Preparation method of coral sand soil water-retaining agent based on sludge

Abstract

The invention discloses a preparation method of a coral sand soil water-retaining agent based on sludge, and belongs to the technical field of materials. The preparation method of the sludge-based coral sand soil water-retaining agent is characterized in that sodium carboxymethyl cellulose and sludge are taken as raw materials for cross-linking modification, and the preparation method specifically comprises the following steps: putting the sodium carboxymethyl cellulose into an alkaline solution to obtain a sodium carboxymethyl cellulose solution; mixing the sodium carboxymethyl cellulose solution with sludge to obtain a sodium carboxymethyl cellulose and sludge mixed solution; and adding a cross-linking agent into the sodium carboxymethyl cellulose sludge mixed solution, and carrying out a cross-linking reaction to complete the preparation. According to the sludge-based water-retaining agent disclosed by the invention, sodium carboxymethyl cellulose is used as a framework, and sludge particles are loaded on the sodium carboxymethyl cellulose to form a three-dimensional network structure. The sludge-based water-retaining agent retains a microporous structure formed by crosslinking of carboxymethyl cellulose, moisture can be fully absorbed, and meanwhile, the pore structure of a three-dimensional network structure is enhanced by adding sludge.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of materials, in particular to a preparation method of a sludge-based coral sandy soil water-retaining agent. BACKGROUND

[0002] At present, soil drought and water resource shortage have become a major challenge faced by human society. In sandy soil distribution areas, due to the uneven particle size distribution, loose structure, large porosity and other characteristics, the soil hydraulic conductivity characteristics are unbalanced, and the water and soil conservation capacity is significantly insufficient. Taking the coral reef island ecosystem as an example, the high salinization substrate, extreme drought stress and alkaline pH environment unique to these areas seriously restrict the growth of vegetation, resulting in low land resource utilization and regional ecological degradation. Under this background, it has become a key technical requirement to develop a water-retaining improvement technology suitable for saline-alkali sandy soil.

[0003] In recent years, hydrophilic polymer materials based on three-dimensional porous network structure have attracted widespread attention in the field of soil improvement. Such materials build molecular networks through physical / chemical crosslinking mechanism, exhibit excellent water and fertilizer retention characteristics, and show significant advantages in promoting plant growth and improving water resource utilization efficiency. However, traditional water-retaining materials are mainly prepared from petroleum-based raw materials, which not only have high cost and degradation difficulty, but also have the problem of insufficient environmental friendliness, making it difficult to meet the requirements of water absorption and water retention performance and ecological adaptability for saline-alkali land improvement. This situation has prompted researchers to develop new environmentally friendly water-retaining materials to address the current ecological and environmental challenges.

[0004] Studies have shown that some natural polymers can significantly improve the water absorption performance of water-retaining agents after chemical modification. Sodium carboxymethyl cellulose (CMC) is prepared by etherification reaction of cellulose and sodium monochloroacetate in alkaline conditions. Due to the introduction of a large number of carboxymethyl groups on the molecular chain, the product exhibits excellent solubility and high sodium ion content. This modified cellulose not only has outstanding water absorption capacity, but also exhibits good stability in salt-containing environments. However, its working performance in the field of soil water-retaining improvement still needs to be further improved. SUMMARY

[0005] The purpose of the present application is to provide a sludge-based coral sandy soil water-retaining agent preparation method to solve the above problems in the background art. The sludge-based water-retaining agent has excellent water absorption rate, water absorption rate, water retention rate, salt tolerance, and the preparation process is simple, the reaction conditions are mild, the raw materials are easy to obtain, and it is green and environmentally friendly.

[0006] To achieve the above purpose, the present application provides the following technical solutions:

[0007] One of the technical solutions of the present application: a preparation method of a sludge-based coral sand soil water-retaining agent is provided, which is prepared by cross-linking modification of sodium carboxymethyl cellulose and sludge as raw materials.

[0008] Preferably, the preparation method of the sludge-based coral sand soil water-retaining agent comprises the following steps:

[0009] The sodium carboxymethyl cellulose is placed in an alkaline solution to obtain a sodium carboxymethyl cellulose solution;

[0010] The sodium carboxymethyl cellulose solution is mixed with sludge to obtain a sodium carboxymethyl cellulose sludge mixture;

[0011] A cross-linking agent is added to the sodium carboxymethyl cellulose sludge mixture, and a cross-linking reaction is carried out to obtain the sludge-based coral sand soil water-retaining agent.

[0012] Preferably, the ratio of the amount of sodium carboxymethyl cellulose to the amount of sodium carboxymethyl cellulose solution is 4g:100mL.

[0013] Preferably, the alkaline solution is a 1-9wt% sodium hydroxide aqueous solution.

[0014] Preferably, the addition amount of sludge is 5-25wt% of the sodium carboxymethyl cellulose.

[0015] Preferably, the cross-linking agent is epichlorohydrin.

[0016] Preferably, the addition amount of the cross-linking agent is 3-6wt% of the sodium carboxymethyl cellulose sludge mixture.

