Papermaking wastewater treatment composite agent as well as preparation method and application thereof

Abstract

The invention discloses a papermaking wastewater treatment composite agent as well as a preparation method and application thereof. The composite agent is formed by compounding core-shell structure composite particles, polyaluminum chloride and a coagulant aid, a core layer of the core-shell structure composite particles comprises an interface compound of a ball-milled activated peroxydisulfate intercalated layered double hydroxide and a catechol lignin-Fe < 3 + > complex, and a shell layer comprises a pH regulator and a shell layer coating material. Defects and active sites are introduced into the layered double-metal hydroxide through ball milling activation, so that the activation efficiency of peroxydisulfate is improved; catechol lignin is obtained by performing quaternization and demethylation modification on industrial lignin, so that the catechol lignin has both a flocculation effect and an electron transfer effect; partitioning and sequential release of components are realized through a core-shell structure, interaction during storage is reduced, and use stability is improved. When the composite agent is used for treating papermaking wastewater, the composite agent has a good removal effect on COD, chromaticity and suspended solids.

Description

Technical Field

[0001] This invention relates to the field of papermaking wastewater treatment technology, and in particular to a composite agent for papermaking wastewater treatment, its preparation method, and its application. Background Technology

[0002] The paper industry is a vital basic industry, but it is also a major consumer of water resources and a major source of wastewater discharge. Papermaking wastewater is characterized by its complex composition, high concentration of organic matter, deep color, and poor biodegradability. It contains large amounts of lignin and its derivatives, cellulose degradation products, residual chemicals, and various additives. Direct discharge without effective treatment will cause serious pollution to aquatic environments. Therefore, developing efficient, economical, and environmentally friendly papermaking wastewater treatment technologies is of significant practical importance.

[0003] Currently, the main methods for treating papermaking wastewater include physicochemical and biochemical methods. Among physicochemical methods, coagulation and sedimentation are widely used due to their simplicity and high treatment efficiency. Commonly used coagulants include polyaluminum chloride, polyferric sulfate, and polyacrylamide. However, the single coagulation and sedimentation method has limited effectiveness in removing dissolved organic matter from papermaking wastewater, especially for lignin-based recalcitrant organic matter, making it difficult to meet increasingly stringent emission standards.

[0004] Advanced oxidation technologies (AOCs), as effective methods for degrading recalcitrant organic pollutants, have garnered significant attention in wastewater treatment in recent years. Among these, persulfate-based AOC processes have become a research hotspot due to their strong oxidizing power, wide applicable pH range, and mild operating conditions. Under activation conditions, persulfate can generate reactive oxides such as sulfate radicals or singlet oxygen, effectively oxidizing and decomposing recalcitrant organic pollutants in wastewater. However, traditional persulfate activation methods, such as thermal activation, ultraviolet light activation, and transition metal activation, suffer from high energy consumption, large equipment investment, and the potential for secondary pollution, limiting their large-scale application in practical engineering.

[0005] Layered bimetallic hydroxides, as a class of inorganic functional materials with a unique layered structure, possess excellent ion exchange and adsorption properties due to the intercalation of various anions between their layers. Intercalating persulfate into the layers of layered bimetallic hydroxides can achieve stable storage and controlled release of persulfate. However, existing persulfate-intercalated layered bimetallic hydroxide materials still need improvement in activation efficiency and selectivity, and their synergistic effect with flocculation processes is not ideal.

[0006] Lignin is one of the main byproducts of the papermaking industry, and it is widely available and inexpensive. Chemically modifying lignin to prepare flocculants can not only realize the resource utilization of papermaking waste but also reduce wastewater treatment costs. Existing lignin-based flocculants mainly improve their charge density and flocculation performance through cationization modification, but research on their synergistic effects with advanced oxidation processes is limited, failing to fully utilize the reductive activation function of the phenolic hydroxyl groups in the lignin molecular structure.

[0007] In addition, existing composite wastewater treatment agents mostly adopt simple physical mixing methods. The components are prone to interaction during storage, which affects the stability of the agent. During use, the release of each component lacks time sequence, making it difficult to achieve effective synergy between flocculation and oxidation processes. Summary of the Invention

[0008] To address the aforementioned problems in the existing technology, the present invention aims to provide a composite agent for treating papermaking wastewater, its preparation method, and its application. This composite agent achieves the sequential release of each component through a core-shell structure design. It performs interfacial composite of ball-milled activated persulfate intercalated layered bimetallic hydroxide and catecholized lignin, giving full play to the synergistic effect of flocculation and advanced oxidation, and effectively improving the removal efficiency of refractory organic matter in papermaking wastewater.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of this invention provides a composite agent for treating papermaking wastewater, comprising the following raw materials in parts by weight: 60-85 parts of core-shell structured composite particles, 15-30 parts of polyaluminum chloride, and 3-10 parts of coagulant aid; wherein the core layer of the core-shell structured composite particles comprises ball-milled activated persulfate intercalated layered bimetallic hydroxide and catecholized lignin-Fe 3+ An interfacial complex of a coordination compound; the shell of the core-shell structured composite particles includes a pH adjuster and a shell coating material.

