A multi-stage filtration system for ultrapure water production process

Abstract

The application discloses a kind of multistage filtration systems for ultrapure water production process, multistage filtration system includes primary filter device and secondary filter device, RO concentrated water stock solution is sequentially transported to primary filter device, secondary filter device;The inside of primary filter device is equipped with colloidal silica filter material, the inside of secondary filter device is equipped with colloidal iron filter material;Colloidal silica filter material is amino functionalized MOFs nanofiber composite membrane, colloidal iron filter material is nanofiber membrane with surface grafted hydroxamic acid group.The multistage filtration system for ultrapure water production process provided by the application has excellent colloidal silica removal effect and excellent colloidal iron removal effect.

Description

Technical Field

[0001] This invention relates to the field of ultrapure water process technology, and in particular to a multi-stage filtration system for ultrapure water production processes. Background Technology

[0002] In the production of ultrapure water, RO (reverse osmosis) membranes are a core water treatment device, widely used in the production of ultrapure water. RO concentrate is unavoidable in the ultrapure water production process, and it often contains a large amount of fine particulate matter such as colloidal silica and colloidal iron. The presence of these substances can adversely affect the operation of subsequent RO membranes and other equipment, thereby reducing the efficiency and stability of the water treatment system.

[0003] Colloidal silica refers to silicon-based fine particles that typically exist in water in a colloidal state, with particle sizes ranging from tens to hundreds of nanometers. Colloidal silica has multifaceted effects on water quality, especially in ultrapure water production, where its presence can easily increase the conductivity of the water, thus affecting the final water quality. Traditional methods for removing colloidal silica include: (1) coagulation and sedimentation, which requires the treatment of large amounts of wastewater and flocculants; (2) ultrafiltration membrane technology, where the pore size of the ultrafiltration membrane limits its complete removal of colloidal silica and it is susceptible to membrane fouling, leading to a shortened membrane lifespan; and (3) reverse osmosis membrane pretreatment, where high concentrations of colloidal silica can cause fouling and clogging of the RO membrane, reducing membrane flux and lifespan.

[0004] Colloidal iron refers to iron ions present in water in colloidal form, usually forming colloidal particles with a size typically ranging from a few nanometers to tens of nanometers. The presence of colloidal iron not only affects the purity of water quality but also causes clogging and scaling of RO membranes, affecting the overall performance of the water treatment system. Traditional methods for removing colloidal iron include: (1) flocculation sedimentation, which usually has problems such as high reagent consumption, complex operation, and wastewater treatment; (2) ion exchange, which requires regular resin regeneration, resulting in high costs and complex operation; and (3) membrane filtration technology, which has limited effectiveness in removing colloidal iron due to the strong adsorption and small particle size of colloidal iron particles, easily leading to membrane fouling and performance degradation.

[0005] Colloidal silica easily adheres to the surface of RO membranes, causing membrane fouling, reducing the effective filtration area, thereby decreasing membrane flux and increasing the frequency of membrane cleaning. In traditional ultrafiltration and RO pretreatment processes, the removal efficiency of colloidal silica remains unsatisfactory, requiring further improvement. The accumulation of colloidal iron not only affects water quality but also leads to fouling and deposition on the RO membrane surface, causing fouling, clogging, and scaling, thus reducing water treatment efficiency. Colloidal iron is even more destructive to RO membranes at high concentrations, especially in ultrapure water production, where higher concentrations exacerbate membrane fouling, often requiring frequent membrane cleaning and increasing operating costs.

[0006] Most current filter materials lack the specific function of filtering colloidal silica and colloidal iron, resulting in insufficient filtration effect; traditional filter materials are often general-purpose, but there is still a lack of materials that can efficiently and selectively remove different types of colloidal pollutants. Summary of the Invention

[0007] In view of this, the present invention proposes a multi-stage filtration system for ultrapure water production processes.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A multi-stage filtration system for ultrapure water production processes includes a primary filtration unit and a secondary filtration unit. RO concentrate is sequentially fed into the primary and secondary filtration units. The primary filtration unit contains a colloidal silica removal filter material, and the secondary filtration unit contains a colloidal iron removal filter material. The colloidal silica removal filter material is an amino-functionalized MOF nanofiber composite membrane, and the colloidal iron removal filter material is a nanofiber membrane with surface-grafted isohydroxamic acid groups.

[0009] Furthermore, the preparation method of the amino-functionalized MOF nanofiber composite membrane includes the following steps: S11: Dissolve PAN in DMF, stir until completely dissolved, and allow to stand to remove bubbles until transparent; use an electrospinning device to form a PAN nanofiber membrane; hot-press the PAN nanofiber membrane; and dry. S12: Immerse the hot-pressed membrane in sodium hydroxide solution for heat treatment, then remove the membrane and soak it in dilute hydrochloric acid to complete the chemical activation of the PAN nanofiber membrane surface; rinse with deionized water until neutral; dissolve ZrCl4 in anhydrous ethanol to prepare a metal solution, immerse the activated membrane in the metal solution, and shake to adsorb at room temperature; finally, remove the membrane and rinse it quickly with anhydrous ethanol. S13: Dissolve ZrCl4 in DMF to form mixture A, and dissolve 2-aminoterephthalic acid in DMF / anhydrous ethanol to form mixture B. Mix mixture A and mixture B together and stir. Add acetic acid as a regulator to prepare the ligand solution. Immerse the Zr-adsorbed membrane in the ligand solution and transfer it to the reaction vessel for reaction. S14: Clean with DMF and anhydrous ethanol alternately using ultrasonic cleaning multiple times, rinse repeatedly with deionized water, and finally dry.

