Modification method of PVDF (Polyvinylidene Fluoride) separation membrane

Abstract

The invention discloses a modification method of a PVDF (Polyvinylidene Fluoride) separation membrane, and belongs to the technical field of water treatment. The modification method provided by the invention comprises the step of impacting the PVDF separation membrane by adopting high-pressure liquid, so that high-pressure micro-droplets are in high-speed friction with the surface of the PVDF separation membrane. According to the modification method provided by the invention, the hydrophilicity, the surface potential and the dye retention rate of the PVDF separation membrane can be effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of water treatment, in particular to a modification method of a PVDF separation membrane. Background Art

[0002] Polyvinylidene fluoride (PVDF) is a membrane material widely used in water treatment processes. It has good chemical stability, wear resistance, fatigue resistance and insulation. However, the surface of PVDF separation membrane has the characteristics of non-polarity and strong hydrophobicity, which makes it easy to pollute and has serious membrane flux attenuation. In recent years, researchers have been trying new modification methods to address the problem of easy pollution of PVDF separation membranes, which can be mainly divided into bulk modification and surface modification. Bulk modification is to prepare modified membranes by adding hydrophilic, amphiphilic additives or inorganic nanoparticles to the casting solution. Commonly used modifiers include polymethyl methacrylate-acrylate, polyvinyl alcohol, nanorods coated with ferric oxide, catechol end groups, and polyethylene oxide and titanium dioxide modified with amphiphilic functional groups. Membrane surface modification is to modify the surface of the base membrane by coating a hydrophilic layer or introducing hydrophilic monomers by surface grafting by physical or chemical methods. Surface modification methods include coating, plasma deposition, ozone oxidation, etc.

[0003] The above modification methods have achieved good results, but each method requires the introduction of other chemical components besides PVDF, which may increase the preparation cost and process difficulty of PVDF separation membrane materials to a certain extent. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides a method for modifying a PVDF separation membrane, which can effectively improve the hydrophilicity, surface potential and dye retention rate of the PVDF separation membrane.

[0005] According to an embodiment of the first aspect of the present invention, a method for modifying a PVDF separation membrane is provided. The modification method comprises impacting the PVDF separation membrane with a high-pressure liquid.

[0006] The modification method according to the embodiment of the present invention has at least the following beneficial effects:

[0007] When the high-pressure liquid impacts the PVDF separation membrane, the high-pressure droplets rub against the surface of the PVDF separation membrane at high speed; during the process, first, electrons and / or ions are transferred to the inside and surface of the PVDF separation membrane, thereby enhancing its charge and hydrophilicity (adsorption of ions, enhancement of hydrogen bonds, adsorption of counterions), and improving its water flux; secondly, the high-pressure liquid and the surface of the PVDF separation membrane generate electrostatic high voltage, polarizing the surface of the PVDF separation membrane, enhancing polarity, and further improving the surface potential of the PVDF separation membrane. When a suitable high-pressure liquid is selected, the negative potential of the surface of the PVDF separation membrane is increased, thereby enhancing the electrostatic repulsion between it and anionic dyes such as Congo red, thereby improving the retention rate of the above-mentioned dyes.

[0008] According to some embodiments of the present invention, the PVDF separation membrane is attached to the surface of the substrate, so that during the impact process, the PVDF separation membrane will not be significantly deformed, and the modification of the PVDF separation membrane is more uniform.

[0009] According to some embodiments of the present invention, in the modification method, there is a relative motion between the PVDF separation membrane and the high-pressure liquid. Thus, the modification method can be carried out continuously, and the relative motion can be controlled to be a uniform motion, thereby improving the uniformity of the modification of the PVDF separation membrane. The speed of the relative motion, to a certain extent, represents the time per unit area of ​​the PVDF separation membrane to be modified, therefore, adjusting the speed of the relative motion can adjust the degree of modification of the PVDF separation membrane.

[0010] According to some embodiments of the present invention, in the modification method, the relative movement speed between the PVDF separation membrane and the high-pressure liquid is 0.8 to 1.2 m / min. For example, it can be about 1.0 m / min. In actual production, the substrate on which the PVDF separation membrane is located and / or the nozzle of the high-pressure liquid can be moved.

