Method for removing algae by coupling photocatalytic oxidation, enhanced coagulation and ultrafiltration membrane

Abstract

The application discloses a kind of photocatalytic oxidation enhanced coagulation coupling ultrafiltration membrane algae removal methods, by using cynao-C3N4 photocatalytic plastic enhanced coagulation coupling ultrafiltration membrane combined process to remove algae in water, cynao-C3N4 plastic material generates H2O2 under illumination and occurs fenton-like reaction with Fe2 (SO4) 3, while coagulation can greatly reduce the degree of cell breakage, effectively remove algae cell, while relieving ultrafiltration membrane pollution, protect the color of effluent, avoid causing secondary pollution;The application is simple, short cycle, cynao-C3N4 photocatalytic plastic is easy to add and recycle, can be repeatedly used, without additional dosing agent, good convenience, high adaptability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of water quality algae removal, and relates to a photocatalytic oxidation enhanced coagulation coupled ultrafiltration membrane algae removal method. BACKGROUND

[0002] Water resources are the first element for the survival of organisms. At present, due to water eutrophication, the phenomenon of large-scale reproduction of algae in lakes and reservoirs frequently occurs in summer. Algal explosion pollutes water bodies and causes a series of serious harmful consequences. Most water treatment plants in developing countries usually use coagulation-sedimentation-filtration as the core treatment process. Iron and aluminum salts are the most commonly used, low-cost and efficient coagulants. However, the diverse morphology of algal cells, low specific density and negatively charged surface can affect the coagulation effect. Although the removal rate of algae can be improved by increasing the amount of coagulant, the residual coagulant can cause secondary pollution to the water body, such as affecting the turbidity of effluent and causing a worldwide public health problem. At present, membrane technology is a safe water purification technology based on physical separation, which does not introduce new pollutants into the water. Among them, the ultrafiltration technology can effectively remove viruses, microorganisms and colloidal substances in water. With the improvement of membrane performance and the reduction of membrane manufacturing cost, the ultrafiltration technology has a broad application prospect. However, the application of membrane technology can cause serious membrane pollution problems, increasing the operation cost; and the ultrafiltration technology is difficult to remove dissolved organic matter, and pre-oxidation can cause a large number of algal cells to rupture, releasing intracellular disinfection by-product precursors, so that the disinfection by-product generation potential is high. In order to solve this problem, researchers usually combine ultrafiltration with other processes, although certain results have been achieved, but there are still problems such as high content of disinfection by-products in effluent, long treatment time and high cost.

[0003] There are some researches on the pretreatment of algae removal and membrane fouling control, such as adsorption, coagulation, pre-oxidation and other methods to alleviate membrane fouling. When using adsorption pretreatment, the membrane fouling is alleviated to a certain extent, but the alleviation degree is small. For example, when using powdered activated carbon as pretreatment, powdered activated carbon can adsorb organic matter and other substances in the solution, but powdered activated carbon will be adsorbed on the surface of the ultrafiltration membrane at the initial stage of filtration, which will aggravate the membrane fouling. When the membrane surface adsorbs a certain amount of activated carbon, a loose filter cake layer will be formed on the membrane surface, which is beneficial to the operation of the ultrafiltration membrane. The filter cake layer can adsorb some organic pollutants in the water, thereby reducing the irreversible pollution of the membrane. However, overall, the adsorption pretreatment has little effect on the improvement of membrane fouling. When using coagulation process as pretreatment method before membrane, membrane fouling can be alleviated to a certain extent. When using iron and aluminum salts as coagulants, pollutants in water can be removed through electric neutralization, adsorption bridging, net capture and sweeping, etc. However, for water containing algae, the removal of algae cells by coagulation is limited due to their strong stability and other characteristics. Moreover, coagulation process cannot remove some hydrophilic small molecular organic matter, and has little effect on alleviating irreversible membrane fouling. Although coagulation has a certain effect on the removal of algae cells, it is not thorough enough, so it has limited effect on alleviating membrane fouling. In addition, when using pre-oxidation as pretreatment, oxidants can destroy algae cells and oxidize large molecular organic matter into small molecular substances to achieve removal effect. However, in order to achieve efficient removal of algae cells, strong oxidants are often used, which can cause the algae cells to break down and release intracellular substances, increasing the generation of disinfection by-products during subsequent chlorination disinfection process, threatening the safety of drinking water. Although simple oxidation pretreatment can achieve certain effect, there are still some problems, so attention should be paid to the selection of oxidants in actual use. Oxidation pretreatment can also be combined with other processes (such as coagulation) as pretreatment method before membrane.