[0010] Furthermore, the method for preparing the nanofiber membrane with surface grafted hydroxamic acid groups includes the following steps: S21: Dissolve PAN in DMF, stir until completely dissolved, and let stand to remove bubbles until transparent; use an electrospinning device to make PAN nanofiber membrane; hot press the PAN nanofiber membrane; dry; and then pre-oxidize the PAN nanofiber membrane in an air atmosphere. S22: Dissolve hydroxylamine hydrochloride in deionized water / anhydrous ethanol to form a hydroxylamine hydrochloride solution, then slowly add sodium carbonate to adjust the pH to 6-7 to form a reaction solution; immerse the pre-oxidized PAN nanofiber membrane in the reaction solution and react; remove the PAN nanofiber membrane and rinse it with deionized water. S23: Immerse the cleaned membrane in hydrochloric acid solution; finally, rinse repeatedly with deionized water until the effluent pH is neutral and the TOC test is qualified.

[0011] Further, PAN is dissolved in DMF to prepare a spinning solution of 8-12 wt%. The operating parameters of the electrospinning equipment are as follows: spinning voltage 15-20 kV, receiving distance 15-20 cm, feed speed 0.8-1.2 mL / h, ambient temperature, relative humidity <45 RH, receiving roller speed 100-300 r / min, to prepare a PAN nanofiber membrane with a thickness of 50-80 μm. The hot pressing temperature is 100-120℃, the hot pressing time is 10-20 min, and the hot pressing pressure is 0.1-1.0 MPa.

[0012] Furthermore, in step S12, the heat treatment is performed on the PAN nanofiber membrane at 55–65°C for 30–60 min; In step S12, the concentration of sodium hydroxide is 1 mol / L; in step S12, the concentration of dilute hydrochloric acid is 1 mol / L, and the dilute hydrochloric acid soaking time is 20–35 min; in step S12, the concentration of the molten metal is 0.05–0.1 mol / L. In step S12, the adsorption is carried out by shaking at room temperature for 2 to 4 hours.

[0013] Further, in step S13, the concentration of mixture A is 0.06–0.08 mol / L; the concentration of mixture B in step S13 is 0.04–0.06 mol / L; the volume ratio of DMF to anhydrous ethanol in step S13 is 4–4.5:1; in step S13, mixture A and mixture B are mixed to form a total mixture, and the volume ratio of the total mixture to acetic acid is 1:0.08–0.1. In step S13, the membrane adsorbed with zirconium is immersed in a ligand solution, transferred to a reaction vessel, and reacted at 110–120°C for 18–30 h.

[0014] Furthermore, in step S14, the drying is carried out in a vacuum drying oven at 100-120°C for 8-12 hours.

[0015] Furthermore, in step S21, a pre-oxidation treatment is performed at 220–240°C for 1–2 hours.

[0016] Further, in step S22, the volume ratio of deionized water to anhydrous ethanol is 3-3.5:1; and the concentration of hydroxylamine hydrochloride solution in step S22 is 1.4-1.6 mol / L. In step S22, the pre-oxidized PAN nanofiber membrane is immersed in the reaction solution and reacted in a constant temperature water bath at 70-80°C for 2-4 hours.

[0017] Furthermore, in step S23, the concentration of the hydrochloric acid solution is 0.1–0.5 mol / L, and the soaking time in the hydrochloric acid solution is 20–35 min.

[0018] Compared with existing technologies, the beneficial effects of this invention are: The multi-stage filtration system for ultrapure water production provided by this invention has excellent removal effect on colloidal silica: the amino groups on the MOF framework are easily protonated in water, transforming into positively charged ammonium ions. Colloidal silica usually has a negatively charged surface in water. The positively charged amino groups electrostatically adsorb the negatively charged surface of colloidal silica. When colloidal silica is electrostatically pulled closer, the oxygen atoms on the silica surface coordinate with the zirconium atoms on the MOFs, forming a very strong and stable Zr-O-Si covalent bond. The high specific surface area of ​​MOFs adsorbs colloidal silica through van der Waals forces. The metal centers in the MOF structure can further adsorb negatively charged siloxy groups on the surface through their vacancy coordination, thereby achieving effective removal of colloidal silica. The multi-stage filtration system for ultrapure water production provided by this invention has excellent removal effect on colloidal iron: the isohydroxamic acid group has a strong chelating effect and can form a stable complex with iron ions; the nitrogen and oxygen atoms on the isohydroxamic acid group form a strong five-membered chelate ring with iron ions; the isohydroxamic acid group has a strong affinity for iron ions, so its effect on removing colloidal iron is outstanding. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0021] The multi-stage filtration system for ultrapure water production processes of the present invention will be described below with reference to specific embodiments. Example 1

[0022] A multi-stage filtration system for ultrapure water production processes includes a primary filtration unit and a secondary filtration unit. RO concentrate is sequentially fed into the primary and secondary filtration units. The primary filtration unit contains a colloidal silica removal filter material, and the secondary filtration unit contains a colloidal iron removal filter material. The colloidal silica removal filter material is an amino-functionalized MOF nanofiber composite membrane, and the colloidal iron removal filter material is a nanofiber membrane with surface-grafted isohydroxamic acid groups.

