Modified chitosan, its preparation method and application in water treatment

Abstract

The application discloses a modified chitosan, a preparation method thereof and application of the modified chitosan in water treatment, relates to the technical field of water treatment, and relates to a composite modified product obtained by sequentially performing substitution modification and quaternary ammonium salt modification on chitosan raw materials and doping polyacrylamide; the preparation method is as follows: firstly, chitosan is dissolved in an acidic solution, an etherification agent and a quaternary ammonium reagent are added, and a quaternary ammonium reaction is performed under alkaline conditions; then, a polyacrylamide solution is added to perform amide copolymerization and doping; finally, dialysis, ethanol precipitation and vacuum drying purification are performed to obtain amide quaternary ammonium salt modified chitosan; the modified chitosan, the preparation method thereof and the application of the modified chitosan in water treatment introduce a large number of cationic groups on the molecular chain of chitosan through amide and quaternary ammonium salt double modification, significantly improve the solubility of the chitosan in water, especially the solubility speed and degree under a low-temperature environment, and effectively solve the problem of poor application effect of natural chitosan in cold region low-temperature low-turbidity surface water.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a modified chitosan, its preparation method, and its application in water treatment. Background Technology

[0002] In the cold regions of northern my country, surface water in winter is generally characterized by low temperature and low turbidity. Low temperatures increase water viscosity, weakening the Brownian motion of colloidal particles and reducing their effective collision probability; while low turbidity means low concentration of suspended particles and large particle spacing, making it difficult to effectively coagulate using traditional flocculation methods. This results in difficulties in floc formation and poor settling performance in conventional water treatment processes, making it difficult to consistently meet effluent quality standards. Currently, water treatment plants commonly use synthetic flocculants such as polyaluminum chloride (PAC) and polyacrylamide (PAM) to treat this type of water. However, the residues of these chemical agents, such as residual aluminum from aluminum salt flocculants and unreacted acrylamide (AM) monomers in PAM, have been proven to have neurotoxic and carcinogenic risks, posing a threat to human health. Furthermore, traditional agents have poor biodegradability, easily causing secondary pollution.

[0003] Chitosan, a natural alkaline polysaccharide, has attracted much attention due to its excellent flocculation properties, unique molecular structure, good biocompatibility, and biodegradability. Its molecular chains are rich in free amino groups that can protonate and become positively charged in water. This charge neutralization destabilizes negatively charged colloidal particles, and the long molecular chains facilitate adsorption bridging, promoting floc sedimentation. It is an ideal natural material to replace traditional chemical flocculants. However, directly applying chitosan to low-temperature, low-turbidity water treatment has significant limitations, such as poor low-temperature solubility, insufficient charge density under extreme water quality conditions, and poor molecular chain flexibility, resulting in unsatisfactory performance when used alone as a primary flocculant. Existing chitosan compounding technologies only achieve basic synergistic flocculation, failing to improve its low-temperature adaptability and flocculation activity through targeted modification. This fails to address the technical bottlenecks of low flocculation efficiency and difficulty in further reducing residual aluminum in effluent under low-temperature, low-turbidity conditions, thus limiting its engineering applications. Therefore, how to improve the adaptability, solubility and flocculation activity of chitosan in low temperature and low turbidity environments through modification is a core technical problem that urgently needs to be solved in this field. Summary of the Invention

[0004] The purpose of this invention is to provide a modified chitosan, its preparation method, and its application in water treatment, so as to solve the problems of poor solubility and insufficient flocculation activity of existing chitosan in the treatment of cold, low-temperature, and low-turbidity surface water, and reduce the health risks caused by chemical residues of existing flocculants.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a modified chitosan, which is a composite modified product obtained by sequentially subjecting chitosan raw material to substitution modification and quaternary ammonium salt modification, and then doping it with polyacrylamide; wherein, the substitution modification and quaternary ammonium salt modification are achieved by reacting chitosan raw material with an etherifying agent and a quaternary ammonium salt under alkaline conditions, the etherifying agent being 3-chloro-2-hydroxypropyltrimethylammonium chloride, the quaternary ammonium salt being glycidyltrimethylammonium chloride, and the doping ratio of polyacrylamide being 0.5% to 5% of the mass of the chitosan raw material; the degree of quaternization substitution of the modified chitosan is ≥15%.

[0006] A method for preparing modified chitosan, which is used for the above-mentioned modified chitosan, specifically includes the following steps:

[0007] S1. Substitution modification and quaternization reaction: Chitosan raw material is dissolved in an acidic solution, then an etherifying agent and a quaternization reagent are added, the pH is adjusted to alkaline with a buffer solution, and the substitution modification and quaternization reaction is carried out under heating conditions to obtain a reaction solution;

[0008] S2, amidation doping: Add polyacrylamide solution to the reaction solution obtained in S1, continue stirring the reaction to carry out amidation copolymerization and doping, and obtain the reaction product;

[0009] S3. Purification: The reaction product obtained in S2 is dialyzed, precipitated with ethanol, and vacuum dried to obtain purified amidated quaternary ammonium salt modified chitosan.