[0010] Further, the layered bimetallic hydroxide is one or more combinations of MgAl-LDH, ZnAl-LDH, CaAl-LDH, and NiAl-LDH. The layered bimetallic hydroxide is a layered structure material prepared by co-precipitation of divalent and trivalent metal ions, wherein the molar ratio of divalent to trivalent metal ions is 2-4:1. MgAl-LDH refers to a material with Mg... 2+ divalent metal ions, Al 3+ It is a layered bimetallic hydroxide composed of trivalent metal ions; the ZnAl-LDH refers to a compound with Zn... 2+ divalent metal ions, Al 3+ It is a layered bimetallic hydroxide composed of trivalent metal ions; the CaAl-LDH refers to a compound composed of Ca...2+ divalent metal ions, Al 3+ It is a layered bimetallic hydroxide composed of trivalent metal ions; the NiAl-LDH refers to a bimetallic hydroxide composed of Ni... 2+ divalent metal ions, Al 3+ It is a layered bimetallic hydroxide composed of trivalent metal ions. Preferably, the layered bimetallic hydroxide is MgAl-LDH, which has good interlayer stability and persulfate intercalation capacity.

[0011] Furthermore, the d(003) interlayer spacing of the ball-milled activated persulfate intercalated layered bimetallic hydroxide is 0.85-0.98 nm, and the specific surface area is 80-200 m². 2 / g. The d(003) interlayer spacing is the interlayer spacing of the (003) crystal planes of the layered bimetallic hydroxide determined by X-ray diffraction. The increase in this interlayer spacing indicates that persulfate anions have successfully inserted into the interlayer of the layered bimetallic hydroxide. The specific surface area is the BET specific surface area determined by nitrogen adsorption-desorption method. Ball milling treatment can significantly improve the specific surface area of ​​the material and increase the number of active sites. Ball milling activation treatment introduces lattice defects and surface active sites into the layered bimetallic hydroxide, changes the activation pathway of persulfate, and is conducive to the generation of active oxide species such as singlet oxygen. Compared with the traditional sulfate radical pathway, it has higher selectivity and helps to reduce the risk of halogenated byproduct formation.

[0012] Further, the persulfate is one or a combination of sodium persulfate, potassium persulfate, and ammonium persulfate. Preferably, the persulfate is sodium persulfate or potassium persulfate, which has good water solubility and stability. The persulfate, also known as persulfate, contains a peroxy bond in its molecule, which can break under activation conditions to produce reactive oxide species.

[0013] Furthermore, the catecholized lignin is an industrial lignin modified by quaternization and demethylation. Quaternization modification involves introducing quaternary ammonium groups onto the lignin molecule, giving it a positive charge and enhancing its adsorption and flocculation capacity for negatively charged pollutants. Demethylation modification involves converting the methoxy group in the lignin molecule into an ortho-dihydroxyl structure, i.e., a catechol structure, which possesses strong reducing power and metal ion coordination ability. The catecholized lignin simultaneously possesses cationic flocculation and electron shuttle functions. The ortho-dihydroxyl group in its molecular structure can act as an electron shuttle, reducing Fe(III) to Fe(II), maintaining the Fe(III) / Fe(II) redox cycle, and continuously activating persulfate to generate active oxide species.

[0014] Further, the industrial lignin is one of alkali lignin, lignin sulfonate, or enzymatically hydrolyzed lignin. Alkali lignin refers to lignin produced during the alkaline pulping process in the papermaking industry, with a molecular weight generally between 1000-10000 Da, containing numerous phenolic and alcoholic hydroxyl groups. Lignosulfonate refers to lignin produced during the sulfite pulping process, containing sulfonic acid groups in its molecule and exhibiting good water solubility. Enzymatically hydrolyzed lignin refers to lignin residue produced during the enzymatic hydrolysis and saccharification of biomass, retaining a relatively intact structure. Preferably, the industrial lignin is alkali lignin, which is widely available, inexpensive, and contains numerous active groups suitable for modification.

[0015] Furthermore, the quaternizing agent used in the quaternization modification is glycidyltrimethylammonium chloride or 3-chloro-2-hydroxypropyltrimethylammonium chloride. The molecular formula of the glycidyltrimethylammonium chloride is C6H. 14 ClNO, whose epoxy group can undergo a ring-opening reaction with the phenolic hydroxyl group in the lignin molecule, thereby grafting a quaternary ammonium group onto the lignin molecule. The molecular formula of the 3-chloro-2-hydroxypropyltrimethylammonium chloride is C6H4. 15 Cl2NO, whose chlorine atom can undergo a nucleophilic substitution reaction with the phenolic hydroxyl group in the lignin molecule, achieving quaternization modification. Preferably, the quaternizing agent is glycidyltrimethylammonium chloride, which has high reactivity and good modification efficiency.

[0016] Furthermore, the demethylating agent used in the demethylation modification is a combination of n-dodecyl mercaptan and sodium hydroxide. The molecular formula of the n-dodecyl mercaptan is C2. 12 H 26 S, whose thiol group can attack the methyl carbon atom of the methoxy group in the lignin molecule under alkaline conditions, undergoing a nucleophilic substitution reaction, causing the methoxy group to lose its methyl group and transform into a phenolic hydroxyl group, thereby forming an ortho-diphenolic hydroxyl structure in the lignin molecule. The sodium hydroxide is used to provide an alkaline reaction environment to promote the demethylation reaction. The mass ratio of the n-dodecyl mercaptan to sodium hydroxide is 1:2-5.

