Capacitive deionization for anion removal in water purification: mechanisms, applications, and prospects

The removal of anions from water remains a major challenge in water treatment and water resource protection. Capacitive deionization (CDI) is an energy-saving and eco-friendly ion removal technology and has gained growing attention. Great advances have been made recently in its electrode materials, system structures, and ion storage principles. This review systematically summarizes the recent advances in CDI for anion removal, with particular emphasis on the roles of carbon-based materials, transition metal compounds, and organic electrode materials in removing different anions from water. Unlike previous reviews, this work comparatively analyzes the removal performance of various electrode materials toward different anions and correlates these performances with the corresponding adsorption and charge-storage mechanisms. In addition, the review highlights the important roles of advanced characterization techniques and computational methods in elucidating ion transport and storage behaviors. Furthermore, optimization strategies for CDI system configurations and the challenges associated with practical large-scale applications are discussed. CDI systems boast prominent strengths in selectivity, treatment efficiency, operational stability, and resource recovery. Nevertheless, multiple key obstacles restrict its practical use in complex water environments. These difficulties involve poor long-term electrode stability during dynamic operation, low selective removal capacity amid mixed ions, and the balance between energy efficiency and large-scale application. Addressing these challenges will be essential for the future development and practical application of CDI technologies in water treatment.