A Gradient Wetting Self-Cleaning Nanopaper and Its Preparation and Application

Abstract

This application relates to the field of high-value utilization technology of lignocellulose biomass, specifically a gradient wettability self-cleaning nanopaper and its preparation and application. This scheme addresses the problems of poor interlayer adhesion, low flux, and membrane fouling in Janus nanopaper by employing amphiphilic lignin nanoparticles (LNPs) with a spherical topology as bridging particles and modified multi-scale cellulose as a heterogeneous substrate. A photo-induced antifouling Janus membrane with microchannel stability is prepared through a particle bridging strategy, enabling rapid and efficient separation of heavy oil-water emulsions. Stable high-flux separation of water-in-oil emulsions (O / W) and water-in-oil emulsions (W / O) is achieved, with separation efficiencies of up to 99.7% and 98.1%, respectively, and fluxes of 6298 L × m³. ‑2 ×h ‑1 ×bar ‑1 and 5499L×m ‑2 ×h ‑1 ×bar ‑1 Its flux is 100 times that of commercially available nanofiltration membranes. It maintains excellent separation flux even after ten cycles. This research provides a new strategy for treating complex oil spill wastewater.

Description

Technical Field

[0001] This application relates to the field of high-value utilization technology of lignocellulose biomass, specifically a gradient wettability self-cleaning nanopaper and its preparation and application. Background Technology

[0002] Oil spills are an international environmental challenge. Oily wastewater contains high concentrations of petroleum hydrocarbons and other toxic and harmful substances, seriously threatening marine ecosystems and human safety. Janus membrane separation technology is a highly promising solution, characterized by high separation efficiency, low energy consumption, and cost-effectiveness, and is applicable to a variety of industrial wastewaters, making it attractive for demulsification. Compared to traditional single-phase permeation membranes, Janus membranes have an asymmetric structure, enabling the integration of multiple functional components into a single unit, resulting in additional anisotropic derivative properties and functions. However, separation flux is still limited by micropore porosity and layer structure bridging stability. On the one hand, in actual preparation processes, the porosity of open microchannels decreases, leading to reduced separation efficiency. On the other hand, the hydrophobic and hydrophilic layers of the separation membrane have weak bonding capabilities. Therefore, designing a fiber-based asymmetric wettability Janus membrane with superwetting properties, tunable porosity, and stable interlayer structure is key to achieving high-flux and highly selective oil-water separation.

[0003] Therefore, finding a method to solve the problems of poor interlayer adhesion, low flux and membrane fouling of Janus nanopaper is currently a key research direction. Summary of the Invention

[0004] To address the aforementioned technical problems in the prior art, this application provides a gradient wettability self-cleaning nanopaper and its preparation and application.

[0005] A method for preparing gradient wettability self-cleaning nanopaper specifically includes the following steps:

[0006] (1) Preparation of nanocellulose (CNF): Poplar cellulose with a moisture content of 20-30 wt% was ground and then homogenized to obtain 2 wt% nanocellulose.

[0007] (2) The CNF prepared in step (1) was anionized by a tetramethylpiperidine oxide (TEMPO) mediated oxidation reaction to obtain carboxylated cellulose nanoparticles (TOCNF); this step was to convert the primary hydroxyl groups in CNF into carboxyl groups.

[0008] (3) Preparation of hydrophobic fiber layer: Take 1-3 mg / mL of TOCNF obtained in step (2) to prepare -1 Add EDS and NHS to the dispersion; stir to activate; add 2-10 mg / mL -1The ODA was reacted; after the reaction, a hydrophobic cellulose derivative (ODANF) was obtained.

[0009] (4) Preparation of hydrophilic fiber layer: Using the TOCNF obtained in step (2) as raw material, prepare 2 mg·mL -1 The dispersion was mixed with EDS and NH4+; the mixture was stirred and activated; AZO was added to carry out the reaction; after the reaction, a photoresponsive cellulose derivative (AZONF) was obtained.

[0010] (5) Preparation of lignin nanoparticles (LNPs): Lignin was dissolved in tetrahydrofuran to obtain a concentration of 2-10 mg·mL. -1 raw lignin solution; at 300-800 rpm·min -1 At a rotation speed of [speed value], the original lignin solution was slowly added dropwise to ultrapure water; magnetic stirring was used, and lignin nanoparticles were obtained after THF evaporated.

[0011] (6) Assembly of Janus nanopaper: Take ODANF, AZONF and LNPs obtained in steps (3), (4) and (5) for later use. Use vacuum filtration method to assemble layer by layer in the order of ODANF, LNPs and AZONF to prepare Janus nanopaper with asymmetric wettability.

