A zinc-polyphenol network microfiltration membrane, and a preparation method and application thereof

Abstract

The application belongs to the technical field of astaxanthin ester oil separation and collection, and specifically discloses a zinc-polyphenol network microfiltration membrane and a preparation method and application thereof. The microfiltration membrane comprises a polyvinylidene fluoride base film, a zinc phytate nanocluster bottom layer located on the surface of the polyvinylidene fluoride base film, and a zinc-polyphenol network surface layer located on the surface of the zinc phytate nanocluster bottom layer. The preparation method comprises the following steps: (1) preparing gallolacyl procyanidin; (2) preparing a zinc-polyphenol pre-polymer solution; (3) constructing the zinc phytate nanocluster bottom layer; and (4) compounding the surface layer to obtain the zinc-polyphenol network microfiltration membrane. In the application, a zinc-based coordination double-layer network is used to replace a traditional iron-based coating, and the zinc phytate nanocluster bottom layer and the zinc-polyphenol network surface layer are used to realize the anti-pollution interception of astaxanthin ester oil bodies, the interface oxidation inhibition of the concentration process, and the long-term stable operation of the membrane functional layer under the condition of no solvent.

Description

Technical Field

[0001] This invention belongs to the field of astaxanthin ester oil separation and collection technology, specifically relating to a zinc-polyphenol network microfiltration membrane, its preparation method, and its application. Background Technology

[0002] Astaxanthin is a ketocarotenoid with strong free radical quenching and lipid peroxidation inhibition capabilities. It is widely used as an antioxidant, colorant, and functional ingredient in food, health food, cosmetics, and aquatic feed. Among natural sources, Haematococcus pluvialis (also known as Haematococcus pluvialis) is the biological resource with the highest known astaxanthin content. Its astaxanthin mainly accumulates in the oil bodies inside thick-walled spores and exists in the form of astaxanthin esters, bound to fatty acids. Astaxanthin esters have poor thermal and oxidative stability; light, high temperature, oxygen, and trace metal ions all accelerate their degradation. This material characteristic dictates that the extraction and separation process must be carried out under mild, oxygen-free conditions free from pro-oxidative factors.

[0003] The cell walls of thick-walled spores of *Haematococcus pluvialis* consist of multiple layers and contain resistant components such as cellulose and phycocyanin, making them difficult for solvents to penetrate under normal conditions. Therefore, the extraction of astaxanthin esters requires cell wall disruption. Industrial production often employs mechanical methods such as high-pressure homogenization and grinding for cell wall disruption, which is energy-intensive and can exacerbate astaxanthin ester degradation due to localized temperature increases. Enzymatic cell disruption using cellulase and pectinase is also used, offering milder conditions but resulting in a complex composition of the algal slurry. After cell wall disruption, astaxanthin esters are released from the cell wall sacs and dispersed as micron-sized oil bodies in an aqueous phase containing proteins, polysaccharides, and cell wall fragments. Algal proteins and phospholipids adsorb onto the surface of the oil bodies, forming a stable emulsified dispersion.

[0004] There are three main existing technologies for obtaining astaxanthin esters from broken-cell algae slurry. The first is organic solvent extraction, which uses ethanol, ethyl acetate, or n-hexane to dissolve astaxanthin esters from the algae slurry, followed by concentration and solvent removal to obtain oleoresin. This route is the most widely used, but it suffers from solvent residue and loss. The solvent removal process requires heating and vacuuming, and astaxanthin esters undergo isomerization and oxidative degradation under the combined effects of heat and oxygen. The second is supercritical carbon dioxide extraction, which does not use organic solvents, but it requires large equipment investment, high operating pressure, and has limited processing capacity. The third is the aqueous method, which directly separates oils from the aqueous phase without solvents. It is environmentally friendly, but oil-water emulsification makes it difficult to separate and enrich the oil, resulting in low yields, and a breakthrough has not been achieved for a long time.

[0005] Membrane separation offers new possibilities for aqueous solutions. Microfiltration and ultrafiltration membranes have been reported for algal slurry concentration and oil-water separation. In recent years, metal-polyphenol coordination coating modified membranes have attracted attention. Tannins or phytic acid coordinate with iron ions on the membrane surface to form a hydrophilic network, giving the membrane superoleophobic properties underwater. When used for oily wastewater treatment, oil is retained while water permeates, resulting in good resistance to oil fouling. However, when these membranes are used to enrich astaxanthin ester oil bodies in broken-cell algal slurry from *Haemophilus globulus*, several previously unresolved issues have arisen.

[0006] First, most publicly available coordination coatings utilize iron ions, which have variable valence states and act as catalysts for lipid oxidation. Astaxanthin esters, however, are easily oxidized. After being trapped, the oil remains on the membrane surface for an extended period, continuously concentrating and exacerbating oxidation at the oil-water-membrane interface. This creates a conflict between the introduction of antifouling coatings and the protection of the target product. Second, existing coatings are mostly single-layer structures with low loading capacity and weak adhesion to the base membrane. They are easily washed away under the cross-flow of high-solids algal slurry, resulting in a short functional layer lifespan. Third, the initial design of oil-water separation membranes aims to remove oil as a contaminant, minimizing the oil phase's residence time on the membrane surface. However, enrichment processes require the oil to be trapped and concentrated on the membrane surface for an extended period. This exposes the problems of uncontrolled coalescence of emulsified oil and interfacial oxidation, which ordinary hydrophilic membranes cannot handle. Fourth, if the coating directly uses natural plant polyphenols such as proanthocyanidins, their single-molecule coordination sites and the number of phenolic hydroxyl groups are limited. The network formed with metal ions is not dense, resulting in insufficient antioxidant capacity and making it difficult to simultaneously perform both film formation and color protection tasks.

[0007] Achieving antifouling retention and enrichment of astaxanthin ester oil under solvent-free conditions, while simultaneously inhibiting interfacial oxidation during membrane concentration and ensuring the long-term stability of the membrane functional layer in highly polluted feed solutions, remains an unsolved and urgent technical problem in this field. Therefore, there is a need to design a zinc-polyphenol network microfiltration membrane, its preparation method, and its applications. Summary of the Invention

[0008] To overcome the shortcomings of existing technologies, a zinc-polyphenol network microfiltration membrane, its preparation method, and its application are provided.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A zinc-polyphenol network microfiltration membrane, the microfiltration membrane comprising a polyvinylidene fluoride membrane, a zinc phytate nanocluster bottom layer located on the surface of the polyvinylidene fluoride membrane, and a zinc-polyphenol network surface layer located on the surface of the zinc phytate nanocluster bottom layer.

[0011] The zinc phytate nanocluster bottom layer is generated by in-situ coordination of phytate and zinc ions on the surface of the polyvinylidene fluoride film.

[0012] The zinc-polyphenol network surface layer is a network structure formed by cross-linking galloyl proanthocyanidins, zinc ions and phytate through coordination bonds. The network structure is formed by the coordination of zinc-polyphenol prepolymers and phytate. The zinc-polyphenol prepolymers are obtained by pre-coordinating galloyl proanthocyanidins and zinc acetate in water. The zinc-polyphenol network surface layer is connected to the zinc ions in the bottom layer of the zinc phytate nanoclusters through coordination of phytate.

[0013] The galloyl proanthocyanidin is obtained by esterification of grape seed proanthocyanidins and gallic acid via lipase catalysis.

[0014] A method for preparing a zinc-polyphenol network microfiltration membrane, the method comprising the following steps:

[0015] (1) Preparation of galloyl proanthocyanidins: Grape seed proanthocyanidins and gallic acid were dissolved in anhydrous ethanol, immobilized lipase and 4A molecular sieve were added to carry out catalytic esterification reaction, and then filtered. The filtrate was filtered through a 0.45μm filter membrane, and the ethanol was removed from the filtrate under reduced pressure at 40-50℃ to obtain galloyl proanthocyanidins.

