Fabrication of a Temperature-Controlled-Release ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Fabrication of a Temperature-Controlled-Release Herbicide Using a Nanocomposite Yu Chi,†,§ Guilong Zhang,†,‡ Yubin Xiang,†,§ Dongqing Cai,*,†,‡ and Zhengyan Wu*,†,‡ †

Key Laboratory of Ion Beam Bioengineering, Hefei Institutes of Physical Science and ‡Key Laboratory of Environmental Toxicology and Pollution Control Technology of Anhui Province, Hefei Institutes of Physical Science, Chinese Academy of Sciences, 350 Shushanhu Road, Hefei, Anhui 230031, People’s Republic of China § University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China ABSTRACT: In this work, temperature-responsive controlled-release herbicide particles (TCHP) with a core−shell structure were developed using a nanocomposite consisting of attapulgite (ATP), NH4HCO3, amino silicon oil (ASO), poly(vinyl alcohol) (PVA), and glyphosate (Gly). Therein, the ATP-NH4HCO3-Gly mixture acts as the core, and ASOPVA acts as the shell. ATP possesses a porous micro/nano networks structure and thus can bind a large amount of Gly molecules. NH4HCO3, as a foaming agent, can produce CO2 and NH3 bubbles to make plenty of micro/nano pores in the ASO-PVA shell, which facilitates the release of Gly. The pore amount can be efficiently adjusted by temperature, meanwhile the PVA shell tends to dissolve in aqueous solution at a high temperature, so that the release of Gly can be easily controlled. Importantly, this technology could effectively decrease the loss of Gly under simulated rainfall and thus improve the control efficiency on weeds. The hydrophobic ASO endows TCHP a high stability in aqueous solution for at least three months. This work provides a promising approach to control the release and loss of pesticide, which has a potential application to enhance the utilization efficiency and thus lower the environmental pollution. KEYWORDS: Temperature-responsive, Controlled-release, Herbicide, Nanocomposite, Micro/nano pore

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INTRODUCTION Pesticides have been widely used in controlling weeds, pests, and diseases all over the world, playing a key role in the protection of crops. Traditional pesticides unabsorbed by crops tend to discharge into the environment through rainwater washing, leaching, and volatilization, causing severe pollution to ecosystems and hazards to the health of human beings.1−3 Particularly, due to the harmful effects such as teratogenicity, carcinogenicity, and so on, herbicides displayed obviously higher risks compared with insecticides4 and bactericides.5−8 Meanwhile, herbicides showed severe hazardous effects on nontarget organisms because of their high mobility, leading to serious ecological problems.9 Therefore, it is rather important to develop facile approaches to reduce the loss of herbicides and enhance the utilization efficiency (UE). In the past few decades, many methods of controlling herbicide loss have been developed such as microcapsule,10 tablet,11,12 organosilicone,13,14 nanoadditive,15−17 and so on. Although these methods can reduce herbicide loss and improve UE in different degrees, they have some disadvantages limiting their applications. Therein, the microcapsule and tablet methods mainly focus on the slow release of pesticides through the coating of polymers; however, the defects of complex procedure and high cost obviously hinder their application.10,11 Besides, the lack of sensitivity to physical and chemical factors © 2017 American Chemical Society

makes it difficult for microcapsules and tablets to control the release of herbicides.12 Organosilicone can increase the adhesion and penetration performance of herbicides to a certain extent, while this method has poor stability, high cost, and low ability to control herbicide release.18 Compared with organosilicones, nanoadditives display a higher stability and can efficiently enhance the adhesion ability of herbicides through networks-structured nanoclay, nevertheless this method is unable to make the release of herbicides well match the need of crops.15−17 Recently, several kinds of controlled-release herbicides have been fabricated using pH-sensitive gels, while they display small application values because of their high cost.19,20 Therefore, it is urgent to develop smart controlledrelease herbicides with a simple procedure and low cost, which can be the main trend for developing the modern pesticide industry. Attapulgite (Mg,Al)4(Si)8(O,OH,H2O)26·nH2O) (ATP), a kind of rod-shaped nanoclay with advantages of abundant, environmentally friendly, low cost, and high stability, and so on, has been widely used as adsorbent, insulation, paint, etc.21,22 Our previous study indicated that ATP with a porous nano Received: February 5, 2017 Revised: April 21, 2017 Published: April 26, 2017 4969

