CN108499539B

CN108499539B - Method for adsorbing heavy metal ions in water by using cellulose ester membrane

Info

Publication number: CN108499539B
Application number: CN201810283109.9A
Authority: CN
Inventors: 陶芙蓉; 史成美; 张通; 崔月芝
Original assignee: Qilu University of Technology
Current assignee: Hunan Dexing electroplating Co.,Ltd.
Priority date: 2018-04-02
Filing date: 2018-04-02
Publication date: 2020-08-18
Anticipated expiration: 2038-04-02
Also published as: CN108499539A

Abstract

The invention discloses a method for adsorbing heavy metal ions in water by using a cellulose ester membrane, which comprises the steps of placing the cellulose ester membrane in an aqueous solution containing the heavy metal ions, adjusting the pH value to 2-12, and balancing for a period of time; the preparation method of the cellulose ester film comprises the following steps: the cellulose ester is obtained by esterification reaction of microcrystalline cellulose and citric anhydride, glutamic acid is introduced into the side chain of the cellulose ester through amide reaction to crosslink the cellulose ester, so that the glutamic acid crosslinked cellulose ester is obtained, and the glutamic acid crosslinked cellulose ester film can be obtained after the dispersion liquid of the glutamic acid crosslinked cellulose ester is placed in a mould to be frozen and dried. The method can adsorb heavy metal ions with low concentration in water.

Description

Method for adsorbing heavy metal ions in water by using cellulose ester membrane

Technical Field

The invention relates to a method for adsorbing heavy metal ions in water by using a cellulose ester membrane.

Background

Currently, with the rapid development of scientific technology, people's living standard is increasing day by day and more environmental pollution problems also appear. Among them, water pollution caused by metal ions has attracted much attention. At present, methods for treating heavy metal ion pollution include an adsorption method, a chemical sedimentation method, a membrane filtration method and the like. Among them, the adsorption method has been a hot point of research because of its high removal efficiency and strong operability.

The adsorption method is to adsorb one or several components in water sample onto the surface with porous solid adsorbent and to desorb the predicted components with proper solvent, heating or blowing to separate and enrich. In the prior research, various modified celluloses are adopted to prepare film-forming materials for adsorbing heavy metal ions. Cellulose is a macromolecular polysaccharide composed of glucose, is insoluble in water and common organic solvents, and is the main component of plant cell walls. Cellulose is the most abundant natural organic matter in the world, and accounts for more than 50% of the carbon content in the plant world, so that the cellulose is an adsorbent membrane raw material which is beneficial to large-scale generation. However, the existing cellulose membrane material has the defects of poor mechanical property and difficulty in realizing the adsorption of a solution with low heavy metal concentration, and is not favorable for the practical application of the cellulose membrane material in the adsorption of heavy metal ions in water.

Disclosure of Invention

In order to solve the defects of the prior art, the invention aims to provide a preparation method of a glutamic acid cross-linked cellulose ester membrane, which can adsorb heavy metal ions with low concentration in water.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a method for adsorbing heavy metal ions in water by using a cellulose ester membrane comprises the steps of placing the cellulose ester membrane in an aqueous solution containing the heavy metal ions, adjusting the pH to 2-12, and balancing for a period of time;

the preparation method of the cellulose ester film comprises the following steps: the cellulose ester is obtained by esterification reaction of microcrystalline cellulose and citric anhydride, glutamic acid is introduced into the side chain of the cellulose ester through amide reaction to crosslink the cellulose ester, so that the glutamic acid crosslinked cellulose ester is obtained, and the glutamic acid crosslinked cellulose ester film can be obtained after the dispersion liquid of the glutamic acid crosslinked cellulose ester is placed in a mould to be frozen and dried.

Another object of the present invention is to provide a method for regenerating a cellulose ester film used in the above method, wherein the cellulose ester film having heavy metal ions adsorbed thereon is subjected to centrifugation in an acidic ethanol solution.

The invention has the beneficial effects that:

1. the cellulose ester film adopted by the invention is a novel cross-linking material obtained by modifying microcrystalline cellulose by using citric anhydride and glutamic acid as cross-linking agents.

