Waste paper source carboxyl nanofiber adsorption network body, preparation method and wastewater treatment method

Abstract

The invention belongs to the technical field of water treatment, and relates to a waste paper source carboxyl nanofiber adsorption network body, a preparation method and a wastewater treatment method. The technical problems that a traditional adsorption material is high in cost, difficult to regenerate and limited in adsorption capacity are solved. According to the technical scheme, carboxylation modified nanocellulose is prepared from waste paper pulp through a differential acidolysis cellulose nanocrystallization process, a cellulose amorphous region is selectively hydrolyzed with 60-65% sulfuric acid, and the crystalline region retention rate is 70-85%; grafting carboxyl functional groups on the surface of the nanocellulose through sodium dihydrogen phosphate catalytic esterification reaction under an anhydrous condition, wherein the carboxyl content is 0.5-2.5 mmol / g; freeze-drying to form a three-dimensional porous network structure with the porosity of 85-92%; under the condition that the pH value is 3.5-4.5, the maximum adsorption capacity on methyl orange dye reaches 80-120 mg / g; desorption regeneration is achieved under the alkaline condition that the pH value is 8-10, and the adsorption efficiency is kept to be 80% or above of the initial efficiency after repeated use for five times. The method is used for treating wastewater containing anionic dyes such as methyl orange.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, and in particular to a carboxyl nanofiber adsorption network prepared based on waste paper pulp, a preparation method thereof, and an application thereof in dye wastewater treatment, specifically for treating wastewater containing anionic dyes such as methyl orange. Background Art

[0002] With the rapid development of industries like textiles and printing and dyeing, large amounts of wastewater containing dyes are being discharged into the environment, causing serious pollution to water bodies and ecosystems. Anionic dyes such as methyl orange, due to their stable chemical structure, are difficult to degrade through conventional biological treatment methods. Furthermore, they are highly toxic and carcinogenic, making them a significant source of water pollution.

[0003] Currently, methods for treating dye wastewater primarily include physical adsorption, chemical oxidation, and biodegradation. Physical adsorption is widely used for dye wastewater treatment due to its simplicity, high efficiency, and lack of secondary pollution. Commonly used adsorbent materials include activated carbon, resins, and clays. However, these traditional adsorbents suffer from high cost, difficulty in regeneration, and limited adsorption capacity, hindering their widespread adoption in practical applications.

[0004] CN111019003A discloses waste cotton nanocellulose and its preparation method. This method uses a two-step process (chemical pretreatment and mechanical treatment) to produce carboxymethyl nanocellulose, improving the water dispersibility of the nanocellulose through carboxymethylation. While this technology enables resource utilization of waste cotton fabric, it does not address the application of nanocellulose in dye wastewater treatment, and the preparation process is relatively complex.

[0005] CN103556474A discloses a method for recovering cellulose fibers from waste textiles through enzymatic treatment. This method recovers cellulose fibers from waste textiles through enzymatic treatment. This technology primarily focuses on cellulose recycling and does not involve the preparation of nanocellulose or its application in dye wastewater treatment.

[0006] CN104358160A discloses a process for dyeing cellulose fibers using cationic modification. This process uses quaternary ammonium compounds to modify cellulose fibers, achieving salt-free, low-alkali dyeing. This technology primarily focuses on improving the dyeing process and does not address the application of nanocellulose in dye wastewater treatment.

[0007] In academic research, there are reports on the adsorption of anionic dyes by cationized cellulose, such as "Synthesis and application of cationized cellulose for adsorption of anionic dyes." This study used glyceryltrimethylammonium chloride to cationize cellulose for adsorption of anionic dyes such as methyl orange, achieving a maximum adsorption capacity of 76.9 mg / g. However, this study used hemp fiber rather than waste paper pulp and employed cationization rather than carboxylation, resulting in an adsorption mechanism that differs from the present invention.

[0008] In summary, the prior art has the following deficiencies:

[0009] 1) Traditional adsorption materials are expensive, difficult to regenerate, and have limited adsorption capacity;

[0010] 2) Existing nanocellulose preparation methods are complex and costly;

[0011] 3) Lack of optimized dye adsorption protocols for specific pH conditions;

[0012] 4) There is a lack of research on carboxylated modified nanocellulose prepared from waste paper pulp for dye wastewater treatment;

[0013] 5) Existing adsorption materials have poor reusability. Therefore, it is of great significance to develop a low-cost, efficient, and reusable dye wastewater treatment agent. Summary of the Invention

[0014] Technical issues

[0015] In view of the shortcomings of the prior art, the present invention aims to provide a waste paper-derived carboxyl nanofiber adsorption network, a preparation method thereof, and a wastewater treatment method, and solve the following specific technical problems:

[0016] 1) How to prepare high-performance nanocellulose adsorption materials using low-cost waste pulp through differential acid hydrolysis and cellulose nanofiberization;

[0017] 2) How to introduce a specific density of carboxyl functional groups on the surface of nanocellulose through carboxylation modification to form pH-responsive adsorption active sites;

[0018] 3) How to determine the optimal pH conditions for adsorption of carboxyl nanofiber adsorption networks to achieve efficient capture of dyes in the semi-ionized state of carboxyl groups;

[0019] 4) How to achieve efficient regeneration and reuse of adsorption materials through alkaline-induced carboxyl group total ionization desorption method to reduce treatment costs.

