Gradient composite light regeneration water treatment flocculant as well as preparation method and application thereof

Abstract

The invention discloses a gradient composite light regeneration water treatment flocculant as well as a preparation method and application thereof. The preparation method comprises the following steps: 1, mixing nano chitin fibers, a lignin-polyphenol complex and photocatalytic microspheres obtained by loading Fe3O4 (at) TiO2 nanoparticles on sodium alginate; 2, cross-linking to form a homogeneous colloid; 3, directionally arranging in a magnetic field; and 4, gradient freeze drying. According to the invention, the nano chitin fibers, the lignin-polyphenol complex and the photocatalytic microspheres are crosslinked to form the gradient composite light regeneration water treatment flocculant, and the flocculant has a four-dimensional adsorption mechanism of electrostatic attraction, pi-pi accumulation, coordination complexation and spatial screening; the efficient removal capability on various pollutants such as heavy metal ions, organic pollutants and suspended solids can be realized. In addition, due to the high crystallinity of the nano chitin fibers, a mechanical supporting effect can be achieved on the interior of the flocculating agent, and the magnetic agglomeration phenomenon of magnetic nuclei containing Fe3O4 is prevented.

Description

Technical Field

[0001] The present invention belongs to the technical field of sewage treatment, and particularly relates to a gradient composite photocatalytic water treatment flocculant, a preparation method thereof, and an application thereof. Background Art

[0002] In the sewage treatment technology system, as a key functional material, the performance of the flocculant directly affects the pollutant separation efficiency and the economy of subsequent processes. The existing flocculation systems are mainly constructed based on two major categories of traditional materials, inorganic and organic, but there are significant technical bottlenecks in the aspects of synergistic removal of pollutants and resource recycling.

[0003] Inorganic flocculants are represented by aluminum-based compounds such as polyaluminum chloride (PAC). Although they have advantages such as easy availability of raw materials and better economy, they have multiple application defects: First, the dosage of the agent needs to reach a relatively high concentration to form effective flocs (usually > 200 mg / L), resulting in a 30%-50% increase in sludge production; Second, the generated flocs have a loose structure, and their sedimentation rate is generally lower than 5 cm / min, seriously affecting the solid-liquid separation efficiency; Third, the complexation and capture ability for dissolved organic pollutants with a molecular weight < 1000 Da and heavy metal ions (such as Cr³⁺, Cd²⁺) is weak, and it is difficult to meet the advanced treatment standards of effluent CODcr < 30 mg / L and heavy metal residue < 0.1 mg / L.

[0004] In the field of organic flocculants, synthetic polymers such as polyacrylamide (PAM) are dominant. Through long-chain bridging, they can form dense flocs (sedimentation rate > 15 cm / min), but there are ecological risks that cannot be ignored: First, the inherent stability of the C-C bonds in the main chain of synthetic polymers results in a biodegradation rate of less than 5%, and the environmental half-life exceeds 180 days; Second, the residual amount of unreacted monomers (such as acrylamide) in the production process can reach 0.05%-0.2%, which has significant neurotoxicity; Third, the conventional process cannot effectively depolymerize the used flocs, resulting in an annual waste volume exceeding one million tons.

[0005] In response to the above problems, researchers have turned to the development of natural biomass-modified flocculants in recent years. Materials such as chitosan and starch have attracted much attention due to their biocompatibility (degradation rate > 90%). However, such materials are limited by a single active site (amino or hydroxyl group), and the synchronous removal rate for heavy metal-organic composite pollution systems is less than 60%, and their thermal stability is poor (thermal decomposition temperature < 200°C), making it difficult to adapt to the high-load conditions of industrial wastewater treatment.

[0006] More notably, existing flocculants generally lack the characteristics of being renewable. According to industry statistics, the recycling rate of traditional flocculants is less than 2%, and the annual amount of wasted resources reaches billions of yuan. Currently, most regeneration technologies focus on chemical elution methods (soaking in hydrochloric acid / EDTA) and high-temperature pyrolysis methods (500 - 800 °C). However, the former requires 3 - 5 times the dosage of regeneration reagents and generates secondary pollution waste liquid containing heavy metals (heavy metal leaching concentration > 50 mg / L); although the latter can achieve the regeneration of more than 80% of the inactivated agents, the high-temperature process leads to the breakage of molecular chains (molecular weight decrease > 60%), and the flocculation efficiency of the regenerated agent only maintains 40% - 50% of the initial value.

[0007] It can be seen that the existing technical system has systematic defects in three dimensions: broad-spectrum removal of pollutants, environmental friendliness, and resource recycling. With the continuous improvement of the effluent index requirements in regulations such as the "Water Pollution Prevention and Control Action Plan" (such as TP < 0.3 mg / L, TN < 10 mg / L), it is urgent to develop new flocculation materials with the characteristics of efficient co-removal of multiple pollutants, biodegradability, and regeneration under mild conditions, which has important strategic significance for promoting the green transformation of the water treatment industry. Summary of the Invention

[0008] In order to meet the above requirements, the purpose of the present invention is to provide a gradient composite photocatalytic regeneration water treatment flocculant, its preparation method and application, to overcome the shortcomings of existing flocculants, achieve efficient removal of multiple pollutants, and at the same time have good magnetic recovery performance, which can reduce the treatment cost and reduce secondary pollution.

