Bipolar membrane electrodialysis technology and its application (preliminary discussion)

1、 Bipolar membrane electrodialysis technology:
It is an efficient separation and conversion technology based on ion exchange membranes, which has wide applications in the fields of electrochemical engineering and environmental engineering. The core component of this technology is a bipolar membrane, which is a special composite membrane structure composed of a cation exchange layer (N-type membrane), an interfacial hydrophilic layer (catalytic layer), and an anion exchange layer (P-type membrane). Under the action of a direct current electric field, bipolar membranes can decompose water into hydrogen ions (H+) and hydroxide ions (OH -), which migrate in opposite directions. When a bipolar membrane is combined with a unipolar membrane (i.e. a cation exchange membrane that only allows one type of ion to pass through), salts in the solution (such as inorganic salts such as sodium sulfate and sodium chloride) can be converted into corresponding acids (such as sulfuric acid and hydrochloric acid) and bases (such as sodium hydroxide and sodium hydroxide) without adding other chemical reagents. This process not only achieves effective separation of anions and cations in salt solutions, but also generates high-purity acid-base products, which have significant economic benefits and environmental value. Bipolar membrane electrodialysis technology has important application potential in many fields such as wastewater treatment, resource recovery, and chemical production due to its high efficiency, energy conservation, and environmental friendliness. For example, it has been promoted and used as a clean production technology in the production process of organic acids, amino acids, vitamin C, and other products. In addition, it can also be used for various industrial process transformations and optimizations such as desulfurizer regeneration, silica sol production, and phenol sodium solution treatment.
The origin of bipolar membrane electrodialysis technology can be traced back to the 1960s and 1970s. During this period, the United States was the first to develop bipolar membranes. Subsequently, in the 1980s, this technology was practically applied and first industrially deployed at a steel plant in Washington, USA, for the treatment of potassium nitrate and potassium fluoride systems. With the passage of time and technological progress, bipolar membrane electrodialysis technology has gradually been adopted by European countries and has been diffused and further developed in Finland, France, Germany, Italy, the Netherlands and other places. Nowadays, this technology has become an important component of the field of electrodialysis. It is not only applied to the acid-base separation and conversion of traditional inorganic saline solutions, but also plays an important role in wastewater treatment, resource recovery, and many chemical production processes, demonstrating broad application prospects and industrial potential.

2、 The main components of the bipolar membrane electrodialysis system are:
1. Bipolar Membrane: The bipolar membrane is the core component of the system, usually composed of three layers: cation exchange layer, intermediate catalytic layer, and anion exchange layer. Under the action of a direct current electric field, bipolar membranes can decompose water into hydrogen ions and hydroxide ions. 2. Anion cation exchange membrane: The cation exchange membrane only allows cations in the solution to pass through and prevents the migration of anions; Anion exchange membranes, on the other hand, only allow anions to pass through and block cations. 3. Spacer: used to separate the anion exchange membrane and bipolar membrane, forming multiple independent water chambers. The partition is equipped with water distribution holes, water distribution channels, and other structures to ensure uniform distribution of solutions and their passage through various membrane stacks. 4. Membrane Stack: A membrane stack is a unit composed of several membrane pairs (including anion exchange membranes and bipolar membranes) assembled in a specific order. It is the core component of an electrodialysis device, which includes an acid chamber, a salt chamber, and a base chamber. 5. Electrodes: including the anode and cathode, which are connected to a power source to provide the potential difference required for driving ion migration, and must have good conductivity, corrosion resistance, and stability. 6. Cell Frame: The cell frame is placed between the electrode and the membrane to support the membrane stack and prevent direct contact between the membrane and the electrode, causing damage or short circuit. 7. Rectifier: The rectifier converts alternating current into direct current and supplies it to the electrodialysis system, controlling the magnitude and direction of the current. 8. Pipes and valves: inlet and outlet pipelines, circulation pumps, flow control valves, etc., to ensure the flow and circulation of the solution. 9. Control system: Monitor system parameters such as temperature, pressure, flow rate, current and voltage, and perform automated control to ensure safe and efficient operation of the system. 10. Compression device: used to compress membrane stacks and other components, maintaining the airtightness and stability of the entire system, and avoiding leakage or loosening caused by internal pressure changes. In summary, the bipolar membrane electrodialysis system is an integrated device composed of multiple precision components that work together to effectively separate and convert ions in the solution.
