A charged heterostructured layered film, its preparation method and application

Abstract

The present invention belongs to the technical field of seawater desalination, and discloses a charged heterostructured layered membrane, a preparation method thereof and an application. The preparation method comprises the following steps: Step 1, preparing charged heteronanoplates; Step 2, diluting the charged heteronanoplates to a certain concentration and then preparing a charged heterostructured layered membrane with regular channels by low-pressure suction filtration. The charged heterostructured layered membrane prepared by the present invention can exhibit significantly excellent seawater desalination performance and mechanical stability compared with traditional positively charged and negatively charged layered membranes by relying on the nanoelectrostatic field formed by positively charged nano-domains and negatively charged nano-domains, and is suitable for seawater desalination applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of seawater desalination, and particularly relates to a charged heterogeneous layered membrane, a preparation method thereof and an application thereof. Background Art

[0002] The selective transport of charged substances is crucial for biological (proton pumps, ion pumps) and chemical engineering (osmotic energy conversion, seawater desalination) processes. However, different from molecular separation, high-selectivity transport and retention can be achieved depending on the differences in polarity and size. For ions, the retention is more difficult due to the deformation of the hydration shell and the dehydration effect. Especially for seawater desalination, as an important means to solve the shortage of fresh water resources, it has attracted extensive attention from researchers. However, the currently commonly used separation processes are multi-effect distillation and multi-effect flash evaporation. These processes rely on phase transformation to achieve excellent water / salt separation and consume a large amount of energy. In recent years, the developed membrane separation technology has brought the possibility of producing fresh water resources with high efficiency and low energy consumption. For example, only 2 kWh of energy is consumed to produce 1 m 3 of fresh water resources by pervaporation technology. However, the pervaporation process poses great challenges to the wettability and structural stability of the membrane.

[0003] As other forms of membrane separation technology, electrodialysis and reverse osmosis have also been widely studied. They enhance ion retention or increase water flux by introducing an electrostatic field and an external pressure respectively. However, the introduction of these strategies not only increases the energy consumption, but also the input of pressure disrupts the ion-channel interaction and reduces the retention rate. The main reason for this phenomenon is that traditional polymer membranes mainly achieve water / salt separation based on size sieving and electrostatic repulsion. There is a serious trade-off effect between water flux and ion retention (i.e., the "trade-off effect" between water flux and desalination rate). At the same time, the channel sizes in the polymer membrane are non-uniform, and the vertical ion transport path is short, resulting in a low ion retention rate. The external electric field can enhance ion retention and has little effect on the water flux, which is also proved by theoretical simulation. However, how to effectively construct channels with a long-range electrostatic nanoscale field to achieve efficient ion retention is still a huge challenge.

[0004] For the layered membrane assembled by two-dimensional nanosheets, since ions flow around between the layers, the transport path is long, and it is easy to realize the regulation of the channel environment by changing the physical and chemical properties of the nanosheets. However, limited by the uneven distribution of surface functional groups on traditional nanosheets and inaccurate and uncontrollable modification, the application of layered membranes is seriously hindered.

[0005] Therefore, it has become very necessary to develop a charged heterogeneous layered membrane with high ion retention performance, high water flux and good mechanical stability for seawater desalination. Summary of the Invention

[0006] In view of the above situation, the object of the present invention is to provide a charged heterostructured membrane, a preparation method and an application thereof. Compared with traditional positively charged and negatively charged layered membranes, the charged heterostructured membrane exhibits higher ion rejection performance and mechanical stability, and has a high water flux, and can be used for seawater desalination.

[0007] According to a first aspect of the present invention, there is provided a method for preparing a charged heterostructured membrane, the preparation method comprising the following steps:

[0008] Step 1, preparing charged heteronanoplates; preferably, the charged heteronanoplates are lateral heteronanoplates, which are composed of alternating distribution of multiple positively charged g-C3N4 nanodomains and negatively charged BN nanodomains, and the charged heteronanoplates are named BN / C3N4 nanoplates.