[0009] In the first aspect, the present invention provides a preparation method of a gradient composite photocatalytic regeneration water treatment flocculant, which includes the following steps: Step 1: Mix nanocrystalline chitin fibers, lignin-polyphenol complexes, and photocatalytic microspheres obtained by loading Fe3O4@TiO2 nanoparticles on sodium alginate in a mass ratio of 1:(0.3 - 0.8):(0.1 - 0.5) and add them to PBS buffer solution.

[0010] Step 2: Add a cross-linking agent to the mixture obtained in Step 1 and carry out ultrasonic reaction to form a homogeneous colloid.

[0011] Step 3: Place the colloid obtained in Step 2 in a magnetic field with a magnetic field strength of 0.3 - 0.8 T to make Fe3O4 in the colloid align directionally.

[0012] Step 4: Perform gradient freeze-drying on the product obtained in Step 3 to form and fix the mesoporous structure to obtain the flocculant.

[0013] The three components in the obtained flocculant can play a synergistic role to achieve efficient pollutant treatment: the positively charged amino groups on the nano-chitin fibers and the negatively charged groups on the lignin-polyphenol complex form a charge gradient field, and colloidal particles and heavy metals are adsorbed through electrostatic interaction; the three-dimensional sieve structure formed by cross-linking of the three components intercepts suspended solids; meanwhile, the lignin-polyphenol complex chelates heavy metals with phenolic hydroxyl groups and improves the COD removal rate by π-π stacking of the lignin aromatic rings. At the same time, the TiO2 in component C generates electron-hole pairs under light irradiation, oxidizing organic substances and reducing heavy metals respectively; calcium alginate generates a gel barrier to protect the active components when the pH value > 6.

[0014] Moreover, the nano-chitin fibers and the lignin-polyphenol complex provide more attachment sites for the photocatalytic microspheres, improving the dispersion uniformity of the photocatalytic microspheres; the lignin-polyphenol complex has a photosensitization effect, which can improve the light absorption efficiency of TiO2, promote the generation of photo-generated carriers, and further enhance the photocatalytic degradation effect, and synergistically degrade organic pollutants with the photocatalytic microspheres.

[0015] Preferably, the preparation process of the nano-chitin fibers is as follows: (1) Crush the shells of crustacean organisms (preferably shrimp shells and crab shells) to obtain chitin raw material particles.

[0016] (2) Remove calcium carbonate from the chitin raw material particles in an acidic system to obtain decalcified particles.

[0017] (3) Deproteinization treatment: Remove proteins from the decalcified particles in an alkaline system to obtain crude white chitin.

[0018] (4) Add the crude chitin and cellulase to a buffer solution and apply microwave radiation to obtain an enzymatic hydrolysis product.

[0019] (5) Pressurize the enzymatic hydrolysis product to form a nanofiber suspension, and perform solid-liquid separation to obtain nano-chitin fibers.

[0020] Preferably, the preparation process of the lignin-polyphenol complex is as follows: (1) Crush lignocellulosic biomass to obtain biomass raw material particles.

[0021] (2) Add the biomass raw material particles to deionized water, adjust the temperature and pressure, and perform subcritical water extraction.

[0022] (3) Perform fractional purification on the product obtained in step (2) to obtain a powdery lignin-polyphenol complex.

[0023] Preferably, the preparation process of the photocatalytic microspheres is as follows: (1) Fe3O4 nanoparticles are obtained by reacting FeCl2·4H2O and FeCl3·6H2O in an alkaline system. After mixing tetrabutyl titanate with absolute ethanol, it is dropped into the ethanol dispersion containing Fe3O4 nanoparticles; the obtained product is calcined to obtain core-shell structured Fe3O4@TiO2 nanoparticles.

[0024] (2) After mixing Fe3O4@TiO2 nanoparticles with sodium alginate solution, it is dropped into CaCl2 solution for cross-linking and curing to obtain photocatalytic microspheres.

[0025] Preferably, when the pollutants in the treatment object of the flocculant are heavy metals, the mass ratio of nano-chitin fiber, lignin-polyphenol complex, and photocatalytic microspheres is 1:(0.7~0.8):(0.1~0.3).

[0026] When the pollutants in the treatment object of the flocculant are organic pollutants, the mass ratio of nano-chitin fiber, lignin-polyphenol complex, and photocatalytic microspheres is 1:(0.3~0.5):(0.4~0.5).

[0027] Preferably, the process of gradient freeze-drying is as follows: the product obtained in step three is pre-frozen at -30~-10°C, deeply frozen at -60~-40°C, and then vacuum-dried.

[0028] In the second aspect, the present invention provides a gradient composite photocatalytic water treatment flocculant, which is prepared by the aforementioned preparation method.

[0029] In the third aspect, the present invention provides an application of the gradient composite photocatalytic water treatment flocculant in the treatment of wastewater containing heavy metals and / or organic pollutants.

[0030] In the fourth aspect, the present invention provides a wastewater treatment method, the process of which is to add the gradient composite photocatalytic water treatment flocculant to the wastewater to be treated; the heavy metals and / or organic pollutants in the wastewater to be treated aggregate to form flocculation products.

[0031] Preferably, light is applied to the wastewater to be treated to degrade the organic pollutants in the flocculation products; after flocculation, the flocculation products are separated and placed in a regeneration system under light conditions to obtain regenerated flocculant. The pH value of the regeneration system is 2.5~4.

