Method for determining number of hydrates carried in process of salt ions passing through polyamide separation membrane

Abstract

The invention provides a method for determining the number of hydrates carried in the process of salt ions passing through a polyamide separation membrane, and belongs to the technical field of membranes. The method comprises the following operation steps: S1, stripping a support layer non-woven fabric and a middle layer of the polyamide separation membrane to obtain a polyamide separation membrane active layer; s2, coating a counter electrode platinum sheet and a gold chip with the polyamide separation membrane active layer obtained in the step S1; s3, placing the counter electrode platinum sheet and the gold chip obtained in the step S2 in an EQCM-D module, connecting the counter electrode platinum sheet and the gold chip with an electrochemical workstation through a three-electrode system, respectively introducing ultrapure water and a salt solution with known concentration until balance, applying constant voltage after balance, recording frequency and current change of the whole process, and calculating the current change of the counter electrode platinum sheet and the gold chip. And quantitatively determining the hydration number carried by salt ions in the process of passing through the polyamide separation membrane. The method is easy and convenient to operate, high in sensitivity and wide in application range, and quantitative determination of the hydration number of salt ions in the polyamide separation membrane process through an electrochemical method is achieved for the first time.

Description

Technical Field

[0001] The invention relates to the field of membrane technology, and in particular to a method for determining the hydration number of salt ions carried through a polyamide separation membrane. Background Art

[0002] Dissolved ions capture water molecules through electrostatic interactions, forming a hydration layer called hydrated ions. Within the hydration layer, water molecules are interconnected by hydrogen bonds, and different ions exhibit different hydration structures (hydrate size and configuration) due to their different ionic properties (e.g., ion size and charge density). The transport of hydrated ions through (sub)nanopores is ubiquitous in nature and in many industrial and biomedical applications. The phenomenal behavior and properties of confined ions differ from those of their bulk solution state due to the amplified interactions of salt ions with the pore wall under (sub)nano conditions. Membrane pores are usually smaller than hydrated ions, and ions inevitably dehydrate during the transmembrane process, which further affects the selective transport of different ions. The extent to which dehydration occurs depends specifically on the hydration energy of the ion, which is a global reflection of the ion's characteristics. Therefore, for ions with the same charge and similar hydrated diameter, dehydration becomes a key factor in determining the selectivity of ions across membranes. Optimizing solution chemistry and focusing on dehydration-dominated transport in membrane design can amplify the differences in ion hydration structures and improve ion selectivity. This research has an important impact on improving the ion selectivity of membrane separation and is of great significance for selective resource recovery and efficient desalination technologies based on membrane technology, such as seawater desalination, lithium extraction, valuable metal recovery and separation of harmful metal ions.

[0003] However, the effect of ion hydration structure on membrane separation has long been ignored, and traditional polyamide separation theory fails to include hydration properties to explain the microscopic ion transport mechanism, and little is known about the complex effects of aqueous solution chemistry on ion hydration structure and corresponding transmembrane transport behavior. It is necessary to clarify the relationship between aqueous solution chemistry, hydration structure and hydration energy to gain a deeper understanding of selective transmembrane transport. Most of the dehydration mechanisms in previous studies have been discussed through theoretical simulations and lack in situ characterization techniques. Therefore, a systematic study of the hydration number during ion selective transport is needed to fully resolve the transmembrane mechanism, which can promote the design of solute-solute selective membranes required for precise separation, thereby improving energy utilization efficiency and process performance efficiency.

[0004] There is an urgent need to develop a method that can in situ determine the hydration number carried by salt ions through polyamide separation membranes in the process, so as to analyze the dehydration behavior of salt ions through polyamide separation membranes and overcome the limitations of existing technologies. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a method for determining the hydration number of salt ions carried through a polyamide separation membrane. The method for determining the hydration number of salt ions carried through a polyamide separation membrane has high sensitivity and a wide range of applications, and can achieve quantitative characterization of the hydration number of salt ions carried through a polyamide separation membrane under different aqueous solution chemical conditions, providing a new strategy and method for clarifying the potential mechanism of salt ion transmembrane mass transfer.

