Preparation method of carboxymethyl chitosan composite adsorbent and application thereof in removal of terramycin

Abstract

This invention discloses a method for preparing a carboxymethyl chitosan composite adsorbent and its application in the removal of oxytetracycline, belonging to the field of wastewater treatment technology. This invention utilizes a simple one-step ionic crosslinking method to remove La... 3+ and Ca 2+ The CMCS-La structure was prepared by complexation with carboxymethyl chitosan. 3+ -Ca 2+ This invention innovatively constructs a carboxymethyl chitosan composite adsorbent with multiple spatial separation sites, which can be used for the efficient adsorption and removal of oxytetracycline through n-π EDA interactions, hydrogen bonds, and La... 3+ Multiple mechanisms, including complexation and cation-π bonding, synergistically participate in the capture and adsorption removal of oxytetracycline. The adsorbent of this invention is simple to prepare, has a wide applicable pH range for the adsorption system (pH=4~7), high adsorption efficiency (580.91 mg / g at 303 K), and strong anti-interference ability, and has great potential for practical application in the removal of oxytetracycline wastewater.

Description

Technical Field

[0001] This invention relates to a method for preparing a carboxymethyl chitosan composite adsorbent and its application in the removal of oxytetracycline, belonging to the field of wastewater treatment technology. Background Technology

[0002] In recent years, with the continuous development of analytical testing technologies, new pollutants, represented by pharmaceuticals and personal care products, have been continuously detected in various environmental media (water, soil, sediments, etc.). Antibiotics, as a typical class of pharmaceuticals and personal care products, are widely used to treat and prevent various bacterial infections due to their low production cost and excellent broad-spectrum antibacterial activity. However, antibiotics cannot be completely absorbed by humans and animals during use and ultimately enter the natural aquatic environment and soil through excrement in the form of domestic sewage, aquaculture wastewater, landfill leachate, and agricultural runoff. Antibiotic residues in water not only accelerate the formation and migration of antibiotic-resistant bacteria and antibiotic resistance genes but also accumulate in farmed organisms and eventually enter the human body through the food chain, posing a serious threat to human life. The environmental and health risks caused by antibiotic abuse have become one of the major challenges in my country's water resource utilization and protection. Therefore, it is urgent to reduce antibiotic pollutants at their source to ensure the safety of drinking water and groundwater.

[0003] Oxytetracycline is one of the most widely used antibiotics in crop cultivation and agriculture. It can be used to prevent and control crop diseases and promote crop growth. Due to the high yield, high dosage and low utilization rate of oxytetracycline, it has become one of the antibiotic pollutants with the highest detection levels in surface water and groundwater. At the same time, oxytetracycline contains a stable aromatic structure, which is difficult to degrade by light or microorganisms in the natural environment. Therefore, it is urgent to develop green, efficient and environmentally friendly oxytetracycline wastewater treatment technologies.

[0004] Currently, the more mature technologies for treating oxytetracycline wastewater include biological treatment, electrocoagulation, advanced oxidation processes, membrane separation, and adsorption. Except for adsorption, other methods have fatal flaws, such as unsatisfactory removal effect, high energy consumption, high cost, generation of toxic byproducts, and difficulty in disposal. In contrast, adsorption has the advantages of flexible operation, stable effect, and low cost, and is suitable for treating a variety of organic pollutants, including oxytetracycline.

[0005] In adsorption methods, the selection of adsorbents is particularly important. In recent years, natural polymers such as chitosan, sodium alginate, and cellulose have been commonly used as raw materials for the preparation of composite adsorbents due to their abundant sources, non-toxicity, and excellent biocompatibility. Among them, chitosan is an aminopolysaccharide produced by the deacetylation of chitin, and it is the second most abundant natural adsorbent after... Chitosan, the second most abundant biopolymer, has oxygen-containing functional groups (-COOH, -OH) in its structure that can interact with N / O groups in organic pollutants such as antibiotics and dyes through electrostatic attraction, hydrogen bonding, coordination / complexation, and ion exchange. It can also form n-π EDA bonds with aromatic structures on organic pollutants. However, the incomplete N-deacetylation of chitosan greatly inhibits its solubility in aqueous solution, which is not conducive to its use as a precursor for the synthesis of adsorbents. At the same time, the low acid stability, weak mechanical strength, low thermal stability, and low specific surface area of ​​chitosan limit its application in wastewater treatment. Therefore, it is necessary to functionalize chitosan to enhance its stability and water solubility, making it more suitable for practical wastewater purification processes.

[0006] Carboxymethyl chitosan is one of the advanced derivatives of chitosan and is a commonly used adsorbent. However, the porosity and specific surface area of ​​powdered carboxymethyl chitosan are not ideal, and achieving complete solid / liquid separation with treated water is extremely challenging.