[0017] Further, the shell coating material is one or a combination of chitosan, sodium carboxymethyl cellulose, and sodium alginate. Chitosan is a natural high-molecular-weight polysaccharide obtained by deacetylation of chitin, possessing good film-forming properties and biocompatibility, with a preferred degree of deacetylation of 80-95%. Sodium carboxymethyl cellulose is a carboxymethyl etherification product of cellulose, possessing good water solubility and thickening properties, with a preferred degree of substitution of 0.6-1.0. Sodium alginate is a natural polysaccharide extracted from brown algae, capable of forming a gel with calcium ions and exhibiting good coating performance. Preferably, the shell coating material is a combination of chitosan and sodium carboxymethyl cellulose in a mass ratio of 1:1-3, forming a composite shell with a suitable dissolution rate.

[0018] Furthermore, the pH adjuster is calcium oxide and / or calcium hydroxide. The calcium oxide reacts with water to generate calcium hydroxide and releases heat, which helps accelerate the dissolution of the shell. The calcium hydroxide is a strong alkali, which can quickly adjust the pH of the wastewater to the alkaline range, facilitating the flocculation reaction. The content of the pH adjuster in the shell is 40-70% of the total mass of the shell, preferably 50-60%. Too low a content of pH adjuster will result in insufficient pH adjustment, while too high a content will affect the shell's formability and mechanical strength.

[0019] Furthermore, the particle size of the core-shell composite particles is 0.5-3 mm, preferably 1-2 mm. Too small a particle size will cause the particles to disperse too quickly in the wastewater, making it difficult to achieve sequential release; too large a particle size will result in a too slow dissolution rate, affecting treatment efficiency. The mass ratio of the core layer to the shell layer is 2-5:1, preferably 3-4:1. This ratio ensures that the shell layer has sufficient thickness to achieve the sequential release function, while the core layer contains sufficient active components to ensure the treatment effect.

[0020] Furthermore, the coagulant aid is one or a combination of anionic and nonionic polyacrylamide. The anionic polyacrylamide has a molecular weight of 8-18 million Da and a degree of hydrolysis of 20-35%. Its carboxyl groups can form coordination bonds with metal ions in the flocs, playing a bridging flocculation role and promoting floc growth and sedimentation. The nonionic polyacrylamide has a molecular weight of 6-15 million Da and is suitable for acidic or neutral wastewater conditions. Preferably, the coagulant aid is anionic polyacrylamide, which has the best coagulant aid effect.

[0021] Furthermore, the polyaluminum chloride has an alumina content of 28-32% and a basicity of 40-90%. The polyaluminum chloride is an inorganic polymeric coagulant containing a large number of hydroxyl and polynuclear aluminum hydroxyl ions in its molecules. It possesses strong charge neutralization and adsorption bridging capabilities, and can synergistically exert flocculation effects with catecholated lignin.

[0022] A second aspect of the present invention provides a method for preparing the above-mentioned composite agent for treating papermaking wastewater, comprising the following steps: S1. Mix layered bimetallic hydroxide and persulfate at a mass ratio of 10:1-3, and ball mill for 1-6 hours at a speed of 300-600 rpm. The ball-to-material mass ratio is 10-30:1 to obtain ball-milled activated persulfate intercalated layered bimetallic hydroxide.

[0023] The layered bimetallic hydroxide can be prepared by co-precipitation: a divalent metal salt and a trivalent metal salt are dissolved in deionized water at a molar ratio of 2-4:1 to prepare a mixed salt solution with a total metal ion concentration of 0.5-2 mol / L; under stirring conditions, the mixed salt solution and the alkaline solution are simultaneously added dropwise to the reaction vessel, controlling the pH of the reaction system to be 9-11, and stirring is continued for 1-4 hours after the addition is complete; the obtained precipitate is filtered, washed until neutral, dried at 60-80℃ for 12-24 hours, ground and sieved to obtain the layered bimetallic hydroxide. The divalent metal salt is one or more of magnesium sulfate, magnesium chloride, magnesium nitrate, zinc sulfate, zinc chloride, zinc nitrate, calcium chloride, calcium nitrate, nickel sulfate, and nickel chloride; the trivalent metal salt is one or more of aluminum sulfate, aluminum chloride, and aluminum nitrate; the alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution with a concentration of 1-4 mol / L.

[0024] The ball milling process is performed using a planetary ball mill or a stirred ball mill. The milling media are zirconia balls or stainless steel balls with a diameter of 3-10 mm. A combination of milling media with different diameters is preferred to improve milling efficiency. The ball milling process introduces lattice defects into the layered bimetallic hydroxide and simultaneously promotes the exchange of persulfate anions with the existing anions between the layers of the layered bimetallic hydroxide, thus achieving persulfate intercalation. The preferred milling time is 2-4 hours. Too short a time will result in incomplete intercalation and insufficient defect introduction, while too long a time will lead to the destruction of the layered structure.

[0025] S2. Industrial lignin is dissolved in sodium hydroxide solution, a quaternizing agent is added and reacted at 60-80℃ for 2-4h for quaternization modification, and then a demethylating agent is added and reacted at 80-120℃ for 4-8h for demethylation modification. After purification and drying, catecholized lignin is obtained.

