A low molecular weight organic acid grafted modified forward osmosis composite membrane and its preparation method and application

Abstract

The present invention relates to a low-molecular-weight organic acid grafted modified forward osmosis composite membrane, its preparation method, and application. The composite membrane comprises a support layer and a separation layer that has been grafted twice. The grafted modified raw materials include polyethyleneimine and / or a low-molecular-weight organic acid. The preparation method comprises the following steps: preparation of the separation layer: on the support layer, interfacial polymerization is carried out using a solution of m-phenylenediamine and a solution of trimesoyl chloride to obtain a polyamide separation layer; primary modification of the separation layer: coating an aqueous solution of polyethyleneimine on the polyamide separation layer to obtain a primary grafted modified composite membrane; and secondary modification of the separation layer: coating an aqueous solution of a low-molecular-weight organic acid on the primary grafted modified composite membrane to obtain a secondary grafted modified low-molecular-weight organic acid grafted modified forward osmosis composite membrane. The composite membrane is used for forward osmosis treatment or recovery of fluorine-containing wastewater systems. Compared with the prior art, the present invention has the advantages of higher water flux and higher fluoride ion rejection rate during the forward osmosis process.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer separation membranes, and in particular to a low-molecular-weight organic acid grafted modified forward osmosis composite membrane, a preparation method thereof, and applications thereof. Background Art

[0002] The continuous development of modern industry has led to increasingly serious wastewater discharge problems, with the discharge of fluoride-containing wastewater increasing year by year. Excessive discharge of fluoride-containing wastewater can cause significant pollution to surrounding organisms and the environment. Drinking water with excessive fluoride levels can cause fluorosis, including dental fluorosis and skeletal fluorosis. Long-term excessive fluoride intake can lead to acute poisoning. The treatment of fluoride-containing wastewater is of vital importance, as it impacts both environmental safety and public health.

[0003] Membrane separation technology has developed rapidly over the past few decades and has been widely used to address various water treatment issues. Forward osmosis, an emerging membrane separation technology, effectively removes ions from water and is used in areas such as desalination and fluoride removal from wastewater. Its simplicity and low energy consumption make it a promising technology for future development. Thin-layer composite membranes are the most widely used type of membrane in membrane separation, and their permeability and separation performance can be further optimized through modification of the membrane material. Summary of the Invention

[0004] The purpose of the present invention is to overcome at least one of the above-mentioned defects of the prior art and to provide a low molecular weight organic acid grafted modified forward osmosis composite membrane with high permeability and high fluoride ion retention rate in the forward osmosis process, as well as its preparation method and application.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] The inventors have learned that surface grafting is a simple and effective modification method. In order to improve the permeability and selectivity of the forward osmosis composite membrane, surface modification of the forward osmosis composite membrane can enable the composite membrane to achieve higher water flux and retention rate. The specific scheme is as follows:

[0007] A low molecular weight organic acid grafted modified forward osmosis composite membrane comprises a support layer and a separation layer that has been grafted and modified twice. The raw materials for the grafting modification comprise polyethyleneimine and / or a low molecular weight organic acid.

[0008] Furthermore, the material of the support layer includes polyvinylidene fluoride, polysulfone, polyethersulfone or polypropylene; the material of the separation layer includes polyamide.

[0009] Furthermore, the low molecular weight organic acid includes glycolic acid, lactic acid or natural organic acid, and the natural organic acid includes malic acid, citric acid or tartaric acid.

[0010] Furthermore, the mass ratio of oxygen element to nitrogen element on the surface of the composite membrane is not less than 1.

[0011] A method for preparing the low molecular weight organic acid grafted modified forward osmosis composite membrane as described above, the method comprising the following steps:

[0012] Preparation of the separation layer: On the support layer, a polyamide separation layer is obtained by interfacial polymerization of a m-phenylenediamine solution and a trimesoyl chloride solution;

[0013] Primary modification of the separation layer: coating a polyethyleneimine aqueous solution on the polyamide separation layer to obtain a primary graft-modified composite membrane;

[0014] Secondary modification of the separation layer: coating a low molecular weight organic acid aqueous solution on the primary graft-modified composite membrane to obtain a secondary graft-modified low molecular weight organic acid graft-modified forward osmosis composite membrane.

[0015] Furthermore, the specific steps for preparing the separation layer are: pouring an aqueous solution of m-phenylenediamine on the surface of the support layer, reacting it first and then pouring off the excess solution on the membrane surface, then taking a n-hexane solution of trimesoyl chloride and pouring it on the surface of the support layer, reacting it again and then pouring off the excess solution on the surface, and heating it to obtain a polyamide separation layer.

