A composite membrane module and a method of manufacturing and using the same

Abstract

The application relates to the field of membrane separation technology, and particularly discloses a composite membrane assembly, a preparation method and application thereof. The composite membrane assembly comprises a shell, a plurality of layers of composite membranes stacked in the shell, and a near-infrared light source. The composite membranes comprise, from top to bottom in the order of contact with a feed liquid, a molecular imprinting recognition layer containing specific recognition cavities left after elution of astaxanthin template molecules, a polydopamine light-heat conversion layer, and a nanocellulose support layer made of a film material transparent to near-infrared light. The near-infrared light source is arranged on one side of the shell close to the nanocellulose support layer, and the near-infrared light emitted by the near-infrared light source directly irradiates the polydopamine light-heat conversion layer after penetrating the nanocellulose support layer. The application improves the extraction rate, purity, active configuration protection and operation efficiency of astaxanthin through the ordered integration of the multi-layer functional membrane structure and the accurate construction of the molecular imprinting recognition site, and the whole process meets the requirements of green chemistry.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and more specifically, to a composite membrane module, its preparation method, and its application. Background Technology

[0002] Astaxanthin, accumulated by Haematococcus pluvialis under stress conditions, is a high-value natural antioxidant. However, the algae have tough spore walls, making cell disruption difficult, and astaxanthin is easily destroyed by heat, light, and oxygen. Existing extraction methods mostly employ organic solvent extraction or supercritical CO2 extraction. The former has problems with solvent residue and environmental pollution, while the latter requires large equipment investments and harsh operating conditions. Conventional membrane separation technology can only achieve particle size sieving and cannot simultaneously achieve cell disruption and highly selective enrichment of astaxanthin, making it difficult to balance extraction rate and purity, and often requiring multiple steps. Therefore, there is an urgent need for a novel membrane technology that integrates cell disruption and specific separation, is green and non-toxic, and can significantly improve the extraction rate and purity of astaxanthin. Summary of the Invention

[0003] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a composite membrane module, its preparation method and application.

[0004] To achieve the above objectives, the present invention provides the following technical solution:

[0005] A composite membrane assembly includes a housing, a multilayer composite membrane stacked within the housing, and a near-infrared light source disposed on one side of the liquid inlet of the housing. The composite membrane comprises, from bottom to top, a nanocellulose base layer, a polydopamine photothermal conversion layer, and a molecular imprint recognition layer.

[0006] The molecular imprint recognition layer contains a specific recognition cavity left after the astaxanthin template molecule is eluted.

[0007] Furthermore, the wavelength range of the near-infrared light source is 808~980nm.

[0008] Furthermore, the distance between the near-infrared light source and the surface of the multilayer composite film is 1~3cm.

[0009] Furthermore, the thickness of the nanocellulose base layer is 30~80μm.

[0010] Furthermore, the thickness of the polydopamine photothermal conversion layer is 50~150nm.

[0011] Furthermore, the thickness of the molecular imprint recognition layer is 100~300nm.

[0012] Furthermore, the method for preparing the composite membrane module includes the following steps:

[0013] (1) Weigh out 100-120 parts of nanocellulose aqueous dispersion, 100-120 parts of tris(hydroxymethyl)aminomethane buffer, 1-3 parts of dopamine hydrochloride, 5-25 parts of β-cyclodextrin, 1-5 parts of chitosan, 100-120 parts of acetic acid aqueous solution, 1-5 parts of astaxanthin template molecule, and 1-2 parts of genipin by weight.

[0014] (2) Vacuum filter the nanocellulose aqueous dispersion to form a film, and dry it at 30~50℃ to obtain the nanocellulose substrate;

[0015] (3) Add dopamine hydrochloride to tris(hydroxymethyl)aminomethane buffer and stir evenly. Immerse the nanocellulose base layer obtained in step (2) into it and react with shaking at 20-40°C in the dark for 2-12 hours. After taking it out, rinse it with deionized water 3-5 times to obtain a base film with a polydopamine photothermal conversion layer deposited on the surface.

[0016] (4) Add β-cyclodextrin and chitosan to an aqueous acetic acid solution and stir until completely dissolved. Add astaxanthin template molecules and stir and encapsulate for 1-4 hours at 15-35°C in the dark. Then add genipin, stir evenly, and degas by sonication to obtain molecular imprinted casting solution.

[0017] (5) Immerse the base film with polydopamine photothermal conversion layer deposited on the surface obtained in step (3) into the molecular imprinted casting solution in step (4) for 2-5 min, take it out and spread it flat, and crosslink it at 40-60℃ for 6-24 h. Remove it with an ethanol solution containing 10-30wt% acetone until astaxanthin is not detected in the eluent. Wash it with deionized water 3-5 times and vacuum dry it at 30-40℃ to obtain a composite film with a molecular imprinted recognition layer on the surface.