[0012] Further, in step (1), poplar cellulose with a moisture content of 25wt% is ground using a 2500-II type KRK high-concentration disc mill to obtain 5wt% microfibrillated nanocellulose (MFC); after homogenization, 2wt% nanocellulose is obtained.

[0013] Furthermore, in steps (2) and (3), the molar ratio of EDS to NHS is (1:1).

[0014] Furthermore, the stirring activation in steps (2) and (3) involves activating cellulose for 1 hour under magnetic stirring at 500 rpm.

[0015] Furthermore, in step (3), 2-10 mg·mL is added. -1 The reaction of ODA was carried out at 60°C for 5 hours.

[0016] Furthermore, in step (4), the addition of AZO for the reaction is carried out by adding 0.1-0.9 mg·mL. -1 The AZO was reacted at 60℃ for 5 hours.

[0017] Furthermore, step (5) specifically involves dissolving 20-100 mg of lignin in 10 mL of tetrahydrofuran to obtain a concentration of 2-10 mg·mL. -1The original lignin solution; at 500 rpm·min -1 At a rotation speed of 1000 rpm, the original lignin solution was slowly added dropwise to 30 mL of ultrapure water; magnetic stirring was performed for 24 h, and lignin nanoparticles were obtained after THF evaporated.

[0018] Further, step (6) is as follows: First, 10 ml of AZONF dispersion (0.2 wt%) is filtered through a PTFE microporous membrane; then, 0.5 mg·mL⁻¹ of LNPs is deposited on top of AZONF; finally, 20 mL of ODANF suspension (0.1 wt%) is deposited on top of the LNPs; after drying at 40 °C for 12 h, a wettability gradient composite membrane (JA / O@LNP) is obtained; wherein, the hydrophobic layer is denoted as the JO layer and the hydrophilic layer is denoted as the JA layer.

[0019] A gradient wettability self-cleaning nanopaper was prepared by the above method.

[0020] The gradient wettability self-cleaning nanopaper described herein is used in oil-water separation.

[0021] Compared with the prior art, the technical effects created by this application are reflected in:

[0022] This method addresses the problems of poor interlayer adhesion, low flux, and membrane fouling in Janus nanopaper. It employs amphiphilic lignin nanoparticles (LNPs) with a spherical topology as bridging particles and modified multi-scale cellulose as a heterogeneous substrate. Through a particle bridging strategy, a photoinduced antifouling Janus membrane with microchannel stability is prepared, enabling rapid and efficient separation of heavy oil-water emulsions. Stable high-flux separation of water-in-oil emulsions (O / W) and water-in-oil emulsions (W / O) is achieved, with separation efficiencies of 99.7% and 98.1%, respectively, and fluxes of 6298 L × m³. -2 ×h -1 ×bar -1 and 5499L×m -2 ×h -1 ×bar -1 Its flux is 100 times that of commercially available nanofiltration membranes. It maintains excellent separation flux even after ten cycles. This research provides a new strategy for treating complex oil spill wastewater. Attached Figure Description

[0023] Figure 1 This is a scanning electron microscope image of the cross-section of Janus nanopaper prepared in Example 1.

[0024] Figure 2 These are cross-sectional scanning electron microscope images and physical images of the hydrophilic JA layer of the Janus nanopaper prepared in Example 1.

[0025] Figure 3 The images show a cross-sectional scanning electron microscope (SEM) image and a physical photograph of the hydrophobic JO layer of the Janus nanopaper prepared in Example 1.

[0026] Figure 4 This is a transmission electron microscope image of the lignin nanoparticles prepared in Example 1.

[0027] Figure 5 This is a physical CT scan image of the Janus nanopaper prepared in Example 1.

[0028] Figure 6 This is a comparison chart of the separation efficiency and throughput of light oil-water mixtures in Example 1 and Comparative Examples 1 and 2.

[0029] Figure 7 This is a comparison chart of the separation efficiency and throughput of heavy oil-water mixtures in Example 1 and Comparative Examples 1 and 2. Detailed Implementation

[0030] The technical solution of this application will be further defined below with reference to specific implementation methods, but the scope of protection is not limited to the description.