[0016] Furthermore, the mass concentration ratio of the m-phenylenediamine solution to the trimesoyl chloride solution is (1.5-2.5):1, the ratio of the time of the first reaction to the second reaction is (1-5):(1-5), and the temperature of the heating treatment is 55-65° C. and the time is 6-10 min.

[0017] Furthermore, the solute concentration of the polyethyleneimine is 1-3 wt %, the solution pH is 3-6, the modification reaction temperature is room temperature, ie 20-40° C., and the coating time is 10-40 min.

[0018] Furthermore, the solute concentration of the low molecular weight organic acid is 0.8-1.2 wt %, the modification reaction temperature is room temperature, ie, 20-40° C., and the coating time is 10-40 min.

[0019] An application of the low molecular weight organic acid grafted modified forward osmosis composite membrane as described above, wherein the composite membrane is used for forward osmosis treatment or recovery of fluorine-containing wastewater systems.

[0020] Compared to existing technologies, the present invention introduces a low-molecular-weight organic acid onto the surface of a forward osmosis composite membrane. This introduced low-molecular-weight organic acid is rich in hydroxyl and carboxyl groups, enhancing its repulsion of anions. The introduction of the low-molecular-weight organic acid reduces the contact angle of water molecules on the composite membrane surface, facilitating solution permeation. Simultaneously, the ionization of carboxyl groups on the membrane surface generates a negative charge, increasing the composite membrane's selectivity for fluoride ions. The method of the present invention can form a forward osmosis composite membrane with high selectivity for fluoride ions. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The forward osmosis performance of the modified composite membrane in the examples is compared with that of the unmodified blank membrane and the single-modified membrane. DETAILED DESCRIPTION

[0022] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0023] A low-molecular-weight organic acid grafted modified forward osmosis composite membrane comprises a support layer, a separation layer, and secondary grafting modification of the separation layer; the support layer comprises polyvinylidene fluoride; the separation layer comprises polyamide; the grafting modification material of the separation layer comprises polyethyleneimine and a low-molecular-weight organic acid; the mass ratio of oxygen and nitrogen on the surface of the composite membrane is greater than 1, and the preparation method comprises the following steps:

[0024] (1) Preparation of polyamide separation layer:

[0025] Secure the polyvinylidene fluoride-based membrane in a square frame. Pour an appropriate amount of m-phenylenediamine aqueous solution onto the upper surface of the base membrane. After 1-5 minutes, remove any excess solution from the membrane surface and wipe the surface dry with filter paper. Next, pour an appropriate amount of trimesoyl chloride solution onto the membrane surface. Allow to react for 1-5 minutes, then remove any excess solution. This will yield a polyamide separation layer.

[0026] (2) Primary modification of the separation layer:

[0027] Pour an appropriate amount of polyethyleneimine aqueous solution onto the upper surface of the separation layer, react at room temperature for 10-40 minutes, and pour off the excess solution on the membrane surface to obtain a polyethyleneimine grafted modified forward osmosis composite membrane.

[0028] (3) Secondary modification of the separation layer:

[0029] Take an appropriate amount of low molecular weight organic acid aqueous solution and pour it on the upper surface of the polyethyleneimine grafted modified forward osmosis composite membrane, react at room temperature for 10-40 minutes, and pour off the excess solution on the membrane surface to obtain a low molecular weight organic acid grafted modified forward osmosis composite membrane.

[0030] Membrane performance test:

[0031] The main indicators of forward osmosis composite membrane performance are permeation flux and retention rate. Permeation flux refers to the volume of solvent that passes through the forward osmosis membrane per unit membrane area per unit time. It can be calculated using formula (1) by recording the mass change of the raw material liquid over time during the forward osmosis operation.

[0032]

[0033] Where Δm (g) is the mass change of the raw material solution under the test time Δt, ρ is the density of the raw material solution (g / mL), Am is the effective membrane area (m 2 ), Δt is the test time (h).

[0034] The retention rate is the ratio of the concentration of the solute retained by the membrane to the concentration of the solute in the original solution. It is generally calculated by measuring the solute concentration in the solution that has been injected. The retention rate is calculated using formula (2).

[0035]

[0036] Among them, C D,M and C F,M are the solute concentrations on the draw side at the end of the test (mg·L -1 ) and the solute concentration of the raw liquid side at the beginning of the test (mg·L -1 ).

