CN116102134B - Multi-membrane stacked electrodialysis system based on modified membrane and chemical salt-containing wastewater treatment method - Google Patents

Abstract

The invention discloses a multi-membrane stacking electrodialysis system based on modified membranes and a method for treating chemical saline wastewater, belonging to the field of water treatment. The system comprises a power supply and an anode, an anode chamber, a cation exchange membrane, a first anion concentration chamber, a repeated negatively charged anion exchange membrane unit, a fresh water chamber, a repeated cation exchange membrane unit, a first cation concentration chamber, a negatively charged anion exchange membrane, a cathode chamber and a cathode, which are arranged in sequence; after dopamine is grafted onto negatively charged monomers on both sides of the anion exchange membrane, the negatively charged anion exchange membrane is obtained; the invention utilizes the different migration rates of inorganic anions and a multi-stage screening mechanism, combines the negatively charged anion exchange membrane, a specifically arranged module unit and an intermittent process combination step, to achieve the separation of monovalent anion inorganic salts, divalent anion inorganic salts and easily ionized organic matter.

Description

Multi-membrane stacking electrodialysis system based on modified membrane and chemical saline wastewater treatment method

Technical Field

The invention belongs to the field of water treatment, and in particular relates to a multi-membrane stacking electrodialysis system based on a modified membrane and a method for treating chemical saline wastewater.

Background technique

In recent years, the chemical industry has developed rapidly worldwide. In the complex production process of chemical products, a large amount of high-salt wastewater is generated. Its main component is sodium chloride, and it also contains impurities such as sulfate and nitrate. In addition, it also contains a large amount of complex and difficult-to-remove organic matter, which poses great environmental hazards. In order to protect the environment and regenerate resources, it is urgent to adopt reasonable disposal technology and resource utilization technology to remove organic pollutants in high-salt wastewater and achieve the purpose of salt separation.

As a highly efficient membrane separation technology for desalination and salt concentration, electrodialysis utilizes the selective permeability of anion and cation exchange membranes and the principle of electric field driving force to separate charged ions. Due to its high water recovery rate, water treatment capacity and flexible operation time, electrodialysis is suitable for a wide range of water treatment occasions such as seawater desalination and industrial wastewater treatment. It has broad application prospects in the corresponding fields and is also a hot topic in membrane separation research in recent years. However, the commonly used electrodialysis technology is mostly used for desalination and concentration and separation of uncharged and difficult to ionize organic matter, but it is difficult to separate charged and easily ionized organic matter.

The Chinese patent document with publication number CN110510712A discloses an electrodialysis system and method for brackish water desalination, including an electrodialysis membrane stack system, a raw water pump, a raw water tank, an anode liquid pump, an anode liquid tank and a DC stabilized power supply. The electrodialysis membrane stack system is formed by connecting a plurality of segmented electrodialysis membrane stacks in sequence. The electrodialysis membrane stack is provided with an anode chamber and a cathode chamber, an anion and cation exchange membrane is provided between the anode chamber and the cathode chamber, the anode chamber is provided with a concentrated water inlet, a fresh water inlet, an anode liquid inlet and an anode liquid outlet, and the cathode chamber is provided with a first outlet, a second outlet, a cathode liquid inlet and a cathode liquid outlet; the electrodialysis membrane stack is equipped with a DC stabilized power supply. The invention adopts non-common electrode connection, and each level of the electrodialysis membrane stack is controlled by a DC stabilized power supply, which can further reduce the energy consumption of the desalination system under the condition of ensuring a certain range of desalination rate, but the above method cannot solve the problem of separating easily ionized organic matter.

The Chinese patent document with publication number CN115448429A discloses a monovalent selective bipolar membrane electrodialysis device, which includes an anode, an anode chamber, a repeating membrane group unit, a monovalent selective anion exchange membrane, a salt chamber, a monovalent selective cation exchange membrane, a cathode chamber, and a cathode; the repeating membrane group unit includes a monovalent selective anion exchange membrane, a salt chamber, a monovalent selective cation exchange membrane, an alkali chamber, a bipolar membrane, and an acid chamber. This invention simplifies the high-salinity wastewater treatment process, but it also cannot solve the problem of separating easily ionized organic matter.

