CN114452835B - Neutral nanofiltration membrane, preparation method and application thereof in small-molecule dye wastewater desalination - Google Patents

Neutral nanofiltration membrane, preparation method and application thereof in small-molecule dye wastewater desalination Download PDF

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CN114452835B
CN114452835B CN202111657674.5A CN202111657674A CN114452835B CN 114452835 B CN114452835 B CN 114452835B CN 202111657674 A CN202111657674 A CN 202111657674A CN 114452835 B CN114452835 B CN 114452835B
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nanofiltration membrane
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interfacial polymerization
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CN114452835A (en
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孙世鹏
唐铭健
李璐
蒋阳琦
刘美玲
邢卫红
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Nanjing Tech University
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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D69/00Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12Composite membranes; Ultra-thin membranes
    • B01D69/125In situ manufacturing by polymerisation, polycondensation, cross-linking or chemical reaction
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D67/00Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002Organic membrane manufacture
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D67/00Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002Organic membrane manufacture
    • B01D67/0006Organic membrane manufacture by chemical reactions
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01DSEPARATION
    • B01D69/00Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/02Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
    • CCHEMISTRY; METALLURGY
    • C02TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02FTREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00Treatment of water, waste water, or sewage
    • C02F1/44Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
    • CCHEMISTRY; METALLURGY
    • C02TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02FTREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2101/00Nature of the contaminant
    • C02F2101/10Inorganic compounds
    • C02F2101/101Sulfur compounds
    • CCHEMISTRY; METALLURGY
    • C02TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02FTREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2101/00Nature of the contaminant
    • C02F2101/30Organic compounds
    • C02F2101/308Dyes; Colorants; Fluorescent agents
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02ATECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A20/00Water conservation; Efficient water supply; Efficient water use
    • Y02A20/124Water desalination
    • Y02A20/131Reverse-osmosis

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Abstract

The invention provides a nanofiltration membrane with a charge property of an electric neutral surface, which is used for micromolecular dye in the printing and dyeing industry<500 Da) concept of wastewater desalination. In the invention, the water phase monomer diethylenetriamine and the oil phase monomer trimesoyl chloride are subjected to interfacial polymerization, and then the interfacial polymerization layer is subjected to secondary amination modification by using short-chain amine (triethylenetetramine, tetraethylenepentamine and the like) in an alcohol phase for the first time, so that an electric neutral selection layer is constructed, and crystal violet (408 Da) and Na in water are realized 2 SO 4 Is effective in separation. While maintaining the crystal violet interception rate of about 99.5% in the mixed solution, na 2 SO 4 The retention rate is only 16.8%, and the separation efficiency reaches 166.4. The prepared nanofiltration membrane is expected to solve the problem of desalting small-molecule charged dyes, and reduce the input cost of enterprises in pollution control.

Description

Neutral nanofiltration membrane, preparation method and application thereof in small-molecule dye wastewater desalination
Technical Field
The invention relates to a nanofiltration membrane for desalting small-molecule dye wastewater and having a characteristic of electric neutrality surface charge and a preparation method thereof, belonging to the technical field of membrane separation materials.
Background
The separation of small molecule charged dyes (< 500 Da) and inorganic salts is a major challenge in the development of today's society, involving many aspects of environmental protection, human health and industrial administration. According to the requirements, the typical high-pollution printing and dyeing industry must consider how to efficiently treat the high-salt small-molecule charged dye wastewater generated by the high-pollution printing and dyeing industry and secondarily utilize the salt component in the wastewater so as to reduce the input cost of enterprises in the aspect of pollution control. As a new part in the field of membrane separation, nanofiltration membranes are expected to solve the desalting problem due to the unique separation scale (200-1000 Da).
In the process of forming nanofiltration membranes, amine monomers diffuse into an oil phase from water in a porous matrix to be condensed with acyl chloride monomers to form a polyamide network. However, the nascent polymeric layer prevents the transport of amine across the membrane, so that excess acid chloride does not participate in the reaction. It brings about two significant problems. First, excess acid chloride is susceptible to irreversible hydrolysis during and after IP, severely interfering with the chemistry of the selective layer formed. Second, the ratio between oppositely charged groups in the film is difficult to control, resulting in a limited charge control range [1-2]. NF membranes previously reported are mostly negatively or positively charged [3-7], and their antifouling properties tend to be undesirable due to the electrostatic attraction between membrane charge and contaminants [8].