[0006] The object of the present invention is to provide a method for determining the hydration number of salt ions carried through a polyamide separation membrane, comprising the following steps: S1, peeling off the supporting layer non-woven fabric and the middle layer of the polyamide separation membrane to obtain the active layer of the polyamide separation membrane; S2, coating the polyamide separation membrane active layer obtained in step S1 on the counter electrode platinum sheet and the QCM-D gold chip; S3. Place the counter electrode platinum sheet and gold chip obtained in step S2 in an EQCM-D module and connect them to an electrochemical workstation through a three-electrode system. Pass water and salt solution into the module until equilibrium occurs. After equilibrium, apply a constant voltage and record the frequency and current changes of the entire process to quantitatively determine the hydration number of salt ions passing through the polyamide separation membrane.

[0007] Specifically, the method of the present invention is as follows: stripping the support layer and the middle layer of the polyamide separation membrane; coating the polyamide separation membrane active layer from which the support layer and the middle layer are stripped onto a quartz microcrystal balance gold chip (QCM-D Sensor 301) and an electrochemical quartz microcrystal balance module (EQCM-D) counter electrode platinum sheet; connecting the assembled EQCM-D module to an electrochemical workstation through a three-electrode system and placing it on a quartz microcrystal balance device (QCM-D), respectively passing ultrapure water and a salt ion solution until equilibrium is reached, and recording frequency changes; using the electrochemical workstation to set a constant voltage, recording current and frequency changes, and quantitatively determining the hydration number carried by salt ions in the process of passing through the polyamide separation membrane.

[0008] In some embodiments of the present invention, in step S1, the polyamide separation membrane is a nanofiltration membrane, a reverse osmosis membrane or a homemade membrane.

[0009] In some embodiments of the present invention, step S1 also includes removing impurities from the polyamide separation membrane: soaking the polyamide separation membrane in an isopropanol solution and then washing it with ultrapure water to remove impurities and the anti-corrosion layer on the surface of the polyamide separation membrane, and peeling off the supporting layer non-woven fabric after drying.

[0010] Further, the concentration of the isopropanol solution is 10-50% v / v; Soaking time is 30~60 min; The number of washes was at least three times.

[0011] In some embodiments of the present invention, in step S2, the method of coating the active layer of the polyamide separation membrane on the counter electrode platinum sheet and the gold chip is as follows: the active layer of the polyamide separation membrane is immersed in an organic solvent, the counter electrode platinum sheet and the gold chip are placed at the bottom of the active layer of the polyamide separation membrane, and then the counter electrode platinum sheet and the gold chip coated with the active layer of the polyamide separation membrane are fished out.

[0012] In some embodiments of the present invention, the organic solvent is selected from one or more of N,N-dimethylformamide, chloroform, and dimethyl sulfoxide.

[0013] In some embodiments of the present invention, step S2 also includes cleaning the counter electrode platinum sheet and the gold chip: using an alcohol cotton pad to wipe off the excess polyamide separation membrane active layer on the back of the counter electrode platinum sheet and the gold chip, and puncturing the polyamide separation membrane active layer at the flow channel of the counter electrode platinum sheet.

[0014] In some embodiments of the present invention, in step S3, the calculation formula for the hydration number carried by salt ions through the polyamide separation membrane is as follows: , , , , Among them, the mass change of hydrated salt ions passing through the polyamide separation membrane is Δm i Defined as the product of the crystal constant C and the frequency change Δf, the relative atomic mass of the hydrated salt ion MW' i Defined as Δm i The product of the Faraday constant F and the charge Q i The ratio of hydration number N i Defined as MW' i Relative atomic mass MW to bare salt ion i The difference between the relative molecular mass of water and MW H2O ratio; n = 5, C = 17.7 ng / (Hz·cm 2 ), Δf is obtained by subtracting the frequency before and after applying constant voltage, F = 96485 C / mol, Q i is the charge required for the i ion to enter the polyamide separation membrane, that is, the total charge Q transferred during the constant voltage application process t Subtract the charge It, Q caused by the double layer t It is obtained by integrating the It curve recorded by the electrochemical workstation, where I is the current corresponding to the system stability, t is the time corresponding to the system stability, and MW i and MW' i is the relative atomic mass of the i ion in its unhydrated and hydrated form, Ni It is the hydration number carried by i ion when entering the polyamide separation membrane.