Summary of the invention

In view of the shortcoming that traditional electrodialysis is difficult to separate easily ionized organic matter and inorganic salts in chemical saline wastewater, the present invention provides a multi-membrane stacked electrodialysis system based on modified membranes. By modifying the surface of the anion exchange membrane, the core component of the electrodialysis, and improving the membrane stack structure, the separation of easily ionized organic matter and inorganic salts can be achieved, and high-purity low-valent anion inorganic salts and high-valent anion inorganic salts can be separated respectively.

The specific technical solutions adopted are as follows:

A multi-membrane stacked electrodialysis system based on a modified membrane comprises a power supply and an anode, an anode chamber, a cation exchange membrane, a first anion concentration chamber, a repeated negatively charged anion exchange membrane unit, a fresh water chamber, a repeated cation exchange membrane unit, a cation concentration chamber, a negatively charged anion exchange membrane, a cathode chamber and a cathode which are arranged in sequence;

The repeated negatively charged anion exchange membrane unit comprises at least three negatively charged anion exchange membranes and at least two anion concentration chambers formed between the membranes, and the repeated cation exchange membrane unit comprises at least three cation exchange membranes and at least two cation concentration chambers formed between the membranes;

After the negatively charged monomers are grafted onto the surfaces of both sides of the anion exchange membrane by dopamine, the negatively charged anion exchange membrane is obtained.

The surface of the anion exchange membrane is modified by negatively charged grafting, which can change the positive charge and hydrophilicity and lipophilicity of the surface of the anion exchange membrane. In electrodialysis, it can reduce membrane pollution (reduce the affinity and electrostatic attraction between the membrane and the easily ionized organic matter), reduce the migration of the easily ionized organic matter through the membrane under the action of the electric field force, and achieve the technical effect of separating the easily ionized organic matter from the inorganic salt.

The device of the present invention arranges the cation exchange membrane and the anode correspondingly, and the negatively charged anion exchange membrane and the cathode correspondingly, which can prevent the inorganic salt ions that can be recovered in the wastewater from entering the cathode chamber or the anode chamber to cause product loss.

The anode and cathode are respectively placed in the anode chamber and the cathode chamber at both ends and are connected to an external power supply.

Preferably, the anode and cathode are both titanium-plated ruthenium-iridium electrodes.

Further preferably, the repeated negatively charged anion exchange membrane group unit includes three negatively charged anion exchange membranes, a second anion concentration chamber and a third anion concentration chamber; the repeated cation exchange membrane group unit includes three cation exchange membranes and a second cation concentration chamber and a third cation concentration chamber.

Preferably, the anode chamber, cathode chamber, concentration chamber and desalination chamber are respectively provided with pumps for promoting the circulation of liquid in the chamber.

Preferably, the method for preparing the negatively charged anion exchange membrane comprises the following steps:

(1) depositing a polydopamine interlayer on both sides of the anion exchange membrane; the polydopamine interlayer can increase the smoothness and group grafting rate;

(2) Grafting the negatively charged monomer onto the anion exchange membrane deposited with the polydopamine intermediate layer through surface quaternization reaction or Michael addition reaction to obtain the negatively charged anion exchange membrane.

The negatively charged monomer is at least one of 2-acrylamide-2-methylpropane sulfonic acid, sodium 3-allyloxy-2-hydroxy-1-propane sulfonate, 2-acrylamide dodecane sulfonic acid, methacrylic acid, sodium methacrylate, acrylic acid and sodium acrylate.

The present invention firstly coats a polydopamine intermediate layer on the surface of an anion exchange membrane, and uses the good adhesion between polydopamine and the anion exchange membrane and the rich functional groups on the surface of the polydopamine layer to fix the organic barrier layer (negatively charged monomer monolayer) on the surface of the anion exchange membrane. The polydopamine and the negatively charged monomer are connected by covalent bonds, and the binding force between the polydopamine-negatively charged monomer modified layer and the original membrane is better than that of the polyelectrolyte layer in a general modification method. In addition, the present invention can also adjust the thickness of the polydopamine intermediate layer and the content of functional groups such as amino groups by controlling the deposition conditions, thereby performing personalized customization on the negatively charged anion exchange membrane.

