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Biomimetic Modification of Super-wetting Electrospun Poly(vinylidene fluoride) Porous Fibers with Organic Dyes and Heavy Metal Ions Adsorption, Oil/Water Separation, and Sterilization Performances Toward Wastewater Treatment
RESEARCH ARTICLE | Updated：2022-06-22
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    • Biomimetic Modification of Super-wetting Electrospun Poly(vinylidene fluoride) Porous Fibers with Organic Dyes and Heavy Metal Ions Adsorption, Oil/Water Separation, and Sterilization Performances Toward Wastewater Treatment

    • Sun De-Xiang

      ,  

      Liao Xiao-Lei

      ,  

      Zhang Nan

      ,  

      Huang Ting

      ,  

      Lei Yan-Zhou

      ,  

      Xu Xiao-Ling

      ,  

      Wang Yong

      ,  
    • Chinese Journal of Polymer Science   Vol. 40, Issue 7, Pages: 738-753(2022)

    • DOI：10.1007/s10118-022-2714-4    

      CLC：

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  • Cite this article

  • De-Xiang Sun, Xiao-Lei Liao, Nan Zhang, et al. Biomimetic Modification of Super-wetting Electrospun Poly(vinylidene fluoride) Porous Fibers with Organic Dyes and Heavy Metal Ions Adsorption, Oil/Water Separation, and Sterilization Performances Toward Wastewater Treatment. [J]. Chinese Journal of Polymer Science 40(7):738-753(2022) DOI： 10.1007/s10118-022-2714-4.

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    Abstract

    Polymeric membranes with the integration of various functional performances toward wastewater treatment are urgently required. However, most of the polymeric membranes only exhibit a single function of highly efficiently removing one kind of pollutants. In this work, a biomimetic modification method was introduced to tailor the chemical and topological structure of the porous poly(vinylidene fluoride) (PVDF) fibers prepared by electrospinning. The polydopamine (PDA) nanoparticles were homogeneously introduced onto the surface of PVDF porous fibers via precisely tailoring the concentration of dopamine, which endowed the fibers with more polar groups and bigger roughness but did not destroy the crystalline structures. The fibrous membranes exhibited switchable superhydrophilicity and superlipophilicity characteristics, excellent adsorption abilities toward organic dyes, heavy metal ions and oils. The highest adsorption capacities achieved 917.4 mg/g toward methylene blue (MB), 42.6 mg/g toward Cr(VI) and 74.6 g/g toward silicone oil, respectively. Specifically, the membrane could rapidly remove the trace MB when water flowed through the membrane. The membrane also exhibited excellent sterilization performances, and the bacterial eliminating rate achieved 99.9% for the E. coli and S. aureus. The excellent light-to-heat conversion ability endowed the membrane with the self-heating ability, furtherly intensifying the wastewater treatment efficiency. This work confirms that the PDA nanoparticles-decorated PVDF porous fibers might be the new generation adsorbents used in wastewater treatment.

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    Keywords

    Poly(vinylidene fluoride) porous fibers; Biomimetic modification; Adsorption; Oil/water treatment; Antimicrobial performance

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    INTRODUCTION

    With the rapid expansion of economy, environmental pollution has become a serious problem that must be resolved, because it greatly affects the sustainability of modern society. Water has been thought as the source of life. However, water pollution has become one of the most serious environmental problems. Many toxic and harmful species, such as organic dyes, heavy metal ions, crude oils and organic solvents, have been disorderly discharged into environment. Some of them may permanently exist in the environment while others may enter the ecological chain and finally affect human health. Undoubtedly, removing toxic and harmful species from industrial wastewater or sanitary sewage before they are being discharged into environment is the most efficient method to reduce the risk of water pollution, and in this condition, developing the materials that can remove toxic and harmful species becomes very important.[

    1]
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    So far, a variety of materials have been exploited and used in wastewater treatment, including various inorganic materials and organic materials, and most of them have the general characteristics,[

    2−4] such as high active surface with many functional groups that can interact with the harmful species in the wastewater, the large specific surface that can provide more adsorption sites, highly porous structure that may endow the materials with high flux or high adsorption capacity, etc. Furthermore, for the oil/water separation process, the lipophilicity or hydrophilicity of the material surface is also very important since it determines the wettability of the materials.[5,6]
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    Membrane separation technologies have been thought one of the highly efficient ways to eliminate the pollutants from water because the microstructure of the membranes could be well manipulated through different methods.[

    7] Generally, it is believed that the polymeric membranes exhibit more promising advantages[8] compared with the common inorganic membranes, such as easy to be fabricated, relatively good chemical resistance and high flexibility. Among those polymeric membranes that have already been commercially manufactured on a large-scale, poly(vinylidene fluoride) (PVDF) membranes attract much more attention of researchers due to their excellent chemical stability, thermal stability, and mechanical properties.[9−11] PVDF porous membranes can be fabricated through different methods, such as the thermal-induced phase separation strategy that usually constructs the finger-like or cell-like porous structure in the membranes,[12,13] the biaxial drawing method that is relating to the lamellar separating under the stress condition,[14] and electrospinning technology that fabricates the non-woven fabrics with irregular or parallel stacking of electrospun fibers.[15] Through tailoring the electrospinning parameters, such as the type of solvent, the compositions of precursor solution, the type of syringe needle, environmental humidity and temperature and electrospinning voltage, the microstructure of the electrospun fibers can be well tailored.[16,17] And therefore, fabricating the PVDF-based membrane via electrospinning technology attracts more and more attention of researchers recently. Specifically, the irregular stacking of fibers constructs large pores in the membrane, which ensures the extremely high flux of the membrane during the oil/water separation process.
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    As a typical non-polar polymer, PVDF-based membranes usually present the hydrophobicity and therefore, they are very appropriate to be used in the treatment of oily waste-water, including removing residual oils from wastewater or rejecting water during the oil/water separation. Recently, many researches have been done to enhance the membrane surface hydrophilicity toward further enlarging the applications of PVDF-based membranes, such as blend-electrospinning with other polar polymers,[

    9,18] incorporating the hydrophilic fillers,[19,20] surface modification of the membrane surface, etc.[21] Other researches have shown that through the combination of the surface topological structure designing and surface chemical modification, the PVDF membranes with switchable superhydrophilicity and superlipophilicity can be acquired.[10,22] It is believed that the switchable superhydrophilic and superlipophilic features can greatly enlarge the application fields of the PVDF-based membranes.
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    However, it is still stressed that during the actual application process, possibly there are different kinds of pollutants in the wastewater. Although the presently developed PVDF-based membranes that prepared through electrospinning technology already exhibit high adsorption ability and/or high removal efficiency toward a certain pollutant, it is still challenging to simultaneously remove different kinds of pollutants. In other words, it is very difficult to achieve the multiple functions for the electrospun PVDF fibrous membranes, which is different from the common PVDF-based membranes prepared through the classical thermal-induced phase separation strategy that can acquire the multifunctional performances easily.[

