Polyamide (PA) nanofiltration (NF) membrane, and preparation method thereof by regeneration from scrapped microfiltration (MF) membrane

Abstract

A polyamide (PA) nanofiltration (NF) membrane and a preparation method thereof by regeneration from a scrapped microfiltration (MF) membrane are provided. The method adopts a cleaning-repairing-interfacial polymerization upgrading strategy, where, sodium hypochlorite and oxalic acid are used for deeply cleaning a scrapped MF membrane. PDA is used as a repairing agent to construct a reaction platform on the membrane surface, and finally a reaction system of piperazine and trimesoyl chloride (TMC) are used to form a PA NF membrane with a PA active layer. The repairing can construct a coating with a given thickness and prominent hydrophilicity on the membrane surface, which provides favorable base membrane conditions for upgrading and preparing an NF membrane.

Claims

1. A preparation method of a polyamide (PA) nanofiltration (NF) membrane by a regeneration from a scrapped microfiltration (MF) membrane, comprising the following steps: 1) subjecting the scrapped MF membrane to a preliminary hydraulic cleaning for 10 min to 30 min to remove large mud cakes attached to a surface of the scrapped MF membrane to obtain a first cleaned MF membrane; 2) performing a deep cleaning on the first cleaned MF membrane by a chemical cleaning, wherein, the first cleaned MF membrane is placed in a cup-shaped reaction vessel and reacted with a solution A for a first reaction for 1 h to 3 h to obtain a second cleaned MF membrane; after the first reaction is completed, the solution A is poured out; and then the second cleaned MF membrane is reacted with a solution B for a second reaction for 1 h to 3 h to obtain a third cleaned MF membrane; 3) placing the third cleaned MF membrane cleaned in step 2) in a cylindrical reaction vessel, pouring a solution C into the cylindrical reaction vessel to obtain a resulting mixture, and subjecting the resulting mixture to a third reaction at 25° C. to 30° C. for 12 h to 36 h under a shaking to obtain a repaired MF membrane, wherein the solution C is a polydopamine (PDA) solution with a concentration of 1 g/L to 5 g/L, a solvent in the solution C is a mixture of an aqueous tris(hydroxymethyl)aminomethane (THAM) solution and an aqueous hydrochloric acid solution with a mass fraction of 36%, and THAM has a concentration of 0.5 g/L to 1.5 g/L in the aqueous THAM solution; and 4) at a porous side of the repaired MF membrane obtained in step 3), preparing an active layer by an interfacial polymerization, wherein preparation steps specifically include: dissolving imine monomer in water to obtain a solution D, soaking the repaired MF membrane in the solution D as a support layer for 1 min to 5 min and then taking out the repaired MF membrane to obtain a resulting composite, and removing excess liquid droplets on a surface of the resulting composite with a rubber roller; soaking the resulting composite in a solution E for the interfacial polymerization for 30 s to 90 s to obtain a membrane product; and finally drying the membrane product in an oven at 60° C. to 80° C. for 1 min to 10 min to obtain the PA NF membrane.

2. The preparation method of the PA NF membrane by the regeneration from the scrapped MF membrane according to claim 1, wherein the solution A is an aqueous sodium hypochlorite solution with a mass concentration of 1.05% to 8.39%.

3. The preparation method of the PA NF membrane by the regeneration from the scrapped MF membrane according to claim 1, wherein the solution B is an aqueous oxalic acid solution with a mass fraction of 0.5% to 4.0%.

4. The preparation method of the PA NF membrane by the regeneration from the scrapped MF membrane according to claim 1, wherein the imine monomer in the solution D is piperazine, and the solution D is an aqueous piperazine solution with a mass volume fraction of 0.04% to 0.4%.

5. The preparation method of the PA NF membrane by the regeneration from the scrapped MF membrane according to claim 1, wherein the solution E is a solution of trimesoyl chloride (TMC) in n-hexane with a mass volume fraction of 0.04% to 0.4%.

6. The preparation method of the PA NF membrane by the regeneration from the scrapped MF membrane according to claim 1, wherein the scrapped MF membrane is a scrapped polyvinylidene fluoride (PVDF) MF membrane or a scrapped polyethersulfone (PES) MF membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be described in more detail below with reference to the examples and the drawings. Particularly:

(2) FIG. 1A shows a scanning electron microscopy (SEM) image of a new membrane;

(3) FIG. 1B shows an SEM image of a scrapped membrane;

(4) FIG. 1C shows an SEM image of an NF membrane prepared in Example 1 of the present invention;

(5) FIG. 1D shows an SEM image of an NF membrane prepared in Example 2 of the present invention;

(6) FIG. 2A shows an SEM image of an NF membrane prepared in Example 3 of the present invention;

(7) FIG. 2B shows an SEM image of an NF membrane prepared in Comparative Example 1 of the present invention;

(8) FIG. 2C shows an SEM image of an NF membrane prepared in Comparative Example 2 of the present invention;

(9) FIG. 2D shows an SEM image of an NF membrane prepared in Comparative Example 3 of the present invention; and

(10) FIG. 3 is a comparison diagram illustrating the water flux and sodium sulfate rejection rate of NF membranes prepared in Example 1 and Comparative Examples 1 to 3 that are determined in Test Example 1 of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(11) The technical solutions in the examples of the present invention are clearly and completely described below with reference to the accompanying drawings in the examples of the present invention. The described examples are merely a part rather than all of the examples of the invention. All other examples obtained by a person of ordinary skill in the art based on the examples of the invention without creative efforts shall fall within the protection scope of the present invention.

