Application of lignin-based flocculant in treatment of wastewater containing anionic dye

Abstract

The invention discloses application of a lignin-based flocculating agent to treatment of wastewater containing anionic dye, and belongs to the technical field of wastewater treatment. According to the invention, an in-situ initiation graft copolymerization process is adopted, a three-dimensional network structure of lignin is taken as a skeleton, methacryloyloxyethyl trimethyl ammonium chloride and an acrylamide monomer are grafted, and the lignin-based flocculant with both a lignin skeleton and a cationic polymer branch chain is synthesized. According to the lignin-based flocculant, anionic dye molecules are unstable through charge neutralization to form tiny flocs, then the tiny flocs grow into large flocs through hydrogen bond combination, and then precipitation separation is conducted under the action of gravity. Compared with commercialized cationic polyacrylamide, the lignin-based flocculant prepared by the method has obviously higher removal rate on anionic dyes, and the removal rate can reach 98% or above.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, and more specifically, relates to the application of a lignin-based flocculant in treating wastewater containing anionic dyes. Background Technology

[0002] Globally, 10,000 kinds of synthetic and natural dyes are produced annually, totaling millions of tons. During production and application, these dyes are discharged into water bodies with industrial wastewater, affecting the photosynthesis of aquatic plants and even causing the death of aquatic organisms. Furthermore, dyes can accumulate through the food chain, posing a potential threat to human health. Therefore, finding economical and effective methods for controlling dye pollution is of paramount importance in protecting human health and the aquatic ecosystem.

[0003] Dye molecules have stable structures; the two most common types are azo dyes and anthraquinone dyes, which are usually anionic dyes (such as alizarin green and Congo red) and exist in water in colloidal form. Coagulation and sedimentation are widely used in water treatment due to their simplicity, wide applicability, low cost, high efficiency, and significant decolorization effect on colloidal pollutants. Commonly used chemical agents are inorganic coagulants and synthetic organic polymeric flocculants. Inorganic coagulants include aluminum salts and iron salts, whose metallic salts hydrolyze to generate positively charged products that can adsorb and neutralize the surface charge of negatively charged colloids in wastewater, promoting destabilization and aggregation to form micro-flocs. However, they have drawbacks such as high dosage, excessive sludge, metal residue, and fragile flocs. Synthetic organic polymeric flocculants are often used as additives to reduce the dosage of inorganic coagulants and sludge production. Their advantages include large flocs and strong resistance to breakage. Organic polymeric flocculants can be classified into four types according to their functional group properties: cationic, anionic, amphoteric, and nonionic, and the appropriate type must be selected based on water quality. Cationic organic polymeric flocculants are widely used in the decolorization of dye wastewater. The positively charged groups on their molecular chains neutralize the surface charge of negatively charged dye colloids in the wastewater, reducing the hydrophilicity of dye particles and destabilizing them. Then, through adsorption bridging, the destabilized particles aggregate into large flocs, thus achieving efficient dye removal. Commonly used cationic organic polymeric flocculants are polyacrylamide and its derivatives. However, they still suffer from problems such as high production costs, strong raw material toxicity, and low effective utilization of charge. Therefore, developing environmentally friendly and efficient organic polymeric flocculants has become a critical issue that urgently needs to be addressed.

[0004] Lignin is a natural polymer material with wide-ranging sources, such as agricultural waste, pulping byproducts, and plants. After modification, it can be prepared into high-performance lignin-based flocculants, showing broad application prospects in wastewater treatment. Chinese invention patent publication number CN109280174A discloses a hyperbranched lignin-grafted cationic polyacrylamide flocculant and its preparation method. This method uses acrylamide and cationic comonomers as raw materials, and prepares a cationic polyacrylamide prepolymer with halogen-terminated ends through free radical polymerization in water. Then, the prepolymer and lignin are mixed and dissolved in an alkaline solution, stirred, heated, and reacted to obtain the hyperbranched lignin-grafted cationic polyacrylamide flocculant. However, the above flocculant does not provide a method for treating wastewater containing anionic dyes. Summary of the Invention

[0005] 1. The problem to be solved

[0006] There is no existing technology for treating wastewater containing anionic dyes using lignin-based flocculants. This invention provides a lignin-based flocculant using an in-situ initiation graft copolymerization process. Its excellent charge neutralization and hydrogen bonding effects enable it to be used efficiently for treating wastewater containing anionic dyes.

