Water-absorbent resin prepared from acrylic acid wastewater, and preparation method and application thereof

Abstract

The application provides a water-absorbing resin prepared from acrylic acid wastewater and a preparation method and application thereof. The preparation method mainly comprises the following steps: firstly, concentrating the acrylic acid wastewater to obtain an acrylic acid concentrated solution; adding acrylic acid monomer, a crosslinking agent, an initiator, biochar and / or zeolite into the acrylic acid concentrated solution, uniformly mixing, and carrying out a redox polymerization reaction at 60-80 DEG C in a closed environment to form sodium polyacrylate, and drying to obtain the water-absorbing resin. The preparation method uses the acrylic acid wastewater and acrylic acid as main raw materials, and a one-pot method is used to prepare the water-absorbing resin, so that the preparation method is simple and the cost is low. The application also provides the water-absorbing resin prepared by the above method, the water-absorbing resin has good water-retaining and fertilizer-retaining properties, and the added value of wastewater utilization is effectively improved. The application further provides an application of the water-absorbing resin in preparing water-retaining and / or fertilizer-retaining materials.

Description

Technical Field

[0001] This invention relates to the field of waste resource utilization technology, specifically to a water-absorbing resin prepared from acrylic acid wastewater, its preparation method, and its application. Background Technology

[0002] Acrylic acid wastewater originates from the production of acrylic acid and the manufacturing of acrylates and water-soluble resins. It contains large amounts of acrylic acid and its esters, and direct discharge into the natural environment without treatment can cause serious pollution to water bodies, soil, and air. Due to its high concentration and toxicity, acrylic acid wastewater is difficult to treat. Currently, the main industrial treatment processes are incineration, electrodialysis, or biochemical treatment.

[0003] Existing incineration methods typically involve first atomizing the wastewater, then spraying the atomized wastewater from the top into a high-temperature furnace, where it is completely vaporized under high temperature to produce harmless carbon dioxide, water, and inorganic ash. Acrylic acid wastewater mainly consists of small-molecule organic matter, making concentration and pretreatment difficult. Combustion cannot be automated, and using combustion methods requires large amounts of fuel oil, resulting in high investment and operating costs.

[0004] Existing electrodialysis methods mainly employ two approaches: one involves circulating acrylic acid wastewater in an electrodialysis unit, separating acrylic acid from other components through a biofilm to concentrate the acrylic acid. The number of biofilms is calculated based on the wastewater treatment capacity corresponding to the membrane area. However, this process requires regular biofilm replacement, resulting in high operating costs, although it effectively treats wastewater and recovers acrylic acid. The other method involves using a portion of the concentrated phase for acrylic acid recovery, while the remaining dilute phase can undergo biochemical treatment. However, this method requires significant electricity. Electrodialysis can treat acrylic acid wastewater, enabling the recovery and utilization of acrylic acid and reducing energy waste. However, this technology suffers from drawbacks such as slow processing speed and large construction area. Furthermore, frequent membrane replacements consume substantial electricity, leading to high operating costs, and the wastewater's inherent odor makes this process unsuitable.

[0005] To address the aforementioned issues, CN112745418A discloses a method for preparing low molecular weight sodium polyacrylate using wastewater generated from the production of acrylic resin. The method includes adjusting the pH of the wastewater from acrylic resin production to 7.5–8.5 with sodium hydroxide or sodium carbonate, filtering to remove impurities, and then concentrating the resulting filtrate by vacuum evaporation to obtain a concentrated sodium acrylate solution. The concentrated sodium acrylate solution, sodium bisulfite, and ammonium persulfate are mixed and subjected to a polymerization reaction at 60–70°C to obtain a reaction solution. The reaction solution is then cooled and the pH is adjusted to 7.0–7.5 to obtain a low molecular weight sodium polyacrylate solution. Although the aforementioned patent application utilizes wastewater from acrylic resin production to prepare low molecular weight sodium polyacrylate, making full use of the acrylic acid and sodium acrylate resources in the wastewater and increasing the added value of the wastewater, it requires adjusting the pH value of the wastewater to 7.5-8.5 with sodium hydroxide or sodium carbonate to prevent subsequent polymerization reactions from failing or causing "explosive polymerization." Then, the wastewater is concentrated and evaporated as a raw material for synthesizing sodium polyacrylate, making the process complex. Moreover, the aforementioned patent application process also requires filtration to remove impurities, mainly some insoluble substances. If these substances are not removed, they may clog the reactor, affecting subsequent reactions and reducing the utilization rate of wastewater generated from acrylic resin production. Summary of the Invention

[0006] In view of this, the main objective of this invention is to provide a method for preparing a water-absorbing resin using acrylic acid wastewater. This method uses acrylic acid wastewater and acrylic acid as the main raw materials, and does not require pH adjustment to remove impurities from the acrylic acid wastewater. The water-absorbing resin is prepared in a one-pot process. This preparation method is simple, low in cost, and the prepared water-absorbing resin has good water retention and fertilizer retention properties, effectively improving the added value of wastewater utilization.

