PRIOR APPLICATION INFORMATION

The instant application claims the benefit of US Provisional Patent Application63/069,859, filed August 25, 2020 and entitled “Dual-functional superwettable nano-structured membrane”, the entire contents of which are incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Rapid industrialization has given rise to a severe deterioration in water quality (1 ). Given that 70% of drinking water comes from rivers and streams, the industrial contamination of these resources can bring about fatal damage to human health (2). The wastewater emitted from industrial processes, partially comprised of immiscible organic solvents/water mixture, has tremendously increased the difficulty of water treatment (3). Therefore, developing an effective water purification process has drawn academic interest and industrial effort to address the issue of clean water crisis.

As one of the solutions, much effort has been devoted to oil/water separation of wastewater discharged from manufacturing processes (4-10). More recently, membranes synthesized to treat immiscible oily emulsions have been extensively studied in oil-water separation techniques (11 -14). However, fouling of membranes caused by irreversible clog of oil droplets in the porous structure has greatly impeded the development of high-flux emulsion separation. To tackle this issue, many studies utilized the synergistic effect of surface morphology and surface chemistry to manipulate wetting behavior of the membrane, thus achieving highly selective oil repellence while allowing easy water permeation. By decorating a multi-scale and hydrophilic layer onto a permeable substrate, many superhydrophilic membranes were successfully constructed with improved anti-fouling performance because of ultralow oil adhesion on the membrane surface (15-18). Atomic layer deposition as one of the surface modification techniques generally enhances the superhydrophilicity by decorating a layer of inorganic nanoparticles onto a surface (19, 20). However, to achieve an homogeneous layer without defects, complicated and strict experimental conditions such as low pressure and high temperature are usually required (21 ). Besides, the use of inorganic compounds increases the potential risk of secondary contamination and threatens the environment and human health (22).

Chemical grafting is another surface modification technique that tailors the surface energy of the membrane by grafting functional groups with special affinity towards water or oil (15,23,24). However, membranes fabricated by this method may suffer from lack of stability under harsh environments containing acid, alkaline or mineral compounds. Once the grafted outer layer is damaged, the substrate material is prone to severe oil fouling.

Biomimetic mineralization is another method to enhance the hydrophilicity property of the membrane, involving construction of an oil-repellent barrier on the surface. By depositing mineral compounds on the polymer substrates, the surface chemistry and architecture are modified to achieve special wettability (25, 26). However, decorating minerals also reduces the pore size of the membrane, resulting in resistance during emulsion separation and consequently lower flux. Aside from the fouling issue in oil/water separation, the mechanical strength of the membranes must also be improved as the materials used to endow oil repellence are far from robust, i.e. hydrogels (27-29), aerogels (12,30,31 ). Furthermore, materials utilizing rigid substrate such as stainless steel (32,33) lack flexibility and deformability. Therefore, robust, anti-fouling and scalable materials are needed to treat highly emulsified oily industrial wastewater.

Apart from treating contaminated water sources, much research has been devoted to increase freshwater supply to address the freshwater crisis. Solar desalination of seawater is a promising methodology with low environmental impact (34-38) in contrast with traditional seawater desalination methods such as thermal distillation and reverse osmosis membrane filtration which are high energy-demand or low efficiency. Recently, plasmonic nanoparticles for solar steam generation has attracted tremendous attention due to its high photothermal conversion efficiency and localized heating (39-41 ). The prominent localized heating is due to the subsequent nonradiative relaxation of metals, causing absorbed energy to be converted into heat (42). However, noble metals used for plasmon-mediated solar desalination, i.e. platinum and gold, are not cost-effective for large scale application (40,43,44). Other more abundant metals generally have higher plasma frequency, resulting in significant plasmonic resonance for only a limited range of the solar spectrum (45).

In that regard, it is highly sought-after to develop a facile method that is simultaneously capable of both efficient immiscible oily wastewater separation with anti-fouling property and large-scale solar steam generation to reduce unnecessary water transportation and energy waste.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method of preparing a water purification membrane comprising: electrospinning a quantity of polymers into nanofibers, said polymers selected from the group consisting of: polyvinyl alcohol (PVA); poly(N-isopropylacrylamide) (PNIPAM); poly(vinylidene fluoride) (PVDF); poly(methacrylic acid) (PMAA); and poly(acrylic acid) (PAA), forming a membrane from the nanofibers by crosslinking the nanofibers; depositing nanoparticles on the membrane by oxidation of pyrrole monomers; and washing the membrane.

According to another aspect of the invention, there is provided a water purification and/or desalination membrane prepared according to the method described above.

According to another aspect of the invention, there is provided a method of purifying contaminated water comprising: flowing the contaminated water through a membrane as described above, said membrane allowing water to flow therethrough, thereby purifying the water.

According to another aspect of the invention, there is provided a method of desalinating water comprising: flowing salt water through a membrane as described above while exposed to light of at least one solar wavelength, said membrane allowing water to flow therethrough and converting the water to steam and condensing the steam, thereby recovering desalinated water. According to another aspect of the invention, there is provided a method of purifying and desalinating contaminated salt water comprising: flowing the contaminated water through a membrane as described above while exposed to light of at least one solar wavelength, said membrane allowing water to flow therethrough, thereby purifying the water; said membrane converting the water to steam and condensing the steam, thereby recovering desalinated water.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. (a) Schematic Illustration of the preparation process of the NPM membrane and the dual functions of the membrane, (b) the on-site set-up illustration for seawater desalination.

Figure 2. Surface morphological changes of the electrospun PVA membrane (a-c) before and (d-f) after coated with PPy nanoparticles, (g) Tensile stress versus strain curves of the crosslinked PVA membranes coated with PPy nanoparticles in situ polymerized for different periods of time, (h) Underwater chloroform contact angle and water contact angle of PVA without PPy coating(NPMI ) or treated with PPy for 5 hours (NPM2), 1 day (NPM3), 2 days (NPM4) and 3 days (NPM5). (i) FT-IR spectrum of as-electrospun NPM1 (red), PPy particles (blue) and NPM5 (black).

Figure 3. SEM images of a) crosslinked electrospun PVA membrane (NPM1 ) and NPMs immersed in pyrrole and Fe(NO ₃ ) ₃ for b) 5 hours, c) 1 day, d) 2 days and e) 3 days. As the in-situ synthesis of pyrrole proceeds, an elastic pyrrole network formed onto the surface of the PVA nanofibers membrane.

Figure 4. Snap shots of (a) water droplets and (b) chloroform droplets spreading on superhydrophilic and underwater superoleophobic NPM5 surfaces, (c) The underwater sliding angle of dichloromethane of NPM 5. d) Droplets of dyed organic solvents on the top of NPM underwater, (e) Underwater Oil Contact Angles and (f) Underwater sliding Angles of the NPM against different oily droplets.

Figure 5. (a) Oil/sea water emulsion separation installation, petroleum ether was selected as the non-soluble organic solvent sample and was tinted with pink pigment, (b) Oil-in-water emulsion was prepared with 10% petroleum ether in distilled water sonicated for 30 min. (d) The filtrated solution. (c,e) Optical microscopy of emulsions before (c) and after (e) separation, (f) The separation flux in response to increasing NPM thicknesses, (g) Particle size distribution of the petroleum ether-in- water microscale mixture.

Figure 6. (a) The separation flux of a sequence of surfactant-free and surfactant-stabilized oil-in-seawater emulsions penetrating through the membrane. The surfactant-stabilized emulsion was prepared by adding 0.01 % SDS into the 10% oil/water mixture followed with sonication for 30 minutes, (b) Performance of flux and separation efficiency in cyclic test using NPM5. In each cycle, emulsion was prepared by sonication of 5 ml hexane in 45 ml water for 30 minutes. NPM5 was washed with ethanol and dried in air after each cycle, (c) The underwater oil contact angles of the NPM5 in response to solution pH ranging from 1 to 13. (d) Separation efficiencies of NPM5 of a series of acidic/alkaline oil-in-water emulsions.

Figure 7. Distribution of oil drops with different diameter ranges with and without emulsifier stabilization, a) DLS results of surfactant-free hexane-in-water emulsion and b) SDS stabilized hexane-in-water emulsion, c) DLS results of surfactant-free petroleum ether-in-water emulsion and d) SDS stabilized petroleum ether-in-water emulsion. Images of the surfactant stabilized 10% oil-in-water emulsions on e) day 1 and f) day 15.

Figure 8. Anti-fouling and resilience characteristics of the NPM. (a) Selfcleaning test of the NPM1 and NPM5. (b) Cyclic surfactant free mechanical pump oil- in-water emulsion separation test for NPM5 and NPM1. (c) Cyclic SDS stabilized mechanical pump oil-in-water emulsions separation, (d) Cyclic humic acid (HA) mixed SDS stabilized mechanical pump oil-in-water emulsion separation and (e) bovine serum albumin (BSA) mixed SDS stabilized mechanical pump oil-in-water emulsion separation.