[0007] 2. Technical Solution To solve the above problems, the technical solution adopted by the present invention is as follows: This invention provides an application of a lignin-based flocculant in treating wastewater containing anionic dyes, comprising the step of adding the lignin-based flocculant to wastewater containing anionic dyes with a pH of 3-12. The preparation method of the lignin-based flocculant includes the following steps: S1. Provide a first lignin solution, wherein the pH value of the first lignin solution is 7.0~8.5; S2. Remove oxygen from the first lignin solution to obtain the second lignin solution; S3. First, add the initiator dropwise to the second lignin solution, then add the methacryloyloxyethyltrimethylammonium chloride solution and acrylamide solution dropwise, and then remove the oxygen in the second lignin solution to obtain the lignin-based flocculant solution. S4. Extract and purify the lignin-based flocculant solution to obtain the lignin-based flocculant.

[0008] Preferably, the lignin-based flocculant has a Zeta potential of 10~50 mV.

[0009] In this invention, methacryloyloxyethyltrimethylammonium chloride provides a quaternary ammonium salt group as a cationic group, which makes the lignin-based flocculant have a positive Zeta potential over a wide pH range, which is beneficial for adsorption and charge neutralization reactions with anionic dyes, causing the anionic dye molecules to become unstable and form micro flocs; acrylamide provides an amide group as a hydrogen bond donor group to form hydrogen bonds with anionic dye molecules, promoting adsorption bridging, and can also improve the water solubility of lignin.

[0010] Preferably, the anionic dye is selected from one or both of alizarin green dye or Congo red dye.

[0011] Preferably, the anionic dye is alizarin green dye, and the pH of the wastewater is 5-7.

[0012] Preferably, the anionic dye is Congo red dye, and the pH of the wastewater is 5-8.

[0013] When the pH of wastewater containing alizarin green dye is 5-7 and the pH of wastewater containing Congo red dye is 5-8, the lignin-based flocculant of the present invention achieves a removal rate of over 98% for both anionic dyes.

[0014] More preferably, the mass ratio of the lignin-based flocculant to alizarin green dye is (0.15~0.20):1.

[0015] More preferably, the mass ratio of the lignin-based flocculant to Congo red dye is (0.39~0.41):1.

[0016] When the mass ratio of lignin-based flocculant to alizarin green dye and Congo red dye is (0.15~0.20):1 and (0.39~0.41):1, respectively, the removal rate of these two anionic dyes in wastewater by the lignin-based flocculant reaches over 80%.

[0017] More preferably, the anionic dye is alizarin green dye, and the concentration of the lignin-based flocculant is 15~25 mg / L.

[0018] More preferably, the concentration of alizarin green dye in the wastewater is 100~150 mg / L.

[0019] More preferably, the anionic dye is Congo red dye, and the concentration of the lignin-based flocculant is 35~45 mg / L.

[0020] More preferably, the concentration of Congo red dye in the wastewater is 75~125 mg / L.

[0021] Preferably, in step S1, the mass concentration of lignin in the first lignin solution is 5~15 g / L; in step S3, the mass ratio of initiator, methacryloyloxyethyltrimethylammonium chloride and acrylamide is 1:(30~180):(10~100); the mass ratio of lignin in step S1 to initiator in step S3 is (10~35):1.

[0022] More preferably, in step S3, the mass ratio of the initiator, methacryloyloxyethyltrimethylammonium chloride and acrylamide is 1:(70~90):(40~60).

[0023] Through orthogonal experiments, the inventors discovered that the influence of the added raw materials in step S3 on the removal rate of alizarin green dye is as follows: acrylamide > methacryloyloxyethyltrimethylammonium chloride > initiator. When the mass ratio of the initiator, methacryloyloxyethyltrimethylammonium chloride and acrylamide is 1:(70~90):(40~60), the lignin-based flocculant has the best removal effect on alizarin green dye.

[0024] Preferably, the lignin comprises one or more of alkali lignin, enzymatically hydrolyzed lignin, and organic solvent lignin; the initiator comprises one or more of potassium persulfate, ammonium persulfate, azobisisobutyronitrile, and azobisisoheptanenitrile.

[0025] As described herein, alkali lignin refers to lignin in alkali pulping waste liquid, enzymatic lignin is lignin separated from plant raw materials after hydrolysis by cellulase, and organic solvent lignin is lignin separated from plant raw materials by organic solvent through dissolution.

[0026] Preferably, in step S3, removing oxygen from the second lignin solution includes heating the second lignin solution to 60-80 °C and purging it with nitrogen for 3-6 h to remove oxygen.

[0027] It should be noted that in step S3, the initiator is added first to activate the hydroxyl groups on the lignin into hydroxyl free radicals. The temperature of 60-80℃ ensures the continuous decomposition of the initiator and the free radical polymerization. The reaction time of 3-6 hours ensures that the monomers fully polymerize and graft onto the lignin molecular chains, improving the grafting rate and monomer conversion rate, and forming a lignin-based flocculant graft copolymer. Dropwise addition avoids excessively high local concentrations of initiator and monomers, which could lead to the deactivation of free radicals and improve the uniformity of grafting sites.

