Enhanced coalescence separation of oil-in-water emulsions using electrospun PVDF nanofibers

Yujie Yang,Lei Li,Qian Zhang,Wenwen Chen,Song Lin,Zaiqian Wang,Wangliang Li,

1 CAS Key Laboratory of Green Process and Engineering,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 Key Laboratory of Magnetic Molecules &Magnetic Information Materials Ministry of Education,The School of Chemical and Material Science,Shanxi Normal University,Linfen 041004,China

3 Innovation Academy for Green Manufacture,Chinese Academy of Sciences,Beijing 100190,China

4 Guizhou Water Fuquan Co.,Ltd,Guiyang 550081,China

Keywords:Coalescence Electrospinning Nanofibrous membrane Oil-in-water emulsions

ABSTRACT A novel and high-efficiency coalescence membrane enhanced by nano-sized polyvinylidene fluoride(PVDF) nanofibers based on polyester (PET) substrate was fabricated using electrospinning method.The properties of the electrospun nanofibers such as roughness and surface morphology greatly affected the oil droplet interception efficiency and surface wettability of the membrane.A series of coalescence units were prepared with different layers of nanofibrous membrane and the separation efficiencies at different initial concentrations,flow rates,and oil types were tested.It is very interesting that the obtained nanofibrous membrane exhibited superoleophilicity in air but poor oleophilicity under water,which was beneficial to the coalescence process.The coalescence unit with four membrane layers had excellent performances under different initial concentrations and flow rates.The separation efficiency of the 4-layers unit remained above 98.2% when the initial concentration reached up to 2000 mg.L-1.Furthermore,the unit also exhibited good performance with the increasing oil density and viscosity,which is promising for large-scale oil wastewater treatment.

1.Introduction

Emulsified oily wastewater,common in oil recovery,oil refinery,pharmaceutical industry,chemical plant,etc.does serious harms to environment and human health[1–3].The emulsified oily wastewater is difficult to be effectively removed because the oil droplet was quite small (<10 μm) and the finely divided oil droplets are uniformly dispersed in water due to the double electric layer.Effective disposal of the oily water is becoming an urgent global problem.The methods such as air flotation [4],chemical flocculation [5,6],adsorption [7,8],and membrane separation[9,10]have been used to solve this problem.However,these methods have the disadvantages of secondary pollution,large space occupation,low separation efficiency and membrane fouling during membrane filtration,which cannot effectively and economically separate oil-in-water emulsions.Coalescence separation is a kind of physical method based on the density difference of oil and water,and the wettability on the solid surface to separate emulsions.Due to its advantages of good structurecontrollability,high separation efficiency and low operation cost,it becomes a hot topic in the field of emulsified oil–water separation [11–14].