[0026] The concentration of the sodium hydroxide solution is 5-20 wt%, and the mass ratio of industrial lignin to sodium hydroxide solution is 1:5-15. The amount of the quaternizing agent is 20-60% of the mass of industrial lignin. The quaternization modification reaction is preferably carried out under nitrogen protection to prevent oxidative degradation of lignin under alkaline high-temperature conditions. The amount of n-dodecyl mercaptan in the demethylating agent is 30-80% of the mass of industrial lignin. The demethylation modification reaction is preferably carried out in a closed container, and the reaction temperature is preferably 90-110℃.

[0027] The purification process includes: cooling the reaction solution to room temperature, adjusting the pH to 2-4 with dilute hydrochloric acid to precipitate the modified lignin; filtering and washing the precipitate to remove unreacted reagents and byproducts; drying the washed precipitate under vacuum at 50-70℃ for 12-24 hours, grinding and sieving to obtain catecholized lignin powder. The obtained catecholized lignin has a quaternary ammonium group content of 0.5-2.0 mmol / g and an ortho-dihydroxyl content of 1.5-4.0 mmol / g. The quaternary ammonium group content can be determined by the Kjeldahl method or elemental analysis; the ortho-dihydroxyl content can be determined by the Folin-Ciocalteu method or UV-Vis spectrophotometry.

[0028] S3. Dissolve catechin-modified lignin in water, add the ball-milled and activated persulfate intercalated layered bimetallic hydroxide obtained in step S1, adjust the pH to 8-9, stir the reaction for 2-6 hours, then add ferric salt solution, adjust the pH to 4-6, stir the reaction for 1-3 hours, collect the precipitate after solid-liquid separation, wash and dry to obtain the core layer material.

[0029] The mass ratio of the catechin-phenolized lignin to the ball-milled and activated persulfate intercalated layered bimetallic hydroxide is 1-5:1, preferably 2-3:1. The concentration of the catechin-phenolized lignin dissolved in water is 5-20 wt%. Under alkaline conditions of pH 8-9, the phenolic hydroxyl groups in the catechin-phenolized lignin molecules partially dissociate and can coordinate with metal ions on the surface of the layered bimetallic hydroxide, thereby achieving interfacial composite formation of catechin-phenolized lignin on the surface of the layered bimetallic hydroxide.

[0030] The ferric salt is ferric sulfate and / or ferric chloride. The molecular formula of the ferric sulfate is Fe2(SO4)3, and the molecular formula of the ferric chloride is FeCl3. The concentration of the ferric salt solution is 0.1-1 mol / L. The molar ratio of catechol functional groups to ferric ions in the catecholized lignin is 3-8:1, preferably 4-6:1. Within this molar ratio range, the catechol functional groups can react with Fe... 3+ A stable coordination structure is formed while retaining sufficient free catechol functional groups for subsequent electron shuttle function. Under acidic conditions of pH 4-6, Fe... 3+ It forms a stable complex with the catechol functional group. During wastewater treatment, the complex can achieve Fe(III) / Fe(II) cyclic transformation through the redox reaction of the catechol functional group, and continuously activate persulfate.

[0031] The solid-liquid separation is performed by centrifugation or filtration, with a centrifugation speed of 3000-8000 rpm and a centrifugation time of 5-20 min. The washing process involves washing 2-5 times with deionized water to remove unreacted reagents and soluble salts. Drying is carried out at 50-80℃ for 12-24 h.

[0032] S4. Prepare a shell slurry by mixing a pH adjuster with a shell coating material, and prepare core-shell composite particles by fluidized bed coating or rolling granulation of the core material and shell slurry. The shell coating thickness is 50-200 μm, and the particles are dried.

[0033] The preparation method of the shell coating material is as follows: when the shell coating material is sodium carboxymethyl cellulose and / or sodium alginate, it is dissolved in water to prepare a solution with a mass concentration of 2-10%; when the shell coating material is chitosan, it is dissolved in an aqueous acetic acid solution with a mass concentration of 1-3% to prepare a solution with a mass concentration of 2-10%, a pH adjuster powder is added, and the mixture is stirred evenly to obtain the shell coating material. The solid content of the shell coating material is 10-40 wt%.

[0034] The fluidized bed coating is performed using a bottom-spray or top-spray fluidized bed coating machine, with an inlet air temperature of 40-70℃, an atomization pressure of 0.1-0.4MPa, and a spray rate of 5-30mL / min. The rolling granulation is performed using a disc granulator or a rotary drum granulator, with a rotation speed of 10-40rpm and a granulation time of 10-60min. Fluidized bed coating is preferred due to its better coating uniformity and thickness controllability.

[0035] The shell coating thickness is preferably 80-150 μm. If the shell thickness is too thin, it will be easily damaged during storage, resulting in poor time-sequential release; if the shell thickness is too thick, the shell dissolution time will be too long, affecting processing efficiency. Drying is carried out at 40-60℃ for 6-12 hours, and the moisture content of the core-shell composite particles after drying should be less than 5%.

[0036] S5. Mix the core-shell composite particles, polyaluminum chloride, and coagulant according to the specified ratio to obtain a composite agent for papermaking wastewater treatment.

[0037] The mixing process is carried out using a V-type mixer, a three-dimensional motion mixer, or a double-cone mixer, with a mixing time of 10-30 minutes and a mixing speed of 10-30 rpm. Vigorous stirring should be avoided during mixing to prevent damage to the shell layers of the core-shell composite particles. The resulting composite agent for papermaking wastewater treatment should be stored in a sealed container in a cool, dry place, avoiding moisture.

