A surface-segregation anti-pollution photocatalytic membrane and a preparation method thereof

Abstract

The application discloses a surface segregation anti-pollution photocatalytic membrane and a preparation method thereof, and the MAX phase is etched by a LiF+HCl mixed solution to obtain a two-dimensional MXene material with clear lamellar structure; a g-C3N4 photocatalytic material is prepared by a thermal polymerization method; PVDF is used as raw material, PVP is used as a hydrophilic agent, DMAc is used as a solvent, positively charged segregation agents MXene@PEI and negatively charged PAA-g-C3N4 with different concentrations are respectively introduced into casting solution and coagulation bath, and a MXene@PEI / PAA-g-C3N4 / PVDF photocatalytic anti-pollution membrane is prepared by a non-solvent induced phase separation (NIPS) method. The photocatalytic anti-pollution membrane material prepared by the application has a double anti-pollution mechanism of'resisting pollutants + degrading pollution', and provides a reference value for developing an ultrafiltration membrane with high hydrophilicity and photocatalytic capacity and a double anti-pollution mechanism. From the perspective of improving the structure of the membrane material, the treatment efficiency and the anti-pollution property of the membrane are improved, and finally, the practical purpose of green, environmental protection and reducing industrial wastewater treatment cost is achieved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of a separation membrane for water treatment, in particular to a surface segregation anti-pollution photocatalytic membrane and a preparation method thereof. BACKGROUND

[0002] Water shortage and pollution widely affect the sustainable development of industry and society, and are one of the most important challenges faced by mankind in the 21st century. Membrane separation technology is considered to be a new generation of water purification and recycling technology, and has the characteristics of being operable at room temperature, high separation efficiency, small equipment volume, and environmental friendliness. In the membrane separation process, suspended solids or soluble substances in the raw material liquid are easy to deposit on the membrane surface or in the pores to form membrane pollution, thereby reducing the separation efficiency of the membrane and limiting the further development and application of the membrane technology, and therefore, preparing a high-performance anti-pollution membrane has become a key problem in the field of membrane separation. The surface segregation technology can realize in-situ three-dimensional modification of an ultrafiltration membrane, has the characteristics of simultaneous film formation and modification, simple operation, and self-repairing, and is a very potential anti-pollution membrane preparation method. However, a single membrane modification method can only enhance the anti-pollution ability of the membrane to a certain extent, and cannot completely remove the pollutants. The combination of the membrane separation technology and the photocatalytic technology can prepare a photocatalytic membrane, and the deposited pollutants on the membrane surface can be directly removed through light irradiation.

[0003] MXene is a new type of two-dimensional (2D) transition metal carbon / nitride, and the general formula is M n+1 X n T x , wherein M is an early transition metal element, X is a carbon or nitrogen element, and T is a surface-attached active group. As a new type of 2D material, MXene has the characteristics of rich surface functional groups, uniform distribution, on-demand design and control, and better rigidity compared with other sheet materials. In recent years, researchers have been keen to use it for the development of high-performance separation membranes. For example, Professor Wang Haihui's research group prepared a MXene membrane by vacuum-assisted filtration stacking of MXene nanosheets on a polyether sulfone (PES) carrier. They found that a MXene membrane with a thickness of greater than 500 nm can realize more than 90% rejection of tetracycline through size sieving effect. Han et al. added different contents of MXene as an additive in a polyimide (P84) polymer matrix, and synthesized a blended nanofiltration membrane with a thickness of 150-200 μm by a phase inversion method. The rejection rate of the membrane to gentian violet dye is close to 100%, and the flux is as high as 268 L m -2 ·h -1 In addition, the sharp edges of the MXene nanosheet damage the bacterial membrane, resulting in loss of the integrity of the bacteria.

