Acid-resistant nanofiltration membrane and preparation method thereof

Abstract

The application relates to the technical field of filtering media, in particular to an acid-resistant nanofiltration membrane and a preparation method thereof, which comprises the following steps in sequence: S1, providing a water-phase solution and an oil-phase solution; the water-phase solution is an aqueous solution in which polyamine monomers are dissolved; the oil-phase solution is an organic solution in which isocyanate monomers and alkyl phosphate salt compounds serving as reaction accelerators are dispersed; S2, contacting the water-phase solution and the oil-phase solution on a support base film to generate an interface polymerization reaction so as to form a polyurea active separation layer on the surface of the support base film; and S3, heat treatment, to obtain the acid-resistant nanofiltration membrane. A small amount of alkyl phosphate salt compounds is added in the oil-phase solution as the reaction accelerators, which can promote the formation of a polyurea active separation layer with a more optimal structure, and the water flux, acid permeation rate and metal ion interception rate of the nanofiltration membrane are simultaneously improved.

Description

Technical Field

[0001] This invention relates to the field of filtration media technology, specifically to an acid-resistant nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration membranes, with their high efficiency in retaining multivalent ions and small organic molecules, have become a core material in many industrial separation processes, including mining, metallurgy, dyeing and printing, and chemical wastewater resource recovery. However, the active layer of traditional polyamide nanofiltration membranes has an amide bond as its core structure. In strongly acidic environments with a pH < 2, these amide bonds are prone to hydrolysis, leading to membrane structure damage and a sharp decline in separation performance. This defect severely limits its industrial application in strongly acidic systems.

[0003] To address the acid resistance limitation of polyamide nanofiltration membranes, existing technologies mainly employ two improvement strategies: First, developing novel acid-resistant monomers, such as reacting multifunctional piperazine derivatives with trimesoyl chloride to construct a polypiperazine amide structure with higher crosslinking degree, thereby enhancing the chemical stability of the molecular backbone; second, post-treatment or surface modification of the membrane, optimizing the acid resistance of the membrane surface through sulfonation, grafting, and other methods. However, both strategies have significant limitations—novel acid-resistant monomers are generally expensive and their reactivity is difficult to precisely control, while post-treatment and surface modification processes are complex and have poor compatibility with existing mature interfacial polymerization membrane production lines, hindering industrial-scale promotion.

[0004] Interfacial polymerization is the mainstream process for preparing high-performance composite nanofiltration membranes. In the preparation of acid-resistant nanofiltration membranes, published research has used aliphatic or alicyclic isocyanates to replace traditional acyl chlorides, reacting them with amine monomers to form a polyurea active layer. The hydrolysis resistance of the polyurea structure is significantly better than that of polyamide, effectively solving the problem of membrane chemical stability. However, the reactivity of isocyanates with amines is much lower than that of the traditional acyl chloride-amine system. Existing technologies can only compensate for the lack of reactivity by increasing the monomer concentration and extending the reaction time to obtain a dense separation layer that meets the requirements of high ion rejection. This remedial measure leads to excessive thickening of the polyurea active layer. Although it can maintain a certain rejection performance, it seriously sacrifices the acid permeation flux of the membrane, causing the membrane performance to fall into the classic trade-off dilemma of "high flux with low rejection, and high rejection with low flux". The core reason is that these existing technologies have not solved the problem of insufficient reactivity of the isocyanate-amine system at the reaction kinetics level, but only caused negative changes in the membrane structure by adjusting macroscopic process parameters.

[0005] In existing technologies, some approaches attempt to improve membrane performance by introducing conventional surfactants or pore-forming agents (such as nonionic surfactants) during interfacial polymerization. The mechanism of action of these additives is primarily to physically hinder monomer diffusion or to introduce more free volume into the polymer network (i.e., irregularly distributed voids or interstitial spaces in the condensed structure formed by the stacking of polymer molecular chains, not occupied by molecular chain atoms / groups). The technological direction is to construct a loose and porous membrane structure to increase water flux. However, this direction fundamentally conflicts with the requirement of a "highly intact and dense active layer" for separation under strong acid conditions. While such methods may increase the media permeation rate, they inevitably come at the cost of sacrificing a high rejection rate for multivalent ions, failing to meet the core requirement of highly selective separation in strong acid systems.

[0006] In recent years, domestic patents have also explored various technologies related to acid-resistant nanofiltration membranes. For example, Chinese patent application CN113509839A discloses an acid / alkali resistant composite nanofiltration membrane, which forms a polyurea separation layer through interfacial polymerization of polyamines and polyisocyanates. The monomer concentrations in both the oil and water phases are high, and only pure isocyanate oil phase solution is used, without introducing specific functional additives into the oil phase. Chinese patent application CN119548987A uses a halogenated five- or six-membered nitrogen-containing heterocyclic compound, a polyisocyanate compound crosslinking agent, and a hyperbranched polyamine compound to prepare an acid-resistant nanofiltration membrane through interfacial polymerization. Chinese patent application CN117181027A constructs a polyurea-polyamide composite structure through interfacial polymerization of trimesoyl chloride, isocyanate, polyvinylpyrrolidone, and linear amine monomers, with the oil phase employing two reactive monomers; another Chinese patent application CN119548987A improves acid resistance by introducing a polyphenol modification layer after the formation of the polyurea separation layer, using a secondary polymerization process; Chinese patent application CN113509840A prepares a polyurea-polyurethane acid-resistant nanofiltration membrane by adding polyol to a polyamine in the aqueous phase and interfacially polymerizing it with polyisocyanate.