[0007] Specifically, a method for preparing a water-absorbing resin using acrylic acid wastewater includes the following steps: first, concentrating the acrylic acid wastewater to obtain an acrylic acid concentrate; adding acrylic acid monomer, crosslinking agent, initiator, biochar, and / or zeolite to the acrylic acid concentrate and mixing them uniformly, then conducting a redox polymerization reaction in a closed environment at 60–80°C to form a polymer, sodium polyacrylate; drying to obtain the water-absorbing resin, wherein the amount of biochar added is 0–8 wt.%, and the amount of zeolite added is 0–3 wt.%, and the content of sodium acrylate in the acrylic acid concentrate is 35–45%.

[0008] To improve the utilization rate of acrylic acid wastewater and ensure the water absorption performance of the superabsorbent resin, the preferred mass ratio of acrylic acid monomer to sodium acrylate in the acrylic acid wastewater is 0.5:1 to 1.5:1. The preferred amount of initiator is 2 wt.% to 6 wt.% of the total mass of acrylic acid monomer and sodium acrylate in the acrylic acid wastewater. The preferred amount of crosslinking agent is 1 wt.% to 3 wt.% of the total mass of acrylic acid monomer and sodium acrylate in the acrylic acid wastewater. The preferred reaction time is 60 to 360 min. The preferred degree of neutralization of the acrylic acid monomer is 50% to 90%. The preferred amount of biochar is 0 to 8 wt.% of the superabsorbent resin. The preferred amount of zeolite is 0 to 3 wt.% of the superabsorbent resin.

[0009] To further improve the water absorption performance of the superabsorbent resin, biochar and zeolite are used for modification. The preferred amount of biochar is 2-5 wt.% of the superabsorbent resin; the preferred amount of zeolite is 0.5-1.5 wt.% of the superabsorbent resin. The acrylic acid wastewater used in this invention is mainly obtained by removing only some obvious suspended solids from the wastewater discharged from the factory. It is weakly acidic and includes approximately 20% acrylate monomer products. These acrylate monomer products are mainly composed of 13 monomer products, including sodium acrylate, dipropylene glycol diacrylate, trimethylolpropane triacrylate, and dipentaerythritol hexaacrylate, with sodium acrylate as the main component. Furthermore, the acrylic acid wastewater used in this invention also includes impurities such as dipropylene glycol diacrylate, trimethylolpropane triacrylate, dipentaerythritol hexaacrylate, methanol, ethanol, acetic acid, and acetone.

[0010] Further, the redox polymerization reaction includes: first, adding acrylic monomer to the concentrated acrylic acid solution and mixing it uniformly; then adding the initiator and crosslinking agent; subsequently adding biochar and zeolite; and then bubbling N2 at a rate of 20–100 mL / min for 3–8 min. The mixture is then placed in a closed environment and heated in a water bath at 60–80°C for 10–16 h to form a hydrogel. The hydrogel is then subjected to initial drying, washing, secondary drying, and pulverization to obtain the water-absorbing resin. The synthesis of acrylic resin under closed conditions is mainly for two reasons: one is to prevent volatile loss—acrylic acid is volatile, and a closed reaction can reduce the loss of raw materials and improve reaction efficiency; the other is to improve product purity—the introduction of nitrogen gas and a closed system can maintain the stability of the reaction environment, avoid the influence of external air factors such as moisture and oxygen on the reaction, and better control the occurrence of side reactions, thereby improving the purity and quality of the final product.

[0011] This invention also provides a water-absorbing resin prepared by the above method. When the raw materials of the water-absorbing resin contain biochar and zeolite, the main component of the biochar and / or zeolite-modified water-absorbing resin is sodium polyacrylate. When the raw materials of the water-absorbing resin contain biochar and zeolite, the water-absorbing resin is a biochar and / or zeolite-modified water-absorbing resin, and its main component is biochar and / or zeolite-modified sodium polyacrylate.

[0012] The initiator used in this invention can be ammonium persulfate, benzoyl peroxide, sodium nitrite, potassium persulfate, etc.; the crosslinking agent used in this invention can be N'N-methylenebisacrylamide, propylene glycol dimethacrylate, trimethylolpropane triacrylate, etc.

[0013] Experimental verification shows that the water-absorbing resin provided by this invention has a water absorption ratio of over 100 g / g for distilled water and over 30 g / g for 0.9 wt.% NaCl, preferably 120–285 g / g and 30–60 g / g. Specifically, the water-absorbing resin without biochar and / or zeolite modification can achieve a water absorption ratio of 200 g / g for distilled water and 37 g / g for 0.9 wt.% NaCl, while the water-absorbing resin modified with biochar and / or zeolite can achieve a water absorption ratio of 285 g / g for distilled water and 60 g / g for 0.9 wt.% NaCl. Therefore, the water-absorbing resin obtained by this invention has good water absorption ratio, water absorption, soil water retention, recyclability, and fertilizer retention performance. Therefore, this invention also provides an application of the above-mentioned water-absorbing resin in the preparation of water-retaining and / or fertilizer-retaining materials.

[0014] This invention also provides a water-retaining slow-release fertilizer, comprising the aforementioned water-absorbing resin and a fertilizer loaded on the water-absorbing resin. The fertilizer can be nitrogen fertilizer, phosphorus fertilizer, etc. This can improve the utilization rate of water resources and fertilizers, alleviate environmental problems, provide new ideas and methods for the preparation of water-retaining and fertilizer-retaining materials, and provide basic theoretical guidance for their application.

