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.