[0015] In some embodiments of the present invention, in step S3, the method for completely adsorbing and balancing the salt ions on the polyamide separation membrane is, illustratively: when the salt ion concentration is 100 mM, the peristaltic pump speed is 30 μL / min, and the equilibrium time is 0.5~2 h.

[0016] In some embodiments of the present invention, in step S3, the pH value of the salt solution is 3-11, the salt solution is a single salt solution, and the ion concentration is 100 mmol / L-1000 mmol / L.

[0017] In some embodiments of the present invention, in step S3, the method for adjusting pH is acid-base titration, and a Leix PHSJ-3F laboratory acidity meter is used to characterize the pH value.

[0018] In some embodiments of the present invention, in step S3, both the water and the salt solution are run for 0.5 to 2 hours to reach a stable equilibrium.

[0019] Specifically, in step S4, the water is ultrapure water produced by a Milli-Q IQ 7000 ultrapure water machine, and the test conductivity is less than 0.2 μS / cm In some embodiments of the present invention, in step S3, the hydration number of anions or cations is determined by controlling the positive and negative voltages of the applied constant voltages. The hydration number of cations is determined by applying a negative voltage, and the hydration number of anions is determined by applying a positive voltage.

[0020] In some embodiments of the present invention, in step S3, the cation of the salt solution is one of lithium, sodium, potassium, calcium or magnesium, and the anion is chloride ion or sulfate ion.

[0021] The above technical solution of the present invention has the following advantages compared with the prior art: The present invention adopts the technology based on quartz microcrystalline balance and electrochemical workstation to quantitatively characterize the hydration number carried by salt ions through polyamide separation membrane. By measuring the mass difference and the amount of charge transferred before and after the constant voltage balance between the polyamide separation membrane and the salt solution, the hydration number carried by the salt ions through the polyamide separation membrane is accurately quantified. This method is simple to operate, real-time and intuitive, and can be widely used in thin-layer composite reverse osmosis membranes, nanofiltration membranes and homemade membranes, providing an important analytical tool for determining the hydration number carried by salt ions through polyamide separation membranes. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein: Figure 1Schematic diagram of the method for determining the hydration number of salt ions carried through a polyamide separation membrane in an embodiment; Figure 2 100 mM Na in Example 1 + Schematic diagram of frequency change under -0.3 V constant voltage condition; Figure 3 100 mM Na in Example 1 + Schematic diagram of current change under -0.3 V constant voltage condition; Figure 4 is the Li measured in the examples + and Na + Schematic diagram of the hydration number carried through the NF90 polyamide separation membrane. DETAILED DESCRIPTION

[0023] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0024] The object of the present invention is to provide a method for determining the hydration number of salt ions carried through a polyamide separation membrane, comprising the following steps: S1, peeling off the supporting layer nonwoven fabric and the middle layer of the polyamide separation membrane, leaving only the active layer; S2, the active layer of the polyamide separation membrane peeled off from the support layer and the intermediate layer in step S1 is evenly coated on the counter electrode platinum sheet and the QCM-D gold chip; S3, placing the counter electrode platinum sheet and gold chip in step S2 in a ventilated place until completely dry; S4. Place the counter electrode platinum sheet and gold chip dried in step S3 in the EQCM-D module and connect them to the electrochemical workstation through a three-electrode system. Pass ultrapure water and a salt solution of known concentration respectively until equilibrium is reached. After equilibrium, apply a known constant voltage, record the frequency and current changes of the whole process, and quantitatively determine the hydration number carried by the salt ions through the polyamide separation membrane.

