An Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane, its preparation method and application

Abstract

This invention discloses an Alg / PVA-based single-atom hydrogel catalytic distillation membrane, its preparation method, and its application, belonging to the field of membrane distillation technology. The preparation method of the Alg / PVA-based single-atom hydrogel catalytic distillation membrane of this invention includes the following steps: uniformly coating a mixed hydrogel precursor solution of sodium alginate, polyvinyl alcohol, and phytic acid onto the surface of a hydrophilically modified hydrophobic microporous membrane, and then wetting and crosslinking it using a transition metal ion crosslinking solution to obtain the Alg / PVA-based single-atom hydrogel catalytic distillation membrane. The Alg / PVA-based single-atom hydrogel catalytic distillation membrane prepared by this invention can exhibit excellent performance in removing volatile organic compounds without sacrificing the permeation flux of the original hydrophobic base membrane, and also demonstrates excellent long-term stability in actual seawater.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of membrane distillation, and in particular to an Alg / PVA-based monatomic hydrogel catalytic membrane for membrane distillation and a preparation method and application thereof. BACKGROUND

[0002] Wastewater generally contains a variety of water-soluble volatile organic compounds (VOCs). As a high-efficiency heat-driven membrane separation technology, membrane distillation (MD) has shown significant advantages in the treatment of high-salinity wastewater, and can theoretically achieve nearly 100% salt rejection. However, when MD is used for VOCs wastewater treatment, the hydrophobic membrane material is easily polluted by organic matter, resulting in membrane performance degradation. Small-molecule VOCs can penetrate the membrane pores due to their hydrophobicity and volatility, which seriously deteriorates the quality of the produced water, and greatly limits the practical application and promotion of MD technology in the treatment of complex wastewater containing VOCs.

[0003] Fenton technology, as a representative of advanced oxidation processes (AOPs), is based on the use of iron-based catalysts (Fe 2+ / Fe 3 + ) to activate hydrogen peroxide (H2O2) to produce strong oxidizing hydroxyl radicals (·OH), which can non-selectively mineralize various organic pollutants (including VOCs) into carbon dioxide and water. Therefore, coupling Fenton catalytic oxidation with MD process (i.e., catalytic membrane distillation) is considered as a very promising solution, which is expected to in-situ degrade VOCs at the membrane interface, thereby simultaneously solving the problems of penetration and pollution, and ensuring the quality of the produced water. However, the traditional homogeneous Fenton technology has two key bottlenecks. One is the pH dependence, which limits the optimal reaction activity window to an acidic environment (pH≈3). Therefore, in actual wastewater treatment, a large amount of acid needs to be added to adjust the pH to meet the reaction conditions, which not only significantly increases the operation complexity and treatment cost, but also introduces an additional salt burden. The second is that iron sludge is produced during the reaction, which leads to low catalyst recycling efficiency: the reduction of Fe 3+ to Fe 2+ is slow, resulting in the accumulation of Fe 3+ and the formation of a large amount of iron sludge (precipitates such as Fe(OH)3) during the reaction, causing secondary pollution and catalyst loss, and affecting the treatment efficiency.

[0004] To overcome the inherent defects of homogeneous Fenton, heterogeneous Fenton-like reactions based on other transition metals (such as Cu, Co, Mn, etc.) have attracted extensive attention. Such reactions can usually activate oxidants such as hydrogen peroxide (H2O2) or persulfate (PMS) to produce reactive oxygen species (ROS) in a wider pH range (especially near neutral conditions), thereby degrading organic matter, and have better environmental adaptability and application potential. However, in the prior art, heterogeneous catalysts are generally used in Fenton-like reactions in a "powder suspension" reaction system, that is, the transition metal-based heterogeneous catalyst (such as Fe3O4, CuO nanoparticles, Co-doped carbon materials, etc.) is directly dispersed in the wastewater in powder form, and H2O2 / PMS is activated to produce reactive oxygen species (ROS) through stirring or fluidized bed device. There are risks such as difficulty in catalyst recovery, low mass transfer efficiency, etc. If the catalyst is integrated on the membrane surface, a "membrane separation (desalination) + catalytic oxidation step-by-step process" needs to be used, which leads to process complication and increased energy consumption, etc. Therefore, researchers have attempted to integrate heterogeneous catalysts on the membrane surface of membrane distillation, realizing a "membrane-catalysis integrated" design to solve the problems of process complication and difficulty in catalyst recovery existing in the prior art.

[0005] The current method for integrating heterogeneous catalysts on the membrane surface of membrane distillation is the "synthesis first and then loading" strategy: first, single-atom catalyst (SACs) powder (metal single-atom catalyst with high atom utilization rate and unique electronic structure) is prepared through high-temperature pyrolysis, etc., and then it is dispersed in a coating liquid and finally fixed on the membrane surface through physical or chemical methods. However, this strategy has significant limitations: 1) The synthesis of SACs usually involves high-temperature calcination, complex templates or carriers, which is complicated, has low yield, is high in cost and is difficult to scale up; 2) When SACs powder is loaded on the membrane surface, it is easy to agglomerate and deactivate, resulting in a decrease in active sites; at the same time, the binding force between the catalyst and the membrane substrate is often insufficient, and it is easy to fall off under long-term operation or complex water chemical environment, and the stability is difficult to guarantee; 3) The traditional loading coating layer can significantly increase the mass transfer resistance or block the membrane pores, resulting in a decrease in MD permeation flux.