2. The cellulose ester film utilized by the invention has better thermal stability; meanwhile, the composite material has higher tensile strength and elongation at break and lower elastic modulus, so that the membrane material can be recycled.

3. The method can adsorb heavy metal ions with low concentration in water, and can adsorb the concentration of the heavy metal ions as low as 0.2 mg.L^-1。

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the application and, together with the description, serve to explain the application and are not intended to limit the application.

FIG. 1 is an infrared spectrum in which a is microcrystalline cellulose, b is the film prepared in comparative example 1, and c is the film prepared in example 1;

FIG. 2 is a thermodynamic analysis curve, wherein A is thermogravimetric analysis (TGA) and B is differential thermal gravimetric analysis (DTG);

FIG. 3 is a scanning electron micrograph, in which a₁Surface of MCC-CAD, b₁Surface of MCC-CAD-5% GA, c₁Surface of MCC-CAD-15% GA, d₁Surface of MCC-CAD-25% GA, a₂Cross section of MCC-CAD, b₂Is the cross section of MCC-CAD-5% GA, c₂Is the cross-section of MCC-CAD-15% GA, d₂Cross-section of MCC-CAD-25% GA

FIG. 4a is a graph of temperature vs. Cd²⁺、Pb²⁺、Cu²⁺、Co²⁺、Ni²⁺Characterization of the effect of the adsorption behavior of ions, FIG. 4b is the contact time versus Cd²⁺、Pb²⁺、Cu²⁺、Co²⁺、Ni²⁺Characterization of the effect of ion adsorption behavior, FIG. 4c is sample dose vs. Cd²⁺、Pb²⁺、Cu²⁺、Co²⁺、Ni²⁺Characterization of the effect of the adsorption behavior of ions, FIG. 4d is the pH vs. Cd²⁺、Pb²⁺、Cu²⁺、Co²⁺、Ni²⁺A profile of the effect of the adsorption behavior of the ions;

FIG. 5 is a graph of initial metal ion concentration versus adsorption of Cd to MCC-CAD-25% GA membrane²⁺、Co²⁺、Ni²⁺、Pb²⁺And Cu²⁺A profile of ions;

FIG. 6 shows the adsorption of Cd on MCC-CAD-25% GA membrane²⁺、Pb²⁺、Cu²⁺、Co²⁺、Ni²⁺A histogram of ion recovery;

FIG. 7a shows the adsorption of Pb on MCC-CAD-25% GA membrane²⁺FIG. 7b is a representation of MCC-CAD-25% GA film adsorbing Cu²⁺FIG. 7c is a representation of MCC-CAD-25% GA film adsorbing Co²⁺FIG. 7d is a chart showing the absorption of Cd by MCC-CAD-25% GA film²⁺FIG. 7e is a representation of MCC-CAD-25% GA film adsorbing Ni² ⁺FIG. 7f is a typical graph of the elemental analysis, showing that the MCC-CAD-25% GA film desorbs Pb²⁺FIG. 7g is a representation of MCC-CAD-25% GA film desorbed Cu²⁺The subsequent elemental analysis is characterized by the graph, FIG. 7h is the Co desorption of MCC-CAD-25% GA film²⁺The subsequent elemental analysis is characterized by the graph, FIG. 7i is the MCC-CAD-25% GA film desorption Cd²⁺FIG. 7j is a representation of the Ni desorbed from the MCC-CAD-25% GA film²⁺The subsequent elemental analysis characterization chart;

FIG. 8 shows the adsorption of Cd on MCC-CAD-25% GA thin films²⁺、Pb²⁺、Cu²⁺、Co²⁺、Ni²⁺A pseudo-kinetic order fit line of the ions, wherein a is a first-order kinetic order and b is a pseudo-second-order kinetic order;

FIG. 9 shows the adsorption of Cd onto MCC-CAD-25% GA membrane²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺Isotherms of ions, where a is Langmuir, b is Freundlich, and c is Elovich.

Detailed Description

It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.