In addition, the separation mechanism of the nanofiltration membrane is complex, and the nanofiltration membrane consists of two parts of pore size screening and a southward effect. As the pore size of the membrane decreases, the southward effect associated with the charge effect will significantly affect the separation performance of the membrane. As the southward effect has practical influence on all charged ions in a separation system, the selectivity of the membrane to the ions is disturbed, and thus the desalting problem of small-molecule charged dyes with molecular weight below 500Da is directly caused. The neutral membrane mainly uses size exclusion instead of the southward equilibrium [9-10] and is important in ultra-high selectivity separation in high pollution environments, but the prior art does not have a better technical means for regulating and controlling the surface charge of the nanofiltration membrane to be neutral.
1.Bandini S.Modelling the mechanism of charge formation in NF membranes:Theory and application.J Membr Sci.2005;264:75-86.
2.Lalia BS,Kochkodan V,Hashaikeh R,Hilal N.A review on membrane fabrication:Structure, properties and performance relationship.Desalination.2013;326:77-95.
3.Cheng XQ,Liu YY,Guo ZH,Shao L.Nanofiltration membrane achieving dual resistance to fouling and chlorine for"green"separation of antibiotics.J Membr Sci.2015;493:156-166.
4.Zhai Z,Jiang C,Zhao N,Dong WJ,Lan HL,Wang M,Niu QJ.Fabrication of advanced nanofiltration membranes with nanostrand hybrid morphology mediated by ultrafast Noria-polyethyleneimine codeposition.J Mater Chem A.2018;6:21207-21215.
5.Jin PR,Zhu JY,Yuan SS,Zhang G,Volodine A,Tian MM,Wang JX,Luis P,Van der Bruggen B.Erythritol-based polyester loose nanofiltration membrane with fast water transport for efficient dye/salt separation.Chem Eng J.2021;406:126796.
6.Wang HF,Zhang QF,Zhang SB.Positively charged nanofiltration membrane formed by interfacial polymerization of 3,3',5,5'-biphenyl tetraacyl chloride and piperazine on a poly(acrylonitrile)(PAN)support.J Membr Sci.2011;378:243-249.
7.Yao YJ,Li M,Cao XZ,Zhang P,Zhang W,Zheng JF,Zhang X,Wang LJ.A novel sulfonated reverse osmosis membrane for seawater desalination:Experimental and molecular dynamics studies.J Membr Sci.2018;550:470-479.
8.Liu LL,Zhang H,Chen XR,Wan YH,Luo JQ.Deconstruction and reconstitution of fouling layer in decolorization of cane juice by nanofiltration membrane.Adv Membr.2021;1:100010.
9.Fan HW,Gu JH,Meng H,Knebel A,Caro J.High-Flux Membranes Based on the Covalent Organic Framework COF-LZU1 for Selective Dye Separation by Nanofiltration.Angew Chem Int Ed.2018;57:4083-4087.
Disclosure of Invention
The present study purposefully designed nanofiltration membranes with electrically neutral surface charge properties, minimizing adverse effects of the daonan effect to break through this separation barrier. The research selects to construct the neutral nanofiltration membrane to solve the problem of small molecular charged dye in the printing and dyeing industry<500 Da) by first reacting diethylenetriamine in the aqueous phase with trimesoyl chloride in the oil phaseInterfacial polymerization, and then secondarily aminating and modifying the interfacial polymerization layer by using short-chain amine (triethylene tetramine, tetraethylene pentamine, and the like) in the alcohol phase for the first time to construct a charge neutral selection layer, thereby realizing crystal violet (408 Da) and Na in water 2 SO 4 Is effective in separation. While maintaining the crystal violet interception rate of about 99.5% in the mixed solution, na 2 SO 4 The retention rate is only 16.8%, and the separation efficiency ratio reaches 166.4.