[0006] Therefore, it is urgent to obtain a catalytic functional layer with a simple and efficient preparation method, low cost and scale-up production, which can stably immobilize high-activity single-atom metal sites on the MD membrane surface, realize efficient in-situ degradation of VOCs under the condition of no need to adjust pH, and at the same time maintain excellent permeation flux, anti-pollution performance and long-term operation stability, thereby promoting the sustainable application of catalytic membrane distillation technology in actual seawater desalination and high-salinity wastewater treatment. SUMMARY

[0007] The purpose of the present application is to provide an Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane and its preparation method and application, to solve the problems existing in the prior art.

[0008] To achieve the above object, the present application provides the following scheme:

[0009] One of the technical solutions of the present application is a preparation method of an Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane, comprising the following steps:

[0010] A mixed hydrogel precursor solution (i.e. an Alg / PVA-based hydrogel precursor solution) of sodium alginate (Alg), polyvinyl alcohol (PVA) and phytic acid (PA) is uniformly coated on the surface of a hydrophilically modified hydrophobic microporous membrane to obtain an Alg / PVA-based pre-gel membrane, and then the Alg / PVA-based pre-gel membrane is infiltrated and crosslinked by a transition metal ion crosslinking solution to obtain the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane.

[0011] Further, the concentration of sodium alginate in the mixed hydrogel precursor solution is 2-4wt%, the concentration of polyvinyl alcohol is 2-5wt%, and the concentration of phytic acid is 0.2-0.5wt%.

[0012] Still further, the preparation method of the mixed hydrogel precursor solution comprises the following steps:

[0013] Polyvinyl alcohol (PVA) and phytic acid (PA) are added to water, heated and stirred until completely dissolved, then sodium alginate (Alg) is added at room temperature under rapid stirring until completely dissolved, and the mixed hydrogel precursor solution (i.e. the Alg / PVA-based hydrogel precursor solution) is obtained by cold storage and standing to remove bubbles.

[0014] Still further, the heating temperature is 90-100℃, and the rapid stirring speed is 500-1000rpm.

[0015] Still further, the uniform coating comprises using a doctor blade with a gap of 100 microns for uniform coating.

[0016] Still further, the preparation method of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane further comprises a drying step after the mixed hydrogel precursor solution of sodium alginate (Alg), polyvinyl alcohol (PVA) and phytic acid (PA) is uniformly coated on the surface of the hydrophilically modified hydrophobic microporous membrane.

[0017] The drying method comprises natural air drying or drying at 30-60℃.

[0018] Further, the material of the hydrophobic microporous membrane comprises one of intrinsic hydrophobic polytetrafluoroethylene (PTFE) and its composite material, and polyvinylidene fluoride (PVDF) and its composite material.

[0019] Further, the method for hydrophilic modification (activation) includes one or more of the following methods:

[0020] Method 1, soaking the hydrophobic microporous membrane in a sodium hydroxide solution;

[0021] Method 2, depositing a mixture of polydopamine or polydopamine and polyethyleneimine on the surface of the hydrophobic microporous membrane;

[0022] Method 3, adhering tannic acid and 3-aminopropyltriethoxysilane on the surface of the hydrophobic microporous membrane;

[0023] Method 4, plasma treatment of the hydrophobic microporous membrane.

[0024] Further, the transition metal ion cross-linking solution is an aqueous solution of a multivalent transition metal salt, and the concentration of transition metal ions is 0.1-1 mol / L.

[0025] Further, the multivalent transition metal salt includes one or more of cobalt chloride (CoCl2), cobalt nitrate (Co(NO3)2), ferrous chloride (FeCl2), ferric chloride (FeCl3), copper chloride (CuCl2), copper nitrate (Cu(NO3)2), and ferrous sulfate (FeSO4).

[0026] Further, the time for infiltration cross-linking is 0.5-24 h.

[0027] Further, the thickness of the Alg / PVA-based hydrogel layer on the Alg / PVA-based pre-gel membrane is 2-10 μm.

[0028] Hydrogel is a hydrophilic material with a three-dimensional network structure formed by hydrophilic polymer chains through chemical cross-linking or physical interaction (such as hydrogen bonding, hydrophobic interaction, ionic cross-linking, etc.). Its abundant hydrophilic groups (such as hydroxyl, carboxyl, amino, etc.) can provide a large number of coordination sites for metal ions, which can effectively fix and disperse metal ions through complexation. This in-situ anchoring of metal ions provides an ideal platform for constructing SACs-based hydrogel coatings. In addition, hydrogel applied to the surface coating of MD membranes has unique advantages: 1) hydrogel contains a large amount of bound water, which can provide a low water evaporation enthalpy when evaporating on the hot side interface, and theoretically can maintain a high water vapor flux; 2) the highly hydrophilic hydrogel layer on the surface can form a dense hydration layer, effectively blocking the adhesion of hydrophobic organic pollutants (such as oils) on the membrane surface, and improving the anti-pollution ability; 3) the synthesis and cross-linking of hydrogel are usually carried out at room temperature or under mild conditions, avoiding high-temperature treatment, and the process is simple and easy to scale up.

