Research Article www.acsami.org
Functionalization of Biodegradable PLA Nonwoven Fabric as Superoleophilic and Superhydrophobic Material for Efficient Oil Absorption and Oil/Water Separation Jincui Gu,† Peng Xiao,† Peng Chen,† Lei Zhang,*,† Hanlin Wang,‡ Liwei Dai,† Liping Song,† Youju Huang,† Jiawei Zhang,*,† and Tao Chen*,† †
Ningbo Institute of Material Technology and Engineering, Key Laboratory of Marine Materials and Related Technologies, Chinese Academy of Science, Ningbo 315201, China ‡ College of Environmental & Resource Sciences, Zhejiang University, 866 Yuhangtang Road, Hangzhou 310058, China S Supporting Information *
ABSTRACT: Although the construction of superwettability materials for oil/water separation has been developed rapidly, the postprocess of the used separation materials themselves is still a thorny problem due to their nondegradation in the natural environment. In this work, we reported the functionalization of polylactic acid (PLA) nonwoven fabric as superoleophilic and superhydrophobic material for efficient treatment of oily wastewater with ecofriendly post-treatment due to the well-known biodegradable nature of PLA matrix.
KEYWORDS: biodegradable, PLA nonwoven fabric, superoleophilicity and superhydrophobicity, oil absorption, oil/water separation
1. INTRODUCTION The increasing consumption of fossil fuels and frequent oil spill accidents have produced lots of oily wastewater, which seriously threatens the sustainable development of human beings.1−3 Functional materials with high absorption capacity are highly anticipated to dispose oily wastewater effectively.4,5 Recently, much attention has been paid on bioinspired superhydrophobic materials with excellent capability of selectively adsorbing oil from water.6−9 Until now, abundant studies have been reported on the superwettability materials, for instance, metal meshes,10−12 freestanding filter films,13 aerogel,14−17 sponges,18,19 manganese nanoparticles,20 and textiles,21−25 which can selectively separate oil or water from oily wastewater. In our previous work, a facile strategy was developed to prepare carbon nanotube hybrid membranes with superwettability for the effective separation (water-in-oil or oil-in-water emulsions).26,27 Although many efforts have been devoted to construct superhydrophobic separation materials for wastewater treatment, the postprocessing of the used materials is still a daunting task because of their nondegradable essence. Conventionally, the polluted separation materials are directly discarded or burnt, inevitably leading to the secondary pollution to the environment. Therefore, eco-friendly separation materials with convenient post-treatment is of great significance for achieving all-around green materials for oil/water separation. In recent decades, polylactic acid (PLA) has been regarded as the most promising eco-friendly materials due to its © 2017 American Chemical Society
outstanding properties and is also expected to be developed into novel biodegradable separation materials.28,29 However, there is still a lack of a versatile and facile approach to prepare efficient PLA-based separation material with robust surface wettability and high mechanical stability for large scale practical applications. To achieve practical biodegradable PLA separation, in the present work, a porous structures PLA nonwoven fabric is fabricated for efficient oil/water separation, which is schematically illustrated in Figure 1. Dopamine, a biomimetic molecule inspired by mussel adhesive proteins that contains catechol and amine groups, can self-polymerize in an alkaline environment based on the aerobic auto-oxidation,30 and it was used first to modify PLA nonwoven fabric (Figure 1, panels A and B). In order to endow the PLA nonwoven fabric with superoleophilic and superhydrophobic properties, hierarchical micro/nanoparticles consisting of hydrophobic polystyrene (PS) microspheres and silica oxide (SiO2) nanoparticles were deposited densely on the polydopamine (PDA)-modified PLA fabric to enhance its surface roughness (Figure 1C). The resultant SiO2/PS/PLA hybrid nonwoven fabric with hierarchical porous structures exhibits high oil-absorption capacity and selectivity separation for oil/water mixtures (Figure 1D). Received: October 24, 2016 Accepted: January 30, 2017 Published: January 30, 2017 5968
