Ion separations are important for resource recovery, water treatment, and energy production and storage. Techniques such as chemical precipitation, selective adsorption, and solvent extraction are effective, but membranes may separate ions continuously with less waste and lower energy costs. Separation of monovalent and multivalent ions with nanofiltration or electrodialysis membranes already enables water softening and edible salt purification. Similar membranes are attractive as separators in vanadium redox flow batteries. Selective partitioning of divalent counter‐ions into ion‐exchange membranes even allows transport of these ions against their concentration gradients in salt mixtures. However, separations of ions with the same charge is more challenging. Recent research demonstrated highly selective ion “sieving” at small scales. Separations using electrical potentials and differences in ion electrophoretic mobilities are promising, but relatively unexplored. Carrier‐mediated transport affords high selectivity in liquid membranes, but these systems are not very stable, and selective transport via hopping between anchored carriers has proven elusive. Finally, this paper discusses how concentration polarization decreases selectivities in many membrane processes. Although development of selective, inexpensive ion‐separation membranes is a work in progress, successes in water softening and edible salt purification suggests that future selective membranes will serve as complementary methods to traditional purification techniques.

中文翻译：

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膜离子分离

离子分离对于资源回收，水处理以及能源生产和存储非常重要。化学沉淀，选择性吸附和溶剂萃取等技术是有效的，但膜可以连续分离离子，从而减少浪费并降低能源成本。用纳滤或电渗析膜分离单价和多价离子已经可以实现水软化和食用盐净化。类似的膜作为钒氧化还原液流电池中的隔板很有吸引力。将二价抗衡离子选择性地分配到离子交换膜中，甚至可以使这些离子按照盐混合物中的浓度梯度进行运输。但是，具有相同电荷的离子分离更具挑战性。最近的研究表明，小规模的高选择性离子“筛分”。利用电势分离和离子电泳迁移率的差异是有希望的，但尚未得到充分研究。载体介导的转运在液膜中具有很高的选择性，但是这些系统不是很稳定，通过锚定载体之间的跳跃选择性转运已被证明是可望而不可及的。最后，本文讨论了浓度极化如何在许多膜工艺中降低选择性。尽管选择性廉价的离子分离膜的开发正在进行中，但水软化和食用盐纯化的成功表明，未来的选择性膜将作为传统纯化技术的补充方法。载体介导的转运在液膜中具有很高的选择性，但是这些系统不是很稳定，通过锚定的载体之间的跳跃选择性转运已被证明难以实现。最后，本文讨论了浓度极化如何在许多膜工艺中降低选择性。尽管选择性廉价的离子分离膜的开发正在进行中，但在水软化和食用盐纯化方面的成功表明未来的选择性膜将作为传统纯化技术的补充方法。载体介导的转运在液膜中具有很高的选择性，但是这些系统不是很稳定，通过锚定载体之间的跳跃选择性转运已被证明是可望而不可及的。最后，本文讨论了浓度极化如何在许多膜工艺中降低选择性。尽管选择性廉价的离子分离膜的开发正在进行中，但水软化和食用盐纯化的成功表明，未来的选择性膜将作为传统纯化技术的补充方法。