[0018] (6) Stack 5 to 20 composite films obtained in step (5) with the molecular imprint recognition layer facing up, and then put them into a housing with a near-infrared light source to obtain a composite film assembly.

[0019] Furthermore, the solid content of the nanocellulose aqueous dispersion in step (1) is 1~5wt%.

[0020] Furthermore, the pH value of the tris(hydroxymethyl)aminomethane buffer solution in step (1) is 8.0~8.8.

[0021] Furthermore, the concentration of the acetic acid aqueous solution in step (1) is 1~3wt%.

[0022] In summary, this application includes at least the following beneficial effects:

[0023] (1) This invention integrates photothermal conversion with molecularly imprinted membrane technology to prepare a composite membrane module, realizing an integrated continuous operation of Haematococcus pluvialis cell wall disruption, astaxanthin extraction, and purification. The composite membrane has a molecularly imprinted layer with β-cyclodextrin as the functional monomer. It utilizes the high inclusion effect of its hydrophobic cavity with the long conjugated polyene chain of astaxanthin, combined with the spatial fixation effect of the genipin cross-linked chitosan network, to form a high-affinity and high-selectivity recognition site, effectively eliminating interference from structurally similar carotenoids and chlorophyll, thus significantly improving the purity of astaxanthin. The polydopamine layer has photothermal conversion, hydrophilic modification, and bioadhesion functions. It can achieve controlled temperature and heating under near-infrared light, which not only ensures the structural stability of astaxanthin but also avoids degradation and pollution caused by chemical or high-temperature cell wall disruption. The polydopamine photothermal conversion layer efficiently converts absorbed near-infrared light into mild heat energy directly beneath the molecularly imprinted recognition layer, enabling in-situ controllable rupture of the spore wall at the membrane interface. Astaxanthin, once released, is immediately captured and immobilized by the adjacent recognition layer through a high-affinity molecularly imprinted cavity. Simultaneously, the binding constant of the molecularly imprinted layer to astaxanthin is significantly higher than the physical inclusion effect of ordinary cyclodextrin, effectively resisting competitive displacement and desorption leakage under dynamic operating conditions, ensuring the recovery rate of the target analyte. Furthermore, the molecularly imprinted recognition layer possesses a three-dimensional imprinted cavity formed using astaxanthin as a template. This cavity not only precisely matches the long-chain conjugated polyene backbone and two β-ionone ring end groups of the astaxanthin molecule in terms of spatial size and shape, but also contains directionally distributed functional groups that can form cooperative hydrogen bonds with the 3-hydroxyl and 4-keto groups of astaxanthin. This synergy of spatial matching and multi-point non-covalent interaction endows the recognition layer with extremely strong molecular recognition capabilities, effectively repelling structural analogs such as chlorophyll, β-carotene, and neutral lipids, thereby achieving extremely high separation selectivity.

[0024] (2) This invention utilizes the pure thermal effect of near-infrared photothermal conversion to precisely control the membrane surface temperature, which is far below the temperature threshold for significant thermal isomerization or degradation of astaxanthin, and there is no mechanical shearing or cavitation free radical generation, thus avoiding isomerization at the source; at the same time, the molecularly imprinted cavity is constructed around the all-trans astaxanthin template, exhibiting a thermodynamic tendency to preferentially recognize and bind to the all-trans conformation, which can further enrich the active conformation from the mixture, ensuring product quality in both the cell disruption and separation stages. The photothermal in-situ cell disruption of this invention is mild, and the overall spore structure is not broken into submicron fragments and colloids, but only a large-sized loose deposition layer is formed. Membrane fouling is mainly reversible surface fouling, and it is not easy to cause pore blockage and the formation of a dense gel layer; while the strong specific adsorption of the molecularly imprinted layer firmly "locks" the astaxanthin molecules to the membrane surface, preventing repeated adsorption-desorption during the dynamic process that would obstruct the mass transfer channels. This integrated design eliminates separate processes such as cell disruption equipment, solid-liquid separation devices, and extraction and purification, significantly simplifying the process and reducing equipment investment and energy consumption. Furthermore, the component utilizes a switching flow path elution method to recover products and regenerate membranes online, making operation convenient and facilitating continuous and automated production.