One of the invention is that: alcohol solvent has two major functions as a modification phase, namely, the reactivity of acyl chloride groups in a new interfacial polymerization layer is protected, the acyl chloride groups are prevented from being hydrolyzed into carboxyl groups, and electronegativity of a selection layer is weakened; secondly, the miscibility of the short-chain amine solute and the n-hexane is utilized to improve the contact capability of the short-chain amine solute and acyl chloride, and the number of amino groups reacted with the acyl chloride is increased so as to control the positive electricity of the selective layer. The two aspects together promote the formation of a neutral charge selective layer, and the small-aperture separation layer formed by interfacial polymerization is used for strengthening the interception capability of small molecular substances, so that the separation of the charged dye with the molecular weight lower than 500Da from salt is realized.
In a first aspect of the invention, there is provided:
a nanofiltration membrane is formed by compounding a base membrane and a selective separation layer, wherein the selective separation layer is formed by interfacial polymerization of polyethylene polyamine and acyl chloride monomers, and a modification layer is further included in the selective separation layer, and the modification layer is obtained by crosslinking short-chain amine and acyl chloride units.
In one embodiment, the nanofiltration membrane surface has electrically neutral electrochemical properties and exhibits a pH in the range of 5.8 to 7.3.
In one embodiment, the polyethylene polyamine is Diethylenetriamine (DETA).
In one embodiment, the acid chloride-based monomer is selected from trimesoyl chloride (TMC).
In one embodiment, the short chain amine is Ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and Pentaethylenehexamine (PEHA).
In one embodiment, the material of the base membrane is Polyimide (PI), polyethersulfone (PES), sulfonated Polysulfone (SPSF), polyetherimide (PEI), or the like, and is an ultrafiltration membrane.
In a second aspect of the invention, there is provided:
the preparation method of the nanofiltration membrane comprises the following steps:
step 1, providing a base film;
step 2, preparing an aqueous phase solution containing polyethylene polyamine; preparing an organic phase solution containing acyl chloride monomers by adopting a first solvent;
step 3, preparing a modification solution containing short-chain amine by adopting a second solvent; the second solvent can be mutually dissolved with the first solvent, and the acyl chloride monomer cannot be hydrolyzed in the second solvent;
step 4, the aqueous phase solution and the organic phase solution are contacted to prepare an interfacial polymerization layer;
and step 5, fully contacting the modification solution with the nascent interfacial polymerization layer to prepare the charge neutral selection layer.
In one embodiment, the second solvent is an alcohol.
In one embodiment, the alcohols are ethanol, isopropanol, and n-butanol.
In one embodiment, the concentration of polyethylene polyamine in the aqueous phase is 0.01 to 0.06mol/L; the concentration of the acid chloride monomer in the organic phase is 0.1wt%; the content of short-chain amine in the modifying solution is 0.1-14wt%.
In a third aspect of the invention, there is provided:
the nanofiltration membrane is applied to desalting of small-molecule dyes.
In one embodiment, the small molecule dye has a molecular weight of 200-600Da.
In one embodiment, the solution of the small molecule dye is acridine orange (302 Da, AO), methylene blue (320 Da, MB), gold orange II (350 Da, oll), crystal violet (408 Da, CV) and indigo carmine (466 Da, IC).
In one embodiment, the anions and cations in the salt are monovalent or divalent ions.
In one embodiment, the concentration of the small molecule dye is 10 to 2000ppm and the salt concentration is 100 to 40000ppm.
In a fourth aspect of the invention, there is provided:
use of a short-chain amine for enhancing the separation performance of nanofiltration membranes in the desalination of small-molecule dyes.
Advantageous effects
Compared with the traditional composite nanofiltration membrane, the composite nanofiltration membrane based on the alcohol phase short-chain amine modification prepared by the invention has the advantages that the water small molecular dye (such as crystal violet) and Na are reacted 2 SO 4 Has better separation effect, the separation efficiency ratio reaches 166.4, and is favorable for the desalination treatment of small-molecule dye wastewater in the printing and dyeing industry. In addition, the characteristic that the positive and negative electric groups in the selective layer can be formed can be controlled simultaneously by utilizing the alcohol modified phase for the first time, so that the nanofiltration membrane with the property of electric neutral surface charge is constructed. The process weakens the adverse effect of the uncontrollable southward effect of the nanofiltration membrane on the accurate separation of the membrane, and improves the separation performance of the membrane.