    23] Obviously, the designing and tailoring of the topological structure and chemical features of the membrane surface are the key points to achieve the multifunctional PVDF fibrous membranes.
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    In this work, a biomimetic modification method based on polydopamine (PDA) nanoparticles deposition is developed to modify the electrospun PVDF fibers. PDA has already been widely used in fabricating the functional PVDF fibers, however, most of the researches are focused on using PDA as the intermediate layer to assist the secondary modification of PVDF fibers by further depositing other polymers[

    24−26] or nanoparticles[27,28] on the surface of the PVDF fibers or using PDA as the interfacial enhancer to improve the interfacial interaction between nanoparticles and PVDF matrix,[29−32] but less researches have been carried out to declare the effect of PDA nanoparticles on the microstructure and performance of the electrospun PVDF porous fibers.[21,33] In this work, different from the common biomimetic modification that coating a thin layer of PDA on the substrate,[21,34−36] here, the porous PVDF fibers with superhydrophilicity and superlipophilicity simultaneously were firstly fabricated according to the blend-electrospinning of PVDF and poly(vinyl pyrrolidone) (PVP) because of the relatively good miscibility between them,[37] and then the oxidation polymerization of PDA was precisely controlled in the Tris-HCl buffer solution to decorate the PDA nanoparticles on the porous PVDF fibers, further tailoring the topological structure of the fiber surface. It is interesting to observe that the super-wettability of the membrane surface is well reserved, while the high adsorption abilities toward various contaminants, including organic dyes, heavy metal ions and oils, are simultaneously achieved. Specifically, with the presence of the PDA nanoparticles on the electrospun fibers, the membrane also exhibits high sterilization performances and the bacterial eliminating rate achieves 99.9% toward E. coli and S. aureus, respectively. In addition, the excellent photothermal conversion ability also endows the fibrous membranes with good self-heating ability, which is believed to be very beneficial to expand the application of the membranes. Undoubtedly, this work provides a new strategy to fabricate the multifunctional PVDF membranes toward wastewater treatment.
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    EXPERIMENTAL

    Materials

    PVDF with tradename of Kynar 720 was purchased from Arkema (France), and it has a density of 1.78 g/cm3 and a molecular weight ( ¯Mw

    ) of 1.05×105 g/mol. Poly(vinyl pyrrolidone) with the ¯Mw
         of 5.8×104 g/mol, dopamine (DA) with a purity of 98%, and Tris-HCl with the purity of 99.5% were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (China). The solvents, organic dyes, and heavy metallic salt were purchased from Kelong Chemical Reagent Co. (China). All the oils were purchased from supermarket. E. coli (ATCC 25922) and S. aureus (ATCC 6538) were provided by Guangdong Institute of Microbiology, China.
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    Membrane Sample Preparation

    The preparation of the membrane sample includes two steps as shown in Fig. 1. The first step is relating to the preparation of the electrospun porous PVDF fibers. First, PVP and PVDF were homogeneously dissolved in the mixed solvents of N,N-dimethylformamide (DMF)/tetrahydrofuran (THF) (1:1, V:V) at 55 °C for 8 h. Subsequently, the electrospun PVDF/PVP fibers were obtained using the mixed solution through the electrospinning technology. During the electrospinning process, the feeding rate was set at 30 μL/min and the voltage was 13 kV. The electrospun fibers were collected using an aluminum foil with the distance between syringe needle and substrate of 15 cm. According to our previous work,[

    37] in the PVDF/PVP solution, the content of PVDF was 18 wt%, while the mass ratio between PVDF and PVP was 1:2. Subsequently, the membrane was soaked in distilled water for 4 days at room temperature for removing PVP. After that, the electrospun PVDF porous fibers were obtained. Here, although some of PVP were still present in the porous fibers and could not be completely removed due to the good mutual solubility between PVDF and PVP, which has already been intensively investigated in our previous work,[37] the composite fibers are still named as PVDF fibers in this work.
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    fig

    Fig 1  Schematic representations showing the fabrication procedures of the biomimetically modified electrospun PVDF porous fibrous membranes.

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    The second step is relevant to the decoration of PDA nanoparticles on the PVDF porous fibers via the oxidation polymerization of DA in the Tris-HCl solution. First, the prepared PVDF porous fibrous membrane was immersed into the Tris buffer solution. Subsequently, a certain amount of DA was added into the system, and the baker was placed in a thermostatic shaker (Jiang Su, China), which was set at 25 °C and a rotate speed of 200 r/min, and the oxidation polymerization time of DA was kept at 24 h. After that, the membrane was washed repeatedly with deionized water to completely remove unreacted DA monomer. The membrane was placed in a freeze-drying machine FD-1A-50 (Boyikang, China) and dried at −60 °C and a pressure of 20 Pa. Here, the concentrations of DA in the Tris-HCl buffer solution were varied at 0.2, 0.5, 1.0 and 1.5 mg/L. For convenience, the prepared membrane samples were abbreviated as PVDF/PDA-x, where x presented the concentration (mg/L) of DA.

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    Morphology, Microstructure and Physicochemical Performance Characterizations

    A scanning electron microscope (SEM, JSM 7800F, JEOL, Japan) was used to characterize the morphologies of the fibers. Prior to observation, all surfaces of membrane samples were sputtered with a gold layer with the thickness of around 10 nm. The surface chemical feature as well as the presence of functional groups of the fibers was detected using a Fourier transform infrared spectroscope (FTIR, Nicolet iS20, Thermo Fisher Scientific, USA). The measurement was carried out with an attenuated total reflection (ATR) mode with the wavenumber range of 4000−400 cm−1 and the resolution was set at 4 cm−1. The crystalline structure of the fibers was characterized using an X-ray diffractometer (XRD, Empyrean, PANalytical B.V., the Netherlands). The diffractometer was equipped with Cu Kα radiation source and the tube current and generator voltage were set as 40 mA and 45 kV, respectively. A thermogravimetric analysis (TGA) TG 209 F1 (NETZSCH, Germany) was used to evaluate the thermal stability of the membrane samples. In the nitrogen atmosphere, about 8 mg membrane sample was heated from 30 °C to 800 °C at 10 °C/min.