Figure 9. (a) Schematic set up for solar-vapor desalination unit used in the experiment, (b) the solar vapor radiation installation setup and the NPM floating on water in a beaker, (c) Mass change over 60 min with and without the NPM5 under 0.5 and 1 solar flux, (d) The comparison of evaporation rates of NPM1 , NPM2, NPM3, NPM4, NPM5 and pure water on Copt from 0.1 to 1 (with dark evaporation subtracted), (e) Solar vapor efficiencies of NPM1 , NPM2, NPM3, NPM4, NPM5 for different values of Copt, (f) The summary of energy efficiencies and evaporation rates of NPM with various degree of PPy coating.

Figure 10. (a) UV-vis NIR spectra comparison of NPM samples with 200 pm thickness, (b) Swelling ratios of NPM1 , NPM3 and NPM5 under a 15 min interval, (c) IR images indicating the thermal dispersion of the NPM5 and water placed above the water surface with 1 kW m ^-2 solar flux irradiance at different time points.

Figure 11. (a) The salinity test result using NPM5 with salinity result before and after desalination. The blue and red lines refer to the World Health Organization (WHO) and Health Canada (HC) salinity standards for drinkable water, (b) Calculated concentrations of four major ions in an artificial seawater sample before and after desalination, (c) The evaporation rates of the recycled rinsed NPM under solar illumination of 1 kW m ^-2 . The blue bar indicates the evaporation rates range between 2.72 and 2.91 kg m ^-2 h ^-1 . (d) The evaporation rate and energy efficiencies of NPM5 under various seawater conditions with 1 sun illumination.

Figure 12. (a) A prototype NPM5 in direct contact with the seawater and (b) a prototype of NPM5 folded as a cone floating on the surface of seawater, (c) COMSOL model of calculated device with 2D indirect contact and (d) 3D artificial transpiration, (e) On-site dual-functional water purification system applying NPM for seawater desalination.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned hereunder are incorporated herein by reference.

Freshwater contamination and scarcity have progressively exacerbated the worldwide water crisis. Developing a feasible method to remediate polluted water as well as converting the abundant seawater to freshwater has become a matter of urgency.

Herein, we demonstrate a hierarchical nano-structured water purification membrane fabricated with polyvinyl alcohol (PVA) nanofibers and polypyrrole (PPy) nanoparticles to achieve efficient emulsion separation with low oil-adhesion property as well as high-rate seawater desalination (Figure 1 ). Graphene oxide (GO), as one of the most important derivatives of graphene, is valued for its non-toxicity, excellent dispersity and has long been used as reinforcement elements to strengthen the material. In some embodiments, 0.2% graphene oxide was mixed into the PVA electrospinning solution as a reinforcement to strengthen the material. The NPM exhibits the integrated properties of mechanical robustness, superhydrophilic/underwater superoleophobic wettability and broadband solar absorption.

The crosslinked electrospun PVA nanofibrous mat acts as a porous skeleton with fine flexibility and internal gaps, leading to a high permeate flux during oil/water separation.

The PPy nanoparticles are deposited on the surface of the PVA nanofibers in a close-packed fashion, giving the membrane a rough surface with superhydrophilicity and underwater superoleophobicity (158°C) and an extremely low oil-adhesion property. The ultra-porous structure with superwettability provides the membrane with the separation ability of surfactant-stabilized immiscible mixtures, with high separation efficiency (oil residue in filtrate after one-time separation lower than 0.01 wt %) and high flux. Unlike many membranes using oleophilic polymer (46, 47) as the substrate, PVA is less prone to oil fouling due to its intrinsic hydrophilicity. The PPy coated on the surface of PVA further creates a hierarchical structure and superhydrophilic chemistry that act as an oil-repellent barrier, thereby enhancing the anti-fouling performance. The NPM exhibited a 98% flux recovery ratio after continuous cyclic surfactant-stabilized oil-in-water emulsion separation. In addition, solar steam generation was realized by placing an NPM on the seawater-air interface with solar irradiation. Owing to the high area-to-volume ratio of nanofibers and the high energy conversion efficiency of PPy, the densely packed PPy nanoparticles on the PVA fibers can quickly harvest the solar light and convert it into thermal energy. Furthermore, the hydrophilic nature of PVA helps efficiently transport water due to capillary effects, sustaining a continuous water supply for steam generation. The efficient solar energy absorption was verified by an over 99% light absorption of the material within a broadband wavelength (250 to 1100 nm). A water evaporation rate of 2.87 kg m ^-2 h ^-1 was realized with one sun irradiation. To verify the practicality of the membrane, a solar steam generator was installed and utilized to collect purified water under natural sunlight and a freshwater collection capability of solar water purification yield of 14.3 I rrr ² daily. The oil-water separation and desalination performance of the NPM is compared with other related materials; the dual-functional NPM not only achieved a remarkably separation efficiency (>99.99%) but also demonstrated an excellent desalination performance (with the evaporation rate of 2.87 kg m ^-2 h ^-1 ). The dualfunctional superwetting membrane represents a new approach to more effective purification of clean, safe drinkable water from any source, whether from the ocean or contaminated industrial supplies.

In one embodiment of the invention, there is provided a method of preparing a water purification membrane comprising: electrospinning a quantity of polymers into nanofibers, said polymers selected from the group consisting of: polyvinyl alcohol (PVA); poly(N-isopropylacrylamide) (PNIPAM); poly(vinylidene fluoride) (PVDF); poly(methacrylic acid) (PMAA); and poly(acrylic acid) (PAA), forming a membrane from the nanofibers by crosslinking the nanofibers; depositing nanoparticles on the membrane by oxidation of pyrrole monomers; and washing the membrane.

As discussed herein, the oxidation of pyrrole monomers is carried out slowly, for example, by carrying the reaction out at a low temperature. It is of note that suitable low temperatures will be readily apparent to one of skill in the art. In one exemplary example, the low temperature may be about 4C.

In some embodiments, the oxidation of pyrrole monomers takes place in the presence of Fe ions. Preferably, the Fe ions are Fe (III) ions, for example, from FeCl ₃ or Fe(NO ₃ ) ₃ .

The membrane may have a thickness of at least 0.2 mm.

The crosslinking agent may be glutaraldehyde.

In some embodiments, prior to crosslinking, the nanofibers may be precrosslinked by exposure to a vaporous crosslinking agent, such as, for example, but by no means limited to glutaraldehyde.

According to another aspect of the invention, there is provided a method of purifying contaminated water comprising: flowing the contaminated water through a membrane prepared as described above, said membrane allowing water to flow therethrough, thereby purifying the water.

The contaminated water may be for example wastewater, salt water, contaminated water from an industrial process or contaminated water from an industrial accident.

According to another aspect of the invention, there is provided a method of desalinating water comprising: flowing salt water through a membrane as described herein while exposed to light of at least one solar wavelength, said membrane allowing water to flow therethrough and converting the water to steam and condensing the steam, thereby recovering desalinated water.

The light of at least one solar wavelength may be for example sunlight or any suitable wavelength capable of sufficient absorption by the membrane so as to sufficiently heat the membrane, as discussed herein.

According to another aspect of the invention, there is provided a method of purifying and desalinating contaminated salt water comprising: flowing the contaminated water through a membrane of claim 13 while exposed to light of at least one solar wavelength, said membrane allowing water to flow therethrough, thereby purifying the water; said membrane converting the water to steam and condensing the steam, thereby recovering desalinated water.

It is of note that “purification” of water, in all its grammatical forms, does not require absolute purity but only that the purity of the sample of water have been improved by the removal of a quantity of a contaminant. It is further noted that methods for determining the purity of water are well-known in the art and may be used for determining the purity of a sample of water both prior to and after exposure to the membrane of the invention.

As will be appreciated by one of skill in the art and as discussed herein, ahigh oil contact angle is an important property because an oil contact angle higher than 150 degrees indicates oleophobicity of a material, which is why the membrane can repel oil droplets.

Similarly, low sliding angles are a crucial property indicative of low oil adhesion, which provides the membrane with anti-fouling properties, meaning that the membrane is suitable for repeated use and long-term separation.

Finally, high flux is also a desirable property as this permits fast separation, as discussed herein.

As discussed herein, the membrane of the invention can absorb a large range of wavelengths from sunlight, which provides the membrane with high sunlight utilization efficiency. Furthermore, the membranes have low calculated equivalent enthalpy, which indicates that the membranes have high solar steam efficiency, as discussed herein.

As discussed herein, higher GO content makes the membrane stronger but also more brittle. Accordingly, as discussed herein, suitable amounts of graphene oxide are added to the membrane depending on the intended use of the membrane. For example, If the membrane is in a fixed position, then flexibility of the membrane is not required. In these cases, GO content can go beyond 2%. As will be appreciated by one of skill in the art, a membrane with GO has the same functionality as a membrane without GO, but the addition of GO strengthens the membrane.