[0007] In summary, existing technologies for preparing acid-resistant nanofiltration membranes either suffer from complex processes, high costs, and incompatibility with existing production lines, or they fail to overcome the performance trade-off between flux and retention in polyurea membranes, making it difficult to simultaneously meet the requirements of high acid resistance, high water flux, and high ion selectivity under strong acid environments. Therefore, developing a simple process that is highly compatible with existing industrial production and can simultaneously improve the acid resistance, water permeability, and ion selectivity of polyurea nanofiltration membranes has significant industrial application value. Summary of the Invention

[0008] (a) Technical problems to be solved

[0009] In view of the aforementioned defects and shortcomings of existing technologies, this invention provides an acid-resistant nanofiltration membrane and its preparation method. This method accelerates the interfacial polymerization reaction between isocyanates and amine monomers by introducing alkyl phosphate salts as reaction promoters into the oil phase, without altering existing industrial interfacial polymerization processes. This chemically accelerates the reaction, allowing for the construction of a thinner, denser, and more structurally complete polyurea separation layer under milder process conditions. The resulting acid-resistant nanofiltration membrane exhibits excellent chemical stability in long-term strong acid environments, while simultaneously achieving a significant increase in acid permeation flux and maintaining a high rejection rate for high-valent salts. This fundamentally resolves the core contradiction in existing polyurea nanofiltration membranes where "high rejection" and "high flux" are mutually exclusive, providing a high-performance key membrane material for the efficient separation and resource recovery of acidic solutions.

[0010] (II) Technical Solution

[0011] In a first aspect, the present invention provides a method for preparing an acid-resistant nanofiltration membrane, comprising the following steps performed sequentially:

[0012] S1. Provide aqueous phase solution and oil phase solution;

[0013] The aqueous solution is an aqueous solution containing dissolved polyamine monomers;

[0014] The oil phase solution is an organic solution in which isocyanate monomers and alkyl phosphate salts as reaction promoters are dispersed.

[0015] S2. The aqueous solution and the oil solution are brought into contact on the supporting base membrane to undergo an interfacial polymerization reaction, so as to form a polyurea active separation layer on the surface of the supporting base membrane.

[0016] S3. Heat treatment to obtain a strong acid-resistant nanofiltration membrane.

[0017] According to a preferred embodiment of the present invention, in S1, the alkyl phosphate salt compound in the oil phase solution has a molecular structure containing at least one C8-C. 18 An alkyl chain hydrophobic segment and a phosphate group (-OPO3) 2- or -OPO3H - Compounds with hydrophilic head groups (i.e., containing C8-C) 18 The phosphate salt is monoalkyl-substituted or dialkyl-substituted, with the cation preferably Na. + K + NH4 + HNEt3 + (No heavy metal residues) The concentration of alkyl phosphate salts added to the oil phase solution is 0.001-0.1 wt%.

[0018] Preferably, the alkyl phosphate salt compound is sodium dodecyl phosphate.

[0019] According to a preferred embodiment of the present invention, in S1, the isocyanate monomer of the oil phase solution is an aliphatic or alicyclic diisocyanate, and the concentration of the isocyanate monomer is 0.01-1.0 wt%; the organic solvent of the oil phase solution is an aprotic inert organic solvent that is immiscible with water.

[0020] According to a preferred embodiment of the present invention, in S1, the organic solvent is n-hexane, cyclohexane, isoparaffin (Isopar G or Isopar L), or a mixed solvent of n-hexane and cyclohexane in a mass ratio of 1:1 to 3:1.

[0021] According to a preferred embodiment of the present invention, in S1, the isocyanate monomer of the oil phase solution is at least one selected from phenyl diisocyanate, 1,4-diisocyanate butane, toluene diisocyanate and isophorone diisocyanate; preferably, the isocyanate monomer of the oil phase solution is 1,4-diisocyanate butane with a mass concentration of 0.2%.

[0022] According to a preferred embodiment of the present invention, in S1, the amine monomer in the aqueous solution is at least one selected from polyethylene polyamine, hyperbranched polyethyleneimine and linear diamine, and the concentration of the amine monomer is 0.1-2.0 wt%; preferably, the aqueous solution is an aqueous solution of ethylenediamine with a concentration of 0.2 wt%.

[0023] According to a preferred embodiment of the present invention, in S2, the contact time between the aqueous solution and the oil solution is 10-300 s, preferably 30-60 s.

[0024] According to a preferred embodiment of the present invention, in step S3, heat treatment is performed at 60-120°C for 3-15 minutes to consolidate and strengthen the polyurea active separation layer, thereby obtaining the acid-resistant nanofiltration membrane of the present invention.

[0025] Preferably, when preparing the aqueous phase solution, the weighed amine monomer compound is dissolved in water and mechanically stirred until completely dissolved to obtain a clear aqueous phase solution. When preparing the oil phase solution, the isocyanate monomer and reaction promoter are weighed and dissolved together in an aprotic organic solvent, and ultrasonically treated to ensure complete dissolution and uniform mixing to obtain an oil phase solution. During the interfacial polymerization reaction, the prepared support film is flattened and fixed on a glass plate or other platform, and the aqueous phase solution is poured onto the film surface to ensure complete coverage. It is allowed to stand for 30-80 seconds. A clean rubber roller is used to roll over the film surface at a uniform speed to carefully scrape away all visible droplets and accumulated liquid on the film surface, leaving only a thin liquid film adsorbed in the micropores of the base film (removing excess aqueous phase). Then, the oil phase solution is immediately poured onto the wet film surface with the aqueous phase attached, and the interfacial polymerization reaction is carried out at room temperature, with the reaction time strictly controlled within 10-300 seconds (preferably 30-60 seconds). Then, the oil phase solution on the film surface is poured off (removing the oil phase). The membrane was rapidly transferred to a preheated oven at 60-120°C and heat-treated for 3-15 minutes to further promote the reaction of residual monomers, remove solvents, and solidify the polyurea network. Finally, the membrane was removed from the oven, cooled to room temperature, and thoroughly rinsed with deionized water to remove any possible residual chemicals. The membrane was then immersed in pure water and stored at 4°C until use. This yielded the high-performance, strong acid-resistant nanofiltration membrane sample.

[0026] Secondly, the present invention provides an acid-resistant nanofiltration membrane, which is prepared by the preparation method of any of the above embodiments.

