Abstract

Water purification remains a challenge across sectors worldwide, especially the efficient removal of specific (toxic or valuable) dissolved ions at low salinity. In this article, shock electrodialysis (SED) is shown for the first time to have this capability, by demonstrating continuous separation of magnesium ions from aqueous mixtures of NaCl and MgCl 2 . By systematically measuring the composition of all input and output streams, the mechanisms that drive selectivity, current efficiency, and desalination are revealed, as well as strategies to improve performance. For solutions initially rich in sodium, highly selective (> 98%) continuous removal of magnesium can be achieved with only moderate (50–70%) total salt removal. This remarkable selectivity is associated with super‐diffusive ion transport, mediated by charged double layers in a porous glass frit, behind a steady deionization shockwave in cross flow.