[0028] Preferably, in step S1, lignin and alkali are dissolved in deionized water, heated to 50-70°C until completely dissolved, cooled to 20-30°C and centrifuged, the supernatant is collected, and the pH is adjusted to 7.5-8.5 to obtain the first lignin solution.

[0029] More preferably, the alkali includes one or both of sodium hydroxide and potassium hydroxide.

[0030] Preferably, in step S2, the first lignin solution is heated to 60-80 °C and nitrogen gas is introduced to remove oxygen for 30-60 min to obtain the second lignin solution.

[0031] It should be noted that graft copolymerization follows a free radical polymerization mechanism. Oxygen quenches the free radicals in the system, leading to initiator failure and termination of the polymerization reaction. Preheating the system to the reaction temperature can reduce temperature fluctuations in subsequent reactions; introducing nitrogen gas can provide an oxygen-free environment for free radical polymerization, ensuring initiation efficiency and the continuity of the grafting reaction.

[0032] Preferably, in step S4, the lignin-based flocculant solution is added to acetone for extraction, centrifuged to remove the supernatant, and then purified with anhydrous ethanol to obtain the lignin-based flocculant.

[0033] More preferably, the volume ratio of the lignin-based flocculant solution to acetone is 1:3. This ratio ensures sufficient precipitation of the product.

[0034] The lignin-based flocculant provided by this invention uses the three-dimensional network structure of lignin as a framework, grafting methacryloyloxyethyltrimethylammonium chloride and acrylamide monomers to form a cationic branched graft copolymer. Lignin, as the material framework, provides the active sites required for modification, and its inherent hydroxyl groups act as hydrogen bond donor groups, contributing to the basic ability of hydrogen bond adsorption. It forms hydrogen bonds with hydrogen bond acceptor groups such as carbon-oxygen double bonds and sulfonic acid groups on anionic dye molecules, promoting adsorption bridging. Methacryloxyethyltrimethylammonium chloride provides quaternary ammonium salt groups as cationic groups, ensuring that the copolymer has a positive zeta potential over a wide pH range, which is conducive to adsorption charge neutralization reactions. Acrylamide provides amide groups as hydrogen bond donor groups, forming hydrogen bonds with anionic dye molecules, promoting adsorption bridging, and also improving the water solubility of lignin. The lignin-based flocculant provided by this invention first destabilizes anionic dye molecules through charge neutralization (adsorption charge neutralization) to form micro-flocs, which then grow into large flocs through hydrogen bonding (adsorption bridging), and finally precipitate and separate under gravity.

[0035] 3. Beneficial effects Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention provides an application of a lignin-based flocculant in the treatment of wastewater containing anionic dyes. Through charge neutralization (adsorption neutralization), the anionic dye molecules become unstable, forming tiny flocs. These flocs then grow into larger flocs through hydrogen bonding (adsorption bridging), and subsequently precipitate and separate under gravity. Compared to commercially available cationic polyacrylamide, the lignin-based flocculant prepared in this invention exhibits significantly higher removal rates for anionic dyes, particularly alizarin green and Congo red, with optimal removal rates reaching 98.66% and 98.00%, respectively. Furthermore, it maintains stable performance over a wide pH range.

[0036] (2) The lignin-based flocculant provided by the present invention maintains a positive Zeta potential over a wide pH range, which expands the pH range in which it is applied and is beneficial to the stable progress of the adsorption charge neutralization reaction.

[0037] (3) When treating wastewater containing anionic dyes, the lignin-based flocculant provided by the present invention achieves the goal of efficiently removing anionic dyes from wastewater with a single agent, compared with traditional polymeric flocculants which need to be used in combination with metal salts to enhance the flocculation effect. This significantly simplifies the water treatment process, leaves no metal residue, and reduces the cost of agent addition and the risk of secondary pollution.

[0038] (4) The preparation method of the lignin-based flocculant provided by the present invention adopts an in-situ initiation graft copolymerization process. First, a well-dispersible lignin solution is prepared by an alkali-dissolution and acid-adjustment process. Then, methacryloyloxyethyltrimethylammonium chloride and acrylamide monomer are grafted and copolymerized to synthesize a lignin-grafted cationic copolymer product that has both a lignin skeleton and cationic polymer branches. This preparation method uses lignin as a base material, which is a natural polymer material with wide availability, non-toxicity, biodegradability, and independence from petroleum resources, meeting the needs of environmental protection and sustainable development.