Drawings
FIG. 1 is an infrared spectrogram;
FIG. 2 is an XPS diagram;
FIG. 3 is a comparison of nanofiltration membrane morphology of the same DETA and different TETA content of the alcohol modified phase in the interfacial polymerization aqueous phase;
FIG. 4 is a comparison of nanofiltration membrane water contact angles for interfacial polymerization of the same DETA and different TETA content of the alcohol modified phase;
FIG. 5 shows the surface potential contrast of nanofiltration membranes with the same DETA and different TETA content in the alcohol modified phase in the interfacial polymerization aqueous phase;
FIG. 6 is a comparison of nanofiltration membrane surface potentials for interfacial polymerization of different short-chain amine species with the same DETA content in the aqueous phase and alcohol modified phase;
FIG. 7 is a comparison of nanofiltration membrane molecular weight cut-off for the same DETA and different TETA content of the alcohol modified phase of the interfacial polymerization aqueous phase;
FIG. 8 is a comparison of nanofiltration membrane retention properties for interfacial polymerization of aqueous phases of different DETA and the same TETA content of the alcohol modified phase;
FIG. 9 is a comparison of nanofiltration membrane retention properties of interfacial polymerization of aqueous phase with different alcohols as the modified phase;
FIG. 10 is a comparison of nanofiltration membrane retention properties for interfacial polymerization of the same DETA and different TETA content of the alcohol modified phase;
FIG. 11 is the rejection performance of electrically neutral nanofiltration membranes for single dye solutes;
FIG. 12 is a long term stability performance of the electrically neutral nanofiltration membrane to separate small molecule charged dye crystal violet from sodium sulfate;
Detailed Description
The present invention will be described in further detail with reference to the following specific embodiments. It will be appreciated by those skilled in the art that the following examples are illustrative of the present invention and should not be construed as limiting the scope of the invention. The specific techniques or conditions are not identified in the examples and are performed according to techniques or conditions described in the literature in this field or according to the product specifications. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention.
The words "comprise," "include," "have" or any other variation thereof, as used herein, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
The concentrations set forth in the present invention are mass concentrations, and the alcohols are ethanol, unless otherwise specified.
The invention provides a nanofiltration membrane for desalting small-molecule dye wastewater. After the nanofiltration membrane is subjected to surface chemical modification by alcohol phase short-chain amine for the first time, the membrane surface electrical property is in a unique electric neutral state. The method obviously weakens the adverse effect of uncontrollable daway balance on the membrane separation performance, improves the separation capability of the membrane on small molecule charged dye and salt, and can be used for the fine desalting process of printing and dyeing wastewater.
The neutral nanofiltration membrane provided by the invention is formed by compounding a base membrane and a selective separation layer formed by an alcohol phase short chain amine modified DETA/TMC interface polymerization layer.
The base membrane used therein may be a suitable ultrafiltration membrane selected according to the actual circumstances, for example: polyimide (PI), polyethersulfone (PES), sulfonated Polysulfone (SPSF), polyetherimide (PEI), and the like. The preparation process of the base film mainly comprises the steps of mixing corresponding polymer particles with a solvent to obtain a casting film liquid, then scraping and coating the casting film liquid on non-woven fabrics, and preparing the porous asymmetric base film through a phase inversion method.
The separation layer is selected to be prepared on the surface of the base film by an interfacial polymerization and alcohol phase modification two-step method, wherein the preparation process comprises the steps of preparing a water phase, an organic phase and an alcohol modification phase solution, and adding polyethylene polyamine (such as Diethylenetriamine (DETA)) into the water phase solution; the organic phase solution is mainly an organic solution of acyl chloride monomers, such as n-hexane solution of trimesoyl chloride; the alcohol phase solution is predominantly an alcohol (e.g., ethanol) solution of a short chain amine (e.g., triethylenetetramine (TETA)). The concentration of the polyethylene polyamine aqueous solution is 0.01-0.06mol/L, the concentration of the acyl chloride monomer n-hexane solution is 0.1wt%, and the content of the short chain amine in the alcohol phase is 0-14wt%.