The present invention also provides a method for treating chemical saline wastewater, wherein the chemical saline wastewater comprises monovalent anion inorganic salts, divalent anion inorganic salts and easily ionized organic matter; the method comprises the following steps:

The modified membrane-based multi-membrane stack electrodialysis system is used. First, a sodium sulfate solution is introduced into the anode chamber and the cathode chamber to form a current loop in the modified membrane-based multi-membrane stack electrodialysis system. A monovalent anion inorganic salt solution is introduced into the anion concentration chamber and the cation concentration chamber, and chemical saline wastewater is introduced into the fresh water chamber to perform the first stage of electrodialysis. After the electrodialysis is completed, a monovalent anion inorganic salt is obtained.

Subsequently, the liquids in the anion concentration chamber and the cation concentration chamber are replaced with divalent anion inorganic salt solutions, and the second stage of electrodialysis is carried out; after the electrodialysis is completed, divalent anion inorganic salts are obtained.

In the first stage of electrodialysis, since both the anion concentration chamber and the cation concentration chamber contain monovalent anion inorganic salt solutions, the migration rate of monovalent anions is the fastest. In this stage, monovalent anions preferentially migrate to the first anion concentration chamber to achieve the purification and separation of monovalent anion inorganic salts. In the second stage of electrodialysis, divalent anion inorganic salts and easily ionized organic matter remain in the fresh water chamber, which are separated. Since the migration rate of divalent anions is the fastest, organic anions are blocked by the negatively charged anion exchange membrane, ultimately achieving the purification and separation of divalent anion inorganic salts.

The present invention utilizes the different migration rates of inorganic anions and a multi-stage screening mechanism, combined with negatively charged anion exchange membranes, specifically arranged module units and intermittent process combination steps, to achieve the separation of monovalent anion inorganic salts, divalent anion inorganic salts and easily ionized organic matter.

Preferably, the chemical saline wastewater includes sodium chloride, sodium sulfate and easily ionized organic matter, the concentration of sodium chloride is 0.1-0.5 mol/L, the concentration of sodium sulfate is 0.01-0.1 mol/L, and the concentration of easily ionized organic matter is 0.5-2 mmol/L.

Preferably, the chemical saline wastewater is pretreated before being added to the fresh water chamber; the pretreatment includes coagulation, sedimentation and filtration.

Preferably, a DC electric field is applied to the modified membrane-based multi-membrane stack electrodialysis system for electrodialysis, the voltage is kept constant at 5 to 10 V, and the current density is controlled at 4 to 8 mA/cm ^2. The use of low-voltage DC can avoid the breakdown voltage of the ion exchange membrane caused by excessive power, thereby causing the membrane structure to change and then causing its function to fail. Appropriate power conditions can make the difference in the migration rate of monovalent anions, divalent anions and easily ionized organic matter reach a larger value, thereby achieving a better separation effect.

Preferably, the flow rate of the liquid in the anode chamber, cathode chamber, concentration chamber and desalination chamber is 75-150 mL/min, and the liquid circulates in the chamber for 15 minutes before applying the DC electric field.

Compared with the prior art, the present invention has the following beneficial effects:

(1) The present invention specifically modifies the anion exchange membrane, which is the core component of electrodialysis, to make it negatively charged, and constructs a multi-membrane stacked electrodialysis system based on the modified membrane. The superposition of electrostatic effects, multi-stage screening effects, and differences in ion migration rates of the stacked negatively charged anion exchange membranes are utilized to achieve the separation of low-valent anion inorganic salts, high-valent anion inorganic salts, and easily ionized organic matter.