DOI: 10.1021/acsami.6b13547 ACS Appl. Mater. Interfaces 2017, 9, 5968−5973
Research Article
ACS Applied Materials & Interfaces
oil models. The absorption capacity, Q, was calculated according to the equation:
Q=
m2 − m1 m1
where m1 and m2 are the weights of the SiO2/PS/PLA nonwoven fabric before and after being used. The ratio of water and oil absorbed by the SiO2/PS/PLA nonwoven fabric was also measured by squeezing it out. To decrease the calculation error, the oil absorption process was completed in a short time to avoid the solvents evaporation. 2.4. Oil/Water Separation. The organic solvents and water were dyed by oil red O and CuSO4, respectively. An oil/water mixture consisting of 20 mL organic solvent and 50 mL water were poured directly onto the separation setup and separated through the prepared SiO2/PS/PLA nonwoven fabric. The separated liquids were collected to determinate the separation efficiency. 2.5. Calculation of Fluxes of Superhydrophobic SiO2/PS/PLA Nonwoven Fabric. The separation efficiency of the SiO2/PS/PLA nonwoven fabric was calculated according to the equation:
flux =
V St
where V represents the volume of permeated organic solvent, S represents the surface area of nonwoven fabric membrane, and t is the operation time. 2.6. Characterizations. The surface wettability of fabric was tested by virtue of static water contact angles (WCA) using Dataphysics OCA 20. The microstructures of prepared PLA nonwoven fabrics were characterized by transmission electronic microscopy (TEM, JEOL JEM-2100F microscope) and scanning electronic microscopy (SEM, JEOL JMS-6700F microscope). The chemical composition of PLA nonwoven fabric was analyzed by X-ray photoelectron spectroscopy (XPS, Shimadzu Axis Ultradld spectroscope). Thermogravimetric analyzer (TGA) measurements were performed on a Netzsch TG 209F1 instrument in N2 atmosphere. The morphology of PLA fiber was observed by optical microscopy (BX 51TF Instec H601). Coefficient of friction (COF) measurements were performed with a friction and wear testing machine (Rtec). Tensile properties were performed on an Instron 5567 Universal Testing System (Instron).
Figure 1. Schematic illustration of the preparation of SiO2/PS/PLA nonwoven fabric and its application for oil/water separation.
More interestingly, it shows stable wettability even after rigorous friction and stretching. The above advantages allow this novel PLA-based hybrid fabric not only separates oily wastewater effectively but also can be conveniently disposed due to its biodegradability. This work provides a new pathway to fabricate oil/water separation materials taking thorough consideration on both high separation efficiency and formidable post-treatment of the used separation materials, which shows attractive potential applications in water purification.
2. EXPERIMENTAL SECTION 3. RESULTS AND DISCUSSION 3.1. Wetting Behaviors. The oil/water separation performances are closely dependent on the surface superhydrophobicity. The surface wettability of PLA nonwoven fabric after modification was investigated by virtue of water contact angle (WCA) measurement (Figure 2). The WCA of the original PLA nonwoven fabric is 117 ± 3.0° (Figure 2A), showing simultaneously oleophilic and hydrophilic surface (Figure 2D), which is not suitable to separate oil/water mixtures. Therefore, appropriated modification on PLA is necessary to endow it with hydrophobicity. Upon modified with PDA, PLA nonwoven fabric becomes more hydrophilic with a WCA of 23 ± 2.3° (Figure 2B) due to the introduction of abundant hydroxyl and amino groups on the surface of PLA fiber.31 With accordance to the Wenzel and Cassie−Baxter models, a pristine hydrophobic surface can become more hydrophobic or even superhydrophobic after being modified with multiscale roughness.32 In our system, different sizes of PS microspheres and SiO2 nanoparticles are introduced simultaneously on the surface of PLA to obtain hydrophobic PLA nonwoven fabric (Figure S1). When the mass proportion of SiO2 nanoparticles and PS microspheres approaches to 18:1, superhydrophobic PLA fabric with a WCA of 152.0 ± 2.1° can be achieved (Figure 2, panels C and E). Under this condition, a water droplet rolls away quickly once in contact with the slightly tilted