Coalescence separation is a process that tiny droplets grow up to large ones and finally oil is separated by gravity difference,which can be subdivided into 3 steps.Firstly,small oil droplets approach to the coalescing material,and are intercepted and captured on the surface by strong adhesion.Next,the captured small droplets coalesce with each other to form the large ones.Finally,the large droplets self-release from the material by buoyancy[15,16].Coalescence separation method has been widely used to dispose emulsions using coalescing bed,in which different kinds of fibers or filter media are packed.Chase and co-workers used the nonwoven glass fiber as the filter packing augmented with polymeric nanofibers to perform the coalescence [16].The result showed that the flow rate had a significant impact on the coalescence performance and the lower flow rates performed better than the higher flow rates.Shou experimented different filter media with a certain bed thickness,which the separation efficiency of the polytetrafluoroetylene-glass fiber (PTFE-GF) filter bed was 95.1% [17].Hu et al.prepared a high-calibre nonwoven filter mat with different kinds of fibers to deal with oil-in-water emulsions,the separation efficiency of which was up to 99.5% [18].Krasinski used the meltblow technique with or without an electric field to jets of molten polypropylene as the coalescing material with the mean diameter of 15.2 μm [19].The prepared nanofibrous structure performed with good efficiency (20 mg.L-1) for the removal of emulsified water from a low sulfur diesel fuel.Matsuyama investigated the effects of the wettability of fibers and fiber diameter (4–16 μm) on the coalescing behaviours for the O/W emulsions through modelled fibrous filters using the lattice Boltzmann method (LBM) [20].The bi-layered filters composed of a small-pore filter was designed to catch the droplets and a large-pore filter to effectively enlarge the droplets.Chen experimentally investigated the effect of pore size on filtration performance of coalescing filters during oil–water separation,using four oleophilic and three oleophobic glass-fiber filter materials with the diameter from 1 to 4 μm [21].In conclusion,the types and diameter of the fibers greatly affected the coalescence separation efficiency.The type of the fibers determines the hydrophilicity/oleophilicity property [22,23].Meanwhile,with the decrease of the fiber’s diameter,the surface area increases and eventually improves the probability of the interception of oil droplets.As a result,the coalescence process can be promoted by decreasing the fibers diameter and balancing their hydrophilicity/oleophilicity(L/H).The value represents the relative wettability.Prashant prepared polypropylene (PP) fibers with three different sizes(300 nm,600 nm,and 900 nm) via electrospinning and the fibers were blended with micro-glass (MG) fibers to form composite media [24].By changing the diameter of polypropylene fiber and the relative content of polypropylene fiber and glass fiber,the media with specific L/H value was prepared.However,the method improving the coalescence separation efficiency by decreasing the fibers diameter has some limitations in practical applications.The smaller the fibers diameter is,the more difficult dispersion became.The media of alternating thin layers of polypropylene(PP) or polyester and micro-glass fibers with diameters of 7 μm was fabricated.However,the two kinds of fibers were not bonded stably[25].Huang reported the framework-8@thiolated grapheme(ZIF-8@GSH) composites-based polyimide (PI) nanofibrous membrane via electrospinning and in situ hydrothermal synthesis,which the average diameter of fibers was 300 nm.The membrane showed good separation efficiency,but the material may be damaged during the separation [26].So,a uniform coalescence bed with ultrathin fibers was difficult to obtain by traditional method.It is necessary to seek and explore a simple and effective method to prepare coalescence material with ultrathin fibers.

Electrospinning is an efficient technique to produce ultrathin polymer fibers [27–32].By random deposition of the nanofibers,membranes with microstructure were generated and could be used as coalescence materials,in which the dispersion process of ultrathin fibers was avoided.What is more,electrospun nanofiber membranes had some advantages such as small pore size,high porosity,large specific surface area and good structure controllability.The electrospun nanofiber membranes have been widely used for the emulsified oil/water separations [33–41].Polyvinylidene fluoride (PVDF) as the common polymer material,has been widely applied in electrospinning field.Sadeghi used PVDF and the copolymer additive to prepare nanofibers via electrospinning,and applied them to separate oil–water mixtures [42].However,the mechanism of the oil/water separation in that work was not coalescence rather than material structures [43,44].Using PVDF nanofibers to separate oil-in-water emulsions by coalescence was rarely reported.

In this research,a kind of nano-sized PVDF fibrous coalescence material with a nonwoven fabric as the substrate was prepared by electrospinning.By introducing the nanofibers,the probability of intercepting oil droplets was greatly improved.Several properties of the membrane such as surface morphology,wettability and pore size were characterized.Uniform pore distribution and suitable wettability led to coalescence separation of oil-in-water emulsion successfully.The layer-by-layer combination of nanofibrous membrane can effectively increase the probability of the colliding of oil droplets and improve the interception to obtain good separation performance.Different flow velocities and initial concentrations of oil-in-water emulsions were tested and their influence for coalescence separation were studied.Moreover,different types of oil were also used to examine the wide applicability of the nanofibrous membrane material to other emulsions.It was found that the membrane still maintained good separation efficiencies under higher flow rate and concentration which indicated to handle the large-scale oil wastewater.