[0007] Therefore, in order to solve the problem of efficient removal of oxytetracycline, and considering the defects of carboxymethyl chitosan in practical applications, such as easy aggregation, low specific surface area, and limited active sites, this invention provides a method for preparing a carboxymethyl chitosan composite adsorbent and its application in removing oxytetracycline from water. Summary of the Invention

[0008] To address the problems of existing carboxymethyl chitosan adsorbents, such as easy aggregation in water, low specific surface area, limited active sites, and difficulty in separation, this invention provides a method for preparing a carboxymethyl chitosan composite adsorbent. The specific preparation steps are as follows:

[0009] (1) Add carboxymethyl chitosan to ultrapure water and stir evenly to obtain carboxymethyl chitosan gel.

[0010] (2) Add all the carboxymethyl chitosan gel obtained in step (1) dropwise to La 3+ / Ca 2+ After the carboxymethyl chitosan gel was added dropwise to the mixed solution, the reaction was allowed to continue for at least 24 hours to obtain a carboxymethyl chitosan composite hydrogel.

[0011] (3) The carboxymethyl chitosan composite hydrogel obtained in step (2) is washed and then freeze-dried. The carboxymethyl chitosan composite adsorbent obtained after the carboxymethyl chitosan composite hydrogel is completely dried has the structure CMCS-La. 3+ -Ca 2+ .

[0012] Preferably, the amount of carboxymethyl chitosan added to the ultrapure water in step (1) is 35~45g / L, and the water is ultrapure water.

[0013] Preferably, in step (2) La 3+ / Ca 2+ The mixed solution is prepared by adding La(NO3)3·6H2O and CaCl2·2H2O to ultrapure water, wherein the amount of La(NO3)3·6H2O added is 5~32 g / L and the amount of CaCl2·2H2O added is 30~80 g / L.

[0014] Preferably, a medical syringe can be used to add carboxymethyl chitosan to ultrapure water in step (2).

[0015] Preferably, the cleaning method in step (3) is to rinse with ultrapure water 3 to 5 times, changing the ultrapure water each time, until the used ultrapure water is neutral.

[0016] Preferably, the freeze-drying temperature in step (3) is -40~-50℃ and the time is 24h.

[0017] Preferably, in step (2), the carboxymethyl chitosan gel and La 3+ / Ca 2+ There are no special requirements for the volume ratio of the mixed solution; carboxymethyl chitosan composite adsorbent can be prepared by mixing in any volume ratio.

[0018] The application of the carboxymethyl chitosan composite adsorbent prepared in this invention in the removal of oxytetracycline.

[0019] Beneficial effects of the present invention

[0020] (1) This invention utilizes La 3+ and Ca 2+ A simple and sustainable one-step ion crosslinking process was used to synthesize a structurally simple carboxymethyl chitosan composite adsorbent for the first time.

[0021] (2) The carboxymethyl chitosan composite adsorbent prepared by the present invention can achieve efficient separation and removal of oxytetracycline in wastewater. The maximum adsorption capacity for oxytetracycline can reach 580.91 mg / g, and it also has a good adsorption effect when the concentration of oxytetracycline in the water is low.

[0022] (3) The carboxymethyl chitosan composite adsorbent prepared by the present invention has the characteristics of good stability, easy recycling, reusability, and good salt and acid resistance.

[0023] (4) The carboxymethyl chitosan composite adsorbent prepared by the present invention is not easy to aggregate in water, has a high specific surface area and many active sites, and therefore can adsorb a large amount of oxytetracycline in water.

[0024] (5) The carboxymethyl chitosan composite adsorbent prepared in this invention can be activated by hydrogen bonding, cation exchange, and La... 3+ Multiple mechanisms, including complexation, cation-π bonds, and n-π EDA, work synergistically to remove oxytetracycline. Attached Figure Description

[0025] Figure 1 The images show SEM images of the three carboxymethyl chitosan composite adsorbents prepared in Example 1, where Figures (a) to (d) show the CMCS-La composite adsorbent. 3+- Ca 2+ SEM images of the carboxymethyl chitosan composite adsorbent; Figures (e) to (h) show the structure CMCS-Ca 2+ SEM images of the carboxymethyl chitosan composite adsorbent; Figures (i) to (l) show the structure CMCS-La 3+ SEM image of the carboxymethyl chitosan composite adsorbent.

[0026] Figure 2 Figure 1 shows the BET diagrams of the three carboxymethyl chitosan composite adsorbents prepared in Example 1. Figure (a) shows the structure CMCS-La. 3+- Ca 2+ BET diagram of the carboxymethyl chitosan composite adsorbent; Figure (b) shows the structure of CMCS-Ca 2+ BET diagram of the carboxymethyl chitosan composite adsorbent; Figure (c) shows the structure CMCS-La 3+ BET diagram of carboxymethyl chitosan composite adsorbent.