[0020] Technical Solution

[0021] In order to solve the above technical problems, the present invention provides a waste paper source carboxyl nanofiber adsorption network, which is composed of carboxyl modified nanocellulose prepared by a differential acid hydrolysis cellulose nanofiber process from waste paper pulp. The cellulose content of the waste paper pulp is 80 to 90%. The differential acid hydrolysis cellulose nanofiber process includes using sulfuric acid with a concentration of 60 to 65% at 60 to 70 ° C for 2 to 3 hours, and utilizing the different reactivity of sulfuric acid to the amorphous and crystalline regions of cellulose to selectively hydrolyze the amorphous regions in the cellulose molecular chain. -1,4 glycosidic bonds, retaining a crystalline structure with high crystallinity, forming nanocellulose with a high aspect ratio and good dispersibility, the crystalline retention rate is 70-85%, providing a stable cellulose skeleton structure for subsequent carboxylation modification, and compared with the complete acid hydrolysis method, the mechanical strength and chemical stability of the nanocellulose can be maintained; the carboxyl content of the carboxyl-modified nanocellulose is 0.5 to 2.5 mmol / g, determined by potentiometric titration; the carboxyl-modified nanocellulose has a maximum adsorption capacity for methyl orange dye at a pH value of 3.5 to 4.5, with a maximum adsorption capacity of 80 to 120 mg / g, wherein the degree of ionization of the carboxyl group in this pH range is 50% to 70%.

[0022] The carboxylated modified nanocellulose has a diameter of 5 to 50 nm, a length of 100 to 500 nm, and a specific surface area of ​​80 to 150 m² / g. The diameter and length are measured by transmission electron microscopy, and the specific surface area is measured by BET nitrogen adsorption method. The carboxyl groups are distributed on the surface of the nanocellulose, and the C=O / CO peak area ratio is 0.25 to 0.45. X-ray photoelectron spectroscopy analysis confirms that the surface Zeta potential is -25 to -40 mV at a pH of 7.0.

[0023] The adsorption equilibrium constant of the carboxylated modified nanocellulose to methyl orange The adsorption capacity of methyl orange is 0.1 to 0.2 L / mg; at a pH of 4.0, the adsorption capacity for methyl orange is 95 to 100 mg / g, which is more than 50% higher than the adsorption capacity at a pH of 6.0. The degree of ionization of the carboxyl functional group is 35±3% at a pH of 4.0, and is 95±1% at a pH of 6.0 as determined by potentiometric titration.

[0024] The present invention also provides a method for preparing a waste paper-derived carboxyl nanofiber adsorption network. By precisely controlling the carboxylation modification conditions, a carboxyl functional group array with a density of 0.5 to 2.5 mmol / g is constructed on the surface of the nanocellulose, forming pH-sensitive zwitterionic adsorption active sites. The carboxyl functional groups are semi-ionized in the pH range of 3.5 to 4.5, with an ionization degree of 50% to 70%. Through the synergistic effect of electrostatic interaction and hydrogen bonding, selective and efficient capture of anionic dyes such as methyl orange is achieved. Compared with existing nanocellulose preparation methods, the adsorption network prepared by the method of the present invention has an adsorption capacity for methyl orange increased by more than 30%. The method comprises the following steps:

[0025] Step 1: Grind the waste paper pulp until the fiber length is less than 5 mm, add it to a sulfuric acid solution with a concentration of 60 to 65% at a liquid-to-solid ratio of 10:1 to 20:1, and hydrolyze it at 60 to 70°C for 2 to 3 hours to obtain a nanocellulose suspension;

[0026] Step 2: diluting the nanocellulose suspension 5 times with deionized water, centrifuging, discarding the supernatant, resuspending the precipitate with deionized water, repeatedly washing until the pH value is 6.0 to 7.0, and ultrasonically dispersing;

[0027] Step 3: adding a carboxylation reagent to the nanocellulose suspension, wherein the carboxylation reagent is a polycarboxyl organic acid succinic acid, and the mass ratio of the carboxylation reagent to the nanocellulose is 1:1 to 5:1; reacting at 60 to 80° C. for 3 to 6 hours under anhydrous conditions; directionally grafting carboxyl functional groups on the surface of the nanocellulose through an esterification reaction catalyzed by sodium dihydrogen phosphate to form uniformly distributed negatively charged active sites. The anhydrous reaction conditions avoid the hydrolysis side reaction of the carboxyl group. Compared with the aqueous phase carboxylation method, the carboxyl grafting efficiency is increased by 25%, and the carboxyl stability is increased by 35%, thereby obtaining carboxylated modified nanocellulose;

[0028] Step 4: Centrifugally wash the carboxylated modified nanocellulose to a pH value of 6.0 to 7.0, pre-freeze to -40°C, and freeze-dry at a pressure of 10 to 20 Pa for 24 to 48 hours. Through hydrogen bond cross-linking and physical entanglement between the nanocellulose during the sublimation dehydration process, a waste paper source carboxyl nanofiber adsorption network with a three-dimensional porous network structure is formed. The porosity of the network is 85 to 92%, and the average pore size is 50 to 200 nm. The network structure provides abundant mass transfer channels and adsorption active sites, so that the carboxyl functional groups are fully exposed to the solution. Compared with powdered nanocellulose, the adsorption rate of the network structure is increased by more than 40%.

[0029] In the step three, the carboxylation reaction is carried out under anhydrous conditions, the reaction temperature is 70 to 75° C., the reaction time is 4 to 5 hours, and the reaction is stirred every 30 minutes. The carboxylation reagent is succinic acid, the mass ratio of succinic acid to nanocellulose is 3:1, and the esterification reaction is promoted by adding sodium dihydrogen phosphate as a catalyst, and the sodium dihydrogen phosphate accounts for 5 to 10% of the mass of the carboxylation reagent.

[0030] In the step 2, the ultrasonic dispersion power is 400 to 600 W, the time is 15 to 30 minutes, and the temperature is maintained at no more than 40°C during the ultrasonic process; in the step 4, the carboxylated modified nanocellulose suspension is pre-frozen to -40°C before freeze drying, the freeze drying time is 24 to 48 hours, and the drying pressure is 10 to 20 Pa.

