Polyamide thin-layer composite membrane modification method

Abstract

The invention discloses a method for modifying a polyamide thin-layer composite membrane. The method is characterized by comprising the following steps: washing or soaking the polyamide thin-layer composite membrane with a cleaning agent to remove impurities; a polyphenol small molecule compound and a penetration enhancer are added into water, the pH of the solution is adjusted to be 8-9, a modifier solution is obtained, the mass concentration of the catechol compound in the modifier solution is 0.05-1%, and the mass concentration of the penetration enhancer is 1-5%; the polyamide thin-layer composite membrane is placed in a filtering device, a modifier solution is adopted as a feeding solution, the filtering pressure is adjusted to be 2-10 kg, filtering is conducted for 10-60 min, then pure water is used for flushing, and the modified polyamide thin-layer composite membrane is obtained and has the advantages that hydrophilicity, pollution resistance, oxidation resistance and interception capacity are all improved.

Description

Technical Field

[0001] The invention relates to the technical field of membrane separation, and in particular to a method for modifying a polyamide thin-layer composite membrane. Background Art

[0002] Polyamide thin-layer composite membrane (abbreviated as PA-TFC) is prepared by interfacial polycondensation of polyamino aqueous monomers and acyl chloride oil-phase monomers. The polyamide thin-layer composite membrane prepared by interfacial polymerization is currently the most important type of reverse osmosis and nanofiltration membrane. Polyamide thin-layer composite membranes have been widely used in sewage treatment, seawater desalination and other fields.

[0003] The modification method of polyamide thin-layer composite membrane is mainly to improve the membrane's anti-fouling and chlorine resistance by changing the hydrophilicity, charge and roughness of the membrane surface through surface modification. Surface modification is generally divided into two methods: surface coating and surface grafting. Surface coating generally uses water-soluble molecules as raw materials. For example, applying polyvinyl alcohol (abbreviated as PVA) to the surface of polyamide reverse osmosis membrane can improve the hydrophilicity of the membrane and reduce the charge density of the membrane. Depositing positively charged polyethyleneimine (abbreviated as PEI) on the surface of polyamide membrane by electrostatic assembly can make the membrane surface positively charged and slow down the deposition of cationic pollutants on the surface. In addition to hydrophilic polymer macromolecules, small molecule modification reagents can also be selected to form a coating on the membrane surface through chemical reactions. For example, the characteristics of dopamine oxidation self-polymerization are used to soak the polyamide membrane in a dopamine hydrochloride solution under slightly alkaline conditions. Under the action of oxygen, dopamine undergoes oxidation self-polymerization to form a super-hydrophilic polydopamine coating on the membrane surface, thereby improving the membrane's anti-fouling properties. Due to the presence of the coating, the surface coating method will increase the mass transfer resistance, and the coating may gradually fall off during long-term use. Therefore, by chemical bonding, the modifier molecules are grafted to the membrane surface, which can greatly improve the stability of the modified membrane. As for the polyamide thin layer composite membrane, there are amino and carboxyl functional groups on the membrane surface. The covalent bond-based modification is mainly carried out around the amino and carboxyl groups. Such grafting methods include surface-initiated atom transfer radical polymerization (ATRP) method, plasma-modified grafting method, photo-initiated modified grafting, oxidant-initiated grafting method, Click chemical reaction method, and carboxyl activation reaction grafting method (EDC+NHS). Although these methods are effective, the modification conditions are relatively harsh, the modification process has many steps, and the equipment requirements are high. Or. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for modifying a polyamide thin-layer composite membrane with improved hydrophilicity, pollution resistance, antioxidant ability and interception ability.

[0005] The technical solution adopted by the present invention to solve the above technical problems is: a method for modifying a polyamide thin layer composite membrane, comprising the following steps:

[0006] Step 1, washing or soaking the polyamide thin layer composite membrane with a cleaning agent to remove impurities;

[0007] Step 2, adding a small molecule polyphenol compound and a penetration enhancer into water and adjusting the pH of the solution to 8-9 to obtain a modifier solution, wherein the mass concentration of the catechol compound in the modifier solution is 0.05-1%, and the mass concentration of the penetration enhancer is 1-10%;

[0008] Step 3: Place the polyamide thin-layer composite membrane obtained in step 1 in a filtration device, use the modifier solution as the feed solution, adjust the filtration pressure to 2-10 kg, filter for 10-60 minutes, and then rinse with pure water to obtain a modified polyamide thin-layer composite membrane.