[0031] Example 1

[0032] This invention provides a method for preparing gradient wettability self-cleaning nanopaper for oil-water separation. The method utilizes amphiphilic lignin nanoparticles (LNPs) with a spherical topology as bridging particles and modified multi-scale cellulose as a heterogeneous substrate. Through interfacial interactions, a longitudinal fluid-directed transport structure of "hydrophilic-amphiphilic-hydrophobic" is formed. A photo-induced antifouling Janus nanopaper with microchannel stability is prepared for oil-water separation using this particle bridging strategy.

[0033] This invention relates to a method for preparing gradient wettability self-cleaning Janus nanopaper for oil-water separation, the specific implementation process of which is as follows:

[0034] (1) Poplar cellulose with a moisture content of 25 wt% was ground using a 2500-II type KRK high-concentration disc mill to obtain 5 wt% microfibrillated nanocellulose (MFC). After homogenization, 2 wt% nanocellulose (CNF) was obtained. CNF was anionized by TEMPO-mediated oxidation to obtain carboxylated nanocellulose (TOCNF).

[0035] (2) Preparation of hydrophobic fiber layer: Add 2 mg·mL -1 EDS and NHS were added to the TOCNF dispersion at a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 4 mg / mL of the solution was then added. -1The ODA was reacted at 60°C for 5 hours. After the reaction, a hydrophobic cellulose derivative (ODANF) was obtained.

[0036] (3) Preparation of the hydrophilic fiber layer: using 2 mg·mL -1 TOCNF was used as a raw material, and EDS and NHS were added in a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 0.3 mg / mL of the solution was added. -1 AZO was reacted at 60°C for 5 hours. After the reaction, a photoresponsive cellulose derivative (AZONF) was obtained.

[0037] (4) Preparation of lignin nanoparticles (LNPs): 40 mg of lignin was dissolved in 10 mL of tetrahydrofuran to obtain a concentration of 4 mg·mL⁻¹. -1 The original lignin solution. At 500 rpm·min -1 At a rotation speed of [speed value missing], the original lignin solution was slowly added dropwise to 30 mL of ultrapure water. The mixture was magnetically stirred for 24 h, and lignin nanoparticles (LNPs) were obtained after THF evaporation.

[0038] (5) Assembly of Janus nanopaper. Janus nanopaper with asymmetric wettability was prepared for oil-water separation by vacuum filtration, layer by layer, in the order of ODANF, LNPs, and AZONF. First, 10 mL of AZONF dispersion (0.2 wt%) was filtered through a PTFE microporous membrane. Then, 0.5 mg·mL⁻¹ of LNPs was deposited on top of AZONF. Finally, 20 mL of ODANF suspension (0.1 wt%) was deposited on top of the LNPs. After drying at 40 °C for 12 h, a wettability gradient composite membrane (JA / O@LNP) was obtained.

[0039] Example 2

[0040] This invention provides a method for preparing gradient wettability self-cleaning nanopaper for oil-water separation. The method utilizes amphiphilic lignin nanoparticles (LNPs) with a spherical topology as bridging particles and modified multi-scale cellulose as a heterogeneous substrate. Through interfacial interactions, a longitudinal fluid-directed transport structure of "hydrophilic-amphiphilic-hydrophobic" is formed. A photo-induced antifouling Janus nanopaper with microchannel stability is prepared for oil-water separation using this particle bridging strategy.

[0041] This invention relates to a method for preparing gradient wettability self-cleaning Janus nanopaper for oil-water separation, the specific implementation process of which is as follows:

[0042] (1) Poplar cellulose with a moisture content of 25 wt% was ground using a 2500-II type KRK high-concentration disc mill to obtain 5 wt% microfibrillated nanocellulose (MFC). After homogenization, 2 wt% nanocellulose (CNF) was obtained. CNF was anionized by TEMPO-mediated oxidation to obtain carboxylated nanocellulose (TOCNF).

[0043] (2) Preparation of hydrophobic fiber layer: Add 2 mg·mL -1 EDS and NHS were added to the TOCNF dispersion at a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 4 mg / mL of the solution was then added. -1 The ODA was reacted at 60°C for 5 hours. After the reaction, a hydrophobic cellulose derivative (ODANF) was obtained.

[0044] (3) Preparation of the hydrophilic fiber layer: using 2 mg·mL -1 TOCNF was used as a raw material, and EDS and NHS were added in a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 0.3 mg / mL of the solution was added. -1 AZO was reacted at 60°C for 5 hours. After the reaction, a photoresponsive cellulose derivative (AZONF) was obtained.