As introduced in the background art, the prior art has the defect that a cellulose membrane is difficult to adsorb a solution with a low heavy metal concentration, and the like, and in order to solve the technical problems, the application provides a method for adsorbing heavy metal ions in water by using the cellulose membrane.

The application provides a typical embodiment of the method for adsorbing heavy metal ions in water by using a cellulose ester membrane, wherein the cellulose ester membrane is placed in an aqueous solution containing the heavy metal ions, the pH value is adjusted to 2-12, and the cellulose ester membrane is obtained after a period of balance;

Preferably, the mass ratio of the microcrystalline cellulose to the citric anhydride is 2: 0.9-1.1.

Preferably, the mass ratio of the microcrystalline cellulose to the glutamic acid is 2: 0.1-0.5.

Preferably, the step of preparing the cellulose ester film is:

(1) preparing microcrystalline cellulose into a cellulose solution, dropwise adding a citric anhydride solution into the cellulose solution, and heating to 65 +/-2 ℃ for reaction to obtain a cellulose ester solution;

(2) adding glutamic acid, carbodiimide and N-hydroxysuccinimide into a cellulose ester solution to perform an amide reaction to obtain a cross-linking solution;

(3) and standing the crosslinking solution, adding propanol, standing to form a film, washing with water to remove salt, and freeze-drying.

Further preferably, the solvent of the cellulose solution in step (1) is lithium chloride and dimethylacetamide.

Further preferably, the reaction time in the step (1) is 24-36 h.

Further preferably, the reaction conditions in step (2) are 24. + -.1 h at room temperature. The room temperature is 15-30 ℃.

Further preferably, the process of step (3) is: standing the crosslinking solution for 30 + -5 min, adding propanol, standing for 1 + -0.1 h, forming a film, washing with water for 8 + -0.1 h to remove salt, and freeze-drying.

Preferably, the concentration of the heavy metal ions in the aqueous solution containing the heavy metal ions is 0.2 to 2 mg.L^-1。

Preferably, the pH is 6 and the equilibration time is 2 h.

In another embodiment of the present invention, there is provided a method for regenerating a cellulose ester membrane used in the above method, wherein the cellulose ester membrane having heavy metal ions adsorbed thereon in the above method is subjected to centrifugation in an acidic ethanol solution.

In order to make the technical solutions of the present application more clearly understood by those skilled in the art, the technical solutions of the present application will be described in detail below with reference to specific embodiments.

Example 1

(1) 2g of dried microcrystalline cellulose (MCC) and 25mL of dimethylacetamide (DMAc) were mixed in a flask and heated at 130 ℃ for 30min, then 2g of lithium chloride (LiCl) was added to the mixture and heating was continued for 10 min. Subsequently, the above mixture was stirred at room temperature for 12 hours to obtain a cellulose solution (abbreviated as MCC solution). Then, 1g of Citric Anhydride (CAD) dissolved in 6mL of DMAc was added dropwise to the MCC solution above with stirring. The final mixture was stirred at room temperature for 30min and then reacted at 65 ℃ for 24h to obtain a cellulose ester solution (abbreviated as MCC ester solution).

(2) 0.2g of 1- (3-dimethylaminopropyl) -3-Ethylcarbodiimide (EDC), 0.1g N-hydroxysuccinimide (NHS) and 0.1g of glutamic acid (L-GA) were dispersed in 5mL of DMAc for 2 h. Finally, the dispersion was added to the cellulose ester solution and stirred at room temperature for 24 hours to obtain a crosslinking solution.

(3) 3.6g of the crosslinking solution was transferred to a polytetrafluoroethylene mold (Φ ═ 8cm), and allowed to stand at room temperature for 30 min. The cast cross-linked cellulose solution was immersed in acetone for 1h, then washed with cold water for 8h to completely remove salts, and the finally obtained cross-linked cellulose ester film was dried on a glass petri dish using freeze-drying, to obtain a cross-linked cellulose ester film of glutamic acid, which was designated as MCC-CAD-5% GA.

Example 2

This example is the same as example 1, except that: 0.2g of glutamic acid is obtained in the step (2), and the finally prepared glutamic acid cross-linked cellulose ester film is marked as MCC-CAD-10% GA.