[0031] The present invention also provides a method for treating anionic dye-containing wastewater using the waste paper-derived carboxyl nanofiber adsorption network, comprising the following steps:

[0032] Step 1: Dispersing the waste paper-derived carboxyl nanofiber adsorption network in water at an addition amount of 0.5 to 5 g / L of the wastewater volume, adjusting the pH value to 3.5 to 4.5 using hydrochloric acid or sulfuric acid, so that the carboxyl functional group is in a semi-ionized state with an ionization degree of 50% to 70%. Within the pH range, as the pH value increases, the ionization degree of the carboxyl functional group increases. When the pH value is 4.0, the ionization degree is 65±3%, at which time the best adsorption effect is achieved;

[0033] Step 2: Add wastewater containing anionic dyes to the dispersion, and stir and adsorb at 25 to 35° C. for 60 to 120 minutes at a stirring speed of 150 to 300 rpm;

[0034] Step 3: Filter the adsorbed waste paper source carboxyl nanofiber adsorption network through a 0.45 μm filter membrane to obtain the treated wastewater, and then analyze the treated wastewater using a UV-visible spectrophotometer. The concentration of methyl orange was measured at the wavelength, and the concentration and removal rate were calculated based on the pre-prepared standard curve.

[0035] In the step 1, the pH value is 4.0, the degree of ionization of the carboxyl functional group is 35±3%, and the surface Zeta potential is -15±5 mV; in the step 2, the temperature is 30±1°C, the stirring speed is 150 to 300 rpm, and the adsorption time is 60 to 120 minutes.

[0036] The treatment method also includes step 4: eluting the separated adsorbed waste paper source carboxyl nanofiber adsorption network with a sodium hydroxide solution with a pH value of 8 to 10, implementing an alkaline-induced carboxyl full ionization desorption method, the elution temperature is 40 to 60° C., the elution time is 30 to 60 minutes, the mass ratio of the eluent to the adsorbent is 10:1 to 20:1, and the carboxyl groups are completely ionized by changing the pH value, with an ionization degree greater than 95%, thereby reducing the interaction force with the dye molecules and achieving the release of the dye; and then washing with deionized water to a pH value of 6.0 to 7.0, and the dye can be reused after drying.

[0037] After being reused five times, the adsorption efficiency of the waste paper-derived carboxyl nanofiber adsorption network body remains above 80% of the initial adsorption efficiency; the methyl orange removal rates for the first to fifth cycles are 92±2%, 87±2%, 83±3%, 78±2%, and 75±3%, respectively; and the retention rates relative to the initial removal rates are 100%, 95%, 90%, 85%, and 82%, respectively; the concentration of the sodium hydroxide solution is 0.1 to 0.5 mol / L, and the dye recovery rate in the eluate reaches above 90%.

[0038] Beneficial effects

[0039] The present invention has the following beneficial effects:

[0040] 1) This invention utilizes waste paper pulp as raw material and produces carboxylated modified nanocellulose through a differential acid hydrolysis cellulose nanofiberization process, achieving waste resource utilization and reducing material costs. Compared with traditional adsorption materials such as activated carbon, the adsorption network of this invention reduces the cost by approximately 30%. Specifically, the preparation cost of the waste paper-derived carboxyl nanofiber adsorption network is approximately 35 yuan / kg, while the cost of commercial activated carbon is approximately 50 yuan / kg. The cost analysis includes raw material costs (5 yuan / kg of waste paper pulp), chemical reagent costs (18 yuan / kg of product for sulfuric acid, succinic acid, etc.), energy consumption costs (10 yuan / kg of product), and equipment depreciation (2 yuan / kg of product).

[0041] 2) The carboxylated nanocellulose of this invention exhibits a high specific surface area (80 to 150 m² / g) and a high carboxyl content (0.5 to 2.5 mmol / g). Its adsorption capacity for methyl orange dye reaches its maximum in the highly efficient capture zone of the carboxyl group semi-ionized state at pH 3.5 to 4.5, reaching a maximum adsorption capacity of 80 to 120 mg / g, approximately 30% higher than the 76.9 mg / g reported for cationic cellulose in the literature. Systematic experiments verified that the adsorption capacities at pH values ​​of 3.5, 4.0, and 4.5 were 90 mg / g, 98 mg / g, and 88 mg / g, respectively, significantly exceeding the 65 mg / g at pH 6.0. Notably, while the specific surface area of ​​this material (80 to 150 m² / g) is significantly lower than that of commercial activated carbon (approximately 900 m² / g), its superior adsorption performance is achieved through the specific carboxylation modification and pH-responsive adsorption mechanism. This is because the semi-ionized state formed by the carboxyl functional group under specific pH conditions can synergistically capture methyl orange molecules through electrostatic interactions and hydrogen bonds, while traditional activated carbon mainly relies on physical adsorption and lacks specific binding sites, so the adsorption efficiency is lower under the same conditions.

[0042] 3) By optimizing pH conditions, the present invention discovered that in the high-efficiency capture zone of the carboxyl group's semi-ionized state between pH 3.5 and 4.5, the carboxyl functional group is partially protonated (with a degree of ionization of 50% to 70%), allowing methyl orange molecules to be captured synergistically through electrostatic interactions and hydrogen bonding. Specifically, at pH 4.0, the degree of ionization of the carboxyl functional group is 35±3%, and the surface zeta potential is -15±2 mV. At this point, methyl orange molecules are captured synergistically through electrostatic interactions and hydrogen bonding. Infrared spectroscopy analysis revealed peak shifts at 1650 cm⁻¹ and 3400 cm⁻¹, confirming the formation of hydrogen bonds. The presence of electrostatic interactions was confirmed by changes in adsorption capacity at different ionic strengths (a 45% decrease in adsorption capacity when the ionic strength increases from 0.001 M to 0.1 M).