[0017] Preferably, the cross-linking reaction is carried out at a temperature of 50-70℃ for 20-60min.

[0018] Preferably, the sludge is residual sludge collected from a domestic sewage treatment plant.

[0019] More preferably, the preparation method of the sludge-based coral sand soil water-retaining agent comprises the following steps:

[0020] (1) Preparation of sodium hydroxide aqueous solution

[0021] Prepare a sodium hydroxide aqueous solution with a mass concentration of 1-9wt%;

[0022] (2) Preparation of sodium carboxymethyl cellulose solution

[0023] The sodium hydroxide aqueous solution is placed on a stirrer, and sodium carboxymethyl cellulose is added at a stirring speed of 200-500r / min to obtain a sodium carboxymethyl cellulose solution;

[0024] (3) Preparation of sodium carboxymethyl cellulose sludge mixture

[0025] In the sodium carboxymethyl cellulose solution, sludge is added and stirred evenly, the sludge addition amount is 5-25% of the mass of sodium carboxymethyl cellulose, to obtain a sludge-sodium carboxymethyl cellulose mixture;

[0026] (4) Preparation of sludge-based water-retaining agent

[0027] In the sludge-sodium carboxymethyl cellulose mixture, a crosslinking agent is added, after uniform stirring at a stirring speed of 200-800 r / min on a stirrer, centrifugation is performed to remove gas bubbles, and then heating is performed at 50-70 DEG C for 20-60 min, to complete the preparation.

[0028] The second technical scheme of the present application provides a sludge-based coral sandy soil water-retaining agent obtained by the above preparation method.

[0029] The third technical scheme of the present application provides an application of the above sludge-based coral sandy soil water-retaining agent in the field of sandy soil water retention.

[0030] When water is sufficient, the sludge-based water-retaining agent can absorb a large amount of water, and form a stable gel-like substance through ion adsorption and internal porous structure, effectively alleviating the problem of water loss of sandy soil. When water is insufficient, the sludge-based water-retaining agent applied to the soil near the plant root system can slowly release the stored water, continuously supplying the required water for the plant root system, thereby guaranteeing the normal growth needs of the plant.

[0031] The beneficial technical effects of the present application are as follows:

[0032] The sludge-based water-retaining agent of the present application takes sodium carboxymethyl cellulose as a skeleton, and sludge particles are loaded on the sodium carboxymethyl cellulose to form a three-dimensional network structure. The sludge-based water-retaining agent retains the microporous structure formed by crosslinking of carboxymethyl cellulose, and can fully absorb water, and the addition of sludge enhances the stability of the three-dimensional network structure. When water enters the three-dimensional network structure, it is locked by the hydrophilic group and the pore structure, and the structural stability of the crosslinked network enables the water-retaining agent to store a large amount of water without dissolving or disintegrating; under drought conditions, the elasticity of the crosslinked network enables the water-retaining agent to maintain structural integrity when releasing water, realizing slow release of water, so that the sludge-based water-retaining agent has good water absorption and water retention performance.

[0033] The sludge-based water-retaining agent prepared by the application is in block or film shape, can effectively fill in the voids of soil, and slow down water loss. The carboxymethyl cellulose sodium contains a large number of carboxyl and hydroxyl groups in the molecular weight, has strong hydrophilicity, thereby improving the osmotic pressure difference inside and outside the water-retaining agent, and therefore the sludge-based water-retaining agent has good salt tolerance. In addition, the sludge contains rich organic matter (such as polysaccharides and proteins) and inorganic components (such as metal ions and minerals), and has a porous structure; the porous structure of the sludge can serve as a water transmission channel, and the organic matter and inorganic components of the sludge can interact with the carboxymethyl cellulose sodium (CMC), thereby enhancing the stability of the water-retaining agent. Therefore, the sludge-based water-retaining agent has good mechanical properties while having salt tolerance, and is not prone to breakage and loss.

[0034] In the carboxymethyl cellulose sodium-containing water-retaining agent system of the application, when the sludge is added, the water absorption and release performance, mechanical properties, salt tolerance and thermal stability of the water-retaining agent are all significantly improved. The sludge components play a physical crosslinking role in the polymerization process, significantly improve the stability of the network structure by strengthening the intermolecular force, and this dense structure endows the material with excellent thermal stability, effectively inhibits the expansion of the structure under high temperature conditions, and enhances the binding of water molecules, thereby reducing the water release rate and prolonging the water retention period.

[0035] The sludge-based water-retaining agent prepared by the application not only has the advantages of high water absorption rate, fast water absorption rate, high water retention rate, good salt tolerance and the like, but also has the advantages of simple preparation process, mild reaction conditions, easy-to-obtain raw materials, green environmental protection, no pollution to the environment after application to the soil, and realization of resource utilization of the sludge. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings described below only show some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0037] Figure 1 The figures (a) and (b) are scanning electron microscope (SEM) photos of the water-retaining agent of Example 1 of the application at different magnifications.

[0038] Figure 2 The figures (a) and (b) are scanning electron microscope (SEM) photos of the water-retaining agent of Example 1 of the application at different magnifications.