[0027] (III) Beneficial Effects

[0028] This invention introduces a specific alkyl phosphate salt as a reaction promoter into the oil phase. Without altering the traditional interfacial polymerization industrial production process, it optimizes reaction kinetics at the molecular level. Leveraging the promoter's interfacial directional adsorption, monomer enrichment, and chemocatalytic effects, it fundamentally accelerates the interfacial polymerization reaction between isocyanates and amine monomers, achieving precise polymerization under milder process conditions. This successfully constructs a thinner, denser, and more structurally complete polyurea active separation layer. The resulting acid-resistant nanofiltration membrane exhibits excellent chemical stability during long-term use in harsh acidic environments (pH < 2). It simultaneously achieves a breakthrough increase in acid permeation flux and maintains a high rejection rate of over 95% for polyvalent metal ions such as iron, calcium, and magnesium ions. This fundamentally overcomes the inherent technical challenge of achieving both permeability and selectivity in existing acid-resistant nanofiltration membranes. It provides high-performance membrane material support for the efficient separation and resource recovery of acidic solutions, and the process is compatible with existing industrial production lines, demonstrating promising industrial application prospects. Detailed Implementation

[0029] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below through specific embodiments.

[0030] This invention provides a high-performance, strong acid-resistant nanofiltration membrane, which has a typical thin-layer composite membrane structure, mainly comprising an active separation layer and a porous polymer substrate layer from top to bottom. The active separation layer is an aliphatic or alicyclic polyurea, formed by an interfacial condensation reaction between amine monomers dissolved in the aqueous phase (such as ethylenediamine or piperazine) and isocyanate monomers dissolved in the oil phase (such as 1,4-diisocyanate butane or isophorone diisocyanate) on the surface of a supporting membrane. Its main chain contains urea bonds -NH-CO-NH-. An alkyl phosphate salt reaction promoter (typically sodium dodecyl phosphate) is contained in the oil phase solution, playing a crucial role in the interfacial polymerization process. This additive promotes the interfacial reaction and does not remain in the active separation layer after the reaction; its mechanism of action lies in altering the degree of crosslinking and structural integrity of the polyurea network by participating in the interfacial polymerization reaction. Therefore, the membrane product prepared using this additive has an active separation layer with structural characteristics distinct from conventional polyurea, exhibiting increased crosslinking density and enhanced network integrity.

[0031] The method for preparing the acid-resistant nanofiltration membrane of the present invention includes the following steps performed sequentially:

[0032] S1. Provide aqueous phase solution and oil phase solution;

[0033] The aqueous phase solution is an aqueous solution containing dissolved polyamine monomers, and the oil phase solution is an organic solution containing dispersed isocyanate monomers and alkyl phosphate salts as reaction promoters.

[0034] Among them, the reaction promoter has a molecular structure containing at least one C8-C. 18 An alkyl chain hydrophobic segment and a phosphate group (-OPO3) 2- or -OPO3H - Compounds with hydrophilic head groups (i.e., containing C8-C) 18 The phosphate salt is monoalkyl-substituted or dialkyl-substituted, with the cation preferably Na. + K + Li + NH4 + HNEt3 + To avoid heavy metal residues, the concentration of alkyl phosphate salts added to the oil phase solution is 0.001-0.1 wt%.

[0035] It should be noted that monoalkyl-substituted phosphate salts are more suitable for the technical objectives of this invention than dialkyl-substituted phosphate salts. The molecular structures and hydrophilic / hydrophobic ratios of monoalkyl-substituted and dialkyl-substituted phosphate salts are completely different, directly determining the molecule's HLB value (hydrophilic-lipophilic balance). Monoalkyl phosphate salts have a basically balanced proportion and a moderate HLB value (approximately 8-12), classifying them as oil-dispersible anionic surfactants. Dialkyl-substituted phosphate salts exhibit an absolutely dominant hydrophobic chain proportion, with a significantly diluted hydrophilic group proportion, resulting in an extremely low HLB value (typically <3), classifying them as superhydrophobic and weakly hydrophilic compounds. Simultaneously, the long hydrophobic chains of the dialkyl groups undergo intermolecular hydrophobic association and aggregation, and the steric hindrance of the phosphate hydrophilic groups exacerbates this aggregation, ultimately preventing the formation of uniformly dispersed monomolecules / micelles in the oil phase. This results in poor oil phase dispersibility (such as stratification, precipitation, and scum), affecting the reaction-promoting effect and nanofiltration membrane performance.

[0036] The above-mentioned alkyl phosphate salts include: ①C8-C 10 Short-chain alkyl compounds, such as sodium monooctyl phosphate, potassium monooctyl phosphate, and sodium monodecyl phosphate, are phosphate esters suitable for low-viscosity oil phase systems and have a fast diffusion rate; ②C 12 ~C 14 Medium-chain alkyl compounds, such as sodium dodecyl phosphate (sodium lauryl phosphate), potassium dodecyl phosphate (potassium lauryl phosphate), ammonium dodecyl phosphate (mono-ammonium dodecyl phosphate, with excellent oil-phase dispersibility), and sodium tetradecyl phosphate (sodium myristyl phosphate, with moderately extended hydrophobic chains and stronger interfacial adsorption), exhibit the best compatibility with isocyanates and provide a balanced reaction-promoting effect; ③C 16 ~C 18 Long-chain alkyl compounds, such as sodium hexadecyl phosphate (sodium palmitate phosphate), sodium octadecyl phosphate (sodium stearyl phosphate), and sodium heptadecanyl phosphate, are suitable for use with high-concentration isocyanate oil phases, resulting in better film uniformity. Preferably, the alkyl phosphate compound is sodium dodecyl phosphate.

[0037] The isocyanate monomer in the oil phase solution is an aliphatic or alicyclic diisocyanate, with a concentration of 0.01-1.0 wt%. The organic solvent in the oil phase solution is a non-protic inert organic solvent immiscible with water. The organic solvent is n-hexane, cyclohexane, isoalkanes (Isopar G or Isopar L), or a mixture of n-hexane and cyclohexane in a mass ratio of 1:1-3:1. Preferably, the isocyanate monomer in the oil phase solution is at least one selected from phenyl diisocyanate, 1,4-diisocyanate butane, toluene diisocyanate, and isophorone diisocyanate; preferably, the isocyanate monomer in the oil phase solution is 0.2% 1,4-diisocyanate butane by mass.