[0027] Figure 3 Figure 4 shows the adsorption capacity and zeta potential of the solution before and after adsorption under different initial pH conditions in Experiment 1 of Example 4. Figure (a) shows the adsorption capacity under different initial pH conditions; Figure (b) shows the zeta potential of the solution before and after adsorption.

[0028] Figure 4 For Example 4, Experiment 2, the structure is CMCS-La under different adsorption times. 3+ -Ca 2+ The effect of carboxymethyl chitosan composite adsorbent on the adsorption capacity of oxytetracycline was shown in Figure (a), which shows the fitting of pseudo-first-order and pseudo-second-order adsorption kinetic models; Figure (b) shows the intraparticle diffusion model.

[0029] Figure 5 The structure in Experiment 3 of Example 4 is CMCS-La 3+ -Ca 2+The adsorption capacity of the carboxymethyl chitosan composite adsorbent for oxytetracycline at different initial concentrations is shown in Figure (a), which is the Redlich-Peterson isotherm model; Figure (b) is the Langmuir isotherm model; Figure (c) is the Freundlich isotherm model; and Figure (d) is the Temkin isotherm model fitting.

[0030] Figure 6 For example, in Experiment 4 of Example 4, different salt concentrations affected the structure CMCS-La 3+ -Ca 2+ The effect of carboxymethyl chitosan composite adsorbent on adsorption performance.

[0031] Figure 7 The structure in Experiment 5 of Example 4 is CMCS-La 3+ -Ca 2+ Cyclic experiment effect of carboxymethyl chitosan composite adsorbent on oxytetracycline absorption.

[0032] Figure 8 The structure is CMCS-La in Example 1. 3+ -Ca 2+ A schematic diagram of the adsorption mechanism of the carboxymethyl chitosan composite adsorbent. Detailed Implementation

[0033] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the following embodiments are merely simple examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention shall be determined by the claims.

[0034] Unless otherwise specified, all reagents used in this invention are commercially available analytical grade reagents.

[0035] Example 1

[0036] A carboxymethyl chitosan composite adsorbent is prepared by the following steps:

[0037] (1) Weigh 4.0g of carboxymethyl chitosan and dissolve it in 100.0mL of ultrapure water. After stirring magnetically for 24h, a uniform carboxymethyl chitosan gel is obtained.

[0038] (2) Weigh 0.666 g of La(NO3)3·6H2O and 3.312 g of CaCl2·2H2O and dissolve them in 100.0 mL of ultrapure water to prepare La 3+ / Ca 2+ Mixed solution.

[0039] (3) Using a 10.0 mL medical syringe, slowly drip all the carboxymethyl chitosan gel obtained in step (1) into the La prepared in step (2). 3+ / Ca 2+ The cross-linking reaction was carried out in the mixed solution. After the carboxymethyl chitosan gel was added, the reaction was allowed to continue at room temperature for 24 hours to obtain the carboxymethyl chitosan composite hydrogel.

[0040] (4) Wash away excess La in the carboxymethyl chitosan composite hydrogel obtained in step (3) with ultrapure water. 3+ and Ca 2+ The washed material is then placed in a freeze dryer at -49.2℃ for 24 hours to obtain a carboxymethyl chitosan composite adsorbent with the structure CMCS-La. 3+ -Ca 2+ .

[0041] In this embodiment, two other types of carboxymethyl chitosan composite adsorbents with different structures were also prepared:

[0042] Weigh 3.998 g of La(NO3)3·6H2O and dissolve it in 100.0 mL of ultrapure water to prepare La 3+ To prepare a solution, weigh 3.974 g of CaCl₂·2H₂O and dissolve it in 100.0 mL of ultrapure water. 2+ Solutions were prepared separately using the same operating steps described above, each containing only La complexes. 3+ or Ca 2+ The carboxymethyl chitosan composite adsorbent has the following structures: CMCS-La 3 and CMCS-Ca 2+ .

[0043] The structure prepared in this embodiment is CMCS-La. 3+ -Ca 2+ A schematic diagram of the adsorption mechanism of the carboxymethyl chitosan composite adsorbent is shown below. Figure 8 As shown, from Figure 8 It can be seen from the data that the carboxymethyl chitosan composite adsorbent can be activated through n-π EDA interactions, hydrogen bonds, and La... 3+ Multiple mechanisms, including complexation and cation-π bonds, work synergistically to capture and adsorb oxytetracycline.