2.Experimental

2.1.Materials

PVDF (molecular weight with 300,000–500,000,500,000–700,000 and 700,000–1000,000) was purchased from Shanghai San Ai Fu New Materials Co.,Ltd.N,N-dimethyl formamide (DMF)was purchased from Beijing Chemical Reagent Factory.N,Ndimethyl acetamide (DMAc) and octane were obtained from Sinopharm Chemical Reagent Co.,Ltd.Hexadecane was purchased from Haltermann GmbH.Rapeseed oil was purchased from a local supermarket.Diesel oil was purchased from the local gas station.Meltblown polyester (PET) nonwoven medium was obtained from Shandong Laifen Nonwoven Co.,Ltd.

2.2.Fabrication of PVDF nanofibrous membranes

Firstly,a series of electrospinning solutions with different PVDF molecular weights,concentrations and solvents were prepared.PVDF was dissolved in the solvent at certain ratios and then,stirred for 3 h at 70 °C to obtain the electrospinning solutions.Secondly,the PVDF solution was used for nanofibers fabricating on an electrospinning apparatus (Ucalery Co.,China).The feed solution was pumped at the rate of 0.05 mm.min-1.The applied voltage was varied from 0 to 30 kV and the tip-to-collector distance was varied from 10 cm to 15 cm.The relative humidity was controlled below 20%,and the temperature was maintained at (20 ± 5) °C.Finally,electrospun nanofibers were collected on the non-woven PET substrate.The composite membranes with different electrospinning times from 15 to 90 min were prepared.

2.3.Preparation of oil-in-water emulsions

The oil-in-water emulsions with different initial concentrations ranging from 500 to 2000 mg.L-1were prepared by mixing oil and water using a homogenizer(Shinetek Instruments Co.,Ltd.)operating at 15,000 r.min-1for 10 min.

2.4.Separation of oil-in-water emulsions

The evaluation of the separation performance was conducted using a dead-end filtration setup equipped with a peristaltic pump.The PVDF membranes were placed into a separator with the diameter of 47 mm and then used to deal with the emulsions.The effects of layers of nanofibrous composite membranes,flow rates,initial concentrations and types of oil on coalescence separation were tested.The concentration of feed solutions and filtrates was determined by the chemical oxygen demand (COD) method and used to evaluate the separation efficiency.The separation efficiency E was calculated using the following formula:

where C0was the initial concentration of the oil in the emulsion,and Cfwas the oil concentration in the filtrate.

2.5.Characterizations

The surface morphologies of the PVDF nanofiber membranes were observed using a cold field emission electron microscope(Hitachi S-6700,JEOL).A contact angle analyzer (Dataphysics OCA20,Dataphysics instruments GmbH)was used to test the contact angle of the composite membranes,in which approximately 2 μl of oil or water droplets were applied to the surface of the membrane for each test.The contact angles at different locations of the film were tested.The pore size and distribution of the membranes were tested by a pore size analyzer(porometer 3Gzh,Quantachrome).The change of emulsion before and after separation were observed by an optical microscope made by Shanghai Optical Instrument Factory.A spectrophotometer was used to measure the chemical oxygen demand in feed solutions and filtrates (Beijing Lianhua Technology Co.,Ltd).

3.Results and Discussion

3.1.Morphologies of PVDF nanofibrous membranes

The schematic diagram of the preparation of the electrospun PVDF nanofibrous membranes was shown in Fig.1.The solution was ejected from the tip of needle and the jet was stretched continuously by electric field force to form the nano-sized fibers and collected on PET substrate.Then,the composite membranes were used to deal with the emulsions.Due to the nano-size and rough surface of the PVDF fibers,the coalescence separation efficiency of the composite membrane was greatly improved.

Fig.1.Schematic diagram of the nanofibrous membranes preparation.