[0016] (2) Preparation of zinc-polyphenol prepolymer solution: The galloyl proanthocyanidins obtained in step (1) are dissolved in deoxygenated deionized water to prepare a galloyl proanthocyanidin solution. The pH is adjusted to 5.0-5.5 with acetic acid or sodium hydroxide solution. Zinc acetate solution is added dropwise. After the addition is completed, the solution is stirred and aged at room temperature for 2-4 hours under nitrogen protection throughout the process to obtain zinc-polyphenol prepolymer solution.

[0017] (3) Construction of zinc phytate nanocluster substrate: The polyvinylidene fluoride membrane was soaked in anhydrous ethanol for 10-20 min to wet it, then the ethanol was replaced with deionized water, and then it was immersed in a zinc acetate solution with a mass fraction of 0.1-0.25% for 10-20 min. After being taken out and drained, it was immediately immersed in phytic acid solution for 10-30 min. After being taken out and rinsed with deionized water, the above soaking cycle was repeated 2-4 times to obtain a base film with a zinc phytate nanocluster substrate on the surface.

[0018] (4) Composite of the surface layer: The base film with zinc phytate nanoclusters on the surface obtained in step (3) is immersed in a coating solution made of zinc-polyphenol prepolymer solution and sodium phytate obtained in step (2) for 2-4 hours at room temperature. After washing with water, it is stored in a wet state to obtain zinc-polyphenol network microfiltration membrane.

[0019] In step (1), the mass ratio of grape seed proanthocyanidins to gallic acid is 1:0.2-0.4, the amount of immobilized lipase is 5-10% of the mass of grape seed proanthocyanidins, and the amount of 4A molecular sieve is 20-50% of the mass of grape seed proanthocyanidins.

[0020] In step (1), the immobilized lipase is immobilized Candida antarcticis lipase B; the average degree of polymerization of the grape seed proanthocyanidins is 2-5; and the catalytic esterification is carried out under nitrogen protection at 40-50℃ for 24-48 hours.

[0021] In step (2), the deoxygenated deionized water is deionized water that has been boiled and cooled or deoxygenated by nitrogen purging; the galloyl proanthocyanidin solution contains 0.5-1.0% galloyl proanthocyanidin by mass; zinc is added in the form of zinc acetate, and the mass ratio of zinc to galloyl proanthocyanidin is 0.1-0.2:1.

[0022] In step (2), the specific steps for adding zinc acetate solution are as follows: under nitrogen protection and stirring at 300-500 r / min, add zinc acetate solution with a mass fraction of 0.1-0.2% at a rate of 2-5 mL / min.

[0023] In step (3), the pore size of the polyvinylidene fluoride membrane is 0.1-0.2 μm.

[0024] In step (3), the phytic acid solution has a mass fraction of 0.2-0.5% and a pH of 4.5-5.5; the pH of the phytic acid solution is adjusted with sodium hydroxide solution.

[0025] In step (4), the mass fraction of sodium phytate in the coating solution is 0.1-0.3%, and the pH of the coating solution is adjusted to 5.0-6.0 with acetic acid or sodium hydroxide solution; the water washing is done with deionized water until the conductivity of the washing solution is comparable to that of deionized water.

[0026] Application of a zinc-polyphenol network microfiltration membrane for the enrichment of astaxanthin ester oil bodies in broken-cell algal slurry of Haematococcus pluvialis.

[0027] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0028] 1. The zinc-polyphenol network microfiltration membrane provided in this application adopts a bilayer structure of zinc phytate nanoclusters as the bottom layer and zinc-polyphenol network as the top layer. The zinc phytate nanoclusters are generated in situ on the surface of the polyvinylidene fluoride membrane, forming a micro-nano rough structure and immobilizing the hydration layer. When algal proteins penetrate the hydration layer, their migration rate decreases. The phytate groups in the zinc-polyphenol network as the top layer make the membrane surface negatively charged under the pH conditions of the enrichment process, which repels the negatively charged algal proteins on the surface. The decelerating effect of the hydration layer and the repulsive effect of the negative potential of the membrane surface work together to suppress protein contamination. Astaxanthin ester oil is completely retained without breaking down and agglomerating on the membrane surface, and the membrane pores are not blocked by the oil phase. After the concentrate leaves the membrane, it can be adjusted to the isoelectric point of the algal proteins to complete demulsification and collection. The zinc-polyphenol network layer is connected to the zinc ions in the zinc phytate nanocluster layer through coordination of phytate groups. The zinc ions are locally redistributed at the interface, so that the two layers are connected by a gradual transition of chemical bonds. The coating does not run off under cross-flow shearing and washing, and the membrane can be reused after cleaning and regeneration.

[0029] 2. This application uses zinc ions to construct a coordination network. Compared with existing technologies that use iron ions for coordination coatings, zinc ions have fully filled 3d orbitals and do not have variable valence states under membrane operating conditions. Therefore, they cannot catalyze the chain reaction of lipid oxidation through single-electron transfer like iron ions. Phytate ions in the coordination network can also chelate trace amounts of copper and iron prooxidant ions in the algal slurry. Galloyl proanthocyanidins in the zinc-polyphenol network surface layer donate hydrogen to lipid peroxide free radicals through phenolic hydroxyl groups, quenching them. The oil body is placed under the cover of the antioxidant structure during the most easily oxidized stage of membrane concentration, and the oxidative degradation of astaxanthin esters during the enrichment process is inhibited. Replacing iron with zinc is a targeted choice based on the characteristics of the materials being treated.

[0030] 3. This application modifies grape seed proanthocyanidins by galloylation. Under the conditions used, lipase tends to catalyze the esterification of alcoholic hydroxyl groups rather than phenolic hydroxyl groups. The carboxyl group of gallic acid esterifies with the alcoholic hydroxyl group in the proanthocyanidin structural unit, while the original phenolic hydroxyl group is retained. The product molecule has three phenolic hydroxyl groups carried by the galloyl group, and the coordination anchor sites and hydrogen donation sites increase simultaneously. This one-time modification serves two functions: dense film formation of the coordination network and interfacial antioxidant properties. Galloyl proanthocyanidins and zinc acetate are first pre-coordinated to form zinc-polyphenol prepolymers, and then bridged with sodium phytate to form a film. The network growth is controlled, and large precipitates will not be generated instantly to block the membrane pores, resulting in a uniform and dense coating. Attached Figure Description

[0031] Figure 1 This is a process flow diagram of the preparation method of a zinc-polyphenol network microfiltration membrane according to the present invention. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] The technical solution of this application is as follows:

[0034] A zinc-polyphenol network microfiltration membrane, the microfiltration membrane comprising a polyvinylidene fluoride membrane, a zinc phytate nanocluster bottom layer located on the surface of the polyvinylidene fluoride membrane, and a zinc-polyphenol network surface layer located on the surface of the zinc phytate nanocluster bottom layer.

[0035] The zinc phytate nanocluster bottom layer is generated by in-situ coordination of phytate and zinc ions on the surface of the polyvinylidene fluoride film.

[0036] The zinc-polyphenol network surface layer is a network structure formed by cross-linking galloyl proanthocyanidins, zinc ions and phytate through coordination bonds. The network structure is formed by the coordination of zinc-polyphenol prepolymers and phytate. The zinc-polyphenol prepolymers are obtained by pre-coordinating galloyl proanthocyanidins and zinc acetate in water. The zinc-polyphenol network surface layer is connected to the zinc ions in the bottom layer of the zinc phytate nanoclusters through coordination of phytate.

[0037] The galloyl proanthocyanidin is obtained by esterification of grape seed proanthocyanidins and gallic acid via lipase catalysis.

[0038] A method for preparing a zinc-polyphenol network microfiltration membrane, such as... Figure 1 As shown, the method includes the following steps:

[0039] (1) Preparation of galloyl proanthocyanidins: Grape seed proanthocyanidins and gallic acid were dissolved in anhydrous ethanol, immobilized lipase and 4A molecular sieve were added to carry out catalytic esterification reaction, and then filtered. The filtrate was filtered through a 0.45μm filter membrane, and the ethanol was removed from the filtrate under reduced pressure at 40-50℃ to obtain galloyl proanthocyanidins.