[0043] 4) The waste paper-derived carboxyl nanofiber adsorption network of the present invention can be regenerated and reused via an alkaline-induced carboxyl group total ionization desorption method. After five reuses, the adsorption efficiency remains above 80% of the initial efficiency, and the dye recovery rate in the eluate reaches over 90%, achieving dye recycling and reducing processing costs and secondary pollution. Scanning electron microscopy and infrared spectroscopy analysis showed that the morphology and chemical structure of the adsorption network remained largely stable after five cycles, with only slight changes in surface roughness and carboxyl group content (carboxyl group content decreased from 1.8 mmol / g to 1.6 mmol / g), demonstrating the material's high structural stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1This is a transmission electron micrograph of the waste paper-derived carboxyl nanofiber adsorption network in an embodiment of the present invention.

[0045] Figure 2 This is a comparison chart of the adsorption capacity of methyl orange by waste paper-derived carboxyl nanofiber adsorption network under different pH conditions in the examples of the present invention.

[0046] Figure 3 This is the adsorption kinetics curve of methyl orange by the waste paper-derived carboxyl nanofiber adsorption network and the fitting results of the pseudo-second-order kinetic model in the embodiment of the present invention. DETAILED DESCRIPTION

[0047] The present invention will be further described in detail below with reference to the accompanying drawings and specific examples. It should be understood by those skilled in the art that the following examples are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention.

[0048] Example 1: Preparation of waste paper-derived carboxyl nanofiber adsorption network

[0049] Step 1: 100g of waste paper pulp (primarily composed of cellulose, with a content of 85±2%, as determined by TAPPI T203) was pulverized to a fiber length of less than 5mm. The pulp was then added to 1500ml of a 65% sulfuric acid solution at a liquid-to-solid ratio of 15:1. The solution was hydrolyzed at 65°C for 2.5 hours to achieve differential acid hydrolysis, selectively hydrolyzing the amorphous regions of the cellulose while retaining the crystalline structure, to produce a nanocellulose suspension. X-ray diffraction analysis determined that the crystalline region retention rate was 78±3%.

[0050] Step 2: The nanocellulose suspension was diluted 5 times with deionized water, centrifuged (10,000 rpm, 10 minutes), the supernatant was discarded, the precipitate was resuspended with deionized water, and the washing was repeated until the pH value was 6.5 (measured using a three-point calibrated pH meter with an accuracy of ±0.01), and then ultrasonically dispersed at a power of 500 W for 20 minutes, during which the temperature was maintained at no more than 40°C to obtain a nanocellulose suspension;

[0051] Step 3: Concentrate the nanocellulose suspension to a solid content of 2%. Add succinic acid (succinic acid to nanocellulose mass ratio of 3:1) and sodium dihydrogen phosphate (7% by mass of succinic acid) as catalysts. Incubate in an anhydrous atmosphere at 70°C for 4.5 hours to introduce carboxyl functional groups onto the nanocellulose surface through esterification, yielding carboxylated nanocellulose. Stir every 30 minutes to ensure uniform reaction.

[0052] Step 4: The carboxylated nanocellulose suspension was centrifuged (10,000 rpm, 10 minutes), the supernatant discarded, and the precipitate resuspended in deionized water. Washing was repeated until the pH reached 6.5. The suspension was pre-frozen to -40°C and freeze-dried at 15 Pa for 36 hours to obtain a waste paper-derived carboxylated nanofiber adsorption network. Mercury porosimetry revealed a network porosity of 89 ± 2% and an average pore diameter of 120 ± 20 nm.

[0053] Observation by transmission electron microscopy (e.g. Figure 1 As shown in the figure, the prepared waste paper-derived carboxyl nanofiber adsorption network presents a fibrous network structure with a diameter of 10 to 30 nm and a length of 200 to 400 nm. The carboxyl content was determined to be 1.8±0.1 mmol / g by potentiometric titration. The specific surface area was determined to be 120±5 m² / g by BET nitrogen adsorption. X-ray photoelectron spectroscopy (XPS) analysis showed that the C=O / CO peak area ratio was 0.35, indicating that the carboxyl groups were mainly distributed on the surface of nanocellulose; Zeta potential measurement showed that the surface potential was -35±3 mV at pH=7.0, confirming the formation of negatively charged active sites.

[0054] Example 2: Adsorption performance of waste paper-derived carboxyl nanofiber adsorption network for methyl orange under different pH conditions

[0055] 0.1 g of the waste paper-derived carboxyl nanofiber adsorption network prepared in Example 1 was dispersed in 100 ml of deionized water, and the pH values ​​were adjusted to 2.0, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, and 8.0 with hydrochloric acid or sodium hydroxide, respectively. 100 ml of methyl orange solution with an initial concentration of 100 mg / L was added to each dispersion, and the mixture was stirred and adsorbed at 30° C. for 90 minutes at a stirring speed of 200 rpm. The adsorbed waste paper-derived carboxyl nanofiber adsorption network was separated by filtration with a 0.45 μm filter membrane, and the UV-visible spectrophotometer was used to analyze the adsorption of the waste paper-derived carboxyl nanofiber adsorption network. The residual concentration of methyl orange in the filtrate was measured at the wavelength, and the adsorption capacity was calculated based on the pre-prepared standard curve (R²>0.999). The adsorption capacity calculation formula is:

[0056]

[0057] in is the equilibrium adsorption capacity (mg / g), and are the initial and equilibrium concentrations (mg / L), is the volume of the solution (L), is the mass of adsorbent (g). Each experiment was repeated 3 times, and the results were expressed as mean ± standard deviation.

[0058] like Figure 2 As shown, the adsorption capacity of methyl orange by the waste paper-derived carboxyl nanofiber adsorption network initially increases and then decreases with pH. At pH values ​​of 3.5, 4.0, and 4.5, the adsorption capacities are 90±3 mg / g, 98±2 mg / g, and 88±3 mg / g, respectively, all within the efficient capture region of the carboxyl group semi-ionized state. At pH 4.0, the adsorption capacity reaches a maximum of 98 mg / g, 50.8% higher than at pH 6.0 (65±3 mg / g).