[0044] SEM images of the three carboxymethyl chitosan composite adsorbents prepared in this embodiment are shown below. Figure 1 As shown, from Figure 1 (a) It can be seen that the structure is CMCS-La 3+ -Ca 2+ The carboxymethyl chitosan composite adsorbent is granular in shape, with small pores of varying sizes on its outer surface. Irregular swelling and fine cracks were observed in some areas of the surface. Figure 1 (e) It can be seen that the structure is CMCS-Ca 2+The surface of the carboxymethyl chitosan composite adsorbent microspheres also has small pores, but fewer internal channels; from Figure 1 (i) It can be seen that the structure is CMCS-La 3+ The surface of the carboxymethyl chitosan composite adsorbent is smoother and has no pores.

[0045] To further investigate the internal structure of the three carboxymethyl chitosan composite adsorbents, the cross-sectional morphology of the materials was observed using scanning electron microscopy. Figure 1 As can be seen in (b), the structure is CMCS-La 3+ -Ca 2+ The pores of the carboxymethyl chitosan composite adsorbent cross-section are uniformly distributed within the adsorbent particles, as shown in the magnified image. Figure 1 (c)~ Figure 1 As shown in (d), the branches and filaments intertwine to form pores, resulting in an adsorbent with a dendritic structure; from Figure 1 As can be seen from (f): the structure is CMCS-Ca 2+ The cross-section of the carboxymethyl chitosan composite adsorbent can be found at a distance from CMCS-Ca 2+ The center of the cross-section has a large number of dense pores, while the pores are fewer near the center. When magnified, as shown... Figure 1 (g)~ Figure 1 As shown in (h), the pores can be observed to be composed of interwoven dendritic filaments; from Figure 1 As can be seen from (j), the structure is CMCS-La 3+ The cross-section of the carboxymethyl chitosan composite adsorbent has a relatively compact "cabbage" shape, as shown when magnified. Figure 1 (k)~ Figure 1 As shown in (l), the surface of the "plates" inside is smooth and flat, and no structure similar to CMCS-Ca is observed. 2+ The carboxymethyl chitosan composite adsorbent has a loose, dendritic pore distribution across its cross-section.

[0046] The BET diagrams of the three carboxymethyl chitosan composite adsorbents prepared in this embodiment are shown below. Figure 2 As shown, by Figure 2 (a) ~ Figure 2 (c) It can be seen that the specific surface area of ​​the samples follows the following order: CMCS-La 3+ -Ca 2+ >CMCS-La 3+ >CMCS-Ca 2 + This indicates that the pore-filling mechanism also participates in the removal of oxytetracycline. Furthermore, combining the SEM and BET experimental results from this embodiment, it was found that: La 3+Small amounts of doping with carboxymethyl chitosan composite adsorbents can increase their specific surface area; La 3+ When ions are in excess, the specific surface area of ​​the sample decreases. According to the "hard acid-base" (HASB) theory, and Ca... 2+ Compared to ions, La 3+ The ion is a "harder" Lewis acid, exhibiting stronger coordination with the N / O functional groups on the carboxymethyl chitosan composite adsorbent. Therefore, when La... 3+ When there is an excess of ions, the synthesized carboxymethyl chitosan composite adsorbent has a relatively denser structure, and the specific surface area of ​​the material is reduced.

[0047] Example 2

[0048] A carboxymethyl chitosan composite adsorbent is prepared by the following steps:

[0049] (1) Weigh 4.0g of carboxymethyl chitosan and dissolve it in 100.0mL of ultrapure water. After stirring magnetically for 24h, a uniform carboxymethyl chitosan gel is obtained.

[0050] (2) Weigh 0.50 g of La(NO3)3·6H2O and 3.477 g of CaCl2·2H2O and dissolve them in 100.0 mL of ultrapure water to prepare La 3+ / Ca 2+ The mixed solution was then slowly dripped dropwise into La using a 10.0 mL medical syringe to extract all the carboxymethyl chitosan gel obtained in step (1). 3+ / Ca 2+ The cross-linking reaction is carried out in the solution. After the carboxymethyl chitosan gel is added dropwise, the reaction is allowed to continue at room temperature for at least 24 hours to obtain the carboxymethyl chitosan composite hydrogel.

[0051] (3) Wash away excess La in the carboxymethyl chitosan composite hydrogel obtained in step (2) with ultrapure water. 3+ and Ca 2+ The washed material is then placed in a freeze dryer at -50℃ for 24 hours to obtain a carboxymethyl chitosan composite adsorbent with the structure CMCS-La. 3+ -Ca 2+ .

[0052] Example 3

[0053] A carboxymethyl chitosan composite adsorbent is prepared by the following steps:

[0054] (1) Weigh 4.0g of carboxymethyl chitosan and dissolve it in 100.0mL of ultrapure water. After stirring magnetically for 24h, a uniform carboxymethyl chitosan gel is obtained.