[0004] There are some researches on the pretreatment of algae removal and membrane fouling control, such as adsorption, coagulation, pre-oxidation and other methods to alleviate membrane fouling. When using adsorption pretreatment, the membrane fouling is alleviated to a certain extent, but the alleviation degree is small. For example, when using powdered activated carbon as pretreatment, powdered activated carbon can adsorb organic matter and other substances in the solution, but powdered activated carbon will be adsorbed on the surface of the ultrafiltration membrane at the initial stage of filtration, which will aggravate the membrane fouling. When the membrane surface adsorbs a certain amount of activated carbon, a loose filter cake layer will be formed on the membrane surface, which is beneficial to the operation of the ultrafiltration membrane. The filter cake layer can adsorb some organic pollutants in the water, thereby reducing the irreversible pollution of the membrane. However, overall, the adsorption pretreatment has little effect on the improvement of membrane fouling. When using coagulation process as pretreatment method before membrane, membrane fouling can be alleviated to a certain extent. When using iron and aluminum salts as coagulants, pollutants in water can be removed through electric neutralization, adsorption bridging, net capture and sweeping, etc. However, for water containing algae, the removal of algae cells by coagulation is limited due to their strong stability and other characteristics. Moreover, coagulation process cannot remove some hydrophilic small molecular organic matter, and has little effect on alleviating irreversible membrane fouling. Although coagulation has a certain effect on the removal of algae cells, it is not thorough enough, so it has limited effect on alleviating membrane fouling. In addition, when using pre-oxidation as pretreatment, oxidants can destroy algae cells and oxidize large molecular organic matter into small molecular substances to achieve removal effect. However, in order to achieve efficient removal of algae cells, strong oxidants are often used, which can cause the algae cells to break down and release intracellular substances, increasing the generation of disinfection by-products during subsequent chlorination disinfection process, threatening the safety of drinking water. Although simple oxidation pretreatment can achieve certain effect, there are still some problems, so attention should be paid to the selection of oxidants in actual use. Oxidation pretreatment can also be combined with other processes (such as coagulation) as pretreatment method before membrane. SUMMARY

[0005] The present application aims to provide a photocatalytic oxidation enhanced coagulation coupled ultrafiltration membrane algae removal method, which removes algae in water by using cynao-C3N4 photocatalytic plastic enhanced coagulation coupled ultrafiltration membrane combined process, cynao-C3N4 plastic generates H2O2 under light and has a Fenton-like reaction with Fe2(SO4)3, which can greatly reduce the degree of cell crushing while enhancing coagulation, effectively remove algae cells, and at the same time alleviate the pollution of ultrafiltration membrane, protect the color of the effluent, and avoid secondary pollution; the algae removal method is simple, the cycle is short, the cynao-C3N4 photocatalytic plastic is easy to add and recover, can be repeatedly used, does not need to add extra reagents, and is convenient.

[0006] The technical scheme of the present application is as follows:

[0007] A photocatalytic oxidation enhanced coagulation coupled ultrafiltration membrane algae removal method is carried out in the following order of steps:

[0008] S1, cage cynao-C3N4 plastic is added to the algae-containing water, simulated sunlight is used for 15 min, then Fe2(SO4)3 solution is added, mixed at a stirring speed of 250 rpm for 2 min, and then mixed at a stirring speed of 50 rpm for 15 min, to obtain A;

[0009] S2, A is left to stand and precipitate for 35 min to obtain B;

[0010] S3, the reaction water is taken from 2 cm below the liquid surface of the upper solution of B, and is transferred to an ultrafiltration device for ultrafiltration treatment to obtain treated algae-removed treated water, and the ultrafiltration membrane is backwashed with pure water for next cycle.

