Manufacturing efficient and economical membrane microfilters for separating suspended particles in solution

Abstract

In this invention, a practical, simple and economical method for synthesizing a new type of polyethersulfone polymer membrane with desirable surface properties and high efficiency at low pressure was introduced for use in the manufacture of membrane microfilters. The prepared membrane was characterized by SEM, AFM, porosity and pore radius size distribution analyses. Low roughness and high porosity of the membrane and the membrane with a uniform radius of 0.2±0.01 μm were obtained through the analysis of the aforementioned analyses. Also, the new membrane, with a porosity of 92.4±2.1%, provided a high pure water flux (14,143 L / m2.h) at low pressure, which was not significantly different from the control membrane (a commercial membrane with similar characteristics) (14,331 L / m2.h). In order to evaluate the membrane performance, two compounds of bovine serum albumin (BSA) and pepsin enzyme were filtered for separation from both the new and control membranes. The results showed 100% pepsin and 73% bovine serum albumin removal for the new membrane, compared to 94% and 65% for the control membrane, respectively. On the other hand, filtering the pepsin enzyme solution in 5 continuous cycles and observing a more stable trend of the new membrane (4.8%) compared to the control membrane (14.4%) confirmed the mechanical and functional stability of the prepared membrane. Therefore, this new membrane prepared with an efficient and economical method compared to imported commercial membranes with similar properties can be used as a suitable option for sample preparation in quality control laboratories of various industries.

