Department of Civil, Construction, and Environmental Engineering, North Carolina State University, 2501 Stinson Drive, Raleigh, NC, 27695, United States; Department of Civil Engineering, King Saud University, Riyadh, 11421, Saudi Arabia.
Department of Civil, Construction, and Environmental Engineering, North Carolina State University, 2501 Stinson Drive, Raleigh, NC, 27695, United States.
Water Res. 2020 Sep 15;183:116044. doi: 10.1016/j.watres.2020.116044. Epub 2020 Jun 16.
Capacitive deionization (CDI) is an electrochemical method of removing salt ions from brackish water. A common assumption in CDI is that monovalent ions (e.g., Na, Cl) are removed in a 1:1 symmetry on the electrodes. Validation of this assumption with techniques such as ion chromatography is not commonly performed, but is important to better understand how parasitic process, such as faradaic reactions, affect ion removals. In this study, we quantified the removals of Na and Cl as a function of electrode orientation in flow-through CDI. When the cathode was positioned upstream, Na and Cl removals approached a 1:1 symmetry, but when the anode was located upstream, we observed a significant drop in Na, but not Cl, removals. We attributed this drop to oxygen reduction reactions at the cathode that competed with Na adsorption. Oxidation of carbon in the upstream anode yielded H that enhanced the reduction of oxygen to HO at the downstream cathode, which in turn diverted electrons from Na adsorption. In the absence of oxygen, Na removals increased in the upstream anode orientation and were comparable to Cl removals, confirming that competition with oxygen reduction reactions was the primary reason for decreased Na removal. In the upstream cathode orientation, we show that HO generated at the cathode can be oxidized at the downstream anode, possibly enhancing Na removals via internal electron recycling. Salt adsorption capacities calculated using actual ion removals did not always agree with those estimated using changes in solution conductivity, with the largest disagreement observed when conductivity data were corrected for pH changes. Our results highlight that faradaic reactions, particularly oxygen reduction reactions, can contribute to asymmetrical removals of monovalent ions in flow-through CDI.
电容去离子 (CDI) 是一种从咸水中去除盐离子的电化学方法。CDI 中的一个常见假设是单价离子（例如 Na、Cl）在电极上以 1:1 的对称性去除。虽然离子色谱等技术通常不会验证此假设，但了解寄生过程（例如法拉第反应）如何影响离子去除对于更好地理解这一点很重要。在这项研究中，我们通过流动式 CDI 定量研究了电极取向对 Na 和 Cl 去除的影响。当阴极位于上游时，Na 和 Cl 的去除接近 1:1 的对称性，但当阳极位于上游时，我们观察到 Na 的去除显著下降，但 Cl 的去除没有下降。我们将这种下降归因于阴极处与 Na 吸附竞争的氧还原反应。上游阳极中碳的氧化产生 H，增强了下游阴极中氧气还原为 HO，从而将电子从 Na 吸附中转移出来。在没有氧气的情况下，上游阳极取向的 Na 去除增加，与 Cl 去除相当，证实与氧还原反应的竞争是 Na 去除减少的主要原因。在上游阴极取向中，我们表明在阴极生成的 HO 可以在下游阳极被氧化，通过内部电子回收可能增强 Na 的去除。使用实际离子去除计算的盐吸附容量并不总是与使用溶液电导率变化估计的容量一致，当对电导率数据进行 pH 变化校正时，观察到最大的不一致。我们的结果强调了法拉第反应，特别是氧还原反应，可导致流动式 CDI 中单价离子的非对称去除。
