High selectivity does not always translate into effective water treatment or the recovery of valuable resources. An international research team, including scientists from Wrocław University of Science and Technology, has proposed a new way to evaluate selective ion separation processes.

The approach is described in the paper “Comparative analysis of selective ion removal in electrochemical technologies reveals the need for a new data representation framework”, published in Nature Water.

Wrocław Tech is represented in the research team by Piotr Śledzik, MSc Eng., and Associate Professor Sławomir Porada from the Faculty of Chemistry. The study was carried out in collaboration with researchers from Wageningen University & Research, Ben-Gurion University of the Negev and Wetsus, European Centre of Excellence for Sustainable Water Technology.

One number is not enough

Selective ion separation is essential for water treatment and reuse, as well as for recovering valuable substances from solutions. The aim may be either to remove an unwanted ion from water or to recover a valuable component from a mixture.

Circular settling tanks at a wastewater treatment plant. Photo: Adobe Stock.

– These processes are often evaluated using a selectivity coefficient, known as the separation factor. The problem is that high selectivity does not necessarily mean effective separation – explains Prof. Porada.

A good example is the removal of a toxic ion from water that also contains other, harmless ions. A process may demonstrate very high selectivity towards the target ion while removing only a small fraction of it. This means that the separation factor alone is not enough to determine whether water treatment is genuinely effective.

– We have therefore proposed a way of presenting results based on changes in the concentrations of individual ions. This makes it possible to assess how much of a particular component has actually been removed or recovered, while also determining parameters such as selectivity, product purity and water recovery. As a result, different separation technologies can be compared more easily and reliably – says Prof. Porada.

The most important parameters depend on the purpose of the process. In water treatment, the key factors are contaminant removal efficiency, water recovery and productivity. When the objective is to recover a valuable component, recovery rate, purity and selectivity become more important.

Two technologies, different capabilities

The researchers applied the new approach to compare two electrochemical processes: electrodialysis (ED) and membrane capacitive deionization (MCDI).

Both processes use ion-exchange membranes and electrical energy, but they operate differently. Electrodialysis is a continuous process, while MCDI operates in cycles, with electrodes alternately charged and discharged. During charging, ions are adsorbed by the electrodes and then released during discharging.

This cyclic operation provides an additional way to control the separation process.

– Individual ions may be adsorbed and released from the electrodes at different rates. By choosing the right moment to switch between the collected streams, we can increase the proportion of one ion while reducing the presence of another – explains Prof. Porada.

This means that, in some cases, selectivity can be improved not only by changing the membrane or electrode material, but also by carefully controlling the timing of the process.

– Before such a solution can be implemented on an industrial scale, however, sufficient electrode stability and long-term process reproducibility must be ensured – Prof. Porada points out. – It will also be crucial to gain a better understanding of ion transport mechanisms and find the right balance between selectivity, recovery, productivity and energy consumption.

Where does “infinite selectivity” come from?

The paper also introduces the concept of “infinite selectivity”. This does not, however, mean that the process has infinite efficiency.

– Mathematically, this occurs when only the target ion is removed, while the concentration of the other ion remains unchanged. In such a case, the selectivity coefficient approaches infinity – explains the researcher.

The authors demonstrated that this effect can be achieved in the cyclic MCDI process by carefully selecting when to start and stop collecting individual streams.

For ions with distinctly different transport rates, such as sodium and calcium, these differences in kinetics can be exploited. When ions behave similarly, as in the case of sodium and potassium, selective materials may also be useful.

Cleaner water and resource recovery

Precise ion separation could play an important role in water treatment and reuse, environmental protection and the recovery of valuable substances from wastewater, brines and other industrial streams.

Aerial view of a wastewater treatment plant. Photo: Adobe Stock.

The analytical framework proposed in the paper is intended to make comparisons between different separation technologies more reliable.

– Instead of reducing the evaluation of a process to a single coefficient, we can see both how much of a selected component has actually been removed or recovered and what kind of product has been obtained. This allows us to assess much more accurately whether a given technology meets the requirements of a specific application – concludes Prof. Porada.

Li K., Shocron A. N., Śledzik P., Rijnaarts H. H. M., Porada S. Dykstra J. E. "Comparative analysis of selective ion removal in electrochemical technologies reveals the need for a new data representation framework". Nature Water. 2026.