(12) The invention provides a preparation method of a PA NF membrane by regeneration from a scrapped MF membrane, including the following steps:

(13) 1) subjecting the scrapped MF membrane to hydraulic cleaning for 10 min to 30 min to remove large mud cakes attached to a surface of the scrapped MF membrane;

(14) 2) deeply cleaning the contaminated MF membrane by chemical cleaning, where, the MF membrane that is a PVDF or PES MF membrane is placed in a cup-shaped reaction vessel and reacted with a solution A for 1 h to 3 h; after the reaction is completed, the solution is poured out; and then the MF membrane is reacted with a solution B for 1 h to 3 h;

(15) 3) placing the scrapped MF membrane cleaned in step 2) in a cylindrical reaction vessel, pouring a solution C into the vessel, and subjecting a resulting mixture to reaction at 25° C. to 30° C. for 12 h to 36 h under shaking to obtain a repaired MF membrane; and

(16) 4) at a porous side of the repaired MF membrane obtained in step 3), preparing an active layer by interfacial polymerization, where, specifically: imine monomer is dissolved in water to obtain a solution D, the repaired MF membrane is soaked in the solution D as a support layer for 1 min to 5 min and then taken out, and excess liquid droplets on a surface of a resulting composite are removed with a rubber roller; the repaired MF membrane that is attached with the solution D and serves as the support layer is soaked in a solution E, and interfacial polymerization is conducted for 30 s to 90 s; and finally the membrane is dried in an oven at 60° C. to 80° C. for 1 min to 10 min to obtain the PA NF membrane.

(17) The solution A is an aqueous sodium hypochlorite solution with a mass concentration of 1.05% to 8.39%. The solution B is an aqueous oxalic acid solution with a mass fraction of 0.5% to 4.0%. The solution C is a PDA solution with a concentration of 1 g/L to 5 g/L; a solvent in the solution C is a mixture of an aqueous THAM solution and an aqueous hydrochloric acid solution with a mass fraction of 36%; and THAM has a concentration of 0.5 g/L to 1.5 g/L in the aqueous THAM solution. The imine monomer in the solution D is piperazine, and the solution D is an aqueous piperazine solution with a mass volume fraction of 0.04% to 0.4%. The solution E is a solution of TMC in n-hexane with a mass volume fraction of 0.04% to 0.4%.

(18) The concentration of each solution and the reaction time and temperature in the preparation method can be selected within the above range according to a required filtration effect of a PA NF membrane.

(19) The present invention also provides a PA NF membrane prepared by regeneration from a scrapped MF membrane, which includes a support layer and an active layer.

(20) The support layer is a repaired MF membrane obtained by subjecting a scrapped membrane to deep cleaning with sodium hypochlorite and oxalic acid after preliminary cleaning, and constructing a reaction platform on the membrane surface with PDA as a repairing agent; and the active layer is a PA active layer obtained by soaking the repaired MF membrane in a reaction system of piperazine and TMC for interfacial polymerization on a surface of the support layer, where, the interfacial polymerization is conducted on a surface at a porous side of the support layer.

Example 1

(21) Cleaning of a scrapped PVDF MF membrane: A 4% sodium hypochlorite (Maclin) solution is diluted with water to 0.5% to obtain a solution A, and a 2 wt % oxalic acid (Maclin) solution is prepared to obtain a solution B. The contaminated PVDF membrane is placed in a cup-shaped reactor, the solution A is first added, and the solution is poured out after standing for 2 h; then the solution B is added to stand for 2 h; and the membrane is taken out and rinsed 3 to 5 times with deionized water.

Example 2

(22) Preparation of a repairing layer: A 1.21 g/L aqueous THAM solution is prepared and titrated with 3.65 g/L hydrochloric acid, a pH is adjusted to 8.5, and dopamine (Sigma) is dissolved into the above solution to obtain a 2 g/L solution C. The membrane is fixed in a cylindrical mold, 40 mL of the dopamine solution is poured on a surface of the membrane, and the cylindrical mold is shaken for 24 h in the dark to form a repairing layer with a given thickness on the surface of the membrane.

(23) It can be seen from the SEM images shown in FIGS. 1A-D that a repairing layer is formed from PDA on the surface.

(24) It can be seen from the comparison in FIGS. 1A-D that, in the preparation method of the PA NF membrane provided by the present invention, cleaning with the solutions A and B and repairing with dopamine shows prominent effects, and a formed NF membrane has high performance.