[0038] The amine monomer in the aqueous solution is at least one selected from polyethylene polyamine, hyperbranched polyethyleneimine and linear diamine, and the concentration of the amine monomer is 0.1-2.0 wt%; preferably, more preferably, it is an aqueous solution of ethylenediamine with a concentration of 0.2 wt%.

[0039] Polyethylene polyamines are linear polyamino aliphatic polymers formed by multiple amination and condensation reactions based on ethylenediamine. The main molecular chain is a linear structure, and the side chains contain only amino functional groups. They belong to multifunctional polyamines and have the general structural formula H2N(CH2CH2NH). n H; n≥2. Specifically, when n=2, the structure is diethylenetriamine (DETA); when n=3, the structure is triethylenetetramine (TETA); when n=4, the structure is tetraethylenepentamine (TEPA), and so on. Polyethylene polyamines contain three or more active amino groups (primary amine + secondary amine), exhibiting high reactivity. When reacting with isocyanates, they can form a highly cross-linked polyurea network, enhancing film density and chemical stability. The amino groups are uniformly distributed, allowing for controllable reaction rates during interfacial polymerization, making them suitable for continuous industrial production.

[0040] Hyperbranched polyethyleneimine is a high-molecular-weight polyamine with a three-dimensional branched topology, obtained by ring-opening polymerization of ethyleneimine monomers. It belongs to the hyperbranched polymer category, distinct from linear polyamines with low branching. Its molecules lack fixed repeating units and relative molecular mass, exhibiting an irregular dendritic branched structure. The molecule contains a dense distribution of primary, secondary, and tertiary amino groups (primary amines are located at the ends of the branches, while secondary / tertiary amines are located at the nodes of the main chain / branch). The primary and secondary amines serve as active sites for interfacial polymerization. The active amino groups have extremely high functionality (containing dozens or even hundreds of active amino groups per molecule), resulting in rapid crosslinking efficiency when reacting with isocyanates, quickly forming a dense and uniformly three-dimensional polyurea separation layer. The steric hindrance effect of the hyperbranched structure can regulate the membrane porosity, balancing density and flux. It also exhibits excellent water solubility, allowing for uniform dispersion in the aqueous phase without the risk of precipitation.

[0041] Linear diamines are diamines with a linear aliphatic / alicyclic backbone and only one primary amino group at each end of the molecule, meaning they contain only two active amino groups and are bifunctional polyamines. Their molecular structure is unbranched and lacks side groups (or contains only alkyl side groups), resulting in a simple and regular structure. The two primary amino groups are located at opposite ends of the molecule, with symmetrical reaction sites. When undergoing interfacial polymerization with isocyanates, they readily form a predominantly linear, slightly cross-linked polyurea structure, resulting in membrane flexibility and permeability superior to other amine monomers. The bifunctionality ensures mild and controllable reactivity, preventing excessive cross-linking during interfacial polymerization and effectively avoiding membrane thickening and flux reduction. The high molecular chain regularity results in a uniformly distributed pore structure in the formed polyurea network, enhancing ion retention selectivity. Linear diamines include aliphatic linear diamines (ethylenediamine, 1,3-propanediamine, 1,4-butanediamine, 1,6-hexanediamine, etc.) and alicyclic linear diamines (1,4-cyclohexanediamine, isophoronediamine, etc.). Ethylenediamine is the preferred linear diamine monomer of this invention because it has the shortest molecular chain, moderate reactivity, and produces a thin and controllable film after formation.

[0042] Among the three polyamine monomers mentioned above, polyethylenepolyamine-mediated interfacial polymerization exhibits high crosslinking efficiency, easily controllable membrane density, excellent separation layer density, and good chemical stability. Hyperbranched polyethyleneimine-mediated interfacial polymerization results in three-dimensional hyperbranching and a random structure, leading to a uniform separation layer network and excellent retention selectivity. Linear diamines (including ethylenediamine)-mediated interfacial polymerization exhibits linear regularity and unbranching, resulting in a thin, flexible separation membrane with excellent flux performance. These three polyamine monomers can be appropriately selected or combined according to the target performance of the nanofiltration membrane to be prepared.

[0043] S2. The aqueous solution and the oil solution are brought into contact on the supporting base membrane for 10-300s, preferably 20-60s, to allow an interfacial polymerization reaction to occur, so as to form a polyurea active separation layer on the surface of the supporting base membrane.

[0044] S3. Perform heat treatment at 60-120℃ for 3-15 minutes to consolidate and strengthen the polyurea active separation layer, thereby obtaining the acid-resistant nanofiltration membrane of the present invention.

[0045] The following is a detailed description of possible substitutions and changes in raw materials and composition:

[0046] Selection of amine monomers:

[0047] The aqueous amine monomers are not limited to ethylenediamine; any compound containing two or more primary or secondary amine groups that can react with isocyanates is suitable. For example, piperazine can be used to form a more cross-linked polypiperazinurea network; m-phenylenediamine can be used to introduce an aromatic ring structure to enhance rigidity; or other aliphatic polyamines such as hexamethylenediamine and diethylenetriamine can be used. Mixtures of these amine monomers can also be used, with the concentration range adjustable from 0.1 wt% to 5.0 wt%, and the optimal range being 0.5-2.0 wt% as described in the examples.

[0048] Selection of isocyanate monomers:

[0049] The oil-phase isocyanate monomer is not limited to 1,4-diisocyanate butane; any aliphatic, alicyclic, or aryliphatic diisocyanate or triisocyanate may be used. For example, isophorone diisocyanate (IPDI) provides better weather resistance and flexibility; hexamethylene diisocyanate (HDI) can form more flexible polyurea chains; derivatives of toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) may also be considered when specific properties are required, with concentrations varying between 0.01 wt% and 1.0 wt%.

[0050] Expansion and Variation of Reaction Promoters:

[0051] The core characteristic of the reaction promoter is the presence of a phosphate hydrophilic head group and a long-chain alkyl hydrophobic tail. Therefore, homologues and analogues of sodium dodecyl phosphate (SDP) can produce similar effects.