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    The lipophilicity and hydrophilicity of the membrane surfaces were roughly tested by a contact angle meter (DSA25E, KRÜSS, Germany). Diiodomethane (CH2I2) and distilled water (H2O) were used as the candidate liquids. During the measurements, the diameters of H2O and CH2I2 were about 1.18 and 0.93 mm, respectively. The membrane porosity was measured using the common solvent impregnation method and the formula for calculating the porosity (AK) can be seen in the electronic supplementary information (ESI). The mechanical properties of the membrane samples were measured using a universal tensile machine (UTM4104X, Suns, China) at a cross-head speed of 1 mm/min at room temperature (23 °C).

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    Organic Dyes and Heavy Metal Ions Adsorption Measurements

    PDA has been confirmed as a highly efficient adsorbent in removing organic dyes and heavy metal ions.[

    38] Here, the removal abilities toward methylene blue (MB) and hexavalent chromium Cr(VI) were intensively investigated. For the purpose of exploring the adsorption behaviors of the membrane toward MB, different adsorption measurements were carried out, including the adsorption isotherms, adsorption kinetics and adsorption thermodynamics, through tailoring the adsorption time (t, h), initial concentration (C0, mg/L) of MB solution, and solution temperature (T, °C), etc. The concentration of MB in the solution during the adsorption process was monitored using a UV-Vis spectroscope UV-1800PC (SPEC-TROPHOTOMETER, Japan) at a wavelength range of 400–800 nm. The adsorption measurements were also conducted on the thermostatic shaker as mentioned above using a membrane sample with the precision-measured mass (m, g) and MB solution with a certain volume (V, mL). During the measurements, the adsorption amount (qt, mg/g) at time t was calculated using the following equation:
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    qt=(C0−Ct)×Vm
         		1

    Once the adsorption achieved the equilibrium state, the MB concentration was also named as the equilibrium concentration (Ce, mg/L), and then the final adsorption capacity was defined as the equilibrium adsorption capacity (qe, mg/g), which was measured through the Eq. (2):

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    qe=(C0−Ce)×Vm
         		2

    In addition, the adsorption selectivity of the membrane sample was also investigated. Three dyes, including Congo red (CR), methyl orange (MO) and rhodamine (RhB), were used to be mixed with the MB solution. The concentrations of MB and other dyes in the mixed solution were 20 mg/L. The mixed solution was poured into a glass funnel and the porous membrane (single layer membrane and the average thickness was about 63 μm) was used as the filter paper. Once the solution flowed through the membrane, MB was adsorbed or rejected by the membrane. Here, the measurement was carried out at 25 °C and the volume of the filtrate was 25 mL. The residual amount of MB in the filter liquor was also monitored using the UV-Vis spectroscope, and the solution colors before and after adsorption were taken photos.

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    The Cr(VI) adsorption of the sample was tested in light of the following steps. Here, potassium dichromate (K2Cr2O7, purity of 99%) was used as the source of Cr(VI), while the pH of the Cr(VI) was tailored by adding nitric acid (HNO3, analytical reagent grade, 65%−68%). The variation of the Cr(VI) concentration in the solution was monitored using an atomic adsorption spectrometer (AAS, PinAAcle 900T, USA). The above Eq. (1) was also used to calculate the adsorption capacity of the Cr(VI). Furthermore, the effects of the initial Cr(VI) concentrations varied from 50 mg/L to 200 mg/L on the adsorption behaviors of the membrane sample were also investigated.

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    Oil/Water Separation Measurements

    To detect the oil/water separation ability of the fibrous membrane, the representative PVDF/PDA-0.5 fibrous membrane was used as the reference object. Two emulsified solutions were prepared, including the emulsified oil/water (E-O/W) system that contained 1 mL of n-hexane, 99 mL of MB solution with a concentration of 10 mg/L, and 0.03 g of Tween 80, and the emulsified water/oil (E-W/O) system that contained 1 mL of MB solution with a concentration of 10 mg/L, 99 mL of n-hexane, and 0.03 g of Span 80. The gravity-driven oil/water separation measurements were conducted on the glass funnel. The emulsions were strongly stirred at 500 r/min for 8 h to achieve the stable emulsions that could be placed for long time without apparent phase separation. The time that used for separating the emulsions were recorded. The oil and water droplets in the emulsions before and after being separated were also investigated using an optical microscope DM2700P (Leica, Germany).

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    Sterilization Performance Experiments

    The sterilization experiments were carried out through the following steps. The first step is relating to the preparation of the bacterial suspension. First, sodium chloride (NaCl) buffer solution was prepared in a conical flask, and then the glass beads with a diameter of about 2−3 mm were added. Subsequently, a certain number of bacteria (E. coli and S. aureus) were added, and the concentration of the bacterial suspension was tailored to 2.5×105−3×105 CFU/mL. The electrospun PVDF and the PVDF/PDA-0.5 porous fibrous membranes were firstly sterilized at high temperature (121 °C). After that, the membranes were placed in the conical flask that contained 25 mL of saline solution and nutrient broth, and then 0.1 mL of bacterial suspension was added and then further treated in the thermostatic shaker for 18 h. Subsequently, 0.5 mL of solution was taken out and diluted according to the ten-fold dilution method. Then, 0.1 mL of solution was further taken out and placed in a petri dish, and 15 mL of nutrient agar medium was added. After solidification at room temperature, the petri dish was placed into an oven set at 37 °C for 24 h. The bacterial eliminating rate (R) can be calculated according to the following equation:

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    R=B−CB×100%
         		3

    where B and C represent the average numbers of colony (CFU/mL) of the control blank sample and PVDF-based fibrous membrane sample, respectively.

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    Light-to-Heat Transition Measurement

    The photothermal conversion ability of the fibrous membrane was also tested. An infrared light lamp (Philips) with a power of 150 W was used as the light source. The surface temperature of the membrane sample under the light irradiation was recorded by an infrared thermal imaging camera (FLIR T620, USA). The measurements were carried out through two ways. One is measuring the temperature evolution of the membrane sample in the air condition and the other is measuring the temperature evolution of the membrane sample in the distilled water that was placed in a watch glass.

