A method for fractionating and analyzing the irreversible fouling of micro / ultrafiltration membranes

Abstract

The application provides a kind of hierarchical extraction and analysis method of unrecoverable pollution in contaminated micro / ultrafiltration membrane, belongs to the field of membrane pollution analysis. Including the following steps: (1) pre-cleaning of contaminated micro / ultrafiltration membrane;(2) after the contaminated membrane is dried after pre-cleaning, immerse in the first solvent extraction liquid, make the membrane and the extraction liquid fully contact, after treatment, solid-liquid separation obtains first extraction liquid;(3) the second solvent extraction liquid is added to the membrane sample treated in step (2), the membrane sample is completely dissolved, and the second extraction liquid is obtained;(4) purification and analysis of extraction liquid: add the first extraction liquid or the second extraction liquid drop by drop to 5-10 times volume of pure water, centrifugal filtration solid after phase inversion to obtain purified extraction liquid, and analyze the physicochemical properties of the extraction liquid. The application extracts the unrecoverable pollution in the membrane under different temperature conditions and solvent concentration gradients by introducing polar organic solvent.

Description

Technical Field

[0001] This invention relates to the field of membrane fouling analysis, and in particular to a method for the graded extraction and analysis of irreversible fouling in fouled micro / ultrafiltration membranes. Background Technology

[0002] One of the main technical obstacles to the long-term stable operation of membrane separation processes such as microfiltration and ultrafiltration is membrane fouling. As the operating time of the membrane system increases, membrane fouling inevitably occurs due to the adsorption and accumulation of pollutants in the water on the membrane surface / pores, leading to a decrease in membrane flux, an increase in operating pressure, and an increase in system energy consumption.

[0003] Based on the adhesion between contaminants and membrane modules and the ease of removal, membrane fouling can be classified into three types: membrane contaminants that can be removed by physical cleaning are called reversible fouling; those that can only be removed by conventional chemical cleaning are called irreversible fouling; and the remaining contaminants that cannot be removed by conventional chemical cleaning are called irreversible fouling. The paper "Membranecleaning in membrane bioreactors: A Review" (Wang et al., Journal of Membrane Science, 2014, 468: 276–307) points out that once the membrane is fouled during long-term operation of a membrane bioreactor, the flux cannot be completely restored to its initial state.

[0004] The accumulation of irreversible fouling is a major cause of membranes failing to meet final permeate yield standards and ultimately reaching the end of their lifespan. However, current membrane fouling component analysis mainly focuses on reversible and irreversible fouling, lacking extraction and component analysis methods for irreversible fouling. Technicians have very little knowledge of the composition and properties of irreversible fouling, which severely restricts the development of efficient cleaning technologies for irreversible fouling and end-of-life membrane regeneration technologies.

[0005] Conventional methods for analyzing reversible and irreversible membrane fouling components often employ a one-step immersion process using acid, alkali, and deionized water to transfer membrane contaminants from the membrane to the liquid phase. The contaminants are then analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES), three-dimensional fluorescence spectroscopy, and liquid-phase organic carbon detection. However, these acid, alkali, and deionized water extraction methods have very limited effectiveness in extracting irreversible fouling.

[0006] In the early stages of this invention's research, our team, based on Hansen's solubility parameter theory, discovered that polar organic solvents can remove most of the irreversible fouling from ultrafiltration membranes at the end of their lifespan. Simultaneously, the team noted that polar organic solvents are also solvents used in the preparation of some polymer membranes (polyvinylidene fluoride, polyethersulfone, etc.); in other words, under anhydrous conditions, some polar organic solvents can dissolve polymer membranes. However, due to the varying compatibility between different polymer materials and polar organic solvents, it is necessary to select appropriate solvents specifically to achieve partial and complete dissolution of the membrane under relatively mild conditions, thereby enabling controlled, fractional extraction of irreversible fouling.

[0007] Therefore, our team needs to study the most suitable solvents and methods to achieve gradient extraction and analysis of membranes with irreversible fouling. Summary of the Invention

[0008] To address the aforementioned problems in existing technologies, this invention provides a method for the graded extraction and analysis of irreversible contaminants in fouled micro / ultrafiltration membranes. This invention utilizes a polar organic solvent to grade and extract irreversible contaminants within the membrane under different temperature conditions and solvent concentration gradients.