In order to obtain the uniform and non-defect PVDF nanofibers,the effects of solution properties and process parameters on electrospinning process were systematically investigated.The formation of electrospun PVDF nanofibers was largely determined by the solution properties,including the solvent,the molecular weight of the polymer and its concentration.Solvent was one of the most important parameters for electrospinning,which was closely related to the solubility of the polymers,volatility and surface tension of the spinning solution.Fig.S1(see Supplementary Material) showed the influence of the type of solvents on the surface morphologies of the nanofibrous membrane.Although both DMF and DMAc were good solvents for PVDF,the solvent systems containing DMAc did not exhibit a better performance.When the ratios of DMF to DMAc were 0:1,1:1 and 2:1,respectively,the surface of the fibers fabricated with 15% (mass) PVDF solution had some defects.The solvent was not evaporated timely during the electrospinning process.Well-aligned uniform shaped PVDF nanofibers were only formed with DMF.The molecular weight of the polymer was related to the length of the molecular chain.When the molecular weight was smaller than 500,000,there were no sufficient intramolecular interactions among the small molecules even in their highly concentrated solutions [45].The products collected were not only nanofibers but also some spheres with big defects (shown in Fig.S2a Supporting Information).However,when the molecular weight was larger than 700,000,the spinning solution viscosity was so high that the solution was difficult to be ejected smoothly.Therefore,a high voltage (30 kV) was required to electrospun the solution.Meanwhile,the surface of the nanofibrous membrane collected were uneven and the number of fibers obviously declined (Fig.S2b,Supporting Information).The effect of the concentration was mainly related to viscosity.The higher concentration resulted in higher viscosity [46,47] (Table S1).When the mass fraction of PVDF to DMF was 15,the continuous nanofibers without defects can be obtained (shown in Figs.2a and S3).Interestingly,the nanofibers obtained had a rough fiber surface which increased the specific surface area,making it possible to increase the probability of the collision between oil droplets.

Table 1Physical properties of oils

The electric field strength is the most significant parameters in the electrospinning process.A certain electric field strength was needed to ensure the ejection of the spinning solution and the formation of the nanofibers.As shown in Fig.S4,when the electric field strength increased from 0.58 kV.cm-1to 1.25 kV.cm-1,the number of PVDF nanofibers gradually increased and the surface of PVDF nanofibrous membranes became uniform.However,when the electric field strength changed from 1.25 kV.cm-1to 2.1 kV.cm-1,the number of nanofibers gradually decreased and the diameters of nanofibers became larger.When the distance between the needle tip and the collector was smaller than 8 cm,the jet flying speed was so fast that the solvent had no time to evaporate completely.The nanofibers were easy to breakdown and the morphologies of nanofibrous membranes had obvious defects in Fig.S5a.The nanofibers flied uncontrollably and the collection efficiency of nanofibers was very low when the distance was larger than 15 cm (Fig.S5b).

Herein,the optimal conditions for electrospun PVDF nanofibers were determined.DMF was selected as the solvent and the concentration of the PVDF (Mw=700,000) was 0.15.The distance between the needle tip and the collector was 12 cm,the voltage 15 kV.As shown in Fig.2b,the evenly distributed nanofibers without defects were obtained under the above these conditions,and there were rough structures on each nanofiber surface.The diameter of PVDF nanofibers was between 0.2 μm to 1um with the mean diameter at 0.4 μm as shown in Fig.2c.

The cross-section image of the nanofibrous membrane was shown in Figs.2d and S6.The nanofibers formed a thin film on top of the nonwoven fabric and the two layers were well bonded.As some nanofibers settled into the void space of the substrate.From the cross-section images,it can be seen that the thickness of the nanofibrous layer clearly increased with the electrospun time increasing.

Fig.2.(a)SEM image of the 15%(mass)PVDF nanofibrous membranes with 700,000 of molecular weights in PVDF;(b)SEM image of the rough surface of PVDF nanofibers;(c)The distribution frequency of the nanofiber diameter under the optimal conditions;(d) Cross-sectional image of the PVDF nanofibrous membrane.