[0011] As a limitation of the present application, in step S1, the cage cynao-C3N4 plastic is prepared according to the following method:

[0012] S11, respectively weigh 150-250 mg of cynao-C3N4 and 1.3-1.7 g of PVDF powder, dissolve in N, N-dimethylacetamide, use an ultrasonic cleaning instrument to ultrasonic disperse and completely dissolve the powder to obtain a;

[0013] S12, add 0.5 g of acetone to a, use the ultrasonic cleaning instrument again to ultrasonic for 0.5 h, mix the acetone and PVDF solution completely, prepare a PVDF printing solution, and obtain b;

[0014] S13, using Solidworks software draws the cynao-C3N4 / CCF / PVDF sample 3D model in.stl format, and uses Cura software to slice the model, exports the printing scheme in.gcode format and inputs 3D printing equipment, then prepares cynao-C3N4 / CCF / PVDF composite material by in-situ impregnation 3D printing process, and gets cage cynao-C3N4 plastic material.

[0015] As a further limitation of the application, in step S11, the total mass of PVDF powder and N,N-dimethylacetamide solvent is 10 g.

[0016] As a second limitation of the application, in step S13, the conditions for 3D printing are as follows:

[0017] The thickness is 0.2-0.5 mm, the flow ratio is 65-95%, the printing temperature is 375-405℃, the bottom plate temperature is 120℃, the nozzle moving speed is 1200 mm / min, the nozzle diameter is 0.4 mm, the filling rate is 100%, and the filling angle is 0°.

[0018] As a third limitation of the application, in step S11, the ultrasonic time is 1 h.

[0019] The application also has a limitation that the amount of Fe2(SO4)3 added is 20-80 μmol / L.

[0020] The mechanism of photocatalytic synthesis of H2O2 by the cage cynao-C3N4 plastic material of the application is a typical proton-coupled electron transfer process involving two-electron reduction of oxygen. Specifically, the valence band electrons of cynao-C3N4 photocatalyst are excited by light and transition to the conduction band to form photo-generated electrons, which then migrate to the surface of the photocatalyst to participate in the reduction of oxygen and form H2O2. The cyan-functionalized g-C3N4 can be uniformly dispersed in water to form a colloid and used for quasi-homogeneous photocatalytic reaction. Compared with the original CN, the cyan-functionalized g-C3N4 increases the H2O2 production rate by two orders of magnitude.

[0021] The application can alleviate the ultrafiltration membrane pollution caused by algae removal. The cynao-C3N4 photocatalytic plastic material produces H2O2 under light, and after adding coagulant Fe2(SO4)3, Fe 3+ As a catalyst to generate Fe-OOH 2+ complex, and then decomposes into HOO· and Fe 2 + , Fe 2+After reacting with H2O2, through a series of reactions, the algae cell removal efficiency is significantly improved through oxidation enhanced coagulation. At the same time, most algae are aggregated and wrapped up or wrapped up by Fe 3+ and its hydrolysis product, the number and area of algae exposed in the solution are both reduced, the oxidation of HO· with algae cells is weakened, so the degree of algae cell breakage is less, the integrity is better, and the release of algae organic matter is reduced.

[0022] After the pre-oxidation is completed, there are still small cell groups, free algae cells that have not reacted and a small amount of algae organic matter in the solution, which are intercepted to the membrane surface by ultrafiltration, the main pollution is mainly reversible pollution, a filter cake layer is formed on the membrane surface, the membrane flux decreases rapidly, and the ultrafiltration membrane is protected to avoid irreversible membrane pollution.

[0023] The above technical solutions are closely related and associated with each other as a whole, and jointly affect the algae removal effect.

[0024] Due to the adoption of the above technical solutions, the beneficial effects achieved by the present application are as follows:

[0025] 1. The present application can efficiently remove algae cells, H2O2 generated by the photocatalytic material can enhance the coagulation effect of Fe2(SO4)3, efficiently remove algae cells, without causing massive rupture of algae cells, reducing the release of algae organic matter, and OH· generated by the Fenton-like reaction of H2O2 and Fe2(SO4)3 can react with algae organic matter, effectively reducing the disinfection by-product generation potential, alleviating membrane pollution, reducing membrane operation load, reducing membrane cleaning cycle, prolonging membrane service life, and the ion content of the effluent is far lower than the national standard, without secondary pollution to the water body.