[0009] The technical solution of the present invention is as follows:

[0010] A method for the fractional extraction and analysis of irreversible contamination in contaminated micro / ultrafiltration membranes, comprising the following steps:

[0011] (1) Pre-cleaning of fouled micro / ultrafiltration membranes: Reversible fouling on the surface of the membrane to be extracted is removed by surface rinsing, and then irreversible fouling on the membrane surface and inside the membrane pores is removed by immersion cleaning.

[0012] (2) Extraction of primary irreversible contamination: After drying the pre-cleaned contaminated membrane at 40-50℃, it is immersed in the first solvent extract and placed in a constant temperature shaking incubator at 25-30℃ for 1-5 hours at a speed of 50-150 rpm to ensure that the membrane and the extract are in full contact. After the treatment is completed, solid-liquid separation is performed to obtain the primary extract.

[0013] (3) Extraction of secondary irreversible contamination: Add the second solvent extract to the membrane sample treated in step (2) and place it in a constant temperature shaking incubator at 50-80℃ for 1-5 hours at a speed of 50-150 rpm until the membrane sample is completely dissolved to obtain the secondary extract;

[0014] (4) Purification and analysis of the extract: The primary or secondary extract is added dropwise to 5-10 times the volume of pure water. After the polymer undergoes phase inversion in the pure water for 12-24 hours, the solid is centrifuged and filtered to obtain the purified extract. The physicochemical properties of the extract are then analyzed.

[0015] Preferably, the component form of the contaminated membrane to be extracted in step (1) includes flat sheet membrane, hollow fiber membrane and spiral wound membrane.

[0016] Preferably, the material of the contaminated membrane to be extracted in step (1) includes polyvinylidene fluoride (PVDF), polyethersulfone (PES), and polysulfone (PS).

[0017] Preferably, the soaking and cleaning method in step (1) is as follows: first, soak and clean with a sodium hypochlorite solution containing 0.2-0.5 wt% NaOH and a concentration of 0.2-1.0 wt% for 12-24 hours, and then soak and clean with citric acid solution containing 0.2-0.5% for 12-24 hours.

[0018] Preferably, the first solvent extract in step (2) is an aqueous solution of one of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide or triethyl phosphate with a volume fraction of 80-95%.

[0019] Preferably, the second solvent extract in step (3) is a pure solution of one of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, or triethyl phosphate.

[0020] Preferably, before adding the second solvent extract to the membrane sample treated in step (2) in step (3), the membrane sample needs to be dried.

[0021] Preferably, the primary or secondary extract in step (4) needs to be added drop by drop to pure water to avoid aggregation.

[0022] Preferably, the analysis of the physicochemical properties in step (4) includes the analysis of contaminants in the purified extract using inductively coupled plasma atomic emission spectrometry, three-dimensional fluorescence spectroscopy, gel chromatography, and liquid chromatography-organic carbon detection.

[0023] The beneficial technical effects of this invention are as follows:

[0024] 1. Existing analyses of membrane fouling components mainly focus on reversible and irreversible fouling, and there is a lack of extraction and component analysis methods for irreversible fouling. The method for graded extraction and analysis of irreversible fouling in fouled micro / ultrafiltration membranes provided by this invention fills the gap in the extraction methods for irreversible fouling and provides strong support for targeted control of membrane fouling and selection of pretreatment technologies.

[0025] 2. This invention first employs hydraulic cleaning and chemical cleaning to remove reversible and irreversible fouling from the membrane surface and pores, respectively. Then, a polar organic solvent is introduced, and the interaction between the solvent and the membrane is controlled by adjusting the membrane's water content to extract irreversible fouling in stages. For irreversible fouling with relatively weak adhesion, extraction is performed under relatively mild wet membrane conditions; while for irreversible fouling with strong adhesion and tightly bound to the membrane, membrane dissolution and deep extraction analysis of the contaminants are performed under dry membrane conditions.