[0052] Alkyl chain variation: Straight-chain alkyl phosphate salts with C8 to C18 carbon atoms can be used, such as sodium decyl phosphate (C10), sodium tetradecyl phosphate (C14), and sodium hexadecyl phosphate (C16).

[0053] Phosphate salt selection: Monoalkyl or dialkyl phosphate salts can be used. Counterion changes: The counterion of phosphate is not limited to sodium ions (Na+). + ), can be replaced by other alkali metal ions such as potassium ions (K). + Lithium ion (Li) + ), or organic ammonium ions such as triethylamine hydrogen ions (HNEt3). + Other phosphorus-containing anionic surfactants with similar interfacial activity and coordination ability, such as alkyl phosphonates, are also within the reasonable scope of this invention. The concentration of the reaction promoter can be adjusted between 0.0005% and 0.5 wt%, preferably between 0.001% and 0.1 wt%.

[0054] Selection of oil phase solvent:

[0055] The oil phase solvent is not limited to n-hexane. Any nonpolar or weakly polar organic solvent that can dissolve isocyanate monomers and reaction promoters and is immiscible with water can be used, such as cyclohexane, n-heptane, isooctane, methylcyclohexane, or mixtures thereof.

[0056] Selection of supporting base film:

[0057] The supporting membrane is not limited to polyethersulfone ultrafiltration membranes; other porous polymer membranes with good chemical stability and mechanical strength can also be used as supports, such as ultrafiltration membranes made of polysulfone (PSF), polyacrylonitrile (PAN), and polyvinylidene fluoride (PVDF). The structure of the supporting membrane can be planar, hollow fiber, or tubular to suit different membrane module configurations.

[0058] The strong acid-resistant polyurea active layer prepared by this invention can be coated not only on a single-layer ultrafiltration membrane, but also on a composite support reinforced with non-woven fabric or other materials, to prepare industrial membrane elements with higher mechanical strength.

[0059] The core working principle of this invention is the specific enhancement and regulation of the interfacial polymerization kinetics of isocyanate and amine monomers by alkyl phosphate salt reaction promoters. By changing the reaction microenvironment and reaction energy barrier at the oil-water interface, the precise construction of the polyurea active layer is achieved, as follows: In the traditional isocyanate-amine interfacial polymerization system without promoters, amine monomers diffuse from the aqueous phase to the oil-water interface, and isocyanate monomers diffuse from the oil phase to the interface. The two only undergo a nucleophilic addition reaction when they meet at the interface. However, isocyanates themselves have low reactivity, resulting in a slow overall polymerization rate. Conventional techniques, in order to form a dense separation layer that can achieve ion retention, have to increase the monomer concentration or extend the reaction time, ultimately causing the polymer to grow excessively at the interface, forming a thick polyurea active layer with a loose cross-linked structure. This structure will significantly increase the mass transfer resistance of acid molecules in a strong acid environment, resulting in low acid permeation flux. When alkyl phosphate salt reaction promoters (such as sodium dodecyl phosphate, SDP) are introduced into the oil-phase system of this invention, their molecules, due to their hydrophobic-hydrophilic structural characteristics, undergo directional and orderly arrangement at the oil-water interface: the hydrophilic phosphate head group faces the oil-water interface region, while the hydrophobic alkyl long chain is embedded in the oil-phase system. This directional arrangement produces two core synergistic effects, fundamentally improving the efficiency of interfacial polymerization: Effect 1: Interfacial catalytic effect: The phosphate head group can undergo dipole interaction or hydrogen bonding with the -N=C=O group of isocyanate, effectively reducing the activation energy of the nucleophilic addition reaction between isocyanate and amine monomer, significantly accelerating the reaction rate of amine monomer and isocyanate monomer diffused to the interface. Effect 2: Interfacial enrichment and microenvironment regulation effect: The directional arrangement of alkyl phosphate salts at the interface optimizes the polar microenvironment of the interfacial region, while efficiently enriching isocyanate monomers in the oil phase at the reaction interface, increasing the local concentration and effective collision probability of monomers at the interface, further accelerating the polymerization reaction.

[0060] The synergistic effect of these two effects allows the interfacial polymerization reaction to proceed rapidly and fully at lower monomer concentrations and shorter reaction times, without relying on conventional methods of compensating for reactivity by adjusting macroscopic process parameters. This ultimately results in a thinner, denser, and more structurally complete polyurea active separation layer on the supporting membrane surface. This optimized microstructure endows the nanofiltration membrane with excellent separation performance in strong acid systems: acid molecules with small hydration radii (H... + (and its associated anions) can rapidly permeate the membrane with lower mass transfer resistance, achieving high acid flux; while multivalent ions with larger hydration radii (such as Mg) can permeate the membrane rapidly, achieving high acid flux. 2+ Fe 2+ The acid flux is effectively retained by the dense and defect-free polyurea cross-linked network through molecular sieving, maintaining a high retention rate and ultimately achieving a synergistic improvement in high acid flux and high ion retention rate.