[0055] (2) Weigh 3.198 g of La(NO3)3·6H2O and 0.795 g of CaCl2·2H2O and dissolve them in 100.0 mL of ultrapure water to prepare La 3+ / Ca 2+ Mixed solution.

[0056] (3) Using a 10.0 mL medical syringe, slowly drip all the carboxymethyl chitosan gel obtained in step (1) into the La obtained in step (2). 3+ / Ca 2+ The cross-linking reaction was carried out in the mixed solution. After the carboxymethyl chitosan gel was added, the reaction was allowed to continue at room temperature for at least 24 hours to obtain the carboxymethyl chitosan composite hydrogel.

[0057] (4) Wash away excess La in the carboxymethyl chitosan composite hydrogel obtained in step (3) with ultrapure water. 3+ and Ca 2+ The washed material is then placed in a freeze dryer at -40℃ for 24 hours to obtain a carboxymethyl chitosan composite adsorbent with the structure CMCS-La. 3+ -Ca 2+ .

[0058] Example 4

[0059] To further investigate the structure of CMCS-La 3+ -Ca 2+ The application of carboxymethyl chitosan composite adsorbent in the adsorption of oxytetracycline in water was investigated through the following experiments and analyses.

[0060] Experiment 1

[0061] The structure is CMCS-La at different initial pH values. 3+ -Ca 2+ The effect of carboxymethyl chitosan composite adsorption on the adsorption effect of oxytetracycline was investigated, including the following steps:

[0062] (1) Prepare a 100.0 mg / L oxytetracycline solution, divide the solution into 6 portions, and adjust the pH of the 6 portions of oxytetracycline solution with 1.0 mol / L NaOH and 1.0 mol / L HCl so that the initial pH of the 6 portions of oxytetracycline solution is 3.0, 4.0, 5.0, 6.0, 7.0 and 8.0 respectively.

[0063] (2) Weigh 0.02g of the CMCS-La structure prepared in Example 1. 3+ -Ca 2+ The carboxymethyl chitosan composite adsorbent was placed in a ground glass conical flask, and two parallel samples were set up under different initial pH conditions.

[0064] (3) To the device equipped with a structure of CMCS-La 3+ -Ca 2+ 30 mL of the solution prepared in step (1) was poured into a ground glass conical flask containing the carboxymethyl chitosan composite adsorbent and placed in a constant temperature shaker. The shaker was shaken for 24 hours at a speed of 180 rpm.

[0065] Experimental results are as follows Figure 3 As shown in (a), the structure can be seen from the figure as CMCS-La 3+ -Ca 2+ The carboxymethyl chitosan composite adsorbent showed adsorption effects in oxytetracycline solutions with initial pH values ​​ranging from 3.0 to 8.0. The adsorption effect was particularly stable within the initial pH range of 4.0 to 7.0, indicating excellent pH adaptability. Specifically, the adsorption capacity q at pH 6.0 was [not specified]. t =112.55mg / g.

[0066] Analysis showed that the structure was CMCS-La under different pH conditions. 3+ -Ca 2+ The reasons for the varying removal efficiency of carboxymethyl chitosan composite adsorbents for oxytetracycline are as follows: In strongly acidic aqueous solutions (pH=3.0), the adsorbent exhibits poor removal efficiency for oxytetracycline because, under these conditions, the protonated -NH3 groups on the adsorbent surface... + -COOH2 + and -C-OH2 + Functional groups and H3OTC + Strong electrostatic repulsion between cations inhibited the adsorption of oxytetracycline. As the initial pH of the solution increased from 3.0 to 4.0, the adsorbent's ability to capture oxytetracycline was greatly enhanced. Within this pH range, the adsorbent surface underwent continuous deprotonation, meaning the number of positive charges on the adsorbent surface decreased with increasing pH. This weakened the electrostatic repulsion between the adsorbent and oxytetracycline, facilitating the adsorption and removal of oxytetracycline. Within the pH range of 4.0–7.0, the removal efficiency of oxytetracycline reached its peak and remained almost constant, indicating that the adsorbent has a wide pH adaptability. Within this pH range, the adsorbent surface remained positively charged, and oxytetracycline mainly existed in the aqueous solution as cations and zwitterions, indicating that oxytetracycline was not removed through electrostatic interactions. Other mechanisms, such as n-π EDA interactions, hydrogen bonding, and La... 3+ The complexation effect synergistically dominates the adsorption process of oxytetracycline; when pH=8, the adsorbent surface is negatively charged, and the adsorbent reacts with HOTC. - The electrostatic repulsion between anions slightly inhibited the removal of oxytetracycline.

