Membrane capacitive deionization (MCDI) for selective ion separation and recovery: Fundamentals, challenges, and opportunities

Membrane capacitive deionization (MCDI) has attracted much attention to selective separation and recovery of various ionic species due to its simplicity, environmental friendliness, excellent removal efficiency, and low energy consumption. However, there have been very few review papers focusing on the challenges and opportunities in this field. Therefore, this review comprehensively summarizes the basic principles and significant development of MCDI processes. The potential applications of MCDI processes in selective separation and recovery of ionic species, e.g., heavy metal removal, water softening, desalination, selective anion decontamination, and resource recovery, are highlighted and benchmarked using various performance indicators. In addition, challenges and novel countermeasures of MCDI processes are further summarized. Future research needs for MCDI processes to reach full-scale potential in practical water treatment are suggested accordingly. This review will provide a solid basis to guide the future development of MCDI-based water treatment technology.