[0010] Furthermore, in S1, the weight ratio of chitosan raw material to etherifying agent and quaternizing agent is 1:0.5-2:0.5-2; the reaction pH is set to 8-10, the reaction temperature is set to 50-80℃, and the reaction time is set to 4-8 hours.

[0011] Furthermore, the amount of polyacrylamide added in S2 is 1% to 3% of the mass of chitosan raw material; the conditions for continued stirring and reaction are: temperature 40 to 60°C, time 1 to 3 hours.

[0012] Furthermore, in step S3, dialysis uses dialysis bags with a molecular weight cutoff of 8000–14000 Da; the vacuum drying temperature is set to 40–60°C, and the time is set to 12–24 hours.

[0013] An application of modified chitosan in water treatment involves using modified chitosan as a coagulant aid, in combination with polyferric salts or polyaluminum salts, to treat low-temperature, low-turbidity surface water in cold regions.

[0014] Furthermore, the conditions for treating low-temperature and low-turbidity surface water are as follows: water temperature 2℃~15℃, raw water turbidity 3 NTU~10 NTU; in the compound system, the dosage of modified chitosan is 0.5 mg / L~3 mg / L, and the dosage of polyaluminum salt and / or polyferric salt is 10 mg / L~30 mg / L.

[0015] Furthermore, the turbidity of the effluent after treatment of low-temperature and low-turbidity surface water is ≤3 NTU.

[0016] Compared with existing technologies, the modified chitosan, its preparation method, and its application in water treatment provided by this invention have the following beneficial effects:

[0017] (1) This invention successfully introduced a large number of ammonium ions into the chitosan molecular chain through dual modification of amidation and quaternary ammonium salt, which significantly improved the positive charge density and its solubility in water, especially the dissolution rate and degree in low temperature environment; at the same time, the doped polyacrylamide long chain further enhanced the adsorption bridging ability of the molecule; the modified chitosan obtained in the end showed excellent charge neutralization and flocculation activity under low temperature and low turbidity conditions, effectively solving the problem of poor application effect of natural chitosan in cold region low temperature and low turbidity surface water.

[0018] (2) In this invention, modified chitosan is used as a coagulant and combined with polyaluminum salt or polyferric salt to produce a significant synergistic flocculation effect. The long chain of modified chitosan can strengthen the interparticle bridging, promote floc growth, and effectively encapsulate aluminum salt hydrolysis products. On the one hand, it greatly improves the turbidity removal rate, and on the other hand, it significantly reduces the residual aluminum concentration in the effluent. At the same time, since only a small amount of PAM is added to the modified chitosan, and the modification process repolymerizes PAM, the health risks of acrylamide monomers are avoided from the source, ensuring the safety of drinking water.

[0019] (3) The modified chitosan and compound solution provided by the present invention have the advantages of simple operation, good treatment effect and green environmental protection. It is specifically designed for surface water with low temperature and low turbidity in cold regions. The turbidity of the treated water can be stabilized below 3 NTU, which fully meets the relevant water quality standards and has broad prospects for industrial application. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0021] Figure 1 This is a flowchart of a method for preparing modified chitosan according to Embodiment 1 of the present invention. Detailed Implementation

[0022] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0023] Example 1:

[0024] Please see Figure 1 A modified chitosan is a composite modified product obtained by sequentially substituting and modifying chitosan raw materials with quaternary ammonium salts, and then doping with polyacrylamide. The substituting and quaternary ammonium salt modifications are achieved by reacting chitosan raw materials with an etherifying agent and a quaternary ammonium salt under alkaline conditions. The etherifying agent is 3-chloro-2-hydroxypropyltrimethylammonium chloride, the quaternary ammonium salt is glycidyltrimethylammonium chloride, and the doping ratio of polyacrylamide is 0.5% to 5% of the mass of the chitosan raw material.

[0025] The preparation method of this modified chitosan specifically includes the following steps:

[0026] S1. Substitution Modification and Quaternization Reaction: Chitosan raw material is dissolved in an acidic solution, then an etherifying agent and a quaternization reagent are added. The pH is adjusted to alkaline with a buffer solution, and the substitution modification and quaternization reaction is carried out under heating conditions to obtain a reaction solution. The weight ratio of chitosan raw material to etherifying agent and quaternization reagent is 1:0.5~2:0.5~2. The reaction pH is set to 8~10, the reaction temperature is set to 50~80℃, and the reaction time is set to 4~8 hours.

[0027] The specific implementation method is as follows: Weigh 10 g of chitosan (degree of deacetylation > 90%) and dissolve it in 500 mL of 2% acetic acid solution, stirring until completely dissolved. Then, add 15 g of 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) and 10 g of glycidyltrimethylammonium chloride (GTMAC) to the above solution. Adjust the pH of the reaction system to 9.0 with a buffer solution (Tris buffer or carbonate-bicarbonate buffer), raise the temperature to 70°C, and react for 6 hours under constant temperature and stirring conditions.

[0028] S2, amidation doping: Add polyacrylamide solution to the reaction solution obtained in S1, continue stirring to carry out amidation copolymerization and doping, and obtain the reaction product; the amount of polyacrylamide added is 1% to 3% of the mass of chitosan raw material; the conditions for continuing stirring are: temperature 40 to 60℃, time 1 to 3 hours.