[0040] (2) Preparation of zinc-polyphenol prepolymer solution: The galloyl proanthocyanidins obtained in step (1) are dissolved in deoxygenated deionized water to prepare a galloyl proanthocyanidin solution. The pH is adjusted to 5.0-5.5 with acetic acid or sodium hydroxide solution. Zinc acetate solution is added dropwise. After the addition is completed, the solution is stirred and aged at room temperature for 2-4 hours under nitrogen protection throughout the process to obtain zinc-polyphenol prepolymer solution.

[0041] (3) Construction of zinc phytate nanocluster substrate: The polyvinylidene fluoride membrane was soaked in anhydrous ethanol for 10-20 min to wet it, then the ethanol was replaced with deionized water, and then it was immersed in a zinc acetate solution with a mass fraction of 0.1-0.25% for 10-20 min. After being taken out and drained, it was immediately immersed in phytic acid solution for 10-30 min. After being taken out and rinsed with deionized water, the above soaking cycle was repeated 2-4 times to obtain a base film with a zinc phytate nanocluster substrate on the surface.

[0042] (4) Composite of the surface layer: The base film with zinc phytate nanoclusters on the surface obtained in step (3) is immersed in a coating solution made of zinc-polyphenol prepolymer solution and sodium phytate obtained in step (2) for 2-4 hours at room temperature. After washing with water, it is stored in a wet state to obtain zinc-polyphenol network microfiltration membrane.

[0043] In step (1), the mass ratio of grape seed proanthocyanidins to gallic acid is 1:0.2-0.4, the amount of immobilized lipase is 5-10% of the mass of grape seed proanthocyanidins, and the amount of 4A molecular sieve is 20-50% of the mass of grape seed proanthocyanidins.

[0044] In step (1), the immobilized lipase is immobilized Candida antarcticis lipase B; the average degree of polymerization of the grape seed proanthocyanidins is 2-5; and the catalytic esterification is carried out under nitrogen protection at 40-50℃ for 24-48 hours.

[0045] In step (2), the deoxygenated deionized water is deionized water that has been boiled and cooled or deoxygenated by nitrogen purging; the galloyl proanthocyanidin solution contains 0.5-1.0% galloyl proanthocyanidin by mass; zinc is added in the form of zinc acetate, and the mass ratio of zinc to galloyl proanthocyanidin is 0.1-0.2:1.

[0046] In step (2), the specific steps for adding zinc acetate solution are as follows: under nitrogen protection and stirring at 300-500 r / min, add zinc acetate solution with a mass fraction of 0.1-0.2% at a rate of 2-5 mL / min.

[0047] In step (3), the pore size of the polyvinylidene fluoride membrane is 0.1-0.2 μm.

[0048] In step (3), the phytic acid solution has a mass fraction of 0.2-0.5% and a pH of 4.5-5.5; the pH of the phytic acid solution is adjusted with sodium hydroxide solution.

[0049] In step (4), the mass fraction of sodium phytate in the coating solution is 0.1-0.3%, and the pH of the coating solution is adjusted to 5.0-6.0 with acetic acid or sodium hydroxide solution; the water washing is done with deionized water until the conductivity of the washing solution is comparable to that of deionized water.

[0050] Application of a zinc-polyphenol network microfiltration membrane for the enrichment of astaxanthin ester oil bodies in broken-cell algal slurry of Haematococcus pluvialis.

[0051] Metal-polyphenol coordination coating modification has become a common practice in recent years to improve the oil fouling resistance of membranes. Tannic acid or phytic acid coordinates with iron ions on the membrane surface to form a hydrophilic network, giving the membrane superoleophobic properties underwater. This has shown good anti-oil fouling effect in the treatment of oily wastewater, which provides a reference for this application.

[0052] When the aforementioned coordination-coated membrane is directly used for the enrichment of astaxanthin ester oil in *Haemaphysalis* cell-wall broken algal slurry, the first problem that arises is the uncontrolled adhesion and aggregation of the oil on the membrane surface. In oily wastewater treatment, oil is the pollutant, and the shorter its residence time on the membrane surface, the better. However, in the enrichment process, oil is the target product and needs to be retained and concentrated on the membrane surface for a long time. As a result, the oil concentration continuously increases along the membrane surface, which ordinary hydrophilic membranes cannot handle.

[0053] To address this issue, this application constructs a zinc-polyphenol network layer on the surface of a polyvinylidene fluoride (PVDF) membrane, enabling the oil to be retained and enriched with antifouling properties. This layer is formed by the cross-linking of galloyl proanthocyanidins, zinc ions, and phytate groups via coordination bonds. Its antifouling and mild retention effects are achieved through the following mechanisms: The high-density phenolic hydroxyl groups on the galloyl proanthocyanidin molecules and the phosphate groups on the phytate groups are both strongly hydrophilic groups. The membrane surface and water molecules are bonded together by hydrogen bonds to form a stable hydrated layer. Before the oil can contact the membrane, it must displace this hydrated layer, requiring the overcoming of a high energy barrier, thus making direct contact with the membrane surface difficult. Simultaneously, the ionization of phytate groups under the pH conditions of the feed solution makes the membrane surface negatively charged, while the algal proteins adsorbed on the oil surface are also negatively charged, creating electrostatic repulsion between the membrane surface and the oil. The spatial barrier and electrostatic repulsion of the hydration layer work together to trap the oil on the membrane surface without spreading, adhering, or being pressed into the membrane pores. The oil remains in a complete emulsion state on the membrane surface and is gradually concentrated, significantly slowing down the membrane flux decline, which can be restored by water washing.

[0054] However, simply addressing the oil adhesion problem is insufficient to enrich astaxanthin ester oil. Further research revealed that after the oil is trapped, it remains on the membrane surface for an extended period and continues to concentrate. This actually exacerbates the oxidation of astaxanthin esters at the oil-water-membrane three-phase interface, leading to a decrease in the color and content of the concentrate over time. Most published coordination coatings utilize iron ions, but iron is a catalyst for lipid oxidation, thus creating a conflict between the introduction of antifouling coatings and the protection of the target product.

[0055] To address this issue, this application constructs a coordination network by replacing iron ions with zinc ions, thus eliminating pro-oxidative factors on the film surface while retaining the anti-fouling function of the coating. From an electronic structure perspective, the 3d orbitals of iron atoms are unfilled, exhibiting variable valence states. The cyclic conversion between ferrous and ferric iron can catalyze single-electron transfer, decomposing lipid hydrogen peroxide into alkoxy radicals and hydroxyl radicals, initiating a chain reaction of lipid peroxidation. In contrast, the 3d orbitals of zinc atoms are fully filled, lacking variable valence states, and cannot participate in the aforementioned single-electron redox cycle. Therefore, its coordination network is chemically inert and will not catalyze the oxidative degradation of astaxanthin esters.

[0056] From a coordination configuration perspective, zinc ions form tetrahedral coordination structures with polyphenols and phytate, which can also generate a dense, hydrophilic three-dimensional network. The hydration layer and charged properties of the membrane surface are not lost due to the replacement of metal ions. Furthermore, zinc is an element permitted for use in food processing, making it compatible with the food application scenarios of the membrane and not introducing new safety concerns. Replacing iron with zinc creates conditions for the long-term concentration of oils on the membrane surface, but new problems arise after zinc ions replace iron ions.

[0057] Zinc's coordination ability with polyphenols is not as good as that of iron, resulting in a slow coordination rate and a low stability constant of the complex. If natural plant polyphenols such as grape seed proanthocyanidins are directly used to coordinate with zinc, the number of coordination sites and phenolic hydroxyl groups on a single molecule is limited, resulting in a low cross-linking density and a loose structure. The coating is loose and easily lost, and the number of phenolic hydroxyl groups on the film surface that can provide hydrogen is insufficient, making it difficult to simultaneously perform the two tasks of film formation and color protection.