[0025] Embodiment 1:

[0026] This embodiment provides a method for determining Na + The method of carrying hydration number during ion passing through NF90 polyamide separation membrane is as follows: (1) After soaking the commercial NF90 polyamide separation membrane in 25% v / v isopropanol for 60 min, the membrane was washed three times with ultrapure water. The support layer of the washed NF90 polyamide separation membrane was peeled off and the membrane was placed in an N,N-dimethylformamide solution to peel off the intermediate layer to obtain the NF90 polyamide separation membrane active layer. (2) Place the counter electrode platinum sheet and gold chip at the bottom of the NF90 polyamide separation membrane active layer immersed in N,N-dimethylformamide solution, remove the counter electrode platinum sheet and gold chip coated with the NF90 polyamide separation membrane active layer, place them in a fume hood until completely dry, wipe off the excess NF90 polyamide separation membrane active layer on the back of the counter electrode platinum sheet and gold chip with an alcohol cotton pad after drying, and pierce the NF90 polyamide separation membrane active layer at the flow channel of the counter electrode platinum sheet with a needle; (3) Prepare a 100 mM NaCl solution with a pH of 5.70 (4) The counter electrode platinum sheet and gold chip obtained in step (2) were placed in the EQCM-D module and connected to the electrochemical workstation. The peristaltic pump speed was set to 30 μL / min and ultrapure water was passed for 1 h to stabilize the frequency. Then, the 100 mM NaCl solution obtained in step (3) was passed for 30 min until the frequency stabilized. After stabilization, a constant voltage of -0.3 V was applied for 10 min. The frequency and current changes of the whole process were recorded to quantitatively determine the Na + The hydration number carried by ions passing through NF90 polyamide separation membrane; (5) Na + Mass changes of ions passing through polyamide separation membrane △m Na + is defined as the product of the crystal constant C and the frequency change Δf during the process of passing the NaCl salt solution, Na + Relative atomic mass of hydrated ions MW ' Na + is defined as △m Na + multiplied by the Faraday constant F and the charge Q Na + ratio, hydration number N Na + is defined as MW ' Na + with Na + Relative atomic mass of ions MW Na The difference between + and the relative molecular mass of water MW H2O The ratio is calculated as follows: , , , , Where n = 5, C = 17.7 ng / (Hz·cm 2 ), Δf is obtained by subtracting the frequency before and after applying constant voltage, F = 96485 C / mol, Q Na + is Na +The amount of charge required to enter the NF90 polyamide separation membrane is the total amount of charge Q transferred during the constant voltage application process. t Subtract the charge It, Q caused by the double layer t It is obtained by integrating the It curve recorded by the electrochemical workstation, where I is the current corresponding to the system stability, t is the time corresponding to the system stability, and MW Na + and MW ' Na + is Na + Relative atomic masses of unhydrated and hydrated ions, N Na+ for Na + The hydration number carried by ions when entering the NF90 polyamide separation membrane; (6) The calculated results show that when a constant voltage of -0.3 V is applied and t is 400 s, the concentration of Na is 100 mM and the pH is 5.70. + △m Na + is 146.0 ng, MW ' Na + is 68.4 g / mol, Q t 3.75×10 -3 C, I is 8.86×10 -6 A, Q Na +2.06×10 - 4 C, N Na + is 2.52; Figure 2 In this example, 100 mM Na + Schematic diagram of frequency change under -0.3 V constant voltage condition. It can be seen from the figure that before applying -0.3 V constant voltage (setting the constant voltage application time to T = 0), 100 mM Na + About 30 minutes until the frequency stabilizes ( Figure 2 The figure shows the introduction of 100 mM Na + After the frequency stabilizes, a constant voltage of -0.3 V is applied, and it is observed that the frequency decreases over time, and the corresponding frequency change Δf corresponds to the mass change.