[0023] The preparation method of amino-functionalized MOF nanofiber composite membrane includes the following steps: S11: Dissolve PAN in DMF and stir magnetically until completely dissolved to prepare a 10wt% spinning solution. Let it stand to degas until transparent. Use an electrospinning device to prepare a PAN nanofiber membrane. The operating parameters of the electrospinning device are: spinning voltage 18kV, receiving distance 15cm, feed speed 1mL / h, ambient temperature, relative humidity 40%RH, and receiving roller speed 200r / min. Prepare a PAN nanofiber membrane with a thickness of 80μm. Hot-press the PAN nanofiber membrane to increase its mechanical strength and prevent the fibers from loosening. The hot-pressing temperature is 120℃, the hot-pressing time is 15min, and the hot-pressing pressure is 0.5MPa. Then place it in a vacuum drying oven at 60℃ for 12h to remove residual solvent.

[0024] S12: The hot-pressed PAN nanofiber membrane was immersed in a 1 mol / L sodium hydroxide solution for heat treatment at 60°C for 45 min. The PAN nanofiber membrane was then removed and soaked in a 1 mol / L dilute hydrochloric acid solution for 30 min to complete the chemical activation of the PAN nanofiber membrane surface. It was then rinsed with deionized water until neutral. ZrCl4 was dissolved in anhydrous ethanol to prepare a 0.05 mol / L metal solution. The activated membrane was then immersed in the metal solution and shaken for adsorption at room temperature for 3 h. Finally, the PAN nanofiber membrane was removed and quickly rinsed with anhydrous ethanol. This step converts the inert cyano groups on the fiber surface into active carboxyl groups, preparing for the anchoring of Zr ions.

[0025] S13: Dissolve ZrCl4 in DMF to form a mixture A with a concentration of 0.07 mol / L. Dissolve 2-aminoterephthalic acid in DMF / anhydrous ethanol (the volume ratio of DMF to anhydrous ethanol is 4:1) to form a mixture B with a concentration of 0.04 mol / L. Mixture A and mixture B are mixed and stirred to form a total mixture. Add acetic acid as a regulator (the volume ratio of the total mixture to acetic acid is 1:0.09) to prepare the ligand solution. Immerse the Zr-adsorbed membrane in the ligand solution, transfer it to the reaction vessel, and react at 120℃ for 24 h.

[0026] S14: Cleaned repeatedly with alternating ultrasonic cleaning of DMF and anhydrous ethanol, rinsed repeatedly with deionized water, and finally dried in a vacuum drying oven at 100℃ for 12 hours to remove solvent molecules from the pores.

[0027] The principle of removing colloidal silica: The amino groups on the MOF backbone are readily protonated in water, transforming into positively charged ammonium ions (-NH3). + Colloidal silica typically carries a negative surface charge in water. Positively charged amino groups (-NH2) electrostatically adsorb the negative charge on the colloidal silica surface. When colloidal silica is electrostatically pulled closer, oxygen atoms on the silica surface coordinate with zirconium atoms on MOFs, forming extremely strong and stable Zr-O-Si covalent bonds. The high specific surface area of ​​MOFs adsorbs colloidal silica through van der Waals forces. The metal centers (such as Zr) in the MOF structure can further adsorb negatively charged siloxy groups on the surface through vacancy coordination, thereby achieving effective removal of colloidal silica.

[0028] A method for preparing nanofiber membranes with surface-grafted isohydroxamic acid groups includes the following steps: S21: Dissolve PAN in DMF and stir until completely dissolved to prepare a 10wt% spinning solution. Let it stand to remove bubbles until transparent. Use an electrospinning device to spin PAN nanofiber membranes. The operating parameters of the electrospinning device are: spinning voltage 18kV, receiving distance 15cm, feed speed 1mL / h, ambient temperature, relative humidity 40%RH, and receiving roller speed 200r / min to prepare a PAN nanofiber membrane with a thickness of 80μm. Hot-press the PAN nanofiber membrane at a temperature of 120℃ for 15min and a pressure of 0.5MPa. Then, dry it in a vacuum drying oven at 60℃ for 12h to remove residual solvent. Next, pre-oxidize the PAN nanofiber membrane in air at 220℃ for 1.5h. This step causes the fibers to undergo cyclization and cross-linking, making them insoluble and non-swelling in subsequent chemical reactions.

[0029] S22: Dissolve hydroxylamine hydrochloride in deionized water / anhydrous ethanol (volume ratio of deionized water to anhydrous ethanol is 3:1) to form a hydroxylamine hydrochloride solution with a concentration of 1.5 mol / L. Then slowly add sodium carbonate to adjust the pH to 7 to form a reaction solution. Immerse the pre-oxidized PAN nanofiber membrane in the reaction solution and react in a constant temperature water bath at 70℃ for 3 hours. Take out the PAN nanofiber membrane and rinse it with deionized water.

