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Abstract

Biodegradable and efficient filtration membrane is essential to practical oil–water separation. Herein, we fabricated highly porous membrane induced by the humidity. The resulting poly(lactic acid) fiber membrane exhibits superhydrophobicity due to more trapped air by the increased roughness of fiber at the relative humidity of 80%. Meanwhile, the increased humidity favors to the membrane porosity improvement from 81 ± 3.3% to 92 ± 4.1% with the humidity change from 40% to 80%. The superhydrophobicity company with the increased membrane porosity makes the functional poly(lactic acid) membrane with enhanced flux for practical oil–water separation.

Keywords

Humidity porous poly(lactic acid)superhydrophobicity oil–water separation

Introduction

Membrane separation technology has become a promising method for treatment of oily wastewater due to facile process, low cost, and high efficiency of oil removal (Li et al., 2016a; Tenjimbayashi et al., 2016).

Special wettability surface, superhydrophobic surfaces and superhydrophilic surfaces, characterized by water contact angle (WCA) larger than 150° or WCA lower than 5° in air, respectively, have been proved to be efficiency in oil–water separation (Liao et al., 2014; Nuraje et al., 2013). The superhydrophobic surface repels the water permeating while allows the oil or other organic matters passing (Li et al., 2016b). Differently and tactfully, superhydrophilic in air and superoleophobic in water surfaces have been achieved in recent years to separate oil–water mixtures (Yuan et al., 2015; Zhang et al., 2014). Compared with underwater superoleophobic surface, constructing the superhydrophobic surface in air is featured with the advantages such as easy fabricating process, separating oil–water mixtures directly (Gupta et al., 2017). Therefore, constructing superhydrophobic surface and applying the functional materials to oil–water separation have drew much attention all the time.

Inspired by the natural creatures, two methods are developed to fabricate superhydrophobic surface: (1) constructing special topographical morphology on hydrophobic surface and (2) modifying hydrophobic materials on special topographical surface (Lin et al., 2011; Wang et al., 2011). According to this principle, various techniques have been applied so far to construct superhydrophobic surfaces, such as template synthesis (Celia et al., 2013), electrochemical deposition (He and Wang, 2011), coating method (Li et al., 2016c; Xue et al., 2012), and electrospinning method (Liu et al., 2016). Among them, electrospinning is believed a facile technology to achieve specific geometrical structure surface membrane. For example, Zheng et al. (2006) reported a microsphere/nanofiber composite film of polystyrene with superhydrophobic features using electrospinning method; Mimicking the rough surface of silver ragwort leaf, Miyauchi et al. (2006) fabricated micro/nano-porous polystyrene with nano-sized grooves along the fiber axis. Furthermore, many inorganic nanofibers with super wettability properties were also obtained by electrospinning, such as SiO₂ nanofiber (Lim et al., 2007), Carbon–silica nanofiber (Tai et al., 2014).

Poly(lactic acid) (PLA) is a biodegradable and renewable polymer with great potential to replace conventional petrochemical-based polymers (Martin and Averous, 2001). Electrospun PLA nanofiber membranes have been intensively researched. Particularly, due to the biodegradability, PLA-based materials are used in biomedicine conventionally (Lasprilla et al., 2012; Santoro et al., 2016; Tyler et al., 2016). However, little attention is drawing to PLA-based materials applied in oil–water separation. Xue et al. (2013) fabricated oil filtration films through non-solvent-induced phase separation and template synthesis method. The resulting PLA films showed superhydrophobicity and high oil separation efficiency. Zhang et al. (2015) prepared PLA foams using thermally induced phase separation method. The foams exhibited excellent oil absorb property. However, these methods suffered the drawbacks of complex fabrication process or lots of solvent consumption.

In the present study, we fabricated highly porous membrane induced by the humidity by electrospinning method. We aimed to (1) the effect of humidity on PLA morphology and membrane wettability and (2) the effect of resulting PLA membrane (different PLA fiber morphology) on the separation performance during oil–water separation.

Experimental

Materials

PLA (Wm = 1.0 × 10⁵) was purchased from Zhejiang Hai Zheng Biological Materials Co., Ltd., Zhejiang, China. Chloroform and N,N-dimethylformamide were supplied by Sinopharm Chemical Reagent Co., Ltd. (Suzhou, China). All reagents were analytical grade and were used as received without further treatment.