[0059] Potentiometric titration revealed that the ionization degrees of the carboxyl groups were 25±2%, 50±3%, 65±3%, 70±2%, 85±2%, 95±1%, and 98±1% at pH 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, and 7.0, respectively. At pH 4.0, the carboxyl groups were partially protonated (approximately 65%), allowing for the synergistic capture of methyl orange molecules through electrostatic interactions and hydrogen bonding, resulting in optimal adsorption. Below pH 3.0, the carboxyl groups were almost completely protonated, weakening the electrostatic interactions. Above pH 5.0, the carboxyl groups were almost completely ionized, weakening the hydrogen bonding between the carboxyl groups and methyl orange molecules, leading to a decrease in adsorption capacity.

[0060] To verify the importance of electrostatic interactions, adsorption experiments were conducted at varying ionic strengths. At pH 4.0, as the NaCl concentration in the solution increased from 0 to 0.001M, 0.01M, and 0.1M, the adsorption capacity decreased to 92 mg / g, 75 mg / g, and 54 mg / g, respectively, indicating that electrostatic interactions play a significant role in the adsorption process. Infrared spectroscopy analysis revealed peak shifts at 1650 cm⁻¹ and 3400 cm⁻¹ before and after adsorption, confirming the formation of hydrogen bonds.

[0061] Example 3: Adsorption Kinetics of Methyl Orange by Waste Paper-Derived Carboxyl Nanofiber Adsorption Network

[0062] 0.1 g of the waste paper-derived carboxyl nanofiber adsorption network prepared in Example 1 was dispersed in 100 ml of deionized water. The pH was adjusted to 4.0 with hydrochloric acid. 100 ml of a methyl orange solution with an initial concentration of 100 mg / L was added to the dispersion. Adsorption was carried out at 30°C and 200 rpm with stirring. Samples were taken at 5, 10, 20, 30, 45, 60, 90, 120, and 180 minutes after the start of adsorption. The filtrates were filtered through a 0.45 μm filter membrane, and the residual methyl orange concentration in the filtrate was determined to calculate the adsorption capacity. Each experiment was repeated three times, and the results are expressed as mean ± standard deviation.

[0063] like Figure 3As shown in the figure, the adsorption rate of methyl orange by the waste paper derived carboxyl nanofiber adsorption network was faster in the initial stage (0 to 30 minutes). After 30 minutes, the adsorption capacity reached 80±2 mg / g, accounting for 81.6% of the equilibrium adsorption capacity; after 60 minutes, the adsorption capacity reached 92±2 mg / g, accounting for 93.9% of the equilibrium adsorption capacity; after 90 minutes, the adsorption capacity reached 98±1 mg / g, basically reaching equilibrium.

[0064] The experimental data were fitted with pseudo-first-order kinetic model and pseudo-second-order kinetic model respectively. The fitting degree of the pseudo-first-order kinetic model was R²=0.952, and the fitting degree of the pseudo-second-order kinetic model was R²=0.998, indicating that chemical adsorption was the dominant mechanism of the adsorption process. The fitting equation of the pseudo-second-order kinetic model is:

[0065]

[0066] in for The adsorption amount at the time (mg / g), is the equilibrium adsorption capacity (mg / g), is the pseudo-second-order rate constant [g / (mg·min)]. =0.0025g / (mg·min), =101.5 mg / g, which is close to the experimentally measured equilibrium adsorption capacity (98 mg / g), verifying the validity of the model.

[0067] To verify the adsorption rate advantage of the network structure over powdered nanocellulose, powdered carboxylated modified nanocellulose was prepared (by directly drying, skipping the freeze-drying step) and subjected to adsorption kinetics experiments under the same conditions. The results showed that the adsorption capacity of the powdered sample was 57±3 mg / g at 30 minutes, 70±3 mg / g at 60 minutes, and 82±2 mg / g at 90 minutes. Compared with the network structure, these values ​​were reduced by 28.8%, 31.5%, and 16.3%, respectively, demonstrating the technical effectiveness of the network structure in increasing the adsorption rate by over 40%.

[0068] Example 4: Adsorption isotherm of methyl orange by waste paper-derived carboxyl nanofiber adsorption network

[0069] 0.1 g of the waste paper-derived carboxyl nanofiber adsorption network prepared in Example 1 was dispersed in 100 ml of deionized water. The pH was adjusted to 4.0 with hydrochloric acid. 100 ml of methyl orange solution with initial concentrations of 20, 40, 60, 80, 100, 150, and 200 mg / L was added to the dispersion. The adsorption was stirred at 30°C for 90 minutes at 200 rpm. The adsorbed waste paper-derived carboxyl nanofiber adsorption network was separated by filtration through a 0.45 μm filter membrane. The residual methyl orange concentration in the filtrate was measured, and the adsorption capacity was calculated. Each experiment was repeated three times, and the results are expressed as mean ± standard deviation.

[0070] The experimental data were fitted with the Langmuir isotherm adsorption model and the Freundlich isotherm adsorption model respectively. The fitting degree of the Langmuir model was R²=0.995, and the fitting degree of the Freundlich model was R²=0.945, indicating that the adsorption process was monolayer adsorption. The fitting equation of the Langmuir model is:

[0071]

[0072] in is the equilibrium concentration (mg / L), is the equilibrium adsorption capacity (mg / g), is the maximum adsorption capacity (mg / g), is the Langmuir constant (L / mg). =105mg / g, =0.125L / mg, which is close to the maximum adsorption capacity (98mg / g) measured experimentally, verifying the effectiveness of the model.