[0039] Figure 3 The figure is the water absorption multiple curve of the water-retaining agent of Examples 1-5 of the application.

[0040] Figure 4The water retention rate of the water-retaining agent of Example 1 of the present application after water absorption as a function of time.

[0041] Figure 5 The repeated water absorption performance of the water-retaining agent of Example 1 of the present application.

[0042] Figure 6 The salt tolerance of the water-retaining agent of Example 1 of the present application.

[0043] Figure 7 The water absorption rate of the water-retaining agent of Example 1 and Comparative Example 1 of the present application in different solutions. (a) is in ultrapure water, (b) is in 0.9 wt% NaCl solution.

[0044] Figure 8 The water retention of the water-retaining agent of Example 1 and Comparative Example 1 of the present application in different solutions. (a) is the water retention curve of the water-retaining agent soaked in ultrapure water at 35°C, (b) is the water retention curve of the water-retaining agent soaked in 0.9 wt% NaCl solution at 35°C, (c) is the water retention curve of the water-retaining agent soaked in ultrapure water at 60°C, (d) is the water retention curve of the water-retaining agent soaked in 0.9 wt% NaCl solution at 60°C.

[0045] Figure 9 The repeated water absorption performance of the water-retaining agent of Example 1 and Comparative Example 1 of the present application in different solutions. (a) is in ultrapure water, (b) is in 0.9 wt% NaCl solution.

[0046] Figure 10 The water absorption performance of the water-retaining agent of Example 1 and Comparative Example 1 of the present application in six salt solutions. DETAILED DESCRIPTION

[0047] Various illustrative embodiments of the present application are described in detail below. The detailed description is presented in terms of specific embodiments which include particular components, materials, and dimensions. Those skilled in the art will recognize that embodiments of the present application can be practiced with

[0048] In addition, for numerical ranges that are expressly recited herein, it is to be understood that every intervening value between the upper and lower limits of the range is also specifically contemplated. In addition, each smaller range that falls within the encompassed range is also specifically contemplated. These smaller ranges are also specifically disclosed herein. The upper and lower limits of these smaller ranges can independently be included or excluded in the range.

[0049] Unless otherwise indicated, all technical and scientific terms have the same meaning as those one of ordinary skill in the art of the invention would understand. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described. It is to be understood that this invention is not limited to particular methods and materials, unless otherwise specified, unless specifically stated otherwise.

[0050] As used herein, the terms "comprise", "comprising", "include", "including", "have" and "having" and the like are used synonymously to mean one or more steps as used in the art and are not intended to exclude additional steps.

[0051] The sludge used in the present invention is derived from a sewage treatment plant on an island.

[0052] The "room temperature" in the present invention is 10-30°C unless otherwise specified.

[0053] The crosslinking agent used in the following examples and comparative examples in the present invention is epichlorohydrin.

[0054] Each raw material used in the following examples and comparative examples in the present invention is a commercially available product.

[0055] Example 1

[0056] A method for preparing a soil water-retaining agent, comprising the following steps:

[0057] (1) Preparation of sodium hydroxide aqueous solution

[0058] Prepare a 5wt% sodium hydroxide aqueous solution.

[0059] (2) Preparation of sodium carboxymethyl cellulose solution

[0060] Place the above sodium hydroxide aqueous solution in a blender, add 4.0 g of sodium carboxymethyl cellulose at a stirring speed of 200 r / min and stir until uniform, to prepare 100 mL of a 4.0% (W / V) sodium carboxymethyl cellulose solution.

[0061] (3) Preparation of sodium carboxymethyl cellulose sludge mixture

[0062] Add 0.4 g of sludge to the above sodium carboxymethyl cellulose solution and stir until uniform, with the sludge addition amount being 10% of the mass of the sodium carboxymethyl cellulose, and stir at a stirring speed of 200 r / min for 15 min to obtain a sodium carboxymethyl cellulose sludge mixture.

[0063] (4) Preparation of sludge-based water-retaining agent

[0064] In the above sodium carboxymethyl cellulose sludge mixture, add 5 mL of crosslinking agent, the crosslinking agent addition amount is 5% of the volume of sodium carboxymethyl cellulose sludge mixture, after uniform stirring on the stirrer at 200 r / min, centrifugal to remove bubbles, then heated at 60℃ for 30 min, dried and then washed repeatedly to clean up residual sodium hydroxide and crosslinking agent, dried, to obtain the sludge-based water retaining agent (recorded as the sample with 10% sludge content).

[0065] Example 2

[0066] A method for preparing a soil water retaining agent, the steps are as follows:

[0067] (1) Preparation of sodium hydroxide aqueous solution

[0068] Prepare a 5wt% sodium hydroxide aqueous solution.

[0069] (2) Preparation of sodium carboxymethyl cellulose solution

[0070] Put the above sodium hydroxide aqueous solution on the stirrer, add 4.0 g of sodium carboxymethyl cellulose under the stirring speed of 200 r / min and stir uniformly to prepare 4.0% (W / V) sodium carboxymethyl cellulose solution 100 mL.