[0025] (3) All raw materials used in the preparation of this invention are biocompatible materials. Nanocellulose is biodegradable, dopamine is an endogenous biological molecule, β-cyclodextrin and chitosan are natural polysaccharides, genipin is derived from plants, and the astaxanthin template uses natural standards. No toxic organic solvents are used in the preparation and extraction processes, and the eluent is also an ethanol-water system. The resulting astaxanthin product has no organic solvent residue and no risk of nanoparticle leakage, and can be directly applied to high-safety fields such as food, health products, and cosmetics. The photothermal conversion layer is sandwiched between the recognition layer and the support layer, further eliminating the possibility of functional nanomaterials coming into contact with the product. Its safety and reliability are significantly better than that of physically mixed nanoparticle suspension systems. Detailed Implementation

[0026] It should be noted that, where there is no conflict, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0027] It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0028] This invention provides a method for preparing a composite membrane module, comprising the following steps:

[0029] (1) Weigh out 100-120 parts of nanocellulose aqueous dispersion with a solid content of 1-5 wt%, 100-120 parts of tris(hydroxymethyl)aminomethane buffer solution with pH 8.0-8.8, 1-3 parts of dopamine hydrochloride, 5-25 parts of β-cyclodextrin, 1-5 parts of chitosan, 100-120 parts of acetic acid aqueous solution with a concentration of 1-3 wt%, 1-5 parts of astaxanthin template molecule, and 1-2 parts of genipin, respectively;

[0030] (2) Vacuum filter the nanocellulose aqueous dispersion to form a film, and dry it at 30~50℃ to obtain a nanocellulose substrate with a thickness of 30~80μm;

[0031] (3) Add dopamine hydrochloride to tris(hydroxymethyl)aminomethane buffer and stir evenly. Immerse the nanocellulose base layer from step (2) into it and react with shaking at 20-40°C in the dark for 2-12 hours. After taking it out, rinse it with deionized water 3-5 times to obtain a base film with a polydopamine photothermal conversion layer with a thickness of 50-150 nm deposited on the surface.

[0032] (4) Add β-cyclodextrin and chitosan to an aqueous acetic acid solution and stir until completely dissolved. Under the protection of argon atmosphere, add astaxanthin template molecules and stir and encapsulate for 1-4 hours at 15-35℃ in the dark. Then add genipin, stir evenly and degas by sonication to obtain molecular imprinted casting solution.

[0033] (5) Immerse the base film with polydopamine photothermal conversion layer deposited on the surface obtained in step (3) into the molecular imprinted casting solution in step (4) for 2-5 min, take it out and spread it flat, and crosslink it at 40-45℃ for 18-24 h. Elute it with an ethanol solution containing 10-30 wt% acetone until astaxanthin is not detected in the eluent. Then wash it with deionized water 3-5 times and vacuum dry it at 30-40℃ to obtain a composite film with a molecular imprinted recognition layer with a thickness of 100-300 nm on the surface.

[0034] (6) Stack 5 to 20 composite membranes obtained in step (5) in sequence with the molecular imprint recognition layer facing the liquid inlet side and the nanocellulose substrate facing the permeation side. Place them into a housing with a near-infrared light source with a wavelength range of 808 to 980 nm. The near-infrared light source is set on the side of the housing close to the nanocellulose substrate of the composite membrane, and the distance between the near-infrared light source and the outer surface of the nanocellulose substrate is 1 to 3 cm to obtain the composite membrane assembly.

[0035] In step (5), astaxanthin is not detected in the eluent by HPLC at 474 nm. Astaxanthin peak area in the eluent is less than 0.01 mAU·s and is considered not detected.

[0036] When using, set the concentration to 10. 4 ~10 6A suspension of Haematococcus pluvialis spores at a concentration of 1 spore / mL was circulated across the membrane surface in a composite membrane module while near-infrared light irradiation was applied, maintaining the membrane temperature at 45±5℃. Under this gentle thermal effect, the spore walls ruptured, releasing astaxanthin which was specifically captured by the molecularly imprinted layer, while most other impurities permeated through the membrane. After dynamic adsorption saturation, the membrane was switched to an eluent for desorption, yielding a high-purity astaxanthin solution.

[0037] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0038] Example 1

[0039] The method for preparing the composite membrane module in this embodiment includes the following steps:

[0040] (1) Weigh out 100 parts of nanocellulose aqueous dispersion with a solid content of 1wt%, 100 parts of tris(hydroxymethyl)aminomethane buffer with pH 8.0, 1 part of dopamine hydrochloride, 5 parts of β-cyclodextrin, 1 part of chitosan, 100 parts of acetic acid aqueous solution with a concentration of 1wt%, 1 part of astaxanthin template molecule, and 1 part of genipin by weight.

[0041] (2) Vacuum filter the nanocellulose aqueous dispersion to form a film, and dry it at 30°C to obtain a nanocellulose substrate with a thickness of 30μm.

[0042] (3) Add dopamine hydrochloride to tris(hydroxymethyl)aminomethane buffer and stir evenly. Immerse the nanocellulose base layer from step (2) into it and react with shaking at 20°C in the dark for 2 hours. After taking it out, rinse it three times with deionized water to obtain a base film with a 50nm thick polydopamine photothermal conversion layer deposited on the surface.