[0039] (5) The preparation method of lignin-based flocculant provided by the present invention has mild reaction conditions (60~80℃, weakly alkaline environment), simple steps, no special operation or equipment, clear and controllable raw material ratio, low energy consumption, and no toxic by-products generated, thus realizing green and efficient preparation. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the preparation process of the lignin-based flocculant in this invention; Figure 2 This is the reaction formula for the preparation of the lignin-based flocculant in this invention; Figure 3 The photoelectron spectra of the lignin used in this invention and the lignin-based flocculant prepared in Example 10 of this invention are shown. Figure 4The image shows a comparison of wastewater containing alizarin green dye before (left) and after (right) treatment using the lignin-based flocculant prepared in Example 10 in Test Example 2. Figure 5 This is a comparison chart of wastewater containing Congo red dye before (left) and after (right) treatment using the lignin-based flocculant prepared in Example 10 in Test Example 9. Detailed Implementation

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Where specific conditions are not specified in the examples, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0042] Concentration, amount, and other numerical data may be presented in range format herein. It should be understood that such range format is used solely for convenience and brevity and should be flexibly interpreted to include not only the values ​​explicitly stated as the limits of the range, but also all individual values ​​or subranges encompassed within the range, as if each value and subrange were explicitly stated. For example, a range of values ​​from about 1 to about 4.5 should be interpreted to include not only the explicitly stated limits of 1 to 4.5, but also individual numbers (such as 2, 3, 4) and subranges (such as 1 to 3, 2 to 4, etc.). The same principle applies to ranges that describe only a single value, such as "less than about 4.5," which should be interpreted to include all the values ​​and ranges described above. Furthermore, this interpretation should apply regardless of the breadth of the range or characteristic described.

[0043] In this invention, lignin (alkali lignin) was purchased from Maclean (No.: 8068-05-1), alizarin green dye was purchased from Maclean (No.: 4403-90-1), Congo red dye was purchased from Maclean (No.: 573-58-0), and polyacrylamide (cationic type) was purchased from Damao (No.: 9003-05-8).

[0044] The present invention will be further described below with reference to specific embodiments.

[0045] Example 1 This embodiment provides a method for preparing a lignin-based flocculant. The schematic diagram of the preparation process and the preparation reaction formula are shown below. Figure 1 and Figure 2 As shown, the specific steps include: S1. Dissolve 2 g of lignin and 1 g of sodium hydroxide in 200 mL of deionized water. Stir magnetically at 60 °C until completely dissolved. After cooling to 25 °C, centrifuge and collect the supernatant. Adjust the pH to 8.0 with 0.1 mol / L HCl to obtain the first lignin solution. S2. Heat the obtained first lignin solution to 70 °C and purge with nitrogen gas to remove oxygen for 30 min to obtain the second lignin solution. S3. First, add 8 mL of potassium persulfate solution (mass fraction of 1%) dropwise to the second lignin solution, then add 8 mL of methacryloyloxyethyltrimethylammonium chloride solution (mass fraction of 75%) and 29.7 mL of acrylamide solution (mass fraction of 10%) dropwise. The reaction is carried out under nitrogen gas continuously at 70 °C for 4 h to obtain lignin-based flocculant solution. S4. Take 10 mL of lignin-based flocculant solution and add it to 30 mL of acetone for extraction. After centrifugation, remove the supernatant and then purify it with anhydrous ethanol to obtain the lignin-based flocculant.

[0046] Example 2 The method is basically the same as in Example 1, except that 8 mL of potassium persulfate solution, 10 mL of methacryloyloxyethyltrimethylammonium chloride solution, and 39.6 mL of acrylamide solution are added.

[0047] Example 3 The method is basically the same as in Example 1, except that 8 mL of potassium persulfate solution, 12 mL of methacryloyloxyethyltrimethylammonium chloride solution, and 49.5 mL of acrylamide solution are added.

[0048] Example 4 The method is basically the same as in Example 1, except that 10 mL of potassium persulfate solution, 10 mL of methacryloyloxyethyltrimethylammonium chloride solution, and 29.7 mL of acrylamide solution are added.

[0049] Example 5 The method is basically the same as in Example 1, except that 10 mL of potassium persulfate solution, 12 mL of methacryloyloxyethyltrimethylammonium chloride solution, and 39.6 mL of acrylamide solution are added.

[0050] Example 6 The method is basically the same as in Example 1, except that 10 mL of potassium persulfate solution, 8 mL of methacryloyloxyethyltrimethylammonium chloride solution, and 49.5 mL of acrylamide solution are added.

[0051] Example 7 The method is basically the same as in Example 1, except that 12 mL of potassium persulfate solution, 12 mL of methacryloyloxyethyltrimethylammonium chloride solution, and 29.7 mL of acrylamide solution are added.