(2) The present invention provides a method for treating chemical saline wastewater, which mainly comprises two stages: in the first stage, electrodialysis is used to preferentially separate low-valent anion inorganic salts from the wastewater; before the second stage, the concentrate in each concentration chamber is replaced with high-valent anion inorganic salts to facilitate separation of high-valent anion inorganic salts from the wastewater in the second stage; and intermittent processes are combined to recover high-purity low-valent anion inorganic salts and high-valent anion inorganic salts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of the multi-membrane stacked electrodialysis system based on the modified membrane.

FIG2 is a SEM image of the anion exchange membrane and the negatively charged anion exchange membrane in Example 1, wherein (a) is the anion exchange membrane and (b) is the negatively charged anion exchange membrane.

FIG3 is a schematic diagram of the process for treating chemical saline wastewater using the multi-membrane stacked electrodialysis system based on the modified membrane.

FIG. 4 is a graph showing changes in ion concentrations in the first anion concentration chamber in Example 2, wherein (a) is a graph showing changes in concentrations of organic anions, and (b) is a graph showing changes in concentrations of chloride ions and sulfate ions.

FIG. 5 is a graph showing changes in ion concentrations in the first anion concentration chamber in Example 3, wherein (a) is a graph showing changes in organic anion concentrations, and (b) is a graph showing changes in sulfate ion concentrations.

FIG6 is a diagram showing the ion concentration distribution of the fresh water chamber and each anion concentration chamber after the electrodialysis is completed.

Figure numerals: 1—power supply, 2—anode, 21—anode chamber, 3—cation exchange membrane, 31—first anion concentration chamber, 4—repeated negatively charged anion exchange membrane group unit, 32—second anion concentration chamber, 33—third anion concentration chamber, 45—fresh water chamber, 5—repeated cation exchange membrane group unit, 63—third cation concentration chamber, 62—second cation concentration chamber, 61—first cation concentration chamber, 6—negatively charged anion exchange membrane, 71—cathode chamber, 7—cathode.

Detailed ways

The present invention is further illustrated below in conjunction with the examples and drawings. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention. The operating methods in the following examples where specific conditions are not specified are usually carried out under conventional conditions or under conditions recommended by the manufacturer; both the anion exchange membrane and the cation exchange membrane are purchased from Hangzhou Lvhe Environmental Protection Technology Co., Ltd.

As shown in Figure 1, the multi-membrane stack electrodialysis system based on the modified membrane provided by the present invention includes a power supply 1 and an anode 2, an anode chamber 21, a cation exchange membrane 3, a first anion concentration chamber 31, a repeated negatively charged anion exchange membrane membrane group unit 4, a fresh water chamber 45, a repeated cation exchange membrane membrane group unit 5, a first cation concentration chamber 61, a negatively charged anion exchange membrane 6, a cathode chamber 71 and a cathode 7 arranged in sequence; wherein the repeated negatively charged anion exchange membrane membrane group unit 4 includes three negatively charged anion exchange membranes 6, a second anion concentration chamber 32 and a third anion concentration chamber 33; the repeated cation exchange membrane membrane group unit 5 includes three cation exchange membranes 3, a second cation concentration chamber 62 and a third cation concentration chamber 63.

The anode 2 and cathode 7 are respectively placed in the anode chamber and cathode chamber at both ends, and are connected to an external power supply 1. The anode 2 and cathode 7 are preferably titanium-plated ruthenium-iridium electrodes, which have better conductivity and corrosion resistance than ordinary electrodes and have a longer service life.

The anode chamber, cathode chamber, concentration chamber and fresh water chamber are respectively provided with pumps to promote the circulation of liquid in the chamber, which is beneficial to the electrodialysis process.

After the negatively charged monomers are grafted onto the surfaces of both sides of the anion exchange membrane by dopamine, the negatively charged anion exchange membrane 6 is obtained.