Description

Description of the invention Title of the invention Manufacturing efficient and economical membrane microfilters for separating suspended particles in solution Technical background of the relevant invention The invention relates to membrane filtration and separation processes. Technical problem and stating the objectives of the invention Membrane filters produced domestically have no or negligible flux at low pressures due to their low porosity. For this reason, it is not possible to use these membranes in microfiltration processes at low pressures. The purpose of this invention is to manufacture a polyethersulfone polymer membrane with high flux capability at low pressures for the preparation of membrane microfilters for the removal of micron-sized particles from solutions in quality control laboratories. A description of the state of the prior art and the history of developments related to the claimed invention. In numerous studies, various pore-forming compounds have been used to fabricate membranes with high porosity and performance for the removal of soluble and insoluble pollutants from water and wastewater. In patent number 98319 in 2017, Shokri et al. (application number 139650140003009171) studied polysulfone membranes for the separation of mineral pollutants at low pressure, which gave a new method for preparing this type of membranes by the adsorption membrane method. In this work, polyethylene glycol polymer (PEG-600) was used to create pores, and mineral clay particles modified with the amino acids arginine and lysine were also used in the manufacture of adsorption membranes, which caused permeation and improved efficiency. The polysulfone membrane manufactured in this study presented better separation behavior in performance than other membranes that were used in the separation of the heavy metal arsenic. On the other hand, efforts to effectively utilize polyethersulfone polymer in recent filtration process research have been developed. As in patent number US10456754B2, in 2019, Hogen-Esch and colleagues prepared graphene oxide modified polymer membranes for water purification. In order to improve hydrophilicity and enhance the performance of the polymer membrane, it was modified with graphene oxide nanomaterials. The synthesized membranes had several advantages such as super hydrophilicity, high flux, high rejection rate and excellent antifouling ability. Also, in patent number 98734 in 2018, Olad et al. (Application number 139750140003002258), introduced a modified polyethersulfone membrane that had advantages such as super hydrophilicity and improved morphology. The modified polyethersulfone membrane showed 100% algal color rejection along with 92.07% COD removal with complete turbidity removal. However, the modified polymer membranes did not have the high porosity and pore radius size as well as the high throughput capability to be applied in the ultra-low pressure membrane processes that are considered in the membrane filtration process. In fact, many previously reported membrane preparation methods have been limited to specific morphologies that have used several different and expensive compounds to modify the properties of the polymer membrane. Hence, numerous preparation methods and pore formers have been developed in research. In 2015, Samuel C. Hess and colleagues reported a novel solvent evaporation-based process that uses particle-stabilized continuous emulsions to form previously unattainable membrane morphologies. Their fabricated membrane, consisting of a solution of polyethersulfone / dimethylacetamide (PES / DMAc), glycerol, and ZnO nanoparticles, allows for simple and precise tuning of pore diameters from 20 nm to a maximum of 100 nm. In addition to structural analysis, water flux rates of over 5600 L / m2.h were reported for the prepared membranes. In another study conducted in 2017 by Adrian Kaiser and colleagues, they reported a solvent evaporation method for preparing cellulose acetate polymer membranes. This method used a three-step fabrication process using calcium carbonate and glycerol to create pores and high porosity in the membrane. Scanning electron microscope (SEM) images showed that the removal of calcium carbonate particles using hydrochloric acid created pores with a diameter of approximately 1–5 μm. The membrane filtration was then qualitatively investigated by filtering solutions containing watercolor pigments and food coloring. Comparison of the filtration results showed that the insoluble watercolor pigments were too large to pass through the membrane pores and were successfully removed by the membrane, while the food coloring was completely soluble in water and passed through the membrane easily. Providing a solution to an existing technical problem along with an accurate, sufficient, and integrated description of the invention In this invention, by using calcium carbonate as a pore-forming agent alongside glycerol, in addition to increasing the porosity of the membrane, the flexibility of the membrane is also improved. 1- Preparation of polyethersulfone membranes 1.1- Preparation of polymer solution The membrane polymer solution was prepared by mixing 6.87 wt% of polyethersulfone polymer with 69.64 wt% of N,N-dimethylacetamide (DMAc) solvent. The resulting mixture was mixed for 2 hours using a magnetic stirrer until the polymer was completely dissolved and a clear solution was obtained. In the next step, 17.38 wt% of calcium carbonate and 6.11 wt% of glycerol were added to the above solution under vigorous stirring and ultrasonicated for 30 minutes to make fine calcium carbonate particles and completely homogenous suspension. After stirring at room temperature for 24 hours, the resulting suspension was ultrasonicated for 15 minutes to remove bubbles. After that, some of the mixture was cast onto a flat glass plate using a 150 μm thick film spreader. The plate coated with the casting solution, the membrane films were partially dried at 60 °C for 5 min. 1.2- Removal of calcium carbonate particles The glass plate containing the membrane sheet was immersed in a hydrochloric acid bath (0.24 M) to cause an exchange between the solvent (DMAc) and the non-solvent (water) in addition to removing the calcium carbonate particles. The membrane sheet detached itself from the mirror plate and gas bubbles indicated the removal of the calcium carbonate particles. After 5 minutes, the membrane sheet was transferred to a distilled water bath and kept in distilled water for 24 hours to complete the washing and phase inversion and to completely wash away the calcium carbonate and residual DMAc solvent. Finally, the membrane sheet was pulled out of the water bath and dried between filter paper for 24 hours. It should be noted that in order to achieve a membrane with desirable characteristics and performance, all parameters, including the percentage composition of the casting solution, ultrasonic time, solvent evaporation time, and membrane thickness, have been carefully optimized through the manufacture of multiple membranes, identification of their characteristics, and evaluation of their performance. 1.3- Identification methods The surface and cross-sectional morphology of the membranes were characterized by transmission electron microscopy (SEM, TESCAN, MIRA III). The surface roughness parameters of the membranes, including Sa, Sy and Sq, which represent the average roughness, the average difference between the highest and lowest points of the pores and the root mean square of the data y, were obtained using atomic force microscopy (AFM, Nanosurf ® MobileS) and the results are presented in Table (1). The membrane porosity (Ɛ) was determined according to the gravimetric method in the following equation: Ɛ = Where, mw is the weight of the membrane in the wet state and md is the dry weight of the membrane. ρ (g / cm3) is the density of water, A (m2) is the surface area and L (m) is the thickness of the membrane. The average pore radius (rm) of the membrane was also predicted through the Guerout-Elford-Ferry equation: rm= Where η and Q are the viscosity of water and the volume of permeated water per unit time (m3 / s), respectively. ΔP is the operating pressure. In addition, in order to minimize experimental errors, the pore size distribution of the membrane surface was examined using NIH Image J analysis software on SEM images of the membrane surface and the results are presented in Table (2). 