[0061] Alkyl phosphate salts are not ordinary anionic surfactants. The unique chemical structure of their phosphate root group and the dual-function effect after directional arrangement are the core of interfacial polymerization kinetic regulation. Common anionic surfactants (such as SDS) cannot achieve the same effect due to the inherent defects in their structure and function. The reasons include: (1) Ordinary anionic surfactants only have physical interfacial effects and no chemical catalytic effect. The core advantage of alkyl phosphate salts is the dual function of physical interfacial regulation and chemical catalysis. Ordinary anionic surfactants such as sodium dodecyl sulfate only have physical interfacial adsorption / microenvironment regulation capabilities and no chemical function of catalyzing interfacial polymerization reactions. The phosphate root group of alkyl phosphate salts contains multiple electron-rich oxygen atoms, which can form dipole interactions or hydrogen bonds with the -N=C=O group of isocyanate, directly reducing the activation energy of the reaction - this is the chemical essence of achieving the reaction rate increase; the sulfate root group of sodium dodecyl sulfate (-OSO3) -(1) As a strongly polar ionic group, it can only achieve interfacial adsorption and microenvironment regulation through electrostatic interaction. It cannot specifically interact with the -N=C=O group of isocyanate, and cannot reduce the reaction activation energy. It can only slightly increase the enrichment degree of monomer interface, and cannot fundamentally solve the problem of low reactivity of isocyanate. (2) Ordinary anionic surfactants are prone to causing defects in the film structure, making it difficult to achieve the characteristics of thin and dense separation layer. The combination of hydrophobic chains and hydrophilic groups in common anionic surfactants such as SDS results in a much lower stability of interfacial orientation compared to alkyl phosphate salts. Furthermore, it easily introduces structural defects during polymerization. The excessive hydrophilicity of the sulfate root group leads to "excessive penetration of the hydrophilic group into the aqueous phase" during interfacial alignment, resulting in poor enrichment of oil-phase monomers. It also easily causes some oil-phase monomers to diffuse into the aqueous phase, leading to bulk homopolymerization and forming loose polymer particles, causing defects such as pinholes and cracks in the film. In contrast, the moderate hydrophilicity of the phosphate root group in alkyl phosphate salts, after orientation, only faces the interfacial region and does not excessively penetrate the aqueous phase. This ensures that the polymerization reaction occurs efficiently only at the interface, avoiding bulk homopolymerization and ensuring the integrity and density of the film structure. (3) Anionic surfactants such as SDS are prone to residue, affecting the chemical stability of the membrane: The sulfate ester bond of SDS has a slight risk of hydrolysis in a strong acid environment. If a small amount remains in the polyurea active layer, the sulfate ions generated by hydrolysis will have a weak interaction with the polyurea layer, which may reduce the chemical stability of the membrane in the long term. In contrast, the phosphate ester bond of alkyl phosphate salts has excellent chemical stability and does not hydrolyze in a strong acid environment with pH < 2. It has good compatibility with the polyurea layer, no risk of residue, and will not affect the long-term acid resistance of the membrane. It can be seen that ordinary anionic surfactants can only achieve physical interfacial adsorption and slight enrichment of monomers, and cannot provide the key interfacial chemical catalytic effect. They cannot fundamentally accelerate the polymerization reaction of isocyanate-amine, and are prone to causing defects in the membrane structure and affecting long-term stability. Therefore, they cannot replace alkyl phosphate salts as the reaction promoter of this invention.

[0062] Alkyl phosphate salt reaction promoters function in the oil phase, but if added to the aqueous phase, they completely lose their promoting effect and may even negatively impact the polymerization reaction and film properties. The specific reasons are as follows:

[0063] 1. Alkyl phosphate salts have a high proportion of hydrophobic alkyl chains, making them unable to disperse stably in the aqueous phase, let alone achieve interfacial directional alignment.

[0064] Alkyl phosphate salts are oil-phase dispersible anionic interfacial activity promoters. Their molecules contain long hydrophobic alkyl chains of C8-C18, with only the hydrophilic phosphate head groups being water-soluble, resulting in an overall hydrophobic ratio far exceeding the hydrophilic ratio. When the reaction promoter is added to the oil phase, the hydrophobic alkyl chains can quickly embed into the oil phase solvent (aliphatic / alicyclic hydrocarbons), with the phosphate head groups facing the oil-water interface, achieving a stable directional arrangement and thus exerting catalytic and enrichment effects. However, if added to the aqueous phase, the long hydrophobic alkyl chains will severely aggregate due to hydrophobic association, failing to form a uniform dispersion system in the aqueous phase, let alone migrate to the oil-water interface to achieve directional arrangement. Ultimately, they exist in the form of precipitate / scum, completely losing their interfacial regulation and catalytic functions.

[0065] 2. The core target of this reaction promoter is the isocyanate monomer in the oil phase, and it needs to be in the same phase as the isocyanate to achieve interfacial enrichment.

[0066] The function of this reaction accelerator is to enrich isocyanate monomers in the oil phase at the oil-water interface. Isocyanates are oil-soluble monomers and insoluble in water. When the reaction accelerator is added to the oil phase, it can mix thoroughly with the isocyanate monomers and "carry" them to the oil-water interface through hydrophobic association, achieving localized high-concentration enrichment. If added to the aqueous phase, it cannot bind with amine monomers in the aqueous phase, nor can it effectively interact with isocyanate monomers in the oil phase, thus failing to achieve interfacial enrichment of isocyanates and losing its core interfacial regulation function.

[0067] 3. To avoid side reactions between the accelerator and the aqueous phase components and ensure the specificity of the polymerization reaction. The aqueous phase system is an aqueous solution of polyamines. If alkyl phosphate salts are added to the aqueous phase, the phosphate head group, being a weakly basic group, may form hydrogen bonds with the amine monomers in the aqueous phase, hindering the diffusion of amine monomers to the interface and thus reducing the concentration of amine monomers at the interface, inhibiting the polymerization reaction; some alkyl phosphate salts have cations (such as Na+). + K + In the aqueous phase, alkyl phosphate salts may undergo weak electrostatic interactions with amine monomers, altering the reactivity of the amine monomers and leading to uncontrolled polymerization rates, easily forming polyurea layers with uneven structures. Therefore, if alkyl phosphate salt reaction promoters are added to the aqueous phase, they will completely lose their promoting effect due to problems such as poor dispersibility, lack of target sites, and the initiation of side reactions, thus failing to achieve the goal of optimizing the construction of the polyurea active layer.

[0068] Example 1

[0069] This embodiment provides a method for preparing an acid-resistant nanofiltration membrane, the steps of which are as follows:

[0070] Step 1: Providing and processing the supporting base film:

[0071] Take a commercially available polyethersulfone ultrafiltration flat sheet membrane, rinse it thoroughly with deionized water to remove the protective agent, and set it aside.

[0072] Step 2: Prepare the aqueous solution:

[0073] Accurately weigh the ethylenediamine monomer, dissolve it in deionized water, and mechanically stir until completely dissolved to prepare a clear aqueous solution with a concentration of 0.2 wt%.

[0074] Step 3: Prepare the oil phase solution:

[0075] Accurately weigh 1,4-diisocyanate butane monomer and sodium dodecyl phosphate (SDP) (reaction promoter), dissolve them together in n-hexane, and sonicate for 10 min to ensure complete dissolution and homogeneous mixing to obtain an oil phase solution. The concentration of 1,4-diisocyanate butane is 0.2 wt%, and the concentration of SDP is 0.01 wt%.