[0029] The specific implementation method is to add 1% of the mass of chitosan raw material in polyacrylamide (PAM) solution to the above reaction solution, and continue to stir the reaction at 50°C for 2 hours to allow PAM to undergo amidation copolymerization and physical doping with modified chitosan.

[0030] S3. Purification: The reaction product obtained in S2 is subjected to dialysis, ethanol precipitation and vacuum drying to obtain purified amidated quaternary ammonium salt modified chitosan; dialysis is performed using a dialysis bag with a molecular weight cutoff of 8000-14000 Da; the vacuum drying temperature is set to 40-60℃ and the time is set to 12-24 hours.

[0031] The specific implementation method is as follows: After the reaction is complete, the mixture is cooled to room temperature, and the pH is adjusted to neutral with 0.1 mol / L HCl. The mixture is transferred to a dialysis bag with a molecular weight cutoff of 12000 Da, and dialyzed in deionized water for 48 hours, changing the water every 8 hours. After dialysis, the liquid in the bag is poured into a beaker, and three volumes of anhydrous ethanol are added for precipitation. The precipitate is obtained by centrifugation. Finally, the precipitate is dried in a vacuum drying oven at 50°C for 24 hours, ground into powder, and the purified amidated quaternary ammonium salt modified chitosan product is obtained.

[0032] Experimental example:

[0033] (I) Optimization experiment of chitosan dosage:

[0034] Simulated water sample preparation:

[0035] Based on the typical surface water quality characteristics of the Songhua River in winter, simulated water samples were prepared using kaolin. The specific procedure was as follows: a certain amount of kaolin was accurately weighed, added to deionized water, thoroughly stirred and dispersed, and allowed to stand overnight. The supernatant was taken as the raw water, and the turbidity was adjusted as needed. Two sets of simulated water samples with different initial turbidities were set up, at 3.46 NTU and 6.87 NTU respectively. The experimental temperatures were controlled at 9.1℃ and 9.5℃ respectively to simulate the water temperature conditions of a cold region with low temperature and low turbidity.

[0036] Different dosage gradients were set up for the experiment: for water samples with an initial turbidity of 3.46 NTU, the dosages were set at 0.5 mg / L, 2 mg / L, and 4 mg / L; for water samples with an initial turbidity of 6.87 NTU, the dosages were set at 1 mg / L, 2 mg / L, and 3 mg / L. Each dosage was tested in duplicate to ensure the reliability of the data.

[0037] Experimental results:

[0038] (1) Experimental results under the condition of initial turbidity of 3.46 NTU:

[0039] Under the conditions of water temperature 9.1℃ and raw water turbidity 3.46 NTU, the turbidity and removal rate of the treated water samples are shown in Table 1 when the chitosan dosage is 0.5 mg / L, 2 mg / L, and 4 mg / L, respectively.

[0040] Table 1 Results of the chitosan dosage optimization experiment for raw water turbidity of 3.46 NTU

[0041] Dosage (mg / L) Parallel experiments Turbidity after treatment (NTU) Removal rate (%) 0.5 Experiment 1 0.76 78.04 0.5 Experiment 2 0.62 82.08 2 Experiment 1 0.43 87.57 2 Experiment 2 0.91 73.70 4 Experiment 1 1.64 52.60 4 Experiment 2 1.21 65.03

[0042] As shown in Table 1, when the dosage was 0.5 mg / L, the removal rates in the two parallel experiments were 78.04% and 82.08%, respectively, with effluent turbidity of 0.76 NTU and 0.62 NTU, indicating good treatment effects. When the dosage was increased to 2 mg / L, the removal rate in the first experiment reached its highest value of 87.57%, and the effluent turbidity decreased to 0.43 NTU; the removal rate in the second experiment was 73.70%, and the effluent turbidity was 0.91 NTU. Although there were some fluctuations, the overall treatment effect remained at a high level. When the dosage was further increased to 4 mg / L, the removal rate decreased to 52.60% and 65.03%, and the effluent turbidity increased to 1.64 NTU and 1.21 NTU, indicating that excessive chitosan can lead to floc restabilization or colloidal protection, which is detrimental to turbidity removal.

[0043] (2) Experimental results under the condition of initial turbidity of 6.87 NTU:

[0044] Under the conditions of water temperature of 9.5℃ and raw water turbidity of 6.87 NTU, the turbidity and removal rate of the treated water samples are shown in Table 2 when the chitosan dosage is 1 mg / L, 2 mg / L and 3 mg / L respectively.

[0045] Table 2 Results of the chitosan dosage optimization experiment for raw water turbidity of 6.87 NTU

[0046] Dosage (mg / L) Parallel experiments Turbidity after treatment (NTU) Turbidity removal rate (%) 1 Experiment 1 3.49 49.21 1 Experiment 2 2.25 67.24 2 Experiment 1 2.40 65.12 2 Experiment 2 2.05 70.23 3 Experiment 1 2.18 68.30 3 Experiment 2 1.96 71.51

[0047] Table 2 shows that when the dosage was 1 mg / L, the removal rates in the two experiments were 49.21% and 67.24%, respectively, and the effluent turbidity was 3.49 NTU and 2.25 NTU, respectively. The treatment effect fluctuated with operating conditions. When the dosage was increased to 2 mg / L, the removal rate increased to 65.12% and 70.23%, and the effluent turbidity decreased to 2.40 NTU and 2.05 NTU. When the dosage was further increased to 3 mg / L, the removal rate stabilized at 68.30% and 71.51%, and the effluent turbidity decreased to a minimum of 1.96 NTU, achieving the best treatment effect.