[0011] According to some embodiments of the present invention, the temperature of the high-pressure liquid is 20-40° C. For example, it may be about 25° C., 30° C. or about 35° C.

[0012] According to some embodiments of the present invention, the pressure of the high-pressure liquid is 2-4 bar. For example, it can be about 2.5 bar, 3.0 bar or about 3.5 bar. If there is no special explanation, the pressure here refers to the pressure given to the liquid by the high-pressure liquid nozzle.

[0013] According to some embodiments of the present invention, the distance between the nozzle that generates the high-pressure liquid and the PVDF separation membrane is 1.5 to 2.5 cm. For example, it can be about 2 cm. Unless otherwise specified, the above distance is the running distance of the high-pressure liquid, not the vertical distance between the nozzle and the PVDF separation membrane. By adjusting this distance, the pressure of the high-pressure liquid when it reaches the surface of the PVDF separation membrane can also be adjusted to a certain extent, thereby adjusting the effect of the modification method.

[0014] According to some embodiments of the present invention, the angle between the high-pressure liquid and the PVDF separation membrane is 45° to 50°, for example, about 46°, 47°, 48° or about 49°.

[0015] According to some embodiments of the present invention, the high-pressure liquid is selected from at least one of water, an acidic aqueous solution, a saline solution, or an alkaline aqueous solution.

[0016] According to some embodiments of the present invention, the water includes at least one of ultrapure water, deionized water and distilled water.

[0017] According to some embodiments of the present invention, when the high-pressure liquid is selected from an acidic aqueous solution, the solute of the acidic aqueous solution is HCl.

[0018] According to some embodiments of the present invention, when the high-pressure liquid is selected from an acidic aqueous solution, the concentration of the acidic aqueous solution is 0.1×10 -4 mol / L~1.0×10 -4 mol / L. For example, it can be about 0.5×10 -4 mol / L or about 0.8×10 -4 mol / L.

[0019] According to some embodiments of the present invention, when the high-pressure liquid is selected from an alkaline aqueous solution, the solute of the alkaline aqueous solution is at least one of NaOH, KOH and LiOH.

[0020] According to some embodiments of the present invention, when the high-pressure liquid is selected from an acidic aqueous solution, the concentration of the alkaline aqueous solution is 0.1×10 -4 mol / L~1.0×10 -4 mol / L. For example, it can be about 0.5×10 -4 mol / L or about 0.8×10 -4 mol / L.

[0021] Limiting the concentration of the alkaline aqueous solution or the acidic aqueous solution can promote ion transfer on the one hand, and prevent excessive free ions from generating a shielding effect and interfering with electron transfer on the other hand. In general, limiting the above concentration can significantly improve the modification effect of the PVDF separation membrane.

[0022] According to some embodiments of the present invention, the water contact angle of the PVDF separation membrane is reduced by 0.1 to 10° by the modification method, for example, by about 1°, 2°, 3°, 4°, 5°, 6°, 7°, 8° or about 9°.

[0023] According to some embodiments of the present invention, the modification method increases the surface negative potential of the PVDF separation membrane by ≥0.5 mV. For example, it can be 1.0 mV, 2.0 mV, 3.0 mV, 4.0 mV, 5.0 mV, 6.0 mV, 7.0 mV, 8.0 mV, 9.0 mV, 10.0 mV, 11.0 mV, 12.0 mV, 13.0 mV, 14.0 mV, 15.0 mV, 16.0 mV, 17.0 mV, 18.0 mV, 19.0 mV, 20.0 mV, 21.0 mV, 22.0 mV, 23.0 mV, 24.0 mV or about 25 mV. The increase in negative potential is reduced from the data. For example, the modification from -15 mV to -30 mV can be expressed as a negative potential increase of 15 mV.

[0024] According to some embodiments of the present invention, the water flux of the PVDF separation membrane increased by the modification method by ≥8%, for example, by about 8.5% or about 9.0%.

[0025] According to some embodiments of the present invention, the modification method can improve the dye retention rate of the PVDF separation membrane by ≥8%, for example, about 8.5% or about 9.0%.