Example 1 Preparation of negatively charged anion exchange membrane

Specifically, in the preparation process of the negatively charged anion exchange membrane of this embodiment, the negatively charged monomer is selected from 2-acrylamide-2-methylpropane sulfonic acid;

First, a dopamine solution with a concentration of 2 g/L is prepared, and an anion exchange membrane is immersed in the above dopamine solution for 24 hours, and an anion exchange membrane with a polydopamine intermediate layer is obtained through dopamine self-polymerization reaction; then a 1wt% 2-acrylamide-2-methylpropanesulfonic acid negatively charged monomer solution is prepared, and the anion exchange membrane with the polydopamine intermediate layer is immersed in the negatively charged monomer solution for grafting reaction (Michael addition reaction) for 120 minutes to obtain the negatively charged anion exchange membrane; scanning electron microscope images of the unmodified anion exchange membrane and the negatively charged anion exchange membrane are shown in Figure 2, wherein (a) is an anion exchange membrane, and (b) is a negatively charged anion exchange membrane. It can be seen that the surface structure of the membrane has undergone certain changes before and after modification.

The present invention coats a polydopamine intermediate layer on the surface of an anion exchange membrane, utilizes the good adhesion between polydopamine and the anion exchange membrane and the abundant functional groups on the surface of the polydopamine layer to fix the organic barrier layer on the surface of the anion exchange membrane, and controls the coating conditions to adjust the thickness of the polydopamine intermediate layer and the surface functional group content.

The surface of the anion exchange membrane is modified by negatively charged grafting, which can change the positive charge and hydrophilicity and lipophilicity of the surface of the anion exchange membrane. In electrodialysis, it can reduce membrane pollution (reduce the affinity and electrostatic attraction between the membrane and the easily ionized organic matter), reduce the migration of the easily ionized organic matter through the membrane under the action of the electric field force, and achieve the technical effect of separating the easily ionized organic matter from the inorganic salt.

Example 2

The multi-membrane stacked electrodialysis system based on the modified membrane is used to treat simulated chemical saline wastewater, where the simulated chemical saline wastewater is homemade in the laboratory and contains 0.2 mol/L sodium chloride, 0.05 mol/L sodium sulfate and 1 mmol/L trisodium 1,3,5-trisulfonate;

As shown in FIG3 , the first stage of electrodialysis is first performed. In the modified membrane-based multi-membrane stack electrodialysis system, 0.1 mol/L sodium sulfate solution is introduced into the anode chamber and the cathode chamber as an electrolyte to form a current loop in the system. Similarly, 0.2 mol/L sodium chloride solution is circulated in the first anion concentration chamber, the second anion concentration chamber and the third anion concentration chamber, and 0.2 mol/L sodium chloride solution is circulated in the first cation concentration chamber, the second cation concentration chamber and the third cation concentration chamber to form a current path. After the solution is introduced into each chamber, the peristaltic pump is turned on at a flow rate of 100 mL/min. The solution is allowed to flow fully for 15 minutes, and a DC power supply is turned on to perform the first stage of electrodialysis. The voltage is kept constant at 5 V and the current density is 4 mA/cm ² . The voltage and current changes at both ends of the modified membrane-based multi-membrane stack electrodialysis system are monitored online, and the conductivity changes of each chamber are monitored using an online conductivity meter.

During the operation, 1 mL of sample was taken from each chamber every hour. The concentration changes of inorganic anions in each chamber during the electrodialysis operation were analyzed by ion chromatography, and the concentration changes of organic anions in each chamber were analyzed by liquid chromatography. The specific results are shown in (a) and (b) in Figure 4. After 18 hours of desalination and separation, the chloride ion concentration in the first anion concentration chamber increased from 0.2 mol/L to 0.42 mol/L, the sulfate ion concentration increased from 0 to 0.0021 mol/L, and the organic matter concentration increased from 0 to 0.005 mmol/L.

After the desalination separation is completed, the peristaltic pump corresponding to the first anion concentration chamber is reversely circulated to pump out until the solution in the chamber is completely pumped out into the container. The solution is a sodium chloride solution, and the purity of the sodium chloride contained therein is 98.8%.

