Selective sodium removal with electrodialysis by modifying concentration gradients using EDTA complexation

Summary

Circular water reuse is often limited by the accumulation of harmful ions and the loss of valuable ions during water desalination. Selective removal of specific ions from water is essential but challenging with conventional desalination technologies, especially for ions with similar properties, such as sodium (Na⁺) and potassium (K⁺). In the present study, the use of electrodialysis in combination with EDTA complexation in the concentrate is proposed to selectively remove Na⁺ ions from a multi-ionic solution containing Na⁺, K⁺, and NO₃⁻ ions. Electrodialysis experiments were conducted at lab-scale at different operational conditions (i.e. solution pH, applied voltage, EDTA/Na⁺ ratio and solution ion composition) to evaluate the selectivity of the proposed process. It was found that a high solution pH (>10) and a low applied voltage (<0.3 V per cell pair) is required to maximize the selective transport of Na⁺ ions, while the presence of other metal ions in solution limits the process efficiency. The effect of the proposed process on the transport mechanisms in electrodialysis, which are electromigration, convection and diffusion, was also examined. The provided analysis concluded that the electromigration and convection mechanisms show the largest contribution to the transport of both Na⁺ and K⁺ ions, while the process selectivity is controlled by selective diffusion of ions which is enhanced by EDTA complexation. Finally, the regeneration and recovery of EDTA with acidification was experimentally evaluated at different pH values and different precipitation times; recoveries of >95 % were achieved with an acidic solution with pH < 2 in 30 min.