Example 3

This example is the same as example 1, except that: 0.3g of glutamic acid is obtained in the step (2), and the finally prepared glutamic acid cross-linked cellulose ester film is marked as MCC-CAD-15% GA.

Example 4

This example is the same as example 1, except that: 0.4g of glutamic acid is obtained in the step (2), and the finally prepared glutamic acid cross-linked cellulose ester film is marked as MCC-CAD-20% GA.

Example 5

Comparative example 1

(2) 3.6g of the MCC ester solution was transferred to a polytetrafluoroethylene mold (phi. 8cm) and allowed to stand at room temperature for 30 min. The cast cross-linked cellulose solution was immersed in acetone for 1h, then washed with cold water for 8h to completely remove the salts, and the finally obtained ester film was dried on a glass petri dish using freeze-drying, to obtain a cellulose ester film, designated MCC-CAD.

The mechanical properties and heavy metal ion adsorption properties of the films prepared in examples 1 to 5 and comparative example 1 were characterized as follows:

mechanical Properties

Mechanical properties are key parameters for the recyclability of materials. In this characterization, the mechanical properties of the film were measured using a computer electronic universal tester (WDL-005, Jinan, China) equipped with a 500N tensile load cell, and three parameters of the mechanical properties, tensile strength, elastic modulus and elongation at break of the crosslinked cellulose ester film, were measured. The crosshead speed was set at 5mm/min before testing. The initial grip length, width and thickness of each film were measured by a vernier caliper (0.02mm/150mm, Shanghai, China) and a micrometer (0-25mm/0-1 "0.001 mm/0.0001", SYTITEK, Zhejiang, China), respectively.

Adsorption process

Bulk adsorption characterization was performed at room temperature of 25 ℃ using known mass of adsorbent. The MCC-CAD-GA membrane pair Cd was studied with 10. + -. 2.5mg MCC-CAD-GA in 20mL of aqueous metal solution²⁺，Co²⁺，Ni²⁺，Pb²⁺And Cu²⁺Adsorption capacity of metal ions. The concentration range of the metal ions is 0.2-2 mg.L^-1The pH value is 2-12, and the balance time is 120 min. The pH of the different solutions was adjusted by adding different concentrations of HCl (0.1M) or NaOH (0.1M). Kinetic studies were performed by changing the contact time from 20 minutes to 120 minutes. The metal ion concentration in the supernatant was determined by UV-Vis spectrophotometer (T6New century). Adsorption Capacity (Q) in adsorption Studies_t，mg·g^-1) And the percentage of metal ion removal (% removal rate) is explained by the following equation (equation 1):

in the formula: q_tThe amount of adsorption (mg. g) at time t^-1)，C₀Is the initial metal ion concentration (mg. L)^-1)，C_tThe concentration of metal ions (mg. L) at time t^-1) M is the mass of MCC-CAD or MCC-CAD-GA (10mg) and V is the volume of the solution (20 mL). All tests were performed at least three times and the mean value was used for analysis.

Recovery of crosslinked cellulose ester films

Recovery is performed in a similar manner to the adsorption procedure. Adsorbing metal and loading on glutamic acid cross-linked cellulose ester film (initial metal ion concentration 1.0 mg. L)^-1pH6, temperature 25 ℃). After equilibrium is reached, the sample membrane is separated from the solution using a centrifuge. And (3) performing reusability research on the regenerated membrane after the acidic ethanol solution is centrifuged for 24 hours at room temperature, drying in a vacuum oven, and repeatedly performing adsorption characterization on heavy metal ions by using a regenerated sample.

Characterization results

Structural characterization

1064cm for all films^-1The nearby absorption band is due to the C-O stretching vibration that symbolizes the glucose ring, and the peak is not changed after modification, which means that the glucose ring is not broken. As can be seen from figure 1, the ir spectra of b and c show all the characteristic peaks of MCC. B is 1722cm compared with a^-1And 1650cm^-1The new absorption peaks observed therein are due to the stretching vibration of the newly formed ester carbonyl and carboxyl absorption peaks, respectively, indicating the successful synthesis of cellulose esters. For MCC-CAD-GA, except at 1628cm^-1And 1650cm^-1Out of the absorption peaks of ester carbonyl and carboxyl, 1710cm^-1A new stretching vibration absorption peak appears, and belongs to the carbonyl absorption peak in amide. Meanwhile, the height of the groove is 1530-1560 cm^-1The characteristic peak of (A) is the absorption of the N-H bending vibration. The above evidence clearly shows that MCC-CAD and L-GA are crosslinked by amidation reaction.