[0038] A third aspect of the present invention provides an application of the above-mentioned composite agent for treating papermaking wastewater in the treatment of papermaking wastewater, wherein the dosage of the composite agent is 200-800 mg / L and the treatment time is 30-120 min.

[0039] The papermaking wastewater includes pulping wastewater, bleaching wastewater, papermaking wastewater, and their mixed wastewater. The COD concentration of the papermaking wastewater is 500-5000 mg / L, the color is 200-1000 times, and the SS concentration is 200-2000 mg / L.

[0040] The dosage of the compound agent is adjusted according to the degree of wastewater pollution. For wastewater with high COD concentration, the upper limit of the dosage is used; for wastewater with low COD concentration, the lower limit of the dosage is used. Preferably, the dosage of the compound agent is 300-600 mg / L.

[0041] The treatment process includes: adding the compound agent to the papermaking wastewater, rapidly stirring for 1-3 minutes to ensure uniform dispersion of the agent, then slowly stirring for 20-60 minutes to induce flocculation, and finally allowing the mixture to settle for 10-30 minutes. The supernatant is then used to test water quality indicators. The rapid stirring speed is 150-300 rpm, and the slow stirring speed is 30-80 rpm.

[0042] During the treatment process, the shell layer of the core-shell composite particles dissolves first, releasing a pH adjuster to adjust the pH of the wastewater to an alkaline range of 8-10, which is conducive to the flocculation reaction. Subsequently, the core layer material is exposed to the wastewater, and the ball-milled and activated persulfate intercalated layered bimetallic hydroxide releases persulfate and catecholizes lignin-Fe 3+ The activation effect of the complex generates active oxide species, which oxidize and degrade recalcitrant organic matter in the wastewater. Simultaneously, catecholized lignin and polyaluminum chloride synergistically exert a flocculation effect, removing suspended solids and colloidal substances from the wastewater through flocculation and sedimentation. Through the above-mentioned sequential release and synergistic mechanism, the composite agent of this invention can achieve highly efficient treatment of papermaking wastewater.

[0043] Compared with the prior art, the present invention provides a composite agent for treating papermaking wastewater, its preparation method and application, which has the following beneficial effects: 1. This invention employs a core-shell structure design, encapsulating the pH adjuster in the shell layer and placing the active oxidizing component and flocculating component in the core layer. This achieves the sequential release of each component, avoiding interactions between components during storage and improving the storage stability of the agent. Simultaneously, it ensures the orderly conduct of the flocculation and oxidation processes, thereby increasing treatment efficiency. Furthermore, this invention uses lignin, a byproduct of the papermaking industry, as a raw material to prepare the flocculant component, realizing the resource utilization of papermaking waste, reducing wastewater treatment costs, and demonstrating significant economic and environmental benefits.

[0044] 2. This invention employs a ball milling activation method to prepare persulfate intercalated layered bimetallic hydroxides. The ball milling process introduces lattice defects into the layered bimetallic hydroxides, which helps to alter the activation pathway of persulfate and improve oxidation selectivity. Simultaneously, this invention performs dual modification of industrial lignin through quaternization and demethylation. The resulting catecholized lignin possesses both cationic flocculation and electron shuttle functions. The ortho-dihydroxyl groups in its molecular structure help promote the Fe(III) / Fe(II) redox cycle, continuously activating persulfate and achieving a synergistic effect of flocculation and advanced oxidation.

[0045] 3. The composite agent of the present invention has excellent treatment effect on papermaking wastewater, with a COD removal rate of 85-95%, a color removal rate of 90-98%, and an SS removal rate of over 94%. Detailed Implementation

[0046] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] All raw materials used in the embodiments of this invention are commercially available products, as detailed below: Alkali lignin: molecular weight approximately 5000 Da; Chitosan: degree of deacetylation 90%; Sodium carboxymethyl cellulose: degree of substitution 0.8; Polyaluminum chloride: alumina content 30%; Anionic polyacrylamide: molecular weight 12 million Da, degree of hydrolysis 25%; Nonionic polyacrylamide: molecular weight 10 million Da; All other products were commercially available. Example 1

[0048] This embodiment provides a composite agent for treating papermaking wastewater, prepared according to the following steps: S1. Preparation of ball-milled activated persulfate intercalated layered bimetallic hydroxides (1) Preparation of MgAl-LDH: Magnesium sulfate hexahydrate and aluminum nitrate nonahydrate were prepared according to the Mg... 2+ With Al 3+ The mixed salt solution with a total metal ion concentration of 1 mol / L was prepared by dissolving it in deionized water at a molar ratio of 3:1. Under stirring conditions, the mixed salt solution and a sodium hydroxide solution with a concentration of 2 mol / L were simultaneously added dropwise to the reaction vessel, and the pH of the reaction system was controlled at 10. After the addition was completed, stirring was continued for 2 hours. The resulting precipitate was filtered, washed until neutral, dried at 70℃ for 18 hours, and ground through a 100-mesh sieve to obtain MgAl-LDH.

[0049] (2) Ball milling activation: MgAl-LDH and sodium persulfate were mixed at a mass ratio of 10:2 and placed in a planetary ball mill. Zirconia balls with a diameter of 5 mm were used as the ball milling medium. The ball-to-material mass ratio was 20:1. The ball milling speed was 450 rpm and the ball milling time was 3 h to obtain ball-milled activated persulfate intercalated MgAl-LDH. XRD analysis showed that the d(003) interlayer spacing was 0.92 nm; BET analysis showed that the specific surface area was 140 m². 2 / g.