[0048] Comparing the two sets of experimental data reveals that the optimal chitosan dosage is closely related to the raw water turbidity. When the raw water turbidity is low (3.46 NTU), the optimal dosage is 2 mg / L, achieving a maximum removal rate of 87.57%. When the raw water turbidity is high (6.87 NTU), the optimal dosage increases to 3 mg / L, achieving a maximum removal rate of 71.51%. This pattern aligns with the general principle of flocculant dosage: the higher the raw water turbidity, the greater the required flocculant dosage to provide sufficient charge neutralization and adsorption bridging capacity.

[0049] In summary, the optimal dosage range of the chitosan prepared by this invention for treating low-temperature and low-turbidity surface water in cold regions is 2 mg / L to 3 mg / L. Within this dosage range, the turbidity of the treated effluent can be reduced to below 2 NTU, and the turbidity removal rate can reach over 70%, with a maximum of 87.57%. This provides reliable process parameters for subsequent optimization of compound systems and engineering applications.

[0050] (II) Optimal pH conditions for chitosan treatment of low-temperature, low-turbidity water:

[0051] Simulated water sample preparation:

[0052] Based on the typical surface water quality characteristics of the Songhua River in winter, simulated water samples were prepared using kaolin. The specific procedure was as follows: a certain amount of kaolin was accurately weighed, added to deionized water, thoroughly stirred and dispersed, and allowed to stand overnight. The supernatant was taken as the raw water, and the turbidity was adjusted as needed. Two groups of simulated water samples with different initial turbidities were set up: the acidic experimental group had a raw water turbidity of 4.38 NTU and a water temperature controlled at 14.5℃; the moderately alkaline experimental group had a raw water turbidity of 6.35 NTU and a water temperature controlled at 13.6℃. The chitosan dosage was fixed at 2 mg / L.

[0053] Test method:

[0054] Take 1 L of simulated water sample, adjust the pH to the set value with 0.1 mol / L HCl or NaOH solution, and add 2 mg / L chitosan. The flocculation stirring program is as follows: rapid stirring (300 rpm, 1 minute), slow stirring (80 rpm, 10 minutes), and settling. At different time points after settling (11 min, 20 min, 38 min), take samples of the supernatant at 2 cm below the liquid surface to measure the turbidity and calculate the removal rate. Perform parallel experiments for each pH condition.

[0055] Experimental results:

[0056] (1) Experimental results under acidic pH conditions:

[0057] Under the conditions of water temperature 14.5℃, raw water turbidity 4.38 NTU, and chitosan dosage 2 mg / L, with pH gradients of 6 and 6.5, the turbidity and removal rate of the water samples after 38 min of treatment are shown in Table 3.

[0058] Table 3. Effects of chitosan treatment under acidic pH conditions

[0059] pH value Turbidity after treatment (NTU) Removal rate (%) 6.0 0.47 89.26 6.5 2.60 40.63

[0060] As shown in Table 3, under acidic conditions, the flocculation effect of chitosan initially increases and then decreases with increasing pH. The optimal treatment effect is achieved at pH 6.0, with an effluent turbidity of only 0.47 NTU and a removal rate as high as 89.26%. At pH 6.5, the removal rate significantly decreases to around 40%, and the effluent turbidity increases to over 2.5 NTU. These results indicate that chitosan exhibits the best flocculation effect under slightly acidic conditions (pH≈6).

[0061] (2) Experimental results under moderately alkaline pH conditions:

[0062] Under the conditions of water temperature 13.6℃, ​​raw water turbidity 6.35 NTU, and chitosan dosage 2 mg / L, pH gradients of 7, 8, and 9 were set. Turbidity was measured at 11 min, 20 min, and 38 min after flocculation and stirring. The results are shown in Table 4.

[0063] Table 4 shows the effects of chitosan treatment at different times under alkaline pH conditions.

[0064] pH value Turbidity (NTU) 11 min after flocculation and stirring Turbidity (NTU) 20 min after flocculation and stirring Turbidity (NTU) 38 min after flocculation and stirring Removal rate (%) at 38 min 7 1.91 1.93 1.66 73.86 8 2.65 1.39 1.54 75.75 9 1.75 1.78 1.60 74.80

[0065] As shown in Table 4, the flocculation effect of chitosan is stable and good in the pH range of 7 to 9. The removal rate is between 73.86% and 75.75% in 38 min, and the turbidity of the effluent is between 1.75 and 2.65 NTU, all of which are <3 NTU. The removal rate is the highest at pH=8 (75.75%).

[0066] Comparing experimental results under acidic and alkaline conditions:

[0067] (1) Under acidic to neutral conditions (pH 6-7), the optimal treatment pH for chitosan is 6.0, at which point the removal rate can reach 89.26%; increasing the pH (≥6.5) will lead to a decrease in the flocculation effect. Under moderately alkaline conditions (pH 7-9), the flocculation effect of chitosan is relatively stable.