[0045] (4) Preparation of lignin nanoparticles (LNPs): 20 mg of lignin was dissolved in 10 mL of tetrahydrofuran to obtain a concentration of 2 mg·mL⁻¹. -1 The original lignin solution. At 500 rpm·min -1 At a rotation speed of [speed value missing], the original lignin solution was slowly added dropwise to 30 mL of ultrapure water. The mixture was magnetically stirred for 24 h, and lignin nanoparticles (LNPs) were obtained after THF evaporation.

[0046] (5) Assembly of Janus nanopaper: Janus nanopaper with asymmetric wettability was prepared by vacuum filtration and layer-by-layer assembly in the order of ODANF, LNPs, and AZONF for oil-water separation. First, 10 mL of AZONF dispersion (0.2 wt%) was filtered through a PTFE microporous membrane. Then, 0.5 mg·mL⁻¹ of LNPs was deposited on top of AZONF. Finally, 20 mL of ODANF suspension (0.1 wt%) was deposited on top of the LNPs. After drying at 40 °C for 12 h, a wettability gradient composite membrane (JA / O@LNP) was obtained.

[0047] Example 3

[0048] This invention provides a method for preparing gradient wettability self-cleaning Janus nanopaper for oil-water separation. The method utilizes amphiphilic lignin nanoparticles (LNPs) with a spherical topology as bridging particles and modified multi-scale cellulose as a heterogeneous substrate. Through interfacial interactions, a longitudinal fluid-directed transport structure of "hydrophilic-amphiphilic-hydrophobic" is formed. A photo-induced antifouling Janus nanopaper with microchannel stability for oil-water separation is prepared using this particle bridging strategy.

[0049] This invention relates to a method for preparing gradient wettability self-cleaning Janus nanopaper for oil-water separation, the specific implementation process of which is as follows:

[0050] (1) Poplar cellulose with a moisture content of 25 wt% was ground using a 2500-II type KRK high-concentration disc mill to obtain 5 wt% microfibrillated nanocellulose (MFC). After homogenization, 2 wt% nanocellulose (CNF) was obtained. CNF was anionized by TEMPO-mediated oxidation to obtain carboxylated nanocellulose (TOCNF).

[0051] (2) Preparation of hydrophobic fiber layer: Add 2 mg·mL -1 EDS and NHS were added to the TOCNF dispersion at a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 6 mg / mL of [the solution was then added]. -1 The ODA was reacted at 60°C for 5 hours. After the reaction, a hydrophobic cellulose derivative (ODANF) was obtained.

[0052] (3) Preparation of the hydrophilic fiber layer: using 2 mg·mL -1 TOCNF was used as a raw material, and EDS and NHS were added in a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 0.6 mg / mL of the solution was added. -1 AZO was reacted at 60°C for 5 hours. After the reaction, a photoresponsive cellulose derivative (AZONF) was obtained.

[0053] (4) Preparation of lignin nanoparticles (LNPs): 40 mg of lignin was dissolved in 10 mL of tetrahydrofuran to obtain a concentration of 4 mg·mL⁻¹. -1 The original lignin solution. At 500 rpm·min -1 At a constant rotation speed, the original lignin solution was slowly added dropwise to 30 mL of ultrapure water. The mixture was magnetically stirred for 24 h, and lignin nanoparticles (LNPs) were obtained after THF evaporation.

[0054] (5) Assembly of Janus nanopaper: Janus nanopaper with asymmetric wettability was prepared by vacuum filtration and layer-by-layer assembly in the order of ODANF, LNPs, and AZONF for oil-water separation. First, 10 mL of AZONF dispersion (0.2 wt%) was filtered through a PTFE microporous membrane. Then, 0.5 mg·mL⁻¹ of LNPs was deposited on top of AZONF. Finally, 20 mL of ODANF suspension (0.1 wt%) was deposited on top of the LNPs. After drying at 40 °C for 12 h, a wettability gradient composite membrane (JA / O@LNP) was obtained.

[0055] Comparative Example 1:

[0056] (1) Poplar cellulose with a moisture content of 25 wt% was ground using a 2500-II type KRK high-concentration disc mill to obtain 5 wt% microfibrillated nanocellulose (MFC). After homogenization, 2 wt% nanocellulose (CNF) was obtained. CNF was anionized by TEMPO-mediated oxidation to obtain carboxylated nanocellulose (TOCNF).

[0057] (2) Preparation of hydrophobic fiber layer: Add 2 mg·mL -1 EDS and NHS were added to the TOCNF dispersion at a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 6 mg / mL of [the solution was then added]. -1 The ODA was reacted at 60°C for 5 hours. After the reaction, a hydrophobic cellulose derivative (ODANF) was obtained.