[0004] The research group of Long et al. prepared a two-dimensional MXene (Ti3C2T x) Nanosheets and zero-dimensional alumina (Al2O3) nanoparticles were simply self-assembled to prepare high-performance MXene membranes. By adjusting the mass ratio of MXene and Al2O3, the internal nanochannels and surface properties of the MXene membrane can be precisely controlled. However, this technology does not discuss the anti-pollution performance and recycling ability of the membrane, which is contrary to the actual application requirements of membrane separation. Second, this technology uses vacuum self-assembly to construct composite membranes, and some of the membrane materials will be lost during long-term permeation and cleaning, resulting in low separation efficiency. The common problem of two-dimensional material membranes is that they cannot be operated by cross-flow filtration, and their application prospects are still far from being realized. Third, the MXene nanosheets prepared by etching MAX phase with a mixture of HCl and LiF are multi-layer structures, which are difficult to stack orderly on the membrane surface, leading to poor anti-swelling performance of the membrane and unclear separation mechanism. Finally, the performance of the pure MXene / Al2O3 composite membrane needs to be further improved. For example, the salt ion rejection rate of this composite membrane is relatively low, with a maximum of only about 25%, so further micro-regulation of MXene is needed to enhance the separation performance of the membrane for different types of pollutants.

[0005] Dang's group successfully synthesized a new type of nitrogen-doped carbon dot composite material (NCD@BMCN) for the degradation of ciprofloxacin (CIP) under visible light irradiation. Zero-dimensional nitrogen-doped carbon dots (NCD) and one-dimensional bismuth molybdate (Bi2MoO6) nanorods are well dispersed on 2D CN nanosheets to form a Z-type heterojunction. NCD can act as a charge carrier, effectively promoting the migration of electron-hole pairs, reducing the band gap of BMCN from 2.64 eV to 2.07 eV for NCD@BMCN. NCD@BMCN can completely degrade CIP under visible light irradiation for 5 min, and still shows good stability after five consecutive light degradation cycles. However, powder photocatalysts have the problems of difficult separation and recovery, easy secondary pollution, and other issues during use, which seriously limit their effectiveness in practical applications. Moreover, this photocatalyst is mainly aimed at the removal of a single pollutant CIP, while the actual water environment is often complex, and the treatment of complex wastewater using photocatalysts needs to be further studied. During the photocatalytic degradation of pollutants, the powder photocatalyst is prone to agglomeration, which reduces the effective specific surface area of the powder and weakens the treatment effect.

[0006] Based on the above analysis, a new type of photocatalytic anti-pollution membrane material with a dual anti-pollution mechanism of "resisting pollutants + degrading pollutants" is urgently needed in the industry. SUMMARY

[0007] In view of the above shortcomings, the present application constructs a new type of photocatalytic anti-pollution membrane material, which has a dual anti-pollution mechanism of "resisting pollutants + degrading pollutants", and provides a reference value for developing an ultrafiltration membrane with a dual anti-pollution mechanism and high hydrophilicity and photocatalytic ability. From the perspective of improving the structure of the membrane material itself, the treatment efficiency and anti-pollution of the membrane are improved, and the practical purpose of green, environmental protection and reducing the cost of industrial wastewater treatment is finally achieved.

[0008] The present application is realized by the following technical means:

[0009] A preparation method of a surface segregation anti-pollution photocatalytic membrane, comprising:

[0010] (1) Preparation of MXene: a two-dimensional MXene material is prepared by chemical etching of a MAX phase with a LiF+HCl mixed solution, specifically comprising:

[0011] ① 8g of LiF is dissolved in 50mL of HCl (12mol / L) solution at room temperature, and 5g of Ti3AlC2 powder is added to the above solution, and magnetically stirred at a temperature of 25℃ for 24h;

[0012] ② The above solution is repeatedly centrifuged (3500rpm), and washed with deionized water (DI) for multiple times to neutralize the remaining acid until the supernatant pH of the solution is 6, and the supernatant is collected to obtain multi-layer MXene nanosheets;

[0013] ③ The multi-layer MXene nanosheets are dispersed in 100mL of deionized water, and continuously ultrasonically stripped under a nitrogen environment for 8h, then the dispersion is centrifuged (3500rpm) for 30min, and the obtained supernatant (single-layer MXene nanosheets) is collected and freeze-dried for storage.