[0032] Preferably, the wastewater to be treated is printing and dyeing wastewater.

[0033] The beneficial effects of the present invention are as follows: The present invention crosslinks nano-chitin fibers (component A), lignin-polyphenol complex (component B), and photocatalytic microspheres (component C) to form a gradient composite photocatalytic water treatment flocculant, constructing a "charge complementarity - adsorption catalysis - magnetic response regeneration" trinity synergistic system. Among them, the protonated amino groups on the surface of component A carry positive charges and attract negatively charged colloidal particles in sewage; component B contains negatively charged groups that can chelate heavy metals through coordination bonds, and the lignin aromatic ring forms π-π stacking with the aromatic rings in organic pollutant molecules. The protonated amino groups on the surface of component A and the phenolic hydroxyl groups / carboxyl groups of component B form a dynamic charge gradient field, and the pollutants are directionally enriched through electrostatic attraction and π-π interaction; component C realizes the simultaneous oxidation and decomposition of the enriched organic pollutants and the reduction of the enriched heavy metal ions through the photogenerated carrier separation effect of the Fe3O4@TiO2 core-shell heterojunction.

[0034] In the present invention, component C is loaded with calcium alginate on the basis of the core-shell structured Fe3O4@TiO2 nanoparticles, and the interaction among the three reduces the effective band gap of TiO2, shifting the light response range towards the long-wavelength visible light region. In addition, the lignin aromatic ring and polyphenol groups of component B not only strengthen the adsorption of pollutants, but their photosensitization effect also expands the light response range of TiO2 towards the visible light region.

[0035] The high crystallinity of the nano-chitin fibers (component A) in the present invention can improve the compressive strength of the flocculant in the water flow impact or stirring environment, and at the same time can provide a mechanical support for the inside of the flocculant, preventing the magnetic aggregation of component C, ensuring the uniform dispersion of component C in the system, and maintaining good magnetic properties.

[0036] The present invention has an "adsorption - photolysis - regeneration" cycle mechanism; in the treatment stage (pH≈7), components A / B rapidly adsorb pollutants through amino / phenolic hydroxyl groups, and the photocatalysis of component C degrades to regenerate the active sites; in the regeneration stage (pH≈3), the calcium alginate gel barrier shrinks, releasing newly formed TiO2 to assist in the regeneration of the flocculant. At the same time, component B chelates Fe 3+ inhibits the corrosion of the magnetic core, ensuring the removal rate of pollutants after the material is recycled. Description of the Drawings

[0037] Figure 1 It is a flow chart of Example 1 of the present invention. Detailed Embodiments

[0038] The following further elaborates on the present invention with reference to the embodiments.

[0039] Example 1 As Figure 1 shown, a preparation method of a gradient composite photocatalytic water treatment flocculant includes the following steps: Step 1: Prepare raw materials for components A, B, and C. Among them, component A is nano-chitin fiber; component B is lignin-polyphenol complex; component C is photocatalytic microsphere.

[0040] The preparation process of the nano-chitin fiber (component A) includes the following steps: (1) Raw material cleaning: Take fresh shrimp shells, remove the residual meat, crush them into particles with a particle size of 2 - 5 mm, wash them 3 times with deionized water to remove impurities. In some other embodiments, the shrimp shells can be replaced with crab shells or the shells of other crustacean organisms, and the shells of multiple crustacean organisms can be mixed.

[0041] (2) Demineralization treatment: Immerse the raw material particles obtained in step (1) in 5% HCL at a liquid-solid ratio of 10:1 and stir magnetically at a stirring speed of 300 rpm for 24 h until no bubbles are generated and the CaCO3 in the particles is completely dissolved; after filtration, wash with deionized water until neutral (pH≈7), and dry at 60 °C to obtain demineralized particles.

[0042] (3) Deproteinization treatment: Immerse the demineralized particles in 3% NaOH solution and react in a reaction vessel equipped with a condensation reflux device at 90 °C for 4 h (liquid-solid ratio 15:1); centrifuge (8000 rpm, 10 min), take the precipitate and wash it until neutral to obtain crude white chitin.

[0043] (4) Microwave-enzymatic hydrolysis coupling treatment: Disperse the crude chitin in an acetic acid-sodium acetate buffer solution with a pH value of 5.0 at a solid-liquid ratio of 1:50; add 0.5% cellulase (enzyme activity ≥500 U / mg), and keep it in a water bath at 50 °C; simultaneously apply microwave radiation (800 W, pulse mode: 2 s on / 5 s off) for 6 h to obtain an enzymatic hydrolysis product. In this step, the local hot spots generated by the microwave can reduce the crystallinity of the chitin crystal structure, make the enzyme molecules more easily penetrate the chitin crystal structure, accelerate the enzymatic hydrolysis reaction, and thus refine the chitin.

[0044] (5) High-pressure homogenization: Circulate the enzymatic hydrolysis product through a high-pressure homogenizer at a pressure of 30 MPa for 5 times, with an interval of 5 min between each treatment to obtain a nano-fiber suspension. The diameter of the nano-chitin fibers in the nano-fiber suspension obtained in this step is 50 - 100 nm. This step uses the action of high-pressure shear force to further strip the enzymatically refined chitin into nano-fibers, exposing a large number of amino groups on the surface.