[0029] The hydrophilic modification treatment of the surface of the hydrophobic microporous membrane can significantly enhance the adhesion of the subsequent hydrogel coating to the surface of the base membrane, effectively preventing the peeling of the coating under long-term operation or hydraulic shear. The carboxyl groups of sodium alginate (Alg), the phosphate groups in phytic acid (PA), and the hydroxyl groups on the molecular chain of polyvinyl alcohol (PVA) can be used as metal ion cross-linking ligands. The transition metal ions (M) can be cross-linked under mild conditions, and the transition metal ions (such as Fe, Cu, Co, etc.) are highly dispersedly anchored in the Alg / PVA-based hydrogel three-dimensional network structure, effectively avoiding the agglomeration of metal ions, forming high-activity and high-density single-atom catalytic sites. This in-situ chelation-crosslinking one-step method is significantly better than the traditional "synthesis first and then loading" single-atom catalyst preparation process. The transition metal ions (M) not only act as active sites, but also act as efficient physical cross-linking agents, forming multiple coordination bonds and ionic bonds with the carboxyl groups of Alg, the hydroxyl groups of PVA, and the phosphate groups of PA, greatly enhancing the mechanical strength and chemical stability of the hydrogel network structure.

[0030] The entire membrane preparation process is completed in an aqueous solution environment at room temperature or near room temperature, completely avoiding the high-temperature calcination, complex templates or carriers required for traditional single-atom catalyst synthesis, and has the advantages of simple preparation steps (activation-coating-crosslinking), easy-to-obtain raw materials (Alg, PVA, PA are all bulk commodities and non-toxic), mild conditions, easy to scale up and produce, etc., which can lay a solid foundation for industrial application.

[0031] The second technical solution of the present application is an Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane prepared by the above preparation method.

[0032] The third technical solution of the present application is the application of the above-mentioned Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane in seawater desalination or VOCs wastewater treatment.

[0033] Further, the application method comprises: taking seawater or VOCs wastewater added with an oxidizing agent (such as H2O2, PMS) as a feed liquid, and contacting the Alg / PVA-based single-atom hydrogel layer (hot side interface) of the Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane with the feed liquid to perform a Fenton catalytic reaction.

[0034] The Alg / PVA-based single-atom hydrogel layer side of the Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane is the feed liquid, and the other side is the permeate liquid; the temperature of the feed liquid is 45-60℃, and the temperature of the permeate liquid is 20℃; the permeate liquid is water.

[0035] The VOCs wastewater is wastewater containing salt and volatile organic compounds (VOCs).

[0036] The Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane of the application can efficiently activate oxidants (such as H2O2 and PMS) to generate strong oxidizing free radicals (·OH, SO4· - The Fenton catalytic reaction performed by the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared by the application adopts a heterogeneous Fenton-like mechanism, can maintain high catalytic activity in a near-neutral or even wider pH range (without the need for additional acid adjustment), and can significantly simplify the operation and reduce the cost.

[0037] The hydrogel layer is rich in bound water, which has a lower evaporation enthalpy than free water when evaporated at the hot side interface of the membrane, thereby minimizing the negative impact on the inherent permeation flux of the base membrane, and even the interface evaporation effect caused by the surface hydrophilization can increase the permeation flux. The hydrophilic Alg / PVA-based hydrogel layer can form a dense hydration layer, effectively preventing the adsorption and deposition of hydrophobic organic pollutants (such as oils) on the membrane surface, and significantly improving the anti-pollution ability of the membrane to ensure long-term stability.

[0038] The application discloses the following technical effects:

[0039] (1) The Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared by the application can exhibit excellent performance in removing volatile organic compounds without sacrificing the permeation flux of the original hydrophobic base membrane, and exhibits excellent long-term stability in actual seawater.

[0040] (2) The application adopts a mild aqueous preparation process, solves the industry problems of poor stability of monatomic catalysts and complex preparation process, and can provide a catalytic membrane distillation solution with high efficiency, stability and engineering potential for high-salinity toxic wastewater treatment.

[0041] (3) The preparation method and the prepared Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane of the application fundamentally solve the core bottleneck problems of traditional supported catalyst membranes and high-temperature calcined monatomic catalysts, such as poor stability, complex preparation process, narrow application pH range and membrane flux decay.

[0042] (4) The application realizes the stable anchoring of high-density and high-dispersity transition metal monatomic sites in the ultra-thin hydrogel network in a mild aqueous environment through the surface hydrophilic modification pretreatment of the hydrophobic membrane and the in-situ coating-metal ion cross-linking strategy of the hydrogel precursor (containing Alg, PVA and PA), avoids the complex steps such as high-temperature calcination and template sacrifice required by traditional monatomic catalysts, greatly reduces the preparation cost and significantly improves the yield, and lays a foundation for large-scale application.