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    RESULTS AND DISSCUSSION

    Morphology, Microstructure, and Physicochemical Performances of the Fibrous Membranes

    Surface topological structure of the fibers plays great role in determining the wettability of the fibrous membranes.[

    39,40] Here, the morphologies of the electrospun fibers were firstly characterized by SEM. As shown in Figs. 2(a) and 2(f), the electrospun PVDF fibers exhibit the typical porous structure. A large quantity of pores in nanoscale are homogeneously distributed in the fibers. The formation of the pores is mainly attributed to the dissolution of PVP during the treatment in distilled water.[37] For the PVDF/PDA fibers, it is clearly shown that the fiber morphology is greatly dependent on the concentration of DA in the Tris buffer solution. At relatively low DA concentration (0.2 wt%) (Figs. 2b and 2g), the PVDF/PDA-0.2 fibers exhibit the similar morphology to that of the PVDF fibers and few PDA nanoparticles can be observed. At DA concentration of 0.5 wt% (Figs. 2c and 2h), many PDA nanoparticles are homogeneously deposited on the fibers. Further increasing DA concentration to 1.0 wt% (Figs. 2d and 2i), the PVDF fibers are completely covered by PDA nanoparticles. However, further increasing DA concentration to 1.5 wt% (Figs. 2e and 2j), many PDA nanoparticles aggregate together in the membrane and few PDA nanoparticles are observed on the fiber surface. The reason may be attributed to the relatively high reaction rate of DA at high concentration. Specifically, it is also found that the diameter of the PDA nanoparticles is also related to the DA concentration. Higher DA concentration leads to larger PDA nanoparticles.
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    fig

    Fig 2  SEM images of the PVDF and PVDF/PDA fibrous membranes obtained at different DA concentrations. (a, f) PVDF, (b, g) PVDF/PDA-0.2, (c, h) PVDF/PDA-0.5, (d, i) PVDF/PDA-1.0, and (e, j) PVDF/PDA-1.5. Here, images (a−e) are characterized at higher magnifications while the images shown in (f−j) are obtained at relatively low magnifications.

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    The porosity of the fibrous membrane was calculated through the classical impregnation method. As shown in Fig. 3(a), all the fibrous membranes exhibit very high porosity. For the PVDF fibrous membrane, the porosity is as high as 91.8%. The high porosity is mainly attributed to the presence of the large pores between fibers and the large quantity of nanopores in the fibers. After being decorated by PDA nanoparticles, the porosity of the PVDF/PDA membranes still maintains at high level. Even for the PVDF/PDA-1.0 sample, the porosity is still 87.7%. Although the nanopores on the fibers may be covered or filled by the PDA nanoparticles, the large pores between fibers still endow the membranes with high porosity. Obviously, the high porosity ensures the high flux during the oil/water separation process. For the PVDF/PDA-1.5 sample, the porosity is higher than that of the PVDF/PDA-1.0 sample, which may be due to the fact that the relatively high self-polymerization rate of PDA at high DA concentration resulting in PDA agglomerates, which results in relatively low fraction of coverage on the PVDF porous fibers and more nanopores on the fibers can be exposed.

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    fig

    Fig 3  (a) Comparison of the porosity of the pristine PVDF and PVDF/PDA fibrous membranes; (b) SEM image of the PVDF/PDA-0.5 membrane after being immersed into the distilled water and ultrasonically treated at 120 W for 30 min.

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    The stability of the PDA nanoparticles on the PVDF porous fibers was also evaluated through ultrasonically treating the membrane in the distilled water. As shown in Fig. 3(b), even if the PVDF/PDA-0.5 fibrous membrane was ultrasonically treated at an ultrasonic power of 120 W for 30 min, many PDA nanoparticles can still be observed on the fibers, indicating the high stability of the PDA nanoparticles on the fiber surface. The good stability of PDA nanoparticles is mainly attributed to the good adhesion ability of PDA.[

    41]
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    The surface chemical characteristics of the fibrous membranes were characterized using FTIR. As shown in Fig. 4(a), the PVDF fibrous membrane does not exhibit apparent characteristic absorption band at wavenumber range of 3000−3700 cm−1. However, for the PVDF/PDA membranes, apparent absorption band is observed in this wavenumber range, which can be assigned to the stretching vibration of the ―NH and/or ―OH groups.[

    42] Furthermore, it can be seen that the absorption band shifts to lower wavenumbers with increasing the DA concentrations, which is attributed to the formation of hydrogen bond between PVDF and PDA, and with the increase of DA concentration, the hydrogen bonding interaction between PVDF and PDA is stronger. Specifically, another characteristic absorption band is also observed at 1660 cm−1 of the PVDF/PDA fibrous membranes as shown in Fig. 4(b), which is related to the stretching vibration of the C＝N.[42] Whatever, the FTIR characterizations clearly confirm that the polar groups are successfully introduced on the nonpolar PVDF fibers. Besides declaring the chemistry structure of samples, FTIR characterization is also a powerful technique to reflect the crystalline structure of polymers. This is because that the lamellar stacking state greatly affects the conformation of the molecular chains, which also influences the vibration modes of some chemical bonds, leading to the apparent change of the characteristic absorption bands, such as the shift of the absorption bands and the variation of the absorption band intensity. PVDF is a typical semicrystalline polymer with multiple crystal forms, such as the nonpolar α-form and the polar β-form, etc. The former crystallites exhibit characte-ristic absorption bands at 1381, 1208, 974, 854, 795 and 764 cm−1, while the latter crystallites show the characteristic absorption bands at 1275 and 840 cm−1.[43−45] As shown in Fig. 4(b), all the membrane samples show the intense absorption band at 840 cm−1, which demonstrates that the membrane samples mainly exhibit the β-form crystallites.
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    fig

    Fig 4  FTIR spectra of the electrospun PVDF and PVDF/PDA fibrous membranes in different wavenumber ranges as indicated.

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    The crystalline structure of the fibrous membranes was also characterized utilizing XRD, and the typical XRD profiles of membrane samples are illustrated in Fig. S1(a) (in ESI). Similar to the other investigations reported in literature, the electrospun PVDF fibers chiefly exhibit the β-form crystalline structure, which has the characteristic diffraction peak at 2θ=20.6°, attributing to the diffractions of the (110)/(200) crystal planes.[

    43] The formation mechanism for the β-form crystallites is mainly attributed to the orientated stacking of PVDF chain segments under the high-voltage electric field during the electrospinning processing.
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    Furthermore, Fig. S1(b) (in ESI) shows the TGA curves of all the samples to characterize the thermal stability. For comparison, the TGA curve of DA monomer is also illustrated. With regard to the PVDF sample, the temperature that weight loss achieves 5 wt% (Td-5wt%) is as high as 450 °C. With regard to the PVDF/PDA samples, the Td-5wt% is obviously reduced, especially at high DA concentrations. For example, the PVDF/PDA-0.2 sample shows Td-5wt% of about 373 °C, while the Td-5wt% of the PVDF/PDA-1.5 sample is dramatically reduced to 325 °C. However, it is worth noting that although the presence of PDA nanoparticles reduces the thermal stability, they enhance the charforming ability of the membrane at high temperature to a certain extent since higher residual weights are obtained in the PVDF/PDA membranes compared with the PVDF membrane.[

