Electrochemical removal of amphoteric ions
Water treatment is necessary for a sustainable supply of drinking water and can be used to harvest valuable elements. The removal of charged, pH-dependent species from polluted water, such as boron, ammonia and phosphate, is crucial for these processes. These species can represent a challenge for conventional technologies. Currently, the removal of boron requires several reverse osmosis steps, combined with the dosage of a caustic agent. Capacitive deionization (CDI) promises to allow the efficient removal of these species without chemical additives, but requires a deep understanding of the coupled interaction of pH dynamics, ion electrosorption and transport phenomena. Here, we provide a detailed theory addressing this topic and show both theoretically and experimentally very counterintuitive design rules governing the removal of pH-dependent ions by CDI.
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.
Author contributions: research designed by ANS, ENG, HHMR, PMB, MES and JED; ANS, ENG, PMB, MES and JED carried out research; ANS, ENG, PMB, MES and JED contributed new reagents / analysis tools; ANS, ENG, PMB, MES and JED analyzed the data; and ANS, ENG, HHMR, PMB, MES and JED wrote the article.
The authors declare no competing interests.
This article is a direct PNAS submission.
This article contains additional information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2108240118/-/DCSupplemental.
All study data is included in the article and / or additional information.