[0043] (5) The Alg / PVA-based monatomic hydrogel catalytic membrane prepared by the application has the following three core advantages: first, the multiple cross-linking network formed by the rich coordination groups (phosphate and carboxyl) of PA and Alg and PVA not only ensures that the metal monatomic does not agglomerate and has little leaching during operation, but also significantly strengthens the binding strength of the coating and the base membrane as a physical barrier; second, the monatomic active sites enriched on the hot side interface of the membrane can directly activate oxidants (H2O2 and PMS), realize the rapid in-situ mineralization of VOCs on the interface, and break through the bottleneck of low degradation efficiency caused by mass transfer limitation in traditional Fenton technology; third, the inherent hydrophilic property of the hydrogel and the low evaporation enthalpy channel provided by the internal bound water make the catalytic layer almost not sacrifice the permeation flux of the base membrane while efficiently intercepting salt and degrading pollutants, and even can improve the water vapor transmission efficiency by optimizing the interface evaporation effect, and the formation of the dense hydration layer on the surface endows the membrane with excellent anti-pollution and anti-wetting performance, which can still operate stably even in extreme polluted feed liquid containing surfactant-stabilized oil-water emulsion. What is particularly key is that the in-situ constructed high-activity monatomic sites can efficiently activate oxidants (such as H2O2 and PMS) in a near-neutral and wide pH range, realize the rapid in-situ degradation of VOCs on the membrane interface, and completely get rid of the dependence of traditional Fenton technology on strong acidic environment (pH≈3), and save the cumbersome pH adjustment step and cost. Therefore, the application provides a revolutionary solution for the engineering application of catalytic membrane distillation technology in the fields of high-salt and toxic wastewater deep treatment and seawater desalination, with the synergistic breakthroughs in process simplicity, cost economy and efficient operation stability. BRIEF DESCRIPTION OF DRAWINGS

[0044] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0045] Figure 1 The surface electron microscope graph of the PTFE hydrophobic microporous membrane used for Example 1;

[0046] Figure 2 The surface electron microscope graph of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 1;

[0047] Figure 3 The degradation performance of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 1 on phenol under different H2O2 concentrations;

[0048] Figure 4 degradation efficiency of phenol under different treatment modes;

[0049] Figure 5 The results of the cyclic stability test of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Examples 1-3;

[0050] Figure 6 The water contact angle in air and the oil contact angle under water of the PVDF hydrophobic microporous membrane (i.e., PVDF membrane) used in Example 4 and the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e., Alg / PVA-Co membrane) prepared in Example 4;

[0051] Figure 7 The degradation efficiency of phenol in phenol solutions with different salt contents by the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e., Alg / PVA-Co membrane) prepared in Example 4;

[0052] Figure 8 The removal efficiency of phenol by the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e., Alg / PVA-Co membrane) prepared in Example 4;

[0053] Figure 9 The removal efficiency of different types of VOCs by the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e., Alg / PVA-Co membrane) prepared in Example 4;

[0054] Figure 10 The water flux and salt rejection rate of seawater containing mineral oil and sodium dodecyl sulfate by the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e., Alg / PVA-Co membrane) prepared in Example 4. DETAILED DESCRIPTION

[0055] Various illustrative embodiments of the present application are now described in detail below. The described embodiments are not intended to limit the scope of the application, but instead are presented as a description of certain aspects, features, and embodiments of the application. Those of skill in the art will, upon reading this disclosure, appreciate the many alternative designs and embodiments that come within the scope of the application.

[0056] It should be understood that the terms used herein are for the purpose of describing particular embodiments and are not intended to limit the application. Additionally, for numerical ranges that are expressed in a range format, it is intended that any numerical value, which is within the range, is also specifically contemplated. For example, a range of 1.0 to 10.0 includes 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0, and any and all whole and fractionals in between, whether the value is in the single or double digits, are contemplated in the application. The same applies to any range that is expressed in a range format.

[0057] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as those commonly understood by one of ordinary skill in the art to which this application pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the preferred methods and materials are described. All publications mentioned in this specification are herein incorporated by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. In case of conflict between the content of the specification and that of any incorporated literature, the content of the specification controls.

[0058] Many modifications and variations of this application can be made in the light of the above teachings without departing from the spirit and scope thereof. Other implementations of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The examples in the specification are illustrative only and not restrictive of the application.

[0059] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having", "contains", "containing", or variations thereof, are intended to be open-ended terms that mean inclusion, but not limited to, the listed materials and methods.

[0060] It should be noted that the present application does not describe in detail the conventional operation means in the art, and is not the focus of the present application.

[0061] Unless otherwise specified, the operating temperature in the embodiments of the present application is room temperature (25°C).

[0062] Example 1

[0063] A preparation method of an Alg / PVA-based monatomic hydrogel catalytic membrane for membrane distillation:

[0064] (1) 0.1 g of dopamine hydrochloride (weight average molecular weight of 189.64 g / mol) and 0.1 g of polyethyleneimine (weight average molecular weight of 600 Da) were dissolved in 50 mL of Tris-HCl buffer solution (pH = 7.8) with a concentration of 50 mmol / L to obtain an activation solution;

[0065] Then, one side of the PTFE hydrophobic microporous membrane (pore size of 0.22 microns, porosity of about 60%) was immersed in the activation solution, and after standing for 10 h, the membrane surface was rinsed clean and naturally dried to obtain a PTFE membrane with one side modified to be hydrophilic.