[0027] Figure 3 In this example, 100 mM Na + Schematic diagram of current change under -0.3 V constant voltage condition. As can be seen from the figure, as time goes on, the current continues to decrease and finally reaches a stable value. The total amount of charge transferred during the constant current application process is Q t The charge corresponding to this stable value is the charge It caused by the double electric layer, and the difference between the two is Na +The amount of charge Q required for ions to enter the polyamide separation membrane Na +.

[0028] Embodiment 2:

[0029] This embodiment provides a method for determining Li + The method of carrying hydration number during ion passing through NF90 polyamide separation membrane is as follows: (1) After soaking the commercial NF90 polyamide separation membrane in 25% v / v isopropanol for 60 min, the membrane was washed three times with ultrapure water, the support layer of the washed NF90 polyamide separation membrane was peeled off, and the membrane was placed in an N,N-dimethylformamide solution to peel off the intermediate layer to obtain the NF90 polyamide separation membrane active layer; (2) Place the counter electrode platinum sheet and gold chip at the bottom of the NF90 polyamide separation membrane active layer immersed in N,N-dimethylformamide solution, fish out the counter electrode platinum sheet and gold chip coated with the NF90 polyamide separation membrane active layer, place the counter electrode platinum sheet and gold chip coated with the NF90 polyamide separation membrane active layer in a fume hood until completely dry, wipe off the excess NF90 polyamide separation membrane active layer on the back of the counter electrode platinum sheet and gold chip with an alcohol cotton pad after drying, and pierce the NF90 polyamide separation membrane active layer at the flow channel of the counter electrode platinum sheet with a needle; (3) Prepare a 100 mM LiCl solution with a pH of 5.70. (4) The counter electrode platinum sheet and gold chip obtained in step (2) were placed in the EQCM-D module and connected to the electrochemical workstation. The peristaltic pump speed was set to 30 μL / min and ultrapure water was passed for 1 h to stabilize the frequency. Then, the 100 mM LiCl solution obtained in step (3) was passed for 30 min until the frequency stabilized. After stabilization, a constant voltage of -0.3 V was applied for 10 min. The frequency and current changes of the whole process were recorded to quantitatively determine the Li + The hydration number carried by ions passing through NF90 polyamide separation membrane; (5) Li + Mass changes of ions passing through polyamide separation membrane △m Li + is defined as the product of the crystal constant C and the frequency change Δf during the process of passing LiCl salt solution, Li + Relative atomic mass of hydrated ions MW ' Li + is defined as △m Li + multiplied by the Faraday constant F and the charge Q Li + ratio, hydration number N Li + is defined as MW ' Li + With Li + Relative atomic mass of ionsMW Li The difference between + and the relative molecular mass of water MW H2O The ratio is calculated as follows: , , , , Where n = 5, C = 17.7 ng / (Hz·cm 2 ), Δf is obtained by subtracting the frequency before and after applying constant voltage, F = 96485 C / mol, Q Li + for Li + The amount of charge required to enter the NF90 polyamide separation membrane is the total amount of charge Q transferred during the constant voltage application process. t Subtract the charge It, Q caused by the double layer t It is obtained by integrating the It curve recorded by the electrochemical workstation, where I is the current corresponding to the system stability, and t is the time corresponding to the system stability. MW Li + and MW ' Li + for Li + Relative atomic masses of unhydrated and hydrated ions, N Li + for Li + The hydration number carried by ions when entering the NF90 polyamide separation membrane; (6) The calculated results show that when a constant voltage of -0.3 V is applied and t is 100 s, the concentration of Li is 100 mM and the pH is 5.70. + △m Li + is 44.4 ng, MW ' Li + is 97.4 g / mol, Q t 5.74×10 -4 C, I is 5.30×10 -6 A, Q Li +4.40×10 - 4 C, N Li + is 5.03.

[0030] Figure 4 is the Li measured in the examples + and Na + Schematic diagram of the hydration number carried through the NF90 polyamide separation membrane. It can be seen from the figure that Li + The average hydration number carried through the polyamide separation membrane process is 5.03, Na +The average hydration number carried through the NF90 polyamide separation membrane process is 2.52.