[0076] Step 4: Interfacial polymerization reaction:

[0077] Coating with the aqueous phase: Smoothly fix the prepared substrate membrane from step 1 onto a glass plate. Pour the ethylenediamine aqueous phase solution prepared in step 2 onto the membrane surface, ensuring complete coverage, and allow it to stand for 60 seconds. Removing excess aqueous phase: Use a clean rubber roller to roll over the membrane surface at a uniform speed, carefully scraping away all visible droplets and accumulated liquid, leaving only a thin liquid film adsorbed in the micropores of the substrate membrane. Applying the oil phase reaction: Immediately pour the oil phase solution containing the reaction promoter prepared in step 3 onto the surface of the wet membrane with the aqueous phase attached, and carry out the interfacial polymerization reaction at room temperature (25°C), strictly controlling the reaction time to 30 seconds. Removing the oil phase: Pour off the oil phase solution removed from the membrane surface.

[0078] Step 5: Post-treatment and film formation:

[0079] Heat treatment: The membrane is quickly transferred to a preheated oven at 100°C and heat-treated for 4 minutes to further promote the reaction of residual monomers, remove solvents, and cure and shape the polyurea network.

[0080] Step 6: Rinsing and Storage

[0081] The membrane was removed from the oven and cooled to room temperature. It was then thoroughly rinsed with deionized water to remove any possible residual chemicals. Subsequently, the membrane was immersed in pure water and stored at 4°C for later use. This yielded the high-performance, strong acid-resistant nanofiltration membrane sample (denoted as membrane M-1).

[0082] Example 2

[0083] In this embodiment, sodium octyl phosphate was selected as the reaction promoter, and its concentration in the oil phase was adjusted as follows:

[0084] Preparation of the aqueous solution: consistent with Example 1, is a 0.2 wt% ethylenediamine aqueous solution.

[0085] Preparation of oil phase solution: Dissolve 1,4-diisocyanate butane (concentration 0.2wt%) and sodium octyl phosphate (concentration 0.005wt%) together in n-hexane solvent, and sonicate for 10 minutes until homogeneous.

[0086] Interfacial polymerization reaction: water phase wetting time 60s, oil phase reaction time 30s.

[0087] Heat treatment: Heat treatment at 100℃ for 4 minutes to obtain an acid-resistant nanofiltration membrane sample (denoted as membrane M-2).

[0088] Example 3

[0089] In this embodiment, sodium tetradecyl phosphate was selected as the reaction promoter, and its concentration in the oil phase was adjusted as follows:

[0090] Preparation of the aqueous solution: consistent with Example 1, is a 0.4 wt% ethylenediamine aqueous solution.

[0091] Preparation of oil phase solution: Dissolve 1,4-diisocyanate butane (0.2 wt%) and sodium tetradecyl phosphate (0.05 wt%) together in n-hexane solvent, and sonicate for 10 minutes until homogeneous.

[0092] Interfacial polymerization reaction: water phase wetting time 60s, oil phase reaction time 30s.

[0093] Heat treatment: Heat treatment at 100℃ for 4 minutes to obtain an acid-resistant nanofiltration membrane sample (denoted as membrane M-3).

[0094] Example 4

[0095] In this embodiment, sodium octadecyl phosphate is selected as the reaction promoter, and its highest preferred concentration is used. The specific parameters are as follows:

[0096] Preparation of the aqueous solution: consistent with Example 1, is a 0.2 wt% ethylenediamine aqueous solution.

[0097] Preparation of oil phase solution: Dissolve isophorone diisocyanate (concentration 0.25wt%) and sodium octadecyl phosphate (concentration 0.1wt%) together in n-hexane solvent, and sonicate for 10 minutes until uniformly mixed.

[0098] Interfacial polymerization reaction: water phase wetting time 60s, oil phase reaction time 30s.

[0099] Heat treatment: Heat treatment at 100℃ for 4 minutes to obtain an acid-resistant nanofiltration membrane sample (denoted as membrane M-4).

[0100] Example 5

[0101] In this embodiment, potassium dodecyl phosphate was selected as the reaction promoter, and its concentration in the oil phase was adjusted as follows:

[0102] Preparation of the aqueous solution: consistent with Example 1, is a 0.2 wt% ethylenediamine aqueous solution.

[0103] Preparation of oil phase solution: Dissolve 1,4-diisocyanate butane (concentration 0.2wt%) and potassium dodecyl phosphate (concentration 0.02wt%) together in n-hexane solvent, and sonicate for 10 minutes until the mixture is homogeneous.

[0104] Interfacial polymerization reaction: water phase wetting time 60s, oil phase reaction time 30s.

[0105] Heat treatment: Heat treatment at 100℃ for 4 minutes to obtain an acid-resistant nanofiltration membrane sample (denoted as membrane M-5).

[0106] Example 6

[0107] In this embodiment, diethylenetriamine (DETA) was selected as the aqueous phase amine monomer, and sodium dodecyl phosphate was used as the reaction promoter. The specific parameters are as follows:

[0108] Preparation of aqueous solution: Accurately weigh the diethylenetriamine monomer, dissolve it in deionized water, and mechanically stir until completely dissolved to prepare a 0.8 wt% diethylenetriamine aqueous solution.

[0109] Preparation of oil phase solution: Dissolve 1,4-diisocyanate butane (concentration 0.2wt%) and sodium dodecyl phosphate (concentration 0.015wt%) together in n-hexane solvent, and sonicate for 10 minutes until homogeneous.

[0110] Interfacial polymerization reaction: water phase wetting time 60s, oil phase reaction time 30s.

[0111] Heat treatment: Heat treatment at 100℃ for 4 minutes to obtain an acid-resistant nanofiltration membrane sample (denoted as membrane M-6).