3、 Classification of common bipolar membrane electrodialysis systems: 1 Modular design: Standardized bipolar membrane electrodialysis unit module: composed of multiple membrane stacks, each stack containing multiple alternating layers of cation exchange membranes and bipolar membranes, combined in series or parallel to form a system with different processing capabilities. 2. Continuous flow and batch processing system: Continuous flow system: suitable for industrial production processes with continuous feeding and discharging, such as continuous preparation of acid and alkali from chemical intermediates. Batch processing system: suitable for intermittent treatment or small-scale production, such as laboratory research and small-scale wastewater treatment. 3. Fixed bed and rotating disc: Fixed bed electrodialysis: The most common form is a fixed membrane component, where the solution flows through the membrane stack for separation. Rotating disc electrodialysis machine (although not commonly used in bipolar membrane electrodialysis, it has applications in certain fields): improves mass transfer efficiency and equipment compactness by rotating the disc. 4. Integrated solution: Customized bipolar membrane electrodialysis systems for specific industries, such as pharmaceuticals, food processing, metallurgy, water treatment, etc., may include a complete set of devices including pre-treatment, post-treatment, and control systems. 5. Multi functional combination system: The combination of bipolar membrane electrodialysis and other separation technologies, such as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), etc., forms a more comprehensive resource recovery or wastewater treatment process. The choice of bipolar membrane electrodialysis system depends on the specific application goals, such as whether it is used for converting saltwater into acid-base, treating high concentration saline wastewater, recycling organic acids and inorganic salts, or other special separation and purification tasks.
4、 The working principle of bipolar membrane electrodialysis technology: 1 Bipolar membrane structure: A bipolar membrane is a special composite membrane material composed of three layers: a cation exchange layer (N-type membrane), an interfacial hydrophilic layer (catalytic layer), and an anion exchange layer (P-type membrane). This membrane has the ability to decompose water into hydrogen ions (H+) and hydroxide ions (OH -) under the action of a direct current electric field. 2. Electric field driven ion migration: When the bipolar membrane electrodialysis system is powered on, under the action of a DC electric field, charged ions in the solution begin to selectively pass through the corresponding ion exchange membrane. The cation moves towards the negative (cathode) direction and enters the concentration chamber through the cation exchange membrane; Anions move towards the positive electrode (anode) and enter the concentration chamber on the other side through the anion exchange membrane. 3. Water decomposition generates acid-base: Within the bipolar membrane, due to the influence of an external electric field, water molecules are dissociated into H+and OH - ions in the catalytic layer. H+passes through the cation exchange layer to reach the cathode side, while OH - passes through the anion exchange layer to reach the anode side, forming acidic and alkaline environments, respectively. 4. Salt water desalination and resource recovery: In the desalination room, the anions and cations in the saline water are continuously removed, resulting in fresh water that has removed most of the salt. In the concentration chamber, as more and more H+and OH - aggregate, they combine with the existing cations or anions in the solution to generate corresponding acid and alkali products. For example, sodium chloride (NaCl) can be converted into hydrochloric acid (HCl) and sodium hydroxide (NaOH). In summary, bipolar membrane electrodialysis technology effectively achieves selective separation of electrolytes in aqueous solutions through cleverly designed composite membrane structures and electrochemical reaction processes, and can convert inorganic salts into acids and bases, providing an efficient and sustainable technological solution for many industrial processes.
5、 Factors affecting the water quality of bipolar membrane electrodialysis technology:
1. Desalination efficiency: Bipolar membrane electrodialysis technology can effectively remove monovalent cations (such as Na+, K+) and anions (such as Cl -, SO4 ^ 2-) in raw water, thus achieving a high desalination effect. The total dissolved solids (TDS) of produced water is usually significantly reduced, resulting in low salinity or even near pure water quality.
2. Organic matter and impurity removal: For the presence of organic matter, trace heavy metals, and other harmful substances in the influent, if an appropriate pre-treatment system is configured in the equipment design, and the bipolar membrane itself has a certain interception effect on these impurities, the organic matter content and impurity level in the produced water can be controlled within a lower range.
3. Acid base generation: When the goal is to convert salts into acids and bases, the produced water may no longer be neutral water, but instead form corresponding acidic or alkaline solutions based on the reaction process.
4. Stability and continuous operation: The stability and continuous operation management of bipolar membrane electrodialysis systems also have a direct impact on the water quality of the produced water. For example, changes in operating current, voltage, influent flow rate, and water quality can affect system performance and the purity and consistency of the final produced water.
5. Residual ion concentration: Although bipolar membranes are mainly used for monovalent salt systems, in actual operation, there may be a small amount of divalent or more high valent cations penetrating or adsorbing on the membrane. Therefore, the ideal water quality requires that the concentration of these cations be extremely low, generally less than 1ppm.
In summary, bipolar membrane electrodialysis technology can provide high-quality water production, especially in the fields of desalination and acid-base preparation, but specific water quality parameters need to be determined based on specific process design, operating conditions, and influent water quality. In order to ensure that the water quality of the produced water meets the needs of users, strict pretreatment and real-time monitoring of the operating status are usually required.
