameter of 75 mm and placed on the top of the soil. The funnels were irrigated 8 times with 1 h intervals, and 40 mL water was used for each irrigation. The leachate was collected after each irrigation, and the AT concentration was determined by UV−vis spectrum.

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RESULTS AND DISCUSSION Characterization of Herbicide-Loaded HNTs. Figure 2a shows the FTIR spectra of HNTs and HNTs-AT. In the

Figure 3. TEM images of (a) pristine HNTs; (b) HNTs-AT before the deloading of AT; and (c) HNTs-AT after the deloading of AT.

Figure 4. TGA curves of HNTs and HNTs-AT.

spectrum of HNTs, the characteristic vibrations of the aluminol groups on the HNTs internal surfaces are observed at 3696 and 3622 cm−1. The peak of O−H bending at 1647 cm−1 originates from the adsorbed water, while the peak at 1035 cm−1 is 10447

DOI: 10.1021/acs.jafc.7b04220 J. Agric. Food Chem. 2017, 65, 10445−10451

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Journal of Agricultural and Food Chemistry

Figure 5. Weight loss during soil burial tests (a) and mechanical properties (b) of PVA/ST films with different contents of ST.

Figure 6. SEM images of biodegradable films: (a,b) PVA/ST/AT; and (c,d) PVA/ST/HNTs-AT.

Figure 8. Cumulative release of AT from biodegradable polymer films in water.

Figure 6b, the surface of PVA/ST/AT film is filled with wirelike AT crystals formed during the drying process of the polymer film. However, as shown in Figure 6c, the surface of the PVA/ST/HNTs-AT film is very flat, and no similar wirelike AT crystals exist. In addition, it is found from Figure 6d that HNTs-AT is uniformly semiembedded instead of being

totally exposed on the polymer surface, which is suggestive of a strong interfacial interaction between HNTs-AT and polymer matrix. As shown in Figure 7a, the strong interfacial interaction should be attributed to the hydrogen-bonding interaction between the surface siloxane of HNTs and polymer matrix.40

Figure 7. (a) Schematic representation of hydrogen bonding interaction between HNTs-AT and polymer matrix; and (b) Si 2p XPS spectra of pristine HNTs and PVA/ST/HNTs-AT film. 10448

DOI: 10.1021/acs.jafc.7b04220 J. Agric. Food Chem. 2017, 65, 10445−10451

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Journal of Agricultural and Food Chemistry Table 1. Release Dynamics Model Fitting Results of Biodegradable Films in Water samples PVA/ST/AT

fitted model

release stage

1/2

Higuchi Hixson− Crowell Hopfenberg Ritger− Peppas

first stage second stage

PVA/ST/ HNTs-AT

Higuchi Hixson− Crowell Hopfenberg Ritger− Peppas

fitted equation

first stage second stage

Mt/M∞ = 0.125t W01/3 − Wt1/3 = 0.0507t 1 − (1 − Mt/M∞)1/2 = 0.0124t ln(Mt/M∞) = 0.538 ln t + 2.722 ln(Mt/M∞) = 0.155 ln t + 3.905 Mt/M∞ = 0.080t1/2 W01/3 − Wt1/3 = 0.0507t 1 − (1 − Mt/M∞)1/2 = 0.006t ln(Mt/M∞) = 1.416 ln t + 1.992 ln(Mt/M∞) = 0.311 ln t + 2.823

r2 0.9687 0.9264 0.9210 0.9927 0.9405 0.9189 0.8261 0.8303

Figure 10. Leaching of herbicide form biodegradable polymer films through a soil layer.

0.9887 0.9726

biodegradable films are shown in Figure 8. Both PVA/ST/AT and PVA/ST/HNTs-AT films display a burst release in the first 4 h. The total release amount exceeds 30%. The release of AT from PVA/ST/HNTs-AT film slows down when compared to that from PVA/ST/AT film. For example, the total release amount of AT from PVA/ST/HNTs-AT film is only 61% after 96 h, while this value reaches 97% in PVA/ST/AT film, indicating the advantage of combining PVA/ST composites and HNTs in the controlled release of pesticides. Actually, as shown in Figure S1, the release of AT from PVA/ST/HNTs-AT film is also slower than that from the PVA/ST film with montmorillonite immobilized AT (MMT-AT), further demon-

The formation of hydrogen bonding will cause the variation of chemical environment, which can be detected by the variation of binding energy of the related atoms via XPS.41,42 From Figure 7b, the binding energy of silicon atom in PVA/ST/ HNTs-AT film, which connects to the oxygen atom in the hydrogen bond directly, is shifted to lower value than that in HNTs (from 103.15 to 102.86 eV), confirming the formation of hydrogen bonding in PVA/ST/HNTs film.43 Controlled Release of Herbicide from Biodegradable Polymer Films in Water. The release profiles of AT from

