High-Efficiency Separation of Mg(2+)/Sr(2+) through a NF Membrane under Electric Field

The efficient separation of Sr(2+)/Mg(2+) through nanofiltration (NF) technology is a great challenge because Sr(2+) and Mg(2+) 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 Mg(2+) and Sr(2+) ions for the first time. The effects of current densities, Mg(2+)/Sr(2+) mass ratios, pH of the feed, and coexisting cations on separation performance were investigated. Dehydration of Sr(2+) or Mg(2+) ions under EF was proved by molecular dynamics simulation. The results showed that a high-efficient separation of Mg(2+)/Sr(2+) was achieved: Mg(2+) removal of above 99% and increase in Sr(2+) permeation with increasing EF. A separation factor reached 928 under optimal conditions, far higher than that without EF. The efficient separation of Mg(2+)/Sr(2+) ions was mainly due to rejection of most Mg(2+) ions by NF membrane and in situ precipitation of partly permeated Mg(2+) ions by OH(−) generated on the cathode under EF. Meanwhile, preferential dehydration of Sr(2+) ions under EF due to lower hydration energy of Sr(2+) compared with Mg(2+) resulted in an increase of permeation of Sr(2+) ions. This work provided a new idea for separation of congener ions with similar valence and chemical properties.