[0015] Therefore, the water-absorbing resin synthesized from acrylic acid wastewater in this invention can solve the problem of acrylic acid wastewater treatment and enable resource utilization. Furthermore, the water-absorbing resin modified with activated carbon / orthorhombic zeolite in this invention exhibits improved water absorption rate, water absorption, soil water retention, recyclability, and fertilizer retention performance. Applying this water-absorbing resin to agriculture saves energy, explores a novel water- and fertilizer-retaining material, and addresses the problems of low fertilizer utilization and increasingly severe water shortages, thus promoting sustainable agricultural development. Attached Figure Description

[0016] Figure 1This is a flowchart illustrating the preparation process of the water-absorbing resin provided in Example 1 of the present invention. Figure 2 The graph shows the effect of monomer and formulation ratio on the water absorption ratio of the water-absorbing resin prepared in Example 1 of the present invention. Figure 3 The graph shows the effect of reaction temperature on the water absorption ratio of the water-absorbing resin prepared in Example 1 of this invention. Figure 4 The kinetic curves of diffusion of distilled water (a) and 0.9 wt.% NaCl solution (b) in the water-absorbing resin prepared in Example 1 of the present invention are shown. Figure 5 The graph shows the effect of initiator dosage on the water absorption ratio of the water-absorbing resin prepared in Example 1 of this invention. Figure 6 The graph shows the effect of the amount of crosslinking agent on the water absorption ratio of the water-absorbing resin prepared in Example 1 of this invention. Figure 7 This is a flowchart illustrating the preparation process of the modified water-absorbing resin provided in Example 2 of the present invention. Figure 8 The graph shows the effect of the degree of neutralization of acrylic acid on the water absorption ratio of the water-absorbing resin prepared in Example 2 of this invention. Figure 9 The effect of biochar dosage on the water absorption ratio of the modified water-absorbing resin prepared in Example 2 of this invention is shown in the figure. Figure 10 The effect of zeolite dosage on the water absorption ratio of the modified water-absorbing resin prepared in Example 2 of this invention is shown in the figure. Figure 11 The Fourier transform infrared (FTIR) spectra of the unmodified and modified water-absorbing resins prepared under optimal conditions according to Examples 1 and 2 of the present invention are shown in Figures a, b, c, and d, which are the FTIR spectra of biochar, clinoptilolite, modified resin, and unmodified resin, respectively. Figure 12 Thermogravimetric analysis spectra of the unmodified and modified water-absorbing resins prepared under optimal conditions according to Examples 1 and 2 of the present invention are shown. Figure 13 SEM images of unmodified water-absorbing resin (a) and modified water-absorbing resin (b) prepared under optimal conditions according to Examples 1 and 2 of the present invention; Figure 14 The graph shows the water absorption ratio curves of the unmodified and modified water-absorbing resins prepared under optimal conditions according to Examples 1 and 2 of the present invention at different pH values. Figure 15The diagram shows the recyclability of the unmodified and modified water-absorbing resins prepared under optimal conditions in distilled water (a) and 0.9 wt.% NaCl solution (b) obtained by Examples 1 and 2 of the present invention. Figure 16 The water retention effect of unmodified and modified water-absorbing resins prepared under optimal conditions according to Examples 1 and 2 of the present invention in soil; Figure 17 The graph shows the slow-release behavior of unmodified and modified water-absorbing resins loaded with urea in distilled water, prepared under optimal conditions according to Examples 1 and 2 of the present invention. Detailed Implementation

[0017] The technical solution of the present invention will be further described in detail below through specific embodiments.

[0018] All terms used in this invention are common terms in the relevant field. Unless otherwise specified, the raw materials, equipment, preparation processes, testing methods, etc., used are all existing technologies in the relevant field.

[0019] In the following embodiments, the amounts of initiator and crosslinking agent are calculated as a percentage of the total mass of acrylic monomer and sodium acrylate in acrylic wastewater, the amounts of biochar and zeolite are calculated as a percentage of the total mass of modified water-absorbing resin, and the degree of neutralization of acrylic monomer is adjusted by sodium hydroxide.

[0020] The methods for measuring the water absorption ratio and salt water absorption ratio in each embodiment are in accordance with the current industry standards for agricultural and forestry water-retaining agents.

[0021] Example 1 Please see Figure 1 This invention provides a method for preparing a water-absorbing resin using acrylic acid wastewater, comprising the following steps: Concentrating sodium acrylate wastewater to 50% of its initial mass using a rotary evaporation method, at which point the sodium acrylate content in the concentrated acrylic acid solution is approximately 40%, and using this solution as the experimental raw material. Adding acrylic acid monomer (AA) to the concentrated acrylic acid solution and mixing thoroughly yields a monomer mixture solution; adding ammonium persulfate (APS) and N'N-methylenebisacrylamide (MBA) to the monomer mixture solution; bubbling N2 into the reaction system for 5 min; sealing the reaction system and placing it in a water bath at 60–80°C; gelling for 12 h; drying the hydrogel; and pulverizing to obtain the water-absorbing resin. In this embodiment, the mass ratio of the added acrylic acid monomer to the sodium acrylate in the sodium acrylate wastewater is (0.5–1.5):1, the degree of neutralization of the acrylic acid monomer is 50%–90%, the amount of initiator APS is 2–6 wt.%, and the amount of crosslinking agent MBA is 1.0–3.0 wt.%.

