Visualizing the local ion concentration in electrodialysis cells via magnetic resonance imaging

Electrodialysis (ED) offers a promising solution to address global freshwater shortages and prevent water pollution caused by brackish wastewater from industrial plants. However, local ion concentrations in an opaque ED module cannot currently be measured, even though this information is essential for efficient desalination.

Here, we introduce Magnetic Resonance Imaging (MRI) as an investigative technique to reveal the concentration profiles within ED modules. The signal intensity correlates with the local copper concentration, enabling us to reconstruct the copper distribution inside the module. In our setup, we used a platinum-coated titanium mesh as the anode and a copper mesh as the cathode. These materials are electrochemically stable and minimize disturbances to the tomograph’s magnetic field. In our example case, we applied a current density of 50 mA cm^-2 with a flow rate of 0.1 mL min^-1. The MRI measurement successfully showed desalination and concentration along the channel length, where the concentration in one diluate channel unexpectedly exhibited a local peak. However, the resolution of the utilized low-field tomograph was not sufficient to image the concentration polarization. Utilizing the proposed desalination module in a high-field tomograph with a higher resolution can deepen our understanding of the in situ process and pave the way for observation-based optimization including the boundary layer at the membranes and unexpected concentration profiles along the channel length.