As demonstrated below, the reaction time is long enough so that all incorporated pyrrole monomer polymerize. As will be appreciated by one of skill in the art, the percentage of the PPy in the membrane is exclusively dependent on the pyrrole monomer added in the reaction. Furthermore, as discussed herein, PPy has a similar effect on the membrane as GO, specifically, that more PPy means higher brittleness. In the examples, a moderate amount of PPy (10%) was used and found to maintain flexibility and strength of the membrane. However, as discussed herein, different percentages may be added, depending on the intended end use and the desired characteristics of the membrane.

Specifically, as shown in Figure 3, higher concentrations of PPv give the membrane a rougher surface, which increases and/or improves anti-fouling properties of the membrane and increase oleophobicity of the membrane, as discussed herein.

Furthermore, PPy is black, which is ideal for light absorption. As discussed herein, shorter reaction times resulted in lower incorporation levels which in turn produced a membrane that is lighter in color (deep green) compared to a membrane with higher PPv incorporation. Membranes with relatively lower incorporation have lower light absorption abilities, which, as discussed herein, is important at least for desalination.

Membrane characterization

To prepare the NPM (Figure 1 ), the electrospun PVA membrane was first precrosslinked with glutaraldehyde vapor rather than solution, because the as- electrospun PVA nanofibers are dissolvable in glutaraldehyde water solution. After 24 hours of vapor treatment, the pre-crosslinked PVA membrane was processed with liquid phase crosslinking in glutaraldehyde acetone solution to further crosslink the PVA nanofibers. Until now, the PVA membrane was fully crosslinked and capable of keeping the integrity under water (referred as NPM1 ). Afterward, the membrane was dipped in a mixture of Fe ³⁺ and pyrrole monomer solution. With the process of the slow oxidation of the pyrrole monomers under 4°C, the scale of the PPy nanoparticle layer on the surface was controlled to be much smaller than the PVA nanofibers to construct a hierarchical nanostructure. As will be apparent to one of skill in the art, this reaction is temperature sensitive. Specifically, higher temperatures will overly accelerate the reaction and will cause the polymerized pyrrole agglomerate deposit as clusters on the membrane. If the reaction is carried out at a temperature below room temperature, for example, about 4C, the nanoparticles are approximately evenly deposited on the membrane which in turn gives rise to a nano-hierarchical structure. When the membrane is coated in this manner, PPy nanoparticles significantly roughen the surface and provide the membrane with superhydrophilicity/superolephobicity, which plays a critical role in the separation of surfactant-stabilized immiscible oil/water mixture as well as fouling-resistant properties, as discussed herein. As can be seen from Figure 2b, the entangled PVA fibers form a porous interconnected 3D network with irregular interstice. After the deposition of in situ synthesized PPy on the PVA nanofiber skeleton, the membrane turned pitch black (Figure 2d) and clusters of particles were observed in between the internal gaps (Figure 2e). The enlarged SEM illustration of the membrane surface (Figure 2f) shows that the added PPy roughened the surface of PVA nanofibers by forming tightly packed nanoscale particles, which is different from the smooth as- electrospun PVA fibers without PPy deposition (Figure 2c). These finer particles are randomly deposited on the surface of nanofibers while closely attached with each other, as a result forming a fairly coarse morphology, which explains the underwater superoleophobicity according to the Cassie’s model in Young’s equation (48). The micro- and nano-scale PPy particles play a critical role as light absorber with low reflectance because of the dark appearance of PPy and this effect of PPy will be discussed later.

Given that the membrane is designed to endure repeated oil/water separation and seawater desalination, the durability of the membrane under aqueous conditions is a crucial factor to be considered. The mechanical properties of the membrane were modified by adding GO during electrospinning based on GO’S high Young’s modulus and strong mechanical properties. The ratio of GO and PVA was investigated to understand its effect on the mechanical properties of the membrane. The mechanical properties of the NPM can be adjusted by varying the ratio of reinforcement (GO) and matrix (PVA). As predicted, by changing the ratio of GO to PVA from 0 to 0.3% led to a decrease of elongation from 60.07% to 17.14% and an increase of modulus from 10.42 MPa to 114.36 MPa. With a decrease of the GO/PVA ratio, the tensile strength of the nanofibers initially increased. At the 0.3% ratio of GO/PVA, the nanofiber membranes exhibited the ultimate tensile strength of 6.08 MPa and the strain at break of 17.14%. However, the high ratio of GO/PVA results in brittleness in mechanical property, which is undesirable in the separation. Given that PPy will further strengthen the material, 0.2% GO/PVA was chosen as an optimal amount which possesses high strength but also a reasonable flexibility. As discussed herein, the strength and flexibility characteristics can be varied according to the intended end use.

To clarify the influence of PPy on mechanical properties and wetting behavior of the membrane, a tensile test and an underwater contact angle test were carried out with various PPy contents. Figure 2g shows typical stress-strain curve of the NPM with different extent of PPy coating. The mechanical property of NPM is tunable by changing the proportion of rigid constituent (PPy) and flexible constituent (PVA).

The crosslinked PVA membranes (NPM1 ) are immersed in pyrrole monomer solution for 5 hours (NPM2), 1 day (NPM3), 2 days (NPM4) and 3 days (NPM5), respectively. As expected, with the PPy in situ polymerization proceeding, more PPy particles were aggregated on PVA nanofibers (Figure 3), resulting in an elongation decrease from 33.75% to 8.75% and a modulus increase from 41.51 MPa to 159.37 MPa. The maximum strength of NPM achieved was 7.62 MPa after 2 days of PPy coating. With the in situ synthesis of PPy reacting for 3 days, the NPM5 shows the highest strength at break of 9.69 MPa and a Young's modulus of 159.37 MPa with an elongation of 8.75%, comparing to the NPM1 with lower modulus (10.42 MPa) and higher flexibility (fracture strain at 33.75%). A hypothesis was raised that the surface wettability is directly influenced by the amount of PPy coated onto PVA nanofibers. Therefore, the effect of PPy on the wetting behavior of membranes was also investigated. The results demonstrate that the roughness of the surface of the NPM, tightly related to the deposition of PPy nanoparticles on the PVA nanofibers, has great impact on the water or oil wettability of the material. As can be observed in Figure 2h, NPM1 exhibits a water contact angle of 13° and an underwater oil contact angle of 123°. After immersing of the membrane in PPy solution for 5h, the water contact angle reduces to 0° and the underwater oil contact angle increases to 132°. With continuous immersion for 2 days, more PPy nanoparticles are synthesized and deposited onto the surface of the PVA nanofibers, leading to higher hydrophilicity and oleophobicity with an underwater oil contact angle of 148° after 1 day. After 2 days, the water contact angle stays at 0° while the underwater oil contact angle increases to approximately 152°.

Once the membrane being immersed in PPy for 3 days, water contact angle still remains a 0° and an average underwater oil contact angle of 158° is observed, demonstrating superhydrophilicity and underwater superoleophobicity. This phenomenon is in accordance with the hypothesis that the deviation of wettability is closely associated with the amount of PPy nanoparticles packed on the PVA nanofibers surfaces. The result can be explained by Wenzel’s model (49), where increased ratio of actual to geometric surface area results in higher roughness factor, which brings about higher affinity toward oil or water. This suggests that for a particle- composed surface, more nanoparticles bring about a greater coarseness, thereby giving the membrane a superhydrophilic wettability. The detailed mechanism will be presented in the following discussion.

To verify whether PPy nanoparticles are successfully deposited onto the PVA fibers, the FT-IR spectrum of the NPM was analyzed (Figure 2i). As can be observed in the PVA spectrum (red line in Figure 2i), the broad band of PVA (3550 - 3200cm ^-1 ) is attributed to O-H stretching from the hydrogen bond. The presence of alkyl groups in PVA results in vibrational band between 3000 and 2850 cm ^-1 . The characteristic peak around 1732 cm ^-1 is attributed to the C=O and C-0 stretches in the acetate groups (50). As can be seen from the PPy spectrum (Figure 2i, blue), absorption at 1 ,552 cm ^-1 is attributed to the in-ring stretching of C=C bonds in the pyrrole ring (51 ), while the absorption at 1 ,315 cm ^-1 is associated with in-plane vibration of =C-H bonds (52). As can be seen in the black curve in Figure 2i, most of the characteristic absorptions of PPy and PVA described above were observed in the FT-IR spectrum of NPM5 without shifts, therefore confirming the existence of PPy and PVA in the membrane.