[0026] 2. The photocatalytic material used in the present application is easy to add and recover, can be repeatedly used, does not need to add additional reagents, and is convenient.

[0027] 3. The algae removal method of the present application is simple, short in cycle, economical and efficient, and low in cost.

[0028] The present application is suitable for treating water containing algae.

[0029] The specific embodiments of the present application will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a graph of algae removal rate under different Fe2(SO4)3 dosages in Example 6;

[0031] Figure 2 is a graph of alleviation of membrane pollution under different Fe2(SO4)3 dosages in Example 6;

[0032] Figure 3 Figure for residual iron in effluent under different Fe2(SO4)3 dosages in Example 6;

[0033] Figure 4 Figure for influence of different Fe2(SO4)3 dosages on generation of disinfection by-products in Example 6. DETAILED DESCRIPTION

[0034] In the following examples, the reagents used are commercially available unless otherwise specified. The experimental methods and detection methods used are conventional unless otherwise specified.

[0035] Example 1

[0036] The present embodiment is a method for removing algae by photocatalytic oxidation, enhanced coagulation and coupling ultrafiltration membrane, which is performed according to the following steps in sequence:

[0037] S1, add caged cynao-C3N4 plastic material into water containing algae, irradiate with simulated sunlight for 15 min, then add Fe2(SO4)3 solution at 80 μmol / L, mix rapidly at a stirring speed of 250 rpm for 2 min, and then mix slowly at a stirring speed of 50 rpm for 15 min, to obtain A;

[0038] The caged cynao-C3N4 plastic material is prepared according to the following method:

[0039] S11, respectively weigh 150 mg of cynao-C3N4 and 1.3 g of PVDF powder into N,N-dimethylacetamide, the total mass of PVDF powder and N,N-dimethylacetamide is 10 g, and use an ultrasonic cleaning instrument to ultrasonic for 1 h to disperse and completely dissolve the powder, to obtain a;

[0040] S12, add 0.5 g of acetone to a, and use an ultrasonic cleaning instrument again to ultrasonic for 0.5 h to completely mix the acetone and the PVDF solution, to prepare a PVDF printing solution, to obtain b;

[0041] S13, use Solidworks software to draw a cynao-C3N4 / CCF / PVDF sample 3D model in.stl format, use Cura software to slice the model, export a printing scheme in.gcode format and input a 3D printing device, and then prepare a cynao-C3N4 / CCF / PVDF composite material by in-situ impregnation 3D printing process, to obtain the caged cynao-C3N4 plastic material;

[0042] The conditions for 3D printing in this step are as follows:

[0043] Thickness: 0.2 mm, flow ratio: 75%, printing temperature: 400℃, bottom plate temperature: 120℃, nozzle moving speed: 1200 mm / min, nozzle diameter: 0.4 mm, filling rate: 100%, filling angle: 0°;

[0044] S2, A is left to stand for 35 min to obtain B;

[0045] S3, the reaction water is taken from 2 cm below the liquid surface of B, and is transferred to an ultrafiltration device for ultrafiltration treatment to obtain treated water after algae removal. After ultrafiltration, the ultrafiltration membrane is backwashed with pure water for next cycle.

[0046] This embodiment uses cynao-C3N4 photocatalytic plastic material to strengthen coagulation coupling ultrafiltration membrane process to remove algae in water. Cynao-C3N4 plastic material generates H2O2 under light, which reacts with Fe2(SO4)3 to generate a Fenton-like reaction. The Fenton-like reaction can greatly reduce the degree of cell rupture while strengthening coagulation, effectively removing algae cells and relieving ultrafiltration membrane pollution. The oxidation of generated H2O2 is weak and will not affect the membrane structure; the Fenton-like reaction between H2O2 and Fe2(SO4)3 is weaker than the Fenton reaction and other oxidants, and the reaction can continuously produce new trivalent iron. The Fenton-like reaction can greatly reduce the degree of cell rupture while strengthening coagulation and effectively removing algae, thereby effectively relieving membrane pollution. At the same time, the amount of disinfection byproduct generated in the subsequent disinfection process can be effectively reduced.