Figure 9. Kinetics fitting lines of different films by Hixson−Crowell and Ritger−Peppas models: (a,b) PVA/ST/AT;and (c,d) PVA/ST/HNTs-AT. 10449

DOI: 10.1021/acs.jafc.7b04220 J. Agric. Food Chem. 2017, 65, 10445−10451

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Journal of Agricultural and Food Chemistry

did free herbicide. Furthermore, the leaching of loaded herbicide in polymer film from soil was also reduced. This work is believed to promote the understanding of pesticide release characteristics in a dual delivery system comprising natural nanotubes and biodegradable polymer, as well provide a new opportunity for the preparation of biodegradable agriculture mulch with pesticide.

strating the advantage of nanotubes as sorbent material of pesticide for controlled release. To better understand the release mechanism of AT, the release data were checked by Higuchi, Hixson−Crowell, Hopfenberg, and Ritger−Peppas models. The fitting results are tabulated in Table 1. Typically, the fitting lines by Hixson− Crowell and Ritger−Peppas models are also provided in Figure 9. In the case of PVA/ST/AT biodegradable film, the values obtained for the correlation coefficients indicate that Higuchi and Ritger−Peppas kinetic models are well fitted. According to the fitting results of the Ritger−Peppas model, the release process of AT from PVA/ST/AT film is divided into two stages. At the first stage, the release exponent n (0.54), which is an important indicator of the diffusion mechanism of a drug,35 is higher than 0.45, indicating that the release of AT is a nonFickian diffusion behavior.44 This can be explained by the fact that the polymer film was not fully swollen during this stage, so the diffusion of AT through the polymer film was restricted. At the second stage, n (0.16) is less than 0.45, indicating that the release of AT is dominated by a mechanism of Fickian diffusion. This is because the polymer film was fully swollen at this stage, leading to the free diffusion of AT in the polymer film.45 As for PVA/ST/HNTs-AT biodegradable film, the release data of AT in water only fit well with the Ritger−Peppas model, and there are also two release stages. However, the release of AT at the first stage is dominated by the mechanism of matrix erosion (n = 1.416 > 0.89),46 which is attributed to the loading of AT into the lumen of HNTs. When the biodegradable polymer film was immersed in water, the flexibility and mobility of polymer chains in the free-swelling state were largely improved. Hence, polymer chains easily adhered at the mouth of the nanotube as stopper, and the release of AT from HNTs lumen was limited before the polymer film was totally dissolved in water. After the polymer layer was fully swollen, the water could permeate into the lumen of HNTs for releasing the AT molecules. When the nanotubes were occupied by water, the diffusion of AT molecules into the surrounding was allowed. During this stage, the release rate mainly depends on the degree of polymer erosion. The n value (0.311) of the second release stage is less than 0.45, indicating that the release of AT from PVA/ST/HNTs-AT biodegradable film follows Fickian behavior and depends on the time and initial concentration of herbicide-loaded nanotubes in the release system. Retarded Leaching of Herbicide in Soil. Figure 10 shows the leaching profiles of AT through the soil layer from different biodegradable films. The amount of herbicide leached after each irrigation for different polymer films is related to its release behavior in water. As can be seen, the herbicide leaching from PVA/ST/HNTs-AT film for each irrigation is lower than that from PVA/ST/AT film. This result proves that the loading of herbicide in the lumen of HNTs is useful for reducing the leaching of herbicide in soil. After 8 cycles of irrigation, there is still residual herbicide in the polymer films. The residual amount is oppositely related to the release rate of herbicide from biodegradable polymer films. The relatively high herbicide residue in PVA/ST/HNTs-AT film will be beneficial for keeping the biological effect of herbicide for a long period, which can simultaneously enhance the utilization efficiency of herbicide and avoid the environmental problems caused by the leaching and runoff of pesticide. In summary, herbicide loaded in nanotubes displayed much slower release from biodegradable polymer film in water than

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.7b04220. Figure S1: Release of AT from different biodegradable polymer films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhixin Jia: 0000-0002-2455-1523 Funding

This work was supported by the National Natural Science Foundation of China (51703063), Provincial Produce-LearnResearch Projects of Guangdong (2013B090500085), Provincial Public Interest Research and Special Capacity Building of Guangdong (2014B030303004), Cooperative Innovation Project of Guangzhou (201508010022), Special Fund for Applied Science and Technology Research of Guangdong (2015B020235010), Fundamental Research Funds for the Central Universities (2017BQ033), and China Postdoctoral Science Foundation (2017M612658). Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.jafc.7b04220 J. Agric. Food Chem. 2017, 65, 10445−10451