[0052] Example 8 The method is basically the same as in Example 1, except that 12 mL of potassium persulfate solution, 8 mL of methacryloyloxyethyltrimethylammonium chloride solution, and 39.6 mL of acrylamide solution are added.

[0053] Example 9 The method is basically the same as in Example 1, except that 12 mL of potassium persulfate solution, 10 mL of methacryloyloxyethyltrimethylammonium chloride solution, and 49.5 mL of acrylamide solution are added.

[0054] Example 10 The method is basically the same as in Example 1, except that 10 mL of potassium persulfate solution, 10 mL of methacryloyloxyethyltrimethylammonium chloride solution, and 49.5 mL of acrylamide solution are added.

[0055] Test Example 1 Figure 3 The images show the photoelectron spectra of the lignin (alkali lignin) used in this invention and the lignin-based flocculant prepared in Example 10 of this invention. Figure 3 It is known that C1s, O1s, and N1s orbitals exist in both lignin and lignin-based flocculants. Furthermore, Cl2p orbitals appear in the lignin-based flocculant, and the relative N content increases from 2.67% to 9.07%. This is because the quaternary ammonium salt group on methacryloyloxyethyltrimethylammonium chloride in the lignin-based flocculant increases the Cl content, and this group, along with the amide group in acrylamide, jointly increases the N content. These changes are consistent with the theoretical trend of elemental content changes during the synthesis process, indirectly proving the successful preparation of the lignin-based flocculant.

[0056] Test Example 2 Alizarin Green dye was added to distilled water to prepare Alizarin Green dye wastewater with an initial concentration of 100 mg / L and an initial pH of 7. The Alizarin Green dye wastewater was then introduced into a flocculation sedimentation tank, with 20 mg of lignin-based flocculant prepared in Examples 1-10 added per liter of wastewater. The mixture was rapidly stirred at 400 rpm / min for 30 s, then slowly stirred at 80 rpm / min for 30 min, followed by settling for 30 min. The dye concentration in the effluent was measured, and the removal rate was calculated according to formula (1). The results are shown in Table 1.

[0057]

[0058] As shown in Table 1, the range analysis of the orthogonal experiment revealed that the influence of each factor on the removal rate of alizarin green was in the order of acrylamide > methacryloyloxyethyltrimethylammonium chloride > potassium persulfate. The optimal level was determined by using the K values ​​from Examples 1 to 9 (in the orthogonal experiment, the K value represents the sum of the experimental indicators of the same factor at different levels) and converted to the optimal mass ratio of potassium persulfate:methacryloyloxyethyltrimethylammonium chloride:acrylamide, which was 0.10:8.29:5. The flocculant prepared using this optimal ratio in Example 10 achieved the highest removal rate, verifying the correctness of this ratio.

[0059] Figure 4 The images show a comparison of photographs before and after the treatment of alizarin green dye wastewater using the lignin-based flocculant prepared in Example 10 in this test case. Figure 4 It can be seen that the color of the dye wastewater was significantly reduced after treatment, proving that the lignin-based flocculant prepared in Example 10 has a good decolorization effect.

[0060] Table 1. Removal rate of alizarin green by the lignin-based flocculants prepared in Examples 1-10

[0061] Test Example 3 10 mg of lignin powder was dissolved in 10 mL of ultrapure water and then fully dispersed and dissolved under magnetic stirring to obtain a lignin dispersion of 1 g / L.

[0062] 10 mg of the lignin-based flocculant prepared in Example 10 was dissolved in 10 mL of ultrapure water and fully dispersed and dissolved under magnetic stirring to obtain a 1 g / L lignin-based flocculant mother liquor.

[0063] The pH of the lignin dispersion and the lignin-based flocculant mother liquor was adjusted to 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0, respectively, using 1 mol / L HCl and 0.1 mol / L NaOH. The Zeta potentials of the lignin dispersion and the lignin flocculant mother liquor were measured using a Zeta potential meter from Malvern Instruments (UK), and the results are shown in Table 2.

[0064] As shown in Table 2, the Zeta potential of the lignin dispersion is negative at pH 5-10. After graft copolymerization modification, the quaternary ammonium salt group provided by methacryloyloxyethyltrimethylammonium chloride acts as a cationic group, making the Zeta potential of the lignin-based flocculant mother liquor positive at pH 5-10, which provides favorable conditions for adsorption and charge neutralization.

[0065] Table 2. Zeta potentials of lignin dispersions and lignin-based flocculant mother liquors at different pH values.

[0066] Test Example 4 Alizarin green dye was added to distilled water to prepare alizarin green dye wastewater with an initial concentration of 100 mg / L. The pH of the dye wastewater was adjusted to 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0, respectively, using 1 mol / L HCl and 0.1 mol / L NaOH. Polyacrylamide and the lignin-based flocculant prepared in Example 10 were used to prepare a polyacrylamide solution with a pH of 7 and a concentration of 1 g / L, and a lignin-based flocculant stock solution, respectively.