[0071] (3) Preparation of sodium carboxymethyl cellulose sludge mixture

[0072] Stir uniformly 0.6 g of sludge in the above sodium carboxymethyl cellulose solution, the sludge addition amount is 15% of the mass of sodium carboxymethyl cellulose, stir at 200 r / min for 15 min to obtain sodium carboxymethyl cellulose sludge mixture.

[0073] (4) Preparation of sludge-based water retaining agent

[0074] In the above sodium carboxymethyl cellulose sludge mixture, add 5 mL of crosslinking agent, the crosslinking agent addition amount is 5% of the volume of sodium carboxymethyl cellulose sludge mixture, after uniform stirring on the stirrer at 200 r / min, centrifugal to remove bubbles, then heated at 60℃ for 60 min, dried and then washed repeatedly to clean up residual sodium hydroxide and crosslinking agent, dried, to obtain the sludge-based water retaining agent (recorded as the sample with 15% sludge content).

[0075] Example 3

[0076] A method for preparing a soil water retaining agent, the steps are as follows:

[0077] (1) Preparation of sodium hydroxide aqueous solution

[0078] Prepare a 5wt% sodium hydroxide aqueous solution.

[0079] (2) Preparation of sodium carboxymethyl cellulose solution

[0080] The above sodium hydroxide aqueous solution was placed in a blender, 4.0 g of sodium carboxymethyl cellulose was added at a stirring speed of 200 r / min and stirred uniformly to prepare 100 mL of 4.0% (W / V) sodium carboxymethyl cellulose solution.

[0081] (3) Preparation of sodium carboxymethyl cellulose sludge mixed solution

[0082] 0.8 g of sludge was added to the above sodium carboxymethyl cellulose solution and stirred uniformly, the sludge addition amount was 20% of the mass of sodium carboxymethyl cellulose, and stirring was carried out at a stirring speed of 200 r / min for 15 min to obtain a sodium carboxymethyl cellulose sludge mixed solution.

[0083] (4) Preparation of sludge-based water-retaining agent

[0084] 5 mL of crosslinking agent was added to the above sodium carboxymethyl cellulose sludge mixed solution, the crosslinking agent addition amount was 5% of the volume of the sodium carboxymethyl cellulose sludge mixed solution, and after uniform stirring at a stirring speed of 200 r / min on a blender, gas bubbles were removed by centrifugation, then heated at 50°C for 30 min, and dried, and then repeatedly washed to clean residual sodium hydroxide and crosslinking agent, and dried to obtain a sludge-based water-retaining agent (denoted as a sample with a sludge content of 20%).

[0085] Example 4

[0086] A method for preparing a soil water-retaining agent, the steps being as follows:

[0087] (1) Preparation of sodium hydroxide aqueous solution

[0088] A sodium hydroxide aqueous solution with a mass concentration of 5 wt% was prepared.

[0089] (2) Preparation of sodium carboxymethyl cellulose solution

[0090] The above sodium hydroxide aqueous solution was placed in a blender, 4.0 g of sodium carboxymethyl cellulose was added at a stirring speed of 200 r / min and stirred uniformly to prepare 100 mL of 4.0% (W / V) sodium carboxymethyl cellulose solution.

[0091] (3) Preparation of sodium carboxymethyl cellulose sludge mixed solution

[0092] 1 g of sludge was added to the above sodium carboxymethyl cellulose solution and stirred uniformly, the sludge addition amount was 25% of the mass of sodium carboxymethyl cellulose, and stirring was carried out at a stirring speed of 200 r / min for 15 min to obtain a sodium carboxymethyl cellulose sludge mixed solution.

[0093] (4) Preparation of sludge-based water-retaining agent

[0094] In the above carboxymethyl cellulose sodium sludge mixture solution, 5 mL of crosslinking agent was added, the crosslinking agent was added in an amount of 5% of the volume of the carboxymethyl cellulose sodium sludge mixture, and after uniform stirring on a stirrer at a stirring speed of 200 r / min, the bubbles were removed by centrifugation, and then heated at 60°C for 30 min, and after drying, the residual sodium hydroxide and crosslinking agent were cleaned by repeated washing, and dried, to obtain the sludge-based water retaining agent (denoted as a sample with a sludge content of 25%).

[0095] Example 5

[0096] The difference from Example 1 is only that step (3) is modified as:

[0097] In the above carboxymethyl cellulose sodium solution, 0.2 g of sludge was added and stirred uniformly, the sludge was added in an amount of 5% of the mass of the carboxymethyl cellulose sodium, and stirred at a stirring speed of 200 r / min for 15 min to obtain a carboxymethyl cellulose sodium sludge mixture. The finally prepared soil water retaining agent is denoted as a sample with a sludge content of 5%.

[0098] Comparative Example 1

[0099] The difference from Example 1 is only that the addition of sludge in step (3) is omitted, and a pure carboxymethyl cellulose sodium hydrogel type water retaining agent is prepared, denoted as CDH.

[0100] Effect verification

[0101] 1. The actual photos of the water retaining agent of Example 1 before and after absorbing water in water for 24 h. The test results are shown in Figure 1 .