[0047] After the above treatment, the ion content of the algae-containing water effluent is far lower than the national standard, and there is no secondary pollution to the water body.

[0048] Examples 2-5

[0049] This embodiment is a photocatalytic oxidation and coagulation strengthening coupling ultrafiltration membrane algae removal method. The process is similar to that of Example 1, except that the technical parameters in the preparation process are different, as follows:

[0050]

[0051] Example 6

[0052] In order to explore the effect of different dosages of Fe2(SO4)3 solution on the removal rate of algae, the following experiments were conducted.

[0053] A method for photocatalytic oxidation enhanced coagulation coupled with ultrafiltration membrane to remove algae, the process is similar to embodiment 1, the difference is only that the dosing amount of Fe2(SO4)3 solution is different, respectively explore the dosing amount of 20, 40, 50, 60, 80 μmol / L, the influence on the removal rate of algae, see Figure 1 .

[0054] (I) by Figure 1 It can be seen that when the dosing amount of Fe(III) is less than 50 μmol / L, the removal efficiency of algae by pretreatment gradually increases with the increase of the dosing amount, and when the dosing amount is higher than 50 μmol / L, the removal efficiency of algae decreases with the increase of the dosing amount, which may be due to the fact that after adding Fe(III) and H2O2, part of Fe(III) and H2O2 have a Fenton-like reaction to produce Fe-OOH 2+ and H + , and part of Fe(III) will react with OH - in water to increase the concentration of H + in water, the solution is acidic, and the surface of the algae cells changes from negative to neutral. When the dosing amount continues to increase, the increase of cations in the solution will make the algae cells generate repulsive force again, so that the solution returns to a stable state, resulting in poor effect of enhanced coagulation, thus the removal effect of algae cells begins to show a downward trend. The removal effect of algae cells by ultrafiltration can reach more than 95%. The better the pretreatment method before the membrane, the better the membrane pollution relief condition may be.

[0055] (II) Figure 2 shows the membrane pollution relief condition when using different doses of Fe2(SO4)3 to treat algae-containing water. From the figure, it can be seen that the membrane pollution relief condition is consistent with the trend of the removal efficiency of algae in Figure 1 . When the concentration is less than 50 μmol / L, the membrane pollution relief effect becomes better with the increase of the concentration, and when the concentration is greater than 50 μmol / L, the membrane pollution relief degree gradually becomes worse with the increase of the Fe(III) dosing amount. When the dosing amount is 50 μmol / L, the membrane pollution relief degree is better than when the Fe(III) dosing amount is 0, and the membrane pollution can be relieved by 66%. It shows that the efficiency of coagulation and algae removal is directly related to the effect of membrane pollution relief, and the better the coagulation effect, the better the membrane pollution relief condition.

[0056] (III) The content of iron in the effluent under different dosing methods of pretreatment reagents is as follows Figure 3The Fe residual concentration in the effluent was far below the national standard of 0.3 mg / L (5.36 mol / L) according to the newly released Standards for Drinking Water Quality (GB5749-2022) in China in 2022. The Fe residual concentration helped to clarify the removal mechanism. When the Fe(III) dosage was 20 μmol / L, the Fe(III) dosage was insufficient, and the coagulation effect was poor, with a Fe residual concentration of 1.526 μg / L in the effluent. When the dosage increased to 50 μmol / L, the Fe residual concentration decreased, mainly because the coagulation effect improved, and most of the Fe was removed by sedimentation with the formation of flocs with algae cells. Moreover, the residual Fe in the water after coagulation reacted with the functional groups on the membrane surface, adsorbed on the membrane surface, and formed a filter cake layer. Therefore, the Fe content in the effluent after ultrafiltration was very low, only 0.023 μg / L. When the dosage increased to 80 μmol / L, the excess Fe(III) could not be complexed, and the membrane surface was covered with enough Fe, which led to a small amount of Fe passing through the ultrafiltration into the effluent. Therefore, an excessively high Fe(III) dosage not only caused serious membrane fouling but also led to an increase in the Fe residual concentration in the effluent. As can be seen from the figure, the amount of Fe(III) added in this process had little effect on the safety of drinking water, and it was safe and feasible.

