Data underlying the research of Enhancing Electrodialysis using Spacer Electrokinetic Effects: The Influence of Bivalent Ions on Spacer Performance

Concentration polarization in electrodialysis (ED) stacks can significantly reduce the process efficiency in the separation of ions. We previously investigated an innovative way of reducing the concentration polarization by inducing electro-osmotic mixing through the use of surface-modified spacers and tested this approach using single monovalent salt streams. Typically, more complex mixtures containing multivalent ionic species occur in practical settings which may lead to divalent ion adsorption on spacers and a reduction in electro-osmotic mixing, therefore reducing the improvement in efficiency observed for the monovalent-only case. To further test the effectiveness of using surface-modified spacers for enhancing electrodialytic transport, we have studied the influence of bivalent cations on the resulting ED process using polyelectrolyte-coated spacers. For this, we compared the monovalent feed solutions with feeds containing either bivalent ions or a mixture of monovalent and bivalent salts. The results showed that when using surface-modified spacers in ED with a mixture of bivalent and monovalent salts (MgCl$_2$ + NaCl) in the feed, a similar enhancement to the monovalent ion only case was obtained whereas when using a feed consisting of only a bivalent salt (MgCl$_2$) there was a negligible difference between the performance of surface-modified and conventional spacers. We show that the loss in performance of surface-modified spacers can be plausibly attributed to a reduction in the zeta potential (surface charge) in the presence of divalent cations leading to a reduction in the electro-osmotic remixing of the boundary layer, obtaining the behaviour of an ED process using uncoated spacers. Our findings also indicate that this performance loss in using surface-coated spacers can be mitigated or avoided if there is an additional monovalent cation in the mixture at similar concentrations. This work highlights that the use of surface-modified spacers can be a viable option for enhancing electrodialysis for realistic feed sources consisting of mixtures of mono and higher valence ions.

电渗析（electrodialysis, ED）膜堆内的浓差极化（concentration polarization）会显著降低离子分离过程的效率。我们此前曾研究出一种可通过使用表面改性隔板（surface-modified spacers）诱导电渗混合（electro-osmotic mixing）以缓解浓差极化的创新方法，并采用单一一价盐（monovalent salt）流对该方法进行了测试。实际应用场景中通常存在更为复杂的多价离子组分（multivalent ionic species）混合体系，这可能导致二价离子吸附（divalent ion adsorption）在隔板表面，削弱电渗混合效果，进而使仅针对一价离子体系的效率提升幅度下降。为进一步验证表面改性隔板用于强化电渗析传质的有效性，我们针对聚电解质涂层隔板（polyelectrolyte-coated spacers），研究了二价阳离子（bivalent cations）对ED过程的影响。为此，我们将纯一价进料溶液，与含二价离子的进料溶液以及一价-二价混合盐进料溶液进行了对比测试。结果表明，当进料为二价与一价盐的混合体系（氯化镁MgCl₂ + 氯化钠NaCl）时，采用表面改性隔板可获得与纯一价离子体系相当的强化效果；而当进料仅为二价盐（氯化镁MgCl₂）时，表面改性隔板与传统隔板的性能差异可忽略不计。我们认为，表面改性隔板性能下降的合理解释为：二价阳离子存在时，泽塔电势（zeta potential，即表面电荷）降低，导致边界层（boundary layer）的电渗再混合效果减弱，最终呈现出与未涂层隔板（uncoated spacers）一致的电渗析过程行为。研究还发现，若混合体系中存在浓度相当的额外一价阳离子，则可缓解甚至避免表面涂层隔板的性能损失。本研究表明，对于包含一价与高价离子混合体系的实际进料源，采用表面改性隔板是强化电渗析过程的可行方案。