2- Evaluation of membrane performance In order to evaluate the performance of the prepared membranes, all the tests performed for the commercial membrane used with a pore diameter of 0.45 μm were also examined and compared with the prepared membrane. The performance tests of the membranes were performed using a vacuum filtration system consisting of a vacuum pump, a vacuum Erlenmeyer flask and a Buchner funnel, the schematic of which is presented in Figure (3). The flux (Flux, L / m2.h) was measured by filtering 100 mL of distilled water through the membrane with a cross-sectional area of ​​12.56 cm2 and measuring the filtration time. The following equation was used to calculate the pure water flux: Flux = Where M is the volume of distilled water flux (L), A is the effective membrane area (m2) and t is the filtration time (h). The separation performance of the membranes was investigated by filtration of bovine serum albumin (BSA, 1000ppm), pepsin enzyme (Pepsin, 100ppm) in the mentioned vacuum filtration system. At least three replicate experiments were performed for all filtration studies and the average of the measured values ​​is reported. The following equation was used to calculate the recovery rate (R): Where Cp and Cf represent the concentrations of the above mentioned substances in the permeate and feed solutions, respectively. The concentration of the solutions was measured using a UV-Vis spectrophotometer (Hach DR 5000) at a specific wavelength (the maximum wavelengths for albumin and pepsin are 280 and 283 nm, respectively). In order to further evaluate the membrane performance, reusability was assessed by passing the pepsin enzyme solution through the membrane in 5 consecutive cycles and examining the recovery rate in each cycle. Explanation of shapes, maps and diagrams 1- Membrane identification The structural features of the prepared membrane were investigated by cross-sectional and top-surface SEM images presented in Fig. (1). The cross-sectional SEM image shows an asymmetric structure consisting of two parts: (1) a dense selective layer on the top surface that determines the membrane resistance and permeability and (2) a porous substrate with finger-like structures that provide mechanical strength. In addition, high porosity and many pores are observed in the membrane structure images, which are caused by the reaction of calcium carbonate with hydrochloric acid, which causes the removal of calcium carbonate particles and the release of carbon dioxide gas according to the following chemical equation: CaCO3(s) + 2H+(aq) Ca2+(aq) + H2O (aq) + CO2(g) As Hess and colleagues showed for a polyethersulfone (PES)-based membrane, the amount of glycerol and the amount and size of the pore-forming agent are key parameters in controlling the pore size and pore gradient along the membrane cross-section. In this work, we optimized the amounts of glycerol and calcium carbonate and finally used the best values ​​for the membrane preparation. As a result, many pores are formed in the membrane with respect to the amount of calcium carbonate. To achieve the desired pore size (resulting particle size), the mixing and ultrasonication step is important. This step is not only required to form a homogeneous particle dispersion in the polymer solution, but also reduces the particle size, leading to the formation of very fine pores and high membrane porosity. The selected mixing time ensures good dispersion and a significant reduction in particle size and pore size. The homogeneous pore size as well as the asymmetric morphology of the membrane are evident in the cross-sectional images.The pore size increases from the top of the membrane to the bottom, which, in addition to the difference in the exchange rate between the non-solvent (water) and solvent (DMAc) phases and the speed of the membrane coagulation process, can be caused by the asymmetry of the drying process, where solvent evaporation occurs only on one side (air side) of the membrane, leading to a gradient of polymer solubility in the material. The 3D AFM images presented in Figure (2) were used to investigate the surface morphology of the membranes. The roughness parameters of the membranes, including the mean difference between the deepest pores and the highest points (Sy), the root mean square (Sq), and the average roughness (Sa), were analyzed using Nanosurf C3000 and the corresponding results are presented in Table (1). The images and roughness parameters indicate a smooth surface and low roughness of the prepared membrane. According to the literature review, the membrane with a rougher surface is more prone to fouling and the roughness of the membrane surface significantly contributes to membrane fouling because the trapping or adsorption of contaminants on the rough membrane surface will lead to further increase in membrane fouling and subsequently reduce membrane performance. The total porosity and average pore radius for the prepared membrane and the control membrane were studied using the water swelling method and the results are shown in Table (2). As can be seen, the porosity of the new membrane (92.4±2.1) was higher than that of the control membrane (89.1±1.8). Since the water swelling method may be associated with errors, in order to minimize experimental errors, the pore size distribution of the membrane surface was also investigated using the NIH Image J analysis software on the SEM images of the membrane surface and the results of both methods are presented in Table (2). It can be seen that the surface pore radius of the newly prepared membrane is generally distributed in the range of 0.2±0.01 μm. 2- Evaluation of membrane performance The first parameter to measure membrane performance in terms of its application is the flux through the membrane. Since the goal of membrane microfilters is fast and efficient filtration and separation, but compared to other membrane process applications, they do not have a significant driving force (low pressure), providing high flux while providing effective separation requires careful design of membrane characteristics, including porosity and membrane pore size. It is important to note that in addition to high performance, mechanical stability, which will be provided by the membrane thickness, is essential. The flux of the new membrane compared to the control membrane is presented in Table (2). As can be seen, the flux of the prepared membrane (14143 L / m2.h) is very close to the control membrane (14331 L / m2.h), indicating the optimal performance of the membrane for use in microfiltration processes. In order to investigate the separation performance of the new membrane, bovine serum albumin (BSA) and pepsin enzyme were passed through the membrane and the results are presented in Figure (4). As can be seen, the results indicate the maximum separation efficiency of the new membrane, i.e., removal of 100% pepsin and 73% bovine serum albumin. This is while the separation efficiency of the mentioned compounds was 94% and 65%, respectively, in the control membrane. Since the surface pore radius (0.2 μm) and porosity (92.4±2.1%) of the membrane were designed and optimized for separation and filtration applications at very low pressure, high flux and efficient separation were achieved. In order to investigate the potential of the novel membrane for industrial applications, the reproducibility of its performance in 5 continuous cycles of filtration of pepsin enzyme solution was evaluated and compared with the control membrane, and the results are presented in Figure (5). The results showed that the pepsin removal during five filtration cycles for the novel membrane was more stable compared to the control membrane. It is clear that the long-term filtration and separation is limited by membrane fouling. According to Figure (5), the pepsin removal decreased by 4.8% (100–95.6%) from cycle 1 to 5, which may be related to the clogging of the membrane pores due to recovery and reuse. The experiments described in this section were performed using the vacuum filtration system in Figure (3). A clear and precise statement of the advantages of the claimed invention over prior inventions. The prepared membrane has a higher porosity than previous membranes. Also, the flux through the membrane at low pressures is much higher than previous samples. The pore size of the synthesized membrane is controllable and uniform. The membrane surface is smooth (low roughness) and has good flexibility. The membrane manufacturing method is simple and inexpensive. Description of at least one implementation method for implementing the invention Using a simple and cost-effective method, a polyethersulfone membrane with high porosity, a defined and uniform pore radius, low roughness, and a flux comparable to similar imported samples was fabricated. The membrane stability was adequate and it showed a favorable performance compared to commercially available membranes under operating conditions (Figures 4 and 5). As a result, the membrane prepared using a cost-effective method has the potential for applications in the manufacture of membrane microfilters for the rapid and efficient separation of micron-sized particles from aqueous and organic samples. Explicit mention of the industrial application of the invention Given the importance and necessity of separating micron-sized particles from laboratory samples before analyzing the samples with analytical devices in quality control laboratories of various industries, including pharmaceuticals, chemicals, etc., membrane microfilters made from polyethersulfone polymer can be used for this application.