[0068] (2) In actual water treatment, appropriate pH conditions need to be selected according to the raw water quality and treatment objectives. If the raw water itself is neutral or weakly acidic, good results can be obtained without additional pH adjustment (e.g., removal rate of 89.26% when pH=6).

[0069] In summary, the optimal pH conditions for treating low-temperature, low-turbidity water with chitosan (dosage 2 mg / L) are: pH=6.0 under acidic to neutral conditions (removal rate 89.26%); and under moderately alkaline conditions, the effect is stable and good within the pH range of 7–9. In practical engineering applications, excellent flocculation and turbidity removal effects can be obtained depending on the water quality characteristics and treatment costs.

[0070] (III) Experiment on the treatment effect of chitosan on low-temperature, low-turbidity water with different initial turbidity:

[0071] Simulated water sample preparation:

[0072] Referring to the typical surface water quality characteristics of the Songhua River in winter, simulated water samples with different turbidities were prepared using raw diatomaceous earth. The specific procedure was as follows: different masses of raw diatomaceous earth were accurately weighed and added to deionized water, thoroughly stirred and dispersed, and allowed to stand overnight. The supernatant was taken as the raw water, and its initial turbidity was measured. Two sets of simulated water samples with different concentration gradients were set up: for Preparation 1, the raw diatomaceous earth dosage was 10 mg / L, 15 mg / L, and 20 mg / L, with an experimental temperature of 14.2℃; for Preparation 2, the raw diatomaceous earth dosage was 5 mg / L, 10 mg / L, and 15 mg / L, with an experimental temperature of 9℃. The chitosan dosage was fixed at 2 mg / L, and each concentration was tested in duplicate to ensure data reliability.

[0073] Experimental results:

[0074] (1) Experimental results under configuration 1:

[0075] Table 5 shows the results of raw water turbidity and treated water turbidity and removal rate corresponding to different raw water diatomaceous earth dosages at a water temperature of 14.2℃ and a chitosan dosage of 2 mg / L.

[0076] Table 5. Chitosan treatment effect under configuration 1 (T=14.2℃)

[0077] Concentration of diatomaceous earth in raw water (mg / L) Raw water turbidity (NTU) Turbidity after treatment (NTU) Removal rate (%) 10 5.76 1.59 72.40 10 5.46 1.94 64.47 15 6.18 4.53 26.70 15 5.38 3.43 36.25 20 8.40 5.26 37.38 20 7.44 3.83 48.52

[0078] As shown in Table 5, chitosan exhibited the best treatment effect when the raw diatomaceous earth dosage was 10 mg / L (corresponding to raw water turbidity of 5.46–5.76 NTU), with removal rates of 72.40% and 64.47% in two parallel experiments, and effluent turbidity decreasing to 1.59 NTU and 1.94 NTU, respectively. When the raw diatomaceous earth dosage increased to 15 mg / L (corresponding to raw water turbidity of 5.38–6.18 NTU), the removal rate significantly decreased to 26.70% and 36.25%, and effluent turbidity increased to 4.53 NTU and 3.43 NTU, respectively. When the dosage was further increased to 20 mg / L (corresponding to raw water turbidity of 7.44–8.40 NTU), the removal rate rebounded to 37.38% and 48.52%, with effluent turbidity of 5.26 NTU and 3.83 NTU, respectively. The results showed that, at a fixed dosage (2 mg / L), chitosan had the best treatment effect on water samples with low turbidity (5-6 NTU); the treatment effect decreased for water samples with high turbidity, suggesting that the dosage needs to be increased appropriately.

[0079] (2) Experimental results under configuration 2:

[0080] Table 6 shows the results of raw water turbidity and treated water turbidity and removal rate corresponding to different raw water diatomaceous earth dosages under the conditions of water temperature 9℃ and chitosan dosage 2 mg / L:

[0081] Table 6. Chitosan treatment effect under configuration 2 (T=9℃)

[0082] Concentration of diatomaceous earth in raw water (mg / L) Raw water turbidity (NTU) Turbidity after treatment (NTU) Removal rate (%) 5 2.00 1.92 4.00 5 2.00 1.72 14.00 10 4.74 3.28 30.80 10 4.74 3.46 27.00 15 6.71 2.75 59.02 15 6.71 3.37 49.75

[0083] As shown in Table 6, under low water temperature (9℃), the treatment effect of chitosan increases with increasing raw water turbidity. When the raw water turbidity is 2.00 NTU (low turbidity concentration 5 mg / L), the removal rate is only 4.00% and 14.00% due to the extremely low particle concentration in the water, resulting in very poor treatment effects. When the raw water turbidity increases to 4.74 NTU (concentration 10 mg / L), the removal rate increases to 30.80% and 27.00%, and the effluent turbidity decreases to 3.28 NTU and 3.46 NTU. When the raw water turbidity further increases to 6.71 NTU (concentration 15 mg / L), the removal rate significantly increases to 59.02% and 49.75%, and the effluent turbidity decreases to 2.75 NTU and 3.37 NTU.