[0067] The flocculation experiment used a six-stage programmable coagulation mixer. Each beaker had a single processing capacity of 200 mL. The specific flocculation steps were as follows: 4 mL of polyacrylamide solution and lignin-based flocculant mother liquor were added to 200 mL of alizarin green dye wastewater, and the mixer was started immediately. The mixture was stirred rapidly at 400 rpm for 30 s, followed by slow stirring at 80 rpm for 5 min. After stirring, the mixture was allowed to settle for 30 min. The supernatant was collected and filtered through a 0.45 μm filter membrane to remove fine flocs. The dye concentration of the supernatant was then measured, which is the dye concentration in the effluent. The entire flocculation process was carried out at room temperature. Each experiment was repeated three times. The removal rate was calculated according to formula (1), and the average value was taken. The results are shown in Table 3.

[0068] As shown in Table 3, within the pH range of 5-10 for alizarin green dye wastewater, the removal rate of alizarin green by lignin-based flocculants was above 65%, with a maximum of 99.72%. In contrast, the highest removal rate of alizarin green by polyacrylamide was only 36.14%.

[0069] pH affects the charge properties (Zeta potential) of flocculants and dye molecules. The lignin-based flocculant prepared in Example 10, due to the introduction of methacryloyloxyethyltrimethylammonium chloride, is always positively charged in the pH range of 5 to 10, which can stably neutralize the negatively charged alizarin green. Therefore, it has a good removal effect on alizarin green and its performance is stable in a wide pH range.

[0070] Table 3. Removal rates of alizarin green from wastewater at different pH values ​​using polyacrylamide and the lignin-based flocculant prepared in Example 10.

[0071] Test Example 5 Alizarin green dye was added to distilled water to prepare alizarin green dye wastewater with an initial concentration of 100 mg / L and an initial pH of 7. The alizarin green dye wastewater was then introduced into a stirred test vessel, and polyacrylamide and the lignin-based flocculant prepared in Example 10 were added at concentrations of 10 mg / L, 15 mg / L, 20 mg / L, 25 mg / L, 30 mg / L, 35 mg / L, and 40 mg / L, respectively. The flocculation experiment was conducted using a six-stage programmable coagulation mixer. The mixture was rapidly stirred at 400 rpm / min for 30 s, then slowly stirred at 80 rpm / min for 5 min, followed by settling for 30 min. The dye concentration in the effluent was measured, and the removal rate was calculated according to formula (1). The results are shown in Table 4.

[0072] As shown in Table 4, the removal rate of alizarin green dye exhibits an inverted U-shape with increasing concentration of lignin-based flocculant. This is because charge neutralization plays a crucial role in the decolorization process. Near the optimal flocculant dosage, the positively charged flocculant interacts with the negatively charged dye molecules, disrupting the stable state of the dye molecules and causing them to aggregate and settle. When excessive flocculant is added, the colloidal particles regain a positive charge and stabilize again through electrostatic repulsion, reducing the decolorization efficiency.

[0073] Within the dosage range of 10–30 mg / L, the lignin-based flocculants achieved a removal rate of over 60% for alizarin green dye. The highest removal rate, reaching 98.66%, was observed at a dosage of 20 mg / L. In contrast, polyacrylamide achieved a maximum removal rate of only 25.51% for alizarin green dye.

[0074] Table 4. Effect of the dosage concentration of polyacrylamide and the lignin-based flocculant prepared in Example 10 on the removal rate of alizarin green.

[0075] Test Example 6 Alizarin green dye was added to distilled water to prepare alizarin green dye wastewater. The initial concentrations of the dye wastewater were 25 mg / L, 50 mg / L, 75 mg / L, 100 mg / L, 150 mg / L, and 200 mg / L, respectively, and the initial pH was 7. The alizarin green dye wastewater was introduced into a stirred test cup, and then polyacrylamide and the lignin-based flocculant prepared in Example 10 were added, with a concentration of 20 mg / L for each. The flocculation experiment was conducted using a six-stage programmable coagulation mixer. The mixture was rapidly stirred at 400 rpm / min for 30 s, then slowly stirred at 80 rpm / min for 5 min, and then allowed to settle for 30 min. The dye concentration in the effluent was measured, and the removal rate was calculated according to formula (1). The results are shown in Table 5.