[0058] (3) Preparation of hydrophobic monolayer separation membrane: 20 mL of 0.1 wt% ODANF suspension was prepared, and a hydrophobic monolayer separation membrane was prepared by vacuum filtration.

[0059] Comparative Example 2:

[0060] (1) Poplar cellulose with a moisture content of 25 wt% was ground using a 2500-II type KRK high-concentration disc mill to obtain 5 wt% microfibrillated nanocellulose (MFC). After homogenization, 2 wt% nanocellulose (CNF) was obtained. CNF was anionized by TEMPO-mediated oxidation to obtain carboxylated nanocellulose (TOCNF).

[0061] (2) Preparation of hydrophilic fiber layer: using 2 mg·mL -1 TOCNF was used as a raw material, and EDS and NHS were added in a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 0.6 mg / mL of the solution was added. -1AZO was reacted at 60°C for 5 hours. After the reaction, a photoresponsive cellulose derivative (AZONF) was obtained.

[0062] (3) Preparation of hydrophilic monolayer separation membrane: 20 mL of 0.2 wt% AZONF suspension was prepared, and a hydrophobic monolayer separation membrane was prepared by vacuum filtration.

[0063] Comparative Example 3:

[0064] (1) Poplar cellulose with a moisture content of 25 wt% was ground using a 2500-II type KRK high-concentration disc mill to obtain 5 wt% microfibrillated nanocellulose (MFC). After homogenization, 2 wt% nanocellulose (CNF) was obtained. CNF was anionized by TEMPO-mediated oxidation to obtain carboxylated nanocellulose (TOCNF).

[0065] (2) Preparation of hydrophobic fiber layer: Add 2 mg·mL -1 EDS and NHS were added to the TOCNF dispersion at a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 2 mg / mL of the solution was then added. -1 The ODA was reacted at 60°C for 5 hours. After the reaction, a hydrophobic cellulose derivative (ODANF) was obtained.

[0066] (3) Preparation of the hydrophilic fiber layer: using 2 mg·mL -1 TOCNF was used as a raw material, and EDS and NHS were added in a molar ratio of 1:1. Cellulose was activated for 1 hour under magnetic stirring at 500 rpm. 0.3 mg / mL of the solution was added. -1 AZO was reacted at 60°C for 5 hours. After the reaction, a photoresponsive cellulose derivative (AZONF) was obtained.

[0067] (4) Preparation of Janus bilayer separation membrane: The Janus bilayer separation membrane was prepared by vacuum filtration, assembling ODANF and AZONF layer by layer in that order. First, 10 mL of AZONF dispersion (0.2 wt%) was filtered through a PTFE microporous membrane. Then, 20 mL of ODANF suspension (0.1 wt%) was deposited on top of the ODANF. After drying at 40 °C for 12 h, a wettability gradient composite membrane (JA / O) was obtained.

[0068] The separation efficiency and flux were determined according to the following steps:

[0069] The prepared membrane was fixed between glass filter funnels. 100 mL of an oil-water mixture was passed through the filter device, and a separation experiment was conducted under water pump pressure (0.7 bar). The separation efficiency (η1) of the immiscible oil-water mixture was calculated using Formula 1:

[0070] η1=m / m0×100% (1)

[0071] Where m and m0 represent the mass of the permeate liquid before and after separation, respectively. To eliminate the influence of membrane adsorption, the separation efficiency is calculated after the initial separation.

[0072] In addition, the prepared membrane was fixed between glass filter funnels, and the volume of liquid passing through the membrane and the transmembrane pressure were recorded over 1 hour. The membrane separation flux was calculated using Equation 2:

[0073] F=V / (A×t×P) (2)

[0074] Where F is the membrane flux, and A(m 2 ) is the effective area of ​​the membrane (14.50 cm²) 2 ), where t(h) is the test time, P(bar) is the pressure, and V(L) is the volume of the separated phase. Each data point is the average of three parallel experiments.

[0075] Finally, it should be noted that the above embodiments are merely representative examples of this application. Obviously, the technical solutions of this application are not limited to the above embodiments, and many variations are possible. All variations that can be directly derived or conceived by those skilled in the art from the content disclosed in this application should be considered within the scope of protection of this application.