[0014] The main chemical reactions are as follows:

[0015] Ti3AlC2 +3LiF+3HCl = AlF3 + 3 / 2H2 + Ti3C2+3LiCl (1-1)

[0016] Ti3C2 + 2H2O = Ti3C2(OH)2 + H2 (1-2)

[0017] Ti3C2 + 2LiF+2HCl = Ti3C2F2 + H2 + 2LiCl (1-3)

[0018] The Al layer of the MAX phase is stripped through reaction (1-1), and the -OH, -F and =O hydrophilic groups are generated on the surface of the MXene through reactions (1-2) and (1-3), and the excess electrons on the Ti metal surface are neutralized, to obtain a stable nanosheet structure.

[0019] (2) Preparation of g-C3N4: The photocatalytic material g-C3N4 is prepared by a thermal polymerization method, specifically including:

[0020] ① 15g of melamine powder is accurately weighed by an analytical balance;

[0021] ② The powder is transferred to a semi-closed alumina crucible and heated at 500℃ for 4h;

[0022] ③ After the crucible is naturally cooled to room temperature, the obtained sample is ground to obtain a yellow powder g-C3N4.

[0023] (3) Construction of surface segregation photocatalytic anti-pollution film, specifically including:

[0024] ① 0.216g of PEI solution is accurately weighed and poured into a reagent bottle containing 77g of DAMc solvent, and ultrasonic stirring is carried out at normal temperature and pressure for 10min;

[0025] ② 0.144g of MXene powder is accurately weighed and dissolved in the above solution, and ultrasonic stirring is carried out at 25℃ until the solution is uniformly dispersed;

[0026] ③ 4g of PVP is accurately weighed and dissolved in the above solution, and ultrasonic stirring is continued for 1h to mix uniformly;

[0027] ④ 19g of PVDF powder is accurately weighed and dissolved in the above solution, and mechanical stirring is carried out at 70℃ for 8h to fully dissolve, to obtain a homogeneous casting solution, and then the casting solution is naturally cooled and degassed at room temperature for 24h;

[0028] ⑤ 400mg, 800mg and 1200mg of PAA powder are respectively weighed and dissolved in 800mL of deionized water, ultrasonic stirring is carried out for 2h to disperse uniformly, and then 40mg of g-C3N4 is added respectively to continue ultrasonic stirring until complete dissolution, to obtain a coagulation bath of PAA-g-C3N4 with different concentrations;

[0029] ⑥ The completely degassed casting solution is taken out and slowly poured on a clean flat glass, the thickness of the coating applicator is adjusted to 200μm, and a film with uniform size and thickness is scraped out. After pre-evaporation in air for 30s, the film is horizontally immersed in the coagulation bath for 5min to cause phase separation, and after the film is completely detached from the glass plate, the film piece is taken out and placed in another container containing deionized water for immersion for 48h;

[0030] ⑦ Finally, the film piece is taken out again and washed with deionized water to remove the residual solvent on the film, and after drying, a surface segregation anti-pollution photocatalytic film is obtained.

[0031] The application also discloses a surface segregation anti-pollution photocatalytic film prepared by the above preparation method.

[0032] The present application has the beneficial effects of:

[0033] The present application constructs a new type of surface segregation anti-pollution film by NIPS method, which brings many excellent effects, mainly as follows:

[0034] 1. The present application realizes the dual anti-pollution of surface segregation anti-pollution and photocatalytic pollution reduction. Membrane pollution is the most intractable problem in membrane separation field, which needs repeated and complex membrane cleaning process to alleviate, which not only forms secondary pollution, but also reduces the separation efficiency of the membrane. The present application uses the excellent hydrophilicity and electrification of MXene-based materials to construct an ultrafiltration membrane with dual anti-pollution mechanism of high hydrophilicity and photocatalytic ability through electrostatic strengthening surface segregation strategy.

[0035] 2. The present application realizes the effective separation of small molecule pollutants and endows the membrane with photocatalytic ability. The separation effect of pure PVDF membrane (M0) on dye Congo red (CR) is not obvious, and the rejection rate is only 17.2%, while after modification, the rejection rates of surface segregation anti-pollution membrane (M1-M3) on CR are increased to 97.0%, 98.6% and 93.2% respectively (b). Figure 2 In addition, the combination of membrane separation technology and photocatalytic technology not only solves the problems of difficult recovery and the need for carriers of photocatalytic materials, but also can directly use the composite membrane for photocatalytic degradation of pollutants.