[0102] Figure 1 The actual photos of the water retaining agent of Example 1 of the present application before and after absorbing water and swelling in water. Among them, (a) is before absorbing water and swelling, and (b) is after absorbing water and swelling.

[0103] As can be seen from Figure 1 , the volume of the sludge-based water retaining agent of Example 1 increases significantly after absorbing water for 24 h. During the process of absorbing water, the block of the sample swells obviously, but there is no dissolution phenomenon, which shows that the sludge-based water retaining agent prepared by the present application has good mechanical properties.

[0104] 2. Figure 2 (a) and (b) in

[0105] As can be seen from Figure 2It can be seen that the water-retaining agent of Example 1 has a rich pore structure. The sludge-based water-retaining agent of the present application has a strong adsorption of water due to the strong binding force of the hydrophilic groups such as -OH, -COOH and -NH2 on the polymer matrix to water, and the physical adsorption of the pore structure to water, so the rich pore structure of the sludge-based water-retaining agent can significantly improve the adsorption capacity of water.

[0106] 3. The water absorption rate of the sludge-based water-retaining agent of Example 1 was tested as follows: dry sludge-based water-retaining agent was immersed in 1000 mL of distilled water at room temperature, and after the gel was swelled and balanced, it was taken out and the surface moisture was quickly wiped off, weighed, and the water absorption multiple was calculated; then the test was re-conducted by replacing only the distilled water with a 0.9wt% NaCl solution. The water absorption multiple (g / g) was calculated by formula (1):

[0107]

[0108] wherein: Q eq is the water absorption multiple, g / g; M0 and M1 are the weights of the dry sample and the swelled sample, respectively, g.

[0109] Figure 3 is the water absorption multiple curve of the water-retaining agent of Examples 1-5 of the present application.

[0110] From Figure 3 It can be seen that the water absorption multiple of the water-retaining agent of Examples 1-5 after swelling equilibrium is 463g / g-812g / g. Among them, when the sludge addition amount in the sludge-based water-retaining agent is 10% of the mass of sodium carboxymethyl cellulose (Example 1), the water absorption rate in ultrapure water is the highest, which can reach 812g / g. In a 0.9wt% NaCl solution, the water absorption multiple can be as high as 147g / g.

[0111] In contrast, the water absorption rate of the water-retaining agents of Examples 2, 3 and 4 (samples with 15%, 20% and 25% of sludge content) relatively decreased. This is mainly because, as the proportion of sludge increases, more metal ions are introduced by the sludge, resulting in too high crosslinking density, limiting the stretching of the polymer chain and reducing the swelling capacity; moreover, the effect of the ash in the sludge on blocking the pores is further enhanced, hindering the penetration of water; and when the sludge addition amount is too high, it will excessively damage the uniformity of CMC-Na, causing local network rupture and reducing the overall water absorption performance; when the sludge is added in excess, the hydrophobic components such as oil and undegraded organic matter in the sludge will cover the hydrophilic groups on the surface of CMC-Na to a greater extent, forming a hydrophobic barrier and hindering the entry of water into the network, resulting in weakened water absorption.

[0112] 4. The water release performance reflects the speed of water release of the material under certain conditions. The slower the water release, the stronger the hydrophilicity and water retention of the material.

[0113] The water-release performance (water retention rate) of the water-retaining agent was tested using the following method: At 25°C, a sample of the sludge-based water-retaining agent was immersed in deionized water until the gel reached swelling equilibrium. The sample was then removed, the surface moisture was quickly wiped off, and the weight was weighed. The sludge-based water-retaining agent sample that had reached swelling equilibrium was then placed at the bottom of a beaker and placed in a 35°C constant-temperature forced-air drying oven for heat treatment to evaluate the water-release performance. The determination of water retention is based on the relationship between mass and time. The weight of the gel was measured at regular intervals, and the water retention rate of the water-retaining agent was calculated using formula (2):

[0114]

[0115] Where: W is the water retention rate, %; M2 is the saturated water absorption of the material, g / g; M t is the water absorption of the material at time t, g / g.

[0116] Figure 4 The graph shows the change in water retention rate over time of the water-retaining agent of Example 1 of the present invention after water absorption.

[0117] like Figure 4 As shown, the sludge-based water-retaining agent of Example 1 maintained a high water retention rate of approximately 52% after being incubated at 35°C for 12 hours. Therefore, when applied to soil, this material can store large amounts of water when moisture is abundant and slowly release the stored water when moisture is scarce, meeting the material's practical application requirements.

[0118] 5. Test the repeated water absorption capacity of the water-retaining agent as follows: Immerse a sample (0.5 g) in 1 L of distilled water until swelling equilibrium is reached. Calculate the water absorption multiple of each sample using formula (1). Then, dry the swollen sample in a 60°C oven to constant weight. Repeat this process five times and test the repeated water absorption efficiency. The repeated water absorption efficiency is calculated using formula (3):

[0119]

[0120] Where: D is the repeated water absorption efficiency, %; X e is the water absorption multiple of the material for the e-th time, g / g; X0 is the water absorption multiple of the material for the first time, g / g.