The nanofiltration membrane prepared by the method can be applied to the desalination process of small molecular dyes of printing and dyeing wastewater, such as crystal violet and Na 2 SO 4 Separating, or separating crystal violet, naCl and other small molecular dye<500 Da) from sodium salt, and the like.
Example 1
1. Preparation of Flat film
Firstly, polyimide polymer material is dissolved in organic solvent, stirring is carried out for 24 hours to ensure sufficient dissolution, standing is carried out for 12 hours, and then the solution is scraped on non-woven fabric by a scraper. Then, the film is formed by a phase inversion method, and the film is put into deionized water for preservation.
2. Preparation of the selection layer
Preparing DETA aqueous solutions (0.01-0.06 mol/L) with different concentrations, and adding the DETA into water to obtain an interfacial polymerization aqueous phase; dissolving trimesoyl chloride (0.1 wt%) in n-hexane to obtain an interfacial polymerization oil phase; the short-chain amine (0-14 wt%) was dissolved in alcohol to give a modified phase. The three solutions were thoroughly stirred before use. Firstly pouring the aqueous phase solution on a membrane, removing excessive aqueous phase by using wiping paper after two minutes, then pouring the aqueous phase into an organic phase, reacting for one minute, removing excessive oil phase solution on the surface, immediately soaking the surface of a new interfacial polymerization layer with a modified phase, reacting for two minutes, and then placing the new interfacial polymerization layer into water for preservation.
For convenience of description, in the film prepared by the present method, ethylenediamine EDA is abbreviated as ED, diethylenetriamine DETA is abbreviated as DT, triethylenetetramine TETA is abbreviated as TT, tetraethylenepentamine TEPA is abbreviated as TP, and pentaethylenehexamine PEHA is abbreviated as PH. On the basis, the membrane prepared by the method is named DT X -M Y A film, wherein X is the DETA content (mol/L) in the aqueous phase and Y is the short chain amine M (ED, DT, TT, TP, PH) content (wt%) in the alcohol modified phase.
Comparative experiments
The difference from example 1 is that: when the secondary short-chain amine modification is carried out, the solvent adopted is water.
Infrared sign
Alcohol phase short chain amine modification significantly alters the chemical composition of the selective layer. In FIG. 1, when there is no alcohol phase modified interfacial polymerization layer (DT 0.015 ) The infrared spectrum shows that it is 1,645cm -1 And 1,560cm -1 The characteristic amide absorption bands (c=o and C-N, respectively) are weak. With the participation of the modification process, the reaction of the short-chain amine TETA in the alcohol and the residual acyl chloride in the interfacial polymerization layer intensifies the characteristic band of the amide. On the other hand, with increasing TETA concentration, at 1,243cm -1 The C-N absorption peak of unreacted amino is obviously enhanced, which indicates that more TETA modifies the surface, thereby being beneficial to improving the electropositivity of the surface of the membrane. Furthermore, all group absorption peaks in the selective layer no longer change significantly at TETA concentrations greater than 7%, demonstrating that at this time the chemical modification of the membrane surface by short chain amines in the alcohol reaches a steady state.
XPS characterization
The XPS characterization of fig. 2 shows the law of element variation in the selection layer. As the TETA concentration in the alcohol-modified phase increases, the N/O ratio in the selective layer increases significantly. Since N is mainly composed of two parts of amide and amino, O is mainly composed ofAmide groups and carboxyl groups, so this change in N/O can account for the increasing trend in positively charged amino content and the decreasing trend in negatively charged carboxyl content in the select layer. Meanwhile, comparing DT 0.015 -TT 7 And DT (DT) 0.015 -TT 14 The N/O changes of the two are weak, which also confirms the conclusion of infrared characterization: chemical modification of the membrane surface by short chain amines in alcohols reached a steady state at TETA concentrations greater than 7%.
SEM characterization
FIG. 3 is DT prepared according to example 1 0.015 、DT 0.015 -TT 3.5 、DT 0.015 -TT 7 And DT (DT) 0.015 -TT 14 Surface topography of the film. By adjusting the content of TETA in the finishing phase, the film surface roughness goes through the process from rough to smooth, and the thickness of the layer is selected to be thinner. From infrared characterization, it can be seen that low concentrations of TETA reduce the degree of crosslinking of the selective layer (corresponding to the amide group content, fig. 2), thus DT 0.015 -TT 3.5 The surface of the film cannot be completely covered by the local cross-linked cluster structure, so that the local thickness of the selective layer is increased. The increase in the TETA addition significantly improves this phenomenon and promotes the formation of a smooth thin selection layer.