[0084] Compare the two sets of experimental data:

[0085] (1) Under higher water temperature (14.2℃), chitosan has the best treatment effect on water samples with lower turbidity (5-6 NTU), with a removal rate of over 70%; however, for water samples with slightly higher turbidity (6-8 NTU), the treatment effect fluctuates greatly under the same dosage, with a removal rate between 26% and 48%, indicating that the dosage needs to be optimized according to the turbidity of the raw water.

[0086] (2) At lower water temperatures (9℃), the treatment effect of chitosan improved significantly with increasing raw water turbidity. When the raw water turbidity was extremely low (2 NTU), the removal rate was less than 15%, with almost no treatment effect. As the raw water turbidity increased to 6.71 NTU, the removal rate increased to 50%–60%, indicating that chitosan has better adaptability to water samples with higher turbidity under low temperature conditions. This pattern is consistent with the principle of flocculation kinetics: the higher the initial particle concentration, the greater the probability of collision between particles, and the easier it is for flocs to form.

[0087] (3) Based on the combined results of configuration 1 and configuration 2, the optimal application range for chitosan (dosage 2 mg / L) is 5-7 NTU for raw water turbidity, within which a turbidity removal rate of 50%-70% can be achieved. For water samples with turbidity below 5 NTU, the chitosan dosage should be appropriately increased or a process of compounding with polyaluminum salts should be adopted to improve treatment efficiency; for water samples with turbidity above 7 NTU, the dosage should also be increased or the compounding scheme should be optimized.

[0088] In summary, chitosan (dosage 2 mg / L) exhibits a certain treatment capacity for low-temperature, low-turbidity water with varying initial turbidity. The highest removal rate reached 72.40% at 14.2℃ and a raw water turbidity of 5.76 NTU; the highest removal rate reached 59.02% at 9℃ and a raw water turbidity of 6.71 NTU. These results demonstrate that the chitosan prepared in this invention can exert effective flocculation effects over a wide range of raw water turbidity and under low-temperature conditions. However, the dosage needs to be adjusted according to the specific water quality to achieve the optimal treatment effect, providing fundamental data support for subsequent optimization of the compound system parameters.

[0089] (iv) Optimization test of chitosan flocculation stirring scheme:

[0090] Simulated water sample preparation:

[0091] Based on the typical surface water quality characteristics of the Songhua River in winter, simulated water samples were prepared using kaolin. The specific procedure was as follows: a certain amount of kaolin was accurately weighed, added to deionized water, thoroughly stirred and dispersed, and allowed to stand overnight. The supernatant was taken as the raw water, and the turbidity was adjusted as needed. Three groups of simulated water samples with different initial turbidities were set up, with experimental temperatures controlled at 12.0℃, 12.1℃, and 10.5℃, respectively. The chitosan dosage was fixed at 2 mg / L. Three different flocculation stirring schemes were investigated in each group of experiments, each scheme including three stages: rapid stirring, low-speed stirring, and slow stirring.

[0092] Experimental results:

[0093] (1) The effect of flocculation stirring scheme on Experiment 1 (T=12.0℃):

[0094] Under the conditions of water temperature 12.0℃ and chitosan dosage of 2 mg / L, three stirring schemes were set up, and the treatment effects are shown in Table 7:

[0095] Table 7. Effects of Flocculation and Stirring Scheme on Experimental Results (T=12.0℃)

[0096] Mixing scheme Raw water turbidity (NTU) Turbidity after treatment (NTU) Removal rate (%) Option A: 300 rpm, 1 min → 100 rpm, 5 min → 30 rpm, 14 min 5.83 1.51 74.10 Option B: Rotation speed 400 rpm, 1 min → 100 rpm, 5 min → 50 rpm, 14 min 5.02 1.75 65.14 Option C: Rotation speed 500 rpm, 1 min → 200 rpm, 5 min → 100 rpm, 14 min 3.94 1.75 55.58

[0097] As shown in Table 7, among the three stirring schemes, Scheme A achieved the highest turbidity removal rate, reaching 74.10%, with the raw water turbidity decreasing from 5.83 NTU to 1.51 NTU after treatment. Scheme B achieved a removal rate of 65.14%, with a treated turbidity of 1.75 NTU. Scheme C had the lowest removal rate at 55.58%, and also the lowest raw water turbidity (3.94 NTU). The results indicate that lower stirring speeds (300 rpm for rapid stirring, 100 rpm for slow stirring, and 30 rpm for settling) are more conducive to floc formation and sedimentation.

[0098] (2) The effect of flocculation stirring scheme on Experiment 2 (T=12.1℃):

[0099] Under the conditions of water temperature 12.1℃ and chitosan dosage of 2 mg / L, three stirring schemes were set up, and the treatment effects are shown in Table 8:

[0100] Table 8. Effects of Flocculation and Stirring Scheme on Experimental Results (T=12.1℃)

[0101] Mixing scheme Raw water turbidity (NTU) Turbidity after treatment (NTU) Removal rate (%) Option D: Original speed 300 rpm, 1 min → 100 rpm, 5 min → 30 rpm, 14 min 4.23 2.90 31.44 Option E: Fast speed 600 rpm, 1 min → 200 rpm, 5 min → 60 rpm, 14 min 5.60 2.84 49.29 Option F: Slow speed 150 rpm, 1 min → 50 rpm, 5 min → 15 rpm, 14 min 5.58 2.02 63.80

[0102] As shown in Table 8, scheme F achieved the highest removal rate of 63.80%, reducing the turbidity to 2.02 NTU after treatment, despite the relatively high turbidity of the raw water (5.58 NTU). Scheme E achieved a removal rate of 49.29%, while scheme D only achieved a removal rate of 31.44%. Experiment 2 further verified the advantages of the low-speed stirring scheme, especially the optimal combination of slow initial stirring (150 rpm) and extremely low settling speed (15 rpm).