[0067] Simultaneously, the Zeta potential of the adsorbent before and after adsorption was measured under different pH conditions, and the test results are as follows: Figure 3 As shown in (b), the Zeta potential of the adsorbent did not change abruptly before and after adsorption, indicating that the structure is CMCS-La. 3+ -Ca 2+ The carboxymethyl chitosan composite adsorbent mainly captures H2O and OTC in water. 0 .

[0068] Method for determining the residual oxytetracycline concentration: The specific steps for determining the residual oxytetracycline concentration are as follows: After filtering the solution through a 0.45 μm filter membrane, measure the absorbance at a wavelength of 275 nm using a UV-Vis spectrophotometer, and calculate the residual concentration according to the calibration mark.

[0069] Experiment 2

[0070] The effect of adsorption time on the removal efficiency of oxytetracycline is analyzed in the following steps:

[0071] (1) Prepare oxytetracycline solutions with concentrations of 100.0 mg / L and 400.0 mg / L respectively, and adjust the pH to 6.0 with 1.0 mol / L NaOH and 1.0 mol / L HCl.

[0072] (2) Since the adsorption time is set to 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 12, 18, 24, 30, 36, 42, 48, 60, 72h, and two parallel sets are required for different adsorption times, the oxytetracycline solutions of different concentrations prepared in step (1) are divided into 36 portions and added to ground glass conical flasks, each portion being 30mL.

[0073] (3) Add 0.02g of the oxytetracycline solution prepared in step (2) to the solution prepared in Example 1, which has a structure of CMCS-La. 3+ -Ca 2+ The carboxymethyl chitosan composite adsorbent was placed in a constant temperature shaker (25℃) and shaken for 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 12, 18, 24, 30, 36, 42, 48, 60, and 72 hours at a speed of 180 rpm.

[0074] Experimental results show that the adsorbent reaches adsorption equilibrium for 100.0 mg / L oxytetracycline after about 48 hours, and for 400.0 mg / L oxytetracycline after about 72 hours.

[0075] Method for determining the residual concentration: The specific steps for determining the residual concentration of oxytetracycline are as follows: After filtering the solution through a 0.45 μm filter membrane, the absorbance is measured at a wavelength of 275 nm using a UV-Vis spectrophotometer, and the residual concentration is calculated according to the calibration mark.

[0076] To further understand the adsorption process and mechanism, the kinetic data were fitted using a pseudo-first-order kinetic model, a pseudo-second-order kinetic model, and an intraparticle diffusion model (Weber-Morris model):

[0077] pseudo-first-order adsorption kinetics equation:

[0078]

[0079] pseudo-second-order adsorption kinetic equation:

[0080]

[0081] Intraparticle diffusion model (Weber-Morris model):

[0082]

[0083] Where, q t (mg / g) and q e (mg / g) represent the adsorption capacity of oxytetracycline at t and equilibrium time, respectively, k1(h -1 ) and k2(g·mg -1 ·h -1 ) represent the adsorption rate constants for the pseudo-first-order kinetic model and the pseudo-second-order kinetic model, respectively. i (mg·g -1 ·h -0.5 C(mg / g) and C(mg / g) represent the rate constant and boundary layer thickness of the intraparticle diffusion kinetics model, respectively.

[0084] The fitting results are as follows Figure 4 As shown, the nonlinear fitting curves for the pseudo-first-order and pseudo-second-order equations are as follows: Figure 4 As shown in (a), calculations show that the R² values ​​(0.992~0.993) of the pseudo-second-order kinetic model are all higher than those of the pseudo-first-order kinetic model (0.972~0.973), indicating that the pseudo-second-order kinetic model can better describe the adsorption process of the adsorbent; from Figure 4(b) The intraparticle diffusion model shows that the adsorption process of oxytetracycline can be divided into three stages: (1) oxytetracycline molecules in the bulk solution diffuse through the boundary layer to the surface of the adsorbent; (2) oxytetracycline molecules slowly diffuse into the pores inside the adsorbent, and the number of available active sites on the adsorbent continuously decreases; (3) the available reaction sites on the adsorbent are insufficient, reaching adsorption saturation. At the same time, since the fitting curve of the intraparticle diffusion model does not pass through the origin, it indicates that the intraparticle diffusion mechanism is not the only mechanism for the removal of oxytetracycline, and it is speculated that other adsorption effects such as surface distribution may also exist.

[0085] Experiment 3

[0086] The effects of initial oxytetracycline concentration and adsorption temperature on adsorption efficiency were investigated using the following experimental steps:

[0087] (1) Prepare oxytetracycline solutions with initial concentrations of 5, 10, 25, 50, 75, 100, 200, 300, 400, 500 and 600 mg / L respectively, and adjust the pH of the solutions to 6.0 with 1.0 mol / L NaOH and 1.0 mol / L HCl.

