Industrial wastewater treatment is an inevitable product in industrial production and manufacturing processes. With the development of the social economy, the proportion of water used in industry continues to increase, resulting in a huge amount of wastewater generated. The demand for industrial wastewater treatment is very urgent. Ion exchange membranes can achieve wastewater treatment by selectively permeating ions, and their current application technology is highly efficient, mature, and environmentally friendly. Therefore, they are widely used in the chemical industry, wastewater treatment, and seawater desalination. In previous articles, we introduced ion exchange membranes involved in the battery industry, mainly proton exchange membranes and anion exchange membranes for batteries. We also reviewed the application of ion exchange membranes in electrolysis of water. This article will focus on the application of ion exchange membranes in water treatment.

In the field of water treatment, such as metal ion wastewater treatment, there are many methods and means currently used. Traditional methods for removing metal ions from wastewater include non membrane treatment methods such as precipitation and solution extraction, as well as membrane water treatment methods such as reverse osmosis, nanofiltration, and electrodialysis. Among them, the electrodialysis method using ion exchange membranes is currently the most researched and promising.

Ion exchange membrane is a type of membrane composed of polymer materials, which consists of a polymer skeleton and exchange groups. Essentially, it is an ion exchange resin containing ionizable groups and has selective permeability to ions in solution. Ion exchange membranes can be divided into cation exchange membranes, anion exchange membranes, and zwitterionic ion exchange membranes based on the types of ions they pass through. The different functions of ion exchange membranes are mainly due to the different ion exchange groups contained in them. Cation exchange membranes contain a large number of anionic groups, which can achieve directional attraction to cations due to charge interactions, while anions are specifically repelled. Anion exchange membranes, on the other hand, have the opposite effect. For zwitterionic membranes, the active groups of anions and cations are uniformly distributed on the surface of the exchange membrane, forming a bipolar membrane. At present, the widely recognized working mechanisms of ion exchange membranes in the academic community mainly include the double layer theory, hole transfer theory, and Donnan equilibrium theory.

In addition, ion exchange membranes can also be classified according to their structure. According to membrane structure, it can be divided into three categories: heterogeneous membranes, homogeneous membranes, and semi homogeneous membranes [1]. Heterogeneous membrane, also known as heterogeneous membrane, is mainly formed by mixing ion exchange agents and binders, and then rolling them into a thin film of about 0.3 millimeters after a certain process of treatment. Combined with actual situations, it is pressed with different quantities of reinforced mesh. Its biggest feature is that the ion exchange groups and binders inside the membrane are often difficult to maintain consistency and continuity in chemical structure due to their simple mixing. Therefore, heterogeneous membranes often have simpler processes, but the resulting film has a higher membrane resistance and poorer selectivity.

Homogeneous films are formed by polymerizing monomers into polymer films and then functionalizing them directly, or functionalizing them before coating them into films. In the preparation process of homogeneous membranes, chemical bonding often occurs between ion exchange groups and film-forming materials, resulting in superior physical and electrochemical properties, which is currently the mainstream research direction. A semi homogeneous membrane, on the other hand, is a type of membrane that falls between a homogeneous membrane and an heterogeneous membrane in terms of function and structure. The ion exchange groups within the membrane are also evenly distributed, but their connection with the film-forming material is not a chemical connection in a homogeneous membrane.

Of course, ion exchange membranes can also be classified according to the type of material, such as organic ion exchange membranes and inorganic ion exchange membranes, depending on their composition. The perfluorosulfonic acid proton exchange membrane and quaternary ammonium anion exchange membrane mentioned in our previous article are both organic ion exchange membranes synthesized from polymer materials.

The main method for using ion exchange membranes in water treatment is electrodialysis. Electrodialysis can be simply summarized as the process of separating charged ions in water driven by an external electric field, and under the action of an ion exchange membrane, the directional migration and separation of ions in the solution can be achieved through different electric field gradients, thereby achieving the goal of water treatment. During this process, due to the different properties of ion exchange membranes, it is possible to achieve targeted migration of specific ions to the vicinity of an electrode electrolyte, thereby forming two electrolyte chambers with different ion concentrations: a concentration chamber and a dilution chamber.