Example 3

As shown in FIG3 , after the sodium chloride solution in each ion concentration chamber is discharged through the peristaltic pump of the independent water circulation system, it is replaced with a 0.018 mol/L sodium sulfate solution and circulated independently for 15 minutes;

The second stage of electrodialysis was carried out. At this time, the fresh water chamber contained the simulated chemical saline wastewater solution remaining after the first stage of electrodialysis, wherein the chloride ion concentration was 0.002 mol/L, the sulfate ion concentration was 0.018 mol/L, and the organic matter concentration was 0.85 mmol/L. As in Example 2, 0.1 mol/L sodium sulfate solution was circulated in the anode chamber and the cathode chamber as an electrolyte to form a current loop in the system. Similarly, 0.018 mol/L sodium sulfate solution was circulated in the first anion concentration chamber, the second anion concentration chamber, and the third anion concentration chamber, and 0.018 mol/L sodium sulfate solution was circulated in the first cation concentration chamber, the second cation concentration chamber, and the third cation concentration chamber to form a current path. After the solution was introduced into each chamber, the peristaltic pump was turned on at a flow rate of 100 mL/min. The solution was allowed to flow fully for 15 minutes, and a DC power supply was turned on to carry out the second stage of electrodialysis, keeping the voltage constant at 5 V and the current density at 4 mA/cm ² , online monitoring of the voltage and current changes at both ends of the multi-membrane stack electrodialysis system based on the modified membrane, and using an online conductivity meter to monitor the conductivity changes of each chamber;

During the operation, 1 mL of sample was taken from each chamber every hour. The changes in the concentration of inorganic anions in each chamber during the electrodialysis operation were analyzed using an ion chromatograph, and the changes in the concentration of organic anions in each chamber were analyzed using liquid chromatography. The specific results are shown in (a) and (b) in Figure 5. After 10 hours of desalination and separation, the sulfate ion concentration in the first anion concentration chamber increased from 0.018 mol/L to 0.039 mol/L, and the organic matter concentration increased from 0 to 0.009 mmol/L.

After the desalination separation is completed, the peristaltic pump corresponding to the first anion concentration chamber is reversely circulated to extract until the solution in the chamber is completely extracted into the beaker. The solution is a sodium sulfate solution, and the purity of the sodium sulfate contained is 99.9%.

After the two-step electrodialysis was completed, the ion concentration distribution diagram of the fresh water chamber and each anion concentration chamber was statistically analyzed, as shown in Figure 6. The final chloride ion concentration in the fresh water chamber was 0.001 mol/L, and the sulfate ion concentration was 0.002 mol/L, which proved that a large number of chloride ions in the fresh water chamber migrated to the first anion concentration chamber, and a large number of sulfate ions also migrated to the first anion concentration chamber; and due to the layer-by-layer electrostatic effect of the negative charge on the negatively charged anion exchange membrane, the organic anions were blocked in the fresh water chamber and the third anion concentration chamber and the second anion concentration chamber. The concentrations of organic anions in the fresh water chamber, the third anion concentration chamber, the second anion concentration chamber and the first anion concentration chamber were 0.6 mmol/L, 0.17 mmol/L, 0.04 mmol/L, and 0.009 mmol/L, respectively, which ensured that there was almost no organic matter in the first anion concentration chamber and the high purity of the collected inorganic salts.

It can be seen from the above results that by designing negatively charged anion exchange membranes, specifically arranged module units and intermittent process combination steps, and utilizing the electrostatic repulsion superposition of negatively charged anion exchange membranes, multi-stage screening effects and differences in the migration rates of various ions, not only the separation of easily ionized organic matter and inorganic salts can be achieved, but also the separation of low-valent salts and high-valent salts such as monovalent anion inorganic salts and divalent anion inorganic salts can be achieved, which greatly simplifies the process flow and reduces costs.