Aside from oil, the wetting behaviors of the membrane with several non-polar organic solvents were also investigated. In under water contact angle test (Figure 4a), the process of the water droplet being completely soaked by the membrane took less than 0.5s. Whereas the membrane resisted the adhesion of organic solvents even after repeated contact (Figure 4b), indicating the low adhesion force between NPM and organic solvents, this can be explained by its rough nano-structured surface that allows for the approaching oils to be repelled by the soaked water. The underwater oil contact angles of canola oil and several versatile organic solvents, including hexane, toluene, and petroleum ether are all above 154°C (Figure 4e). Wettability test of the membrane using non-soluble organic solvents with densities higher than water (i.e. chloroform, dichloromethane and carbon disulfide), excellent underwater superoleophobicity were also carried out, with underwater oil contact angle of over 156°. The stability of this wetting behavior was tested by immersing NPM5 in water for 30 consecutive days. The membrane maintained its underwater superoleophobic property, indicating exceptional durability in an aqueous environment. The underwater superoleophobicity also indicates that the NPM possesses anti-fouling propensity due to its superior surface hydration and low adhesion force towards oil.

To further investigate the oil-repellent property of NPM, sliding angle tests were carried out. It can be observed from Figure 4c that the dichloromethane droplet easily rolled off the surface of NPM5 with a sliding angle of ~1 °. In contrast, the dynamic sliding angle of dichloromethane on NPM1 is ~7° due to the lack of PPy nanoparticles on the PVA nanofibers. The hydrophilicity of NPM was enhanced by the presence of PPy nanoparticles due to the synergistic effect of surface energy and the nanoscale topography. The dynamic sliding angles of various organic solvents on NPM5 were comprehensively studied, as shown in Figure 4f. The sliding angles range from 1 ° (chloroform and dichloromethane) to 3° (canola oil), showing outstanding underwater superoleophobicity.

The separation efficiencies of the as-described NPM during oil/water separation process were tested via a series of experiments presented in Figure 5. To demonstrate the separation efficiency of the membrane, six types of insoluble organic solvents were chosen based on their massive use in manufacturing: petroleum ether, hexane, chloroform, dichloromethane, toluene and carbon disulfide.

A surfactant-free oil/water mixture was prepared by sonicating a 10% petroleum ether-in-water mixture for 10 minutes in a water bath. As shown in Figure 5a, NPM5 with a diameter of 8 mm and a thickness of 200 pm was fixed in between a funnel and an Erlenmeyer flask. The oil-in-water emulsion was transferred into the funnel, which was in direct contact with NPM5. Water instantaneously soaked into the membrane. The oil-water emulsion was demulsified once in contact with the membrane as water permeated through NPM5 and oil remained above. The filtered water was examined (Figure 5d) to study the separation efficiency by counting the oil droplets residue after separation. On observing the collected filtrate using optical microscopy, no visible droplet was observed in images obtained from the same batch (Figure 5e). The diameter distribution of the oil droplets before separation demonstrates that the diameter of oil droplets is most likely to fall into a range of 5-20 pm (Figure 5g). The flux reduced proportionally to the thickness increasing of the membrane. The flux can reach 12740 L m ^-2 h ^-1 when testing a single layer of the NPM, but decreases to 3981 L m ^-2 h ^-1 with a tri-layered membrane (Figure 5f). This test determined that a membrane with a thickness of at least 0.2 mm is enough to make the membrane durable while maintaining a high flux rate. In addition, the separation efficiency and consistency were measured to assess the oil/water separation performance.

Similar phenomenon was observed among other organic solvents/water mixture. After organic solvent/water mixtures were poured into the funnel, water quickly permeated through the membrane, whilst the organic solvents remained above the membrane, showing excellent separation capability of the material among a wide range of immiscible solutions.

Membrane permeability was comprehensively investigated. The fluxes of sundry solvent-in-water emulsions were measured as shown in Figure 6a. As oceanic crude oil spills are one of the most frequently encountered scenarios in industrial oil/water separation, artificial sea water was prepared to simulate real-life separation condition. Among all the organic solvents and oils used, the separation fluxes are all above 2638 L m ^-2 h ^-1 for surfactant free emulsion and 1271 L m ^-2 h ^-1 for surfactant- stabilized emulsion. The flux of hexane-in-water emulsion is the highest (12689 L m ^-2 while the flux of surfactant-stabilized hexane-in-water emulsion is 3962 L m ^-2 h ^-1 . There is a slightly higher difference of fluxes between surfactant-free and surfactant- stabilized emulsion in hexane than other organic solvent. This phenomenon is possibly due to higher impact of SDS (surfactant used in this report) on emulsification of hexane compared to the impact of SDS on the dispersion of other non-soluble organic solvents in water, which results in smaller hexane droplets in the surfactant- stabilized emulsion. With smaller oil droplets, it is harder to demulsify the emulsion at the surface of NPM, hence the lower flux and more significant difference between the surfactant-free and surfactant-stabilized emulsion. To further confirm this assumption, dynamic light scattering (DLS) was carried out to test the droplet size of hexane and other organic solvents-in-water surfactant-stabilized emulsion. Here, we will compare hexane and petroleum ether as an example. As can be seen from the DLS results in Figure 7a-d, the average droplet size of surfactant free hexane-in-water emulsion is 2327 nm while the average droplet size of surfactant stabilized hexane-in-water emulsion is 510.4 nm, which is a 78% reduction in hexane droplet size in the presence of surfactant. Meanwhile, the average droplet size of surfactant free petroleum ether-in-water emulsion is 1873 nm while the average droplet size of surfactant stabilized petroleum ether-in-water emulsion is 713.4 nm, which is a 62% reduction in petroleum ether droplet size in the presence of surfactant. The moderately larger change of hexane micelle size after inducing surfactant is consistent with the bigger difference of fluxes of separating surfactant-free and surfactant-stabilized emulsions. In addition, when comparing surfactant free hexane- in-water emulsion and petroleum ether-in-water emulsion, the larger droplet size of hexane is consistent with the result of slightly higher flux during the separation compared to petroleum ether.

To characterize the consistency of the separation performance, repeated separation was carried out. The membrane exhibited high separation efficiency (>99.99%) in oil-in-water emulsions after 20 separations, indicating the stability of the membrane (Figure 6b). Furthermore, the 0.01 % sodium dodecyl sulfate (SDS) stabilized emulsions are left unshaken for a prolonged period of time. As can be seen from Figure 7e-f, no stratification was observed after 15 days, exhibiting excellent separation efficiency of stabilized emulsions of the membrane. As can be observed from the above separation results, the NPM shows prominent performance in oil or organic solvents/water separation for surfactant-free or surfactant-stabilized oil-in- water emulsion. The membrane can quickly absorb water as soon as it is in contact with the emulsion mixture. Afterwards, the superhydrophilic and underwater superoleophobic properties allow water to instantly permeate through the membrane while the oil phase remains above.

Emulsion separation under harsh environment

Another feature of this invention is the excellent functionality under harsh conditions. The NPM is capable of resisting acidic/alkaline conditions in severe pH environments. Figure 6c shows water contact angles of NPM in artificial sea water with varying pH. When the membrane was immersed in acid/alkaline seawater with pH ranging from 1 to 13, the oil droplets exhibit spherical shape and all the underwater contact angles are above 151°, showing consistent superoleophobic characteristic. The oil-in-water acidic or alkaline emulsion was prepared by mixing 10% of organic solvent and artificial seawater with the addition of HCI or NaOH solution to adjust the pH. The filtration device was the same as described above, with the membrane fixed in between the rims of flask and the funnel. After the equipment was set up, the emulsion was poured into the funnel under 0.2 bar pressure. In this case, hexane-in-water emulsion was selected as an example.

During the separation, the cloudy emulsion transformed into transparent, clear water in the flask, indicating potent separation. The as-prepared NPM can separate stabilized micro and nanoscale oil-in-water emulsions effectively under acid, basic and briny conditions. Owing to the chemical-resistant nature of PVA, the as-described membrane can retain the superhydrophilicity/underwater superoleophobicity property under harsh environments. To gauge the separation efficiency under severe environment, the water purity after separation was measured as can be seen from Figure 6d. The membrane shows extremely high efficiency (>99.6%) under a wide range of pH value, indicating its stability under complex separation environments. The above results indicate that the NPM has the capability of resisting acidic/basic environment under a series of pH conditions.