Preparation of PLA membrane

PLA was dissolved in binary solvent of chloroform/N,N-dimethylformamide with weight ratio of 9:1. Then, they were stirred for about 2 hours at room temperature, to achieve transparent solutions with PLA concentration of 8 wt.%. In electrospinning experiment, a high electric potential of 20 kV was applied to the droplet of PLA solution at the tip of a syringe needle (0.8 mm in internal diameter). The electrospun PLA nanofibers were collected on flat aluminum foil, which was placed at a distance of 15 cm from the syringe tip. A constant volume flow rate of 1.0 mL/h was maintained using a syringe pump. Two hours were controlled to obtain electrospun PLA fiber membrane. The temperature used in the spinning process were 25 ± 2°C and kept constant. The ambient relatively humidity (RH) was 40%, 60%, and 80%. Then, the resulting PLA membrane under three different RH is called H-40, H-60 and H-80, respectively.

Characterization

The morphology of electrospun PLA fiber membrane was observed using a scanning electron microscope (Hitachi S-4800, Tokyo, Japan) at 20°C, 60 RH. Samples were sputter-coated with gold layer prior to imaging. The diameters of PLA fibers were calculated by measuring at least 100 fibers at random using Image J program.

The wetting properties of water/oil droplets on the resultant PLA surface were evaluated using an optical contact angle meter system (Krüss DSA100, Germany). The volumes of droplets used for the WCA were 6 µL. The resultant values were the average of five droplets at different locations.

An Olympus TH4-200 microscope was used to capture the optical microscopy images of PLA fibers at RH of 80%. The Brunauer–Emmett–Teller (BET) surface area was measured at 77K using a nitrogen adsorption–desorption MicromeriticsASAP-2020 analyzer (Micromeritics Co., USA).

The porosity of all PLA porous membrane was calculated by the following equation (Wang et al., 2015)

ε = (1 - \frac{m}{Z \times A \times ρ}) \times 100 %

where ε is the porosity of the relevant fibrous membrane; m, Z, and A are the mass, thickness, and area of per unit measured membrane, respectively. ρ is the density of polymer raw material, and the density of PLA is 1.27 g/cm³.

The as-prepared PLA membranes were positioned in an all glass vacuum filter holder (vacuum resistant). The oil–water mixtures were prepared by mixing n-hexane, olive oil, and lubricant oil with water in a volume ratio of 1:1, respectively. In the separation process, 100 mL oil–water mixtures permeated through the PLA membrane (solely by the gravity). The flux was calculated by the following equation

J = \frac{V}{t \times s}

where J is the flux, t is the time, and s is the area of the PLA membrane.

The separation efficiency was determined using the formula below

Separation efficiency = (1 - \frac{C_{p e r m e a t e}}{C_{f e e d}}) \times 100 %

where

C_{p e r m e a t e}

and

C_{f e e d}

are the oil concentrations of permeate collected and the original feed, respectively. The

C_{p e r m e a t e}

was characterized by a Shimadzu TOC-Vcsh TOC analyzer.

The morphology and wettability stability are checked by two methods. The first is immersing the porous H-60 PLA membrane (2 × 2 cm²) into water in a glass container and then stirring the water for 30 minutes at the rotor rate of 400 r/min. The second is leaving the H-80 PLA membrane in an ultrasonicator for 40 minutes at the sonication power 50 W. Then character the morphology and wettability change after these two processes.

Results and discussion

Morphology of PLA membrane under different relative humidity

In electrospinning, humidity has great effect on the morphology of electrospun fibers (De Vrieze et al., 2009; Fashandi and Karimi, 2012). Therefore, we fabricated electrospun PLA fibers with different humidity (40%, 60%, and 80%, respectively), but same other parameters such as PLA concentration, voltage supply, and temperature (Table 1). Interestingly, with the increase in humidity, it shows two significant changes: (1) the fiber diameter increases from 0.73 ± 0.08 µm to 1.78 ± 0.16 µm and (2) the fiber exhibits increased surface roughness, that is, the higher humidity is the bigger of pore size on fiber surface, as shown in Figure 1.

Figure 1.

The morphology of porous PLA fiber at different RH. (a) H-40 at magnification of ×5000, ×20,000, and ×40,000; (b) H-60 at magnification of ×5000, ×20,000, and ×40,000; and (c) H-80 at magnification of ×5000, ×20,000, and ×40,000.

Table 1.

The spinning parameters: Voltage, temperature, and relative humidity.

Sample	Voltage	Temperature (°C)	Relative humidity (%)
H-40	12	25 ± 2	40
H-60	12	25 ± 2	60
H-80	12	25 ± 2	80

The humidity effects on the solvent evaporation. With the increase in humidity, the more water vapor hinders the solvent evaporation from charged jets, resulting in increased diameter of PLA fiber, which is accordance with the previous researches (Pelipenko et al., 2013; Thompson et al., 2007). In the spinning process, on one hand, solvent evaporates from charged jets; on the other hand, water vapor occupies the position of the evaporated solvent. As a result, pores formed on fiber surface after the polymer phase solidification. Therefore, bigger pores formed at higher humidity due to more water vapor replacing the position of the evaporated solvent. What’s more, the H-80 fiber showed extremely rough surface even with notches on fiber surface due to the big diameter of pores, as shown in Figure 1(c) (marked by the arrows).