[0022] During the redox polymerization process, the initiator APS is reduced to generate sulfate ion radicals (Equation 1). These sulfate ion radicals can attract hydrogen from the hydroxyl groups of biochar to form alkoxy groups or attract AA molecules to form AA radicals (Equations 2 and 3). Biochar or AA molecules near the reaction site become free radical acceptors (Equations 2 and 4). After accepting free radicals, they become the next free radical donors for adjacent molecules, initiating chain growth reactions. The chain growth of sodium acrylate may terminate through the bonding of two polyacrylic acid (PAA) chains or by connection with biochar (Equation 5). During chain growth, the vinyl groups at the ends of the MBA, acting as crosslinking agents, can react synchronously with the polymer PAA chains to form a three-dimensional network structure.

[0023] Therefore, it can be seen that in the process of preparing water-absorbing resin using acrylic acid wastewater, the amount of each raw material, the degree of AA neutralization, the reaction temperature, and the reaction time all affect the synthesis and performance of the water-absorbing resin. The following is a further explanation of the water absorption performance of the water-absorbing resin synthesized using the preparation method provided in this embodiment, in response to the above-mentioned influencing factors.

[0024] 1.1 Effect of the mass ratio of acrylic acid monomer to sodium acrylate in acrylic acid wastewater on the properties of water-absorbing resins The reaction parameters were as follows: the mass ratio of acrylic acid monomer to sodium acrylate in acrylic acid wastewater (hereinafter referred to as "monomer ratio") was 0.5:1, 0.75:1, 1:1, 1.25:1, and 1.5:1, respectively; the reaction temperature was 60℃; the amount of initiator APS was 5 wt.%; the amount of crosslinking agent MBA was 1 wt.%; and the reaction time was 180 min. The effect of monomer ratio on the water absorption ratio of the superabsorbent resin is as follows: Figure 2 As shown.

[0025] Figure 2 The results show that as the monomer ratio is changed, the water absorption ratio of the superabsorbent resin continuously increases, reaching a maximum value before slightly decreasing. This is because when the monomer concentration is too low, the polymerization rate is relatively slow, and the resulting molecular chains are too short, allowing only a few active groups to be adsorbed. When the monomer concentration is too high, the polymerization rate is too fast, and the reaction time is significantly shortened, resulting in insufficient cross-linking between monomers under the action of the cross-linking agent to form a three-dimensional network structure, thus greatly reducing the water absorption ratio. Therefore, the monomer ratio is preferably 0.9:1 to 1.25:1, more preferably 1:1 to 1.25:1, and when the monomer ratio is 1:1, the synthesized superabsorbent resin has the highest water absorption ratio, reaching 185 g / g in distilled water and 35 g / g in 0.9 wt.% NaCl solution.

[0026] 1.2 Effect of reaction temperature on the properties of water-absorbing resins The reaction parameters were as follows: monomer ratio 1:1, reaction temperatures of 60℃, 65℃, 70℃, 75℃, and 80℃, initiator dosage of 5 wt.%, crosslinking agent dosage of 1 wt.%, and reaction time of 180 min. The effect of reaction temperature on the water absorption ratio of the superabsorbent resin is shown below. Figure 3 As shown.

[0027] Figure 3 The results show that the water absorption ratio of the superabsorbent resin increases with increasing reaction temperature, reaching a maximum at 70°C. At lower reaction temperatures, the reaction rate is slower and then decreases with further increases in temperature. This is mainly because the polymerization reaction involves both the self-polymerization and grafting of acrylic acid, and the grafting rate affects the water absorption performance of the superabsorbent resin. Higher temperatures promote the self-polymerization of acrylic acid, thereby reducing the grafting rate. Therefore, the preferred reaction temperature is 65–75°C, with an optimal temperature of 70°C. Superabsorbent resins synthesized at this optimal temperature achieve water absorption ratios of 191 g / g in distilled water and 36 g / g in 0.9 wt.% NaCl solution.

[0028] 1.3 Effect of reaction time on the properties of water-absorbing resin The reaction parameters were: monomer ratio 1:1, reaction temperature 70℃, reaction time 0–1000 min, initiator dosage 5 wt.%, and crosslinking agent dosage 1 wt.%. The effect of reaction time on the water absorption ratio of the superabsorbent resin is as follows: Figure 4 As shown.

[0029] Figure 4 The results show that the water absorption ratio increases rapidly in the initial stage, and then increases slowly in distilled water (a) and 0.9 wt.% NaCl solution (b) to reach an equilibrium state. This result may be related to the change in osmotic pressure difference during the swelling process. Due to the electrostatic repulsion between hydrophilic and anionic groups in the resin structure, the osmotic pressure difference is high, allowing water molecules to easily enter the superabsorbent network, and the swelling rate increases rapidly. However, as more water molecules join the network, the osmotic pressure difference gradually decreases, the swelling rate decreases, and eventually an equilibrium state is reached. Therefore, the reaction time is preferably 60–360 min, more preferably 120–240 min.

[0030] 1.4 Effect of initiator dosage on the properties of water-absorbing resin The reaction parameters were as follows: monomer ratio 1:1, reaction temperature 70℃, APS initiator dosages of 2 wt.%, 3 wt.%, 4 wt.%, 5 wt.%, and 6 wt.%, crosslinking agent dosage of 1 wt.%, and reaction time of 180 min. The effect of APS initiator on the water absorption ratio of the superabsorbent resin is as follows: Figure 5 As shown.