Anti-fouling capability of the NPM 6

Filtration efficiency in emulsion separation is often impeded by the fouling of oil on the membrane surface or clogging in pores. The oil/organic solvent residue adheres to the membrane and weakens the superoleophobicity of the membrane, affecting the separation efficiency as well as permeation flux. Therefore, low adhesion between oil and the membrane is an important characteristic to remediate oil fouling and ensure highly efficient oil/water emulsion separation (53-55). A surface dynamic adhesion test was carried out to assess the adhesion behavior between oil droplets and the NPM underwater. A dichloromethane droplet was pressed onto a NPM5 and lifted up. During the pressing process, the droplet was deformed into an ellipsoid while during the lifting process, the droplet shape returned to a sphere and no further deformation was observed when the droplet was detached from the membrane surface. Similar phenomenon was observed when highly viscous mechanical pump oil was utilized in the test. The results demonstrate the extremely low adhesion between the oil droplet and the NPM, which is beneficial for relieving oil-fouling during separation. In addition, the NPM5 also possesses easy-cleaning properties. During the self-cleaning experiment, the NPM1 and NPM5 were immersed in mechanical pump oil, followed by shaking in DI water. As can be seen from Figure 8a, oil droplets were easily shed from the NPM5 after shaking in water and the oil droplets can be observed from surface of the water. In contrast, the NPM1 was immersed the mechanical pump oil, and after shaking in water, almost no oil droplet was observed at the top of the water surface, indicating oil fouling on the membrane. The results indicate low adhesion of oil on NPM5 and easy cleaning once fouled with oil.

To test the long-term anti-fouling property of the NPM, a cyclic filtration test was carried out on the NPM with several foulants including surfactant (SDS), Bovine Serum Albumin (BSA) and humic acid (HA). The flux recovery ratio (FRR) was calculated after each cycle of emulsion separation and washing. Mechanical pump oil was utilized in this test in view of its similar composition to crude oil (petroleum distillates) and high viscosity. Firstly, surfactant free mechanical pump oil-in-water emulsion was tested and the relative flux result is shown in Figure 8b. The FRR of NPM5 maintained beyond 98% after 420 min of repeated separation and washing processes, compared to the FRR of 87% of NPM1 . The relative flux of NPM5 dropped to 0.51 after 6 cycles, while the relative flux of NPM1 dropped to 0.37, demonstrating that the lower hydrophilicity of NPM1 due to the lack of PPy nanoparticles coating results in more severe fouling even without the surfactants or other contaminants.

0.1 mg ml ^-1 surfactant (SDS) was mixed into the mechanical pump oil and water and as can be seen from Figure 8c, the FRR of NPM5 remained at 97% while the FRR of NPM1 dropped to 85% after 420 min of cyclic separation and washing procedure. The relative flux of NPM1 decreased from 0.47 after the first cycle of separation to 0.34 after 6 cycles, suggesting more severe fouling during the surfactant stabilized emulsion separation. The results for two other contaminants, HA and BSA, are shown in Figure 8d and Figure 8e, in which 1 mg ml’ ¹ HA and 1 mg ml’ ¹ BSA were added to the surfactant stabilized emulsion, respectively. The FRR of NPM 5 maintained at the level of 95% after cyclic test of both HA mixed emulsion and BSA mixed emulsion. In contrast, the FRR of NPM1 decreased to 71 % for HA mixed emulsion and 73% for BSA mixed emulsion, respectively.

The results confirmed the effect of PPy nanoparticles on the fouling-resistant property of the membrane against heavy oil and multiple contaminants. The NPM5 shows outstanding anti-fouling property as well as longevity for heavy-duty emulsion separation.

Super wettability mechanism

To thoroughly explain the ultrahigh oil-water separation performance, the underwater superoleophobicity/superhydrophilicity of the NPM was studied. According to Young’s equation, the solid surface wetting behavior is generally calculated by the contact angle as follows: (1 ) in which y _sg is the solid-vapor interfacial energy, y _sl is the solid-liquid interfacial energy and ylg is the liquid-vapor interfacial energy.

Young’s equation is generally applied to oil on a solid interface with the presence of air. In this case, it is also reasonable to apply the equation to the oil droplet on a solid surface in water. Thus, the following equation can be generated (2)

Where Y _{oi i-g} is the oil-gas interface tension, is the oil contact angle with the presence of air, v _water-g is the water/gas interface tension, is the water contact angle with the presence of air, v _oii-water is interface tension of oil and water interface, and is the oil contact angle with the presence of water.

According to Equation 2, the phenomenon of a hydrophilic surface in air becoming oleophobic can be explained. If we use dichloromethane as an example, the water surface tension , while the interfacial tension of gas The dichloromethane-water interfacial tension Y _oii-water is 28.3 mN With the presence of air, the dichloromethane contact angle on NPM ^measured is almost 0, while water contact angle θ _-was 57.5. As indicated in Equation -0.49, therefore, θ ₃ equals to 155°, indicating the NPM behaved as a superoleophobic surface in water. This theoretical speculation aligns with the experimental results, in which the membrane is oleophilic in air while showing superoleophobicity property under water. In a regular water contact angle situation, the material will show a hydrophobic property only if liquid surface tension is larger than 4 times the solid surface tension. Since typical surface tensions of non-polar organic solvents and oils are generally 20 to 30 mN m ^-1 , the surface tension of a material should be smaller than 10 mN m ^-1 to make it oleophobic. Nonetheless, the air phase was exchanged by water phase in this study, which implemented oleophobic characteristic on materials with high surface tension. Cassie et al. (48) established a model related to the contact angle in a solid/water/air system. When the air phase was displaced by the water phase, Cassie’s model can be written as: where area fraction of solid is referred to as f, contact angle of an oil droplet on a smooth surface in water is referred to as θ ₃ , and the contact angle of an oil droplet on a rough surface in water is referred to as The rougher the solid phase is, the lower the area fraction is. In this study, the PPy particles coated on the electrospun PVA nanofibers, resulted in lower area fraction of the solid substrate.

Water was trapped in the grooves of the rough membrane surface, forming a cushion between NPM5 and the oil phase. Consequently, oil droplets on NPM5 stand on the peaks of the rough surface. Thus, the PPy coated PVA membrane (NPM5) shows higher underwater superoleophobicity than PVA membranes prior to PPy coating (NPM1 ). This is validated by the experiment results, where the underwater contact angle of PVA membrane increased from 123° to 158° with coated PPy nanoparticles.

The solar steam installation under various illumination concentration.

Our hierarchical nano-structure membrane possesses many special characteristics suitable for high energy efficiency seawater desalination. The broad band absorption of light and high energy conversion provides localized water heating and high energy efficiency. In water distillation, localized heating is considered an important quality as the traditional bulk heating of water leads to unnecessary energy loss to the unevaporated bulk portion of the water (58). The experiment installation of a solar vapor generation is shown in Figure 9a. The installation is composed of a water supply, a sunlight simulator and an NPM as the functional light converter. The membrane is floating on the surface of a container filled with water, while the sun simulator is perpendicularly illuminating the whole system from above. Under solar irradiance, the water molecules at the interface of membrane and air will escape the pores and channels on the membrane and later condense in the chamber as distilled water. The gaps and pores between different layers of the PVA nanofibers (typically 400 nm in diameter) give rise to channels for sufficient water supply. Considering the characteristics of NPM including broadband light absorption, low thermal conductivity and high light-to-heat energy conversion, such material is exceptionally suited for solar desalination processes. In this example, NPM5 samples with a diameter 10 mm and a thickness of 200 pm are utilized for solar steam generation.

Figure 9b shows the temperature change with respect to time under 1 sun irradiation. With ambient temperature of 22°C, bulk water started with a temperature of 18.9°C and reached 27.3°C after 5 min of illumination. On the contrary, with the presence of NPM5, the temperature of water rapidly increased to 39.4°C after 5 min of irradiation, demonstrating a remarkable solar-to-heat conversion ability. It is noticeable that with the presence of NPM5 in the water, the temperature reached equilibrium after 10 min of sun illumination, demonstrating the rapid response time of this water distillation system. Figure 9c demonstrates the effect of PPy on mass change under 1 sunlight and 0.5 sun illumination with respect to time. Once the solar steam reaches an equilibrium state, the evaporation rates under various Copt were recorded in Figure 9d. As can be observed in Figure 9d, solar vapor generation with the NPM is more substantial than without NPM present in the bulk water under all solar concentration conditions. In particular, the evaporation rate with the NPM structure for Copt = 1 reaches ~2.87 kg m ^-2 h ^-1 , which is almost 7 times higher than the evaporation rate of pure water. Figure 9e represents the experimental thermal efficiency of the solar vapor system. The efficiency of solar vapor generation of the NPM5 reaches up to 76.1 % under 0.8 kW m ^-2 solar flux, 87.5% under 1 kW m ^-2 solar flux, contrasting 22.7% of pure water.

The equilibrium evaporation rates and energy efficiencies of NPM1 , NPM2, NPM3, NPM4 and NPM5 under 1 sun irradiation are summarized in Figure 9f. As can be observed, the evaporation rate of NPM increase along with the reaction time of PPy polymerization. Accordingly, the equilibrium evaporation rates of NPM1 , NPM2, NPM3, NPM4 and NPM5 are 1.568, 2.257, 2.464, 2.71 and 2.872 kg nr ² h" ¹ , and the energy efficiencies are 52.21 %, 73.37%, 79.09%, 85.1 % and 87.5%, respectively.

