Ion Exchange Conversion of Na-birnessite to Mg-buserite for Enhanced Cu2+ Removal via Membrane Capacitive Deionization (MCDI)

Layered manganese oxides (LMOs) have recently been demonstrated to be one of the most promising redox-active material platforms for electrochemical removal of heavy metal ions from solution via capacitive deionization (CDI). However, the impacts of phase transformation behaviors of the LMOs minerals, especially during the desalination operations, on the deionization performance of the LMOs/C electrodes have yet to be extensively evaluated. In this study, Mg-buserite derived from ion exchange of fresh Na-birnessite, Na- and K-birnessite were systematically evaluated as active electrode materials for removal of copper (Cu2+) ions from synthetic saline in a symmetric membrane capacitive deionization (MCDI) cell. In the cases of Na+, K+, Mg2+, and Ca2+ ions, the Mg-buserite/C electrode demonstrated the best deionization performance in terms of the salt and/or ion adsorption capacity, electrosorption rate, charge efficiency, and cycling stability, followed by K-birnessite/C, and then Na- birnessite/C. More importantly, the Mg-buserite/C electrode also exhibited the highest Cu2+ ion adsorption capacity (IAC) of 89.3 mg Cu2+/g active materials at a cell potential of 1.2 V in 500 mg L−1 CuCl2 solution, with an IAC retention as high as 96.3% after 60 electrosorption/desorption cycles. The underlying mechanisms for Cu2+ sequestration were investigated via ex-situ X-ray diffraction, indicating that the improving deionization performance toward Cu2+ from solution is mainly attributed to the expanded interlayer spacing through ion exchange of the original stabilizing Na+ ions of Na-birnessite with foreign Mg2+ ions, leading to a phase transformation from Na-birnessite into Mg-buserite that has larger ion diffusion channels and a higher ion storage capacity. Our work has demonstrated that expansion of interlayer spacing of LMOs minerals via ion exchange is a reliable and solid strategy for improving the desalination performance in CDI platforms, and provided insight for the rational design of smart electrodes for CDI applications towards heavy metal ion sequestration.