[0009] Step 2, after diluting the charged heteronanoplates to a certain concentration, preparing a charged heterostructured membrane with regular channels by low-pressure suction filtration. Preferably, the concentration of the charged heteronanoplates after dilution is 0.005-0.01 g L -1 ; more preferably, the concentration of the charged heteronanoplates after dilution is 0.01 g L -1 ; preferably, the pressure of the low-pressure suction filtration is 0.2-1.0 bar. More preferably, the pressure of the low-pressure suction filtration is 0.2-0.5 bar. Preferably, the pressure of the low-pressure suction filtration is 0.2 bar, 0.3 bar, 0.4 bar, 0.5 bar.

[0010] Preferably, in step 1, the preparation of the charged heteronanoplates includes:

[0011] Step 1-1, preparing a precursor of charged heteronanoplates by molecular-level pre-assembly at a certain temperature;

[0012] Step 1-2, preparing a layered nanomaterial by high-temperature polycondensation of the precursor of charged heteronanoplates;

[0013] Step 1-3, preparing charged heteronanoplates by ultrasonic-assisted exfoliation of the layered nanomaterial.

[0014] Preferably, in step 1-1, the process of pre-assembly at the molecular level is as follows: the temperature is controlled at 95-100 °C. Preferably, 95 °C, 95.5 °C, 96 °C, 96.5 °C, 97 °C, 97.5 °C, 98 °C, 98.5 °C, 99 °C, 99.5 °C, 100 °C can be selected, and further preferably 95 °C. The cyanuric acid aqueous solution and boric acid aqueous solution are added to the melamine aqueous solution in multiple alternating batches, stirred until complete precipitation, and the precipitate obtained after filtration is vacuum dried at 40 °C for 12 h to obtain a white powder-like charged heterostructured nanosheet precursor; in step 1-2, during the high-temperature polycondensation process of the charged heterostructured nanosheet precursor, ammonia gas is introduced throughout the process, and the ammonia gas flow rate is 15 mL min -1 ; in step 1-3, a certain amount of layered nanomaterial is dispersed in a water-ethanol mixed solution for ultrasonic-assisted exfoliation.

[0015] Preferably, in step 1-1, the molar ratio of cyanuric acid, boric acid, and melamine is 1:1:2. When 0.1 mol of cyanuric acid, 0.1 mol of boric acid, and 0.2 mol of melamine are taken as monomers respectively, the volumes of the solvent water are 3 mL, 3 mL, and 8 mL respectively; in step 1-2, the conditions for high-temperature polycondensation are to keep the temperature constant at 540 °C for 4 h, and then continue to heat up to 750 °C for 2 h, and the entire heating rate is kept constant at 3 °C min -1 ; in step 1-3, the volume ratio of water to ethanol in the water-ethanol mixed solution is 6:4, the ultrasonic-assisted exfoliation time is 10 h, and the obtained charged heterostructured nanosheet is named BN / C3N4 nanosheet.

[0016] According to the second aspect of the present invention, a charged heterostructured layer membrane is provided, which is prepared by using the preparation method described in the first aspect. Preferably, the thickness of the charged heterostructured layer membrane is 0.2-2.0 μm, and further preferably, the thickness is 1.0-2.0 μm. The specific thickness can be controlled by controlling the volume of the nanosheet dispersion liquid.

[0017] According to the third aspect of the present invention, the use of the charged heterostructured layer membrane of the second aspect of the present invention is provided, that is, its application in seawater desalination.

[0018] The operating steps and parameters not defined in the present invention can be conventionally selected according to the prior art.

[0019] The working principle of the present invention is that a heterostructured nanomaterial is used to cleverly couple the opposite electrochemical characteristics of two materials, providing an ideal platform for constructing a long-range electrostatic nanochannel to achieve efficient seawater desalination.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The charged heteronanoplates of the present invention successfully construct lateral heteronanoplates containing positively charged g-C3N4 nanodomains and negatively charged BN nanodomains through a precursor pre-assembly strategy at the molecular level. The charged heteronanoplates effectively couple the electrochemical properties of two different materials, avoiding the precipitation phenomenon caused by the mutual attraction of positively and negatively charged nanoplates during the conventional self-assembly process, and also avoiding the problem of inaccurate and uncontrollable functional modification of nanoplates.