Effect of PPy on solar vapor efficiency

To understand why NPM5 shows such outstanding solar vapor efficiency, the optical properties of NPM1 and NPM5 were measured via a UV-vis Spectrometer Ultraspec 4300 pro from 250-1100 nm. As can be seen from Figure 10a, the elestrospun PVA membrane (NPM1 ) shows mediocre light absorption properties due to the intrinsically high reflectivity of PVA nanofibers. After the PVA nanofibers are deposited with a layer of closely packed PPy nanoparticles, the surface becomes less reflective (the sunlight cannot be reflected, thus it shows a dark black appearance in macro scale) because of the excellent light absorbability of the PPy. The light absorption of the NPM5 can reach 99.9% within a broad wavelength range from 250 nm to 1100 nm. In contrast, the absorption of PVA membrane can only reach 65% at 250 nm and 90% absorption along most wavelengths.

The effect of the PPy nanoparticles on the swelling ratios of the NPM is shown in Figure 10b. It can be observed that an increase in the deposition of PPy nanoparticles on the PVA fibers yields a slight increase in the swelling ratio of the membrane. This phenomenon can be explained that the presence of PPy increases the hydrophilicity of the membrane, which results in slightly higher equilibrium swelling ratio. To systematically evaluate the energy efficiency of the NPM solar vapor device, the corresponding energy efficiency (q) for solar steam generation was calculated using the following formula: where m refers to the mass flux, hv refers the evaporation enthalpy of water stored in the membrane, Po refers to the solar radiation power of 1 sun (1 kW m ^-2 ), while Copt is the optical concentration of the sunlight simulator. It is observed that the evaporation enthalpy of bulk water is larger than that in the NPM. Here, water cluster theory can be used to understand this increase in evaporation enthalpy. Either as one molecule or as clusters with multiple molecules, water can be evaporated and escape the liquid-air interface. Compared to a single water molecule, a cluster of water molecules trapped in a porous structure is more likely to escape the matrix. Therefore, with the NPM, the water is more likely to be evaporated from the porous matrix as a result of lower enthalpy.

Figure 10c is the IR images of heat distribution of the NPM floating at the airliquid interface while sunlight irradiation starts from 0 s to 300 s. Starting from an 18.9°C water temperature, the surface temperature of the NPM rapidly increases to 41.6°C during the first 300 s of irradiation. The thermal distribution of the membrane surface shows minor changes after 300 s, ultimately reaching the highest at 42.3 °C, compared to the water temperature underneath the membrane which shows insignificant increase due to the low thermal conductivity of the material. These data suggest that the membrane has a highly localized heating property which is beneficial to energy efficiency and steam generation.

To quantitatively gauge the distilled water quality, four artificial seawaters of various salinities were used and carefully evaluated by conductivity tests: the Baltic sea (0.8 wt%), the average salinity of the world ocean (3.5 wt%), the Red Sea (4 wt%) and the Dead Sea (10 wt%), respectively (59). It is obvious that the Na ⁺ concentrations in the collected desalinized water are all distinctively decreased compared with the ion concentrations prior to the desalination. The salinity results of the four samples are all below the WHO and Health Canada drinking water standards (Figure 1 1 a). Moreover, a real sea water sample (from North Pacific Ocean) was utilized for on-site desalination test with NPM5. The concentration of Na, Ca, Mg and K ions are monitored to measure the desalination efficiency. From Figure 1 1 b, it is observed that the ion concentrations after desalination are 2 orders of magnitude below the original ion concentrations in seawater among all four samples. Durability is another feature of the NPM, which enables a durable and consistent desalination performance. Figure 1 1 c illustrates the evaporation efficiency of a NPM under 20 cycles. The test was held under 1 sun irradiance and the membrane was washed and dried after each use. The evaporation rates of the 20 cycles fit in a narrow range between 2.72 and 2.91 kg m ^-2 h ^-1 . The stability of solar vapor generation performance was also tested under acidic/base harsh environment, where seawater was treated to achieve specific pH (ranging from 1 to 13). As shown from Figure 1 1 d, the evaporation rates of NPM5 are all above 2.74 L m ^-2 h ^-1 with the energy efficiencies all above 83.02%, presenting excellent solar vapor generation ability over a wide range of pH conditions.

A prototype water purification system utilizing NPM5 was used for on-site purification testing. Here, an NPM5 with 10 cm diameter and 200 pm thickness was fabricated (Figure 11 e). A brine tank with a floating NPM5 at the surface was placed in a plastic chamber for steam condensation. The purified water was recovered by a tube connecting condensation chamber and a water bottle. To test the desalination function of the water purification system, the system was then exposed to sunlight. The natural sunlight led to an average per day purified water yield of ~1.2 L m ^-2 h ^-1 . The temperature of seawater in the flask remained around 30°C under sun exposure, despite the fact that the temperature inside of the condenser climbed up to 42°C, demonstrating a prominent localized heating effect of the NPM. In addition, the fact that the system is still collecting purified water during cloudy days proved that the NPM supported solar vapor generation under periods of various solar exposure.

Additionally, to demonstrate a thermal distribution of the solar vapor system, a COMSOL Multiphysics (a simulation software for multiphysics simulation) was constructed to simulate the heat loss of the system (Figure 12c, d). A steady-state heat transfer module was used because the actual experiment occurred under quasisteady conditions, with constant evaporation rate. Some boundary conditions and parameters were used in the COMSOL model are listed below.

The results of the COMSOL model are shown in Figure 12 c,d. With these boundary and parameters matched, the temperature mapping in the COMSOL model matches the experimental results, and the fitted evaporation rate are 2D direct contact (53%) and cone-shape transpiration device (83%) respectively. The radiation losses, convection losses and conduction loss of 2D direct contact account for -7% and -30%. 3D artificial transpiration device account for -5% and -1 %. These data are consistent with the results of experiment.

We have successfully established a highly effective water purification system with facile synthesis of NPM as light absorber and oil/water separator. Instead of representing oil/water separation or solar steam generation as the single feature of the purification system, for the first time we combined these two elements together and created a dual-functional purification system. The deposition of PPy nanoparticles onto PVA nanofiber provides NPM5 with superhydrophilicity which facilitates rapid water permeation. Making most use of the superhydrophic/underwater superoleophobic hierarchical nano-structure and ultra-high light absorption of NPM5, the system is capable of firstly, separating the mixed oil and water and secondly, generating a solar steam for water desalination. The oil/water separation efficiency is over 99.99% with the highest flux of 12740 L m ^-2 h ^-1 for surfactant free oil/water emulsions and 6503 L m ^-2 h ^-1 for surfactant-stabilized emulsions with sundry organic solvents emulsions. Due to the porous architecture of NPM5 and its high thermal insulation, the conductive heat loss to bulk water during sunlight illumination was reduced, thereby increasing the solar energy efficiency. The PPy coated PVA membrane demonstrate the ability to absorb a wide range of wavelengths (250-1100 nm) of sunlight to thermal energy with an absorbance of over 99.9%. Thanks to the low thermal conductivity of PVA, the thermal energy was restrained in the NPM, achieving high energy localization. Interestingly, the pores and gaps in between the PVA nanofibers serves as a 3D network, where continuous water supply was provided to guarantee adequate water for solar vapor generation. The NPM solar steam system demonstrates a solar steam generation efficiency around 87.5% with an evaporation rate of 2.87 kg nr ² h~ ¹ under 1 sun irradiation. The proof-of-concept method toward oil/water separation and seawater desalination provides a solution for the real-life application of efficient, scalable oil/water separation, desalination and wastewater purification.

Materials and methods

Materials

PVA (Mw = 205,000, Mowiol®, Millipore Sigma), glutaraldehyde (GA, 25% aqueous solution, Alfa Aesar), pyrrole (98%+, Alfa Aesar) and Iron(lll) nitrate (anhydrous, 98%, Alfa Aesar), hydrochloric acid (HCI, 37% reagent grade, Sigma Aldrich), GO (Tanfeng Tech. Inc., China), BSA (Sigma-Aldrich), humic acid (Millipore Sigma). Deionized water was obtained water from EASYpure II LF ultrapure water system.

Electrospinning of PVA

10 g of PVA powder was dissolved in 90 g of water under vigorous stirring for 6 h to prepare 10% PVA solution. Different concentrations of GO were homogeneously mixed into the PVA solution with vortex mixer. The PVA-GO mixture was added in a 10 ml syringe fitted with 18G flat end stainless steel needle. Electrospinning of the PVA-GO mixture was set up with a 15-18 KV high voltage and the distance between the tip of the needle and the collector was 165 mm. The PVA solution was pumped out by a syringe pump (PHD2000, Harvard Apparatus) at a flow rate of 5 mL/h to achieve a continuous nanofiber deposition and the NPM were formed onto a textiled nickel mesh (10 cm X 10 cm) as a collector.

