Abstract

Several harmful or valuable ionic species found in seawater, brackish water and wastewater are weak amphoteric acids or bases, and therefore their properties are dependent on the local pH of the water. Efficient removal of these species can be difficult for conventional membrane technologies, requiring chemical dosing of the feedwater to adjust the pH. A striking example is boron, which is considered toxic at high concentrations and often requires additional membrane passages to be removed during seawater desalination. Capacitive deionization (CDI) is an emerging membrane-less technique for the treatment and desalination of water, based on the electrosorption of salt ions in microporous charging electrodes. CDI cells exhibit large pH changes generated internally during operation and therefore CDI can potentially eliminate pH dependent species without chemical assay. However, the development of this technique is inhibited by the complexities inherent in the coupling of pH dynamics and ionic properties in a CDI cell under charge. Here, we present a theoretical framework predicting the electrosorption of pH-dependent species in continuous flow electrode CDI cells. We demonstrate that such a model provides a better understanding of the factors affecting the electrosorption of species and conclude that the important design rules for such systems are very counterintuitive. For example, we show both theoretically and experimentally that for boron removal, the anode must be placed upstream and the cathode downstream, an electrode order that goes against popular belief in the field. permanent contracts. Overall, we show that to achieve target separations based on complex coupled phenomena, such as amphoteric species removal, a theoretical CDI model is essential.