[0026] If there is no special explanation, the actual meaning of “about” in the present invention is that the error is allowed to be within the range of ±2%, for example, about 100 is actually 100±2%×100.

[0027] Unless otherwise specified, “between” in the present invention includes the number itself, for example, “between 2 and 3” includes the endpoint values ​​2 and 3.

[0028] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0030] Figure 1 It is a schematic diagram of the mechanism of the modification method provided in Example 1 of the present invention.

[0031] Figure 2 It is the angle between the moving directions of the high pressure liquid and the PVDF separation membrane in the modification method adopted in Example 1 of the present invention.

[0032] Figure 3 This is a graph showing the changes in water flux and dye retention rate of the PVDF separation membrane before and after modification in Example 2 of the present invention. DETAILED DESCRIPTION

[0033] The following will be combined with the embodiments to clearly and completely describe the concept of the present invention and the technical effects produced, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0034] In the description of the present invention, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0035] Example 1

[0036] refer to Figures 1-2 This example provides a method for modifying a PVDF separation membrane, specifically:

[0037] The deionized water at a temperature of about 25°C is pressurized to 3 bar to impact the PVDF separation membrane; wherein,

[0038] During the impact process, the nozzle that sprays high-pressure liquid remains in place and is 2 cm away from the PVDF separation membrane (the running distance of the high-pressure liquid from the nozzle to the PVDF separation membrane); the PVDF separation membrane is attached to the substrate and moves with the substrate at a speed of 1 m / min.

[0039] The angle between the running direction of the PVDF separation membrane and the pressurized deionized water is 48°.

[0040] In the PVDF separation membrane used in this example, the number average molecular weight of PVDF is 60W.

[0041] Example 2

[0042] This example provides a method for modifying a PVDF separation membrane, which is different from Example 1 in that:

[0043] Replace the deionized water with a concentration of 10 -4 mol / L NaOH aqueous solution.

[0044] Example 3

[0045] This example provides a method for modifying a PVDF separation membrane, which is different from Example 1 in that:

[0046] Replace the deionized water with a concentration of 10 -4 mol / L HCl aqueous solution.

[0047] Test Example 1

[0048] In this example, a KruSS / DSA25 contact angle meter and a Sur-PASS solid surface Zeta potential meter were used to measure the contact angle (contact angle to deionized water) and surface potential of the surface of the modified PVDF separation membrane in the example.

[0049] The specific test results are shown in Tables 1 and 2.

[0050] Table 1 Surface potential and contact angle of PVDF separation membrane before and after modification of Examples 1 to 3

[0051]

[0052] The above test results show that the modification method provided in the embodiment of the present invention can significantly improve the hydrophilicity and surface negative potential of the PVDF separation membrane; precisely because of the optimization of the above performance, it can be expected that the modified PVDF separation membrane can effectively improve the water flux and the retention efficiency of anionic dyes.

[0053] Specifically, the modification effects of high-pressure liquids on PVDF separation membranes are ranked as follows: alkaline aqueous solution > deionized water > acidic aqueous solution.

[0054] The reason is that there are almost no active groups on the surface of the PVDF separation membrane, and it is inert as a whole; the surface modification of the PVDF separation membrane requires the help of active hydroxide in the high-pressure liquid. When the concentration is the same, the concentration of hydroxide in the high-pressure liquid is alkaline aqueous solution> deionized water> acidic aqueous solution. From another perspective, in the HCl solution process, hydrogen ion adsorption and electron transfer are competing processes. The hydrogen ion adsorption force is strong, which inhibits the electron transfer between water molecules and the PVDF separation membrane. Since the concentration of hydrogen ions in the HCl aqueous solution is very low, the PVDF separation membrane can still be modified, but the overall effect is not as good as that of the deionized water solution. In the NaOH solution process, hydroxide can also activate the PVDF inert membrane and change its hydrophilicity; compared with deionized water, the charge transfer amount is much larger, so the effect of NaOH solution is better than deionized water.

[0055] Test Example 2

[0056] This example tests the water flux and dye retention rate of the PVDF separation membrane before and after modification. The specific test conditions are: using a pressure of 1 bar to make the Congo red solution pass through the PVDF separation membrane before and after modification.