[0031] Obviously, the above embodiments are only examples for clear explanation and are not intended to limit the implementation methods. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from this are still within the protection scope of the invention.

Claims

1. A method for determining the hydration number of salt ions carried through a polyamide separation membrane, characterized in that: The following steps are involved: S1, peeling off the supporting layer nonwoven fabric and the middle layer of the polyamide separation membrane to obtain the active layer of the polyamide separation membrane; S2, coating the polyamide separation membrane active layer obtained in step S1 on the counter electrode platinum sheet and gold chip; S3. Place the counter electrode platinum sheet and gold chip obtained in step S2 in an EQCM-D module and connect them to an electrochemical workstation through a three-electrode system. Pass water and salt solution into the module until equilibrium occurs. After equilibrium, apply a constant voltage and record the frequency and current changes of the entire process to quantitatively determine the hydration number of salt ions passing through the polyamide separation membrane.

2. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S1, the polyamide separation membrane is a nanofiltration membrane, a reverse osmosis membrane or a homemade membrane.

3. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: Step S1 also includes removing impurities from the polyamide separation membrane: soaking the polyamide separation membrane in an isopropanol solution and then washing it with ultrapure water to remove impurities and the anti-corrosion layer on the surface of the polyamide separation membrane, and peeling off the supporting layer non-woven fabric after drying.

4. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S2, the method of coating the active layer of the polyamide separation membrane on the counter electrode platinum sheet and gold chip is as follows: the active layer of the polyamide separation membrane is immersed in an organic solvent, the counter electrode platinum sheet and gold chip are placed at the bottom of the active layer of the polyamide separation membrane, and then the counter electrode platinum sheet and gold chip coated with the active layer of the polyamide separation membrane are fished out.

5. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 4, characterized in that: The organic solvent is selected from one or more of N,N-dimethylformamide, chloroform and dimethyl sulfoxide.

6. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: Step S2 also includes cleaning the counter electrode platinum sheet and gold chip: using an alcohol cotton pad to wipe off the excess polyamide separation membrane active layer on the back of the counter electrode platinum sheet and gold chip, and piercing the polyamide separation membrane active layer at the flow channel of the counter electrode platinum sheet.

7. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S3, the calculation formula of the hydration number carried by salt ions in the process of passing through the polyamide separation membrane is as follows: ， ， ， ， Among them, the mass change of hydrated salt ions passing through the polyamide separation membrane is Δm i Defined as the product of the crystal constant C and the frequency change Δf, the relative atomic mass of the hydrated salt ion MW' i Defined as Δm i The product of the Faraday constant F and the charge Q i The ratio of hydration number N i Defined as MW' i Relative atomic mass MW to bare salt ion i The difference between the relative molecular mass of water and MW H2O ratio; n = 5, C = 17.7 ng / (Hz·cm 2 ), Δf is obtained by subtracting the frequency before and after applying constant voltage, F = 96485 C / mol, Q i is the charge required for the i ion to enter the polyamide separation membrane, that is, the total charge Q transferred during the constant voltage application process t Subtract the charge It, Q caused by the double layer t It is obtained by integrating the It curve recorded by the electrochemical workstation, where I is the current corresponding to the system stability, t is the time corresponding to the system stability, and MW i and MW' i is the relative atomic mass of the i ion in its unhydrated and hydrated form, N i It is the hydration number carried by i ion when entering the polyamide separation membrane.

8. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S3, both the water and the salt solution are run for 0.5 to 2 h to reach a stable equilibrium.

9. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S3, the hydration number of anions or cations is measured by controlling the positive and negative of the applied constant voltage. The hydration number of cations is measured by applying a negative voltage, and the hydration number of anions is measured by applying a positive voltage.

10. The method for determining the hydration number of salt ions carried through a polyamide separation membrane according to claim 1, characterized in that: In step S3, the cation of the salt solution is one of lithium, sodium, potassium, calcium or magnesium, and the anion is chloride ion or sulfate ion.

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