[0030] S23: Immerse the cleaned membrane in a 0.3 mol / L hydrochloric acid solution for 30 minutes; finally, rinse repeatedly with deionized water until the effluent pH is neutral and the TOC test is qualified.

[0031] The principle of removing colloidal iron: The isohydroxamic acid (-C(=NOH)-OH) group has a strong chelating effect and can react with metal ions (such as Fe). 3+ Forming stable complexes; the nitrogen (-NH) and oxygen (-OH) atoms on the isohydroxamic acid group react with iron ions (Fe) to form stable complexes. 3+ Forms a strong five-membered chelate ring; the isohydroxamic acid group affects Fe. 3+ It has a strong affinity for other metal ions (such as Ca). 2+ Mg 2+ Its adsorption capacity is relatively low, so its effect on removing colloidal iron is more prominent. Example 2

[0032] The multi-stage filtration system of the ultrapure water production process in this embodiment is the same as that in Embodiment 1, and the specific details are as described in Embodiment 1.

[0033] The difference is that in step S13 of this embodiment, the concentration of mixture B is 0.05 mol / L.

[0034] The difference is that in step S22 of this embodiment, the concentration of hydroxylamine hydrochloride solution is 1.4 mol / L. Example 3

[0035] The multi-stage filtration system of the ultrapure water production process in this embodiment is the same as that in Embodiment 1, and the specific details are as described in Embodiment 1.

[0036] The difference is that the concentration of mixture B in step S13 of this embodiment is 0.06 mol / L.

[0037] The difference is that the concentration of hydroxylamine hydrochloride solution in step S22 of this embodiment is 1.4 mol / L. Example 4

[0038] The multi-stage filtration system of the ultrapure water production process in this embodiment is the same as that in Embodiment 1, and the specific details are as described in Embodiment 1.

[0039] The difference is that in step S13 of this embodiment, the concentration of mixture B is 0.05 mol / L.

[0040] The difference is that the concentration of hydroxylamine hydrochloride solution in step S22 of this embodiment is 1.6 mol / L.

[0041] Comparative Example 1 A multi-stage filtration system for ultrapure water production processes includes a primary filtration unit to which RO concentrate is delivered; the primary filtration unit is equipped with colloidal silica removal filter material, which is an amino-functionalized MOF nanofiber composite membrane.

[0042] The preparation method of amino-functionalized MOF nanofiber composite membrane includes the following steps: S11: Dissolve PAN in DMF and stir magnetically until completely dissolved to prepare a 10wt% spinning solution. Let it stand to degas until transparent. Use an electrospinning device to prepare a PAN nanofiber membrane. The operating parameters of the electrospinning device are: spinning voltage 18kV, receiving distance 15cm, feed speed 1mL / h, ambient temperature, relative humidity 40%RH, and receiving roller speed 200r / min. Prepare a PAN nanofiber membrane with a thickness of 80μm. Hot-press the PAN nanofiber membrane to increase its mechanical strength and prevent the fibers from loosening. The hot-pressing temperature is 120℃, the hot-pressing time is 15min, and the hot-pressing pressure is 0.5MPa. Then place it in a vacuum drying oven at 60℃ for 12h to remove residual solvent.

[0043] S12: The hot-pressed PAN nanofiber membrane was immersed in a 1 mol / L sodium hydroxide solution for heat treatment at 60°C for 45 min. The PAN nanofiber membrane was then removed and soaked in a 1 mol / L dilute hydrochloric acid solution for 30 min to complete the chemical activation of the PAN nanofiber membrane surface. It was then rinsed with deionized water until neutral. ZrCl4 was dissolved in anhydrous ethanol to prepare a 0.05 mol / L metal solution. The activated membrane was then immersed in the metal solution and shaken for adsorption at room temperature for 3 h. Finally, the PAN nanofiber membrane was removed and quickly rinsed with anhydrous ethanol. This step converts the inert cyano groups on the fiber surface into active carboxyl groups, preparing for the anchoring of Zr ions.

[0044] S13: Dissolve ZrCl4 in DMF to form a mixture A with a concentration of 0.07 mol / L. Dissolve 2-aminoterephthalic acid in DMF / anhydrous ethanol (the volume ratio of DMF to anhydrous ethanol is 4:1) to form a mixture B with a concentration of 0.04 mol / L. Mixture A and mixture B are mixed and stirred to form a total mixture. Add acetic acid as a regulator (the volume ratio of the total mixture to acetic acid is 1:0.09) to prepare the ligand solution. Immerse the Zr-adsorbed membrane in the ligand solution, transfer it to the reaction vessel, and react at 120℃ for 24 h.

[0045] S14: Cleaned repeatedly with alternating ultrasonic cleaning of DMF and anhydrous ethanol, rinsed repeatedly with deionized water, and finally dried in a vacuum drying oven at 100℃ for 12 hours to remove solvent molecules from the pores.

[0046] Comparative Example 2 A multi-stage filtration system for ultrapure water production processes includes a secondary filtration unit, to which RO concentrate is delivered; the secondary filtration unit is equipped with a colloidal iron removal filter material; the colloidal iron removal filter material is a nanofiber membrane with surface grafted isohydroxamic acid groups.