DOI: 10.1021/acssuschemeng.7b00348 ACS Sustainable Chem. Eng. 2017, 5, 4969−4975

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ACS Sustainable Chemistry & Engineering

given intervals, and the Gly concentration in the solution (1 mL) was detected. Afterward, 1 mL of deionized water with the same pH was added to the system. Finally, the release ratio (RR) of Gly was calculated by eq 1

networks structure possessed a high adsorption ability on pesticide molecules and thus could be used as an ideal carrier for herbicide.15 However, ATP has low sensitivities to physical and chemical factors; therefore, it is difficult to fabricate a smart controlled-release herbicide using ATP alone.16 In this work, a temperature-responsive controlled-release herbicide particle (TCHP) with a core−shell structure was developed using a nanocomposite consisting of ATP, NH4HCO3, amino silicon oil (ASO), poly(vinyl alcohol) (PVA), and glyphosate (Gly), wherein the ATP-NH4HCO3-Gly mixture acts as the core, and ASO-PVA acts as the shell. Gly was selected as the model herbicide because it is one of the most widely used herbicides.23 In this system (Figure 1), ATP acts as an adsorbent for Gly,

⎛ C · V + ∑t − 1 C · V ⎞ t total t t 0 ⎟ × 100% RR (%) = ⎜⎜ ⎟ m 0 ⎠ ⎝

where Ct (mg/mL) and Vt (1 mL) are the concentration and volume of the solution taken out at time t, respectively, Vtotal (50 mL) is the volume of the total solution, and m0 (mg) is the initial loading amount of Gly. To investigate the thermosensitivity of TCHP, the release of Gly from TCHP at temperatures of 25, 40, and 50 °C was measured respectively according to the same method as above. Stability Investigation. 0.02 g of AGNP or TCHP was soaked in 5 mL of deionized water, respectively. Then the system was kept steadily at room temperature, and the system was filtered through a net with pore size of 0.5 mm after 1, 7, 30, and 90 d, respectively. The weight of the remaining sample was measured after air drying at room temperature. Ultimately, the collapse ratio (CR) of sample was calculated by eq 2

CR = (1 − Wt /W0) × 100%

(2)

where Wt is the weight of remaining sample at time t, and W0 is the initial weight of the sample. Pot Experiments. TCHP (150 mg) and Gly (50 mg) were evenly sprayed onto ZM in a Petri dish (diameter of 150 mm) at temperatures of 25 and 40 °C, respectively. Additionally, 25 mL of deionized water was sprayed onto the Petri dish every 2 days. Herein, ZM sprayed with deionized water was used as the control. After 1, 3, and 7 d, the area of dead ZM in each Petri dish was measured, and the control efficiency (CE) of the sample on ZM was calculated by eq 3

Figure 1. Schematic illustration of fabrication procedure and mechanism of TCHP.

meanwhile NH4HCO3 acts as a foaming agent to produce CO2 and NH3 bubbles and make micro/nano pores in the ASO-PVA shell. The micro/nano pore number and the solubility of the PVA shell can be effectively adjusted by temperature, so that the release of Gly can be controlled. Additionally, ASO, as a hydrophobic agent, can effectively enhance the stability of TCHP in aqueous solution. The interactions in the TCHP system and the control efficacy on weeds were also investigated. This work provides a promising route with low cost and simple procedures to reduce pesticide loss and improve the UE.

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(1)

CE = St /S0 × 100%

(3)

where St is the area of dead ZM at time t, and S0 is the initial area of ZM. Particularly, to investigate the control efficiency of TCHP on ZM under rainfall conditions at 40 °C, 200 mL of deionized water was sprayed immediately to simulate a rainfall after the spraying of TCHP (150 mg) and Gly (50 mg) onto ZM in a Petri dish. Other experiment processes are the same as given above. Characterization. The morphologies of samples were observed on a scanning electron microscope (SEM) (Sirion 200, FEI Co., USA). Thermogravimetry-differential thermal analysis (TG-DTA) curves were obtained at a temperature from 24 to 800 °C using air atmosphere with a heating rate of 10 °C min−1 on a DSCQ2000 thermogravimetric analyzer (TA Co., USA). The structure and interaction analyses were performed using a TTR-III X-ray diffractometer (XRD) (Rigaku Co., Japan) and a Fourier transform infrared spectrometer (FTIR) (iS10, Nicolet Co., USA). The concentration of Gly was measured using an UV−vis spectrophotometer (UV 2550, Shimadzu Co., Japan) at a wavelength of 243 nm.