Schematic diagram of electrodialysis device

Taking the removal of fluoride ions from drinking water as an example [2], fluoride ions are a monovalent anion that can pass through anion exchange membranes (AEMs), but not through cation exchange membranes (CEMs). In the process of electrodialysis, as shown in the figure below, in the electrodialysis chamber where the anion exchange membrane and cation exchange membrane are alternately placed, a negatively charged fluoride ion moves towards the anode under the action of the electric field (from right to left), but cannot pass through the cation exchange membrane. Therefore, under the action of the electric field, a concentration chamber containing high concentrations of fluoride ions, as shown by the red line in the figure, is formed. At the same time, sodium ions present in the water move towards the cathode and enter the concentration chamber, which can result in a very low ion concentration in the electrolyte shown by the green line, forming a dilution chamber and achieving the purpose of water treatment.

Schematic diagram of an electrodialysis device for removing fluoride ions from drinking water

In water treatment, ion exchange membranes need to have certain performance and requirements in order to be better applied in water treatment. These requirements can be summarized in three aspects: high mechanical performance, outstanding electrochemical performance, and excellent ion separation performance [3]. The mechanical properties of ion exchange membranes mainly require fracture strength, which mainly depends on the inherent structure of the ion exchange membrane, especially its degree of polymer polymerization (crosslinking degree). However, the crosslinking degree should not be too high. Within an appropriate range, the higher the crosslinking degree, the better its mechanical properties will be. However, if it exceeds the maximum limit that the ion exchange membrane can withstand, it will cause the membrane itself to lose its flexibility and become brittle, thereby losing its ion exchange capacity.

The electrochemical performance of ion exchange membranes mainly depends on their surface resistance, which can reflect the conductivity and ion penetration ability of ion exchange membranes. The faster the ion penetrates the ion exchange membrane, the smaller its resistance and better its electrochemical performance. Conversely, the worse its conductivity and ion penetration ability.

The ion separation ability of ion exchange membranes depends on the selective permeability of ion exchange groups within the membrane towards opposite charged ions and the repulsive performance towards ions with the same charge. The permeability of this choice largely depends on the number of active groups contained in the ion exchange membrane and the water content of the ion exchange membrane. Generally speaking, the more active groups there are, the stronger their selective permeability. The higher the water content of the ion exchange membrane, the more free water molecules there are, which to some extent causes the same charged ions to pass through the ion exchange membrane, resulting in a decrease in its selective permeability.

There are many specific applications of ion exchange membranes in the field of water treatment. Below are also some examples of using ion exchange membranes for electrodialysis to achieve water treatment and future development directions. Electrodialysis can be used for desalination of brackish water. As a common water source in the northwest inland region, drinking brackish water directly is not conducive to health. Long term use for agricultural irrigation can cause crop yield reduction or even withering. Therefore, using ion exchange membranes for desalination of brackish water is crucial for alleviating freshwater shortages in inland areas and ensuring water safety. However, membrane fouling is an important factor that restricts the development of brackish water desalination. Membrane fouling can significantly increase membrane surface resistance, and even reduce membrane desalination performance and service life. Currently, membrane surface modification or optimization of membrane production processes are mainly used to improve membrane fouling resistance. Compared to cation exchange membranes, the surface fouling and pollution problems of anion exchange membranes are particularly serious. This is mainly because most natural organic pollutants in brackish water have negative charges and are easily adsorbed on the surface of anion exchange membranes with positive fixed groups through electrostatic attraction. Therefore, the anti pollution performance can be significantly improved by increasing the negative charge density and hydrophilicity of the membrane surface, reducing the roughness of the membrane surface, and other methods. In order to improve the anti fouling performance of anion exchange membranes, negative polymeric electrolyte modification is usually applied to the surface of the membrane to prevent membrane fouling through electrostatic repulsion. Commonly used modified materials include dopamine, sodium 4-phenylenesulfonate, metal organic frameworks (MOFs), zwitterions, graphene oxide, molybdenum disulfide, etc. However, this method of modifying through negative polymeric electrolytes undoubtedly increases membrane resistance, leading to a decrease in desalination performance and an increase in desalination energy consumption. Meanwhile, the electrostatic neutralization reaction of negative polymers with positive fixed groups can also lead to a decrease in ion exchange capacity. Therefore, balancing the improvement of anti pollution performance and the decrease of desalination performance during the modification process is an important development direction in the field of using ion exchange membranes for desalination of brackish water in the future.