[0047] A method for preparing nanofiber membranes with surface-grafted isohydroxamic acid groups includes the following steps: S21: Dissolve PAN in DMF and stir until completely dissolved to prepare a 10wt% spinning solution. Let it stand to remove bubbles until transparent. Use an electrospinning device to spin PAN nanofiber membranes. The operating parameters of the electrospinning device are: spinning voltage 18kV, receiving distance 15cm, feed speed 1mL / h, ambient temperature, relative humidity 40%RH, and receiving roller speed 200r / min to prepare a PAN nanofiber membrane with a thickness of 80μm. Hot-press the PAN nanofiber membrane at a temperature of 120℃ for 15min and a pressure of 0.5MPa. Then, dry it in a vacuum drying oven at 60℃ for 12h to remove residual solvent. Next, pre-oxidize the PAN nanofiber membrane in air at 220℃ for 1.5h. This step causes the fibers to undergo cyclization and cross-linking, making them insoluble and non-swelling in subsequent chemical reactions.

[0048] S22: Dissolve hydroxylamine hydrochloride in deionized water / anhydrous ethanol (volume ratio of deionized water to anhydrous ethanol is 3:1) to form a hydroxylamine hydrochloride solution with a concentration of 1.5 mol / L. Then slowly add sodium carbonate to adjust the pH to 7 to form a reaction solution. Immerse the pre-oxidized PAN nanofiber membrane in the reaction solution and react in a constant temperature water bath at 70℃ for 3 hours. Take out the PAN nanofiber membrane and rinse it with deionized water.

[0049] S23: Immerse the cleaned membrane in a 0.3 mol / L hydrochloric acid solution for 30 minutes; finally, rinse repeatedly with deionized water until the effluent pH is neutral and the TOC test is qualified.

[0050] Comparative Example 3 A multi-stage filtration system for ultrapure water production processes includes a primary filtration unit and a secondary filtration unit. RO concentrate is sequentially fed into the primary and secondary filtration units. The primary filtration unit contains a colloidal silica removal filter material, and the secondary filtration unit contains a colloidal iron removal filter material. The colloidal silica removal filter material is a terephthalic acid-functionalized MOF nanofiber composite membrane, and the colloidal iron removal filter material is a nanofiber membrane with surface-grafted isohydroxamic acid groups.

[0051] A method for preparing terephthalic acid-functionalized MOF nanofiber composite membranes includes the following steps: S11: Dissolve PAN in DMF and stir magnetically until completely dissolved to prepare a 10wt% spinning solution. Let it stand to degas until transparent. Use an electrospinning device to prepare a PAN nanofiber membrane. The operating parameters of the electrospinning device are: spinning voltage 18kV, receiving distance 15cm, feed speed 1mL / h, ambient temperature, relative humidity 40%RH, and receiving roller speed 200r / min. Prepare a PAN nanofiber membrane with a thickness of 80μm. Hot-press the PAN nanofiber membrane to increase its mechanical strength and prevent the fibers from loosening. The hot-pressing temperature is 120℃, the hot-pressing time is 15min, and the hot-pressing pressure is 0.5MPa. Then place it in a vacuum drying oven at 60℃ for 12h to remove residual solvent.

[0052] S12: The hot-pressed PAN nanofiber membrane was immersed in a 1 mol / L sodium hydroxide solution for heat treatment at 60°C for 45 min. The PAN nanofiber membrane was then removed and soaked in a 1 mol / L dilute hydrochloric acid solution for 30 min to complete the chemical activation of the PAN nanofiber membrane surface. It was then rinsed with deionized water until neutral. ZrCl4 was dissolved in anhydrous ethanol to prepare a 0.05 mol / L metal solution. The activated membrane was then immersed in the metal solution and shaken for adsorption at room temperature for 3 h. Finally, the PAN nanofiber membrane was removed and quickly rinsed with anhydrous ethanol. This step converts the inert cyano groups on the fiber surface into active carboxyl groups, preparing for the anchoring of Zr ions.

[0053] S13: Dissolve ZrCl4 in DMF to form a mixture A with a concentration of 0.07 mol / L. Dissolve terephthalic acid in DMF / anhydrous ethanol (the volume ratio of DMF to anhydrous ethanol is 4:1) to form a mixture B with a concentration of 0.04 mol / L. Mix the mixture A and the mixture B together and stir to form a total mixture. Add acetic acid as a regulator (the volume ratio of the total mixture to acetic acid is 1:0.09) to prepare the ligand solution. Immerse the Zr-adsorbed membrane in the ligand solution, transfer it to the reaction vessel, and react at 120℃ for 24 h.

[0054] S14: Cleaned repeatedly with alternating ultrasonic cleaning of DMF and anhydrous ethanol, rinsed repeatedly with deionized water, and finally dried in a vacuum drying oven at 100℃ for 12 hours to remove solvent molecules from the pores.