EXPERIMENTAL SECTION

Materials. Natural ATP powder (100−200 mesh) was provided by Mingmei Co., Ltd. (Anhui, China). Gly (100−200 mesh, purity ≥95%) was produced by Fengyuan Pesticide Co. (Henan, China). PVA, NH4HCO3, and other chemicals were of analytical reagent grade and were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Weeds (zoysia matrella (ZM)) were provided by Chunsemanyuan Flower Co. (Zhejiang, China). Deionized water was used throughout this work. Preparation of TCHP. ATP, Gly, and NH4HCO3 with a weight ratio of 1:1:1 were mixed evenly, and the resulting mixture was granulated by a BY400 pelletizer (Taizhou Changjiang Medicine Machinary Limited Co., Jiangsu, China) to obtain ATP/Gly/ NH4HCO3 particles (AGNP) with a diameter of 0.5−1 mm. Afterward, 3 g of AGNP was added to 30 mL of ASO/ethanol solution with a VASO:Vethanol ratio of 1:40, and the resulting system was shaken (120 rpm) for 2 h to obtain AGNP@ASO particles (AAP) with diameters of 0.5−1 mm after air drying at room temperature. Then, 3 g of AAP was added to PVA (10 g/L) aqueous solution, and the resulting system was shaken (120 rpm) for 20 min to obtain TCHP with a diameter of 0.5−1 mm after air drying at room temperature. Release Ratio. 1.5 g of AGNP, AAP, and TCHP was soaked in deionized water (50 mL), respectively. Then the system was kept steadily at room temperature, 1 mL of the solution was taken out at

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RESULTS AND DISCUSSION Structure and Interaction Analyses. ATP displayed a fibrous structure consisting of numerous nano rods (Figure 2a) and thus could adsorb plenty of Gly molecules on the surface (arrow I in Figure 2b) of ATP nano rods and in the pores (arrow II in Figure 2b) formed by the cross-link of ATP rods. Interestingly, after adsorption of Gly, the fibrous structure transformed to micro/nano networks (Figure 2b), which was probably because of the electrostatics repulsion among ATPGly rods. There are a great number of −OH on the surface of ATP, which is favorable for binding Gly molecules through hydrogen bonds between ATP (−OH) and Gly (−COOH and −NH−).24 The ATP-Gly was evenly mixed with NH4HCO3 4970


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Figure 4. (A) Digital images of AGNP in aqueous solution after 1 d (a), TCHP in aqueous solution after 1 (b), 7 (c), 30 (d), and 90 d (e), respectively; (B) collapse ratios of AGNP (a) and TCHP (b) with time.

mainly of NH3 and CO2 derived from the decomposition of NH4HCO3, transported through the ASO-PVA shell to form these pores with a diameter of 0.1−1 μm. It was estimated that there were approximately 15000 pores in each TCHP at 40 °C, which were beneficial for the release of Gly. Additionally, after release at 40 °C, TCHP displayed an obviously rough surface compared with that at 25 °C, which was probably attributed to the dissolution of PVA in aqueous solution. The dissolution of PVA could probably make the ASO-PVA shell thinner, and this also contributed to the release of Gly. In order to obtain the interactions in TCHP, FTIR measurement was performed. As shown in Figure 3A(a), the peaks at 1091 and 979 cm−1 for AGNP should be assigned to the antisymmetrical and symmetrical stretching vibrations of P−O bonds, and the peak at 1327 cm−1 was ascribed to the stretching vibration of −C−P, suggesting that Gly was loaded in AGNP successfully. In Figure 3A(b), the peaks at 1568 and 1405 cm−1 ascribed to the bending vibration of −NH2 in AAP

Figure 2. SEM images of ATP (a), ATP-Gly (b), AAP (c), TCHP at 25 °C (d), and TCHP after release at 40 °C (e) with a magnified image (f). Digital images of TCHP at room temperature (g) and TCHP in aqueous solution (h) at 40 °C.