[0031] from Figure 5 It can be seen that the water absorption ratio of the water-absorbing resin increases with the increase of APS dosage, reaching a maximum at 4 wt.% APS, with a water absorption ratio of 196 g / g in distilled water and 37 g / g in 0.9 wt.% NaCl solution. This is mainly because more free radicals and more active sites are generated on the sodium acrylate backbone. However, further increasing the amount of APS leads to a decrease in absorbency, because the large number of free radicals generated by excessive initiator leads to a shortening of the grafted chain length, thus affecting absorbency. Therefore, the preferred amount of initiator is 3 wt% to 5 wt%.

[0032] 1.5 Effect of crosslinking agent dosage on the properties of water-absorbing resin The reaction parameters were as follows: monomer ratio 1:1, reaction temperature 70℃, initiator APS dosage 4 wt.%, crosslinking agent dosages 1.0 wt.%, 1.5 wt.%, 2.0 wt.%, 2.5 wt.%, and 3.0 wt.%, respectively, and reaction time 180 min. The effect of crosslinking agent MBA on the water absorption ratio of the superabsorbent resin is as follows. Figure 6 As shown.

[0033] from Figure 6 It can be seen that the water absorption ratio of the superabsorbent resin increases with the increase of the crosslinking agent dosage. The water absorption ratio is highest when the crosslinking agent mass fraction is 2.0 wt.%, with absorption ratios of 200 g / g in distilled water and 35 g / g in 0.9 wt.% NaCl solution. The amount of crosslinking agent is directly related to the crosslinking density, thus affecting the formation of the polymer network and mechanical strength. Clearly, a higher level of crosslinking agent produces more crosslinks, leading to the formation of additional networks. These networks reduce the available free volume within the polymer, thereby decreasing the water absorption ratio. Furthermore, insufficient crosslinking agent MBA reduces the crosslinking density, leading to an increase in soluble substances and thus a decrease in the water absorption ratio. Therefore, the preferred amount of crosslinking agent is 1.5 wt% to 2.5 wt%.

[0034] Therefore, the optimal synthesis conditions for the water-absorbing resin provided in this embodiment are: monomer ratio of 1:1, acrylic acid neutralization degree of 70%, reaction temperature of 70°C, reaction time of 180 min, initiator APS dosage of 4 wt.%, and crosslinking agent dosage of 2.0 wt.%.

[0035] Example 2 Please see Figure 7This invention provides a method for preparing modified absorbent resin using acrylic acid wastewater. This method is essentially the same as the method provided in Example 1, except that in this example, after adding APS, sodium bisulfite, and MBA to the monomer mixture solution, biochar and / or zeolite are added before introducing N2 into the reaction system. Other steps and parameters are the same, resulting in a biochar and / or zeolite-modified absorbent resin. In this example, the degree of neutralization of acrylic acid (AA) is 50%–90%, the amount of biochar is 0 wt.%–8 wt.% of the modified absorbent resin, and clinoptilolite is used, with an amount of 0 wt.%–3 wt.% of the modified absorbent resin.

[0036] Biochar and zeolite are modifying materials for the water-absorbing resin provided in Example 1. Their dosage has a significant impact on the modified water-absorbing resin obtained in this example, which will be further explained in detail below.

[0037] 2.1 Effect of AA neutralization degree on the properties of water-absorbing resin The reaction parameters were: monomer ratio 1:1, AA neutralization degrees of 50%, 60%, 70%, 80%, and 90%, reaction temperature 60℃, APS initiator dosage 5 wt.%, MBA crosslinking agent dosage 1 wt.%, and reaction time 180 min. The effect of AA neutralization degree on the water absorption ratio of the superabsorbent resin is as follows: Figure 8 As shown.

[0038] from Figure 8 It is evident that the degree of AA neutralization significantly affects the water absorption capacity of the superabsorbent resin. When the AA neutralization degree increases from 50% to 70%, the water absorption capacity of the synthesized superabsorbent resin increases significantly. This observation can be explained by the increase in charge density, which leads to increased electrostatic repulsion of the COO− groups in the polymer matrix. Therefore, the resin network becomes larger, with the absorption rate in distilled water increasing from 136 g / g to 279 g / g, and in 0.9 wt.% NaCl solution from 37 g / g to 56 g / g. At higher neutralization degrees (greater than 70%), the water absorption capacity decreases, possibly due to increased chain stiffness and condensation of counterions on the polyions. Therefore, the preferred AA neutralization degree is 60%–90%, more preferably 65%–80%.

[0039] 2.2 Effect of Biochar Dosage on the Properties of Water-Absorbent Resin The reaction parameters were as follows: monomer ratio 1:1, AA neutralization degree 70%, reaction temperature 70℃, APS initiator dosage 4 wt.%, crosslinking agent dosage 2.0 wt.%, reaction time 180 min, biochar dosage 0, 2, 4, 6, and 8 wt.%, and zeolite dosage 2 wt.%. The effect of biochar dosage on the water absorption ratio of the biochar / zeolite modified water-absorbing resin is shown below. Figure 9 As shown.