[0066] (2) 1.5 g of PVA (polymerization degree 1750±50, alcoholysis degree (mol / mol) 98.0-99.0%) and 0.15 g of PA were added to 46.8 g of deionized water, heated and stirred at 95°C for 6 h until completely dissolved, then 1.5 g of Alg powder was added under the condition of normal temperature and fast stirring (stirring speed 500 rpm), completely dissolved, and then cold storage (temperature 4°C) was used for standing and defoaming to obtain an Alg / PVA-based hydrogel precursor solution.

[0067] (3) The PTFE film modified on one side with hydrophilic prepared in step (1) was fixed to a coated sample stage, and the Alg / PVA-based hydrogel precursor solution prepared in step (2) was cast on the surface of the PTFE film modified on one side with hydrophilic, and a doctor blade with a gap of 100 microns was used for uniform coating, and after uniform coating, natural air drying was performed to obtain an Alg / PVA-based pre-gel film; wherein the thickness of the Alg / PVA-based hydrogel layer on the Alg / PVA-based pre-gel film was about 7 microns.

[0068] (4) The Alg / PVA-based pre-gel film prepared in step (3) was immersed in a 0.5 mol / L cobalt chloride (CoCl2) ion solution for ion crosslinking, and the crosslinking time was 24 h, then the excess ion solution was poured out, and the film surface was thoroughly washed with deionized water to obtain an Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e. Alg / PVA-Co hydrogel catalytic membrane distillation membrane or Alg / PVA-Co membrane).

[0069] Effect Example 1

[0070] The surfaces of the PTFE hydrophobic microporous membrane used in Example 1 and the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 1 were characterized by scanning electron microscopy, and the results are shown in Figure 1 and Figure 2 , Figure 1 is a PTFE hydrophobic microporous membrane, Figure 2 is an Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane.

[0071] From Figure 1 and Figure 2 , it can be seen that the surface of the PTFE hydrophobic microporous membrane exhibits a porous morphology of reticular fiber nodules, and the surface of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 1 is relatively dense and smooth.

[0072] Effect Example 2

[0073] The performance of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 1 was evaluated using a direct contact membrane distillation system, and the specific method was as follows:

[0074] The concentration of the phenol solution with a concentration of 10 mg / L added with different concentrations of H2O2 (0-30 mM) was used as the feed liquid, the temperature of the feed liquid was 65℃, the function layer of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane was heated on the side of the feed liquid, and the flow rate was 300 mL / min; the degradation efficiency of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 1 on phenol under different H2O2 concentrations was determined, and the results are shown in Figure 3 . Figure 3 The ordinate C / C0 in the above table refers to the ratio of the concentration of phenol in the feed liquid at time t to the concentration of phenol in the initial feed liquid.

[0075] As can be seen from Figure 3 , the degradation efficiency of phenol is significantly positively correlated with the concentration of H2O2: when the concentration of H2O2 increases from 0 mM to 20 mM, the degradation rate of phenol increases significantly; when the concentration of H2O2 continues to increase to 30 mM, the degradation efficiency tends to be saturated. This phenomenon shows the dependence of H2O2 concentration, which proves that the monatomic cobalt sites on the surface of the hydrogel catalytic membrane distillation membrane can be gradually activated, and 20 mM H2O2 can fully occupy the available active sites to achieve nearly complete degradation. When the concentration of H2O2 exceeds 20 mM, the degradation efficiency cannot be significantly improved due to the depletion of active sites. This shows that the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane can achieve efficient degradation of 10 ppm phenol under the condition of 20 mM H2O2, which verifies the advantage of the active site density and the utilization efficiency of the oxidant, and provides a key basis for the optimization of the oxidant dosage in practical applications.

[0076] Example 2

[0077] The same as Example 1, except that the 0.5 mol / L cobalt chloride (CoCl2) ion solution in step (4) was replaced by a 0.5 mol / L Cu(NO3)2 ion solution; an Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e., an Alg / PVA-Cu hydrogel catalytic membrane distillation membrane or an Alg / PVA-Cu membrane) was prepared.

[0078] Example 3

[0079] The same as Example 1, except that the 0.5 mol / L cobalt chloride (CoCl2) ion solution in step (4) was replaced by a 0.5 mol / L FeCl2 ion solution; an Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e., an Alg / PVA-Fe hydrogel catalytic membrane distillation membrane or an Alg / PVA-Fe membrane) was prepared.

[0080] Effect Example 3

[0081] The Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in the examples was evaluated for performance using a direct contact membrane distillation system. The specific method is as follows:

[0082] Control group 1: The phenol solution with a concentration of 10 mg / L was used as the feed liquid, the temperature of the feed liquid was 65℃, the functional layer of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (prepared in Example 1, Example 2 or Example 3) was heated to the side of the feed liquid, and the flow rate was 300 mL / min; the change of the phenol concentration in the feed liquid treated by the three membranes with time was tested, and the average value was calculated to evaluate the degradation efficiency of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane on phenol.

[0083] Control group 2: H2O2 was added to the phenol solution (feed liquid) with a concentration of 10 mg / L to make the concentration of H2O2 20 mM, and then heated to 65℃ for thermal activation to degrade phenol.

[0084] Alg / PVA-Co membrane group: The phenol solution with a concentration of 10 mg / L added with 20 mM H2O2 was used as the feed liquid, the temperature of the feed liquid was 65℃, the functional layer of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 1 was heated to the side of the feed liquid, and the flow rate was 300 mL / min; the change of the phenol concentration in the feed liquid with time was tested to evaluate the degradation efficiency of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 1 on phenol.