[0009] Furthermore, the cleaning agent described in step 1 is pure water or an ethanol aqueous solution with a volume percentage of 5 to 10%.

[0010] Furthermore, the small molecule polyphenol compound described in step 2 is one or more of catechol, pyrogallol, 3,4,5-trihydroxybenzoic acid, 3,4,5-trihydroxybenzaldehyde, 4,5-dimethylcatechol, 3-methoxycatechol, 1,2,4-benzene and tannic acid.

[0011] Furthermore, the penetration enhancer in step 2 is one or more of ethanol, ethylene glycol, glycerol and polyoxyethylene ether. Furthermore, the polyamide thin layer composite membrane includes a polyamide nanofiltration membrane and a polyamide reverse osmosis membrane.

[0012] Compared with the prior art, the advantages of the present invention are as follows: the present invention provides an in-situ filtration modification method for a polyamide thin-layer composite membrane. The surface of the polyamide thin-layer composite membrane has unreacted primary amines and amide bonds (secondary amines). Catechol compounds can be converted into quinones with high reaction activity under oxidative conditions. Coupled with intermolecular hydrogen bonds and π-π effects, the catechol compounds will undergo self-polymerization, and the quinone and the amino group can undergo Michael addition or Schiff base reaction. Generally, the thickness of the polyamide thin-layer composite membrane is within 200nm, and the coating and grafting modification of the hydrophilic macromolecules are mainly on the surface. The method of the present invention is based on the "phenol-amine" chemical reaction, and uses a polyphenol small molecule compound containing a catechol group to modify the polyamide thin-layer composite membrane. Under the promotion of the permeation agent, the small molecule modifier is embedded in the pore size of the thin-layer composite membrane by in-situ filtration, which plays a role in increasing hydrophilicity and further reducing the pore size. After modification by the method of the present invention, the hydrophilicity, pollution resistance, antioxidant ability, and interception ability of the polyamide thin-layer composite membrane are all improved. DETAILED DESCRIPTION

[0013] The present invention is further described in detail below with reference to the embodiments.

[0014] Example 1

[0015] The polyamide reverse osmosis membrane was soaked and cleaned in pure water to remove residual impurities in the membrane. A 0.1% mass concentration of catechol solution was used as the feed solution (10wt% ethanol was added as a permeation enhancer), and the pH was adjusted to 8.5. The operating pressure of the filtration system was adjusted to 6 kg, and the produced water and concentrated water were refluxed to the feed tank. The filtration modification lasted for 30 minutes. After that, the modifier solution was replaced with pure water to rinse the membrane. The performance of the membrane before and after modification was tested with a 2g / L NaCl solution. The water flux of the membrane before modification was 7.5Lm-2h-1bar-1, and the retention rate of NaCl was 92.5%. The water flux of the modified membrane was 5.5Lm-2h-1bar-1, and the retention rate of NaCL was 97.7%. The water contact angle of the membrane before modification was 42°, and the water contact angle of the membrane after modification was 35°, which was slightly reduced, indicating that the hydrophilicity was improved.

[0016] Using 1g / L bovine serum albumin solution for anti-pollution test, the modified membrane flux decreased slowly compared with the unmodified membrane. After 2h of pollution, the membrane was cleaned in situ with pure water for 20min. The pure water flux recovery rate of the modified membrane was 90%, while the flux recovery rate of the unmodified membrane was only 76%, indicating that the pollution resistance was enhanced. Sodium hypochlorite (0.1wt% of effective chlorine) was added to the NaCl feed solution. After 2h of filtration, the NaCl retention rate of the unmodified membrane dropped from the initial 92.5% to 78%, a decrease of about 15%. The flux dropped to 40% of the initial flux. The NaCl retention rate of the modified membrane only dropped by 4%, from the initial 97.7% to 94%, and the flux dropped to 60% of the initial flux. The results show that the chlorine resistance of the modified membrane has been improved to a certain extent.