Crosslinking of electrospun PVA-GO membrane

The obtained PVA-GO membranes were instantaneously dissolved in water as PVA has a high swell ratio and fibers can fuse with each other. To stabilize the membrane, the electrospun GO-PVA mat was pre-crosslined with glutaraldehyde vapor. The pre-crosslink was carried out by exposure of the as-electrospun PVA-GO mat to with 600 pl GA(2.6M) and 200 pl HCI (37%) in a sealed container for 24 hours. To further stabilize the membrane, the GO-PVA membrane was immersed in 40 ml acetone solution with 500 pl GA for further crosslinking. The originally separate nanofibers merged and entangled among each other and bonding was formed at the intersection points between the nanofibers. Afterwards, the membranes were washed with distilled water 5 times, before drying under ambient atmosphere. The membrane was referred to as NPM1 .

Preparation of NPM5

The crosslinked PVA-PPy-GO membrane was coated with PPy via in situ polymerization of pyrrole. The crosslinked membrane was soaked in 10 ml solution of 8 wt% Fe(lll) nitrate for 20 min under 4°C. Afterwards, another solution containing 340 pl of pyrrole dissolved in 10 ml of deionized water was added under 4°C. After polymerization for 3 days, the PPy coated membrane was taken out of the solution, washed with deionized Milli-Q water and dried at ambient atmosphere.

Membrane Characterization

Field-emission scanning electron microscopy (JEOL, JSM-5900LV, SEM Tech Solution) was conducted at an accelerating voltage of 20 eV. All specimens were sputter-coated with gold.

Fourier-transform infrared (FTIR) spectroscopy analysis was performed using a Thermo Nicolet iS10 FTIR Spectrometer (Thermo Nicolet, Madison, Wl, USA) from 4000 to 400 cm ^-1 with a resolution of 2 cm ^-1 and an accumulation of 32 scans. Tensile tests were conducted at room temperature with samples of 20 mm in length, 5 mm in width and 200 pm in thickness on a tensile testing machine (LS5, Instron, Inc., USA). The membranes were fixed by the grips and subjected to an crosshead speed of 10 mm/min using a gauge length of 20 mm. Tensile test for each group were carried out with 3 specimens. Underwater oil contact angle and water contact angle were tested on a goniometer (JY-PHA, Shengding, China). To test the underwater oil contact angle, a 5 pl chosen organic solvent or oil was dispensed on a NPM placed in a water-filled petri-dish. To test the dynamic sliding angle of the NPM, a 15 pl chosen organic solvent or oil droplet was placed on the membrane and the sliding angle was the minimum tilt angle required for the droplet to move.

The anti-fouling capability of the NPM was evaluated by a self-cleaning experiment: a NPM was immersed in canola oil, followed by transferring the NPM to a container filled with DI water. The container was gently shacked to remove the oil from the NPM. Dynamic oil adhesion test was carried out to examine the capability of the NPM to repel oil with high viscosity, in which a 5 pl of highly viscous mechanical pump oil was pressed on the surface of NPM and subsequently detached from the membrane surface.

Oil/water emulsion preparation and filtration evaluation

Surfactant-free oil/water emulsion was prepared by sonicating a 10% chosen organic solvent-in-water mixture for 10 minutes in water bath. The surfactant- stabilized emulsion was prepared by mixing 0.01 % SDS into 500 ml of DI water, followed by adding 50 ml chosen organic solvent or oil into the solution. A homogenized emulsion was obtained after sonicating the oil/surfactant/water mixture for 2 h in water bath. The organic solvent/oil droplet size was measured by a dynamic light scattering machine (Zetasizer, Nano-ZS, Malvern, UK) and optical microscopy (Micromaster, Fisher Scientific). The oil content was measured by a total organic carbon (TOC) analyzer (7000RMS, Mettler Toledo, Canada).

For the fouling resistant testing, DI water was first poured into the filtration equipment and the flux was recorded as Fi. After 10 min of pure water filtration, oil/water emulsion was poured into the filtration equipment for 60 min of continuous filtration. The NPM was subsequently released from the equipment and was with DI water to remove the oil residue. The flux of pure water was tested again with the same NPM and the flux was recorded as F2. The anti-fouling capability of the NPM was measured by the flux recovery ratio (FRR) according to the following equation:

The separation efficiency of the NPM was calculated by comparing the oil content before (Co) and after (C _P ) filtration according to the following equation:

Solar vapor generation experiment

A sun tester (Suntest XLS+, ATLAS) was utilized to test out the solar vapor generation performance of NPM. The solar irradiance was set as 1 kW m ^-2 . The NPM5 sample was cut into a round shape with diameter of 8 cm ^-2 . A NPM5 (-200 pm in thickness) was placed and floated on simulated seawater water (or DI water as control group) in a beaker. The set-up was located in the beam spot with various solar irradiance. A weight balance was utilized to measure the water evaporation performance. Prior to radiation of the experimental set-up, the evaporation rate without any light source was calculated with the beaker placed in dark environment for 1 h. The evaporation rate without any light source was deducted from the solar irradiated evaporation rate.

All solar vapor generation rates were calculated after reaching equilibrium state under 1 kW m ^-2 solar irradiance for 1 h.

COMSOL heat transfer simulation

Boundary conditions:

1 ) input solar energy(1000 W m ^-2 ),

2) temperature of the underlying bulk water, and

3) temperature of evaporating surface.

Parameters:

1 ) the surface emissivity of 0.95 was used on absorber,

2) A natural convection heat transfer coefficient of 5 W/m2K was used on evaporation surfaces. 3) The evaporation heat transfer coefficient of the evaporating surface was fit to match the boundary conditions.

4) The parameters of porous materials were based on the water because the materials were filled with large amount of water, (the thermal conductivity ~ 0.6 W m ^-2 K ^-1 , specific heat 4.2 J g ^-1 K ^-1 , and density 1000 kg m ^-3 ).

The scope of the claims should not be limited by the preferred embodiments set forth in the examples but should be given the broadest interpretation consistent with the description as a whole.

REFERENCES

1. Ebenstein, A. The consequences of industrialization: evidence from water pollution and digestive cancers in China. Review of Economics and Statistics 94, 186- 201 (2012).

2. Postel, S. L., Daily, G. C. & Ehrlich, P. R. Human appropriation of renewable fresh water. Science 271 , 785-788 (1996).

3. Yuan, J. et al. Superwetting nanowire membranes for selective absorption. Nature Nanotechnology 3, 332 (2008).

4. Yang, Y. et al. 3D-printed biomimetic super-hydrophobic structure for microdroplet manipulation and oil/water separation. Advanced materials 30, 1704912 (2018).

5. Li, F., Wang, Z., Huang, S., Pan, Y. & Zhao, X. Flexible, durable, and unconditioned superoleophobic/superhydrophilic surfaces for controllable transport and oil-water separation. Advanced Functional Materials 28, 1706867 (2018).

6. Song, Y. et al. Hydrophilic/oleophilic magnetic janus particles for the rapid and efficient oil-water separation. Advanced Functional Materials 28, 1802493 (2018).

7. Fu, Q., Ansari, F., Zhou, Q. & Berglund, L. A. Wood nanotechnology for strong, mesoporous, and hydrophobic biocomposites for selective separation of oil/water mixtures. ACS nano 12, 2222-2230 (2018).

8. Du, R. et al. Microscopic dimensions engineering: Stepwise manipulation of the surface wettability on 3D substrates for oil/water separation. Advanced Materials 28, 936-942 36 (2016).

9. Zhou, H. et al. A waterborne coating system for preparing robust, self-healing, superamphiphobic surfaces. Advanced Functional Materials 27, 1604261 (2017).

10. Li, J. et al. Underoil superhydrophilic desert sand layer for efficient gravity- directed water-in-oil emulsions separation with high flux. Journal of Materials Chemistry A 6, 223-230 (2018).

11. Liu, M., Hou, Y., Li, J. & Guo, Z. Stable superwetting meshes for on-demand separation of immiscible oil/water mixtures and emulsions. Langmuir 33, 3702-3710 (2017).

12. Li, Y. et al. C02-responsive cellulose nanofibers aerogels for switchable oilwater separation. ACS applied materials & interfaces 11 , 9367-9373 (2019).

13. Qing, W. et al. Robust superhydrophobic-superoleophilic polytetrafluoroethylene nanofibrous membrane for oil/water separation. Journal of Membrane Science 540, 354- 12 361 (2017).

14. Zhu, Z. et al. Superamphiphilic silicon wafer surfaces and applications for uniform polymer film fabrication. Angewandte Chemie 129, 5814-5818 (2017).

15. Zhu, Y. et al. Zwitterionic nanohydrogel grafted PVDF membranes with comprehensive antifouling property and superior cycle stability for oil-in-water emulsion separation. Advanced Functional Materials 28, 1804121 (2018).