[0055] A method for preparing nanofiber membranes with surface-grafted isohydroxamic acid groups includes the following steps: S21: Dissolve PAN in DMF and stir until completely dissolved to prepare a 10wt% spinning solution. Let it stand to remove bubbles until transparent. Use an electrospinning device to spin PAN nanofiber membranes. The operating parameters of the electrospinning device are: spinning voltage 18kV, receiving distance 15cm, feed speed 1mL / h, ambient temperature, relative humidity 40%RH, and receiving roller speed 200r / min to prepare a PAN nanofiber membrane with a thickness of 80μm. Hot-press the PAN nanofiber membrane at a temperature of 120℃ for 15min and a pressure of 0.5MPa. Then, dry it in a vacuum drying oven at 60℃ for 12h to remove residual solvent. Next, pre-oxidize the PAN nanofiber membrane in air at 220℃ for 1.5h. This step causes the fibers to undergo cyclization and cross-linking, making them insoluble and non-swelling in subsequent chemical reactions.

[0056] S22: Dissolve hydroxylamine hydrochloride in deionized water / anhydrous ethanol (volume ratio of deionized water to anhydrous ethanol is 3:1) to form a hydroxylamine hydrochloride solution with a concentration of 1.5 mol / L. Then slowly add sodium carbonate to adjust the pH to 7 to form a reaction solution. Immerse the pre-oxidized PAN nanofiber membrane in the reaction solution and react in a constant temperature water bath at 70℃ for 3 hours. Take out the PAN nanofiber membrane and rinse it with deionized water.

[0057] S23: Immerse the cleaned membrane in a 0.3 mol / L hydrochloric acid solution for 30 minutes; finally, rinse repeatedly with deionized water until the effluent pH is neutral and the TOC test is qualified.

[0058] Comparative Example 4 A multi-stage filtration system for ultrapure water production processes includes a primary filtration unit and a secondary filtration unit. RO concentrate is sequentially fed into the primary and secondary filtration units. The primary filtration unit contains colloidal silica removal material, and the secondary filtration unit contains colloidal iron removal material. The colloidal silica removal material is an amino-functionalized MOF nanofiber composite membrane, and the colloidal iron removal material is an oxalic acid-grafted nanofiber membrane.

[0059] The preparation method of amino-functionalized MOF nanofiber composite membrane includes the following steps: S11: Dissolve PAN in DMF and stir magnetically until completely dissolved to prepare a 10wt% spinning solution. Let it stand to degas until transparent. Use an electrospinning device to prepare a PAN nanofiber membrane. The operating parameters of the electrospinning device are: spinning voltage 18kV, receiving distance 15cm, feed speed 1mL / h, ambient temperature, relative humidity 40%RH, and receiving roller speed 200r / min. Prepare a PAN nanofiber membrane with a thickness of 80μm. Hot-press the PAN nanofiber membrane to increase its mechanical strength and prevent the fibers from loosening. The hot-pressing temperature is 120℃, the hot-pressing time is 15min, and the hot-pressing pressure is 0.5MPa. Then place it in a vacuum drying oven at 60℃ for 12h to remove residual solvent.

[0060] S12: The hot-pressed PAN nanofiber membrane was immersed in a 1 mol / L sodium hydroxide solution for heat treatment at 60°C for 45 min. The PAN nanofiber membrane was then removed and soaked in a 1 mol / L dilute hydrochloric acid solution for 30 min to complete the chemical activation of the PAN nanofiber membrane surface. It was then rinsed with deionized water until neutral. ZrCl4 was dissolved in anhydrous ethanol to prepare a 0.05 mol / L metal solution. The activated membrane was then immersed in the metal solution and shaken for adsorption at room temperature for 3 h. Finally, the PAN nanofiber membrane was removed and quickly rinsed with anhydrous ethanol. This step converts the inert cyano groups on the fiber surface into active carboxyl groups, preparing for the anchoring of Zr ions.

[0061] S13: Dissolve ZrCl4 in DMF to form a mixture A with a concentration of 0.07 mol / L. Dissolve 2-aminoterephthalic acid in DMF / anhydrous ethanol (the volume ratio of DMF to anhydrous ethanol is 4:1) to form a mixture B with a concentration of 0.04 mol / L. Mixture A and mixture B are mixed and stirred to form a total mixture. Add acetic acid as a regulator (the volume ratio of the total mixture to acetic acid is 1:0.09) to prepare the ligand solution. Immerse the Zr-adsorbed membrane in the ligand solution, transfer it to the reaction vessel, and react at 120℃ for 24 h.

[0062] S14: Cleaned repeatedly with alternating ultrasonic cleaning of DMF and anhydrous ethanol, rinsed repeatedly with deionized water, and finally dried in a vacuum drying oven at 100℃ for 12 hours to remove solvent molecules from the pores.

[0063] The principle of removing colloidal silica: The amino groups on the MOF backbone are readily protonated in water, transforming into positively charged ammonium ions (-NH3). + Colloidal silica typically carries a negative surface charge in water. Positively charged amino groups (-NH2) electrostatically adsorb the negative charge on the colloidal silica surface. When colloidal silica is electrostatically pulled closer, oxygen atoms on the silica surface coordinate with zirconium atoms on MOFs, forming extremely strong and stable Zr-O-Si covalent bonds. The high specific surface area of ​​MOFs adsorbs colloidal silica through van der Waals forces. The metal centers (such as Zr) in the MOF structure can further adsorb negatively charged siloxy groups on the surface through vacancy coordination, thereby achieving effective removal of colloidal silica.

