Membrane Charge Weakly Affects Ion Transport in Reverse Osmosis
Membrane Charge Weakly Affects Ion Transport in Reverse Osmosis
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DOI：
10.1021/acs.estlett.0c00291
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发表时间：
2020-06-09
影响因子：
10.9
通讯作者：
Freger, Viatcheslav
中科院分区：
文献类型：
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作者：
Stolov, Mikhail;Freger, Viatcheslav
聚酰胺反渗透（RO）膜一直是水脱盐的“黄金标准”，芳香族聚酰胺层提供了盐截留率和水渗透率之间的良好平衡。然而，在反渗透中的盐传输的理解仍然存在差距，密切相关的性质和形成的膜电荷及其对离子渗透的影响。在这里，我们报告了一个系统的调查，检查标称膜电荷之间的相关性，确定通过离子结合方法，和关键的离子渗透特性，盐渗透性，和膜电导率，在不同的pH值。在中等pH值范围内，最重要的脱盐实践中，观察到的电导率，盐渗透率和标称电荷之间的关系显示出比唐南模型预测的弱得多的依赖。这表明聚酰胺膜内的固定带电基团在很大程度上失活，从而膜表现为有效中性或弱带电。然而，通过吸收H+和OH-离子，在极端酸性和碱性pH下可能形成大量电荷，这增加了离子吸收和电导率，促进了聚合物溶胀并削弱了盐排斥。总的来说，结果强烈表明，固定电荷在反渗透中的盐截留率的影响很弱，它是由介电和空间位阻机制。
Polyamide reverse osmosis (RO) membranes have been a "golden standard" in water desalination, with an aromatic polyamide layer providing an excellent balance between salt rejection and water permeability. Yet, there still are gaps in understanding the salt transport in RO, closely related to the nature and formation of membrane charge and its effect on ion permeation. We report here a systematic investigation that examines the correlation between the nominal membrane charge, determined by ion-binding methods, and key ion permeation characteristics, salt permeability, and membrane conductivity, at different pH values. In the mid-pH range, the most important in the desalination practice, observed relations between the conductivity, salt permeability and nominal charge show a much weaker dependence than the Donnan model predicts. This indicates that fixed charged groups inside polyamide films are largely deactivated, thereby the membrane behaves as effectively neutral or weakly charged. However, a substantial charge may form at extreme acidic and basic pH via uptake of H+ and OH- ions, which increases ion uptake and conductivity, promoting polymer swelling and weakening salt rejection. Overall, the results strongly suggest that fixed charges weakly affect salt rejection in RO, and it is dominated by dielectric and steric mechanisms.