[0026] 3. This invention uses polar organic solvents to achieve controlled stepwise leaching of irreversible contamination, while using phase inversion to remove dissolved polymers from the extract, resulting in a purified irreversible contamination stepwise leachate. The operation is simple and enables the extraction and analysis of irreversible contamination in contaminated membranes, which helps to deepen the academic and industrial understanding of irreversible membrane contamination components.

[0027] 4. The method for graded extraction and analysis of irreversible contaminants in fouled micro / ultrafiltration membranes proposed in this invention is based on Hansen's solubility parameter theory and membrane preparation theory. It innovatively introduces polar organic solvents into membrane fouling analysis and controls the interaction between polar organic solvents and membranes by controlling the membrane water content, thereby achieving graded extraction of irreversible contaminants within the membrane.

[0028] 5. The method for graded extraction and analysis of irreversible contaminants in fouled micro / ultrafiltration membranes proposed in this invention is simple to operate, requires no special equipment, and has a wide range of applications. It represents a significant technological breakthrough in the field of membrane fouling component analysis and can effectively promote the research and development of efficient cleaning technology for irreversible contaminants and end-of-life membrane regeneration technology. Attached Figure Description

[0029] Figure 1 This is a comparison chart of the inorganic element content in the reversible and primary / secondary non-reversible contamination membranes of Example 1.

[0030] Figure 2 The image shows the three-dimensional fluorescence spectra of the primary and secondary extracts purified from the contaminated membrane in Example 1.

[0031] Figure 3 The image shows the three-dimensional fluorescence spectra of the purified primary and secondary extracts from the contaminated membrane in Example 2.

[0032] Figure 4 The image shows the three-dimensional fluorescence spectra of the purified primary and secondary extracts from the contaminated membrane in Example 3.

[0033] Figure 5 The image shows the three-dimensional fluorescence spectra of the purified primary and secondary extracts from the contaminated membrane in Example 4. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0035] Example 1:

[0036] This embodiment provides a method for the graded extraction and analysis of irreversible contaminants in contaminated micro / ultrafiltration membranes. The specific steps are as follows:

[0037] (1) Take a certain amount of PVDF hollow fiber fouling membrane, remove the reversible fouling on the surface by surface rinsing, then soak it in a sodium hypochlorite solution containing 0.5wt% NaOH for 24h, and then soak it in citric acid solution containing 0.5wt% for 24h to remove the irreversible fouling on the membrane surface and inside the membrane pores.

[0038] (2) After drying the pre-cleaned contaminated membrane at 40°C, it is immersed in the first solvent extraction solution (volume ratio: N,N-dimethylacetamide: water = 9:1) and placed in a constant temperature shaking incubator at 25°C for 4 hours at a speed of 100 rpm to ensure that the membrane and the extraction solution are in full contact for primary irreversible contamination extraction.

[0039] (3) After drying the membrane sample treated in the above steps at 40°C, add the second solvent extract N,N-dimethylacetamide and place it in a constant temperature shaking incubator at 60°C for 4 hours at a speed of 100 rpm. The membrane sample is completely dissolved and secondary irreversible contamination extraction is performed.

[0040] (4) Take 5 mL of primary and secondary extracts respectively and add them dropwise to 30 mL of pure water. After the PVDF polymer has undergone phase inversion in pure water for 24 h, centrifuge and filter the solids to obtain purified primary and secondary extracts.

[0041] Example 2:

[0042] This embodiment provides a method for the graded extraction and analysis of irreversible contaminants in contaminated micro / ultrafiltration membranes. The specific steps are as follows:

[0043] (1) Take a certain amount of PVDF hollow fiber fouling membrane, remove the reversible fouling on the surface by surface rinsing, then soak it in a sodium hypochlorite solution containing 0.2wt% NaOH and 0.5wt% sodium hypochlorite for 24h, and then soak it in citric acid solution with 1.0wt% concentration for 12h to remove the irreversible fouling on the membrane surface and inside the membrane pores.

[0044] (2) After drying the pre-cleaned contaminated membrane at 45°C, it is immersed in the first solvent extraction solution (volume ratio: N-methylpyrrolidone: water = 9:1) and placed in a constant temperature shaking incubator at 25°C for 4 hours at a speed of 100 rpm to ensure that the membrane and the extraction solution are in full contact for primary irreversible contamination extraction.