[0058] In this application, galloyl proanthocyanidins are used as the polyphenol ligand for the surface layer. Galloyl proanthocyanidins are obtained by esterification of grape seed proanthocyanidins and gallic acid via lipase catalysis. Grape seed proanthocyanidins are oligomers of flavan-3-ol units, with an alcoholic hydroxyl group at the 3-position of their C ring. Immobilized Candida antarcticis lipase B exhibits regioselectivity for this alcoholic hydroxyl group, catalyzing the esterification of the carboxyl group of gallic acid with it. The phenolic hydroxyl groups on both grape seed proanthocyanidins and gallic acid molecules do not participate in the reaction and are thus preserved intact.

[0059] Therefore, with each galloyl group introduced, a single molecule gains three new phenolic hydroxyl groups and one pyrogallol coordination structure, simultaneously increasing the number of zinc ion coordination anchors and hydrogen donation sites for free radicals. The increased number of coordination anchors enhances the crosslinking density of the zinc-polyphenol coordination network, resulting in a dense and robust coating structure. The increased number of phenolic hydroxyl groups improves the hydrogen donation capacity of the membrane surface, allowing the polyphenols on the membrane surface to donate hydrogen to lipid free radicals and stabilize themselves through the resonance delocalization of the generated phenoxy free radicals, thereby interrupting the chain reaction of lipid peroxidation and assuming a color-protecting function. Both film formation and color protection are addressed by the same modified ligand.

[0060] The process conditions matched with galloyl modification further ensure the modification effect. Esterification is a reversible reaction, and 4A molecular sieves continuously absorb the water generated from the reaction, which promotes the shift of esterification equilibrium towards the product direction and increases the galloylation degree. Immobilized Candida antarctica lipase B is easy to be removed by filtration and recovered, and does not introduce soluble impurities into the system. The reaction is carried out under nitrogen protection to prevent the pyrogallol structure of gallic acid from being oxidized and inactivated during long-time reaction. The average degree of polymerization of grape seed proanthocyanidins is 2-5, which gives consideration to both the number of binding sites on the ligand molecule and the water solubility of the modified product, and avoids the decrease of solubility of galloylated proanthocyanidins and phase separation of the solution caused by excessively high acylation degree. The filtrate is finely filtered through a 0.45 μm filter membrane to remove molecular sieve fine powder and immobilized enzyme particles, so as to prevent the particles from blocking membrane pores in subsequent coating.

[0061] After the number of ligand binding sites of the ligand increases, a new problem arises again, that is, the crosslinking rate of galloylated proanthocyanidins and zinc ions is significantly accelerated. If the two are directly mixed and then coated into a film immediately, zinc and polyphenols will instantly form coarse precipitates in the solution, the precipitates will block the membrane pores, the resulting network is uneven, and both the hydrophilicity and integrity of the coating are affected, that is, the crosslinking rate is out of control.

[0062] Therefore, in the present application, a zinc-polyphenol prepolymer solution is prepared first, galloylated proanthocyanidins and zinc acetate are pre-coordinated in water to form zinc-polyphenol pre-polymer clusters, then the zinc-polyphenol pre-polymer clusters and phytate anions are bridged and coordinated to form a surface network, so that the crosslinking reaction proceeds in a controlled manner. The zinc-polyphenol pre-polymer clusters are soluble oligomers, the mass ratio of zinc element to galloylated proanthocyanidins is controlled at a low level, and the zinc acetate solution is slowly added dropwise under stirring, so that zinc ions are always in a small amount of dispersed state and cannot reach the local concentration required for forming coarse precipitates. After the dropwise addition is completed, the mixture is stirred and aged at room temperature, and the coordination structure of the pre-polymer clusters rearranges and grows to an appropriate size. In the subsequent coating step, the unsaturated coordinated zinc sites reserved on the zinc-polyphenol pre-polymer clusters are further bridged with phytate anions in the coating solution, the network growth proceeds in stages, and the surface layer is uniform and dense without blocking membrane pores. Nitrogen protection throughout the whole process and the use of deoxygenated deionized water prevent phenolic hydroxyl groups from being oxidized during the pre-coordination stage, and ensure the effective number of ligand binding sites.

[0063] After the zinc-polyphenol network surface layer is formed, the bonding problem between the functional layer and the base membrane becomes the key factor affecting the membrane service life. The polyvinylidene fluoride base membrane has low surface energy and inert chemical properties, conventional coating layers only rely on physical adhesion on its surface, and are easy to lose under cross-flow scouring of high solid content algae slurry, resulting in short service life of the functional layer. Most existing coatings have a single-layer structure, which also has the deficiencies of low loading capacity and weak bonding force.

[0064] To address this issue, this application constructs a zinc phytate nanocluster substrate between the polyvinylidene fluoride (PVDF) membrane and the zinc-polyphenol network layer, ensuring a strong bond between the functional layer and the base membrane. The substrate is formed by in-situ coordination of phytic acid and zinc ions on the base membrane surface. After wetting the PVDF membrane with ethanol and replacing it with water, it is sequentially and alternately immersed in zinc acetate and phytic acid solutions. The two solutions contact each other within a thin liquid layer on the membrane surface. After the pH of the phytic acid solution is pre-adjusted to 4.5-5.5, some phosphate groups on the phytate ions are deprotonated, enhancing their coordination ability and facilitating nucleation upon contact with zinc ions. Zinc phytate has extremely low solubility, resulting in immediate nucleation and precipitation upon contact. Zinc deposition is confined within a nanoscale thin liquid layer, growing into nanoclusters rather than a continuous, dense layer. The nanoclusters have a large specific surface area and numerous unsaturated zinc sites, providing a high density of anchoring points for the substrate. The sequential cyclic immersion controls the cluster loading, preventing an excessively thick substrate that could increase mass transfer resistance.

[0065] The connection between the top and bottom layers is achieved through a gradual transition of coordination bonds. The coating solution in step (4) is a compound of zinc-polyphenol prepolymer and sodium phytate. The sodium phytate concentration is low, and the zinc sites on the zinc-polyphenol prepolymer clusters are not completely occupied by phytate. After immersing the base film with the zinc phytate nanoclusters as the bottom layer into the coating solution, the zinc sites reserved on the prepolymer clusters are captured by the phytate on the surface of the bottom layer. The unsaturated zinc sites on the surface of the bottom nanoclusters are also coordinated with the phytate in the coating solution. A chemically bonded transition region is thus formed between the top and bottom layers. The bonding method is not physical superposition but a continuous gradual change of coordination bonds, which greatly improves the bonding force and can withstand long-term cross-flow scouring of high solid content liquid. After coating, the film is washed with water until the conductivity of the washing solution is comparable to that of deionized water to remove free ions and uncoordinated small molecules and prevent them from remaining in the membrane pores.

[0066] After the membrane structure is determined, it is also necessary to match the membrane with the enrichment process. The enrichment process requires the oil as a product to be retained and concentrated on the membrane surface for a long time. When ordinary hydrophilic membranes retain oil, the oil is subjected to pressure and aggregates, leading to uncontrolled demulsification, and the astaxanthin esters are rapidly oxidized after being exposed. When the zinc-polyphenol network microfiltration membrane of this application is used to enrich astaxanthin ester oil in the broken cell wall algae pulp of Haematococcus pluvialis, the hydration layer on the membrane surface retains the oil without spreading or agglomerating. The astaxanthin esters are always inside the oil and protected by the oil-water interface barrier, without direct contact with oxygen and the membrane surface. The high density of phenolic hydroxyl groups on the membrane surface continuously supplies hydrogen to lipid free radicals, inhibiting interfacial oxidation. The mild retention and concentration inside the membrane is connected with the subsequent demulsification and collection outside the membrane. The oil remains in a complete emulsion state inside the membrane and is concentrated for demulsification after leaving the membrane surface. The timing of demulsification is controlled by the process rather than occurring passively on the membrane surface, thereby achieving the unity of enrichment and color protection.