[0022] 2. Based on the charged heteronanoplates as the building units, the present invention successfully prepares a two-dimensional layered membrane with a nanoelectrostatic field through low-pressure filtration. Here, the low-pressure filtration process ensures the orderly stacking of the charged heteronanoplates, forming regular interlayer channels. The orifices with a nanoelectrostatic field can severely inhibit the entry of ions into the nanochannels. At the same time, the ions entering the channels will also be affected by the electrostatic field, disturbing the ion movement and increasing the transport resistance. However, the transport of water molecules is hardly disturbed, and finally, highly selective water / salt separation is achieved. In addition, due to the electrostatic interaction between the charged heteronanoplates, the mechanical stability of the membrane is effectively enhanced, which is very promising for practical industrial applications. Moreover, the electrostatic repulsion between the charged heteronanoplates also provides sufficient interlayer spacing for water penetration, avoiding the problem of weakened interlayer spacing and decreased water flux caused by electrostatic attraction. Finally, the charged heterolayered membrane prepared by the present invention can exhibit more excellent ion rejection performance, water flux, and mechanical stability.

[0023] 3. The preparation method of the charged heteronanoplates of the present invention is simple, and the sizes of the positive and negative charge regions are easy to control. The charged heterolayered membrane prepared by the preparation method of the present invention is applied to the field of seawater desalination, which can effectively improve the ion rejection ability while maintaining a high water flux and excellent mechanical stability. Description of the Drawings

[0024] Figure 1 Transmission electron microscope image of the charged heteronanoplates prepared in Example 1.

[0025] Figure 2 Cross-sectional scanning electron microscope image of the charged heterolayered membrane prepared in Example 1.

[0026] Figure 3 Comparison chart of ion permeation rates of the layered membranes prepared in each example and comparative example in the forward osmosis mode.

[0027] Figure 4 Comparison chart of NaCl rejection and water flux of the layered membranes prepared in each example and comparative example.

[0028] Figure 5 Long-term operation stability performance chart of the layered membrane prepared in Example 1. Detailed Embodiments

[0029] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the accompanying Figures 1 to 5 drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0030] Example 1

[0031] A preparation method of a charged heterostructured layered film, the preparation method comprising the following steps:

[0032] 1) Preparation of charged heteronanoplates. Mainly through molecular-level pre-assembly to prepare a charged heteronanoplate precursor, and then under an ammonia atmosphere, high-temperature polycondensation is carried out, followed by ultrasonic-assisted exfoliation. The specific steps are as follows:

[0033] Dissolve 0.1 mol of cyanuric acid, 0.1 mol of boric acid, and 0.2 mol of melamine in beakers containing 3 mL, 3 mL, and 8 mL of deionized water respectively. Then heat the three beakers in an oil bath to 95 - 100 °C until the monomers are completely dissolved, and add the cyanuric acid aqueous solution and the boric acid aqueous solution alternately 15 times (about 0.2 mL each time) to the melamine aqueous solution in sequence. Continue heating and stirring until complete precipitation. The precipitate obtained after filtration is vacuum dried at 40 °C for 12 h to obtain a charged heteronanoplate precursor. Subsequently, the dried charged heteronanoplate precursor powder is placed in a porcelain boat. Under an ammonia atmosphere (the ammonia flow rate is 15 mL min -1 ), the temperature is raised to 540 °C at a heating rate of 3 °C min -1 and heated for 4 h, and then continue to heat to 750 °C and heat for 2 h. After the porcelain boat is cooled to room temperature, take 0.1 g of the product and uniformly disperse it in 1 L of a water-ethanol (the volume ratio of water to ethanol is 6:4) mixed solution. Then perform ultrasonic-assisted exfoliation for 10 h, centrifuge at a rotation speed of 5000 rpm for 20 min to remove the unexfoliated multi-layer charged heteronanoplates, and obtain charged heteronanoplates with a thickness of about 1.2 nm. The charged heteronanoplates are lateral heteronanoplates, composed of multiple positively charged g-C3N4 nanodomains and negatively charged BN nanodomains alternately distributed. Among them, the sizes of the g-C3N4 nanodomains and the BN nanodomains are about 20 - 40 nm, and its transmission electron microscope is as Figure 1 shown.

