The history and development of membrane electrolysis for the treatment of drinking water

Today, the need to supply safe drinking water to communities and regions all over the world continues to grow. Although great progress has been made in infrastructures and more efficient water purification systems, factors such as population growth, industrialization, climate change and increasingly severe desertification and drought are all working against Goal 6 of the SDGs: ¨To ensure the availability and sustainable management of water and sanitation for all.¨

Against this backdrop, it has become critical to develop technologies that are efficient and sustainable, both in terms of production and cost, so that access to safe drinking water does not have a negative impact on other environmental considerations.

In this article we are going to look at one of these technologies, which is incorporated into our electrolysis plants at Welysis and which represents a significant innovation in the field of electrochemistry: ion exchange membrane electrolysis. This process offers important advantages over previous methods such as the diaphragm cell process.

What is ion exchange membrane electrolysis?

Ion exchange membrane electrolysis is a method of breaking down chemical compounds into their constituent elements using an ion exchange membrane. This membrane allows for the selective passage of specific ions while preventing the mixing of different reaction products.

In this process, a brine (salt water) solution reacts with water to form chlorine gas, sodium hydroxide (caustic soda) and hydrogen. The brine is fed into an electrochemical cell containing two electrodes (anode and cathode) separated by an ion exchange membrane. Electric current is passed through the solution, triggering reactions that produce the desired elements. The chlorine gas is in turn reacted with the previously obtained sodium hydroxide (caustic soda), producing through a new exothermic reaction in a totally differentiated process, the desired sodium hypochlorite.

Due to its fungicidal and bactericidal properties the main use of sodium hypochlorite is specifically in the purification of water for human consumption, although it is also used in other sectors, for example, as a bleaching agent in the textile industry, or in other sectors for cleaning and disinfecting.

The history and evolution of electrolysis in the production of Sodium Hypochlorite

The conceptual origin of this method can be traced back to the 19th century mercury cell process, also known as the Castner-Kellner process. This was one of the first methods used for brine electrolysis. This process involved a mercury cathode where sodium ions would combine with mercury to form an amalgam or mixture. This amalgam would then react with water to produce sodium hydroxide and hydrogen gas, while chlorine gas would be released at the anode. However, this process brought environmental and health concerns due to the use, disposal and handling of the mercury.

Cell room of a chlorine-caustic soda plant – six banks ok 74 cells each (Edgewood, Maryland). Copyright McGraw-Hill Pub. Co. (Albany, N.Y.).

In the mid-20th century, the diaphragm cell process evolved as an alternative to the mercury cell process. In diaphragm cells, a permeable diaphragm separated the anode and cathode compartments. Sodium ions would pass through the diaphragm to the cathode, where they would combine with water to form sodium hydroxide and hydrogen gas. Chlorine gas was produced at the anode. This process was an improvement over the mercury cell method in terms of environmental impact, as it did not involve the use of mercury, but also had several disadvantages, including the low purity of sodium hydroxide and the need to handle large volumes of hazardous chlorine and hydrogen gas.

The membrane cell process, also known as ion exchange membrane electrolysis, emerged as an advance over the diaphragm cell process. In the late 1970s and early 1980s, membrane electrolysis began to gain acceptance due to its operational and environmental advantages. Ion exchange membranes, originally developed for nuclear power and desalination applications, proved to be extremely effective for ionic separation in electrolysis processes.

Instead of using a physical barrier such as a diaphragm, a selective ion exchange membrane is used to separate the anode and cathode compartments. This allows the selective passage of ions and at the same time prevents mixing of the chemicals produced at each electrode. The membrane cell process is more efficient and environmentally friendly than previous methods, as it reduces energy consumption and eliminates the need for a physical diaphragm.

Benefits of ion exchange membrane electrolysis

  1. Increased product purity: The ion exchange membrane prevents the mixing of reaction products, allowing for the production of high purity sodium hydroxide and chlorine.
  2. Energy efficiency: This process requires less energy compared to the diaphragm cell process, reducing operating costs.
  3. Reduced environmental impact: Membrane technology reduces emissions and hazardous waste, improving environmental safety.
  4. Safer operation: Effective product separation minimizes the risks associated with handling hazardous gases such as chlorine and hydrogen.
  5. Lower maintenance: Ion exchange membranes have a longer service life and require less maintenance than the porous diaphragms used in previous methods.

Applications of ion exchange membrane electrolysis

As mentioned previously, the most relevant application of ion exchange membrane electrolysis is in the indirect production of sodium hypochlorite for the purification of water for human consumption and industrial uses, although it is not the only one. It can also be applied to:

  • Caustic soda production: In addition to sodium hypochlorite, caustic soda is a by-product of our electrolysis plants, and both are essential chemicals in a variety of industries, including papermaking, textiles, detergents and pharmaceuticals.
  • Hydrogen production: Another by-product of our plants, the hydrogen produced can be used as a clean fuel in fuel cells and other industrial processes.
  • Production of Hydrochloric Acid (HCl or Hydrogen Chloride): Can also be obtained indirectly as a downstream process after electrolysis,

It should also be noted that the food industry benefits greatly from electrolysis and its associated by-products for the manufacture of whiteners and sweeteners.

In conclusion it can be said that the ion exchange membrane electrolysis process has revolutionized the electrochemical industry by offering a more efficient, safer and cleaner alternative to the diaphragm cell process. With its numerous benefits and wide applications, this technology remains a cornerstone in the production of essential chemicals and especially in water treatment.