Properties of L-GA Cross-Linked cellulose ester film

Thermal stability

As shown in FIG. 2, two stages of thermal degradation of mass loss were observed at 50-150 ℃ and 250-360 ℃. The first weight loss that occurs at temperatures from 50 ℃ to 150 ℃ is associated with elimination of the bounded water molecules and degradation of the side chains (e.g., hydroxyl groups) from the cellulose backbone of all polymers. The second weight loss (from 250 to 360 ℃) is due to thermal degradation of the cellulose. Above this temperature, there is a complex dehydration process of the sugar rings. As shown in FIG. 2B, the maximum weight loss temperatures for MCC-CAD, MCC-CAD-5% GA, MCC-CAD-15% GA, and MCC-CAD-25% GA were 347.6, 354.4, 356.1, and 358.5 deg.C, respectively. This indicates that the crosslinking effect improves the thermal stability of the material to some extent. In one aspect, the crosslinking reaction between the cellulose ester and L-GA forms amide bonds, making the macromolecular structure more stable and stronger. On the other hand, hydrogen bonding and electrostatic interactions of functional groups in MCC-CAD and L-GA further enhance the structure. All of these factors provide an effective reinforcement layer to withstand thermal degradation. The above evidence demonstrates that the thermal stability of MCC-CAD-GA is enhanced.

Scanning Electron Microscope (SEM)

From FIG. 3a₁It can be seen that SEM photographs of the surface topography of the MCC-CAD film show a dense, smooth and compact structure without any embossing or holes. To cross-link the surface of the membrane (FIG. 3 b)₁FIG. 3c₁And FIG. 3d₁) More irregular and rough, which is advantageous for adsorption. And as the L-GA content in the film increases, the surface of the crosslinked film becomes rougher. For the cross-section of the blank film (FIG. 3 a)₂) A clear and clean structure was observed. While the composite membrane is shown in FIG. 3b₂FIG. 3c₂And FIG. 3d₂The layer structure shown. And delamination was more pronounced with increasing L-GA content. Thus, it can be shown that crosslinking has occurred.

Mechanical Properties

Table 1 shows the thickness of the crosslinked film, TS, EM and EaB. The results show that the addition of L-GA improves the mechanical properties of the cross-linked MCC-CAD-GA membrane. The tensile strength of the MCC-CAD-GA film is increased compared to the MCC-CAD film, wherein the TS of MCC-CAD is 29.65MPa, and the tensile strength of MCC-CAD-25% GA is 107.44 MPa. This is because crosslinking results in a more rigid structure, giving it the ability to resist external stresses, resulting in an increase in the tensile strength of the composite film. As shown in Table 1, the EaB of MCC-CAD-5% GA is 42.1%, which is about twice that of the pure MCC-CAD complex, while the EM of MCC-CAD-5% GA is 62.000MPa, which is more than one twentieth that of the pure complex, 1378.558 MPa. EM and EaB tend to predict new materials that exhibit elasticity and flexibility and are no longer brittle. This is because highly crosslinked composites have less tendency to elastically deform when returning to their original shape. All the above evidence confirms that the mechanical properties of the crosslinked membranes are enhanced. This is due to the cross-linking of L-GA with MCC-CAD. For reusable adsorbents, MCC-CAD-GA membranes are highly desirable.