3.2.Properties of PVDF nanofibrous membranes

The pore sizes and their distribution of the membrane were crucial to coalescence separation of oil-in-water emulsions.In general,decreasing the pore size could greatly increase the probability of droplet capture by interception,which finally led to an increase of separation efficiency.By adjusting the electrospinning time,the pore sizes and their distribution of the electrospun nanofibrous membranes were well controlled and shown in Fig.3a.As the electrospinning time increased from 15 min to 90 min,the average pore diameter of the PVDF nanofibrous membranes gradually decreased from about 82 μm to 35 μm and the pore size became more uniform.Unfortunately,when the spinning time was over 120 min,the nanofibers layer was peeled off from the substrate.Thus,the electrospun time was fixed as 90 minutes.Generally,the oil droplet size in emulsified oily wastewater was smaller than 10 μm,therefore,the pore size of the 90 min-membrane was still relatively larger than the oil droplets,which may cause low coalescence separation efficiency.In order to increase the coalescence separation efficiency,a series of coalescence units were prepared by stacking the layered nanofibrous membrane.The pore sizes and their distributions of the units were measured and shown in Fig.3b.With the layer of membrane increased from 1 to 5,the pore size of the coalescing unit decreased from 35 μm to 3.2 μm.The small pore size of the units made them more suitable for coalescence separation.Hence,the coalescence units with different layers were used to deal with the emulsions later and their coalescence separation performance was explored.

Fig.3.(a)Pore sizes and their distribution of PVDF nanofibrous membranes with different electrospun times.(b)Pore sizes and their distribution of the coalesce units with different layers of PVDF nanofibrous membranes.

The affinity of the material to oil and water was crucial in the process of the coalescence separation [48].The coalescence materials should have the lipophilic property to remove oil from emulsified water.To investigate the wettability of PVDF nanofibrous membranes to oil and water,the contact angle of water/oil in air and oil under the water were measured and shown in Fig.4.It can be seen that the water contact angle of the nanofibrous membranes was 130°.When the small oil droplet (hexadecane) of 2 μl dripped on the surface of the PVDF nanofibrous membrane,it spread out immediately on surface of the membrane.The oil contact angle in air was 0°,indicating the whole nanofibers had the lipophilicity.However,the oil contact angle was increased to 65.7° under the water,which was favorable to separating oil-inwater emulsion during the coalescence separation process [15].The poor lipophilicity of the nanofibrous membrane under water made the grown-up oil droplets detaching from the fibers surface more easily after coalescing.

Fig.4.(a) Photograph of the water droplet in air on the membranes.(b–c) Photograph of the oil (hexadecane) droplet in air and under water on the membrane.

3.3.Separation performances of PVDF nanofibrous membranes

As shown above,the prepared nanofibrous composite membranes had different affinities for oil and water.When the oil-inwater emulsion passed through the membrane material,the oil droplets were coalesced and grew up.Subsequently,the oil separated from water due to the different densities of oil and water.In order to study the coalescence separation performance of PVDF nanofibrous membranes,hexadecane-in-water emulsions with different concentrations were prepared.The particle size of oil droplets of the obtained emulsion ranged from 2 μm to 4 μm,as shown in Fig.5.Next,the coalescence performances of the nanofibrous membranes to separate oil-in-water emulsion were studied.The coalescence separation efficiency of one layer of the nanofibrous membrane was only 73.0%.The coalescence performance was not very ideal because the pore diameter of the PVDF nanofibrous membrane was too larger than the size of oil droplets.The possibility of collision and coalescence of the oil droplets on the material was very low.In order to improve the coalescence separation efficiency,a series of coalescing unit were prepared by stacking the nanofibrous membranes layer by layer.As shown in Fig.6,the coalescence separation efficiency increased from 73.0% to 99.5% as the number of layers increasing from 1 to 4.With the increasing of the layers,the pore size of the coalescing unit gradually decreased,and emulsified oil droplets became more likely to be intercepted and collided,which was beneficial to the coalescence separation process.The treated solution became clear and transparent and no oil droplets was observed in the optical microscopic image,which was shown in Fig.5.As presented in Fig.7,the surface morphology of PVDF nanofibrous membrane before and after separation was almost unchanged and still remained the surface roughness.However,the coalescence separation efficiency decreased to 95.8% as the number of layers was further increased to 5.The pore size of the 5-layer unit was very small and close to the size of oil droplets,which led to a significant increase in resistance.And a phenomenon was found that the separator with 5-layer coalescing unit happened to side leakages during the coalescence separation experiment.The separation mechanism changed coalescence to screening with the pore size of the unit decreasing.In summary,the 4-layer coalescing unit exhibited the highest coalescence separation efficiency and was selected to treat the emulsions in the following experiments.