2.1. Materials. Pump oil, vegetable oil, and soybean oil were obtained from the local supermarket. Dopamine (98%) was received from Alfa Aesar China (Tianjin) Co., Ltd. Styrene (99%), 2,2′-azobis(2methylpropionitrile) (AIBN, 98%), tetraethoxysilane (TEOS, 98%) and (3-aminopropyl)triethoxysilane (APTES, 98%) were obtained from Aladdin China (Shanghai) Co., Ltd. The raw PLA nonwoven fabric (diameter 20−25 μm) was purchased from Zhejiang Hisun Biomaterials Co., Ltd. Other chemicals are purchased from Sinopharm Group Co., Ltd. 2.2. Preparation of Superhydrophobic SiO2/PS/PLA Nonwoven Fabric. Superhydrophobic SiO2/PS/PLA nonwoven fabric was synthesized by a hierarchical micro/nanoparticles assistant strategy. Briefly, purified PLA nonwoven fabric was rinsed thoroughly in ethanol by sonication for 3 h. After drying, the PLA nonwoven fabric was submerged in a glass tube containing 2 mg/mL dopamine (10 mM, Tris-buffer solution). Subsequently, a certain amount of the as-prepared SiO2 nanoparticles and PS microspheres were added and stirred at 25 °C for 5 h. Finally, the resultant SiO2/PS/PLA nonwoven fabric was rigorously rinsed with ethanol to remove undecorated SiO2 nanoparticles and PS microspheres. 2.3. Absorption of Organic Solvents. To distinctly distinguish different liquids, the organic solvents were colored by oil red O dye (16 wt %) and were added into water immediately. Phase-separated oil/water mixture was formed due to their different density. The as-prepared SiO2/PS/PLA nonwoven fabric were placed on the interface of the organic solvent phase for absorbing selectively organic solvents. During the experiments, hexane, toluene, tetrachloromethane, pump oil, vegetable oil, and soybean oil were selected as 5969
DOI: 10.1021/acsami.6b13547 ACS Appl. Mater. Interfaces 2017, 9, 5968−5973
Research Article
ACS Applied Materials & Interfaces
clearly show the macroscopic rough surfaces of PLA fibers (Figure S7). The particulate morphology endows hierarchical rough structures to the nonwoven fabric, which is essential to build robust superhydrophobic PLA materials. 3.3. Chemical Composition. The chemical composition of the SiO2/PS/PLA nonwoven fabric was further characterized by XPS. For the pristine PLA nonwoven fabric, only peaks of C (Figure S8 and S9A) and O (Figure S8 and S9B) are detected and no other impurities can be observed. In the XPS pattern of SiO2/PS/PLA nonwoven fabric, typical N 1s and Si 2p peaks appear at around 408 and 102 eV (Figure 3, panels E and F),33,34 mainly assigning to the PDA and SiO2, respectively. From the XPS elemental spectra of C, O, N, and Si (Figure S10), all are distributed on the SiO2/PS/PLA nonwoven fabric surface, which suggests that SiO2 nanoparticles and PS microspheres are coated on the PLA nonwoven fabric. 3.4. Absorption of Organic Solvents. Surface superhydrophobic SiO2/PS/PLA nonwoven fabric consists of rough porous structures, showing great promise as oil absorption and oil/water separation material. As shown in Figure 4A, n-hexane (dyed with oil red O) floating on water (dyed with CuSO4) can be absorbed quickly by the SiO2/PS/PLA nonwoven fabric within 2 s (Movie S2), even high-density of tetrachloromethane can also be rapidly absorbed within 3 s (Figure 4B). Significantly, the water is hardly absorbed during the oil-absorption process, showing the excellent selective separation capacity of the SiO2/PS/PLA nonwoven fabric. To further investigate the absorption capacity, the weight ratio (Q) was employed to evaluate quantitatively oil-absorption capacity. Various organic solvents and oils are selected as common model pollutants. It is indicated that the SiO2/PS/ PLA nonwoven fabric has a very high oil-absorption capacity (Figure 4C). Moreover, after the absorption process, the superhydrophobic nonwoven fabric can be recovered by successive alcohol/water rinsing and drying at 40 °C for 10 min. The separation process can be recycled more than 10 times (Figure 4D). 