[0034] 2) Dilute 20 mL of the charged heteronanoplate dispersion to 200 mL, and perform vacuum filtration under a pressure of 0.2 - 0.5 bar to prepare a charged heterostructured layered film on a porous nylon substrate with a pore size of 200 nm. Subsequently, immediately immerse it in an aqueous solution for use. The thickness is 1.9 μm, denoted as film-1, and its cross-sectional scanning electron microscope (SEM) image is as Figure 2 shown.

[0035] Example 2

[0036] In step 2), 10 mL of the charged heteronanoplatelet dispersion was diluted to 200 mL, that is, the concentration of the charged heteronanoplatelets was diluted to 0.005 g L -1 , and vacuum filtration was carried out under a pressure of 1 bar to prepare a charged heterolayered membrane on a porous nylon substrate membrane. The remaining steps were the same as those in Example 1. The measured membrane thickness of the obtained charged heterolayered membrane was 0.98 μm, denoted as membrane-2.

[0037] Comparative Example 1

[0038] 1) Preparation of positively charged g-C3N4 nanosheets. Using the same pre-assembly method as in Example 1, 0.1 mol of melamine and 0.1 mol of cyanuric acid were respectively dissolved in 4 mL and 3 mL of aqueous solutions, and heated to 95-100 °C until the monomers were completely dissolved. Subsequently, the aqueous solution of cyanuric acid was slowly added to the aqueous solution of melamine, stirred until complete precipitation, and the obtained precipitate after filtration was vacuum dried for 12 h. Subsequently, the obtained powder was placed in an ammonia atmosphere and calcined at 540 °C for 4 h. Then, 0.1 g was dispersed in isopropanol and ultrasonically assisted for exfoliation for 10 h, and centrifuged at 5000 rpm for 20 min to prepare positively charged g-C3N4 nanosheets.

[0039] 2) A positively charged layered membrane was prepared using the same method as in Example 1, with a thickness of about 1.9 μm. The obtained membrane was denoted as membrane-3.

[0040] Comparative Example 2

[0041] 1) 0.1 g of boron nitride powder with a size of 5-10 μm was dissolved in 1 L of isopropanol solvent, ultrasonically assisted for exfoliation at 20 °C for 30 h, and centrifuged at 5000 rpm for 20 min to prepare negatively charged BN nanosheets.

[0042] 2) A negatively charged layered membrane was prepared using a method similar to that in Example 1, and the thickness was adjusted to be close to that in Example 1 by controlling the filtration volume. The obtained membrane was denoted as membrane-4.

[0043] Comparative test:

[0044] The membranes -1, -2, -3, and -4 prepared in Example 1, Example 2, Comparative Example 1, and Comparative Example 2 were subjected to performance tests:

[0045] 1. Ion permeation rate test of the layered membrane

[0046] The ion permeation performance was evaluated using a U-shaped device. 0.2 mol L was injected into the feed side and the permeation side respectively -1A salt solution (such as KCl, NaCl, CaCl2, etc.) and 50 mL of deionized water. To minimize the influence of concentration polarization, magnetic stirring was continuously carried out throughout the experiment, and the ion conductivity of the permeate was measured in real time with a conductivity meter.

[0047] The ion conductivity can be converted to ion concentration by the following formula:

[0048]

[0049] Where C, κ, and Λ m represent the ion concentration (mol L -1 ) on the permeate side, the ion conductivity (S cm -1 ), and the molar conductivity (S cm -1 / mol L -1 ), respectively. The ion diffusion rate can be calculated by the following formula:

[0050]

[0051] Where P i , V, C, A, and t represent the ion diffusion rate (mol m -2 h -1 ), the volume (L) of the permeate side and the ion concentration (mol L -1 ), the effective membrane area (1.767×10 -4 m 2 ), and the permeation time (h), respectively.