[0037] Example 1

[0038] A low molecular weight organic acid grafted modified forward osmosis composite membrane comprises a support layer and a twice-modified separation layer; the support layer is made of polyvinylidene fluoride, the separation layer is made of polyamide, and the grafted modified raw materials include polyethyleneimine and low molecular weight organic acid-malic acid. The preparation method comprises the following steps:

[0039] (1) Prepare a 3.0% m-phenylenediamine aqueous solution and a 0.15% trimesoyl chloride / n-hexane solution. Fix the polyvinylidene fluoride-based membrane in a square frame. Pour an appropriate amount of the m-phenylenediamine aqueous solution onto the upper surface of the base membrane. After 3 minutes, pour off the excess solution on the membrane surface and wipe the surface dry with filter paper. Then, pour an appropriate amount of trimesoyl chloride / n-hexane solution onto the membrane surface, react for 1 minute, and pour off the excess solution on the surface to obtain a polyamide separation layer.

[0040] (2) Prepare a polyethyleneimine solution with a mass fraction of 3.0%, take an appropriate amount of polyethyleneimine aqueous solution and pour it on the upper surface of the polyamide separation layer, react at room temperature for 40 minutes, take it out and rinse it with deionized water several times to obtain a composite membrane with a single grafting modification.

[0041] (3) Prepare a malic acid aqueous solution with a mass fraction of 1.0% and a pH of about 3. Pour an appropriate amount of malic acid aqueous solution onto the surface of the primary grafted modified composite membrane. After reacting at room temperature for 40 minutes, pour off the excess solution on the membrane surface. After the reaction is complete, rinse again with deionized water, dry at room temperature for 10 minutes, and then transfer to a 40°C oven and dry for 10 minutes to obtain a secondary grafted modified composite membrane. Then, store in deionized water.

[0042] Membrane performance: The low molecular weight organic acid secondary grafted modified forward osmosis composite membrane prepared by the above method was -1 Fluoride ion aqueous solution is the raw material solution, 1.0 mol·L -1 The ammonium sulfate aqueous solution was used as the draw solution, and the permeate flux measured at room temperature was 29.5 L·m -2 ·h -1 The retention rate of fluoride ions is 99.61%.

[0043] Example 2

[0044] A low molecular weight organic acid grafted modified forward osmosis composite membrane consists of a support layer and a twice-modified separation layer; the support layer is made of polyvinylidene fluoride, the separation layer is made of polyamide, and the grafted modified raw materials include polyethyleneimine and low molecular weight organic acid-hydroxyacetic acid.

[0045] (1) Prepare a 3.0% m-phenylenediamine aqueous solution and a 0.15% trimesoyl chloride / n-hexane solution. Fix the polyvinylidene fluoride-based membrane in a square frame. Pour an appropriate amount of the m-phenylenediamine aqueous solution onto the upper surface of the base membrane. After 3 minutes, pour off the excess solution on the membrane surface and wipe the surface dry with filter paper. Then, pour an appropriate amount of trimesoyl chloride / n-hexane solution onto the membrane surface, react for 1 minute, and pour off the excess solution on the surface to obtain a polyamide separation layer.

[0046] (2) Prepare a polyethyleneimine solution with a mass fraction of 3.0%, take an appropriate amount of polyethyleneimine aqueous solution and pour it on the upper surface of the polyamide separation layer, react at room temperature for 40 minutes, take it out and rinse it with deionized water several times to obtain a composite membrane with a single grafting modification.

[0047] (3) Prepare a 1.0% hydroxyacetic acid aqueous solution with a pH of about 3. Pour an appropriate amount of the hydroxyacetic acid aqueous solution onto the surface of the primary grafted modified composite membrane. After reacting at room temperature for 40 minutes, pour off the excess solution on the membrane surface. After the reaction is complete, rinse again with deionized water, dry at room temperature for 10 minutes, and then transfer to a 40°C oven and dry for 10 minutes to obtain a secondary grafted modified composite membrane. Then, store in deionized water.

[0048] Membrane performance: The low molecular weight organic acid secondary grafted modified forward osmosis composite membrane prepared by the above method was -1 Fluoride ion aqueous solution is the raw material solution, 1.0 mol·L -1 The ammonium sulfate aqueous solution was used as the draw solution, and the permeate flux measured at room temperature was 22.8 L·m -2 ·h -1 The retention rate of fluoride ions is 99.73%.