[0050] S2. Preparation of catechin-modified lignin Alkali lignin was dissolved in a 10% sodium hydroxide solution at a mass ratio of 1:10. Under nitrogen protection, 40% (by weight) of glycidyltrimethylammonium chloride (by weight) of the alkali lignin was added, and the mixture was reacted at 70°C for 3 hours for quaternization modification. Then, 50% (by weight) of n-dodecyl mercaptan (by weight) of the alkali lignin and 150% (by weight) of sodium hydroxide (by weight) of the alkali lignin were added, and the mixture was reacted at 100°C for 6 hours for demethylation modification. After the reaction was completed, the mixture was cooled to room temperature, and the pH was adjusted to 3 with dilute hydrochloric acid to precipitate the modified lignin. The precipitate was filtered, washed, vacuum dried at 60°C for 18 hours, and ground through a 100-mesh sieve to obtain catecholized lignin. The content of ortho-dihydroxyl groups was determined to be 2.8 mmol / g by the Folin-Ciocalteu method.

[0051] S3. Preparation of core layer materials Catechinized lignin was dissolved in deionized water to prepare a 12% (w / w) solution. The ball-milled and activated persulfate-intercalated MgAl-LDH obtained in step S1 was added, with a mass ratio of 2.5:1 between the catechinized lignin and the ball-milled and activated persulfate-intercalated MgAl-LDH. The pH was adjusted to 8.5 with sodium hydroxide solution, and the reaction was stirred for 4 hours. Then, a 0.5 mol / L ferric sulfate solution was added to control the molar ratio of catechol functional groups to ferric ions in the catechinized lignin to be 5:1. The pH was adjusted to 5 with dilute hydrochloric acid, and the reaction was stirred for 2 hours. The mixture was centrifuged at 5000 rpm for 10 minutes, and the precipitate was collected, washed three times with deionized water, and dried at 65°C for 18 hours to obtain the core layer material.

[0052] S4. Preparation of core-shell composite particles Chitosan was dissolved in a 2% (w / w) aqueous solution of acetic acid, and sodium carboxymethyl cellulose was added at a mass ratio of 1:2 to prepare a 5% (w / w) solution. Calcium oxide powder was then added and stirred until homogeneous to obtain a shell slurry, in which calcium oxide accounted for 55% of the total solid mass of the shell layer. The core layer material was placed in a fluidized bed coating machine with an inlet air temperature of 55℃, an atomization pressure of 0.25MPa, and a spraying rate of 15mL / min to spray the shell slurry. The mass ratio of the core layer to the shell layer was controlled at 3.5:1, and the shell coating thickness was 120μm. The mixture was dried at 50℃ for 8 hours to obtain core-shell composite particles with a particle size of 1.5mm.

[0053] S5. Preparation of composite agents for papermaking wastewater treatment 72 parts of core-shell composite particles, 22 parts of polyaluminum chloride, and 6 parts of anionic polyacrylamide were placed in a three-dimensional motion mixer and mixed at 20 rpm for 20 min to obtain a composite agent for papermaking wastewater treatment. Example 2

[0054] This embodiment provides a composite agent for treating papermaking wastewater: In step S1, the mass ratio of MgAl-LDH to sodium persulfate was 10:1, the ball milling time was 1 hour, the ball milling speed was 300 rpm, and the ball-to-material mass ratio was 10:1. The d(003) interlayer spacing was measured to be 0.86 nm, and the specific surface area was 82 m². 2 / g.

[0055] In step S2, the quaternization modification temperature is 60℃ and the time is 2h; the demethylation modification temperature is 80℃ and the time is 4h.

[0056] In step S3, the pH was adjusted to 8 and the reaction was stirred for 2 hours; the molar ratio of catechol functional groups to ferric ions was 3:1, the pH was adjusted to 4, and the reaction was stirred for 1 hour.

[0057] In step S4, the calcium oxide content in the shell is 40% of the total solid mass of the shell, the mass ratio of the core to the shell is 2:1, the shell coating thickness is 50 μm, and the particle size is 0.5 mm.

[0058] In step S5, there are 60 parts of core-shell composite particles, 30 parts of polyaluminum chloride, and 3 parts of anionic polyacrylamide. Example 3

[0059] This embodiment provides a composite agent for treating papermaking wastewater: In step S1, the mass ratio of MgAl-LDH to sodium persulfate was 10:3, the ball milling time was 6 hours, the ball milling speed was 600 rpm, and the ball-to-material mass ratio was 30:1. The d(003) interlayer spacing was measured to be 0.97 nm, and the specific surface area was 198 m². 2 / g.

[0060] In step S2, the quaternization modification temperature is 80℃ and the time is 4h; the demethylation modification temperature is 120℃ and the time is 8h.

[0061] In step S3, the pH was adjusted to 9 and the reaction was stirred for 6 hours; the molar ratio of catechol functional groups to ferric ions was 8:1, the pH was adjusted to 6, and the reaction was stirred for 3 hours.

[0062] In step S4, the calcium oxide content in the shell is 70% of the total solid mass of the shell, the mass ratio of the core to the shell is 5:1, the shell coating thickness is 200 μm, and the particle size is 3 mm.