TABLE 1 mechanical Properties of MCC-CAD and MCC-CAD-GA films

Factors affecting the adsorption of metal ions

Influence of the temperature of the adsorption Medium

FIG. 4a shows Cd on the surface of a cross-linked cellulose ester film (MCC-CAD-25% GA)²⁺、Co²⁺、Ni²⁺、Pb²⁺And Cu²⁺Representative plots of the adsorption capacity of ions at different temperature ranges from 20 to 40 ℃. It was found that the amount of ion adsorption on the surface increases with increasing temperature when the temperature is increased from 20 c to 25 c. The increase in ion removal with increasing temperature can be explained by adsorption sites that are more active at moderate temperatures. Therefore 25 ℃ was chosen as the solution temperature. Similar trends have been observed for adsorption of heavy metal ions on other membrane adsorbents.

Influence of adsorption time

FIG. 4b shows Cd²⁺、Co²⁺、Ni²⁺、Pb²⁺And Cu²⁺Adsorption capacity of ions according to contact time (20 to 120 min). As shown in FIG. 4b, Cd as the contact time increased to 120min²⁺、Co²⁺、Ni²⁺、Pb²⁺And Cu²⁺The adsorption capacity of the ions increases. After 120min, no significant change in the adsorption capacity was observedEquilibration indicated that equilibrium was reached and the fiber surface was saturated. At this time, the amount of ions adsorbed at 120min under the experimental conditions showed the maximum adsorption capacity. This time is defined as the equilibrium time. Notably, the adsorption of ions is rapid in the initial stages and then becomes slower and approaches equilibrium. This is because, in the initial stage, a large number of vacant surface sites are available for adsorption, and after a certain period of time, the remaining vacant surface sites are hardly occupied due to the repulsive force between solute molecules on the solid phase and bulk phase. Meanwhile, as can be seen from FIG. 4b, the adsorption capacity gradually increases as the L-GA content increases. MCC-CAD-25% GA film pair Cd²⁺、Pb²⁺、Cu²⁺、Co²⁺、Ni²⁺The maximum adsorption capacities of the adsorption media were 1.910, 1.982, 1.840, 1.901 and 1.674mg/g, respectively. Thus, MCC-CAD-25% GA membrane and duration (120 min) were selected as the optimal membrane and contact time for all further experiments.

Effect of sample dose

FIG. 4c shows the amount of MCC-CAD-GA thin film vs. Cd²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺Influence of ion adsorption capacity. This characterization is intended to assess the importance of this factor and to determine the relationship between the mass of the adsorbent and the metal ion interaction. As can be seen from fig. 4c, the metal adsorption capacity of the ions is characterized to follow a trend inversely proportional to the MCC-CAD-GA film mass. Cd [ Cd ]²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺The adsorption amounts of ions in 1mg of MCC-CAD-25% GA membrane were 13.872, 17.775, 16.981, 14.162 and 15.147mg g^-1. The high adsorption capacity of these five ions using low MCC-CAD-25% GA membrane doses is mainly due to partial aggregation occurring at high adsorbent content, resulting in a reduction of active sites and hence adsorption. In addition, it was found that an increase in sample dose forces more availability of additional surface area and functional groups for possible binding and adsorption of the target metal ions, while the number of metal ions in the solution remains constant. Thus, Cd when a high dose of MCC-CAD-GA membrane (10mg) was applied during this procedure²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺The metal capacity values of the ions were 1.806, 1.959, 1.542, 1.726 and 1.804mg g, respectively^-1。

Influence of the pH value

FIG. 4d shows pH vs Cd²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺The effect of ion adsorption on the surface of the cross-linked cellulose. From the results shown in fig. 4d, the adsorption capacity of these five ions on the MCC-CAD-GA membrane increases dramatically as the pH of the solution increases from 2 to 6, since the functional groups are deprotonated and can freely exchange or complex metal ions. The pH of the solution is the most important variable affecting the adsorption of metal ions. This is partly because hydrogen ions themselves compete strongly with metal ions. Minimal adsorption potential observed at low pH with H⁺The higher concentration of ions is related to the mobility, which is supported by the high solubility and ionization of the metal salt in acidic media. Cd as the pH value of the solution increases²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺The amount of ions adsorbed on the MCC-CAD-GA membrane surface increased until pH6, as shown in figure 4 d. At low pH values, hydronium ions have high concentrations and greater adsorption tendencies. Therefore, hydronium ions are adsorbed more than other ions. However, as the pH increases, the hydronium ion concentration decreases, resulting in more adsorption of other ions in solution. And at pH>At 6, a decrease in the adsorption of metal ions was observed due to the occurrence of metal precipitation. The metal ions are present in the alkaline medium in the form of hydroxyl species and this minimizes the availability of metal ion species to bind to active sites present on the surface of the MCC-CAD-GA membrane. Thus, it is evident from FIG. 4d that the adsorption process of the metal ions is significantly dependent on the pH of the solution. Cd on MCC-CAD-GA membranes was observed at pH6²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺Maximum adsorption capacity of ions. Therefore, pH6 was chosen as the optimum pH for further adsorption experiments.