[0045] (3) After drying the membrane sample treated in the above steps at 40°C, add the second solvent extract N-methylpyrrolidone and place it in a constant temperature shaking incubator at 50°C for 5 hours at a speed of 120 rpm. The membrane sample is completely dissolved and then the secondary irreversible contamination is extracted.

[0046] (4) Take 5 mL of primary and secondary extracts respectively and add them dropwise to 30 mL of pure water. After the PVDF polymer has undergone phase inversion in pure water for 18 h, centrifuge and filter the solids to obtain purified primary and secondary extracts.

[0047] Example 3:

[0048] This embodiment provides a method for the graded extraction and analysis of irreversible contaminants in contaminated micro / ultrafiltration membranes. The specific steps are as follows:

[0049] (1) Take a certain amount of PVDF hollow fiber fouling membrane, remove the reversible fouling on the surface by surface rinsing, then soak it in a sodium hypochlorite solution containing 0.3wt% NaOH and 0.5wt% sodium hypochlorite for 20h, and then soak it in citric acid solution containing 0.7wt% for 20h to remove the irreversible fouling on the membrane surface and inside the membrane pores.

[0050] (2) After drying the pre-cleaned contaminated membrane at 50°C, it is immersed in the first solvent extraction solution (volume ratio: dimethyl sulfoxide: water = 9:1) and placed in a constant temperature shaking incubator at 30°C for 2 hours at a speed of 100 rpm to ensure that the membrane and the extraction solution are in full contact and to carry out the extraction of primary irreversible contamination.

[0051] (3) After drying the membrane sample treated in the above steps at 40°C, add the second solvent extract dimethyl sulfoxide and place it in a constant temperature shaking incubator at 70°C for 3 hours at a speed of 90 rpm. The membrane sample is completely dissolved and then extracted for secondary irreversible contamination.

[0052] (4) Take 5 mL of primary and secondary extracts respectively and add them dropwise to 30 mL of pure water. After the PVDF polymer has undergone phase inversion in pure water for 20 h, centrifuge and filter the solids to obtain purified primary and secondary extracts.

[0053] Example 4:

[0054] This embodiment provides a method for the graded extraction and analysis of irreversible contaminants in contaminated micro / ultrafiltration membranes. The specific steps are as follows:

[0055] (1) Take a certain amount of PVDF hollow fiber fouling membrane, remove the reversible fouling on the surface by surface rinsing, then soak it in a sodium hypochlorite solution containing 0.5wt% NaOH for 12h, and then soak it in citric acid solution containing 1.0wt% for 24h to remove the irreversible fouling on the membrane surface and inside the membrane pores.

[0056] (2) After drying the pre-cleaned contaminated membrane at 40°C, it is immersed in the first solvent extraction solution (volume ratio: triethyl phosphate: water = 9:1) and placed in a constant temperature shaking incubator at 30°C for 3 hours at a speed of 100 rpm to ensure that the membrane and the extraction solution are in full contact and to carry out the extraction of primary irreversible contamination.

[0057] (3) After drying the membrane sample treated in the above steps at 40°C, add the second solvent extractant triethyl phosphate and place it in an 80°C constant temperature shaking incubator for 2 hours at 60 rpm. The membrane sample is completely dissolved and then subjected to secondary irreversible contamination extraction.

[0058] (4) Take 5 mL of primary and secondary extracts respectively and add them dropwise to 30 mL of pure water. After the PVDF polymer has undergone phase inversion in pure water for 24 h, centrifuge and filter the solids to obtain purified primary and secondary extracts.

[0059] Test Example 1:

[0060] The specific methods for characterizing inorganic contamination in the extract are as follows:

[0061] After acid digestion of the chemical cleaning solution obtained in Example 1, the purified primary and secondary extracts, and other solutions, the contents of five inorganic elements (calcium, magnesium, aluminum, iron, and silicon) were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). The results are as follows: Figure 1 As shown.