Contact angle characterization
Fig. 4 is the contact angle of the film prepared as in example 1. As can be seen from the figure, the increase in the amount of TETA added weakens the hydrophilicity of the film surface. Although TETA is a hydrophilic substance, the reduction in surface roughness (fig. 3) more significantly increases the hydrophobicity of the film surface. The effect of this variation is manifested in permeation flux and entrapment experiments.
Potential characterization
FIG. 5 is a graph of the change in surface potential of nanofiltration membranes with the same DETA in the interfacial polymerization aqueous phase and different TETA contents in the alcohol modified phase. With increasing TETA concentration in alcohol, the isoelectric point of the membrane gradually shifts to the right, meaning that the amount of unreacted amino on the surface is increased and the electropositivity of the membrane surface is enhanced. At a TETA concentration of 7%, the selective layer exhibits an electrically neutral character at pH5.8-pH 7.3. The reasons are divided into two parts. On the one hand, the decreasing trend of the O element content in the XPS characterization demonstrated a decrease in the carboxyl content of the film surface (fig. 2), demonstrating that the alcohol modified phase can avoid hydrolysis of the residual acid chloride, promoting an increase in TETA reaction (fig. 1). On the other hand, due to the miscibility of the alcohol with N-hexane, the TETA monomer in the alcohol can react fully with the acid chloride group, corresponding to the rising trend of the N element in the selective layer, indicating an increase in the amino content of the film surface. This cancellation in two ways promotes the appearance of electrically neutral surface charges.
Fig. 6 replaces the TETA equimolar amount of fig. 5 with other short chain amine combinations. As can be seen from the figure, all membranes showed weak surface charge or neutrality at near neutral pH, further confirming the conditions of appearance and feasibility of preparation of the electrically neutral nanofiltration membranes in this experiment.
Characterization of molecular weight cut-off
The molecular weight cut-off of the membrane in fig. 7 decreases with increasing TETA concentration in the alcohol phase, corresponding to the enhancement of the amide band formed by crosslinking in fig. 1. In addition, electrically neutral DT 0.015 -TT 7 The molecular weight cut-off of the membrane is 351Da, which is favorable for small molecular dye<500 Da).
Permeation flux and entrapment experiments
The retention rate tests all used salt concentrations of 2000ppm and the small molecule charged dye concentrations of 100ppm. The test was carried out at 6bar, 25 ℃, acidity of the deionized water for experiment (ph=6), concentration polarization was prevented by stirring during the test.
The film formulations were optimized in FIGS. 8,9 and 10, and the films used were prepared as in example 1. First, fig. 8 determines the optimal aqueous DETA concentration. DT as the concentration of interfacial polymerization DETA decreases when the TETA content in the alcohol is constant 0.015 -TT 7 Solution permeability of membrane compared to DT 0.06 -TT 7 The membrane was increased by approximately 2.5 times. Na (Na) 2 SO 4 And NaCl rejection drops due to the formation of a loose interfacial polymerized layer with lower DETA concentration. In order to obtain higher water permeability and lower sodium salt rejection rate, the water phase is provided withThe final DETA concentration of (C) was finally set to 0.015mol/L.
Next, fig. 9 compares the modified membrane properties in the conventional aqueous phase with those in ethanol, isopropanol or n-butanol; in the comparative experiment, only the solvent type of the preparation process in example 1 under the above-described optimal conditions was replaced. As the traditional aqueous phase modification process is easy to hydrolyze a large amount of residual acyl chloride groups of the interfacial polymerization layer, the electronegativity of the membrane surface is enhanced, and the prepared membrane has low flux and higher rejection rate of sodium salt, is not beneficial to desalting of small-molecule dye, and proves the superiority of alcohol phase modification. It can be seen from the figure that ethanol can increase the membrane permeability by approximately 3 times, and can be used as the optimal modification solvent. Finally, the experiment determines the optimal TETA concentration in the alcohol modified phase under controlled and constant DETA concentration in the aqueous phase (fig. 10). An increase in TETA concentration significantly reduces salt solution permeability and due to DT 0.015 -TT 7 Electroneutral character of the film, the film is opposite to Na 2 SO 4 And the retention rate of NaCl is the lowest. The decrease in solution permeability is caused by two aspects: one is to form a more dense selection layer (fig. 7); the other is that the film surface becomes smoother, more hydrophobic (fig. 3 and 4). In pursuit of low sodium rejection to face the need for small molecule charged dye desalination, electrically neutral DT 0.015 -TT 7 Nanofiltration membranes are the optimal solution for this experiment, na of the membrane 2 SO 4 And NaCl reach 13L m -2 h -1 bar -1 The above (fig. 10).