[0052] Membrane-1 ion permeation results: In the forward osmosis mode, with a 0.2 mol L -1 salt solution as the feed, the permeation rates of K + , Na + , Li + , Ca 2+ , and Mg 2+ are 0.00857 mol m -2 h -1 , 0.00683 mol m -2 h -1 , 0.00573 mol m -2 h -1 , 0.00326 mol m -2 h -1 , and 0.00213 mol m -2 h -1 , respectively.

[0053] Membrane-2 ion permeation results: In the forward osmosis mode, with a 0.2 mol L -1 salt solution as the feed, the permeation rates of K+ , Na + , Li + , Ca 2+ and Mg 2+ have permeation rates of 0.0138 mol m -2 h -1 , 0.0113 mol m -2 h -1 , 0.00944 mol m -2 h -1 , 0.00646 mol m -2 h -1 and 0.00552 mol m -2 h -1 .

[0054] Membrane - 3 ion permeation results: In the forward osmosis mode, with a 0.2 mol L -1 salt solution as the feed, the permeation rates of K + , Na + , Li + , Ca 2+ and Mg 2+ are 0.283 mol m -2 h -1 , 0.191 mol m -2 h -1 , 0.140 mol m -2 h -1 , 0.055 mol m -2 h -1 and 0.034 mol m -2 h -1 .

[0055] Membrane - 4 ion permeation results: In the forward osmosis mode, with a 0.2 mol L -1 salt solution as the feed, the permeation rates of K + , Na + , Li + , Ca 2+ and Mg 2+ are 0.652 mol m -2 h -1 , 0.546 mol m -2 h -1 , 0.467 mol m -2 h -1 , 0.331 mol m -2 h -1 and 0.287 mol m -2 h -1 .

[0056] The comparison of the ion permeation rates of Membrane-1, Membrane-2, Membrane-3, and Membrane-4 is shown in Figure 3 the figure. In the figure, Permeantionrate is the ion permeability, and Hydrated diameter is the hydrated diameter of the ion.

[0057] 2. Evaluation of the ion rejection and water permeation performance of the layered membrane

[0058] Membrane-1, Membrane-2, Membrane-3, and Membrane-4 were respectively placed in a U-shaped device. The ion concentration on the feed side was 0.1 mol L -1 of NaCl solution, and the driving side was a sucrose solution with a concentration of 2.0 mol L -1 . According to the Van't Hoff equation, the effective osmotic pressure on both sides of the osmotic cell was 44.5 bar. The water flux (J w , L m -2 h -1 bar -1 ) can be calculated by the following formula:

[0059]

[0060] where ΔV, A, ΔP, and t represent the change in liquid level difference (L) between the permeate side and the driving side, the effective membrane area (m 2 ), the effective osmotic pressure (bar), and the test time (h), respectively.

[0061] By monitoring the ion concentration on the permeate side, the ion rejection rate can be calculated, and the formula is as follows:

[0062]

[0063] where J r , C d , and C f represent ion rejection, the concentration on the driving side, and the concentration on the feed side (mol L -1 ), respectively.

[0064] Ion permeation results of Membrane-1: In the forward osmosis mode, with a 0.1 mol L -1 NaCl solution as the feed, the NaCl rejection rate was 99.7%, and the water flux was 77.1 mL m -2 h -1 bar -1 .

[0065] Ion permeation results of Membrane-2: In the forward osmosis mode, with a 0.1 mol L -1 NaCl solution as the feed, the NaCl rejection rate was 96.2%, and the water flux was 84.8 mL m -2 h -1bar -1 .

[0066] Ion permeation results of Membrane-3: In the forward osmosis mode, with a 0.1 mol L -1 NaCl solution as the feed, the NaCl rejection rate was 73.6%, and the water flux was 83.5 mL m -2 h -1 bar -1 .

[0067] Ion permeation results of Membrane-4: In the forward osmosis mode, with a 0.1 mol L -1 NaCl solution as the feed, the NaCl rejection rate was 62.5%, and the water flux was 95.0 mL m -2 h -1 bar -1 .