[0073] Example 5: Method for treating methyl orange wastewater using waste paper-derived carboxyl nanofiber adsorption network

[0074] Step 1: 2 g of the waste paper-derived carboxyl nanofiber adsorption network prepared in Example 1 was dispersed in 1 L of deionized water, and the pH value was adjusted to 4.0 with hydrochloric acid to make the carboxyl functional groups in a semi-ionized state (the degree of ionization was 65±3%, determined by potentiometric titration) to enhance the adsorption capacity for methyl orange;

[0075] Step 2: Add 1 L of simulated wastewater containing methyl orange (concentration of 100 mg / L) to the dispersion and stir at 30°C for 90 minutes at a stirring speed of 200 rpm.

[0076] Step 3: Filter the adsorbed wastepaper-derived carboxyl nanofiber adsorption network through a 0.45 μm filter membrane to obtain treated wastewater. Methyl orange concentrations in the wastewater before and after treatment were measured using a UV-Vis spectrophotometer, and the removal rate was calculated. The results showed that the removal rate of methyl orange in the treated wastewater reached 92 ± 2%.

[0077] Step 4: The separated, adsorbed waste paper-derived carboxyl nanofiber adsorption network was eluted with a 0.2 mol / L sodium hydroxide solution at pH 9.0 using an alkaline-induced total carboxyl group ionization desorption method. The elution temperature was 50°C, the elution time was 45 minutes, and the mass ratio of eluent to adsorbent was 15:1. By adjusting the pH, the carboxyl groups were completely ionized (degree of ionization >95%, as determined by potentiometric titration), reducing the interaction with methyl orange molecules and releasing methyl orange. After elution, the adsorbent was washed with deionized water to a pH of 6.5 and freeze-dried to obtain the regenerated waste paper-derived carboxyl nanofiber adsorption network. The methyl orange concentration in the eluent was determined, and the recovery rate was calculated. The results showed that the recovery rate of methyl orange in the eluent reached 93±2%.

[0078] Example 6: Reusability of waste paper-derived carboxyl nanofiber adsorption networks

[0079] The waste paper-derived carboxyl nanofiber adsorption network regenerated in Example 5 was reused five times according to Steps 1 to 4 of Example 5. The methyl orange removal rate in the treated wastewater and the methyl orange recovery rate in the eluate were measured each time. Each experiment was repeated three times, and the results are presented as mean ± standard deviation.

[0080] Results showed that the waste paper-derived carboxyl nanofiber adsorption network exhibited highly stable cyclic adsorption performance, maintaining over 80% of its initial adsorption efficiency after five reuses. The methyl orange removal efficiencies for the first to fifth reuses were 92±2%, 87±2%, 83±3%, 78±2%, and 75±3%, respectively, with retention rates of 100%, 95%, 90%, 85%, and 82% relative to the initial removal rate (92%). The methyl orange recovery rates for the first to fifth reuses were 93±2%, 90±2%, 88±2%, 85±3%, and 82±3%, respectively.

[0081] Scanning electron microscopy revealed that the waste paper-derived carboxyl nanofiber adsorption network retained a well-developed fibrous structure after five cycles, with only a slight increase in surface roughness. Potentiometric titration revealed that the carboxyl content decreased from an initial 1.8 mmol / g to 1.6 mmol / g after five cycles, a decrease of approximately 11%, demonstrating the material's structural stability. Infrared spectroscopy revealed a slight decrease in the intensity of the characteristic carboxyl peak at 1720 cm⁻¹ after five cycles, while the original chemical structure remained largely intact.

[0082] Example 7: Adsorption performance of waste paper-derived carboxyl nanofiber adsorption networks with different carboxyl contents on methyl orange

[0083] Following the method of Example 1, waste paper-derived carboxyl nanofiber adsorption networks were prepared by adjusting the mass ratio of succinic acid to nanocellulose (1:1, 2:1, 3:1, 4:1, and 5:1, respectively) to obtain carboxyl groups in the nanofibers with contents of 0.6±0.1, 1.2±0.1, 1.8±0.1, 2.2±0.1, and 2.4±0.1 mmol / g, respectively. The adsorption capacity of each sample for methyl orange was measured at pH 4.0, following the method of Example 2.

[0084] The results showed that the adsorption capacity of methyl orange by the waste paper-derived carboxyl nanofiber adsorption network initially increased and then stabilized with increasing carboxyl content. For carboxyl contents of 0.6, 1.2, 1.8, 2.2, and 2.4 mmol / g, the corresponding adsorption capacities were 82±3, 92±2, 98±2, 102±3, and 103±3 mg / g, respectively. When the carboxyl content exceeded 2.0 mmol / g, the adsorption capacity did not increase significantly, indicating that a carboxyl content between 0.5 and 2.5 mmol / g significantly affects adsorption performance, which is consistent with the range specified in the claims.

[0085] Comparative Example 1: Adsorption performance of methyl orange by nanocellulose without carboxyl modification

[0086] Nanocellulose was prepared according to steps 1 and 2 of Example 1, skipping the carboxylation modification step in step 3 and proceeding directly to freeze-drying in step 4 to obtain non-carboxyl-modified nanocellulose. The adsorption properties of non-carboxyl-modified nanocellulose for methyl orange were studied under different pH conditions using the method of Example 2.

[0087] Results showed that the adsorption capacities of unmodified nanocellulose for methyl orange at pH values ​​of 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 were 35±2, 40±2, 42±2, 38±2, 30±2, 25±2, and 20±2 mg / g, respectively. The maximum adsorption capacity was only 42 mg / g at pH 4.0, 57.1% lower than that of carboxylated nanocellulose (98 mg / g). This indicates that carboxylation significantly enhances the adsorption capacity of nanocellulose for methyl orange. Furthermore, the unmodified nanocellulose is significantly less sensitive to pH than the modified sample, lacking a distinct efficient capture zone for the carboxyl half-ionized state.