[0063] In step S5, there are 85 parts of core-shell composite particles, 15 parts of polyaluminum chloride, and 10 parts of anionic polyacrylamide. Example 4

[0064] This embodiment provides a composite agent for treating papermaking wastewater, which differs from Embodiment 1 in that: In step S1, ZnAl-LDH is used instead of MgAl-LDH. The preparation method of ZnAl-LDH is as follows: zinc nitrate hexahydrate and aluminum nitrate nonahydrate are mixed according to Zn... 2+ With Al 3+ The solution was dissolved in deionized water at a molar ratio of 3:1, and the remaining preparation conditions were the same as those for the preparation of MgAl-LDH in Example 1.

[0065] The remaining steps and parameters are the same as in Example 1. Example 5

[0066] This embodiment provides a composite agent for treating papermaking wastewater, which differs from Embodiment 1 in that: In step S1, potassium persulfate is used instead of sodium persulfate.

[0067] The remaining steps and parameters are the same as in Example 1. Example 6

[0068] This embodiment provides a composite agent for treating papermaking wastewater, which differs from Embodiment 1 in that: In step S2, sodium lignin sulfonate is used instead of alkali lignin, and 3-chloro-2-hydroxypropyltrimethylammonium chloride is used instead of glycidyltrimethylammonium chloride as the quaternizing agent.

[0069] The remaining steps and parameters are the same as in Example 1. Example 7

[0070] This embodiment provides a composite agent for treating papermaking wastewater, which differs from Embodiment 1 in that: In step S4, sodium alginate is used as the shell coating material instead of the combination of chitosan and sodium carboxymethyl cellulose, and calcium hydroxide is used instead of calcium oxide as the pH adjuster.

[0071] The remaining steps and parameters are the same as in Example 1. Example 8

[0072] This embodiment provides a composite agent for treating papermaking wastewater, which differs from Embodiment 1 in that: In step S5, nonionic polyacrylamide is used as the coagulant aid instead of anionic polyacrylamide.

[0073] The remaining steps and parameters are the same as in Example 1.

[0074] Comparative Example 1 This comparative example provides a papermaking wastewater treatment agent, which differs from Example 1 in that: In step S1, ball milling was not performed. MgAl-LDH and sodium persulfate were directly mixed in deionized water at a mass ratio of 10:2 for 4 hours, filtered, and dried to obtain persulfate-intercalated MgAl-LDH. The d(003) interlayer spacing was determined to be 0.78 nm, and the specific surface area was 45 m². 2 / g.

[0075] The remaining steps and parameters are the same as in Example 1.

[0076] Comparative Example 2 This comparative example provides a papermaking wastewater treatment agent, which differs from Example 1 in that: In step S2, only quaternization modification is performed, without demethylation modification, to obtain quaternized lignin.

[0077] The remaining steps and parameters are the same as in Example 1.

[0078] Comparative Example 3 This comparative example provides a papermaking wastewater treatment agent, which differs from Example 1 in that: In step S3, no ferric salt solution is added.

[0079] The remaining steps and parameters are the same as in Example 1.

[0080] Comparative Example 4 This comparative example provides a papermaking wastewater treatment agent, which differs from Example 1 in that: Instead of preparing core-shell composite particles, the core layer material obtained in step S3 is directly physically mixed with polyaluminum chloride, anionic polyacrylamide, and calcium oxide in the same ratio to obtain a papermaking wastewater treatment agent.

[0081] Comparative Example 5 This comparative example provides a papermaking wastewater treatment agent, using a combination of conventional coagulants: A papermaking wastewater treatment agent was obtained by physically mixing 80 parts of polyaluminum chloride, 10 parts of anionic polyacrylamide, and 10 parts of calcium oxide. Test methods

[0082] Wastewater discharged from the pulping workshop of a paper mill was taken as a test water sample. The raw water quality indicators were: COD 2800mg / L, color 600 times, SS 850mg / L, pH 6.5.

[0083] The papermaking wastewater treatment agents obtained in Examples 1-8 and Comparative Examples 1-5 were used to treat the above-mentioned papermaking wastewater. The dosage was 500 mg / L. The mixture was stirred rapidly (200 rpm) for 2 min, stirred slowly (50 rpm) for 40 min, and allowed to settle for 20 min. The supernatant was taken to determine COD, color and SS, and the removal rate was calculated.

[0084] (1) COD determination: The determination shall be carried out in accordance with HJ 828-2017 "Determination of Chemical Oxygen Demand in Water by Dichromate Method". When the COD of the water sample exceeds the upper limit of the method, the water sample shall be appropriately diluted according to the standard requirements before determination. (2) Color determination: The color was determined according to HJ 1182-2021 "Determination of Color in Water - Dilution Method"; (3) SS determination: The determination shall be carried out in accordance with GB / T 11901-1989 "Determination of suspended solids in water by gravimetric method". Test data

[0085] Table 1. Wastewater treatment effects of the examples and comparative examples: .

[0086] As shown in Table 1, Example 1, using the optimal parameter combination, achieved a COD removal rate of 92.5%, a color removal rate of 96.8%, and a SS removal rate of 98.2%, demonstrating the best treatment effect. Examples 2 and 3, using the lower and upper limits of all parameters respectively, achieved COD removal rates of 85.6% and 87.8%, both lower than Example 1, indicating that the synergistic effect was optimal when the parameters were at their median values. Examples 4-8 verified the substitutability of different types of layered bimetallic hydroxides, persulfate, lignin raw materials, shell materials, and coagulants, with COD removal rates all between 88% and 92%, indicating that the present invention has good applicability and flexibility in raw material selection.

