Ion-selectivity advancements in capacitive deionization: A comprehensive review

In the past decade, the applications of capacitive deionization have expanded to encompass resource recovery and targeted ion separation in complex solutions, besides water desalination. This comprehensive review discusses the underlying mechanisms of selective ion removal from mixed salt solutions using various electrode materials and membranes. Various mechanisms of ion selectivity are explored, considering different factors (for example, electrode pore characteristics, ion valence, operational parameters) and different chemical modifications. It also investigates ion selectivity in flow-electrode capacitive deionization. A meticulous analysis of the relevant literature from the first publications to the most recent findings allowed describing the progress in electrode and membrane preparation, theoretical knowledge, and respective roles. Furthermore, this review explores the definition and utilization of ion selectivity in different studies. Finally, a broad spectrum of ions is covered, examining the selective removal mechanisms employed for each of them. This review presents for the first time, an extensive evaluation of selective removal methods for different ions (uranium, arsenic, copper, sulfate, nitrate, phosphate, and lithium) in the capacitive deionization field.