[0062] As can be seen from the figure, compared to irreversible fouling, the content of Fe and Si contaminants in irreversible fouling is significantly increased, indicating that Fe and Si contaminants have become enriched. Therefore, for the fouled membrane in Example 1, Fe and Si are prone to enrichment, becoming the main inorganic components of irreversible fouling. Targeted removal of Fe and Si can be considered to alleviate the accumulation of irreversible fouling in the membrane, thereby extending the membrane's service life.

[0063] Test Example 2:

[0064] The organic contamination characterization of the irreversibly contaminated extracts in Examples 1-4 is performed using the following methods:

[0065] The purified primary and secondary extracts were characterized by three-dimensional fluorescence spectroscopy. After eliminating Rayleigh scattering, the three-dimensional fluorescence spectra of the primary and secondary irreversible contamination in Examples 1-4 were obtained (e.g., Figure 2-5 (As shown).

[0066] As shown in the figure, primary irreversible contamination is mainly humic acid, while secondary irreversible contamination, in addition to humic acid, also contains small amounts of soluble microbial products and proteins. Therefore, for the contaminated membranes in Examples 1-4, humic acid is the main component of irreversible contamination. Targeted removal of humic acid can be considered to alleviate the accumulation of irreversible contamination in the membrane, thereby extending the membrane's service life.

[0067] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, and for those of ordinary skill in the art, various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, the present invention is not limited to the specific details without departing from the general concept defined by the claims and their equivalents.

Claims

1. A method for the graded extraction and analysis of irreversible contaminants in contaminated micro / ultrafiltration membranes, characterized in that, Includes the following steps: (1) Pre-cleaning of fouled micro / ultrafiltration membranes: The reversible fouling on the surface of the membrane to be extracted is removed by surface rinsing, and then the irreversible fouling on the membrane surface and inside the membrane pores is removed by immersion cleaning. (2) Extraction of primary irreversible contamination: After drying the pre-cleaned contaminated membrane at 40-50°C, it is immersed in the first solvent extract and placed in a constant temperature shaking incubator at 25-30°C for 1-5 h at a speed of 50-150 rpm to ensure that the membrane and the extract are in full contact. After the treatment is completed, the solid and liquid are separated to obtain the primary extract. (3) Extraction of secondary irreversible contamination: Add the second solvent extract to the membrane sample treated in step (2) and place it in a constant temperature shaking incubator at 50-80°C for 1-5 h at a speed of 50-150 rpm until the membrane sample is completely dissolved to obtain the secondary extract; (4) Purification and analysis of the extract: The primary or secondary extract is added dropwise to 5-10 times the volume of pure water. After the polymer undergoes phase inversion in pure water for 12-24 h, the solid is centrifuged and filtered to obtain the purified extract. The physicochemical properties of the extract are then analyzed. The soaking and cleaning method described in step (1) is as follows: first, soak and clean with a sodium hypochlorite solution containing 0.2-0.5 wt% NaOH and a concentration of 0.2-1.0 wt% for 12-24 h, and then soak and clean with citric acid solution containing 0.2-0.5% for 12-24 h; The first solvent extract in step (2) refers to an aqueous solution with a volume fraction of 90% of one of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide or triethyl phosphate. The second solvent extract in step (3) is the pure solvent in the first solvent extract.

2. The method according to claim 1, characterized in that, The components of the contaminated membrane to be extracted in step (1) include flat sheet membranes, hollow fiber membranes, and spiral wound membranes.

3. The method according to claim 1, characterized in that, The materials of the contaminated membrane to be extracted in step (1) include polyvinylidene fluoride (PVDF), polyethersulfone (PES), and polysulfone (PS).

4. The method according to claim 1, characterized in that, Before adding the second solvent extract to the membrane sample treated in step (2) in step (3), the membrane sample needs to be dried.

5. The method according to claim 1, characterized in that, The primary or secondary extract described in step (4) needs to be added drop by drop to pure water to avoid aggregation.

6. The method according to claim 1, characterized in that, The analysis of physicochemical properties described in step (4) includes the analysis of contaminants in the purified extract using inductively coupled plasma atomic emission spectrometry, three-dimensional fluorescence spectroscopy, gel chromatography, and liquid chromatography-organic carbon detection.

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