[0045] The preparation process of the lignin-polyphenol complex (component B) includes the following steps: (1)Raw material pretreatment: The corncobs are crushed to 60 mesh and dried to constant weight at 105 °C to obtain the B-component raw material. In some other embodiments, the corncobs can be replaced with bagasse or other lignocellulosic biomasses, and multiple lignocellulosic biomasses can be used in combination.

[0046] (2)Subcritical water extraction: The B-component raw material obtained in step (1) and deionized water are added to a high-pressure reactor at a mass ratio of 1:15, and the temperature is set at 180 °C and the pressure is 6 MPa to make the deionized water reach the subcritical state for 30 min. In this step, the dielectric constant ε of water in the subcritical state is approximately 20, which can selectively dissolve the lignin-hemicellulose cross-linked structure, and its mild conditions can retain the polyphenol antioxidant groups in the lignin-hemicellulose cross-linked structure.

[0047] (3)Fractionation and purification: The cooled crude extract is filtered through a 0.22 μm ceramic membrane to remove suspended particles; then the extract is concentrated to 1 / 5 of the original volume with a 10 kDa ultrafiltration membrane (tangential flow rate 2 m / s); finally, the concentrated solution is placed in a vacuum freeze dryer and dried for 24 h at -50 °C and 0.1 Pa to obtain a dark brown powder.

[0048] The synthesis method of the photocatalytic microspheres (C component) described above includes the following steps: (1)Preparation of Fe3O4@TiO2 nanoparticles with core-shell structure: ①.Synthesis of Fe3O4 magnetic core: FeCl2·4H2O and FeCl3·6H2O are dissolved in deionized water at a molar ratio of 1:2, and 25% NH3·H2O is added dropwise at 80 °C until the pH value = 10, and aged for 1 h; magnetic separation is carried out, washed with ethanol 3 times, and vacuum dried at 60 °C to obtain 50 nm Fe3O4 particles. ②.Coating of TiO2: Tetrabutyl titanate and absolute ethanol are mixed at a ratio of 1:4 and added dropwise to the ethanol dispersion containing Fe3O4; the molar ratio of Fe to Ti is 1:3; calcined at 500 °C for 2 h to obtain core-shell structured Fe3O4@TiO2 nanoparticles. Fe3O4@TiO2 is the main component of the photocatalytic microspheres. Under visible light irradiation, Fe3O4 will produce a surface plasmon resonance effect, which can enhance the absorption of visible light, enhance the local electromagnetic field, promote the generation and separation of photo-generated carriers, and make the system respond to visible light.

[0049] (2)Calcium alginate loading: Ultrasonically mix Fe3O4@TiO2 nanoparticles with a 2% sodium alginate solution at a mass ratio of 1:0.3 for 30 min; use a syringe pump to drip the resulting mixture into a 2% CaCl2 solution at a rate of 2 mL / min and crosslink and solidify for 2 h to form microspheres with a particle size of 200-400 μm. In this step, after the Fe3O4@TiO2 nanoparticles are loaded with calcium alginate, the three interact, changing the electronic structure and energy level distribution of the system, narrowing the effective bandgap of TiO2, and shifting the light response range to the long-wavelength visible light region; in addition, calcium alginate provides a stable dispersion environment for Fe3O4@TiO2, and under illumination, the photo-generated carriers of Fe3O4@TiO2 are transferred to calcium alginate through the interface, reducing the recombination probability. At the same time, its functional groups interact with Fe3O4@TiO2, promoting the separation and transport of photo-generated carriers and enhancing the photocatalytic activity in the visible light region.

[0050] The aforementioned reduction of the recombination probability specifically refers to reducing the probability that photo-generated carriers (i.e., photo-generated electrons and holes) recombine after generation. In the photocatalytic reaction, the recombination of photo-generated carriers will lead to a decrease in the photocatalytic efficiency because a part of the photo-generated carriers will be consumed during the recombination process and cannot participate in the subsequent redox reactions. Therefore, reducing the recombination probability can improve the utilization rate of photo-generated carriers and thus enhance the photocatalytic performance.

[0051] Step 2: Gradient composite assembly of components A, B, and C, including the following steps: 2-1. Mixing ratio: Mix components A, B, and C at a mass ratio of 1:0.6:0.3 and add PBS buffer; the pH value of the PBS buffer is 7.4 and the solid content is 5%.

[0052] 2-2. Interface crosslinking: Add a 0.5% glutaraldehyde crosslinking agent to the mixture obtained in step 2-1 and ultrasonically disperse for 30 min at 40 kHz to form a homogeneous colloid.

[0053] 2-3. Magnetic field forming: Inject the colloid obtained in step 2-2 into a polytetrafluoroethylene mold and place it in a permanent magnet array with a magnetic field strength of 0.5 T, and let it stand for 1 h to orient the Fe3O4 in the colloid.

[0054] In this step, the permanent magnet array refers to multiple permanent magnets arranged and combined in a specific manner to generate a magnetic field with an intensity of 0.5T, so as to achieve the regulation of specific properties of the material. The magnetic field intensity of 0.5T can provide sufficient magnetic force to enable the Fe3O4 magnetic moment to overcome interference such as thermal motion and be arranged orderly in the direction of the magnetic field, thereby laying a foundation for constructing the three-dimensional network structure of the flocculant. When Fe3O4 is arranged under a rotating magnetic field to form an electron transport channel, it also enables the entire material system to maintain good magnetic response characteristics. Using this characteristic, the material can be easily separated and recovered through an external magnetic field, realizing the recycling of the material and reducing the treatment cost.