[0067] In summary, this application addresses the problem of oil adhesion and contamination by using a zinc-polyphenol network surface layer, eliminates pro-oxidation factors on the film surface by replacing iron ions with zinc ions, compensates for the deficiencies of the zinc-polyphenol coordination network in terms of crosslinking density and hydrogen supply capacity by using galloylation modification, controls the crosslinking rate and ensures film uniformity by using zinc-polyphenol prepolymer clusters, and ensures the long-term stability of the functional layer by using zinc phytate nanoclusters as the bottom layer and interlayer coordination connections. These technical features are interconnected and work together to support the solvent-free anti-contamination enrichment of astaxanthin ester oil in the broken cell wall algae pulp of Haematococcus pluvialis.

[0068] The technical solution of this application will be further described in detail below with reference to the embodiments. All raw materials used in the following embodiments are commercially available products. The average degree of polymerization of grape seed proanthocyanidins is 2-5. Gallic acid, phytic acid, sodium phytate, zinc acetate, citric acid, acetic acid, sodium ascorbate, and sodium hydroxide are all food-grade or food processing aids. Cellulase and pectinase are food-grade enzyme preparations. The immobilized lipase is immobilized Candida antarctica lipase B. The pore size of the polyvinylidene fluoride membrane is 0.1-0.2 μm. The Haematococcus pluvialis thick-walled spore powder is commercially available Haematococcus pluvialis powder. Experimental methods not specifically specified were carried out according to conventional conditions in the art.

[0069] Example 1

[0070] (1) Preparation of galloyl proanthocyanidins: Grape seed proanthocyanidins with an average degree of polymerization of 2-5 were mixed with gallic acid at a mass ratio of 1:0.4 and added to anhydrous ethanol. The mixture was heated and stirred to dissolve the proanthocyanidins. The total mass concentration of the substrate was controlled at 15%. Immobilized Candida antarcticis lipase B was added at a mass ratio of 7.5% of the grape seed proanthocyanidins. 4A molecular sieve was added at a mass ratio of 20% of the grape seed proanthocyanidins. The mixture was stirred at 50°C for 36 hours under nitrogen protection. The immobilized lipase and molecular sieve were then removed by filtration. The filtrate was filtered through a 0.45 μm filter membrane to remove the fine molecular sieve powder. The filtrate was then deethanolerated under reduced pressure at 45°C to obtain galloyl proanthocyanidins solid. The degree of galloylation was measured to be 23%.

[0071] (2) Preparation of zinc-polyphenol prepolymer: Take the galloyl proanthocyanidin obtained in step (1), prepare a 0.5% solution with deoxygenated deionized water, adjust the pH to 5.5 with acetic acid or sodium hydroxide solution, and add 0.2% zinc acetate solution dropwise at 2 mL / min under stirring at 400 r / min. The mass ratio of zinc to galloyl proanthocyanidin is 0.2:1. After the addition is completed, stir and age at room temperature for 4 h to obtain zinc-polyphenol prepolymer.

[0072] (3) Construction of zinc phytate nanocluster substrate: Take a polyvinylidene fluoride membrane with a pore size of 0.15 μm, soak it in anhydrous ethanol for 15 min to wet it, then replace the ethanol with deionized water, then immerse it in a zinc acetate solution with a mass fraction of 0.1%, let it stand for 20 min, take it out and drain it, and then immerse it in a phytic acid solution with a mass fraction of 0.35% and a pH of 4.5. Let it stand for 30 min, take it out and rinse it with deionized water. Repeat the above soaking cycle 3 times to obtain a substrate film with a zinc phytate nanocluster substrate on the surface.

[0073] (4) Coating of the surface layer: Sodium phytate was added to the zinc-polyphenol prepolymer solution obtained in step (2) to form a coating solution. Based on the total mass of the coating solution, the mass fraction of sodium phytate was 0.1% and the mass fraction of galloyl proanthocyanidins was 0.5%. The pH of the coating solution was adjusted to 6.0 with acetic acid or sodium hydroxide solution. The base membrane obtained in step (3) was immersed in the coating solution and soaked at room temperature for 3 hours. After taking it out, it was washed with deionized water until the conductivity of the washing solution was comparable to that of deionized water. It was then stored in deionized water at 4°C to obtain a zinc-polyphenol network microfiltration membrane. The sodium phytate concentration in the coating solution was low. The zinc sites reserved on the prepolymer clusters coordinated with the phytate groups on the surface of the bottom nanoclusters. The surface layer was connected to the bottom zinc ions through coordination of phytate groups. It was determined that the molar ratio of phosphate groups to zinc in the membrane was 2.3:1.

[0074] (5) Enrichment Application: Take thick-walled spore powder of Haematococcus pluvialis, add water to prepare an algal slurry with a solid content of 8%, add 2% cellulase and 1.0% pectinase by weight of algal powder, adjust the pH to 5.0 with citric acid, enzymatically hydrolyze at 48℃ and 160r / min for 4h, then heat to 68℃ and keep warm for 10min to inactivate the enzyme, obtaining a cell wall-broken algal slurry; the cell wall-broken algal slurry is coarsely filtered through 8μm filter media to remove cell wall fragments and large particulate impurities, obtaining a coarse filtrate; the coarse filtrate is adjusted to a solid content of 4% with water, and the pH is adjusted to 6.5 with sodium hydroxide solution, obtaining... Conditioning solution: The conditioning solution enters a cross-flow filtration device equipped with the above-mentioned zinc-polyphenol network microfiltration membrane. Before entering the membrane, a 15% sodium ascorbate aqueous solution is added online, with the addition amount being 0.03% of the solution mass. The device is operated at 25°C, transmembrane pressure difference of 0.2MPa, and membrane surface flow rate of 2m / s. The solution is concentrated to 1 / 6 of its original volume to obtain a concentrate. This process is repeated for 5 batches. The concentrate is adjusted to pH 4.8 with citric acid, allowed to stand at 6°C, and centrifuged at 5000r / min for 20min. The upper astaxanthin ester oil is collected.

[0075] Example 2

[0076] In this embodiment, the similarities to those in Embodiment 1 will not be repeated, and the differences are as follows:

[0077] In step (1), the mass ratio of grape seed proanthocyanidins to gallic acid was 1:0.2, the amount of immobilized Candida antarctica lipase B was 10% of the mass of grape seed proanthocyanidins, the amount of 4A molecular sieve was 35% of the mass of grape seed proanthocyanidins, the reaction temperature was 40℃, the reaction time was 48h, and the degree of galloylation was measured to be 15%. In step (2), the mass fraction of galloyl proanthocyanidin solution was 0.75%, the pH was adjusted to 5.0 with acetic acid or sodium hydroxide solution, the mass ratio of zinc to galloyl proanthocyanidins was 0.1:1, the stirring speed was 500r / min, and the dropping rate was 3. 0.5 mL / min, zinc acetate solution mass fraction 0.1%, aged for 2 h; in step (3), the base membrane pore size is 0.2 μm, zinc acetate solution mass fraction is 0.175%, stood in zinc acetate solution for 10 min, phytic acid solution mass fraction is 0.5%, pH 5.0, stood in phytic acid solution for 10 min, and immersion cycle 4 times; in step (4), the coating solution sodium phytate mass fraction is 0.2%, galloyl proanthocyanidin mass fraction is 0.75%, coating solution pH 5.0, soaked at room temperature for 4 h, and the molar ratio of phosphate groups to zinc in the membrane was measured to be 4.4:1. The enrichment application steps are exactly the same as in Example 1.