[0103] (3) The effect of flocculation stirring scheme on Experiment 3 (T=10.5℃):

[0104] Under the conditions of water temperature 10.5℃ and chitosan dosage of 2 mg / L, three stirring schemes were set up, and the treatment effects are shown in Table 9:

[0105] Table 9. Effects of Flocculation and Stirring Scheme on Experimental Results (T=10.5℃)

[0106] Mixing scheme Raw water turbidity (NTU) Turbidity after treatment (NTU) Removal rate (%) Option G: Original speed 300 rpm, 1 min → 100 rpm, 5 min → 30 rpm, 14 min 5.54 2.39 56.86 Option H: Fast speed 400 rpm, 1 min → 130 rpm, 5 min → 40 rpm, 14 min 5.66 3.56 37.10 Option I: Slow speed 200 rpm, 1 min → 70 rpm, 5 min → 20 rpm, 14 min 4.51 2.18 51.66

[0107] As shown in Table 9, Scheme G had the highest removal rate at 56.86%, with a turbidity of 2.39 NTU after treatment; Scheme I had a removal rate of 51.66%, with a turbidity of 2.18 NTU after treatment; and Scheme H had the lowest removal rate at only 37.10%. Under relatively low water temperature (10.5℃), the medium and low speed schemes still showed good treatment effects.

[0108] In summary, for treating low-temperature, low-turbidity water with chitosan (dosage 2 mg / L), the recommended optimal flocculation and stirring scheme is: rapid stirring at 150 rpm for 1 min, slow stirring at 50 rpm for 5 min, and a settling stage at 15 rpm for 14 min. This scheme achieves a turbidity removal rate of 63.80% at a temperature of 12.1℃ and a raw water turbidity of 5.58 NTU. In practical engineering applications, a low-speed, long-duration, multi-stage decreasing stirring strategy is preferable to promote sufficient floc growth and reduce shear damage, thereby improving flocculation efficiency.

[0109] Example 2:

[0110] Modified chitosan, used as a coagulant aid, is combined with polyaluminum chloride to treat low-temperature, low-turbidity surface water in cold regions.

[0111] Simulated water sample: Prepared with kaolin clay, referencing the winter water quality of the Songhua River, with a water temperature of 5±1℃, turbidity of 15±1 NTU, and pH of 7.0±0.2.

[0112] Compound treatment experiment: With a fixed PAC dosage of 20 mg / L, the modified chitosan dosage gradient was 0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mg / L, with the modified chitosan having a quaternization substitution degree ≥15%. Stirring procedure: First, add PAC and stir at 300 rpm for 30 seconds; then add modified chitosan, stir at 300 rpm for 1 minute, then stir at 80 rpm for 10 minutes, and let stand for 15 minutes. Turbidity and residual aluminum in the effluent were measured. The compound treatment effects under different modified chitosan dosages are shown in Table 10.

[0113] Table 10 Effect of modified chitosan combined with PAC on treatment (water temperature 5℃, raw water turbidity 15 NTU).

[0114] Modified chitosan dosage (mg / L) PAC dosage (mg / L) Effluent turbidity (NTU) Turbidity removal rate (%) Residual aluminum (mg / L) 0 (PAC only) 20 6.50 56.7 0.22 0.5 20 3.20 78.7 0.18 1.0 20 2.10 86.0 0.12 1.5 20 1.80 88.0 0.08 2.0 20 1.75 88.3 0.07 2.5 20 1.90 87.3 0.07 3.0 20 2.20 85.3 0.06

[0115] The results showed that the effluent turbidity of PAC alone was 6.50 NTU, and the residual aluminum was 0.22 mg / L. After adding modified chitosan, when the dosage was ≥1.0 mg / L, the effluent turbidity was ≤2.10 NTU, and the residual aluminum was ≤0.12 mg / L. The optimal dosage of modified chitosan was 1.5–2.0 mg / L.

[0116] In summary, the combination of modified chitosan (1.0–3.0 mg / L) and PAC (10–30 mg / L) can achieve effluent turbidity ≤3 NTU and residual aluminum ≤0.12 mg / L under low temperature and low turbidity conditions in cold regions.

[0117] Example 3:

[0118] Modified chitosan, used as a coagulant aid, is combined with polyferric chloride to treat low-temperature, low-turbidity surface water in cold regions.

[0119] Simulated water sample: Based on the winter water quality of the Songhua River, a simulated water sample was prepared using kaolin and humic acid, with a water temperature of 5±1℃, turbidity of 15±1 NTU, and pH of 7.0±0.2.