Fig.5.Optical microscopy images and photographs of the emulsions before and after separation and droplets distribution frequency in the hexadecane-in-water emulsion.

Fig.6.Separation efficiency of the coalescing unit with different layers.

Fig.7.SEM images of nanofibrous membranes before and after separation.

The initial concentration of the emulsion was one of the most important factors influencing coalescence efficiency.When the initial emulsion concentration was in the range from 500 mg.L-1to 2000 mg.L-1,the coalescence separation performance was investigated and shown in Fig.8.The coalescing unit exhibited an excellent separation efficiency within the test range.When the emulsion concentration was less than 1000 mg.L-1,the coalescing unit with 4-layer nanofibrous membranes had excellent coalescence performances with the efficiency of above 99.0%.However,when the emulsion concentration was higher than 1000 mg.L-1,the coalescence separation efficiency decreased slightly.This phenomenon can be explained by the excessive dose of small emulsified oil droplets,which passed through the unit before coalescing to large ones.

Fig.8.Separation efficiency of the 4-layer nanofibrous membranes unit for different concentrations of the emulsion.

The separation coalescence performance of the PVDF nanofibrous membrane for various oil-in-water emulsions was evaluated.In order to further evaluate the applicability of the nanofibrous coalescing unit,four kinds of oils with different viscosities and surface tensions were chosen to prepare emulsions.The properties of hexadecane,octane,diesel oil and rapeseed oil were listed in Table 1.The separation efficiencies of the coalescing unit for the four oil–water emulsion systems were tested.As shown in Fig.9,the coalescing unit had good separation efficiencies above 95.0%except the rapeseed oil-in-water emulsion.The viscosity of rapeseed oil was relatively large resulting the difficulty of the oil droplet colliding and coalescing on the nanofibrous membrane.However,the coalescence separation efficiency of the unit for rapeseed oil-in-water emulsion still kept above 92.0%.In general,the nanofibrous membranes unit had the ability to treat different oilin-water emulsions.

From the above results,compared to the PET substrate,the presence of nanofibers greatly improved the coalescence efficiency.In order to explain the specific mechanism of nanofibrous membranes,a schematic diagram was drawn to demonstrate the separation process.As shown in Fig.10,when using PVDF nanofibers with rough surface had large specific surface area,high porosity and mean pore size as coalescing material,the probability of emulsified oil droplets to be intercepted and coalesced on the membrane unit greatly increased.When the emulsions passed through the coalescing unit,the small oil droplets were intercepted by the sub-micrometer skin layer of PVDF nanofibrous membrane due to the affinity of the nanofibers to the oil droplets.Then,the small oil droplets collided on the rough surface of the nanofibers and quickly adhered then the small oil droplets spread on surfaces.After that,the oil droplets grew up to large ones and detached from the surface of nanofibers along with the flow.Finally,oil and water were separated by the difference of densities.

Fig.9.Separation efficiency of the unit for different types of oils.

Fig.10.Schematic diagram illustrating the separation mechanism of the oil-inwater emulsion.

4.Conclusions

Nano-sized PVDF fibers were obtained by electrospinning technology and used as coalescing materials.The optimal conditions to prepare PVDF nanofibers with rough surface and uniform size were determined by the solution properties and process parameters.The reduction in fibers diameter and rough structure of the fiber surface greatly increased the surface area of fibers,which the coalescence separation performance was improved.By using the coalescing unit to deal with oil-in-water emulsions,the coalescence separation efficiency of the nanofibrous membranes reached 99.5%.When the emulsion concentration was up to 2000 mg.L-1,the coalescence separation efficiency still remained higher than 98.0%.It was worth noting that the coalescing unit had a very good separation performance for different types of emulsions,indicating that it had the wide applicability to dispose different oily wastewaters.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (No.2017YFB0308000),the National Natural Science Foundation of China (No.21706259),the State Key Laboratory of Heavy Oil Processing(SKLOP201903001),Guizhou Science Technology Support Program([2019]2839)and the Natural Science Youth Foundation of Shanxi Province (No.201701D221033) and Program of Innovation Academy for Green Manufacture,CAS (IAGM2020C04).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.08.037.