6、 Scope and application areas of bipolar membrane electrodialysis technology: 1 Conversion and recovery of inorganic salts: In the treatment of high salt wastewater, inorganic salts such as sodium chloride, sodium sulfate, and sodium nitrate are converted into corresponding acids (such as HCl, H2SO4) and bases (such as NaOH), achieving zero emissions while recovering valuable chemicals. 2. Preparation of organic acid salts and organic base salts: It can be used for the conversion of organic acid salts or organic base salts generated in the food industry, pharmaceutical industry, and fine chemical production processes, such as decomposing sodium tartrate, sodium citrate, sodium gluconate, etc. into corresponding organic acids and bases. 3. Resource recycling and utilization: In industries such as coal chemical industry, bipolar membrane electrodialysis technology can be applied to further treat nanofiltration produced water or concentrated water, extracting and purifying useful chemical substances from wastewater. 4. Environmental protection: Desalination of low concentration brackish water, although relatively high energy consumption for large-scale seawater desalination projects, is still effective under specific conditions. The removal and recovery of heavy metal ions and other harmful substances in sewage treatment. 5. New energy and energy storage: In some cases, it can be used in fields such as proton exchange membrane fuel cells (PEMFCs) and water electrolysis tanks in the process of electrolyzing water to produce hydrogen. 6. Other industrial applications: - Product purification in the food and beverage industry;
-Ultra pure water manufacturing in the semiconductor industry; -Separation and purification of intermediates in chemical synthesis processes; -Regeneration or product separation of enzyme reaction solution in biochemical engineering. The bipolar membrane electrodialysis technology, due to its ability to directly decompose water molecules into acids and bases under the action of an electric field, is widely used in various situations that require acid-base separation, salt conversion, and resource recycling.
7、 The main functions and functions of bipolar membrane electrodialysis technology:
1. Ion separation and conversion: Under the action of a DC electric field, bipolar membranes can directly electrolyze water molecules into hydrogen ions (H+) and hydroxide ions (OH -), thereby achieving the separation of anions and cations in aqueous solutions. When combined with other unipolar membranes (cation exchange membranes and anion exchange membranes), inorganic salts such as sodium chloride and sodium sulfate can be converted into corresponding acids (such as hydrochloric acid and sulfuric acid) and bases (such as sodium hydroxide and potassium hydroxide).
2. Resource recovery: In the treatment of high salt wastewater or chemical wastewater, bipolar membrane electrodialysis can effectively remove and convert salt, achieving the recovery and reuse of salt resources while reducing the impact on the environment.
3. Environmental protection treatment: This technology is widely used in various industrial wastewater treatment fields, helping enterprises achieve zero discharge of wastewater through efficient desalination and the conversion of harmful substances, and in some cases, it can generate valuable by-products.
4. Chemical production: In organic and inorganic chemical processes, it is used to prepare high-purity acids, bases, and other compounds, replacing traditional chemical synthesis methods and reducing waste generation.
5. Clean production: In industries such as food processing and pharmaceuticals, pure organic acids and corresponding bases can be separated from solutions containing organic acid salts through bipolar membrane electrodialysis technology, promoting the development of green manufacturing and circular economy.
In summary, bipolar membrane electrodialysis technology plays an important role in environmental protection, resource recovery, and the preparation of various chemicals due to its unique chemical reactivity and separation performance.
8、 The main impact of pretreatment on bipolar membrane electrodialysis technology:
1. Reduce membrane fouling: If suspended solids, colloids, organic matter, and microorganisms in water are not removed through pre-treatment, they will directly adhere to the surface of bipolar membranes and supporting cation exchange membranes and anion exchange membranes, forming a fouling layer, leading to a decrease in membrane flux and separation efficiency.
-Pre treatment can reduce the blockage of membrane pores caused by these impurities and maintain high membrane permeability.
2. Prevent concentration polarization: When untreated influent contains a large amount of interceptable substances, a concentration polarization layer is easily formed on the membrane surface, increasing local resistance, increasing energy consumption, and may accelerate membrane performance degradation. Pre treatment can effectively control the concentration of particulate matter in the influent, thereby reducing the occurrence of concentration polarization phenomenon.
3. Extending membrane life: Membrane materials are very sensitive to certain chemicals (such as inorganic salts with high hardness, heavy metal ions, or strong oxidizing/reducing substances). Pre treatment can reduce the damage of harmful substances to the membrane and extend the service life of membrane components through softening, turbidity removal, deoxygenation, decolorization, and other steps.
4. Optimize current efficiency and energy consumption: Pre treatment can reduce the concentration of impurities in the solution that cause incomplete or side reactions in electrolysis, improve the current efficiency in the electrodialysis process, and thus save energy consumption.
5. Ensure safety and stability: For some specific industrial wastewater, such as wastewater containing acid, alkali, toxic and harmful substances, pre-treatment can reduce these substances to a safe range, ensuring the safe and stable operation of the entire electrodialysis system.
Good pre-treatment can significantly improve the practical application effect of bipolar membrane electrodialysis technology, which contributes to its economy and reliability in long-term operation. Common pretreatment methods include sedimentation, coagulation, filtration, adsorption, biodegradation, disinfection, and sterilization.
9、 Common pretreatment methods for bipolar membrane electrodialysis technology:
1. Precipitation method: By adding chemical agents such as lime, aluminum sulfate, etc., heavy metal ions and partially soluble solids (such as calcium, magnesium, iron, manganese, etc.) in water are formed into insoluble precipitates, which are then removed by sedimentation or filtration. 2. Coagulation and flocculation: Use coagulants and coagulants to aggregate suspended particles and colloidal substances in water into larger flocs, facilitating subsequent solid-liquid separation processes. 3. Filtration: After coagulation and sedimentation, physical filtration is carried out using sand filters, multimedia filters, etc. to remove impurities