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（ED）堆中的浓差极化会显著降低离子分离过程的效率。我们此前曾研究过一种创新方法，通过使用表面改性隔网诱导电渗混合来减少浓差极化，并利用单一单价盐流测试了该方法。在实际应用中，通常会遇到含有多价离子种类的更复杂混合物，这可能导致二价离子在隔网上吸附，从而降低电渗混合效果，进而削弱仅含单价离子时所观察到的效率提升。为进一步验证使用表面改性隔网增强电渗析传递的有效性，我们研究了二价阳离子对采用聚电解质涂层隔网的ED过程的影响。为此，我们将单价进料溶液与含有二价离子或单价与二价盐混合物的进料进行了对比。结果表明，当在进料中含有二价与单价盐混合物（MgCl$_2$ + NaCl）的ED中使用表面改性隔网时，获得了与仅含单价离子情况相似的效率提升；而当进料仅由二价盐（MgCl$_2$）组成时，表面改性隔网与传统隔网的性能差异可忽略不计。我们发现，表面改性隔网的性能损失可合理归因于二价阳离子存在下zeta电位（表面电荷）的降低，这导致边界层的电渗再混合减少，从而表现出与使用未涂层隔网的ED过程相似的行为。我们的研究结果还表明，若混合物中存在浓度相近的额外单价阳离子，则使用表面涂层隔网时的这种性能损失可得到缓解或避免。本研究强调，对于由单价与高价离子混合物组成的实际进料源，使用表面改性隔网是增强电渗析性能的可行方案。