[0040] from Figure 9 It is evident that with the increase of biochar dosage, the number of active groups such as carboxyl and hydroxyl groups in the reaction system increases rapidly. This provides more opportunities for acrylic acid monomers to undergo graft polymerization with biochar, significantly improving the water absorption ratio of the resin. Further increasing the biochar dosage results in excessive biochar not reacting completely with the acrylic acid monomers, but rather being physically incorporated into the resin structure, essentially introducing other impurities and disrupting the original three-dimensional network structure, thus greatly reducing the water absorption ratio of the modified water-absorbing resin. Considering all these factors, the preferred biochar dosage is 2–5 wt.%, and the modified water-absorbing resin achieves its highest water absorption ratio in distilled water and 0.9 wt% NaCl solution at a biochar dosage of 4 wt.%, reaching 281 g / g and 57 g / g, respectively.

[0041] 2.3 Effect of zeolite dosage on the properties of water-absorbing resin The reaction parameters were as follows: monomer ratio 1:1, AA neutralization degree 70%, reaction temperature 70℃, initiator APS dosage 4 wt.%, crosslinking agent dosage 2.0 wt.%, reaction time 180 min, biochar dosage 4 wt.%, and zeolite dosage 0.5–3 wt.%. The effect of zeolite dosage on the water absorption ratio of the biochar / zeolite modified water-absorbing resin is shown below. Figure 10 As shown.

[0042] Zeolites possess suitable pore diameters and large specific surface areas, resulting in excellent adsorption properties. (From the curve...) Figure 10 It can be seen that the water absorption performance of the modified superabsorbent resin is significantly improved with the addition of clinoptilolite; even a small amount of clinoptilolite can enhance the water absorption performance. Simultaneously, the negative charge on the surface of clinoptilolite repels the carboxyl groups in the modified resin, promoting gel network expansion. However, excessive clinoptilolite can lead to incomplete cross-linking of some polymer networks and also reduce the number of hydrophilic groups per unit volume of polymer, thus decreasing the resin's water absorption capacity. Considering all factors, the preferred amount of zeolite is 0.5–1.5 wt.%, and the modified superabsorbent resin achieves its highest water absorption ratios in distilled water (284 g / g) and 0.9 wt% NaCl solution (59 g / g) when the zeolite content is 1 wt.%.

[0043] Therefore, the optimal synthesis conditions for the biochar / zeolite modified water-absorbing resin in this embodiment are: monomer ratio of 1:1, acrylic acid neutralization degree of 70%, reaction temperature of 70℃, reaction time of 180 min, initiator APS amount of 4 wt.%, crosslinking agent amount of 2.0 wt.%, biochar amount of 4 wt.%, and zeolite amount of 1 wt.%.

[0044] Structural characterization The following characterization methods were used to analyze the water-absorbing resin synthesized under optimal conditions in Example 1 (hereinafter referred to as "unmodified resin") and the biochar zeolite-modified water-absorbing resin synthesized under optimal conditions in Example 2 (hereinafter referred to as "modified resin"), including infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy.

[0045] 3.1 Infrared Spectroscopy Analysis Figure 11 The infrared spectrum shown indicates that in the spectrum of biochar (a), at 3400 cm⁻¹ −1 The broad peak at 2900 cm⁻¹ is related to the stretching vibration of the hydroxyl group. −1 Belongs to CH, 1571 cm -1 The peak at 1384 cm⁻¹ is the absorption peak of the stretching vibration of the carbonyl conjugated double bond (C=O); -1 The peaks at 1183 and 1037 cm⁻¹ represent the vibrational peaks of the hydroxyl group (-OH). -1 The peak at 1008 cm⁻¹ represents the stretching vibration of CO in the ether and alcohol structures of biochar. (b) Spectrum of clinoptilolite (b), 1008 cm⁻¹ -1 The absorption peak is due to the Si-O (Si) bond.

[0046] Observation of the infrared spectra of unmodified resin (d) and modified resin (c) shows that at 1557 cm⁻¹ -1 A strong absorption peak appeared at 3200–3500 cm⁻¹, which is the overlapping stretching vibration peak of the carbonyl group (C=O) in acrylic acid. Furthermore, a strong absorption peak was observed in the 3200–3500 cm⁻¹ range. -1 A broad absorption peak for OH appeared at 1637 cm⁻¹. Compared to SAP, BC-SAP showed a broader absorption peak for OH at 1637 cm⁻¹. -1 The stretching vibration peak of the aromatic conjugated double bond (C=C) of biochar appeared at 1365 cm⁻¹. -1 The hydroxyl (-OH) vibration peak from biochar was observed at 1190 cm⁻¹. -1 and 1053 cm -1 The peaks at 988 cm⁻¹ all showed slight shifts, indicating that biochar participated in the graft copolymerization reaction. -1 The peak at 1008 cm⁻¹ shows stretching vibrations of the Si-O (Si) group, compared to the 1008 cm⁻¹ peak of zeolite. -1The peak shifted, which proves that clinoptilolite participated in the polymerization reaction.