Claims

1. A method for preparing a gradient-wettability self-cleaning nanopaper, characterized by, Specifically comprising the following steps: (1) Preparation of nanocellulose CNF: grinding poplar cellulose with water content of 20-30 wt%, and then obtaining 2 wt% nanocellulose after homogenization process; (2) Anionization of CNF prepared in step (1) by TEMPO-mediated oxidation reaction, obtaining carboxylated nanocellulose TOCNF; (3) Preparation of the hydrophobic fiber layer: 1-3 mg·mL of the dispersion obtained in step (2) was taken, and EDC and NHS were added; stirring was activated; 2-10 mg·mL of ODA was added for reaction; after the reaction, the hydrophobic cellulose derivative ODA NF was obtained; -1 (3) Preparation of the hydrophobic fiber layer: 1-3 mg·mL of the dispersion obtained in step (2) was taken, and EDC and NHS were added; stirring was activated; 2-10 mg·mL of ODA was added for reaction; after the reaction, the hydrophobic cellulose derivative ODA NF was obtained; -1 (3) Preparation of the hydrophobic fiber layer: 1-3 mg·mL of the dispersion obtained in step (2) was taken, and EDC and NHS were added; (4) Preparation of the hydrophilic fiber layer: Take the TOCNF obtained in step (2) as raw material to prepare a dispersion liquid of 2 mg·mL -1 , add EDC and NHS; stir and activate; add AZO to react; after reaction, obtain the photoresponsive cellulose derivative AZONF; (5) Preparation of lignin nanoparticles LNPs: lignin was dissolved in tetrahydrofuran THF to obtain a lignin original solution with a concentration of 2-10 mg·mL -1 ; the lignin original solution was slowly added to ultrapure water at a rotation speed of 300-800 rpm·min -1 ; magnetic stirring was performed, and lignin nanoparticles were obtained after the volatilization of tetrahydrofuran THF; (6) Assembly of Janus nanopaper: ODANF, AZONF and LNPs obtained in steps (3), (4) and (5) are prepared, and vacuum filtration method is used to assemble them layer by layer in the order of ODANF, LNPs and AZONF, thereby preparing Janus nanopaper with asymmetric wettability.

2. The method of claim 1, wherein the gradient-wettability self-cleaning nanopaper is prepared by the steps of: In the step (1), specifically grinding poplar cellulose with water content of 25 wt%, and obtaining 5 wt% microfibrillated nanocellulose MFC; and then obtaining 2 wt% nanocellulose after homogenization process.

3. The method for preparing gradient wettability self-cleaning nanopaper according to claim 1, characterized in that, In the step (3) and step (4), the molar ratio of EDC and NHS added is 1:

1.

4. The method for preparing gradient wettability self-cleaning nanopaper according to claim 1, characterized in that, In the step (3) and step (4), the stirring activation is to activate the cellulose under the condition of magnetic stirring at 500 rpm for 1 h.

5. The method of claim 4, wherein the gradient-wettability, self-cleaning nanopaper is prepared by the steps of: The step (3) of adding 2-10 mg-mL -1 ODA to react is reacted at 60°C for 5 h.

6. The method for preparing gradient wettability self-cleaning nanopaper according to claim 1, characterized in that, The step (4) of adding AZO to react is to add 0.1-0.9 mg·mL -1 of AZO to react at 60°C for 5 h.

7. The method for preparing gradient wettability self-cleaning nanopaper according to claim 1, characterized in that, Specifically, step (5) involves dissolving 20-100 mg of lignin in 10 mL of tetrahydrofuran (THF) to obtain a concentration of 2-10 mg / mL. -1 raw lignin solution; at 500 rmp·min -1 At a rotation speed of 1000 rpm, the original lignin solution was slowly added dropwise to 30 mL of ultrapure water; magnetic stirring was performed for 24 h, and lignin nanoparticles were obtained after the tetrahydrofuran (THF) evaporated.

8. The method for preparing gradient wettability self-cleaning nanopaper according to claim 1, characterized in that, The step (6) is specifically as follows: first, 10 ml of 0.2 wt% AZONF dispersion is filtered through a PTFE polytetrafluoroethylene microporous membrane; then, 0.5 mg·mL -1 of LNPs are deposited above the AZONF; finally, 20 mL of 0.1 wt% ODANF suspension is deposited above the LNPs; after drying at 40 ℃ for 12 h, a wetting gradient gradient composite film J-A / O@LNP is obtained; wherein the hydrophobic layer is denoted as the J-O layer, and the hydrophilic layer is denoted as the J-A layer.

9. A gradient-wettability self-cleaning nanopaper, characterized by, Prepared by the method of any one of claims 1-8.

10. Application of the gradient wettability self-cleaning nanopaper of claim 9 in oil-water separation.

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