Dia-nanofiltration-electrodialysis hybrid process for selective removal of monovalent ions from Mg²⁺ rich brines

A hybrid nanofiltration-electrodialysis process was developed and tested for highly-selective separation of MgSO₄ from seawater. The magnesium-rich solution was produced primarily for replenishing Mg²⁺ into desalinated water within the post-treatment step in seawater desalination plants. The new three-step process consists of: (1) seawater nanofiltration (NF) for generation of a magnesium-rich retentate; (2) a Dia-NF step in which the NF retentate is used as feed, which is aimed at minimizing unwanted solutes (primarily B, Na⁺ and Cl⁻); and (3) a selective electrodialysis (ED) step on the retentate from step 2, aimed at further lowering the Cl⁻:Mg²⁺ mass ratio to below 0.2, to produce a high-purity MgSO₄-rich product solution. Results are presented with an emphasis on the ED step, for which the effects of flow velocity, current density and two operation modes (constant-current and constant-potential) were assessed. The results show a highly-dependent selectivity of the monovalent-selective ion-exchange membrane to the cell potential, suggesting that it can serve as a fine-tuning tool for selective ED processes. The cost of adding 20 mgMg/L to one m³ of desalinated water was estimated at ~$0.014/m³, which is competitive compared to other separation techniques, and certainly versus the alternative of direct dosage of food-grade magnesium salts.