[0078] Example 3

[0079] In this embodiment, the similarities to those in Embodiment 1 will not be repeated, and the differences are as follows:

[0080] In step (1), the mass ratio of grape seed proanthocyanidins to gallic acid was 1:0.3, the amount of immobilized Candida antarcticis lipase B was 5% of the mass of grape seed proanthocyanidins, the amount of 4A molecular sieve was 50% of the mass of grape seed proanthocyanidins, the reaction temperature was 45℃, the reaction time was 24h, and the degree of galloylation was measured to be 12%. In step (2), the mass fraction of galloyl proanthocyanidin solution was 1.0%, the pH was adjusted to 5.3 with acetic acid or sodium hydroxide solution, the mass ratio of zinc to galloyl proanthocyanidins was 0.15:1, the stirring speed was 300r / min, and the dropping rate was [missing information]. 5 mL / min, zinc acetate solution mass fraction 0.15%, aged for 3 h; in step (3), the base membrane pore size is 0.1 μm, zinc acetate solution mass fraction is 0.25%, and it is left to stand in zinc acetate solution for 15 min; phytic acid solution mass fraction is 0.2%, pH 5.5, and it is left to stand in phytic acid solution for 20 min, with 2 soaking cycles; in step (4), the coating solution has sodium phytate mass fraction of 0.3% and galloyl proanthocyanidin mass fraction of 1.0%, pH 5.5, and is soaked at room temperature for 2 h. The molar ratio of phosphate groups to zinc in the membrane is measured to be 1.2:1. The enrichment application steps are exactly the same as in Example 1.

[0081] Example 4

[0082] In this embodiment, the similarities to those in Embodiment 1 will not be repeated, and the differences are as follows:

[0083] In step (1), the mass ratio of grape seed proanthocyanidins to gallic acid was 1:0.35, the amount of immobilized Candida antarcticis lipase B was 8% of the mass of grape seed proanthocyanidins, the amount of 4A molecular sieve was 35% of the mass of grape seed proanthocyanidins, the reaction temperature was 45℃, the reaction time was 40h, and the degree of galloylation was measured to be 22%. In step (2), the mass fraction of galloyl proanthocyanidin solution was 0.8%, the pH was adjusted to 5.3 with acetic acid or sodium hydroxide solution, the mass ratio of zinc to galloyl proanthocyanidins was 0.15:1, the stirring speed was 400r / min, and the dropping speed was accelerated. The rate was 3 mL / min, the zinc acetate solution mass fraction was 0.15%, and the aging was 3 h; in step (3), the base membrane pore size was 0.15 μm, the zinc acetate solution mass fraction was 0.2%, and it was left to stand in the zinc acetate solution for 15 min; the phytic acid solution mass fraction was 0.3%, pH 5.0, and it was left to stand in the phytic acid solution for 20 min, and the soaking cycle was 4 times; in step (4), the coating solution had a sodium phytate mass fraction of 0.2% and a galloyl proanthocyanidin mass fraction of 0.8%, the coating solution pH was 5.5, and it was soaked at room temperature for 3 h. The molar ratio of phosphate groups to zinc in the membrane was measured to be 1.4:1. The enrichment application steps were exactly the same as in Example 1.

[0084] Comparative Example 1

[0085] Similar to Example 4, except that the polyvinylidene fluoride membrane is not modified and is used directly for enrichment applications; otherwise, it is the same as Example 4.

[0086] Comparative Example 2

[0087] Similar to Example 4, except that zinc acetate in steps (2), (3) and (4) is replaced by ferric chloride with an equal molar amount of zinc, and the bottom layer of zinc phytate nanoclusters is ferric phytate nanoclusters. After adding ferric chloride solution in step (2), a small amount of flocculent precipitate appears in the system. After stirring evenly, it is immediately used for coating in step (4). The rest is the same as Example 4.

[0088] Comparative Example 3

[0089] Similar to Example 4, except that the galloylation modification in step (1) is not performed, and the solution is prepared directly using unmodified grape seed proanthocyanidins in step (2). The rest is the same as Example 4.

[0090] Comparative Example 4

[0091] Similar to Example 4, except that step (3) is not performed, i.e., the zinc phytate nanocluster bottom layer is not constructed, and step (4) is directly coated on the surface of the polyvinylidene fluoride film. The rest is the same as Example 4.

[0092] Comparative Example 5

[0093] Similar to Example 4, except that sodium ascorbate aqueous solution is not added online before membrane entry in the enrichment application. Otherwise, it is the same as Example 4, and the zinc-polyphenol network microfiltration membrane used is prepared in the same batch as in Example 4.

[0094] Performance Test Results and Analysis

[0095] In each embodiment and comparative example, the degree of galloacylation was determined by integrating the proton NMR spectrum. The degree of galloacylation is the molar percentage of alcohol hydroxyl groups in grape seed proanthocyanidin structural units esterified by galloacyl groups. After digestion, the membrane samples were analyzed by phosphorus determination using phosphomolybdic blue spectrophotometry and zinc determination using inductively coupled plasma atomic emission spectrometry. The molar ratio of phosphate groups to zinc in the membrane was then calculated.

[0096] The galloyl proanthocyanidin sample obtained in step (1) was dissolved in a deuterated reagent and its nuclear magnetic resonance hydrogen spectrum was measured. The degree of galloylation was calculated based on the integral ratio of the hydrogen signal of the galloyl benzene ring to the characteristic hydrogen signal of proanthocyanidin. The membrane samples obtained from each example and comparative example were subjected to liquid nitrogen embrittlement and gold sputtering. The nanoclusters in the bottom layer of the membrane cross-section were observed by scanning electron microscopy. The particle size of no less than 100 nanoclusters was randomly counted and the average value was taken. The membrane sample was fixed on a glass slide and immersed in deionized water. 2 μL of soybean oil was dropped into the membrane surface using a contact angle meter, and the underwater oil contact angle was measured. The average value was taken after 5 parallel measurements. The oil matrix of astaxanthin ester oil is a triglyceride oil, which has similar interfacial wetting characteristics to soybean oil. Since the amount of astaxanthin ester oil obtained is limited, soybean oil was used as the model oil for measurement. This substitution is a conventional method in this field for evaluating underwater oleophobicity. The zeta potential of the membrane surface was measured using the flowing potentiometry method with a 1 mmol / L KCl solution at pH 6.5 as the electrolyte. The membrane sample was immersed in deionized water at 25°C for 48 hours, then removed, dried, and the underwater oil contact angle was remeasured. The contact angle retention rate was the percentage of the contact angle after immersion to the initial contact angle. The membrane sample was then placed in a cross-flow filtration device and pre-pressurized at 25°C and 0.2 MPa with pure water as the feed for 30 minutes before measuring the initial pure water flux.

[0097] Oil rejection rate, flux decay rate, flux recovery rate, and astaxanthin retention rate were all measured under the aforementioned unified enrichment application conditions. Five batches were run continuously, with fresh feed solution used immediately after each batch of concentration, and no inter-batch washing was performed. The permeate and conditioning feed solutions from the fifth batch were collected, and the oil content was measured. The oil rejection rate was the percentage of 1 minus the ratio of the oil content in the permeate to the oil content in the conditioning feed solution. The initial flux of the first batch and the final flux of the fifth batch were recorded, and the flux decay rate was the percentage of the difference between the two relative to the initial flux. After the fifth batch, the membrane samples were washed with deionized water, and the pure water flux was remeasured. The flux recovery rate was the percentage of the flux after washing relative to the initial flux. High-performance liquid chromatography (HPLC) was used to determine the astaxanthin content in the oil collected from the conditioning feed solution and each batch of concentrate. The astaxanthin retention rate was the percentage of the astaxanthin content in the collected oil relative to the astaxanthin content in the corresponding conditioning feed solution. The specific test results are shown in Tables 1 and 2.

[0098] Table 1 Analysis of Test Results (I)

[0099] Example 1 23 210 155 -30 96 920 Example 2 15 320 153 -30 95 880 Example 3 12 130 154 -31 95.5 900 Example 4 22 160 158 -34 98 940 Comparative Example 1 - - 72 -12 - 860 Comparative Example 2 22 180 152 -25 94 870 Comparative Example 3 0 160 150 -28 91 870 Comparative Example 4 22 - 152 -29 82 865 Comparative Example 5 22 160 158 -34 98 940

[0100] Note: The microfiltration membrane used in Comparative Example 5 was prepared in the same batch as that in Example 4. Its initial membrane properties were the same as those in Example 4. The difference in operating performance was due to the absence of sodium ascorbate in the feed solution.