    46]
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    Generally, the wettability of the membrane surface can be evaluated by testing the contact angles of H2O and CH2I2 on the membrane surface. As shown in Fig. 5, PVDF sample shows the super-wettability feature, and the H2O and CH2I2 droplets both spread out completely when they touch the membrane surface, confirming the switchable superhydrophilicity and superlipophilicity simultaneously. Furthermore, the variations of contact angel of H2O (and CH2I2) with time were recorded when H2O (and CH2I2) were dripped on the surfaces of the PVDF, PVDF/PDA-0.5 and PVDF/PDA-1 membrane samples, respectively, and the results can be seen in Movies S1, S2 and S3 (in ESI) (for H2O) and Movies S4, S5 and S6 (in ESI) (for CH2I2), respectively, which further indicates that the membrane surfaces exhibit super-wettability. All the measurements show that in several seconds the H2O and CH2H2 can be completely adsorbed by the membranes and the contact angles are zero on the one hand. On the other hand, the PVDF/PDA membranes also show relatively higher adsorption rate for CH2I2 compared with the PVDF membrane, further confirming that decorating PDA nanoparticles on PVDF porous fibers intensify the superlipophilicity of the membrane surface due to the increased surface roughness. The super-wettability of the PVDF sample is attributed to the porous structure of the fibers and the residual PVP in the fibers that cannot be removed due to the high miscibility between PVP and PVDF.[

    37] Here, the high miscibility between PVP and PVDF is mainly attributed to the strong hydrogen bonding interaction and the good mutual solubility between them. Consequently, some of the PVP that locates in the amorphous region between PVDF lamellae cannot be removed and they endow the porous fibrous membrane with switchable superhydrophilicity and superlipophilicity features because of the amphipathic feature of PVP. More explanation about the achievement of the superhydrophilicity and superlipophilicity for the PVP-incorporated PVDF fibrous membrane can be seen in our previous work.[37]
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    fig

    Fig 5  Photos showing the super-wettability of the PVDF and PVDF/PDA membranes obtained at different DA concentrations. Here, H2O and CH2I2 were used as the probe liquids.

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    The presence of PDA nanoparticle has little effect on the super-wettability of the samples. Here, PVDF is hydrophobic while PVP is amphipathic. PDA exhibits hydrophilic feature because of the large number of hydroxyl and amino groups, which are favorable for the improvement of the hydrophilicity.[

    47] However, different from the common PDA coating layer, in this work, the PDA nanoparticles are decorated on the PVDF porous fibers, which apparently changes the roughness of the fiber surface. Consequently, the membrane samples still maintain the superhydrophilicity and superlipophilicity features, which is of great significance for expanding the application of the membranes in wastewater treatment.
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    The effect of PDA nanoparticles on the mechanical properties of the membrane samples were also evaluated by measuring the stress-strain curves, and the results are shown in Fig. S2 (in ESI). Compared with the PVDF membrane sample, largely enhanced tensile strength, tensile modulus and elongation at break are obtained for the PVDF/PDA-0.5 membrane sample. This ensures the application of the membranes in wastewater treatment.

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    Adsorption Behaviors

    In the field of polymer composites, due to the good adhesive ability on any substrate and the presence of the amino and hydroxyl groups of the PDA molecules, PDA has been widely used to modify the fillers and then intensify the interfacial interaction between fillers and polymer matrix.[

    48] Similarly, in wastewater treatment, since most of the organic dyes have the polar groups and aromatic ring structure in the molecules, there are different kinds of interaction between PDA and organic dyes,[38,49] such as hydrogen bonding interaction and π-π interaction. Specifically, in acidic condition, protonation usually occurs for the amino groups while in alkaline condition, deprotonation occurs, which may bring the charge interaction between PDA and organic dyes.[21] Here, as mentioned above, through controlling the polymerization of PDA, a large number of PDA nanoparticles are successfully deposited on the porous PVDF fibers. The adsorption behavior toward organic dye (MB) was systematically investigated. It is worth noting that all the performance measurements were carried out using the PVDF/PDA-0.5 sample.
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    Fig. 6(a) exhibits the variation of MB adsorption capacity (qe, mg/g) from the MB solution with increasing initial MB concentration (C0, mg/L). It is found that with increasing initial MB concentration, the qe increases gradually with the increasing C0. At C0 of 20 mg/L, the qe achieves 57.3 mg/g. To quantitatively describe the removal efficiency of the MB, a parameter of removal ratio that is defined as the ratio of (1 −Ct/C0) (%) is named, where Ct represents the MB concentration at adsorption time of t. From Fig. 6(a), one can see that the PVDF/PDA-0.5 membrane has extremely high removal ratio toward MB, bigger than 95%. Specifically, at relatively low MB concentration (2 mg/L), the removal ratio is as high as 99.9%, and in this condition, the residual concentration of MB in the solution is only 0.01 mg/L.

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    fig

    Fig 6  (a) Variation of the MB adsorption capacity of the PVDF/PDA-0.5 sample with increasing initial MB concentration in the solution, and (b) the corresponding removal ratios of MB from the MB solutions. During the adsorption measurements, membrane (10 mg) was soaked in 40 mL MB solution at 25 °C for 24 h.

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    Furthermore, the adsorption capacities of the MB at other different initial concentrations (varied from 10 mg/L to 200 mg/L) and different initial solution volumes (10 and 40 mL) were also comparatively investigated. And in this condition, the adsorption time was maintained at 24 h at 25 °C. As shown in Fig. S3 (in ESI), similar phenomenon is observed. Namely, increasing the C0 of MB leads to higher qe. At the C0 of 200 mg/L, the qe achieves 486.2 mg/g. However, it is worth noting that higher C0 leads to lower removal ratio.

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    For the purpose of exploring the adsorption behaviors toward MB by the PVDF/PDA fibrous membrane, the adsorption kinetics and isotherms were investigated. Fig. 7(a) shows the variation of the qe with increasing adsorption time. Here, the initial concentration of MB was 150 mg/L. The results obtained at other initial concentrations (20, 50 and 100 mg/L) are illustrated in Fig. S4 (in ESI). From Fig. 7(a) one can see that in the early stage of the adsorption process, qe increases dramatically with increasing adsorption time, while it slightly increases at the later stage of the adsorption process. In the early stage, the concentration of the MB in the solution is re-latively high and simultaneously, there are many adsorption sites on the fibrous membrane, and therefore, high adsorption rate is achieved. However, in the later stage of the adsorption, on one hand, the concentration of MB in the solution is reduced, on the other hand, most of the adsorption sites have already been occupied by the MB molecules and in this condition, the adsorption rate is reduced and the increase of qe becomes smaller. Through the simulations based on the pseudo-first-order and pseudo-second-order,[

    50,51] the adsorption kinetics is researched too, and the results are shown in Figs. 7(b) and 7(c). The corresponding kinetics parameters are also listed in the graphs. It is clearly seen that the pseudo-second-order model simulation gives higher R2 (0.99999) compared with the pseudo-first-order model simulation. This indicates that the adsorption kinetics of the MB by the PVDF/PDA-0.5 fibrous membrane can be well described by the pseudo-second-order model. More explanation about the adsorption kinetics can be seen in ESI.
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    fig

    Fig 7  (a) The adsorption capacity toward MB of the PVDF/PDA-0.5 sample as a function of adsorption time, and the corresponding (b) pseudo first-order model and (c) pseudo second-order model, and in this measurement, the initial concentration of MB was 150 mg/L; (d) Variation of equilibrium adsorption capacity with increasing MB equilibrium concentration, and the corresponding (e) Langmuir fitting plot and (f) Freundlich fitting plot.