[0121] Figure 5 This is the repeated water absorption performance curve of the water-retaining agent of Example 1 of the present invention.

[0122] like Figure 5 As shown, the sludge-based water-retaining agent of Example 1 still maintained a high water absorption efficiency of 75.14% after the fifth repeated water absorption. This is because the addition of sludge strengthens the three-dimensional network structure, mitigating the damage to the carboxymethyl cellulose microparticle network structure after water absorption and swelling, resulting in excellent repeated water absorption performance.

[0123] 6. The salt tolerance of the water-retaining agent was tested according to the following method: dry samples were immersed in 0.9wt% aqueous solutions of MgCl2, CaCl2, FeCl3, NH4Cl, KCl, NaCl respectively at room temperature to achieve swelling equilibrium, then the swollen samples were filtered with 100 mesh nylon mesh until no water droplets fell, then weighed, and the water absorption multiple was calculated by method (1).

[0124] Figure 6 The salt tolerance of the water-retaining agent of Example 1 of the present application.

[0125] As shown in Figure 6 , the water absorption multiples of the sludge-based water-retaining agent of Example 1 in 0.9wt% NH4Cl, KCl, NaCl, CaCl2, MgCl2 and FeCl3 solutions were 141, 157, 143, 112, 111 and 56g / g respectively. It can be seen that the water-retaining agent designed in the present application has certain water absorption performance in different salt solutions, so the water-retaining agent of the present application has good salt tolerance. As can be seen from the figure, under the condition of the same anion, the influence of different cations on the water absorption performance of the sludge-based water-retaining agent has certain differences, and the influence degree of the several cation salt solutions on the water absorption multiple of the sludge-based water-retaining agent is in the order of: 3+ Mg 2+ > Ca 2+ > NH4 + > Na + > K + It can be seen that at the same concentration, the more the number of charges carried by the cation, the more significant the influence on the water absorption performance of the sludge-based water-retaining agent.

[0126] 7. The water absorption rate of the water-retaining agent was tested according to the following method: dry water-retaining agents of Example 1 (denoted as SDH) and Comparative Example 1 (denoted as CDH) were taken and immersed in 1000mL ultrapure water and 0.9wt% aqueous NaCl solution respectively at room temperature, after a period of soaking, part of the samples were taken and the surface moisture was quickly wiped off, weighed, and the water absorption multiple (Q t , g / g) at that time was calculated, until the gel swelling equilibrium was reached and stopped, to draw the water absorption rate curve.

[0127] Figure 7 The water absorption rate curves of the water-retaining agents of Example 1 and Comparative Example 1 of the present application in different solutions. Among them, (a) is in ultrapure water, (b) is in 0.9wt% NaCl solution.

[0128] From Figure 7As shown in the figure, the swelling kinetics of the two hydrogels have the same trend. They absorb water and swell rapidly in the first 50 minutes, and then gradually slow down in the later period until the water absorption saturation reaches equilibrium. The water absorption capacity of SDH is significantly better than that without adding sludge water-retaining agent. The water absorption rate of SDH in ultrapure water increases from 446g / g to 531g / g, and the water absorption rate in 0.9% NaCl solution increases from 130g / g to 161g / g, indicating that the addition of sludge significantly increases the water absorption rate of the hydrogel.

[0129] 8. The water retention performance of the water-retaining agent was tested according to the following method: At 25°C, samples of the water-retaining agent of Example 1 (denoted as SDH) and Comparative Example 1 (denoted as CDH) were immersed in ultrapure water and a 0.9 wt% NaCl aqueous solution, respectively. After the gel reached swelling equilibrium, the sample was removed, the surface moisture was quickly wiped dry, and the gel was weighed. The swollen and equilibrium water-retaining agent sample was placed at the bottom of a beaker and then placed in a constant temperature forced air drying oven at 35°C and 60°C, respectively, to evaluate the water retention performance. The water retention performance was determined based on the relationship between mass and time. The weight of the gel was measured at regular intervals, and the water retention rate of the water-retaining agent was calculated using Formula (2).

[0130] Figure 8 The water retention curves of the water-retaining agents of Example 1 and Comparative Example 1 of the present invention in different solutions are shown. (a) shows the water retention curve of the water-retaining agent after being immersed in ultrapure water at 35°C, (b) shows the water retention curve of the water-retaining agent after being immersed in a 0.9 wt% NaCl solution at 35°C, (c) shows the water retention curve of the water-retaining agent after being immersed in ultrapure water at 60°C, and (d) shows the water retention curve of the water-retaining agent after being immersed in a 0.9 wt% NaCl solution at 60°C.