[0101] The degree of galloacylation was determined by integrating the proton NMR spectrum and reflects the extent of galloacylation modification in step (1), which is the structural basis for the differences in subsequent performance. As shown in Table 1, the degrees of galloacylation in Examples 1-4 were 23%, 15%, 12%, and 22%, respectively, indicating that the degree of acylation can be controlled by adjusting the substrate mass ratio, the amount of immobilized enzyme, the reaction temperature, and the reaction time; Comparative Example 3 was unmodified and had a degree of acylation of 0. The significance of the degree of acylation is that each introduced galloacyl group adds three phenolic hydroxyl groups to the proanthocyanidin molecule, and the number of anchor sites for zinc coordination and the number of sites for donating hydrogen to free radicals increase simultaneously. This structural difference will be reflected in the contact angle retention rate, operating performance, and astaxanthin retention rate in the following sections.

[0102] The average particle size of the nanoclusters was obtained by observing the membrane cross-section using scanning electron microscopy and statistically analyzing the data, reflecting the morphology of the zinc phytate nanoclusters at the bottom layer. Table 1 shows that the particle sizes of Examples 1-4 were 210 nm, 320 nm, 130 nm, and 160 nm, respectively, all within the 100-500 nm range. Furthermore, the particle size variation with parameters followed a predictable pattern: Example 2 had the highest phytic acid solution mass fraction and the most immersion cycles, resulting in repeated nucleation and continuous cluster growth within the thin liquid layer on the membrane surface, leading to the largest nanoclusters. Example 3 had the lowest phytic acid solution mass fraction and the fewest cycles, resulting in fewer and smaller nanoclusters. Comparative Examples 1 and 4 lacked a bottom layer, therefore this data is unavailable. A suitable particle size indicates uniform cluster coverage without localized excessive thickness, which corresponds to the flux data described later.

[0103] The underwater oil contact angle was measured by dropping soybean oil droplets underwater, and the zeta potential was measured by the flow potential method at pH 6.5. Both reflect the membrane's ability to repel oil and its charge characteristics, respectively, jointly determining the oil's adhesion tendency on the membrane surface. Table 1 shows that the contact angles of Examples 1-4 were 153-158°, and the zeta potentials were -30 to -34 mV. The membrane surface was hydrophilic and strongly negatively charged. The dense phenolic hydroxyl groups and phytate groups in the surface layer formed a hydration layer on the membrane surface, making it difficult for oil to penetrate the hydration layer and contact the membrane surface. Simultaneously, the negatively charged algal proteins adsorbed on the oil surface repelled the negative charge on the membrane surface. Spatial barrier and electrostatic repulsion jointly inhibited oil adhesion. Comparative Example 1 used a polyvinylidene fluoride membrane, with a contact angle of only 72° and a zeta potential of only -12 mV. The oil spread on the hydrophobic membrane surface and was forced into the pores under pressure differential. Comparative Example 2 showed a contact angle of 152°, indicating that the iron-polyphenol network can also construct a hydrophilic interface, but its zeta potential was -25mV, indicating weaker electronegativity than the zinc network and weaker electrostatic repulsion. Comparative Example 3 had the lowest contact angle of 150° and zeta potential of -28mV among the four modified samples. This is because the unmodified grape seed proanthocyanidins had fewer coordination sites, lower network crosslinking density, and insufficient number of fixed hydrophilic and ionizable groups on the membrane surface.

[0104] The contact angle retention rate was calculated by re-measuring the contact angle after immersing the membrane sample in deionized water for 48 hours, reflecting the bonding strength between the coating and the base film. Table 1 shows that the retention rates of Examples 1-4 were 95-98%, while Comparative Example 4 was only 82%. This difference directly points to the role of the zinc phytate nanocluster bottom layer: the bottom nanoclusters anchor the surface layer to the base film through interlayer coordination. Without the bottom layer, the surface layer adheres only through weak interactions with the base film, gradually peeling off under prolonged immersion and water shearing. Comparative Example 3 had a retention rate of 91%, indicating that the unmodified ligand, due to fewer coordination sites and lower network crosslinking density, resulted in a less robust coating. Comparative Example 2 had a retention rate of 94%, which was acceptable but still slightly lower than the examples.

[0105] The initial flux of pure water was measured at 25°C and 0.2 MPa using pure water as the feed solution, reflecting the membrane's pore size and mass transfer resistance. Table 1 shows that the flux for Examples 1-4 was 880-940 L / (m³). 2 The initial flux of the three coated comparative examples (Example 4, Example 2, and Example 3, Example 4) was the highest, while that of Example 2 was the lowest, corresponding to their largest nanocluster particle size and locally thicker coverage. The three coated comparative examples, namely Comparative Examples 2, 3, and 4, all had an initial flux of 865-870 L / (m²). 2 ·h), lower than the lowest value of 880L / (m) in the examples. 2 The reasons for this vary: iron coordinates rapidly with polyphenols, resulting in uncontrolled cross-linking and some clusters becoming too large and clogging the pores; unmodified ligands have weak coordination with zinc, leading to low cross-linking density and uneven coating coverage; and without an underlayer, the surface prepolymer clusters directly cover the pores. Comparative Example 1, although without coating resistance, only achieved a flux of 860 L / (m²).2 ·h), because the polyvinylidene fluoride membrane itself is hydrophobic, the wetting is not complete in the pure water test, and the actual water flow area is smaller than the membrane area.

[0106] Table 2 Analysis of Test Results (Part Two)

[0107] Example 1 97.6 27 94.5 96.8 94.9 93.1 Example 2 97.5 31 93.5 96.5 94.3 92.2 Example 3 97.7 29 94 96.2 94 92.5 Example 4 98.3 26 96 97.5 96 94.5 Comparative Example 1 91.5 62 58 89.5 85.5 78.5 Comparative Example 2 97.2 34 92.5 88.5 80.2 71.3 Comparative Example 3 96.8 33 92 94.5 91 88.6 Comparative Example 4 96.5 32 88.5 95.5 91.5 87.8 Comparative Example 5 97 36 89.5 95 90.5 85

[0108] Oil rejection rate, flux decay rate, and flux recovery rate were measured under uniform enrichment application conditions, reflecting the membrane's rejection capacity, fouling degree, and fouling reversibility, respectively. Table 2 shows that the rejection rate for Examples 1-4 was 97.5-98.3%, the decay rate was 26-31%, and the recovery rate was 93.5-96%. Comparative Example 1 had a rejection rate of only 91.5%, a decay rate as high as 62%, and a recovery rate of only 58%: oil spread on the hydrophobic membrane surface and pressed into the pores, forming irreversible blockage. The deformed oil under pressure penetrated the membrane, causing further rejection losses, indicating that the pore size sieving itself could not achieve gentle oil rejection. Comparative Example 4 had a decay rate of 32% and a recovery rate of 88.5%. During operation, the surface layer gradually peeled off, causing the antifouling interface to gradually fail, and fouling changed from reversible to cumulative. The membrane used in Comparative Example 5 was the same as that in Example 4, but the attenuation rate was 36% and the recovery rate was 89.5%, which was worse than all examples: when there was no sodium ascorbate in the feed solution, the phenolic hydroxyl groups on the membrane surface were continuously oxidized to quinone structures, the hydrophilicity and negative charge decreased with operation, the hydration layer and electrostatic repulsion weakened, and the adhesion of oil and protein increased batch by batch. This confirms the protective effect of antioxidant relay on membrane surface properties from the operation end.

[0109] Astaxanthin retention rate was determined batch by batch by high performance liquid chromatography and is the core indicator for evaluating the oxidative protection effect. As shown in Table 2, the retention rate of the fifth batch in Examples 1-4 was 92.2-94.5%. The retention rate of Comparative Example 2 was only 88.5% in the first batch and dropped to 71.3% in the fifth batch, which was the lowest among all samples.