Optical microscope was applied to further observe the morphology of porous PLA fiber. As can be seen in Figure 2(b), the parts of light-blue spread on the dark surface, indicating pores on the fiber surface. The rough surface of pores and notches (Figure 2(c) and (d)) on fiber surface attribute to the increase in fiber specific surface area. The BET of H-40, H-60, and H-80 is 31.25%, 35.34%, and 42.51%, respectively (Table 2), indicating that the increase in surface roughness with the increase in humidity. Meanwhile, both the bigger pores and higher diameter favor to the increase in PLA membrane porosity. The increase in membrane porosity was proved by the membrane porosity characterization. The porosity of H-40 is 81 ± 3.3%. The membrane porosity of H-60 increased to 84 ± 2.5%. For H-80, the membrane porosity dramatically improved to 92 ± 4.1%. The increased membrane porosity is helpful to the flux improvement in filtration (Gai et al., 2014).

Figure 2.

Morphology of H-80. (a and b) Optical image of H-80 at different magnification; (c) scanning electron microscope of H-80; and (d) diagram image of H-80.

Table 2.

The fiber diameter, membrane BET, and porosity of H-40, H-60, and H-80.

Sample	Fiber diameter (µm)	BET (%)	Porosity (%)
H-40	0.73 ± 0.08	31.25	81 ± 3.3
H-60	1.21 ± 0.11	35.34	84 ± 2.5
H-80	1.78 ± 0.16	42.51	92 ± 4.1

BET: Brunauer–Emmett–Teller.

Rough surface always exhibits super anti-wettability. Figure 3 shows the WCA of electrospun PLA membrane with different humidity. As shown in Figure 3, the WCA increased from 126.5° to 150.3° with the increase in humidity. The diameter and surface morphology of PLA fibers play a crucial role in the wet ability of resulting PLA membrane. Generally, bigger diameter of fiber shows higher WCA due to more air trapped between membrane and water (Zhou and Wu, 2015). Therefore, the H-40 PLA membrane presents relatively low WCA (126.5°), while the H-80 PLA membrane shows higher WCA of 150.3° (Figure 3). Cassie and Baxter law argue that hydrophobicity is related to the contact area between solid surface and water, that is, the less contact area leading to larger contact angle (McHale et al., 2004). The extremely rough surface of PLA with pores and notches on fiber surface reduces the contact area between membrane surface and water (Figure 4(a)), endowing the H-80 PLA membrane superhydrophobicity with WCA higher than 150° (Figure 4(c)). It is noteworthy that the WCA of nonporous PLA membrane is 120.8°, further conforming that porous surface favors to the increase in membrane hydrophobicity (Figure 3). The Figure 4(b) shows that water droplets stand on PLA membrane, suggesting good hydrophobicity. Moreover, the PLA membrane exhibits superoleophilicity with oil contact angle of nearly 0° (Figure 4(d)). Therefore, superhydrophobicity and superoleophilicity PLA membrane can be used in oil–water separation.

Figure 3.

The water CA in air of nonporous, H-40, H60, and H-80 PLA membrane. Water CA: water contact angle.

Figure 4.

The wet ability of PLA membrane at RH of 80%. (a) The morphology of H-80; (b) the optical image of water droplets on PLA membrane (the volume of water is 10 µL); (c) the WCA of H-80; and (d) the OCA of H-80. OCA: oil contact angle; WCA: water contact angle.

The different properties such as fiber morphology and porosity of H-40, H-60, and H-80 may lead to different results in oil–water separation. Several types of oil–water mixtures were prepared using n-hexane, olive oil, and lubricant oil to investigate the separation ability of PLA membranes. The separation process of the oil–water mixtures is demonstrated in Figure 5. The insert is the PLA membrane with diameter of 5 cm. In the separation process, oil–water mixtures were discharged into the setup. Subsequently, oil phase permeates into the bottle due to the membrane superoleophilicity, concurrently repelling the water due to the hydrophobicity of the membrane. 100 mL oil–water mixtures (the weight ratio of oil and water is 1:1) permeated through the PLA membrane. Then, the flux and separation efficiency are shown in Figure 5. Overall, the flux of n-hexane is higher than the other two oils for three PLA membranes, which is contributed by the viscosity of n-hexane lower than olive oil and lubricant oil. The flux of n-hexane of three PLA membranes is 8231 ± 83 L/m²h, 9870 ± 75 L/m²h and 12,326 ± 103 L/m²h, respectively. For olive oil, the flux is 1311 ± 62 L/m²h, 1873 ± 58 L/m²h, and 2785 ± 73 L/m²h, respectively. And for lubricant oil, the flux of three PLA membranes is 987 ± 63 L/m²h, 1125 ± 74 L/m²h, and 1636 ± 87 L/m²h, respectively. Obviously, for three types of oil, the oil flux of three membranes increased with the increase in porosity induced by humidity change. Two factors favor to this result (1) with the increase in humidity, the resulting PLA fiber diameter increased, leading the bigger of membrane pore size; (2) with the increase in humidity, the extremely rough surface with many notches on PLA fiber makes the increase in membrane porosity. As a result, the increase in membrane pore size and porosity contributes to the increase in oil permeation flux. Moreover, the separation efficiency of the H-60 and H-80 is all higher than 99.98% for three oil types, indicating good separation efficiency.