[0088] (2) Since the adsorption temperatures are set to 20℃ (293K), 25℃ (298K), 30℃ (303K), and 35℃ (308K), and two parallel samples are made at the same temperature, it is necessary to divide each different concentration of oxytetracycline solution in step (1) into 8 portions and add them to ground glass conical flasks, each portion being 30mL.

[0089] (3) Add 0.02g of the CMCS-La structure prepared in Example 1 to the oxytetracycline solution separated in step (2). 3+ -Ca 2+ The carboxymethyl chitosan composite adsorbent was placed in constant temperature shakers with different temperature settings (20℃ (293K), 25℃ (298K), and 30℃ (303K) respectively), and the adsorption time was 96h with a rotation speed of 180rpm.

[0090] Experimental results show that the Redlich-Peterson isotherm model has the largest correlation coefficient R2 under different temperature conditions, and the maximum adsorption capacity obtained in the experiment is 580.91 mg / g.

[0091] Method for determining the residual concentration: The specific steps for determining the residual concentration of oxytetracycline are as follows: After filtering the solution through a 0.45 μm filter membrane, the absorbance is measured at a wavelength of 275 nm using a UV-Vis spectrophotometer, and the residual concentration is calculated according to the calibration mark.

[0092] To further describe CMCS-La 3+ -Ca 2+The adsorption behavior of oxytetracycline was nonlinearly fitted to the experimental data using Redlich-Peterson (a combination of Langmuir and Freundlich), Langmuir, Freundlich, and Temkin models. The equations for each model are as follows:

[0093] Redlich-Peterson isotherm model:

[0094]

[0095] Langmuir isotherm model:

[0096]

[0097] Freundlich isotherm model:

[0098]

[0099] Temkin model:

[0100]

[0101] Among them, C e (mg / L) and q e (mg / g) represent the equilibrium concentration and the adsorption capacity at equilibrium of oxytetracycline, respectively; A (L / g) and B (L / mg) g ], K L (L / mol), K F (mg (1−1 / n) ·L 1 / n ·g −1 ) and K T (L / mg) represent the equilibrium constants of the Redlich-Peterson, Langmuir, Freundlich, and Temkin models, respectively; g, n F and B l denoted as the dimensionless exponents of the Redlich Peterson model, Freundlich model, and Temkin model, respectively.

[0102] Depend on Figure 5 (a) ~ Figure 5 (d) It can be obtained that the equilibrium adsorption capacity q is obtained under different temperature conditions (293, 298, 303 and 308 K). e With C eThe adsorption capacity increases with increasing temperature, but this increase is not linear, indicating that intraparticle diffusion is not the controlling factor of the adsorption process. Furthermore, the equilibrium adsorption capacity also increases significantly with increasing temperature, indicating that the adsorption process of oxytetracycline is endothermic. Correlation coefficients of different isotherm models were obtained through fitting. The Redlich-Peterson isotherm model showed the highest correlation coefficient R² under different temperature conditions. Therefore, the Redlich-Peterson adsorption isotherm model, which combines monolayer and multilayer coverage characteristics, is more suitable than the Langmuir, Freundlich, and Temkin isotherm models for explaining the adsorption process of oxytetracycline. In addition, at 303 K, the saturated adsorption capacity of oxytetracycline calculated based on the Langmuir model is 580.91 mg / g.

[0103] Experiment 4

[0104] The effect of salt concentration in oxytetracycline solution on adsorption efficiency is investigated through the following steps:

[0105] (1) Prepare a 10.0 mg / L oxytetracycline solution and adjust the pH of the solution to 6.0 with 1.0 mol / L NaOH and 1.0 mol / L HCl.

[0106] (2) Weigh 0.01, 0.025, 0.05, 0.10, 0.25, 0.50 and 1.00 g of NaCl and CaCl2 into beakers respectively.

[0107] (3) Since different salt solutions of different concentrations need to be prepared in parallel, the oxytetracycline solution prepared in step (1) is divided into 28 portions and added to the beaker in step (2), with 100.0 mL added to each portion, to prepare NaCl solution and CaCl2 solution with concentration gradients of 0.1, 0.25, 0.5, 1.0, 2.5, 5.0 and 10.0 g / L.

[0108] (4) Add the solution obtained in step (3) to ground glass conical flasks, 30.0 mL for each, and add 0.02 g of the CMCS-La structure prepared in Example 1. 3+ -Ca 2+ The carboxymethyl chitosan composite adsorbent was placed in a constant temperature shaker at 25℃ with a rotation speed of 180 rpm for 48 hours.