[0055] 2 - 4. Gradient freeze-drying: Pre-freeze at -20°C for 4h to form large-size ice crystal templates by the directional growth of ice crystals; deep-freeze at -50°C for 2h to further fix the mesoporous structure; finally, place it in a vacuum drying oven and dry for 24h under the condition of 0.1Pa to obtain a flocculant with a three-dimensional network structure.

[0056] In the flocculant obtained in this example, there is a significant functional synergistic effect among the three components A, B, and C: First, the charge complementarity mechanism between the A / B components. The protonated amino groups on the surface of component A carry positive charges and can attract negatively charged colloidal particles in sewage through electrostatic interaction. At the same time, the three-dimensional sieve formed by its high aspect ratio can intercept suspended substances in water. Component B contains negatively charged groups such as phenolic hydroxyl groups and carboxyl groups, which can chelate heavy metals through coordination bonds. The lignin aromatic ring can form π - π stacking with the aromatic ring in organic pollutant molecules, thereby improving the COD removal rate. The positive charge region on component A and the negative charge region on component B can jointly form a charge gradient field, promoting the directional migration of pollutants under the action of the electric field and accelerating the flocculation reaction.

[0057] Second, the adsorption-photocatalysis linkage mechanism driven by component C. Under light irradiation conditions, TiO2 in component C generates electron-hole pairs. The photo-generated holes have strong oxidizing properties and can directly oxidize and decompose organic pollutants in sewage; the photo-generated electrons have strong reducing properties and can reduce high-valent heavy metal ions such as Cr(VI) to low-valent Cr(III). In addition, the calcium alginate network of component C will form a gel barrier when the pH value > 6, which can effectively block the attack of free radicals on components A and B and improve the ability of the flocculant to continuously play its role. At the same time, components A and B can provide more attachment sites for component C, enabling it to be more evenly dispersed in water and increasing the contact area of the photocatalytic reaction. Moreover, component B has a photosensitization effect, which can improve the light absorption efficiency of TiO2, promote the generation of photo-generated carriers, and thus enhance the photocatalytic degradation effect, and cooperate with component C to degrade organic pollutants.

[0058] During the flocculation process, components A and B first adsorb and enrich pollutants by virtue of their respective charge characteristics and chemical structures. Subsequently, component C undergoes photocatalytic in-situ degradation under light conditions, regenerating the adsorption sites, realizing the "adsorption-degradation-regeneration" cycle, and continuously exerting the purification efficiency of the flocculant.

[0059] Moreover, in the flocculant prepared in this example, the three components A, B, and C are spatially intercalated, and different pore structures cooperate with each other, having a selective removal function for pollutants with different molecular weights, which can improve the removal ability of the flocculant for complex pollutants. The high crystallinity of the nanofibers of component A endows the flocculant with good compressive strength and can maintain a stable structure even in a wet environment, ensuring that the flocculant does not break under external forces such as water flow impact or stirring during the water treatment process. The Fe3O4 magnetic core of component C endows the flocculant with magnetic response characteristics. Under the action of an external magnetic field, the flocculant can quickly aggregate and achieve rapid recovery. The mechanical support of component A can prevent the magnetic agglomeration of component C, ensuring the uniform dispersion of component C in the system and maintaining good magnetic properties. The phenolic hydroxyl groups of component B can chelate with Fe 3+ in component C to form a stable complex, inhibiting the photocorrosion of Fe3O4 during the photocatalytic process and protecting the activity of component C. The magnetism of component C can make up for the deficiency of the sedimentation performance of components A and B, enabling the flocculant to be separated from the treated water in a short time, facilitating subsequent treatment operations.

[0060] In addition, the flocculant prepared in this example has a unique regeneration synergistic effect: during the light irradiation stage (pH ≈ 7), the phenolic hydroxyl groups of component B provide hydrogen atoms, which can react with excessive hydroxyl radicals (·OH) and combine, thereby preventing the oxidation of the amino groups in component A and ensuring the stability and activity of component A during the water treatment process; during the regeneration stage (pH ≈ 3), calcium alginate can also protect the internal photocatalytic structure of component C to a certain extent and has a slow-release characteristic in an acidic environment, which can continuously expose fresh TiO2 active sites, creating conditions for the regeneration of the flocculant and thus realizing the recycling of the flocculant. On the other hand, the TiO2 shell and Fe3O4 magnetic core of component C can form a type-II heterojunction. Under light conditions, photo-generated electrons are transferred from TiO2 to Fe3O4, while holes are enriched on the surface of TiO2, thereby reducing the recombination probability of photo-generated carriers and improving the separation efficiency of photo-generated carriers. In addition, during the preparation of the flocculant, a rotating magnetic field (0.5 T) is used to induce the alignment of Fe3O4 to form a high-speed electron transmission channel, which can promote the transmission of photo-generated carriers, significantly improve the photocatalytic efficiency, and further promote the regeneration and recycling of the flocculant.

[0061] Comparative Example 1 A flocculant made of chitosan flocculant with a purity of 90%.