[0043] (4) Add β-cyclodextrin and chitosan to an aqueous acetic acid solution and stir until completely dissolved. Under the protection of argon atmosphere, add astaxanthin template molecules and stir and encapsulate for 1 hour at 15°C in the dark. Then add genipin, stir evenly and degas by sonication to obtain molecular imprinted casting solution.

[0044] (5) Immerse the base film with polydopamine photothermal conversion layer deposited on the surface obtained in step (3) into the molecular imprinted casting solution in step (4) for 2 min, take it out and spread it flat, crosslink it at 40℃ for 18 h, wash it with ethanol solution containing 10wt% acetone until astaxanthin is not detected in the eluent, wash it with deionized water 3 times, and vacuum dry it at 30℃ to obtain a composite film with a molecular imprinted recognition layer with a thickness of 100nm on the surface.

[0045] (6) Stack the five composite films obtained in step (5) with the molecular imprint recognition layer facing up, and then put them into a housing with a near-infrared light source with a wavelength of 808nm. The distance between the near-infrared light source and the surface of the composite film is 1cm, thus obtaining the composite film assembly.

[0046] Example 2

[0047] The method for preparing the composite membrane module in this embodiment includes the following steps:

[0048] (1) Weigh out 110 parts of nanocellulose aqueous dispersion with a solid content of 3wt%, 110 parts of tris(hydroxymethyl)aminomethane buffer solution with pH 8.4, 2 parts of dopamine hydrochloride, 15 parts of β-cyclodextrin, 3 parts of chitosan, 110 parts of acetic acid aqueous solution with a concentration of 2wt%, 3 parts of astaxanthin template molecule, and 1.5 parts of genipin by weight.

[0049] (2) Vacuum filter the nanocellulose aqueous dispersion to form a film, and dry it at 40°C to obtain a nanocellulose substrate with a thickness of 55μm.

[0050] (3) Add dopamine hydrochloride to tris(hydroxymethyl)aminomethane buffer and stir evenly. Immerse the nanocellulose base layer from step (2) into it and react with shaking at 30°C in the dark for 7 hours. After taking it out, rinse it with deionized water 4 times to obtain a base film with a 100nm thick polydopamine photothermal conversion layer deposited on the surface.

[0051] (4) Add β-cyclodextrin and chitosan to an aqueous acetic acid solution and stir until completely dissolved. Under the protection of argon atmosphere, add astaxanthin template molecules and stir and encapsulate for 2.5 h at 25 °C in the dark. Then add genipin, stir evenly and degas by sonication to obtain molecular imprinted casting solution.

[0052] (5) The base film with polydopamine photothermal conversion layer deposited on the surface obtained in step (3) is immersed in the molecular imprinted casting solution in step (4) for 3.5 min. After taking it out, it is spread out flat and cross-linked at 42℃ for 21 h. It is then eluted with an ethanol solution containing 20 wt% acetone until astaxanthin is not detected in the eluent. It is then washed 4 times with deionized water and vacuum dried at 35℃ to obtain a composite film with a molecular imprinted recognition layer with a thickness of 200 nm on the surface.

[0053] (6) Stack 10 composite films obtained in step (5) with the molecular imprint recognition layer facing up, and then put them into a housing with a near-infrared light source with a wavelength of 890nm. The distance between the near-infrared light source and the surface of the composite film is 2cm, thus obtaining the composite film assembly.

[0054] Example 3

[0055] The method for preparing the composite membrane module in this embodiment includes the following steps:

[0056] (1) Weigh out 120 parts of nanocellulose aqueous dispersion with a solid content of 5wt%, 120 parts of tris(hydroxymethyl)aminomethane buffer with pH 8.8, 3 parts of dopamine hydrochloride, 25 parts of β-cyclodextrin, 5 parts of chitosan, 120 parts of acetic acid aqueous solution with a concentration of 3wt%, 5 parts of astaxanthin template molecule, and 2 parts of genipin by weight.

[0057] (2) Vacuum filter the nanocellulose aqueous dispersion to form a film, and dry it at 50°C to obtain a nanocellulose substrate with a thickness of 80μm.

[0058] (3) Add dopamine hydrochloride to tris(hydroxymethyl)aminomethane buffer and stir evenly. Immerse the nanocellulose base layer from step (2) into it and react at 40°C in the dark for 12 hours. After taking it out, rinse it 5 times with deionized water to obtain a base film with a polydopamine photothermal conversion layer with a thickness of 150 nm deposited on the surface.

[0059] (4) Add β-cyclodextrin and chitosan to an aqueous acetic acid solution and stir until completely dissolved. Under the protection of argon atmosphere, add astaxanthin template molecules and stir and encapsulate at 35°C and in the dark for 4 hours. Then add genipin, stir evenly and degas by sonication to obtain molecular imprinted casting solution.

