Russian scientists improve water purification membranes using metal ions

Researchers have proposed using polymer membranes modified with copper, zinc, and chromium metal ions for water purification. These polymers were used for the first time in water purification via electrodialysis. Copper-based membranes demonstrated record selectivity for monovalent ions, opening new possibilities for sustainable water recycling. The study has been published in the Journal of Membrane Science. 

Electrodialysis is a water purification method that removes contaminants without chemical reagents by moving ions through a membrane under an electric field. The efficiency of purification depends on the membrane’s properties, particularly its selectivity—the ability to allow certain ions to pass while blocking others. Polymer membranes typically allow multivalent ions, such as sulphates, to pass through more easily than monovalent ions like nitrates. This results in precipitate forming on the membrane surface, which block ion transfer and can lead to system failure. Therefore, scientists are developing ways to enhance membrane selectivity for monovalent ion transport.

Electrodialysis membranes are usually made of polymers containing channels with charged walls that selectively permit ions to pass. A conventional polymer consists of a chain of carbon atoms with charged functional groups attached via strong covalent bonds. These groups are part of the polymer’s structure because they are fixed to its side chain.

HSE scientists have demonstrated how a simple modification can enhance the selectivity of polymer membranes. They used polybenzimidazole (PBI)-based membranes embedded with metal ions—copper, zinc, and chromium. In this structure, metals bonded with nitrogen atoms from the polymer through coordination bonds, which are easier to form and break compared to strong covalent bonds. This property allows the material’s structure to be modified and adapted for specific applications. 

‘A typical polymer can be imagined as a string of lights, where the main chain is the wire and the functional groups are bulbs attached to it. In regular membranes, the bulbs remain fixed in place, whereas in our case, they appear to levitate. We can adjust their number by adding more or fewer metal ions during synthesis, thus regulating the membrane’s properties,’ explains study author Andrey Manin, a master's student in the Chemistry of Molecular Systems and Materials programme at the HSE Faculty of Chemistry and a researcher at the Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences (IGIC RAS).

Copper-ion-enhanced membranes demonstrated record-breaking selectivity in separating nitrates from sulphates. Scientists suggest two possible explanations. Firstly, the ion channel size within the membrane structure is ideally suited for nitrate transport, while larger sulphate ions get trapped. Secondly, nitrate ions may interact with metal ions in the coordination sphere, facilitating their passage through the membrane.

Experiments confirmed that copper-infused membranes are not only effective but also stable, retaining their properties over extended use. This is a significant advantage over zinc-based membranes, where zinc ions leached from the polymer matrix, contaminating water with heavy metals. Scientists are exploring several ways to scale the technology for industrial application.

‘In our experiment, we used substituted polybenzimidazole synthesised by our colleagues at INEOS RAS. However, for industrial production, a more accessible and widely manufactured polymer, such as unsubstituted polybenzimidazole, could be used. Another option is to apply a thin layer of the synthesised polymer with metal ions onto commercially available electrodialysis membranes. This approach would enhance selectivity while maintaining conductivity due to the base material. Since these membranes are already produced using well-established processes, adding one more stage would not significantly impact production costs but could greatly improve membrane efficiency,’ says Andrey Manin.