[0085] Alg / PVA-Cu membrane group: The phenol solution with a concentration of 10 mg / L added with 20 mM H2O2 was used as the feed liquid, the temperature of the feed liquid was 65℃, the functional layer of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 2 was heated to the side of the feed liquid, and the flow rate was 300 mL / min; the change of the phenol concentration in the feed liquid with time was tested to evaluate the degradation efficiency of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 2 on phenol.

[0086] Alg / PVA-Fe membrane group: The phenol solution with a concentration of 10 mg / L added with 20 mM H2O2 was used as the feed liquid, the temperature of the feed liquid was 65℃, the functional layer of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 3 was heated to the side of the feed liquid, and the flow rate was 300 mL / min; the change of the phenol concentration in the feed liquid with time was tested to evaluate the degradation efficiency of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 3 on phenol.

[0087] The degradation efficiency determination results are shown in Table 1. Figure 4 . Figure 4The ordinate C / C0in the graph indicates the ratio of the phenol concentration in the feed liquid at time t to the initial phenol concentration in the feed liquid.

[0088] As can be seen from Figure 4 It can be seen that, in the control group 1 without the addition of H2O2, the phenol concentration in the feed liquid did not change significantly over time (C / C0≈1), excluding the effects of adsorption of phenol by the Alg / PVA-based monatomic hydrogel catalytic membrane and pipeline in the MD (membrane distillation) process and thermal degradation of phenol itself. The control group 2 was to degrade phenol by thermal activation of H2O2 at 65℃ without the Alg / PVA-based monatomic hydrogel catalytic membrane for membrane distillation, and it can be found that the phenol concentration almost did not change within 40 min. The phenol degradation efficiency of the Alg / PVA-based monatomic hydrogel catalytic membrane cross-linked by different ionic cross-linking solutions was in the order of Alg / PVA-Cu membrane > Alg / PVA-Co membrane > Alg / PVA-Fe membrane. The high catalytic activity of the Alg / PVA-Cu membrane was due to the Cu + / Cu 2+ Rapid circulation of activated H2O2 is conducive to the generation of a large number of active free radicals (such as ·OH). The relatively low phenol degradation efficiency of the Alg / PVA-Fe membrane is because, at near neutral pH, the Fe 2+ in the hydrogel complex is rapidly oxidized to Fe 3+ , and the energy barrier for the reduction regeneration of Fe 3+ and the tendency of Fe 3+ to catalyze non-radical pathways.

[0089] Example 4

[0090] The cyclic stability of the Alg / PVA-based monatomic hydrogel catalytic membrane for membrane distillation (Alg / PVA-Co membrane, Alg / PVA-Cu membrane, and Alg / PVA-Fe membrane) prepared in Examples 1 to 3 was determined, and the determination method was as follows:

[0091] To evaluate the cyclic stability of Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane, in-situ continuous circulation test was adopted: after completing a single experiment (experimental method same as Example 3), the membrane was kept in the membrane module, the feed liquid was switched to 25℃ deionized water to flush at a constant flow rate (300 mL / min) for 5 min, and the residual metal ions, VOCs and H2O2 on the pipeline and membrane surface were completely removed to eliminate the interference of cross contamination on the subsequent data; then the feed liquid was updated (phenol solution with a concentration of 10 mg / L added with 20 mM H2O2), the temperature was 65℃, and the next cycle was started under the same hydrodynamic conditions. No reduction regeneration treatment was applied to the Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane during the period to simulate the natural state in actual working conditions, so as to truly reflect the stable anchoring ability of the hydrogel network to the metal active sites. The results are shown in Figure 5 .

[0092] As can be seen from Figure 5 , the degradation efficiency of phenol by the Alg / PVA-Cu membrane decreases with the increase of the number of cycles, because the Cu ions in the hydrogel are based on Cu 2+ / Cu + redox cycle, and Cu + is weakly combined with the hydrogel due to the mismatch of valence, resulting in leaching and loss of active sites. The Co ions in the hydrogel layer of the Alg / PVA-Co membrane are based on Co 2+ / Co 3+ redox cycle, and the conversion of Co 3+ is firmly coordinated with the carboxyl / phosphoryl groups of the hydrogel, therefore, the cyclic stability is better. The Alg / PVA-Fe membrane has good cyclic stability, because the strong chelation of the hydrogel active groups (such as carboxyl) to Fe 3+ effectively inhibits its hydrolysis precipitation and loss, so that it can exist stably.

[0093] Example 4

[0094] A preparation method of an Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane:

[0095] (1) The PVDF hydrophobic microporous membrane was immersed in anhydrous ethanol and ultrasonically cleaned for 10 min, and then taken out and blown dry with nitrogen; then it was fixed on the sample table of the plasma treatment chamber, the electrode spacing was controlled to be 20 mm, oxygen (purity ≥ 99.9%) was introduced into the reaction chamber until the gas pressure stabilized at 30 Pa, and plasma irradiation was carried out at a frequency of 40 kHz of the radio frequency power source with a power of 80 W. The treatment time was 120 s. After the treatment was completed, nitrogen was continuously introduced, and the sample was placed in a protective atmosphere for 5 min to obtain a PVDF membrane with one side modified by hydrophilic groups.