Influence of initial ion concentration

Using metal ions of different concentrations, e.g. 0.2, 0.4, 06, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 and 2 mg.L^-1The results are shown in FIG. 5. FIG. 5 shows the adsorption capacity and C₀In an obvious proportional relationship therebetween, C₀The increase in (b) results in a concentration of 0.2 to 1 mg.L^-1The adsorption capacity in the range increases in direct proportion. This behavior is directly related to the presence of large amounts of available metal ions in solution, compared to the active sites available on the MCC-CAD-GA membrane surface. However, at 1 mg. L^-1Cd in MCC-CAD-GA membranes, supra²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺The ion concentration becomes saturated, balancing the adsorption capacity with C₀Is irrelevant.

Recovery study

These five metal ions Cd of the adsorption-desorption cycle are shown in FIG. 6²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺Repeat up to five times. As can be seen from fig. 6, the adsorption capacity of five cycles gradually decreased, but the decrease was small. This behavior may be due to the destruction of the cross-linked network by repeated treatment with acidic ethanol solution during the regeneration of these five metal ion desorption and adsorption sites, in other words, the membrane starts to degrade. For better understanding of desorption and desorption, elemental analysis was performed on the above five ions adsorbed and desorbed on the MCC-CAD-25% GA membrane as shown in FIGS. 7a to 7 j. FIGS. 7 a-7 j show the results of elemental analysis of the initial and desorbed MCC-CAD-GA membranes, showing C and O peaks as the major elemental components. On the other hand, five ions adsorbed on the cross-linked MCC ester membrane are clearly shown. Furthermore, no ion peaks were shown after desorption on the cross-linked MCC ester membrane, which means that the MCC-CAD-25% GA membrane was completely desorbed in ethanolic acid medium. These results indicate that MCC-CAD-GA films with good adsorption properties can be regenerated efficiently and can be reused for adsorption of metal ions at least five times.

Study of kinetics

From fig. 8 it was found that the first order kinetics of these five ions did not correctly explain the adsorption process. By contrast, by t/Q_tThe pseudo-second order kinetics, represented by a linear plot of t (equation (2)), accurately describes the adsorption process. At the same time, Table 2Showing the pseudo-second order kinetic correlation coefficient (R)²>0.999) higher than the pseudo first-order model (R)²<0.990). On the other hand, the Δ Q value of the pseudo quadratic model is lower than that of the pseudo primary model. In other words, Q_e,expIs closer to the value calculated from the pseudo quadratic model. In summary, the results show that the pseudo-secondary kinetic model is best suited to describe the adsorption of these five ions onto the adsorbent of cellulose ester-based films.

TABLE 2 adsorption of Cd on MCC-CAD-25% GA membranes²⁺，Pb²⁺，Cu²⁺，Co²⁺And Ni²⁺Kinetic parameters of ions

Adsorption isotherm

The ability of the adsorbent to adsorb ions and the interaction between the adsorbent and the ions can be described in terms of adsorption isotherms. In this study, the adsorption isotherm models used to quantify adsorption capacity were Langmuir, Freundlich and Elovich isotherms.