3.5. Separation of Oil and Water Mixtures. The separation performances of SiO2/PS/PLA nonwoven fabric were investigated using different organic solvents/water mixtures, as shown in Figure 5 and Movie S4. A certain volume of water and oil mixture is poured into the separation setup; the oil quickly penetrates through the modified PLA nonwoven fabric, while the water is inhibited on the fabric. Almost no water can be observed in the collected oil, showing efficient separation behavior. In addition, the separation process of SiO2/PS/PLA nonwoven fabric can be recycled stably. After each cycle, the surface WCA of changes within 3−6°, indicating SiO2/PS/PLA nonwoven fabric possess of excellent reusability. 3.6. Surface Mechanical Stability. The surface mechanical stability of SiO2/PS/PLA nonwoven fabric is significant for preserving stable superhydrophobicity, which directly determines practicality of PLA nonwoven fabric as oil/water separation materials. The mechanical properties of the SiO2/PS/PLA nonwoven fabric were evaluated by the measurement of friction resisting and tensile. As shown in Figure 6A, after being rubbed for 10 min under 5 N, the purified PLA nonwoven fabric is worn out, whereas the SiO2/PS/PLA nonwoven fabric still keeps its macro morphology, indicating these micro/nano particles can improve the frictional resistance (Movie S3). Moreover, compared with unmodified PLA nonwoven fabric, SiO2/PS/PLA nonwoven fabric shows larger coefficient of friction (COF) value and presents a rising tendency (Figure 6B). These results indicate the micro/nano particles have improved the surface
Figure 2. Static WCA measurements for (A) PLA nonwoven fabric, (B) PDA@PLA nonwoven fabric,and (C) SiO2/PS/PLA nonwoven fabric. (D) A PLA nonwoven fabric with water droplets on the surface. (E) An as-prepared superhydrophobic SiO2/PS/PLA nonwoven fabric with water droplets on the surface (dyed with KMnO4, CuSO4, and FeCl3, respectively). (F) Photograph of a water column squirted on the SiO2/PS/PLA nonwoven fabric.
superhydrophobic SiO2/PS/PLA nonwoven fabric (Figure 2F and Movie S1). 3.2. Surface Morphology. Furthermore, the microstructures of prepared PLA nonwoven fabricwas characterized by scanning electronic microscopy (SEM). As shown in Figure 3,
Figure 3. (A and B) SEM images of PLA nonwoven fabric and (C and D) SiO2/PS/PLA nonwoven fabric. XPS spectrum of (E) SiO2/PS/PLA nonwoven fabric and (F) Si 2p.
the PLA nonwoven fabric displays three-dimensional porous network structures consisting of smooth microfibers (20−25 μm) (Figure 3, panels A, B, and Figure S3). Upon being attached by PS microspheres (∼2 μm, Figure S4) and SiO2 nanoparticles (∼200 nm, Figure S5), these granules distribute randomly on the surface of PLA fibers (Figure 3, panels C and D). Highresolution SEM images indicate that these micro/nanoparticle overlapped densely and form poly dispersive aggregates that even deeply imbed inside the nonwoven fabric uniformly attributed to the high aperture size of the PLA nonwoven fabric (Figure S6). In addition, the optical microscopy images also 5970
DOI: 10.1021/acsami.6b13547 ACS Appl. Mater. Interfaces 2017, 9, 5968−5973
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Figure 4. Photographs of the removal of (A) n-hexane and (B) tetrachloromethane by SiO2/PS/PLA nonwoven fabric. (C) Absorption capacity of the SiO2/PS/PLA nonwoven fabric for various organic oils (1, hexane; 2, toluene; 3, silicone oil; 4, pump oil; 5, vegetable oil; and 6, soybean oil). (D) The recycled absorption of the SiO2/PS/PLA nonwoven fabric for n-hexane.