[0096] (2) 1.5 g of PVA (polymerization degree 1750±50, alcoholysis degree (mol / mol) 98.0-99.0%) and 0.2 g of PA were added to 46.8 g of deionized water, heated and stirred at 95°C for 6 h until completely dissolved, then 1.5 g of Alg powder was added under the condition of normal temperature and rapid stirring (stirring speed 1000 rpm), completely dissolved, and then cold storage (temperature 4°C) was used for standing and defoaming to obtain an Alg / PVA-based hydrogel precursor solution.

[0097] (3) The PVDF film modified on one side with hydrophilicity prepared in step (1) was fixed to a coated sample stage, and the Alg / PVA-based hydrogel precursor solution prepared in step (2) was cast on the surface of the PVDF film modified on one side with hydrophilicity, and a doctor blade with a gap of 100 microns was used for uniform coating, and after uniform coating, it was placed in a drying oven at 40°C for 12 h to obtain an Alg / PVA-based pre-gel film; wherein the thickness of the Alg / PVA-based hydrogel layer on the Alg / PVA-based pre-gel film was about 8 microns.

[0098] (4) The Alg / PVA-based pre-gel film prepared in step (3) was immersed in a 0.5 mol / L cobalt chloride (CoCl2) ion solution for ion crosslinking, and the crosslinking time was 24 h, then the excess ion solution was poured out, and the surface of the film was thoroughly washed with deionized water to obtain an Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e. Alg / PVA-Co hydrogel catalytic membrane distillation membrane or Alg / PVA-Co membrane).

[0099] Effect Example 5

[0100] The wettability of the PVDF hydrophobic microporous membrane (i.e. PVDF membrane) used in Example 4 and the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane (i.e. Alg / PVA-Co membrane) prepared in Example 4 was tested by a contact angle measuring instrument, and the results are shown in Figure 6 .

[0101] As can be seen from Figure 6 , the PVDF hydrophobic microporous membrane (i.e. PVDF membrane) has the characteristics of hydrophobicity in air and lipophilicity under water, with a water contact angle in air of about 134.3° and an oil contact angle under water of about 66.3°. The Alg / PVA-Co membrane prepared in Example 4 exhibits hydrophilicity in air and super-oleophobicity under water, with a water contact angle in air of about 84.6° and an oil contact angle under water of about 141.9°.

[0102] Effect Example 6

[0103] The performance of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 4 was evaluated using a direct contact membrane distillation system, and the specific method was as follows:

[0104] Solutions containing different concentrations of sodium chloride (0-600 mM), 10 mg / L phenol, and 20 Mm H2O2 were used as feed solutions at a temperature of 65 °C. The functional layer of the Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane prepared in Example 4 was fed hot-side into the feed solution at a flow rate of 300 mL / min. The degradation efficiency of the Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane for phenol was evaluated by testing the change in phenol concentration in the feed solution over time. The results are shown in […]. Figure 7 .

[0105] from Figure 7 As can be seen, when different concentrations of sodium chloride are added to the feed liquid (simulating a high-salt environment), the efficiency of Alg / PVA-Co membrane in degrading phenol gradually increases, proving that Alg / PVA-Co membrane has the ability to resist the inhibitory effect of salt ions on catalytic activity under high ionic strength conditions.

[0106] Example 7

[0107] The performance of the Alg / PVA-based single-atom hydrogel catalytic membrane distillation membrane prepared in Example 4 was evaluated using a direct contact membrane distillation system, as follows:

[0108] A solution containing 3.5 wt% sodium chloride and 10 mg / L phenol was used as the feed liquid at a temperature of 65°C. The functional layer of the Alg / PVA-based monoatomic hydrogel catalytic membrane was fed hot-side into the feed liquid. The other side of the membrane was the condensation side, with deionized water as the permeate at a temperature of 20°C. The flow rate on both sides was 300 mL / min. The phenol concentration in the permeate solution was tested with and without the addition of 20 mM H2O2 to the feed liquid. The phenol removal efficiency of the Alg / PVA-Co membrane prepared in Example 4 was evaluated by testing the phenol concentration in the permeate. The results are shown in [Figure 4]. Figure 8 .

[0109] from Figure 8 As can be seen, when no H2O2 was added to the feed solution, the removal rate of phenol at 10 mg / L by the Alg / PVA-Co membrane ranged from 5.3% to 19.9%, which is partly attributed to the physical retention of phenol by the Alg / PVA-Co membrane. When H2O2 was added to the feed solution, the removal rate of phenol at 10 mg / L by the Alg / PVA-Co membrane approached 100%. This confirms that the single-atom cobalt active sites were successfully activated by H2O2 to undergo a Fenton-like reaction, achieving near-complete mineralization of phenol at the membrane interface. Under simulated seawater salinity, the Alg / PVA-Co membrane has achieved a breakthrough in solving the problem of VOCs retention failure in traditional membrane distillation, simultaneously ensuring high throughput and deep purification efficiency.