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    The adsorption isotherms were further studied by investigating the variation of the qe

    with increasing Ce
         , and then the data were simulated using the Langmuir and Freundlich models.[52,53] From Fig. 7(d) it can be seen that qe gradually increases with increasing Ce. The corresponding Langmuir and Freundlich plots are illustrated in Figs. 7(e) and 7(f). One can see that the Langmuir model simulation gives the R2 of 0.95586, which is higher than that of the Freundlich model simulation (0.91316), which also indicates that the Langmuir adsorption model is more appropriate to describe the MB adsorption behavior by the PVDF/PDA-0.5 membrane sample. According to the theoretical background of the two models, it can be deduced that the adsorption of MB by the PVDF/PDA-0.5 sample is a monolayer and homogeneous process. Similarly, more description about the Langmuir and Freundlich model simulation can be seen in ESI. Specifically, according to the Langmuir adsorption model, the maximum MB adsorption capacity of the PVDF/PDA-0.5 sample achieves 917.4 mg/g, which is much higher than the adsorption capacities of the most of the fibrous membranes reported in literatures[54-62] as shown in Fig. 8.
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    fig

    Fig 8  Comparison of the maximum adsorption capacity toward MB between the PVDF/PDA-0.5 membrane sample and other membrane samples reported in literature.[

    54-62] (BP/PVA: buckypaper/polyvinyl alcohol; PVA/CMC: polyvinyl alcohol/carboxymethyl cellulose; CNT: carbon nanotube; GO: graphene oxide; DA: deacetylated cellulose acetate; PLA: poly(lactic acid)).

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    Achieving the removal of dyes from the running wastewater is very significant, because it can greatly improve the treatment efficiency of the wastewater. Here, to detect the effect of the PVDF/PDA fibrous membrane on the removal of MB from the running water, a simple filtering unit was designed as shown in Fig. 9(a), and the PVDF/PDA membrane samples were used as the filter papers, and then the MB solution was poured into the filtering unit. The water flux was measured and the filtrate was collected and taken photos. The movie that recorded the adsorption process are presented in Movie S7 (in ESI). From Fig. 9(b) one can see that different from the brilliant blue of the MB solution before adsorption, the color of the filtrate becomes very shallow, proving that MB in the solution is almost removed. In other words, the PVDF/PDA fibrous membrane exhibits a rejection role for the MB molecules. Specifically, after being filtered by the PVDF/PDA-0.5 sample, the filtrate becomes nearly colorless, indicating that most of MB is removed during the flow process. Fig. 9(c) shows the change of the water flux during the measurements. The PVDF/PDA-0.2 sample shows the water flux of 746.42 L·m−2·h−1. The water flux of PVDF/PDA-0.5 sample is largely reduced (331.74 L·m−2·h−1). The reduction of the water flux was possibly related to the deposition of PDA nanoparticles on the fiber surface. However, due to the aggregation of PDA nanoparticles in the fibrous membrane, the membrane samples prepared at higher DA concentrations show increased water flux compared with the PVDF/PDA-0.5 sample. Specifically, the PVDF/PDA-1.5 sample shows the similar water flux (734.16 L·m−2·h−1) to that of the PVDF/PDA-0.2 sample. The variation of the water flux among different membrane samples indirectly confirms the different morphologies and the dispersion states of the PDA nanoparticles.

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    fig

    Fig 9  (a) Photos presenting the quick adsorption of MB using the PVDF/PDA fibrous membranes as the filter papers and (b) the solution color changes before and after being filtered by the different membranes. In the photo, the numbers of 0.2, 0.5, 1 and 1.5 represent that the MB solution was filtered by the PVDF/PDA-0.2, PVDF/PDA-0.5, PVDF/PDA-1, PVDF/PDA-1.5 fibrous membranes, respectively; (c) The variation of the water flux using MB solution as the probe liquid with increasing DA concentration during the oxidation-polymerization of PDA.

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    The selective adsorption behavior of the membranes was also tested. Different organic dyes including CR, MO and RhB were used to mix with MB as shown in Fig. 10. It is well known that MB and RhB are typical cationic dyes while CR and MO are anionic dyes. MB exhibits typical absorption peak at wavelength of 663 nm, while CR, MO and RhB exhibit characteristic absorption peaks at 466, 463 and 554 nm, respectively. From Figs. 10(a) and 10(b), it can be seen that when the cationic and anionic dyes are mixed together, the PVDF/PDA-0.5 membrane sample exhibits high adsorption selectivity toward cationic MB, while the adsorption of anionic dyes is very small, especially in the mixed MB/CR solution. It should be stressed that the adsorption occurs during the flow of solution through the membrane and therefore, the adsorption time is relatively short. While if the cationic dyes are mixed together, Fig. 10(c) shows that the MB and RhB can be simultaneously adsorbed by the PVDF/PDA-0.5 membrane sample. According to the change of the intensity of the characteristic absorption peak, it can be deduced that the membrane exhibits high MB removal ratios from the MB/CR, MB/MO and MB/RhB solutions during the filtering process, i.e., 68.8%, 89.9% and 93.4%, respectively. By the way, the PVDF/PDA-0.5 membrane sample also shows high adsorption ability toward RhB, and the removal ratio is about 69.1%. Obviously, the PVDF/PDA-0.5 membrane sample still shows higher adsorption selectivity in the mixed cationic dyes and MB can be easier to be adsorbed compared with the RhB. The reason is possibly attributed to the fact that the steric hindrance of the ethyl groups that link on the nitrogen atoms of the RhB are larger than those of the methyl groups of the MB, which weakens the electrostatic interaction between membrane and RhB molecules. The removal mechanisms of the dyes by the PVDF porous fibers membrane are mainly attributed to the π-π stacking interaction, electrostatic interaction and hydrogen bonding interaction between PDA and dye molecules. Specifically, according to literatures,[

    63] PDA-decorated surface usually exhibits the negative charge feature, which is possibly the main reason why the PVDF/PDA membranes show high adsorption ability toward MB. Furthermore, PVDF membranes provide the hydrophobic adsorption sites, which is also conducive to the adsorption of dyes on the membranes.[64] The schematic representations showing the removal mechanisms toward MB are shown in Fig. S8 (in ESI). The adsorption selectivity measurements further confirm that the PVDF/PDA fibrous membrane is a promising adsorbent to remove cationic dyes from the waste water.
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    fig

    Fig 10  Selective removal of MB from different solutions containing two dyes by the representative PVDF/PDA-0.5 fibrous membrane. (a) Mixed MB/CR solution, (b) mixed MB/MO solution, and (c) mixed MB/RhB solution. Here, the concentration of each dye in the solution was 20 mg/L.