Figure 5. (A) Photograph of hexane/water separation using SiO2/PS/PLA nonwoven fabric. (B and C) are the photographs of water (blue) and hexane (red), respectively, after separation. (D) Permeate flux for various mixtures of SiO2/PS/PLA nonwoven fabric (1, hexane; 2, toluene; 3, silicone oil; 4, pump oil; and 5, vegetable oil). (E) The recycled separation fluxes of hexane/water mixture passing through SiO2/PS/PLA nonwoven fabric. Inset: WCA of SiO2/PS/PLA nonwoven fabric before and after separation.
roughness of the PLA nonwoven fabric and realized the stability of wettability.35,36 In addition, the tensile measurement was also conducted for studying the effect of stretching deformation on the surface superhydrophobicity of SiO2/PS/PLA nonwoven fabric. It is found that the tensile property of SiO2/PS/PLA nonwoven fabric is higher than that of PLA nonwoven fabric (Figure 6, panels C and D). When the tensile force reaches 1.0 MPa, the SiO2/PS/PLA nonwoven fabric suffers from severe fracture
while the broken fabric still sustains its superhydrophobicity (Figure S12). The stretching is considered to not destroy the surface rough structures and composites of the nonwoven fabric coated with PDA and micro/nano particles, which leads to the robust surface wettability and stable separation peformances.22
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CONCLUSION In summary, we have reported a biodegradable PLA nonwoven fabric with rough surfaces and porous structures that 5971
DOI: 10.1021/acsami.6b13547 ACS Appl. Mater. Interfaces 2017, 9, 5968−5973
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ACS Applied Materials & Interfaces
Tao Chen: 0000-0001-9704-9545 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (51603216), Ningbo Natural Science Foundation (2015A610022), Open Research Fund of Key Laboratory of Marine Materials and Related Technologies (2016Z01 and 2017K03) and Zhejiang Nonprofit Technology Applied Research Program (2015C33031), Key Research Program of Frontier Science, Chinese Academy of Sciences (QYZDB-SSW-SLH036), and Ningbo Science and Technology Bureau (2016C50009). We also thank Prof. Xuefen Wang in Donghua University for her support for providing PLA matrix and fabricating PLA nonwoven fabric, and the open foundation from State Key Laboratory for Modification of Chemical Fibers and Polymer Materials Donghua University (LK1615).
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Figure 6. (A) Photograph of (a) PLA nonwoven fabric and (b) SiO2/ PS/PLA nonwoven fabric after friction for 10 min. (B) The COF of (a) PLA nonwoven fabric and (b) SiO2/PS/PLA nonwoven fabric after friction for 10 min. (C) Stretching test images of SiO2/PS/PLA nonwoven fabric. (D) Mechanical properties of (a) PLA nonwoven fabric and (b) SiO2/PS/PLA nonwoven fabric. Inset: photographs of water droplets on the broken fabric.
could be used for efficient oil absorption and oil/water separation. A hierarchical micro/nanoparticles assistant strategy is developed for the fabrication of biodegradable superhydrophobic PLA nonwoven fabric for a gravity driven oil−water separation. Interestingly, the as-prepared SiO2/PS/PLA nonwoven fabric maintains its own superhydrophobicity after rigorous friction and stretching tests. Furthermore, it also exhibits an efficient separation for oil and water mixtures, which will make it a good candidate for oil-polluted water treatments.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13547. Synthetic procedures of SiO2 nanoparticles, PS microspheres, and SiO2/PS/PLA nonwoven fabric and its chemical structure and characterization (PDF) Movie S1 shows water droplets quickly rolls away on the SiO2/PS/PLA nonwoven fabric (AVI) Movie S2 shows the absorption of n-hexane from water by SiO2/PS/PLA nonwoven fabric (AVI) Movie S3 shows the surface mechanical and superhydrophobic stability of the SiO2/PS/PLA nonwoven fabric (AVI) Movie S4 shows the hexane/water separation behavior of the SiO2/PS/PLA nonwoven fabric (AVI)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Jiawei Zhang: 0000-0002-3182-9239 5972
DOI: 10.1021/acsami.6b13547 ACS Appl. Mater. Interfaces 2017, 9, 5968−5973
Research Article
ACS Applied Materials & Interfaces
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