[0076] As shown in Table 5, when the dosage of lignin-based flocculant is 20 mg / L, the removal rate of alizarin green dye exhibits an inverted U-shaped pattern as the initial concentration of alizarin green dye wastewater increases. This is because, with a fixed flocculant dosage, when the initial dye concentration is lower than the charge equivalence ratio, the total amount of negative charge increases, improving neutralization efficiency and collision probability, thus increasing the removal rate. However, when this ratio is exceeded, excess negative charge leads to incomplete neutralization and enhanced electrostatic repulsion, resulting in a decrease in the removal rate.

[0077] When the concentration of alizarin green dye wastewater was 100 mg / L, the lignin-based flocculant achieved the highest removal rate of alizarin green dye, reaching 98.66%. Under the same conditions, polyacrylamide only achieved a removal rate of 25.51% for alizarin green dye.

[0078] Table 5. Removal rates of alizarin green dye wastewater with different initial concentrations using polyacrylamide and the lignin-based flocculant prepared in Example 10.

[0079] Test Example 7 Congo red dye was added to distilled water to prepare Congo red dye wastewater with an initial concentration of 100 mg / L. The pH of the wastewater was adjusted to 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0, respectively, using 1 mol / L HCl and 0.1 mol / L NaOH. Polyacrylamide and the lignin-based flocculant prepared in Example 10 were used to prepare polyacrylamide solution and flocculant mother liquor with a pH of 7 and a concentration of 1 g / L, respectively.

[0080] The flocculation experiment used a six-stage programmable coagulation mixer. Each beaker had a single processing capacity of 200 mL. The specific flocculation steps were as follows: 8 mL of polyacrylamide solution and flocculant mother liquor were added to 200 mL of Congo red dye wastewater, and the mixer was started immediately. The mixture was stirred rapidly at 400 rpm for 30 s, followed by slow stirring at 80 rpm for 5 min. After stirring, the mixture was allowed to settle for 30 min. Finally, the supernatant was collected and filtered through a 0.45 μm filter membrane to remove fine flocs. The dye concentration in the supernatant was then measured, which is the dye concentration in the effluent. The entire flocculation process was carried out at room temperature. Each experiment was repeated three times. The removal rate was calculated according to formula (1), and the average value was taken. The results are shown in Table 6.

[0081] As shown in Table 6, within the pH range of 5-10, the lignin-based flocculant achieved a removal rate of over 80% for Congo red dye, with a maximum of 98.19%. In contrast, polyacrylamide achieved a maximum removal rate of only 55.94% for Congo red dye. Therefore, the lignin-based flocculant prepared in this invention exhibits superior removal performance for Congo red dye and maintains stable performance over a wider pH range.

[0082] Table 6. Removal rates of Congo red from wastewater at different pH values ​​using polyacrylamide and the lignin-based flocculant prepared in Example 10.

[0083] Test Example 8 Congo red dye was added to distilled water to prepare Congo red dye wastewater with an initial concentration of 100 mg / L and an initial pH of 7. The Congo red dye wastewater was then introduced into a stirred test vessel, and polyacrylamide and the lignin-based flocculant prepared in Example 10 were added at concentrations of 20 mg / L, 25 mg / L, 30 mg / L, 35 mg / L, 40 mg / L, 45 mg / L, and 50 mg / L, respectively. The flocculation experiment was conducted using a six-stage programmable coagulation mixer. The mixture was rapidly stirred at 400 rpm / min for 30 s, then slowly stirred at 80 rpm / min for 5 min, followed by settling for 30 min. The dye concentration in the effluent was measured, and the removal rate was calculated according to formula (1). The results are shown in Table 7.

[0084] Table 7 shows that when treating Congo red dye wastewater, the removal rate of Congo red dye exhibits an inverted U-shaped pattern with increasing concentration of lignin-based flocculant. Within the concentration range of 35–45 mg / L, the removal rate of Congo red dye by lignin-based flocculants is consistently above 60%. The highest removal rate, reaching 98.00%, is achieved with lignin-based flocculants at a concentration of 40 mg / L. In contrast, polyacrylamide achieves a maximum removal rate of only 28.34% for Congo red dye.

[0085] Table 7. Effect of the dosage concentration of polyacrylamide and the lignin-based flocculant prepared in Example 10 on the Congo red removal rate.

[0086] Test Example 9 Congo red dye was added to distilled water to prepare Congo red dye wastewater. The initial concentrations of the dye wastewater were 25 mg / L, 50 mg / L, 75 mg / L, 100 mg / L, 150 mg / L, and 200 mg / L, with an initial pH of 7. The Congo red dye wastewater was introduced into a stirred test cup, and then polyacrylamide and the lignin-based flocculant prepared in Example 10 were added at a concentration of 40 mg / L. The flocculation experiment was conducted using a six-stage programmable coagulation mixer. The mixture was rapidly stirred at 400 rpm / min for 30 s, then slowly stirred at 80 rpm / min for 5 min, and then allowed to settle for 30 min. The dye concentration in the effluent was measured, and the removal rate was calculated according to formula (1). The results are shown in Table 8.