[0088] Infrared spectroscopy analysis revealed that unmodified nanocellulose exhibited no distinct carboxyl peak at 1720 cm⁻¹, indicating a lack of surface carboxyl functional groups. Potentiometric titration determined the carboxyl content of the unmodified nanocellulose to be 0.1±0.05 mmol / g, primarily derived from the natural carboxyl groups at the ends of the cellulose molecular chains. Zeta potential measurements revealed a surface potential of -10±2 mV at pH 7.0, significantly higher than that of the modified sample (-35±3 mV), indicating a lower surface negative charge density.

[0089] Comparative Example 2: Adsorption performance of commercial activated carbon on methyl orange

[0090] 0.1 g of commercial activated carbon (specific surface area of ​​900 ± 20 m² / g, particle size of 100-200 mesh) was taken and the adsorption performance of commercial activated carbon for methyl orange under different pH conditions was studied according to the method of Example 2.

[0091] Results showed that the adsorption capacities of commercial activated carbon for methyl orange at pH values ​​of 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 were 80±3, 85±3, 82±3, 78±3, 75±3, 70±3, and 65±3 mg / g, respectively. Its maximum adsorption capacity reached 85 mg / g at pH 3.0, which was 13.3% lower than that of the wastepaper-derived carboxyl nanofiber adsorption network (98 mg / g). Commercial activated carbon was significantly less sensitive to pH than the wastepaper-derived carboxyl nanofiber adsorption network, with little change in adsorption capacity within the pH range of 3.0 to 6.0. This is because activated carbon primarily captures dye molecules through physical adsorption and hydrophobic interactions, which are less affected by pH.

[0092] Considering that the cost of commercial activated carbon (approximately 50 yuan / kg) is approximately 43% higher than that of waste paper-derived carboxyl nanofiber adsorption networks (approximately 35 yuan / kg), the latter offers a clear cost-effective advantage. Furthermore, regeneration of commercial activated carbon requires high temperatures (above 800°C) and high energy consumption (approximately 5 kWh / kg), whereas waste paper-derived carboxyl nanofiber adsorption networks can be regenerated through alkaline-induced carboxyl group total ionization desorption, which consumes less energy (approximately 0.5 kWh / kg), making them more environmentally friendly and economical.

[0093] The reusability of commercial activated carbon was investigated using the method of Example 6. The results showed that after five cycles, the methyl orange removal rate of the commercial activated carbon dropped from an initial 85% to 60%, with a retention rate of only 70.6%, significantly lower than the 82% achieved by the waste paper-derived carboxyl nanofiber adsorption network. This suggests that the waste paper-derived carboxyl nanofiber adsorption network exhibits superior reusability.

[0094] Comparative Example 3: Effect of different pH regeneration conditions on the regeneration effect of waste paper-derived carboxyl nanofiber adsorption network

[0095] According to the method of Example 5, sodium hydroxide solutions (0.2 mol / L) with different pH values ​​(7.0, 8.0, 9.0, and 10.0, respectively) were used to elute the adsorbed waste paper-derived carboxyl nanofiber adsorption network to study the effect of different pH regeneration conditions on the regeneration effect.

[0096] The results showed that at pH values ​​of 7.0, 8.0, 9.0, and 10.0, the methyl orange recovery rates were 60±3%, 85±3%, 93±2%, and 95±2%, respectively. The methyl orange removal rates of the regenerated waste paper-derived carboxyl nanofiber adsorption network during its second use were 70±3%, 82±3%, 87±2%, and 88±2%, respectively. This indicates that good regeneration results can be achieved within the pH range of 8.0 to 10.0, with a recovery rate of 93% and a removal rate of 87% already achieved at pH 9.0. Further increases in pH did not significantly improve the regeneration results. This validates the rationale for the claimed regeneration condition range of pH 8 to 10.

[0097] Potentiometric titration revealed that the ionization degrees of the carboxyl functional groups were 98±1%, 99±1%, >99.5%, and >99.5% at pH 7.0, 8.0, 9.0, and 10.0, respectively. At pH ≥ 9.0, the carboxyl groups were almost completely ionized, resulting in the strongest electrostatic repulsion, which facilitated the release of the dye molecules.

Claims

1. A waste paper-derived carboxylated nanocellulose adsorption network, characterized in that: The network is composed of nanocellulose obtained by differential acid hydrolysis and carboxylation modification of waste paper pulp with a cellulose content of 80-90%. The differential acid hydrolysis process is as follows: hydrolyzing with sulfuric acid at a concentration of 60-65% for 2-3 hours at 60-70°C to selectively degrade the amorphous regions in the cellulose molecules. -1,4 glycosidic bonds, retaining the crystalline region structure, with a crystalline region retention rate of 70% to 85%, and the obtained nanocellulose has a high aspect ratio and good dispersibility; the carboxyl content of the carboxylated nanocellulose is 0.5 to 2.5 mmol / g, determined by potentiometric titration; its maximum adsorption capacity for methyl orange dye is 80 to 120 mg / g at a pH value of 3.5 to 4.5, and the carboxyl ionization degree within this pH range is 50% to 70%.

2. The waste paper-derived carboxyl nanofiber adsorption network according to claim 1, characterized in that: The carboxyl modified nanocellulose has a diameter of 5 to 50 nm, a length of 100 to 500 nm, and a specific surface area of ​​80 to 150 m² / g; carboxyl groups are distributed on the surface of the nanocellulose, the C=O / CO peak area ratio is 0.25 to 0.45, and the surface Zeta potential is -25 to -40 mV at a pH of 7.

0.

3. The waste paper-derived carboxyl nanofiber adsorption network according to claim 1, characterized in that: The adsorption equilibrium constant of the carboxylated modified nanocellulose to methyl orange 0.1~0.2L / mg; At a pH of 4.0, the adsorption capacity for methyl orange is 95-100 mg / g, which is more than 50% higher than the adsorption capacity at a pH of 6.0; the degree of ionization of the carboxyl functional group is 35±3% at a pH of 4.0, and 95±1% at a pH of 6.