[0017] Example 2

[0018] The membrane components of the waste polyamide reverse osmosis membrane (measured water flux of 3.2Lm-2h-1bar-1, NaCl retention of 85%) in the coking wastewater treatment project were planed open, the membrane sheets were cut off, and the membrane sheets were immersed in chemical cleaning to remove surface dirt.

[0019] The performance of the cleaned membrane was tested with a 2g / L NaCl solution under a pressure of 10 kg. The water flux and NaCl retention rate were 4.5Lm-2h-1bar-1 and 93.3%, respectively. A 0.1wt% tannic acid solution was used as the feed solution (10%wt ethanol was added as a permeation enhancer), the operating pressure of the filtration system was adjusted to 6 kg, the produced water and concentrated water were refluxed to the feed tank, and the filtration modification lasted for 30 minutes. After that, the modifier solution was replaced with pure water and the membrane was rinsed. After that, the performance of the modified membrane was tested with a 2g / L NaCl solution. The water flux was 3.6Lm-2h-1bar-1, and the retention rate of NaCl was 97.5%.

[0020] Example 3

[0021] The 600G reverse osmosis membrane (3013 type membrane assembly) in the household water purifier has a desalination rate of 84% for tap water after two years of use (calculated based on the conductivity of tap water and the conductivity of produced water). The performance of the membrane has declined. The method of the present invention is used to repair it. A 0.1wt% pyrogallol solution is used as the feed liquid (10wt% ethanol is added as a permeation enhancer) and the pH is adjusted to 8.5. The operating pressure of the filtration system is adjusted to 6 kg, and the produced water and concentrated water are refluxed to the feed, and the filtration modification lasts for 60 minutes. Before the modification, the produced water conductivity of the reverse osmosis membrane used for tap water purification was 52μS / cm. After the modification, the produced water conductivity dropped to 24μS / cm, and the desalination rate was 92.5%. There was no obvious change in the produced water flux of the membrane.

[0022] The above method can be used for modifying polyamide thin-layer composite membranes, and can also be used for repairing waste polyamide thin-layer composite membranes. When used for repairing polyamide thin-layer composite membranes, the old membranes need to be chemically cleaned to remove pollutants on the membrane surface.

[0023] The above description is not intended to limit the present invention, and the present invention is not limited to the above examples. Any changes, modifications, additions or substitutions made by a person skilled in the art within the spirit and scope of the present invention shall also fall within the protection scope of the present invention.

Claims

1. A method for modifying a polyamide thin layer composite membrane, characterized in that The following steps are involved: Step 1, washing or soaking the polyamide thin layer composite membrane with a cleaning agent to remove impurities; Step 2, adding a polyphenol small molecule compound and a penetration enhancer into water and adjusting the solution pH to 8-9 to obtain a modifier solution, wherein the mass concentration of the catechol compound in the modifier solution is 0.05-1%, and the mass concentration of the penetration enhancer is 1-10%; Step 3: Place the polyamide thin-layer composite membrane obtained in step 1 in a filtration device, use the modifier solution as the feed solution, adjust the filtration pressure to 2-10 kg, filter for 10-60 minutes, and then rinse with pure water to obtain a modified polyamide thin-layer composite membrane.

2. A polyamide thin layer composite membrane modification method according to claim 1, characterized in that: The cleaning agent described in step 1 is pure water or an ethanol aqueous solution with a volume percentage of 5 to 10%.

3. A polyamide thin layer composite membrane modification method according to claim 1, characterized in that: The polyphenol small molecule compound described in step 2 is one or more of catechol, pyrogallol, 3,4,5-trihydroxybenzoic acid, 3,4,5-trihydroxybenzaldehyde, 4,5-dimethylcatechol, 3-methoxycatechol, 1,2,4-benzene and tannic acid.

4. A polyamide thin layer composite membrane modification method according to claim 1, characterized in that: The penetration enhancer described in step 2 is one or more of ethanol, ethylene glycol, glycerol and polyoxyethylene ether.

5. A method for modifying a polyamide thin layer composite membrane according to any one of claims 1 to 4, characterized in that: Polyamide thin layer composite membranes include polyamide nanofiltration membranes and polyamide reverse osmosis membranes.