The embodiments described above provide a detailed description of the technical solutions of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements or similar substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims (8)

1. A method for treating chemical saline wastewater, characterized in that a multi-membrane stacked electrodialysis system based on a modified membrane is used. The modified membrane-based multi-membrane stack electrodialysis system comprises a power supply (1) and an anode (2), an anode chamber (21), a cation exchange membrane (3), a first anion concentration chamber (31), a repeated negatively charged anion exchange membrane module (4), a fresh water chamber (45), a repeated cation exchange membrane module (5), a first cation concentration chamber (61), a negatively charged anion exchange membrane (6), a cathode chamber (71) and a cathode (7) arranged in sequence; The repeated negatively charged anion exchange membrane group unit (4) comprises at least three negatively charged anion exchange membranes (6) and at least two anion concentration chambers formed between the membranes, and the repeated cation exchange membrane group unit (5) comprises at least three cation exchange membranes (3) and at least two cation concentration chambers formed between the membranes; After the negatively charged monomers are grafted onto the surfaces of both sides of the anion exchange membrane by dopamine, the negatively charged anion exchange membrane (6) is obtained; The chemical saline wastewater includes monovalent anion inorganic salts, divalent anion inorganic salts and easily ionized organic matter; The method for treating chemical saline wastewater comprises the following steps: First, a sodium sulfate solution is introduced into the anode chamber and the cathode chamber to form a current loop in the multi-membrane stack electrodialysis system based on the modified membrane; a monovalent anion inorganic salt solution is introduced into the anion concentration chamber and the cation concentration chamber, and chemical saline wastewater is introduced into the fresh water chamber to perform the first stage of electrodialysis; after the electrodialysis is completed, a monovalent anion inorganic salt is obtained; Subsequently, the liquid in the anion concentration chamber and the cation concentration chamber is replaced with a divalent anion inorganic salt solution, and a second stage of electrodialysis is performed; after the electrodialysis is completed, a divalent anion inorganic salt is obtained; A direct current electric field is applied to the multi-membrane stack electrodialysis system based on the modified membrane to perform electrodialysis, the voltage is kept constant at 5-10 V, and the current density is controlled at 4-8 mA/cm ² . 2. The method for treating chemical saline wastewater according to claim 1, characterized in that the anode (2) and cathode (7) are respectively placed in the anode chamber and cathode chamber at both ends and are externally connected to a power supply (1). 3. The method for treating chemical saline wastewater according to claim 1, characterized in that the anode (2) and cathode (7) are both titanium-plated ruthenium-iridium electrodes. 4. The method for treating chemical saline wastewater according to claim 1 is characterized in that the repeated negatively charged anion exchange membrane group unit (4) includes three negatively charged anion exchange membranes (6), a second anion concentration chamber (32) and a third anion concentration chamber (33); the repeated cation exchange membrane group unit (5) includes three cation exchange membranes (3), a second cation concentration chamber (62) and a third cation concentration chamber (63). 5. The method for treating chemical saline wastewater according to claim 1 is characterized in that pumps for promoting the circulation of liquid in the anode chamber, cathode chamber, concentration chamber and desalination chamber are respectively provided. 6. The method for treating chemical saline wastewater according to claim 1, characterized in that the preparation method of the negatively charged anion exchange membrane comprises the following steps: (1) Depositing a polydopamine intermediate layer on both sides of the anion exchange membrane; (2) grafting the negatively charged monomer onto the anion exchange membrane deposited with the polydopamine intermediate layer through a surface quaternization reaction or a Michael addition reaction to obtain the negatively charged anion exchange membrane; The negatively charged monomer is at least one of 2-acrylamide-2-methylpropane sulfonic acid, sodium 3-allyloxy-2-hydroxy-1-propane sulfonate, 2-acrylamide dodecane sulfonic acid, methacrylic acid, sodium methacrylate, acrylic acid and sodium acrylate. 7. The method for treating chemical saline wastewater according to claim 1, characterized in that the chemical saline wastewater comprises sodium chloride, sodium sulfate and easily ionized organic matter, the concentration of sodium chloride is 0.1~0.5 mol/L, the concentration of sodium sulfate is 0.01~0.1 mol/L, and the concentration of easily ionized organic matter is 0.5~2 mmol/L. 8. The method for treating chemical saline wastewater according to claim 1, characterized in that the flow rate of the liquid in the anode chamber, cathode chamber, concentration chamber and desalination chamber is 75-150 mL/min.