Membrane Charge Weakly Affects Ion Transport in Reverse Osmosis,Environmental Science & Technology Letters

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Membrane Charge Weakly Affects Ion Transport in Reverse Osmosis
Environmental Science & Technology Letters ( IF 8.9 ) Pub Date : 2020-04-30 , DOI: 10.1021/acs.estlett.0c00291
Mikhail Stolov ₁ , Viatcheslav Freger _{1,

2,

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Affiliation

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.

中文翻译：

膜电荷弱影响反渗透中的离子转运。

聚酰胺反渗透（RO）膜已成为水脱盐的“黄金标准”，其芳香族聚酰胺层可在脱盐率和透水性之间实现出色的平衡。然而，在理解反渗透中盐的迁移方面仍然存在差距，这与膜电荷的性质和形成及其对离子渗透的影响密切相关。我们在这里报告了一项系统研究，该研究检查了在不同pH值下通过离子结合方法确定的标称膜电荷与关键离子渗透特性，盐渗透性和膜电导率之间的相关性。在海水淡化实践中最重要的中pH值范围内，电导率，盐渗透性和标称电荷之间的关系比Donnan模型预测的依赖性弱得多。这表明聚酰胺薄膜内的固定带电基团被大大失活，因此该膜表现为有效的中性或弱电。但是，在极强的酸性和碱性pH条件下，可能会通过吸收H形成大量电荷⁺和OH ^–离子，可增加离子吸收和电导率，促进聚合物溶胀并减弱盐分排斥。总体而言，结果强烈表明固定电荷对反渗透中的盐分排斥具有微弱的影响，并且它主要受介电和位阻机制的影响。

更新日期：2020-04-30

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