[0087] Comparative Example 1, without ball milling activation, achieved a COD removal rate of only 72.5%, a decrease of 20 percentage points compared to Example 1, indicating that ball milling activation is a key step in improving the activation efficiency of persulfate. Comparative Example 2, without demethylation modification, and Comparative Example 3, without the addition of ferric salt, achieved COD removal rates of 76.8% and 74.2%, respectively, indicating that the ortho-diphenolic hydroxyl structure and Fe... 3+ The complex plays a crucial role in promoting the redox cycle. Comparative Example 4, without a core-shell structure design, achieved a COD removal rate of 80.5%, indicating that the sequential release function of the core-shell structure is beneficial for improving the synergistic effect of flocculation and oxidation. Comparative Example 5, using a conventional coagulant combination, achieved a COD removal rate of only 65.2%, demonstrating that the composite agent of this invention has significant technical advantages compared to conventional coagulants.

[0088] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A composite agent for treating papermaking wastewater, characterized in that, The raw materials include the following parts by weight: 60-85 parts of core-shell composite particles, 15-30 parts of polyaluminum chloride, and 3-10 parts of coagulant aid; The core layer of the core-shell composite particles comprises ball-milled activated persulfate intercalated layered bimetallic hydroxide and catecholized lignin-Fe 3+ Interfacial complexes of coordination compounds; The shell of the core-shell composite particle includes a pH adjuster and a shell coating material.

2. The composite agent for treating papermaking wastewater according to claim 1, characterized in that, The layered bimetallic hydroxide is one or a combination of MgAl-LDH, ZnAl-LDH, CaAl-LDH, and NiAl-LDH; the d(003) interlayer spacing of the ball-milled activated persulfate intercalated layered bimetallic hydroxide is 0.85-0.98 nm, and the specific surface area is 80-200 m². 2 / g.

3. The composite agent for treating papermaking wastewater according to claim 1, characterized in that, The persulfate is one or more combinations of sodium persulfate, potassium persulfate, and ammonium persulfate.

4. The composite agent for treating papermaking wastewater according to claim 1, characterized in that, The catechin-modified lignin is an industrial lignin that has undergone quaternization and demethylation modification; the industrial lignin is one of alkali lignin, lignin sulfonate, and enzymatically hydrolyzed lignin.

5. The composite agent for treating papermaking wastewater according to claim 4, characterized in that, The quaternization reagent used in the quaternization modification is glycidyltrimethylammonium chloride or 3-chloro-2-hydroxypropyltrimethylammonium chloride; the demethylation reagent used in the demethylation modification is a combination of n-dodecyl mercaptan and sodium hydroxide.

6. The composite agent for treating papermaking wastewater according to claim 1, characterized in that, The shell coating material is one or more of chitosan, sodium carboxymethyl cellulose, and sodium alginate; the pH adjuster is calcium oxide and / or calcium hydroxide, and the content of the pH adjuster in the shell is 40-70% of the total mass of the shell.

7. The composite agent for treating papermaking wastewater according to claim 1, characterized in that, The core-shell composite particles have a particle size of 0.5-3 mm and a mass ratio of core to shell of 2-5:1; the coagulant is one or a combination of anionic polyacrylamide and nonionic polyacrylamide.

8. A method for preparing a composite agent for treating papermaking wastewater as described in any one of claims 1-7, characterized in that, Includes the following steps: S1. The layered bimetallic hydroxide and persulfate are mixed at a mass ratio of 10:1-3 and ball-milled for 1-6 hours at a speed of 300-600 rpm. The ball-to-material mass ratio is 10-30:1 to obtain ball-milled activated persulfate intercalated layered bimetallic hydroxide. S2. Industrial lignin is dissolved in sodium hydroxide solution, quaternization reagent is added and reacted at 60-80℃ for 2-4h for quaternization modification, then demethylation reagent is added and reacted at 80-120℃ for 4-8h for demethylation modification, purified and dried to obtain catecholized lignin. S3. Dissolve catecholized lignin in water, add the ball-milled activated persulfate intercalated layered double metal hydroxide obtained in step S1, adjust the pH to 8-9, stir the reaction for 2-6 hours, then add ferric salt solution, adjust the pH to 4-6, stir the reaction for 1-3 hours, collect the precipitate after solid-liquid separation, wash and dry to obtain the core layer material. S4. Prepare a shell slurry by mixing a pH adjuster with a shell coating material, and prepare core-shell composite particles by fluidized bed coating or rolling granulation of the core material and shell slurry. The shell coating thickness is 50-200 μm, and the particles are then dried. S5. Mix the core-shell composite particles, polyaluminum chloride, and coagulant according to the specified ratio to obtain a composite agent for papermaking wastewater treatment.

9. The preparation method according to claim 8, characterized in that, In step S3, the ferric salt is ferric sulfate and / or ferric chloride, and the molar ratio of catechol functional groups to ferric ions in the catecholized lignin is 3-8:

1.

10. The application of a composite agent for papermaking wastewater treatment as described in any one of claims 1-7 in the treatment of papermaking wastewater, characterized in that, The dosage of the compound agent is 200-800 mg / L, and the treatment time is 30-120 min.