[0062] Comparative Example 2 A flocculant made of polyaluminum chloride with an Al2O3 content of ≥ 30%.

[0063] The following comparative experiments were conducted on the gradient composite photocatalytic water treatment flocculant prepared in Example 1 and the flocculants in Comparative Examples 1 and 2: (1) Preparation of simulated wastewater. The wastewater in this example was 1 L of simulated wastewater, a kaolin suspension containing heavy metal components and organic components. The heavy metal components included Cr(VI) at a concentration of 50 mg / L and Cu at a concentration of 40 mg / L 2+ . The organic components included phenol. The COD value of the simulated wastewater was 100 mg / L, and the turbidity was 150 NTU (2) Flocculation experiment The dosage of the flocculant was 0.5 g / L, the pH value was 7.0, the flocculation temperature was 25 ± 1 °C; there was no light; the reaction was carried out by stirring at a speed of 150 rpm for 30 min

[0064] (3) Determination of the removal effects of Cr(VI), Cu 2+ , COD, and turbidity are shown in Table 1 below: Table 1 Comparison of pollutant removal performance of Example 1, Comparative Example 1, and Comparative Example 2 Example 2: A method for treating mixed wastewater containing heavy metals and organic matter, using the gradient composite photocatalytic water treatment flocculant prepared in Example 1; the specific steps of the method are as follows: (I) Preparation of the flocculant Prepare component A (nano chitin fiber), component B (lignin-polyphenol complex), and component C (photocatalytic microsphere) according to the scheme in Example 1 above

[0065] Then perform gradient composite assembly on components A, B, and C: (1) Mixing ratio: Mix components A, B, and C in a mass ratio of 1:0.6:0.3, and add PBS buffer with a pH of 7.4 and a solid content of 5%. (2) Interface cross-linking: Add 0.5% glutaraldehyde cross-linking agent and disperse ultrasonically at 40 kHz for 30 min to form a homogeneous colloid. (3) Magnetic field shaping: Inject the colloid into a polytetrafluoroethylene mold and place it in a 0.5 T permanent magnet array, and let it stand for 1 h to align Fe3O4. (4) Gradient freeze-drying: Pre-freeze at -20 °C for 4 h, deep-freeze at -50 °C for 2 h, and finally vacuum-dry at 0.1 Pa for 24 h to obtain a flocculant with a three-dimensional network structure

[0066] (II) Use the flocculant to remove pollutants in the wastewater

[0067] Flocculation treatment: Add 0.5 g of the prepared flocculant to the wastewater with a pH value of 7.0. Place the resulting mixture in a photocatalytic reactor, irradiate it with simulated sunlight, and stir it at a speed of 150 rpm for 30 min at 25 °C for the reaction. The wastewater in this example uses 1 L of simulated wastewater, specifically a mixed wastewater containing 30 mg / L Cu 2+ , 50 mg / L Cr(VI) and 100 mg / L phenol.

[0068] In this example, the processes of flocculation treatment and photocatalytic degradation can be carried out synchronously or successively.

[0069] (3) Separation and detection: After the reaction, apply an external magnetic field to the mixture to rapidly aggregate the flocculation products in the mixture and separate the supernatant.

[0070] (III) Flocculant regeneration.

[0071] Cleaning pretreatment: Wash the separated saturated flocculant with deionized water multiple times to remove large particle impurities and easily soluble pollutants on the surface to improve the efficiency of subsequent regeneration.

[0072] Light irradiation treatment: Place the washed saturated flocculant and appropriate deionized water (adjust the pH value to about 3) in a photocatalytic reactor, turn on the dual-band light source, 365 nm (ultraviolet light) and 450 nm (visible light), set the light intensities to 50 mW / cm 2 and 100 mW / cm 2 , stir at a speed of 100 rpm, maintain the temperature at 35 °C, and irradiate for 120 min; meanwhile, monitor the ORP in real time, and stop the light irradiation treatment when ∆ORP < 5.

[0073] Physical cleaning and magnetic separation: Rapidly separate the flocculant suspension after photocatalytic regeneration through a magnetic field, and then ultrasonically clean it with deionized water for 10 minutes to remove the loosely attached pollutants and photolysis products on the surface.

[0074] Surface activity repair: Immerse the washed flocculant in a 0.5% lignin-polyphenol complex (Component B) solution for 30 minutes to repair the surface adsorption sites. (The polyphenolic hydroxyl groups of Component B can supplement the active groups oxidized during the regeneration process and restore the chelating ability for pollutants.) Drying and structure rearrangement: Vacuum freeze-drying (-50 °C, 0.1 Pa) to prevent the nanofiber network from collapsing due to capillary force.

[0075] Performance verification and activation: Take 0.5 g of the regenerated flocculant to treat the simulated sewage, same as step (II), and the results are shown in Table 2 below: Table 2 Comparison table of wastewater treatment efficiency before and after flocculant regeneration Flocculant Removal rate of Cr (VI) <![CDATA[Cu 2+ Removal rate]]> Removal rate of phenol Before regeneration 91.6% 94.7% 82% After regeneration 90.2% 94% 80.7% Determine the concentrations of Cu 2+ and Cr(VI) in the supernatant obtained in step (2) using an atomic absorption spectrometer, and determine the concentration of phenol using a high-performance liquid chromatograph. The test results are as follows: Cu 2+ The concentration decreased from 30 mg / L to 1.6 mg / L, and the removal rate was 94.7%; the Cr(VI) concentration decreased from 50 mg / L to 4.2 mg / L, and the removal rate was 91.6%; the phenol concentration decreased from 100 mg / L to 18 mg / L, and the removal rate was 82%.