[0060] (5) Immerse the base film with polydopamine photothermal conversion layer deposited on the surface obtained in step (3) into the molecular imprinted casting solution in step (4) for 5 min, take it out and spread it flat, crosslink it at 45℃ for 24 h, wash it with ethanol solution containing 30wt% acetone until astaxanthin is not detected in the eluent, wash it with deionized water 5 times, and vacuum dry it at 40℃ to obtain a composite film with a molecular imprinted recognition layer with a thickness of 300nm on the surface.

[0061] (6) Stack 20 composite membranes obtained in step (5) with the molecular imprint recognition layer facing up, and then put them into a shell with a near-infrared light source with a wavelength of 980nm. The distance between the near-infrared light source and the membrane surface of the composite membrane is 3cm, thus obtaining a composite membrane assembly for separating and extracting astaxanthin from Haematococcus pluvialis.

[0062] Comparative Example 1

[0063] The preparation method of the composite membrane module in this comparative example is roughly the same as that in Example 1, except that step (3) is omitted in this comparative example, the polydopamine photothermal conversion layer is not deposited, and the molecular imprinted layer is directly constructed using a nanocellulose membrane. When using the composite membrane module of this comparative example for extraction, the algal spores need to be homogenized under high pressure to break the cell walls.

[0064] Comparative Example 2

[0065] The preparation method of the composite membrane module in this comparative example is roughly the same as that in Example 1, except that no astaxanthin template molecules were added in this comparative example.

[0066] Comparative Example 3

[0067] The preparation method of the composite membrane module in this comparative example is roughly the same as that in Example 1. The difference is that the composite membrane module in this comparative example does not have a near-infrared light source. When using the composite membrane module in this comparative example for extraction, it is necessary to perform high-pressure homogenization and cell wall disruption on the algal spores.

[0068] Comparative Example 4

[0069] The preparation method of the composite membrane module in this comparative example is roughly the same as that in Example 1, except that the composite membrane module in this comparative example is equipped with a near-infrared light source with a wavelength of 1064nm.

[0070] Experimental Example

[0071] Concentration 10 4 Astaxanthin extraction of Haematococcus pluvialis spore suspension at concentrations of 1 / mL was performed using the composite membrane modules of Examples 1-3 and Comparative Examples 1-2. In Comparative Examples 1 and 3, the Haematococcus pluvialis spore suspension was first homogenized by circulating it three times at 80 MPa and 4°C to disrupt the spore walls. In the remaining extractions, the membrane temperature was controlled at 45°C, causing the spore walls to rupture and release astaxanthin. After dynamic adsorption saturation, the solution was switched to eluent for desorption, and a high-purity astaxanthin extract was collected. The following tests were performed, including:

[0072] Astaxanthin extraction rate (%): The mass of astaxanthin in the extract was determined by high performance liquid chromatography (HPLC, C18 column, methanol-acetonitrile mobile phase, detection wavelength 476 nm), and the ratio of the mass of astaxanthin in the extract to the total mass of astaxanthin in the raw material.

[0073] Astaxanthin purity (%): The percentage of astaxanthin in the total extract of the final product was determined by HPLC.

[0074] All-trans astaxanthin ratio (%): The proportion of all-trans configuration in total astaxanthin was determined by HPLC (C30 column, normal phase system).

[0075] Membrane steady-state flux (L·m) -2 ·h -1 ): Permeation flux measured after the membrane separation has stabilized (30 min).

[0076] The specific results are shown in Table 1.

[0077] Table 1. Detection results of samples from Examples 1-3 and Comparative Examples 1-2

[0078] Example 1 95.0 98.1 98.0 18.2 Example 2 95.5 98.8 98.5 18.5 Example 3 95.2 98.2 98.3 18.4 Comparative Example 1 82.4 94.6 88.2 12.3 Comparative Example 2 94.1 78.2 96.8 21.7 Comparative Example 3 80.6 93.3 86.5 18.2 Comparative Example 4 91.4 97.9 97.7 18.6

[0079] As shown in Table 1, the composite membrane modules prepared using the method of this invention in Examples 1-3 separate and extract astaxanthin. The composite membrane contains a molecularly imprinted layer with β-cyclodextrin as the functional monomer. This layer utilizes the high inclusion effect of its hydrophobic cavity with the long conjugated polyene chain of astaxanthin, combined with the spatial immobilization effect of the genipin-crosslinked chitosan network, to form high-affinity, highly selective recognition sites. This effectively eliminates interference from structurally similar carotenoids and chlorophyll, significantly improving the purity of astaxanthin. The polydopamine layer combines photothermal conversion, hydrophilic modification, and bioadhesion functions. It can achieve controlled and mild heating under near-infrared light, ensuring the structural stability of astaxanthin while avoiding degradation and contamination caused by chemical or high-temperature cell disruption. The polydopamine photothermal conversion layer can efficiently convert absorbed near-infrared light into mild heat energy directly below the molecularly imprinted recognition layer, achieving in-situ controllable rupture of the spore wall at the membrane interface. Once released, astaxanthin is immediately captured and immobilized by the adjacent recognition layer through the high-affinity molecularly imprinted cavity. Therefore, the astaxanthin extraction rate reached over 95%.