crystals to obtain AGNP which was then coated by ASO to obtain AAP (Figure 2c). Therein, several cracks were clearly found in NH4HCO3 because of its decomposition under a high temperature. Subsequently, AAP was well coated by PVA to form spherical (diameter of 0.5−1 mm) TCHP which possessed a smooth surface without any pores (Figure 2d and 2g), wherein the ASO-PVA shell played a key role in hindering the release of Gly. When TCHP was soaked in aqueous solution at 40 °C, a number of bubbles appeared on the surface of TCHP (Figure 2h), and plenty of micro/nano pores were found in the ASO-PVA shell (Figure 2e and f). These bubbles,

Figure 3. (A) FTIR spectra of AGNP (a), AAP (b), and TCHP (c); (B) XRD spectra of ATP (a), NH4HCO3 (b), Gly (c), AGNP (d), AAP (e), and TCHP (f); (C) TGA pattern of TCHP. 4971


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Figure 5. (A) Release ratios of AGNP (a) and TCHP (b) at 25 °C; (B) release ratios of TCHP at different temperatures.

−OH proved the existence of PVA coating on the surface of AAP.26 Besides, XRD measurements were performed to investigate the crystal structure of TCHP. As shown in Figure 3B, the characteristic peaks of ATP (2θ = 27.52°), NH4HCO3 (2θ = 24.58°), and Gly (2θ = 20.28°, 22.94°, and 23.48°) were displayed in the spectrum of AGNP, which exhibited that NH4HCO3 and Gly were successfully loaded in ATP. Additionally, the peak (2θ = 29.78°) of AAP was intensified compared with AGNP, which was probably because AAP could form large bunches via H-bonds between ASO and ATP, and the amount of ATP rods in the same direction increased. However, some reflections (2θ = 20.92° and 22.84°) of TCHP and AAP were weakened compared with AGNP, which was attributed to the lower crystallinity and content of Gly in TCHP and AAP. The thermal stability of TCHP was also investigated. As shown in Figure 3C, the mass loss (7.310%) between 40.44 and 129.22 °C was attributed to the removal of free water, −OH2, and NH4HCO3. The mass loss (8.767%) between 129.22 and 303.55 °C corresponded to the removal of −OH2 and the decomposition of ASO, PVA, and Gly to small molecules.27 The mass loss (11.64%) between 303.55 and 799.91 °C was probably because of the pyrolysis of those small molecules of ASO, PVA, and Gly. Stability Investigation in Aqueous Solution. The stability performance of TCHP in aqueous solution was investigated compared with AGNP. As shown in Figure 4, AGNP collapsed completely in 1 day because of the high hydrophilicity of Gly, ATP, and NH4HCO3. In contrary, TCHP was rather stable in aqueous solution. Although the volume of TCHP increased by approximately 4-fold after three months because of the swelling effect, the collapse ratio of TCHP still increased very slowly with time and reached a value of only 5% after three months. This was because the hydrophobic ASO coating can effectively hinder the access of water molecules into TCHP. Such stability facilitates the long-term application of TCHP in field. Release Behavior Investigation. The release behavior of TCHP in aqueous solution at 25 °C was investigated in comparison with AGNP. As shown in Figure 5A, TCHP exhibited a significantly slower release than AGNP, achieving release ratios of 12% and 91% after 13 h, respectively, which was attributed to the protection effect of the ASO-PVA shell in TCHP. Noteworthily, the release ratio of AGNP increased rapidly from 0% to 72% during the initial 1 h, while that of TCHP increased rather slowly from 0% to 10%, which was

Figure 6. SEM images of TCHP after release at different temperatures.

indicated that ASO was introduced to the surface of AGNP.25 As seen in Figure 3A(c), the peaks at 3617, 3583, and 3553 cm−1 for TCHP corresponded to the vibrations of Si−OH, and the peak at 3380 cm−1 ascribed to the bending vibration of 4972


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Figure 7. Digital images of weeds treated with deionized water (control), TCHP, and Gly at different temperatures.

Figure 8. (A) Schematic illustration of the process of the pot experiment and (B) the control efficiency on weeds.