[0109] The concentration of residual oxytetracycline in the solution was measured after 48 hours, and the results are as follows. Figure 6 As shown, from Figure 6The results show that even at a salt concentration as high as 10.0 g / L, neither NaCl nor CaCl2 inhibits the removal of oxytetracycline, and the removal efficiency remains stable under different salt concentrations. This further proves that the adsorption of oxytetracycline is not controlled by an electrostatic mechanism. The salt concentration in actual wastewater is often below 100.0 mg / L, far below the experimentally set limit concentration (10.0 g / L), indicating that the carboxymethyl chitosan composite adsorbent has salt tolerance when treating complex wastewater.

[0110] Experiment 5

[0111] The structure is CMCS-La 3+ -Ca 2+ The cyclic performance test of the carboxymethyl chitosan composite adsorbent was conducted, and the specific experimental steps are as follows:

[0112] (1) Prepare a 10.0 mg / L oxytetracycline solution and adjust the pH of the solution to 6.0 with 1.0 mol / L NaOH and 1.0 mol / L HCl.

[0113] (2) Add the oxytetracycline solution prepared in step (1) into a ground glass conical flask, set up two parallel samples, add 30.0 mL to each sample, and add 0.04 g of the CMCS-La structure prepared in Example 1. 3+ -Ca 2+ The carboxymethyl chitosan composite adsorbent was placed in a constant temperature shaker at 25℃ with a rotation speed of 180 rpm for 24 hours.

[0114] (3) After the adsorption process is terminated, the used carboxymethyl chitosan composite adsorbent is easily separated from the aqueous solution by filtration. The oxytetracycline solution containing the adsorbent is filtered with a 0.45 μm filter membrane to obtain the supernatant. The supernatant contains the recovered adsorbent. The method for determining the remaining concentration is as follows: The specific steps for determining the remaining concentration of oxytetracycline are as follows: After filtering the solution with a 0.45 μm filter membrane, the absorbance is measured at a wavelength of 275 nm using a UV-Vis spectrophotometer, and the remaining concentration is calculated according to the calibrated line.

[0115] (4) The carboxymethyl chitosan composite adsorbent recovered in step (3) is soaked in 40.0 mL of methanol and shaken in a constant temperature shaker at 25 °C for 24 h under the same conditions to achieve regeneration. Then, the regenerated carboxymethyl chitosan composite adsorbent is rinsed with ultrapure water 5 to 6 times to completely remove the residual methanol. Then, the carboxymethyl chitosan composite adsorbent is added to 30.0 mL of the oxytetracycline solution prepared in step (1) for the next adsorption-desorption cycle.

[0116] The experimental results of five adsorption-desorption cycles of the carboxymethyl chitosan composite adsorbent are as follows: Figure 7 As shown, from Figure 7 As can be seen, after five regenerations, the removal rate of oxytetracycline decreased from 97.48% to 95.94%, with a loss of less than 3%, indicating that the carboxymethyl chitosan composite adsorbent has exceptional stability, repeatability, and recyclability when treating complex wastewater.

Claims

1. The application of a carboxymethyl chitosan composite adsorbent in the removal of oxytetracycline, characterized in that: The specific preparation steps of the carboxymethyl chitosan composite adsorbent are as follows: (1) Add carboxymethyl chitosan to ultrapure water and stir evenly to obtain carboxymethyl chitosan gel; (2) The carboxymethyl chitosan gel obtained in step (1) is added dropwise to La 3+ / Ca 2+ After the carboxymethyl chitosan gel was added dropwise to the mixed solution, the reaction was allowed to stand for at least 24 hours to obtain a carboxymethyl chitosan composite hydrogel. (3) The carboxymethyl chitosan composite hydrogel obtained in step (2) is washed and then freeze-dried. The carboxymethyl chitosan composite adsorbent obtained after the carboxymethyl chitosan composite hydrogel is completely dried has the structure CMCS-La. 3+ -Ca 2+ ; In step (2), La 3+ / Ca 2+ The mixed solution is prepared by adding La(NO3)3·6H2O and CaCl2·2H2O to ultrapure water, wherein the amount of La(NO3)3·6H2O added is 5~32 g / L and the amount of CaCl2·2H2O added is 30~80 g / L.

2. The application according to claim 1, characterized in that: Step (1) The amount of carboxymethyl chitosan added to the ultrapure water is 35~45g / L.

3. The application according to claim 1, characterized in that: The cleaning method in step (3) is to rinse with ultrapure water 3 to 5 times, changing the ultrapure water each time, until the used ultrapure water is neutral.

4. The application according to claim 1, characterized in that: In step (3), the freeze-drying temperature is -50~-40℃.

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