[0047] 3.2 Thermogravimetric Analysis from Figure 12 The thermogravimetric curves of the unmodified and modified resins show that their decomposition occurs in three steps. First, as the resin heats from room temperature to 100°C, approximately 4% of its mass is lost, primarily due to the evaporation of free water. Second, from 100°C to 420°C, the mass losses for the unmodified and modified resins are 30.72% and 31.68%, respectively, caused by the degradation of the cross-linked network structure and the decomposition of the polymer chains. Third, from 420°C to 480°C, a rapid loss of 22.10% and 24.93% of mass occurs. These losses are due to the evaporation of volatile components and the carbonization of the polymer chains. Above 480°C, the rate of mass loss is very rapid, with the unmodified and modified resins losing 24.99% and 21.37%, respectively, indicating that most of the resin is converted to carbon and completely loses its properties. With further increases in temperature, the mass loss of both resins tends to slow down. Comparing the thermogravimetric curves of the unmodified and modified resins, the two curves show similar trends, with a final residue mass difference of 4.85%. This indicates that the addition of biochar and clinoptilolite slightly improves the stability of the superabsorbent polymer. This may be because, at higher temperatures, the acrylic polymer is more stable than biochar and decomposes more completely at high temperatures, resulting in a lower residue mass for the modified resin compared to the unmodified resin. In summary, both superabsorbent resins exhibit good thermal stability, and the effect of temperature on the resin structure can be ignored when applied in agriculture.

[0048] 3.3 Scanning electron microscopy analysis SEM images of the appearance of unmodified and modified resins in the dry state are shown below. Figure 13 As shown in the image, the dry, unmodified resin (a) has a rigid structure and a smooth, porous surface. This porous structure allows water molecules to quickly penetrate the resin, facilitating rapid swelling equilibrium. The smooth surface helps to slow the release rate of water molecules. This observation indicates that the unmodified resin can absorb and retain a large amount of water without losing its structural integrity, a finding confirmed by scanning electron microscopy (SEM). The SEM image (b) of the modified resin shows a surface with numerous wrinkles and an uneven shape, indicating a larger specific surface area. This allows water molecules to penetrate the polymer network more easily and quickly, while simultaneously absorbing more water molecules. Furthermore, these interconnected pores can delay the dissolution of urea in the swelling medium and improve the resin's sustained-release behavior.

[0049] Performance verification 4.1 Water Absorbency By observing the unmodified and modified resins of the product in the dry state Figure 13 The SEM images show that the unmodified resin can absorb and retain a large amount of water without losing its structural integrity. The modified water-absorbing resin has numerous wrinkles on its surface, exhibiting an uneven shape and a larger specific surface area. This facilitates the easier and faster penetration of water molecules into the polymer network, while simultaneously enabling the absorption of more water molecules.

[0050] (2) Effect of pH value on water absorption ratio The water absorption ratios of the unmodified and modified resins synthesized under optimal conditions were tested using distilled water with a pH range of 2–12, where the pH was adjusted using hydrochloric acid and sodium hydroxide, respectively. The effects of distilled water pH on the unmodified and modified resins are shown in the following figures. Figure 14 As shown.

[0051] Figure 14 The results show that the water absorption of unmodified and modified resins exhibits similar trends across a wide pH range, increasing sharply from pH 3 to 7 and decreasing significantly from pH 9 to 12. Under acidic conditions, the water absorption ratio is lower and the water absorption is poorer, as most -COO- groups are converted to -COOH groups, weakening the repulsive force between anions and causing the three-dimensional network structure to shrink. On the other hand, due to higher protonation and increased network crosslinking, hydrogen bonding interactions are enhanced, leading to a decrease in the water absorption ratio. As the pH increases, the negative impact of hydrogen ions on the electrostatic repulsion of carboxylate anions decreases, and hydrogen bonding interactions are limited. Therefore, the network structure of both unmodified and modified resins is further expanded, resulting in enhanced water absorption and a larger absorption ratio, reaching their maximum values ​​of 190 g / g and 300 g / g, respectively, at pH 7. However, at higher pH values ​​(pH > 9), due to excess Na... + The shielding effect of ions reduces water absorption and prevents repulsive electrostatic interactions, thus decreasing the water absorption ratio. On the other hand, the water absorption ratio of the modified resin is significantly higher than that of the unmodified resin. This is mainly because the addition of biochar and clinoptilolite increases the number of active groups such as hydroxyl and carboxyl groups in the system. This indicates that the addition of biochar and clinoptilolite greatly improves the internal network structure of the resin, thereby enhancing its water absorption and retention performance. Therefore, both the unmodified and modified resins are preferably used in distilled water with a pH of 6–10, and the water absorption ratio is above 140 g / g.

[0052] (3) Recyclability The recyclability of absorbent resin products, i.e., their ability to absorb moisture after multiple saturation absorptions and thorough drying, determines their lifespan. To examine the recyclability of the absorbent resin prepared according to the embodiments of this invention, 1.00 g of both the unmodified and modified resins were taken and subjected to five liquid absorption cycle tests. Their water absorption ratios were measured, and the results are as follows: Figure 15 As shown.

[0053] from Figure 15 It can be seen that after five cycles, the water absorption ratios of the unmodified resin (a) and the modified resin (b) in distilled water were 107 g / g and 204 g / g, respectively, which are approximately 53.5% and 72.86% of their initial water absorption capacity (based on the maximum water absorption ratio in distilled water). The water absorption ratios of the unmodified resin and the modified resin in 0.9 wt.% NaCl solution were 28 g / g and 45 g / g, respectively, which are approximately 70% and 75% of their initial water absorption capacity (based on the maximum water absorption ratio in 0.9 wt.% NaCl solution). These results indicate that biochar and zeolite effectively improve the three-dimensional network structure of the water-absorbing resin and enhance its recyclability.