[0064] The preparation method of oxalic acid-grafted nanofiber membrane includes the following steps: S21: Dissolve PAN in DMF and stir until completely dissolved to prepare a 10wt% spinning solution. Let it stand to remove bubbles until transparent. Use an electrospinning device to spin PAN nanofiber membranes. The operating parameters of the electrospinning device are: spinning voltage 18kV, receiving distance 15cm, feed speed 1mL / h, ambient temperature, relative humidity 40%RH, and receiving roller speed 200r / min to prepare a PAN nanofiber membrane with a thickness of 80μm. Hot-press the PAN nanofiber membrane at a temperature of 120℃ for 15min and a pressure of 0.5MPa. Then, dry it in a vacuum drying oven at 60℃ for 12h to remove residual solvent. Next, pre-oxidize the PAN nanofiber membrane in air at 220℃ for 1.5h. This step causes the fibers to undergo cyclization and cross-linking, making them insoluble and non-swelling in subsequent chemical reactions.

[0065] S22: Dissolve oxalic acid in deionized water / anhydrous ethanol (volume ratio of deionized water to anhydrous ethanol is 3:1) to form an oxalic acid solution with a concentration of 1.5 mol / L. Then slowly add sodium carbonate to adjust the pH to 7 to form a reaction solution. Immerse the pre-oxidized PAN nanofiber membrane in the reaction solution and react in a constant temperature water bath at 70℃ for 3 hours. Take out the PAN nanofiber membrane and rinse it with deionized water.

[0066] S23: Immerse the cleaned membrane in a 0.3 mol / L hydrochloric acid solution for 30 minutes; finally, rinse repeatedly with deionized water until the effluent pH is neutral and the TOC test is qualified.

[0067] The same RO concentrate was filtered using the multi-stage filtration systems for ultrapure water production processes described in Example 1 and Comparative Examples 1-4. The effluent from Example 1 and Comparative Examples 1-4 was collected, and the concentrations of colloidal silica and colloidal iron in the effluent were measured. The initial concentrations of colloidal silica and colloidal iron in the RO concentrate of Example 1 and Comparative Examples 1-4 were: 60–80 mg / L for colloidal silica and 2–4 mg / L for colloidal iron. These initial concentrations are suitable for simulating the levels of colloidal silica and colloidal iron contamination commonly found in RO concentrate, are suitable for testing under laboratory conditions, and can demonstrate the removal effect of the filter material.

[0068] Table 1 shows the removal effects of colloidal silicon and colloidal iron in Example 1 and Comparative Examples 1-4.

[0069] Table 1. Removal rates of colloidal silica and colloidal iron in Example 1 and Comparative Examples 1-5 project Colloidal silica removal rate (%) Colloidal iron removal rate (%) in conclusion Example 1 93.7 95.2 Both colloidal silica and colloidal iron showed excellent removal effects. Comparative Example 1 93.4 Basically not removed Excellent performance only in removing colloidal silica Comparative Example 2 Basically not removed 94.9 Excellent effect only in removing colloidal iron Comparative Example 3 73.8 94.6 Colloidal silica shows good removal performance, while colloidal iron shows excellent removal performance. Comparative Example 4 92.4 75.1 Colloidal silica shows excellent removal performance, while colloidal iron shows good removal performance. As shown in Table 1, the multi-stage filtration system used in Example 1 for ultrapure water production processes exhibits excellent removal effects for both colloidal silica and colloidal iron. The principles for removing colloidal silica and colloidal iron have been explained in detail. Comparative Example 1 shows excellent removal effects only for colloidal silica; however, since the colloidal iron particle size is typically smaller than the membrane pore size, colloidal iron is essentially not removed. Comparative Example 2 shows excellent removal effects only for colloidal iron; there is no electrostatic attraction between the isohydroxamic acid groups and the negatively charged colloidal silica, and the isohydroxamic acid does not chemically react with the colloidal silica. Colloidal silica was basically not removed; in Comparative Example 3, the removal effect of colloidal iron was excellent, and the removal effect of colloidal silica was relatively good. This is because after replacing 2-aminoterephthalic acid with terephthalic acid, colloidal silica can also be removed through the adsorption of functionalized groups with colloidal silica, but its removal efficiency is slightly lower than that of amino groups; in Comparative Example 4, the removal effect of colloidal silica was excellent, and the removal effect of colloidal iron was relatively good. This is because after replacing hydroxylamine hydrochloride with oxalic acid, the hydroxyl groups in oxalic acid can also play a complexing role with colloidal iron, but the complexing effect is weaker than that of isohydroxamic acid groups.

[0070] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A multi-stage filtration system for ultrapure water production processes, characterized in that, The multi-stage filtration system includes a primary filtration unit and a secondary filtration unit. The RO concentrate is sequentially fed into the primary filtration unit and the secondary filtration unit. The primary filtration unit is equipped with a colloidal silica removal filter material, and the secondary filtration unit is equipped with a colloidal iron removal filter material. The colloidal silica removal filter material is an amino-functionalized MOF nanofiber composite membrane, and the colloidal iron removal filter material is a nanofiber membrane with surface grafted isohydroxamic acid groups.