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    Fig. 11 shows the adsorption behaviors toward Cr(VI) of the PVDF/PDA-0.5 sample. Considering that the removal of Cr(VI) in the wastewater by the adsorbents mainly relies on the electrostatic interaction and the chelation effect,[

    65] the dependence of the adsorption capacity on pH value of the Cr(VI) solution was firstly investigated. As shown in Fig. 11(a), the maximum adsorption capacity (42.6 mg/g) is achieved at pH value of 2. Generally, Cr(VI) in the solution is mainly present in the different states, including H2CrO4 at very low pH (pH<1.0), HCrO4− and Cr2O72− ions at pH of 2.0–6.0, and Cr2O72− at pH>7.0.[66] On the other hand, the amino groups of the PDA have the strong protonation effect at pH<7.0.[67] The smaller the pH value, the stronger the protonation effect of the PDA is. As a consequence, the surface of the membrane sample is positively charged. Correspondingly, there is a strong electrostatic attraction between PDA and negative charged Cr(VI) ions. This is the reason why the PVDF/PDA-0.5 sample exhibits the maximum adsorption capacity at pH value of 2.
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    fig

    Fig 11  (a) Variation of the adsorption capacity toward Cr(VI) by the PVDF/PDA-0.5 membrane sample measured at different solution pH values with the initial Cr(VI) concentration of 200 mg/L; (b) The adsorption capacity of Cr(VI) by the PVDF/PDA-0.5 sample with different initial Cr(VI) concentrations in the solution. During the adsorption measurements, the adsorption time was 24 h at 25 °C.

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    The variation of the adsorption capacity toward Cr(VI) with increasing initial Cr(VI) concentration in the solution is illustrated in Fig. 11(b). It can be seen that the adsorption capacity of PVDF/PDA-0.5 membrane sample toward Cr(VI) increases with the increase of the initial Cr(VI) concentration in the solution and the qe reaches 42.6 mg/g when the initial Cr(VI) concentration is 200 mg/L.

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    Oil/Water Separation

    The superlipophilicity feature and the porous structure of the fibers endow the membrane with high oil adsorption ability. Here, to confirm the oil adsorption capacities of the PVDF/PDA membrane samples, different oils and organic solvents were used and the adsorption capacities were measured. As shown in Fig. 12(a), the PVDF/PDA-0.5 membrane sample shows high adsorption ability toward silicone oil and engine oil, and the adsorption capacities are 74.6 and 64.2 g/g, respectively. However, the membrane sample exhibits relatively low adsorption capacities toward organic solvents, i.e., 13.3 and 17.6 g/g toward n-hexane and cyclohexane, respectively. The different adsorption capacities toward different oils and organic solvents are mainly attributed to the combined effects of the influencing factors, such as the density, surface tension, viscosity of oils and the interfacial interaction between membrane and adsorbates.[

    68,69] In order to further explore the adsorption ability of the samples, Fig. 12(b) simply compares the adsorption capacities toward silicone oil by the different adsorbents that prepared in this work and reported in literatures.[70−75] It is found that the PVDF/PDA-0.5 membrane sample exhibits relatively high adsorption capacity among these adsorbents, confirming the good adsorption ability of the membrane sample.
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    fig

    Fig 12  (a) Comparison of the adsorption capacities for the adsorption of different oils and/or organic solvents onto PVDF/PDA-0.5 membrane, (b) comparison of maximum adsorption capacities for the adsorption of silicone oil onto different membranes.[

    37,70−75] (PLA: poly(lactic acid); PVA: poly(vinyl alcohol); PS: polystyrene)

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    The oil/water separation measurements were carried out through continuously separating the E-O/W and E-W/O systems using the glass funnel. It is worth noting that the membrane was firstly wetted by the distilled water (or n-hexane) when separating the E-O/W emulsion (or E-W/O emulsion). As shown in Figs. 13(a), 13(b) and 13(c), for the E-O/W system, there are many oil droplets in the emulsion before separating. After being separated by the PVDF/PDA-0.5 membrane sample, the oil droplets can be hardly observed from the water, indicating that most of n-hexane have been rejected by the membrane. Furthermore, it is also observed that different from the brilliant blue of the emulsion, the filtrate becomes colorless. Since MB is a water-soluble molecule and it mainly presents in the water phase rather than in the oil phase, it can be concluded that MB can be also removed from the water. This is very significant because the simultaneous removal of MB and n-hexane is achieved during the separating process using the PVDF/PDA-0.5 membrane sample. Similarly, as shown in Figs. 13(d), 13(e), and 13(f), the membrane sample also shows high oil/water separation effect toward the E-W/O system, and water droplets can be completely rejected by the membrane sample, and in this condition, the filtrate (n-he-xane) becomes colorless too, totally different from the epinephelos state of the emulsion before separating. The movies that recorded the detailed oil/water separation process are presented in Movies S8 and S9 (in ESI). According to the movies, the water flux and oil flux of PVDF/PDA-0.5 sample are calculated, which reach 1107.7 and 1132.3 L·m−2·h−1, respectively. The n-hexane (or water) droplet aggregation states in the emulsion after a half of emulsion were separated was also taken photos and shown in Fig. S5 (in ESI).

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    fig

    Fig 13  OM images showing the n-hexane and water droplets in the E-O/W system (a, b, c) and E-W/O system (d, e, f) before (a, d) and after (c, f) being separated by the PVDF/PDA-0.5 membrane sample, respectively. (b, e) Photos showing the E-O/W and E-O/W emulsions before and after being separated. For the E-O/W system, a small amount of MB was added into the system so that the water was colored.