[0054] (4) Soil water retention capacity The water retention capacity of unmodified and modified resin samples in soil was determined to investigate the water retention performance of the two water-absorbing resins after 30 days within their polymer network structure. For this purpose, the soil was initially dried in an oven at 80°C for two days and sieved through a 10-mesh nylon sieve. Then, 1.00 g of the prepared unmodified and modified resin samples were mixed with 10 g of dried soil in plastic beakers, and distilled water was added to the resin-soil mixture. The beakers were then weighed and placed at room temperature. A blank test without added resin was also conducted (beakers containing the same soil but without resin). The beakers were weighed every two days for 30 consecutive days, and the results are shown below. Figure 16 As shown.

[0055] Figure 16 The water retention behavior of soils containing unmodified resin, modified resin, and no added resin was demonstrated over 30 days. The results showed that the water evaporation rate of the resin-added soil was significantly lower than that of the unmodified soil. Soils treated with unmodified and modified resins retained 29% and 38% of their initial moisture content, respectively, after 30 days. These results indicate that the water evaporation rate of soils using the absorbent resin provided in this embodiment of the invention is significantly lower than that of blank soils, and the water retention capacity of the modified resin in the soil is significantly higher than that of the unmodified resin.

[0056] (5) Fertilizer retention performance Experimental Method: Accurately weigh 1.00 g of unmodified resin and 1.00 g of modified resin respectively, and immerse them in a 5 g / L urea solution for 24 h. After reaching swelling equilibrium, dry the resins separately in an oven to constant weight, accurately weigh the resins, and calculate the urea loading mass. Immerse the urea-loaded resins separately in 400 mL of distilled water. Carefully remove 2 mL of solution every 1 h, and obtain the urea content using a UV spectrophotometer. Immediately add the same amount of fresh distilled water to the culture medium for the next test. Determine the urea content according to GB / T23348-2009. The results are as follows: Figure 17 As shown.

[0057] Figure 17 The results showed that the urea release rates of unmodified and modified resin-loaded urea were 75% and 57% respectively after 10 h. This indicates that the network of the water-absorbing resin is sufficiently dense to retain urea molecules internally. Therefore, water and urea molecules are inhibited, their release is delayed, and the fertilizer retention performance of the modified water-absorbing resin is higher than that of the unmodified water-absorbing resin over a longer period.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A method for preparing a water-absorbing resin using acrylic acid wastewater, comprising the steps of: First, remove the suspended solids from the acrylic acid wastewater. Without adjusting the pH value, concentrate the solution using rotary evaporation to obtain an acrylic acid concentrate with a sodium acrylate content of 35-45%. Acrylic acid monomers, crosslinking agents, initiators, biochar, and zeolite are added to the concentrated acrylic acid solution and mixed evenly. A redox polymerization reaction is then carried out in a sealed environment at 60–80°C to form sodium polyacrylate. After drying, a water-absorbing resin is obtained. The degree of neutralization of the acrylic monomer is 50% to 90%; The mass ratio of acrylic acid monomer to sodium acrylate in acrylic acid wastewater is 0.5:1 to 1.5:1, the amount of crosslinking agent added is 1 wt.% to 3 wt.% of the total mass of acrylic acid monomer and sodium acrylate in acrylic acid wastewater, the amount of biochar added is 2 to 5 wt.%, and the amount of zeolite added is 0.5 to 1.5 wt.%.

2. The method according to claim 1, characterized in that, The reaction time of the redox polymerization reaction is 60 to 360 min, and the amount of initiator added is 2 wt% to 6 wt% of the total mass of the acrylic monomer and sodium acrylate in the acrylic wastewater.

3. The method according to claim 2, characterized in that, The mass ratio of acrylic acid monomer to sodium acrylate in acrylic acid wastewater is 0.9:1 to 1.25:

1. The reaction temperature of the redox polymerization reaction is 65 to 75°C, the reaction time is 120 to 240 min, the amount of initiator added is 3 wt% to 5 wt% of the total mass of acrylic acid monomer and sodium acrylate in acrylic acid wastewater, and the amount of crosslinking agent added is 1.5 wt.% to 2.5 wt.% of the total mass of acrylic acid monomer and sodium acrylate in acrylic acid wastewater.

4. The method according to claim 3, characterized in that, The mass ratio of the acrylic acid monomer to the sodium acrylate in the acrylic acid wastewater is 1:1 to 1.25:

1.

5. The method according to any one of claims 1 to 4, characterized in that, The degree of neutralization of the acrylic monomer is 65% to 80%.

6. The method according to claim 1, characterized in that, The initiator is ammonium persulfate, benzoyl peroxide, sodium nitrite, or potassium persulfate; the crosslinking agent is N'N-methylenebisacrylamide, propylene glycol dimethacrylate, or trimethylolpropane triacrylate.

7. The water-absorbing resin prepared by the method according to any one of claims 1 to 6, wherein the main component is sodium polyacrylate.

8. The water-absorbing resin according to claim 7, characterized in that, The main components are biochar and zeolite-modified sodium polyacrylate.

9. The use of the water-absorbing resin according to claim 7 or 8 in the preparation of water-retaining and / or fertilizer-retaining materials.

10. A water-retaining slow-release fertilizer, comprising the water-absorbing resin as described in claim 7 or 8 and a fertilizer loaded on the water-absorbing resin.

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