Figure 5.

(a) The setup of oil–-water separation, the inset image is the PLA membrane; (b) the flux of n-hexane, olive oil, and lubricant oil based on H-40, H-60, and H-80, respectively; and (c) the separation efficiency of the H-60 and H-80 porous PLA membrane.

Additionally, the flux compared with non-porous PLA fiber with similar diameter conforms the pores contributing to the increase in flux. As shown in Figure 6(a) and (b), the non-porous PLA fiber presented similar fiber diameter to the H-60 fiber. However, the flux of three oil types is 8637 ± 93 L/m²h, 1326 ± 63 L/m²h, 987 ± 49 L/m²h, respectively (Figure 6(c)), suggesting that the porous PLA fiber favors to higher flux in separation process. This result was further demonstrated by the flux between non-porous PLA fiber and H-80. As illustrated in Figure 6(d) to (f), the flux of non-porous PLA fiber is 9789 ± 87 L/m²h, 1984 ± 49 L/m²h, 1335 ± 37 L/m²h, which are lower than the flux of porous PLA fiber 12,326 ± 103 L/m²h, 2785 ± 73 L/m²h, 1636 ± 87 L/m²h, respectively, though they have similar fiber diameter. These two results synergistically show that the porous PLA fiber enhanced the flux in oil–water separation.

Figure 6.

(a and b) The non-porous PLA fiber with similar diameter to H-60; (c) the relevant flux of three type oils; (d and e) the non-porous PLA fiber with similar diameter to H-80; and (f) the relevant flux of three type oils.

To further check the morphology and wettability stability of the porous fiber membrane, firstly, the H-60 PLA membrane was immersed in water and was stirred with the rotation rate of 400 r/min (Figure 7(a)). The resulting fiber morphology is shown in Figure 7(b) and (c). It is can be seen that the fiber hold porous surface morphology after stirring in the present stirring rate. What’s more, the WCA is 150.2° (Figure 7(d)), exhibiting the PLA membrane good wettability stability in harsh conditions. Secondly, the porous H-80 PLA membrane was immersed in an ultrasonicator (Figure 7(e)). After 40 minutes, no obvious change was observed of the fiber morphology (Figure 7(f) and (g)). Meanwhile, the porous fiber membrane preserves the superhydrophobic property with the WCA higher than 150° (Figure 7(h)), suggesting good morphology and surface wettability stability.

Figure 7.

The morphology and wettability stability of H-60 and H-80 porous fiber membrane. (a) The optical image of H-60 membrane before and after rotation; (b and c) the morphology of H-60 membrane after rotation; (d) the WCA of H-60 membrane after rotation; (e) The optical image of H-80 immersed in the ultrasonicator; (f and g) the morphology of H-80 membrane after ultrasonic treatment; (h) the WCA of H-80 membrane after ultrasonic treatment.

Conclusions

In summary, we have successfully fabricated PLA membrane with pores and notches on fiber at high humidity. The extremely rough surface endows the H-80 PLA membrane with superhydrophilic and superoleophobic characteristics. Subsequently, the H-80 PLA membrane achieves enhanced oil permeation flux for n-hexane, olive oil, and lubricant oil. The flux improvement is contributed by the increase in membrane pore size and porosity at high humidity. Therefore, this facile method provides an alternative approach to constructing special material surface and enhancing flux in oil–water separation.

Footnotes

Acknowledgements

The authors would like to thank the support by Talent start-up foundation of Anhui Polytechnic University (2017YQQ012), Open Project Program of Anhui Province College Key Laboratory of Textile Fabrics (2018AKLTF07) and Natural Science Research Project of Universities of Anhui under Grant No. KJ2017A103.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Humidity-induced porous poly(lactic acid) membrane with enhanced flux for oil–water separation

Abstract

Keywords

Introduction

Experimental

Materials

Preparation of PLA membrane

Characterization

Results and discussion

Morphology of PLA membrane under different relative humidity

Conclusions

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

References