[0036] In general, the electrostatic strengthening surface segregation anti-pollution strategy cooperates with the photocatalytic pollution reduction strategy, which not only enhances the anti-pollution property of the composite membrane, but also enhances the reusability of the composite membrane, and improves the effective separation of small molecule pollutants in water. In addition, the introduction of photocatalytic materials also endows the membrane with photocatalytic performance, realizes the effective degradation of pollutants by the membrane, and shows good application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 It is a preparation route diagram of MXene@PEI / PAA-g-C3N4 / PVDF membrane.

[0038] Figure 2 It is (a) pure water flux, (b) CR and BSA rejection rate, (c) BSA flux change and FRR under four cycles and (d) Rh B removal rate (visible light irradiation for 8h) of different membranes. DETAILED DESCRIPTION

[0039] Abbreviations and key term definitions

[0040] MAX phase (Ti3AlC2), MXene (Ti3C2T xLiF (lithium fluoride), HCl (hydrochloric acid), DMAc (N,N-dimethylacetamide), PVP (polyvinylpyrrolidone), PAA (polyacrylic acid), PVDF (polyvinylidene fluoride), g-C3N4 (graphitic carbon nitride), PEI (polyethyleneimine)

[0041] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not intended to limit the present application.

[0042] Example 1

[0043] A preparation method of a surface-segregation anti-pollution photocatalytic film, comprising:

[0044] (1) Preparation of MXene: two-dimensional MXene material is prepared by chemical etching of MAX phase using LiF+HCl mixed solution, specifically comprising:

[0045] ① At room temperature, 8 g of LiF is dissolved in 50 mL of HCl (12 mol / L) solution, and 5 g of Ti3AlC2 powder is added to the above solution, and the solution is magnetically stirred at a temperature of 25°C for 24 h.

[0046] ② The above solution is repeatedly centrifuged (3500 rpm), and washed with deionized water (DI) multiple times to neutralize the remaining acid until the supernatant pH of the solution is 6, and the supernatant is collected to obtain multi-layer MXene nanosheets.

[0047] ③ The multi-layer MXene nanosheets are dispersed in 100 mL of deionized water, and ultrasonic stripping is performed under a nitrogen environment for 8 h, then the dispersion is centrifuged (3500 rpm) for 30 min, and the obtained supernatant (single-layer MXene nanosheets) is collected and freeze-dried for storage.

[0048] The main chemical reactions are as follows:

[0049] Ti3AlC2 + 3LiF + 3HCl = AlF3 + 3 / 2H2 + Ti3C2 + 3LiCl (1-1)

[0050] Ti3C2 + 2H2O = Ti3C2(OH)2 + H2 (1-2)

[0051] Ti3C2 + 2LiF + 2HCl = Ti3C2F2 + H2 + 2LiCl (1-3)

[0052] The Al layer of the MAX phase is peeled off by reaction (1-1), and the surface of the MXene is generated with hydrophilic groups such as -OH, -F and =O by reactions (1-2) and (1-3), and the excess electrons on the surface of the Ti metal are neutralized, to obtain a stable nanosheet structure.

[0053] (2) Preparation of g-C3N4: The photocatalytic material g-C3N4 is prepared by a thermal polymerization method, specifically including:

[0054] ① 15 g of melamine powder is accurately weighed with an analytical balance.

[0055] ② The powder is transferred to a semi-closed alumina crucible and heated at 500°C for 4 h.

[0056] ③ After the crucible is naturally cooled to room temperature, the obtained sample is ground to obtain a yellow powder g-C3N4.

[0057] (3) Construction of surface segregation photocatalytic anti-pollution film

[0058] ① 0.216 g of PEI solution is accurately weighed and poured into a reagent bottle containing 77 g of DAMc solvent, and ultrasonic stirring is performed at room temperature and normal pressure for 10 min.

[0059] ② 0.144 g of MXene powder is accurately weighed and dissolved in the above solution, and ultrasonic stirring is performed at 25°C until the solution is uniformly dispersed.

[0060] ③ 4 g of PVP is accurately weighed and dissolved in the above solution, and ultrasonic stirring is continued for 1 h to mix uniformly.

