High-Efficiency Separation of Mg²⁺/Sr²⁺ through a NF Membrane under Electric Field

The efficient separation of Sr²⁺/Mg²⁺ through nanofiltration (NF) technology is a great challenge because Sr²⁺ and Mg²⁺ ions are congeners with the same valence and chemical properties. In this work, an NF membrane under an electric field (EF) was successfully employed to separate Mg²⁺ and Sr²⁺ ions for the first time. The effects of current densities, Mg²⁺/Sr²⁺ mass ratios, pH of the feed, and coexisting cations on separation performance were investigated. Dehydration of Sr²⁺ or Mg²⁺ ions under EF was proved by molecular dynamics simulation. The results showed that a high-efficient separation of Mg²⁺/Sr²⁺ was achieved: Mg²⁺ removal of above 99% and increase in Sr²⁺ permeation with increasing EF. A separation factor reached 928 under optimal conditions, far higher than that without EF. The efficient separation of Mg²⁺/Sr²⁺ ions was mainly due to rejection of most Mg²⁺ ions by NF membrane and in situ precipitation of partly permeated Mg²⁺ ions by OH^- generated on the cathode under EF. Meanwhile, preferential dehydration of Sr²⁺ ions under EF due to lower hydration energy of Sr²⁺ compared with Mg²⁺ resulted in an increase of permeation of Sr²⁺ ions. This work provided a new idea for separation of congener ions with similar valence and chemical properties.