[0057] (Four) Effect of different Fe2(SO4)3 dosages on the formation potential of chlorinated disinfection byproducts in algal water Figure 4 TCM (trichloromethane) had the highest content in algal water, mainly because the algal liquid contained a large amount of phycocyanin, which was the precursor of TCM; TCM and HAAs were the main disinfection byproducts generated during the chlorination of humic acid. After adding H2O2 and Fe2(SO4)3 to the original algal liquid, the content of DBPs decreased. This was mainly due to the removal of a portion of proteins, humic acid, and cell metabolites by coagulation, as well as the removal of a portion of AOM by oxidation through pre-oxidation. Ultrafiltration had little effect on the formation potential of disinfection byproducts. Therefore, the reduction of disinfection byproduct content was mainly due to the pre-oxidation of coagulation, which strengthened the coagulation effect, and the algae cells were aggregated by bridging and entrapment. A large number of algae cells were wrapped up, and OH· with oxidation effect was more likely to react with AOM than algae cells, which oxidized AOM and reduced the formation potential of DBPs. At the same time, the oxidation of algae cells caused cell rupture, and some disinfection byproduct precursors in the cells were removed with the coagulation of algae cells and were not released, which reduced the formation potential of DBPs compared with other pretreatment methods.

[0058] Finally, it should be noted that the above only describes the preferred embodiments of the present application and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art will appreciate that the technical solutions described in the foregoing embodiments can be modified or some technical features thereof can be replaced by equivalent ones. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the scope of protection of the present application.

Claims

1. A method for photocatalytic oxidation enhanced coagulation coupled with ultrafiltration membrane for algae removal, characterized in that, The steps are performed in the following order: S1, add the caged cynao-C3N4 plastic material into the algae-containing water, irradiate with simulated sunlight for 15 min, then add Fe2(SO4)3 solution, mix rapidly at a stirring speed of 250 rpm for 2 min, and then mix slowly at a stirring speed of 50 rpm for 15 min to obtain A; S2, let A stand for 35 min to obtain B; S3, take the reaction water 2 cm below the liquid surface of the upper solution of B, and transfer it to an ultrafiltration device for ultrafiltration treatment to obtain treated water after algae removal, and after ultrafiltration, the ultrafiltration membrane is backwashed with pure water for next cycle; In step S1, the caged cynao-C3N4 plastic material is prepared by the following method: S11, respectively weigh 150-250 mg of cynao-C3N4 and 1.3-1.7 g of PVDF powder into N,N-dimethylacetamide, and use an ultrasonic cleaning instrument to ultrasonic disperse and completely dissolve the powder to obtain a; S12, add 0.5 g of acetone to a, and use an ultrasonic cleaning instrument to ultrasonic for 0.5 h again to make the acetone completely mixed with the PVDF solution to prepare a PVDF printing solution, and obtain b; S13, use Solidworks software to draw a cynao-C3N4 / CCF / PVDF sample 3D model in.stl format, use Cura software to slice the model, export the printing scheme in.gcode format and input into a 3D printing device, and then prepare a cynao-C3N4 / CCF / PVDF composite material by in-situ impregnation 3D printing process to obtain the caged cynao-C3N4 plastic material.

2. The method according to claim 1, wherein the method is characterized by, In step S11, the total mass of PVDF powder and N,N-dimethylacetamide is 10 g. 3.The method according to claim 1, wherein the method is characterized in that, In step S13, the conditions for 3D printing are as follows: Thickness: 0.2-0.5 mm, flow ratio: 65-95%, printing temperature: 375-405℃, bottom plate temperature: 120℃, nozzle moving speed: 1200 mm / min, nozzle diameter: 0.4 mm, filling rate: 100%, filling angle: 0°.

4. The method according to claim 1, wherein the method is characterized by, In step S11, the ultrasonic time is 1 h.

5. The method according to any one of claims 1-4, wherein the method is characterized in that, The amount of Fe2(SO4)3 added is 20-80 μmol / L.

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