In addition, the use of ion exchange membranes for fluoride ion removal, as mentioned earlier, is also a very important application. Due to the fact that fluoride ions are an important pollutant in drinking water, appropriate measures need to be taken to remove excessive fluoride in drinking water based on human health and the maximum allowable fluoride ion mass concentration (1.0-1.5 mg/L) in various countries around the world. The removal technology of fluoride ions has always been one of the challenges in drinking water treatment technology. Electrodialysis technology has gradually matured in its development process, and it can not only be used to treat high salt wastewater, but also has been well studied in the treatment of low concentration drinking water, such as treating fluorinated groundwater. However, there is still a problem of difficulty in achieving selective removal of low concentration fluoride ions during electrodialysis treatment of fluorinated groundwater. Research has found that the removal efficiency of fluoride in fluorinated groundwater using electrodialysis depends on factors such as the operating conditions of the electrodialysis device, the properties of the raw water, and the properties of the ion exchange membrane. Therefore, higher fluoride removal efficiency can be achieved through parameter optimization and membrane optimization of the influencing factors of electrodialysis, and the competitive migration rate of fluoride in the co removal process can be improved to enhance the selective fluoride removal effect of electrodialysis. Of course, ion exchange membrane water treatment is also widely used in high salt wastewater, nitrogen and ammonia wastewater treatment, heavy metal wastewater, radioactive wastewater, organic wastewater, etc., playing an increasingly important role in these fields.

At present, the development of high-performance ion exchange membranes is the key to the advancement of electrodialysis technology and the use of electrodialysis for water treatment. An ion exchange membrane with selectivity for ions with the same electrical properties but different valence states can effectively improve the efficiency of electrodialysis separation. In recent years, the emerging membrane materials in China have to some extent promoted the development of electrodialysis technology. Ion exchange membranes with high selectivity and good pollution resistance will be the focus of development and research in electrodialysis technology. The application of ion exchange membranes for electrodialysis in wastewater resource utilization and resource recovery will also receive more attention in the future.

更多阅读：

液流电池用离子交换膜的国产化及非氟化进展

离子交换膜在液流电池及燃料电池的技术及市场概况

氢燃料电池膜的要求与进化过程

低成本电解水制氢的阴离子交换膜机理及类型

用于电解水制氢的质子交换膜机理及类型

Reference materials:

[1] Guan Wenxue, Wang Sanfan, Li Yanhong. Overview of the Development and Future Applications of Ion Exchange Membranes [J]. Applied Chemistry, 2019,48 (04): 888-892

DOI: 10.16581/j.cnki.issn1671-3206.20190125.002. (School of Environmental and Municipal Engineering, Lanzhou Jiaotong University; Ministry of Education Engineering Research Center for Comprehensive Utilization of Water Resources in Cold and Drought Regions

[2] Luo Sheng, Zhu Ming, Tian Binghui, Theekshana MALALAGAMA. Research progress and main influencing factors of fluoride removal in electrodialysis water treatment [J/OL]. Industrial water treatment: 1-21 [2022-06-26] DOI: 10.19965/j.cnki.iwt.2021-1159. (Ecological Environment Research Center, Chinese Academy of Sciences, State Key Laboratory of Environmental Water Quality; School of Environment and Municipal Engineering, Tianjin Urban Construction University, Tianjin Key Laboratory of Water Quality Science and Technology

[3] Liu Yuping Preparation of Polypropylene Homogeneous Ion Exchange Membrane and Its Application in Electrodialysis [D]. Suzhou University, 2020. DOI: 10.27351/d.cnki.gszhu.2020.001837. (Suzhou University)

[4] Dong Lin, Chen Qingbai, Wang Jianyou, Li Pengfei, Wang Jin. Research progress in electrodialysis brackish water desalination technology [J]. Chemical Progress, 2022, 41 (04): 2102-2114

DOI: 10.16085/j.issn.1000-6613.2021-0811. (Tianjin Key Laboratory of Cross Media Composite Pollution Environmental Control Technology, School of Environmental Science and Engineering, Nankai University)