[0131] Depend on Figure 8 As shown, the water retention performance of SDH under different temperature and salt concentration environments is better than that of CDH, showing significantly optimized water release efficiency, indicating that the introduction of sludge enhances the water holding capacity of the hydrogel. The main reasons may be the following aspects: First, it promotes the rigid structure of the hydrogel. The organic-inorganic composite components in the sludge are embedded in the CMC chain to form a denser and more rigid three-dimensional network structure; this structure not only inhibits the excessive swelling of the network structure, but also provides additional water storage space through the multi-level pores of micropores and mesopores, delaying the diffusion and loss of water molecules. Secondly, the interaction between ion shielding and surface, the silicates and metal oxides in the sludge can adsorb Na through cation exchange + , reducing its competitive binding to carboxyl groups, thereby weakening the salt-induced network contraction effect. Simultaneously, the hydroxyl and carboxyl groups on the sludge surface form high-strength hydrogen bonds with water molecules, further securing them. Furthermore, the inorganic minerals in the sludge maintain network integrity through their thermally stable skeletons, preventing chain segment relaxation caused by high temperatures, while the organic components reduce water evaporation through their hydrophobic microdomains.

[0132] 9. The repeated water absorption capacity of the water-retaining agent was tested using the following method: 0.5 g of the water-retaining agent samples from Example 1 (denoted as SDH) and Comparative Example 1 (denoted as CDH) were immersed in 1 L of ultrapure water and a 0.9 wt% NaCl aqueous solution, respectively, until swelling equilibrium was reached. The water absorption multiple of each sample was calculated using formula (1). The swollen samples were then dried in a 60°C oven to constant weight. This process was repeated five times, and the repeated water absorption efficiency was measured. The repeated water absorption efficiency was calculated using formula (3).

[0133] Figure 9 The repeated water absorption performance curves of the water-retaining agents of Example 1 and Comparative Example 1 in different solutions are shown, wherein (a) is in ultrapure water, and (b) is in 0.9 wt% NaCl solution.

[0134] Depend on Figure 9 As shown, the repeated water absorption of the hydrogels was measured through repeated swelling-drying experiments. Although the water absorption capacity of both materials decreased after multiple cycles, SDH exhibited superior stability, with a significantly lower performance decay rate than CDH. In ultrapure water, after five cycles, the water absorption of CDH remained at 68.5% of its initial value, while SDH's absorption rate was higher, remaining at 75.1% of its initial value, with the water loss rate decreasing from 31.5% to 24.9%. In 0.9% NaCl solution, the water absorption of CDH dropped to 52.1% of its initial value, while SDH maintained 65.8%, with the water loss rate decreasing from 47.9% to 34.2%. This is primarily due to the sludge strengthening the hydrogel network and improving its stability. On the one hand, the sludge particles act as nanofillers embedded between the polymer chains, enhancing the mechanical strength of the three-dimensional framework through hydrogen bonding and van der Waals forces, reducing network damage during water absorption and release. On the other hand, the sludge's porous structure provides additional water storage space, alleviating the impact of cyclic stress on the main network. In a salt solution environment, the cation exchange sites in the sludge can partially adsorb Na + , weakening its competitive binding with carboxylate, thereby delaying the shrinkage of the hydrogel network. Therefore, the introduction of sludge improves the reuse performance of SDH. Especially in salinized soils such as those containing Na + In coral sand soil, SDH can maintain a longer-lasting water retention capacity, thereby promoting plant growth while reducing irrigation frequency, and its enhanced structural stability can extend the material life and increase the service life of SDH.

[0135] 10. The salt tolerance of the water-retaining agent was tested according to the following method: dry samples of the water-retaining agent of Example 1 (denoted SDH) and Comparative Example 1 (denoted CDH) were immersed in 0.9 wt% aqueous solutions of MgCl2, CaCl2, FeCl3, NH4Cl, KCl, NaCl at room temperature to achieve swelling equilibrium, then the swollen samples were filtered with a 100-mesh nylon mesh until no water droplets fell, then weighed, and the water absorption multiple was calculated by method (1).

[0136] Figure 10 Water absorption performance of the water-retaining agent of Example 1 and Comparative Example 1 in six salt solutions.

[0137] Figure 10 After comparing the swelling behavior of CDH and SDH in different salt solutions, it was found that the swelling performance of the hydrogel showed a significant negative correlation with the valence of the cation. Both hydrogels showed higher water absorption in monovalent salt solutions (KCl, NH4Cl, NaCl), while in divalent (CaCl2, MgCl2) and trivalent salt (FeCl3) solutions, the water absorption decreased. This trend indicates that the higher the valence of the cation, the stronger the charge neutralization ability of the -COO - group in the hydrogel network, and the high-valence cation is more likely to form a stable complex with the carboxylate (-COO - ) group, hindering the swelling of the hydrogel.

[0138] SDH showed better salt solution adaptability than CDH, and the degree of improvement in water absorption performance varied with the solution system. This may be due to the fact that the organic matter and inorganic particles rich in sludge increase the porosity of the hydrogel, while providing additional adsorption sites, enhancing the physical adsorption and ion exchange capacity. In addition, the hydroxyl and carboxyl groups in the sludge may optimize the network structure through hydrogen bonding or weak interactions, thereby improving the accommodation of monovalent ions. In the medium-valence cation solutions CaCl2, MgCl2, the water absorption of SDH decreased, which was significantly lower than that in monovalent salt environments, but still higher than that of CDH, indicating that the introduction of sludge partially alleviated the compression effect of high-valence cations on the gel network. This phenomenon may be attributed to the adsorption of Ca 2+ / Mg 2+ ions in the sludge, reducing their direct binding to the polymer chains, thereby delaying network contraction. In the high-valence cation (FeCl3) system, the water absorption performance of SDH decreased significantly, which was attributed to the high complex stability of Fe 3+ with carboxylate groups (log K≈11.4), and its complexing ability was Ca 2+ (log K≈4.5) and Mg 2+(log K≈3.4) of 107 and 108, which can induce stronger molecular cross-linking, significantly inhibit the water absorption of hydrogel. The addition of sludge disperses Fe 3+ .