[0061] ④ 19 g of PVDF powder is accurately weighed and dissolved in the above solution, and mechanical stirring is performed at 70°C for 8 h to fully dissolve it, to obtain a homogeneous casting solution, and then the casting solution is naturally cooled and degassed at room temperature for 24 h.

[0062] ⑤ 400 mg, 800 mg and 1200 mg of PAA powder are respectively weighed and dissolved in 800 mL of deionized water, ultrasonic stirring is performed for 2 h to disperse uniformly, and then 40 mg of g-C3N4 is added respectively to continue ultrasonic stirring until complete dissolution, to obtain a coagulation bath of different concentrations of PAA-g-C3N4.

[0063] ⑥ The completely degassed casting solution is taken out and slowly poured onto a clean flat glass, the coating film thickness is adjusted to 200 μm, and a film with uniform size and thickness is scraped out. After pre-evaporation in air for 30 s, the film is immersed horizontally in the coagulation bath for 5 min to undergo phase separation, and after the film is completely detached from the glass plate, the film is taken out and placed in another container containing deionized water for immersion for 48 h.

[0064] ⑦Finally, the membrane was taken out again and washed with deionized water to remove the residual solvent on the membrane, and after drying, MXene@PEI / PAA-g-C3N4 / PVDF membrane (M1~M3) was obtained.

[0065] ⑧The permeation flux and the rejection and removal rate of protein molecules (bovine serum albumin (BSA)) and dye molecules (rhodamine B (Rh B) and Congo red (CR)) of the surface segregation anti-pollution photocatalytic membrane prepared at each different concentration coagulation bath were tested at room temperature and normal pressure, respectively, as performance evaluation indicators of the membrane. At the same time, the pure PVDF membrane (M0) was also obtained by the same method.

[0066] Table 1 Casting solution and coagulation bath parameters of different membranes

[0067]

[0068] Combined with the results of Table 1 and Figure 2 (a~c), the permeation flux of the pure PVDF membrane (M0) was 500.8 L / (m 2 ·h), the rejection rate of BSA was 45.2%, and the flux recovery rate (FRR) of the membrane was 79.3% after four cycles. The permeation flux of the surface segregation anti-pollution membrane (M1~M3) was 81.2, 146.5 and 187.3 L / (m 2 ·h), respectively, the rejection rate of BSA was increased to 96.8%, 95.8% and 94.7%, respectively, and the FRR of the membrane was increased to 97.0%, 93.5% and 92.8%, respectively, after four cycles. The separation effect of the pure PVDF membrane (M0) on the dye Congo red (CR) was not obvious, and the rejection rate was only 17.2%, while after modification, the rejection rate of CR of the surface segregation anti-pollution membrane (M1~M3) was increased to 97.0%, 98.6% and 93.2%( Figure 2 (b)), respectively. After 8h visible light irradiation, the removal rate of Rh B was as high as 90.0%, 92.4% and 91.5%( Figure 2 (d)), respectively.

[0069] The above only describes some preferred embodiments of the present application, and does not limit the present application in any form. Those skilled in the art can make many equivalent changes and improvements and modifications under the inspiration of the present application without departing from the inventive concept, and these all belong to the protection scope of the present application. In the complete technical scheme of the present application, the following ways can still be used to prepare the surface segregation anti-pollution photocatalytic membrane to achieve the purpose of the present application:

[0070] 1. Except for etching MAX phase by LiF+HCl mixed reagent, if other people use HF, NH4HF2, molten fluoride salt, NaOH and H2SO4 and other methods to etch to prepare MXene, other steps consistent with the technical scheme of the application will also prepare surface segregation photocatalytic anti-pollution film, and achieve the purpose of the application.

[0071] 2. The high molecular material used in the application is PVDF, if other people use cellulose acetate (CA), polyether sulfone (PES) or polysulfone (PSF) and other organic polymers as casting film material, other steps consistent with the technical scheme of the application will also prepare surface segregation photocatalytic anti-pollution film, and achieve the purpose of the application.