[0049] Example 3

[0050] The difference from Example 1 is that malic acid is replaced by tartaric acid.

[0051] Example 4

[0052] The difference from Example 1 is that malic acid is replaced by lactic acid.

[0053] Comparative Example 1

[0054] A forward osmosis composite membrane, the preparation method of which comprises the following steps:

[0055] Prepare a 3.0% m-phenylenediamine aqueous solution and a 0.15% trimesoyl chloride / n-hexane solution. Secure the polyvinylidene fluoride-based membrane in a square frame. Pour an appropriate amount of the m-phenylenediamine aqueous solution onto the upper surface of the membrane. After 3 minutes, remove the excess solution from the membrane surface and wipe the surface dry with filter paper. Next, pour an appropriate amount of the trimesoyl chloride / n-hexane solution onto the membrane surface. Allow to react for 1 minute, remove the excess solution from the surface, remove the membrane from the frame, and heat in a 60°C oven for 8 minutes to obtain the polyamide separation layer. The membrane is then stored in deionized water.

[0056] Membrane performance: The unmodified forward osmosis composite membrane prepared by the above method was -1 Fluoride ion aqueous solution is the raw material solution, 1.0 mol·L -1 The ammonium sulfate aqueous solution was used as the draw solution, and the permeate flux measured at room temperature was 12.7 L·m -2 ·h -1 The rejection rate for fluoride ions is 99.83%.

[0057] Comparative Example 2

[0058] A one-step graft-modified forward osmosis composite membrane, the preparation method of which comprises:

[0059] (1) Prepare a 3.0% m-phenylenediamine aqueous solution and a 0.15% trimesoyl chloride / n-hexane solution. Fix the polyvinylidene fluoride-based membrane in a square frame. Pour an appropriate amount of the m-phenylenediamine aqueous solution onto the upper surface of the base membrane. After 3 minutes, pour off the excess solution on the membrane surface and wipe the surface dry with filter paper. Then, pour an appropriate amount of trimesoyl chloride / n-hexane solution onto the membrane surface, react for 1 minute, and pour off the excess solution on the surface to obtain a polyamide separation layer.

[0060] (2) Prepare a polyethyleneimine solution with a mass fraction of 3.0%, pour an appropriate amount of polyethyleneimine aqueous solution onto the upper surface of the polyamide separation layer, react at room temperature for 40 minutes, remove it, rinse it with deionized water several times, dry it at room temperature for 10 minutes, and then transfer it to a 40°C oven and dry it for 10 minutes to obtain a primary grafted modified composite membrane. Then, store it in deionized water.

[0061] Membrane performance: The single graft modified forward osmosis composite membrane prepared by the above method was -1 Fluoride ion aqueous solution is the raw material solution, 1.0 mol·L -1 The ammonium sulfate aqueous solution was used as the draw solution, and the permeate flux measured at room temperature was 14.2 L·m -2 ·h -1 The rejection rate for fluoride ions is 99.46%.

[0062] The present invention emphasizes the introduction of a low-molecular-weight organic acid into the surface of a forward osmosis composite membrane for surface modification. The introduced low-molecular-weight organic acid is rich in hydroxyl and carboxyl groups, enhancing its repulsion of anions. The introduction of the low-molecular-weight organic acid reduces the contact angle of water molecules on the composite membrane surface, facilitating solution permeation. Simultaneously, the ionization of carboxyl groups on the membrane surface generates a negative charge, enhancing the composite membrane's selectivity for fluoride ions. The method of the present invention can form a forward osmosis composite membrane with high selectivity for fluoride ions.

[0063] The difference between Comparative Example 2 and Comparative Example 1 lies in whether the forward osmosis composite membrane undergoes primary grafting modification. The membrane undergoing primary grafting modification exhibits better permeability. Compared with Comparative Examples 1 and 2, Examples 1 and 2 exhibit improved permeability due to the increased permeation flux of the forward osmosis composite membrane, while maintaining substantially no decrease in fluoride ion rejection, demonstrating superior separation performance.

[0064] Figure 1 Comparison of the forward osmosis performance of the modified composite membrane, the unmodified blank membrane, and the primary modified membrane in the examples (a) water flux and reverse salt flux, (b) fluoride ion rejection. The draw solution was a 1.0M (NH4)2SO4 aqueous solution, and the feed solution was 300mg·L -1NaF aqueous solution, AL-FS (the separation layer (active layer) of the membrane faces the feed solution) operation mode, 30°C, MA-malic acid, TA-tartaric acid, LA-lactic acid, GA-glycolic acid.

