Modern technology makes it possible to chlorinate water for disinfection without using a single gram of chlorine. The substances needed are produced by electrolysis and special catalysts. And the use of advanced metals promises to make the process both cheaper and more efficient.
Not just drinking water, but any water that people come into contact with, whether in a swimming pool or during wet cleaning, needs to be disinfected. Chlorine, a mixture of hypochlorite and other chlorine salts, a pungent smell that has been part of our daily lives since most of us were children, is widely used around the world for this purpose. Chlorite dissolves in water to form highly active ions that kill bacterial cells and dangerous protozoa. Neither ozonation nor ultraviolet radiation have this effect and can only be used as an adjunct to the main technique, chlorination.
This treatment is harmless to humans if used with care. The permitted concentration of hypochlorite in pure chlorine is 4 mg/litre, although in practice the maximum concentration is 1.5 mg/l and often less, depending on the level of contamination. However, it is not a very pleasant substance to work with. Concentrated hypochlorite is toxic, carcinogenic, environmentally hazardous and corrosive. That is why a new method of water disinfection by electrolysis has been gaining momentum in recent years, and there is a market for it both worldwide and in the Russian Federation.
Common salt, a cheap and completely safe substance, is added to the water to be treated. An electric current is passed through electrodes into the water tank. The chloride ions are converted into active chlorine, which is the disinfectant. All that is needed for the reactions to take place are special electrodes made of a specific metal with a special catalytic layer on their surface. One of the advantages of this technology is that it allows the concentration of disinfectant ions to be flexibly adjusted simply by regulating the intensity of the current applied to the electrolyser unit.
“This electrochemical technology is advantageous because, firstly, there is no need to transport and store large stocks of pre-prepared sodium hypochlorite,” says Sergei Saltykov, product manager for catalysts at Nornickel. “Secondly, the chlorine is not added to the water, which means you don’t have to employ a special person to do it and worry about their safety. It also eliminates many environmental issues, such as the disposal of unused reagent and many others. The disinfectant is produced on site in exactly the amount needed”.
The catalyst problem
But this technology cannot work without the necessary catalysts. The electricity in the aqueous solution of table salt causes other side processes to occur, and the catalysts are the ones that accelerate and “amplify” the required processes many times over. The platinum group metals, in particular ruthenium, iridium and/or platinum itself, are used for this purpose. They do not react chemically and are hardly consumed in the process, which means that these valuable elements are needed in very small quantities to form a thin coating on the surface of the robust titanium electrodes. But there is another metal in the platinum family that has very promising catalytic properties: palladium.
According to project manager Dmitry Korolev, the fact that palladium is not widely used in electrolysers for water disinfection is only temporary. “To create a competitive technology, you need to experiment, and this metal is more expensive than platinum,” Dmitry explains. “When there is a ready and proven solution, few are willing to spend money to develop a new one. It is a very ambitious task to combine the high catalytic activity of palladium with the required lifetime characteristics in a single product. In this sense, we are lucky, because Nornickel is not only a palladium producer, but also has a direct interest in making products that are technologically more sophisticated and have a higher return on investment”.This work has recently started in collaboration with the scientific community. So far, the scientists are busy determining the optimal composition of a catalyst containing palladium in combination with other platinum metals, and finding the best technology for manufacturing electrodes with a palladium-containing catalytic layer. However, this phase is expected to be completed by the end of 2023 and the catalyst will be sent for “field” testing through a company that manufactures electrolysers for water treatment. The finished solution will then be scaled up for mass application.
Beyond the pool
It is estimated that the use of palladium-based catalysts will require a modest amount of palladium, at least compared to the amount available in Russia. When the electrodes reach the end of their useful life – which is likely to be several years – they will need to be replaced. In the meantime, other valuable materials can be recycled and partially recovered for reuse. The main benefit, however, is that the new palladium-based catalysts allow water treatment to be carried out more quickly and efficiently.
This means that despite the higher cost of the metal itself, the process will be more cost-effective and profitable. “But we are not just talking about a higher speed of water treatment, we are potentially talking about a higher degree of purification. So far we are only talking about water disinfection for swimming pools, homes and water treatment plants. This technology is just gaining momentum, but over time it will become mainstream,” concludes Sergey Saltykov. “And in the future, why not try to produce drinking water?”