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    Sterilization Performance

    Besides the application in wastewater treatment that involves high concentration of pollutants, the fibrous membrane can also be used in the purification of the drinking water. In this condition, the sterilization performance of the membranes becomes very important. PDA has already been demonstrated as an efficient sterilization material due to its abundant structure characteristics, and several sterilization mechanisms have been proposed,[

    76] including the direct sterilization methods and the indirect sterilization methods. For example, through the electrostatic and chelation effects of ions/proteins when PDA contacts the bacteria[77] or releasing reactive oxygen species,[78] the cell structure of bacteria can be destroyed. The excellent light-to-heat conversion ability of PDA under the near-infrared light irradiation leads to the apparent enhancement of temperature of the PDA-coated substrate, which is also unfavorable for the survive of the bacteria.[79]
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    In this work, the sterilization performance of the membrane samples was also investigated. For comparison, the PVDF and PVDF/PDA-0.5 membrane samples were comparatively researched. Fig. 14 shows the plate count results of the E. coli and S. aureus. Different from the presence of large number of bacteria in the control sample as shown in Figs. 14(a) and 14(d), fewer bacteria can be observed for the solution treated by PVDF fibrous membrane (seen in Figs. 14b and 14e). Interestingly, when the PVDF/PDA-0.5 membrane is present in the solution, nearly no bacteria is detected out in the petri dish. And in this condition, the bacterial eliminating rate (R) toward E. coli and S. aureus achieves 99.9%, indicating the excellent sterilization performance of the PVDF/PDA-0.5 membrane sample. Whatever, the sterilization performance measurements confirm that the PVDF/PDA fibrous membrane has the role of greatly reducing the bacteria concentration in the solution. This is very significant since the membrane can be used to separate the bacteria for providing the clean drinking water, especially in the field conditions.

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    fig

    Fig 14  Plate count results of E. coli (a−c) andS. aureus (d−f) in the bacteria suspension. (a, d) Reference bacteria suspension, (b, e) after being treated by the PVDF fibrous membrane, and (c, f) after being treated by the PVDF/PDA-0.5 fibrous membrane.

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    Light-to-Heat Conversion

    As mentioned above, PDA exhibits the excellent light-to-heat conversion ability,[

    79] which can be used to enhance the surface temperature of membrane under the irradiation. This feature endows the PVDF/PDA fibrous membranes with at least two advantages that the other common fibrous membrane cannot have.
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    First, the enhanced surface temperature of the membrane is favorable for reducing the viscosity of the oils on the membrane surface, which facilitates the flow of oil through the fibrous membrane. Here, the variation of the oil viscosity with increasing temperature was measured using silicon oil as the research object, and the results are shown in Fig. S6 (in ESI). It can be seen that at 80 °C that is very close to the surface temperature of the PVDF/PDA-0.5 membrane sample after being irradiation as mentioned below, the viscosity of the silicone oil is greatly decreased from 452 mPa·s at 25 °C to 236 mPa·s. Since the flow ability of oil is greatly dependent upon the viscosity, and lower viscosity leads to higher flow ability. Therefore, it can be deduced that relatively higher membrane temperature may bring higher oil flux, accelerating the oil/water separation process. Second, the enhanced membrane temperature is unfavorable for the survive of bacteria on the membrane surface. Once the membrane adsorbs the bacteria, it can be directly irradiated under the infrared light to kill the bacteria. Here, the light-to-heat conversion ability of the samples was also evaluated through using the infrared light. As shown in Fig. 15, the PVDF fibrous membrane exhibits very weak light-to-heat conversion ability. Even after being irradiated for about 70 s, the surface temperature is slightly enhanced by 11.8 °C due to the fact that the infrared light source emits a certain amount of heat. However, under the completely same irradiation conditions, largely enhanced surface temperature is achieved for the PVDF/PDA-0.5 membrane sample, and the temperature is enhanced from 27.8 °C to 79.1 °C in air condition. The photos showing in Fig. 15(b) clearly confirm the temperature evolution of the membrane surface under the irradiation condition. Furthermore, the light-to-heat conversion ability of the PVDF/PDA-0.5 sample in distilled water was also measured, and the results are shown in Fig. S7 (in ESI). It can be seen that even if the membrane sample was soaked in distilled water that was placed in a watch glass, the membrane sample still shows the gradually increased surface temperature with increasing irradiation time, and the maximum surface temperature achieves 60 °C at the irradiation time of 70 s, higher than the temperature that most of the bacteria can survive.[

    80] Whatever, the PVDF/PDA-0.5 sample exhibits excellent light-to-heat conversion ability, which is of great significance for expanding the potential applications of the membrane.
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    fig

    Fig 15  (a) Surface temperature evolutions of the PVDF and PVDF/PDA-0.5 fibrous membranes under the infrared light irradiation of 150 W with increasing the irradiation time, and (b) infrared thermal images showing the light-to-thermal transition abilities of the membrane samples. The measurements were carried out in air condition.

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    CONCLUSIONS

    In summary, the PDA nanoparticles-decorated electrospun PVDF porous fibers have been fabricated in this work. Through tailoring the DA concentration during the self-polymerization in the Tris buffer solution, the homogeneous deposition of PDA nanoparticles with high adhesion stability on the electrospun PVDF porous fibers is achieved. The presence of PDA nanoparticles does not change the microstructure of the fibers but increases the number of polar groups on the fibrous membrane. The membranes exhibit switchable superhydrophilicity and superlipophilicity. The fibrous membranes exhibit various excellent performances. Using the PVDF/PDA-0.5 membrane sample as the example, it exhibits extremely high adsorption ability toward MB, and the maximum adsorption capacity achieves 917.4 mg/g, which is much higher than that of most of the fibrous membranes reported in literature. Specifically, the membrane sample can reject most of MB when MB solution flows through the membrane. Furthermore, the membrane sample also exhibits high adsorption selectivity when treating the solution containing different organic dyes. During treating the wastewater containing heavy metal ions, the PVDF/PDA-0.5 membrane sample shows a adsorption capacity of 42.6 mg/g at pH of 2 towards Cr(VI). The oil/water separation measurements confirm that the PVDF/PDA-0.5 membrane sample has excellent oil adsorption ability and outstanding oil/water separation ability. The maximum adsorption value achieves 74.6 g/g during adsorbing silicone oil. The membrane also exhibits high sterilization performance and the bacterial eliminating rates achieve 99.9% toward E. coli and S. aureus. Finally, the membrane sample also exhibits high light-to-heat conversion ability and in a very short time under the irradiation condition, the surface temperature of the membrane sample achieves 79.1 °C, which provides more chances for the applications of the membrane not only in wastewater treatment but also in cleaning of the drinking water. This work concludes that the PDA nanoparticles-decorated PVDF porous fibers membranes are a new generation of fibrous membranes and exhibit great potential applications in wastewater treatment.

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