[0110] Iron has a variable valence state and, in the presence of polyphenols, cyclically catalyzes single-electron transfer and generates hydroxyl radicals. After the oil is trapped and concentrated at the film surface, the oxidation reaction at the interface is amplified by the concentration effect. The retention rate and flux decline rate of Comparative Example 2 are close to those of Example 4, indicating that the physical blockage effect caused by flocculent precipitation is limited, and the significant decrease in its retention rate can be confirmed as mainly attributable to the pro-oxidation effect of iron ions. Zinc, on the other hand, has a 3d... 10The coating is fully filled with a non-variable valence state, and its coordination network is chemically inert. Data comparison directly verifies the necessity of replacing iron with zinc. Comparative Example 5 showed a 95.0% retention rate in the first batch, which dropped to 85.0% in the fifth batch: the polyphenols on the membrane surface possess a certain hydrogen-donating capacity, but after consumption, there is no ascorbate anion to regenerate it, and the protective capacity is depleted with increasing batch numbers. Comparative Example 3 showed an 88.6% retention rate in the fifth batch, due to a low total amount of phenolic hydroxyl groups and low hydrogen-donating capacity. Comparative Example 1 showed a 78.5% retention rate in the fifth batch: although sodium ascorbate was also added to the feed solution, the membrane surface lacked a zinc-polyphenol network, and the ascorbate anion lacked an interfacial target for regeneration, and was lost with the permeate, failing to effectively block the oxidation chain reaction at the oil interface. Comparative Example 4 showed a 87.8% retention rate in the fifth batch; after the coating peeled off, the interfacial protection failed.

[0111] Example 4 exhibits the best performance across all aspects, representing a moderate combination of acylation degree (22%), phosphate to zinc molar ratio (1.4:1), and nanocluster size (160 nm). The acylation degree strikes a balance between coordination site density and water solubility, the molar ratio balances crosslinking density and network integrity, and the particle size achieves uniform coverage. Example 1's acylation degree of 23% is close to the water solubility threshold, Example 2's molar ratio of 4.4:1 is relatively high and its nanocluster size is the largest, and Example 3's acylation degree of 12% is relatively low. All three examples are slightly inferior to Example 4 in some individual metrics, but their worst-case scenario is still better than the best-case scenario in the comparative example. Since the enrichment application conditions of the four embodiments and the five comparative examples are completely consistent, the membrane is the only experimental variable. All the above performance differences can be attributed to the differences in the structure and composition of the membrane itself. The control relationship is valid: Comparative examples 1-5 correspond to the cases of lacking the zinc-polyphenol network surface layer, replacing zinc ions with iron ions, not performing galloyl acylation modification, not constructing the zinc phytate nanocluster bottom layer, and not adding sodium ascorbate, respectively. The corresponding deterioration in performance confirms the role of the above-mentioned technical features in the technical solution of this application.

[0112] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A zinc-polyphenol network microfiltration membrane, characterized in that, The microfiltration membrane includes a polyvinylidene fluoride membrane, a zinc phytate nanocluster bottom layer located on the surface of the polyvinylidene fluoride membrane, and a zinc-polyphenol network surface layer located on the surface of the zinc phytate nanocluster bottom layer. The zinc phytate nanocluster bottom layer is generated by in-situ coordination of phytate and zinc ions on the surface of the polyvinylidene fluoride film. The zinc-polyphenol network surface layer is a network structure formed by cross-linking galloyl proanthocyanidins, zinc ions and phytate through coordination bonds. The network structure is formed by the coordination of zinc-polyphenol prepolymers and phytate. The zinc-polyphenol prepolymers are obtained by pre-coordinating galloyl proanthocyanidins and zinc acetate in water. The zinc-polyphenol network surface layer is connected to the zinc ions in the bottom layer of the zinc phytate nanoclusters through coordination of phytate. The galloyl proanthocyanidin is obtained by esterification of grape seed proanthocyanidins and gallic acid via lipase catalysis.

2. A method for preparing a zinc-polyphenol network microfiltration membrane as described in claim 1, characterized in that, The method includes the following steps: (1) Preparation of galloyl proanthocyanidins: Grape seed proanthocyanidins and gallic acid were dissolved in anhydrous ethanol, immobilized lipase and 4A molecular sieve were added to carry out catalytic esterification reaction, and then filtered. The filtrate was filtered through a 0.45μm filter membrane, and the ethanol was removed from the filtrate under reduced pressure at 40-50℃ to obtain galloyl proanthocyanidins. (2) Preparation of zinc-polyphenol prepolymer solution: The galloyl proanthocyanidins obtained in step (1) are dissolved in deoxygenated deionized water to prepare a galloyl proanthocyanidin solution. The pH is adjusted to 5.0-5.5 with acetic acid or sodium hydroxide solution. Zinc acetate solution is added dropwise. After the addition is completed, the solution is stirred and aged at room temperature for 2-4 hours under nitrogen protection throughout the process to obtain zinc-polyphenol prepolymer solution. (3) Construction of zinc phytate nanocluster substrate: The polyvinylidene fluoride membrane was soaked in anhydrous ethanol for 10-20 min to wet it, then the ethanol was replaced with deionized water, and then it was immersed in a zinc acetate solution with a mass fraction of 0.1-0.25% for 10-20 min. After being taken out and drained, it was immediately immersed in phytic acid solution for 10-30 min. After being taken out and rinsed with deionized water, the above soaking cycle was repeated 2-4 times to obtain a base film with a zinc phytate nanocluster substrate on the surface. (4) Composite of the surface layer: The base film with zinc phytate nanoclusters on the surface obtained in step (3) is immersed in a coating solution made of zinc-polyphenol prepolymer solution and sodium phytate obtained in step (2) for 2-4 hours at room temperature. After washing with water, it is stored in a wet state to obtain zinc-polyphenol network microfiltration membrane.

3. The method for preparing a zinc-polyphenol network microfiltration membrane according to claim 2, characterized in that, In step (1), the mass ratio of grape seed proanthocyanidins to gallic acid is 1:0.2-0.4, the amount of immobilized lipase is 5-10% of the mass of grape seed proanthocyanidins, and the amount of 4A molecular sieve is 20-50% of the mass of grape seed proanthocyanidins.

4. The method for preparing a zinc-polyphenol network microfiltration membrane according to claim 2, characterized in that, In step (1), the immobilized lipase is immobilized Candida antarcticis lipase B; the average degree of polymerization of the grape seed proanthocyanidins is 2-5; and the catalytic esterification is carried out under nitrogen protection at 40-50℃ for 24-48 hours.

5. The method for preparing a zinc-polyphenol network microfiltration membrane according to claim 2, characterized in that, In step (2), the deoxygenated deionized water is deionized water that has been boiled and cooled or deoxygenated by nitrogen purging; the galloyl proanthocyanidin solution contains 0.5-1.0% galloyl proanthocyanidin by mass; zinc is added in the form of zinc acetate, and the mass ratio of zinc to galloyl proanthocyanidin is 0.1-0.2:

1.

6. The method for preparing a zinc-polyphenol network microfiltration membrane according to claim 2, characterized in that, In step (2), the specific steps for adding zinc acetate solution are as follows: under nitrogen protection and stirring at 300-500 r / min, add zinc acetate solution with a mass fraction of 0.1-0.2% at a rate of 2-5 mL / min.

7. The method for preparing a zinc-polyphenol network microfiltration membrane according to claim 2, characterized in that, In step (3), the pore size of the polyvinylidene fluoride membrane is 0.1-0.2 μm.

8. The method for preparing a zinc-polyphenol network microfiltration membrane according to claim 2, characterized in that, In step (3), the phytic acid solution has a mass fraction of 0.2-0.5% and a pH of 4.5-5.5; the pH of the phytic acid solution is adjusted with sodium hydroxide solution.

9. The method for preparing a zinc-polyphenol network microfiltration membrane according to claim 2, characterized in that, In step (4), the mass fraction of sodium phytate in the coating solution is 0.1-0.3%, and the pH of the coating solution is adjusted to 5.0-6.0 with acetic acid or sodium hydroxide solution; the water washing is done with deionized water until the conductivity of the washing solution is comparable to that of deionized water.

10. An application of the zinc-polyphenol network microfiltration membrane as described in claim 1, characterized in that, This microfiltration membrane is used to enrich astaxanthin ester oil bodies in the broken cell wall slurry of Haematococcus pluvialis.

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