[0110] Example 8

[0111] The Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 4 was evaluated for performance using a direct contact membrane distillation system, in accordance with the following specific method:

[0112] A solution containing 3.5wt% sodium chloride, 10mg / L of different types of VOCs (phenol, aniline, pyridine or dimethyl sulfoxide) was used as the feed liquid, and the temperature of the feed liquid was 65°C. The functional layer of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane was used for hot-side feeding, with the feed liquid facing the functional layer. The other side of the membrane was the condensation side, and the permeate liquid on the condensation side was deionized water, with a temperature of 20°C. The flow rates on both sides were 300mL / min. The VOCs concentration in the solution on the permeation side was tested with and without the addition of 20mM H2O2 to the feed liquid, and the VOCs removal efficiency of the Alg / PVA-Co membrane prepared in Example 4 was evaluated by testing the VOCs concentration in the permeate liquid, and the results are shown in Figure 9 .

[0113] As can be seen from Figure 9 , when the feed liquid does not contain H2O2, the removal rate of the Alg / PVA-Co membrane for 10mg / L of different volatile organic compounds (VOCs) is only maintained at 4.6-14.9%, which is attributed to the physical interception mechanism of the hydrogel layer. When the feed liquid contains H2O2, the removal rate of the Alg / PVA-Co membrane for representative VOCs (phenol, aniline, pyridine and dimethyl sulfoxide) is significantly improved to 94.2-99.8%, which proves that the in-situ generated reactive oxygen species (ROS) achieves near-complete mineralization of VOCs. This comparative experiment fully reveals the key contribution of the activation of H2O2 by the Alg / PVA-Co membrane to the improvement of the VOCs removal efficiency.

[0114] Example 9

[0115] The Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared in Example 4 was evaluated for performance using a direct contact membrane distillation system, in accordance with the following specific method:

[0116] Seawater containing 1000ppm mineral oil and 0.1mM sodium dodecyl sulfate (sodium chloride content of about 3.5wt%, conductivity of about 61mS / cm) was used as the feed liquid, and the temperature of the feed liquid was 65°C. The functional layer of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane was used for hot-side feeding, with the feed liquid facing the functional layer. The other side of the membrane was the condensation side, and the permeate liquid on the condensation side was deionized water, with a temperature of 20°C. The flow rates on both sides were 300mL / min. The anti-fouling and anti-wetting properties of the Alg / PVA-Co membrane prepared in Example 4 were tested, and the results are shown in Figure 10 .

[0117] As can be seen from Figure 10As can be seen, the Alg / PVA-Co membrane prepared in Example 4 still has a stable water flux and a very high salt rejection rate (>99.9%) after 50 hours of operation. This result shows that the Alg / PVA-Co membrane prepared in Example 4 has excellent anti-fouling and anti-wetting properties at the same time, mainly due to the formation of a dense hydration layer on the surface of the hydrogel layer of the Alg / PVA-Co membrane, which can effectively block the adsorption of oil pollution, thereby preventing the contact of the hydrophobic base membrane with the pollutants. The results show that the Alg / PVA-Co membrane can simultaneously achieve almost zero flux decay, high salt rejection rate and long-term stable operation in complex actual seawater with high salt, oil and surfactant, breaking through the industry bottleneck of the traditional membrane distillation facing the "pollution-wetting" synergistic failure.

[0118] The above-described embodiments are only to describe the preferred modes of the present application, and not to limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A method for preparing an Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane, characterized in that, The method comprises the following steps: A mixed hydrogel precursor solution of sodium alginate, polyvinyl alcohol and phytic acid is uniformly coated on the surface of a hydrophilically modified hydrophobic microporous membrane, and then the membrane is infiltrated and crosslinked by a transition metal ion crosslinking solution to obtain the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane.

2. The production method according to claim 1, characterized by, The concentration of sodium alginate in the mixed hydrogel precursor solution is 2-4 wt%, the concentration of polyvinyl alcohol is 2-5 wt%, and the concentration of phytic acid is 0.2-0.5 wt%.

3. The production method according to claim 1, characterized by, The material of the hydrophobic microporous membrane comprises one of polytetrafluoroethylene and its composite material, polyvinylidene fluoride and its composite material.

4. The preparation method according to claim 1, characterized in that, The method for hydrophilic modification comprises one or more of the following methods: Method 1: soaking the hydrophobic microporous membrane in a sodium hydroxide solution; Method 2: depositing a mixture of polydopamine and polyethyleneimine on the surface of the hydrophobic microporous membrane; Method 3: adhering tannic acid and 3-aminopropyltriethoxysilane on the surface of the hydrophobic microporous membrane; Method 4: performing plasma treatment on the hydrophobic microporous membrane.

5. The preparation method according to claim 1, characterized in that, The transition metal ion crosslinking solution is an aqueous solution of a multivalent transition metal salt, and the concentration of transition metal ions is 0.1-1 mol / L.

6. The production method according to claim 5, wherein The multivalent transition metal salt comprises one or more of cobalt chloride, cobalt nitrate, ferrous chloride, ferric chloride, copper chloride, copper nitrate and ferrous sulfate.

7. The preparation method according to claim 1, characterized in that, The time for infiltrating and crosslinking is 0.5-24 h.

8. An Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane prepared by the preparation method of any one of claims 1-7.

9. The use of the Alg / PVA-based monatomic hydrogel catalytic membrane distillation membrane of claim 8 in seawater desalination or VOCs wastewater treatment.

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