Figure 9. (A) Schematic illustration of the pot experiment under simulated rainfall; (B and C) digital images of weeds and control efficiency on weeds treated by TCHP and Gly at 40 °C under simulated rainfall.

because high temperature could promote the decomposition of NH4HCO3. Noteworthily, some pits appeared on the surface of TCHP at temperatures higher than 30 °C, and their number and area increased with temperature, which was because of the dissolution effect of PVA. At temperatures of 40 and 50 °C, some ATP-Gly particles even appeared obviously on the surface of TCHP. Moreover, the dissolution of PVA makes the ASOPVA shell thinner in the regions of pits, which is beneficial for the formation of pores and thus release of Gly. Interestingly, some bubbles were found on the surface of TCHP at temperatures of 35 and 40 °C, which probably resulted from the migration of CO2 and NH3 from inside of TCHP. In a

because AGNP had an obviously higher collapse ratio than TCHP in aqueous solution. Additionally, the release of Gly from TCHP in aqueous solution with temperature was investigated. Figure 5B clearly illustrated that the release ratio of TCHP increased significantly with temperature, displaying values of 13% (25 °C), 22% (40 °C), and 33% (50 °C), respectively, after 72 h. This result indicated that the release of TCHP can be efficiently adjusted by temperature because of the temperature-responsive structure of TCHP. Additionally, the morphologies of TCHP after release at different temperatures were observed. As shown in Figure 6, the pore number of TCHP increased obviously with temperature 4973


ACS Sustainable Chemistry & Engineering word, high temperature could facilitate the formation of pores and pits on the surface of TCHP, which could help the release of Gly. Pot Experiment. In order to obtain the control efficiency of TCHP, the pot experiment was carried out. As shown in Figures 7 and 8, the control efficiency of TCHP on ZM increased obviously with temperature from 25 to 40 °C, while that of Gly increased slowly, which was in accordance with the result of the release behavior. Additionally, deionized water seemed to display a little influence on ZM. Besides, the control efficiency of TCHP on ZM at 40 °C increased slowly with time compared with Gly because of the slow release of TCHP. These results proved the excellent temperature-responsive controlled-release performance of TCHP. Besides, the antiwashing ability of TCHP was investigated at 40 °C under simulated rainfall in comparison with traditional Gly. During the initial period after spraying, when the ZM was treated by traditional Gly, all the Gly molecules existing on the leaf surface were exposed to the rainfall and tended to be washed off to soil. However, owing to the slow release performance of TCHP, only a small amount of Gly molecules was released and existed on the leaf surface of ZM, so that just a few Gly molecules were washed off by water. After the simulated rainfall, there were still a great number of Gly molecules that remained in TCHP on the TCHP-treated ZM, while quite a few Gly molecules were left on the ZM treated by traditional Gly (Figure 9A). Noteworthily, the Gly in soil tended to become inactive because of the chelating effect with metal ions.28 As a result, the control efficiency of TCHP was obviously higher than that of traditional Gly after 3 and 7 days (Figure 9B and C). This result indicated that this technology could efficiently increase the antiwashing ability, decrease the loss amount, and enhance the utilization efficiency of Gly.

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ACKNOWLEDGMENTS

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REFERENCES

The authors acknowledge financial support from the National Natural Science Foundation of China (No. 21407151), the Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2015385), the Key Program of Chinese Academy of Sciences (No. KSZD-EW-Z-022-05), the Science and Technology Service Programs of Chinese Academy of Sciences (Nos. KFJ-EW-STS-083 and KFJ-EW-STS-067), and the Grant of the President Foundation of Hefei Institutes of Physical Science of Chinese Academy of Sciences (No. YZJJ201502).

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CONCLUSIONS In this work, a core−shell structured TCHP was fabricated based on a nanocomposite. Therein, ATP could adsorb plenty of Gly molecules in the micro/nano networks through hydrogen bonds, meanwhile NH4HCO3 could produce CO2 and NH3 bubbles and make micro/nano pores in the ASO-PVA shell under high temperature. The micro/nano pore number and the solubility of the PVA shell can be efficiently controlled by temperature, so that TCHP displayed a high temperaturecontrolled-release performance, which was proved by the pot test. Importantly, this technology could significantly control the loss of Gly under simulated rainfall and thus enhance the control efficiency on ZM. Additionally, TCHP was rather stable in aqueous solution owing to the protection of hydrophobic ASO. This work provides a low cost and simple approach to control pesticide release and improve the UE, which may have a promising application prospect.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-551-65595143. Fax: +86-551-65595012. E-mail: [email protected] (D.C.). *Phone: +86-551-65595012. Fax: +86-551-65595012. E-mail: [email protected] (Z.W.). ORCID

Zhengyan Wu: 0000-0002-8142-1848 Notes

The authors declare no competing financial interest. 4974


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