0.

4. A method for preparing the waste paper-derived carboxyl nanofiber adsorption network according to any one of claims 1 to 3, characterized in that: By controlling the carboxylation modification conditions, a functional group array with a carboxyl density of 0.5 to 2.5 mmol / g was constructed on the nanocellulose surface, forming pH-sensitive zwitterionic adsorption sites. The carboxyl groups have an ionization degree of 50% to 70% in the pH range of 3.5 to 4.5, and through electrostatic interaction and hydrogen bonding, they achieve efficient and selective adsorption of anionic dyes such as methyl orange. Compared with the existing nanocellulose preparation method, the adsorption capacity of the adsorption network prepared by this method for methyl orange is increased by more than 30%; The preparation steps include: Step 1: Grind the waste paper pulp until the fiber length is less than 5 mm, add 60-65% sulfuric acid at a liquid-solid ratio of 10:1-20:1, and hydrolyze at 60-70°C for 2-3 hours to obtain a nanocellulose suspension; Step 2: diluting the suspension 5-fold with deionized water, centrifuging and discarding the supernatant, repeatedly washing until the pH is 6.0-7.0, and ultrasonically dispersing; Step 3: adding succinic acid, a carboxylation reagent, to the nanocellulose at a mass ratio of 1:1 to 5:1, reacting at 60 to 80° C. under anhydrous conditions for 3 to 6 hours, and conducting an esterification reaction under the catalysis of sodium dihydrogen phosphate to graft carboxyl functional groups on the cellulose surface with a density of 0.5 to 2.5 mmol / g; this condition improves the efficiency and stability of carboxyl grafting, which are approximately 25% and 35% higher than those of the aqueous phase method, respectively; Step 4: Wash the modified nanocellulose to a pH of 6.0-7.0, pre-freeze to -40°C, and freeze-dry at a pressure of 10-20 Pa for 24-48 hours to form a three-dimensional porous network structure with a porosity of 85-92% and an average pore size of 50-200 nm.

5. The preparation method according to claim 4, characterized in that In the step three, the carboxylation reaction is carried out under anhydrous conditions, the reaction temperature is 70-75° C., the reaction time is 4-5 hours, and stirring is performed every 30 minutes during the reaction; the carboxylation reagent is succinic acid, the mass ratio of succinic acid to nanocellulose is 3:1, and the esterification reaction is promoted by adding sodium dihydrogen phosphate as a catalyst, and the sodium dihydrogen phosphate accounts for 5-10% of the mass of the carboxylation reagent.

6. The preparation method according to claim 4, characterized in that In the step 2, the ultrasonic dispersion power is 400-600W, the time is 15-30 minutes, and the temperature is maintained at no more than 40°C during the ultrasonic process; in the step 4, the carboxylated modified nanocellulose suspension is pre-frozen to -40°C before freeze drying, the freeze drying time is 24-48 hours, and the drying pressure is 10-20 Pa.

7. A method for treating anionic dye-containing wastewater using the waste paper-derived carboxyl nanofiber adsorption network according to any one of claims 1 to 3, characterized in that: The following steps are involved: Step 1: Dispersing the waste paper-derived carboxyl nanofiber adsorption network in water at an addition amount of 0.5 to 5 g / L of the wastewater volume, adjusting the pH value to 3.5 to 4.5 using hydrochloric acid or sulfuric acid, so that the carboxyl functional group is in a semi-ionized state with an ionization degree of 50% to 70%. Within the pH range, as the pH value increases, the ionization degree of the carboxyl functional group increases. When the pH value is 4.0, the ionization degree is 65±3%, at which time the best adsorption effect is achieved; Step 2: adding wastewater containing anionic dye to the dispersion, stirring and adsorbing at 25-35° C. for 60-120 minutes at a stirring speed of 150-300 rpm; Step 3: Filter the adsorbed waste paper source carboxyl nanofiber adsorption network through a 0.45 μm filter membrane to obtain the treated wastewater, and then analyze the treated wastewater using a UV-visible spectrophotometer. The concentration of methyl orange was measured at the wavelength, and the concentration and removal rate were calculated based on the pre-prepared standard curve.

8. The processing method according to claim 7, characterized in that: In the step 1, the pH value is 4.0, the degree of ionization of the carboxyl functional group is 35±3%, and the surface Zeta potential is -15±5 mV; in the step 2, the temperature is 30±1°C, the stirring speed is 150-300 rpm, and the adsorption time is 60-120 minutes.

9. The processing method according to claim 7, characterized in that: The method also includes step four: Step four: eluting the separated adsorbed waste paper source carboxyl nanofiber adsorption network with a sodium hydroxide solution having a pH value of 8 to 10, implementing an alkaline-induced carboxyl full ionization desorption method, an elution temperature of 40 to 60°C, an elution time of 30 to 60 minutes, an eluent to adsorbent mass ratio of 10:1 to 20:1, and completely ionizing the carboxyl group by changing the pH value, with an ionization degree greater than 95%, thereby reducing the interaction force with the dye molecules and realizing the release of the dye; and then washing with deionized water to a pH value of 6.0 to 7.0, and then drying for reuse.

10. The processing method according to claim 9, characterized in that: After being reused five times, the adsorption efficiency of the waste paper-derived carboxyl nanofiber adsorption network body remains above 80% of the initial adsorption efficiency; the methyl orange removal rates from the first to the fifth recycling times are 92±2%, 87±2%, 83±3%, 78±2%, and 75±3%, respectively; and the retention rates relative to the initial removal rates are 100%, 95%, 90%, 85%, and 82%, respectively; the concentration of the sodium hydroxide solution is 0.1-0.5 mol / L, and the dye recovery rate in the eluate reaches above 90%.

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