[0139] 11. The improved soil is obtained by adding the sludge-based water-retaining agent prepared in Example 1, 2, 3, and 4 into the air-dried coral sandy soil without adding a water-retaining agent at a proportion of 2.0wt%, 1wt%, 0.5wt%, and 0.2wt% (the mass of the air-dried coral sandy soil is taken as the calculation unit), respectively. Then, the improved soil with a weight of W1 is filled into a ring knife, and then subjected to 1 hour of rainfall (the concentrations of calcium, magnesium, potassium, and sodium ions in the simulated rainwater are 10.3, 2.00, 4.97, and 8.96 mg / L, respectively) under the action of a peristaltic pump. The rainfall intensity is 40 mm / h. After the termination of the simulated rainfall, the ring knife is left to stand for 8 hours, and the excess gravitational water is discharged to evaluate the rainfall interception capacity of the soil. Then, the ring knife is weighed (the mass of the wet soil, W2), and the soil moisture content is calculated. The soil moisture content is calculated by formula (4):

[0140]

[0141] In the formula, C is the soil moisture content, %; W1 is the mass of the dry soil, g; and W2 is the mass of the wet soil, g.

[0142] Then, a test of 10 days of continuous water evaporation loss is performed (the test environment simulates the actual environment of an island: the temperature is 30°C, and the relative humidity is 70%). The soil moisture content after the continuous water evaporation loss during the period is still calculated by formula (4). At this time, W2 is the mass of the real-time moisture content of the soil.

[0143] It is found through measurement that, compared with the pure coral sandy soil, the soil moisture content of the coral sandy soil added with the sludge-based water-retaining agent of Example 1 is increased from 15.6% to 66.0% (the moisture content is increased by 3.23 times), and the soil moisture content after 10 days of continuous water evaporation loss is still 27.6%.

[0144] Compared with the pure coral sandy soil, the soil moisture content of the coral sandy soil added with the sludge-based water-retaining agent of Example 2 is increased from 15.6% to 60.3% (the moisture content is increased by 2.87 times), and the soil moisture content after 10 days of continuous water evaporation loss is still 19.7%.

[0145] Compared with the pure coral sandy soil, the soil moisture content of the coral sandy soil added with the sludge-based water-retaining agent of Example 3 is increased from 15.6% to 52.4% (the moisture content is increased by 2.36 times), and the soil moisture content after 10 days of continuous water evaporation loss is still 12.6%.

[0146] Compared with pure coral sand soil, the moisture content of the coral sand soil with the addition of the sludge-based water-retaining agent of Example 4 increased from 15.6% to 37.0% (the moisture content increased by 1.37 times). After 10 days of continuous evaporation of water, the soil moisture content was still 8.0%.

[0147] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a coral sand soil water retaining agent based on sludge, characterized in that: It is prepared by cross-linking and modifying sodium carboxymethyl cellulose and sludge as raw materials.

2. The preparation method according to claim 1, characterized in that The following steps are involved: placing sodium carboxymethyl cellulose in an alkaline solution to obtain a sodium carboxymethyl cellulose solution; mixing the sodium carboxymethyl cellulose solution with sludge to obtain a sodium carboxymethyl cellulose sludge mixed solution; A cross-linking agent is added to the sodium carboxymethyl cellulose sludge mixed solution to undergo a cross-linking reaction to obtain the sludge-based coral sand soil water-retaining agent.

3. The preparation method according to claim 2, characterized in that The dosage ratio of the sodium carboxymethyl cellulose to the sodium carboxymethyl cellulose solution is 4g:100mL.

4. The preparation method according to claim 2, characterized in that The alkaline solution is a 1-9 wt% sodium hydroxide aqueous solution.

5. The preparation method according to claim 2, characterized in that The added amount of the sludge is 5 to 25 wt% of the sodium carboxymethyl cellulose.

6. The preparation method according to claim 2, characterized in that The cross-linking agent is epichlorohydrin.

7. The preparation method according to claim 2, characterized in that The added amount of the cross-linking agent is 3-6 wt% of the sodium carboxymethyl cellulose sludge mixed solution.

8. The preparation method according to claim 2, characterized in that The cross-linking reaction temperature is 50-70° C., and the time is 20-60 minutes.

9. A sludge-based coral sand soil water-retaining agent obtained according to the preparation method according to any one of claims 1 to 8.

10. Use of the sludge-based coral sand soil water-retaining agent according to claim 9 in the field of sandy soil water retention.

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