FIG. 11 shows a charge neutral DT prepared according to example 1 0.015 -TT 7 The membrane handles the retention properties of different dyes. The membrane has 90% trapping effect on most of positive and negative dyes with molecular weight between 300 and 500Da, which basically corresponds to the test result of trapping molecular weight (figure 7). With the increase of the molecular weight of the dye, the retention rate is greatly increased, and when the molecular weight reaches more than 400Da (CV, IC), the retention rate of the dye with positive and negative charges of the neutral nanofiltration membrane is more than 99%, thereby meeting the removal requirement of the dye in practical application occasions.
FIG. 12 shows the charge neutrality obtained in accordance with example 1DT 0.015 -TT 7 Separation performance of the membrane against 2000ppm CV and 40000ppm mixed solution. After 5 days of stability separation test (the membrane is soaked in the solution to be tested at night), the retention rate of the membrane to crystal violet in the mixed solution is kept to be about 99.5 percent, and meanwhile, na is used as a catalyst 2 SO 4 The rejection rate is only 16.8%. According to the separation efficiency formulaAnd R is the retention rate), and the separation efficiency reaches 166.4. Therefore, the research is expected to break through the difficult problem of desalting the small-molecule charged dye.

Claims (1)

1. Application of neutral nanofiltration membrane in solution desalination of small molecule charged dye, wherein the small molecule charged dye is crystal violet, and the salt is Na 2 SO 4 The preparation method of the electrically neutral nanofiltration membrane is characterized by comprising the following steps of:
step 1, providing a base film;
step 2, preparing an aqueous phase solution containing 0.015mol/L diethylenetriamine; preparing an organic phase solution containing 0.1 weight percent of trimesic chloride by adopting n-hexane;
step 3, preparing a modified phase solution containing 7wt% of triethylene tetramine by adopting ethanol;
and step 4, firstly pouring the aqueous phase solution on the base film, removing redundant aqueous phase by using wiping paper after two minutes, then pouring the organic phase solution, reacting for one minute, removing redundant organic phase solution on the surface, immediately soaking the surface of the newly generated interfacial polymerization layer with the modified phase solution, reacting for two minutes, and then placing the solution into water for preservation to obtain the neutral nanofiltration membrane.
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CN105727763A (en) * 2016-03-07 2016-07-06 天津大学 Preparation method of fluorine-containing polyamide composite nano-filtration membrane
CN111545083A (en) * 2020-03-24 2020-08-18 南京工业大学 Nanofiltration membrane, preparation method and application of nanofiltration membrane in fermentation liquor concentration
CN112642305A (en) * 2021-01-12 2021-04-13 天津工业大学 Acid-resistant composite nanofiltration membrane and preparation method thereof

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KR101692784B1 (en) * 2015-04-29 2017-01-17 고려대학교 산학협력단 Method of Preparing Membrane Using Active Layer Prepared by Support-free Interfacial Polymerization in Free Surface

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Publication number Priority date Publication date Assignee Title
CN105727763A (en) * 2016-03-07 2016-07-06 天津大学 Preparation method of fluorine-containing polyamide composite nano-filtration membrane
CN111545083A (en) * 2020-03-24 2020-08-18 南京工业大学 Nanofiltration membrane, preparation method and application of nanofiltration membrane in fermentation liquor concentration
CN112642305A (en) * 2021-01-12 2021-04-13 天津工业大学 Acid-resistant composite nanofiltration membrane and preparation method thereof

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