Example 3

(25) Preparation of an NF membrane by regeneration from a scrapped PVDF MF membrane through cleaning-repairing-upgrading: The moisture on the surface of the repaired membrane is removed with absorbent paper, then the MF membrane is treated for 2 min with a 0.2 wt/v % aqueous piperazine (Aladdin) solution, and excess droplets on the surface of the MF membrane are removed with a rubber roller; the MF membrane is treated for 30 s with a solution of TMC in n-hexane (0.16 wt/v %, Aladdin), and then the solution is poured out; and the membrane is air-dried for 1 min and then subjected to a thermal treatment at 60° C. for 5 min.

(26) It can be seen from the SEM images shown in FIG. 2A that a continuous, uniform, and dense PA trapping layer is formed on the surface of the scrapped MF membrane.

Comparative Example 1

(27) Preparation of an NF membrane by regeneration from a scrapped PVDF MF membrane through cleaning-upgrading: The moisture on the surface of the MF membrane cleaned in Example 1 is removed with absorbent paper, then the MF membrane is treated for 2 min with a 0.2 wt/v % aqueous piperazine (Aladdin) solution, and excess droplets on the surface of the MF membrane are removed with a rubber roller; the MF membrane is treated for 30 s with a solution of TMC in n-hexane (0.16 wt/v %, Aladdin), and then the solution is poured out; and the membrane is air-dried for 1 min and then subjected to a thermal treatment at 60° C. for 5 min.

(28) It can be seen from the SEM images shown in FIG. 2B that a uniform and dense PA trapping layer is not formed on the surface of the scrapped MF membrane.

Comparative Example 2

(29) Preparation of an NF membrane by regeneration from a scrapped PVDF MF membrane through upgrading alone: The moisture on the surface of a contaminated PVDF MF membrane is removed with absorbent paper, then the MF membrane is treated for 2 min with a 0.2 wt/v % aqueous piperazine (Aladdin) solution, and excess droplets on the surface of the MF membrane are removed with a rubber roller; the MF membrane is treated for 30 s with a solution of TMC in n-hexane (0.16 wt/v %, Aladdin), and then the solution is poured out; and the membrane is air-dried for 1 min and then subjected to a thermal treatment at 60° C. for 5 min.

(30) It can be seen from the SEM images shown in FIG. 2C that a uniform and dense PA trapping layer is not formed on the surface of the scrapped MF membrane.

Comparative Example 3

(31) Preparation of a conventional NF membrane: The moisture on the surface of an unused PVDF MF membrane is removed, then the MF membrane is treated for 2 min with a 0.2 wt/v % aqueous piperazine (Aladdin) solution, and excess droplets on the surface of the MF membrane are removed with a rubber roller; the MF membrane is treated for 30 s with a solution of TMC in n-hexane (0.16 wt/v %, Aladdin), and then the solution is poured out; and the membrane is air-dried for 1 min and then subjected to a thermal treatment at 60° C. for 5 min.

(32) It can be seen from the SEM images shown in FIG. 2D that a continuous PA trapping layer is not formed on the surface of the scrapped MF membrane.

(33) It can be seen from FIGS. 2A-D that a dense, continuous, and uniform PA trapping layer is formed in Example 3 of the present invention, while the PA layers formed in Comparative Examples 1, 2, and 3 exhibited some defects.

Test Example 1

(34) Laboratory performance test of NF membranes: At 25° C., the cross-flow filtration is adopted, with a cross-flow rate of 20 cm/s. Deionized water and a 0.01 mol/L aqueous sodium sulfate solution are used as test solutions. The performance of the NF membranes prepared in Example 3 and Comparative Examples 1 to 3 are tested. As tested (FIG. 3), the Comparative Examples 1 to 3 has water fluxes of 56.6 L/(m.sup.2.Math.h.Math.bar), 28.0 L/(m.sup.2.Math.h.Math.bar), and 116.7 L/(m.sup.2.Math.h.Math.bar), respectively, and sodium sulfate rejection rates of 33.7%, 58.9%, and 4.3%, respectively, exhibiting poor HPM formation effect; and the NF membrane prepared in Example 3 has a water flux of 31.7 L/(m.sup.2.Math.h.Math.bar) and a sodium sulfate rejection rate of 90.3%, exhibiting significantly-improved film formation stability and effect.

(35) The above examples are merely intended to describe the technical solutions of the present invention, rather than to limit the present invention. Although the present invention is described in detail with reference to the above examples, persons of ordinary skill in the art should understand that modifications may be made to the technical solutions described in the above examples or equivalent replacements may be made to some or all technical features thereof, which do not make the essence of corresponding technical solutions depart from the scope of the technical solutions in the examples of the present invention.

(36) In addition, those skilled in the art can understand that, although some examples herein include some features included in other examples but no other features, a combination of features of different examples falls within the scope of the present invention and forms a different example. For example, in the claims, any one of the claimed examples can be used in any combination. The information disclosed in this background section is only intended to deepen the understanding of the general background of the present invention, and should not be regarded as an acknowledgement or any form of suggestion that this information constitutes the prior art commonly known to those skilled in the art.