[0112] Comparative Example 1

[0113] This comparative example is based on the previous example, with the preparation of the aqueous solution, treatment of the supporting membrane, and aqueous wetting operations all consistent with Example 1. The only difference is that no reaction promoter was added to the oil phase solution, and the interfacial polymerization reaction time was extended to 120 s (90 s longer than the reaction time in Example 1). All other operations remained unchanged. Heat treatment and post-treatment: consistent with Example 1, heat treatment was carried out at 100°C for 4 minutes to obtain the control membrane sample (denoted as membrane D-1).

[0114] Comparative Example 2

[0115] This comparative example is based on the example, with the aqueous solution preparation, substrate membrane treatment, and aqueous wetting operation all consistent with Example 1. The only difference is that no reaction promoter was added to the oil phase solution, and the concentration of 1,4-diisocyanate butane in the oil phase solution was increased to 2.0 wt% (10 times that of Example 1). All other conditions are the same as in Example 1. Finally, the control membrane sample (denoted as membrane D-2) was obtained by heat treatment at 100°C for 4 minutes.

[0116] Performance Tests and Results:

[0117] Membranes M-1 to M-6 were tested in the same cross-flow filtration apparatus as membranes D-1 to D-2. The test conditions were: operating pressure 1.0 MPa, temperature 25°C, and a feed solution of 2000 mg / L MgSO4 mixed with 15 wt% sulfuric acid solution. After the flux stabilized, the acid permeate flux (LMH / MPa) was recorded, and the permeate was analyzed by acid-base titration and ion chromatography to calculate the acid permeate flow rate and MgSO4 content. 2+ Retention rate. Test results are shown in Table 1.

[0118] Table 1: Performance of acid-resistant nanofiltration membranes prepared in the examples and comparative examples

[0119]

[0120] The test results show that the nanofiltration membrane prepared by the method of this invention has significantly better performance than the samples prepared in the comparative example. Specifically, the water flux of the nanofiltration membrane prepared by this invention reaches more than 49.2 LMH / MPa, which is at least 1.96 times that of the comparative membrane samples D-1 (25.1 LMH / MPa) and 2.48 times that of D-2 (19.8 LMH / MPa), achieving a breakthrough improvement; the acid permeability is increased: the acid permeability of the nanofiltration membrane prepared by this invention reaches more than 75.2%, which is at least 7.9% higher than that of the comparative membrane samples; the magnesium ion rejection rate is also improved: the magnesium ion rejection rate of the nanofiltration membrane prepared by this invention reaches more than 98.2%, which is at least 5.4% higher than that of the comparative membrane samples.

[0121] The above experimental results clearly demonstrate that even with conventional methods such as extending the reaction time and significantly increasing the monomer concentration, removing the reaction promoter only slightly improves the rejection rate, but leads to a significant decrease in water flux and acid permeability, and the performance is far inferior to all embodiments containing the promoter. However, adding a small amount of alkyl phosphate salt compound as a reaction promoter to the oil phase solution can promote the formation of a polyurea active separation layer with a superior structure, simultaneously improving the membrane's acid permeability and ion selectivity. Therefore, this invention achieves a simultaneous increase in nanofiltration membrane water flux, acid permeability, and metal ion rejection rate by adding a small amount of reaction promoter to the oil phase solution, without any performance trade-offs. This stands in stark contrast to the system without the promoter, highlighting the technical advantages of this invention.

[0122] Finally, it should be noted that the above specific embodiments are merely examples of the best implementation of the present invention and are not intended to limit the present invention. Those skilled in the art, understanding the core concept of the present invention—namely, optimizing the polyurea active layer structure by adding specific alkyl phosphate salt reaction promoters to enhance the isocyanate / amine interfacial polymerization activity—can make various changes, substitutions, combinations, or omissions to the various technical elements and process steps of the embodiments. These modifications or substitutions, or combinations where the technical features in the above embodiments do not conflict with each other, can be made according to the manner described in the embodiments. These modifications, substitutions, or combinations do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing an acid-resistant nanofiltration membrane, characterized in that, This includes the following steps performed sequentially: S1. Provide aqueous phase solution and oil phase solution; The aqueous solution is an aqueous solution containing dissolved polyamine monomers; The oil phase solution is an organic solution in which isocyanate monomers and alkyl phosphate salts as reaction promoters are dispersed. The alkyl phosphate salt compound in the oil phase solution is a C8-C soluble compound. 18 Monoalkyl-substituted phosphate salts, wherein the concentration of alkyl phosphate salts added to the oil phase solution is 0.001-0.1 wt%; S2. The aqueous solution and the oil solution are brought into contact on the supporting base membrane to undergo an interfacial polymerization reaction, so as to form a polyurea active separation layer on the surface of the supporting base membrane. S3. Heat treatment to obtain a strong acid-resistant nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that, In S1, the alkyl phosphate salt compound is sodium dodecyl phosphate.

3. The preparation method according to claim 1, characterized in that, In S1, the isocyanate monomer in the oil phase solution is an aliphatic or alicyclic diisocyanate, and the concentration of the isocyanate monomer is 0.01-1.0 wt%; the organic solvent in the oil phase solution is an aprotic inert organic solvent that is immiscible with water.

4. The preparation method according to claim 3, characterized in that, In S1, the organic solvent is n-hexane, cyclohexane, isoalkanes, or a mixture of n-hexane and cyclohexane in a mass ratio of 1:1 to 3:

1.

5. The preparation method according to claim 1, characterized in that, In S1, the isocyanate monomer of the oil phase solution is at least one selected from phenyl diisocyanate, 1,4-diisocyanate butane, toluene diisocyanate and isophorone diisocyanate.

6. The preparation method according to claim 1, characterized in that, In S1, the amine monomer in the aqueous solution is at least one selected from polyethylene polyamine, hyperbranched polyethyleneimine and linear diamine, and the concentration of the amine monomer is 0.1-2.0 wt%.

7. The preparation method according to claim 1, characterized in that, In S2, the contact time between the aqueous solution and the oil solution is 10-300 s.

8. The preparation method according to claim 1, characterized in that, In S3, heat treatment is performed at 60-120℃ for 3-15 minutes to consolidate and strengthen the polyurea active separation layer.

9. An acid-resistant nanofiltration membrane, which is prepared by the preparation method according to any one of claims 1-8.

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