[0057] Test temperature: 25℃;

[0058] Test pressure 1 bar;

[0059] The effective tested area of ​​PVDF separation membrane is 22.06 cm 2 .

[0060] Before the test, the PVDF separation membrane was pre-pressed at a pressure of 1.5 bar for 0.5 h to reach a stable state, that is, deionized water was passed through it for 0.5 h under the above pressure conditions.

[0061] The test solution is a 0.05 g / L aqueous solution of Congo red dye;

[0062] During the test, the Congo red dye aqueous solution was passed through the PVDF separation membrane obtained in the example (or the raw material used) under the test pressure. The volume of liquid passing through the PVDF separation membrane was collected every 0.5 hours, and the volume of the permeate was recorded, and the test was performed for a total of 2 hours. The modified PVDF separation membrane obtained in each example was checked at least three times, and after eliminating the obviously abnormal results, the remaining results were averaged to prevent any errors.

[0063] The calculation formula of dye interception rate is: R = (1-C p / C r )×100%, where R represents the interception rate, C p and C rare the concentrations of the liquid passing through the PVDF separation membrane and the feed Congo red dye aqueous solution, respectively.

[0064] The water flux calculation formula is: F = V / A × t × p, F is the permeability (water flux) of the PVDF separation membrane at a given pressure, in units of L·m -2 ·h -1 bar -1 ; V is the volume of the permeate solution (liquid passing through the PVDF separation membrane), A is the effective area of ​​the PVDF separation membrane, and p is the filtration pressure.

[0065] The above test results are as follows Figure 3 And as shown in Table 3.

[0066] Table 3 Water flux and dye retention rate of PVDF separation membrane before and after modification

[0067]

[0068]

[0069] The above results show that the modification method provided by the present invention can significantly improve the water flux and dye retention rate of the PVDF separation membrane, thereby facilitating the further promotion and use of the PVDF separation membrane.

[0070] In summary, the present invention adopts a simple modification method, without changing the PVDF separation membrane matrix / surface material and without adding additional chemical reagents, and significantly improves its hydrophilicity, surface potential, water flux and dye retention rate, thereby greatly improving the practicality of the PVDF separation membrane. Most importantly, the modification method provided by the present invention is simple to operate, has good repeatability, low cost, and is easy to carry out large-scale production.

[0071] The embodiments of the present invention are described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the purpose of the present invention. In addition, the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

Claims

1. A method for modifying a PVDF separation membrane, characterized in that: The modification method comprises using high-pressure liquid to impact the PVDF separation membrane.

2. The modification method according to claim 1, characterized in that: In the modification method, the relative movement speed between the PVDF separation membrane and the high-pressure liquid is 0.8 to 1.2 m / min.

3. The modification method according to claim 1, characterized in that: The pressure of the high-pressure liquid is 2-4 bar.

4. The modification method according to claim 1, characterized in that: The angle between the high-pressure liquid and the PVDF separation membrane is 45° to 50°.

5. The modification method according to claim 1, characterized in that: The high-pressure liquid is selected from at least one of water, an acidic aqueous solution, a saline solution and an alkaline aqueous solution.

6. The modification method according to claim 5, characterized in that: When the high-pressure liquid is selected from an acidic aqueous solution, the solute of the acidic aqueous solution is HCl; and / or, when the high-pressure liquid is selected from an acidic aqueous solution, the concentration of the acidic aqueous solution is 0.1×10 -4 mol / L~1.0×10 -4 mol / L.

7. The modification method according to claim 5, characterized in that: When the high-pressure liquid is selected from an alkaline aqueous solution, the solute of the alkaline aqueous solution is at least one of NaOH, KOH and LiOH; and / or, when the high-pressure liquid is selected from an acidic aqueous solution, the concentration of the alkaline aqueous solution is 0.1×10 -4 mol / L~1.0×10 -4 mol / L.

8. The modification method according to any one of claims 1 to 7, characterized in that: The modification method reduces the water contact angle of the PVDF separation membrane by 0.1 to 10 degrees.

9. The modification method according to any one of claims 1 to 7, characterized in that: The modification method increases the surface negative potential of the PVDF separation membrane by ≥0.5 mV.

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