[0068] The comparison of ion rejection and water flux of Membrane-1, Membrane-2, Membrane-3 and Membrane-4 is as Figure 4 shown. In the figure, Waterpermeance is the water flux, and Salt rejection is the desalination rate.

[0069] 3. Mechanical stability test of the layered membrane

[0070] The conductivity meter was used to detect the concentration of ions permeating through Membrane-1 at different times to evaluate the stability of the channels, that is, the mechanical stability of the membrane. The results are as Figure 5 shown, where Concentration is the concentration of ions on the permeate side, Time is the effective permeation time, and Permeantion rate is the ion permeation rate.

[0071] Through the above comparative experiments, it can be seen that the charged heterostructure layered membrane prepared by the present invention, relying on the nano-electrostatic field formed by the positively charged g-C3N4 nano-domains and the negatively charged BN nano-domains, can exhibit significantly excellent seawater desalination performance and mechanical stability compared with traditional positively charged and negatively charged layered membranes.

[0072] The embodiments of the present invention have been described above. The above description is exemplary and not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations are obvious to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for preparing a charged heterolayered membrane, characterized in that, The preparation method includes: Step 1: Prepare charged heteronanoplates. The charged heteronanoplates are lateral charged heteronanoplates, which are composed of alternating distribution of multiple positively charged g-C3N4 nanodomains and negatively charged BN nanodomains, and the preparation steps include: Step 1-1: Prepare a charged heteronanoplate precursor by molecular-level pre-assembly at 95-100 °C; Step 1-2: Prepare a layered nanomaterial by high-temperature polycondensation of the charged heteronanoplate precursor; Step 1-3: Prepare charged heteronanoplates by ultrasonic-assisted exfoliation of the layered nanomaterial; Step 2: Dilute the charged heteronanoplatelets to a concentration of 0.005 - 0.01 g L -1 , and prepare a charged heterolayered membrane with regular channels and a nanoelectrostatic field by suction filtration under a low pressure of 0.2 - 0.5 bar.

2. The method for preparing a charged heterolayered film according to claim 1, wherein: In Step 1-1, the process of pre-assembly at the molecular level is as follows: the temperature is controlled at 95-100 °C, and the aqueous solutions of cyanuric acid and boric acid are alternately added to the aqueous solution of melamine in multiple portions and stirred until complete precipitation. The precipitate obtained after filtration is vacuum dried at 40 °C for 12 h to obtain a white powder-like charged heterostructured nanosheet precursor; in Step 1-2, ammonia gas is introduced throughout the high-temperature polycondensation process of the charged heterostructured nanosheet precursor, and the ammonia gas flow rate is 15 mL / min -1 ; in Step 1-3, a certain amount of layered nanomaterials is dispersed in a water-ethanol mixed solution for ultrasonic-assisted exfoliation.

3. The method for preparing a charged heterolayered film according to claim 2, wherein In Step 1-1, the molar ratio of cyanuric acid, boric acid, and melamine is 1:1:

2. When 0.1 mol of cyanuric acid, 0.1 mol of boric acid, and 0.2 mol of melamine monomers are taken respectively, the volumes of the solvent water are 3 mL, 3 mL, and 8 mL; in Step 1-2, the conditions for high-temperature polycondensation are to keep the temperature constant at 540 °C for 4 h and then continue to heat up to 750 °C for 2 h, and the heating rate is constant at 3 °C min -1 ; in Step 1-3, the volume ratio of water to ethanol in the water-ethanol mixed solution is 6:4, and the ultrasonic-assisted exfoliation time is 10 h, obtaining a charged heteronanoplate composed of alternating distribution of multiple positively charged g -C3N4 nanodomains and negatively charged BN nanodomains.

4. The method for preparing a charged heterolayered film according to claim 3, wherein In Step 2: the concentration of the charged heteronanoplates after dilution is 0.01 g L -1 .

5. A charged heterolayered membrane, characterized in that, Prepared by using the preparation method described in any one of claims 1 to 4.

6. The charged heterolayered film according to claim 5, characterized in that, The thickness of the charged heterolayered film is 0.2-2.0 μm.

7. Application of the charged heterolayered film according to claim 5 or 6 in seawater desalination.

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