Abstract 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.

中文翻译：

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从富含 Mg2+ 的盐水中选择性去除单价离子的渗析-纳米过滤-电渗析混合工艺

摘要 开发并测试了混合纳滤-电渗析工艺，用于从海水中高选择性分离 MgSO4。生产富含镁的溶液主要是为了在海水淡化厂的后处理步骤中将 Mg2+ 补充到淡化水中。新的三步工艺包括：(1) 海水纳滤 (NF) 以产生富镁滞留物；(2) Dia-NF 步骤，其中使用 NF 截留物作为进料，旨在最大限度地减少不需要的溶质（主要是 B、Na+ 和 Cl-）；(3) 对来自步骤 2 的渗余物进行选择性电渗析 (ED) 步骤，旨在进一步降低 Cl-:Mg2+ 质量比至低于 0.2，以生产高纯度富含 MgSO4 的产品溶液。结果以 ED 步骤为重点，其中流速的影响，评估了电流密度和两种操作模式（恒流和恒电位）。结果显示单价选择性离子交换膜对细胞电位的高度依赖性选择性，表明它可以作为选择性 ED 过程的微调工具。将 20 mgMg/L 添加到 1 m3 淡化水中的成本估计约为 0.014 美元/m3，与其他分离技术相比具有竞争力，当然也与直接添加食品级镁盐的替代方案相比。