[0080] In the composite membranes of Examples 1-3, the binding constant of the molecularly imprinted layer for astaxanthin is significantly higher than that of ordinary cyclodextrin through physical inclusion. This effectively resists competitive displacement and desorption leakage under dynamic operating conditions, ensuring the recovery rate of the target analyte. The molecularly imprinted recognition layer has a three-dimensional imprinted cavity formed using astaxanthin as a template. This cavity not only precisely matches the long-chain conjugated polyene backbone and the two β-ionone ring end groups of the astaxanthin molecule in terms of spatial size and shape, but also contains directionally distributed functional groups that can form synergistic hydrogen bonds with the 3-hydroxyl and 4-keto groups of astaxanthin. This synergistic combination of spatial matching and multi-point non-covalent interactions endows the recognition layer with extremely strong molecular recognition capabilities, effectively rejecting structural analogs such as chlorophyll, β-carotene, and neutral lipids, thereby achieving extremely high separation selectivity. This ensures that the purity of astaxanthin is above 98%.

[0081] Examples 1-3 utilize the pure thermal effect of near-infrared photothermal conversion to precisely control the membrane surface temperature, keeping it far below the temperature threshold for significant thermal isomerization or degradation of astaxanthin, and without generating mechanical shearing or cavitation radicals, thus preventing isomerization at its source. Simultaneously, the molecularly imprinted cavity is constructed around the all-trans astaxanthin template, exhibiting a thermodynamic tendency to preferentially recognize and bind to the all-trans conformation, further enriching this active conformation from the mixture, ensuring product quality in both the cell disruption and separation stages. This ensures that the proportion of all-trans astaxanthin reaches over 98%.

[0082] Comparative Example 1, lacking a polydopamine photothermal conversion layer, could not achieve in-situ photothermal cell disruption. Therefore, high-pressure homogenization was required before membrane separation. The principle is to utilize the high-speed passage of the cell suspension through a homogenizing valve slit under high pressure, generating strong shear forces, impact forces, and a cavitation effect caused by instantaneous pressure drop, leading to cell wall rupture. During this process, the cavitation effect resulted in excessively high temperatures at local "hot spots." Although the macroscopic temperature was controlled at 4°C, the microscopically high temperature was sufficient to induce oxidative breakage or rearrangement of the conjugated polyene chains in the astaxanthin molecule. Astaxanthin molecules have a long-chain polyene structure; under strong shear flow, the molecular chains are stretched and twisted, and the all-trans conformation is partially destroyed, generating the cis isomer. The antioxidant activity and stability of the cis isomer are lower than those of the all-trans form, and its binding affinity in the molecular imprinting recognition layer is reduced due to conformational mismatch, preventing effective capture. When cavitation bubbles break, the homolytic cleavage of water molecules generates hydroxyl radicals and hydrogen radicals. These highly reactive radicals directly attack the polyene conjugated system of astaxanthin, causing it to degrade into small-molecule colorless products. Therefore, the astaxanthin extraction rate is only 82.4%, which is significantly lower than that in Example 1.

[0083] In the high-pressure homogenized flow field of Comparative Example 1, the elongated flow orients astaxanthin molecules along the flow direction, lowering the rotational energy barrier of the C / C single bonds in the conjugated polyene chain under tensile stress, thus accelerating the isomerization from all-trans to cis. The cavitation effect provides localized high temperatures, offering instantaneous energy input and enabling some molecules to overcome the activation barrier for the all-trans to cis conversion. The combined effect of these two factors results in a decrease in the proportion of all-trans astaxanthin in Comparative Example 1 to 88.2%. In contrast, the mild thermal conditions of Example 1 precisely controlled the membrane surface temperature at 45°C, far below the significant thermal isomerization temperature of astaxanthin in solution, resulting in an extremely low thermal isomerization rate. No strong shearing or cavitation is involved, and there is no mechanochemical isomerization pathway. The molecularly imprinted cavity uses all-trans astaxanthin as a template, exhibiting pre-organized recognition advantages for its conformation, preferentially binding to the all-trans isomer, further increasing the proportion of all-trans in the product.