[0120] Compound treatment experiment: With a fixed PFC (calculated as Fe) dosage of 20 mg / L, the modified chitosan dosage gradient was 0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mg / L, with a quaternization substitution degree of ≥15% for the modified chitosan. Stirring procedure: First, add PFC and stir at 300 rpm for 30 seconds; then add modified chitosan, stir at 300 rpm for 1 minute, then stir at 80 rpm for 10 minutes, and let stand for 15 minutes. The combined treatment effects of different modified chitosan dosages on effluent turbidity and total iron residue are shown in Table 11.

[0121] Table 11 Effect of modified chitosan combined with PFC on treatment (water temperature 5℃, turbidity 15 NTU)

[0122] Modified chitosan (mg / L) Effluent turbidity (NTU) Turbidity removal rate (%) Total iron residue (mg / L) 0 7.20 52.0 0.35 0.5 3.50 76.7 0.28 1.0 2.30 84.7 0.20 1.5 1.95 87.0 0.15 2.0 1.88 87.5 0.12 2.5 2.05 86.3 0.11 3.0 2.40 84.0 0.10

[0123] The results showed that the effluent turbidity of PFC alone was 7.20 NTU, and the total iron residue was 0.35 mg / L. After adding modified chitosan, when the dosage was ≥1.0 mg / L, the effluent turbidity was ≤2.30 NTU, and the total iron residue was ≤0.20 mg / L; when the dosage was ≥1.5 mg / L, the effluent turbidity was ≤1.95 NTU, and the total iron residue decreased to below 0.15 mg / L. The optimal dosage for the combination was 1.5–2.0 mg / L of modified chitosan and 20 mg / L of PFC.

[0124] In summary, the combination of modified chitosan and polyferric salt (PFC) can effectively remove turbidity and reduce total iron residue in effluent under cold, low-temperature, and low-turbidity conditions in cold regions. When the dosage of modified chitosan is 1.0–3.0 mg / L and the dosage of PFC is 10–30 mg / L, the effluent turbidity is ≤3 NTU, proving that the combination of modified chitosan and polyferric salt is equally effective.

[0125] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A modified chitosan, characterized in that, Modified chitosan is a composite modified product obtained by sequentially substituting and modifying chitosan raw materials with quaternary ammonium salts, and then doping with polyacrylamide. The substituting and quaternary ammonium salt modifications are achieved by reacting chitosan raw materials with an etherifying agent and a quaternary ammonium salt under alkaline conditions. The etherifying agent is 3-chloro-2-hydroxypropyltrimethylammonium chloride, the quaternary ammonium salt is glycidyltrimethylammonium chloride, and the doping ratio of polyacrylamide is 0.5% to 5% of the mass of the chitosan raw material. The degree of quaternization substitution of the modified chitosan is ≥15%.

2. A method for preparing modified chitosan, characterized in that, It is used to prepare the modified chitosan according to claim 1, and the preparation method specifically includes the following steps: S1. Substitution modification and quaternization reaction: Chitosan raw material is dissolved in an acidic solution, then an etherifying agent and a quaternization reagent are added, the pH is adjusted to alkaline with a buffer solution, and the substitution modification and quaternization reaction is carried out under heating conditions to obtain a reaction solution; S2, amidation doping: Add polyacrylamide solution to the reaction solution obtained in S1, continue stirring the reaction to carry out amidation copolymerization and doping, and obtain the reaction product; S3. Purification: The reaction product obtained in S2 is dialyzed, precipitated with ethanol, and vacuum dried to obtain purified amidated quaternary ammonium salt modified chitosan.

3. The method for preparing modified chitosan according to claim 2, characterized in that, In S1, the weight ratio of chitosan raw material to etherifying agent and quaternizing agent is 1:0.5-2:0.5-2; the reaction pH is set to 8-10, the reaction temperature is set to 50-80℃, and the reaction time is set to 4-8 hours.

4. The method for preparing modified chitosan according to claim 2, characterized in that, The amount of polyacrylamide added in S2 is 1% to 3% of the mass of chitosan raw material; the conditions for continued stirring and reaction are: temperature 40 to 60°C, time 1 to 3 hours.

5. The method for preparing modified chitosan according to claim 2, characterized in that, In step S3, dialysis uses dialysis bags with a molecular weight cutoff of 8000–14000 Da; the vacuum drying temperature is set to 40–60°C, and the time is set to 12–24 hours.

6. The application of modified chitosan in water treatment according to claim 1, characterized in that, This application involves using modified chitosan as a coagulant aid, combined with polyferric salts or polyaluminum salts, to treat low-temperature, low-turbidity surface water in cold regions.

7. The application of modified chitosan in water treatment according to claim 6, characterized in that, The conditions for the low-temperature, low-turbidity surface water treatment are as follows: water temperature 2℃~15℃, raw water turbidity 3 NTU~10 NTU; in the compound system, the dosage of modified chitosan is 0.5 mg / L~3 mg / L, and the dosage of polyaluminum salt and / or polyferric salt is 10 mg / L~30 mg / L.

8. The application of modified chitosan in water treatment according to claim 6 or 7, characterized in that, The turbidity of the effluent after the treatment of the low-temperature and low-turbidity surface water is ≤3 NTU.

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