[0076] It can be seen from this that the flocculant obtained by gradient composite assembly of components A, B, and C in the present invention can remove heavy metals and organic pollutants in wastewater.

[0077] In some embodiments, since the nano-chitin fibers form a dense three-dimensional network, the shear strength is improved. Therefore, for sewage treatment scenarios with relatively turbulent water flow, the proportion of component A can be increased to improve the structural strength of the obtained flocculant.

[0078] Comparative Example 3 A preparation method of a flocculant. The difference between this comparative example and Example 1 is that only component A is prepared as the flocculant.

[0079] Comparative Example 4 A preparation method of a flocculant. The difference between this comparative example and Example 1 is that only component B is prepared as the flocculant.

[0080] Comparative Example 5 A preparation method of a flocculant. The difference between this comparative example and Example 1 is that only component B is prepared as the flocculant.

[0081] Comparative Example 6 A preparation method of a flocculant. The difference between this comparative example and Example 1 is that component C is not prepared in step 1, and only components A and B are assembled in step 2. The assembly process is as follows: Mix components A and B in a mass ratio of 1:0.6; add a PBS buffer solution with a pH value of 7.4 and a solid content of 5%; add a glutaraldehyde cross-linking agent with a mass fraction of 0.5%, and disperse by ultrasonic wave at 40 kHz for 30 min, and then perform gradient freeze-drying (-20°C pre-freezing for 4 h, -50°C deep-freezing for 2 h, vacuum drying at 0.1 Pa for 24 h).

[0082] Test the flocculation effects of the flocculants prepared in Example 1, Comparative Examples 3, 4, 5, and 6. The test process is the same as that in step (2) of Example 2.

[0083] Collect the concentrations of Cu 2+ , Cr(VI), and phenol in each experimental group. The results are shown in Table 2 below.

[0084] Table 2 Comparison Table of Pollutant Removal Performance of Example 1, Comparative Examples 3, 4, 5, and 6 Flocculant Removal rate of Cr (VI) <![CDATA[Cu 2+ Removal rate]]> Removal rate of phenol Comparative example 3 (only component A) 43.1% 36.7% 19.5% Comparative example 4 (only component B) 22.4% 80.6% 58.2% Comparative example 5 (only component C) 65.3% — 36.8% Comparative example 6 (components A + B) 75.9% 83.5% 60.1% Example 1 (components A + B + C) 91.6% 94.7% 82% Example 3 A method for treating printing and dyeing wastewater, using the gradient composite photocatalytic water treatment flocculant prepared in Example 1; (I) Preparation of Flocculant Prepare component A (nano chitin fiber), component B (lignin-polyphenol complex), and component C (photocatalytic microsphere) according to the scheme in Example 1 above, and then perform gradient composite assembly on components A, B, and C; the specific process is the same as that in Example 1.

[0085] (II) Removal of Pollutants in Wastewater by Flocculant.

[0086] (1) Flocculation treatment: Take 0.6 g of the flocculant and add it to 1 L of printing and dyeing wastewater, and stir at a speed of 180 rpm for 40 min at 25 °C for reaction. The wastewater in this example is taken from the effluent of the secondary sedimentation tank of a textile factory, and its initial COD is measured to be 450 mg / L and the chromaticity is 500 times.

[0087] (2) Photocatalytic degradation: Place the obtained mixed solution in a photocatalytic reactor, irradiate it with simulated sunlight for 150 min, and keep stirring at the same time.

[0088] (3) Separation and detection: After the reaction, apply an external magnetic field to the mixed solution to make the flocculation products in the mixed solution aggregate quickly and separate the supernatant.

[0089] (III) Regeneration of Flocculant.

[0090] This step is the same as step (III) of Example 2.

[0091] Detect the COD value of the supernatant obtained in step (II). The detection result is: the COD value drops from 450 mg / L to 69 mg / L, and the removal rate is 84.7%; the chromaticity drops from 500 times to 25 times, and the removal rate is 95%.

[0092] It can be shown from this that the flocculant obtained by gradient composite assembly of components A, B, and C in the present invention can achieve efficient and sufficient treatment of printing and dyeing wastewater Example 4 A preparation method of a gradient composite photocatalytic water treatment flocculant. The difference between this example and Example 1 is only that: the mass ratio of components A, B, and C is different; in this example, the mass ratio of components A, B, and C is 1:0.6:0.3. The flocculant prepared in this example is applicable to the treatment of conventional industrial wastewater. Component B provides high-density adsorption sites, and the proportion of component C is moderate to ensure the visible light response efficiency during the regeneration stage.

[0093] Example 5 A preparation method of a gradient composite photocatalytic water treatment flocculant. The difference between this example and Example 1 is only that: the mass ratio of components A, B, and C is different; in this example, the mass ratio of components A, B, and C is 1:0.8:0.2. The flocculant prepared in this example is applicable to the treatment of surface water mainly polluted by heavy metals. The proportion of component B is increased to 40%, the content of polyphenol hydroxyl groups increases, and the adsorption capacity for heavy metal ions is improved. The amount of component C is reduced, and the photocatalytic regeneration time needs to be extended to 3 h.