2. The multi-stage filtration system for ultrapure water production process according to claim 1, characterized in that, The preparation method of the amino-functionalized MOF nanofiber composite membrane includes the following steps: S11: Dissolve PAN in DMF, stir until completely dissolved, and allow to stand to remove bubbles until transparent; use an electrospinning device to form a PAN nanofiber membrane; hot-press the PAN nanofiber membrane; and dry. S12: Immerse the hot-pressed membrane in sodium hydroxide solution for heat treatment, then remove the membrane and soak it in dilute hydrochloric acid to complete the chemical activation of the PAN nanofiber membrane surface; rinse with deionized water until neutral; dissolve ZrCl4 in anhydrous ethanol to prepare a metal solution, immerse the activated membrane in the metal solution, and shake to adsorb at room temperature; finally, remove the membrane and rinse it quickly with anhydrous ethanol. S13: Dissolve ZrCl4 in DMF to form mixture A, and dissolve 2-aminoterephthalic acid in DMF / anhydrous ethanol to form mixture B. Mix mixture A and mixture B together and stir. Add acetic acid as a regulator to prepare the ligand solution. Immerse the Zr-adsorbed membrane in the ligand solution and transfer it to the reaction vessel for reaction. S14: Clean with DMF and anhydrous ethanol alternately using ultrasonic cleaning multiple times, rinse repeatedly with deionized water, and finally dry.

3. A multi-stage filtration system for ultrapure water production process according to claim 1, characterized in that, The method for preparing the nanofiber membrane with surface grafted isohydroxamic acid groups includes the following steps: S21: Dissolve PAN in DMF, stir until completely dissolved, and let stand to remove bubbles until transparent; use an electrospinning device to make PAN nanofiber membrane; hot press the PAN nanofiber membrane; dry; and then pre-oxidize the PAN nanofiber membrane in an air atmosphere. S22: Dissolve hydroxylamine hydrochloride in deionized water / anhydrous ethanol to form a hydroxylamine hydrochloride solution, then slowly add sodium carbonate to adjust the pH to 6-7 to form a reaction solution; immerse the pre-oxidized PAN nanofiber membrane in the reaction solution and react; remove the PAN nanofiber membrane and rinse it with deionized water. S23: Immerse the cleaned membrane in hydrochloric acid solution; finally, rinse repeatedly with deionized water until the effluent pH is neutral and the TOC test is qualified.

4. A multi-stage filtration system for ultrapure water production processes according to claim 2 or 3, characterized in that, PAN was dissolved in DMF to prepare a spinning solution of 8–12 wt%. The operating parameters of the electrospinning equipment were as follows: spinning voltage 15–20 kV, receiving distance 15–20 cm, feed speed 0.8–1.2 mL / h, ambient temperature, relative humidity <45 RH, receiving roller speed 100–300 r / min, to prepare a PAN nanofiber membrane with a thickness of 50–80 μm. The hot pressing temperature was 100–120 °C, the hot pressing time was 10–20 min, and the hot pressing pressure was 0.1–1.0 MPa.

5. A multi-stage filtration system for ultrapure water production process according to claim 2, characterized in that, In step S12, the heat treatment is performed at 55-65°C for 30-60 minutes on the PAN nanofiber membrane. In step S12, the concentration of sodium hydroxide is 1 mol / L; in step S12, the concentration of dilute hydrochloric acid is 1 mol / L, and the dilute hydrochloric acid soaking time is 20–35 min; in step S12, the concentration of the molten metal is 0.05–0.1 mol / L. In step S12, the adsorption is carried out by shaking at room temperature for 2 to 4 hours.

6. A multi-stage filtration system for ultrapure water production process according to claim 2, characterized in that, In step S13, the concentration of mixture A is 0.06–0.08 mol / L; the concentration of mixture B in step S13 is 0.04–0.06 mol / L; the volume ratio of DMF to anhydrous ethanol in step S13 is 4–4.5:1; in step S13, mixture A and mixture B are mixed to form a total mixture, and the volume ratio of the total mixture to acetic acid is 1:0.08–0.

1. In step S13, the membrane adsorbed with zirconium is immersed in a ligand solution, transferred to a reaction vessel, and reacted at 110–120°C for 18–30 h.

7. A multi-stage filtration system for ultrapure water production process according to claim 2, characterized in that, In step S14, the drying process involves drying in a vacuum drying oven at 100–120°C for 8–12 hours.

8. A multi-stage filtration system for ultrapure water production process according to claim 3, characterized in that, In step S21, a pre-oxidation treatment is performed at 220–240°C for 1–2 hours.

9. A multi-stage filtration system for ultrapure water production process according to claim 3, characterized in that, In step S22, the volume ratio of deionized water to anhydrous ethanol is 3–3.5:1; the concentration of hydroxylamine hydrochloride solution in step S22 is 1.4–1.6 mol / L. In step S22, the pre-oxidized PAN nanofiber membrane is immersed in the reaction solution and reacted in a constant temperature water bath at 70-80°C for 2-4 hours.

10. A multi-stage filtration system for ultrapure water production process according to claim 3, characterized in that, In step S23, the concentration of hydrochloric acid solution is 0.1–0.5 mol / L, and the soaking time in hydrochloric acid solution is 20–35 min.