16. Yang, H.-C. et al. Crude-oil-repellent membranes by atomic layer deposition: oxide interface engineering. ACS nano 12, 8678-8685 (2018).

17. Zhang, J. et al. Taro leaf-inspired and superwettable nanonet-covered nanofibrous membranes for high-efficiency oil purification. Nanoscale Horizons 4, 1174-1184 (2019).

18. Yang, H. C. et al. Janus membranes: creating asymmetry for energy efficiency. Advanced Materials 30, 1801495 (2018).

19. Huang, A. et al. Nanoarchitectured design of porous ZnO@ copper membranes enabled by atomic-layer-deposition for oil/water separation. Journal of Membrane Science 582, 26 120-131 (2019).

20. Weber, M. et al. Novel and facile route for the synthesis of tunable boron nitride nanotubes combining atomic layer deposition and annealing processes for water purification. Advanced Materials Interfaces 5, 1800056 (2018).

21. Bielinski, A. R. et al. Rational design of hyperbranched nanowire systems for tunable superomniphobic surfaces enabled by atomic layer deposition. ACS nano 11 , 478-489 32 (2017).

22. Dong, X. et al. A self-roughened and biodegradable superhydrophobic coating with UV shielding, solar-induced self-healing and versatile oil-water separation ability. Journal of Materials Chemistry A 7, 2122-2128 (2019).

23. Mi, H.-Y., Jing, X., Huang, H.-X. & Turng, L.-S. Controlling superwettability by microstructure and surface energy manipulation on three-dimensional substrates for versatile gravity-driven oil/water separation. ACS applied materials & interfaces 9, 37529-37535 (2017).

24. Liao, Z., Wu, G., Lee, D. & Yang, S. Ultrastable underwater anti-oil fouling coatings from spray assemblies of polyelectrolyte grafted silica nanochains. ACS applied materials & interfaces 11 , 13642-13651 (2019).

25. Xiao, C. et al. Total morphosynthesis of biomimetic prismatic-type CaCO 3 thin films. Nature communications 8, 1 -9 (2017).

26. Yang, D. et al. Combination of redox assembly and biomimetic mineralization to prepare graphene-based composite cellular foams for versatile catalysis. ACS applied materials & interfaces 9, 43950-43958 (2017).

27. Gao, S. et al. A robust polyionized hydrogel with an unprecedented underwater anti-crude-oil-adhesion property. Advanced Materials 28, 5307-5314 (2016).

28. Zhu, Y. et al. Seawater-Enhanced Tough Agar/Poly (N isopropylacrylamide)/ Clay Hydrogel for Anti-Adhesion and Oil/Water Separation. Soft Matter (2020).

29. Wang, Z. et al. Facile preparation of antifouling hydrogel architectures for drag reduction and oil/sea water separation. Materials Today Communications 21 , 100618 (2019). 30. Gao, S., Li, X., Li, L. & Wei, X. A versatile biomass derived carbon material for oxygen reduction reaction, supercapacitors and oil/water separation. Nano Energy 33, 334-342 (2017).

31. He, M. et al. Reduced graphene oxide aerogel membranes fabricated through hydrogen bond mediation for highly efficient oil/water separation. Journal of Materials Chemistry A 7, 11468-11477 (2019).

32. Yin, K. et al. Femtosecond laser induced robust periodic nanoripple structured mesh for highly efficient oil-water separation. Nanoscale 9, 14229-14235 (2017).

33. Zhang, W. et al. Superwetting porous materials for wastewater treatment: from immiscible oil/water mixture to emulsion separation. Advanced Materials Interfaces 4, 1600029 (2017).

34. Zhou, L. et al. 3D self-assembly of aluminium nanoparticles for plasmon enhanced solar desalination. Nature Photonics 10, 393 (2016).

35. Pal, A., Natu, G., Ahmad, K. & Chattopadhyay, A. Phosphorus induced crystallinity in carbon dots for solar light assisted seawater desalination. Journal of Materials Chemistry A 6, 4111 -4118 (2018).

36. Yao, J. & Yang, G. An efficient solar-enabled 2D layered alloy material evaporator for seawater desalination. Journal of Materials Chemistry A 6, 3869-3876 (2018).

37. Chen, C. et al. Highly flexible and efficient solar steam generation device. Advanced Materials 29, 1701756 (2017).

38. Li, T. et al. Scalable and Highly Efficient Mesoporous Wood-Based Solar Steam Generation Device: Localized Heat, Rapid Water Transport. Advanced Functional Materials 28, 1707134 (2018).

39. Zhu, M. et al. Plasmonic Wood for High-Efficiency Solar Steam Generation. Advanced Energy Materials 8, 1701028 (2018).

40. Liu, T. & Li, Y. Photocatalysis: Plasmonic solar desalination. Nature Photonics 10, 361 36 (2016).

41. Ye, M. et al. Synthesis of black TiOx nanoparticles by Mg reduction of TiO2 nanocrystals and their application for solar water evaporation. Advanced Energy Materials 7, 1601811 (2017).

42 Yao, J., Zheng, Z. & Yang, G. Layered tin monoselenide as advanced photothermal conversion materials for efficient solar energy-driven water evaporation. Nanoscale 10, 42 2876-2886 (2018).

43. Xu, N. et al. Mushrooms as Efficient Solar Steam-Generation Devices. Advanced Materials 29, 1606762 (2017).

44. Ren, H. et al. Hierarchical graphene foam for efficient omnidirectional solarthermal energy conversion. Advanced Materials 29, 1702590 (2017).

45. Morciano, M. et al. Efficient steam generation by inexpensive narrow gap evaporation device for solar applications. Scientific reports 7, 11970 (2017).

46. Zhu, Y., Xie, W., Zhang, F., Xing, T. & Jin, J. Superhydrophilic in-situ-cross- linked zwitterionic polyelectrolyte/PVDF-blend membrane for highly efficient oil/water emulsion separation. ACS applied materials & interfaces 9, 9603-9613 (2017).

47. Ge, J., Zong, D., Jin, Q., Yu, J. & Ding, B. Biomimetic and superwettable nanofibrous skins for highly efficient separation of oil-in-water emulsions. Advanced Functional Materials 28, 1705051 (2018).

48. Cassie, A. Contact angles. Discussions of the Faraday society 3, 11 -16 (1948).

49. Wenzel, R. N. Resistance of solid surfaces to wetting by water. Industrial & Engineering Chemistry 28, 988-994 (1936).

50. Alhosseini, S. N. et al. Synthesis and characterization of electrospun polyvinyl alcohol nanofibrous scaffolds modified by blending with chitosan for neural tissue engineering. International journal of nanomedicine 7, 25 (2012).

51. Lopez-Garcia, F., Canche-Escamilla, G., Ocampo-Flores, A., Roquero-Tejeda, P. & Ordonez, L. Controlled size nano-polypyrrole synthetized in micro-emulsions as PT support for the ethanol electro-oxidation reaction. Int. J. Electrochem. Sci 8, 3794- 3813 18 (2013).

52. Liu, Y., Chu, Y. & Yang, L. Adjusting the inner-structure of polypyrrole nanoparticles through microemulsion polymerization. Materials chemistry and physics 98, 304-308 21 (2006).

53. Gao, S. et al. Layer- by- Layer Construction of Cu2 ⁺ /Alginate Multilayer Modified

Ultrafiltration Membrane with Bioinspired Superwetting Property for High-Efficient Crude-Oil-in-Water Emulsion Separation. Advanced Functional Materials 28, 1801944 25 (2018).

54. Meng, F.-N., Zhang, M.-Q., Ding, K., Zhang, T. & Gong, Y.-K. Cell membrane mimetic PVDF microfiltration membrane with enhanced antifouling and separation performance for oil/water mixtures. Journal of Materials Chemistry A 6, 3231 -3241 (2018).

55. Zhang, L., Cheng, L., Wu, H., Yoshioka, T. & Matsuyama, H. One-step fabrication of robust and anti-oil-fouling aliphatic polyketone composite membranes for sustainable and efficient filtration of oil-in-water emulsions. Journal of Materials Chemistry A 6, 24641 - 32 24650 (2018).

56. Vargaftik, N., Volkov, B. & Voljak, L. International tables of the surface tension of water. Journal of Physical and Chemical Reference Data 12, 817-820 (1983).

57. Ghosh, P. Shape of the interface. NPTEL — Chemical Engineering — Interfacial

Engineering, IIT Guwahati, Guwahati India, https ://nptel/. ac. in/courses/103103033/module2/lecture3. pdf (2012).

58. Zhang, L., Tang, B., Wu, J., Li, R. & Wang, P. Hydrophobic Light-to-Heat Conversion Membranes with Self-Healing Ability for Interfacial Solar Heating. Advanced Materials 27, 4889-4894 (2015).

59. Boyer, T. P. et al. World Ocean Database 2013. (2013).