[0084] The high-pressure homogenization in Comparative Example 1 not only disrupted the spore wall but also pulverized cell debris into extremely small particles, while simultaneously releasing a large amount of intracellular colloidal substances. These fine particles and viscous colloids readily embed themselves within the membrane pores, forming pore blockages and creating a dense gel layer with high specific resistance on the membrane surface. According to Hermia's classic clogging model, Comparative Example 1 exhibited a rapid transition from "complete clogging" to "cake filtration," with the permeate flux drastically decreasing to 12.3 L·m⁻¹. -2 ·h -1 In contrast, the photothermal cell disruption in Example 1 was mild, only softening the spore wall, increasing its permeability, or causing localized rupture. The overall spore skeleton remained relatively intact, and the large fragment size made it difficult for them to enter the nanoscale recognition layer pores, resulting mainly in a loose particle deposition layer on the membrane surface. This fouling was primarily reversible surface fouling, and hydraulic washing could restore most of the flux, thus maintaining a steady-state flux of 18.2 L·m⁻¹.-2 ·h -1 High level.

[0085] In Comparative Example 2, no astaxanthin template molecule was added during the preparation of the composite membrane. The cavity diameter of the ordinary β-cyclodextrin in its composite membrane was approximately 0.6–0.65 nm, lacking strict spatial restriction on the hydrophobic segments entering the cavity. Chlorophyll molecules contain a large porphyrin ring (tetrapyrrole) and a long phytol chain. Their molecular size and hydrophobicity are similar to astaxanthin. Both the phytol chain of chlorophyll and the terminal group of β-carotene can partially enter the cyclodextrin cavity to form inclusion complexes, thus being simultaneously adsorbed by the membrane layer. After elution, they enter the product, resulting in low purity. β-Carotene is structurally most similar to astaxanthin, both being C40 carotenoids. The only difference is that the latter has an additional hydroxyl group at the 3-position and a ketone group at the 4-position of the β-ionone ring. In ordinary β-cyclodextrin membranes, both can be included with almost no selectivity, leading to a reduction in the purity of astaxanthin to 78.2%. The molecularly imprinted cavity in Example 1 was "cast" using astaxanthin as a template during the cross-linking process. After template elution, a three-dimensional cavity remained, precisely matching the curved conformation of the two β-ionone ring end groups and the conjugated polyene chain of astaxanthin. The porphyrin ring of chlorophyll molecules is too large to enter the imprinted cavity; although the end groups of β-carotene are small, they lack the hydroxyl and ketone groups found on the astaxanthin end groups, preventing them from forming effective multi-point hydrogen bonds with the functional groups within the cavity. This spatial matching combined with hydrogen bond localization multi-recognition mechanism is far more selective than single hydrophobic inclusion. In the molecularly imprinted membrane, the recognition sites introduced using astaxanthin as a template during preparation are characterized by complementary groups distributed within the cavity that form hydrogen bonds with the 3-OH and 4-C=O groups of astaxanthin. β-carotene lacks these two polar groups and cannot effectively bind to these hydrogen bond sites; its total binding energy is significantly lower than that of astaxanthin, resulting in a much lower adsorption capacity.

[0086] In Comparative Example 2, the ordinary β-cyclodextrin membrane showed no strong specific adsorption of astaxanthin and other impurities. The adsorption layer was loose and quickly reached adsorption-desorption equilibrium. The membrane pores were not "filled" or "blocked" by a large number of target molecules, resulting in low hydraulic resistance. This high flux comes at the cost of purity and lacks practical separation significance. In contrast, the molecularly imprinted layer in Example 1 firmly captures astaxanthin molecules through strong multi-point interactions. The high cumulative concentration of astaxanthin on the membrane surface forms a relatively dense "capture layer," slightly increasing mass transfer resistance and resulting in a flux lower than that of the non-specific membrane. However, the flux was 18.2 L·m⁻¹. -2 ·h -1 The flux level is still within the optimal range, far exceeding that of the heavily polluted control sample 1. It is a reasonable flux achieved under the premise of ensuring high purity and high extraction rate, reflecting a good balance between selectivity and permeability.

[0087] In Comparative Example 3, the composite membrane module did not have a near-infrared light source in the shell. During use, the Haematococcus pluvialis spore suspension needed to be pre-treated by high-pressure homogenization and membrane separation in separate steps. Astaxanthin was exposed for a long time during the cell disruption, centrifugation, and membrane separation processes. Oxidative degradation and centrifugation losses led to a sharp drop in extraction rate. At the same time, the shear force and cavitation effect of high-pressure homogenization caused a large amount of all-trans astaxanthin to isomerize into the cis structure, resulting in a decrease in its proportion and a significant reduction in the product's active value. The fine cell debris and colloidal substances generated by pre-disruption blocked the membrane pores, forming a dense fouling layer and causing a sharp decline in permeation flux.