[0087] Table 8 shows that when the dosage of lignin-based flocculant is 40 mg / L, the removal rate of Congo red dye exhibits an inverted U-shaped pattern as the initial concentration of the wastewater increases. The highest removal rate, reaching 98.00%, is achieved when the concentration of the Congo red dye wastewater is 100 mg / L. In contrast, polyacrylamide achieves a maximum removal rate of only 36.28% for Congo red dye.

[0088] Table 8. Removal rates of polyacrylamide and the lignin-based flocculant prepared in Example 10 for Congo red dye wastewater with different initial concentrations.

[0089] Test Case 10 Prepare 1 L of 0.5% (w / w) kaolin suspension, add 6 mg of the lignin-based flocculant prepared in Example 10, and stir at 200 rpm / min for 2 minutes at room temperature, then stir at 40 rpm / min for 10 minutes. After standing for 15 minutes, take 30 mL of liquid from 3 cm above the liquid surface and place it in a sample bottle for multiphase light scattering analysis. The flocculation performance is judged by comparing the transmittance at 5 mm above the liquid surface; higher transmittance indicates better flocculation performance. Repeat the above experiment 3 times, and take the average transmittance. After treatment with the lignin-based flocculant prepared in Example 10, the transmittance of the kaolin suspension is 96.38%.

[0090] When the amount of lignin-based flocculant prepared by the Chinese invention patent with publication number CN109280174A added in the above experiment was not less than 10 mg, the transmittance of the kaolin suspension treated with it was as high as 92%.

[0091] Therefore, the lignin-based flocculant prepared by this invention has a better flocculation effect on kaolin suspension with a smaller dosage.

[0092] The above description provides an illustrative overview of the present invention and its embodiments. This description is not restrictive, and the embodiments shown are merely one example of the invention's implementation. Actual implementations are not limited to these examples. Therefore, if those skilled in the art are inspired by this description and design similar implementations and examples without departing from the spirit of the invention, such designs should fall within the scope of protection of the present invention.

Claims

1. The application of a lignin-based flocculant in treating wastewater containing anionic dyes, characterized in that, The method includes the step of adding a lignin-based flocculant to wastewater containing anionic dyes with a pH of 3 to 12; The preparation method of the lignin-based flocculant includes the following steps: S1. Provide a first lignin solution, wherein the pH value of the first lignin solution is 7.0~8.5; S2. Remove oxygen from the first lignin solution to obtain the second lignin solution; S3. First, add the initiator dropwise to the second lignin solution, then add the methacryloyloxyethyltrimethylammonium chloride solution and acrylamide solution dropwise, and then remove the oxygen in the second lignin solution to obtain the lignin-based flocculant solution. S4. Extract and purify the lignin-based flocculant solution to obtain the lignin-based flocculant.

2. The application according to claim 1, characterized in that: The zeta potential of the lignin-based flocculant is 10~50 mV.

3. The application according to claim 1, characterized in that: The anionic dye is selected from one or both of alizarin green dye or Congo red dye.

4. The application according to claim 3, characterized in that: The anionic dye is alizarin green dye, and the pH of the wastewater is 5-7.

5. The application according to claim 3, characterized in that: The anionic dye is Congo red dye, and the pH of the wastewater is 5-8.

6. The application according to claim 3 or 4, characterized in that: The mass ratio of the lignin-based flocculant to alizarin green dye is (0.15~0.20):

1.

7. The application according to claim 3 or 5, characterized in that: The mass ratio of the lignin-based flocculant to Congo red dye is (0.39~0.41):

1.

8. The application according to claim 1, characterized in that: In step S1, the mass concentration of lignin in the first lignin solution is 5~15 g / L; In step S3, the mass ratio of the initiator, methacryloyloxyethyltrimethylammonium chloride and acrylamide is 1:(30~180):(10~100); preferably, the mass ratio of the initiator, methacryloyloxyethyltrimethylammonium chloride and acrylamide is 1:(70~90):(40~60). The mass ratio of lignin in step S1 to initiator in step S3 is (10~35):

1.

9. The application according to claim 1 or 8, characterized in that: The lignin includes one or more of alkali lignin, enzymatically hydrolyzed lignin, and organic solvent lignin; The initiator includes one or more of potassium persulfate, ammonium persulfate, azobisisobutyronitrile, and azobisisoheptanenitrile.

10. The application according to claim 1 or 8, characterized in that: In step S3, removing oxygen from the second lignin solution includes heating the second lignin solution to 60-80 °C and purging it with nitrogen for 3-6 h.

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