[0065] Will Figure 1 The water flux and reverse salt flux of the forward osmosis composite membrane modified with a small molecular weight organic acid for the second time in (a) are compared with those of the unmodified blank membrane and the first modified membrane. It can be seen that the water flux of the forward osmosis composite membrane modified with a small molecular weight organic acid for the second time has been greatly improved, while the reverse salt flux has been significantly reduced. Figure 1 (b) shows the fluoride ion rejection rates of the composite membrane after secondary modification with a small molecular weight organic acid, the unmodified blank membrane, and the single-modified membrane. It can be seen that the fluoride ion rejection rates of the composite membrane after secondary modification are basically the same as those of the single-modified membrane, and are slightly higher than those of the single-modified membrane. The fluoride ion rejection rates of the forward osmosis composite membrane after secondary modification are all higher than 99.5%, still showing a very high fluoride ion rejection rate, indicating that the use of polyethyleneimine primary modification plus low molecular weight organic acid secondary modification of the forward osmosis composite membrane is very effective in improving the comprehensive performance of the forward osmosis membrane, among which malic acid secondary modification has the best effect.

[0066] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other manner. Any person skilled in the art may utilize the above-disclosed technical content to modify or modify the present invention into equivalent embodiments. However, any simple modifications, equivalent variations, and modifications to the above embodiments that do not depart from the technical content of the present invention and are based on the technical essence of the present invention remain within the scope of protection of the present invention.

Claims

1. An application of a low molecular weight organic acid grafted modified forward osmosis composite membrane, characterized in that: The composite membrane is used for forward osmosis treatment or recovery of fluorine-containing wastewater systems; The composite membrane comprises a support layer and a separation layer which has been grafted and modified twice, wherein the raw materials for the grafting and modification comprise polyethyleneimine and a low molecular weight organic acid; The preparation method of the composite film comprises the following steps: Preparation of the separation layer: On the support layer, a polyamide separation layer is obtained by interfacial polymerization of a m-phenylenediamine solution and a trimesoyl chloride solution; Primary modification of the separation layer: coating a polyethyleneimine aqueous solution on the polyamide separation layer to obtain a primary graft-modified composite membrane; Secondary modification of the separation layer: coating a low molecular weight organic acid aqueous solution on the primary graft-modified composite membrane to obtain a secondary graft-modified low molecular weight organic acid graft-modified forward osmosis composite membrane; Wherein, the low molecular weight organic acid includes glycolic acid, lactic acid or natural organic acid, and the natural organic acid includes malic acid, citric acid or tartaric acid; The solute concentration of the low molecular weight organic acid is 0.8-1.2 wt %, the reaction temperature of the low molecular weight organic acid aqueous solution modification is 20-40° C., and the coating time of the low molecular weight organic acid aqueous solution is 10-40 min.

2. The use of a low molecular weight organic acid grafted modified forward osmosis composite membrane according to claim 1, characterized in that: The material of the support layer includes polyvinylidene fluoride, polysulfone, polyethersulfone or polypropylene; the material of the separation layer includes polyamide.

3. The use of a low molecular weight organic acid grafted modified forward osmosis composite membrane according to claim 1, characterized in that: The mass ratios of oxygen and nitrogen on the surface of the composite film are not less than 1.

4. The use of a low molecular weight organic acid grafted modified forward osmosis composite membrane according to claim 1, characterized in that: The specific steps for preparing the separation layer are: pouring an aqueous solution of m-phenylenediamine on the surface of the support layer, reacting it first and then pouring off the excess solution on the membrane surface, then taking a n-hexane solution of trimesoyl chloride and pouring it on the surface of the support layer, reacting it again and then pouring off the excess solution on the surface, and heating it to obtain a polyamide separation layer.

5. The use of a low molecular weight organic acid grafted modified forward osmosis composite membrane according to claim 4, characterized in that: The mass concentration ratio of the m-phenylenediamine solution to the trimesoyl chloride solution is (1.5-2.5):1, the ratio of the time of the first reaction to the second reaction is (1-5): (1-5), and the temperature of the heating treatment is 55-65°C and the time is 6-10 minutes.

6. The use of a low molecular weight organic acid grafted modified forward osmosis composite membrane according to claim 1, characterized in that: The solute concentration of the polyethyleneimine is 1-3 wt %, the solution pH is 3-6, the reaction temperature of the polyethyleneimine modification is 20-40° C., and the polyethyleneimine coating time is 10-40 min.

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