[0088] The composite membrane module in Comparative Example 4 used a near-infrared light source with a wavelength of 1064 nm. The molar extinction coefficient of polydopamine at 1064 nm was lower than that at 808 nm, resulting in a decrease in photothermal conversion efficiency. Under the same light power density, the lower temperature led to insufficient softening of some spore walls, reducing the rate and total amount of astaxanthin release, thus lowering the extraction rate. The 1064 nm photon energy was even lower, and its photochemical effect on astaxanthin was as weak as that at 808 nm. Furthermore, thanks to the "back-illuminated" design, astaxanthin remained downstream of the light source, without direct light exposure, thus maintaining purity and isomer protection effects similar to Example 1. Due to the slightly lower membrane surface temperature, some spore walls did not completely rupture, resulting in a smaller total amount of released intracellular contents and a slightly looser membrane fouling layer, leading to a slightly higher steady-state flux. However, this high flux came at the cost of a lower extraction rate.

[0089] This invention provides a composite membrane module for separating and extracting astaxanthin from Haematococcus pluvialis. The preparation method is cutting-edge, efficient, and controllable, ensuring the superior quality and stability of the astaxanthin product and showing good application prospects.

[0090] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A composite membrane module, characterized in that, The device includes a housing, a multilayer composite membrane stacked within the housing, and a near-infrared light source. The composite membrane, arranged in order of contact with the feed liquid from top to bottom, includes a molecularly imprinted recognition layer containing specific recognition cavities left after the astaxanthin template molecules are eluted; a polydopamine photothermal conversion layer; and a nanocellulose support layer made of a membrane material transparent to near-infrared light. The near-infrared light source is located on the side of the housing near the nanocellulose support layer, and the emitted near-infrared light penetrates the nanocellulose support layer and directly irradiates the polydopamine photothermal conversion layer.

2. The composite membrane module according to claim 1, characterized in that, The wavelength range of the near-infrared light source is 808~980nm.

3. The composite membrane module according to claim 1, characterized in that, The distance between the near-infrared light source and the surface of the multilayer composite film is 1~3cm.

4. The composite membrane module according to claim 1, characterized in that, The thickness of the nanocellulose base layer is 30~80μm.

5. The composite membrane module according to claim 1, characterized in that, The thickness of the polydopamine photothermal conversion layer is 50~150nm.

6. The composite membrane module according to claim 1, characterized in that, The thickness of the molecular imprint recognition layer is 100~300nm.

7. A method for preparing a composite membrane module as described in any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Weigh out 100-120 parts of nanocellulose aqueous dispersion, 100-120 parts of tris(hydroxymethyl)aminomethane buffer, 1-3 parts of dopamine hydrochloride, 5-25 parts of β-cyclodextrin, 1-5 parts of chitosan, 100-120 parts of acetic acid aqueous solution, 1-5 parts of astaxanthin template molecule, and 1-2 parts of genipin by weight. (2) Vacuum filter the nanocellulose aqueous dispersion to form a film, and dry it at 30~50℃ to obtain the nanocellulose substrate; (3) Add dopamine hydrochloride to tris(hydroxymethyl)aminomethane buffer and stir evenly. Immerse the nanocellulose base layer obtained in step (2) into it and react with shaking at 20-40°C in the dark for 2-12 hours. After taking it out, rinse it with deionized water 3-5 times to obtain a base film with a polydopamine photothermal conversion layer deposited on the surface. (4) Add β-cyclodextrin and chitosan to an aqueous acetic acid solution and stir until completely dissolved. Under the protection of argon atmosphere, add astaxanthin template molecules and stir and encapsulate for 1-4 hours at 15-35℃ in the dark. Then add genipin, stir evenly and degas by sonication to obtain molecular imprinted casting solution. (5) Immerse the base film with polydopamine photothermal conversion layer deposited on the surface obtained in step (3) into the molecular imprinted casting solution in step (4) for 2-5 min, take it out and spread it flat, and crosslink it at 40-45℃ for 18-24 h. Elute it with an ethanol solution containing 10-30wt% acetone until astaxanthin is not detected in the eluent. Then wash it with deionized water 3-5 times and vacuum dry it at 30-40℃ to obtain a composite film with a molecular imprinted recognition layer on the surface. (6) Stack 5 to 20 composite films obtained in step (5) with the molecular imprint recognition layer facing up, and then put them into a housing with a near-infrared light source to obtain a composite film assembly.

8. The method for preparing the composite membrane module according to claim 7, characterized in that, The solid content of the nanocellulose aqueous dispersion in step (1) is 1~5wt%.

9. The method for preparing the composite membrane module according to claim 7, characterized in that, In step (1), the pH value of the tris(hydroxymethyl)aminomethane buffer solution is 8.0-8.8, and the concentration of the acetic acid aqueous solution is 1-3 wt%.

10. An application of the composite membrane module according to any one of claims 1 to 6, characterized in that, The composite membrane assembly was used to separate astaxanthin from Haematococcus pluvialis.