[0094] Example 6 A preparation method of a gradient composite photocatalytic water treatment flocculant. The difference between this example and Example 1 is only that: the mass ratio of components A, B, and C is different; in this example, the mass ratio of components A, B, and C is 1:0.3:0.5. The flocculant prepared in this example is applicable to the treatment of high-organic-load sewage that needs to be regenerated frequently; in this example, the proportion of component C is increased to 28%, the loading of Fe3O4@TiO2 is increased, the density of photo-generated carriers is increased, and the regeneration time is shortened to 1.5 h. The decrease in the amount of component B leads to a decrease in the adsorption capacity.

[0095] Example 7 A preparation method of a gradient composite photocatalytic water treatment flocculant. The difference between this example and Example 1 is only that: the mass ratio of components A, B, and C is different; in this example, the mass ratio of components A, B, and C is 1:0.3:0.1. The flocculant prepared in this example is applicable to the treatment of high-turbulence and low-concentration sewage; in this example, the proportion of component A is relatively high, and the nano-chitin fibers form a dense three-dimensional network, improving the shear strength; the amounts of components B and C are reduced, and the total production cost is reduced.

Claims

1. A method for preparing a gradient composite photoregeneration water treatment flocculant, characterized in that: The following steps are involved: Step 1: Nano-chitin fibers, lignin-polyphenol complexes, and Fe3O4@TiO2 nanoparticles loaded on sodium alginate to obtain photocatalytic microspheres are mixed in a mass ratio of 1: (0.3-0.8): (0.1-0.5) and added into PBS buffer; Step 2: adding a crosslinking agent to the mixture obtained in step 1 and subjecting it to ultrasonic reaction to form a homogeneous colloid; Step 3, placing the colloid obtained in step 2 in a magnetic field with a magnetic field strength of 0.3-0.8T, so that the Fe3O4 in the colloid is oriented; Step 4: performing gradient freeze drying on the product obtained in step 3 to obtain a flocculant.

2. The preparation method according to claim 1, characterized in that: The preparation process of the nano chitin fiber is as follows: (1) crushing the shells of crustaceans to obtain chitin raw material particles; (2) removing calcium carbonate from chitin raw material particles in an acidic system to obtain decalcified particles; (3) Deproteinization: The decalcified particles are deproteinized in an alkaline system to obtain a crude white chitin product; (4) adding crude chitin and cellulase into a buffer and applying microwave radiation to obtain an enzymatic hydrolysis product; (5) The enzymatic hydrolysis product is pressurized to form a nanofiber suspension, and the nano-chitin fibers are obtained by solid-liquid separation.

3. The preparation method according to claim 1, characterized in that: The preparation process of the lignin-polyphenol complex is as follows: (1) crushing lignocellulosic biomass to obtain biomass raw material particles; (2) adding biomass raw material particles into deionized water, adjusting the temperature and pressure, and performing subcritical water extraction; (3) The product obtained in step (2) is subjected to graded purification to obtain a powdery lignin-polyphenol complex.

4. The preparation method according to claim 1, characterized in that: The preparation process of the photocatalytic microspheres is as follows: (1) FeCl2·4H2O and FeCl3·6H2O are reacted in an alkaline system to obtain Fe3O4 nanoparticles; tetrabutyl titanate is mixed with anhydrous ethanol and then added dropwise to an ethanol dispersion containing Fe3O4 nanoparticles; the obtained product is calcined to obtain Fe3O4@TiO2 nanoparticles with a core-shell structure; (2) Fe3O4@TiO2 nanoparticles were mixed with sodium alginate solution and then dropped into CaCl2 solution for cross-linking and curing to obtain photocatalytic microspheres.

5. The preparation method according to claim 1, characterized in that: When the pollutants in the treatment object of the flocculant are heavy metals, the mass ratio of the nano-chitin fiber, the lignin-polyphenol complex, and the photocatalytic microspheres is 1:(0.7-0.8):(0.1-0.3); When the pollutants in the treatment object of the flocculant are organic pollutants, the mass ratio of the nano-chitin fiber, the lignin-polyphenol complex, and the photocatalytic microspheres is 1:(0.3-0.5):(0.4-0.5).

6. The preparation method according to claim 1, characterized in that: The process of gradient freeze drying is as follows: the product obtained in step 3 is sequentially pre-frozen at -30 to -10°C, deep-frozen at -60 to -40°C, and vacuum-dried.

7. A gradient composite photo-regenerated water treatment flocculant, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 6.

8. Use of a gradient composite photo-regeneration water treatment flocculant as described in claim 7 in the treatment of wastewater containing heavy metals and / or organic pollutants.

9. A method for treating wastewater, characterized in that: The gradient composite photo-regeneration water treatment flocculant as claimed in claim 7 is added to the wastewater to be treated; the heavy metals and / or organic pollutants in the wastewater to be treated aggregate to form flocculation products.

10. A wastewater treatment method according to claim 9, characterized in that: Applying light to the wastewater to be treated to degrade organic pollutants in the flocculation products; After the flocculation is completed, the flocculation product is separated and placed in a regeneration system under light conditions to obtain a regenerated flocculant; the pH value of the regeneration system is 2.5-4.

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