Claims

Claims What is claimed: Claim 1) A membrane microfilter with high flux at low pressures was prepared by adding calcium carbonate and glycerol to a polyether sulfone membrane matrix. Claim 2) The agents that create pores in the membrane microfilter of claim 1 are calcium carbonate and glycerol. Claim 3) The membrane microfilter referred to in claim 1 is an asymmetric polymer membrane made of polyethersulfone. Claim 4) The percentage of calcium carbonate and glycerol used to prepare the membrane microfilter of claim 1 is 17.38 and 11.6% by weight, respectively. Claim 5) The polymer concentration for making the membrane microfilter of claim number 1 is 87.6% by weight. Claim 6) The pore radius of the membrane microfilter mentioned in claim 1 was determined to be within the range of 0.2 ± 0.01 μm using SEM images. Claim 7) The porosity of the membrane microfilter in claim 1 was determined to be 92.4±1.2%. Claim 8) The flow rate of the membrane microfilter in claim 1 is equal to 14143 L / m2.h. Claim 9) The efficiency of removing bovine serum albumin and pepsin enzyme by the membrane microfilter in claim 1 is 73 and 100 percent, respectively. Claim 10) The efficiency of removing the pepsin enzyme by the membrane microfilter of claim number 1 is at least 95% after 5 consecutive uses.

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