[0072] 3. The application uses g-C3N4 as a photocatalyst added to the coagulation bath, if other people use other photocatalytic materials such as bismuth-based photocatalysts (BiOCl, BiOBr) or metal oxides (TiO2, ZnO) added to the coagulation bath, other steps consistent with the technical scheme of the application will also prepare surface segregation photocatalytic anti-pollution film, and achieve the purpose of the application.

[0073] 4. The application uses PEI to modify MXene, if other people use chitosan (CS), 3-aminopropyl triethoxysilane (APTES) or polyethylene glycol (PEG) and other modifiers to modify MXene to be positively charged, other steps consistent with the technical scheme of the application will also prepare surface segregation photocatalytic anti-pollution film, and achieve the purpose of the application.

Claims

1. A method for preparing a surface segregation anti-pollution photocatalytic membrane, comprising: (1) Preparation of MXene nanosheets: MXene nanosheets are prepared by chemical etching of MAX phase with LiF+HCl mixed solution; (2) Preparation of g-C3N4: photocatalytic material g-C3N4 is prepared by thermal polymerization method; (3) Construction of surface segregation anti-pollution photocatalytic membrane: ① PEI solution is mixed with DMAc solvent, ultrasonic stirring for 10 min, then MXene powder is added, ultrasonic stirring at 25℃ until the solution is uniformly dispersed, to obtain a first mixed solution; ② PVP is added to the first mixed solution, ultrasonic stirring for 1 h, then PVDF powder is added, mechanical stirring and natural cooling, then degassing at room temperature to obtain a degassed casting solution; ③ 400 mg, 800 mg and 1200 mg of PAA powder are weighed and dissolved in 800 mL of deionized water, ultrasonic stirring for 2 h, then 40 mg of g-C3N4 is added and ultrasonic stirring is continued until complete dissolution, to obtain PAA-g-C3N4 coagulation bath with different concentrations; ④ The degassed casting solution is poured on a flat glass plate, a membrane with a thickness of 200 μm is scraped out, the membrane is pre-evaporated in air, then immersed horizontally in the PAA-g-C3N4 coagulation bath, and after it completely falls off from the glass plate, the membrane is taken out and immersed in deionized water for 48 h; ⑤ Finally, the membrane is taken out again and washed with deionized water, and after drying, a surface segregation anti-pollution photocatalytic membrane is obtained.

2. The method according to claim 1, wherein: in step ①, the mass of PEI solution, DMAc solvent and MXene powder is 0.216 g, 76.64 g and 0.144 g, respectively.

3. The method according to claim 1, wherein: in step ②, the mass of PVP and PVDF powder is 4 g and 19 g, respectively.

4. The method according to claim 1, wherein: in step ②, the mechanical stirring temperature is 70℃ and the stirring time is 8 h; the degassing time is 24 h.

5. The method according to claim 1, wherein: in step ④, the pre-evaporation time is 30 s; the immersion time in the coagulation bath is 5 min; the immersion time in deionized water is 48 h.

6. The method according to claim 1, wherein: in step (1), the preparation of MXene includes: 8 g of LiF is dissolved in 50 mL of HCl, then 5 g of Ti3AlC2 powder is added, magnetic stirring at 25℃ for 24 h to obtain a first solution; the first solution is repeatedly centrifuged and washed with deionized water, the supernatant is collected to obtain multi-layer MXene nanosheets; the multi-layer MXene nanosheets are dispersed in deionized water, ultrasonic stripping under nitrogen environment, then the dispersion liquid is centrifuged to obtain MXene nanosheets.

7. The method according to claim 6, wherein: the first solution is washed with deionized water until the pH value of the supernatant is 6.

8. The method according to claim 6, wherein: The ultrasonic peeling time is 8 h, and the centrifugal treatment time of the dispersion liquid is 30 min.

9. The preparation method of claim 1, wherein: The preparation of g-C3N4 in step (2) comprises: 15 g of melamine powder is weighed, the powder is transferred into a semi-closed alumina crucible, heated at 500 °C for 4 h, and the sample is ground after the crucible is naturally cooled to room temperature to obtain g-C3N4.

10. A surface-segregated anti-pollution photocatalytic film prepared by the preparation method of any one of claims 1-9.

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