FIG. 9 shows Cd on MCC-CAD-GA membrane²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺Adsorption isotherms of the ions ((a) Langmuir, (b) Freundlich and (c) Elovich) and the data are shown in Table 2. As can be seen from fig. 9 and table 3, the adsorption of the above ions by the MCC-CAD-GA membrane is better described by the Langmuir isotherm. This indicates that the active sites on the surface of the MCC-CAD-GA membrane are uniformly distributed. It can be seen from the table that in the case of the Langmuir model, its significant correlation is evident, which determines the factor (R)_L ²>0.999) higher than other isotherm ranges (R)_F ²<0.997，R_E ²<0.999). The exposed carboxylate ensures surface uniformity at the molecular level, based on the chemical structure and properties of the film, and then the Langmuir type is more advantageous. In addition, the value of Freundlich constant (1/n)<1) Indicating the favorable affinity of the metal ion for the adsorbent surface. In summary, the adsorption of these five ions on the cross-linked MCC membrane better matched the Langmuir model. TABLE 3 adsorption of Cd on MCC-CAD-25% GA membranes²⁺、Pb²⁺、Cu²⁺、Co²⁺、Ni²⁺Parameters of isothermal model of ions

And (4) conclusion:

in conclusion, a novel crosslinked cellulose ester film using microcrystalline cellulose, citric anhydride and L-glutamic acid as raw materials, which adsorbs heavy metal ions (Cd) with low concentration in water, has been successfully developed²⁺、Pb²⁺、Cu²⁺、Co²⁺And Ni²⁺Ions)) are used. The increased maximum weight loss temperature indicates improved thermal stability of the film. SEM pictures show favorable adsorption surface morphology. Increased tensile strength and elongation at break, and decreased modulus of elasticity indicates that the film exhibits elasticity and flexibility and is no longer brittle. The adsorption experiment result shows that the room temperature, 2 hours of adsorption and the pH6 are the optimal adsorption conditions, and under the optimal adsorption conditions, Cd²⁺、Pb²⁺、Cd²⁺、Cd²⁺、Ni²⁺The optimum adsorption amounts of (A) are 1.910, 1.982, 1.840, 1.901 and 1.674mg/g, respectively.

The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for adsorbing heavy metal ions in water by using a cellulose ester membrane is characterized in that the cellulose ester membrane is placed in an aqueous solution containing the heavy metal ions, the pH value is adjusted to 2-12, and the cellulose ester membrane is obtained after a period of balance;

2. The method for adsorbing heavy metal ions in water by using the cellulose ester film as claimed in claim 1, wherein the mass ratio of the microcrystalline cellulose to the citric anhydride is 2: 0.9-1.1.

3. The method for adsorbing heavy metal ions in water by using the cellulose ester film as claimed in claim 1, wherein the mass ratio of the microcrystalline cellulose to the glutamic acid is 2: 0.1-0.5.

4. The method for adsorbing heavy metal ions in water by using the cellulose ester film as claimed in claim 1, wherein the step of preparing the cellulose ester film comprises:

5. The method for adsorbing heavy metal ions in water by using the cellulose ester membrane according to claim 4, wherein the solvent of the cellulose solution in the step (1) is lithium chloride and dimethylacetamide;

or, the reaction time in the step (1) is 24-36 h.

6. The method for adsorbing heavy metal ions in water by using the cellulose ester membrane as claimed in claim 4, wherein the reaction condition in the step (2) is 24 +/-1 h at room temperature, wherein the room temperature is 15-30 ℃.

7. The method for adsorbing heavy metal ions in water by using the cellulose ester membrane as claimed in claim 4, wherein the step (3) comprises the following steps: standing the crosslinking solution for 30 + -5 min, adding propanol, standing for 1 + -0.1 h, forming a film, washing with water for 8 + -0.1 h to remove salt, and freeze-drying.

8. The method as claimed in claim 1, wherein the concentration of the heavy metal ion in the aqueous solution containing the heavy metal ion is 0.2-2 mg-L-¹。

9. The method for adsorbing heavy metal ions in water by using the cellulose ester film as claimed in claim 1, wherein the pH is 6 and the equilibrium time is 2 hours.

10. A method for regenerating a cellulose ester film used in the method according to any one of claims 1 to 9, wherein the cellulose ester film having heavy metal ions adsorbed thereon in the method according to any one of claims 1 to 9 is centrifuged in an acidic ethanol solution.